SLM process parameters effects on the fatigue strength of AMed Inconel 718

Selective Laser Melting (SLM) emerged as a technology suitable for the industrial production of structural components featuring complex geometries. In the field of elevated temperature applications, the possibilities offered by the SLM can be successfully used to produced complex geometries as internal cooling channels or lattice structures, as long as the process doesn’t jeopardize the mechanical properties, in particular the fatigue strength. The SLM process parameters play a fundamental role in determining the mechanical performances of the component but are also the key parameter for increasing the productivity of the process and thus the industrial spread of the technology. In the present work, it is presented an experimental assessment of the effects produced by different sets of productivity-oriented SLM process parameters on the Wöhler curves of cylindrical plain specimens. The adopted process parameters were defined on the basis of a previously developed thermal analytical model aimed to predict the melt pool dimensions and shape. HCF tests were carried out at room temperature in an axial load configuration with a stress ratio of 0.05 and a loading frequency of about 150 Hz, by using a resonant testing machine. Test frequency was monitored to detect the occurrence of crack nucleation and monitor the propagation phase. In order to understand the causes of the fatigue behavior, metallographic analyses were carried out to investigate the microstructural properties or the presence of internal defects, i.e. porosity and hot tearing cracks, produced by each set of process parameters. The surface quality was also investigated in detail through optical microscope analyses. Fractographic analyses were used to identify the nucleation and crack propagation region, as well as the presence of the defects in proximity to the fracture onset. The experimental data, along with an analytical model of the thermal field produced by a single scan line, allowed to define a preliminary feasible region for the SLM process on Inconel 718, in terms of scan velocity, laser power, layer thickness, and scan strategy.