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    Microstructural and mechanical characterization of fiber laser welded quench-partitioning steels
    (Walter De Gruyter Gmbh, 2025) Celik, Hafize; Saray, Onur
    Advanced high-strength steels (AHSS) are increasingly used in the automotive industry for lightweight components due to their superior mechanical properties. Quench and partitioning (QP) steels provide an optimal balance between strength and formability, but their susceptibility to liquid metal embrittlement (LME) during resistance spot welding presents challenges. Laser welding, with its low heat input and high efficiency, offers a promising solution for reducing LME risks while ensuring strong, reliable joints for automotive applications. This study investigates the microstructural changes and mechanical performance of laser-welded joints between QP and dual phase (DP) steels. The fusion zone (FZ) and supercritical heat-affected zone (HAZ) primarily exhibited martensitic microstructures, while the midcritical and subcritical HAZ contained tempered martensite and ferrite on the DP side and a combination of tempered martensite, ferrite, and retained austenite on the QP side. These microstructural transformations contributed to enhanced FZ and HAZ regions, resulting in defect-free welds. Fractures occurred within the softer base metal (BM) regions, exhibiting ductile fracture characteristics without significant strength loss. However, joint ductility was slightly reduced compared to BMs due to strain localization caused by microstructure and thickness variations. The results demonstrate that laser welding is an effective method for joining QP steels in automotive manufacturing.
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    Microstructure and Mechanical Behaviors of Fiber-Laser-Welded QP980-QP1180 Steels
    (Mdpi, 2025) Celik, Hafize; Saray, Onur
    Advanced high-strength steels are considered the first choice when manufacturing lighter vehicles. Quench-partitioning (QP) steels are good candidates that fulfill manufacturing and performance requirements with their outstanding strength and formability. Laser welding offers a productive solution to the challenges of liquid metal embrittlement due to a low heat input and higher welding efficiency. This study investigated the microstructural evolution and mechanical performance of dissimilar laser-welded joints between QP980 and QP1180 steels. The microstructure of the joint mainly consisted of martensite phase in the fusion zone (FZ) and super-critical heat-affected zone (HAZ). In the mid and sub-critical HAZ, the microstructure consisted of tempered martensite along with ferrite and retained austenite on both sides. Due to these microstructural evolutions, FZ and HAZ are strengthened, and thus, laser welds can be achieved without the formation of a visible soft zone. Fracture of the joints occurred in softer base metal (BM) with ductile characteristics without any considerable strength loss. However, the ductility of the joints was lower than that of BMs because of deformation localization due to microstructure, yield strength, and thickness variations in the tensile and Erichsen test specimens. These results show that laser welding can be considered an effective alternative for joining QP steels.