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Plasma arc cutting has come a long way since it emerged as large, expensive machines in the 1950s, then as new technology that attempted to make cutting metal more efficient than the oxyfuel torch.
Back then, it really wasn't. But times have changed, thanks in large part to developments around two elements: the electrode and the plasma arc itself.
EARLY DEVELOPMENTS
In the early days, the plasma-cutting systems were quite simple, and, as with any new technology, problems abounded. The tungsten electrode and nozzle had to survive long enough and attain a quality cut at a reasonable speed so more people could actually use it in the market.
Gas remained the crux of the problem; the technology could not use oxidizing gases like air or oxygen. To blame was the wear mechanics behind the tungsten electrode. Also to blame was the nozzle orifice, which wasn't small enough to sufficiently constrict the plasma to emit an arc with higher energy density that could cut metal better and faster.
The ability to use air and, later, oxygen came about through the use of better water-cooling systems, encasing the electrode in a copper holder and, most significantly, moving away from tungsten and toward novel materials—including zirconium and hafnium—as the electrode materials of choice. This made the electrode last longer and, hence, the process more efficient and cost-effective.
EXTENDING ELECTRODE LIFE
What made tungsten such a problem? Consider this: When running with oxidizing gas like air or oxygen, the tungsten forms an oxide layer, as does zirconium and hafnium. But during cutting, tungsten's oxide-layer is volatile at temperatures lower than the operating temperature of the solid tungsten. Because the tungsten remains at a higher temperature, the tungsten oxide vaporizes readily. This leaves the pure tungsten underneath, which again forms an oxide layer, which vaporizes, and so on—literally eroding the electrode and, ultimately, limiting its life-span. The cycle happens in seconds if running with pure oxygen as the cutting gas, and after enough time the torch fails.
Hafnium's temperature during plasma cutting, however, remains significantly lower, since hafnium melts at much lower temperature compared to tungsten. In fact, hafnium forms a molten puddle when the arc is in operation. The wear mechanisms of hafnium are directly related to its molten state. Imagine a spinning bucket of water; spin it fast enough and some water will spill over. This represents, basically, what happens with the liquid hafnium. Control of the gas around the plasma arc and the current minimizes "spilling" some of that liquid. This electrode wear phenomenon occurs when the arc is turned off. With every "spill" comes a reduction in electrode life. The fewer spills of that liquid, the longer electrode life can be.
Moreover, the industry also started to use different electrode-holder material. Where in the past the technology used copper, more recently the industry has used a silver enclosure, which disperses heat much faster than copper, quickly taking away heat from the hafnium. Because this removes heat, it also helps push electrode life even further.
Along with these improvements, engineers began looking at how to ignite the plasma arc itself, smoothly ramping up the current and gas in a coordinated manner to minimize thermal shock. This is much like slowly opening the floodgates in a dam instead of waiting for the water to blow the whole dam apart. This is far superior to, say, putting a thousand amps immediately through a cold electrode with layers of oxide and other compounds left over from the previous cut. Instead of this layer melting slowly, the high amperage causes cracking and disintegration of that layer immediately—again leading to premature electrode wear.
The "ramp up" also means the electrode is started in a different environment, with an optimum amount of gas. The arc should never be ignited in pure oxygen, because oxygen is (naturally) quite oxidizing, creating an excessively thick oxide layer on the electrode that "eats into" the electrode body and, again, causes premature wear. So to start, most use less oxidizing gases like nitrogen or air.
WHAT MAKES A HIGH-PRECISION CUT?
What does "high precision" plasma mean? As the science of plasma evolves, the definition changes; what was "precision" decades ago wouldn't be called the same today. But quality and speed do offer some objective benchmarks.
Quality in plasma cutting entails several aspects. For one, a system should create cuts that are as square as possible, with a minimal bevel angle. What creates this bevel angle is, in part, how the plasma forming gas swirls around the arc. When cutting, a user can see one side that has a very slight bevel angle, 0 to 2 degrees, while the other side has a higher angle. This means that, when precision-cutting, the operator must note the cut direction to ensure the quality cut ends up on the part edge; most nesting programs account for this.
The kerf represents another cut-quality aspect. The top and bottom of the kerf face should be very well-defined, a sharp edge without any rounding nor dross, and a smooth face.
A "well-defined" arc describes, primarily, the diameter of the plasma arc as it leaves the torch. The smaller the diameter, the higher the energy density, which, in turn, allows faster cutting. The smaller diameter also creates a smaller kerf, a characteristic well-appreciated in metal fabrication, and therefore a smaller heat-affected zone. The arc must also be very stable, without fluctuations that could create roughness on the cut surface.
WHAT MAKES A HIGH-PRECISION PLASMA TORCH?
To create these arc characteristics requires careful design inside the torch using a mix of electrical engineering, thermal and fluid dynamics. For one, it requires tight control over the cutting gas swirling around the arc. In basic terms, the cutting gas creates a "tornado" inside the arc chamber. The better this tornado—meaning lower pressure inside and higher pressure on the outside—the better the gas will help concentrate and constrict the arc to a narrow column.
Exactly how this gas "tornado" flows can make the difference between high-precision and conventional cutting. This tornado is governed by the interplay of gas pressure, current and the geometry of the chamber, formed by the electrode geometry relative to the nozzle's inside surface geometry. Optimizing that geometry can improve cut quality greatly.
The nozzle orifice represents another important part of the equation. It could be seen as the "lens" that helps focus the plasma onto the work. The nozzle shape determines the "focusing length," or torch height above the work. The higher the torch can be without disturbing the arc the better, since a higher torch height means less thermal stress is put into the torch. Depending on the nozzle design, the material, thickness, speed and other application-specific variables, torch height can be between 0.050 and 0.300 inch. Thinner plate requires shorter torch heights than thicker plates.
Once the plasma exits the nozzle, the shielding gas enters the equation. This gas can be "tuned," through the way it is injected and its chemistry, so that the plasma arc experiences minimal disturbance from the atmosphere. The shielding gas also helps guide the plasma as directly as possible toward the workpiece surface. When applied in the right way, the shielding gas can increase momentum so that the arc penetrates through and removes material for a clean, dross-free cut.
As the arc comes out of the nozzle, it wants to spread. Consider top-edge rounding within the cut, which can be common in plasma cutting. This occurs from a spread of the arc on top of the plate, from an "unfocused" plasma arc jet due to shielding gas that doesn't effectively guide it into the metal (among other factors).
Shielding is all about buffering. The arc is very hot, the surrounding atmosphere relatively cold, and excessive mixing between the two can drastically cool the arc and significantly reduce energy density. Tuning the shielding gas, the buffer between the arc and atmosphere, involves careful control over the gas-flow properties—somewhere in between the high-speed, hot plasma plume and the low-speed, relatively cold ambient atmosphere. Whenever any gas is introduced around the plasma, the arc will be disturbed to a point. The key is to minimize that disturbance as much as physically possible.
Water has also helped further define the arc. For some aluminum and stainless-steel applications, for example, a nitrogen cutting gas may be used with a water shielding. As an analogy, think about a hot plate with a droplet of water on it. The droplet bubbles and moves but does not immediately evaporate. This happens because between the liquid and hot plate exists a film of water vapor that insulates the droplet from the hot plate and delays the droplet's evaporation. This is similar to what happens between the water shielding and plasma. Here, water not only acts as a shield but further constricts the arc, acting as a "liquid nozzle." The end result is an increase in energy density.
Behind this lies an important phenomenon in plasma physics. Cooling the plasma with water or by any other means does indeed take energy out of the plasma. Too much cooling can be detrimental. But moderate cooling, carefully controlled, can be beneficial. Under these conditions, the plasma—on its own—will shrink its volume and increase its temperature to survive, to stay in the plasma state of matter. The decreased size of the column further increases its energy density and, in the right application, makes a better, faster cut.
A FABRICATOR'S PANACEA
The future of plasma cutting holds more improvements, particularly through automation. To further improve performance, there is a need to further understand the phenomena that happens within the arc itself and how all the elements interact, from the electrode and nozzle to gases and the workpiece material. The drive for this is coming from the higher expectations of the end users.
The bottom line: The arc must be controlled from the electrode to the bottom of the work. If the current and gas are controlled precisely and automatically, then the system can boast both increased speed and performance at a reduced cost—indeed, a fabricator's panacea.
Editor's Note: Nakhleh Hussary, Ph.D., and Thierry Renault, Ph.D., are principal arc process engineers at Thermal Dynamics, a brand of Thermadyne, St. Louis, Mo. Artwork courtesy of Thermadyne.
author: By Nakhleh Hussary, Ph.D, and Thierry Renault, Ph.D.