American Welding Society (AWS), ASME, and ISO. Mechanics of Plasma Gouging Plasma is essentially a superheated, electrically conductive, ionized gas. First developed in the late 1960s as a cutting technology, plasma has long since gained mainstream acceptance as a viable cutting alternative to oxyfuel, laser, electro-discharge machining, or waterjet. Plasma can be used effectively for a variety of common gouging applications that have traditionally been reserved for technologies such as carbon arc gouging (CAG) or mechanical grinding. At a system level, plasma shares several key features with CAG. Elements such as high-current arc melting and high air flow are common between both CAG and plasma. Figure 4 diagrams a typical high-frequencystart plasma system: Unlike with CAG or grinding, plasma consumables do not directly interact with the workpiece. As a result, there is no contamination of the base material by the consumable material. Speeds are comparable depending on the application. To illustrate the primary difference between cutting and gouging with plasma, the mechanics can be viewed simply. Like other fluid systems, the plasma can be constricted in its crosssectional area to increase its velocity and resultant energy density. Typical velocities in the nozzle bore reach supersonic speeds, though this parameter can vary significantly depending on the bore design. In theory, as the plasma stream becomes more constricted and velocity increases, the higher its cutting capabilities are in terms of material thickness, speed, and kerf minimization. The inverse is true with gouging. Less constriction will result in less air velocity but a higher volumetric flow rate. This will limit the cutting action. Figures 5 and 6 illustrate a simplified cold flow computational fluid dynamics analysis showing the differences in the velocity profiles of the respective nozzle bores. In gouging, a diffuse plasma stream coupled with a high shield gas flow rate provides the correct combination of elements to produce partial 40 WELDING JOURNAL / DECEMBER 2016 Fig. 4 — Plasma gouging system diagram. Fig. 5 — Typical plasma cutting nozzle bore. Fig. 6 — Typical plasma gouging nozzle bore. Fig. 7 — Cold flow computational fluid dynamics analysis of a plasma cutting nozzle.
Welding Journal | December 2016
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