The basic formula for spot welding is related to heat generation and weld current:

Where:

(Q) = heat generated (joules)
(I) = welding current (amperes)
(R) = electrical resistance at the weld spot (ohms)
(t) = welding time (seconds)

This formula shows that heat increases with higher current, resistance, or welding time, which is critical for forming a strong spot weld.

Formula for Spot welding:

Here’s a detailed explanation of the formula and calculations used in spot welding, including heat generation, weld nugget sizing, and key parameters:

1. Fundamental Principle

Spot welding is based on resistance heating (Joule heating). The heat ( Q ) generated at the contact point depends on:

Electrical current ( I )
Resistance ( R ) of the metal at the contact interface
Time ( t ) the current is applied

The basic formula is:

Where:

( Q ) = heat energy generated (Joules)
( I ) = welding current (Amperes)
( R ) = electrical resistance of the metal at the weld spot (Ohms)
( t ) = welding time (seconds)

This formula shows that heat increases with the square of the current, which is why precise control of current is critical in spot welding.

2. Heat Required to Melt the Metal

To create a weld nugget, the generated heat must at least equal the energy required to raise the metal temperature to melting:

Where:

( Q_m ) = heat required to melt the metal (Joules)
( m ) = mass of metal at the weld spot (kg)
( c ) = specific heat capacity of the metal (J/kg·°C)
( \Delta T ) = temperature rise needed to reach melting point (°C)
( L_f ) = latent heat of fusion of the metal (J/kg)

The resistance heating formula ( Q = I^2 R t ) must match or exceed ( Q_m ) to create a proper weld nugget.

3. Electrode Force and Contact Resistance

The electrode force ( F ) affects the contact resistance ( R_c ):

Higher pressure reduces contact resistance → more current flows through the metal rather than the electrodes → more localized heating.
Low force → higher resistance at the contact → inconsistent heating or poor nugget formation.

4. Weld Nugget Size Calculation

The weld nugget diameter ( D ) depends on the current, time, and sheet thickness. An empirical formula often used is:

Where:

( D ) = nugget diameter (mm)
( I ) = welding current (kA)
( t ) = weld time (seconds)
( k ) = material constant (depends on steel, aluminum, thickness, electrode type)