Creep expansion of porous Ti-6Al-4V sandwich structures

Low-density titanium alloy sandwich structures consisting of a porous core and fully dense face sheets can be produced by consolidating argon gas charged powder compacts followed by hot rolling and annealing to expand the gas-filled pores. Little is known about the rate of pore expansion, its dependence upon temperature, and the morphological evolution of the pore shape during expansion. In situ eddy current and laser ultrasonic sensors have been combined with metallographic and texture measurements to measure the relative density, the elastic moduli, and the microstructural evolution of Ti-6Al-4V sandwich panels during the annealing stage of low-density core (LDC) processing. The eddy current data indicated that expansion began during the heating phase, reached a maximum expansion rate (Delta-dot) of 2 x 10 exp -5/s at approximately 685 C, and had almost ceased (Delta-dot less than 1 x 10 exp -6/s) after annealing for 4 h at 920 C. The elastic moduli were found to decrease with increasing temperature and volume fraction of porosity. The initial (as-rolled) microstructure consisted of a lamellar alpha + beta microstructure with an alpha-phase lath thickness of 2.0 microns, and contained a distribution of oblate-shaped pores with aspect ratios of up to 10. During the expansion process, it recrystallized into an equiaxed alpha + beta structure with an alpha-phase grain diameter of 7.5 microns with spheroidal pores with aspect ratios of up to 3. The combination of the two sensors was found to enable the in situ determination of both the porous core's relative density and its elastic properties. These are the two material indices that govern the elastic response of a sandwich structure. (Author)