Abstract:
Alloy Inconel 718 has found wide use in high-temperature aerospace applications (up to 700°C), due to a combination of good mechanical properties, environmental resistance, and workability at moderate cost.  These properties have led Alloy 718 to early adoption by selective laser melting (SLM) additive manufacturing.  This presentation will share results from a comprehensive industry survey of 718 powder feedstock for SLM and focuses on the impact of variability in powder characteristics on build quality, microstructure, tensile and high cycle fatigue in the as-fabricated and machined surface conditions. The investigation includes sixteen powder lots from eight suppliers that are produced by gas atomization or rotary atomization in argon or nitrogen.  Although similar in regularity in shape, these powders showed distinct differences in flow and spreading behavior due to variation in the percentage of fines, particle size distributions and degree of agglomeration.  Compositional differences expectedly had the strongest impact on microstructure with N and C contents leading to minor phase formation on the grain boundaries that prevented recrystallization during heat treatment.  Optimal processing led to fully dense test bars and consistent tensile behavior between the as-fabricated and machined surface conditions.  For as-fabricated surface condition, the fatigue lives were statistically equivalent across powder lots with failures mostly initiating at stress concentrators at the surface.  The fatigue behavior for the machined surface was more varied and may be understood in context of failure mechanisms common to conventionally processed Alloy 718.  More detail will be provided on the linkages to powder feedstock variability and unique features associated with SLM processing.  This research was previously presented and published at the 2018 Superalloys 718 & Derivatives conference and was supported by the SLS Liquid Engine office with NASA HEOMD.