Resistance Welding Principle

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Brittney Cole
5 years ago
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Resistance Welding Principle In resistance welding heat is generated by the electrical resistance of the work pieces and the interface between them.

1 Resistance Welding Principle In resistance welding heat is generated by the electrical resistance of the work pieces and the interface between them. Pressure is supplied externally and is varied throughout the weld cycle. Heat generated is proportional to I2R t, where I is value of current, R is resistance and t is the time during which current flows In resistance welding, due to pressure, a lower temperature needed than oxy-fuel or arc welding processes. Features The overall resistance in the welding circuits can be quite low, high currents are generally required to produce a resistance weld. Power transformers convert the high-voltage, lowcurrent line power to the high-current (up to 100,000 A) low-voltage (0.5 to 10 V) power required for welding. While smaller machines may utilize single-phase circuitry. FIG. The fundamental resistance-welding circuit Advantages of Resistance welding Limitations 1.Very rapid. 2.Fully automation possible. 3.Conserve material; no filler metal, shielding gases, or flux is required. 4. Skilled operators are not required. 5.Dissimilar metals can be easily joined. 6. High reliability and High reproducibility

2 1. The equipment has a high initial cost. 2. There are limitations to the type of joints that can be made (mostly lap joints). 3. Skilled maintenance personne1 are required: to service the control equipment. 4. For some materials, the surfaces must receive special preparation prior to welding. Applications The resistance welding processes are among the most common technique for high volume joining. Automobile bodies, sheet metal joining. Types of Resistance Welding 1. Resistance spot welding 2. Resistance seam welding 3. Projection welding 4. Butt welding 5. Flash welding 6. Percussion welding 1. Resistance Spot Welding This is essentially done to join two sheet-metal jobs in a lap joint, forming a small nugget at the interface of the two plates. Spot welding consists of joining two pieces by placing them between two electrodes and passing a heavy current through them for a very short duration. This causes the material just below the electrodes to heat up quickly due to the intervening resistance to the flow of electric current. When coalescence temperature is reached, the current is switched off and a pressure is applied on the two electrodes. The pressure is released when the spot weld cools off. The portion of the material just below the electrodes gets pressure welded. The weld joint is usually in the form of a round spot (if the electrodes have circular tips), hence the name spot weld. The electrodes are usually made of copper and are water cooled. One of them may be fixed and the other one is movable. Normally A.C. power is used along with a step down transformer. The two terminals of secondary winding of transformer are connected to the two copper electrodes to complete the circuit. Usually spot welding (as also other ERW machines) are automatic and work on the following weld cycle:

3 1. Squeeze the two metal pieces together with a light pressure. 2. Pass heavy electric current for a very brief time to obtain coalescence temperature, 3. Apply pressure and hold for some time. 4. Remove pressure. Figure : Setup of Spot welding Figure : Setup of Spot welding

4 Advantages of Resistance welding Limitations 1.Very rapid. 2.Fully automation possible. 3.Conserve material; no filler metal, shielding gases, or flux is required. 4. Skilled operators are not required. 5.Dissimilar metals can be easily joined. 1. Joints in spots only. 2. Thick plates can not be welded easily 3. Only lap joint welding is possible 2. Seam Welding Weld is made between overlapping sheets of metal. The seam is a series of overlapping spot welds. The basic equipment is the same as for spot welding. except that the electrodes are now in the form of rotating disks (As shown below). Timed pulses of current pass to form the overlapping welds. As the metal passes between the electrodes, timed pulses of current pass through it to form the overlapping welds. Weld is made between overlapping sheets of metal, and the process is used to produce liquid- or gastight sheet metal vessels, such as gasoline tanks, automobile mufflers, and heat exchangers. The seam is actually a series of overlapping spot welds such as those in Figure below. The welding current is usually a bit higher than conventional spot welding, to compensate for the short circuit of the adjacent weld, and external cooling of the work, by air or water, is often employed. In a variation of the process, a continuous seam is produced by passing a continuous current through the rotating electrodes. A typical welding speed is about 1.5 m/min for thin sheet.

Upset Butt Welding In upset welding (UW), the pieces to be joined are brought together to mate with each other in a butt joint compared to lap

5 Advantages 1. Faster than spot welding. 2. Continuous spots can be generated. 3. Gastight joints are prodiced. 3. Upset Butt Welding In upset welding (UW), the pieces to be joined are brought together to mate with each other in a butt joint compared to lap joint. The two pieces are held tightly together and current is applied, so that the heat is generated through the contact area between the two plates. Because of the joint being under pressure, the ends of the two pieces get slightly upset and hence its name. This is useful for joining the two ends of rods or similar pieces.

4. Flash Welding Flash welding (FW) is similar to upset welding except that the heat required for melting is obtained by means of an arc rather than the simple resistance heating.

6 4. Flash Welding Flash welding (FW) is similar to upset welding except that the heat required for melting is obtained by means of an arc rather than the simple resistance heating. The flash-welding equipment consists of essentially two platens to which the two pieces to be joined are clamped. One of the platens is fixed while the other is movable, the movement being controlled by means of a cam, as shown in Fig. below. The ends of the two pieces need not be parallel and as prepared as in the case of upset welding. The two pieces are brought together and the power supply is switched on. Momentarily the two pieces are separated to create the arc to melt the ends of the two pieces. Then again the pieces are brought together and the power switched off while the two ends are fused under force. Most of the metal melted would flash out through the joint and forms like a fin around the joint. It is generally a faster operation compared to that of upset welding and would be automatically controlled by a cam arrangement.

7 5. Percussion Welding Percussion welding is a similar process where the, heating is obtained by an arc produced by a rapid discharge of stored electrical energy, which is followed by a rapid application of force to expel the metal and produce the joint. In percussion welding, the arc duration is only 1 to 10 ms. while the heat is intense, it is also highly concentrated. Only a small amount of weld metal is produced, little or no upsetting occurs at the joint, and the heat-affected zone is quite small. Application is generally restricted to the butt welding of bar or tubing where heat damage is a major concern

COURSE: ADVANCED MANUFACTURING PROCESSES. Module No. 4: ADVANCED WELDING PROCESSES

COURSE: ADVANCED MANUFACTURING PROCESSES Module No. 4: ADVANCED WELDING PROCESSES Lecture No-2: Resistance Welding Process Resistance welding process makes use of the electrical resistance for generating