mfe final - CHP 35 Joining types - Welding o The permanent...

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CHP 35 Joining types - Welding o The permanent joining of 2 materials usually metals, by coalescence, which is induced by a combination of temperature, pressure, and metallurgical conditions o Dominant method of joining in manufacturing o Number of different processes have been developed - Brazing/soldering - Adhesives o Polymeric materials and the need for low temperature joining of composite materials - Mechanical methods o Rivets, nuts, bolts, screws Welding types - Oxyfuel, arc, resistance - Solid state o Contaminated layers are generally removed by mechanical or chemical cleaning prior to welding or by causing sufficient metal flow along the interface so that the impurities are squeezed out of the joint - Thermit, laser beam, electroslag, flash, induction, electro beam Types of Welds - Bead o Surfacing welds; made directly onto a flat surface and therefore require no edge penetration o Used primarily for joining thin sheets of metal, building up surfaces, and depositing hard-facing (wear resistant) material - Groove o Used when full thickness strength is desired on thicker material o Usually depends upon the thickness of the joint, the welding process to be employed, the position of the work o Objective is to obtain a sound weld throughout the full thickness with a minimum amount of additional weld metal - Fillet o Used for tee, lap, and corner joints, require no special edge preparation - Plug o Attach one part on top of another and are often used to replace rivets or bolts o Hole is made on the top plate and welding is stated at the bottom of this hole Types of Joints - Butt, tee, lap, corner, edge Multiple pass welds Weld Zones - Weld metal - HAZ
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o Adjacent to the fusion zone, wholly within the base material; undesirable o The parent metal is not melted, but I subjected to elevated temperatures for a brief period of time; the adjacent metal may well experience sufficient heat to bring about structure and property change - Base metal Total heat input - Processes with low rates of heat input tend to produce high total heat content within the metal, slow cooling rates, large HAZ, and resultant structures with lower strength and hardness but higher ductility - High heat input processes have low total heat heats, fast cooling rates, and small HAZ Residual stresses - Affect of heat and cooling; effects can be observed in the form of dimensional changes, distortion, and cracking - Are the result of restraint to thermal expansion and contraction by the pieces being welded - To deal with residual stresses o Total heat input should be minimized o Initial components can also be oriented out of position so that the subsequent distortion will move them to the desired final shape o Warping can be reduced by peening; as the weld bead surface is being hammered with the peening tool or material, the metal is flattened and tries to spread o Surface rolling of the weld bead area can have the same effect o To minimize the possibility of fracture in welds, the joints should be designed to
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mfe final - CHP 35 Joining types - Welding o The permanent...

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