Thermal Science of Weld Bead Defects: A Review

Abstract

Mechanisms for the formation of bead defects, such as humping, gouging, rippling, and other unexpected surface patterns, encountered in welding or drilling are interpreted and reviewed from thermal-fluid science viewpoint. These defects usually accompanying with porosity, undercut, segregation, stress concentration, etc., seriously reduce the properties and strength of the joint or solidification. Even though different mechanisms for formation of the defects have been extensively proposed in the past, more systematical understanding of pattern formations from thermal, fluid, physics, electromagnetic, pattern selections, and metallurgy sciences is still limited. The effects of working parameters and properties on humping and rippling, for example, can be systematically and quantitatively interpreted from scale analysis presented in this work. Good comparison with experimental results reveals mechanisms of different surface patterns. The mechanistic findings for bead defects are also useful for other manufacturing and materials processing.

The Physics of Weld Bead Defects

“…In this work, it is proposed that a free surface heated from above and a negative surface tension coefficient may still cause instability, as illustrated in figure 14(b) [1]. Provided that the surface is strongly deformed toward the bottom, significant heat conduction is transport from the interior to free surface.…”

“…As illustrated in figure 15, an increase in liquid pressure due to a decreased surface flow from the central to rear edge of the pool give rise to deformation of the free surface near the solidification front. Subtracting Young-Laplace equation at two locations which are away and near the edge of the pool surface, and introducing the pressure difference between two locations from Bernoulli's equation,, the amplitude of ripples can be found to be [1,20] …”

“…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 aims of this work are to provide rigorous and pictorial interpretation based on thermal science concepts, and clarify and propose some physics involved in the mechanisms of weld bead formation. 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].…”

Mechanisms of Spiking and Humping in Keyhole Welding

“…Humping is also a common defect in arc-welding processes [15]- [19]. Spiking represents unexpected increase in the local penetration in periodic successions.…”