Influence of FSW process parameters on thermal cycles and mechanical properties of welded joints of AA7075 T651

Abstract

With excellent characteristics and mechanical properties, aluminum alloys meet the growing demands of the transportation sector for higher level vehicles with a focus on greater autonomy. Aluminum alloys of the 7XXX series are widely used in the aeronautical industry; however, their applicability becomes limited when subjected to conventional welding processes. To overcome this difficulty, the Friction Stir Welding (FSW) process, as a process that occurs in the solid state, is a viable alternative to promote the joining of these alloys. Since the manipulation of the mechanical properties of welded joints of thermally protected alloys occurs upon exposure to the welding heat, it is important to investigate the temperature distribution during joining by the FSW process in order to relate the results obtained with the thermal cycles experienced by the welded joints. In this work, AA7075 T651 alloy joints were welded at welding speeds of 117 and 47 mm/min and tool rotation speeds of 1585 and 470 rpm, with a threaded cylindrical welding tool made of H13 steel. Uniaxial tensile tests, macro and microstructure analysis, Vickers hardness tests were performed, and thermal cycles were measured using K type thermocouples. It was possible to verify that the highest temperature peaks, in the order of 375 ºC, were collected when higher rotation speeds (117 mm/min) and tool rotation speed (1585 rpm) were used. In addition, these joints experienced higher cooling rates, in the order of 8 ºC/s. The combination of higher temperature peaks and high renewal rates caused these joints to present brittle behavior linked to low mechanical strength (176 MPa of UTS). On the other hand, welded joints with lower rotation speeds (47 mm/min) and tool rotations (470 rpm) provided lower peak temperatures, around 324 ºC, lower abrupt cooling rates (3 ºC/s) and better mechanical strength 353 MPa.