Microstructure and mechanical properties of 316L/Inconel 625 gradient multi-material additively manufactured by laser powder bed fusion

• Multi-material laser powder bed fusion enabled manufacturing of 316L/Inconel 625 gradient parts. • Step change in 316L/Inconel 625 ratio leads to smooth chemical composition gradient. • Microcracks and precipitates in mixing gradient zones do not cause embrittlement. • Strength of mixing zones higher than 316L due solution strengthening by Nb and Mo. • Tensile fracture initiates in gradient zones with higher 316L content. Gradient multi-materials combining 316L steel and Inconel 625 superalloy are needed for components operating at variable temperature and load conditions. This study investigates for the first time the laser powder bed fusion manufacturing of 316L/Inconel 625 gradient multi-materials by step-like varying the composition. Tomographic and microscopy analysis revealed low porosity and few microcracks. Its presence does not affect mechanical properties. Mixing 316L with Inconel 625 favors the microstructure with small equiaxed grains and precipitates of the Laves phase and carbides along cell and grain boundaries. The phase analysis of precipitates by electron diffraction is in line with Thermo-Calc calculations and shows that higher Nb and Mo content promotes precipitation of M 23 C 6 carbides instead of M 6 C. The increase in hardness across the gradient is mainly attributed to solid solution strengthening by Nb and Mo. The transverse arrangement of the gradient sample in tensile test results in higher strength than in a longitudinal orientation. This study shows that the designed process conditions and gradient scheme reduce the risk for abrupt element segregation, leading to brittleness, typical for directed energy deposition. The interplay between manufacturing, chemical composition, microstructure and mechanical properties of gradient multi-material is determined.