Resistance welding sequence — force and current timing: In resistance spot/seam welding, when should electrode pressure be applied relative to the welding current?

Introduction / Context:
Resistance welding depends on heat generated by electrical resistance at the faying surfaces, combined with mechanical force to ensure intimate contact and containment of the molten nugget. Correct timing of electrode force and current is vital for nugget formation and joint quality.

Given Data / Assumptions:

• Standard spot or seam welding with copper alloy electrodes.
• Weld cycle includes squeeze time, weld time, and hold time.
• Goal: produce a consistent solidified nugget with minimal expulsion.

Concept / Approach:
The classic sequence is: apply electrode force first (squeeze) to ensure close contact and stable resistance; then pass current for a set weld time to form the molten nugget; finally hold the force for a hold time while current ceases so the nugget solidifies under pressure, preventing porosity and expulsion. Applying force only after current would cause arcing and spatter, while releasing force during current destabilises the nugget.

Step-by-Step Solution:
Squeeze: apply and stabilise force before current.Weld: maintain force while current flows to form the nugget.Hold: maintain force briefly after current to consolidate and solidify.Therefore, pressure must be applied and held before, during, and briefly after current flow.

Verification / Alternative check:
Most weld controllers let you program squeeze, weld, and hold times explicitly, reflecting this standard sequence.

Why Other Options Are Wrong:
Applying pressure only after current risks surface arcing and expulsion.

No-pressure claims contradict the fundamental principle of resistance welding.

Applying force only after solidification prevents proper nugget formation.

Releasing force during current destabilises the nugget and increases spatter.

Common Pitfalls:
Insufficient squeeze time; too short hold time causing shrinkage voids; worn electrode tips increasing contact area and lowering current density.

Final Answer:
pressure is applied and held before, during, and briefly after current flow