Natural Ageing of Al–Fe(Zn,Mg) Alloy Castings for Structural Automotive Application

Abstract.Novel Al–Fe-based eutectic alloys with Zn and Mg as precipitation strengtheners were used in the manufacturing of test plates using high-vacuum high-pressure die casting to evaluate the feasibility of manufacturing structural automotive components with a higher strength to weight ratio than the present. Natural ageing at room temperature is a typical behaviour of this new alloy due to the continuous precipitation reactions of the Zn and Mg atoms dissolved in the primary solid Al phase in the microstructure. The sequential stages of precipitation include the formation of self-clusters of atoms, dissolution of self-clusters, formation of GP-I zones, dissolution of GP-I zones, formation of GP-II zones and stabilization of GP-II zones; all these stages distinctly affect the tensile properties. Uniaxial tensile tests of castings coupled with the differential scanning calorimetry experiments were carried out to elaborate the mechanisms of natural ageing in these alloys. The results show that the elongation of the castings is unaffected during natural ageing and the strength increases to its double, with time, before stabilization. The properties of as-cast and stabilized state were significantly improved when compared to the contemporary Al–Si–Mg alloy such as the Aural 5.