Research on Powder Metallurgy of Titanium Alloy

Ti-2007 Science and Technology, edited by M. Ninomi, S. Akiyama, M. Ikeda, M. Hagiwara, K. Maruyama The Japan Institute of Metals (2007)

Zhiqiang Chen, Hongqiang Zhou

Department of Titanium, Luoyang Ship Material Research Institute, Luoyang 471039, China

Powder metallurgy is a near net shape processing technology of materials. It is highly advantageous in the manufacture of high material cost and high machining cost parts. Titanium parts is of high material cost and high machining cost. Therefore, powder metallurgy of titanium alloys should be highly emphasized and developed. However, till now it is far from actual demand, with application mainly in filtering parts but only a few in mechanical parts. This paper presents our research and review on the powder metallurgy of titanium alloys, mainly on effects of warm compaction and lubrication. Both warm compaction and lubrication can improve the density of pressed green compacts and sintered compacts. Some suggestions are made for future development of low cost titanium P/M.

Keywords: titanium (Ti), powder metallurgy, warm compaction, lubrication

1. Introduction target of research to get low cost and high density parts is set Powder metallurgy is a near net shape processing technology based on HDH powders and blended elemental P/M of materials. It is highly advantageous in the manufacture of technology. Typically the blend of titanium powder and master high material cost and high machining cost parts. Titanium alloy powder is compacted by mechanical die pressing and parts is of high material cost and high machining cost. then vacuum sintered. Usually the density after vacuum Therefore, powder metallurgy of titanium alloys should be sintering may reach over 95%, but post-sinter HIP is necessary highly emphasized and developed. However, till now it is far to get over 99% dense compacts. HIP is expensive and causes from actual demand, with application mainly in filtering parts extra size variation. If one pressing and one sintering process but only a few in mechanical parts. The main problem is cost, can attain over 99% dense compacts, blended elemental P/M particularly the cost of post-sintering conversions such as titanium parts would find much more application. forging or HIP and machining, which accounts for more than To achieve high sintered density, high density green compacts 50% of the whole process. If titanium P/M parts consolidated must be attained first. The warm compaction process has got by simple cold or warm compacting and sintering could have great success in producing high density iron-based P/M high performance and exact shape, they are going to be quite components2-4). Both temperature and lubrication can cost competitive for application. contribute to the enhancement of green and sintered density. In general, the mechanical properties of titanium P/M products But few report was seen on the research of warm compaction depend on alloy composition, density and final microstructure. process in titanium-based P/M components. Ultra-high The final density and microstructure depend on the nature of pressure warm compaction for P/M titanium components the powder, on the specific consolidation technique and on post research had been proved successful in this field5). It realizes a consolidation thermal or mechanical treatment1). high precision and eliminates costs of post sintering Commercially used titanium powders can be classified as conversions. However, ultra-high pressure compaction sponge fine, HDH powder, PREP powder and GA powder by employs very expensive cemented carbide die, which limits its their production methods. Different powders have different application in turn. Here we tried to research the influences of process characteristics. The powder morphology can be divided temperature and lubrication on density of titanium P/M into two types: irregularly shaped powder and spherical components under relatively low pressure of 500 MPa. powder. Irregularly shaped powders can be cold pressed into green shapes and subsequently sintered to higher density. 2. Experiment procedures Spherical powders can not be cold pressed to green shapes. They are usually containerized and consolidated by HIP. 2.1 Raw Powders Sponge fines and HDH powders are generally irregular shaped Ti-6Al-6V-2Sn-0.5Cu-0.5Fe was selected as the alloy for and low cost. PREP and GA powders are generally spherical experiments. Table 1 shows the chemical compositions and and high cost. Considering the cost and availability, HDH particle sizes of HDH titanium powder, Al-V master alloy powder is preferred for current research work. powder and other elemental powders. This alloy composition is Titanium powder metallurgy techniques can be divided into believed to have better transient liquid phase sintering which is two general categories: Blended elemental P/M and Prealloyed helpful for higher density. Availability limited size distribution P/M. Considering the process cost, blended elemental P/M and chemical composition control of powders. technology is preferred. Therefore, the

1181 Table 1. The chemical compositions and particles of powders used

2.2 Lubrication temperature of 1250°C when compacted at room temperature. Lubricants of MoS2, Lithium Stearate(Li-St.) and HDPE are But when compacted at over 140°C , the difference is about applied for die wall lubrication and powder lubrication. Spray 16%, as is smaller than that of cold compacting. was applied for die wall lubrication with particle size less than 1μm. Powder lubricants with average particle size of 8µm were applied for titanium powder lubrication. Lubrication is believed to be a good way to increase density of green compacts.

2.3 Compacting pressure and temperature All specimens are made under the same pressure of 500Mpa, which is considered to be the upper limit to avoid unusual heat treated steel die damage.. The compacting temperature ranges from room temperature to 240°C .

2.4 Vacuum sintering Sintering is made at 1250°C in vacuum of 1*10-3Pa for 4 hours.

2.5 Experiment evaluation The compaction experiment employed steel die having an inside diameter of 15mm. Each specimen weighs 4 gram. The die and blended powders are simultaneously pre-heated to pressing temperature and hold for 1 hour in an oven. All compacts are hold at given temperature and pressure for 50 seconds. Green density and sintered density are used as main evaluation parameter. Size tolerance, ejection force, microstructure, hardness and chemical composition change are also analyzed. Figure 1. Green density changes under different compacting temperature and die wall lubrication 3. Results and Discussion Figure 1. shows the relationship between density and compacting temperature under different die wall lubrication. It can be seen that warm compacting can effectively enhance green density. When MoS2 was used as die wall lubricant, the green density is 3.30g/cm3 at room temperature, but it increases to 3.58 g/cm3 at 140°C ,with an improvement of 8.48%. When the compacting temperature is 240°C , the relative density reaches 80.57%, 10.6% higher than at room temperature. In the temperature range of 140 °C and 180°C , HDPE seems to be the most effective die wall lubricant, with the highest density of 81.46% attained. Li-St. acts stably in the test experiment range. Figure 2 shows the relationship among compacting temperature, green density and sintered density when the die wall lubricant is MoS2. The higher the green density is, the higher the sintered density. The difference between green density and sintered density is about 20% at sintering Figure 2. Relationship among compacting temperature, green density and sintered density

1182 It is also found that the ejection force increases with the When HDPE was used as die wall lubricant, the relative increasing of compacting temperature, and the biggest ejection sintered density was 94.59%. When 0.06% HDPE was added force happens earlier while ejecting, as is shown in Figure 3. as powder lubricant, the relative sintered density achieved was 96.36%. Lubricant admixed with metal powder can also lubricate the die wall, so the higher density is connected with both die wall lubrication and powder lubrication. But apparently powder lubrication is more effective for enhancing the final density. To further increase the density, warm compacting experiments with both die wall lubrication and powder lubrication were made. Table 2. shows the results. MoS2 spray was chosen as the die wall lubricant. Density was further slightly increased. However, further chemical analyses reveal contamination of lubricants on the alloy. MoS2 decomposes during vacuum sintering, part of it evaporates away and part of it is left in the sintered alloy. When 0.2% MoS2 was added, which means 0. 12%Mo and 0.08%S, the sintered alloy contains 0.045%Mo and 0.054%S. When Li-St. and HDPE were used as powder lubricants, C and O contamination were very serious. The actual C and O contents are higher than calculated, which Figure 3. Relationship Among ejection force, ejection movement and means not only no evaporation loss but also extra compacting temperature contamination. H was found decreased after vacuum sintering. No H contamination happened although both Li-St. and HDPE The size precision is also found improved with the increasing contain H. Obviously there is something strange in the of compacting temperature, as is shown in Figure 4. With the chemical composition. Where does the chemical contamination increasing of compacting temperature, the size shrinkage come from? Does it come from powder mixing process or decreased. Clearly high green compact density is critical for sintering process? This needs to be clarified in the future high quality M/P products. research. Lubrication is necessary but contamination must be Higher temperature is preferable but vacuum compacting or managed to be avoided. inert gas protection compacting is necessary to protect titanium powder from oxidizing. At present time, the so-called warm compacting means pressing at around 150°C. Most experiments are made at the temperature range of 120°C to 180°C.

Table 2. Relative Densities with Powder Lubrication

4. Conclusions and Summary 1) Warm compacting can effectively increase the green density and sintered density of compacts. The higher the compacting is, the denser the compacts. 2) Die wall lubrication and powder lubrication are effective in enhancing the green density and sintered density of compacts. 3) HDPE is most effective for die wall lubrication among the lubricants tested. 4) MoS2 decomposes into Mo and S under vacuum sintering. 5) The lubrication may cause chemical contamination to titanium alloy, particularly powder lubrication. The research results show the possibility of getting high density parts by lubrication and warm compacting. But the density gained in experiments is only around 97%, which is

Figure 4. Effect of compacting temperature on size shrinkage

1183 still not satisfactory, not to mention mechanical properties. REFERENCES The relative density desired is over 99%. To achieve this 1)Rodney Boyer, Gerhard Welsch and E.W. Collings: Materials Properties Handbook: Titanium Alloys, (ASM International, Materials Park, Ohio, 1994) goal, the compressibility of the blended elemental powders pp.1137-1143. should be improved. Mainly that is to say, high purity 2)Anon: Metal powder Report 8(1996)pp.38-40. titanium powders should be used. Vacuum pressing at 3)Xiao Zhiyu, Chen weiping et al: Transactions of Nonferrous Metals Society higher temperature might be worth to try. Lubrication is of China 14(2003)pp.756-761. 4) Wang Shanglin, Li Yuanyuan et al: Powder Metallurgy Technology 23 effective but more work needs to be done to avoid (2005)pp195-198 contamination. 5)Hiroyuki Takamiya, Mikio Kondoh and Takashi Saito: Cost-affordable Titanium Symposium Dedicated to Professor Harvey Flower (2004)pp.185-192

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