In recent years, the automotive industry has promoted thin sheets for the purpose of improving long-term quality and lightweighting, and the galvanizing rate of galvanized steel sheets has continued to increase. However, compared with ordinary steel plates, the weldability of galvanized steel plates is very poor: first, pore defects caused by zinc evaporation during welding (including pits that appear on the surface of the weld bead and internal pores remaining in the weld bead); second, the zinc The steam blows the molten droplets and molten pool, causing excessive spatter (Figure 1). In response to these problems, many improvements in welding materials or power sources have been made in the past, but it is far from being able to solve the problems of porosity resistance and spatter at the same time. Particularly, the pore defects are speculated to be caused by the intrusion of zinc vapor generated from the electroplated layer of the overlapping portion of the steel plate into the molten pool, but the mechanism of its occurrence is not yet clear.
Mechanism of pore defect generation
1.1 Experimental methods
The base material is an alloyed hot-dip galvanized steel plate with a thickness of 2.3mm (galvanizing amount 45 g/m2). Spot welding is used to ensure close contact between the overlapping parts of the steel plates to prevent zinc vapor from being discharged from the gaps between the steel plates. MAG welding was performed using the specifications shown in Table 1, and the impact of the welding position on pits and pores was analyzed. A high-speed camera with a frame rate of 6000 fps was used to observe the surface of the molten pool, and a high-brightness X-ray imager with a frame rate of 500 fps (equipment of the Osaka University Joint Science Institute) was used to dynamically observe the formation of pores inside the molten pool.
1.2 Observation of pore defects
It is known from experience that pore defects are greatly affected by the welding position. The number of pits and internal pores during flat welding and downhill welding increased significantly during downhill welding. A high-speed camera observes the surface state of the molten pool. For downslope welding, when the zinc vapor generated by the evaporation of the galvanized layer escapes through the interior of the molten pool behind the arc, there will be a large number of pits and internal pores; for flat welding, when the zinc vapor escapes directly below the arc, it is not only difficult to have concavities. pits, and the number of internal pores will also be reduced, and this tendency is not affected by changes in welding current and voltage. The characteristic of downhill welding is the position where the molten pool meets the arc. The results of this experiment show that the subsidence phenomenon directly under the arc has a great relationship with the formation of pore defects.
