3-2 Spot Welding
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Transcript of 3-2 Spot Welding
Spot WeldingLesson ObjectivesWhen 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
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
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.
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]
Typical Equipment of Resistance Spot Welding
(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
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 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•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.
Heat = I2 RTK
W hereIs a function of:
I = Current (Am ps) Transform er Tap SettingR = Resistance (Ohm s) M aterial Prop., & PressureT = Tim e (Cycles 1/60
Second)Control Setting
K = Heat Losses Conduction, Convection,Radiation
Heat = I2 RTK
W hereIs a function of:
I = Current (Am ps) Transform er Tap SettingR = Resistance (Ohm s) M aterial Prop., & PressureT = Tim e (Cycles 1/60
Second)Control Setting
K = Heat Losses Conduction, Convection,Radiation
Contact-Resistance Measurement
ContactArea
Electrode Force
Electrode Force
Small CurrentRec
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
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.
Surface ConditionSteel
Steel
Steel
Steel
Oils/DirtOxide
OxideOils/Dirt
(a) Pickled Conditions
(b) Rusted Conditions Rusty
PickledPolished
Electrode ForceResistivity
Volume-Resistance Measurement
ContactArea
Electrode Force
Electrode Force
Small CurrentRec
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
Resistivity as a Function of Temperature
100 200 300 400 500 600 700 800102030405060708090
100110120130
HSLA
Low Carbon
Temperature, °C
Resistivity,
-cm
[Reference: Welding in the Automotive Industry, D.W. Dickinson, p.125]
Heat = I2 RTK
W hereIs a function of:
I = Current (Am ps) Transform er Tap SettingR = Resistance (Ohm s) M aterial Prop., & PressureT = Tim e (Cycles 1/60
Second)Control Setting
K = Heat Losses Conduction, Convection,Radiation
Upslope/Downslope, Hold Time, & Temper
Weld CurrentTemper Current
Electrode
Pressure
Current
Squeeze Time Weld Time Off Time Hold TimeUpslope Downslope Temper
Heat = I2 RTK
W hereIs a function of:
I = Current (Am ps) Transform er Tap SettingR = Resistance (Ohm s) M aterial Prop., & PressureT = Tim e (Cycles 1/60
Second)Control Setting
K = Heat Losses Conduction, Convection,Radiation
Heat Dissipation
Weld Nugget
Water-Cooled Copper Alloy Electrode
Water-Cooled Copper Alloy Electrode
Base Metal
Base Metal
Heat = I2 RTK
W hereIs a function of:
I = Current (Am ps) Transform er Tap SettingR = Resistance (O hm s) PressureT = Tim e (Cycles 1/60
Second)Control Setting
K = Heat Losses Conduction, Convection,Radiation
Let’s put it all together
Initial Resistance Through Weldment
Top ElectrodeWater
WeldNugget
Bottom ElectrodeResistance
Distance
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)
After 20%welding time
At the end ofwelding time
Temperature
Elec
trod
eElec
trod
e
Work
piec
eTemperaturedistribution atvarious locationduring welding.
Temperature Distribution