Influence of impurity elements on the casting properties of Al - Mg based alloys

Increasing demand for industrial applications of aluminum alloys achieved a new aspect; nowadays the demand for corrosion-resistant alloys largely exceeds the one for other high-strength, wear-resistant and heat-resistant aluminum alloys. Moreover, the relatively low-cost of casting technology provides an increased share of demand for foundry corrosion-resistant alloys in comparison with the deformable ones. Another advantage of foundry Al alloys is their excellent recyclability. However, recycling of scrap metals causes an increase in the content of impurity elements. In this study, the influence of individual and combined addition of impurity elements, such as Si, Fe, Cu, Zn, Pb, Sn, Ni and Mn, on the casting properties of corrosion-resistant Al - Mg alloy with the Mg content ranging between 4-8% was investigated. While the microstructure of the alloys in as-cast state was characterized using optical and scanning electron microscopes, the fractured surfaces of tensile test specimens were investigated using the latter microscope. The results of the experimental investigations of the effect of the main impurities on casting properties, such as fluidity, hot cracking, shrinkage and pre-shrinkage expansion of Al - Mg based alloys have been systematically summarized. It has been found that among various impurity elements, Si, Fe and Cu can exert much greater influence on fluidity and hot tearing. All these elements tended to improve fluidity; however, it occurred at the expense of a considerable reduction in hot tearing of Al -Mg alloys. The casting properties of die castings, castings obtained under pressure and die castings after hot isostatic pressing (HIP) have been studied and compared with each other and it has been found that isostatic pressing causes reduction of porosity in alloys, but slightly affects the mechanical properties. Casting with excess gas pressure (P = 0.7 atm) exerted almost no apparent influence on density (porosity) of the castings from various alloys. In general, the increased content of impurities has an insignificant effect on the casting properties of Al -Mg based alloys, so it is concluded that low-grade (secondary or recycled) aluminum can be used in the fabrication of corrosion-resistant applications.