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Plasma arc welding
Plasma Arc Welding (PAW) is an advanced arc welding process that utilizes a highly ionized gas (plasma) to melt and fuse metals, either through a transferred arc between an electrode and the workpiece or a non-transferred arc to a nozzle. The technique offers several advantages, including faster metal deposition, high weld quality, and the ability to weld a variety of materials, but it also requires specialized equipment and safety precautions due to high power needs and radiation emissions. Applications of PAW span various industries, such as aerospace, automotive, and construction.
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Plasma arc welding
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- 2. Content 1. Introduction 2. Howplasma welding works 3. Equipment 4. Welding modes in PAW 5. Advantages 6. Disadvantages 7. Applications
- 3. 1 - Introduction Arcwelding process that produces coalescence of metals by heating them with a constricted arc between an electrode and the work piece (transferred arc) or between the electrode and the water- cooled constricting nozzle (non transferred arc). Plasma: A gaseous mixture of positive ions, electrons and neutral gas molecules.
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- 5. 2 - HowPlasma Welding Works Plasma • Gas which is heated to an extremely high temperature and ionized so that it becomes electrically conductive • PAW process uses this plasma to transfer an electric arc to the work piece • The metal to be welded is melted by the intense heat of the arc and fuses together Objective of PAW • To increase the energy level of the arc plasma in a controlled manner • This is achieved by providing a gas nozzle around a tungsten electrode operating on direct current electrode negativity
- 6. Variants of PAW Transferredarc mode • Arc is struck between the electrode(-) and the work piece(+) • Used for high speed welding • Used to weld Ceramics, steels, Aluminum alloys, Copper alloys, Titanium alloys, Nickel alloys Non-transferred mode • Arc is struck between the electrode(-) and the nozzle(+), thus eliminating the necessity to have the work as a part of the electrical system • Arc process produces plasma of relatively low energy density • Since the work piece in non-transferred plasma arc welding is not a part of electric circuit, the plasma arc torch may move from one work piece to other without extinguishing the arc
- 7. Transferred and Non-TransferredARC Welding
- 8. 3 - Equipment PowerSupply • A DC power source (generator or rectifier) having drooping characteristics and open circuit voltage of 70 volts or above is suitable for PAW • Rectifiers are generally preferred over DC generators • Working with He as an inert gas needs open circuit voltage above 70 volts. This voltage can be obtained by series operation of two power sources or the arc can be initiated with argon at normal open circuit voltage and then helium can be switched on... • High frequency generator and current limiting resistors used for arc ignition
- 9. Shielding gases • Shieldsthe molten weld from the atmosphere. • Two inert gases or gas mixtures are employed. • Argon(commonly used), Helium, Argon+Hydrogen and Argon+Helium, as inert gases or gas mixtures. • Helium is preferred where a broad heat input pattern and flatter cover pass is desired. • A mixture of argon and hydrogen supplies heat energy higher than when only argon is used and thus permits higher arc alloys and stainless steels. • For cutting purposes a mixture of argon and hydrogen (10¬ 30%) or that of nitrogen may be used. • Hydrogen, because of its dissociation into atomic form and thereafter recombination generates temperatures above those attained by using argon or helium alone.
- 10. Welding Parameters • Current50 to 350 Amps, • Voltage 27 to 31 Volts, • Gas flow rates 2 to 40 liters/min. (lower range for orifice gas and higher range for outer shielding gas), • Temp of Jet 50000°F (28000°C)
- 11. 4 - Modesin PAW Micro-plasma welding • Welding Current from 0.1A to 15A • Arc Length is varied up to 20mm • Used for welding thin sheets (0.1mm thick), and wire and mesh sections Medium-plasma welding • Welding current from 15A to 100A. Keyhole welding • Welding Current above 100A, where the plasma arc penetrates the wall thickness. • Widely used for high-quality joints in aircraft/space, chemical industries to weld thicker material (up to 10mm of stainless steel) in a single pass.
- 12. 5 - Advantages •Permits faster metal deposition rate and high arc travel speed as compared to Gas tungsten arc welding • Uniform penetration with high welding rate is possible • Stability of arc and Excellent weld quality • Can produce radiographic quality weld at high speed • Can weld steel pieces up to about half inch thick, square butt joint • Useful for semi automatic and automatic processes. • Process is very fast and clean • Requires less operator skill due to good tolerance of arc to misalignments • High penetrating capability (keyhole effect)
- 13. 6 - Disadvantages •Special protection is required as Infrared and UV Radiations is produced • Needs high power electrical equipment • Gives out ultraviolet and infrared radiation. • Operation produces a high noise of the order of 100dB • Expensive equipment; • Can weld only up to 25mm thickness. • High distortions and wide welds as a result of high heat input (in transferred arc process). • More chances of Electrical hazards.
- 14. 7 - Applications •Aerospace Industries • Foodstuff and Chemical Industries • Machine and Plant Construction • Automobiles and Railways • Ship Construction • Tank Equipment and Pipeline Construction etc.