MECHANISM OF RIPPLE FORMATION DURING WELD SOLIDIFICATION

The formation of ripples on the surfaces of GTA spot and seam weld surfaces in thin metal sheet was investigated by high speed motion pictures. The ripples are observed to form solely due to oscillation of the weld pool during solidification; no other mechanism for ripple formation was found. Weld pools were melted through thin metal sheets while a high speed camera observed the melting and solidification events on the bottom surface of the pool opposite the arc. When welding power is supplied from storage batteries, the weld pool surface shows virtually no oscillation while the arc is on. Shutting off of this arc suddenly releases the plasma pressure which was stretching the pool surfaces, setting the pool into oscillation like a struck drumskin. Solidification during this oscillation results in rippled surfaces. When a single phase full-wave rectified conventional dc welding power supply is used, the pool surface is observed to oscillate at 120 hertz. When this arc is shut off, the pool changes oscillation frequency in a fraction of a second from the frequency imposed by the pulsating plasma pressure to its own natural frequency. Pool oscillation periods after the arc is shut off are measured on the film strips. If the pool is considered to be a stretched membrane with surface tension providing the stretching force, a theoretical surface tension can be calculated from the oscillation period, pool mass and pool geometry. Agreement of the calculated surface tension values with published surface tension values for several metals demonstrates that the stretched membrane model adequately describes the pool oscillations. By counting the number of pool oscillations in the films and counting the number of ripples on solidified spot welds afterwards, a one-to-one correlation is established between ripples and pool oscillations. When a seam weld is made using storage batteries as the welding random disturbances of the pool surface are observed. Each disturbance leaves a few ripples before it damps out.