Phase formation and microstructural evolution during sintering of Al–Zn–Mg–Cu alloys

Al–5·6Zn, Al–5·6Zn–2·5Mg and Al–5·6Zn–2·5Mg–1·6Cu (wt-%) powder blends were compacted at 350 MPa and sintered in nitrogen at different temperatures to study microstructural evolution during sintering. Densification, dimensional changes and mechanical properties of the Al alloys were investigated. Microstructural analyses were performed by scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and simultaneous thermal analysis (STA) in order to determine the temperature and chemical composition of the phases formed during sintering. It was shown that various liquids and intermetallic phases including Al_0·71Zn_0·29 at 438°C, Al_0·58Mg_0·42 and Al₃₂(Mg,Zn)₄₉ at 450°C, Al₃Mg₃Zn₃ at 500°C, and most probably η and S phases at 600°C were formed during the heating cycle. Magnesium was found to be the most effective alloying element on the densification and microstructural development during sintering, and eventually on the mechanical properties of the investigated systems sintered below 550°C. At higher temperatures, copper also becomes effective by the formation of a liquid phase. In the present paper, the phase formation during the sintering of the 7075 Al alloy prepared from elemental powders is addressed.