Powder Metallurgy Processes of Ti and TiNi Alloys

• Main Authors: Shiau-Han Wu, 吳筱涵
• Other Authors: 黃坤祥
• Format: Others
• Language: zh-TW
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/18522740220010624190

碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 96 === The metal injection molding (MIM) process has been applied to produce net-shape or near net-shape parts due to its advantages of fewer processing steps and waste. It is particularly attractive to Ti alloys due to their machinability. Generally speaking, it is difficult to make fine Ti powders. However, fine TiH2 particles can be produced brittle enough to be pulverized easily. It is also chemically stable and inexpensive. If TiH2 powder can be used in the MIM process, it will open up Ti alloy market. The challenge of this thesis is thus to find a way. In order to alleviate the contamination, it is necessary to optimize the thermal debinding parameter. In this study, the lowest carbon content after thermal debinding was about 0.08wt% ,which meets the ASTM standard (<0.1wt%). For sintering, specimens were sintered in a boat with cover at 1100℃ for 2 hours in high vacuum environment (10-4~10-5torr) in the graphite vacuum furnace. The carbon content of the as-sintered specimen still maintained about 0.08wt%. Both pressed and MIM part that were made of TiH2 powders were brittle and weak. The elongation was about 0.94% and the tensile strength was about 160MPa. Dry compacts using Ti powders instead of TiH2 had much higher ductility and strength 11% of elongation,and 360MPa strength. But, the elongation was reduced to 4% when MIM process was employed. The results also indicated that TiH2 was to be more prone to reactions with carbon and oxygen than did Ti. Moreover, the MIM part had higher carbon and oxygen content than did the dry compact. The impurities mainly came from the binder system. TiNi shape memory alloy (SMA) was also prepared with TiH2 and Ti powder. Ti-50at%Ni that was made of TiH2 powder showed weak shape memory effect (SME, ΔH=13.4J/g). A little more Ti over 50at% in SMA improved the SME, probably due to the presence of Ti2Ni. This is probably because Ti2Ni could absorb carbon and oxygen during the process and purifies TiNi. The other reason was that the TiNi matrix becomes Ni-rich and not uniform when Ti2Ni existed in Ti-50at%Ni. Thus, to improve the SME, increasing Ti content slightly helps producing more TiNi phase. The results showed that Ti-44.92at%Ni (Ti-50wt%Ni) using TiH2 powder had an improved latent heat (ΔH=23.8J/g) and Ti-49at%Ni that were made of Ti powder had the highest latent heat(ΔH=28.7J/g).