The Effect of the Addition of Transition Metals on Double Oxide Film Defects in an Al–Si–Mg Alloy

Abstract

Double oxide film defects (bifilms) are associated with a reduction in mechanical properties in Al and Mg alloys, but also with a variability in mechanical properties that is difficult to compensate for when designing castings. This paper reports the effect of the addition of 0.45 wt% Mo, or of 0.45 wt% W, to an Al-7Si-0.3 Mg aerospace and automotive alloy. The additions resulted in an increase in the Weibull modulus of the UTS (i.e. a reduction in the scatter of the measured mechanical property data), by a factor of about two (about from m = 16 to m = 32). The observation of AlN on the fracture surfaces of some tensile test bars suggests the additions of Mo or W were associated with the acceleration of the reaction between the oxygen and nitrogen in the internal atmosphere of the double oxide defects and the surrounding liquid metal. This may alter the morphology of the defects, reducing their size and reducing their impact of mechanical properties. The greatest improvement in reproducibility of tensile properties (UTS) was obtained when an addition of 0.45 wt% W was coupled with a well-designed running system, resulting in a Weibull modulus of m = 96.