| The Al Mg alloy with high Mg content has low density and high specific strength,but the plasticity and welding coefficient of the joint are very low when melting welding,which is very unfavorable for its appl ication.Therefore,it is of great significance to study and solve this bottleneck problem for the promotion and application of such materials.Friction stir welding(FSW)is a new solid-state welding technology which can obtain high quality and high perfo rmance welds.In recent years,scholars at home and abroad have carried out extensive research on the FSW technology of 5xxx series aluminum alloy with medium and low Mg content,but the research on the FSW of 5A12 aluminum alloy with high Mg content has n ot been reported.In this paper,taking the hot-rolled 5A12 aluminum alloy sheets with thickness of 2 mm as the research object,we explored and optimized the FSW processing parameters of the 5A12 aluminum alloy,studied the characteristics of joint structure,discussed the influence of process parameters on structure and mechanical properties of joints,and clarified the relevant mechanism.These results can provide the basis for optimizing the parameters of FSW 5A12 aluminum alloy and improving the quality of joint.This paper first studied the effects of the inserted depth of the shoulder,rotation rate and welding speed on the welding quality,and the better welding process parameters range were optimized: the inserted depth of the shoulder was controlled at 0.15 mm.When the welding speed was set to 50 mm/min,the appropriate rotation rate was 750-1500 rpm,respectively.When the rotation speed was set to 1000 rpm,the appropriate welding speed was 25-100 mm/min,respectively.Nugget zone(NZ)formed fine equiaxed grains due to the dynamic recrystallization(DRX).Thermo-mechanically affected zone(TMAZ)formed grains arranged along the edge of the stirring pin due to dynamic recovery(DRV)and DRX and the action of friction shear stress.Heat affected zo ne(HAZ)went through grain coarsening under the action of thermal cycle and formed equiaxed-grain structure due to the occurred static recrystallization in HAZ.The DRX mechanism in NZ was continuous dynamic recrystallization(CDRX)that the subgrains grew and developed into the equiaxed grains by absorbing dislocations to increase misorientation angle,formed the high angle grain boundary(HAGB).In addition,from the HAZ to the TMAZ and to the NZ,the HAGBs fractions increased,low angle grain boundary(LAGB)changed to the HAGB.This paper studied the effects of rotation rate and welding speed on the microstructure and mechanical properties.The results showed that NZs obtained high fraction of HAGB,the fine second phase particles with uniform distribut ion and the significantly weakened joint texture at various rotation rates and welding speeds,respectively.With the increase of rotation rate,the grain size and the second phase particle size first decreased and then increased,in addition,the tensile properties of the joint increased first and then decreased.With increasing welding speed,the grain size and the second phase particle size first decreased and then increased,and the tensile properties of the joint increased first and then decreased.For the 5A12 aluminum alloy sheet with 2 mm thickness,based on the systematic process experiment,the optimal welding process parameters were optimized: the inserted depth of the shoulder was 0.15 mm,the rotate rate was 1000 rpm,and the welding speed was 50 mm/min.Under these conditions,this joint had the highest ultimate tensile strength(?477 MPa)and the biggest elongation to rupture(?22%),about 45% higher than the base metal.The excellent mechanical properties can be ascribed to the combined effects of the fine DRX grains with high fraction of HAGB,the fine second phase particles with uniform distribution and the weakened texture of NZs. |
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