Precipitation hardening stainless steel samples processed by additive manufacturing: Process parameters and thermo mechanical treatments effects on microstructure and corrosion resistance

The effects of different process parameters and thermo mechanical treatments on 17-4 PH samples processed by Selective Laser Melting have been investigated in this study. Overlap, scanning radius and laser beam defocusing were varied while all the other process parameters were kept constant. So, the role of process parameters was analysed in terms of microstructure, defectiveness and response to heat treatment. Specifically, the as built samples were subjected to stress relaxation heat treatment (650 degrees C-2 h) and H 900 heat treatment (solution treatment at 1100 degrees C 1 h + 480 degrees C-1 h). On the as built, stress relieved and H 900 heat treated samples the corrosion resistance, microstructural evolution and hardness were studied. Moreover, Ultrasonic Peening treatment (UPT) was applied and its effect corrosion resistance was evaluated. The study was carried out by optical, scanning and transmission electron microscopes, Vickers hardness, DRX and Image analysis software. It was found that the amount of austenitic phase increases with the decreasing of the scanning radius from 5000 mu m to 1250 mu m; with positive defocusing of laser beam (+1mm) respect to in focus processed sample and with increasing of the overlap from 25 mu m to 50 mu m.Each samples exhibit pores lower than 0,3% in terms of area percentage and mostly circular in morphology. In the process variability window, no significant difference in defects occurrence were observed. In the as built state, different values of hardness were found according to the different microstructure and average grain size. After the stress relaxation heat treatments at 650 degrees C-2 h all the samples exhibit similar hardness values while the most austenitic and finer grained as built samples exhibit higher increasing in hardness after H900 heat treat-ment. Corrosion resistance changes as function of the different process parameter and heat treatment and is improved by Ultrasonic Peening mechanical treatment.