Microstructures and mechanical properties of Invar/MnCu functionally graded material fabricated by directed energy deposition

Directed energy deposition (DED) is an ideal approach for the direct fabrication of metallic functionally graded materials (FGMs) and has attracted great interest during the past decade. However, researches about DED-fabricated FGMs are mainly focused on structural metals, such as iron/nickel, and titanium/aluminum material systems. It is significant to develop new systems and give useful insight for a wider application field of FGMs. Invar and MnCu alloys are important materials for thermal stability and vibration suppression in high-precision equipment. In this study, the crack-free FGM from 100% Invar to 100% MnCu with a 10 wt% linear increment was prepared by DED. In the gradient region, grains were refined (size from 46 μm to 20 μm) with the increasing content of MnCu, owing to the combination of columnar to equiaxed transition (CET) and recrystallization. The ultimate tensile strength and elongation of Invar/MnCu FGM are 359 MP and 5.7%, respectively. The deformation behavior of FGM shows a “cask effect”, most of the plastic deformation is concentrated at the 90%Invar-10%MnCu position, which is the part with the lowest hardness value. This study will provide a useful reference to aid in the selection of materials for the designing of new FGMs and extend the application field.