Graded Inconel-stainless steel multi-material structure by inter- and intralayer variation of metal alloys

Additively manufactured multi-metal hybrid structures can be designed as functionally graded materials providing an optimized response at specific positions for particular applications. In this study, liquid dispersed metal powder bed fusion is used to synthesize a multi-metal structure based on Inconel 625 (IN625) and stainless steel 316L (S316L) stainless steel regions, built on a S316L base plate. Both alloys alternate several times along the build direction as well as within the individual sublayers. The multi-metal sample was investigated by optical microscopy, scanning electron microscopy, microhardness measurements, nanoindentation and energy-dispersive X-ray spectroscopy. Cross-sectional synchrotron X-ray micro-diffraction 2D mapping was carried out at the high-energy material science beamline of the storage ring PETRAIII in Hamburg. Sharp morphological S316L-to-IN625 interfaces along the sample's build direction are observed on the micro- and nanoscale. A gradual phase transition encompassing about 1 mm is revealed in the transverse direction. Mechanical properties change gradually following abrupt or smooth phase transitions between the alloys where a higher strength is determined for the superalloy. The two-dimensional distribution of phases can be assessed indirectly as S316L and IN625 in this multi-metal sample possess a <110> and a <100> fiber crystallographic texture, respectively. Tensile residual stresses of ∼900 and ∼800 MPa in build direction and perpendicular to it, respectively, are evaluated from measured residual X-ray elastic strains. Generally, the study indicates possibilities and limitations of liquid dispersed metal powder bed fusion for additive manufacturing of functionally graded materials with unique synergetic properties and contributes to the understanding of optimization of structurally and functionally advanced composites.