Effects of precipitates strengthening on vacuum inductive melting copper mould water-cooled casting alloys

such as Al-8.2Zn-2.05Mg（10Zn-2.5Mg

12Zn-3Mg）-2.2Cu-0.1Mn-0.25Zr

and subgrains strengthening introduced by thermo-mechanical treatment on the Al-8.2Zn-2.05Mg-2.2Cu-0.1Mn-0.25Zr alloy were investigated by optical microscope （OM）

XRD phase analysis

transmission electron microscope（TEM） and tensile tests. The results show that there exist uniform dispersedly strengthening phase η′ in three kinds of alloys with T6 treatment（ageing 120 ℃ at 24 h）

where precipitation strengthening plays a dominant role. With the increase of Zn、Mg content

the amount of precipitated phases is increased gradually

resulting in simultaneous enhancement of precipitation strengthening effect. The tensile strength of the three alloys reaches 646.2 MPa

697.4 MPa

732.5 MPa

respectively

corresponding to its precipitation strengthening effect

and the elongation is 13.0%

10.6%

7.1%

respectively. Al-8.2Zn-2.05Mg-2.2Cu-0.1Mn-0.25Zr alloy can obtain subgrains structure by using thermo-mechanical treatment including pre-ageing at 120 ℃ for 12 h +30% warm deformation at 120 ℃+final-ageing at 120 ℃ for 10 h

where the strengthening mechanisms of the alloy is characterized by co-action of subgrains strengthening and precipitation strengthening

and tensile strength and elongation reach up to 752.3 MPa and 6.7%

respectively. Subgrains with uniform dispersedly strengthening phases distributing inside are more favorable to the properties of alloy than increasing number of precipitated phases only.