| Aluminum alloy is widely distributed on the earth and has good physical and chemical properties,so it is widely used in the fields of shipbuilding,aerospace and automobile manufacturing.2024 aluminum alloy belongs to the Al-Cu-Mg series of high-strength aluminum alloys.It is a hard aluminum alloy that can be strengthened by heat treatment.It is widely used in aerospace fields,such as aircraft propellers and skins.However,its application is severely restricted due to poor weldability.Laser welding and laser-MIG hybrid welding are a new type of welding method,which can greatly improve the performance of 2024 aluminum alloy joints,and can improve the softening phenomenon of joints through postweld heat treatment.In this regard,this thesis takes 3mm thick 2024 aluminum alloy as the object to carry out laser self-fusion welding and laser-MIG hybrid welding process and postweld heat treatment.The main research results are as follows:When the laser power P=3.6kW and the welding speed v=24mm/s,the laser self-fusion welding seam formation quality is the best,and the surface has no obvious defects;welding current I=120A,laser power P=2.8k W,welding speed v=24mm/s and the laser-arc distance Dla=2mm,the welding seam forming quality of laser-MIG hybrid welding is the best.In the two types of welds,a columnar crystal structure perpendicular to the fusion line grows near the fusion line,the center of the weld is equiaxed crystal,and a large number of precipitated phases are distributed on the matrix.The weld of laser-MIG hybrid welding is divided into arc zone and laser zone.The grain size of the arc zone is larger than that of the laser zone.SEM analysis shows that the main strengthening phase of laser self-fusion welding seam center is θ,and the main strengthening phase of laser-MIG hybrid welding is S+θ phase.The hardness distribution of the two types of joints is: base metal>heat affected zone>weld seam area.The hardness of the weld zone is significantly lower than that of the base material,and softening occurs.The tensile strength of laser self-fusion welding joints can reach 262 MPa,and the tensile strength of laser-MIG hybrid welding can reach 335 MPa,the latter is significantly higher than the former.The heat-affected zone is the weakest area of the entire joint,and the fracture positions of the tensile specimens are all in the heat-affected zone.From the fracture analysis,the two types of joints are both ductile and brittle mixed fractures.The mechanical properties of the joints are significantly improved after solution aging treatment.The microhardness of the weld zone of the laser self-fusion welding joint increased from 97HV0.5 to 146HV0.5,and the tensile strength increased to 417 MPa.The microhardness of the weld zone of the laser-MIG hybrid welding joint increased from 94HV0.5 to 155HV0.5,and the tensile strength increased to 421 MPa.From the fracture morphology,there are a large number of large and deep dimples in the fracture,and there are second phase particles at the bottom of the dimples,but they are still ductile and brittle fractures.SEM results showed that the main strengthening phase of laser self-fusion welding changed to S+θ phase,and the main strengthening phase of laser-MIG hybrid welding is S phase.The solution aging treatment causes the original continuous precipitated phase to transform to the discontinuous phase,which blocks the corrosion channel and improves the corrosion resistance of the joint.Finally,through electrochemical tests and intercrystalline corrosion tests,the corrosion resistance of the four joints is arranged from small to large: laser self-fusion welding without heat treatment<laser-MIG hybrid welding without heat treatment<laser self-fusion welding solid solution aging state< laser-MIG hybrid welding solution and aging state. |
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