Density and mechanical properties in selective laser melting of Invar 36 and stainless steel 316L

In this study, the process-structure-property relationship for selective laser melting of Invar 36 and stainless steel 316L is discussed. Invar 36 and stainless steel 316L have been used in various industrial applications for their unique properties, especially in the aerospace industry. Invar 36 offers a very low coefficient of thermal expansion while stainless steel 316L offers high corrosion resistance. Since both materials are weldable, but hard to machine, this study is aimed at finding the optimum laser process parameters for producing dense components from both alloys. A full factorial design of experiments was formulated in this paper to study a wide range of process parameters for both materials. The bulk density, tensile mechanical properties, fractography, material composition, and residual stresses of the parts produced were investigated. An optimum process window has been suggested based on experimental work. The induced residual stresses were categorized into two categories: microscopic residual stresses and macroscopic residual stresses. The microscopic residual stresses were measured using X-ray diffraction method and the macroscopic residual stresses were measured using cantilever deflection method and finite element simulations. The paper proposes two laser energy densities for each material: brittle-ductile transition energy density, ET, and critical laser energy density, EC. Below the brittle-ductile transition energy density, the parts exhibited void formation, low density, and brittle fracture. Above the critical energy density, the parts showed vaporization of some alloying elements that have low boiling temperatures. Stable melting ranges were found to occur between these two laser energy densities: 52.1–86.8 J/mm3 for Invar 36 and 62.5–104.2 J/mm3 for stainless steel 316L.