Study On Microstructure And Properties Of Dissimilar 6061/2A12 Aluminum Alloys Welding Joints Of Laser Beam Scanning

6061 and 2A12 aluminum alloys are widely used in the fields of artificial satellites,aerospace,military equipment,railway systems,marine industry,etc.,therefore,there are a lot of demands for the dissimilar aluminum alloys welding.However,due to the differences of alloy composition,segregation,porosity and other problems are easy to occur in the weld.Laser beam scanning welding can stir the molten pool through the high frequency oscillation of the beam to improve the metallurgical behavior of the weld,providing a new method for solving the welding problem of 6061/2A12 dissimilar aluminum alloy.Therefore,this paper systematically studies the laser beam scanning welding process of 3 mm thin plate and 20 mm thick plate 6061/2A12 dissimilar aluminum alloys,and discusses the influence of beam scanning parameters on the microstructure and mechanical properties of weld joint.The main research results are as follows:For 3 mm thin plate welding,laser beam scanning behavior can refine grain,promote the generation of equiaxed grains and improve the solute segregation of weld.Firstly,the increasing of laser scanning frequency and scanning amplitude promote the flow of molten pool and have destructive effect on the growth of columnar grains and promote the formation of broken dendrites.Secondly,because of the laser beam scanning,the temperature gradient of molten pool becomes smaller and smaller,and the molten pool supercooling becomes larger and larger.These two factors promote the heterogeneous nucleation of welds and the formation of equiaxed grains.A large amount of nucleations lead to the refinement of weld grains.For laser scanning welding,the highest proportion of equiaxed grains was 42.7%,which is 80.9% higher than that without beam scanning weld,and the minimum average diameter of equiaxed grains is at 21.2 ?m,which is 43.3% lower than that without beam scanning weld.The rapid stirring of keyhole in the molten pool promotes the molten flow with high Cu in the side of 2A12 aluminum alloy to more diffuse to the side of 6061 aluminum alloy,so that the Cu element distribution in the welding seam is more uniform,inhibiting the solute segregation.The grains refinement and the increasing of equiaxed grains zone have great effect on the toughness of welded joint.When the laser beam scanning frequency increases from 0 Hz to 500 Hz,the elongation of the tensile specimen increases from 3.0% to 6.6%,and when the laser scanning amplitude increases from 0 mm to 1.6 mm,the elongation of the tensile specimen increases from 3.0% to 6.8%.For 20 mm thick plate welding,the laser power and welding current have a great influence on the lack of fusion of narrow gap welding,while the laser scanning frequency has a relatively small influence.The concave weld is beneficial to restrain the lack of fusion defect,and the increasing of laser power promotes the high temperature melting flow to the edge of the weld,melts the side wall of the narrow gap,and forms the concave upper surface of the weld with the action of surface tension.When the current is more than 100 A,the molten pool is relatively stable,which is conducive to forming a concave welding layer on the surface.The laser scanning frequency has little effect on the concavity of the weld surface.Laser scanning welding technology can solve the problem of porosity in aluminum alloy welds.With the increasing of the beams canning frequency,the porosity gradually decreases.The porosity is 10.6% without beam oscillation weld,however,when the scanning frequency is greater than 400 Hz,the pores in the weld completely disappear.The generation and elimination of bubbles in molten pool is a dynamic process.In laser scanning wire filler welding,the bubbles are generated by hydrogen precipitation in the molten pool and keyhole collapse,the elimination of the bubbles result from its escape and keyhole capture.Only when the bubbles removal ability is greater than the bubbles generation ability will not appear porosity defects in the weld.For 20 mm thick plate welding,the beam scanning frequency with optimization weld joints is 300-500 Hz,and the differences of mechanical properties are small.As the softening of 6061 aluminum alloy heat affected zone is serious,most of the fractures are in heat affected zone of the side of 6061 aluminum alloy.The differences of tensile strength of each welding layer are small,and the difference value is within 20 MPa.The maximum tensile strength of the welded joint reaches 227.3MPa,which is 72.9% of the 6061 base metal.The differences in the elongation of each layer are also small,within 2.2%,with the maximum elongation of 9.1%,reaching 65% of the base metal of 6061.This is due to the fact that in the range of 300-500 Hz,there are few or no porosity defects in the weld and the joint has good performance.