| Welding and corrosion protection have a close relation with the applacaiton of Mg alloys. At present, many methods have already been applied to welding Mg alloy-Mg alloy, Mg alloy-Al alloy and Mg alloy-steel, obtaining good strength. However, corrosion problem of weld joints is inevitable. Many sduties focus on the investigation of corrosion and protection of Mg alloy substrate, but the investigation on the weld joint is absent. Therefore, it has important theoretical significance and practical application value to study the corrosion mechanism and protection technology of weld joints.In this study, corrosion behaviours of Mg-Mg joints, Mg-Al joints and Mg-steel joints are investigated, and corrosion mechanisms of joints are clarified. It is found that the proper distribution of Mg-Al intermetallic compounds is benefit for enhancing the corrosion resistance of weld joint. Based on this, arc spray technology is applied to enhance the performance of joints. After post-heat treatment, the protection results could be improved further. The main research contents are as follows:1. The corrosion characteristics of laser-TIG hybrid welded Mg alloys are investigated including the butt and lap joints of similar Mg alloy AZ31-AZ31 and butt joint of dissimilar Mg alloys AZ31-AZ91. The results indicate that the corrosion resistance of butt joints is increased due to the grain refinement (current density of fusion zone from 2.324 X 10-5A·cm-2 to 6.741×10-6A·cm-2) or interactions of grain refinement and continuous net-shapedβphases (current density of fusion zone from 1.624×10-5A·cm-2 to 1.213 X 10-6A·cm-2). At the same time, for similar Mg alloy AZ31-AZ31 lap joint, its micro-structural variation is similar to that of butt joint. The corrosion resistance of lap joint is as good as butt joint, but the reliability of the lap joint is lower than that of butt joint due to the severe local corrosion of zone H in the lap joint during corrosion process.2. The corrosion characters of dismiss metal joints are investigated including Mg-Al joints and Mg-steel joints. The results show that the effect of galvanic corrosion on the joint could not be neglected. The potential difference between Mg and steel is much larger than that between Mg and Al, therefore, the Mg-steel joint suffers the more corrosion damage than that of Mg-Al joint. In addition, the corrosion extent of Mg frontier adjacent to the Al alloy or steel side is more serious. For the Mg-steel joint, the phases with high potential in the FZ play an important role in decreasing the corrosion resistance of joint, while the effect of grain refinement on the corrosion is slight in the Mg-steel joint. 3. Mg-Mg weld joint are protected by the arc prayed Al coating and its post heat treatment (PHT). The microstructure, interface behaviors and protective mechanisms of the coating are investigated. The results display that the surface of TIG weld joint and laser-TIG weld joint could be deposited an Al coating with high corrosion resistance by arc spray technology. Al coating shows a mechanical bonding with the weld joint. After PHT, the bonding strength between coating and weld joint is improved from 4MPa to 6MPa by the mode transformation from mechanical to metallurgical bonding. In addition, Mg-Al diffusion layer composed of intermetallic compounds of Al12Mg17 and Al3Mg2 enhances the corrosion resistance of Al coating.4. Mg-Al and Mg-steel weld joints are protected by Al-Mg composite coating prepared by the wires combination of Mg-cathode and Al-anode and composite coating+PHT. The microstructure, interface behaviours and protective mechanisms of the composite coating are investigated. The results exhibit that the composite coating has a high corrosion resistance and low potential (300 mV lower than that of Al alloy). Al lamellas as a barrier play an important role in improving the corrosion resistance of the Mg-rich Al coating. Mg lamellas play an important role in decreasing the potential of the coating. After PHT, the intermetallic compounds layer composed of Al3Mg2 and Al12Mg17 with a high mircohardness (200HV0.5) forms on the Mg-steel weld joint due to the phase transition of arc-sprayed Al-Mg composite coating, which provides a good protection for the weld joint. |
PDF Full Text Request