Advances in the Sintering of Titanium Powders

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1 Euro PM2004 PM Lightweight and Porous Materials Advances in the Sintering of Titanium Powders Donald F. Heaney and Randall M. German The Center for Innovative Sintered Products, The Pennsylvania State University, 118 Research Building West University Park, PA 16866, USA Abstract: Titanium metal is difficult to process because of its reactive nature. Starting with the highest purity material is required to obtain high quality sinterings. In this paper, titanium powders with different sizes and chemistries are sintered using both argon and vacuum atmospheres. The density, physical behaviour, and final chemistries have been analysed. The solid state sintering behaviour of this metal is characterized. Background: Titanium is a metal of engineering interest because of its high strength to weight ratio and corrosion resistance. However, it is very expensive to process due to its high reactive nature. In fact, titanium requires 431x10 6 Btu/ton to process from metal ore, whereas, iron requires only 203x10 6 Btu/ton to process [1]. Recently, much work has been devoted to lowering the cost of titanium metals because of its engineering attributes [2]. One option for cost reduction is to directly fabrication components from powder, since titanium is produced from ore as a powder. The primary issue with fabrication of components directly from powder is the purity of the powder. The general observation is that impurity content will increase strength, due to solid solution strengthening and decrease ductility. Although, little data exists for the actual mechanical properties and their relationship with impurity content for P/M produced components. This data is available for wrought products [3], as shown in Table 1, however, it has not been thoroughly addressed for P/M products, where porosity can influence the properties. This brief paper is aimed at bridging this gap in property information. Table 1: Mechanical properties of different grades of wrought titanium. ASTM Grade Tensile Strength (Mpa) Max Carbon Max Oxygen Max Iron

2 Experimental: In this set of experiments, three titanium powders from Sumitomo were processed and analysed. Table 1 shows the powders and their characteristics. The logic in the selection of these powders is as follows to compare a hydride/dehydride powder with a gas atomised powder and to compare large gas atomised powder with a small gas atomised powder. The first being with regards to cost and the second being with regards to surface area and the potential for contamination. Table 2: Powder Characteristics Impurity Levels Product Size (micron) Ti (% min) Fe O C A TSP350 (Hydride/Dehydride) B TILOP150H (Gas) C TILOP45 (Gas) These powders were compacted into MPIF Standard 10 tensile bars at 372 MPa (27 tsi) using a zinc stearate die wall lubricant and upper punch hold down. The upper punch hold down was required because of the spherical nature of the powder and their subsequent low green strength. The tensile bars were then sintered in both vacuum and flowing argon gas. The vacuum furnace was an MRF all metal hot zone furnace capable of 10-6 torr vacuum level. The argon furnace was an all graphite KYK Oxynon furnace. This furnace has oxygen reduction capability as a result of the oxygen reacting with the carbon to produce carbon monoxide. Samples were placed on zirconia substrates and were shielded in a molybdenum box. The tensile specimens were characterized for density, tensile strength, tensile ductility, hardness, oxygen, and carbon. Density was determined using the Archimedes technique. Tensile behaviour was determined using a strain rate of cm/minute and a 15 kip load cell. Hardness was measured on the B scale with a 100 kg load. Oxygen and carbon were measured using LECO equipment. All reported values are the average of at least 3 measurements.

3 Results: Density: The Tilop 45 had the greatest green density while the TSP350 had the least density. The TSP350 and Tilop 45 had comparable sintered densities. Whereas, the Tilop150 had the highest sintered densities. Interestingly, the largest powder had the highest as-measured density, however, the microstructure shows both larger pores and greater porosity C 1290C Density (g/cc) 1290C GreenArgon Vac Vac GreenArgon Vac Vac GreenArgon Vac Vac A A A A B B B B C C C C Figure 1: Green and Sintered Density of Ti Samples. Hardness: The TSP350 had the greatest hardness followed by Tilop 45 and Tilop150H. This is logical since the TSP350 had greater initial impurity content and the porous microstructure of the Tilop 150H would produce the least hardness. Hardness (Rockwell B) C 1290C 1290C Argon Vac Vac Argon Vac Vac Argon Vac A A A B B B C C Figure 2: As-sintered hardness of Ti samples.

4 Tensile Strength: The TSP350 had the greatest strength followed by the Tilop 45 and then the Tilop150H. This is to be expected since the TSP had the greatest impurity content and the Tilop 150H has large pores and the least amount of initial impurities. Tensile Strength (MPa) C 1290C 1290C Argon Vac Vac Argon Vac Vac Argon Vac A A A B B B C C Figure 3: Tensile Strength of as-sintered Ti powders. Tensile Elongation: The Tilop45 showed the greatest elongation, followed by the TSP350 and the Tilop150H. The 6% elongation TSP350 result is suspect due to contamination from a sodium reduced Ti powder that was sintered beside this sample. The low elongation of the Tilop 150H is believed to be a result of not impurities, but rather the large pore size. % Elongation C 1290C 1290C Argon Vac Vac Argon Vac Vac Argon Vac A A A B B B C C Figure 4: Tensile Elongation of as-sintered Ti powders.

5 Carbon Content: In general, all samples had approximately the same carbon level, within measurement capability. The vacuum atmosphere had slightly higher carbon levels than the argon atmosphere. This is unexpected due to the argon being in a graphite hot zone, however, the vacuum furnace may have contamination from previous sintering or vacuum oil residue. Table 3: Carbon levels on 1300ºC sintered Ti powders. Powder Carbon Level in Vacuum Carbon Level in Argon A TSP B Tilop 150H C Tilop Oxygen Content: The vacuum atmosphere consistently had higher oxygen levels than the argon atmosphere. This is unexpected due to the argon being in a graphite hot zone, however, the vacuum furnace may have contamination from previous sintering or vacuum oil residue. Table 4: Oxygen levels on 1300ºC sintered Ti powders. Powder Oxygen Level in Vacuum Oxygen Level in Argon (wt%) (wt%) A TSP B Tilop 150H C Tilop Microstructures: The microstructure clearly shows the large pores and large pore volume of the Tilop 150H, as compared to the TSP350. (a) (b) Figure 5: (a) TSP ºC, vacuum, (b) Tilop 150H 1300ºC, vacuum.

6 Discussion: The mechanical strength compares similarly to wrought properties of similar impurity content. However, ductility is clearly inferior to wrought product, the best obtained in this study was 17.50%, whereas, wrought product can have as much as 50% elongation. Elongation could be increased by HIPing and/or annealing of the samples. This study shows that compaction and sintering for both vacuum sintering and argon sintering pick up about 800 to 1000 ppm of carbon. The oxygen content remains similar to initial powder oxygen content for the TSP350 and the Tilop 45, however, the Tilop 150H shows greater oxygen pickup. The reason for this behaviour is believed to be an artifact of the density of the sample and the testing technique. The measured density shows these samples to have higher density, however, the microstructure shows a large amount of porosity that is most likely interconnected. The TSP350 has the higher strength and hardness due to solid solution strengthening of the titanium material by the higher oxygen content. The Tilop 150H shows the lower strength and ductility due to the high porosity of the samples. The Tilop 45 has the highest ductility due to a relatively high density and low oxygen content. Reference: [1] ASM Metals Reference Book, 3 rd Edition, 1993, ASM International, p [2] Scientific American, vol. 289, no. 4, October 2003, pp [3] Metals Handbook Desk Edition, 2 nd Edition, 1998, ASM International, pp