Formation mechanism of porosity in high power YAG laser welding

With the objectives of clarifying the formation mechanism of porosity and producing a sound weld bead, welding conditions of porosity formation were investigated in A5083 alloy and Type 304 steel welded with a high power YAG laser, and the behavior of a keyhole, bubbles and porosity as well as liquid flows were observed during laser welding through X-ray transmission imaging system using markers. It was confirmed that a lot of bubbles and pores were formed in 3.5 kW YAG laser weld beads produced in Ar, He and N₂ gases except Type 304 in N₂ gas. Porosity was reduced at high welding speed in Type 304 steel even in He and Ar gases. A lot of bubbles were formed by the evaporation of metals from the bottom tip of the keyhole and flowed upwards in front of the solid-liquid interface. Some bubbles disappeared out of the molten surface especially in A5083 alloy welded at low welding speed, but the majority of bubbles were trapped at the solidifying front of the weld beads in most cases. The shielding gas was also included in the porosity. This mechanism is similar to that in high power CO₂ laser welding. Fast liquid flows occurred circularly from the bottom keyhole to the rear upper part of the molten pool, from the rear to the front near the pool surface, and from the top to the bottom behind the keyhole in weld molten pools of both A5083 alloy and Type 304 steel in He, Ar or N₂ shielding gas. Slightly different flows were noticed in the molten pool of Type 304 steel between YAG and CO₂ lasers.