The shape of a weld is important because it can influence the soundness or performance of the joint.
If a weld is sectioned transversely and then polished and etched, the cross-sectional shape of the weld can be seen. Measurements of the width of the cap and the depth of penetration of individual weld beads, used to make up the joint, can be made. A shape factor, defined by the width to depth ratio, sometimes known as 'form factor', can be determined. This factor is particularly important in MIG/MAG (GMA) and submerged-arc welding where a ratio of 3:2 is optimal for sound welds. A larger ratio, say 3:1, gives shallow beads which are prone to surface cracking, while a smaller ratio, say 1:2, results in centreline cracking. For other processes, which use key-hole techniques (e.g. electron beam welding), the single pass bead may be narrow and deep but still satisfactory.
Externally, the surface shape can influence both performance and costs. For example, if the weld face has too much excess weld metal, the weld is said to be 'peaked'. This causes a sharp change in section forming notches at the weld toes that act as stress-raisers, promoting early fatigue failure. Excess weld metal also raises the cost of making the weld in terms of both time and consumables, without adding value.
Other weld shape imperfections, such as undercut, root gap and insufficient penetration also influence joint performance.
It is important, therefore, that both the external and internal shape of a weld are controlled.
The required shape and size of a weld is primarily the responsibility of the designer who might be aided and guided by reference to Standards; for example, EN ISO 5817 Arc-welded joints in steel - Guidance on quality levels for imperfections. The welding engineer specifying the welding parameters, the welder and the post-weld inspector also have crucial roles to play in achieving an acceptable weld shape and hence quality.
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