Solid-state foaming of Ti-6A1-4V by creep or superplastic expansion of argon-filled pores

Ti-6Al-4V foams are produced by the expansion of pressurized argon pores trapped in billets created by powder metallurgy. Pore expansion during thermal cycling (840-1030 °C, which induces transformation superplasticity in Ti-6Al-4V) improves both the foaming rate (by reducing the flow stress) and the final porosity (by delaying fracture of the pores and subsequent escape of the gas), as compared to isothermal pore expansion at 1030 °C, where Ti-6Al-4V creep is the controlling mechanism. Raising the argon content in the billet increases the foaming rates for both creep and superplastic conditions, in general agreement with an analytical model taking into account the non-ideal behavior of high-pressure Ar and the pore size dependence of surface tension. Superplastically foamed Ti-6Al-4V with 52% open porosity exhibits a combination of high strength (170 MPa) and low stiffness (18 GPa), which is useful for bone implant applications.