| Spray formed 7055 Al alloy has aroused general concern in the aerospace and automotive industries due to its brilliant property of high strength and high toughness. The welding technology which restricts the applications of spray formed 7055 alloy has been a focus topic among the industries as well. Because of the particularity of spray formed technology, quality of spray formed 7055 alloy joint is hard to ensure by traditional Friction Stir Welding(traditional FSW). By introducing underwater Friction Stir Welding(underwater FSW), a fine and defect-free joint has been achieved under the parameters of 1550 rpm and 155mm/min. Compared with traditional joint, underwater joint gets a far better properties. The tensile strength of underwater joint is 402.48 MPa which is equivalent to 75% of base metal and the elongation is 1.52%. The curve of hardness in underwater joint presents a symmetrical “W” shape which possesses more uniformity and narrower soften than traditional joint. The microstructure of underwater joint was studied by SEM&EDS, DSC, XRD and TEM compared with traditional joint. The results show that underwater FSW with a high temperature gradient has reduced the dark-grey clusters and EDS detects Mg:Zn in these clusters are nearly 1:2 which is close to that in base metal. The peaks in DSC curves of underwater joint have been identified referring to phase diagrams and it reveals that the main changes in microstructure coming from the evolution of G.P. zone and Mg-Zn phases. Mg Zn2 and Mg2Zn3 have been detected as the strengthening phrases through XRD analysis and Mg Zn2 is in the most so most brilliant property of base metal has been reserved by the underwater joint. TEM analysis confirms the main strengthening phase Mg Zn2 in underwater joint is semi-coherent with Al matrix that brings the optimum strengthening effect. All in all, the mechanism of strengthening and toughening of underwater joint can be attributed to the change of cooling method by underwater FSW and a fine microstructure with the characters of good strengthening effect can be achieved in joint so that the joint is improved by coherent strengthening mechanism.Based on the mechanism of strengthening and toughening of underwater joint of spray formed 7055 Al alloy, a further research of adjustment mode of UFSW has been carried out. In the paper, two ways of adjustment for underwater joint have been studied, namely changing the parameters and changing the temperature of cooling medium. If it is intended to improve the property of joint through changing the welding parameters, increasing the rotation speed of tool pin is a better way than changing its travelling speed. The changing of parameters mainly causes the evolution of Mg-Zn and Al-Cu phrases and high rotation speed + high travelling speed is recommended for achieving a joint with a high property. As for changing the temperature of cooling media, high temperature medium can ensure the property of underwater joint. Joint achieved in the high temperature medium shows a high tensile strength which is over 85% of base metal as well as a highly improved elongation. The outstanding performance of the joint is benefited from the strengthening phase of i-type Al32(Mg,Zn)49 besides Mg Zn2 and Mg2Zn3. |
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