Influence Of Welding Conditions On Temperature Distributions And Quality Of Friction Stir Welding Butt Joints

In this paper, different welding tests on 7050 aluminum alloy plates of 6mm thickness in air, hot water and cold water were carried out via CNC friction stir welding equipment separately. Eight channels temperature collection system was employed to measure temperature field in different technological conditions. The distribution of micro-hardness measured through Vickers hardness tester combined the results of temperature field and DSC analysis of secondary particles solution and precipitate, to investigate the relationship of distribution between micro-hardness and secondary particles. In addition, after welded by different welding procedure in air and in cold water, the metallographic structure and the quality of weld joints were characterized by optical microscopy.The result of temperature field observation show that in three thermal boundary conditions, featured points go through the process of "heating-peak temperature-cooling ", the farther away from seam center, the lower of the transverse distribution characteristic points temperature is and the longer the time of reaching peak temperature is. And their heating rate is higher than the cooling rate in the condition of air and cold water, but nearly the same under the condition of hot water. Furthermore, the heating rate in hot water is higher than that in air, and the lowest in cold water; the cooling rate in hot water is also the highest, but the lowest in air. The peak temperature rises with the increased rotational speed of principal axis with fixed welding speed in cold water, by contrast, which falls when the speed reaches 800r/min in air and cold water.The result of hardness test indicates that not only the size and distribution of strengthening phase affected the micro-hardness values, but also the grain size have the efforts on it, however, the strengthening phase plays a main role in the distribution of hardness.The quality of weld joints show that flash forms on the surface of seam with exceeded pressure, however, void and other internal defects will emerge if the axial force is lower. It's easily to form defects if the principal axis is in severe conditions such as at high rotation speeds and low welding speeds or at high welding speeds and low rotation speeds. The excellent weld joints will be formed when principal axis rotating is counter-clockwise or weld under water.