Mechanisms of Spiking and Humping in Keyhole Welding

The effects of power density, concentration of volatile alloying element (magnesium, Mg), and welding speed on the mechanisms of fusion-zone defects, i.e., spiking and humping (and coarse rippling) during keyhole mode electron-beam welding of Al 6061, Al 5083, and SS 304 are investigated experimentally. Spiking represents a sudden increase in penetration beyond the average penetration line. Rippling exhibits rather regular, arc-shaped topographic features on a solidified surface, whereas humping shows an irregular surface contour consisting of a series of swelled protuberance. These defects seriously reduce the properties and strength of the joint. The quantitative variations of humping, coarse rippling, and spiking defects with the beam-focusing characteristics, volatile element, and scanning speed are quite limited in the literature. The experiments in this paper confirm that average pitch of the humps or coarse ripples is approximately identical to that of the spikes. Thus, the frequency of spiking can be determined from the observation of the weld surface. The results show that the average pitches and amplitudes of humping or coarse rippling and spiking increase with decreasing welding speed and increasing content of volatile element Mg from Al 5083. The frequency and amplitude of spiking, however, are increased by lowering the focal-spot location. The measured amplitude and frequency of spiking and humping and fusion-zone depth are confirmed from good agreement with available scaled analysis. This paper provides quantitative results useful for understanding mechanisms of these defects, so that preventing spiking and humping during keyhole mode welding becomes possible.