Development and Influence of Fusion Boundary Microstructure on Resistance Spot Welds in 3rd Generation Advanced High Strength Steels
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Date
2024-05-08
Authors
Ramachandran, Dileep Chandran
Journal Title
Journal ISSN
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Publisher
University of Waterloo
Abstract
The use of advanced high-strength steels (AHSS) is one of the design solutions for making new-generation auto bodies to build economical and environmentally friendly structures without compromising vehicle safety. Among various types of third generation AHSSs, quenched and partitioned (Q&P) steels, which are one of the material solutions due to their exceptional combination of strength and ductility compared to the conventional dual-phase (DP) steels. To produce body-in-white structures, resistance spot welding (RSW) is the predominant joining process used in the automotive industry. Hence, it is essential to investigate the resistance spot weldability of Q&P steels.
Recent investigations show that during welding, partitioning of alloying elements occurs at the weld fusion boundary (FB), especially in the steels with higher carbon equivalents. A narrow region around the nugget was transformed, which has a softer microstructure than the fusion zone (FZ) and heat-affected zone (HAZ), referred to as the “halo ring”, leading to premature failures in this weakened zone. When welded (AWS recommended single pulse schedule) and tested in tensile shear and cross tension geometries, it was seen that the fracture path in Q&P RSWs occurred through the halo ring. Moreover, the fracture surfaces of such welds show an intergranular fracture, which is contrary to the previous observations.
Based on these preliminary observations, the focus of the present research is to characterize the halo microstructure, find a phenomenological link between thermal history and the halo ring, and solve the discrepancies in failure behaviors by providing robust methods to eliminate halo. Modified welding schedules and post-weld heat treatments were implemented to eliminate the halo ring. The modified double-pulse schedule exhibited improved cross tensile strength (CTS) values by 33%, with an associated 110% increase in absorbed energy than the single-pulse weld. Similarly, the paint baking (PB) process was also implemented in single pulse weld, and it shows 34%, and 102% improvements in CTS and absorbed energy. Both methods improved the mechanical properties by shifting the crack path to the upper-critical heat affect zone (UCHAZ) from the halo ring. The former method modifies the grain structure at the fusion boundary, whereas the latter one redistributes the elements segregated at the grain boundaries to the grain interiors.
The elemental diffusion in the halo ring has been discovered by making spot welds sandwiching low carbon (LC steel weld) and high carbon (HC steel weld) steels, respectively with Q&P steels on both sides, which is to tailor the chemical composition of the FZ. The halo formation is more prominent in welds made with LC steel rather than HC steel. It was found that the transient softened zone can be affected by differences in chemical composition between the FZ and UCHAZ. Furthermore, the mechanism of halo formation has been studied by characterizing the halo microstructure with transmission electron microscopic (TEM) analysis. TEM investigation disclosed that the microstructure within the halo ring is characterized as bainitic ferrite accompanied by needle-like cementite, specifically identified as upper bainite. Carbon diffusion towards the FZ in LC steel weld is attributed to higher activity resulting from differences in the chemical potential of carbon. This also accompanied by varying substitutional solute content towards the FZ from the UCHAZ is the reason for the halo formation. These advances in knowledge facilitated the development of strategies to mitigate halo formation through alloying adjustments (alloying with greater and lesser compositions of steels), paint baking, and welding schedule modifications.
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Keywords
Advanced High Strength Steels, Resistance Spot Welding, Halo ring, Microstructure, Fracture, Paint baking, Double pulsing