Plasma Arc Machining process

PAM - Unit - 5

1. PLASMA ARC MACHINING D.PALANI KUMAR, Assistant Prof. / Mech. Engg ., Kamaraj College of Engg . & Tech. Virudhunagar.

2. PRINCIPLE OF PLASMA ARC MACHINING : plasma is considered as the fourth state of matter , beside of the solid, liquid and gas states. when gas is heated to a high temperature of order 2000 o C ,the molecules separate out as atoms. If the temperature is raised to 3000 o C , the electrons from some of the atoms dissociate and the gas become ionized consisting of ions and electrons. This state of gas is called as plasma. Plasma becomes electrically conductive as well responsive to magnetism. The source of heat generation in plasma is the recombination of electrons and ions into atoms , or recombination of atoms into molecules . This liberated bonding energy is responsible for increased kinetic energy of the atoms formed by recombination.

3. The temperature of plasma can be of the order of 33,000 o C .when such a high temperature source reacts with work material ,work melts out and vaporizes. Many material like aluminium , stainless steel have high thermal conductivity ,large heat capacity and good oxidation resistance. As a result ,such materials cannnot be cut using conventional techniques like oxyfuel cutting.But these material can be easily cut by plasma arc machning . characteristics of plasma High electric conductivity Very good interaction capacity with the electric and magnetic fields Permanent source of electromagnetic radiations

4. Working principle Plasma arc cutting is a very complex process that is used in order to cut steel and other metals(or sometimes other materials) using a plasma torch. In this process, an inert gas is blown at high speed out of a nozzle. At the same time an electrical arc is formed through that gas from the nozzle to the surface being cut, turning some of that gas to plasma. Due to the height concentration of the energy in a limited space, the material of the workpiece is wormed till the melting of a layer and the melted material is removed from the cut by the plasma jet

5. Pilot arc ignition Arc transfer The cutting procedure begins with arc ignition, which is a process consisting of several steps. It starts with igniting a pilot arc, an arc discharge between the electrode (cathode) and nozzle, done in two different ways . one is with a short HF pulse applied to the electrode-nozzle gap, another is retract starting.

6. Retract starting uses the pressure of the air supplied to the torch to drive back a piston to which the electrode is ultimately connected. The electrode and nozzle start out in contact with an electric current running through them. When the electrode retracts a pilot arc is created Once the pilot arc is created, the gas flow blows the pilot arc out of the nozzle thus creating an arc loop protruding out of the nozzle. If the work-piece is in position (typically within 5–15mm from the end of the nozzle), the arc will attach to the work-piece, the power supply will sense arc transfer and the nozzle will be removed from the circuit. The work piece becomes the anode and the main current establishes in the electrode-work gap.

7. TRANSFERRED AND NON-TRANSFERRED MODES A plasma jet can be operated in the transferred mode , where the electric current flows between the plasma torch electrode (cathode) and the workpiece (anode). It can also be operated in the non-transferred mode where the electric current flows between the electrode and the torch nozzle Although a stream of hot plasma emerges from the nozzle in both modes of operation, the transferred mode is invariably used in plasma cutting because the usable heat input to the workpiece is more efficiently applied when the arc is in electrical contact with the workpiece .

8. PLASMA ARC MACHINING

9. MECHANISM OF MATERIAL REMOVAL In this process, an inert gas is blown at high speed out of a nozzle. At the same time an electrical arc is formed through that gas from the nozzle to the surface being cut. Due to the height concentration of the energy in a limited space, the material of the workpiece is warmed till the melting of a layer; afterwards, the melted material is removed from the cut by the plasma jet. ARC GENERATION It starts with igniting a pilot arc, an arc discharge between the electrode (cathode) and nozzle. Once the pilot arc is created, the gas flow blows the pilot arc out of the nozzle thus creating an arc loop protruding out of the nozzle. If the work-piece is in position typically within 5–15mm from the end of the nozzle, the arc will attach to the work-piece ,

10. ELEMENTS OF PLASMA ARC MACHNING Power supply Plasma torch Gas supply Control console Cooling water system 1.Air plasma torch 2.Oxygen injected torch 3.Dual gas system 4.Water injected torch

11. AIR PLASMA TORCH: It uses a compressed air as the gas that ionizes and does cutting.The air to be used should be uncontaminated. The nozzle of this torch may result in prematured failure because of double arcing ie arcing between the electrode and the nozzle and between the nozzle and the workpiece . Air plasma cutting results in a high degree of tapered machined surface. Zirconium or halfnium are used as electrode material because of their higher resistance to oxidation.Because of poor oxidation resistance,tungsten electrode doesnot last for more than few seconds.

12. DUAL FLOW PLASMA ARC(1962) The dual flow technique was developed and patented by Thermal Dynamics Corporation and James Browning,in 1963. It involved a slight modification of the conventional plasma cutting process. Essentially, it incorporated the same features as conventional plasma cutting, except that a secondary gas shield was added around the plasma nozzle.

13. Usually, in dual flow operation the cutting or plasma gas was nitrogen and the secondary shielding gas was selected according to the metal to be cut. Secondary shield gases typically used were air or oxygen for mild steel, carbon dioxide for stainless steel, and an argon/hydrogen mixture for aluminium . The major advantage of this approach was that the nozzle could be recessed within a ceramic gas cup or shield cup as shown in Figure above, preventing the nozzle from shorting with the workpiece , and reducing the tendency for double arcing. The shield gas also covered the cutting zone, improving cut quality and speeds as well as cooling the nozzle and shield cap.

14. WATER INJECTION CUTTING In the water injection plasma cutting process, water was radially injected into the arc in a uniform manner as shown in Figure below. The radial impingement of the water at the arc provided a higher degree of arc constriction than could be achieved by just the copper nozzle alone. cutting gas – nitrogen .(1MPa)

15. WATER CONSTRICTION GIVES MANY ADVANTAGES : Improved cut squareness , increased cutting speeds Single gas requirement made the process more economical and easier to use Nozzle life was greatly increased with the water injection technique because the steam boundary layer insulated the nozzle from the intense heat of the arc. Water Muffler and Water Table (1972) Since the plasma arc process was a highly concentrated heat source of up to 50,000K, there were some negative side effect,to reduce side effects water muffler was used Water Muffler The Water Muffler system created a high flow water shield around the torch which produced the following benefits •  The high noise level of the plasma arc was sharply reduced through the muffling effect of the water curtain. • smoke and toxic gases were confined . •  Arc glare was reduced to a level that was less dangerous to the eyes. •  With the proper dye in the water, ultraviolet radiation was diminished.

16. Electrical parameters: Plasma arc current intensity Plasma arc voltage Properties of the material workpiece Thickness of the workpiece Other factors : Plasmogen gas type and flow Working speed , Working distance PLASMA CUTTING PROCESS PARAMETERS Roughness of the obtain surface, Ra; The shape of the obtained surfaces; Thickness of heat affected zone. ZIT; Cutting precision; Productivity; The wear of the nozzle; Cutting width

17. PLASMA ARC CURRENT INTENSITY, IP , The value of plasma arc current, Ip , is depending on : 1. The geometrical parameters of the plate to be cut (b - cutting width average, s – thickness of the workpiece ) 2 . The cutting speed , vt 3.The gas flow , Q plasma arc current intensity values till 800 – 1000 A . PLASMA ARC VOLTAGE, Vp , The value of plasma arc voltage, Vp , is establish function of the nature of the plasmogen gases, the ionization voltage of the gas and the gas flow , Dgp . The maximum value for the Vp could reach 300 V .

18. CUTTING SPEED,Vt As the cutting speed increases ,volumetric material removal rate is found to attain a maximum value and then starts decreasing. Factors affecting cutting speed : The thickness of the material (t) The type of material being cut Current(I) TOLERANCE Poor tolerance: + 0.8 mm (T<25mm) ; + 3 mm ( T >25mm ) SURFACE FINISH : 5 – 75 µm WIDTH OF CUT : 2.5- 9 mm HAZ : 0.75 TO 5 mm φ ( t,material type , cutting conditions) TAPER : 5- 7

19. ADVANTAGES OF PLASMA ARC MACHINING Rapid Cutting Speeds: plasma arc cutting is faster than oxyfuel for cutting steel up to 50 mm thick and is competitive for greater thickness. Plasma cutting achieves speeds greater than those of laser cutting systems for thickness over 3 mm. The fast cutting speeds result in increased production, enabling systems to pay for themselves in as little as 6 months for smaller units.

20. Wide Range of Materials and Thickness: Plasma cutting systems can yield quality cuts on both ferrous and nonferrous metals. Thickness from gauge to 80 mm 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 thickness under 25 mm, and comparable up to about 50 mm. For example, for 12 mm steel, plasma cutting costs are about half those of oxyfuel .

21. DISADVANTAGES OF PLASMA ARC MACHINING The cutter's electrode and nozzle sometimes require frequent replacement which adds to the cost of operation. Non-conductive materials such as wood or plastic cannot be cut with plasma cutters with transferred arc type. Another minor drawback is that the plasma arc typically leaves a 4-6 degree bevel on the cut edge, although this angle is almost invisible on thinner material, it is noticeable on thicker pieces.

22. APPLICATIONS: Multiple torch system –varieties of shapes on plate Bevel cut on the end of a pipe as edge preparation before welding CNC PAC system – punching type operation on light duty plates made of steel aluminium and copper

23. SUMMARY The plasma process for cutting was developed approximately thirty years ago, for metals difficult to be cut by classic operations, and uses a high energy stream of dissociated, ionized gas, known as plasma, as the heat source. Material removal mechanism – melting & vaporization .molten material blow off by high velocity gas Capable of cutting high-alloy refractory and stainless steels and other materials of high thermal conductivity ,high oxidation resistance with maximum productivity, through the automation capacity, through the low expenses towards traditional techniques Types of plasma torch – air plasma, dual gas ,water injection cuuting plasma torch preferred due to the quality of the cut and low thickness of the thermal influence zone (TIZ), within 1.50 mm and avg.surface roughness of 40 µm . .