Proprietà microstrutturali e meccaniche della lega IN718 prodotta per additive manufacturing e sottoposta a trattamento termico

This research work investigated the effect of the standard heat treatment for IN718 alloy on microstructure and mechanical properties of additively manufactured samples, fabricated using laser powder bed fusion technique (L-PBF). In order to investigate the above-mentioned properties of the specimens, many experimental techniques have been employed, namely optical microscopy, scanning electron microscopy, electron backscattered diffraction, Vickers hardness measurements and compression tests. The investigated thermal treatment started from the standard industrial treatment usually performed on IN718 products. It includes a solution annealing at 980 degrees C for 1h then water quenching, followed by a first ageing at 720 degrees C for 8h, a controlled cooling for 2 h down to 620 degrees, and then once this temperature is reached a second ageing for 8h. The resulting microstructure shows inside the grains, some fine gamma'/ gamma" precipitates dispersed in the gamma matrix, while along the grain boundaries coarser precipitates were continuously distributed, attributable to both 6 and Laves phases. The results of the compression tests, performed on lattice structures, show an overall embrittlement due to the presence of the above-mentioned phases. In order to reduce or avoid formation of the formation of these brittle phases, some modifications on the heat treatment are needed.