Abstract
The effect of an automotive paint bake (PB) thermal cycle on the microstructural evolution and the mechanical properties of resistance spot welded advanced high strength steel is presented in this work. Mechanical behavior of the heat-treated welds reveals an increase in maximum cross-tension strength, displacement and subsequently energy absorption capability when 453 K (180 °C)-20 minutes a bake thermal cycle is applied after welding. The microstructures of resistance spot welds with and without a PB heat treatment were characterized using scanning and transmission electron microscopy (TEM). TEM analysis revealed that the weld nugget and HAZ of the resistance spot welds consist of a martensitic microstructure. The microstructural analysis of the post-weld heat-treated samples shows the presence of ε carbides in a martensitic matrix within the weld nugget and the HAZ. It is shown that the improved mechanical response of the paint-baked welds is associated with carbide precipitation during heat treatment.
Highlights
THE use of advanced high strength steels (AHSSs) is increasingly preferred in the automotive industry as a result of an enhanced strength–ductility combination and superior crashworthiness compared with conventional high strength steels
Carbides were found in the weld microstructures of the low-temperature post-weld heat-treated samples (single pulse-paint bake (PB) and double pulse-PB, Figures 2(c) through (f)); the scanning electron microscopy (SEM) spatial resolution is not sufficient to confirm the type of carbides
We describe the effect of the PB thermal cycle on the microstructural features of the single and double pulse welds
Summary
THE use of advanced high strength steels (AHSSs) is increasingly preferred in the automotive industry as a result of an enhanced strength–ductility combination and superior crashworthiness compared with conventional high strength steels. These properties rely on advanced multiphase microstructures, based on selectively designed alloying concepts and thermal–mechanical process cycles. The higher alloying compositions required to achieve the desired mechanical properties can have a negative effect on the weldability of these steels. The most widely used joining method in automotive applications is resistance spot welding (RSW) due to its high speed, robustness, and low-cost.[1,2,3] The high.
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