Laser are now established as industrial tool that cut, weld, drill and modify the surface properties of materials. Laser-processes offer many advantages over conventional processes e.g. speed, quality, and increased production. The use of zinc-coated steel in automobile industries as well as in various industry fields has increased due to low price and high corrosion resistance.
Recently the laser welding technology has been considered the application for shipbuilding structure. However, when this technology is applied to primer-coated steel, good quality weld beads are not easily obtained. Because that the primer-coated layer caused the spatter, humping bead and porosity, that are main part of the weld defects, attributed to the powerfully vaporizing pressure of zinc.
We were performed experiment with objectives of understanding spatter and porosity formation mechanism and producing sound weld beads in 6mmt primer coated steels by a CO2 CW laser. The effects of welding parameters were investigated in the bead shape and penetration depth in bead and lap welding.
Laser welding of zinc-coated steel in lap joint leads to a problem that is related to the low boiling point of zinc(907 oC) compared with high melting point of steel(1530 oC). When the keyhole penetrates at the interface between the two sheets in lap joint, the layer rises to a very high temperature. At this temperature, the zinc vapour pressure is so high that it explodes out of the melt pool and generated the spatter and humping bead. Namely serious effects of primer-coated position was lap side rather than surface.
Therefore, introducing a small gap clearance in the lap position, the zinc vapor could escape through it and sound weld beads can be acquired.
And mechanism of porosity formation has searched by analysis the composition of inner surface of porosity and the vaporizing particle according to weld conditions. There is a lot of zinc in inclusion of inner surface of porosity. But there is not zinc in dimple structure. These results are indicated that the porosity was formed by zinc vaporized pressure.
Then, the dynamic behaviors of laser-induced plasma and spatter phenomena were photographed by high speed video camera during CO2 CW laser welding with gap or no-gap clearance. Laser-induced plasma has been accompany with the vaporizing pressure of zinc ejecting from keyhole to surface of primer coated plate. In result, the weld defects such as spatter, porosity and humping bead have formed. This dynamic behavior of plasma was very unstable and this instability was closely related to the unstable motion of keyhole during laser welding. Therefore, as observing the behavior of plasma, it should be estimated how the keyhole was fluctuated and the weld defect was formed. defocused distance, welding speed, coated thickness, coated position