Microstructure-based modelling of spot weld failure in third generation advanced high strength steels

为了提高乘客的安全性和燃油经济性，汽车公司一直在将强度越来越高的钢材整合到汽车结构中。最新的材料是第三代先进高强度钢（3G AHSS），它具有优异的成形和抗碰撞性能，但含有高碳和合金添加剂，会在焊接区域内产生很大的机械性能变化。3G AHSS组件的结构完整性和碰撞性能最终由这些控制故障路径和能量释放的局部异构特性控制。由于3G AHSS最近才开始商业化，因此对焊缝关键子区域的本构和断裂性能的了解不足，无法准确预测其在整车碰撞模拟中的行为。围绕3G AHSS焊缝断裂行为的相当大的不确定性导致汽车公司过度设计3G AHSS部件或避免采用它们。该研究将优化极限抗拉强度（UTS）分别为980 MPa和1180 MPa的两种等级3G AHSS电阻点焊的力学性能，然后建立碰撞失效行为的详细理解和预测模型。这项研究将导致：(i)工业上可行的点焊工艺；（ii）焊缝微观结构和性能的详细描述；（iii）了解3G AHSS点焊失效的机制；(4)开发适合工业仿真的焊缝断裂模型，为下一代轻量化汽车结构提供设计工具。项目成果将直接提升参与该项目的加拿大钢铁、零部件、模具、焊接设备和汽车制造商的竞争力。优化后的焊接参数将被加拿大的零部件和装配厂采用，新的模拟工具将被纳入汽车车辆设计和碰撞安全模拟，这将为这些新钢材创造市场需求。

To improve passenger safety and fuel economy, automotive companies have been integrating steels of increasing strength into the vehicle architecture. The newest materials of interest are third generation advanced high strength steels (3G AHSS) that have excellent forming and crash properties, but contain high carbon and alloying additions that create large variations in the mechanical properties within the weld zone. The structural integrity and crash performance of a 3G AHSS component is ultimately governed by these local heterogeneous properties that control the failure path and energy release. Since 3G AHSS have only recently become commercially available, there is insufficient knowledge of the constitutive and fracture properties of the critical sub-regions of the weld to accurately predict their behavior in full vehicle crash simulations. The considerable uncertainty surrounding the fracture behavior of 3G AHSS welds has led automotive companies to over-design 3G AHSS parts or to avoid their adoption.The proposed research will optimize the mechanical performance of resistance spot welds in two grades of 3G AHSS with ultimate tensile strengths (UTS) of 980 MPa and 1180 MPa, then develop a detailed understanding and predictive model of the failure behaviour during crash. This research will result in: (i) industrially-viable spot welding processes; (ii) a detailed characterization of weld microstructure and properties; (iii) an understanding of the mechanisms responsible for 3G AHSS spot weld failure; and (iv) the development of weld fracture models appropriate for industrial simulation, which will provide design tools for future generation lightweight automotive structures.The project outcomes will directly contribute to the competitiveness of the Canadian steel, parts, tooling, welding equipment and automotive manufacturers partnering on this project. Optimized welding parameters will be adopted into Canadian parts and assembly plants and the new simulation tools will be incorporated in automotive vehicle design and crash safety simulation, which will create market demand for these new steels.