基于流固耦合分析的搅拌摩擦焊接孔洞缺陷形成机理及调控方法研究

Friction stir welding (FSW) has been widely used for fabrication of aluminum alloy structural components. However, void defects sometimes occur in FSW joints if improper parameters or relatively high welding speeds were used, which significantly reducing the properties of FSW joints. The formation mechanism of void defects in FSW joints has not been comprehensively and profoundly revealed till now due to complexity and instantaneity of FSW process, which limits the process optimization and the increase of critical welding speed. This project will gain deep insight into the underling interaction mechanism between the FSW tool and plastically deformed materials at the contact interface through a coupled fluid-solid modelling methodology with experimental measurement. The thermo-mechanical experience (dynamic behaviors of shear deformation, strain/strain rate, temperature, etc. along different streamlines) and flow characteristics (acceleration, deceleration and stagnation along different streamlines) of the plastic materials near FSW tool in different positions are quantitatively analyzed and experimentally tested under different welding processes, and its effects on the final formation of the FSW joints are also studied. The formation mechanism of void defect and its influence factors are revealed and illuminated by quantitatively analyzing the thermo-mechanical experience and flow characteristics of the plastic materials in different positions of the FSW joints. The distinct material flow behaviors in the case of void defect formation will be studied systematically in order to propose a critical characteristic parameter for predicting the void defect formation with different welding parameters. The quantitative relationship among welding process parameters, the FSW tool-workpiece interface interaction, heat generation mechanism, plastic material flow behaviors and joint formation are realized. Finally, based on the research mentioned above, methods to control the plastic material flow behaviors in the stir zone to prohibit void defects and to improve the critical welding speed in FSW will put forward and be validated by preliminary experimental tests. It will lay solid foundation for optimizing the FSW process, improving the critical welding speed and further perfecting the FSW process theoretical system.

搅拌摩擦焊在铝合金构件制造成形领域得到越来越多的应用，但当其工艺参数不匹配或采用较高焊接速度时，焊缝中易产生孔洞缺陷，严重降低了接头性能。迄今，尚未系统定量研究FSW接头孔洞缺陷的形成机理及其影响因素，限制了工艺参数的优化及临界焊接速度的提高。本项目基于流固耦合分析与实验测试，深入研究搅拌头与塑性流变材料之间的动态交互作用机理，定量分析和测试不同部位的塑性材料沿流线填充搅拌头后方空腔的热力变形经历（剪切变形、应变/应变速率、温度等的动态变化）和流动特征（加速、减速和停滞等），揭示和阐明孔洞的产生机理及其影响因素，提出预测孔洞产生的临界特征参量，构建“工艺参数-界面接触与摩擦动态行为-产热机制-材料流变行为-孔洞产生倾向”之间的定量关系，提出消除孔洞缺陷、提高临界焊速、调控接头成形成性的工艺措施。预期成果将完善FSW焊接工艺知识体系，为提高临界焊速和优化FSW工艺奠定坚实基础和提供理论依据。

搅拌摩擦焊在铝合金构件制造成形领域得到越来越多的应用，但当其工艺参数不匹配或采用较高焊接速度时，既会导致接头内部产生孔洞缺陷，严重影响焊接接头的使役性能；又易导致搅拌头受力过大而发生断裂，严重影响搅拌头的服役寿命。这些问题都限制了搅拌摩擦焊接临界速度和焊接效率的提高。本项目基于搅拌头与工件之间的交互作用规律，提出了定量描述搅拌头与工件材料之间接触界面压力的非均匀分布模型，阐明了搅拌头与塑性流变材料之间的动态交互作用机理。建立了搅拌摩擦焊接过程的“热-流-固”多物理场耦合数值模型，揭示了焊缝孔洞缺陷的产生机理及其影响因素，提出了预测接头出现孔洞缺陷的临界特征参量。研究发现，当焊接速度较低时，各平面上的材料均呈水平迁移，能够较好地填充搅拌头后方的空腔，接头无孔洞缺陷；随着焊接速度的提高，搅拌头后方的摩擦驱动力显著减小，塑性材料的流动性降低，导致焊缝下部的塑性材料从搅拌头后退侧绕过搅拌头后，难以向前进侧迁移，最终使得焊缝中下部的前进侧区域形成孔洞缺陷。同时，随着焊接速度的提高，搅拌针根部出现明显的应力集中，且前方受拉应力、后方受压应力，导致搅拌头易从其根部断裂。提出了辅加超声和急冷外场辅助搅拌摩擦焊接的方法，研究了施加超声和急冷外场对搅拌摩擦焊缝成形成性的影响规律，发现施加超声振动能量，促进了焊缝金属的塑性流动，扩大了焊接工艺窗口，接头的成形、组织均匀性与综合力学性能均得以提高；而施加急冷，能够抑制焊接热输入较高条件下的接头软化，细化接头焊核区的晶粒组织，显著提高接头的抗拉强度，但降低了接头的断后伸长率，而在热输入较低时，施加急冷容易引起焊接缺陷，弱化接头的力学性能，使得焊接工艺窗口变窄。此外，针对中厚板异种金属搅拌摩擦焊缝孔洞缺陷的问题，提出了双面搅拌摩擦焊接方法，实现了中厚板异种金属的高质量焊接。项目研究为提升我国轻质合金构件焊接成形制造水平奠定了坚实基础。本项目已发表期刊论文16篇（其中，SCI收录11篇，EI收录4篇），授权发明专利3项。应邀在国内国际会议做分会场主旨报告1次，特邀及邀请报告10次。