Microstructure And Mechanical Behavior Of 7075 Aluminum Alloy Joint By Friction Stir Welding

The butt welding was carried out on the thickness of 6mm 7075 aluminum alloy plate using FSW(friction stir welding) under the rotating speed of 500r/min and welding speed of 150mm/min. The microstructure of welded joints was observed and the mechanical properties were tested. The results show that the FSW joints of 7075 aluminum alloy are made up of WNZ(weld nugget zone), TMAZ(thermo-mechanically affected zone) and HAZ( heat affected zone). On the advancing side of the joint, the interface line between WNZ and TMAZ is obvious, while on the retreating side, it is relatively obscure. The WNZ is made up of recrystallized equiaxed grains and the microstructure of WNZ refined obviously. The WNZ also has the “onion rings” structures. With the increasing of the distance to the center of welding metal, the space between the “onion rings” structures decreases. Because the TMAZ was affected by heat and mechanical power, greater deformation was occurred in the microstructure. The HAZ was only affected by heating and its microstructure was similar to that of the base material and was coarsened. The welded joint can’t achieve the requirements as the micro-hardness and tensile strength of welded joint is far less than that of the base metal which can only reach 50% of the base metal. It is necessary to research the welding processes of 7075 aluminum alloy joints by FSW.The welding was carried out on the 7075 aluminum alloy using FSW under different pin rotating speed and welding speed in order to obtain high-quality welded joints. The influence of different FSW parameters on the microstructure and mechanical properties of welded joints was investigated. The results show that with the increasing of pin rotating speed and the decreasing of welding speed, the grain sizes of the WNZ and TMAZ become big while the width of the WNZ becomes small. The micro-hardness of the joints under different FSW parameters shows an asymmetric “W” shape. The micro-hardness at retreating side is higher than advancing side and the HAZ softened in different degree. With the increasing of pin rotating speed and the decreasing of welding speed, the micro-hardness of the joints increases, but the range of fluctuation is small. With the increasing of rotating speed and welding speed, the ultimate strength firstly decreases and then increases. When the welding parameter is the rotating speed of 400r/min and the welding speed of 120mm/min, the ultimate strength of FSW joints reaches maximum 417 MPa. For the thickness of 6mm 7075 aluminum alloy, the best welding parameter is the rotating speed of 400r/min and the welding speed of 120mm/min.The dual diameter samples were designed and tested by hydraulic servo testing machine in a high tensile speed. The deformation and failure process of FSW joints were observed by Olympus microscopes and scanning electron microscopic. The results show that double necking appears on the FSW joint during the process of loading. The necking firstly appears in the retreating side and then in the retreating side with the progress of loading. Micro-cracks initiate in the boundary of grain in the advancing side and retreating side by the coalescence of micro-voids. With the increasing of strain, the amount of micro-voids increases. When the strain is large enough, the micro-cracks appear due to coalescence of micro-voids. The micro-cracks propagate towards the WNZ along the 45° angle to the loading direction and result in fracture of FSW joints. The location of fracture lies in the WNZ and the fracture mode of the joints is shear fracture mixed dimple.Different heat treatment processes(solution temperature, aging time) were used for heat treatment to welding joint to solve the issue of HAZ-softening. The microstructure of welded joints under different heat treatment processes was observed and the micro-hardness of welded joints under different heat treatment processes was tested. The results show that heat treatment can significantly improve the issue of HAZ-softening. The grains of the WNZ are coarsened after heat treatment. With the increasing of solution temperature, the grain size of WNZ and HAZ increases. With the extending of aging time, the grain coarsening has little change. The micro-hardness of the FSW joints has greatly increased after heat treatment. With the extending of aging time, the micro-hardness of the joints firstly increases and then decreases while the solution temperature selected in the paper has little influence on the micro-hardness of joints. When the heat treatment processes is the solution temperature of 490℃ and the aging time of 24 h, the micro-hardness reaches maximum which is 195 HV. For the thickness of 6mm 7075 aluminum alloy, the best heat treatment processes is the solution temperature of 490℃, aging temperature of 120℃ and the aging time of 24 h.