Phase Composition and Microstructure of High Strength AA6xxx Aluminium Alloys with Nickel Additions

Abstract

High strength AA6XXX series aluminium alloys have become the material of choice for body structures of lightweight automotive vehicles. The properties can be tailored through thermomechanical processing and strict adherence to impurity level. Although some nickel-containing AA6XXX Al alloys were introduced previously, the effect of Ni addition is still out of significant attention. Presently, an AlMgSiCu alloys with 0.1, 0.2 and 0.3 wt.% Ni addition was studied along with thermodynamic analysis. The predicted intermetallic phases in the as-cast base alloy were Al15(Fe, Mn)3Si2, Al5FeSi, Mg2Si and Q phases and also Al3Ni and Al3(CuNi)2 in the as-cast Ni-added alloys. The Al3(CuNi)2 phase with irregular shape was found in all as-cast Ni-added alloys and Al9FeNi phase was also found around dendritic cells in the as-cast alloy containing 0.3 wt.% Ni. After homogenization the Al3(CuNi)2 phase became much leaner in Cu content showing a possible phase transformation occurred. It was found that even at the low Ni content the excess Ni atoms could interact with neighbouring Fe atoms upon heat treatment to form Al9FeNi phase and the Cu atoms were released into the matrix. Thus, the Ni addition could not only lead to the loss of Cu atoms available for subsequent age hardening but also reduce Si entrapment associated with Fe-rich intermetallics. Moreover, improved circularity of intermetallics was found due to appearance of the Al9FeNi phase that could potentially mitigate the detrimental effects of Fe.