| 7075 aluminum alloy,as a heat treatmentable high-strength aluminum alloy,has been widely used in aerospace,aviation,military and automotive industries. However,it is generally considered difficult to be the fusion welding process due to the poor solidification microstructure and porosity in the fusion zone. As a new type of solid state welding method, Friction Stir Welding provides an effective processing methods for welding and processing of 7075 aluminum alloy. But,possibly due to practical application status of 7075 aluminum alloy, the studies have been mainly focused on the processing microstructures and mechanical properties of the age-treated 7075 aluminum alloy. There is much less data on the microstructure evolution and mechanical properties of solution treated 7075 alloy. In this paper,through a large number of micro-hardness and OM experiments and combined with ECC,TEM and EBSD study for specific location of hardness profiles, the solution and age treated 7075 aluminum alloy were investigated by friction stir processing respectively. The relationship among temperature gradient,deformation mechanism,the precipitates distribution and texture evolution are discussed.The effect of processing modes on the microstructures of the two alloys was compared. The optical micro-structures that were processed paralleling to the roll direction were studied in more details. The results showed that the solution treated alloy have defects found at the intersection between Shoulder Flow Zone,Stir Zone and Advancing Side, and the intersection of Swirl Zone,Stir Zone and TMAZ on transvere cross-section of processing zone, while no defects were found in the age treated alloy. Interface between the SFZ and AD changes smoothly in the solution treated alloy while the interface transition in the age treated alloy changes steeply.Micro- hardness on the cross section of both alloy were test systemetically and the maps of hardness distribution were established. Micro-hardness profiles at the certain planes of solution-treated and age-treated 7075 Al exhibits Letter W morphology, typically as that in the most friction stir welds. Hardness in the center of the WZ gradually increased from bottom to surface. Meanwhile,apparent low hardness zone were found at the 3mm plane of the age treated alloy while no softened bands weren't found in the solution treated alloy. In addition,two hardness tests showed that the natural aging has great influence on the hardness distribution in the two alloys. Microstructures at specific location of hardness profiles were studied by SEM-ECC and TEM. The results showed that the sizes of fine equiaxed grains in the WZ increase from bottom to top of sheet. The amounts of intergruanlar precipitates gradually decreased from bottom to top while that of intragranular precipitates increased from bottom to top. The amount of intragranular precipitates in the age treated alloy were more than that of the solution treated alloy. However,in the 3mm level of hardness distribution,distribution of the second phase is more uniform in the alloy under solution heat treatment more than aging.EBSD experiments show that the two alloys have similar grain orientation in the TMAZ and WZ. Low angle boundaries ( LABs) and high angle boundaries ( HABs) in the TMAZ of retreating side increase and decrease, respectively, and grain texture becomes weak when the detected site is approaching to the WZ. The grains are constituted by the HABs that accounted for more than 80% in the WZ. Due to shearing of the pin,the weak shear textures are formed in the WZ by rotation of the grains after the FSP. |
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