Inter-relação entre solidificação, tratamento térmico, corrosão eletroquímica, microestruturas e propriedades mecânicas de uma liga automobilística AlSiMg

Abstract

Al-7wt%Si-0.3wt%Mg alloy belongs to an important group of materials widely used whether casted or wrought in the automotive industry. When heat treated the mechanical strength/weight ratio is increased as well as the corrosion resistance of castings based on this alloy. Thus in this study were conducted directional solidification enexperiments with alloy Al- 7wt%Si-0.3wt%Mg and heat treatment T6 (T6-HT). A water-cooled horizontal directional solidification device was used, and samples of the resulting ingot were subjected to T6-HT as well as electrochemical corrosion tests. T6-HT consisted of three steps: (I) solution heat treatment at 520°C with residence time of 3 hours; (II) quenching with rapid cooling at 70°C in water for 30 seconds, and (III) hardening by artificial aging at 155°C with residence time varying at 1, 2, 3 and 4 hours. The analyzed solidification parameters include growth rate (VL), cooling rate (TR), secondary dendritic spacing (λ2) and formed intermetallic phases. The resulting solidification microstructure was analyzed by optical and scanning electron microscopes. The evaluated mechanical properties of both cast and heat treated samples were microhardness (HV), ultimate tensile strength (UTS) and elongation (E%). The investigated electrochemical parameters of the as-cast samples were the open circuit potential, anodic and cathodic polarization curves with the respective corrosion potentials and currents (ECORR and iCORR) corresponding to the corrosion kinetics in a 0.2 mol L-1 HCl solution with an immersion time of 10 min at 25°C. The results showed that the primary and secondary ramifications of the dendritic microstructure were not affected by heat treatment. Hardening precipitation was observed in the aluminum matrix. Corrosion resistance was found to be higher at the position closest to the heat transfer interface, where the values of VL and TR are higher and those of λ2 lower.