Annealing strategies for enhancing mechanical properties of additively manufactured 316L stainless steel deposited by cold spray

Abstract

Cold spray is a solid-state metal deposition and additive manufacturing (AM) technology. The low processing temperatures in cold spraying result in less favorable properties of the deposits as compared to their fusion-based AM counterparts, particularly for materials with high strength such as 316L stainless steel. Therefore, annealing is generally required for the cold sprayed deposits to improve their mechanical properties. In this paper, three different annealing strategies are systematically investigated in order to optimize the microstructure and mechanical properties of cold sprayed 316L stainless steel deposits: air annealing, vacuum annealing, and hot isostatic pressing (HIP), the latter representing a method widely used for densification of components in fusion-based metal AM. The results indicate that the three annealing treatments improve the mechanical properties of the cold sprayed 316L stainless steel deposits through grain recrystallization and diffusion at oxide-free inter-particle interfaces. Such improvement is less pronounced for the air annealing as a formation of oxide inclusions impedes full inter-particle metallurgical bonding. This effect is suppressed in the vacuum annealing, resulting in a significant improvement in the tensile strength and ductility. Despite the significant improvement in the deposits' density, the HIP results in mechanical properties equivalent to those after vacuum annealing. The experimental results suggest that the strengthening of cold sprayed deposits is mainly dominated by the improved inter-particle bonding and particle grain structure rather than through a reduction of porosity.