Effect of Foaming Agent and Endothermic Agent Addition on Foaming Behavior of NiTi Alloy Made by Combustion Synthesis

Closed cell nickel titanium (NiTi) alloy foams were made by combustion synthesis with the help of both foaming agent and endothermic agent powders. Nickel, titanium, the foaming agent (ZrH2) and the endothermic agent (TiB2) powders were mixed and pressed at room temperature into cylindrical compacts. By heating these powder compacts up to an ignition temperature of the combustion reaction, NiTi alloy specimens are made. In this method, pores in the specimens are made by the gas generation from the foaming agent during melting of NiTi alloy due to the high heat of combustion reaction. Cell wall rupture of these foams caused by the gas generation is successfully prevented by the endothermic agent addition which increases the viscosity of molten NiTi alloy. An optimum amount of endothermic agent addition turned out to be in between 30 and 35 vol%. This volume fraction range is close to the one that makes adiabatic temperature of the combustion reaction below melting point of NiTi alloy. Specimens made by the addition of 40 vol% endothermic agent did not foam sufficiently. Porosity of the NiTi alloy foams increased by increasing the additive amount of the foaming agent by 1 mass%. Cell wall rupture and large pore formation became remarkable by increasing the foaming agent addition.