Using an agar-based binder to produce porous NiTi alloys by metal injection moulding

Porous NiTi alloy samples were injection moulded from feedstocks formulated with an agar-based gel binder system. The gelling condition, including gelling agent and temperature of this binder system were investigated by the measurement of gel strength and viscosity change. Two feedstocks were prepared from elemental powder mixture: Feedstock A (Ni/Ti) and Feedstock B (Ni/TiH2), with a solid loading being 65 and 60 vol.%, respectively. The injection moulded NiTi compacts were vacuum sintered at 1100 °C for 2 h. It is found that the use of the TiH2 powder in the feedstock not only promotes densification but also enhances chemical homogenisation in the sintered samples. The sintered samples made from the Ni/TiH2 feedstock exhibited a lower porosity, smaller pore size and a higher fracture strength, as compared to those made from the Ni/Ti feedstock. The sintered porous NiTi alloys demonstrated a one-way shape memory effect and the phase transformation temperatures were determined. Under tension, the porous NiTi alloys exhibited brittle fracture features.