Profile Map of Weld Beads and Its Formation Mechanism in Gas Metal Arc Welding - MDPI
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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; firstname.lastname@example.org * Correspondence: email@example.com; 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 . 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 , which has motivated many researchers to study the bead formation in detail. Meng et al.  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 . 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.  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 . 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
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.  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.  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.  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.  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.  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.  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  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.  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
Metals 2019, 9, 146 3 of 10 Metals 2019, 9, x FOR PEER REVIEW 3 of 10 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 current 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 , which is characterized by a cold metal transfer (CMT) + pulse process , 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 ininwhich whichthe themaximum maximumwidth widthofofreinforcement reinforcementisisalso also doublethat thatofofthetheweld weldpool pooland andthethecontact contactangle angleisisnearly nearly90°. 90 . ◦ double Figure 2. Cont.
Metals 2019, 9, 146 4 of 10 Metals 2019, 9, x FOR PEER REVIEW 4 of 10 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, ,with with permissionofofElsevier, permission Elsevier,2016); 2016);and and(i) (i)bead beadof ofTIG-CMT TIG-CMT(reproduced (reproducedfrom from, ,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 , 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
Metals 2019, 9, 146 5 of 10 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. Wang Moreover, indicated et al.that 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 . With . 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 . In our case, when the bottom wall is in a solid state, leading edge of the weld pool . 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
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
droplet impingement. Cheon et al.  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 .driving forces Moreover, are et Chen small, it gains al.  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 , , 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 , with 13 mm) (reproduced frompermission , 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 =, with (reproduced 8 mm) permission offrom Springer, ,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  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
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. , 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.  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 . Moreover, Chen et al.  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  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
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.  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.
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] © 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
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