Effects of powder loading on the rheological properties of feedstock and sintered density of injection molded NiTi alloy

【Objective】 This study aimed to investigate the effects of powder loading on the rheological behavior of wax-based feedstocks and the as-sintered density of NiTi shape memory alloys fabricated by metal injection molding (MIM), in order to identify an optimal powder loading for defect-free molding and high densification.
【Method】 Gas-atomized pre-alloyed NiTi powder (Ni_50.47Ti_49.53, at%) with a median particle size of 11.8  μm was mixed with a wax-based binder system consisting of 60% paraffin wax, 40% polypropylene, and a minor amount of stearic acid. Three feedstock formulations with powder loadings of 55 vol% (TN55), 60 vol% (TN60), and 65 vol% (TN65) were prepared by mixing under argon protection at 160-180 ℃ for 3 hours. Rheological properties were evaluated using a capillary rheometer at temperatures of 150  ℃, 170  ℃, and 190  ℃ across shear rates ranging from 100 to 1 200 s^-1. The viscosity data were employed to calculate the power-law exponent n (strain sensitivity), the viscous flow activation energy E (temperature sensitivity), and the comprehensive rheological factor α~STV~. Green parts were injection-molded, subjected to solvent debinding in dichloromethane at 38  ℃ for 12  h, thermally debound in flowing argon at 600  ℃, and finally sintered in vacuum (10^-4  Pa) at 1 240  ℃ for 6  h with a heating rate of 5  ℃/min. Weight loss during solvent debinding, carbon and oxygen contents after thermal debinding and sintering, defect rates, and sintered relative densities were systematically recorded and analyzed.
【Result】 The viscosity of all feedstocks decreases monotonically with increasing temperature and shear rate, exhibiting typical pseudoplastic behavior. The TN60 feedstock exhibits the highest strain sensitivity with a lower power-law exponent *n* value (approximately 0.51-0.55), indicating better fluidity and shear-thinning response under identical external shear conditions. In contrast, higher powder loadings result in higher viscous flow activation energy E values, TN65 possesses E values around 42.76 kJ·mol^-1 at low shear rates, implying greater temperature sensitivity and potential viscosity variation within the mold cavity. The comprehensive rheological factor α~STV~, which integrates viscosity, temperature sensitivity, and strain sensitivity, is largest for the TN65 feedstock (207.86 at low shear rate and 352.76 at high shear rate), demonstrating its superior overall rheological performance among the three formulations. During solvent debinding, the weight loss of TN65 reaches approximately 4.15% after 12 h, approaching the theoretical maximum of 4.186% based on the wax content. After thermal debinding, the TN65 feedstock achieves the highest yield of defect-free parts (99.8%), with negligible cracks or bubbles observed. The carbon and oxygen impurity contents after sintering are not significantly influenced by powder loading and are primarily controlled by process optimization, post-sintering oxygen and carbon contents are as low as 0.16 wt% and 0.03 wt%, respectively, representing a substantial reduction compared with previous studies. Sintered density increases markedly with powder loading, from 92.4% for TN55 to 94.4% for TN60 and finally 96.7% of theoretical density for the TN65 feedstock. The higher green density associated with increased powder loading promotes more effective pore elimination and interparticle bonding during solid-state sintering of the pre-alloyed powder.
【Conclusion】 A powder loading of 65 vol% is the optimal design for MIM NiTi alloys fabricated with the wax-based binder system. This formulation provides the best comprehensive rheological stability, minimizes molding defects during injection, and yields the highest sintered density (96.7%) with low interstitial impurity levels, thereby facilitating the production of high-performance NiTi shape memory components with improved mechanical integrity and dimensional precision.