Using theoretical analysis, experiments and numerical modelling, a mechanism of penetration of a doubled-sided TIG welding has been investigated. The results show that the welding current passes directly through the zone of welding, leads to a concentrated distribution of current, forms the stronger electromagnetic field and causes the stronger fluidity in the weld pool that contributes to penetration. In the process of TIG double-sided welding a zone of heat accumulation is formed at a partial penetration of weld between lower parts of two weld pools, where temperature can be quickly increased even at a small amount of heat supplied. In the lower weld pool, the forcing out force creates the flow directed inside, which can displace the hot fluid from the weld pool surface to its lower part, thus forming a deep weld pool.
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