Interfacial characteristics of austenitic 316 L and martensitic 15–5PH stainless steels joined by laser powder bed fusion

Abstract

Laser powder bed fusion (LPBF) is an additive manufacturing (AM) technology capable of producing complex geometry components from a range of metals and alloys. The static mechanical strength of LPBF manufactured materials can rival that of the equivalent cast and wrought materials, but are more susceptible to fatigue failures due to stress concentrating roughness and porosity defects. The ability to process and join multiple powder materials within a single LPBF build process is an emerging capability that is now becoming commercially available. This new capability offers the possibility of compositional complexity, in addition to the geometric complexity offered by AM, and can help to eliminate the need for additional processing to join different materials. This study focuses on the combination of 316 L austenitic stainless steel (SS) and precipitation hardening 15–5PH martensitic SS by LPBF. The interfacial characteristics and microhardness variation at the interface were investigated by optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, electron backscatter diffraction, and microhardness testing. Good apparent bonding was observed at the interface without any visible cracks or defects. A finer-grain region was observed at a distance of 115 μm below the interface with a grain size of about 25% of that in the surrounding 15–5PH SS. A narrow compositional transition distance of 7 μm along the building direction (less than the 30 μm LPBF layer thickness) and a wavey-morphology interface with an amplitude of about 66 μm (about twice the LPBF layer thickness) were found. A sharp change of hardness was measured within ±200 μm from the interface. Regions far from the interface exhibited similar microstructure and hardness as the corresponding single material components. The results suggest that LPBF joining between 316 L SS and 15–5PH SS can achieve each material's distinct microstructure and properties at far-interface regions, with a narrow wavey region (∼115 μm) at the interface that exhibits high densification and a sharp transition in microstructure and properties.