【作者】 徐荣正；

【导师】 刘黎明；

【作者基本信息】 大连理工大学 ， 材料连接技术， 2011， 博士

【摘要】 焊接和腐蚀、防护问题与镁合金的应用密切相关,随着焊接技术的快速发展,已逐步实现了镁合金与同质及异质材料之间的可靠连接。而镁合金焊接结构的应用,必然面临焊接接头在服役中的腐蚀与防护问题。但是目前关于镁合金腐蚀机制与防护技术的研究主要集中在基体上,针对具有复杂微观组织演变以及结构差异特征的镁合金焊接接头的腐蚀行为与防护机制的研究开展较少。因此系统开展镁合金同质与异质焊接接头的腐蚀行为与防护技术的研究,对于揭示接头的腐蚀机制、评价接头在服役过程中的可靠性、科学指导接头防护技术的选择以及提高接头的服役寿命,具有重要的理论研究意义和实际应用价值。本文以镁合金-镁合金、镁合金-铝合金和镁合金-钢焊接接头作为研究对象,分析了镁-铝系镁合金同质与异质焊接接头在NaCl作为腐蚀介质中的腐蚀行为,总结了威胁焊接接头性能的关键部位的腐蚀规律,阐明了焊接接头的腐蚀机制；在此基础上,采用电弧喷涂(热扩散作用)技术,通过在镁合金同质与异质接头表面制备金属间化合物涂层或镁-铝异质双丝涂层,实现了对焊接接头的有效防护,并系统研究了涂层对接头的防护机制。主要研究内容和结果如下：1.从焊接过程中的组织演变对于接头腐蚀行为影响的角度,系统研究了低功率YAG激光+钨极氩弧焊(TIG)复合热源焊接的镁合金-镁合金接头在NaCl溶液中的腐蚀特征。研究发现：焊缝、热影响区中的组织演变和接头差异对接头腐蚀行为的变化产生重要影响,但是接头各区均以点腐蚀特征为主,焊缝、热影响区的腐蚀产物仍以Mg(HO)2为主。首先,AZ31-AZ31接头中焊缝内部的晶粒细化作用导致该区在3.5%NaCl溶液中虽然腐蚀电位变化不大,但腐蚀电流密度由基体2.324×10-5A·cm-2下降至6.741X 10-6A·cm-2;其次,铝含量较高的镁-铝系镁合金焊缝中β相呈连续网状分布,有助于在腐蚀过程中形成耐腐蚀屏障,对焊缝腐蚀行为的提高起到了决定性作用,例如AZ91-AZ91焊缝在3.5%NaCl溶液中虽然电位变化不大,但腐蚀电流密度由基体1.624×10-5A·cm-2下降至1.213×10-6A·cm-2；再次,接头结合形式的差异也会影响接头的腐蚀行为,搭接接头焊缝边角处的高腐蚀倾向性,直接威胁接头在服役中的可靠性。此外,镁合金-镁合金焊接接头中,热影响区附近极易受到腐蚀侵蚀,威胁接头性能,在接头服役过程中需要重视。2.从焊接过程中组织演变、电偶腐蚀对镁合金异质接头腐蚀行为,尤其是接头界面腐蚀行为影响的角度,研究了镁合金-铝合金、镁合金-钢异质金属焊接接头的腐蚀特征。研究发现：对于镁-钢焊接接头,高电位阴极相在焊缝中的形成,增加了其附近区域的点腐蚀倾向,降低了焊缝自身的耐腐蚀性,晶粒细化对于接头腐蚀行为的影响几乎可以忽略,例如镁-钢焊接接头中,钢飞溅的卷入使焊缝在3.5%NaCl溶液中,不仅电位降低,且腐蚀电流密度由镁合金基体区域2.135×10-5A.cm-2升高至2.741×10-4A.cm-2；对于镁-铝接头,金属间化合物过渡层的生成,使其结合区表现出了极高的耐腐蚀性。此外,电偶腐蚀作用对接头的腐蚀破坏,尤其是界面区域的腐蚀破坏不容忽视。无论对于机械连接为主的镁-钢焊接接头,还是冶金连接为主的镁-铝焊接接头,腐蚀破坏最为严重的界面均发生在镁合金靠近高电位相或金属一侧。3.鉴于镁-铝金属间化合物在提高镁-镁接头中焊缝、镁-铝接头结合区耐腐蚀性方面发挥的重要作用,提出电弧喷涂铝涂层及后续热扩散技术,实现了镁-铝金属间化合物层在镁-镁焊接接头表面的均匀层状分布,研究了涂层对于镁-镁接头的防护机制。研究发现：镁-铝金属间化合物层的形成,改善了单纯铝涂层对接头的防护效果,使涂层在3.5%NaCl溶液中腐蚀电位较热扩散作用前升高了0.08V左右,且腐蚀电流密度下降了1个数量级(即3.60×10-4 A.cm-2下降至1.60×10-5A.cm-2)；同时涂层与焊接接头之间的结合方式由机械结合转变为冶金结合,断裂位置发生在靠近铝涂层一侧的金属间化合物过渡层内,结合强度由不足4MPa升高到6MPa以上。此外,由于涂层的腐蚀电位要高于镁合金焊接接头,因此涂层主要依靠自身高耐腐蚀性对焊接接头起到物理隔离作用。4.提出镁-铝异质双丝电弧喷涂及后处理技术,利用涂层自身中镁、铝片层之间的扩散反应,实现镁-铝金属间化合物在镁合金异质金属接头表面的均匀分布,研究涂层对于镁合金异质接头的防护机制。研究发现：镁-铝异质双丝电弧喷涂涂层均匀致密,由镁、铝交错的片层组织构成。涂层具有高耐腐蚀性和低电位的特征,铝片层在腐蚀过程中会形成耐腐蚀性屏障,起到提高整个涂层耐腐蚀性的作用；而镁片层起到降低整个涂层电位的作用,使涂层的开路电位比铝合金还低300mV。热扩散处理后,可以形成以镁-铝金属间化合物为主的涂层,提高了涂层的耐腐蚀性以及微观硬度(200HVo.5)。此外,采用镁-铝异质双丝涂层作为物理隔离保护层进行防护的镁-铝焊接接头,在3.5%NaCl溶液腐蚀2h后,接头承受载荷几乎未发生变化；采用热扩散后形成的镁-铝金属间化合物层保护的镁-钢接头,在3.5%NaCl溶液腐蚀3h后,接头载荷波动不大(由4.8 kN降为4.5kN),表现出良好的稳定性。更多还原

【Abstract】 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.更多还原