Finite element simulation of plasticity and fracture for Inconel 718 deposited by laser powder bed fusion – Chances, use and challenges

Abstract

Laser Powder Bed Fusion (LPBF), is used to produce a component in layers by selective re-melting of a metal powder bed. A powder layer of an alloy is applied to a metal substrate within a process chamber filled with an inert gas. This powder layer is locally heated by a laser to a temperature above the melting point of the used alloy. Only regions of the applied powder which belong to the desired component are treated by the laser beam. After heating the layer is complete, the build platform is lowered by 30–90 µm and a new powder layer is applied. The procedure is repeated until the part is completed. After suitable post processing the part can be used. The relative density of the finished component is close to 100%. Depending on the deposition system the laser spot diameter measures approximately 40–80 µm, therefore the size of the resulting melt region is relatively small while cooling rates and temperature gradients are very high. Hence some resulting material properties of certain laser-melted alloys differ from the properties which can be achieved by conventional production routes. Furthermore, studies show that adjustment of certain process parameters can be used to influence the material condition, e.g., the anisotropy of the mechanical material properties. On the one hand this degree of freedom might offer opportunities to realize designed component properties, but on the other hand the modeling of the material condition and the resulting mechanical properties is challenging. A testing program covering different stress states as well as different material orientations has been performed at COMTES FHT. The tests revealed a significant orthotropy of plasticity and fracture which both are considered in the derived material card for the material model MF GenYld+CrachFEM – a material model developed by MATFEM. A good agreement between physical tests and finite element simulations of the performed tests was found. Chances and challenges in using the full potential of AM deposited materials based on detailed numerical modeling considering material orthotropic behavior are presented here for Inconel 718 and are supported by experimental testing.