Resistance spot welding was engineered around steel. Point it at aluminium and everything works against you: the oxide layer destabilises contact resistance, the metal’s conductivity demands enormous currents, electrodes foul and wear within hundreds of shots, and nugget quality varies shot to shot. Friction stir spot welding (FSSW) makes the same joint by forging, not melting: consistent solid-state spots, no electrode dressing cycle, and energy consumption reported at around one-twentieth of resistance welding equipment in Mazda’s pioneering production application. Resistance welding keeps the edge on steel bodies-in-white, where a century of infrastructure and know-how still rules.
Why resistance spot welding and aluminium do not get along
Every problem resistance spot welding has with aluminium comes from the same three material facts:
- The oxide layer. Aluminium’s surface oxide has high and variable electrical resistance, so the heat generated at the faying surface varies from shot to shot — and weld nugget quality varies with it.
- Conductivity. Aluminium conducts electricity and heat several times better than steel, so forming a molten nugget takes far higher currents (tens of kiloamps), bigger transformers and heavier cooling.
- Electrode attack. Molten aluminium alloys with copper electrodes. Tips foul and mushroom quickly, forcing frequent dressing and replacement — a maintenance cadence steel lines never see.
Add the fusion-welding classics — porosity and solidification cracking inside the nugget, invisible without NDT — and it is clear why aluminium sheet assembly historically fell back on rivets and clinching instead.
How friction stir spot welding changes the equation
Friction stir spot welding replaces the electrical nugget with a mechanical one: a rotating tool plunges through the top sheet, stirs the interface in the solid state, and forges the two sheets together. No current through the stack, no melt pool, no electrodes to dress.
| Criterion | Resistance spot welding (Al) | Friction stir spot welding |
|---|---|---|
| Joint formation | Molten nugget | Solid-state forged bond |
| Sensitivity to oxide / surface state | High — variable nugget quality | Low — oxide is mechanically dispersed |
| Porosity / cracking in the spot | Present in the cast nugget | Eliminated by mechanism |
| Electrode / tool life | Hundreds of shots between dressing | Thousands of welds per tool |
| Energy demand | Very high peak currents | Around 1/20th of resistance-welding equipment (Mazda, production data) |
| Infrastructure | Large transformers, water cooling | Machine-tool spindle class |
| Shot-to-shot consistency | Process drift with electrode wear | Parameter-controlled, monitorable |
| Witness mark | Surface indentation both sides | Keyhole in top sheet |
| Steel body-in-white | Unbeatable — stay with resistance welding | Niche |
Where this substitution has already happened
Mazda introduced friction stir spot welding into aluminium body production in the early 2000s (RX-8 rear door and bonnet assemblies), publicly citing equipment energy consumption around 5% of resistance welding and dramatically lower capital in cabling and cooling. The approach spread through aluminium closures and, more recently, into EV hardware — battery enclosures, cooling plates and busbar connections — where consistent, low-energy aluminium spot joints are exactly what the platform needs.
Refill friction stir spot welding — the variant that closes the keyhole as part of the cycle — was demonstrated in the publicly funded OASIS programme (Clean Sky 2, CORDIS 785557) through uncured sealant, Alclad AA2024 and anodised surfaces. The tool breaks the interface and disperses those particles in the stir zone. Welding through paint was not successful. Sealant has to be uncured so that process force can expel it from the interface.
The same solid-state spot has a second life beyond sheet assembly: electrical joints. A forged aluminium-aluminium or aluminium-copper spot is a low-resistance, metallurgically continuous connection that does not rely on clamping force or suffer the intermetallic brittleness of fused aluminium-copper joints — increasingly relevant for busbars, battery interconnects and power-transmission hardware where every micro-ohm is an efficiency loss.
And because the process is local and solid-state, it doubles as a repair process: an individual failed joint on a brazed or bonded assembly can often be re-made in place with a single spot, without re-processing the whole part. See repairing failed brazed heat exchangers.
When resistance welding is still the right answer
- Steel. On steel bodies-in-white, resistance spot welding is fast, cheap, robust and supported by a century of infrastructure. There is no case for change.
- Cycle times below a second at automotive line rate, where resistance-welding guns and robots are already amortised.
- Access. A resistance-welding gun reaches around a flange from both sides; friction stir spot welding needs anvil support behind the joint and stiffness in the fixturing.
Frequently asked questions
Does the friction stir spot welding keyhole weaken the joint?
The exit hole sits in the centre of a forged annulus that carries the load. Lap-shear and fatigue performance is set by the bonded area around it. Refill variants close the hole where a flush surface is required.
Can friction stir spot welding join through sealant, Alclad or anodising?
Refill friction stir spot welding through uncured sealant, Alclad AA2024 and anodised surfaces was demonstrated in the publicly funded OASIS programme (Clean Sky 2, CORDIS 785557). The tool breaks the interface and disperses those particles. Paint was not successful. Sealant has to be uncured so that process force can expel it. That is not the same as 3003/4045 braze cladding.
Can friction stir spot welding join aluminium to copper?
Yes. Solid-state joining avoids the brittle intermetallic layers that make fused aluminium-copper joints unreliable, which is why it is used for electrical connections.
When is resistance spot welding still the right answer?
On steel bodies-in-white, at cycle times below a second where resistance-welding guns are already amortised, and where a gun can reach a flange from both sides but anvil support for a stir tool cannot be provided.
Next step A structured feasibility trial answers the question on your geometry, with measurements rather than argument.