| Al-Mg dissimilar metals has a very wide range of application in the military armored laminate, aerospace aircraft wing, pods, and various rocket shell casing, aerospace engines and components as well as the automotive industry and other aspects of the link. However, the intermetallic compounds (IMCs) are usually formed in the Al/Mg welding joint, which is a key factor for the shaping quality and properties of the welded joints. Therefore, in order to improve the properties of the Al-Mg dissimilar FSW (Friction Stir Welding) joint. It is important to study the formation mechanism of the IMC and propose possible control methods. In this paper, we adopt thermodynamics, diffusion kinetics, physical simulation and numerical simulation methods to investigate the formation mechanism of the IMC in the Al-Mg dissimilar FSW joint, which can provide a theoretical basis for the parameters optimization and IMC regulation for the FSW process of dissimilar metal joints.First, we study the FSW process of 6061 aluminum alloy and AZ31 magnesium alloy, the defect-free joints is obtained when the rotating speed is 800rpm and the welding speed is 120mm/min. The microstructure and properties of the FSW joint were analyzed. Meanwhile, the IMC, such as Al3Mg2 and Al12Mg17 were found in the FSW joints, which caused the defects, poor performance and welding difficult of the joint. Therefore, we adopt the thermodynamic method to calculate the Gibbs free energy of the IMC in Al-Mg binary alloy and Al-Zn-Mg ternary alloy system. Meanwhile, non-equilibrium solidification process of the Al-Mg and Al-Zn-Mg system were calculated.Finally, the DSC experiments were conducted to compare the calculated results. The results show that, the Gibbs free energy of Al12Mg17 is less than the one of Al3Mg2. Meanwhile, the Al12Mg17 phase generate in Mg-rich region while the Al3Mg2 phase in Al-rich region. Besides, the MgZn2 phase is found in Al-Zn-Mg ternary alloy whose Gibbs free energy is much less than Al3Mg2 and Al12Mg17 ones. Therefore, the addition of zinc may well inhibit the formation of Al3Mg2 and Al12Mg17 and the same results can be found in DSC results.Secondly, the thermal compression simulation of the Al-Mg dissimilar metal was conducted by using Gleeble 3800.The effects of local composition, thermal and plastic deformation on the formation mechanism of the IMC were studied. The results showed that, the IMC phase Al3Mg2 and Al12Mg17 are formed when the strain rate reaches 10S"1 though the temperature is lower than the Al-Mg eutectic temperature. That is to say, the plastic deformation may accelerate the formation process of the IMC in the deformation zone. Besides, we found the liquid metal formed at the Al-Mg interfere even the temperature is only 380℃. The liquid metal is mainly composed of Mg-Zn, Al-Mg binary compound and Al-Mg-Zn ternary compound. The result indicates that the addition of Zn in the Al-Mg interface can inhibit the Al3Mg2 and Al12Mg17 phase, which is in accordance with the thermodynamic calculated results.Finally, the temperature field of the FSW process is simulated by using ANSYS software. The calculated results are in accordance with the experiment ones. Based one the simulation results, kinetic model of the IMC in the Al-Mg FSW joints is created. Meanwhile, the relationship between FSW parameters and the layer thichness of the IMC were investigated. The results show that, the thickness of the IMC layer increases with the decreases of the welding speed and the increases of the rotational speed. The predict results are in agreement with the experiment ones. |
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