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In addition to the melt‐in mode adopted in conventional arc welding processes (such as GTAW), the keyholing mode can also be used in PAW in certain ranges of metal thickness (e.g. 2.5–6.4 mm). With proper combinations of the orifice gas flow, the travel speed, and the welding current, keyholing is possible. Keyholing is a positive indication of full penetration and it allows the use of significantly higher welding speeds than GTAW. For example, it has been reported [7] that PAW took one‐fifth to one‐tenth as long to complete a 2.5‐m‐long weld in 6.4‐mm‐thick 410 stainless steel as GTAW. GTAW requires multiple passes and is limited in welding speed. As shown in Figure 1.17, 304 stainless steel up to 13 mm (1/2 in.) thick can be welded in a single pass [8]. The wine‐cup‐shaped weld is common in keyholing PAW.

Figure 1.17 A plasma arc weld made in 13‐mm‐thick 304 stainless steel with keyholing.

Source: Lesnewich [8]. © Welding Research Council.