A literature report on heat treatment of SLMed Ti6Al4V parts. To the point and very useful for the engineers looking to optimize heat treatment process for SLMed Ti6Al4V parts for industrial applications.

1. Heat Treatment of Ti6Al4V parts
produced by selective laser
melting: A literature survey
By: Dr. Khuram Shahzad
Email: engineerkhuram@gmail.com

2. Introduction to Ti and Ti alloys
Justin Mezzetta, Process-Property relationships of Ti6AL4V fabricated through selective laser melting, Masters thesis, Department of Mining and Materials Engineering, McGill University, Montreal, Quebec, Canada. 2016.
• Properties of pure Ti
• Melting point 1670 °C
• Crystal structure
• α - Hexagonal close packed (hcp), RT to 882.5 °C
• Densely packed planes are basal (0002),
prismatic {1010} and pyramidal {1011}.
• a = 0.295 nm, c = 0.468 nm
• Β – Body centred cubic (bcc), 882.5 °C to m.p.
(1670 °C)
• Six planes found in {110} are most densely
packed planes
• a = 0.332 nm
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3. Introduction to Ti and Ti alloys
Justin Mezzetta, Process-Property relationships of Ti6AL4V fabricated through selective laser melting, Masters thesis, Department of Mining and Materials Engineering, McGill University, Montreal, Quebec, Canada. 2016.
A.A. Antonysamy, Mcirostructure, Texture and Mechanical Property Evaluation during addtitive manufacturing of Ti6Al4V alloy for Aerspace applications, PhD thesis, School of Materials, University of Manchester, 2012.
• Effect of alloying elements
• α – stabilizers: Al, O, N, C. Increasing the solute contents with
these elements in solid solution will effectively raise the β
transus. Mainly Al is used, commercially available and has large
solubility in both α and β, helping in solid solution strengthening.
• β – stabilizers: are either β-isomorphous or β-eutectoid. β-
isomorphous stabilizing include V, Mo, Nb. Alloying with β-
isomorphous allows both α and β phases to be retained. α-β
alloys are heat treatable and allow for good mixture of strength
and ductility through post processing.
• Types of Ti alloys:
• Based on the chemical composition and microstructure at room
temperature Ti alloys are classified in to five major categories
i. α-alloys
ii. Near α-alloys
iii. α+ β alloys, only this alloy will be discussed
iv. Metastable β-alloys
v. β-alloys
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4. Introduction to Ti and Ti alloys
• α+β alloys, typically contain 4-6 wt.% of β stabilizers, which improves the strength and
ductility. These are heat treatable alloys and microstructure can be tailored to
produce different mechanical properties.
• Ti6Al4V:
• α+β alloy
• Most widely used, 50-60% of the all Ti alloys
• Shows exceptionally good balance of strength, ductility, fatigue and fracture
properties
• Can only be used up to 300 °C because of low creep performance
• Heat treatable
• AT 800°C, β ia 15%. At room temperature α is dominating phase.
• β transus is 995 °C
• Al is added to increase the strength of the alloys by solid solution hardening and
α phase stabilization
• Addition of V stabilizes β phase, which improves the room temperature ductility
by providing the right balance by the presence of both phases α and β. The
addition of β stabilizers can retard the formation of α phase and promotes the β
phase transformation to martensite (β  martensite), or remain as retained β.
A.A. Antonysamy, Mcirostructure, Texture and Mechanical Property Evaluation during addtitive manufacturing of Ti6Al4V alloy for Aerspace applications, PhD thesis, School of Materials, University of Manchester, 2012.
R. Reda, et. al., Optimizing the mechanical properties of Ti6Al4V castings, IJMPERD, 5 (1) (2015), 83-104.
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5. Phase transformations in Ti6Al4V
• β  α transformation, development
of lamellar microstructure
• β  α transformation is a diffusion
controlled transformation and occurs
by nucleation and growth of α phase
on the extent of β. This transformation
takes place when alloy is cooled from
β phase field to room temperature at
moderate cooling rates.
• During cooling through α-β region, α
nucleates on the grain boundaries of β
grains. Further cooling leads to
nucleation of α plates nucleate at grain
boundaries and grows in to β grains.
• Further cooling leads to homogeneous
nucleation and growth of α phase in β
grains leading to woven basket like
microstructure.
Justin Mezzetta, Process-Property relationships of Ti6AL4V fabricated through selective laser melting, Masters thesis, Department of Mining and Materials Engineering, McGill University, Montreal, Quebec, Canada. 2016.
A.A. Antonysamy, Mcirostructure, Texture and Mechanical Property Evaluation during addtitive manufacturing of Ti6Al4V alloy for Aerspace applications, PhD thesis, School of Materials, University of Manchester, 2012.
P.B. Vila, Phase transformation kinetics during continuous heating of α+β and metastable β titanium alloys, PhD thesis, TU Wien, Austria, 2015.
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6. Phase transformations in Ti6Al4V
• β  α’, martensitic transformation
• A diffusionless or displacive
transformation which can be achieved
by high cooling rates, quenching in
water or oil is needed.
• Depending on the alloy composition and
cooling rates martensitic transformation
can be classified in to:
• Acicular martensite: needle like structure,
forms at cooling rates higher than 410 °C/S
• Massive (packet) martensite: consists of
large irregular regions without any features,
formed at cooling rates between 20 to 410
°C/S
A.A. Antonysamy, Mcirostructure, Texture and Mechanical Property Evaluation during addtitive manufacturing of Ti6Al4V alloy for Aerspace applications, PhD thesis, School of Materials, University of Manchester, 2012.
R. Reda, et. al., Optimizing the mechanical properties of Ti6Al4V castings, IJMPERD, 5 (1) (2015), 83-104.
6

7. Ti6Al4V: Continuous cooling phase diagram
Critical cooling rate to make full martensite = 18 °C/S
R. Dabrowski, The kinetics of phase transformation during continuous cooling of the Ti6Al4V alloy from the single-phase β range, DOI: 10.2478/v10172-011-0077-x
J. Sieniawski, et. al., Microstructure and mechanical properties of high strength two-phase titanium alloys, http://dx.doi.org/10.5772/56197
3.4 °C/min56.4 °C/min
150 °C/min
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8. Conventional heat treatment of Ti6Al4V parts
• Heat treatment is applied to Ti6Al4V for stress relief and
to optimize mechanical properties. Conventional heat
treatments can be divided in to four categories
• Stress Relief
• Annealing
• Solution treating and Aging
• Hot isostatic pressing
• It is important to note that every heat treatment, above
427 °C, should be performed under vacuum or Ar.
Becasue above 427 °C oxgen can enter in to solid solution
of Ti and make an alpha case around the component
which is brittle and can cause craking at surface and
reduce fatigue life.
F.F. Schmidt and R.A. Wood, Heat treatment of titanium and titanium alloys, NASA Technical Memorandum, NASA TM X-53445, 1966
Don Jordan, Study of alpha case formation on heat treated Ti64 alloy, Heat Treating Progress MAY/JUN 2008, 45-47.
Ti-alloys – Towards achieving enhanced properties for diversified applications, Ed. By Dr. A.K.M. Nurul Amin, Published by InTech, 2012, page 32. 8

9. Conventional heat treatment of Ti6Al4V parts
• Stress Relief
• Stress relieving is performed at
temperatures between 540 to
650 °C for 0.5 to 1 hr.
• The aim is to avoid any phase
transformation
• Another study suggests that heat
treating at 730 °C for 2 hrs can
completely reduce the residual
stresses
260°C
371°C
482°C
593°C
F.F. Schmidt and R.A. Wood, Heat treatment of titanium and titanium alloys, NASA Technical Memorandum, NASA TM X-53445, 1966
T. Vilaro, C. Colin and J.D. Bartout, As-Fabricated and Heat-Treated Mcirostructure of the Ti6Al4V Alloy Produced by Selective laser Melting, Metallurgical and Materials Transaction s A, 42 A (2011), 3190-3199.
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10. Conventional heat treatment of Ti6Al4V parts
• Annealing
• Annealing treatments usually involve holding for 1 to
2 hours at 700°C to 815 °C, furnace cooling to about
600 °C and then air cooling to room temperature.
• Time, temperature and cooling rate are selected so
that the annealing treatment achieves a mixed α-β
structure that is ductile and stable.
F.F. Schmidt and R.A. Wood, Heat treatment of titanium and titanium alloys, NASA Technical Memorandum, NASA TM X-53445, 1966
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11. Conventional heat treatment of Ti6Al4V parts
• Solution Treating and Aging (STA)
• Generally STA heat treatment is applied to obtain
maximum strength levels. The idea behind STA is to
obtain high volume fractions of β phase in final
composition to achieve high strengths.
• Solution treatment slightly below β transus
temperature will posses high strengths and still retain
adequate ductility.
• STA treatments that bring temperatures to above the
β transus will have excellent strengths with a
significant loss in ductility. A trade-off in ductility for
strength with increasing solution treatment
temperature is shown in table.
• STA is carried out in 3 steps:
• Solution treating (Heating above/below β tarsus) resulting
in higher β phase partition
• Quenching in water or Oil, preserves the β phase partition
• Aging for 4 to 8 hrs at temperature ranging between 480
to 595 °C. This will decompose any residual matersite and
unstable β phase brought on by quenching, and effectively
strengthen the material.
F.F. Schmidt and R.A. Wood, Heat treatment of titanium and titanium alloys, NASA Technical Memorandum, NASA TM X-53445, 1966Justin Mezzetta, Process-Property relationships of Ti6AL4V fabricated through selective
laser melting, Masters thesis, Department of Mining and Materials Engineering, McGill University, Montreal, Quebec, Canada. 2016.
11

12. Microstructure and mechanical properties of
as built Ti64 SLM parts
• Two important works are discussed
1. T. Vilaro et al., Engineers from Polyshape and Materials
center of Mines Paris Tech, France.
2. Bey Vranken et. al., MTM and Mechanical Engineering
dept., KUL, Belgium
• As built parts microstructure consists of martensite and
coulmars grains (~200 µm in width and several mm long)
parallel to build direction.
• The parts show anisotropy in mechanical properties, due
to alignment of defects with loading direction and
texturing of microstructure.
• UTS is higher than 1200 MPa, Yield strength is higher
than 1100 MPa. Much higher compared to reference
material. However, the ductility is much lower, due to
difference in microstructure.
• The ductility is very poor for the samples built in
transverse direction, due to sharp process defects
aligned to the loading direction in such a way that flaws
can open to form cracks.
T. Vilaro, C. Colin and J.D. Bartout, As-Fabricated and Heat-Treated Mcirostructure of the Ti6Al4V Alloy Produced by Selective laser Melting, Metallurgical and Materials Transaction s A, 42 A (2011), 3190-3199.
Bey Vranken, et. al., Heat treatment of Ti6Al4V produced by selective laser melting: Microstructure and Mechanical properties, Journal of Alloys and Compounds, 541 (2012), 177-185.
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13. Effect of solution treatment temperature on
morphology of grains
• Solution temperature is very
important parameter. It
determines the amount and
composition of β phase present
when cooling starts
• Both studies confirm that for
solution treatment temperature
higher than β transus (995 °C)
the columnar structure is
converted into equiaxed grains.
The columnar morphology is not
changed when temperature is
below β transus.
T. Vilaro, C. Colin and J.D. Bartout, As-Fabricated and Heat-Treated Mcirostructure of the Ti6Al4V Alloy Produced by Selective laser Melting, Metallurgical and Materials Transaction s A, 42 A (2011), 3190-3199.
Bey Vranken, et. al., Heat treatment of Ti6Al4V produced by selective laser melting: Microstructure and Mechanical properties, Journal of Alloys and Compounds, 541 (2012), 177-185.
13

14. Influence of residence time
• Both α and β phase will tend to
coarse by increasing the time
at higher temperature.
However, these phases are
competing each other. The
effect is more severe when
parts heated above or close to
β transus, where amount of β
increase with less hindrance to
grain growth.
Bey Vranken, et. al., Heat treatment of Ti6Al4V produced by selective laser melting: Microstructure and Mechanical properties, Journal of Alloys and Compounds, 541 (2012), 177-185.
14

15. Influence of cooling rate
• When cooling from lower temperatures,
not much difference in microstructure is
observed, for example when cooled from
850 ° C, there is not much difference
noted in microstructure for furnace
cooled, air cooled and quenched
samples.
• Cooling rate has much profound effect
when cooling from the temperature
around β transus. At higher cooling rates
smaller α coloney size and finer spacing
between α plates. Furnace cooling
results in lamellar α+ β. Air cooling
results in an α-widmanstatten or basket
weave structure. Wehere as quenching
results in α’, martensitic structure.
Bey Vranken, et. al., Heat treatment of Ti6Al4V produced by selective laser melting: Microstructure and Mechanical properties, Journal of Alloys and Compounds, 541 (2012), 177-185. 15

16. Effect of aging (tempering) temperature
• The third step in solution treatment is
aging or tempering. By increasing the
ageing temperature, α’ phase transforms
to more stable α+β (α’α+β).
• The complete decomposition of α’
appears around 800 °C (for 2 hrs).
• The thickness of the lamellar α phase
increases by increasing the tempering
temperature, 1 µm at 750 °C to 2.3 µm at
950 °C
• The β phase, formed by transformation
from α’, is more stable when
transformation temperature is lower due
to high V contents which stabilizes β. This
increases the ductility as this phase does
not transform to α’ during cooling.
T. Vilaro, C. Colin and J.D. Bartout, As-Fabricated and Heat-Treated Mcirostructure of the Ti6Al4V Alloy Produced by Selective laser Melting, Metallurgical and Materials Transaction s A, 42 A (2011), 3190-3199. 16

17. Influence of heat treatment on mechanical
properties
• Tables, showing the effect of heat treatment
parameters on mechanical properties,
performed by Vilaro et al.
• In general by applying heat treatment strength
decreased and ductility increased. A high
difference in LD and TD built sample is present
for as fabricated and low temperature
temperature heat treated samples.
• For high temperature treatment the samples
were built using different parameters. The
results show an improvement in TD built
samples, but still lower than LD. The
elongation for LD is limited to 9.5% which is
still lower than what is achieved by Vrancken
et al. ~ 14%. Vilaro examined the fracture
surface of tensile samples and concluded that
presence of defects on the fracture surface is
the main reason for the low elongation.
Pointing out the importance of producing
defect free parts.
17T. Vilaro, C. Colin and J.D. Bartout, As-Fabricated and Heat-Treated Mcirostructure of the Ti6Al4V Alloy Produced by Selective laser Melting, Metallurgical and Materials Transaction s A, 42 A (2011), 3190-3199.

18. Influence of heat treatment on mechanical
properties
• In general high temperature heat treatment increased the ductility (7
to 14%) on the expense of strength.
• HT 2 and 7 are best as giving a good combination of strength and
ductility.
• HT 2, heating upto 850 °C for 2 hrs and furnace cooling. The final
microstructure consists of α and β phases. Strain before fracture is ~
13% and yield strength is 909 MPa. However, nothing is mentioned
about anisotropy of microstructure.
• HT 2 is much more easier to adapt as it involves no quenching step.
This treatment is very close to the treatment we are currently using,
with only difference that dwelling time is 1 hr. For our HT the
strength is slightly higher (969 ± 17 MPa) and value of strain before
failure is much lower (6.2 ± 1.9). Please note that we can not make 1
on 1 comparison as the samples are produced using different
parameters and can possess different levels of defects, which can
influence the strain before fracture.
• HT 7 also showing the good mechanical properties how can be tricky
to adapt this kind of heat treatment due to quenching step involved.
This will require special type of furnaces and requires discussion with
potential suppliers.
18Bey Vranken, et. al., Heat treatment of Ti6Al4V produced by selective laser melting: Microstructure and Mechanical properties, Journal of Alloys and Compounds, 541 (2012), 177-185.

19. Use of Hot isostatic pressing
• Use of Hot isostatic pressing
(HIPing) can be used to close
the isolated pores. Open or
surface pores can not be
eliminated.
• Ductility is improved however
the strength can be decreased,
due to modification of
microstructure.
• Distortion of parts during HIPing
can be another problem we
need to be careful while
adapting HIPing.
• HIP furnace is very expensive ~
0.7 million Euros
19S.T.Williams, et al., The Effectiveness of Hot Isostatic Pressing for Closing Porosity in Titanium Parts Manufactured by Selective Electron Beam Melting, Metallurgical and Materials Transaction A, 47 (5) (2016), 1939-1946.
S. Siddiqui, et al., Computed tomography for characterization of fatigue performance of selective laser melted parts, Materials and Design, 83 (2015), 661-669.

20. Important notes
• Ti-6-4 parts produced by SLM show martensetic structure, which results in
high strength and very low ductility. Using HT microstructure and
mechanical properties can be modified to achiever good ductility and good
strength. However, correct heat treatment cycle should be selected.
• It is also important to highlight that the strain before fracture is also
dependent on the defects in the parts. So to see the real potential of HT
can be realized by using parts with best known SLS parameters.
• HIPing can be applied to close isolated gas pores and process defects.
Improvement in ductility and fatigue properties is quoted whereas the
strength decreases. The equipment cost is very high and investigations to
conclude on its effectiveness.
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