Profile Map of Weld Beads and Its Formation Mechanism in Gas Metal Arc Welding

Profile Map of Weld Beads and Its Formation Mechanism in Gas Metal Arc Welding
metals
Article
Profile Map of Weld Beads and Its Formation
Mechanism in Gas Metal Arc Welding
Zhanhui Zhang          and Jiaxiang Xue *
 School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510641,
 China; mezhangzh@mail.scut.edu.cn
 * Correspondence: mejiaxue@scut.edu.cn; Tel.: +86-020-2223-6360
                                                                                                  
 Received: 19 December 2018; Accepted: 27 January 2019; Published: 29 January 2019                


 Abstract: In order to investigate the profile map of weld beads and its formation mechanism in gas
 metal arc welding (GMAW), bead-on-plate welding was carried out with different average currents,
 and the dimensions of a weld bead were measured. The results show that the profile of weld beads
 can be divided into three stages according to the volume relationship between the melted filler metal
 and the weld pool. During the stages, the top surface of the reinforcement consisting of the central
 plane and the side plane mainly goes through inversed-U, W, V, and U shapes, which are mainly
 attributed to the change of flow pattern. Moreover, the role played by the bottom wall in determining
 the flow pattern and the resultant bead profile has been investigated as well. The experiment
 results show that with the bottom wall changing from the solid to the totally melted state, the role
 of redirecting, redistributing and bearing the molten metal played by the bottom wall gradually
 disappears. As a result, the side reinforcement is no longer covered by the liquid flowing backward.
 The top reinforcement is thoroughly collapsed when the width of the bottom reinforcement exceeds
 that of the top one by 1.1 mm.

 Keywords: profile map; gas metal arc welding; reinforcement; flow pattern; weld pool wall




1. Introduction
     Gas metal arc welding (GMAW) is a metal joining process in which droplets are melted at the
tip of electrode then transferred to a weld pool and solidified to form a bead on the base metal [1].
The mechanical properties of a weld bead are significantly affected by its profile, which mainly
comprises depth of penetration, bead width, reinforcement height, and contact angle, etc. The
formation of a bead profile in GMAW is complicated because of the involvement of droplets [2],
which has motivated many researchers to study the bead formation in detail.
     Meng et al. [3] studied the formation mechanism of humping defects in GMAW, which provides
useful knowledge about the reinforcement formation, but the humping has a periodic undulate profile,
which differs from the ordinary bead appearance. Furthermore, the momentum of the backward flow
metal is the direct reason for the humping bead formation by capturing the tracer particle movement
simultaneously [4]. However, the experiment was carried out with the base metal not fully penetrated.
A fully penetrated bead is needed in fabricating metal parts. In regard to the fully penetrated beads,
Liu et al. [5] studied the stabilization of the weld pool through jet flow argon gas backing, but they
did not point out how the jet flow argon affects the bead profile. Moreover, the high crown in a
fully penetrated Mg butt joint weld can be reduced by deepening the groove in the backing plate [6].
Unfortunately, they did not explore the mechanism of how the backing plate affects the reinforcement.
Furthermore, the current literature related to the bead profile sets the welding current in a narrow
range to achieve either a partially penetrated bead or a fully penetrated one instead of a wide range



Metals 2019, 9, 146; doi:10.3390/met9020146                                    www.mdpi.com/journal/metals
Profile Map of Weld Beads and Its Formation Mechanism in Gas Metal Arc Welding
Metals 2019, 9, 146                                                                                                       2 of 10



to achieve beads ranging from a shallow penetration to a collapsed profile, which hinders a more
comprehensive understanding of the whole stages of forming a weld bead.
      Pang et al. [7] pointed out that the formation of a weld bead in the GMAW process is closely
associated with the heat input and the weld pool behavior, which was studied by many researchers.
Wang et al. [4] demonstrated that during the welding process, the surface of a weld pool is significantly
depressed under the combined effects of droplet impingement and the arc pressure, which leaves a thin
liquid pool area between the arc and the base metal. The thin liquid pool and the droplet impingement
plays a significant role in determining the flow pattern in the weld pool. When the accelerated droplet
impinges into the thin weld pool, the liquid in the weld pool is driven downward to the bottom and
redirected to the tail where the molten metal solidifies to form a bead [8,9]. Regarding the role played
by droplet impingement in the reinforcement formation, Chen et al. [10] compared the difference
in bead profile by simulating the process with and without the droplet impingement, and results
showed that the reinforcement is higher when there is droplet impingement. Moreover, the profile
of a weld bead is also impacted by the direction of droplet impingement. Zhang et al. [11] reported
Metals 2019, 9, x FOR PEER REVIEW                                                                                          2 of 10
that the center lines of the lower part and the upper portion no longer coincides once the welding
torch is inclined. Although the molten behavior has been studied quite adequately by the mentioned
      Pang et al. [7] pointed out that the formation of a weld bead in the GMAW process is closely
researches, all the weld pool walls involved above are almost in their solid state, and the role played
associated with the heat input and the weld pool behavior, which was studied by many researchers.
by the wall in affecting the flow pattern and the resultant bead profile is rarely reported.
Wang et al. [4] demonstrated that during the welding process, the surface of a weld pool is
      The present study investigated the various profiles of weld beads by setting the welding current
significantly depressed under the combined effects of droplet impingement and the arc pressure,
to a wide range. The objectives were as follows: (1) Map and divide into stages the profile of weld
which leaves a thin liquid pool area between the arc and the base metal. The thin liquid pool and the
beads; (2) reveal the formation mechanism of a weld bead and figure out the role that the wall of a
droplet impingement plays a significant role in determining the flow pattern in the weld pool. When
weld pool plays in the profile formation process.
the accelerated droplet impinges into the thin weld pool, the liquid in the weld pool is driven
downward
2. Materialsto   andtheMethods
                           bottom and redirected to the tail where the molten metal solidifies to form a bead
[8, 9]. Regarding the role played by droplet impingement in the reinforcement formation, Chen et al.
      Bead-on-plate
[10] compared               welding experiments
                      the difference     in bead profile wereby conducted
                                                                  simulating onthe
                                                                                 aluminum
                                                                                     process alloy
                                                                                              with andAA6061   basethe
                                                                                                          without    metal  (250
                                                                                                                         droplet
×  60 ×   3 mm    3 ) using ER4043 filler wire. A schematic of the experimental setup for GMAW is given in
impingement, and results showed that the reinforcement is higher when there is droplet
Figure   1a. The chemical
impingement.         Moreover,     compositions
                                     the profile of  of AA6061      alloysisand
                                                          a weld bead            ER4043
                                                                              also         are provided
                                                                                    impacted               in Table 1.
                                                                                                 by the direction    of Because
                                                                                                                         droplet
of the  low   energy       input   for thin  sheets,    the  Pulsed-GMAW         (P-GMAW)       process
impingement. Zhang et al. [11] reported that the center lines of the lower part and the upper portion     was  employed,    and
the parameters         are  presented     in Table    2.  The  average   current,    labelled
no longer coincides once the welding torch is inclined. Although the molten behavior has been  as  current  A/B/C/D/E       and
ranging
studied from
           quite 72     A to 136 A,
                    adequately       bywas
                                         the obtained
                                               mentioned   by researches,
                                                               changing the  allbase
                                                                                 the current
                                                                                      weld poolfrom   30 Ainvolved
                                                                                                    walls   to 110 A above
                                                                                                                      while the
                                                                                                                              are
peak
almostcurrent    wassolid
          in their      kept state,
                                constantandat the
                                               280 role
                                                   A as played
                                                           shown in byFigure
                                                                        the wall1b. Pure  argon was
                                                                                    in affecting    thecoaxially  supplied
                                                                                                        flow pattern    and asthea
shielding    gas   at  a  flow   rate of
resultant bead profile is rarely reported.18 L/min.      For  each   process  setting,  three samples     which  are at least  15
cm were
      Thewelded,
            present and study  those  with the best
                                  investigated           typical appearances
                                                  the various     profiles of weldwere   chosen
                                                                                       beads by as   the research
                                                                                                  setting          object.
                                                                                                          the welding      After
                                                                                                                         current
welding,
to a wide range. The objectives were as follows: (1) Map and divide into stages the profile of were
            the  specimens         were  cut  along    the  transverse   cross-sections    of the   welds.  The  samples    weld
ground
beads; (2)and   polished
              reveal           with colloidal
                         the formation            silica and
                                            mechanism        of were
                                                                 a weldsubsequently
                                                                         bead and figureetchedoutforthe
                                                                                                      metallographic   analysis
                                                                                                        role that the wall   of a
using   the  standard       Keller’s   agent   for
weld pool plays in the profile formation process.  20   s. Then,   the profiles  of the  cross  section  were  observed    using
the LIOO SZ850T microscope (Beijing-Jinghao Co. Ltd., Beijing, China) and the dimensions were
measured
2. Materials  using
                 andthe      included software.
                         Methods




             Figure 1. Schematic
             Figure 1. Schematic of
                                 of (a)
                                    (a) experimental
                                        experimental setup
                                                     setup and
                                                           and (b)
                                                               (b) the
                                                                   the adjustment
                                                                       adjustment of
                                                                                  of average
                                                                                     average current.
                                                                                             current.

     Bead-on-plate welding experiments were conducted on aluminum alloy AA6061 base metal (250
× 60 × 3 mm3) using ER4043 filler wire. A schematic of the experimental setup for GMAW is given in
Figure 1a. The chemical compositions of AA6061 alloys and ER4043 are provided in Table 1. Because of
the low energy input for thin sheets, the Pulsed-GMAW (P-GMAW) process was employed, and the
Profile Map of Weld Beads and Its Formation Mechanism in Gas Metal Arc Welding
Metals 2019, 9, 146                                                                                                                  3 of 10


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                                Table 1. Chemical compositions of AA6061 and ER4043 (wt%).
            Material
                 Material           MgMg            Si
                                                    Si            Fe
                                                                 Fe           CuCu        Mn Mn         CrCr Al              Al
            AA6061                  0.96           0.52          0.25           0.25           0.12      0.26                Bal.
                 AA6061               0.96         0.52         0.25         0.25         0.12      0.26      Bal.
            ER4043ER4043            0.05
                                      0.05         5.60
                                                   5.60          0.80
                                                                0.80            0.30
                                                                             0.30         0.05 0.05  –    – Bal.             Bal.

                                     Table
                                      Table2.2.Welding
                                                Weldingprocess
                                                        processparameters
                                                                parametersfor
                                                                           forP-GMAW.
                                                                               P-GMAW.
                                 Process parameters                    Value
                                  Process Parameters                      Value
                                  Mean voltage (V)                      24.3
                                    Mean voltage (V)                       24.3
                                  Mean   current (A)
                                    Mean current (A)
                                                         A: 72 B: 88 C: 104 D: 120 E: 136
                                                             A: 72 B: 88 C: 104 D: 120 E: 136
                               Wire
                                Wirefeeding
                                     feeding rate  (mm/s) A: 61
                                             rate (mm/s)      A: B:
                                                                 6171  C: C:
                                                                    B: 71 82 82
                                                                              D:D:
                                                                                9494E:E:103
                                                                                         103
                                 Wire
                                  Wirediameter
                                        diameter (mm)
                                                  (mm)                   1.21.2
                                 Welding speed
                                Welding   speed (mm/s)
                                                 (mm/s)                  10 10

3.3.Results
     Resultsand
             andDiscussion
                 Discussion

 3.1.Profile
3.1.  ProfileMap
             Mapand
                 andStages
                     StagesofofWeld
                               WeldBeads
                                    Beads
        Figure22shows
      Figure         showsthe  themapmapofofweldweldbeads,
                                                       beads,whichwhichdisplays
                                                                           displaysclearly
                                                                                        clearlythatthatwithwiththe
                                                                                                                 theincrease
                                                                                                                       increaseofofmeanmean
  current,  the   profile   can   be   divided   into  three    stages   according     to   the  volume
current, the profile can be divided into three stages according to the volume relationship between the      relationship      between     the
  meltedfiller
melted     fillermetal
                   metalandandthe theweld
                                        weldpool:
                                               pool:the
                                                      themelted
                                                           meltedfiller
                                                                      fillermetal
                                                                              metal(i)(i)overflowing
                                                                                           overflowing(Figure
                                                                                                            (Figure2a), 2a),(ii)
                                                                                                                             (ii)matching
                                                                                                                                  matching
(Figure 2b–d), and (iii) inadequate (Figure 2e) for the weld pool when the average current is atata alow,
  (Figure  2b–d),     and  (iii)  inadequate      (Figure    2e)  for  the  weld   pool     when    the   average     current    is      low,
medium, and high level, respectively. In our cases, when the average current is smaller than 88 A,A,
  medium,      and    high   level,   respectively.     In  our   cases,   when    the    average     current     is smaller     than   88
  althoughthe
although       theheat
                    heatinput
                          inputisislow lowfor
                                            forboth
                                                 boththethefiller
                                                              fillerwire
                                                                     wireand andthe
                                                                                  thebase
                                                                                        basemetal,
                                                                                               metal,the  themelted
                                                                                                               meltedvolume
                                                                                                                          volumeofoffiller
                                                                                                                                        filler
  metal  is significantly      excessive    for  that  of the    base   metal   which     composes
metal is significantly excessive for that of the base metal which composes the weld pool, thus leading   the  weld    pool,   thus   leading
totoaavolume
       volumelack   lackfor
                          forthethe
                                  weldweld   pool
                                          pool   to to  accommodate
                                                    accommodate          thethe   excessive
                                                                              excessive           melted
                                                                                             melted    fillerfiller
                                                                                                               metal.metal.
                                                                                                                         As a As    a result,
                                                                                                                                result,  the
  the excess
excess   fillerfiller
                  metal metal    overflows
                          overflows       the the
                                                weldweld
                                                       poolpool andand    spreads
                                                                      spreads     ontoonto thethe  surface
                                                                                                surface     of of
                                                                                                                thethe  solidbase
                                                                                                                      solid     basemetal.
                                                                                                                                       metal.
  Hence,asasshown
Hence,          shownby   bythe
                              thegreen
                                     greenlines
                                            linesininFigure
                                                       Figure2a,  2a,the
                                                                       thetypical
                                                                            typicalprofile
                                                                                       profilecharacteristic
                                                                                                 characteristicininthis   thisstage
                                                                                                                                stageisisthe
                                                                                                                                          the
  excesswithin
excess    withinthe   themaximum
                          maximumwidth    widthofofdeposited
                                                       depositedfillerfillermetal
                                                                              metalon onthe thebase
                                                                                                 basemetal
                                                                                                        metaloveroverthatthatofofthe
                                                                                                                                   theweld
                                                                                                                                        weld
  pool.  Furthermore,         under     the  combined       effect   of  poor    wettability
pool. Furthermore, under the combined effect of poor wettability resulting from a small welding   resulting     from    a  small    welding
  current[12]
current    [12]andandsurface
                        surfacetension,
                                    tension,aasmaller
                                                 smallercontact
                                                           contactangle           114.5◦will
                                                                       angleofof114.5°       willbebeformed
                                                                                                      formedafter afterthethemolten
                                                                                                                               moltenwirewire
  spreadsand
spreads     andaccumulates
                   accumulateson      onthe
                                          thesurface
                                               surfaceofofthethebase
                                                                   basemetal,
                                                                          metal,asasrepresented
                                                                                       representedby     byFigure
                                                                                                              Figure2f,g.
                                                                                                                        2f,g.The
                                                                                                                               Thesmaller
                                                                                                                                     smaller
  the welding      current,    the   greater   the volume      deviation      of the melted      wire
the welding current, the greater the volume deviation of the melted wire over the melted base metal,    over    the  melted     base   metal,
  and  thus   the  greater    volume     of the   overflowing       melted    wire,  which      leads
and thus the greater volume of the overflowing melted wire, which leads to a greater width deviation    to  a greater    width    deviation
  betweenthe
between      thereinforcement
                   reinforcementand     andthetheweld
                                                   weldpool.
                                                          pool.An  Anexample
                                                                         examplefor forthisthisviewpoint
                                                                                                 viewpointisisthe   thebead
                                                                                                                         beadwelded
                                                                                                                                 weldedby  by
  cold  metal    transfer   (CMT)      + pulse   process    [7],  which    is characterized       by  a
cold metal transfer (CMT) + pulse process [7], which is characterized by a significant width deviation:  significant     width    deviation:
  Themaximum
The    maximumwidth    widthofofreinforcement
                                     reinforcementisis219% 219%ofofthat thatofofthe
                                                                                 theweld
                                                                                       weldpoolpoolandandthe thecontact
                                                                                                                  contactangle
                                                                                                                             angleisisonly
                                                                                                                                         only
  26.8 degrees,      as can   be   seen  from    Figure   2h.    Another     example       is the
26.8 degrees, as can be seen from Figure 2h. Another example is the bead welded by tungsten inert  bead    welded      by  tungsten     inert
  gaswelding
gas   welding(TIG) (TIG)++CMT CMThybridhybridwelding
                                                welding[12] [12]ininwhich
                                                                      whichthe  themaximum
                                                                                    maximumwidth     widthofofreinforcement
                                                                                                                  reinforcementisisalso  also
  doublethat
           thatofofthetheweld
                          weldpool  pooland
                                          andthethecontact
                                                     contactangle
                                                                angleisisnearly
                                                                            nearly90°.
                                                                                    90 . ◦
double




                                                              Figure 2. Cont.
Profile Map of Weld Beads and Its Formation Mechanism in Gas Metal Arc Welding
Metals 2019, 9, 146                                                                                                     4 of 10
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                  Profilesofofweld
       Figure2.2.Profiles
      Figure                  weldbead
                                   beadand
                                         andits
                                              itsdimensions.
                                                  dimensions.(a)
                                                               (a)7272A;A;(b)
                                                                           (b)8888A;
                                                                                   A;(c)
                                                                                      (c)104
                                                                                          104A;A;(d)
                                                                                                  (d)120
                                                                                                      120A;
                                                                                                          A;(e)
                                                                                                             (e)136
                                                                                                                 136A;
                                                                                                                     A;
       (f)schematic
      (f)  schematicofofmeasurement;
                          measurement;(g)(g)contact
                                             contactangle;
                                                     angle;(h)
                                                             (h)bead
                                                                 beadof  ofCMT
                                                                            CMT++PP(reproduced
                                                                                         (reproducedfromfrom[7],
                                                                                                              [7],with
                                                                                                                  with
       permissionofofElsevier,
      permission       Elsevier,2016);
                                 2016);and
                                        and(i)
                                             (i)bead
                                                 beadof
                                                      ofTIG-CMT
                                                        TIG-CMT(reproduced
                                                                      (reproducedfrom from[12],
                                                                                             [12],with
                                                                                                   withpermission
                                                                                                         permissionofof
       Elsevier, 2017).
      Elsevier, 2017).

        Thewidth
       The   widthdifference
                      differencebetween
                                   betweenreinforcement
                                              reinforcementand   andweld
                                                                      weldpoolpooldecreases
                                                                                    decreasesgradually
                                                                                                 graduallywith
                                                                                                            withincreasing
                                                                                                                    increasing
 average current
average    current andand finally
                            finallydiminishes
                                     diminisheswhen  whenthethecurrent  is up
                                                                  current       to 120
                                                                            is up       A. Although
                                                                                    to 120             the heat
                                                                                             A. Although    the input    grows
                                                                                                                  heat input
 with   the increase     of average   welding      current  and   a larger   volume     is melted  for
grows with the increase of average welding current and a larger volume is melted for both filler wire   both  filler wire  and
 base   metal   [13], the  diminished     width    difference,   however,    indicates    that the
and base metal 13 , the diminished width difference, however, indicates that the base metal has a  base  metal   has  a higher
 meltingmelting
higher     rate than    thethan
                     rate   fillerthe
                                   wire.  Aswire.
                                      filler  we can Assee
                                                         wefrom
                                                              can Figure
                                                                   see from3a, Figure
                                                                                 when the 3a,current increases
                                                                                              when the   current from   72 A to
                                                                                                                    increases
 136  A,  the  increased    melted    area  for  filler wire  and   base  metal    are 12.82   mm  2 and 28.05 mm2 . Thus,
from 72 A to 136 A, the increased melted area for filler wire and base metal are 12.82 mm and 28.05               2

 with
mm   2. the weld
        Thus,       pool
                 with  thegaining
                            weld poola larger   room
                                          gaining       at a higher
                                                     a larger   roomspeed      than the
                                                                       at a higher         melted
                                                                                       speed   thanspeed  for filler
                                                                                                     the melted       wire,for
                                                                                                                    speed   its
 capacity
filler  wire,to its
                 accommodate
                    capacity to the    melted fillerthe
                                   accommodate          wire  grows,filler
                                                           melted      which wireresults
                                                                                    grows,in awhich
                                                                                               gradual  decrease
                                                                                                      results  in aingradual
                                                                                                                       volume
 difference    between    the  melted   filler wire   and  the  base  metal.   The   decreased
decrease in volume difference between the melted filler wire and the base metal. The decreased    volume   difference    makes
volume difference makes most of the melted filler wire held by the growing weld pool, thus leaving
less melted filler wire overflowing the weld pool. Hence, less melted filler wire spreads and
Profile Map of Weld Beads and Its Formation Mechanism in Gas Metal Arc Welding
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  Metals 2019, 9, x FOR PEER REVIEW                                                                                                                      5 of 10
most of the melted filler wire held by the growing weld pool, thus leaving less melted filler wire
  accumulates
overflowing    theoutside    the weld
                    weld pool.    Hence,pool
                                           lesson  the base
                                                 melted    fillermetal,  which, as
                                                                  wire spreads     anda accumulates
                                                                                         result, displays     the diminished
                                                                                                          outside   the weld
  width
pool      difference.
      on the            The difference
              base metal,     which, as ainresult,
                                             width displays
                                                    will decrease      further withwidth
                                                                 the diminished        the increasing
                                                                                              difference. current   and finally
                                                                                                             The difference
indisappears
   width will when
                decreasethe further
                             averagewith
                                       current
                                            the is up to thecurrent
                                                 increasing      critical and
                                                                          valuefinally
                                                                                 of 120disappears
                                                                                          A, and it willwhennotthe
                                                                                                                 emerge    once
                                                                                                                     average
  the average
current  is up tocurrent    is beyond
                   the critical  value the   critical
                                        of 120        value.
                                                 A, and         Namely,
                                                          it will          although
                                                                   not emerge     once melted     filler current
                                                                                         the average     wire accumulation
                                                                                                                   is beyond
  oncritical
the   the beads
             value.remains,
                      Namely,the    width melted
                                although    of the reinforcement        is equal to
                                                    filler wire accumulation        onthat   of theremains,
                                                                                        the beads     weld pool.     However,
                                                                                                                the width   of
  because
the          of the is
    reinforcement      weakened      accumulation,
                         equal to that                  the height
                                        of the weld pool.       However,decreases
                                                                            becauseand       theweakened
                                                                                        of the    contact angle       increases
                                                                                                              accumulation,
  accordingly,
the               as shown
    height decreases       andbytheFigure  2g angle
                                     contact   and Figure      3b. Therefore,
                                                       increases     accordingly,withas filler
                                                                                         shown  metal   accumulated
                                                                                                   by Figures     2g and on3b.
                                                                                                                            the
  base metal,
Therefore,      beads
             with  filler at thisaccumulated
                          metal   stage are characterized
                                                 on the base by      the beads
                                                                 metal,   matched    width
                                                                                 at this  stagebetween     the reinforcement
                                                                                                 are characterized     by the
  and thewidth
matched     weld pool.
                   between the reinforcement and the weld pool.




       Figure    Dimensions
              3. 3.
         Figure     Dimensions ofof
                                  beads welded
                                    beads      with
                                          welded    different
                                                 with         average
                                                      different       currents.
                                                                average         (a)(a)
                                                                        currents.    Area ofof
                                                                                       Area  different parts.
                                                                                               different      (b)
                                                                                                         parts. (b)
       Height ofof
         Height   different parts.
                    different parts.

       With
          With   thetheaverage
                           average   current
                                         current increasing
                                                      increasing  further,   moremore
                                                                        further,      heat heat
                                                                                              is brought
                                                                                                    is broughtto melt  to themeltbase
                                                                                                                                    themetal,       and then
                                                                                                                                           base metal,       and
more     liquid     is  produced         and   held    by   the   weld    pool.    Meanwhile,
   then more liquid is produced and held by the weld pool. Meanwhile, the driving force and the arc  the   driving       force   and    the    arc  pressure
is pressure
    also strengthened              owing to the
                   is also strengthened                 higher
                                                    owing           current,
                                                               to the    higher  which
                                                                                   current,propels
                                                                                                 which  the   liquidthe
                                                                                                           propels         in the   weld
                                                                                                                               liquid     in pool
                                                                                                                                              the weldto flow
                                                                                                                                                            pool
upwards         and    outwards,         creating     a thin    liquid    film   between       the
   to flow upwards and outwards, creating a thin liquid film between the arc and solid base metal   arc   and    solid     base   metal      [14,15].   Thus,[14,
the   heat    brought        by   the   arc  can   be   more     easily   transferred       to  the  base
   15]. Thus, the heat brought by the arc can be more easily transferred to the base metal in a solid state, metal      in   a solid  state,     which    will
bewhich
     meltedwill  faster.    Moreover,
                       be melted             Wang
                                        faster.        et al. [16]Wang
                                                   Moreover,          indicated
                                                                              et al.that
                                                                                       [16]droplets
                                                                                              indicated with thatabundant
                                                                                                                      dropletsheat  with would      impinge
                                                                                                                                             abundant       heat
into   the weld
   would       impinge  poolintoat high
                                     the weldspeed, poolwhich
                                                           at high willspeed,
                                                                          easilywhich
                                                                                   pass through
                                                                                            will easily thepassthinthrough
                                                                                                                       liquid layer,
                                                                                                                                   the thin  crash    against
                                                                                                                                                 liquid   layer,
the   solidagainst
   crash       bottom,the    and    create
                                  solid        a crater.
                                           bottom,      and The    penetration
                                                               create     a crater.isThe thenpenetration
                                                                                                deepened further,  is thenand        when the
                                                                                                                                deepened             average
                                                                                                                                                 further,    and
current      reaches       104   A,   the   base    metal    is  fully   penetrated.
   when the average current reaches 104 A, the base metal is fully penetrated.
       If Ifthetheaverage
                     averagecurrent currentcontinues
                                                continuestotogrow,   grow,  reinforcement
                                                                               reinforcementthen      thenwillwill  become
                                                                                                                       becomeflat      oror
                                                                                                                                    flat     even
                                                                                                                                                even  sunken
                                                                                                                                                        sunken
duedue to the greater heat input and the combined effect of all kinds of driving forces exertedononthe
       to   the    greater      heat    input     and    the   combined        effect     of  all kinds     of   driving       forces     exerted             the
weld
   weld poolpool  [7].  With
                     [7].  With   the
                                    thebasebasemetal
                                                  metal  fully
                                                            fullypenetrated,
                                                                    penetrated,    thethe bottom
                                                                                            bottom   wall
                                                                                                       wall  ofofa weld
                                                                                                                     a weld   pool
                                                                                                                                pool is is
                                                                                                                                         nono  longer
                                                                                                                                                 longer  inin
                                                                                                                                                            itsits
solid
   solidstate
            state andand  it it
                             is is
                                 inindelicate
                                        delicate  balance
                                                     balance   between
                                                                  between    thethesurface
                                                                                      surface  tension
                                                                                                  tension  ofof liquid
                                                                                                                    liquid  bottom
                                                                                                                               bottom  and and  the
                                                                                                                                                  the driving
                                                                                                                                                        driving
forces    acted      on    weld     pool.    Because      the    confining      effect    provided
   forces acted on weld pool. Because the confining effect provided by the bottom wall is significantly  by   the    bottom      wall     is  significantly
weakened
   weakened      due due to to
                             thethetransition
                                       transition  of the
                                                       of thebottom
                                                                 bottom  wall  from
                                                                             wall   froma solid   state
                                                                                             a solid      to liquid
                                                                                                       state    to liquid metal,    the flow
                                                                                                                                metal,            pattern
                                                                                                                                           the flow          of
                                                                                                                                                        pattern
liquid    metal      in  the    weld     pool   changes.       Although       the   flowing      molten
   of liquid metal in the weld pool changes. Although the flowing molten metal has the ability to scour     metal      has    the  ability     to  scour   out
orout
    “dig”      a channel
         or “dig”               with its
                         a channel           thermal
                                          with            energyenergy
                                                  its thermal          and momentum,
                                                                               and momentum,     the flow  thepattern        is determined
                                                                                                                  flow pattern        is determined not only  not
byonly
     the “digging”
           by the “digging”   ability but      alsobut
                                         ability      by also
                                                           the confining        effect the
                                                                  by the confining              weldthe
                                                                                            effect     poolweld wall     canwall
                                                                                                                      pool      provide.       Thus, when
                                                                                                                                     can provide.         Thus,
the   bottom
   when      the wall
                    bottom  is inwall
                                    a solid
                                          is instate,   it can
                                                  a solid         provide
                                                             state,    it canaprovide
                                                                                 strong confining           effect to force
                                                                                              a strong confining                   thetoliquid
                                                                                                                              effect        force metal      to
                                                                                                                                                     the liquid
flow
   metalalong      the channel
             to flow      along the    which
                                          channelconsists
                                                       which   ofconsists
                                                                   a solid wall.      For example,
                                                                              of a solid     wall. For the       formation
                                                                                                           example,               mechanism
                                                                                                                            the formation             of bead
                                                                                                                                                  mechanism
hump
   of bead hump in high-speed GMAW is ascribed to the redirecting role provided by theedge
          in   high-speed          GMAW        is  ascribed     to  the   redirecting      role   provided       by   the    sloping     leading             of
                                                                                                                                                        sloping
the   weld      pool    [17].   In   our    case,   when      the  bottom      wall   is  in  a  solid   state,
   leading edge of the weld pool [17]. In our case, when the bottom wall is in a solid state, it can resist        it can    resist  the     digging     from
thermal
   the diggingliquid and fromredirect
                                   thermal   theliquid
                                                   liquid andbackward.
                                                                     redirect Thethebackward        liquid then accumulates
                                                                                         liquid backward.              The backward          andliquid
                                                                                                                                                   solidifies
                                                                                                                                                            then
ataccumulates
    the rear of the         weld     pool    to  form    reinforcements.          However,        when     the
                         and solidifies at the rear of the weld pool to form reinforcements. However, when the   base     metal    is fully     melted    and
penetrated,
   base metalthe           surface
                      is fully    meltedtension
                                              and provided
                                                     penetrated,    bythethesurface
                                                                              back surface
                                                                                        tensionisprovided
                                                                                                      far from by    enough
                                                                                                                          the backto bear
                                                                                                                                       surface theisdigging
                                                                                                                                                       far from
effect
   enoughand to  redirect
                       bear thethe molten
                                     diggingmetal.  effectTherefore,
                                                             and redirect   under thethe    combined
                                                                                        molten      metal.impact         of arc under
                                                                                                                Therefore,         pressure,  thepowerful
                                                                                                                                                    combined
  impact of arc pressure, powerful droplet impingement, and the weakened confining of liquid bottom,
  more molten metal is driven downwards with little redirection backwards. As a result, a little molten
  metal flows backwards, which means a little metal accumulates at the rear of the weld pool, and then
Profile Map of Weld Beads and Its Formation Mechanism in Gas Metal Arc Welding
Metals 2019, 9, 146                                                                                                                    6 of 10



droplet
Metals    impingement,
       2019,                and the
             9, x FOR PEER REVIEW     weakened confining of liquid bottom, more molten metal is driven         6 of 10
downwards with little redirection backwards. As a result, a little molten metal flows backwards, which
the reinforcement
means                is accumulates
        a little metal  flat. If the arcat
                                         pressure
                                           the rearcontinues  to grow
                                                    of the weld  pool, with   the increase
                                                                        and then           of the average
                                                                                   the reinforcement       current,
                                                                                                      is flat. If the
more   moltencontinues
arc pressure     metal will      be driven
                            to grow           downwards
                                       with the  increase ofand
                                                              the less  willcurrent,
                                                                   average    flow backwards.
                                                                                       more moltenThus,
                                                                                                     metalthewill
                                                                                                                back
                                                                                                                   be
reinforcement
driven downwards sticksand
                         outless
                              highwill
                                     andflow
                                          a sunken  top surface
                                              backwards.    Thus,isthe
                                                                     created.
                                                                       back reinforcement sticks out high and a
     If the
sunken   topaverage    current
              surface is  created.continues to grow, the base metal will burn through, which means that
the back
     If thesurface
             averageloses  its delicate
                      current    continuesbalance  due
                                             to grow, thetobase
                                                            the metal
                                                                oversized    driving
                                                                        will burn     forces which
                                                                                   through,  exerted on the
                                                                                                   means       weld
                                                                                                            that  the
pool.
back surface loses its delicate balance due to the oversized driving forces exerted on the weld pool.

3.2. Formation Mechanism of Reinforcement
      When the  thecurrent
                      currentisisatat  a low
                                    a low        level,
                                             level,   the the
                                                            top top     surface
                                                                  surface           of reinforcement
                                                                             of reinforcement        displaysdisplays      an inverted
                                                                                                                  an inverted      U-shaped  U-
shaped(Figure
profile   profile (Figure
                     2a). As 2a).   As we
                                we can    seecanfrom seeFigures
                                                           from Figure2c and2c4a,  and   Figure
                                                                                      when      the4a,  when the
                                                                                                     average           average
                                                                                                                  current     is upcurrent
                                                                                                                                     to 88 A, is
up  to 88 A, reinforcement
reinforcement        is divided intois divided       into three
                                          three parallel              parallel longitudinal
                                                                 longitudinal        parts by two    parts  by two
                                                                                                         grooves         grooves
                                                                                                                      (the           (the red
                                                                                                                             red arrows      in
arrows
Figure 4b)in Figure
               along the4b) welding
                             along thedirection.
                                          welding direction.
                                                         The three The         three longitudinal
                                                                         longitudinal       parts lie inparts      lie in the
                                                                                                           the central       and central   and
                                                                                                                                    both side
both side
planes,       planes, respectively,
          respectively,                     which represents
                             which represents          a W-shapeda profile.W-shaped    With profile.    With the
                                                                                               the current            currentfurther,
                                                                                                                increasing        increasingthe
further,
height ofthe     height of reinforcement
             reinforcement                         in all three
                               in all three longitudinal             longitudinal
                                                                   planes   decreases   planes
                                                                                           (Figure decreases
                                                                                                     3b). Once(Figure         3b). Once
                                                                                                                    the current             the
                                                                                                                                    increases
current   increases     to  120  A,  the  reinforcement           height    in  the   central
to 120 A, the reinforcement height in the central plane goes down even below the top surface of  plane   goes    down      even    below    the
top
basesurface
      metal, of    the base
                leaving        metal,
                           merely   theleaving     merely the
                                         reinforcement               reinforcement
                                                                 in the   side plane abovein the the
                                                                                                   sidebase
                                                                                                         plane    above
                                                                                                               metal.    Thetheshape
                                                                                                                                 base metal.
                                                                                                                                         of the
The  shape of the
reinforcement       at reinforcement       at this stage
                       this stage is V-shaped                 is V-shaped
                                                       (Figures     2d and 4c).  (Figure    2d and
                                                                                     However,       theFigure    4c). However,will
                                                                                                         side reinforcement          the side
                                                                                                                                           also
reinforcement
begin to sink and   willfinally
                          also begin    to sink
                                  diminish          andcurrent
                                               if the      finallycontinues
                                                                      diminish to   if the  current
                                                                                       increase,        continues
                                                                                                     which    indicatesto increase,
                                                                                                                            that the which
                                                                                                                                        whole
indicates
reinforcementthat the   whole
                    above        reinforcement
                            the top   surface hasabove            the top Instead,
                                                        disappeared.         surface has      disappeared.
                                                                                         a thoroughly      sunken Instead,
                                                                                                                        weldapoolthoroughly
                                                                                                                                       surface
sunken
emergesweld        pool2esurface
            (Figures       and 4d).emerges
                                       The profile(Figureof the 2etop
                                                                    andsurface
                                                                           Figureis4d).     The profile
                                                                                       U-shaped,      as theofterm the reinforcement
                                                                                                                        top surface ishas    U-
shaped,     as the term
lost its meaning      withreinforcement
                             the height of the   hasweldlostpool
                                                               its meaning
                                                                     above thewith  basethe    height
                                                                                           metal         of the
                                                                                                    having         weld pool
                                                                                                              thoroughly           above the
                                                                                                                                disappeared.
base metal having
According                thoroughly between
               to the relationship       disappeared.          According to the
                                                       the reinforcement           andrelationship
                                                                                         the base metal   between      the reinforcement
                                                                                                               as described       above, the
and  the base metal
reinforcement             as undergoes
                    profile   described above,
                                            the shapes the reinforcement
                                                             of inverted U, profileW, V, and undergoes
                                                                                                  U periods.theThe shapes    of inverted
                                                                                                                       formation             U,
                                                                                                                                      of these
W, V, and
profiles       U periods.
           is ascribed    to The   formation effect
                              the combined        of these      profiles
                                                            of heat         is ascribed
                                                                       input,    driving to      the combined
                                                                                            forces    in the weld    effect
                                                                                                                        pool,ofandheatthe
                                                                                                                                        input,
                                                                                                                                           role
driving
that the forces
           weld poolin the   weld
                          wall      pool,
                                 plays       and the role
                                        in bearing,              that the weld
                                                          constraining,        and pool      wall plays
                                                                                      redirecting           in bearing,
                                                                                                       the liquid     flow. Theconstraining,
                                                                                                                                    inputted
and
heat redirecting
      has the ability the liquid
                           to meltflow.     The metal,
                                     the base      inputted       heat then
                                                              which      has theturnsability
                                                                                        intoto the melt
                                                                                                     weldthepool.
                                                                                                               base Themetal,    whichforces
                                                                                                                             driving      then
turns
exerted into
          on the
               the weld
                    weld pool.
                           pool areTheable
                                         driving      forces
                                              to propel      theexerted
                                                                   moltenon      the weld
                                                                              metal    in thepool
                                                                                                weldare    able
                                                                                                        pool       to propel
                                                                                                               to flow    alongthe     molten
                                                                                                                                   a channel,
metal in
which       the weld
         consists        poolweld
                      of the    to flow
                                      poolalong
                                              wall.a Although
                                                        channel, which the wall consists    of the
                                                                                     provides        weld pool
                                                                                                   a channel      forwall.    Although
                                                                                                                       the liquid           the
                                                                                                                                       to flow
wall
along,provides
         the liquid,a channel
                        however, forbrings
                                     the liquid
                                              bothto     flow along,
                                                       thermal      energy theandliquid,   however,
                                                                                     momentum            brings
                                                                                                      that          both and
                                                                                                            will melt      thermal     energy
                                                                                                                                 corrode    the
and momentum
weld                   that willthe
       pool wall (namely,          melt   and corrode
                                      channel).      The heat the input
                                                                    weld pool       wall (namely,
                                                                            and driving       forces arethedirectly
                                                                                                             channel).      The heatby
                                                                                                                         controlled      input
                                                                                                                                            the
and  driving
current,         forces
           whereas     theare  directly
                            channel    is controlled
                                          not only affectedby the by  current,     whereas
                                                                          the current     but the
                                                                                                alsochannel
                                                                                                      determined is notby only
                                                                                                                            the affected     by
                                                                                                                                  state of the
the
basecurrent     but also determined by the state of the base metal wall.
      metal wall.




      Figure 4. Appearances
                Appearances of the top surface
                                         surface for bead
                                                     bead welded
                                                          welded with different average current. (a) Less than
      88A; (b) 104 A; (c) 120 A; (d) 136 A.

      The flow pattern is simulated by Na and his coworkers as shown in Figure 5a, from which we
can see
    see that
         thatthe
              theliquid
                   liquiddriven
                           drivenbyby
                                    droplet
                                      dropletimpingement
                                                 impingement   flows  downwards,
                                                                  flows   downwards, backwards  (redirected),
                                                                                        backwards              and
                                                                                                     (redirected),
thenthen
and   upwards.   When
          upwards.       the average
                      When            current
                             the average        is small,
                                           current         bothboth
                                                     is small,   the heat  input
                                                                      the heat    and and
                                                                                input driving forces
                                                                                          driving     are at
                                                                                                   forces    a low
                                                                                                           are at a
level which  melts   only a small amount    of base  metal   and  does   not propel the molten
low level which melts only a small amount of base metal and does not propel the molten metal   metal   powerfully.
Combined with
powerfully.       the bottom
             Combined      withwall
                                the in its solid
                                    bottom    wallstate, the
                                                    in its   melted
                                                           solid      metal
                                                                  state,      propelled
                                                                         the melted     by driving
                                                                                     metal propelledforces cannot
                                                                                                       by driving
forces cannot flow deeply downwards, and meanwhile, because the driving forces are small, it gains
no ability to drive the liquid flow further backwards. Accordingly, the molten metal accumulates
Profile Map of Weld Beads and Its Formation Mechanism in Gas Metal Arc Welding
droplet impingement. Cheon et al. [18] indicated that the droplet impingement momentum strikes
 the bottom of the weld pool first, and it detours backwards at a deep level where, finally, the liquid
 propelled by the droplet impingement flows upwards along the rear solidified wall (Figure 5a). As
 mentioned earlier, the top surface of the weld pool is depressed deep by the arc pressure, and the
 deepest
Metals 2019,deformation
             9, 146          lies at the surface right under the arc center, which is also exactly the7place    of 10
 where the upward liquid flows intensively. Hence, it can be seen from Figure 2b and Figure 5c that
 the upward flow driven by droplet impingement gains the ability to break the constraint acted on the
flow   deeply by
 top surface     downwards,
                    the surfaceand    meanwhile,
                                  tension           because the
                                          and arc pressure  [19].driving forces
                                                                  Moreover,     are et
                                                                             Chen   small,   it gains
                                                                                       al. [10]        no abilitythat
                                                                                                 also indicated    to
drive  the  liquid   flow  further  backwards.   Accordingly, the  molten  metal accumulates      high
 the droplet impingement had the ability to elevate the reinforcement (Figure 5d). The reinforcement    near the arc
center  and plane
 in central   the inverted     U-shape(Figure
                       then emerges     of the 5c).
                                               reinforcement comes into being, as shown in Figure 5b.




Metals 2019, 9, x FOR PEER REVIEW                                                                                        8 of 10




     Figure   5. The
      Figure 5.  The formation
                      formation mechanism
                                  mechanism for   for different
                                                      different weld
                                                                 weld beads.
                                                                       beads. (a)
                                                                               (a) Simulated
                                                                                   Simulated flow
                                                                                                flow pattern
                                                                                                      pattern of
                                                                                                               of droplet
                                                                                                                  droplet
     impingement
      impingement (reproduced
                     (reproduced from
                                    from [18],
                                           [18], with
                                                  with permission
                                                         permission of
                                                                     of Elsevier,
                                                                        Elsevier, 2016);
                                                                                  2016); (b)
                                                                                          (b) schematic
                                                                                              schematic ofof bead
                                                                                                             bead profile
                                                                                                                   profile
     welded   with  current  no  more than   88  A;  (c)  schematic of bead  profile welded    with 104 A;
      welded with current no more than 88 A; (c) schematic of bead profile welded with 104 A; (d) profile   (d) profile of
     simulation   withwith
      of simulation     and without  droplet
                             and without        impingement
                                             droplet            (X = 13 mm)
                                                         impingement    (X = (reproduced    from [10], with
                                                                              13 mm) (reproduced        frompermission
                                                                                                               [10], with
     of Springer, 2018);
      permission           (e) schematic
                   of Springer,    2018); of(e)bead  profile welded
                                                 schematic    of beadwith   120 A;
                                                                        profile     (f) profile
                                                                                 welded    withof120
                                                                                                   simulation   with and
                                                                                                       A; (f) profile   of
     without   droplet
      simulation   withimpingement
                          and without(Xdroplet
                                            = 8 mm)impingement
                                                        (reproduced from
                                                                       (X =[10],  with (reproduced
                                                                              8 mm)     permission offrom
                                                                                                        Springer,
                                                                                                               [10],2018);
                                                                                                                     with
     (g) schematic
      permission   ofofSpringer,
                         bead profile welded
                                  2018);          with 136ofA;bead
                                         (g) schematic          (h) width
                                                                    profileexcess
                                                                            welded  of with
                                                                                       the top
                                                                                            136reinforcement     over the
                                                                                                 A; (h) width excess    of
     bottom   one.
      the top reinforcement over the bottom one.

     Because the arc pressure obeys a Gaussian distribution, the regions adjacent the central plane
are also depressed heavily and only slightly inferior to that in the central plane. These regions,
however, gain far less intensive upward flow because of the limited effect of the impingement on the
liquid adjacent to the central plane, and the adjacent liquid is not able to negate the surface tension
[19] and remains at a low level. Hence, the interface of the sunken surface and the raised central
reinforcement, namely the grooves, forms. Thus, as illustrated by Figure 5c, the reinforcement in
central plane emerges on a depressed top surface.
     The reason why the reinforcements in the side planes are above the base metal is attributed to
Profile Map of Weld Beads and Its Formation Mechanism in Gas Metal Arc Welding
Metals 2019, 9, 146                                                                                 8 of 10



3.2.1. W-Shaped Reinforcement
     With the increase of the average current, the heat input and the driving forces consequently
increase. The more heat inputted means more base metal melted and stored, and a larger weld pool
forms. Thus, there is more liquid metal that can be driven to flow in the weld pool. Moreover, the
greater driving forces have the ability to propel a more intense convection in the weld pool. As
simulated by Wang et al. [16], the arc pressure increases significantly with the current augmented, and
the arc pressure for the peak current, 220 A, is almost six times higher than that at base current of
90 A. Because of the dominant role played by arc pressure in depressing the weld pool surface, the
greater arc pressure will no doubt cause a deeper weld pool surface deformation, which may affect
the reinforcement height in two ways: (1) The deeper weld pool surface, combined with the larger
weld pool, lowers the whole reinforcement height; and (2) the reinforcement maintains low height
when it solidifies at the rear part of the weld pool, which highlights the liquid flow driven by the
droplet impingement. Cheon et al. [18] indicated that the droplet impingement momentum strikes
the bottom of the weld pool first, and it detours backwards at a deep level where, finally, the liquid
propelled by the droplet impingement flows upwards along the rear solidified wall (Figure 5a). As
mentioned earlier, the top surface of the weld pool is depressed deep by the arc pressure, and the
deepest deformation lies at the surface right under the arc center, which is also exactly the place where
the upward liquid flows intensively. Hence, it can be seen from Figures 2b and 5c that the upward
flow driven by droplet impingement gains the ability to break the constraint acted on the top surface
by the surface tension and arc pressure [19]. Moreover, Chen et al. [10] also indicated that the droplet
impingement had the ability to elevate the reinforcement (Figure 5d). The reinforcement in central
plane then emerges (Figure 5c).
     Because the arc pressure obeys a Gaussian distribution, the regions adjacent the central plane are
also depressed heavily and only slightly inferior to that in the central plane. These regions, however,
gain far less intensive upward flow because of the limited effect of the impingement on the liquid
adjacent to the central plane, and the adjacent liquid is not able to negate the surface tension [19] and
remains at a low level. Hence, the interface of the sunken surface and the raised central reinforcement,
namely the grooves, forms. Thus, as illustrated by Figure 5c, the reinforcement in central plane emerges
on a depressed top surface.
     The reason why the reinforcements in the side planes are above the base metal is attributed to the
combined effect of heat input and arc pressure. First, the heat input follows a Gaussian distribution,
which determines that less base metal is melted far from the central plane. Therefore, the bottom wall
of the weld pool in the side plane is at a higher level than the central plane, which will resultantly
hold the liquid accumulated here at a high level. Meanwhile, Gaussian-distributed arc pressure also
has a weak distribution far from the central plane. In other words, the surfaces in the side planes are
not heavily depressed, and the molten metal that flows here will not be driven away and is able to
accumulate to form a reinforcement higher than the base metal. The reinforcements in the side planes
survive under the combined effect of heat input and arc pressure. The reinforcements in side and
central planes add up to form a W-shaped reinforcement.

3.2.2. V-Shaped Reinforcement
     It should be pointed out that the base metal has been thoroughly penetrated discontinuously
during the W-shape stage, but there is still liquid flowing back because the heat input and arc pressure
are at a low level. With the current increasing to 120 A, the heat input and driving forces including arc
pressure climb to a higher level. During this stage, the base metal has been completely and steadily
penetrated, which indicates a thorough transition of the bottom wall from a solid state to a liquid state.
In addition, this complete transition suggests that the role of bearing, constraining, and redirecting the
fluid flow at a deep level played by the solid bottom wall has totally gone with the liquefying of the
solid wall. Thus, when the droplet impinges and strikes the frontier weld pool wall, the liquid wall can
no longer redirect the molten metal, and the liquid is driven straight down. With little liquid redirected
Profile Map of Weld Beads and Its Formation Mechanism in Gas Metal Arc Welding
Metals 2019, 9, 146                                                                                 9 of 10



backwards, little metal accumulates in the central plane to form the reinforcement. Therefore, as we
can see from Figure 5e, the reinforcement in the central plane disappears, leaving the crater formed by
droplet impingement solidified before being filled up again. Chen et al. [10] simulated the formation
process of the top surface profiles with and without droplet impingement, and the results showed
that the surface in the central plane is sunken when the droplet impinges the weld pool (Figure 5f).
The profile then becomes a V-shape, with the side reinforcements surviving alone.

3.2.3. U-Shaped Reinforcement
      When the average current is up to 136 A, the weld width on the bottom surface is slightly wider
than that on the top surface, with 1.1 mm excess as shown by Figures 2e and 5g,h. Under this condition,
the excess in weld width of the bottom surface over that of the top surface indicates the loss of the
side solid wall, which plays the role of bearing and bracing the molten metal high when accumulating
until it solidifies to form a reinforcement higher than the base metal. Therefore, when the foundation
of bracing the liquid is missing because of the excessive heat input and arc pressure, the liquid
can no longer “stay” and accumulate at the side planes. The weld pool, which has lost the central
reinforcement in the previous stage, then collapses entirely, and the U-shape top surface (Figure 5g)
finally comes into being.

3.2.4. Four Stages of the Top Surface
     The four stages of inversed-U, W, V, and U shape for the top surface that is gone with the increase
of the average current reveals that the reinforcement consists of the central plane and the side planes,
and the shape of the reinforcement is determined by the combined effect of heat input and the driving
forces exerted on a weld pool. At first, when the average current is small, because of the excess
accumulation of filler wire on the base metal and the low-level arc pressure, the reinforcement in
the side plane is covered by the accumulated molten filler metal. However, with the increase of
current, when the base metal melts more and the arc pressure depresses the weld pool greatly, the
side reinforcement emerges out from the top surface. Whereas once the bottom wall melts, it loses
the ability to redirect the liquid backwards, which leaves a groove in the central plane by the droplet
impingement. Moreover, the side reinforcement will also disappear because of the further increase of
heat input and arc pressure.
     From the above analysis, it can also be seen that the essence of the role played by the bottom wall
is redistributing the molten metal, which is determined by the state of the bottom wall. When the
bottom wall is in a solid state, most of the molten metal in the weld pool is redirected backwards and
then flows upwards to form the reinforcement. However, once the bottom is thoroughly melted, which
means the loss of the ability to redirect the liquid, most of the molten metal flows downwards directly
and little is redirected backwards to form the reinforcement.

4. Conclusions
(1). The profile of weld beads obtained over a wide range of welding current in GMAW can be divided
     into three stages: (i) the filler metal overflowing the weld pool; (ii) the filler metal matching the
     weld pool; and (iii) the filler metal inadequate for the weld pool.
(2). Both the flow pattern in the weld pool and the profile of a weld bead are determined by the
     combined effect of driving forces and the significant role played by the weld pool wall, such
     as bearing, constraining, and redirecting the liquid. The essence of the bottom wall’s role is
     redirecting and redistributing the molten metal.
(3). The top surface of the reinforcement goes through stages of inversed-U, W, V, and U shapes, which
     consists of two parts: the central plate and the side plate. The side reinforcement is covered by
     the central one when the current is at a low level and emerges gradually with increasing current.
Profile Map of Weld Beads and Its Formation Mechanism in Gas Metal Arc Welding
Metals 2019, 9, 146                                                                                          10 of 10



Author Contributions: Conceptualization, Z.Z. and J.X.; methodology, Z.Z.; formal analysis, Z.Z.;
writing—original draft preparation, Z.Z. and J.X.; project administration, J.X.; funding acquisition, J.X.
Funding: This research was funded by the National Natural Science Foundation of China (51875213),
The High-level Leading Talent Introduction Program of GDAS (2016-GDASRC-0106), Natural Science Foundation
of Fujian (2018J01503), Longyan Science and Technology Project (2017LY68).
Conflicts of Interest: The authors declare no conflict of interest.

References
1.    Pal, K.; Pal, S.K. Effect of Pulse Parameters on Weld Quality in Pulsed Gas Metal Arc Welding: A Review.
      J. Mater. Eng. Perform. 2010, 20, 918–931. [CrossRef]
2.    Rao, Z.H.; Zhou, J.; Liao, S.M.; Tsai, H.L. Three-dimensional modeling of transport phenomena and their
      effect on the formation of ripples in gas metal arc welding. J. Appl. Phys. 2010, 107, 054905. [CrossRef]
3.    Meng, X.; Qin, G.; Zou, Z. Characterization of molten pool behavior and humping formation tendency in
      high-speed gas tungsten arc welding. Int. J. Heat Mass Transf. 2018, 117, 508–516. [CrossRef]
4.    Wang, L.; Chen, J.; Wu, C.; Gao, J. Backward flowing molten metal in weld pool and its influence on humping
      bead in high-speed GMAW. J. Mater. Process. Technol. 2016, 237, 342–350. [CrossRef]
5.    Liu, Z.; Fang, Y.; Qiu, J.; Feng, M.; Luo, Z.; Yuan, J. Stabilization of weld pool through jet flow argon gas
      backing in C-Mn steel keyhole TIG welding. J. Mater. Process. Technol. 2017, 250, 132–143. [CrossRef]
6.    Chai, X.; Yang, Y.K.; Carlson, B.E.; Kou, S. Gas Metal Arc Welding of Magnesium Alloys: Oxide Films, High
      Crowns, and Fingers. Weld. J. 2015, 94, 16S–33S.
7.    Pang, J.; Hu, S.; Shen, J.; Wang, P.; Liang, Y. Arc characteristics and metal transfer behavior of CMT + P
      welding process. J. Mater. Process. Technol. 2016, 238, 212–217. [CrossRef]
8.    Fan, H.G.; Kovacevic, R. A unified model of transport phenomena in gas metal arc welding including
      electrode, arc plasma and molten pool. J. Phys. D Appl. Phys. 2004, 37, 2531–2544. [CrossRef]
9.    Cho, M.H.; Farson, D.F. Understanding Bead Hump Formation in Gas Metal Arc Welding Using a Numerical
      Simulation. Metall. Mater. Trans. B 2007, 38, 305–319. [CrossRef]
10.   Chen, X.; Yu, G.; He, X.; Li, S.; Miao, H. Effect of droplet impact on molten pool dynamics in hybrid laser-MIG
      welding of aluminum alloy. Int. J. Adv. Manuf. Technol. 2018, 96, 209–222. [CrossRef]
11.   Zhang, Z.; Xue, J.; Jin, L.; Wu, W. Effect of Droplet Impingement on the Weld Profile and Grain Morphology
      in the Welding of Aluminum Alloys. Appl. Sci. 2018, 8, 1203. [CrossRef]
12.   Liang, Y.; Hu, S.; Shen, J.; Zhang, H.; Wang, P. Geometrical and microstructural characteristics of the TIG-CMT
      hybrid welding in 6061 aluminum alloy cladding. J. Mater. Process. Technol. 2017, 239, 18–30. [CrossRef]
13.   Pickin, C.G.; Williams, S.W.; Lunt, M. Characterisation of the cold metal transfer (CMT) process and its
      application for low dilution cladding. J. Mater. Process. Technol. 2011, 211, 496–502. [CrossRef]
14.   Chen, S.; Xu, B.; Jiang, F. Blasting type penetrating characteristic in variable polarity plasma arc welding of
      aluminum alloy of type 5A06. Int. J. Heat Mass. Transf. 2018, 118, 1293–1306. [CrossRef]
15.   Mendez, P.F.; Eagar, T.W. Penetration and defect formation in high current arc welding. Weld. J. 2003, 82,
      S296–S306.
16.   Wang, L.L.; Lu, F.G.; Cui, H.C.; Tang, X.H. Investigation of molten pool oscillation during GMAW-P process
      based on a 3D model. J. Phys. D Appl. Phys. 2014, 47, 465204. [CrossRef]
17.   Wang, L.; Wu, C.S.; Gao, J.Q. Suppression of humping bead in high speed GMAW with external magnetic
      field. Sci. Technol. Weld. Join. 2016, 21, 131–139. [CrossRef]
18.   Cheon, J.; Kiran, D.V.; Na, S.J. CFD based visualization of the finger shaped evolution in the gas metal arc
      welding process. Int. J. Heat Mass Transf. 2016, 97, 1–14. [CrossRef]
19.   Silwal, B.; Santangelo, M. Effect of vibration and hot-wire gas tungsten arc (GTA) on the geometric shape.
      J. Mater. Process. Technol. 2018, 251, 138–145. [CrossRef]

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