Effect of Laser Emission Mode and Layer Thickness on the As-Built Microstructure of a Selective Laser Melted Maraging Steel

Selective laser melting (SLM) or laser powder-bed fusion (L-PBF) is a powder-based additive manufacturing process that enables to produce steel parts with complex designs and a minimum amount of material. In this work, a high performance 300 maraging steel were processed by SLM, where the layer thickness and the laser emission mode were modified. The microstructure was studied analyzing each of the process steps: melting, solidification and phase transformations. Advanced techniques such as electro-backscattered diffraction, X-ray diffraction and electron probe micro analysis were used. Results evidenced the importance of layer thickness as a key parameter to modify the solidification cell size. X-Ray Diffraction enabled to identify three different phases in the microstructure: bct, bcc and fcc. Although the studied processing parameters did not present a significant effect on either the volume fractions of these structures, their lattice parameters, crystallite size or microstrain, several questions were raised. Finally, the performed crystallographic analysis proved that growth happens epitaxially and competitively during solidification. For it to happen, side branching phenomena were identified