The Welding Processes: Resistance Welding

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Introduction to Resistance Welding Technology

Resistance welding represents a fundamental group of welding processes where coalescence occurs through heat generated by the workpiece's resistance to electric current within a circuit that includes the workpiece itself, combined with applied pressure. This welding methodology encompasses at least seven important processes, each sharing common principles while maintaining distinct characteristics that make them suitable for specific applications.

The seven primary resistance welding processes include flash welding, high-frequency resistance welding, percussion welding, projection welding, resistance seam welding, resistance spot welding, and upset welding. While these processes share fundamental similarities in their use of electrical resistance and pressure, they differ significantly in their application methods, equipment requirements, and optimal use cases.

Resistance Spot Welding: Foundation of Modern Manufacturing

Resistance spot welding (RSW) stands as one of the most widely used resistance welding processes, producing coalescence at faying surfaces in a single spot through heat generated by electrical resistance. The current flows through work parts held together under pressure by electrodes, creating localized heating that forms the weld nugget.

The size and shape of individual welds depend primarily on electrode dimensions and contour. Equipment for resistance spot welding ranges from relatively simple and inexpensive single-spot machines to extremely large multiple spot welding systems designed for high-volume production.

Equipment Types and Configurations

Stationary single spot welding machines fall into two general categories: horn or rocker arm type and press type machines. Horn type machines feature a pivoted or rocking upper electrode arm actuated by pneumatic power or manual operation. These machines accommodate a wide range of work but are typically limited to 50 kVA capacity and are most suitable for thinner material gauges.

For applications requiring larger machines, typically exceeding 50 kVA capacity, press type machines provide superior performance. In these systems, the upper electrode moves within a slide mechanism, with pressure and motion provided through hydraulic or pneumatic systems, or motor operation.

High-volume production environments, particularly in the automotive industry, utilize multiple spot welding machines configured as presses with individual guns carrying electrode tips. These systems create welds in sequential order, ensuring that all electrodes do not carry current simultaneously, which optimizes power distribution and weld quality.

Projection Welding: Enhanced Electrode Life and Precision

Projection welding (RPW) produces coalescence through heat generated by electrical resistance, with the distinctive feature of localized heating at predetermined points created by projections, embossments, or intersections. This localization occurs through specially designed projections or embossments on one or both parts being welded.

Projection Types and Applications

Several projection types serve different welding requirements:

• Button or dome projections, typically round in configuration
• Elongated projections for linear applications
• Ring projections for circular joints
• Shoulder projections for stepped assemblies
• Cross wire welding configurations
• Radius projections for curved applications

The primary advantage of projection welding lies in increased electrode life due to larger contact surfaces between electrodes and workpieces. A common application involves special nuts with projections on the portion designed for welding to assemblies, providing strong, reliable connections in manufacturing operations.

Resistance Seam Welding: Continuous Joint Formation

Resistance seam welding (RSEW) creates coalescence at faying surfaces through heat generated by electrical resistance, producing continuous joints through a series of overlapping resistance spot welds made progressively along the joint using rotating electrodes.

When spots do not overlap sufficiently to produce gastight welds, the process becomes a variation known as roll resistance spot welding. This process differs from standard spot welding primarily through its use of wheel electrodes rather than stationary tips.

Equipment and Control Systems

Both upper and lower electrode wheels receive power, with pressure applied similar to press type welders. Wheel orientation can be either in-line with the machine throat or transverse. In-line configurations are typically called longitudinal seam welding machines.

Welding current transfers through roller electrode wheel bearings, and since internal water cooling is not provided, the weld area receives flood cooling to maintain proper electrode wheel temperatures.

Seam welding requires complex control systems managing travel speed and current flow sequences to ensure proper weld overlap. Welding speed, spots per inch, and timing schedules are interdependent variables that must be carefully coordinated. Welding schedules specify pressure, current, speed, and electrode wheel size parameters.

Applications and Variations

This process commonly creates flange welds and watertight joints for tanks and similar applications. Mash seam welding represents another variation where the lap joint is relatively narrow and the electrode wheel is at least twice the width used for standard seam welding. Pressure increases to approximately 300 times normal levels, resulting in final weld thickness only 25% greater than the original single sheet thickness.

Flash Welding: Complete Surface Coalescence

Flash welding (FW) produces coalescence simultaneously across the entire area of abutting surfaces through heat generated by electrical resistance between the two surfaces, followed by pressure application after heating completion.

The process involves intense flashing and metal expulsion from the joint. During welding operations, an intense flashing arc heats metal surfaces that abut each other. After a predetermined time, the two pieces are forced together, creating coalescence at the interface.

Process Mechanics

Current flow occurs through light contact between parts being flash welded. Heat generation through flashing localizes in the area between the two parts, bringing surfaces to melting point and expelling material through the abutting area. As material flashes away, small arcs form continuously until entire abutting surfaces reach melting temperature. Pressure application then extinguishes arcs and initiates upsetting.

Upset Welding: Controlled Pressure Application

Upset welding (UW) produces coalescence simultaneously over entire abutting surface areas or progressively along joints through heat generated by electrical resistance in contact areas between surfaces.

Pressure application occurs before heating begins and continues throughout the heating period. Equipment used for upset welding closely resembles flash welding equipment but can only be used when parts have equal cross-sectional areas. Abutting surfaces require careful preparation to ensure proper heating.

Process Differences

Unlike flash welding, parts are clamped in the welding machine with force applied to bring them tightly together. High-amperage current passes through the joint, heating abutting surfaces. When surfaces reach suitable forging temperature, upsetting force is applied and current stops. High temperature at abutting surfaces combined with high pressure causes coalescence. After cooling, force is released and the weld is completed.

Percussion Welding: Rapid Energy Discharge

Percussion welding (PEW) produces coalescence of abutting members using heat from arcs created by rapid electrical energy discharge, with pressure applied progressively during or immediately following electrical discharge.

This process resembles flash welding and upset welding but is limited to parts with identical geometry and cross-sections. The process complexity exceeds other methods because heat comes from arcs produced at abutting surfaces through very rapid stored electrical energy discharge across rapidly decreasing air gaps, immediately followed by pressure application creating progressive percussive impact.

The primary advantage lies in extremely shallow heating depth and very short time cycles. Applications are limited to parts with relatively small cross-sectional areas.

High-Frequency Resistance Welding: Specialized Applications

High-frequency resistance welding (HFRW) produces metal coalescence through heat generated by workpiece resistance to high-frequency alternating current in the 10,000 to 500,000 hertz range, followed by rapid upsetting force application after heating completion. Current path control occurs through proximity effect.

Applications and Process Control

This process is ideally suited for manufacturing pipe, tubing, and structural shapes, as well as other items made from continuous material strips. High-frequency welding current is introduced into metal at surfaces to be welded before their contact with each other.

Current introduction occurs through sliding contacts at joint edges. High-frequency welding current flows along one seam edge to the welding point between pressure rolls and returns along the opposite edge to the other sliding contact.

The extremely high frequency causes current to flow along metal surfaces to depths of several thousandths of an inch. Each joint edge serves as a current conductor, concentrating heating on edge surfaces. At the area between closing rolls, material reaches plastic temperature, and applied pressure creates coalescence.

Conclusion

Resistance welding processes provide versatile solutions for modern manufacturing challenges, each offering specific advantages for particular applications. From the widespread use of spot welding in automotive assembly to the specialized applications of high-frequency resistance welding in pipe manufacturing, these processes continue to evolve with advancing technology and changing industrial requirements. Understanding the fundamental principles and distinctive characteristics of each process enables manufacturers to select optimal welding methods for their specific applications, ensuring quality, efficiency, and cost-effectiveness in production operations.