Microstructures, tensile properties, and fracture mechanisms of Inconel 718 formed by HDR-LMD with PREP and GA powders

Abstract

For the purpose of improving the productivity of laser metal deposition (LMD), the focus of current research is set on increasing the deposition rate for it, in order to develop high-deposition-rate LMD (HDR-LMD). Plasma rotating electrode process (PREP) and gas atomization (GA) are the most commonly used processes for producing LMD powders. In the presented work, the microstructures, the tensile properties, and the fracture mechanisms of Inconel 718 formed by HDR-LMD with PREP and GA powders have been examined. Initially, blocks were built up with different powders under identical experimental conditions. Afterwards, the samples built were heat treated, and then manufactured to tensile test specimens, after which they were tested in order to determine their tensile properties. Finally, microstructure and fracture analysis has been conducted. According to the results of the optical microscope observations for grain morphology, the EBSD analysis for grain orientations, and the analysis of twin boundary and grain boundary orientation, the GA sample has a higher recrystallization degree than the PREP sample. In addition, the intergranular δ phases are needle-like in the PREP sample, and some spherical δ phases have been observed in the GA sample. The tensile test shows that the quasistatic properties of the GA sample are slightly higher than those of the PREP sample. Based on the fracture analysis, different fracture mechanisms have been observed: in PREP sample, intergranular fracture has been identified; in GA sample, mixed fracture - both intergranular fracture and trans-granular fracture - has been observed. Furthermore, the pores found in the GA sample may be not definitely detrimental for the tensile properties. This hypothesis is supported by the following facts: the GA samples have significantly higher porosity, but meanwhile higher tensile properties; the dimples on the fracture surface that are around the pores develop well and fine; cracks formed in the pores did not extend to the outside of the pores.