Resistance Spot Welding (RSW)

Overview

In RSW, copper alloy electrodes clamp the sheets together and pass a high current through the joint. The resistance at the faying surfaces generates heat, melting a small volume of metal that solidifies into a weld nugget. The process is fast, repeatable, and easily automated, making it ideal for mass production of thin‑gauge sheet metal assemblies.

Apparatus and Working

Apparatus

• Power source: Transformer delivering low‑voltage, high‑current AC or DC.
• Electrodes: Copper alloy tips shaped to concentrate current and pressure.
• Electrode holders and arms: Mechanism to apply force and conduct current.
• Control system: Regulates current, time, and electrode force.
• Cooling system: Water cooling for electrodes and arms to prevent overheating.

Working Steps

1. Clamping: Electrodes press the sheets together with a set force.
2. Current application: High current flows through the contact area for a controlled time.
3. Heat generation: Resistance at the interface produces localized melting.
4. Nugget formation: Molten metal solidifies under pressure to form a weld nugget.
5. Hold and release: Current stops, electrodes hold until solidification, then release.

Principle

RSW is based on Joule’s law of heating: Heat (Q) = I² × R × t, where I is current, R is resistance, and t is time. The resistance at the sheet interface is highest, so most heat is generated there. Combined with electrode pressure, this produces a localized weld nugget without melting the entire sheet.

Advantages and Disadvantages

Advantages

• Very fast process; cycle times in milliseconds.
• Easily automated for mass production.
• No filler metal or flux required.
• Clean process with minimal fumes or spatter.
• Consistent weld quality with proper controls.

Disadvantages

• Limited to lap joints of thin sheet metals (typically up to 3 mm each).
• Electrode wear requires maintenance and dressing.
• Not suitable for non‑conductive or very thick materials.
• Weld nugget is small; not ideal for high‑strength structural joints alone.
• Access required from both sides of the workpiece.

Applications

• Automotive industry: Body panels, frames, and assemblies.
• Aerospace: Lightweight sheet metal structures.
• Appliances: Refrigerators, washing machines, and enclosures.
• Electronics: Battery tabs and small sheet metal parts.
• General fabrication: Sheet metal furniture, cabinets, and ducts.

Key Process Parameters

• Welding current: Determines heat input; too low causes weak welds, too high causes expulsion.
• Weld time: Duration of current flow; must be optimized for nugget size.
• Electrode force: Ensures contact and controls nugget shape; too low causes expulsion, too high reduces resistance.
• Electrode tip size: Affects current density and nugget diameter.
• Cooling rate: Influences microstructure and mechanical properties.

Common Defects in RSW

• Expulsion: Molten metal ejected due to excessive current or low force.
• Porosity: Gas entrapment in the nugget from contamination.
• Lack of fusion: Inadequate current or time leading to weak joints.
• Electrode indentation: Excessive force or worn electrodes leaving marks.
• Cracks: Solidification or stress cracks in the nugget or HAZ.

Procedure and Quality Control

• WPS compliance: Follow qualified welding procedures for material and thickness.
• Electrode maintenance: Regular dressing and replacement to maintain consistency.
• Parameter monitoring: Control current, time, and force with programmable controllers.
• Inspection: Destructive testing (peel, chisel, tensile shear) and NDT (ultrasonic, eddy current).
• Documentation: Record weld schedules, electrode life, and inspection results.