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Argon, helium, and their mixtures are used for nonferrous metals as well as stainless and alloy steels. The arc energy is less uniformly dispersed in an Ar arc than in a He arc because of the lower thermal conductivity of Ar. Consequently, the Ar arc plasma has a very high energy core and an outer mantle of lesser thermal energy. This helps produce a stable, axial transfer of metal droplets through an Ar arc plasma. The resultant weld transverse cross section is often characterized by a papillary‐ (nipple‐) type penetration pattern [10] such as that shown in Figure 1.19 (left) and subsequently in Figure 16.3a. With pure He shielding, on the other hand, a broad, parabolic‐type penetration is often observed.

With ferrous metals, however, He shielding may produce spatter and Ar shielding may cause undercutting at the fusion lines. Adding O₂ (e.g. 3%) or CO₂ (e.g. 9%) to Ar can help reduce the problems. Carbon and low‐alloy steels are often welded with CO₂ as the shielding gas (e.g. as shown subsequently in Fig. 5.16b), resulting in higher welding speed, greater penetration, and lower cost. In some applications, a relatively low voltage can be used to maintain a short buried arc to minimize spatter – that is, the electrode tip is actually below the workpiece surface [10].