Spot Welding

Heating Value of Current = RMS Current Irms=0.707 Ipeak
Upslope/Downslope, Hold Time, & Temper
Electrode Pressure
Current
Weld Current Temper Current
Upslope Downslope Temper Squeeze Time Weld Time Off Time Hold Time
Heat = I2 RTK Where I = Current (Amps) R = Resistance (Ohms) T = Time (Cycles 1/60 Second) K = Heat Losses Is a function of: Transformer Tap Setting Material Prop., & Pressure Control Setting Conduction, Convection, Radiation

Resistance Welding
• Resistance welding depends on three factors:
– Time of current flow (T). – Resistance of the conductor (R) – Amperage (I).
• Heat generation is expressed as Q = I2R T, Q = Heat generated.
Contact-Resistance Measurement
Electrode Force Rec = contact resistance between electrode and sheet surface
Rec
Small Current Rec Rsc Rv
Rtotal
Rec
Rv
Rsc = contact resistance at the faying surface
Generally have higher cost than most arc welding equipment

Produces unfavorable line power demands
Low tensile and fatigue strength The full strength of the sheet cannot prevail across a spot welded joint Eccentric loading condition
Heat = I2 RTK Where I = Current (Amps) R = Resistance (Ohms) T = Time (Cycles 1/60 Second) K = Heat Losses Is a function of: Transformer Tap Setting Material Prop., & Pressure Control Setting Conduction, Convection, Radiation
Heat Dissipation
Water-Cooled Copper Alloy Electrode
Base Metal
Weld Nugget
Base Metal
Water-Cooled Copper Alloy Electrode
Heat = I RTK Where I = Current (Amps) R = Resistance (Ohms) T = Time (Cycles 1/60 Second) K = Heat Losses Is a function of: Transformer Tap Setting Pressure Control Setting Conduction, Convection, Radiation
Heat = I2 RTK Where I = Current (Amps) R = Resistance (Ohms) T = Time (Cycles 1/60 Second) K = Heat Losses Is a function of: Transformer Tap Setting Material Prop., & Pressure Control Setting Conduction, Convection, Radiation
Spot Welding
Spot Welding
Lesson Objectives When you finish this lesson you will understand: • Basics of Resistance Welding Processes • Heat Generation & Control • Spot Welding Process and Applications Learning Activities 1. View Slides; 2. Read Notes, 3. Listen to lecture 4. View Demo 5. Do on-line workbook
Principal Types of Resistance Welds
Electrodes or Welding Tips Electrodes or Welding Wheels Electrodes or Dies
Projection Welds
Spot Weld
Seam Weld
Eleessslidestheweldinginstituteadvantagesofresistancespotwelding?adaptabilityforautomationinhighrateproductionofsheetmetalassemblies?highspeed?economical?dimensionalaccuracylimitationsofresistancespotwelding?difficultyformaintenanceorrepair?addsweightandmaterialcosttotheproductcomparedwithabuttjoint?generallyhavehighercostthanmostarcweldingequipment?producesunfavorablelinepowerdemands?lowtensileandfatiguestrength?thefullstrengthofthesheetcannotprevailacrossaspotweldedjoint?eccentricloadingconditionresistancewelding?resistanceweldingdependsonthreefactors
Heating Value of Current = RMS Current Irms=0.707 Ipeak
Block Diagram of Single-Phase Spot Welder
Contactor
Main Power Line
Spot Weld
N=np/ns Vs= Vp/N Is = I p N
Definition of Resistance Welding
• Resistance welding is a fusion welding process in which coalescence of metals is produced at the faying surfaces by the heat generated at the joint by the resistance of the work to the flow of electricity. • Force is applied before, during, and after the application of current to prevent arcing at the work piece. • Melting occurs at the faying surfaces during welding.
Rv = volume resistance of the sheets
Contact Area Rec
Electrode
Force
Factors Affecting Heat Generation (Q):
• Welding pressure – as welding pressure increases both R and Q decrease. • Electrodes – deformation of electrodes increases contact area. As contact area increases, both R and Q decrease.
Oxide Oils/Dirt Steel
Electrode Force
Resistance Varies with Pressure
Low Pressure
Medium Pressure
High Pressure
(a)
(b)
(c)
Volume-Resistance Measurement
Electrode Force Rec = contact resistance between electrode and sheet surface

High Speed
Economical Dimensional Accuracy


Limitations of Resistance Spot Welding

Difficulty for maintenance or repair
Adds weight and material cost to the product, compared with a butt joint
Projection Weld
Upset Weld
Flash Weld
After Welding
After Welding
[Reference: Resistance Welding Manual, RWMA, p.1-3]
Typical Equipment of Resistance Spot Welding
Small Current Rec Rsc Rtotal Rec Rv
Rv
Rsc = contact resistance at the faying surface
Rv = volume resistance of the sheets
Contact Area Rv
Electrode
Force
Resistivity as a Function of Temperature
Link to electrode force demo
(a) Pickled Conditions
Surface Condition
Steel
(b) Rusted Conditions
Oils/Dirt Oxide
Resistivity
Steel
Steel
Rusty Polished Pickled
2
Let’s put it all together
Initial Resistance Through Weldment
Top Electrode Water Weld Nugget
Distance
Resistance Bottom Electrode
Temperature Readings of A Spot Welding Process
(a)
(b)
[Reference: Welding Process Slides, The Welding Institute]
Advantages of Resistance Spot Welding

Adaptability for Automation in High-Rate Production of Sheet Metal Assemblies
Heat = I2 RTK Where I = Current (Amps) R = Resistance (Ohms) T = Time (Cycles 1/60 Second) K = Heat Losses Is a function of: Transformer Tap Setting Material Prop., & Pressure Control Setting Conduction, Convection, Radiation
Keywords: Resistance Spot Welding, Heat Generation, Equipment Control, Contact Resistance, Upslope, Downslope, Hold Time, Temper, Squeeze Time, Electrode
130 120 110 100 90 80 70 60 50 40 30 20 10
Resistivity, mW-cm
HSLA
Low Carbon
100 200 300 400 500 600 700 800
Temperature, °C
[Reference: Welding in the Automotive Industry, D.W. Dickinson, p.125]