基于控制非平衡冶金反应条件的铝/镁异种合金搅拌摩擦焊技术研究

The friction stir welding (FSW) has great advantages in welding dissimilar metal materials, but for some kind of materials that may form eutectic materials with low melting point or the intermetallic compound (IMC), the suitable welding parameters range is very narrow, especially when welding thick plates. The main reasons are: 1) the component of the alloy elements in weld may be located in a composition range in which the IMC or the eutectic materials can be precipitated and the weld temperature is rather high, and then the brittleness of the material is increased; 2) the heat during FSW is mainly generated from the friction interface between the tool shoulder and the upper surface of the plate. When welding the thick plate, the temperature between the upper surface and the bottom surface of the plate possesses a big difference. Therefore, when the material at upper surface generates some liquid phase, the material at bottom surface is still at lower temperature and cannot be welded. Aiming at this problem presented in the FSW of dissimilar metal materials, this project selects aluminum alloy and magnesium alloy for the research object. And according to the Mg-Al alloy phase diagram, the FSW of dissimilar materials with the precipitation of IMC and the formation of low melting point eutectic is investigated. The purpose is to clarify the function mechanism of chemical composition in the weld and the non-equilibrium metallurgical reaction in the FSW process, to reveal the affection of the low melting point eutectic on the friction state between the shoulder of the welding tool and the welded material, and the affection of the tool shape, the relative location between the tool center and the welding interface on the weld formation. Finally, the formation of the IMC and low melting eutectic in the weld metal can be controlled, a fundamental basis can be provided for the FSW of dissimilar materials with a good weld formation.

采用搅拌摩擦焊（FSW）技术焊接异种材料具有较大优势，但对于可能形成金属间化合物（IMC）或低熔点共晶的异种材料，适用的焊接工艺范围窄，尤其是焊接厚板时更为困难。主要原因是：1）焊缝成分处于可析出IMC的区间且焊缝温度较高，析出低熔点共晶或IMC，增加了材料的脆性；2）FSW的热量主要是搅拌头轴肩与材料上表面之间的摩擦，厚板焊接时板材上下温度差很大，容易在焊缝上部出现液相而焊缝下部温度较低而不能焊接的现象。项目针对异种材料FSW出现的困难，选用铝和镁合金作为对象，依据Mg-Al二元相图，研究具有析出IMC和形成低熔点共晶的异种材料FSW技术，阐明焊缝金属成分、非平衡冶金反应在FSW过程中的作用，揭示低熔点共晶对轴肩与被焊材料间摩擦状态变化和焊缝成形的影响及搅拌头形貌、搅拌头与焊接面相对位置等对焊缝成形的影响规律，控制焊缝金属中IMC及低熔共晶体的形成，为异种材料FSW接头提供理理论依据。

搅拌摩擦焊(Friction stir welding, FSW)作为一种固相连接方法，在异种金属焊接领域具有明显的优势。然而，对于可能析出金属间化合物(Intermetallic compounds, IMCs)或低熔点共晶的异种金属，面临焊接工艺窗口窄和接头成形差等问题，在焊接厚板时更为严重。目前，关于厚板异种金属FSW的基础研究仍较为匮乏，且缺少相关试验与理论支撑。本文采用理论与试验相结合的方法，对厚板铝/镁FSW接头成形和温度分布规律进行研究，深入分析焊缝成分和温度场控制对接头成形及组织的影响。主要研究内容和结果如下：(1) 接头界面上部反应层的平均厚度与旋转速度呈正相关关系，与焊接速度呈负相关关系；接头界面底部的未焊合区宽度与旋转速度呈负相关关系，而与焊接速度呈正相关关系。当镁合金置于前进侧时，铝合金与镁合金混合不充分，接头存在孔洞缺陷。(2) 控制焊缝中铝与镁的相对含量，可以使焊缝避开发生共晶反应和生成IMCs所需要的成分区间，减少低熔点共晶和IMCs的析出。随着焊缝中镁合金含量的增加，接头界面上部和底部反应层的平均厚度呈增加趋势。(3) .搅拌针上的螺旋凹槽有利于增强厚度方向上塑性金属流动，减小接头厚度方向上温差，改善接头界面冶金结合。在双螺旋凹槽搅拌针作用下，焊缝上部高温金属向焊缝底部流动的金属量理论上达到561.0mm3，达到减小厚度方向上温差的作用。(4) 改变轴肩和被焊材料之间的摩擦界面、施加底部加热和外部冷却等方法能够减小接头厚度方向上温差，实现接头界面有效冶金结合。轴肩下方摩擦材料的加入提高了焊缝底部温度；底部辅助加热有助于提高焊缝底部温度，降低厚度方向上的温差；液氮冷却能有效降低焊缝上部峰值温度，抑制界面冶金反应。(5) 铝/镁FSW接头中裂纹的形成与共晶反应、界面错配度和应变梯度有关。搅拌区上部达到共晶反应条件，液化裂纹在铝固溶体和Al3Mg2相交界处产生。微裂纹易沿错配度最高和应变梯度最大的位置萌生。该研究成果不仅为厚板铝/镁FSW焊缝成分和温度场调控提供试验与理论指导，还可以拓展到具有析出IMCs或低熔点共晶特征的其它异种金属焊接。