Electromagnetic force near the anode boundary and double convection in a TIG arc weldpool

Summary

This paper describes observations by video camera of weldpool phenomena during He‐TIG arc welding. The results of observing the distributions of the current density and electromagnetic force near the anode area have identified the mechanism of interaction between the anode area, electromagnetic force, and convection. The results obtained may be summarised as follows.

1. During He‐TIG arc welding, the centre of the weldpool is characterised by a brighter, red‐tinged area being visible at its boundary. From the behaviour of the slag floating at this boundary, this area is named the inner zone, being the radial flow zone.

2. The electromagnetic force distribution is determined on the assumption of a uniform current density on the anode area and on a cylindrical surface (thin plate) or hemispherical surface (thick plate) having the same radius R as the former. The absolute value of the electromagnetic force is maximum at the anode boundary and shows a singularity with an abrupt 90° direction change.

3. On the assumption of Sozou and Pickering that the current flows from an infinitely remote location in another direction across the boundary path, the current is concentrated near the boundary, and the singularity becomes more pronounced. In the disc‐like and ring‐like inner zone, the current density appears to be uniform or concentrated.

4. If the inner zone coincides with the anode area:

   1. At the centre, the centripetal electromagnetic force of the anode area is zero, and there is no obstruction of the surface flow.

   2. Nearer the boundary, the centripetal electromagnetic force intensifies, the radial flow is retarded, and the pressure is raised.

   3. The downward electromagnetic force concentrated at the anode boundary becomes the downward driving force.

   4. The liquid in the low‐temperature zone outside the boundary is adsorbed, with an inward directed flow being generated by desorption.

   5. Factors 1–4 jointly drive an internal‐radial and external‐axial type of double flow.

   6. The penetration change can be explained by the inner zone size.

5. The paper also includes a discussion of the surface flow driving force and arc current concentration on the basis of the foregoing results.