Welding formation, microstructure and mechanical property evolutions for the laser-CMT hybrid welded joints under various welding speeds

The effect of welding speed on the microstructure and mechanical properties of laser-CMT hybrid welded joints of 6 mm thick low carbon steel (Q235) was investigated in this research. The weld zone (WZ) primarily comprised proeutectoid ferrite, side-plate ferrite, and acicular ferrite. For heat-affected zone (HAZ), the microstructure consisted of ferrite, pearlite, and widmanstatten. As the welding speed increased from 1.2 m/min to 2.1 m/min, the following trends were observed: weld appearance initially stabilized but deteriorated at higher speeds; bainite microstructure was found in WZ; the average grain size decreased from 2.47 to 1.35 μm; the proportion of high angle grain boundaries was increased from 46.5% to 69.4%; the average value of KAM increased from 0.48° to 1.17°. This phenomenon is due to the increase in cooling rate and the reduction in heat input. Besides, the tensile strength of welded joints showed a trend of initial increase and subsequent decrease, while the microhardness of the WZ showed a gradual increase. The maximum average hardness was achieved at a welding speed of 2.1 m/min; however, a significant deterioration in toughness was observed at this speed. Within the range of welding speeds tested, the most favorable welding speed was 1.5 m/min, with the tensile strength of welded joint reaching 717 MPa ±11 MPa, elongation reaching 12.5% ± 0.4%and average microhardness of the WZ reaching 197 HV, which was much higher than the base metal.