Study On Microstructure And Properties Of Aluminum/Steel Dissimilar Metal Laser Brazing Joint Controlled By Composite Field

The high-quality welding of aluminum alloy and steel was one of the key means to carry out the lightweighting of automobiles,rail transportation,marine vessels and even high-end equipment,and achieving a high-quality connection between the two can bring great economic and environmental benefits.A large number of experimental studies had focused on the laser welding process and the choice of welding materials,hoping to regulate the generation of intermetallic compounds by these two means,while less research had been reported on physical field modulation.For the sake of solving the problems of forming Al/Fewelding and forming IMCs,the laser brazing test was carried out,and Er2319 was used as welding filler metal.The effects of laser power,welding speed and wire feed speed on the joint were studied in detail.Then the orthogonal test was carried out to optimize the welding process parameters.The effects of ultrasonic vibration power and magnetic field intensity on the microstructure and mechanical properties of joint were studied by introducing ultrasonic vibration and magnetic field into weld pool.Finally,the influence of AlFeNiCrCoTi0.5 powder on the forming microstructure and mechanical properties of the joint was studied by precoating high entropy alloy powder on the surface of the weld passage.The results were as follows:（1）The influence of laser brazing parameters on aluminum/steel joints was studied.The results showed that the change of process parameters had limited control on the types and properties of interface products,and a variety of Al-Feintermetallic compounds were formed at the interface.The wetting spreading property was greatly affected by the heat input,and the optimum brazing area（the"bite"of aluminum to steel）can be obtained with appropriate heat input,while the spreading property of aluminum alloy to steel can be reduced with excessive heat input.The results of orthogonal test showed that welding speed had the greatest influence on weld forming,followed by defocus,laser power and wire feed speed.With the increase of heat input,the thickness of IMCs increased.The morphology of IMCs was mainly smooth on the steel plate side and acicular,flocculent and serrated on the aluminum alloy weld side.The main product phases of the interfacial layer were FeAl,FeAl₂,FeAl₃,Fe₂Al₅ and Fe₄Al₁₃.After optimization,when the laser power was 1300W,the welding speed was 15mm/s,the wire feed speed was 30mm/s,the defocus was 0mm,the laser offset was 0.3mm,and the laser bias was 5°to the aluminum alloy side,the joint with the maximum tensile strength was 135MPa can be obtained.（2）The influence of ultrasonic vibration power on the joint was studied.The results showed that:After adding ultrasonic vibration,the wetting and spreading performance of aluminum alloy was further strengthened with the increase of ultrasonic vibration power.When the vibration power was 400W,the joint forming reached the best.It was easy to cause weld penetration by increasing ultrasonic vibration power because of the cavitation and flow effects of ultrasonic waved.With the introduction of ultrasonic vibration,some IMCs were broken into the weld,resulting in the gradual thinning of the IMCs on the steel surface,with the thinnest being 12.8μm.When the ultrasonic power was high,the IMCs would be peeled off from the IMCs layer on the steel surface into large blocks and enter the molten pool,and the liquid aluminum would continue to react with the iron to generate IMCs.The thickness of IMCs increased rapidly,reaching about 30μm,which leaded to the rapid deterioration of the joint performance under too high ultrasonic power.After ultrasonic vibration was added,the IMCs at the interface were mainly composed of FeAl3and Fe2Al5,which greatly reduced the types of IMCs generated.Therefore,the tensile strength of the joint was increased by 7.5%to 145.2MPa compared with the optimal parameter.Because some IMCs enter the weld,the microhardness of the joint area was significantly higher than that of the aluminum alloy base metal area,with an increase of about 10-15HV.（3）The influence of different magnetic field intensity on joint was studied.The results showed that the magnetic field had a significant influence on the macro forming of the weld.With the addition of the magnetic field,the fishscale pattern on the surface of the pass was significant.With the strengthening of the magnetic field,the fishscale pattern was destroyed,and the wettability of the aluminum alloy was also worse.When the magnetic field intensity was 50m T,the joint formability was best.The introduction of magnetic field significantly eliminated the typical acicular crystals in the weld,which were mainly equiaxed crystals.The analysis of IMCs at the interface showed that the introduction of magnetic field can significantly change the diffusion behavior of iron and aluminum,and the thickness of IMCs decreased gradually with the increase of magnetic field intensity.Compared with ultrasonic vibration,IMCs mainly consists of FeAl phase.Under the optimal parameters,The thickness of the IMCs can be controlled to 11μm,and the tensile strength of the joint was increased by 10%compared with the optimal parameter,to about 148MPa.Thanked to the stirring effect of the magnetic field on the molten pool,the microhardness was also increased by 7-10 HV.（4）The influence of preset AlFeNiCrCoTi_0.5 high entropy alloy powder on the aluminum/steel joint was studied.The results showed that the high entropy alloy powder was difficult to enter the weld pool and more difficult to control the interface when the preset method was used for welding.Further addition of various energy fields,it was found that the formation of pores in the weld can be effectively inhibited by ultrasonic vibration.After the introduction of ultrasonic vibration,the high entropy alloy powder was partially involved in the weld pool,but it was still difficult to control the butt interface.In the weld,the high entropy alloy was scattered in the aluminum alloy matrix in the shape of particles.The addition of high entropy alloy powder had little effect on the tensile strength of the joint,but the microhardness of the weld zone was increased by 20HV.