Microstructural Evolution and Tensile Strength of Laser-Welded Butt Joints of Ultra-High Strength Steels: Low and High A

    
    Ultra-high strength steel (UHSS) grades are becoming the preferred materials of choice in structural and transportation applications due to their outstanding properties such as excellent strength to weight ratio and higher ductility. The frequent welded joining of sheets and plates of UHSS is crucial to create integral structures with maximum mechanical performance. Therefore, it is essential to understand the microstructural evolution and welding-induced residual stresses in order to ensure satisfactory performance of the welded butt joint specimens.
    
    The mechanical behavior of the welded joints is influenced by the microstructural evolution of the weld metal, heat affected zone and the adjacent base metals. In laser welding, the rapid heating and cooling rates enable fine-grained microstructures with higher strength and toughness. On the other hand, the weld process will induce residual stresses, which can deteriorate the overall mechanical properties at room temperature.
    
    This microstructural study shows the development of microstructures in the laser-welded butt joints of UHSS materials. It focuses on the detailed microstructural and mechanical characteristics—including the weld metal, HAZ and joint strength—of two commercially available UHSS materials: low (X80) and high (X95) alloy steels.
    
    The welding experiments were performed using a 3 kW fiber laser system at various laser powers and welding speeds. Samples with a thickness of 15 mm were welded in butt joint configuration with a laser beam perpendicular to the weld direction. The microstructural analysis was performed using scanning electron microscopy (SEM).
    
    This study presents the examination of the weld metal and heat-affected zones (HAZ) of the welded specimens. The macrostructure and fine-grained microstructures of the HAZ and weld metal of low and high alloyed steels were studied. The welds for low and high alloyed steel had a distinct microstructure with finer grains in the weld metal. The microstructures in the HAZ were a combination of grain-refined ferrite, bainite and martensite depending on the alloy content and welding conditions.
    
    The tensile strength of welded samples was found to be strongly dependent on the microstructure, heat input, and HAZ width. It was observed that both low and high alloyed steel specimens showed higher tensile strength when welding at a low welding power and speed. The greater weld joint strength was achieved by the finer grain sizes, reduced porosity, and more uniform microstructural characteristics in the weld metal and HAZ.
    
    Overall, this study has presented useful insight into the microstructural evolution and mechanical behavior of laser-welded butt joint specimens of UHSS materials. The results suggest that the successful fabrication of UHSS welded joints with improved properties requires a careful selection of welding parameters and suitable alloy content. This research will be important for the further development of UHSS welds.

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