Microstructure Thermal Stability Of Stirring Zone Of Friction Stir Welded/Processed Aluminum Alloy

The principle of Friction Stir Welding(FSW)/Friction Stir Processing(FSP)is that the welding tool/stirring pin consisting of the shoulder and the stirring pin is rotated at high speed to squeeze the stirring pin into the workpiece,the friction between the tool and the workpiece surface makes the workpiece locally heated and softened,resulting in strong plastic deformation and sufficient mixing,and through a dynamic recrystallization process,a weld zone/processing zone(also known as a stirring zone)with fine-grained structure characteristics is formed.In general,the fine-grained structure of the aluminum alloy FSW/FSP stirring zone has relatively low thermal stability under high temperature conditions(such as post-weld heat treatment and arc welding repair)and is prone to abnormal grain growth(AGG).This phenomenon will inevitably lead to the deterioration of the mechanical properties of the metal in the stirring zone,reducing the strength,fatigue performance,superplasticity and corrosion resistance of the stirring zone.Therefore,how to improve the thermal stability of the grain structure in the stirring zone has become one of the hot topics in the field of FSW/FSP research.At present,the cause of AGG in fine-grained structures under high temperature conditions is not fully understood,and there is no uniform conclusion.In this paper,the microstructure characteristics and grain growth behavior of three typical aluminum alloy FSW/FSP stirring zones under different high temperature conditions are experimentally studied.And the key factors affecting the thermal stability of the grain structure of the aluminum alloy FSW/FSP stirring zone were discussed.The FSP method was used to modify the 5356 alloy straight wall formed by arc additive manufacturing(WAAM),and the coarse as-deposited structure was transformed into fine equiaxed structure.When the metal is layered on the stirring area(modified area),the SDZ and TZ are remelted,and a deposition layer is formed after solidification;The PDZ reaches a higher peak temperature during thermal cycling,and obvious grain growth/abnormal growth occurs;However,the peak temperature reached during the thermal cycling in the SWZ is relatively low,and there is no obvious change in the microstructure.Applying forced cooling during the arc additive process can effectively reduce the peak temperature and suppress the grain growth/abnormal grain growth in the PDZ.In addition,as the FSP speed(heat input)increases,the degree of recrystallization in the PDZ increases,and the thermal stability of the structure in the SZ increases.The 5356 alloy samples formed by the WAAM+interlayer FSP process were respectively annealed under different conditions,And microstructural characterization was used to observe and analyze the microstructure characteristics and grain growth behavior of different microregions in the SZ.At 350?×30 min annealing condition,the AGG feature was observed only on the surface of the SDZ,and the fine-grained structure in the PDZ and SWZ was stable;Under 400?×30 min annealing conditions,The characteristics of AGG were observed in both the PDZ and SWZ.The AGG in these two regions point to the middle of the SZ.When the annealing temperature is?450?(annealing time=30 min),the grain of the SZ is abnormally grown.The above results show that the order of the thermal stability in the different micro-regions of the SZ from high to low is:PDZ>SWZ>SDZ;The experimental results that 400? annealing at different times(1 min-20 min)also prove that the thermal stability of the SDZ and SWZ is relatively low.The EBSD analysis results show that the heterogeneity of the structure from the surface to the bottom of the SZ is the key factor that causes the thermal stability of each microzone to differ.The extreme surface layer of the SDZ experiences the highest temperature and maximum strain during the welding process,and the grains are fine,which provides a large driving force for recrystallization and grain growth.At the same time,it contains a high-density sub-crystalline structure,so recrystallization takes place first under high temperature conditions;the SWZ is subject to the heat dissipation of the substrate during the FSP process,and the crystal grains are small and the density of the subcrystal structure is second only to the surface layer,so its thermal stability is also low;The thermal stability of the PDZ is the best because the grain size is uniform,the substructure density is low,and the proportion of completely recrystallized structure is large,Butt welding of 6 mm thick 7075-T6 aluminum alloy plates at a welding speed of 30 mm/min and a spindle speed of 1200 rpm(low heat input)and 1500 rpm(high heat input),respectively solution treatment at 485? and 500?(solid solution time=60min).After the joints with low heat input were respectively solution treated,AGG occurred in the surface layer of the SDZ,while the microstructure of the joints with high heat input was stable.Mainly due to the small and large number of second phase particles on the grain boundaries of the SDZ of the low-heat input joint,the dissolution occurred during the solid solution process,which caused the pinning force that hindered the grain growth to decrease;The particles of the second phase on the grain boundary of the influence area of the input joint shoulder are larger in size and smaller in number,and less dissolved in the solid solution process.In addition,the grain size of the SDZ of the low heat input joint is smaller than that of the high heat input joint,which is also a possible cause of AGG.Therefore,the thermal stability of the SZ of the joint with higher welding heat input is higher.