Metal Injection Molding (MIM) for Near Net Shape Manufacturing of NiTi Shape Memory Alloys

摘要

One of the most commonly used shape memory alloy (SMA) is near equiatomic NiTi. Until now, semifinished parts of this alloy were preferably made by melting processes followed by hot and cold working. By cutting, drilling or drawing parts get their final dimensions. The melt process may lead to formation of segregation. Rapid grain growth occurs during subsequent high temperature working resulting in poorer fatigue properties. In addition, NiTi alloys show material inherent problems such as high wear of cutting tools when machined. To avoid these problems, powder metallurgical (PM) manufacturing processes are alternatively under development. In the present investigation, the near net shape Metal Injection Molding (MIM) process - well known for small complex shaped parts - was applied to prealloyed NiTi powder to overcome the machining problems. The first prototype geometry was a tube of about 30 mm length, outer diameter 14 mm and 2 mm wall thickness provided e. g. as coupling element. After debinding, sintering and heat treatment, chemical analysis of impurities, phase composition and properties such as martensite reversible austenite phase transformation - the basic requirement for shape memory effect - were determined in dependence of the sintering temperature. Mechanical properties were determined on net-shape tensile test samples also prepared by MIM.