Mechanisms of Spiking and Humping in Keyhole Welding

Abstract

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, arcshaped 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 beamfocusing 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 fusionzone 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.

The Physics of Weld Bead Defects

“…Different morphologies therefore can be revealed by simply interchanging the liquid and gas phases. Figure 3(d) shows humping in EBW of Al 5083 [2]. Figure 4 also shows the a "star-like" or finger structure develops as well as the number of rays increases with laser power in etching of regular holes in Mo films with an Ar  laser beam immersed in 2 Cl atmosphere [7].…”

“…Notice tiny little "fingers" in lower rim of fringes and asymmetry in fringe profile taken in downward direction. [2] On the other hand, more complicated humping shows an irregular and unpredictable surface contour consisting of a series of swelled protuberance, as can be seen in Figures 3(a)-(d). Morphologies of humped welds are quite complicated, which were roughly categorized into the gouging region and beaded cylinder morphologies [4].…”

“…This strategy, however, is limited by the appearance of surface defects such as rippling, humping, undercutting, etc [1]. Weld ripples exhibit rather regular, arc-shaped topographic features on a solidified surface, for example, as shown in Figure 1 in EBW of Al 6061 [2]. The ripples slightly elevate above the surface.…”

“…Clear physical concepts associated with quantitative scale analysis are important and beneficial for predicting, controlling and avoiding the occurrence of surface defects [1]. [5], (c) cylinder beads in GMAW [6], and (d) humps in EBW [2]. Surface roughness including rippling, gouging, humping, fingers, etc., therefore can be affected by mechanisms shown in Table 3.…”

Laser-based manufacturing concepts for efficient production of tailor welded sheet metals

“…However, at very high welding speed the keyhole process tends to become unstable. This phenomenon is noticed as a periodic undulation of the weld bead profile, also known as humping [7,8]. The humping phenomenon manifests itself by the repeated formation of metal droplets on top and bottom surface as well as weld segments with severe undercut.…”