7A52铝合金双丝MIG焊接头的不均匀性对其表面微弧氧化膜腐蚀防护作用的影响

Influence of Inhomogeneity of Tandem Welding Joint of 7A52 Al-alloy on Protectiveness of Micro-arc Oxidation Films on Its Surface

以装甲用7A52铝合金双丝MIG焊接接头易于腐蚀,需要进行必要的防护为背景,采用金相显微镜、扫描电镜、能谱分析和极化曲线为表征手段,研究了焊缝组织和成分的不均匀性对表面微弧氧化膜腐蚀防护作用的影响。结果表明:焊缝区为树枝状的铸态组织;熔合区靠近焊缝一侧为细小的等轴晶组织,紧邻热影响区一侧为柱状晶;热影响区的组织部分发生了再结晶;母材为典型的轧制态纤维组织。微弧氧化膜在焊缝表面生长速率慢,Mg参与形成焊缝表面的氧化膜,使得氧化膜的钝化性减弱,致密性和均一性变差,其腐蚀防护作用不及母材和热影响区表面氧化膜的。短期浸泡时,焊缝区、热影响区及母材表面微弧氧化膜的腐蚀防护作用差别较小;长期浸泡后,焊缝表面微弧氧化膜的腐蚀防护作用明显不及母材和热影响区的。

Taking the susceptibility of the tandem welding joint of 7A52 Al-alloy into account,it is of significance to reveal the effect of the microstructural heterogeneity of the tandem metal inert gas welding joints of 7A52 Al-alloy on the corrosion performance of the micro-arc oxidation（MAO） films prepared on the surface of the weld plate by means of potentiondynamic polarization measurement, OM and SEM with EDS. It follows that the rolling base metal presents a typical fiber-like microstructure. The microstructure of the weld seam is as-cast dendritic structure. Grains in the fusion zone are equiaxed crystallites on the side adjacent to the weld, but are columnar on the other side near the heat-affected zone（HAZ）. Recrystallization of grains in the heat affected zone has occurred. The growth rate of the MAO film on the weld seam is slower and Mg can be detected in the film, which may be responsible to the poor compactness and uniformity, and thus the weak passivity of the prepared MAO film, as a result, the protectiveness of the MAO film on the weld seam is worse than that on the HAZ and base metal. In general, for short-term immersion, the difference of the protectiveness of the MAO films on the weld seam, HAZ and base metal is not obvious, however, for long-term immersion, the protectiveness of the MAO film on the weld seam is obviously inferior to that on the HAZ and base metal.