Plasma arc welding

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Welding ppt


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Plasma arc welding :

Plasma arc welding Arc welding process similar to gas tungsten arc welding (GTAW). The electric arc is formed between an electrode (which is usually but not always made of sintered tungsten) and the workpiece . The key difference from GTAW is that in PAW, by positioning the electrode within the body of the torch, the plasma arc can be separated from the shielding gas envelope. The plasma is then forced through a fine-bore copper nozzle which constricts the arc and the plasma exits the orifice at high velocities (approaching the speed of sound) and a temperature approaching 28,000 °C (50,000 °F) or higher. Arc plasma is the temporary state of a gas. The gas gets ionized after passage of electric current through it and it becomes a conductor of electricity.

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In ionized state atoms break into electrons(-) and ions(+) and the system contains a mixture of ions, electrons and highly exited atoms. The degree of ionization may be between 1% and greater than 100% i.e.; double and triple degrees of ionization. Such states exist as more number of electrons are pulled from their orbits.

Principle of Operation :

Principle of Operation Plasma arc welding is a constricted arc process. The arc is constricted with the help of a water-cooled small diameter nozzle which squeezes the arc, increases its pressure, temperature and heat intensely and thus improves arc stability, arc shape and heat transfer characteristics. Plasma arc welding process can be divided into two basic types

Non transferred :

Non transferred The arc is formed between the electrode(-) and the water cooled constricting nozzle(+). Arc plasma comes out of the nozzle as a flame. The arc is independent of the work piece and the work piece does not form a part of the electrical circuit. Just as an arc flame (as in atomic hydrogen welding), it can be moved from one place to another and can be better controlled.\ The non transferred arc plasma possesses comparatively less energy density as compared to a transferred arc plasma and it is employed for welding and in applications involving ceramics or metal plating (spraying). High density metal coatings can be produced by this process. A non-transferred arc is initiated by using a high frequency unit in the circuit.

Transferred arc process: :

Transferred arc process : ( The arc is formed between the electrode(-) and the work piece(+). In other words, arc is transferred from the electrode to the work piece. A transferred arc possesses high energy density and plasma jet velocity. For this reason it is employed to cut and melt metals. Besides carbon steels this process can cut stainless steel and nonferrous metals also where oxyacetylene torch does not succeed. Transferred arc can also be used for welding at high arc travel speeds.

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For initiating a transferred arc, a current limiting resistor is put in the circuit, which permits a flow of about 50 amps, between the nozzle and electrode and a pilot arc is established between the electrode and the nozzle. As the pilot arc touches the job main current starts flowing between electrode and job, thus igniting the transferred arc. The pilot arc initiating unit gets disconnected and pilot arc extinguishes as soon as the arc between the electrode and the job is started. The temperature of a constricted plasma arc may be of the order of 8000 - 25000 0 C.

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T he size and the type of nozzle tip are selected depending upon the metal to be welded, weld shapes and desired penetration height.

Ultrasonic welding :

A plastics welding process, in which two work pieces are bonded as a result of a pressure exerted to the welded parts combined with application of high frequency acoustic vibration (ultrasonic). Ultrasonic vibration transmitted by a metal tool (horn, sonotrode ) causes oscillating flexing of the material and friction between the parts, which results in a closer contact between the two surfaces with simultaneous local heating of the contact area. The plastic melts in the contact area, the polymer molecules are cross-linked, forming a strong joint. Ultrasonic welding

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Ultrasonic Welding cycle takes about 1 sec. The frequency of acoustic vibrations is in the range 20 to 70 kHz (commonly 20-40 kHz). The ampltude of the acoustic vibrations is about 0.002” (0.05 mm). Thickness of the welded parts is limited by the power of the ultrasonic generator. Ultrasonic Welding is used mainly for processing amorphous polymers ( Polysterene (PS), Acrylonitrile -Butadiene-Styrene (ABS) )


INTRODUCTION It is solid state joining process. Mechanical friction between a moving workpiece and a stationary component. Lateral force (upset) is applied to plastically displace and fuse the materials

Types of Friction Welding:

Types of Friction Welding Spin Welding: A rotating chuck along with flywheel. After reaching to required speed motor is removed form flywheel.

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Linear Friction Welding: Oscillating Chuck is used. Use for non-round shapes as compare to Spin welding. Material should be of high shear strength.

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Friction Surfacing: It is a surface coating process. Coating material Mechtrode is rotated under pressure over substrate.

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Friction Stir Welding: A cylindrical shouldered tool, with a profiled probe (nib or pin) is used. Friction is between tool shoulder, nib and workmetal.

Advantages of Friction :

Advantages of Friction Simplicity of operation and simple equipment. Less time requirement. Low Surface impurities and oxide films. Compare to resistance butt welding produces improved welds at higher speed and lower cost, less electric current is required. Heat affected zone is small as compare to conventional flash welding. Less shortening of the component, as compared to that in flash or butt welding. There is no flux, gas, filler metal or slag present to cause imperfections in welds.

Disadvantages of Friction :

Disadvantages of Friction Process is restricted to flat and angular butt welds. Use only for joining small parts. In case of tube welding process becomes complicated. In case of high carbon steels it is difficult to remove flash. It require heavy rigid machine due to high Thrust pressure.


Applications: Automobile: Bimetallic engine valve, universal joint yoke, gear hub etc. Aerospace: Turbine blade joining, seamless joining etc. Consumer: Hand tools, sports equipment Industrial: Spindles, tapers, tools Military Medical: Stainless steel joining of containers Marine: Shipping Industry Mining/Drilling: Twist drill etc. Hydraulic equipments

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