Fabrication of SS316L to Ni80Cr20 graded structures by 3D plasma metal deposition

Abstract

Powder metallurgy and thermal spraying has been used traditionally to manufacture functionally graded materials (FGMs). However, only simple geometries can be made with these processes and the compositional gradients cannot be tailored. These disadvantages can be overcome by employing additive manufacturing (AM). 3D plasma metal deposition (3DPMD) is a new technique that combines the advantages of plasma powder and additive processes. This process allows the production of structures with mixtures of different materials and powder fraction targeting changes in local properties and microstructures. For example, up to four powders, which can be different in terms of chemistry and powder fraction, can be mixed within one layer to adapt the local properties of the structure. The feasibility of functionally graded structures of stainless steel 316L (SS316L) and Ni80-20 alloy was studied. Two configurations, transition between the steel and the Ni-based alloy, were tested. The first one is hard transition, SS316L on the bottom and Ni80-20 on the top. The second one, a smooth transition between both materials was created with 50% of steel and 50% of Ni-based alloy. Optical microscopy and scanning electron microscopy were used to characterize the microstructures. The manufactured part showed good appearance, without any external defects and acceptable geometric accuracy. The layer thickness was around z = 1 mm for both structures. Regarding the microstructural characterization, both materials displayed a dendritic structure. In the SS316L, the microstructure was composed by an austenitic matrix with δ-ferrite located in the grain boundaries. The microstructure of the Ni80-20 was characterized as an austenitic matrix, with some M7C3 and M23C6 precipitates. In addition, the Laves phase was also observed.