Effect of defocus amount on microstructure and properties of laser welded joints of Q235/304 dissimilar steels

To optimize the welding process for dissimilar metals, this study explores the influence of defocus amount on the microstructure and properties of laser-welded joints between Q235 and 304 stainless steel. Welding experiments were performed using an EFE-LWM-1500 laser welding machine at a fixed laser power of 1200 W with five defocus amounts: –10 mm, –5 mm, 0 mm, +5 mm, and +10 mm. The welded joints were characterized by optical microscopy, X-ray diffraction (XRD), universal tensile testing, scanning electron microscopy (SEM), and electrochemical workstation to analyze the microstructure, phase composition, tensile strength, fracture morphology, and corrosion resistance. Metallographic analysis shows that the 304 stainless steel side is dominated by austenite, the weld center exhibits lath martensite, and the heat-affected zone of Q235 carbon steel contains Widmanstatten structure, pearlite, and acicular ferrite. The weld grains are most uniform and fine at 0 mm defocus. XRD confirms the presence of Fe-Ni solid solution in the joint. Mechanical tests reveal that the average microhardness of the weld reaches a peak of 373.05 HV and the tensile strength reaches a maximum of 309.46 MPa at 0 mm defocus. Electrochemical tests show that the self-corrosion current density is as low as 2.24×10 -5 A/cm 2 and the capacitive arc radius is the largest under this condition, indicating the best corrosion resistance. SEM analysis of the fracture surface shows typical necking and dimpled structures, confirming ductile fracture. The defocus amount significantly regulates the properties of Q235/304 dissimilar steel laser-welded joints. A defocus amount of 0 mm achieves the optimal microstructure and comprehensive properties, providing an important reference for optimizing laser welding parameters of dissimilar metals.