Effect of powder bed preheating on the crack formation and microstructure in ceramic matrix composites fabricated by laser powder-bed fusion process

Abstract

Ceramic matrix composites like cermets are difficult candidates to be fabricated by additive manufacturing (AM). Green cermets like the TiC-430 L AISI ferritic stainless steel (430 L-FSS) have been fabricated using the AM process like laser powder-bed fusion (LPBF) process or also known as selective laser melting (SLM). Multiple laser scanning was employed as a tool to preheat and melt the powder bed with different laser scanning parameters for each (two laser scans namely pre-heating scan and melting scan) scan to fabricate crack-free cermets. As a part of the preheating scanning (PHS) and melting scans (MS), different laser energy parameters are used to study the microstructure evolution and to study their densification process and mechanical properties. Lowering the preheating and melting scan speed results in prolonged laser exposure and higher temperature in the powder bed and melt pool, which led to the refinement of the microstructure due to higher cooling rate and generation of the cracks in the as-built parts caused by thermal gradient in the melt pool. Scanning electron microscopy and X-ray diffraction were performed to study the microstructure and phase analysis of the fabricated sample. Mechanical properties like density, hardness, fracture toughness, and compression tests were performed and discussed in detail. The results suggest that the laser PHS can effectively eliminate the presence of cracks. On the other hand, increasing the energy density of both PHS and MS results in a coarser microstructure with porosity, which also hampers the fracture toughness of these SLM fabricated cermets.