超声振动钢/铝对接接头激光-MIG复合熔钎焊工艺研究

Ultrasonic Vibration Steel/Aluminum Butt Joints by Laser-MIG Hybrid Fusion-Brazing Welding

针对钢/铝熔钎焊过程中焊接头金属间化合物(IMCs)厚度较厚、接头抗拉强度较低的问题,采用超声波辅助激光-MIG复合熔钎焊(MIG焊,熔化极惰性气体保护焊)的方法,获得了不同超声功率下的钢/铝熔钎焊接头。研究了超声波功率的变化对钢/铝熔钎焊接头的焊缝组织和界面IMCs的影响。结果表明,随着施加的超声功率的增加,超声对熔池的搅拌作用增强,焊缝晶粒明显细化。超声作用下熔池的最高温度降低,温度梯度减小,这使得焊接接头界面处的IMCs的厚度减小,并且FeAl_(3)相的含量降低;当超声功率增加到200~210 W时,IMCs中仅含Al_(8)Fe_(2)Si相。当功率为130~140 W时,去余高接头的抗拉强度可达172 MPa,相较于未施超声时增加了12%。

Objective Steel/aluminum is a widely used lightweight composite material.However,because the difference in the physical and chemical properties between these two metals is large,cracks are prone to appear after welding due to stress concentration,and there exist brittle and hard Fe-Al intermetallic compounds(IMCs)at the joint interface.The mechanical properties of welded joints are seriously influenced by the thickness and grain size of IMCs.It is difficult to obtain highquality and stable welded joints for the steel/aluminum dissimilar metal connection.Therefore,to obtain high-quality steel/aluminum welded joints is a general demand of the society.The application of ultrasonic waves can increase the flow performance of the molten pool during the welding process and improve the temperature field and flow field of the molten pool.The formation of IMCs is mainly related to the temperature field during the welding process.Therefore,to regulate the temperature field of the molten pool is a powerful tool to improve the mechanical properties of the joints.This experiment investigates the effect of ultrasonic power on the IMCs at the steel-aluminum interfaces of the weld brazed joints.Methods Laser and melt inert gas(laser-MIG)hybrid welding is used to perform the butt welding of 2 mm thick 6061 aluminum and 304 steel combined with fusiorrbrazing welding,and ultrasound is introduced in the welding process.The ultrasonic powers set in the experiment are 0 W,130-140 W,200-210 W,and the welded joints under different parameters are obtained.After welding,the metallographic phase is cut,the scanning electron microscope and the built-in energy dispersive spectroscopy(EDS)are used to observe the micro-interfacial structures and micro-area compositions,and the ultra-depth-of-field metallographic microscope is used to observe the grain sizes of the steel/aluminum welded joints.The microcomputer-controlled electronic universal tensile testing machine is used to test the tensile strengths of joints after removing reinforcement,and the experimental results are eompared and analyzed to explore the effect of ultrasonic usage on the steel/aluminum fusion brazed joints.Results and Discussions Comparing the welded joints under three ultrasonic powers,it can be seen from Fig.3 that as the ultrasonic power increases,the pores in the welded joints appear to gather,grow,and float,and the porosity value in the weld first decreases and then increases,which is mainly caused by the cavitation effect of ultrasound.The effect of ultrasonic breaking can increase the nucleation rate of the molten pool,which in turn refines the grains in the weld(Fig.4).It is found that ultrasound can significantly reduce the thickness of the IMCs layer and change the composition contents of the IMCs(Fig.5).As the ultrasonic power increases,the thickness of the IMCs layer decreases,the content of FeAlwith poor mechanical properties in IMCs decreases,and the content of AlFeSi with a better performance increases,which improves the tensile performance of the joints(Fig.7).The analysis shows that the samples are all broken on the IMCs layer,and as the ultrasonic power increases,the AlFeSi phase in the section increases and the FeAlphase content decreases(Fig.8).This is because the ultrasound changes the temperature field of the weld pool and subsequently the compositions and thicknesses of IMCs are changed,which improves the performance of joints.Conclusions The ultrasonicrassisted laser-MIG hybrid welding can realize the connection of steel/aluminum dissimilar metals,and the ultrasonic power has a significant effect on the porosity of the welded joints.When the ultrasonic power increases from 0 W to 130-140 W,the porosity decreases from 0.25%to 0.21%.When the power increases to 200-210 W,the weld porosity increases to 0.93%instead.When the ultrasonic vibration is not applied,the average diameter of weld grains is 5.2μm.When the ultrasonic power is 130-140 W,the average diameter of crystal grains is reduced to 4.1μm.When the ultrasonic power is 200-210 W,the average diameter is reduced to 3.4μm.With the increase of ultrasonic power,the mechanical stirring effect of ultrasound is gradually strengthened,and its effect on the refinement of weld grains is more obvious.With the increase of applied ultrasonic power,the content of FeAlphase in IMCs gradually decreases,which in turn increases the tensile strength of welded joints.When the applied ultrasonic power is 130-140 W,the tensile strength of the welded joint reaches 172 MPa,which is 12%higher than that of the joint without ultrasound.