Heat treatment of twin-belt cast EN AW 7075 alloy

Abstract

Abstract 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 Mg 2 Si and Al 12 (Fe,Cr,Mn) 3 Si 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 Al 2 CuMg. 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.