Correlation between microstructure heterogeneity and multi-scale mechanical behavior of hybrid LPBF-DED Inconel 625

Abstract : The two additive manufacturing processes Powder Bed Fusion (LPBF) and Directed Energy Deposition (DED) have different geometrical resolutions and production flexibilities, making their hybridization attractive. LPBF microstructure displays fine grains, with weak preferential crystal orientation. DED generates a highly textured and inhomogeneous microstructure with equivalent grains diameters ranging from a few micrometers to over a millimeter. The microstructure of the hybrid LPBF-DED sample is the addition of these two microstructures with an interface free from cracks or particular pores. The effect of this strong heterogeneity of the hybrid microstructure on mechanical behavior is analyzed by tensile tests instrumented with local strain gauges, others using digital image correlation method and finally on samples tested inside a scanning electron microscope. This multi-scale characterization showed that the difference in the elastic properties causes the localization of the strain field and generates a plastic incompatibility at the interface. An optimized heat treatment leads to isotropic and homogeneous hybrid microstructure, with a larger DED grain size. It leads to identical plasticity mechanisms during tensile tests and lowers the strain gradient around the interface.