Multi-alloying effect of Ti, Mn, Cr, Zr, Er on the cast Al-Zn-Mg-Cu alloys

Cast aluminum alloys have been widely used in aerospace and automotive lightweight fields due to their high specific strength, low density and high fracture toughness. In this paper, six alloys are designed to reveal the role of the individual and joint addition of minor Ti, Mn, Cr, Zr and Er in the cast Al-Zn-Mg-Cu alloys. The results show that the addition of Ti can significantly refine the ingot grains. The addition of Mn and Cr forms Al(Fe,Mn, Cr) and Al5(Fe,Mn)Si2 phases and consequently reduces the impurity elements Fe and Si in the matrix. The joint addition of Ti, Mn, Cr, Zr, and Er produces a poison effect, forming a bulk composite phase containing Al8Cu4Er, Al12(Mn,Cr), and Al3Ti. After solution treatment, a large amount of Al12(Mn,Cr) precipitates are formed in the alloy containing Cr and Mn, and plenty of dispersed spherical Al3(Er,Zr) particles are precipitated out of the alloy with Zr and Er, playing a second phase strengthening effect on the alloys. However, the most important strengthening phase in all alloys is still eta' phase precipitated in the aging process, and its precipitation behavior is not affected by the addition of these alloying elements, so the aging hardening curves of all the alloys show a similar trend. As a result, the hardness of the alloy with joint addition of Ti, Mn, Cr, Zr, and Er is the highest after both solution and aging treatments, which is attributed to the collaborative strengthening effects of Al12(Mn,Cr) and Al3(Er,Zr) particles formed upon solution treatment and eta' phase precipitated upon aging.