Understanding and Controlling Transformations in the HAZ of 3G AHSS

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The Intergovernmental Panel on Climate Change reported that greenhouse gas (GHG) emissions must decrease to net-zero by 2055 to limit global warming to 1.5°C above pre-industrial levels. Automakers plan to decrease the GHG emissions of their vehicles by using ultra high strength steels (UHSS), with ultimate tensile strength (UTS) greater than 1 GPa, in their vehicles. UHSS allows thinner steels to be used for parts, reducing part weight, while retaining part strength. However, when UHSS are resistance spot welded (RSW), their complex composition and microstructure can lead to hardness peaks, softening, and liquid metal embrittlement (LME) cracks near the weld. These phenomena are not fully understood, but, it is known that thermal history will affect both weld hardness and LME.The proposed CRD project will better understand transformations and LME formation occurring during RSW of UHSS. This knowledge will be used to modify the RSW process to maximize joint performance and minimize LME cracking. To facilitate this, the weldability steels with UTS of 980 MPa and 1180 MPa. In-use steels with dual-phase (DP) microstructure and state-of-the-art Quench & Partition (QP) steels will be compared, enabling an understanding of how base material strength and microstructure affect transformations during welding and LME. The project will characterize transformations in the heat affected zone (HAZ) through changes in observed structures and mechanical properties. LME sensitivity in these steels will also be characterized. LME sensitivity will be mapped as with respect to welding parameters and in-situ observations of crack formation will be made. Understanding how thermal history affects HAZ transformations and LME will be used to optimize the welding process to maximize post-weld quality, informing a model to predict post-weld properties and cracking severity. These results will benefit Canada by increasing adoption of new UHSS steels into vehicles, creating a demand for Canadian steelmakers to make these high value steels, that will be used in Canadian automotive production.