Resistance Spot Welding (RSW), Projection Welding (RPW), Resistance Seam Welding (RSEW), Flash Welding (FW), Upset Welding (UW), Percussion Welding (PEW), and High Frequency Resistance Welding (HFRW) are the seven important resistance welding processes.  

• Resistance welding is a group of welding processes in which coalescence is produced by the heat obtained from resistance of the work piece to electric current in a circuit of which the work piece is a part and by the application of pressure. There are at least seven important resistance-welding processes.
• Resistance spot welding (RSW) is a resistance welding process which produces coalescence at the faying surfaces in one spot by the heat obtained from resistance to electric current through the work parts held together under pressure by electrodes.
• Projection welding (RPW) is a resistance welding process which produces coalescence of metals with the heat obtained from resistance to electrical current through the work parts held together under pressure by electrodes.
• Flash Welding (FW) is a resistance welding process which produces coalescence simultaneously over the entire area of abutting surfaces, by the heat obtained from resistance to electric current between the two surfaces, and by the application of pressure after heating is substantially completed.
• Upset welding (UW) is a resistance welding process which produces coalescence simultaneously over the entire area of abutting surfaces or progressively along a joint, by the heat obtained from resistance to electric current through the area where those surfaces are in contact.
• Percussion welding (PEW) is a resistance welding process which produces coalescence of the abutting members using heat from an arc produced by a rapid discharge of electrical energy.
• High frequency resistance welding (HFRW) is a resistance welding process which produces coalescence of metals with the heat generated from the resistance of the work pieces to a high-frequency alternating current in the 10,000 to 500,000 hertz range and the rapid application of an upsetting force after heating is substantially completed. The path of the current in the work piece is controlled by the proximity effect.

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