Effects of hydrogen and oxides on tensile properties of Al–Si–Mg cast alloys

Gas porosities and entrapped double oxide film (hereafter: oxides) are known to be the most detrimental defects in cast Al–Si–Mg alloys. This study investigated the effects of dissolved hydrogen (hereafter: H) and oxides on reproducibility of tensile properties in Al–7Si–0.35 Mg alloys. Also the effects of H and oxides content on the morphology of defects were studied. Four different casting conditions (low oxide–low H, low oxide–high H, high oxide–low H, high oxide–high H) were tested using tensile test bars that were cast in a metallic mold. Results of tensile test that were obtained for each casting condition were analyzed using Weibull two-parameter analysis. Microstructure and fracture surface of samples were examined by optical microscopy, SEM and EDS. Results showed that H has slightly larger negative impact on tensile properties of this alloy compared to oxides. On the other hand, oxides caused considerably higher scattering of tensile properties than H. Examination of microstructure showed that increasing of initial H of melt changed morphology of oxides from two dimensional (2-D) to three dimensional (3-D) one. It was explained through the diffusion of H into the atmosphere of oxides that expanded them. The effects of this morphological change on Von Mises stress and equivalent plastic strain was simulated by using finite element analysis. Results of finite element analysis showed that the maximum values of both Von Mises stress and equivalent plastic strain around the oxides decreased with changing their morphology from 2-D to 3-D. Also, it was shown that increasing of the H in high oxide samples, decreased the average tensile properties of test bars, but increased the reproducibility of tensile properties.