Specification of resulting microstructure from additive manufacturing by Direct Laser Deposition of SS316L-Si on stainless steel base materials

Abstract

The purpose of this study was to investigate the macro- and microstructural constitution of Direct Laser Deposited (DLD) SS316L-Si on stainless steel base materials. The characterisation of the deposited material was carried out using light optical microscopy (LOM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), electron backscattered diffraction (EBSD), transmission- and scanning transmission electron microscopy (TEM and STEM) together with Vickers hardness measurements. The DLD process produced a crack-free, almost fully dense (porosity >99.5%) metallic part. The investigation revealed a duplex microstructure containing a primary austenitic matrix with secondary δ-ferrite interdendritic and along sub-grain boundaries. The δ-ferrite has formed due to segregation of ferrite stabilizing alloying elements and has helped voiding the material from hot-cracking during the thermal cycles. The austenitic dendrites have formed preferentially along <001> direction generating a solidification texture and large columnar grains along the thermal gradient being 5-10 times larger in longitudinal- than latitudinal direction. A microstructural variation exists throughout the samples with cellular- and columnar dendritic structures depending on solidification rate, with a localized heat-affect zone for each layer. While the process has formed a solid metallurgical bond to the base material in the case where the cleaning procedure was followed thoroughly, with a small heat-affect zone in the base material. Hardness values was found to be almost uniform in parallel- and normal to build directions, ~170HV. The elevated hardness compared to conventional manufactured SS316L is due to silicate nano-inclusions that generate large concentrations of dislocations and the uniform distribution of δ-ferrite facilitating the dislocation pile-ups. The investigation has also reveal defects such as entrapped gas which has created spherical pores, inter-track porosity along laser travel direction and segregation of alloying elements. Various solutions to improve the defects has been discussed so the study may serve as a guide to improve the process parameters.