spot welding
DESCRIPTION
heat generation in spot weldingTRANSCRIPT
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Spot Welding
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Spot Welding
Lesson ObjectivesWhen you finish this lesson you will understand:• Basics of Resistance Welding Processes • Heat Generation & Control• Spot Welding Process and Applications
Learning Activities1. View Slides; 2. Read Notes, 3. Listen to lecture4. View Demo5. Do on-line
workbook
Keywords: Resistance Spot Welding, Heat Generation, Equipment Control, Contact Resistance, Upslope, Downslope, Hold Time, Temper, Squeeze Time, Electrode
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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.
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Principal Types of Resistance Welds
Electrodesor WeldingTips
Electrodesor WeldingWheels
Electrodesor Dies
ProjectionWelds
Electrodes or DiesSpot Weld Seam Weld Projection Weld
Upset Weld Flash Weld
After Welding After Welding[Reference: Resistance Welding Manual, RWMA, p.1-3]
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Typical Equipment of Resistance Spot Welding
(a) (b)[Reference: Welding Process Slides, The Welding Institute]
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Advantages of Resistance Spot Welding
Adaptability for Automation in High-Rate
Production of Sheet Metal Assemblies
High Speed
Economical
Dimensional Accuracy
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Limitations of Resistance Spot Welding
Difficulty for maintenance or repair Adds weight and material cost to the 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
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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.
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Heat = I2 RTK
WhereIs a function of:
I = Current (Amps) Transformer Tap SettingR = Resistance (Ohms) Material Prop., & PressureT = Time (Cycles 1/60
Second)Control Setting
K = Heat Losses Conduction, Convection,Radiation
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Heating Value of Current = RMS Current
Irms=0.707 Ipeak
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Block Diagram of Single-Phase Spot Welder
Spot Weld
Main Power Line
Contactor
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N=np/ns
Vs= Vp/N
Is = Ip N
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Heat = I2 RTK
WhereIs a function of:
I = Current (Amps) Transformer Tap SettingR = Resistance (Ohms) Material Prop., & PressureT = Time (Cycles 1/60
Second)Control Setting
K = Heat Losses Conduction, Convection,Radiation
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Contact-Resistance Measurement
ContactArea
Electrode Force
Electrode Force
Small Current
Rec
Rec
Rsc
Rv
Rv
Rec
Rec
Rtotal
Rec = contact resistance between electrode and sheet surface
Rsc = contact resistance at the faying surface
Rv = volume resistance of the sheets
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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.
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Link to electrode force demo
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Surface ConditionSteel
Steel
Steel
Steel
Oils/DirtOxide
OxideOils/Dirt
(a) Pickled Conditions
(b) Rusted ConditionsRusty
Pickled
Polished
Electrode ForceR
esis
tivity
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Resistance Varies with Pressure
Low Pressure Medium Pressure High Pressure
(a) (b) (c)
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Volume-Resistance Measurement
ContactArea
Electrode Force
Electrode Force
Small Current
Rec
Rec
Rsc
Rv
Rv
Rv
Rtotal
Rec = contact resistance between electrode and sheet surface
Rsc = contact resistance at the faying surface
Rv = volume resistance of the sheets
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Resistivity as a Function of Temperature
100 200 300 400 500 600 700 800
102030405060708090
100110120130
HSLA
Low Carbon
Temperature, °C
Res
istiv
ity,
-cm
[Reference: Welding in the Automotive Industry, D.W. Dickinson, p.125]
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Heat = I2 RTK
WhereIs a function of:
I = Current (Amps) Transformer Tap SettingR = Resistance (Ohms) Material Prop., & PressureT = Time (Cycles 1/60
Second)Control Setting
K = Heat Losses Conduction, Convection,Radiation
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Heating Value of Current = RMS Current
Irms=0.707 Ipeak
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Upslope/Downslope, Hold Time, & Temper
Weld Current
Temper Current
Ele
ctro
de
Pre
ssu
reC
urr
ent
Squeeze Time Weld Time Off Time Hold TimeUpslope Downslope Temper
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Heat = I2 RTK
WhereIs a function of:
I = Current (Amps) Transformer Tap SettingR = Resistance (Ohms) Material Prop., & PressureT = Time (Cycles 1/60
Second)Control Setting
K = Heat Losses Conduction, Convection,Radiation
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Heat Dissipation
Weld Nugget
Water-Cooled Copper Alloy Electrode
Water-Cooled Copper Alloy Electrode
Base Metal
Base Metal
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Heat = I2 RTK
WhereIs a function of:
I = Current (Amps) Transformer Tap SettingR = Resistance (Ohms) PressureT = Time (Cycles 1/60
Second)Control Setting
K = Heat Losses Conduction, Convection,Radiation
Let’s put it all together
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Initial Resistance Through Weldment
Top ElectrodeWater
WeldNugget
Bottom Electrode
Resistance
Dis
tan
ce
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Temperature Readings of A Spot Welding Process
Workpiece
This illustration was taken about 4/60th of a secondafter the welding currentstarts.
(Note: Temp at Electrode Sheet Interface Higher than Bulk)
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After 20%welding time
At the end ofwelding time
Temperature
Ele
ctro
deE
lect
rode
Wor
kpie
ceTemperaturedistribution atvarious locationduring welding.
Temperature Distribution
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Link to nugget growth demo
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Nugget Solidification
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