| The purpose of this thesis is trying to find out an optimized technics to improve the strength and toughness of 7050 high strength aluminum alloy friction stir welding joint. Using differently controlled cooling method, friction stir welding was done in air,20℃water and 100℃water conditions to investigate the effect of weld condition on the mechanical property of different zone at the joint. The conclusions are as followed:Under differently controlled cooling condition, the microstructure of welding nugget zone,thermal-mechanical affect zone and heat affect zone all changed remarkably. The grain size of welding nugget zone is smaller than that of the base material which is smallest in 20℃water condition,then 100℃water and last in air. The range of heat affect zone at joint is wider with increasing environmental temperature and obvious boundary with base material emerged. In the DSC curve of welding nugget zone and heat affect zone at joint, we find that the population of second-phase particle precipitation increased with increasing rotational speed and environmental temperature.Under differently controlled cooling conditions, obvious changes occurred to the property of friction stir welding joint. In the same cooling condition, the tensile strength was improved with increasing rotational speed, and, the tensile strength and toughness of the joint were both improved with increasing environmental temperature. The highest elongation percentage of joint is in the 100℃water condition, then in air, and last 20℃water. The tensile fracture position is always at the heat affect zone and the tensile fracture surface exhibit ductile fracture feature dominated by dimples. In 100℃water condition with 800rpm, the joint of friction stir welding obtains the highest tensile strength and elongation percentage. Therefore, controlled cooling friction stir welding is an efficient method for improving strength and toughness of the joint. |
PDF Full Text Request