Mechanical and microstructural behavior of Inconel 625 deposits by high-pressure cold spray and laser assisted cold spray

Abstract

Thick deposits of Inconel 625 (IN625) by high-pressure cold spray (HPCS) have been described as cold spray additive manufacturing (CSAM). However, given the solid solution and numerous precipitation phases of the IN625 superalloy, these deposits can exhibit low ductility and high residual stresses due to severe deformation of the particles upon impact against the substrate. To counteract this phenomenon, laser-assisted cold spray (LACS) was used here to soften the material using two laser surface temperatures (650 and 900 °C), and to elucidate their effect on the microstructure and mechanical properties, specifically elastic modulus, hardness, and stress relaxation behavior of the deposits. For comparison, IN625 was also deposited by HPCS. The microstructure of the deposits was characterized by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Mechanical properties were determined by nanoindentation. The HPCS and LACS 650 °C deposits showed an interdendritic network of intermetallics, a feature also present in the powder feedstock material. However, the deposit produced with LACS at 900 °C showed a microstructure suggesting a dissolution of this network, a critical surface laser temperature for inducing an evolution on the microstructure in LACS processes. The EBSD maps show strong grain deformation in the material deposited using HPCS, whereas the sample made with LACS at 900 °C showed significant recrystallization and grain growth (3.5 μm in average). Mechanical properties, including stress relaxation phenomena, were measured using nanoindentation. The hardness of the LACS 900 °C sample was found to be 7.2 GPa, which is a decrease from the 8.9 GPa found in the HPCS sample and 9.1 GPa found in the LACS 650 sample. This decrease in hardness can be attributed to the recrystallized microstructure due to the localized laser heating. In contrast, the elastic modulus remained practically the same for all three samples, around 265 GPa.