Research On Laser Welding-Brazing Characteristics Of Mg Alloy/Galvanized Steel Dissimilar Materials

For realizing the goal of lightweight of automobile, the application of Mg alloy and high strengh galvanized steel are more and more wide, so joining Mg alloy to galvanized steel is inevitable. However, joining Mg alloy to steel is very difficult due to great differences of their thermophysical and thermochemical performances, and with the influence of Zn coating, the welding process is more complex.In this thesis, laser welding-brazing method was adopted to join Mg alloy to steel, and Mg alloy wire was chosen as brazing filler metal. Due to the diameter of laser beam is very minor, the requirement of welding process is quiet strict, so the welding process was monitored on line, and the factor that result in the unstability of welding process was analyzed. By ajusting welding parameter, well-appearance seam was gained.Because of local heating of laser beam,and brazing flux's effect on the wettability of filler metal, laser beam type,brazing flux and welding process's influence on filler's spreading ability and joint efficiency was studied. The experiment results showed that dual-beam welding wih flux has the highest joint efficiency, which reaches 73.1% of AZ31 base metal.Interfacial layer is the weak area of the joint. Because of local heating of laser beam, the thickness of interfacial layer is very uneven. The thickness of interfacial layer in Zn-rich area is highest, which is 200～250μm. Laser exposure area has the thinest interfacial layer, which is only 0～5μm. The phase compositions were identified by XRD analysis, which showed that interfacial layer has many phases, such asβ-MgZn,γ-Mg7Zn3,η-MgZn2,θ-Mg2Zn11 and so on. The micorstructure of well-distributed area is confirmed,which areα-Mg+β-MgZn eutectoid structure. The micorstructure of the end of Zn-rich area areη-MgZn2 primary idiomorphs andα-Mg+β-MgZn eutectoid structure, and the micorstructure of the tip of Zn-rich area areη-MgZn2 primary idiomorphs andθ-Mg2Zn11 peritectic structure.The shear test fracture was analyzed, and found Mg-galvanized steel joint has three fracture way. The crack initiation and propagation behavior at the interface was investigated by SEM in-situ observation during tensile test. And found continuous distributed interfacial layer has no effect on preventing crack's propagation. The exsistance of minor fusion U type area in laser exposure area changes the position of crack's propagation, which inproves the property of the joint at last.