| Friction stir welding(FSW)technique of aluminum alloy has been widely used in aerospace and shipbuilding,but conventional friction stir welding process will inevitably produce root flaws and thickness reduction,resulting in a decrease in the mechanical properties of the joint.Therefore,a non-root defects and no thinning FSW technology is needed.In the present study,the influence of different groove dimension and plunge depth on the microstructure and properties of the joint when only the grooved backing plate used was studied firstly,so as to optimize the better groove dimension.Secondly,based on the optimized groove size and the upper sheet which was the same as base material(BM),the orthogonal experiment was designed to analyze the influence of different parameters on the forming,microstructure and properties of the joint,so as to optimize the optimum process parameters.Finally,the joint under the optimal process was selected and post-weld heat treatments(PWHT)(artificial aging,solid solution and aging treatment)were carried out to analyze the effect of the heat treatment process on the microstructure and mechanical properties of the joints.When only the grooved backing plate was used for friction stir welding of aluminum alloy,it could be found that only when the plunge depth matched the groove depth,the joint with sound surface and no macro defects inside could be obtained,otherwise tunnel defect or voids would be produced in the joint,which was because of insufficient heat input or excessive formation of flash.In addition,the vertical length of root flaws increased with the increase of groove depth,but decreased as plunge depth increased.When groove depth was 0.40 mm and plunge depth was 0.45 mm,after removing the excess weld metal(EWM)of the back of the weld,there were no defects inside the joint,and the fracture was located in the lowest hardness of the heat affected zone during the tensile test.The maximum tensile strength was 246 MPa,which was about 86.9% of the base metal.The joint fracture morphology was composed of a large number of dimples and tearing edges,which presented typical ductile fracture.Furthermore,some Mg2 Si precipitates were distributed at the bottom of the dimples.The bend testing showed root defects in the welded joint had little effect on the face bending resistance of the joint.However,during the root bend test,the root defect became the source of the crack,resulting in a significant decrease in the bending resistance of the joint.Based on the optimized groove dimension,the effect of the rotation speed,welding speed and tilt angle of the welding tool on the forming of the welded joint was studied by adding the upper sheet of the same material as the BM.After all the joints had removed the EWM at the root of weld,no root defects and no thinning in the weld could be achieved,and even different degrees of thickening occurred.Due to the existence of the upper sheet and groove,the material flow was changed,so that the macroscopic morphology of the cross section of the joint was able to be divided into two layers: layer structure and onion ring structure.Through the range analysis of the orthogonal results,it was found that the tilt angle had the greatest influence on the tensile strength of the joint,and the welding speed had the least influence.The optimum parameters were rotation speed of 1000r/min,welding speed of 65mm/min,and tilt angle of 2.5o.The tensile strength of the joint reached 238 MPa,approximately 84.1% of the BM.Tensile fracture could be divided into two types: the fracture at the weld nugget zone(WNZ)and the fracture at the lowest hardness of the heat-affected zone(HAZ).When the WNZ fracture occurred,the joint was a mixed fracture of quasi-cleavage fracture and ductile fracture,otherwise it was a typical pure ductile fracture.The results of post-weld heat treatment experiments showed that artificial aging had no significant effect on the macroscopic morphology and grain sizes of the cross-section of the joint,while the solution treatment and aging could effectively improve the microstructure of the thermal mechanically affected zone(TMAZ)and make the boundary between the TMAZ and the WNZ fuzzy.The microstructure of the HAZ was similar to that of the BM and the two heat treatment processes had no significant effect on the root flaws in the joint.The mechanical properties test results showed that the STA not only significantly improved the tensile strength of the joint,its value reached 269 MPa,which was about 95.1% of the BM,but also improved the plasticity of the joint and made the hardness distribution of the joint more uniform.The fracture morphology showed the joints under both heat treatment processes presented typical ductile fractures. |
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