Effect of residual stress on pore formation in multi-materials deposited via directed energy deposition

A multi-layered deposit consisting of a surface layer, buffer layer, and substrate was created using directed energy deposition. The study aimed to determine how residual stress influences pore formation in the surface layer. The size of the pores in the surface layer varied based on the defects present in the buffer layer. When the buffer layer had cracks, the surface layer did not exhibit any large pores. Conversely, in the deposits where the buffer layer was free of cracks, the surface layer had numerous larger pores. Factors such as elevated carbon content and residual stress led to both hot and cold cracks in the buffer layer. The residual stress that wasn't alleviated by cracks in the buffer layer propagated to the surface layer across the buffer/surface layer boundaries. This resulted in an increase in tensile residual stress in the surface layer, amplifying the equilibrium pressure inside gas pores and prompting the expansion of these pores as a means of stress relief.