| Welding of Mg/Al dissimilar metals has attracted many attentions, since it was proposed by former Soviet Union scholars in 1970s. Especially in the last two decades, with the popularization of magnesium alloy application in industries, studies on Mg/Al welding are accelerated. The connection of the two materials, not only can utilize the functional property of the two kind metals, but also can further reduce the structure weight. Welding of Mg/Al dissimilar metals easily results in intermetallic compounds (IMC) in the joint, such as, Al12Mg17, Al3Mg2, etc. Intermetallic compounds are hard and brittle, and usually lead to very low the mechanical properties of the joint. Therefore, to improve the mechanical properties of the joint, the distribution of Mg-Al intermetallic compounds has to be improved, or the amount of the intermetallic compounds has to be reduced. Current studies on Mg/Al welding mostly focus on reducing the amount of intermetallic compounds by adjusting the welding process. And mostly they are fusion welding and solid phase welding with lap joint, thin plates by fusion welding with butt joint has rarely been studied. In particular, research and development of welding wires for Mg/Al dissimilar metal welding is almost blank. In this paper butt joint welding of Mg/Al is studied, by a TIG. By adding different elements or alloys into the Mg/Al weld joint, a multi-alloy welding seam is formed. By designing the components of the filler metal with calculation methods, the weak area composition of the joint can be precisely controlled. As a result the purpose to control the Mg-Al intermetallic compounds in the weld metal is reached. Based on this, the influence of modifier on the microstructure and mechanical properties of Mg/Al weld seam was researched. The filler metal for welding Mg/Al dissimilar metals is developed. The effect of the weld seam formation process and the matching degree on the tensile strength is established. The influences of the grain size and the content of the intermetallic compounds on the tensile strength are also summarized. The main results of this paper are as follows:(1) Utilizing Pure Zn as a filler metal, a TIG is used to weld AZ31B magnesium alloy and 6061 aluminum alloy. The results show that the weld seam is composed of MgZm, Zn and Al based solutions, and Mg-Al intermetallic compounds are not found in the joint. The hardness of the MgZn2 is high and MgZn2 is in particle form. But these compounds are surrounded by Zn and Al based solutions. When the joint is under tensile tests, these Zn and Al solutions can eliminate strain concentration that caused by MgZn2. However, there are many MgZn2 compounds in the weld bead near the Mg base material, causing mechanical properties of the joint are weak at this location and the fracture occurs in this place. The maximum tensile strength of the joint utilizing Zn as the filler metal, reaches 93MPa. which is higher than that of Mg/Al joint directly welded by tungsten arc.(2) Adopting proper thick Al foil can increase the content of solid solution in the joint near the Mg alloy base material. By placing Al foil of different thickness on magnesium alloy base material before welding, the content of Al based solid solution in the joint near magnesium alloy base metals can be regulated. Studies show that the tensile strength increases firstly and then decreases with the increase of the Al foil thickness. And the maximum tensile strength reaches 104 MPa when Al foil is 0.2 mm thick. However, when the Al foil is with excessive thickness (over 0.4 mm), it is easy to form a rich Al region in the weld seam near the Mg alloy base material, that the crack defect is generated at this site.(3) By designing and utilizing Zn-xAl filler metals, AZ31B magnesium alloy and 6061 aluminum alloy are welded. And infrared cameras are used to detect the solidification status. The results are as follows. The Zn-xAl filler metals can accurately control the composition of mechanical property weak area. When using Zn-10Al, Zn-20Al as filler metals, MgZn2 precipitates firstly at the mechanical property weak area. Therefore the joint tensile strength is weak. When using Zn-30Al as the filler metal, a-Al precipitates firstly in the weak area, and the intermetallic compound content decreases. The average tensile strength of the joint is significantly increased, reaching 120 MPa. When using Zn-40Al as filler metals, a MgZn2 compounds layer of transition zone appears in the joint, which significantly reduced the tensile strength of joints.(4) In order to further improve the tensile strength of the joint on the basis of Zn-30Al, the influence of modifier, such as Ce, SiC nano-particles and of Ti, on weld microstructure and mechanical properties is studied. The results show that, SiC nanoparticles can eliminate the beneficial eutectic structure in the weld. Therefore the tensile strength of the joint reduces.. Weld microstructure is also sensitive to Ce content. When 0.05 wt.% of Ce is added to Zn-30A1, the highest tensile strength of the joint reaches 132 MPa. Ti could significantly affect the microstructure and mechanical properties the joint. When Zn-30Al is added with 0.5 wt.%Ti, a-Al that precipitated firstly is fined from average grain size of 35?m to 10?m. The average tensile strength of weld joints significantly increases, reaching 151 MPa.(5) In order to study the improvement mechanism of the tensile strength of Mg/Al dissimilar metal joints, this paper attempts to establish a matching model between intermetallic compounds and the base material and solid solution in the weld. The relationship between the matching degree and the tensile strength of the joiont is revealed. And the influence of matching degree on the tensile strength is clarified, when different kinds of metal compounds are contained in the joint. And when the same kind of metal compounds is contained in the joint, the influence of the content of the intermetallic compound on the tensile strength is explained. And when the joint contains similar contents of the same kind of metal compounds, the influence of grain size on the tensile strength of weld joints is discussed. |
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