Resistance welding (RW)

Transcriptie

1 Resistance welding (RW) Heat required for welding is produced by mean of the electrical resistance between the two members to be joined Major advantages, such as not requiring consumable electrodes, shielding gas, or flux Heat generated in resistance welding is H = I 2 RtK

2 RW The total resistance in these processes is the sum of the following; Resistance of the electrodes Electrode-workpiece contact resistances Resistances of the individual parts to be welded Workpiece-workpiece contact resistances (faying surfaces) The actual temp rise at the joint depends on the specific heat and thermal conductivity of the metals to be joined

3 Resistance spot welding (RSW) The tips of two opposing solid cylindrical electrodes contact the lap joint of two sheet metals, and resistance heating produces a spot weld To obtain a good bond in the weld nugget, pressure is also applied until the current is turned off Accurate control and timing of the electric current and pressure are essential in resistance welding

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7 RSW process capabilities Simplest and most commonly used resistance welding process Welding may be performed by means of single or multiple electroded, and the required pressure is supplies through mechanical or pneumatic means Widely used for fabricating sheet metal Attaching handles to ss cookware Rapid spot welding of automobile bodies with multiple electrode (10,000 spots)

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9 Resistance seam welding (RSEW) Is modification of spot RSW, wherein the electrodes are replaced by rotating wheels or roller With continuous ac power supply, the electrically conducting rollers produce continuous spot welds whenever the current reaches a sufficiently high level in the ac cycle These are actually overlapping spot welds and produce a joint that is liquid tight and gas tight With intermitten application of current to the rollers, a series of spot welds at various intervals can be made along the length of the seam Making the longitudinal (side) seam of cans for household product, mufflers,gasoline tanks, and containers

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11 Resistance projection welding (RPW) High electrical resistance at the joint is developed by embossing one or more projection (dimples) one one of the surfaces to be welded High localized temperature are generated at the projections, which are in contact with the flat mating part The electrode - made of copper-base alloys and water cooled to keep their temperature low are large and flat Weld nugget, similar to those in spot welding, are formed as the electrodes exert pressure to compress the projections The projection may be round or oval for design or strength purpose

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13 RPW process capabilities Spot welding equipment can be used for RPW by modifying the electrodes Although embossing workpieces is an added expense, this process produces a number of weld in one stroke, extends electrode life, and is capable of welding metals of different thicknesses Nuts and bolts are also welded to sheet and plate by this process, with projections that are produced by machining or forging Joining a network of wires such as metal baskets, grills, ovenracks, and shopping carts is also considered RPW because the small contact area between crossing grids

14 Flash welding (FW)