Unidirectional columnar microstructure and its effect on the enhanced creep resistance of selective electron beam melted Inconel 718

Abstract

In this study, the selective electron beam melting (SEBM) process was used to fabricate Inconel 718 superalloy, and their microstructures and creep resistances were investigated. The proper SEBM condition ensuring a relative density higher than 99.9% was found in electron beam current range of 24–30 mA at a scanning speed of 4500 mm/s and a preheat temperature of 1025 °C, from which the SEBMed Inconel 718 exhibited an unidirectional columnar grain microstructure with a strong <100> texture along building direction. It was suggested that the local thermal condition during solidification after SEBM facilitates a continuation of epitaxial growth of crystals on layer beneath. The γ″ precipitates formed in the as-built condition regardless of focus offset, and their size increased with increasing focus offset from 1 to 12 mA. The creep rupture life of the as-built sample fabricated with focus offset of 12 mA showed 2 times longer than that of conventional forged Inconel 718 subjected to full heat treatment under creep condition of 650 °C/700 MPa. EBSD analysis on the crept samples indicated that strain accumulation was significant at the porosities and the high-angle grain boundaries. Additionally, the grain boundaries inclined to loading axis were more damaged during creep. The γ″ particles were occasionally sheared by matrix dislocations with Burger vector of a/2<110>, without leaving stacking faults behind. The enhanced creep resistance of SEBMed Inconel 718 superalloy can be attributed to the columnar microstructure with a strong <100> texture and the high density of dislocations resulted from strong interaction with the γ″ particles.