레이저 용접현상의 유동가시화에 의한 고찰

Abstract

The main purpose of this study was to achieve optimum bead welding conditions when weld the STS304 stainless steel sheet with the pulse- Nd:YAG laser. The conducting characteristics of plasma/plume using the schlieren system and a high speed camera -a flow visualization technique- were observed and analyzed.

The laser output characteristics of the STS304 stainless steel sheet were investigated from multiple viewpoints as follows:

1) the relation of laser output energy and peak power on pulse width,

2) the relation of laser output energy and peak power on lamp voltage,

3) the relation of welding bead width and penetration depth on defocused distance.

Afterwards, the effects of the shield gases while laser welding under actual conditions were closely analyzed: in air, static, and dynamic shield gas atmosphere.

In air, the occurrence of many spatters was observed, even in the low-end laser output condition. But, in a shield gas atmosphere, the spatters rarely occurred, even in the high-end laser output condition, and the plasma stabilized quickly. The laser outputs in a static shield gas atmosphere, were measured under three voltages (243V, 250V, 280V)

the occurrences under actual conditions of the plasma/plume were compared and analyzed.

Measuring for various types of shield gas atmospheres (nitrogen, argon, and helium) revealed a gradual weakening in plasma intensity from argon to nitrogen to helium. The plasma volume gradually decreased, and the gas flow was virtually undetectable in the helium shield gas atmosphere.

Afterwards, the occurrences of spatter and plasma phenomena with nozzle diameters of 2, 4, 6, 8, and 10 mm, at the distances of 13, 16, 19, and 21 mm from the end of nozzle to the welding point, at nozzle angles of 0, 20, 40, and 60 degrees, and shield gas flow-rates of 5, 15, and 25 liters/min were observed and analyzed.

The optimum laser welding condition, using a nozzle in a dynamic shield gas atmosphere, was determined by the nozzle diameter, the distance from the nozzle end to the welding point, the nozzle angle, and the flow rate of the shield gas.

Using the STS304 stainless steel sheet as a test material, the optimum bead welding condition with the pulse-Nd:YAG laser was confirmed to be a nozzle diameter of 6 mm, at the distance of 13 mm from the nozzle end to the welding point, at a nozzle angle of 40 degrees, and at a shield gas flow-rate of 25 liters/min.