A comprehensive study on the mechanical properties and microstructure of 18Ni-300 maraging steel produced with various process parameters via powder bed fusion technique

Abstract

A comprehensive study was conducted to investigate the mechanical properties and microstructure of additively manufactured 18Ni-300 maraging steel. The gas-atomized powder was used as the feedstock, and the morphology and microstructure were examined to reveal the cellular structure with a majority of spherical powder particles. Cubic samples were built with 40 and 50 μm powder layer thicknesses. The 40 μm samples were built using ceramic and brush recoater blades. The three variations were then heat-treated, and all six sets of samples were studied to measure porosity in different layers. All samples showed relatively low porosity levels. The porosity level was shown to increase a few millimetres underneath the finished surface and then decrease gradually. The heat-treatment process was shown to decrease the porosity level due to the evolution of the microstructure. The hardness was also measured for different layers, and no variation was observed through the depth. The aged samples also showed higher hardness due to precipitations. Uniaxial tensile tests were conducted on test coupons by varying powder layer thickness, build direction, and recoater blades. The mechanical properties and microstructure of the specimens were studied before and after the deformation. In the as-built case, horizontal samples showed higher strength and ductility due to lower thermal stress from the manufacturing process. The powder layer thickness showed not to affect the mechanical properties significantly. Besides, the ceramic recoater showed slightly better mechanical properties due to the powder particle’s better spreadability over the build plate. The heat-treatment increased the mechanical strength significantly for both horizontal and vertical samples, while the ductility dropped as a payoff. The aged samples showed to have similar strength regarding their building direction with lower ductility of the vertical samples. Lower ductility was related to the building orientation due to the grain growth direction, while the strength was related to precipitation, regardless of the build orientation. All cases showed a phase transformation through the deformation process. The as-built cases turned into a fully martensitic structure, and the aged samples transformed a portion of the austenite into martensite.