Additive manufacturing of lightweight Al-Si-Mg and Ti-Al-V alloys via laser-powder bed fusion: post-heat treatment Optimization on Microstructure and Mechanical Properties

AlSi7Mg, AlSi10Mg and Ti6Al4V lightweight alloys are widely used within automotive, aerospace, and biomedical fields where high mechanical performance and safety factors are required. In fact, the age-hardening aluminium alloys are characterized by high specific tensile strengths and good corrosion resistance, as well as the titanium alloy that show higher tensile strengths, corrosion resistance and cell biocompatibility. Adding the advantages conferred by the Laser-Powder Bed Fusion manufacturing technology, the application of these alloys can significantly increase. The present doctoral thesis analyses the effects induced by different heat treatments to evaluate their optimization on AlSi7Mg, AlSi10Mg and Ti6Al4V samples manufactured via Laser-Powder Bed Fusion. Both the AlSi7Mg and AlSi10Mg were firstly studied in the as-built conditions; in which the effects induced by the build platform pre-heated at 150 °C were carefully investigated along the total height (300 mm) of the samples. At the same time, the microstructural effects and the resultant mechanical properties obtained by the layer thickness (+ 40 μm) and hatch spacing (– 150 μm) variations were also investigated on the AlSi10Mg samples. Secondly, the effects induced by direct aging at 175 °C, 200 °C and 225 °C were also examined in order to homogenize the mechanical properties and HV microhardness from the bottom to the top regions of the manufactured samples. After just the 1–2 h treatment at 175 °C and 200 °C for the AlSi7Mg and AlSi10Mg alloys, respectively, the mechanical properties were already homogenized and optimized. The homogenization in mechanical properties was also evaluated after the solution heat treatment at 505 °C and artificial aging at 175 °C for both aluminium alloys. Even considering the samples in peak-aging conditions, the yield and ultimate tensile strength were always lower than those obtained by samples heat-treated in optimized direct aging conditions. These worsening effects were also reflected on the ductility, whose values were lowered of 11 %. Ti6Al4V samples manufactured via Laser-Powder Bed Fusion in different build orientations were analysed in as-built and heat-treated conditions. In order to reduce the anisotropy of the mechanical properties, the effects of two different annealing heat treatments at 704 °C and 740 °C were carefully investigated through SEM, XRD and EBSD measurements. Despite the different cooling pathways between both annealing heat treatments, and the consequent coarsening effects of the α-phase laths (+ 23 %), no variation in terms of yield and ultimate tensile strengths were obtained. EBSD and SEM investigations did not show a preferential orientation of the α-phase, but highlighted that the same α-laths maintained the same α’-martensite orientations during the α → α + β. The effective decrease of the tensile strengths was obtained after the solubilization heat treatment at 1050 °C × 60 min, due to the recrystallized microstructure. In fact, the ultimate tensile strength and the yield strength decreased to 925 and 825 MPa, respectively. At the same time, elongation values did not significantly due to the formation of an α-case layer on account of the oxygen diffusion, on the sample surface, during the solubilization heat treatment. Moreover, the different thickness of the samples promoted several cooling pathways and, consequently, the formation of different microstructures at room temperature, despite the same fast cooling in argon gas (60 min). Lastly, the sand-blasting process decreased the surface roughness (– 50 %) but increases the plastic strain thanks to the impact of glass spheres on the sample surface. No variations in terms of tensile strengths were detected. In relation to the AlSi10Mg alloys in as-built and heat-treated conditions, the correlation between the HV microhardness and tensile strengths can be good approximated by Cahoon’s equations. From the comparison of the theoretical and experimental Vickers microhardness values, and those calculated by the yield strength, a correction factor of 0.891 was obtained to adjust Cahoon’s equation. On the other hand, Ti6Al4V samples heat-treated at 704 °C and 740 °C show a discrepancy between the real and calculated HV values of 21–22 %. Therefore, the α-phase increase of about 21–22 % the HV values compared to those calculated with the Cahoon equation.