【作者】 王响群；

【导师】 张福全； 陈振华；

【作者基本信息】 湖南大学 ， 材料加工工程， 2005， 硕士

【摘要】 本文对厚度为1.2mm AZ31镁合金薄板的钨极交流手工氩弧焊（GTAW）工艺进行了研究,分析了各种焊接因素（焊接电流、材料的表面状态、保护气体流量、有无焊丝、焊后热处理等）对焊缝成型和焊接质量的影响及其规律。利用金相显微镜、X射线衍射仪、万能拉伸试验机和扫描电子显微镜等技术对焊接接头显微组织、焊缝相组成、接头力学性能、断口形貌特征以及微区成分等进行了分析;并对焊接过程中气孔和裂纹产生的原因和防止措施进行了讨论。 试验结果表明,AZ31镁合金氩弧焊时,焊接工艺条件对焊缝表面形状和焊缝成型质量有很大影响。镁合金材料表面存在氧化膜时,焊接熔深浅,熔宽大;焊缝的熔宽和熔深随焊接电流的增大而增大;焊缝的熔宽随保护气体流量的增大先加宽、后变窄,焊接熔深随保护气体流量的增大先增大、后减小。采用合适的焊接工艺能得到外观成型良好,内部无气孔和裂纹的焊接接头。 AZ31镁合金氩弧焊接头的热影响区和焊缝分界明显,热影响区晶粒较粗大,焊缝为细小的等轴晶,焊缝主要存在α-Mg和β-Mg₁₇Al₁₂两种相。拉伸断口形貌分析表明,焊接接头呈韧性—脆性混合断裂,断裂一般发生在晶粒较粗大的热影响区。与母材相比,AZ31镁合金焊缝中镁含量有所降低,铝含量有所增大。 焊接电流对AZ31镁合金薄板氩弧焊的接头力学性能影响很大,电流太大或者太小,都会使接头的力学性能降低。不使用填充焊丝,当焊接电流为45A、钨针直径为1.6mm、保护气体流量为15L/min时,AZ31镁合金薄板氩弧焊接头的力学性能可达到母材的79%左右。添加填充焊丝能提高AZ31镁合金接头的力学性能,焊后经热处理可进一步改善接头的组织性能。 AZ31镁合金氩弧焊接头中气孔的形成原因主要是冷却过程中由于氢在镁中的溶解度急剧下降导致氢大量析出而形成氢气孔,其次是镁合金中低熔点合金元素蒸发烧损而形成的气孔。AZ31镁合金氩弧焊接头中热裂纹的产生主要与镁合金中存在低熔点共晶体有关。更多还原

【Abstract】 In this thesis, the influence of welding factors （welding current, surface coating and oxide film, shielding gas, welding wire, post weld heat treatment） on geometry and quality of welded joints of gas tungsten arc （GTAW） welded magnesium alloy AZ31 plates with 1.2 thickness was discussed. The microstructure was investigated with optical microscopy. Mechanical properties were determined by standard tensile tests on small-scale specimens. Fractography characteristics were observed by scanning electron microscopy （SEM）. Meanwhile, the factors leading to the forming of porosity and cracking in welded joints were analyzed and the methods to avoid porosity and cracking were described.The result indicated that welding parameters have great impact on the shape and microstructure of the joints. When oxide film on the surface of magnesium alloy is not erased, the weld is wide and shallow. The width and penetration of weld increases as the welding current increasing. The weld broadens firstly, then narrows as the shielding gas increasing, and the same is the penetration. When the welding parameters are feasible, it can get well weld formation, no porosity and cracking exists in the welded joints.The microstructure is fine equiaxed grains in welded zone, which is shown to consist of a-Mg and Mg₁₇Al₁₂, while the microstructure of heat affected zone is coarser. Tensile specimens fracture at heat affected zone. The content of Mg in welded zone is a little reduced, while the content of Al increased.Welding current has great impact on mechanical properties of the joints. Mechanical properties of the joints decrease at low or high welding current. Without welding wire, the joint strength is about 79 percent of the base material at 45A welding current. The joints strength is increased with welding wire. The joints strength increases with the content of Al in welding wire. The joints strength can be improved with post weld heat treatment.Magnesium has higher hydrogen solubility in liquid than in solid. The difference in the solubility between the solid and liquid phases is major cause of the porosity of magnesium alloys during welding. The low melting element of magnesium alloy is easily to vaporize and the presence of porosity in the base metal coalesced and expanded, which is another cause. Because of low melting更多还原