On the cyclic deformation behavior of wire-based directed energy deposited Fe-Ni Invar alloy

Fe-basis alloys with Ni concentrations of approximately 36 wt.-% are widely used for components with high requirements on mechanical properties and dimensional accuracy. Due to the remarkably low coefficient of thermal expansion, these so-called Invar alloys provide for dimensional accuracy over a wide temperature range. Generally, additive manufacturing technologies have already been qualified for processing of Invar alloys in terms of maintaining mechanical as well as functional properties. However, the mechanical behavior under cyclic loading still is not comprehensively understood. Therefore, the present study focuses on the fatigue properties of the Fe-Ni36 Invar alloy manufactured by wire-based directed energy deposition (DED) employing either an electric arc (DED-Arc) or a laser beam (DED-LB) as energy source. Here, a wire-based DED-LB Invar condition is comprehensively studied for the first time in open literature. For initial assessment, specimens were analyzed using optical and scanning electron microscopy as well as computed tomography and tensile testing. The DED-LB material is characterized by a heterogeneous grain size distribution, less pronounced crystallographic texture as well as increased tensile strength and ductility compared to the DED-Arc counterparts. Despite increased strength and ductility, the DED-LB Invar consistently shows inferior fatigue properties. This behavior is discussed based on microstructural heterogeneity, cyclic deformation response, process-induced porosity and the contribution of plastic strain. Furthermore, investigations on the thermal expansion behavior show that the DED Invar retains its outstanding thermal properties, respectively.