Metallographic preparation of titanium
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
Metallographic
preparation of
titanium Application
Titanium is a relatively new metal
and expensive to produce, but its
titanium production and processing
is extremely important.
Notes
outstanding properties of high
strength to weight ratio and excellent This is why metallography of
corrosion and heat resistance have titanium is an integrated part of
made titanium and its alloys well quality control of titanium, from
established engineering materials. monitoring the initial production
process, to porosity checks on
Titanium is exceptionally resist- cast parts and controlling heat
ant to corrosion by a wide range treatment processes. In addi-
of chemicals. Its high affinity for tion, metallography plays a role
oxygen results in a thin, but dense, in research and development of
self-healing stable oxide layer, which titanium alloys and products.
provides an effective barrier against
incipient corrosion. In addition, it Titanium is a very ductile metal
is the high strength to weight ratio and prone to mechanical de-
maintained at elevated tempera- formation. For the abrasive
tures, which makes titanium and processes in metallographic
its alloys attractive cutting, grinding and polish-
for many critical ing, this aspect has to be
applications. taken into consideration.
Titanium and its al-
loys are widely used
in the aerospace
and aircraft, chemical and medical
Martens
industry, where high safety is essen- on a tub
itic stru
cture in
e weld. the heat
tial. Consequently, quality control of Etchant:
Weck's
influenc
ed zone
reagent,
1000 x
Difficulties during metallographic preparation
Cutting: Titanium can easily overheat Grinding and polishing:
during cutting and large burrs can Due to its ductility titanium deforms
occur: and scratches easily:
Solution:
Special cut-off wheel
for titanium.
Chemical-Mechanical
polishing.
Electrolytical polishing.
Fig. 1 Fig. 2 DIC, 50 x
Production and Fig. 5: Screws and bone
plate electro-chemically
applications
oxidized for colour coding.
The colours are the result
of different thickness
of titanium of the
oxide layer.
The production of titanium is a
three-step process:
1. The first step is the manufacture of
titanium sponge and involves the chlori-
nation of rutile ore (TiO2).
Chlorine gas and coke are combined Fig. 3: Grey oxide rolled into surface of bar.
with the rutile and react to form tita-
nium tetrachloride. Secondary fabrication for produc-
This is purified by distillation and then ing parts from mill products includes
reduced with magnesium to titanium manufacturing processes such as die
sponge and magnesium chloride. forging, extrusion, hot and cold forming Fig. 6: Hip implant with CaP-coating
etc. Hot forming of titanium is not only
2. This titanium sponge is then crushed a shaping procedure, but a method to tions, seawater, body fluids, fruit and
into grainy powder, mixed with scrap produce and control the microstructure. vegetable juices. Typical products are
and/or alloying metals such as vana- reaction vessels, heat exchangers,
dium, aluminium, molybdenum, tin and The high strength/low density of tita- valves, and pumps; prosthetic devices
zirconium, and melted in a vacuum arc nium make it a crucial material in the such as implants, artificial bones, artifi-
reduction furnace to produce titanium aerospace industry. Its main gas turbine cial heart pumps and valve parts.
ingots. engine applications include, compressor The most widely used alloy for these
rings, discs, and spacer casings, ducts products is Ti-6Al-4V.
3. These ingots from the first melt are and shrouds. In aeroplane structural
then used in a second melt as consum- frames titanium alloys are used in under Its light weight combined with aesthetic
able electrodes. This process is called carriage parts, engine mountings, and design has made it a favorite for high
“double consumable-electrode vacuum control mechanism parts, sheet and fas- class consumer goods such as jewel-
melting process”. For very pure and teners for outer body construction. lery, golf clubs, eyeglasses, bikes and
clean titanium with very homogenous watches, and in architecture it is used
structures, an additional third melt can Titanium’s superior corrosion resistance for decorative façades (Fig. 4)
be carried out. and biocompatibility make it an ideal
material for the chemical, medical and
In a first fabrication step, the cast in- food industry, and for ocean research
gots, either cylindrical and 15 metric and development. With its passive oxide
tons, or square and 10 metric tons, are film it has a high corrosion resistance
hot forged into general mill products against salt solutions, nitric acid solu-
such as smaller billets, slabs, bar and
plate. As cast ingots can have an inher-
ent coarse microstructure, which makes
them sensitive to cracking, close tem-
perature and process control are main-
tained during forging operation.
The finished products consist of forg-
ings for aerospace applications as well
as slab, bar, and other feedstock for
further processing into bar, rod, wire,
sheet or plate. Fig. 3 shows a defect
on a rolled titanium bar, grey oxide has
been rolled into the surface.
Fig. 4: Entrance Experience Music
Project, Seattle,
Difficulties in the Preparation Method
preparation of Grinding
titanium Step PG FG
Surface MD-Mezzo MD-Largo
Type Diamond Diamond
Abrasive
Size #220 9 μm
uspension /
S Water DiaPro
The main problem of preparing titanium The first step is a plane Lubricant Allegro/Largo
for microscopic observation is its high grinding with silicon car-
ductility, which makes titanium difficult bide Foil. Plane grinding is rpm 300 150
to cut, grind and polish. In the follow- followed by a single fine
ing recommendations specific advice grinding step on a hard F orce [N]/ 40 *** 30
is given how to overcome this typical surface such as MD-Largo specimen
behavior of titanium. or MD-Plan. As abrasive
either 9 µm diamond sus- Time (min) Until Plane 5
Recommendations for the pension or DiaPro Allegro/
Largo 9 or DiaPro Plan 9
preparation of titanium and are used. Polishing
its alloys
Step OP
When harder titanium such
Cutting: Due to its high ductility, titanium
as α/β or β alloys are pro-
produces long chips when machined or
cessed, the plane grinding Surface MD-Chem
cut, which makes metallographic cutting
with a MD-Piano 120 or
with regular aluminium oxide cut-off Type Collodial Silica
220 may be more efficient, Abrasive
wheels very ineffective. Heat damage can Size 0.04 μm
especially when unmounted
occur easily (see Fig.1) and there-
large samples in high vol- Suspension / OP-S*
fore special silicon carbide cut-off Lubricant
umes have to be prepared.
wheels have been developed spe-
cifically for sectioning of titanium rpm 150
Pure titanium, should
(e.g. 20S30 and 20S35).
always be ground using sili-
F orce [N]/ 30 per sample
Cutting titanium also produces con carbide Foil for plane specimen
a characteristic smell that can become grinding.
pronounced when cutting large pieces Time (min) 5 **
or quantities. In these cases it is recom- The third and final step is
mended to connect a fume extraction to a chemical-mechanical
Table 1 shows a general, automatic preparation method for titanium and its alloys with 6
the cut-off machine. polishing with a mixture of unmounted samples, 30 mm dia. clamped in a holder. Please be aware that the polishing
colloidal silica (OP-S) and time can vary depending on the purity of the titanium and the area of the sample surface.
Mounting: In primary production control hydrogen peroxide (30%). * Mix 90 ml OP-S with 10 ml H2O2 (30%).
** The polishing time depends on the sample area. Very large
labs, which check mainly ingots, billets The concentration can vary samples require more polishing time than small ones.
and slabs, large samples of titanium are between 10-30%. *** Decrease to 25 N to aviod pencil shapes in single sample preparation.
metallographically processed Note: during the last 10 seconds of the preparation step with OP-S, the rotating cloth is
flushed with water. This will clean the samples, holder and cloth.
unmounted. For smaller manu- Unlike some other colloidal
factured parts that need to be silica, OP-S was developed
mounted, such as wires or fas- to accommodate chemical These reagents may work faster, how-
teners, hot compression mount- additions without transform- ever, Struers does not recommend to
ing with phenolic resin (Multi- ing into a gel-like consistency, and is use them for polishing, because they are
Fast) or cold mounting with slow curing therefore well suited for polishing titani- more corrosive than hydrogen peroxide
epoxy (EpoFix) are recommended. um. During chemical-mechanical polish- and proper precautions have to be ob-
ing, the reaction product of the hydrogen served when handling these acids in the
Grinding and polishing: Its extreme peroxide with titanium is continuously polishing procedure. When working with
ductility makes titanium prone to me- removed from the sample surface with hydrogen peroxide it is recommended to
chanical deformation and scratching, the silica suspension and leaves the wear rubber gloves.
which necessitates a chemical-mechan- surface free of mechanical deformation.
ical polish. The three-step, automatic References in the relevant literature also If this chemical-mechanical polish is not
method described in Table 1 is a proven mention nitric and hydrofluoric acid mix- used, the surface of the titanium sample
procedure, which gives excellent, repro- tures for chemical-mechanical polishing will exhibit a very scratched appearance,
ducible results. of titanium. and it is almost impossible to achieve a
good polish with diamond only.
Preparation Method
Grinding
Step PG FG OP
Surface SiC Foil #320 MD-Largo MD-Chem
Type SiC Diamond Collodial Silica
Abrasive
Fig. 7: Titanium after 3 µm diamond polish, Size #320 9 μm 0.04 μm
deformation and scratches are not removed.
Suspension/ Water DiaPro OP-S*
Contrary to the usual procedure of using Lubricant Allegro/Largo
finer and finer diamond for polishing,
diamond polishing actually introduces rpm 300 150 150
continuously mechanical deformation
which leaves scratches and smearing Force [N] 15 20 20
specimen
on the surface (see Fig. 7). Once intro-
duced, this layer of deformation is dif-
Time (min) Until Plane 5 10-25 (or longer)
ficult to remove even with the colloidal
silica and hydrogen peroxide mixture.
*80% OP-S + 10% H2O2 (30%) + 10% NH4OH (25%)
Therefore diamond polishing should be Electrolytic Polishing
avoided, especially with commercially
pure titanium. Equipment: LectroPol-5 are particularly appropriate for the fol-
lowing reasons: speed of results, ease
The preparation time depends on the of operation, reproducibility. Also, elec-
Electrolyte: A3
sample area and the alloy. trolytical polishing leaves no mechanical
deformation on the sample surface.
The larger the sample and the purer the Mask size: 1 cm2 This could be especially relevant for
titanium the longer the preparation time research applications. α alloys, which
of the final oxide polishing step, which Temperature: Room temperature have a homogenous structure, are
18-20°C particularly well suited for electrolytical
can take up to 10 minutes for commer-
cially pure titanium even up to 45 min. polishing, but also α-β alloys can be
Flow rate: 10-15
A properly polished, unetched surface polished electrolytically.
of titanium looks white when examined
in the optical microscope, and polishing Voltage: 35-45 V The electrolytical polishing procedure
has to continue until this state of the requires a fine ground surface to #1200
surface is reached. Time: 20-30 sec.
or finer. Table 2 shows a general pro-
cedure for titanium and titanium alloys.
Due to the production process, Table 2 shows a general, automatic preparation method After the electrolytical polishing, the
titanium and its alloys are very clean,
for pure titanium with 6 unmounted samples, 30 mm dia. polished specimen is examined in polar-
clamped in a holder.
which means, that little black dots ized light or etched chemically (etchants
appearing on a polished surface are then dried with ethanol and a strong see under “Etching and Interpretation”).
remains from grinding deformation and stream of air. If after the cleaning step
not inclusions or part of the structure. residue of OP-S is seen on the surface
of the sample, the cleaning has not been
This artefact needs to be removed with carried out properly and has to be re-
further chemical-mechanical polishing. peated. An efficient and repetable clean-
Once the surface is polished sufficiently, ing method can be done by automatic
the structure can already be seen in cleaning equipment like Lavamin.
polarised light without etching. (See
Fig. 8).
Note: When working with colloidal silica
(OP-S) it is important to wet the cloth
with water before starting the polishing.
To clean the samples, it is essential to
flush the rotating cloth with water ap-
proximately 10-15 seconds before the
machine stops. The water washes off As an alternative to mechanical polish-
the OP-S from the samples, holder and ing, electrolytical polishing can be
cloth. The samples are then cleaned recommended when fast results are
again individually under tap water, and required. Electrolytic polishing methods
Fig. 8: Cross-section of bar, commercially pure
titanium, electrolytically polished, 100 x, polarized
light.
Etching and Fig. 9:
Grain structure of a commercially
interpretation
pure titanium, polarized light, 100 x
As mentioned before, the surface of Fig.10:
Structure of a forged α-β
a well-polished titanium sample can Ti-6Al-4V, 400 x
already be observed unetched with polar-
ized light. The contrast with this illumina-
tion is not always very pronounced, but
it is ideal for a general check to see if the
polish is sufficient.
The most common chemical etchant for
titanium is Kroll’s reagent:
100 ml water
1-3 ml hydrofluoric acid
2-6 ml nitric acid
The concentration can vary depending Fig.11:
α-β Ti-6Al-4V with a white, brittle
on the alloy and can be adjusted individ- “α-case” surface layer, 50 x
ually. It colors the β phase dark bown.
Titanium can be colour etched with
Weck’s reagent:
100 ml water
5 g ammonium bifluoride
Metallurgy and microstructures
Commercial titanium grades and alloys
are divided into four groups: commer-
cially pure titanium; α and near α alloys
such as Ti-6Al-2Sn-4Zr-2Mo; α-β
alloys, of which Ti-6Al-4V is the most Fig.12:
β Ti-15V-3Al-3Sn-3Cr,
well known, and β alloys that have a heat tinted, 50 x
high content of vanadium, chromium
and molybdenum.
Titanium undergoes an allotropic
change from a low temperature close
packed hexagonal structure (α) to a
body centred cubic phase (β), at a tem-
perature of 882°C.
This transformation allows alloys with
α, β, or mixed α/β microstructures, and
the possibility of using heat treatment Consequently metallo-graphy has an Fig. 9 shows the grain structure of a
and thermo-mechanical treatment. important role in ensuring that prod- commercially pure titanium part that has
ucts have the correct microstructure, been mechanically deformed through
Consequently a wide range of properties which in turn reflects the appropriate bending. Twinning due to mechanical
can be obtained from a relatively small degree of process control. The relations deformation is visible.
number of alloy compositions. between hot forming, heat treatment,
To ensure the desired combination of microstructure and physical properties Fig.10 shows the structure of a forged
microstructure and properties, close in the production of titanium are very α-β Ti-6Al-4V of an orthopaedic im-
process control has to be maintained. complex. In the following only a few plant in the annealed condition, etchant:
examples of the most common types of Kroll’s reagent.
titanium microstructures are described.
Struers A/S
Pederstrupvej 84
DK-2750 Ballerup,
Denmark
Phone +45 44 600 800
Fax +45 44 600 801
struers@struers.dk
www.struers.com
AUSTRALIA & NEW ZEALAND JAPAN
Fig.11 shows an α-β Ti-6Al-4V with Application Struers Australia Marumoto Struers K.K.
a white, brittle “α-case” surface layer, Notes 27 Mayneview Street
Milton QLD 4064
Takara 3rd Building
18-6, Higashi Ueno 1-chome
etchant: Weck’s reagent. Although hot Australia Taito-ku, Tokyo 110-0015
Metallographic preparation of titanium Phone +61 7 3512 9600 Phone +81 3 5688 2914
forming processes are carried out under Fax +61 7 3369 8200 Fax +81 3 5688 2927
controlled atmosphere, titanium can Bill Taylor, info.au@struers.dk struers@struers.co.jp
BELGIQUE (Wallonie) NETHERLANDS
absorb oxygen already at lower tem- Struers Ltd, Glasgow Struers S.A.S. Struers GmbH Nederland
peratures, which results in a surface 370, rue du Marché Rollay Elektraweg 5
Elisabeth Weidmann, F- 94507 Champigny 3144 CB Maassluis
hardened zone, “α-case”. This is a very sur Marne Cedex Telefoon +31 (10) 599 7209
Struers A/S, Copenhagen Téléphone +33 1 5509 1430 Fax +31 (10) 5997201
brittle layer and can only be removed Télécopie +33 1 5509 1449 netherlands@struers.de
mechanically. (Note: α-case does not struers@struers.fr
ÖSTERREICH
Acknoledgements: BELGIUM (Flanders) Struers GmbH
show up with Kroll’s etchant, but with Struers GmbH Nederland Zweigniederlassung Österreich
We wish to thank Fa. Aesculap, Tuttlingen,
the bifluoride). Elektraweg 5 Betriebsgebiet Puch Nord 8
Germany, for supplying information on 3144 CB Maassluis
Telefoon +31 (10) 599 7209
5412 Puch
Telefon +43 6245 70567
Fig. 12 shows the β structure of the titanium, and the permission to reproduce Fax +31 (10) 5997201 Fax +43 6245 70567-78
netherlands@struers.de austria@struers.de
longitudinal section of a Ti-15V-3Al- Figs 5, 6 and 11. CANADA POLAND
3Sn-3Cr alloy plate. It is used in the We thank Wah Chang North, Albany, Oregon, Struers Ltd.
7275 West Credit Avenue
Struers Sp. z o.o.
Oddział w Polsce
aerospace industry because of its su- USA for supplying sample material, and Mississauga, Ontario L5N 5M9
Phone +1 905-814-8855
ul. Jasnogórska 44
31-358 Kraków
perior mechanical properties. Etchant: Mr. Paul Danielson from Albany Research Fax +1 905-814-1440 Phone +48 12 661 20 60
info@struers.com Fax +48 12 626 01 46
heat tinting. Centre, Albany, Oregon, USA for the color poland@struers.de
CHINA
micrographs on the front page and Fig. 12. Struers Ltd. ROMANIA
No. 1696 Zhang Heng Road Struers GmbH
Summary We thank Lindberg, Aabyhøj, Denmark, Zhang Jiang Hi-Tech Park Sucursala Sibiu
Titanium is a very ductile, low weight for letting us reproduce the image of
Shanghai 201203, P.R. China
Phone +86 (21) 6035 3900
Str.Scoala de Inot, nr. 18
RO-550005 Sibiu
- high strength metal with an excellent Fax +86 (21) 6035 3999 Phone +40 269 244 558
the eyeglasses, model Air Titanium, and struers@struers.cn Fax +40 269 244 559
corrosion resistance and biocompat- romania@struers.de
Experience Music Project, Seattle, USA for CZECH REPUBLIC
ibility. Its ductility requires a specific Struers GmbH SCHWEIZ
reproducing Fig. 4. Organizační složka Struers GmbH
metallographic preparation, using spe- Havlíčkova 361 Zweigniederlassung Schweiz
cial cut-off wheels for sectioning and a CZ-252 63 Roztoky u Prahy Weissenbrunnenstraße 41
Bibliography: Phone +420 233 312 625 CH-8903 Birmensdorf
chemical-mechanical polish with a mix- Fax +420 233 312 640 Telefon +41 44 777 63 07
Metals Handbook, Desk edition, ASM, 1984 czechrepublic@struers.de Fax +41 44 777 63 09
ture of hydrogen peroxide and colloidal switzerland@struers.de
DEUTSCHLAND
silica. This polishing method, carried Struers GmbH SINGAPORE
Carl-Friedrich-Benz-Straße 5 Struers Singapore
out with automatic equipment, gives D- 47877 Willich 627A Aljunied Road,
consistently excellent and reproducible Telefon +49 (0) 2154 486-0
Fax +49 (0) 2154 486-222
#07-08 BizTech Centre
Singapore 389842
results. verkauf@struers.de Phone +65 6299 2268
Fax +65 6299 2661
FRANCE
struers.sg@struers.dk
Struers S.A.S.
370, rue du Marché Rollay SUOMI
F-94507 Champigny Struers Suomi
sur Marne Cedex Kalevankatu 43
Téléphone +33 1 5509 1430 00180 Helsinki
Télécopie +33 1 5509 1449 Puhelin +358 (0)207 919 430
struers@struers.fr Faksi +358 (0)207 919 431
finland@struers.fi
HUNGARY
Struers GmbH SWEDEN
Magyarországi Fióktelep Struers Sverige
Tatai ut 53 Ekbacksvägen 22
2821 Gyermely 168 69 Bromma
Phone +36 (34) 880546 Telefon +46 (0)8 447 53 90
Fax +36 (34) 880547 Telefax +46 (0)8 447 53 99
hungary@struers.de info@struers.se
IRELAND UNITED KINGDOM
Struers Ltd. Struers Ltd.
Unit 11 Evolution@ AMP Unit 11 Evolution @ AMP
Whittle Way, Catcliffe Whittle Way, Catcliffe
Rotherham S60 5BL Rotherham S60 5BL
Tel. +44 0845 604 6664 Tel. +44 0845 604 6664
Fax +44 0845 604 6651 Fax +44 0845 604 6651
info@struers.co.uk info@struers.co.uk
Details of a titanium frame for eye- glasses. ITALY USA
The high strength and ductility of titanium Struers Italia Struers Inc.
make screws and solder on these frames Via Monte Grappa 80/4 24766 Detroit Road
20020 Arese (MI) Westlake, OH 44145-1598
obsolete. Tel. +39-02/38236281 Phone +1 440 871 0071
Fax +39-02/38236274 Fax +1 440 871 8188
struers.it@struers.it info@struers.com
10.2014 / 62140202 Printed in Denmark
You can also read
