Soap film analogue of a weld pool
surface subjected to significant arc (or beam) pressures. (Click image to enlarge) |
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The natural state of a normal weld pool would be flat, like any puddle of liquid. During high power welding processes the puddle is subjected to increasing amounts of pressure and is partially displaced. This is a reason for the increased penetration of processes such as laser, electron beam, keyhole plasma and high current TIG. This process can be likened to inflating a soap bubble as in Figure 1. |
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Close up of the high current GTAWprocess showing the crater formed as a resultof the strong arc forces. (Click image to enlarge)
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Intuitively we see that this structure is unstable –surface tension is working against the process. For example, the bubble shown is not symmetric, and tends to ‘wobble about’. This behaviour is evident in the close-up of a high current TIG weld pool in Figure 2. Such instability leads to problems including variable penetration and porosity. |
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Soap film analogue of a GTAW keyhole, illustrating the stabilising effect ofsurface tension when the keyhole surface isanchored to both front and root faces.(Click image to enlarge) |
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In keyhole TIG the pool displacement is encouraged to the point that a hole forms through the plate. Under appropriate conditions the weld pool surface becomes anchored to both top and bottom faces of the weldment. This structure is illustrated in Figure 3. |
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End-on view of a keyhole GTA weld on 8mm wall thickness AISI 304 (stainlesssteel) pipe.
(Click image to enlarge) |
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Unlike the former case, this structure is very stable, and conveys many advantages for welding quality and productivity. This is the pool structure developed in keyhole TIG (Figure 4) |
