Micro Structure And Mechanical Properties Of Ultra-high Strength7A55Al Alloy Sheet Friction Stir Welded Joint

The ultra-high strength Al-Zn-Mg-Cu alloys have been widely used in aviation aerospace; industry for their high specific strength. But it is difficult to join these alloys using traditional fusion welding technique. The friction stir welding could be well applied in ultra-high strength aluminum alloys for its advantages. Hardness test tensile test, optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to investigate the effects of welding parameters, including pin rotation speed, welding speed, cooling approach, and subsequent ageing treatment on the microstructure and mechanical properties of friction stir welded joint of7A55Al alloy. The main conclusions are given as follows:(1) The welding parameters of600rpm/150mm/min、1200rpm/100mm/min and1200rpm/150mm/min, may lead to high mechanical properties and the maximum tensile strength was466MPa, with welding coefficient of0.74and elongation of5.2%. The hardness curves of the welded joint exhibited a W-shape, and the minimum hardness value appeared in the thermal-mechanical affected zone. With the increase of rotation speed and welding speed, the hardness of nugget zone tended to increase; the welding parameter of1850rpm/150mm/min resulted in the maximum hardness value of about173HV.(2) Fine recrystallized grains existed in nugget zone. Their size received little effect from welding speed, but tended to increase with the increase of pin rotation speed. It was about1μm for600rpm and increased to4μm for1850rpm. There was a number of coarse η phase (100-250nm) without hardening effect in the nugget zone. Therefore, the hardness of this zone was lower. In the heat-affected zone, η’phase particles coarsened with increasing rotation speed, therefore hardness decreased.(3) The hardness curve of water-cooled joint exhibited a U-shape, which that of air-cooled joint had a W-shape. Mechanical properties of the water-cooled joint were higher than that of the air-cooled joint, and tensile strength and elongation were457MPa and4.0%, and417MPa,3.4%respectively; after ten days’nature ageing. The grain size is smaller than1μm and about2μm in the nugget zone in the water-cooled and air-cooled joint respectively. Most η’ phase transformed into η phase in the water-cooled joint, which lowered supersaturation in the nugget zone and thus subsequent precipitation hardening potential. The transition region between the heat-affected zone and the nugget zone was narrower in the water-cooled joint and η’ phase in the heat-affected zone did not change much.(4) The hardness of the air-cooled joint increased dramatically after nature ageing and artificial ageing. The80days’nature ageing and artificial ageing gave rise to an increase of10HV and20HV in the hardness of the nugget zone, respectively. While there was little change in the hardness of the water-cooed joint. The mechanical properties of water-cooled joint and air-cooled joint were similar after60days’nature ageing and120℃X12h artificial ageing, and tensile strength is434MPa and431MPa respectively, elongation is3.1%and2.4%, respectively,.