Influence of surface layer on properties of hipped Ti-6Al-V-4

The nature of the surface region formed between Ti-6Al-4V and the mild steel tooling used to define the shape of net shape hot isostatically pressed (hipped) Ti-6Al-4V components has been evaluated. The surface topography has been assessed and analytical scanning electron microscopy (SEM) has been used to define the local changes in composition obtained under several hipping conditions. The tensile and fatigue properties of net shape hipped samples of Ti-6Al-4V have been compared with those obtained from samples produced by machining samples from larger hipped cylinders so that the surface layer is removed. The as hipped surface is irregular on the scale of the original powder particles and a 150 mm thick diffusion layer is produced under the standard hipping conditions. It has been found that the tensile properties of samples with the as hipped or machined surfaces are very similar. The fatigue limits of samples which were tested with the as hipped surface were significantly below those of samples which were machined to remove the as hipped surface. Electropolishing the as hipped samples to a depth of,150 mm leads to fatigue properties which are slightly higher than the properties of machined samples. These results are discussed in terms of the failure mechanisms during fatigue and issues in the production of net shape hipped products.