The Effect of Holding Liquid Aluminum Alloys on Oxide Film Content

Melts of commercially pure liquid aluminum, and an Al-7Si-0.3Mg alloy, were cast into molds designed to produce entrainment of oxide film defects. The melts were held for periods of up to 20 minutes to investigate whether changes in the oxide film defects in the melt could occur, once sufficient time had elapsed for consumption of their internal atmosphere. The alloys were characterized by the determination of their Weibull modulus, examination of fracture surfaces under scanning electron microscopy (SEM), and determination of their porosity characteristics. The Weibull moduli of the ultimate tensile strength (UTS) values of the Al-7Si-0.3Mg alloy were reduced initially by holding in the liquid state for 10 minutes, but then the values increased after holding for 20 minutes. This high Weibull modulus was found despite oxide films being observed on the fracture surfaces. In the case of the commercial purity Al, the UTS Weibull moduli increased only slightly with holding for 20 minutes. The results suggested that holding of Al alloys in the liquid state influenced the scatter of mechanical properties by influencing the porosity content of the castings, which was related to their oxide film content. Some evidence for healing of a double oxide film defect with time was also found in the commercial purity Al alloy.