Spot Spot W Weldi elding ng
Spot Welding Lesson Objectives When you finish this lesson you will understand: Basics of Resistance Welding Processes Heat Generation & Control Spot Welding Welding Process and Applications •
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Learning Activities 1. View Slides; 2. Read Notes, 3. Listen to le lecture 4. View Demo 5. Do on-line workbook
Keywords: Resistance Spot Welding, Heat Generation, Equipment Control, Contact Resistance, Upslope, Downslope, Hold Time, Temper, Temper, Squeeze Time, Electrode El ectrode
Definition of Resistance Welding •
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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.
Principal Types of Resistance Welds Electrodes or Welding Tips
Electrodes or Welding Wheels
Spot Weld
Electrodes or Dies
Seam Weld
Projection Welds
Projection Weld
Electrodes or Dies
Upset Weld
After Welding Welding
Flash Weld
After Welding Welding
Typical Equipment of Resistance Spot Welding
(a)
(b)
Advantages of Resistance Spot Welding
Adaptability Adaptability for Automation in High-Rate Production of Sheet Metal Assemblies
High Speed
Economical
Dimensional Accuracy
Limitations of Resistance Spot Welding
Difficulty for maintenance or repair Adds weight and material cost to the product, product, compared with a butt joint
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
Resistance Welding •
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Resistance welding depends on three factors: –
Time of current flow (T).
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Resistance of the conductor (R)
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Amperage (I).
Heat generation is expressed as Q = I2R T, Q = Heat generated.
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
Heating Value of Current = RMS Current Irms=0.707 Ipeak
Block Diagram of Single-Phase Spot Welder Contactor
Main Power Line
Spot Weld
N=n N=np /ns Vs= Vp /N Is = Ip N
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
Contact-Resistance Contact-Resistance Measurement Electrode Rec
Force
Small Current Rec Rsc
Rtotal
Rec
Rv Rv
Contact Area Rec Electrode
Force
Rec = contact resistance between electrode and sheet surface Rsc = contact resistance at the faying surface Rv = volume resistance of the sheets
Factors Affecting Heat Generation (Q): •
Welding pressure –
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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.
Link to electrode force demo
Surface Condition
(a) Pickled Conditions
Steel
Steel (b) Rusted Conditions Oils/Dirt Oxide
Steel
y t i v i t s i s e R
Rusty Polished Pickled
Oxide Oils/Dirt
Electrode Force Steel
Resistance Varies with Pressure
Low Pressure
(a)
Medium Pressure
(b)
High Pressure
(c)
Volume-Resistance Measurement Electrode
Force
Small Current Rec Rsc Rtotal
Rec
Rv Rv
Contact Area Rv Electrode
Force
Rec = contact resistance between electrode and sheet surface Rsc = contact resistance at the faying surface Rv = volume resistance of the sheets
Resistivity as a Function of Temperature 130 120 110 m 100 c W 90 m 80 , y t 70 i v 60 i t s 50 i s 40 e R 30 20 10
HSLA
Low Carbon
100 200 300 400 500 600 700 800
Temperature, emperatur e, °C
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
Heating Value of Current = RMS Current Irms=0.707 Ipeak
Upslope/Downslope, Upslope/Downslope, Hold Time, & Temper e d e r o r u s t c s e e l r E P
t n e r r u C
Weld Current Temper Current Curren t
Upslope Downslope Downslo pe 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
Heat Dissipation Water-Cooled Water-Cooled Copper Alloy Electrode
Base Metal Weld Nugget
Base Metal
2
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
Let’s put it all together
Initial Resistance Through Weldment Top Electrod E lectrode e Water Weld Nugget
e c n a t s i D
Resistance Bottom Electrode
Temperature Readings of A Spot Welding Process (Note: Temp at Electrode Sheet Interface Higher than Bulk)
Workpiece
This illustration was taken about 4/60th of a second after the welding current starts.
Temperature Distribution
Temperature distribution at various location during welding.
e d o r t c e l E
At the end of welding time
After 20% welding time
e c e i p k r o W
Temperature
Link to nugget growth demo
Nugget Solidification