thermo-mechanical processing of titanium alloys by … · waikato centre for advanced materials...

Waikato Centre for Advanced Materials

Thermo-mechanical Processing of Titanium

Alloys by Powder Metallurgy

Brian Gabbitas


Faculty of Science and Engineering The University of Waikato

New Zealand


Benefits of a PM approach

• Increased interest in using PM to make Ti

components:

– Reduced production costs

– Near net-shape forming

– Minimised machining costs

• Drawbacks to processing:

– Availability of affordable Ti powder

– Additional precautions required because Ti has an affinity

for O, N and H

– Dealing with this difficulties to enable economic

processing


Challenges and opportunities [1] [1] Peters et al, Current Status of Ti PM: Progress, Opportunities and Challenges, Key Engineering Materials, 520, pp1-7, 2012

• Cost: a perception that Ti is too expensive. A legacy

of aerospace applications.

• Powder Quality: – Powder cleanliness

– Contaminants introduced during post processing

– Paucity of information on long production runs of a single

part

• Industrial Grade Ti


Powder compact forging and extrusion

Typical PM processes for Ti and its alloys :

Raw material

Green powder compact

Sintering

Thermal-mechanical

processing

PA powder

BE powder

Cold compaction

Warm compaction

CIP

Induction sintering

Vacuum sintering

Forging

Extrusion


Powder compaction

Effect of H/D ratio on density/pressure curves

for Ti powder compaction. A density/pressure

curve for aluminum powder compaction is also

shown for comparison [1].

[1] Z. Razavi Hesabi, et al., "An investigation on the compressibility

of aluminum/nano-alumina composite powder prepared by blending

and mechanical milling," Materials Science and Engineering: A, vol. 454,

pp. 89-98, 2007.

The relative density of HDH Ti and HDH

and GA Ti-6Al-4V powder compacts as

functions of temperature under constant

pressure: 544 MPa applied to HDH Ti powder,

726 MPa applied to HDH and GA Ti-6Al-4V powders.


Particle size distributions

HDH Ti6Al4V

GA Ti6Al4V

Powder Particle size d(0.1) d(0.5) d(0.9)

GA Ti -100 mesh 70.530 97.008 133.547

HDH Ti -200 mesh 20.843 47.559 90.759

GA Ti64 -100 mesh 72.290 104.085 149.479

HDH Ti64 -200 mesh 10.234 45.645 97.630

Al-40wt%V -325 mesh 6.658 39.320 89.396


Powder Compositions

Powder Particle size H O N C Fe V Al Ti

HDH Ti -200 mesh 0.023 0.35 - 0.07 - - - Bal

GA Ti -100 mesh 0.027 0.11 0.02 0.01 0.11 - - Bal

HDH Ti64 -200 mesh 0.039 0.50 0.18 0.02 0.05 3.9 6.00 Bal

GA Ti64 -100 mesh 0.0045 0.13 0.02 0.02 0.05 4.1 6.07 Bal

Al-40%V -250 mesh - 0.24 - 0.003 0.16 42.24 Bal -


Powder Sintering

Stress-strain curves of specimens cut from as-sintered compacts:

(a) HDH Ti, relative density of 91%;

(b) GA Ti-6Al-4V, relative density of 85.8%;

(c) HDH Ti-6Al-4V, relative density of 80.8%. (AS=as-sintered)


Schematic of powder compact forging

35 mm a

45 mm b


Cross-section of a forged valve rocker

HDH Ti GA Ti6Al4V HDH Ti6Al4V


Mechanical Properties of Forgings (Valve Rocker As-Forged Condition)

Powder Oxygen

content %

Yield Stress

σys/MPa

Tensile Strength

σu/MPa

Ductility

% elongation to fracture

HDH Ti 0.41 631-663 767-800 9-14.3

HDH Ti

(5 mins holding)

0.42 597-658 770-800 16.3-27.1

GA Ti 0.12 431-500 538-590 14.0-27.3

HDH Ti6Al4V 0.52 1145-1296 1217-1372 0.7-7.9

GA Ti6Al4V 0.14 737-960 845-1070 1.7-10.9

Grade 1 Ti ingot [2] ≤0.18 170 240 24

Grade 4 Ti ingot [2] ≤0.40 480 550 15

Ti6Al4V ingot [2] 0.08-0.2 800-1100 900-1200 13-16

[2] E.W.C. Rodney Boyer, Materials Properties Handbook: Titanium Alloys, in: ASM International, 1994.


Mechanical Properties of Forgings (Valve Rocker As-Forged + Heat Treated

Condition)

Yield Stress σys/MPa Tensile Strength σu/MPa Ductility

% elongation to fracture

HDH Ti6Al4V (0.52% oxygen)

As-forged 1160 1292 6.2

Duplex anneal 1206 1262 3.3

Solution and aging 1352 1422 7.2

Recrystallization 1092 1221 12.5

Beta anneal 1163 1265 4.8

GA Ti6Al4V (0.14% oxygen)

As-forged 955 1063 9.3

Duplex anneal 997 1046 14.3

Solution and aging 1103 1195 9.9

Recrystallization 811 983 15.1


Stress-strain curves for test-pieces taken from the diving helmet component

45 mm b

As-forged Titanium As-forged and annealed at 750oC for 2 hours


Powder Compact Extrusion


Advantages and Disadvantages of Powder Compact

Extrusion using a BE approach

• Advantages

It is a lower cost process compared with using alloy powder.

There is a high degree of freedom in selecting an alloy

composition and in microstructural design.

A potential for reducing oxygen content compared with using

pre-alloyed powder.

• Disadvantages

Oxide scale on the surface of elemental and master alloy

powder.

An inhomogeneous distribution of elements which causes a

non-uniform microstructure.


Ti

allo

y

Microstructure Observation

(OM, SEM, TEM)

Property Test (UTS, YS, δ)

Table 1 characterization of parent powders

Parent

powder

Particle size O (wt. %)

Ti

(HDH)

-200mesh 0.33

Al 40μm

Al-V -75μm 0.22

Experimental Procedure (Blended Elemental)

1200℃,1250℃, 1300℃

Schematic map of Processing of Ti-6Al-4V rods by powder compact extrusion of powder mixture

Ti-6Al-4V rods : 0.40 wt.%O

100 tonne press


1200 ℃ 2min 1250℃ 2min 1300 ℃ 2min

Yield strength(MPa) - 1061 1255

Tensile strength(MPa) 886 1135 1300

Ductility (% elongation to fracture)

1.2 2.0 7.0%

0 1 2 3 4 5 6 7 8 90

200

400

600

800

1000

1200

1400

Strain, ％

Ten

sile

str

ength

, M

Pa

1200℃ 2min

1250℃ 2min

1300℃ 2min

Tensile properties of extruded bars-effect of

temperature.

Tensile properties of Ti-64 rods extruded at different conditions

The powder compacts were

held at the extrusion

temperature during induction

heating for 2 mins before

extrusion.


1300 ℃ 2min 1300℃ 5min 1300 ℃ 10min

Yield strength(MPa) 1255 1216 1180

Tensile strength(MPa) 1300 1254 1215

Ductility (% elongation to fracture)

7.0% 8.0% 10.0%

Tensile properties of Ti-64 rod extruded at 1300℃ with different holding time

Effect of holding time on tensile properties

The powder compacts

were held at the

extrusion temperature

during induction heating

for 2, 5 and 10 mins

before extrusion.


Effect of extrusion temperature on microstructure

1200℃ 2min 1250℃ 2min 1300℃ 2min

Optical microstructure of Ti-64 rods extruded at different conditions

1300℃ 2min 1200℃ 2min 1250℃ 2min

Undissolved particle

Undissolved particle size is about 70μm

Fully lamellar structure


300 Tonne Press Extrusions

Effect of different extrusion speeds

Alloy: BE Ti6Al4V containing 0.43 % oxygen

Extrusion

speed

mm/s

Yield Stress

σys/MPa

Tensile

Strength

σu/MPa

Ductility

% elongation to

fracture

17 (outer part) 1170 1255 10

17 (inner part) 1170 1210 4

68 1200 1272 10

122 1140 1170 6

Ti 6Al 4V

Hot pressed ar 1300C under argon Held 5 mins at temperature before hot pressing Extruded in air at 1150C


Tensile properties of as-extruded pure titanium

produced using two manufacturing routes

Condition Yield

Stress

σys/MPa

Tensile

Strength

σu/MPa

Ductility

% elongation to

fracture

As-vacuum-sintered at 1300oC for

2h + extruded at 900oC (0.43%

oxygen)

650 705 20

As-extruded at 1300oC (0.38 %

oxygen)

654 798 27


Impact Toughness

Material Impact Toughness (J) Comment Reference

Ingot Titanium

146.0

-

[3]

Ingot Ti-6Al-4V 17.0-20.0 < 0.2 wt% oxygen [4]

Annealed Ingot Ti-6Al-4V 33.0 - [3]

Powder Compact Extruded Ti-

6Al-4V

27.3 Hot Pressed and Extruded, Oxygen level 0.42 wt% Unpublished data

Powder Compact Extruded Ti-

6Al-4V

14.0 Vacuum Sintered and Extruded, Oxygen level 0.32 wt% Unpublished data

Selective laser melting Ti-6Al-4V

11.5

- [5]

HIPed PREP Ti-6Al-4V 60.8 - [6]

[3] Impact Properties of Titanium and Ti-6Al-4V Produced by Powder Compact Forging and Extrusion, Unpublished Report,

Waikato Centre for Advanced Materials, School of Engineering, University of Waikato, Hamilton, New Zealand, (2011).

[4] Titanium Alloy Guide, RMI Titanium Company, (2000) 28.

[5] E.Ye et al, Experimental Investigation of Charpy Impact Tests on Metallic SLM Parts,

Innovative Development in Design and Manufacturing: Advanced Research in Virtual and Rapid Prototyping,

ed. P.J. D. Bartolo et al. (Boca Raton, FL:CRC), (2010) 207-214.

[6] L. Wang, Z. B. Lang, H. P. Shi, Properties and Forming Process of Prealloyed Powder Metallurgy Ti-6Al-4V Alloy,

Transactions of Nonferrous Metals Society of China, 17 (2007) 639-643.


Summary

• Titanium PM is a viable technology for manufacturing low

cost titanium parts.

• The results obtained from PCF and PCE reinforce the concept

of an Industrial Grade of titanium suggested by W. Peter et al.

• Fast processing times and few processing steps are needed to

ensure that processing from compact to final part can be done

economically.

• Although the levels of tensile strength and ductility are

suitable for many applications, more work is needed for a

better understanding of high strain rate and impact properties.

• Much more fracture toughness data is needed, and a good

understanding of the relationship between structure and

fracture properties.


Acknowledgements

Funding to support this work from the Ministry for Business,

Innovation and Employment (MBIE), New Zealand is gratefully

acknowledged.

I am also very grateful for the contribution made by research staff

and students in my research group and the support form our

industrial collaborators.

TiDA

South Auckland Forgings Engineering

Accord Precision Ltd

E. Goddard Ltd

Callaghan Innovation

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