Effects of Scan Strategy Induced Microstructural Differences on Thin-Wall SLM IN718 Fatigue Performance

The as-printed microstructure of additively manufactured parts is a function of many variables that span from scan strategy to part geometry. This is particularly relevant in precipitation strengthened alloys (e.g. IN718), as thermal history—which itself varies across a build—dictates the distribution of precipitates in the microstructure. Elucidation of the complex relationship between geometry, scan strategy, and resultant microstructure is necessary to optimize future scan strategies. In this study, a series of IN718 dogbone samples were printed via selective laser melting at 0.8 mm in thickness using a contour + hatching scan strategy with both rotating (67°) and static beam directions. To quantify the effect of subsurface porosity on fatigue crack initiation, the as-built surface was left intact on the thinwall samples. As a reference, a 10 mm thick “bulk” sample was also printed and had 0.8 mm thick dogbones removed from it. To investigate the influence of scan strategy on the low-cycle fatigue (LCF) resistance of AM IN718, these samples were tested to failure using a custom micromechanical test setup equipped with a linear actuator and digital image correlation (DIC) to identify the onset of plasticity and the initiation and propagation of fatigue cracks across the sample surfaces. The driving hypothesis for this study is that the rotated scan strategy increases the fatigue resistance due to a more uniform microstructure with smaller grains. As-printed defects and fracture surfaces were characterized utilizing SEM imaging and DIC analysis and compared to observations from mechanical testing.