resistance-welding

name	resistance-welding
description	Resistance welding — spot (RSW), seam, projection, flash butt, nugget formation, heat generation Q=I²Rt, electrode force, weld schedule, lobe curve, AWS C1.1, automotive applications.
metadata	{"priority":7,"promptSignals":{"phrases":["resistance welding","spot welding","RSW","resistance spot weld","nugget formation","electrode force","seam welding"],"minScore":3}}

Resistance Welding — Complete Skill

Heat Generation Principle

Joule heating: Q = I² × R × t [J]

I = welding current [A]; R = contact resistance [Ω]; t = weld time [s] Heat localized at interface (highest resistance) → nugget forms

Total resistance: R_total = R_electrode/workpiece + R_workpiece,1 + R_interface + R_workpiece,2 + R_electrode/workpiece

R_interface >> R_workpiece initially: surface asperities → concentrated heating As temperature rises: R_interface drops → bulk material resistivity dominates (positive feedback if ρ increases with T)

Heating efficiency: Only fraction Q at interface produces nugget; rest goes to electrodes and workpiece bulk Efficiency η_thermal ≈ 40–80% (higher for short time, low force, high R material)

Spot Welding (RSW) — AWS C1.1

Process Variables

Weld schedule: 3-phase (squeeze → weld → hold)

• Squeeze time: ~10–30 cycles (60Hz AC); allows electrode to seat before current
• Weld time: 5–30 cycles; controls nugget growth
• Hold time: 5–15 cycles; nugget solidifies under pressure; prevents ejection

Welding current: typically 5,000–30,000 A (low voltage 2–10 V across secondary) Electrode force F: typically 1–5 kN depending on material and thickness

Nugget Size

Minimum nugget diameter (AWS C1.1): d_n_min = 4√t_sheet [mm; t_sheet in mm] for t < 3 mm d_n_min = 3√t_sheet for t ≥ 3 mm

Typical nugget penetration: 20–80% of each sheet thickness (target 30–60%)

Weld shear strength (tensile-shear test): F_ts = τ × π/4 × d_n² [N] τ ≈ 0.6 × S_u,material (conservative for nugget)

Cross-tension strength: 30–60% of tensile-shear; peel mode

Lobe Curve (Process Window)

Current vs. weld time plot showing:

• Lower boundary: minimum current for nugget formation (cold weld)
• Upper boundary: maximum current before expulsion (spatter)
• Process window between boundaries → robust welding

Wide lobe: robust process; narrow lobe → tight process control required Electrode condition, coating, fit-up affect lobe position and width

Electrode Materials and Wear

Copper alloys (RWMA Classes): Class 1 (99.9% Cu): highest conductivity; low strength; for aluminum Class 2 (CuCr or CuCrZr): best balance; most common for steel Class 3 (CuBe or CuCo): high strength; for higher hardness materials

Electrode tip shapes: Truncated cone (Type C or D): flat face; standard for steel Dome (Type B): rounded; for non-flat workpieces Offset (Type E, F, G): access to corners and flanges

Electrode wear: tip mushrooms → diameter increases → current density drops → cold welds Dress cycle: every 200–500 welds for steel; every 50–100 for Zn-coated (galvanized) → electrode pickup of Zn destroys tip faster

Coated Steel (Galvanized/Aluminized)

Coating melts first → surface flooding → higher contact resistance variation Solutions: higher current, longer squeeze, electrode conditioning, slotted electrodes

Seam Welding (RSEW)

Continuous weld from overlapping spot welds using rotating disk electrodes Applications: fuel tanks, radiators, hermetic containers Parameters: peripheral speed of electrode rolls + weld frequency → controls overlap

Weld speed (mm/s) = electrode speed × cos(pitch angle) Overlap: 50–70% (hermetic seam); 20–40% (structural)

Projection Welding (RPW)

Embossed projections on one workpiece concentrate current and force Projection size: ~40–60% of sheet thickness in height; diameter ≈ 2× height Advantage: multiple simultaneous welds; better heat localization; longer electrode life Applications: threaded fasteners, brackets, multiple weld points in one stroke

Flash Butt Welding (FBW)

Workpieces brought to contact → arc flashing → heating → parts upset together Applications: rail joining, pipe butt welding, chain links, billets

Process:

1. Pre-heat (low current, light contact)
2. Flash (steady arcing; metal ejected as flash)
3. Upset (high force, extinguish arc, forge joint)

Weld quality: flash removes oxides; upset creates forged nugget Post-weld: remove flash bead (trim or grind)

Pulsation (Multi-Impulse) Welding

Multiple current pulses with cool time between → controls heat input Used for: high-carbon steel, hot-dip galvanized, dissimilar thickness Reduces spatter; allows higher peak current without burning through

Thermal Analysis (Simplified)

Nugget growth model: r_n(t) ≈ r_electrode × (1 - exp(-t/τ_thermal)) τ_thermal = ρ_metal × c_p × V_nugget / Q_rate

Temperature at interface: T_interface = T_ambient + Q_rate / (ρ c_p × A_electrode × δ_heat) [very rough] Nugget forms when T_interface > T_solidus of material

RSW Weld Quality Inspection

Destructive: Peel test: tear apart lap joint; nugget buttons out → pass (required button diameter) Chisel test: wedge between sheets; peel to reveal nugget Tensile-shear test: measure F_ts; compare to acceptance

Non-destructive: Ultrasonic (phased array PAUT): image nugget; detect cold welds, holes, sticking IR thermography: monitor surface T during welding → correlates to Q deposited Process monitoring: current, voltage, force traces → machine learning for weld quality

ASTM B833: copper alloy electrodes for resistance welding AWS C1.1M: recommended practices for resistance welding AWS D8.1M: automotive steel sheet welding; nugget criteria for various thicknesses

Material-Specific Considerations

Material	Weld Issue	Solution
AHSS (DP, TRIP)	Expulsion, interface fracture	Pulsation, upslope; quench by hold time
Aluminum	Low resistivity; oxide; sticking	High current (30–50 kA); dome electrode; conditioning
Stainless SS	Low conductivity; sensitization	Short weld time; cool electrodes
Coated steel	Zn/Al pickup; inconsistent R	Frequent electrode dressing; compound tip
Dissimilar thickness	Current path asymmetry	Offset electrodes; rebalanced force

Output

Provide: weld current I [kA], weld time t [cycles at 60Hz], electrode force F [kN], nugget diameter d_n [mm] vs. AWS minimum, tensile-shear strength F_ts [kN], lobe curve boundaries (min/max current at design time), electrode type (RWMA class and tip shape), electrode dress frequency, weld schedule (squeeze/weld/hold/cool times), applicable standard (AWS C1.1, D8.1M for automotive).