Effect of Al2O3 film on the mechanical properties of a welded high-strength (AW 7020) aluminium alloy

Abstract

The use in motor vehicles of lightweight metals such as aluminium and titanium provides a high strength-to-weight ratio, thereby lowering overall weight and reducing energy consumption and CO2 emissions. Aluminium alloys have thus become an important structural material especially high strength and ultra-high strength alloys such as AW 7020. Many studies have shown that the presence of an aluminium oxide (Al2O3) thin film formed naturally on aluminium alloys is detrimental to welding. This article further investigates the specific effect of the Al2O3 thin film on welding AW 7020 alloy. An analytical experiment of welded AW 7020 alloy using a pulsed metal inert gas (MIG) robotic weld machine is carried out. Four specimens were cut, butt welded, and examined. The weld parameters included pre-weld cleaning of the Al2O3, pre-, and post-weld heat treatment. Al2O3 was removed by wire brushing; preheating was conducted at a temperature of 130 °C; and natural ageing was conducted by post-weld heating at 480 °C for 2 h, followed by quenching in water at 90 °C for 8 h, reheated, and sustained at 145 °C for 15 h. The result shows that the presence of Al2O3 layer appears not to be detrimental to the weld with new welding technologies, therefore suggesting that it is not necessary to grind off the Al2O3 layer before welding. This finding implies that welding costs can be lowered and weld quality improved when new welding technologies are applied in the welding of high-strength aluminium alloys.