Effect of layer thickness and laser emission mode on the microstructure of an additive manufactured maraging steel

In this work, a high-performance maraging steel M300 was processed by laser powder bed fusion, where the layer thickness and the laser emission mode were modified. As-built microstructures were studied by considering the metallurgical phenomena taking place during printing: melting of the material deposited over previous layers; rapid solidification of the melt pool; and martensite-to-austenite reversion and precipitation of intermetallic phases induced by the combined effects of repeated heating and cooling cycles in the process. After printed, parts were subjected to ageing treatments at temperatures ranging from 480 to 540 °C. Microstructural characterization results evidenced the importance of layer thickness as a key parameter to modify the solidification cell size of the as-built samples. Two different phases were identified from the X-ray diffraction patterns in the microstructure: BCT martensite and FCC austenite. It was found that the tetragonality of martensite remained constant across different layers for all as-built conditions regardless of the successive reheating cycles, which are experienced during subsequent melting passes. No significant effect of the studied printing parameters was observed on the evolution of the microstructure during ageing. Finally, it was observed that the hardness of aged microstructures is comparable to that obtained by conventional manufacturing methods.