Microstructural and mechanical anisotropy of selective laser melted IN718 superalloy at room and high temperatures using small punch test

Abstract

The nature of melting and solidification phenomena during selective laser melting (SLM) process leads to a strong crystallographic texture in the as-built SLM components which causes mechanical anisotropy. This anisotropy is usually reduced by either optimization of the laser scanning strategy during the SLM process or high-temperature heat treatment. Microstructure and mechanical anisotropy of SLMed Inconel 718 (IN718) superalloy were investigated in this study by means of XRD, SEM, EBSD and small punch testing (SPT) method at room and high temperatures. The SPT data were validated against uniaxial tensile testing and corresponding equations were proposed to determine yield and ultimate tensile strengths of the fabricated parts. Significant differences in mechanical behavior of the side and top views were mainly achieved due to the contribution of (100) columnar grains texture. It was found that the IN718 SLM parts possess 54% greater maximum force in the side view compared to the top one at room temperature. However, the side view maximum force lowered about 37% at 650 °C, while no significant decrease was observed for the top view sample. Fractography of small punched specimens showed ductile fracture surface for the side view compared to transgranular brittle surface for the top view at room temperature. Both views exhibited more brittle fracture at elevated temperature with different fracture surface morphologies, intergranular in the side view and cleavage along with Laves phases in the top view. The present work clearly demonstrates that SPT can successfully predict mechanical anisotropy in the as-built SLM IN718 superalloy.