Heating treatments for an AlSi10Mg-Cu alloy processed by Laser Powder Bed Fusion

Additive Manufacturing (AM) has introduced new perspectives in parts production, such as design freedom, part customization, weight reduction. Nonetheless, many problems need to be further investigated. For instance, conventional heat treatments, which have been optimized to improve cast parts, must be adapted for AM parts. This work follows a previous one, in which the process parameters to process a new AlSi10Mg+4Cu powder for Laser Powder Bed Fusion (LPBF) have been carried out. The principal aim of the present thesis is to find the optimized parameters for a T6 treatment of LPBFed AlSi10Mg+4Cu alloy. Conventional T6 heat treatment consists of three stages: solution heat treatment (SHT), quenching and artificial aging. The main purposes of SHT are to homogenize the microstructure and to dissolve as much intermetallic phases as possible. The following step is the quenching. It must be performed immediately after SHT, to ensure the crystallization of the structure, generating a supersaturated solid solution. This is very important to allow a better aging response, since the last step, also known as artificial aging, consists of controlled phases precipitation. A properly calibrated artificial aging assures an increase in hardness and strength of the part, at the expense of ductility. In the first part of the work, SHT treatments at different times and temperatures were performed, selected according to ASTM protocol: 9 treatments were performed at temperatures of 485, 500 and 515 °C, and times of 4, 6 and 8 hours. Porosity, hardness and microstructure were fully analyzed after each heat treatment in order to identify the parameters that allowed the better solutioning treatment, for instance seeking for the highest solubility after SHT. Porosity was evaluated through both Archimede’s method and images analysis: the results were in agreement with the previous study, with a porosity lower than 2%, but for a higher porosity level found in the sample heat treated at 515°C/6h. Other defects, like keyholes and, presumably, incipient fusion of Al-Cu rich phases, were also spotted. The microstructure was studied through different analyses: analysis of images taken from optical microscope and FESEM, whereas the composition was evaluated by EDS, XRD and DSC analyses. Precipitation and dissolution of elements were studied for all cases. A higher dissolution of Cu-rich phases was noted at 515 °C compared to the other temperatures, while Si size tended to increase with the heat treatment temperature. XRD allowed to identify the precipitates previously detected with EDS, mainly θ-Al2Cu, N-Al7Cu2Fe and Q-Al4Cu2Mg8Si7. The study of DSC completed the analysis, allowing to understand the entity of elements that have yet to precipitate and to hypnotize aging behavior. At last, Vickers hardness was measured on all samples in order to detect the level hardness after SHT heat treatments. By analyzing the results, it was possible to define the parameters of the proper SHT treatment that could ensure the best aging result during artificial aging heat treatment.