Resistance spot welding

In an ideal I²Rt model with 100 µΩ constant resistance and 0.100 s current-on, 5,000 A gives 250 J, and 5,500 A gives 302.5 J, a 21% increase. Real contact resistance changes, and heat flows into the electrodes and sheets. Verify nugget geometry and strength separately.

Current passes through the stack under electrode force. Resistance heating forms a local nugget, while electrodes contain the joint and remove heat. Qualify material, geometry, force, current and time together. AMADA WELD TECH, Resistance Welding Fundamentals, undated, AWS, electrode guidance, July 2025

Define the stack and access

Specify every sheet's grade, condition, thickness and coating, the stack order, fit-up, electrode-contact surfaces and available access. A schedule qualified on two equal bare sheets is not automatically qualified for a three-sheet coated stack or an unequal-thickness joint. Identify the required weld location and the load path before choosing a gun or electrode configuration.

AWS explains that electrode material and face geometry follow the materials and application. Its July 2025 guidance identifies applying force, containing the molten nugget and dissipating heat as electrode functions; it also describes using different face diameters to influence nugget location in unequal thicknesses. Thus an electrode is a thermal/electrical/mechanical process element, not merely a replaceable contact. Confirm its alloy, face, cooling and access for the actual stack. AWS, Recommendations for Selecting Spot Welding Electrodes

Current, force and timing

AMADA's fundamentals document identifies electrode force, squeeze time, weld pulse and hold time as critical schedule parameters and depicts the squeeze–heat–hold–off sequence. Force improves mating and changes contact resistance. A repeatable current command does not establish repeatable heat distribution if contact and force change. Define the complete schedule and the actual force delivered at the electrodes rather than only the controller's current setting. AMADA WELD TECH, weld schedule and force sections

Squeeze establishes contact before current; hold retains force after the heating pulse while the joint cools. These intervals must remain distinguished from current-on time in cycle estimates and procedure records. The schedule should specify the relevant timing units and power-source waveform; “cycles” without its frequency basis is ambiguous.

Establish a welding window

The European Aluminium Automotive Manual, version 2015, explains a weld lobe for a fixed material combination, joint configuration, electrode arrangement and force. Its typical axes are current and weld time. The lower boundary represents minimum acceptable button size and the upper boundary expulsion. This is a useful qualification concept, not a universal chart of safe settings for another alloy or stack. European Aluminium, resistance-welding chapter, printed p. 11

A point inside a demonstrated lobe does not establish unlimited process tolerance or electrode life. Agree how input variation and electrode condition are represented in the trials. Record whether a reported window was obtained with fresh tips, dressed tips or a defined production interval. Treat excessive expulsion as an outcome to investigate, not proof that a weld has become sufficiently strong.

Coatings and electrode condition

TWI explains that zinc-coated steel differs from bare steel in required current/force and in electrode deterioration: the coating can alloy with copper tips. The actual change depends on coating and thickness combination. Its illustrative adjustments should not be copied as a universal percentage increase. Qualify the ordered coating, not only the underlying steel grade. TWI, coated-steel spot welding, undated

TWI separately discusses electrode wear and dressing, including automatic tip changing, tip-size/alignment checks and force/current measurement. Include tip maintenance and replacement in the production process and demonstrate the intended service interval. Correcting a worn tip by increasing current is not equivalent to proving that the original process remains capable. TWI, electrode-wear guidance, undated

Weld spacing, sequence and repair

Existing welds can provide an alternative electrical path. TWI identifies current shunting through nearby spots as a consideration when placing additional welds. It also explains that over-welding an existing spot changes contact conditions and can require a different, tested process. A repair is not simply another execution of the original schedule. TWI, over-welding and repair guidance, undated

Define spacing, edge distances and sequence on the component or manufacturing plan. Where an altered sequence, nearby weld or rework changes the current path, test representative assemblies. Do not infer total assembly capacity by multiplying a single coupon's result by weld count; load sharing and the actual joint geometry require their own assessment.

Original electrical-heating example

Use a deliberately simplified constant-resistance model, E = I²Rt, with invented values: 5,000 A, 100 µΩ = 0.000100 Ω, and 0.100 s current-on time:

E = 5,000² × 0.000100 × 0.100 = 250 J

At 5,500 A, with the same assumed resistance and time:

E = 5,500² × 0.000100 × 0.100 = 302.5 J

A 10% current increase gives a 21% increase in this modeled electrical heat term. AMADA describes this square-law relationship while also explaining how force and contact resistance interact. AMADA, resistance-welding fundamentals

Resistance changes with contact, heating and deformation. Heat also flows into electrodes and sheets. Qualify the actual current and time combination with nugget and strength evidence.

Inspection and applicable scope

ISO 14373:2024, edition 3, January 2024, concerns resistance spot welding of uncoated and coated low-carbon steels. Use its applicable scope and the ordered requirements rather than treating its number as a procedure for every stainless steel, aluminium or dissimilar stack. The public publisher record establishes the edition; detailed application requires the applicable document and contract. ISO, title and edition

TWI distinguishes insufficient weld size from a correctly fused but low-ductility nugget and from cracking/porosity-related failures. Fracture mode and appearance must be interpreted with material and loading conditions, not used as one universal pass/fail rule. Its guidance calls for destructive confirmation when establishing suitable conditions. TWI, interface failures, undated

Specify external indentation/surface limits separately from nugget and mechanical requirements. Agree the examination method, sample location, loading mode, acceptance threshold and sampling interval; a visible electrode mark does not establish internal fusion or assembly strength. Provide the complete stack, weld map, electrode details, qualified schedule, maintenance plan and repair restrictions in the handoff. This planning structure and the numerical example are original; the drawing, contract and applicable procedure set actual acceptance.