Effects of laser melting plus remelting on interfacial macrosegregation and resulting microstructure and microhardness of laser additive manufactured H13/IN625 bimetals

Macrosegregation at the interface in Multi-Material (MM) structures fabricated by Selective Laser Melting (SLM) has evidenced deleterious effects on interfacial bonding reliability, but its fundamental understanding is less revealed. Therefore, this work vertically prepared MM parts of H13 tool steel and IN625 superalloy by SLM, as an example, to explore the interfacial macrosegregation mechanism. According to the mechanism, a liquid layer of unmixed H13 can exist at the molten pool bottom due to the no-slip boundary condition in fluid mechanics and solidifies as an H13 macrosegregation beach. Considering the higher liquidus temperature of H13 (1483 degrees C) than IN625 (1300 degrees C), a region cooler than the H13 solidification temperature exists within the IN625 molten pool; therefore, when the melted but unmixed H13 liquid is entrained into the cooler region by fluid flows, it can solidify quickly, forming H13 macrosegregation peninsulas/islands. The laser melting+remelting strategy developed to mitigate interfacial macrosegregation reveals its significant contribution to efficient migration of H13 into IN625 molten pools in a layer-by-layer manner, forming a broad interface. As the direct reflection of interfacial macrosegregation, the microstructure experiences a cellular-columnar-cellular transition across the H13/IN625 interface. Even a peninsula with H13 microstructural features is observed in an IN625 molten pool, verifying the rapid solidification of intruded H13 in IN625. Finally, the microhardness values with varying standard deviations across the interface successfully evaluate the interfacial macrosegregation on a macro-scale. These results can provide theoretical support to mitigate interfacial macrosegregation and benefit the development of MM structures with good interfacial bonding reliability by SLM.

Automated summary

This study vertically prepared Multi-Material parts of H13 tool steel and IN625 superalloy by SLM, revealing the interfacial macrosegregation mechanism. The laser melting+remelting strategy developed can mitigate interfacial macrosegregation and efficiently migrate H13 into IN625 molten pools.