The oscillatory bending fatigue limit of titanium condenser tubes: Dworrak, G., Lindemann, J. and Vogt, G. VGB Kraftwerkstechnik July 1989 69, (7), 718–726 (in Germany)

Abstract

The selective laser melting (SLM) technology applied to Inconel 718 produces near-net shape parts of unlimited geometrical complexity directly from a CAD model and a metal powder bed processing system. With respect to conventional manufacturing, SLM potentially fabricates parts that can be lighter in weight, cheaper and have complex geometries that are difficult or impossible to produce otherwise. However, the surface quality of SLM parts with as-built surfaces negatively affects their fatigue performance and post fabrication surface finishing may be cost-wisely unacceptable in most applications and often impossible because of surface inaccessibility.Two aspects significantly affecting the fatigue performance of SLM Inconel 718 parts have been investigated and are reported here: (i) the directional knock-down factor of the as-built surface state with respect to the surface machined condition, and (ii) the role of a geometrical notch in the as-built surface state as commonly found in complex SLM parts.Eight batches of un-notched and notched miniature specimens of heat treated SLM Inconel 718 with as-built surfaces were tested in cyclic plane bending. The unnotched fatigue strength was found to be sensitive to direction of the applied stress with respect to build direction. The as-built notch fatigue strength of heat treated SLM Inconel 718 is also highly directional in nature. Four SLM-technology-dependent factors are introduced and determined. The un-notched and notched fatigue behavior of SLM Inconel 718 with as-built surfaces obtained here compares satisfactorily with recently published data for the same material and SLM technology but different specimen geometry and test method. Therefore, the proposed experimental methodology based on the use of miniature specimens in cyclic bending may a viable tool for the efficient evaluation of the fatigue response of SLM metals.