狭開先MAG溶接における溶込形状の観察

抄録

In this paper, the cross-sectional shape of I-groove weld beads in DC electrode negative and in DC electrode positive (shielding gas composition: 30%Ar+70%CO₂) MAG welding were studied, and the penetration angle (θ) of the side wall, the contact angle (α) of beads and the wetting height (h) were measured.
Special attention was focussed on clarifying the cause of undercut by direct observation of the flow of molten metal using high speed photography.
The results are as follows:
1) Mechanism of origination of under-cut
Dry under-cut
The arc moves around in I-groove rapidly and gouges the side walls of the base metal above the molten pool, exposing fresh solid surface. Moreover, owing to the mild flow of molten metal on the side walls, the solid surface exposed by the arc is not covered by molten metal, and under-cut of toe originates.
Wet under-cut
The arc ignites towards the groove bottom mainly, and gouges the side walls of the base metal below the surface of the molten pool. Moreover, the flow of molten metal on the side walls is strong and melts the side walls above the molten pool, covering the lower portion of the side walls already gouged by the arc. But on account of the side walls being melted too high, the molten metal on the side wall flows down and under-cut of toe originates.
2) Cross-sectional shape of beads
In cases where under-cut does not originate, (α+θ) becomes, with a little dispersion, a constant value unrelated to welding conditions and h becomes higher with increasing θ.
The bead surface shape has been discussed using surface tensional theory applied to the two dimensional model.