9.4 Nugget Sizing, Seam and Projection Welding, and Lobe Curve Process Windows

Nugget Penetration, Sizing & Expulsion Mechanics

Minimum Nugget Diameter

Under AWS D8.9 (Automotive), AWS D17.2 (Aerospace), and AWS C1.1, the nominal acceptable weld nugget diameter ($d_n$) must satisfy:

$$d_{\text{min}} = 4 \sqrt{t}$$

where $t$ is the single-sheet thickness of the thinner sheet in millimeters. For safety-critical automotive crash applications, specifications often enforce $d_{\text{critical}} = 5 \sqrt{t}$.

• Target Penetration: Molten penetration must extend between $20%$ and $80%$ into each sheet. Penetration $<20%$ indicates cold bonding, while $>80%$ leads to surface overheating, severe indentation, and electrode degradation.

                         [ Upper Electrode ]
                         +-----------------+
               ==========|   Indentation   |========== Sheet 1 (t)
             (           \                 /           )
            (             |  Nugget (d_n) |             ) Penetration = 20% to 80% of t
             (           /                 \           )
               ==========|                 |========== Sheet 2 (t)
                         +-----------------+
                         [ Lower Electrode ]

Expulsion Mechanics

Expulsion (spatter) is the violent ejection of liquid metal from the weld nugget during the weld cycle. The physics of expulsion are governed by a containment force balance:

$$P_{\text{internal}} > \sigma_{\text{containment}}$$

where:

• $P_{\text{internal}}$ is the hydrostatic expansion pressure of the superheated liquid molten pool plus trapped gases.
• $\sigma_{\text{containment}}$ is the mechanical compressive seal provided by the plastic solid ring of metal surrounding the nugget, held shut by electrode clamping force $F$.

Primary Causes of Expulsion:

1. Excessive welding current ($I$) driving superheating and volumetric liquid expansion.
2. Insufficient electrode clamping force ($F$), weakening the solid containment ring.
3. Excessive weld time, allowing the molten nugget diameter to exceed the electrode contact face diameter.
4. Edge proximity (flange width too narrow), providing an easy escape path for molten metal.
5. Surface contamination (heavy oils, mill scale, excessive primer).

Consequences of Expulsion: Expulsion dramatically reduces nugget mass, generates severe internal void porosity, causes excessive electrode indentation, damages sheet surfaces, and cuts tensile-shear and cross-tension fatigue life by $20%\text{ to }50%$.

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Resistance Seam Welding (RSEW)

Resistance Seam Welding replaces pointed spot electrodes with motor-driven rotating copper alloy wheel electrodes (typically RWMA Class 2).

                            [ Upper Rotating Wheel ]
                                     ( ) -> Travel Direction
                         ============================== Workpiece Sheets
                                     ( )
                            [ Lower Rotating Wheel ]

Operating Modes

1. Continuous Motion with Interrupted Current (Roll-Spot / Seam): Current pulses on and off while wheels rotate continuously. Each pulse creates an overlapping nugget. An overlap of $20%\text{ to }50%$ yields a hermetic, pressure-tight joint suitable for automotive fuel tanks, radiators, and pressure vessels.
2. Continuous Motion with Continuous Current: Current flows uninterrupted. Used for high-speed can manufacturing, but generates high heat accumulation that demands continuous water spray cooling directly onto the wheel-workpiece contact zone.
3. Step-Motion Seam Welding: Wheels stop rotation during the weld and hold pulses, then index forward during off time. Eliminates nugget tearing during cooling in crack-sensitive alloys (e.g., Inconel, stainless steel).

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Resistance Projection Welding (RPW)

Resistance Projection Welding localizes current and force at pre-embossed dimples, coined buttons, or natural intersections, utilizing large, planar copper dies (RWMA Class 3 or Class 11).

                   +----------------------------------+  Flat Upper Die
                   +----------------------------------+
                   ==================  ================ Top Sheet
                                     \/
                   ==================/\================ Embossed Projection
                   +----------------------------------+  Flat Lower Die
                   +----------------------------------+

Projection Configurations & Variants

1. Embossed Sheet Metal Projections: Hemispherical dimples stamped into one sheet. Multiple projections can be welded simultaneously in a single machine stroke, ensuring uniform load distribution without individual spot electrode tracking.
2. Solid / Machined Projections: Solid annular rings, chamfers, or pyramid projections machined into heavy fasteners, weld nuts, and studs.
3. Cross-Wire Welding: The orthogonal intersection of round wires naturally creates a point contact with minimal initial surface area. Applied in wire mesh, rebar cages, and shopping carts.

Collapse Dynamics

During the weld cycle, high current density heats the projection to its plastic forging temperature. Under die force, the projection yields and collapses flush into the parent sheet, producing solid-state forge bonding around a concentrated central molten nugget.

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Lobe Curves & Process Windows

A weld lobe curve defines the envelope of acceptable operating parameters (typically Current vs. Time at a constant Force). It represents the certified operating window for quality assurance:

     Current (kA)
          |
          |        /-----------------------------/  <-- Upper Lobe Boundary (Expulsion Limit)
          |       /                             /
          |      /     ACCEPTABLE PROCESS      /
          |     /            WINDOW           /
          |    /                             /  <-- Lower Lobe Boundary (Min Nugget: d >= 4*sqrt(t))
          |   /-----------------------------/
          +--------------------------------------------> Weld Time (Cycles)

• Lower Boundary (Minimum Nugget Size): Below this line, heat input is insufficient to achieve $d_n \ge 4\sqrt{t}$, resulting in undersized nuggets or partial cold bonding.
• Upper Boundary (Expulsion Limit): Above this line, excessive heat generation induces metal expulsion, internal voids, and electrode pitting.
• Process Window Width: A wider lobe width (e.g., $>1.5\text{ kA}$ current margin) indicates a robust process tolerant of tip wear and fit-up variations. Narrow lobe windows require closed-loop adaptive MFDC feedback controls.

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Comprehensive Worked Numerical Example: Thermal Energy, Resistance & Efficiency Calculation

Problem Statement

An automotive body structure joins two $1.20\text{ mm}$ thick sheets of mild steel ($t = 1.20\text{ mm}$) using a Medium-Frequency Direct Current (MFDC) resistance spot welding machine. The qualified welding schedule specifies:

• Welding current: $I = 9,500\text{ A}$
• Weld time: $t_w = 200\text{ ms} = 0.200\text{ s}$
• Clamping force: $F = 3.5\text{ kN}$
• Average dynamic electrical resistance: $R_{\text{dyn}} = 110\ \mu\Omega = 1.10 \times 10^{-4}\ \Omega$
• Steel properties: Density $\rho = 7.85 \times 10^{-3}\text{ g/mm}^3$, specific heat $c_p = 670\text{ J/(kg}\cdot\text{K)}$, ambient temperature $T_0 = 20^\circ\text{C}$, melting point $T_m = 1530^\circ\text{C}$, latent heat of fusion $L_f = 270\text{ kJ/kg} = 2.70 \times 10^5\text{ J/kg}$.

Calculate:

1. The minimum required nugget diameter ($d_{\text{min}}$) per AWS D8.9.
2. The total electrical energy input ($Q_{\text{electrical}}$) delivered by the machine.
3. The theoretical thermal energy ($Q_{\text{melt}}$) required to heat and melt a cylindrical nugget of diameter $d_n = 5.0\text{ mm}$ and total fused height $h = 1.68\text{ mm}$ ($70%$ of total stack thickness $2t$).
4. The thermal efficiency ($\eta_{\text{thermal}}$) of the spot welding process.

Step-by-Step Solution

Step 1: Calculate minimum allowable nugget diameter ($d_{\text{min}}$)

$$d_{\text{min}} = 4 \sqrt{t} = 4 \sqrt{1.20\text{ mm}} = 4 \times 1.0954 = 4.38\text{ mm}$$

Because the actual targeted nugget is $d_n = 5.0\text{ mm}$, it comfortably meets the AWS requirement.

Step 2: Calculate total electrical energy input ($Q_{\text{electrical}}$)

$$Q_{\text{electrical}} = I^2 \cdot R_{\text{dyn}} \cdot t_w = (9500\text{ A})^2 \times (1.10 \times 10^{-4}\ \Omega) \times 0.200\text{ s}$$ $$(9500)^2 = 90,250,000\text{ A}^2$$ $$Q_{\text{electrical}} = 90,250,000 \times 1.10 \times 10^{-4} \times 0.200 = 9,927.5 \times 0.200 = 1,985.5\text{ J}$$

Step 3: Calculate theoretical thermal energy to form the molten nugget ($Q_{\text{melt}}$)

• Nugget Volume: $$V_n = \frac{\pi d_n^2}{4} \cdot h = \frac{\pi (5.0\text{ mm})^2}{4} \times 1.68\text{ mm} = 19.635 \times 1.68 = 32.987\text{ mm}^3$$
• Nugget Mass: $$m_n = V_n \cdot \rho = 32.987\text{ mm}^3 \times (7.85 \times 10^{-3}\text{ g/mm}^3) = 0.2589\text{ g} = 2.589 \times 10^{-4}\text{ kg}$$
• Sensible heat to raise temperature from $20^\circ\text{C}$ to $1530^\circ\text{C}$ ($\Delta T = 1510\text{ K}$): $$Q_{\text{sensible}} = m_n \cdot c_p \cdot \Delta T = (2.589 \times 10^{-4}\text{ kg}) \times (670\text{ J/kg}\cdot\text{K}) \times 1510\text{ K} = 262.0\text{ J}$$
• Latent heat of fusion: $$Q_{\text{latent}} = m_n \cdot L_f = (2.589 \times 10^{-4}\text{ kg}) \times (270,000\text{ J/kg}) = 69.9\text{ J}$$
• Total required melting energy: $$Q_{\text{melt}} = Q_{\text{sensible}} + Q_{\text{latent}} = 262.0\text{ J} + 69.9\text{ J} = 331.9\text{ J}$$

Step 4: Calculate thermal efficiency ($\eta_{\text{thermal}}$)

$$\eta_{\text{thermal}} = \frac{Q_{\text{melt}}}{Q_{\text{electrical}}} = \frac{331.9\text{ J}}{1985.5\text{ J}} = 0.1671 = 16.71\%$$

Engineering Interpretation: Approximately $16.7%$ of the total electrical energy goes directly into melting the weld nugget. The remaining $\sim 83.3%$ is conducted away into the water-cooled copper electrodes and surrounding sheet steel, demonstrating why electrode cooling water flow ($>4\text{ L/min}$ per tip) is vital.

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Real-World Engineering Scenarios & Exam Pitfalls

Industrial Scenario: Galvanized High-Strength Steel Tip Erosion

An automotive plant launching a new crossover body line transitioned from uncoated steel to hot-dip galvanized (HDG) Dual-Phase steel (DP600). Within two production shifts, weld quality failed: severe expulsion occurred, nugget sizes dropped below $4\sqrt{t}$, and electrode tips showed heavy cratering. The plant metallurgist identified that zinc melts at $420^\circ\text{C}$ and forms brass alloys (Cu-Zn) with the RWMA Class 2 copper tips. The lower melting temperature of brass caused electrode face pitting and mushrooming, increasing electrode tip area by $40%$ and reducing current density below the nugget threshold. The welding engineer resolved the failure by:

1. Introducing automated pneumatic tip dressers every 150 spots to restore tip diameter.
2. Switching to dispersion-strengthened copper (${\text{Cu-Al}}_2\text{O}_3$, RWMA C15760), which resists zinc alloying.
3. Incorporating a pre-pulse schedule to disperse the zinc coating before main current firing.

Common Exam Traps

Exam Trap 1: The Function of Hold Time A recurring exam trap asks what defect occurs if Hold Time is too short. Candidates often assume incomplete fusion or small nugget size. Nugget diameter is determined during Weld Time. Prematurely releasing hold force causes solidification cracking, internal shrinkage cavitation, and expulsion, because the nugget is still molten and lacks mechanical consolidation during freezing.

Exam Trap 2: Squeeze Time vs. Weld Initiation Squeeze time does not involve electric current. Initiating current during squeeze time before electrode force stabilizes creates severe surface arcing, destroying electrode faces and blowing holes through the workpiece.

Exam Trap 3: Electrode Material Selection for Projection Welding Candidates frequently select RWMA Class 2 electrodes for all resistance applications. For heavy projection welding dies, RWMA Class 3 or Group B refractory compositions (Class 10, 11) are required. Class 2 copper alloys lack sufficient compressive yield strength at elevated temperatures and deform (mushroom) rapidly under projection welding tonnages.