高强铝合金电弧增材制造弱拘束条件下凝固裂纹自愈合理论研究

Wire and Arc Additive Manufacturing (WAAM) is the most potential method for manufacturing aviation monolithic component with high strength aluminum alloys, but mass WAAM production of high strength alloy component is still under development because the cracking susceptibility of the high strength aluminum alloys is too high and it is difficult to manufacture high strength aluminum alloy wires. Aiming at solving those two problems, the metallurgical process of the weak WAAM constraint welding pool of high strength aluminum alloys will be researched systematically. Firstly, based on the in-situ alloying theory, the control method on chemical content of WAAM pool with two dissimilar wires designed, so as to achieve the WAAM pool with Al-Zn-Mg-Cu alloy content. Then, the weak constraint state of the WAAM pool and the refilling theory of solidification cracking will be mainly studied..The stress distribution and variation model of the weak constraint WAAM pool will be established to reveal the mechanism of solidification cracking. Meanwhile, the solidification path will be calculated to analyze the liquid amount and the liquid channel shape during terminal solidification. And, an evaluation criterion for refilling property of high strength aluminum alloys solidification cracking will be proposed, so that the cracking theory of high strength aluminum alloys during WAAM will be more comprehensive. Finally, the methods to decrease the cracking susceptibility will be put forward by controlling the weld metal composition and the solidification structure based on the refilling theory of solidification cracking. The results of this project will provide theoretical foundation for the improvement of crack-resistant property of high strength aluminum alloys during WAAM.

电弧增材制造是高强铝合金航空整体结构件最有潜力的制造途径，但是高强铝合金裂纹敏感性过高，严重妨碍高强铝合金增材制造的发展。针对高强铝合金丝材成型困难和凝固裂纹敏感性高两大问题，本项目以高强铝合金电弧增材制造弱拘束熔池的冶金过程为研究对象，首先以原位合金化理论为依据，研究异质双丝熔池调质工艺，实现Al-Zn-Mg-Cu合金熔池成分的量化控制，突破高强铝合金不易成丝的瓶颈；然后，重点针对电弧增材制造熔池的弱拘束状态和凝固裂纹的自愈合理论展开研究，构建电弧增材制造弱拘束熔池应力分布及演变模型，揭示电弧增材制造凝固裂纹产生机制，计算熔池凝固末期残余液相回流通道形状及液相性能，提出凝固裂纹自愈合能力评价标准，完善高强铝合金电弧增材制造裂纹理论；最后以凝固裂纹自愈合理论为依据，从熔池成分控制和凝固组织控制两个角度提出高强铝合金电弧增材制造抗裂纹方法；为解决高强铝合金电弧增材制造裂纹问题提供理论依据。

电弧增材制造是高强铝合金航空整体结构件最有潜力的制造途径，但是在航空航天领域应用广泛的高强铝合金裂纹敏感性过高，严重妨碍高强铝合金增材制造的发展。本项目针对高强铝合金电弧增材制造凝固裂纹，从四个部分展开研究，包括：基于熔池调质技术的异质多丝电弧增材制造系统设计；基于熔池调质技术的原位合金化理论研究；电弧增材制造弱拘束熔池条件下凝固裂纹产生机制研究；高强铝合金凝固裂纹敏感性评价及裂纹控制方法，取得一系列进展。. 首先，本项目设计研发基于TIG熔丝工艺的多丝共熔平台，实现多元合金的灵活设计与增材制造。从单层单道和单壁墙两个层面上研究了主要工艺参数对增材制造构件组织性能的影响规律，提出铝合金多丝增材制造最优工艺窗口。. 针对电弧增材制造中烧损严重的Mg、Zn元素的损失规律展开研究。发现Mg、Zn元素的损失改变了铝合金基体的晶格常数，固溶强化效果被弱化，合金硬度和强度下降，但延伸率有一定提高。基于异质多丝增材制造系统，结合上述工艺探究，通过控制多丝的成分与送丝速率，成功设计并获得预期Al-Mg-Cu-Zn合金。. 通过对增材制造构件裂纹产生机制的研究发现，在熔池凝固过程中，Al-Zn-Mg-Cu合金凝固末期的液相含量较少；同时由于合金元素含量高，凝固温度区间大，以糊状形式凝固，残余液相无法弥补晶间微空隙和微裂纹。已沉积部分在周期性重复受热过程中，沿晶界分布的低熔共晶物受热液化，晶间连接变弱。增材制造构件在沉积过程中产生较大残余应力，形成起始于微裂纹并沿晶界扩展的宏观裂纹。. 最后，利用热力学计算软件量化了合金的凝固路径，发现合金元素的含量对凝固路径具有显著影响。基于上述规律，结合元素烧损规律，设计了新型丝材（合金），通过调整关键合金元素的含量，改变了合金的凝固路径，调整了合金凝固末期的液相含量，成功避免了Al-Zn-Mg-Cu合金增材制造构件的裂纹。. 综上所述，本项目从四个方面展开研究，取得多项关键进展，顺利完成本项目申报书与计划书中规定的研究任务。