9.3 Resistance Welding Physics, Electrode Materials & Spot Weld Sequence

9.2 Resistance Welding: Spot (RSW), Seam (RSEW) & Projection Dynamics

Quick Answer: Resistance Welding (RW) generates localized coalescence by combining mechanical clamping force with resistive Joule heating ($Q = I^2 R t$) produced by passing high electrical currents ($5\text{ to }>25\text{ kA}$) across overlapping sheet workpieces. Total resistance is a dynamic series sum consisting of bulk material resistance and three contact interfaces: electrode-to-sheet (top and bottom) and the faying interface between sheets. Liquid nugget nucleation begins at the faying interface where contact resistance peaks. Controlled electrode forging force consolidates the molten pool during the hold cycle, preventing solidification shrinkage cracks and catastrophic expulsion.

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Resistance Welding Physics & Joule's Law

Unlike arc welding, Resistance Welding (AWS C1.1) requires no shielding gas, flux, or filler metal. Thermal energy is generated internally within the workpieces by resistive dissipation of electric current per Joule's Law:

$$Q = \int_0^t I^2(t) R(t) \, dt \approx I^2 R t$$

where:

• $Q$ = Heat energy generated (Joules, $\text{J}$)
• $I$ = Welding current (Amperes, $\text{A}$)
• $R$ = Total dynamic electrical resistance of the circuit (Ohms, $\Omega$)
• $t$ = Weld duration (seconds, $\text{s}$ or AC cycles at $60\text{ Hz}$, where $1\text{ cycle} = 1/60\text{ s} \approx 16.67\text{ ms}$)

Because heat generation scales quadratically with current ($I^2$), current is the primary parameter governing nugget growth. However, resistance ($R$) dictates where that heat is localized.

                    [ (+) Upper Water-Cooled Electrode ]
      - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - R_1: Electrode-to-Sheet Interface
                             Sheet 1 (t_1)
      - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - R_2: Bulk Resistance of Sheet 1
       ~~~~~~~~~~~~~~~~~ [ Molten Nugget ] ~~~~~~~~~~~~~~~~~~~~ R_3: Faying Interface (Highest Initial R)
      - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - R_4: Bulk Resistance of Sheet 2
                             Sheet 2 (t_2)
      - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - R_5: Electrode-to-Sheet Interface
                    [ (-) Lower Water-Cooled Electrode ]

Series Circuit Resistance Breakdown

The total resistance $R_{\text{total}}$ between the electrode connection faces is the sum of five resistances in series:

$$R_{\text{total}} = R_1 + R_2 + R_3 + R_4 + R_5 = 2 R_{\text{electrode-sheet}} + 2 R_{\text{bulk}} + R_{\text{faying}}$$

1. $R_1$ and $R_5$ (Electrode-to-Sheet Contact Resistance): Undesirable parasitic resistances. High values cause surface burning, localized melting at the electrode tip, and rapid electrode degradation. Controlled by water-cooling the copper electrodes and applying sufficient electrode clamping force.
2. $R_2$ and $R_4$ (Bulk Material Resistance): Governed by material resistivity $\rho_e(T)$, sheet thickness $t$, and current-conducting cross-sectional area $A$: $$R_{\text{bulk}} = \frac{\rho_e(T) \cdot t}{A}$$ As temperature increases toward the melting point, $\rho_e$ of steel increases by $4\times$ to $6\times$, increasing bulk Joule heating.
3. $R_3$ (Faying Interface Contact Resistance): The critical resistance required to initiate the weld nugget. At the microscale, surfaces touch only at microscopic asperities. Contact resistance depends inversely on applied electrode force ($F$) and material hardness ($H_v$): $$R_{\text{contact}} \propto \frac{\rho_e}{F^n}$$ where $n$ typically ranges from $0.5\text{ to }1.0$.

The Dynamic Resistance Curve

During a 12-to-16 cycle AC or Medium-Frequency Direct Current (MFDC) weld, $R_{\text{total}}$ changes continuously:

• Stage I (Asperity Breakdown): In the first 1–2 cycles, high local current densities heat and soften surface asperities. Under electrode force, asperities yield plastically, breaking oxide films. Contact resistance $R_3$ drops sharply.
• Stage II (Thermal Expansion & Bulk Heating): As current flows through virgin metal, temperature climbs. The positive temperature coefficient of electrical resistivity dominates, causing $R_{\text{total}}$ to rise to an intermediate peak.
• Stage III (Nugget Nucleation & Molten Growth): When the core reaches melting temperature, a liquid pool nucleates at the faying interface. Although liquid metal has high resistivity, the surrounding plasticized ring expands the effective electrical contact area significantly, causing total resistance to drop steadily.
• Stage IV (Steady State / Solidification): Thermal equilibrium is established between Joule heat generation and heat dissipation into the water-cooled electrodes.

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Electrode Materials & RWMA Classifications

The Resistance Welding Manufacturing Alliance (RWMA) classifies electrode materials into two distinct metallurgical groups:

Group A: Copper-Base Alloys

Group B: Refractory Metal Compositions

Principle of Thermal Balance

When welding dissimilar metals with vastly different electrical/thermal conductivities (e.g., copper alloy to stainless steel), the weld nugget will form entirely inside the higher-resistivity stainless steel. To balance heat generation:

1. Use a high-conductivity electrode (Class 1 or 2) against the low-conductivity (stainless) workpiece.
2. Use a low-conductivity, refractory electrode (Class 10, 11, or 13) against the high-conductivity (copper) workpiece to reflect and concentrate heat.

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Resistance Spot Welding (RSW) Sequence & Kinematics

A complete resistance spot weld cycle consists of four primary chronological phases governed by pneumatic or servo-driven mechanical actuations and precise firing controls:

  Electrode
    Force  ________/-------------------------------------\________
           |       |   Squeeze   |  Weld  | Hold (Forge) |  Off   |
  Current  |       |             |~~~~~~~~|              |        |
           +-------+-------------+--------+--------------+--------+---> Time

1. Squeeze Time: The electrode tips close upon the workpieces, applying forging force ($F$). Mechanical pressure forces the plates into intimate mechanical fit-up, flattens local gaps, and crushes surface oxide films. Current is zero ($I = 0$). Premature current initiation before reaching target force produces catastrophic expulsion and electrode damage.
2. Weld Time: High-amperage current flows through the stack-up. Heat generated by $I^2 R t$ initiates melting at the faying interface within 2–4 cycles. The liquid nugget expands radially until heat generation balances conductive heat losses to the water-cooled copper tips.
3. Hold (Forge) Time: Current is terminated ($I = 0$), but electrode force is maintained (or boosted via a forge pulse). The water-cooled electrodes rapidly quench the nugget at cooling rates exceeding $10^3\text{ to }10^4\text{ }^\circ\text{C/s}$. The applied force consolidates the solidifying pool, preventing centerline shrinkage porosity, gas cavitation, and solidification hot cracking.
4. Off Time: Electrodes release clamping pressure and retract, allowing automated transfer to the next weld pitch location.

Advanced Multi-Pulse Schedules

• Pre-Heat / Upslope: A low-current pulse softens surface zinc or aluminum coatings, displacing them before the main weld current fires to prevent liquid metal embrittlement (LME).
• In-Situ Quench and Temper: In Advanced High-Strength Steels (AHSS, martensitic steels, boron steel 22MnB5), rapid cooling produces brittle martensite. A secondary lower-amperage pulse reheats the nugget above the martensite tempering range without remelting, restoring toughness.

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