Microstructure and mechanical properties of high strength Al-Zn-Mg-Cu alloys used for oil drill pipes

Three Al-Zn-Mg-Cu alloys used for oil drill pipes (Alloy A: Al-6.9Zn-2.3Mg-1.7Cu-0.3Mn-0.17Cr; Alloy B: Al-8.0Zn-2.3Mg-2.6Cu-0.2Zr, Alloy C: Al-8.0Zn-2.3Mg-1.8Cu-0.18Zr) were studied by hardness tests, tensile tests and transmission electron microscopy (TEM). The results show that the ultimate tensile strength, yield strength and elongation for Alloys A, B and C are 736 MPa, 695.5 MPa and 7%; 711 MPa, 674 MPa and 12.5%; 740.5 MPa, 707.5 MPa and 13%, respectively after solid solution treatment ((450 °C, 2 h)+(470 °C, 1 h)) followed by aging at 120 °C for 12 h. The dominant strengthening phases in Alloy A are GPII zone and η′ phase, the main precipitate in Alloy B is η′ phase, and the main precipitates in Alloy C are GPI zone, GPII zone and η′ phase, which are the reason for better comprehensive properties of Alloy C. The increase of zinc content leads to the improvement of the strength. The increase of copper content improves the elongation but slightly decreases the strength. Large second-phase particles formed by the increase in the manganese content induce a decrease in the elongation of alloys.