Microstructure And Mechanical Properties Of Friction Stir Processed/Welded Typical Aluminum Alloys

On account of the high strength to weigh ratio,good electrical and thermal conductivities,aluminum alloys have been widely used in industry.However,the aluminum alloys are difficult to weld by traditional melting methods due to the existence of porosity and slag.Moreover,the joints are often distorted and softened.The formability of high strength aluminum at room temperature is poor,and the spring-back during the plastic forming is obvious,which is not beneficial for fabricating precise aluminum components.As a result,the above two problems limit the applications of aluminum alloys.Friction stir welding(FSW)is a relatively new solid state joining technique,which solves the welding problem of aluminum.Friction stir processing(FSP)is developed based on FSW,aiming at refining the microstructures of aluminum alloy.After FSP,the plasticity of aluminum increases,which is in favor of the aluminum component fabrication.Therefore,it has a great scientific significance and industry values to research the microstructures and mechanical properties of friction stir processed/welded aluminum alloys.In the present work,firstly,the A1 7B04 was friction stir processed,and the following aspects were studied:(1)The Al 7B04 with different initial base metal tempers were friction stir processed,and the evolution of the strengthening precipitates during FSP was illustrated by the Gribbs-Thomson principle and thermodynamics.Moreover,the relationship between the morphology of strengthening precipitates and mechanical properties of stir zone was analyzed.For the Al 7B04 under artificial aging and natural aging states,the initial strengthening precipitates in the matrix were fine and unstable.These precipitates dissolved during FSP,leading to a decrease in the precipitation strengthening,and hence the stir zone was softened.For the Al 7B04 under annealed state,the precipitates became coarse and stable.During FSP,the variation in the morphology of the strengthening precipitates was unconspicuous,and the stir zone was strengthened due to the grain refinement.(2)Compared with the stir zone using air cooling,the grains in the stir zone were further refined when water cooling was applied,however,the impact of water cooling on the mechanical properties of stir zone was dependent on the initial base metal temper.For the A1 7B04 under artificial aging state,the evolution of strengthening precipitates during the FSP processing was independent on the cooling speed,and the effect of cooling speed on the mechanical properties of stir zone was negative.By contrast,for the A1 7B04 under annealed state,the coarsening of strengthening precipitates was limited by water cooling,leading to the strengthening of the stir zone.(3)Three-pass overlapping friction stir processing was applied in the annealed A1 7B04 by varying the moving distance of the stirring pin.The interaction between the neighbouring passes was drastic,and the regions produced by the previous passes were overaged during the processing of the subsequent passes.The interaction between the neighbouring passes could be eliminated by increasing the moving distance of the stirring pin,moreover,the microstructure and mechanical properties of the overlapped zone became homogeneous by elevating the rotational speed.Secondly,three-pass friction stir processing with 100%overlapping(M-FSP)was applied in Al 5083,and the results obtained are summarized as following:(1)For the stir zone produced by M-FSP,the microstructures and mechanical properties varied dependent on the initial base metal temper.For the annealed Al 5083-0,the stir zone was strengthened after single-pass FSP due to the grain refinement.Moreover,a further grain refinement occurred during the second-pass FSP,leading to an increment in the mechanical properties.The effect of the subsequent pass on the microstructure and mechanical properties of the stir zone was negative.By contrast,for the cold rolled Al 5083-H,the microstructure of stir zone became stable after the single-pass FSP.And the microstructure and mechanical properties of stir zone kept unchanged during the second-and third-pass.The stir zone was softened due to the decrease in the amount of dislocations,while an increment in the elongation was achieved.(2)For the Al 5083-H produced by M-FSP,the stability of the stir zone at high temperature was researched.The variation of the grains in the stir zone at high temperature can be illustrated and predicted by the Humphreys' model.And it was indicated that the stability of the stir zone was elevated with increasing the rotational speed.The grains were coarsened as increasing the rotational speed,and the relative pinning force was improved with increasing the grain size,which led to higher stability of the stir zone at high temperature.The region of abnormal grain growth(AGG)had been expanded with prolonging the holding period of high temperature.Increasing the processing passes could not inhibit the occurrence of AGG,however,the microstructure of the stir zone became uniformed as the processing passes increasing,limiting the expanding of AGG region.Thirdly,dissimilar friction stir welding was conducted between the Al 5083-0 and the Al 7B04 using different initial base metal tempers,furthermore,the material flow,microstructures and the mechanical properties of joints were studied:(1)The A1 5083 in the advancing side was transferred to the retreating side during welding,however,the amount of the transferred A1 5083 depended on the mechanical properties of the Al 7B04 in the retreating side.More Al 5083 transferred to retreating side when the Al 7B04 was softer.Two modes of material flow were suggested during welding,sheared flow and slided flow.When the mechanical properties of materials were high while the sheared force provided by the stirring pin was insufficient,the mode was slided flow,otherwise,the mode was sheared flow,(2)The atoms diffusion bet,ween the two materials was not obvious for dissimilar friction stir welding,and the failure location of the joints was dependent on not only the mechanical properties of welded materials but also the defects in the nugget zone.The joints fractured in the defective nugget zone or the base metal region of the soft material.A second-pass FSP with 100%overlapping was beneficial to mixing the materials,and the mechanical properties of the nugget zone increased as the fraction of hard alloy in the nugget zone elevated.At last,for the friction stir welded Al 2024,the microstructures and mechanical properties of the nugget zone under different aging states were researched.When the welding heat input was low,the hardness of the nugget zone was mainly affected by the rotational speed,which increased as the rotational speed increased.When the welding heat input was high,the hardness was mainly controlled by the welding speed and it elevated as the welding speed increased.Under natural aging state,the mechanical properties of the nugget zone elevated as the amount of GP zone increased.During the artificial aging,the mechanical properties of the nugget zone decreased due to overaging.The resistance to overaging of the nugget zone was dependent on the amount of fine strengthening precipitates,which was enhanced as the amount of fine strengthening precipitates increasing.