| Friction stir processing is a new material processing method developed on the basis of friction stir welding.Due to the severe deformation and heat input during processing,the structure of the welded nuclei after processing is fine crystals after recrystallization.At present,it has been found that the abnormal growth of grains will occur during the subsequent heat treatment of the weld zone structure,which will lead to the instability of the weld structure and performance,affecting the promotion of friction stir welding and processing methods.In order to grasp the law of thermal stability of the structure after welding,this paper studies the effect of deformation on thermal stability by means of deformation after welding.The 7075 aluminum alloy was subjected to friction stir processing under different process parameters(advance speed is 80 mm / min,rotation speed is 400rpm/800 rpm / 1200 rpm,referred to as 400-80 / 800-80 / 1200-80),and the original samples after friction stir processing(referred to as FSPed)were heat-treated.At the same time,the FSPed samples were rolled with different reductions and then heat-treated.The effect of 7075 aluminum alloy on microstructure and properties is as follows:(1)The analysis of metallographic structure shows that the thermal stability of the structure after friction stir processing shows a certain difference due to different processing parameters.After 1 hour,2 hours,and 3 hours of heat treatment of the processed structure at 430 ° C and 470 ° C,it was found that under the parameters of400-80 and 800-80,abnormal grain growth occurred in the structure of the nugget zone.While the parameters of 1200-80,no abnormal grain growth was observed in the microstructure of the nugget.(2)The post-weld rolling deformation improves the thermal stability of the weld structure.According to the analysis of EBSD data,normal grain growth occurred in the microstructure of the processing area after rolling reductions of 30% and 50%,and no abnormally grown grains occurred after direct heat treatment after welding.The grain size increases when heated at 470?.Under the condition of 30% deformation and the same heating system,the grain size of the 400-80 parameter nugget zone shows the following rule: SFZ> SZ> SWZ.At 50% deformation,this rule is not shown.Under the1200-80 parameter,the grain size change in the weld nugget area after 50% deformation is the smallest after heat treatment.(3)The heat treatment process after rolling causes recrystallization of the nugget zone structure,and the low-angle grain boundary content in the structure is reduced to a certain extent.Both the B texture and the (?) texture C texture in the structure after the friction stir processing have decreased in strength with the progress of the heat treatment.Heat treatment causes the discontinuous second phase particles in the grain boundary area to dissolve out and then precipitate out,evenly distributed into the crystal,which correspondingly improves the stability of the material structure performance.(4)In terms of mechanical properties,the hardness distribution of the400-80-FSPed specimen after rolling is in the shape of "W".Rolling improves the micro-hardness performance and tensile strength of the welded structure,and reduces the plasticity of the material.Heat treatment weakens the strength of the rolled material and increases the elongation of the material. |
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