Blending an elemental approach to volume titanium manufacture

special feature

Blending an elemental approach to volume titanium manufacture The excellent strength-to-density ratio, stiffness and high temperature corrosion resistance of titanium alloys make them attractive to engineers working in sectors such as aerospace, automotive and sporting goods. High processing costs usually limit titanium alloys’ use in high-volume applications, but American researchers believe they can overcome this using blended elemental titanium powders…

T

itanium alloys possess a unique combination of high strength, low density and good corrosion resistance which makes them very attractive for many structural applications. However, the cost of titanium produced by conventional ingot technology is high compared to steel and aluminum, thus limiting their use in automotive applications [1].

A powder metallurgy approach is a viable and promising route for costeffective fabrication of titanium alloys [2-5]. A blended elemental (BE) method is potentially the lowest-cost titanium components manufacturing process for the titanium-aluminium-vanadium alloy Ti-6Al-4V, especially if any secondary compaction step such as hot pressing or hot isostatic pressing (HIP) can be eliminated. A research group drawn from

giant US auto corporations and their major suppliers has produced encouraging results that were reported at the MPIF’s PowderMet 2006 conference and exhibition in San Diego. The most cost-effective PM processes are based on the use of low-cost blended elemental (BE) technology where alloying elements are added to titanium as elemental or master alloy powders [6,7]. Traditionally this method includes the preparation of powder blends, their consolidation at room temperature, and sintering in vacuum for transformation of initial heterogeneous powder compacts into massive homogeneous alloys. Consolidation at room temperature may be performed by low-cost conventional powder metallurgy process-

The Authors

Figure 1. Microstructure of BE Ti-6Al-4V from sodium reduced titanium powder (a,b) and hydrogenated magnesium reduced titanium powder (c,d) after die-pressing + sintering; (99 per cent density)

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This article is drawn from Low-cost powder metallurgy Ti-6Al-4V components for automotive application, a paper by Vladimir A Druz1, V S Moxson1, Russell Chernenkoff2, William F Jandeska Jnr3 and Jean Lynn4, who presented it at PowderMet 2006, the MPIF conference and exhibition held in San Diego. 1ADMA Products Inc; 2Ford Motor Company; 3Midwest Metallurgical Ltd; and 4DaimlerChrysler Corporation.

0026-0657/06 ©2006 Elsevier Ltd. All rights reserved.

Table 1. Tensile properties for pure Ti, TiC/Ti composites and CermeTi-10C tested at a strain rate of 4.2 x 10-5 s-1 (10). TiC (vol.%)

298 K

723 K

973 K UTS (MPa)

εµ (%)

10

19

30

271

17

28

53

81

246

10

32

45

101

190

5.5

25

44

31

49

148

220

2.5

30

48

163

215

3.6

36

24

37

44

184

6.9

YS (MPa)

UTS (MPa)

εµ (%)

YS (MPa)

UTS (MPa)

εµ (%)

0.0

230

386

12.4

43

85

2.1

414

565

18.5

133

3.4

437

514

2.3 1.7

3.5 4.1

418

516

2.8

5.7

346

405

2.2

7.6 8.5

405

474

2.1

49

194

6.0

CermeTi-10C

617

617

0.9

87

523

4.1

es such as die pressing, cold isostatic pressing (CIP), or by direct powder rolling [8]. In order to achieve desired levels of mechanical properties (such as strength, ductility and fatigue strength) sintered materials should not only have a homogeneous chemical composition and microstructure, but also a relative density of greater than 98 per cent of theoretical [6,7]. However, the relative densities of titanium alloys produced by a blended elemental approach normally do not exceed 95 per cent [9]. In order to increase density, a sintered material can be subjected to HIP or other hot deformation process [1011]. However, this increases the number of production steps, which increases part cost and negates the advantages of the PM approach. Thus a desirable goal is to produce titanium PM alloys, in particular the “work horse” Ti-6Al-4V alloy, with a low residual porosity (99 per cent density) using the simple BE method with no subsequent hot defor-

mation step. It has been demonstrated that fully dense “chunky” components with mechanical properties exceeding the ASTM requirements can be produced by die pressing and sintering [7], CIP and sintering [6], and flat components by direct powder rolling and sintering process [12, 13]. Another advantage of the BE powder metallurgy approach is an ability to produce low-cost metal matrix composites (MMCs), reinforced by ceramic particles. MMCs have drawn interest for automotive applications due to their improved stiffness, wear resistance, yield strength, creep resistance and work hardening [14]. Discontinuously reinforced titanium

YS (MPa)

50

alloys containing ceramic particles are considered as possible candidate materials for advanced applications such as connecting rods. In designing the particle reinforced composite systems, chemical compatibility of the constituents and differences in coefficient of thermal expansion (CTE) are important considerations for processing suitability and minimisation of residual stresses after fabrication. Titanium carbide has a high free energy of formation and a wide range of chemical stability: the chemical formula TiCx where x ranges from 1.0 to 0.5 [15, 16], or even lower values [17, 18]. This implies that titanium carbide can be formed in situ by the blending of a suitable carbide

Acknowledgements The authors acknowledge Professor Orest Ivasishin and his group for providing ADMA Products with hydrogenated titanium powder and useful contributions during discussions of the results. The direct powder-rolling activities were supported by the US Army Research Laboratory under Co-operative Agreement No 911NF05-2-0004. The assistance of Jane W Adams in monitoring this study is appreciated.

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Figure 2. Microstructure of MMC from sodium-reduced Ti sponge fines after die pressing + sintering (a,b); cold isostatic pressing (c,d); and direct powder rolling + sintering (e,f ); (density is 4.25g/cm3).

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Table 2. Chemical composition of titanium powder %, weight

Ti Powder

Fe

Si

Ni

C

Cl

N

O

H

Ti

Sodium reduced

0.044

<0.005

<0.003

0.011

0.18

<0.002

0.096

0.008

bal

Hydrogenated magnesium reduced

0.018

0.01

0.014

0.061

0.12

0.015

0.21

3.90

bal

ceramic with the titanium powder into the titanium matrix. Titanium carbide also shows a minimal difference in CTE with Ti matrix (7.7 x 10-6 K-1 of TiC against 10.1 x 10-6 K-1 of Ti) [19]. This benefits good interfacial bonding. Ti/TiC and Ti-6Al-4V/TiC MMCs have been fabricated from various materials [17, 19-21]. There are a few known processes to produce Titanium MMC alloys. For example, the method for manufacturing the Ti-6Al-4V/TiC composite disclosed in US Patent No 5,722,037 [22] provides a density of the resulting material only at about 93 per cent of the theoretical value even after vacuum sintering for four hours at 1300oC. The method proposed includes formation of reinforcing TiC particles in the titanium matrix by chemical reaction with hydrocarbon gas that is more effective in a porous matrix than in a dense one. The parts produced by this method would require an expensive encapsulation for high temperature densification to achieve acceptable properties characteristic of a near full density alloy. T. Kaba, et al in their US Patent No 5,534,353 [33] proposed compacting a powdered component blend by cold isostatic pressing, atomising the product by melting and spraying, and finally, sintering the atomised powder by HIP at 1100oC (2012oF). The final product

has improved bending strength at room temperature, but includes atomising in a protective atmosphere, and it still has an interconnected porosity that requires an additional encapsulating step for the HIP with a consequent increase in production costs. V de Castro, et al evaluated the effect of the content, size and morphology of the TiC particles in discontinuously reinforced titanium matrix composites including CermeTi10C alloy described above and in situ reinforced Ti matrix composites (produced by blending/milling 99.5 per cent pure Ti sponge and graphite powder followed by moulding the pellets and casting) on mechanical properties at various temperatures (10). The tensile test results indicated that the presence of coarse TiC particles degrade the mechanical properties. All of the tensiletested samples of CermeTi-10C show TiC particle cracking, and cracks in the alloy matrix developed from cracks initiated in cracked TiC particles. The sizes of the TiC particles in CermeTi10C are remarkably larger (approximately 20 microns) than those observed in the TiC/Ti in situ reinforced composites. It appeared that cracking along the reinforcement colonies controls the failure of this material. In contrast to the results for TiC/Ti in situ reinforced composites, tensile deformation in the

CermeTi-10C samples does not exhibit cracking of the TiC particles. The tensile properties of these materials tested at various temperatures are shown in Table 1. The major objective for this study was to demonstrate a unique ability to produce the low-cost re-enforced Ti alloy components for automotive application by applying a cost-effective roomtemperature consolidation (die pressing, cold isostatic pressing and direct powder rolling processes) powder metallurgy approach to achieve near full density titanium alloys for potential automotive applications. Low-cost sodium-reduced titanium sponge fines (-100 mesh) and hydrogenated magnesium-reduced titanium powder TiH2 (-100 mesh) were used in this study. Chemical compositions of these powders are shown in Table 2. The blended elemental approach with an aluminium-vanadium (6Al-4V) master alloy was used to achieve the Ti-6Al-4V composition. In order to fabricate the composite samples, the mixtures of Ti, Cr, and C powders with particle sizes less than 70 µm were prepared first by ball milling for four hours and then by blending with BE Ti-6Al-4V powder [34]. Pre-mixed blends were consolidated at room temperature by die-pressing, cold isostatic pressing or direct

Table 3. Room Temperature Tensile Tests ASTM E8-04 Sample No.

Ultimate Strength (KSI)

Yield Strength (0.2%)

Elongation %

Reduction of Area, %

Hardness, HRC

Modulus of Elasticity MSI

1

143.20

123.10

12.2

18.24

29.70

16.90

2

143.60

123.70

13.8

19.56

29.70

17.30

3

142.60

122.80

11.4

16.50

29.70

16.69

4

142.70

123.10

11.8

23.31

29.70

16.78

Ti6A1-4V hydrogenated Ti powder

144.90

133.90

10.0

27.00

-

-

ASTM B34797

130.00

120.00

10.0

25.0

Composition

Ti-6A1-4V sodium reduced Ti powder

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powder rolling, and sintered in vacuum under identical conditions. Estimated volume fraction of TiC particles was varied between 0 per cent and 20 per cent. The density of compacts before and after sintering was measured with Archimedes’ technique and additionally controlled with analysis of polished cross-sections. Microstructure of materials was evaluated with light microscopy. Scanning electron microscopy (SEM) was used to study the microstructure, and EDX analysis was performed on the samples to determine the chemical composition of particles. X-ray diffraction analysis on the polished surfaces was also carried out to identify the TiC reinforcement (phase composition). Tensile properties of alloys were determined using standard techniques for samples machined from rectangular sintered compacts. A lab-scale 13-inch wide direct powder rolling mill was used for room temperature rolling of Ti-6Al4V and MMC strip. The major objective for this study was to demonstrate a unique ability to produce the low-cost re-enforced Ti alloy components for automotive application by applying a cost-effective roomtemperature consolidation (die pressing, cold isostatic pressing and direct powder rolling processes) powder metallurgy approach to achieve near full density titanium alloys for potential automotive applications. Low-cost sodium-reduced titanium sponge fines (-100 mesh) and hydrogenated magnesium-reduced titanium powder TiH2 (-100 mesh) were used in this study. Chemical compositions of these powders are shown in Table 2. The blended elemental approach with an aluminium-vanadium (6Al-4V) master alloy was used to achieve the Ti-6Al-4V composition. In order to fabricate the composite samples, the mixtures of Ti, Cr, and C powders with particle sizes less than 70 µm were prepared first by ball milling for four hours and then by blending with BE Ti-6Al-4V powder [34]. Pre-mixed blends were consolidated at room temperature by die-pressing, cold isostatic pressing or direct powder rolling, and sintered in vacuum under identical conditions. Estimated volume fraction of TiC particles was varied between 0 per cent and 20 per cent. The density of compacts before

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Figure 3. Microstructure of MMC from hydrogenated titanium powder die pressing + sintering (a,b); cold isostatic pressing (c,d); and direct powder rolling + sintering (e,f ); (Density 4.30 g/cm3).

and after sintering was measured with Archimedes’ technique and additionally controlled with analysis of polished cross-sections. Microstructure of materials was evaluated with light microscopy. Scanning electron microscopy (SEM) was used to study the microstructure, and EDX analysis was performed on the samples to determine the chemical composition of particles. X-ray diffraction analysis on the polished surfaces was also carried out to identify the TiC reinforcement (phase composition). Tensile properties of alloys were determined using standard techniques for samples machined from rectangular sintered compacts. A lab-scale 13-inch wide direct powder rolling mill was used for room temperature rolling of Ti-6Al4V and MMC strip. Microstructure, density and mechanical properties of BE MMC All known processes for manufacturing the dense titanium matrix compos-

ites from Ti- alloy matrix and reinforcing powders have considerable drawbacks that make them undesirable in terms of resulting properties (density, strength and ductility) and processing issues (sufficient protection from oxidation, cost and production capacity). The interconnected porosity causes very rapid oxidation of the reactive titanium powder to a substantial depth during hot consolidation (HIPing, hot pressing), and capsules or cases (which are required for subsequent consolidation to near full density) do not fully protect the sintered article from rapid oxidation, and also increases production cost. A significant difference in structural and mechanical properties between sintered material and material used for encapsulation that are produced from nonreactive wrought metal results in nonuniform deformation and stress concentration in the TMMC during the hot deformation. Cracks occur in various

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areas of the sintered material during the first cycles of hot deformation due to interconnected porosity and stress concentration. These cracks do not allow maintaining a reliable and reproducible manufacturing process through forging or hot rolling. To resolve these manufacturing issues, the following requirements were considered in production of the composite materials in this investigation: (a) a highstrength and near-fully dense titanium matrix composites having less than 2 per cent discontinuous porosity after sintering, (b) a cost-effective method for producing such composites using blended elemental powders and/or combination of pre-alloyed and elemental metal powder blends, and (c) using in-situ formation of reinforced particles by co-attrition of graphite with elemental metals, blending this co-attrited powder with titanium and master alloy to achieve the required titanium alloy composition, room-temperature consolidation and forming carbides during sintering. The composite material exhibits acceptable mechanical properties in the as-sintered condition. If more complicated shapes with improved size control of the finished parts or improved properties are required, hot deformation can be performed without any encasing, canning or encapsulating. The microstructures of BE Ti-6Al4V composite materials manufactured from sodium reduced titanium powder, as well as from a hydrogenated magnesium reduced titanium powder by simplest room-temperature consolidation and sintering approach, are presented in Figures 2 and 3. Homogeneous lamellar

α+β microstructure with average grain size of 100-120 µm and fine TiVCrC (5-20 µm) reinforcements uniformly distributed was observed in all three roomtemperature consolidation techniques performed in this study, ie die pressing, CIP and direct -powder rolling. The relatively small grain size is caused by the pinning effect of particulates. EDS analysis and X-ray diffraction analysis (Figure 4) on the polished surfaces confirmed that the TiVCrC particles were formed in both types of based material. Metallographic observation together with X-ray analysis confirmed that carbon diffuses into Ti-Al-V matrix and forms the carbide particles during sintering. Chromium content in precipitates does not exceed 2.0wt/%, which is consistent with previously reported results [37]. The densities of sintered material were close to full theoretical density for all methods of room-temperature consolidations (die pressing, CIPing or direct powder rolling). Room temperature tensile properties of Ti-MMC studied are shown in Table 4. Blended elemental Ti-6Al-4V reinforced with 10 per cent and 20 per cent particulates exhibit high strength, but the ductility is still below the ASTM requirements. Reinforcement with the particulates increased the modulus from 17 MSI for non-reinforced Ti-6Al-4V alloy to 19 MSI for 10 per cent reinforcement and to 20 MSI for 20 per cent reinforcement.

sodium-reduced titanium powder and hydrogenated magnesium-reduced titanium powder. 2. The sintered BE Ti-6Al-4V alloys exhibited good mechanical properties due to high density, chemical homogeneity and relatively fine microstructure. 3. BE powder metallurgy approach is very attractive for creating new high-performance titanium-based materials that could not be manufactured via conventional ingot metallurgy. Titanium MMCs reinforced with titanium carbide particles were fabricated by simplest BE PM room-temperature consolidation and sintering approach. 4. Blended elemental Ti-6Al-4V reinforced with 10 per cent and 20 per cent particulates exhibit high strength, but the ductility is still below the ASTM requirements. Heat treatment optimisation is required for improvement of mechanical property combination. Reinforcement with the particulates increased in the modulus from 17 MSI for non-reinforced Ti-6Al-4V alloy to 19 MSI for 10 per cent reinforcement and to 20 MSI for 20 per cent reinforcement.

CONCLUSIONS 1. The cost-effective Blended Elemental PM technology has been developed for the titanium alloys. Nearly dense Ti-6Al-4V alloys were synthesised with press-and-sinter approach using

Figure 4. X-ray diffraction analysis of MMC from hydrogenated titanium powder (die pressing + sintering

Table 4. Room temperature tensile tests ASTM E8-04MMC from sodium reduced titanium powder Composition

Ti-6A1-4V +10% Ti, V, Cr, C

Ti-6A1-4V +20% Ti, V, Cr, C

20

Sample No

Tensile Strength. (psi)

Yield Point (psi)

Yield Strength (0.2%). psi

Elongation, %

Modulus, MSI

1

150.30

-

143.50

3.0

17.70

2

151.40

-

137.90

3.0

17.70

3

150.40

-

142.40

3.0

17.90

4

150.10

-

143.00

2.0

18.10

5

148.50

-

141.10

2.0

17.50

1

115.00

115.00

-

2.0

18.30

2

119.20

119.20

-

1.0

19.00

3

124.50

124.50

-

2.0

20.10

4

122.30

122.30

-

2.0

19.50

5

121.50

121.50

-

2.0

19.60

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