INTRODUCTION: Plasma arc cutting was developed 20 years ago primarily for cutting stainless steel and aluminum. Although favorable economically, mild steel was seldom cut with this process becaus becausee of three three fundam fundament ental al limit limitati ations ons:: relati relatively vely poor cut quality quality,, equipme equipment nt reliability, and inability of the earlier cutting machines to handle plasma cutting speeds. As a result of these limitations, plasma cutting did not encounter rapid growth until after Water-injection Plasma Cutting was introduced in 1970. The plasma arc process has always been seen as an alternative to the oxy-fuel process. In this part of the series the process fundamentals are described with emphasis being placed on the operating features and the advantages of the many process variants. THE basic principl principlee behind behind plasma plasma arc cuttin cutting g is that the arc formed formed between between the electrode and the workpiece is constricted by a fine bore, copper nozzle. This increases the temperature temperature and velocity of the plasma emanating from the nozzle. nozzle. The temperature temperature of the plasma is in excess of 20,000°C and the velocity can approach the speed of sound. When used for cutting, the plasma gas flow is increased so that the deeply penetrating plasma jet cuts through the material and molten material is removed in the efflux plasma. The process differs from the oxy-fuel process in that the plasma process operates by using the arc to melt the metal whereas in the oxy-fuel process, the oxygen oxidises the metal and the heat from the exothermic reaction melts the metal. Thus, unlike the oxyfuel process, the plasma process can be applied to cutting metals which form refractory oxides such as stainless steel, aluminium, cast iron and non-ferrous alloys.
The Plasma arc cutting process
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A plasma cutter cutter is a relati relativel vely y easyeasy-toto-use use tool tool to cut steel steel and other other electr electrical ically ly-conduct conductive ive metals metals.. These These cutter cutterss work work by using using a high-v high-volt oltage age electr electrica icall arc and a compressed gas, usually air. An electrical arc generated by an internal electrode ionizes gas passing through a nozzle, creating a concentrated arc of plasma at the cutter's tip. The arc's contact with the working surface makes a high heat circuit which melts a section less than 1/16" (1.6mm) wide. The force of the plasma flow then literally blows out the molten area on the work piece, creating a fairly clean cut with little or no slag slag.. The plasma arc travels through the nozzle at a speed of up to 20,000 feet per second, and at temperatures as high as 30,000 degrees Fahrenheit (16,600 Celsius)!
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PLASMA: In physics and chemistry, plasma is a gas in which a certain portion of the particles are ionized. The presence of a non-negligible number of charge carriers makes the plasma electrically conductive so that it responds strongly to electromagnetic fields. Plasma, therefore, has properties quite unlike those of solids, liquids, or gases and is considered to be a distinct state of matter. Like gas, plasma does not have a definite shape or a definite volume unless enclosed in a container; unlike gas, in the influence of a magnetic field, it may form structures such as filaments, beams and double layers Some common plasmas are flame, lightning, and the Sun. Plasma was first identified in a Crookes tube, and so described by Sir William Crookes in 1879 (he called it "radiant matter").[1] The nature of the Crookes tube "cathode ray" matter was subsequently identified by British physicist Sir J.J. Thomson in 1897,[2] and dubbed "plasma" by Irving Langmuir in 1928,[3] perhaps because it reminded him of a blood plasma. Langmuir wrote:
Plasma lamp, illustrating some of the more complex phenomena of a plasma, including filamentation. The colors are a result of relaxation of electrons in excited states to lower energy states after they have recombined with ions. These processes emit light in a spectrum characteristic of the gas being excited.
Degree of ionization For plasma to exist, ionization is necessary. The term "plasma density" by itself usually
refers to the "electron density", that is, the number of free electrons per unit volume. The degree of ionization of a plasma is the proportion of atoms which have lost (or gained) electrons, and is controlled mostly by the temperature. Even a partially ionized gas in
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which as little as 1% of the particles are ionized can have the characteristics of a plasma (i.e., response to magnetic fields and high electrical conductivity). The degree of ionization, α is defined as α = ni/(ni + na) where ni is the number density of ions and na is the number density of neutral atoms. The electron density is related to this by the average charge state of the ions through ne = ni where ne is the number density of electrons. Plasm a cutters work by sending a pressurized gas, such as nitrogen, argon, or oxygen, through a small channel. In the center of this channel, you'll find a negatively charged electrode . When you apply power to the negative electrode, and you touch the tip of the
nozzle to the metal, the connection creates a circuit . A powerful spark is generated between the electrode and the metal. As the inert gas passes through the channel, the spark heats the gas until it reaches the fourth state of matter. This reaction creates a stream of directed plasma, approximately 30,000 F (16,649 C) and moving at 20,000 feet per second (6,096 m/sec) that reduces metal to molten slag. For example Whe n energy, in the form of heat, is applied to ice, the ice melts becoming water. The H2O transforms from the solid-state, ice, to the liquid state, water. When more heat is applied to the water, the water vaporizes becoming steam. The H2O transforms from the liquid state, water, to the gas state, steam (H2& O2). Finally, when additional heat is applied to the individual gases, the gases ionize. The ionization of the gases is the final change in states. The gases are now in an electrically conductive state called a plasma.
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PLASMA ARC TERMINOLOGY :
PLASM ARC TORCH TERMINOLOGY A plasma torch works by projecting an electric arc produced by polarized electrodes through gas that is being forced through a small opening, the nozzle. As the gas becomes electrically charged its temperature elevates until the gas enters the plasma state. The electric charge is transferred to the metal causing it to melt, and the high velocity gas cuts through the molten material. Nitrogen, oxygen and argon can all be used, but the most popular gas is simply forced air. A plasma torch can cut cheaper, faster and more accurately than the traditional oxy-acetylene torch.
A plasma torch can be effective for both cutting and welding. As with cutters, plasma welding torches have high speeds and high quality performance for creating pore-free welding seams. These tools come with a standard electrical cord, a chamber for the air or gas, and a nozzle with an electrode placed behind it. A gas source, such as an air
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compressor or bottled air or gas, is attached to the torch. For most metals, compressed air is sufficient, though nitrogen or argon may be used for cutting stainless steel or other exotic metals. The automotive and construction industries use a computer numerical control (CNC) plasma torch to make custom auto shapes required for chassis or frames, and to cut large steel beams. The versatility and accuracy of this device makes it a great tool for the creating metal art, jewelry and ornamental iron work. Many CNC torches are large, expensive equipment designed to operate on the assembly line of a large shop, such as an automobile factory; however, smaller and more affordable models are now available for small shops and individual craftsmen. For applications which do not require CNC equipment, hand-operated units are available at a relatively inexpensive price. The air pressure capacity required varies depending upon the size of the plasma torch, so the purchaser should select the torch prior to determining what size air compressor to buy. Replacement parts, such as air filters, nozzles and electrodes are available from a number of sources. A plasma torch, like any other power tool, can cause serious injury if appropriate safety measures are not followed. The light from the plasma torch can seriously damage the naked eye, so safety glasses with side shields should be worn at all times. Gloves and fire-resistant clothing can protect against sparks. These cutters operate at extremely high temperatures which can burn through gloves, so hands should be kept away from the nozzle, arc and metal workpiece. To prevent fires or explosions, the torch should only be used in well-ventilated areas away from any other flammable material.
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PLASMA ARC CUTTING SYSTEM: A plasma jet can either be operated in the transferred mode, where the power supply is connected between the electrode and the workpiece, or in the nontransferred mode wherethe power supply is connected between the electrode and the nozzle. Both modes of Operation are illustrated in Figure 2. Although a stream of hot plasma emerges from the Nozzle in both modes of operation, the transferred mode is always used in plasma cutting because the usable heat input is most efficiently applied when the arc is in electrical contact with the workpiece.
Schematic diagram for non transferred and transferred arcs The thermal efficiency is low for non transferred up to 65-75% while for transferred arcs it is up to 85-90%.
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CONVENTIONAL PLASMA ARC CUTTING: The plasma jet that is generated by conventional "dry" arc constriction techniques can be used to sever any metal at relatively high cutting speeds. The thickness of plate can range from 1/8 inch to a maximum thickness depending on both the current capacity of the torch and the physical properties of the metal. A heavy duty mechanized torch with a current capacity of 1000 amps can cut through 5 inch thick stainless steel and 6 inch thick aluminum. However, in most industrial applications the plate thickness seldom exceeds 1-1/2 inch. In this thickness range, conventional plasma cuts are usually beveled and have a rounded top edge.
Fig.- Pojitive cut angle
Beveled cuts are a result of an imbalance in heat input into the cut face. As shown in Figure , a positive cut angle will result if the heat input into the top of the cut exceeds the heat input into the bottom. One obvious approach to reduce this heat imbalance is to apply the arc constriction principle described in Figure 1: increased arc constriction will cause the temperature profile of the plasma jet to become more uniform and, correspondingly, the cut will become squarer. Unfortunately, the conventional nozzle is limited by the tendency to establish two arcs in series---electrode to nozzle, and nozzle to
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work. This phenomenon is known as "double arcing" and can damage both the electrode and nozzle. Conventional plasma cutting can be cumbersome to apply if the user is cutting a wide
variety of metals and plate thickness. For example, if the conventional
plasma process is
used to cut stainless steel, mild steel and aluminum, it will be
necessary to have three different cutting gases on hand if optimum cut quality is to be obtained. This requirement not only complicates the process, but necessitates stocking expensive cutting gases such as 65% argon - 35% hydrogen.
PROCESS VARIENT OR PROCESS REFINEMENT:
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The process variant have principally been designed to improve cut quality and arc stability, reduce the noise and fume or to increase cutting speed.
DUAL GAS PLASMA ARC: The same features as conventionalplasma cutting except that a secondary shield gas is added around the nozzle. Usually the cutting gas is nitrogen and the secondary shielding gas is selectedaccording to the metal to be cut. Secondary shield gases typically used are: mild steel--either air or oxygen; stainless steel--CO2; aluminum--argon--hydrogen mixture. Cutting speeds are slightly better than with conventional cutting on mild steel; however, cut quality is inadequate for many applications. Cutting speed and quality on stainless steel and aluminium are essentially the same as with the conventional process. The major advantage of this approach is that the nozzle can be recessed within a ceramic shield gas cup as shown in Figure 4, thereby protecting the nozzle from double arcing. If no shield gas were present, the ceramic shield gas cup could deteriorate because of the high radiative heat load produced by the plasma jet.
Schemetic diagram of dual gas plasma torch Air cutting was introduced in the early 1960's for cutting mild steel. The oxygen in the air
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provides additional energy from the exothermic reaction with molten steel. This additionalenergy increases cutting speeds by about 25%. Although the process can be used tocut stainless steel and aluminum, the cut surface will be heavily oxidized and unacceptablefor many applications. Special electrodes, made of zirconium or hafnium, must be used since tungsten will erode in seconds if the cutting gas contains oxygen. Even with these special electrodes, the service life is must less than what can be achieved with the conventional plasma cutting process. The beneficial effects of the secondary gas are increased arc constriction and more effective 'blowing away' of the dross. The plasma forming gas is normally argon, argon-H2 or nitrogen and the secondary gas is selected according to the metal being cut. Steel
air, oxygen, nitrogen Stainless steel
nitrogen, argon-H 2, CO 2 Aluminium
DUALGAS TORCH argon-H2, nitrogen / CO 2 The
advantages compared with conventional
plasma are: •
Reduced risk of 'double arcing'
•
Higher cutting speeds
•
Reduction in top edge rounding
WATER INJECTED PLASMA TORCH: 11
Earlier it was stated that the key to achieving improved cut quality is through increasing arc constriction. In the Water-injection Plasma Cutting process, water is radially injected into the arc in a uniform manner as shown in Figure 6. The radial impingement of the water around the arc provides a higher degree of arc constriction than can be achieved by conventional means. Arc temperatures in this region are estimated to approach 50,000 degrees K, or roughly nine times the surface temperature of the sun. The net result is improved cut squareness and increased cutting speeds. This radial water-injection techniquewas developed and patented by Hypertherm, Incorporated.Another approach to constricting the arc with water is to develop a swirling vortex ofwater around the arc. This technique does not perform as well as radial injection becausethe degree of arc constriction is limited by the high swirl velocities needed to produce astable water vortex: the centrifugal force created by the high swirl velocity tends to flatten the annular film of water against the inner bore of the nozzle.
Schemetic diagram of water injection plasma torch
Despite the extremely high temperatures generated at the point where the water impinges
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the arc, less than 10% of the water is vaporized. The remaining 90% of the water exits from the nozzle in the form of a conical spray which cools the top surface of the workpiece. This additional cooling prevents the formation of oxides on the cut surface. Little water is evaporated at the arc because an insulating boundary layer of steam forms between the plasma and the injected water. This steam boundary layer, usually referred to as a "Lindenfrost Layer", is the same principle that allows a drop of water to dance around on a hot skillet rather than immediately vaporizing. Nozzle life is greatly increased with the Water-injection technique beca use the steam boundary layer insulates the nozzle from the intense heat of the arc, and the water cools the nozzle at the point of maximum arc constriction. The protection afforded by the water-steam boundary layer also allows a unique design innovation: the entire lower portion of the nozzle can be ceramic. Consequently, double arcing from the nozzle touching the workpiece--the major cause of nozzle destruction--is virtually eliminated. Nitrogen is normally used as the plasma gas. Water is injected radially into the plasma arc, Fig. , to induce a greater degree of constriction. The temperature is also considerably increased, to as high as 30,000°C. The advantages compared with conventional plasma are: •
Improvement in cut quality and squareness of cut
•
Increased cutting speeds
•
Less risk of 'double arcing'
•
Reduction in nozzle erosion
WATER SHROUD PLASMA TORCH: 13
The plasma can be operated either with a water shroud, Fig. 2c, or even with the workpiece submerged some 50 to 75mm below the surface of the water. Compared with conventional plasma, the water acts as a barrier to provide the following advantages: •
Fume reduction
•
Reduction in noise levels
•
Improved nozzle life
In a typical example of noise levels at high current levels of 115dB for conventional plasma, a water shroud was effective in reducing the noise level to about 96dB and cutting under water down to 52 to 85dB. As the water shroud does not increase the degree of constriction, squareness of the cut edge
and
the
cutting
speed
are
AIR PLASMA TORCH:
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not
noticeably
improved.
The inert or unreactive plasma forming gas (argon or nitrogen) can be replaced with air but this requires a special electrode of hafnium or zirconium mounted in a copper holder, Fig. The air can also replace water for cooling the torch. The advantage of an air plasma torch is that it uses air instead of expensive gases. It should be noted that although the electrode and nozzle are the only consumables, hafnium tipped electrodes can be expensive compared with tungsten electrodes.
Schematic diagram of air plasma torch
OXYGEN INJECTED PLASMA TORCH: This process refinement circumvented the electrode life problem associated with air cutting by using nitrogen as the cutting gas and introducing oxygen downstream in the nozzle bore as shown in Figure
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Schematic diagram of oxygen-injected torch construction This process is used exclusively on mild steel and increases cutting speed by about 25% if the optimum gas mixture is used (80% N2 - 20% O2). The major disadvantages are lack of cut squareness, short nozzle life, and limited versatility (mild steel only). Although this process is still being used at some locations, it has been almost entirely displaced by Water-injection cutting.
HIGH TOLERANCE PLASMA: In an attempt to improve cut quality and to compete with the superior cut quality of laser systems, High Tolerance Plasma Arc cutting (HTPAC) systems are available which operate
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with a highly constricted plasma. Focusing of the plasma is effected by forcing the oxygen generated plasma to swirl as it enters the plasma orifice and a secondary flow of gas is injected downstream of the plasma nozzle, Fig. 2e. Some systems have a separate magnetic field surrounding the arc. This stabilises the plasma jet by maintaining the rotation induced by the swirling gas. The advantages of HTPAC systems are: •
•
•
Cut quality lies between a conventional plasma arc cut and laser beam cut Narrow kerf width Less distortion due to smaller heat affected zone HTPAC is a mechanised technique requiring precision, high-speed equipment.
The main disadvantages are that the maximum thickness is limited to about 6mm and the cutting speed is generally lower than conventional plasma processes and approximately 60 to 80% the speed of laser cutting.
NON SHIELDING PART ISSUE: Double Arcing:
During the pierce, droplets of the molten metal can form a conductive path to the nozzle, causing the nozzle to be at positive potential.This can cause a “path of least resistance”from the electrode to the nozzle to the plate known as a “double arc”. This can also occur if the nozzle contacts the plate during a cut.
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Nozzle Damage: Contact to work piece damage and blow-back of spatter during cutting damages the nozzle by pitting and ovaling the orifice.normalovaling
TORCH SHIELDING TECHNOLOGY Shielded Front End • Nozzle is protected by an electrically isolated shield•Significantly reduces double
arcing•Prolongs parts life•Facilitates drag cutting and template tracing•Dramatic improvement in nozzle life
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ISOLATED SHIELD
Shielded Front End Benefits •During mechanized torch cutting: •Greater nozzle life • Lower cost of operation • Longer service life with consistent cut quality •Thicker Pierce Capacity • Protects the nozzle from occasional contact with plate.
CUTTING TERM: Kerf:
Opening created by the metal removed by the plasma arc. The width of the kerfis determined by: •amperage •gases •nozzle orifice size
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•consumable condition •torch-to-work distance Cutting
Dross:
The resolidifiedmetal on the bottom or top of the cut. Dross formation and it’s condition is determined by many factors :•travel speeds •amperage • gases used •type and thickness of metal • torch-to-work distance •material surface coatings
LAG LINES: These are the ripples on the cut face or surface. The more consistent the power produced by power supply is, the smoother the cut. Depending on the process, normal lag lines are curved and slanted at about 15°with proper speeds.
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TORCH STATING METHOD: High Frequency
•High voltage (5,000V -10,000V), high frequency AC forms a spark between electrode and nozzle •Gas is forced to flow through this spark, raising it to it’s ionization temperature •This is an effective starting method, but generates electrical “noise” that may affect sensitive electronic equipment Contact Starting
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•Electrode and nozzle are in contact or “shorted” before the gas reaches the torch •Gas flow causes the electrode and nozzle to move apart creating a short circuit spark •The gas is forced to flow through this spark, raising the gas to it’s ionization temperature. •This process works well at power levels under 100 Amps, and is well suited around sensitive electronic equipment
PHYSICAL CONFIGRATION: In PAM, variable such as torch stand off, angle, depth of cut, feed and speed of the work toward the torch are important .The feed and depth of cut determine the volume of metal removal.FIG.show metal removal as a function of the torch angle.
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MATERIAL TO BE CUT
GAS OR GAS MEXTURE USED
Aluminum
Nitrogen,nitrogen-hydrogen.argonhydrogen
magnesium
Nitrogen,nitrogen-hydrogen.argonhydrogen
Stainless steel and some other Nitrogen-hydrogen. argon hydrogen non ferrous metals Carbon and alloy steels, cast iron
Nitrogen-hydrogen. Compressed air
COMPARISON B/W PAM AND LASER CUTTING: Fundamental process difference:
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Safety consideration and operating environment :
PLASMA ARC CUTTING ADVANTAGE: Automated plasma arc cutting systems provide several advantages over other cutting methods such as oxyfuel and laser.
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Rapid cutting speed: Plasma arc cutting is faster than oxyfuel for cutting steel up to 2 inches thick and is competitive for greater thicknesses. Plasma cutting achieves speeds greater than those of laser cutting systems for thicknesses over 1/8 inch. CNC controls allow speeds of up to 500 inches per minute (ipm) to be achieved on gauge thicknesses. These fast cutting speeds result in increased production, enabling systems to pay for themselves in as little as 6 months for smaller units.
Wide range of material thikness Plasma cutting systems can yield quality cuts on both ferrous and nonferrous metals. Thicknesses from gauge to 3 inches can be cut effectively.
Easy to use Plasma cutting requires only minimal operator training. The torch is easy to operate, and new operators can make excellent cuts almost immediately. Plasma cutting systems are rugged, are well suitable for production environments, and do not require the potentially complicated adjustments associated with laser cutting systems. Economical Plasma cutting is more economical than oxyfuel for thicknesses under 1 inch, and comparable up to about 2 inches. For example, for ½ inch steel, plasma cutting costs are about half those of oxyfuel.
LIMITATION AND CONCERN: A chief concern about plasma arc technology is ensuring that gaseous emissions are kept to a minimum and cleaned before being released to the atmosphere.
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Concerns have been raised regarding the reliability of plasma torch technology. The water-cooled copper torch must be replaced periodically to prevent burn-through at the attachment point of the arc and a subsequent steam explosion due to rapid heating of the released cooling water. other limitation are •
Large heat affected zone.
•
Rough Surfaces
•
Difficult to produce sharp corners.
•
Smoke and noise.
•
Burr often results.
APPLICATION: Automated plasma cutting systems are being chosen over oxyfuel, hand tools, and laser cutting in the following areas:
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Sheetmetal
Plasma cutting is commonly used to cut sheet metals from 24 gauge up to 1/8 inch thick at high speeds on carbon steels, aluminum, and stainless steels. •
Plasma cutting is widely used in the transportation industry to form the outer skins of tractor trailers, buses, and agricultural equipment.
•
Plasma cutting systems are also used in the heating, ventilating, and air conditioning industry to cut complex duct work. PlateThicknesses
Industries involved in cutting plate thicknesses also find many applications for plasma cutting. Plasma systems cut plate thicknesses from 1/8 to 3 inches, but the most common applications are for carbon steel plate ¼ to ¾ inch thick. •
Steel service centers cut large plates of steel down to size with plasma.
•
Makers of large construction machinery, mining equipment, and material handling equipment utilize plasma cutting to produce cranes, bulldozers, and other large equipment.
•
Plasma cutting also produces structural steel framework for railroad cars, trucks, and other heavy equipment.
•
Other applications include cutting metal for ship building and the production of pressure vessels. OtherApplications
Plasma cutting is not limited to flat sheets of metal. Plasma torches placed on robots are being used increasingly for contour cutting of pipes and vessels, removal of sprues and risers from castings, and cutting of formed shapes, angles, and curves in various planes.
BIBLIOGTAPHY: •
Advance Machining Process ------
VIJAY K. JAIN
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•
Modern Machining Process ---------
P.C. PANDYE,H.S. SHAN
•
Plasma Arc Cutting-----------------
AHMAD HASSAN SAYED
•
Plasma Processes of Cutting and Welding------ J.A. Hogan and J.B. Lewis
•
Plasma Arc Cutting-----------------
Bill Lucas in collaboration with Derrick
Hilton, •
Plasma Arc Cutting------------------
WIKIPADIA
•
Basic Plasma Theory---------------------
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HYPERTHERM PRIVATE LIMITED