Machinery's Handbook, 31st Edition
1598 Gas Tungsten Arc Welding compositions have been designed that utilize other alloys and rare-earth metals. These electrodes are designed for longer lives in both GTAW and plasma welding. Pure Tungsten: Pure tungsten electrode material provides good arc stability with alter nating current (AC). Tungsten has low current capacity and low resistance to electrode contamination. Pure tungsten is good for low-amperage welding of aluminum and magnesium alloys. On medium- to high-current ferrous applications, there is a potential for tungsten inclusions in the weld. With DC, the current capacity of pure tungsten is lower than with the alloyed tungsten electrodes. During AC welding, a molten ball shape forms at the pure tungsten electrode tip, and this formation is desirable for welding aluminum. Table 16. Selection of Gas Tungsten Arc Welding (GTAW) Electrodes Base Metal Electrode Current Recommendations Carbon, low-alloy, stainless, and nickel steels Thoriated DCEN Use EWZr electrodes with AC on thin materials Aluminum Zirconium or pure tungsten AC Use EWZr on critical applications Aluminum Thoriated zirconium DCEP Use EWZr or EWP electrodes with DCEP on thin sections Copper and copper alloys Thoriated DCEN Use EWZr or EWP with AC on thin sections Magnesium Zirconium AC Use DCEP with same electrode on thin sections Titanium Thoriated DCEN … Table 17. Common Tungsten Electrode Compositions Classification Color Tungsten (%) Thorium Oxide (%) Zirconium Oxide (%) EWP Green 99.50 … … EWTh-1 Yellow 98.50 0.8–1.2 … EWTh-2 Red 97.50 1.7–2.2 … EWTh-3 Blue 98.95 0.35–0.55 … EWZr Brown 99.20 … 0.15–0.4 In the classification column, E = electrode; W = tungsten; P = pure; Th = thoriated (thorium oxide); Zr = zirconiated (zirconium oxide). The colors are codes used by manufacturers to identify the material. Tungsten percentages are minimum requirements. The EWTh-3 is also called striped tungsten because it is made with a strip of thoriated material along the length. This electrode needs to be preheated by striking an arc to melt the tip, providing for the thorium and the tungsten to combine before welding is started. The electrode recommendations in Table 16 are a guide to attaining good-quality GTAW welds from the venous types of polarities available. Electrode and Current Selection.— Table 18 and Table 19 show approximate current recommendations for common electrode types and diameters. The GTAW electrode size should be selected so that its midrange current provides the energy required for the intended application. If the electrode is too thin, excess current may be required, causing the electrode to wear too quickly or melt and contaminate the weld. If the electrodes used are found to be constantly at the top end of the current range, a change should be made to the next larger size. Table 20 and Table 21 show recommended sizes of electrodes and filler metal rods or wires for welding various thicknesses of carbon, low-alloy, and stainless steels and aluminum. Table 18. Recommended Current Ranges for Thoriated GTAW Electrodes Electrode Current Range (amps) 1 ∕ 16 in. (1.6 mm) 60–150 3 ∕ 32 in. (2.4 mm) 150–250 1 ∕ 8 in. (3.2 mm) 250–400 5 ∕ 32 in. (4 mm) 400–500
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