Effect of processing parameters on microstructure and tensile properties of austenitic stainless steel 304L made by directed energy deposition additive manufacturing

The effect of processing parameters on the mechanical properties of AISI 304L stainless steel components fabricated using laser-based directed energy deposition additive manufacturing (AM) was investigated. Two walls were fabricated, with high linear heat inputs of 271 and 377 J/mm, to determine the effect of processing parameters on microstructure and mechanical properties of 304L made by AM. Uniaxial tension tests were performed on samples extracted from the walls in longitudinal and transverse directions. The yield strength, ultimate tensile strength, and ductility, were higher in the lower linear heat input wall compared to the higher linear heat input wall. The ductility in the longitudinal direction was less than that in the transverse direction, while there was no clear anisotropy in strength. A grain growth model adapted from welding was used to interpret and predict the grain sizes in the walls as a function of processing parameters and position. A Hall-Petch relationship was used to explain the effect of local grain size and morphology on the location- and direction-dependent yield strengths in each wall. The ultimate tensile strengths and elongations of the material made by AM were less than those of annealed 304L plate since a microstructural phase transformation from austenite to martensite, which provides a mechanism for significant macroscopic strain hardening, occurred in the annealed material, but not the material made by AM. Chemical analysis showed that walls made by AM had higher nitrogen content, which stabilizes the austenitic phase, than the annealed plate.