Reproducibility Study of NiTi Parts Made by Metal Injection Molding

Powder metallurgy (P/M) is an attractive manufacturing process for net-shaped NiTi parts considering the limited machinability of NiTi alloys. Nevertheless, the industrial implementation of P/M processing for NiTi alloys is not trivial. To become competitive to manufacturing of NiTi alloys based on established ingot metallurgy, combination of fully pronounced shape memory behavior with sufficient mechanical properties is required. Successful use of P/M technology is strongly influenced by high affinity of NiTi alloys for uptake of oxygen and carbon, which leads to the formation of oxygen-containing Ti2Ni and TiC phases coupled with increase of Ni content in the matrix. In the case of Ni-rich NiTi alloys, this increase leads to a shift of phase transformation temperatures to lower values. Furthermore, precipitation of Ni4Ti3 during cooling from sintering temperature is difficult to avoid. Even if these precipitates might be used to decrease the Ni:Ti ratio of the matrix balancing oxygen and carbon uptake, significant loss of ductility arises, especially in the case of finely dispersed Ni4Ti3 precipitates. In the present work, each step of P/M manufacturing is discussed regarding its influence on the specific properties of NiTi alloys. The work is based on the application of prealloyed, gas atomized NiTi powders. Metal injection molding was used for net-shaped manufacturing of tensile samples, which enabled detailed study of sintering behavior combined with investigation of shape memory and mechanical properties depending on particle size, oxygen and carbon content as well as precipitation of Ni4Ti3 phase.