Abstract
In the last years, additive manufacturing (AM) has turned into an emerging technology and an increasing number of classes of material powders are now available for this manufacturing process. For large-scale adoption, an accurate knowledge of the mechanical behaviour of the resulting materials is fundamental, also considering that reliable data are often lacking and dedicated standards are still missing for these AM alloys. In this regard, the aim of the present work is to characterize both the high-cycle-fatigue (HFC) and the low-cycle-fatigue (LCF) behaviour of AM 17–4 PH stainless steel (SS). To better understand the performance of the selected alloy, four series of cylindrical samples were manufactured. Three series were produced via selective laser melting (SLM), better known as laser-based powder bed fusion of metals technology using an EOS M280 machine. The first series was tested in the as-built condition, the second was machined before testing to obtain a better surface finishing, while the third series was post-processed via hot isostatic pressing (HIP). Finally, a fourth series of samples was produced from the wrought 17–4 PH material counterpart, for comparison. The understanding and assessment of the influence of surface finishing on the fatigue behaviour of AM materials are fundamental, considering that in most applications the AM parts may present reticular or lattice structures, internal cavities or complex geometries, which must be set into operation in the as-built conditions, since a surface finishing postprocess is not convenient or not feasible at all. On the other side, a HIP process is often suggested to reduce the internal porosities and, therefore, to improve the resulting mechanical properties. The high-cycle-fatigue limits were obtained with a short staircase approach according to the Dixon statistical method. The maximum number of cycles (run-out) was set equal to 50,00,000. The part of the Wöhler diagram relative to finite life was also characterized by means of additional tests at higher stress levels. On the other side, the low-cycle tests allowed to tune the Ramberg–Osgood cyclic curves and the Basquin–Coffin–Manson LCF curves. The results obtained for the four different series of specimens permitted to quantify the reduction of the mechanical performance due to the actual limits of the laser-based powder bed fusion technology (surface quality, internal porosity, different solidification) with respect to traditional manufacturing and could be used to improve design safety and reliability, granting structural integrity of actual applications under elastic and elasto-plastic fatigue loads.