melting and effective subsequent clearing of slag. To accomplish this, the plasma gouging nozzle bore diameter will be greater than two times the diameter of a cutting nozzle bore, while much shorter in length. Additionally, there will be a sizable counterbore in the nozzle to encourage relaxation of the flow. Lastly, a shielding gas at high flow rates is necessary to evacuate the molten material to reveal the gouge. In Figs. 7 and 8, the same input and output parameters were used for each simulation (pressures, temperatures, densities). While not reflective of a true plasma stream profile (i.e., a helical vector field in the nozzle cavity), the contrast is obvious. Note that the difference in respective velocities is roughly 40%. Process Variables Many process variables influence the features and characteristics of the gouge. Naturally, these relationships play a large role in understanding how to match plasma gouge features to applications. Note that many of these relationships are inversely proportional. Table 1 relates general input process parameters with gouge output features: A key influencing factor on gouge shape is system amperage. As the amperage increases, so will the gouge depth in a proportional fashion. In one example where the amperage was between 35 and 50 A, a fixed gouge width of ½-in. was created at a static speed, while the amperage increased by steps to influence the depth of the gouge. Another key process parameter that influences gouge shape is the angle of the torch relative to the workpiece. While a primary angle is used to gain a basic gouge shape, a second angle can be introduced to increase the gouge width. This is known as offset angle gouging. In this case, the torch is rotated about the “z” axis such that it is positioned at an offset angle relative to the vertical plane of the gouge centerline. When = 0, this is referred to as straight-line gouging. As increases, the gouge widens. However, as approaches 90 deg, the gouge profile tends to become an asymmetric shape. Furthermore, there will be a pronounced tangent surface feature on the near side of the gouge. This feature will exhibit a flat face as DECEMBER 2016 / WELDING JOURNAL 41 Fig. 8 — Cold flow computational fluid dynamics analysis of a gouging nozzle bore. Table 1 — Process Input/Output Parameters Inputs Outputs Width Depth Groove Tangent Surface Transfer Arc Angle Surface Area Texture Height Stretch Amperage Decreases Increases Increases n/a n/a Increases Increases Increases Linear Travel n/a Decreases n/a n/a n/a n/a n/a Speed Increases TWPD Increases Increases Decreases Decreases n/a n/a n/a n/a Primary Torch Decreases Increases Increases n/a n/a n/a n/a Ang. Increases Secondary Torch Increases n/a Decreases Increases n/a n/a n/a Ang. Increases Nozzle Bore Increases Decreases n/a n/a n/a Decreases n/a Diameter Increases
Welding Journal | December 2016
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