Effect of Surface Layer on the Fatigue Strength of Selectively Laser Melted 17-4 PH Steel

The effect of surface layer properties on the fatigue strength of selectively laser melted 17-4 PH stainless steel flat specimens is presented within this work. X-ray surface residual stress measurements in loading direction show that the residual stresses are negligible if a final heat-treatment is applied, whereas values of only about 3% of the nominal yield strength of the base material are evaluated. The presented additively manufactured surfaces highlight irregular surface topographies, at which an area-based assessment of the surface roughness parameters should be applied instead of a line-based evaluation using defined profiles which may lead to an underestimation. Fatigue tests under cantilever bending at a load stress ratio of R = −1 demonstrate a fundamental impact of the surface layer on the fatigue strength especially within the high-cycle fatigue region. A comparison to the test results of machined specimens reveals a significant increase in the fatigue strength by 29% at ten million load-cycles due to the machining process. Finally, a fracture surface analysis highlights that not only the surface topography itself, but also the surface layer is of great importance in terms of fatigue. Thereby, surface-near imperfections or lack of fusion of the contour layer to the bulk material can act as origin for fatigue crack initiation, which illustrates the necessity to consider the surface layer properties in the fatigue design of additively manufactured 17-4 PH steel structures.