铝合金/超高强钢异种材料平模自冲摩擦铆焊机理与方法研究

Mixed use of high strength-to-weight ratio materials such as aluminum alloy and ultra-high strength steels has been recognized as an inevitable trend of vehicle body lightweighting. However, due to the large difference of thermo-physical properties between aluminum and steel as well as the poor compatibility of the two elements, a reliable joining of aluminum to steel cannot be achieved using resistance spot welding, which is the dominant process in assembling the traditional steel vehicle body. In this project, the advantages of traditional self-piercing riveting and friction stir spot welding process are combined and a new flat-die based friction self-piercing riveting (FD-FSPR) method using semi-tubed rivet, was proposed. A thermo-mechanical coupled finite element model and a self-developed friction self-piercing riveting system will be established to investigate the evolution of temperature field, structural field and the interface condition during the joint formation, aiming at revealing the formation mechanism of the hybrid joint under the complex multi-body contact and thermo-mechanical coupling conditions. The effects of process parameters and rivet properties on the characteristics of the ring-shaped solid-phase nugget between the rivet and the steel sheet, the mechanical interlock and the trimming occlusion between the rivet and the aluminum sheet will be studied to obtain the coordination control method of hybrid rivet-welded characteristics. The effects of the hybrid joining properties of FD-FSPR on the mechanical performance and fracture mode of the joints will also be studied to obtain the controlling mechanism of the hybrid joining properties of FD-FSPR on its joint performance. This project will provide a stable and reliable hybrid spot joining method for the application of dissimilar material of aluminum and UHSS in vehicle body lightweighting.

铝合金和超高强钢等高比强度材料的混合应用是当前车身轻量化技术的发展趋势。由于铝钢热物理属性差异巨大且两种元素相容性差，导致传统钢制车身焊装中占主导地位的电阻点焊工艺难以实现铝钢的可靠连接。本项目融合传统自冲铆接和搅拌摩擦点焊各自的优势，提出基于半空心铆钉的平模自冲摩擦铆焊新方法。建立热力耦合数值仿真模型并结合自主开发的自冲摩擦铆焊试验平台，研究平模自冲摩擦铆焊接头形成过程中温度场、结构场和界面状态的动态演化规律，获得多体接触和热力耦合条件下铆焊复合接头的形成机理；探索工艺参数和铆钉特性对铆钉/钢板环状固相焊核、铆钉/铝板机械互锁和铆钉/铝板飞边咬合特征的影响规律，获得铆焊复合连接工艺的协调控制方法；研究铆焊复合连接特征对接头力学性能和失效模式的影响规律，获得铆焊复合连接特征对接头性能的控制机理。为轻量化车身中铝合金/超高强钢异种材料匹配的批量应用提供有效的复合点连接技术。

面向汽车结构轻量化需求，铝合金和高强钢混用车身结构成为车身制造的主流趋势，这为传统点连接工艺带来巨大挑战。铝/钢物理属性和冶金相容性的巨大差异使传统电阻点焊工艺面临挑战；而依赖材料塑性变形能力的自冲铆接工艺随着钢强度等级的提高而难以形成可靠接头。面对这一困境，提出了铝钢平模自冲摩擦铆焊（FD-FSPR）技术，通过驱动半空心铆钉的旋转、进给运动，刺穿上层铝合金板后与下层钢板搅拌摩擦焊形成环形固相连接区。FD-FSPR工艺通过回避铝板和钢板的直接连接，最终获得了以固相连接为主要承载、机械连接辅助锁紧的固相-机械复合连接接头。.本项目首先建立了适用于铝/钢FD-FSPR工艺特征的工业原型机。通过结构拓扑优化，获得了轻量化的C型框架，采用双伺服电机驱动铆钉旋转和进给运动。机械结构优化设计过程中采用数值模拟和受载测设结合的方法，充分保证工艺过程的稳定性和可靠性。以车身制造中广泛应用的6061-T6铝合金和DP600高强钢板材组合为研究对象，通过半空心铆钉结构优选和工艺参数正交优化，实现了稳定的上层板穿刺、“管-板”搅拌摩擦焊和板材机械锁紧的接头形成过程。所获接头无未焊合、板间缝隙和接头大变形/开裂等缺陷，具有良好的接头形貌。通过分段实验，揭示了工艺过程各个阶段、各复合连接特征的成形过程。对接头固相焊区中的夹杂搅拌区进行了夹杂物溯源，明确了作为接头薄弱环节的夹杂搅拌区产生的原因，并基于此提出了铆钉结构设计的“冗余体积原则”，以避免工艺后期塑性态截留铝合金随着铆钉体的摩擦消耗导致的体积不足而挤入钢固相焊区。基于Abaqus软件，采用“欧拉-拉格朗日”复合方法建立FD-FSPR热力耦合数值仿真模型，揭示了铆钉旋转、进给运动下，多体、多界面摩擦接触机制，以及接头温度场、应变场和结构场的动态演化行为，为后续工艺参数优化和应用研究奠定了理论基础。