Laser welding has the characteristics of a high energy density, low heat input, and high welding efficiency; however, conventional laser welding has a small focused spot and high requirements for the welding assembly gap. To solve this problem, scholars have developed laser wire filling welding technology. Based on this, some scholars have developed laser hot wire welding technology, which can effectively improve the absorptivity of the welding wire by preheating the welding wire in advance, reduce the requirements for laser power, and improve the welding speed; however, there are still problems such as high requirements for the alignment of the laser focus and the tip of the welding wire, and an uneven weld height. In this study, the process characteristics of Q235 steel by scanning laser hot wire welding are systematically studied, and the mechanism of the influence of the scanning laser on the solidification process of weld metal is clarified, which provides technical guidance for expanding the industrial application of laser welding.

The base material used in this study is the Q235 steel plate. The size is 50 mm×120 mm×2 mm, and the structure is massive ferrite at normal temperature. The flat surfacing welding method is used in the research on the weld surface and section forming. The docking method is adopted in the study of the microstructure and properties of welded joints. According to the previous research and accumulation of this research group, the fixed wire feeding method is front wire feeding, the tilt angle of the welding torch is 45°, and the laser focus is located on the surface of the plate, that is, the defocus quantity is 0 mm. In the welding process, the shielding gas is argon with purity (volume fraction) greater than 99.99%. The gas pipe angle is 60° and the gas flow rate is approximately 20 L/min. In the butt welding experiment, the fixed laser power is 1.8 kW, the welding speed is 1.0 m/min, the preheating current of laser cold wire welding is 0 A, the preheating current of laser hot wire welding is 100 A, the scanning amplitude ranges from 0.6 mm to 1.0 mm, and the scanning frequency ranges from 100 Hz to 200 Hz.

Under different scanning parameters, the distribution of the laser energy is different, which affects the temperature field distribution of the weld pool, and then affects the macro forming, microstructure, and properties of the weld. Compared with that in non-scanning laser hot wire welding, the weld forming in scanning laser hot wire welding is smoother and straighter, and the splash is less (Fig. 5). The weld structure in non-scanning laser hot wire welding is dominated by thick side lath ferrite. Because the scanning laser enhances the flow of the molten pool through the stirring effect and breaks the coarse columnar crystals, the weld structure in scanning laser hot wire welding is dominated by fine crystalline ferrite and acicular ferrite with finer grains (Fig. 7). The tensile strength (578.8 MPa) of the scanning laser hot wire welded joint is basically the same as that (574.7 MPa) of the non- scanning laser hot wire welded joint, but the elongation is increased from 8.4% to 13.1% (Table 3). The dimple size of the tensile fracture surface of the scanning laser hot wire welded joint is more uniform, and the dimple size difference between the laser hot wire welded joint and the laser cold wire welded joint is larger; moreover, there is obvious inclusion precipitation at the bottom of the dimple, indicating that the scanning laser improves the homogeneity of the weld structure (Fig. 10). Simultaneously, the scanning laser improves the gap tolerance during butt welding. In the butt welding experiment of the Q235 steel plate with a thickness of 2 mm, the scanning laser hot wire welding ensures good weld formation without defects when the gap is 1.3 mm (Fig. 12).

In the experiment of scanning laser hot wire welding, by optimizing the process parameters, when the scanning diameter is 0.4?1.0 mm and the scanning frequency is 50?200 Hz, the welds obtained are well formed, smooth, no defects and nearly no splash, which proves that the scanning laser has a good improvement effect on the weld formation. Simultaneously, the scanning laser improves the gap tolerance of laser hot wire welding, which is conducive to achieve stable welding when the gap is uneven and obtain a weld with good fusion with the base metal side wall and no surface collapse. At the microstructure level, the stirring effect of the scanning laser on the weld pool can promote the flow of the weld pool and refine the grain. In terms of mechanical properties, compared with that in non- scanning laser hot wire welding, when the tensile strength is basically unchanged, the fracture elongation increases to 13.1% in scanning laser hot wire welding, indicating that the addition of the scanning laser can effectively improve the toughness of the weld, which is also proved by the deeper dimples in the electron microscope image of the fracture. The hardness of the fusion zone in the laser hot wire welding is the highest, followed by that of the heat affected zone, whereas the hardness of the base metal is the lowest. The hardness of the fusion zone in the scanning laser hot wire welding is lower than that in the non- scanning laser hot wire welding, mainly because the fusion zone in the non- scanning laser hot wire welding is easy to produce segregation, and the generated inclusions increase the microhardness.