Reproducibility Study of NiTi Parts Made by Metal Injection Moulding

Powder metallurgy is an attractive manufacturing process for net shaped NiTi parts considering the limited machinability of NiTi alloys. Nevertheless, the industrial implementation of this process for NiTi alloys is not trivial due to the need of demonstrating the reproducibility of the shape memory behaviour combined with mechanical properties, which are comparable to the state of the art achieved with conventional manufacturing methods. The successful implementation is strongly influenced by the high affinity of NiTi for oxygen and carbon leading to O- and C-rich phases which change the NiTi ratio in the matrix and thereby the phase transition temperatures. Furthermore, the formation of Ni-rich Ni₄Ti₃-precipitates, which might occur during the sintering process, can influence the transformation behaviour as well. On the other hand, defined precipitation of Ni₄Ti₃ phases by carrying out a heat treatment can be used to adjust the transformation temperatures. In the present work, the influence of each step of the powder metallurgical production-chain on the reproducibility of the specific properties is discussed. The production-chain involves the production of prealloyed NiTi powder, which is followed by shaping via metal injection moulding (e.g. the prototype of a food staple), debinding, sintering and finally conducting a thermal treatment.