超高强镀锌钢板电阻点焊过程液态金属脆化裂纹演化机理及抑制

Resistance spot welding (RSW) is a necessary technique to connect steel sheets in automobile industry. However, the presence of the liquid metal embrittlement (LME) crack becomes a common issue during RSW of ultra-high-strength galvanized steel sheets, and it has been an obstacle to realize high quality joining of car body. The formation process of LME crack during RSW is coupling with multi-field and multi-phase dynamic activity which involves thermo, stress, liquid and solid. So far, the underlying mechanism of LME crack in RSW is still undisclosed. In this proposal，a systematic study will be implemented aiming at uncovering the evolution mechanism of LME crack during RSW of ultra-high strength steel hot-dip galvanized steel sheets, by adopting comprehensive application of numerical modeling of multiple physical fields, thermomechanical tests and some state-of–the-art technologies such as in-situ hot tensile test and observation by Laser scanning Confocal Microscope (LSCM), 3D reconstruction and analysis of LME cracks by full-automatic serial section 3D microscope and Focused Ion Beam Scanning Electron Microscopes (FIB-SEM), to illuminate the dynamic changes of stress and temperature field in micro-region of welding joints and their influence on the position, morphology and susceptibility of the multidimensional LME crack. Based on these experiments, the internal correlation mechanism between welding process parameter and initiation and propagation of LME crack will be fully interpreted, and the comprehensive dynamic evolution mode of LME during multi-scale RSW will be established to reveal the thermodynamic evolution mechanism of the formation and propagation of LME cracks during RSW. As a result, the critical condition and controlling factors of the emergence of LME cracks, and fracture failure modes of the welding joint will be revealed, thus the effective strategy to suppress the LME cracks in RSW joints will be recognized.

电阻点焊为汽车连接的必要手段，超高强镀锌钢板点焊液态金属脆化(LME)裂纹是车身高质量连接所面临的共性难题和瓶颈。点焊LME裂纹形成过程涉及热、力、液、固等多场、多相的动态耦合，其形成机制尚未完全明晰。本项目拟开展超高强钢热镀锌钢板电阻点焊LME裂纹演化行为的系统性研究，综合运用多物理场数值建模、热模拟试验、激光共聚焦显微镜高温原位拉伸及观测、全自动连续切片3D显微镜裂纹三维重构、聚焦离子束扫描电子显微镜三维微观裂纹表征等研究手段，阐明点焊接头各微区应力场、温度场等的动态变化过程及其对多维的LME裂纹位置、形态和敏感性影响的独特规律，诠释点焊过程因素与LME裂纹萌生及扩展梯度条件的内在关联机制，构建多尺度点焊LME动态演化综合模型，揭示点焊LME裂纹萌生及生长过程的热、动力学演化机理，明确点焊LME裂纹产生的临界条件、控制要素及焊点断裂失效机理，提出抑制点焊LME裂纹的有效策略。

超高强镀锌钢板点焊液态金属脆化(LME)裂纹是车身高质量连接所面临的关键问题之一。本项目针对热、力、液、固等多场、多相的动态耦合的电阻点焊加工过程，综合运用点焊过程数值模拟、加工过程热模拟试验、三维表征等技术手段，探明各加工参量因素与裂纹敏感性之间的关联，揭示了点焊过程中锌元素扩散导致钢晶界的弱化及裂纹产生的应力状态、裂纹扩展条件等的作用机理，明确了LME裂纹对点焊接头静态拉伸性能、疲劳寿命及断裂失效模式的影响规律，阐明了电阻点焊中LME裂纹的产生机制为应力辅助扩散机制，针对点焊LME裂纹产生的临界条件提出了相应的抑制策略。为有效控制不同钢种的电阻点焊过程LME裂纹缺陷、推动超高强镀锌钢板在汽车领域的应用提供理论支撑和试验依据提供理论支持。