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 alpha(Al)-Mg(Zn,Cu,Al)(2) eutectic and predominantly Mg2Si and Al-12(Fe,Cr,Mn)(3)Si particles at interdendritic sites. The alpha(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 degrees 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 degrees C while the hot band has entirely recrystallized. Subsequent ageing at 121 degrees 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 degrees C. (C) 2011 Elsevier Inc. All rights reserved.