Effect of initial preform porosity on solid-state foaming of titanium

Titanium foams were produced by the expansion of pressurized argon pores trapped within a preform during a previous powder-consolidation step. Compared with creep expansion at 903 °C, superplastic expansion (induced by a 830–980 °C cycling around the allotropic temperature of titanium) increases foaming rate and final porosity. The pore size and fraction in the preforms were varied by using a range of initial titanium powder sizes and argon pressures. As initial preform porosity increases from 0.06 to 2.7%, foaming rate increases in the early stages of creep and superplastic foaming. However, at a later stage, foaming ceases prematurely for preforms with high initial porosity, as pores connect to the surface, allowing the escape of the pressurized argon. Preforms with 0.40% initial porosity result in foams with an optimal combination of high foaming rate, high final porosity (up to 47%), tailorable open or closed porosity, and Young's modulus as low as 23 GPa.