Weld Heat (Joule's Law)
H = I² × R × t
H = heat energy (J), I = current (A), R = resistance (Ω), t = time (s). Current has the biggest effect since it is squared.
AKWA TECHNIC TECHNICAL LIBRARY
Technical library.
A comprehensive reference center on resistance spot welding technologies, parameters and applications.
7reference sections
15glossary terms
5interactive tools
3,800+data points
INTERACTIVE WELDING GUIDE
Parameters, defects, formulas and cylinder sizing.
The references welding engineers reach for most, in one place.
Nugget Diameter Estimate
d ≈ 4√t – 5√t (mm)
d = nugget diameter, t = thickness of the thinner sheet (mm). E.g. for 1 mm sheet the nugget should be roughly 4–5 mm.
Cycle–Millisecond Conversion
1 cycle = 1000 / f (ms)
On 50 Hz power, 1 cycle = 20 ms; on 60 Hz power, 1 cycle ≈ 16.7 ms.
Cylinder Force
F = P × A
F = force, P = pressure, A = piston area (A = π×(D/2)²). Try it in the calculator below.
Current Density
J = I / A_electrode
J = current density, I = current, A_electrode = electrode tip contact area. A too-small tip causes excessive density and expulsion.
Types of resistance welding
Resistance welding is a family of joining processes that fuse metal parts using the heat generated by applied current and force. Akwa Technic manufactures both standard and custom machines for all four core types.
- Spot welding
- The most common resistance welding process, joining two or more sheets point by point. Current passed between two opposing electrodes clamped onto the parts creates a localized molten zone (the nugget) at the interface. Widely used in automotive body, white-goods and metal furniture manufacturing.
- Projection welding
- One or more pre-formed projections (embossments) on a part concentrate current and force at specific points, allowing multiple welds to be made simultaneously in a single stroke. Improves throughput for welding nuts, studs and brackets.
- Seam welding
- Continuous or pulsed current is applied as parts pass between rotating wheel electrodes, producing a series of overlapping spot welds that form a leak-tight, continuous seam. Used for tanks, drums and enclosures that require a sealed joint.
- Butt / flash welding
- The end faces of two parts are brought into contact and heated by current flow, then forged together under axial force to form a full-section joint. Preferred for joining bars, tubes, chain links and rolled profiles.
Fundamental welding parameters
Resistance spot welding quality is governed by three interdependent variables — often called the "Three Golden Rules" of resistance welding. Every process setup is a balance between these three.
- Welding current
- The current driven through the joint generates heat by I²R (Joule) heating at the faying interface. It is the single most influential variable on nugget size — small changes in current produce large changes in heat input.
- Weld time
- The duration current flows, expressed in cycles of mains frequency (e.g. 10 cycles at 50 Hz = 200 ms) or milliseconds on inverter controls. Longer time increases heat input but also heat loss into the surrounding material and electrodes.
- Electrode force
- The mechanical force clamping the parts together. It sets the contact resistance at the interfaces, contains the molten nugget, and forges the joint as it solidifies. Insufficient force causes expulsion; excessive force reduces contact resistance and heat generation.
| Parameter | Typical effect if too low | Typical effect if too high |
|---|---|---|
| Welding current | Undersized or missing nugget, cold weld | Expulsion, deep electrode indentation, surface burning |
| Weld time | Insufficient fusion, weak joint | Expulsion, excessive heat-affected zone, electrode sticking |
| Electrode force | Expulsion, high contact resistance, spatter | Reduced heat generation, undersized nugget, deep indentation |
How to set welding parameters
Finding the right welding parameters is a systematic process, not trial and error. The steps below help you establish safe, repeatable starting settings for a new part or material.
- 1. Identify material and thickness
- Determine the material type (low-carbon, galvanized, stainless, etc.) and thickness of both sheets — starting current, time and force are selected primarily from these. For dissimilar-thickness stacks, base settings on the thinner sheet.
- 2. Take starting values from machine and electrode data
- Electrode or machine manufacturer weldability charts and weld lobe curves show the safe current-time window for a given material and thickness. Treat these as a starting point for trial welds, not a fixed recipe.
- 3. Set electrode force
- Set electrode force appropriate to the material and thickness, and confirm it stays stable throughout the weld. Force fluctuation is one of the most overlooked causes of inconsistent weld quality.
- 4. Make a trial weld and inspect visually
- Run a trial weld at the starting values. Check for excessive expulsion, surface burning or electrode sticking — early signs that current or time is set too high.
- 5. Verify with destructive testing
- Measure nugget diameter with a peel or chisel test and confirm it meets the minimum required for the material thickness per your applicable standard.
- 6. Map the weld window (lobe curve)
- Step current up and down to find the lower limit (insufficient fusion) and upper limit (expulsion). Running near the center of this window in production gives a safety margin against process variation.
- 7. Re-verify periodically in production
- Electrode wear and material batch variation can affect weld quality over time — even with fixed parameters, confirm quality periodically with destructive testing or monitoring.
Spot welding cycle
A complete resistance spot weld is made of four sequential phases, controlled by the welding timer or the machine's programmable logic controller.
- Squeeze time
- The interval between electrode closure and the start of current flow. It allows the electrode force to fully build up and stabilizes contact before current starts, preventing expulsion at weld initiation.
- Weld time
- The period during which welding current flows and the nugget forms. May be split into multiple current pulses ("upslope" or multi-pulse schedules) on thicker or coated materials to control heat input.
- Hold time
- Current stops, but electrode force is maintained so the molten nugget solidifies under pressure, refining grain structure and preventing porosity or cracking.
- Off time
- The electrodes release and the machine resets — either to move to the next weld point or complete the cycle. Also called interval or break time.
Electrode selection & maintenance
Electrodes conduct current and force into the joint while dissipating heat; their material, geometry and condition directly determine weld consistency and tool life.
- CuCr (copper-chromium)
- The most widely used electrode alloy for general steel welding. Good balance of electrical conductivity, hardness and resistance to mushrooming at moderate duty cycles.
- CuCrZr (copper-chromium-zirconium)
- Higher strength and better resistance to softening at elevated temperature than CuCr, making it preferred for high-volume production, galvanized steel, and higher-force applications.
- Face geometry
- Common face profiles include truncated cone, domed/radius, flat and offset designs — see the diagram below. Face diameter is selected relative to sheet thickness to control current density.
- Mushrooming
- Progressive flattening and diameter growth of the electrode face caused by thermal softening and mechanical loading. It increases contact area, reduces current density, and produces undersized nuggets if not corrected.
- Dressing & redressing
- Periodic machining of the electrode face restores its original geometry and diameter. Dressing intervals should be set from weld count or destructive testing, not fixed calendar time, since wear rate depends on material and duty cycle.
- Cooling
- Internal water cooling channels remove heat from the electrode tip and holder. Adequate, unobstructed flow is essential — scale build-up or low flow accelerates mushrooming and shortens electrode life.
Weldability of materials
Each base material presents a different resistance-welding "process window" due to its electrical resistivity, thermal conductivity and surface condition.
- Low-carbon steel
- The benchmark material for resistance welding: moderate resistivity, wide acceptable parameter range, and forgiving of minor process variation.
- Galvanized steel
- The zinc coating has lower melting point and resistivity than steel, requiring higher current and contaminating electrode faces faster — expect shorter dressing intervals and slightly wider current settings.
- Stainless steel
- Higher electrical resistivity and much lower thermal conductivity than carbon steel concentrate heat at the interface, allowing lower current and shorter time — but overheating risks chromium-carbide precipitation and reduced corrosion resistance near the weld.
- Aluminum
- Very high thermal and electrical conductivity require significantly higher current in a much shorter time. The tenacious oxide surface layer must be controlled (cleaning or specialized electrode alloys), and process windows are narrower than steel.
| Material | Relative resistivity | Thermal conductivity | Typical current level | Key challenge |
|---|---|---|---|---|
| Low-carbon steel | Moderate | Moderate | Baseline | Few — wide process window |
| Galvanized steel | Moderate (coating varies) | Moderate | Baseline + 10–30% | Electrode contamination, faster wear |
| Stainless steel | High | Low | Lower, shorter time | Carbide precipitation if overheated |
| Aluminum | Low | Very high | Much higher, very short time | Oxide layer, narrow process window |
Common defects & troubleshooting
Most resistance-welding defects trace back to an imbalance between current, time and force, or to worn electrodes and contaminated surfaces.
- Expulsion (spatter)
- Molten metal ejected from the weld interface, caused by excess current, excess time, insufficient electrode force, or contaminated/misaligned surfaces. Reduces effective nugget size and can damage part surfaces.
- Undersized nugget
- The fused weld zone is smaller than specification, from insufficient current or time, worn/mushroomed electrodes, or excessive electrode force limiting heat generation. Verified by peel or chisel destructive testing.
- Deep electrode indentation
- Excessive marking of the sheet surface from too much force, too much current/time, or a worn electrode face with reduced diameter concentrating pressure.
- Cold weld (lack of fusion)
- Insufficient heat input — often from low current, short weld time, high contact resistance from dirty or oxidized surfaces, or poor electrode alignment — leaves the interface unfused despite surface appearance looking acceptable.
| Defect | Likely cause | Corrective action |
|---|---|---|
| Expulsion / spatter | Excess current or time, low force, dirty surface | Reduce current/time, increase force, clean surfaces, check alignment |
| Undersized nugget | Low current/time, mushroomed electrode, excess force | Increase current/time, dress or replace electrodes, verify force setting |
| Deep indentation | Excess force, worn electrode face, excess heat | Reduce force, dress electrode face, check current/time |
| Cold weld | Low current/time, contact resistance, misalignment | Increase current/time, clean surfaces, verify electrode alignment |
Glossary of terms
Quick definitions of the technical vocabulary used across this library.
- Welding current
- The electrical current passed through the workpieces to generate resistive heat at the joint.
- Weld time
- Duration current flows during a weld, measured in cycles (mains frequency) or milliseconds.
- Electrode force
- Mechanical clamping force applied by the electrodes during welding.
- Squeeze time
- The delay after electrode closure, before current starts, used to stabilize force and contact.
- Hold time
- Post-weld period where force is maintained while the nugget solidifies.
- Nugget
- The fused, cast metal zone at the center of a resistance spot weld.
- Mushrooming
- Deformation and diameter growth of an electrode face from thermal and mechanical wear.
- Dressing
- Machining an electrode face back to its original geometry and diameter.
- CuCr
- Copper-chromium electrode alloy, the general-purpose standard for steel welding.
- CuCrZr
- Copper-chromium-zirconium electrode alloy with higher strength and thermal resistance than CuCr.
- Expulsion
- Molten metal ejected from the weld interface, usually due to excess heat or insufficient force.
- Indentation
- The mark left in the sheet surface by the electrode face after welding.
- Cold weld
- A weld with insufficient fusion at the interface despite an acceptable external appearance.
- Contact resistance
- Resistance at the interface between the electrode and the sheet, or between the two sheets.
- Duty cycle
- The proportion of time a welding machine or electrode is actively under load versus resting.