| Friction stir welding（FSW）is a novel solid-state joining technology,which has the advantages of simple welding process and few weld defects.It can be used in the welding of high-strength aluminum alloy and widely used in transportation and aerospace industry.In this paper,the friction stir welding joint of 2A12-T42 aluminum alloy with thickness of2mm is taken as the research object.The macro-morphology observation,microstructures characterization,mechanical properties and residual stress measurement of the joint at different welding parameters are carried out.The evolution law of the microstructure at different process and the relationship with its performance are studied.The research results are as follows:（1）Through the observed of macroscopic morphology and microstructure,when the welding speed is 200mm?min-1,the macro morphology is good.The thermo-mechanically affected zone is very narrow and the microstructures are obviously distorted,whose direction is tangent to the welding direction.The divide line on the advancing side is more obvious than that on the retreating side.The structure at the heat-affected zone is rougher than that of the base materia,and the microstructure of nugget zone is smaller.（2）According to the residual stress analysis determined by X-ray diffraction,the base metal shows a obvious<200>orientation,and the weld zone exhibits a strong<111>orientation compared to the parent metal.When the welding speed is constant,with the increased of the spindle speed,the<111>orientation intensity shows a first increase and then a decrease trend.When the lattice distortion degree decreased,the compressive stress in the weld zone is decreased first and then increased,and the compressive stress at the heat affected zone increases continuously.The spindle speed is constant,and as the welding speed increases,the<111>orientation intensity decreased,and when the lattice distortion increased,the weld zone stress increased continuously,and the longitudinal stress of the heat-affected zone decreased first and then increased,and the lateral direction decreased continuously.Under the same welding process parameters,the residual stress at the heat affected zone is higher than that at the weld zone,and the longitudinal stress（mainly compressive stress）of each zone is higher than the transverse stress.（3）The analysis of mechanical properties shows that the micro-hardness distribution of the welded joint is"W"type.The hardness of the weld nugget is increased with the increase of the spindle speed.The hardness is 142HV at 1000r/min.The grains at heat-affected zone are coarsened and the hardness is relatively reduced.When the rotational speed is 1000r/min,the tensile strength is 415MPa.The elongations are higher,they are 9.94%at 800r/min and 9.52%at 1000r/min.When the tensile fracture location is located at the interface between the weld nugget zone on the advancing side and the heat-affected zone.The experimental and theoretical analysis shows that there is a great difference in the size of microstructure and the content of S strengthening phase between the nugget zone and the thermo-mechanically affected zone.A smooth transition in the middle is lacked,which is prone to stress concentration and leads to weak mechanical properties. |
PDF Full Text Request