Heat treatment of twin-belt cast EN AW 7075 alloy

The potential of twin-belt cast EN AW 7075 sheet for structural sheet applications was investigated with a particular emphasis on its response to the standard industrial T6 heat treatment. The twin belt cast EN AW 7075 strip is dendritic with an α(Al)–Mg(Zn,Cu,Al)2 eutectic and predominantly Mg2Si and Al12(Fe,Cr,Mn)3Si particles at interdendritic sites. The α(Al)–Mg(Zn,Cu,Al)2 eutectic has largely transformed into a Mg(Zn,Cu,Al)2 precipitate-rich network during the break down pass. The coarse intermetallic particles have fragmented while the fine intragranular precipitates have coarsened upon further hot rolling to a gauge of 2 mm. The predominant restoration process during rolling at 350 °C was dynamic recovery with no evidence of recrystallization. A fraction of Mg(Zn,Cu,Al)2 was replaced with Al2CuMg. Both phases dissolved during the solution heat treatment at 470 °C while the hot band has entirely recrystallized. Subsequent ageing at 121 °C for 16 h has produced a T6 hardness of 170 HB, considerably higher than that reported for the conventionally processed counterpart (150 HB). In addition to a remarkable age hardening capacity, twin belt cast EN AW 7075 alloy has enjoyed a very high resistance to age softening as inferred from a nearly T6 hardness after the industrial T73 heat treatment with an additional ageing cycle at 160 °C.