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

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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.