Eliminating porosity defects, promoting equiaxed grains and improving the mechanical properties of additively manufactured Ti-22V-4Al with super-transus hot isostatic pressing

High energy input additive manufacturing (AM) such as wire-based directed energy deposition technologies are capable of high deposition rates but can result in coarse microstructures along with potentially large defects such as gas porosity. The presence of defects and the adverse effect on mechanical properties is further exacerbated when low-quality or contaminated feedstock materials are used. This work demonstrates a pathway for producing high-quality additively manufactured products with low-quality feedstock materials that ordinarily would be considered unsuitable for use and relegated to scrap. A large number of macropores up to and exceeding 2 mm in diameter are intentionally introduced using a contaminated wire feedstock in a novel metastable β-Ti alloy (Ti-22V-4Al). Following super transus hot isostatic pressing (HIPing), it was found that porosity free parts with reduced texture, refined grain size and excellent ductility (exceeding 32% elongation to failure) can be obtained. Such improvement is achieved by eliminating pore defects while forming fine equiaxed grains through dynamic recrystallisation within the pore region. This approach to producing AM products with desirable microstructures and mechanical properties from low-quality feedstock materials is applicable to a wide variety of metallic alloys produced by other AM techniques and can be used to engineer completely homogenous equiaxed grain microstructures.