Additive manufacturing of a carbon-martensitic hot-work tool steel using a powder mixture – Microstructure, post-processing, mechanical properties

This work examines the processing of a hot-work tool steel using laser-based powder bed fusion of metals (PBF-LB/M). The hot-work tool steel was produced using a low-cost powder mixture consisting of pure iron and other elemental powders as well as ferroalloys. Furthermore, a prealloyed starting powder with the same nominal chemical composition as the powder mixture was produced by inert-gas atomization. Besides, a reference steel was produced by casting to compare the microstructures and mechanical properties resulting from the different processing routes. The first step examined the application of a chemically homogeneous and dense layer of the powder mixture prior to PBF-LB/M densification. In addition to evaluate suitable process parameters for PBF-LB/M processing of the starting materials, the microstructure formation was comprehensively examined using electron microscopy and the processes adapted to it. To eliminate defects (cracks, pores) and chemical inhomogeneities, thermal posttreatments, namely supersolidus liquid phase heat-treatment (SLPHT) and hot isostatic pressing (HIP) were performed. Suitable heat-treatment parameters were evaluated. Finally, the obtained microstructures and the associated properties of the post-processed PBF-LB/M samples were compared with those in the reference states. As a main result, it was possible to achieve full redensification and simultaneous chemical homogenization of the PBF-LB/M-processed powder mixture by SLPHT post-processing. The hardness of the additively manufactured and SLPHT-post-processed specimens exceeds that of the cast reference. •PBF-LB/M of a hot-work tool steel powder mixture leads to undissolved raw material particles in the microstructure.•Chemical inhomogeneities locally vary the aimed martensite start temperature and locally promote cold cracking.•Hot isostatic pressing (HIP) increases the relative density but could not improve the chemical homogeneity.•Supersolidus liquid phase heat-treatment (SLPHT) simultaneously improves the density and the chemical homogeneity.•The secondary hardness of the redensified and homogenized powder mixture exceeds that of a cast reference.