Simulation of the creep expansion of porous sandwich structures

Recently developed sandwich structures consist of a porous metal core sandwiched between two fully dense face sheets. These structures are produced by pressurizing a metal powder compact with an inert gas prior to consolidation by hot isostatic pressing ("hipping"). After consolidating and hot rolling the compact to a sheet form, a high-temperature annealing step is used to expand the internally pressurized gas-filled micropores. This expansion results in a porous core sandwich structure with integrally bonded face sheets. Recent experimental studies([1]) with a Ti-6Al-4V porous core sandwich have indicated that the expansion rate exhibits a maximum during thermal ramping to 920 degreesC but then continued to expand over many hours at a constant temperature. Significant grain growth also accompanied the expansion. A microstructure-dependent creep model has been developed for a body containing a distribution of spheroidal pores. The body's constitutive behavior is described by microstructure-dependent creep potentials for dislocation (power law) and diffusion-accommodated grain-boundary sliding (DAGS). It has been used to simulate the expansion of Ti-6Al-4V sandwich structures subjected to thermal cycles similar to those studied experimentally. The simulated response compared well with experimental results. The model was then used to identify an attainable core porosity as a function of the initial gas pressure and initial core relative density at the completion of the expansion process step.