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Introduction:- welding is one of the material addition operations where components made by various processes are permanently joined together to obtain the desired configuration .other techniques of joined the components are brazing, soldering, mechanical joining ( riveting ,bolting and keying ) and adhesive bonding. Here we shall consider only welding and some of the allied processes like brazing and soldering .
In welding processes the components are joined permanently by coalescence. To obtain coalescence or permanent bonding , the surfaces to be welded together must come into intimate contact so that activities between atoms result in the formation of common metallic crystals.
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when two atoms come sufficiently close to each other , an attractive force is excreted between them . this interacting attractive force is negligible when the distance between these atoms ( inter-atomic distance ) is greater then few atomic spacings ( that is , a few angstroms ) . As the interatomic distance is decreased ,the attractive force increase rapidly ( figure ) .The atoms ,however , do not collapse because a repulsive force is manifested as the interatomic distance decreases .These two forces interact and are equal in magnitude at the equilibrium interatomic distance .At the intersection point ( figure ). the slope of the repulsive force is always greater than that of the attractive force,resulting in a stable equilibrium .
Thus if two metallic bodies are brought sufficiently close to each other ( less than a few angstroms apart ) , then they will adhere together due to a large attractive force. The resulting coalescence or formation of common metallic crystals joins the two bodies permanently. The process of joining metals in this manner has come to be known as welding .
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The formation of ideal metallurgical bond requires smooth and flat surface which are free from oxides , absorbed gases and other contaminants , and the metals should have single crystals with identical structure and orientation with no internal impurities. Such an ideal situation can never be achieved in practice and various techniques have been developed to overcome this impossibility so that welding may be achieved .For example smooth and flat surfaces can be obtained by melting the surfaces or through application of pressure. Similarly , contaminants can be removed by causing sufficient metal flow along the interface through heating or application of pressure so that they are able to squeeze out. Thus, two metallic surfaces can be brought into close proximity through the application of temperature and pressure .In fact , in welding processes a combination of temperature and pressure ranging from high temperature and no pressure , to high pressure and no heating is applied.
When high temperatures are used for welding , the metallurgical structure and quality of metal may get affected by the heating and cooling cycle. At high temperatures , most metals also get adversely aspects by the environment near the welding zone. These aspects should be considered by the designer before recommending the process to be used .
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