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Mechanical Engineering

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Mechanical Engineering

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  1. Introduction • Arc welding 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

  2. How Plasma 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 DCEN

  3. Variants of PAW

  4. Equipment Power Supply • 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

  5. Equipment High frequency generator and current limiting resistors • Used for arc ignition Plasma Torch • Either transferred arc or non transferred arc typed

  6. Shielding gases • Shields the 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

  7. Permits faster metal deposition rate and high arc travel speed as compared to TIG • 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) Advantages

  8. Special protection is required as Infrared and UV Radiations is produced • Consumption of Inert Gas is high • 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 upto 25mm thickness • High distortions and wide welds as a result of high heat input (in transferred arc process) • More chances of Electrical hazards Disadvantages

  9. Applications • Food Industry • Cryogenics • Automobiles • Ship Construction

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