Ageing Behaviour of Spray Cast Al-Zn-Mg-Cu Alloys


Abstract: Three 7xxx series aluminium SS70, N707 and 7075 alloys containing copper, zinc and magnesium were spray deposited using rapid solidification. Ageing of the 7075 alloy resulted in a peak hardnesses of 147, 186 and 191 MPa after 2, 24 and 48 h at temperatures of 175^oC, 130^oC and 120^oC respectively. The other two alloys, SS70 and N707, were aged at 120^oC, and both reached peak hardness after 24 h; SS70 had a peak hardness of 229 MPa and N707 of 228 MPa. A double ageing treatment at 105^oC for 5 h then at 175^oC for 1 h significantly increased the peak hardness to 185.6 MPa compared to a single stage of ageing at the higher temperatures in the 7075 alloy. TEM analysis of aged specimens revealed two types of precipitate that contributed to age hardening: \eta' and \eta (MgZn_{2}).

Keywords: 7xxx series aluminium alloys, Spray casting, Age hardening, Ageing precipitates, TEM.

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