Transcript Spot Welding - ::Gateway Engineering Education Coalition

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
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
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
Flash Weld
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
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=np/ns
Vs= Vp/N
Is = I p 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 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
– 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
Steel
Steel
(b) Rusted Conditions
Oils/Dirt
Oxide
Steel
Resistivity
(a) Pickled Conditions
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, mW-cm
Resistivity as a Function of Temperature
130
120
110
100
90
80
70
60
50
40
30
20
10
HSLA
Low Carbon
100 200 300 400 500 600 700 800
Temperature, °C
[Reference: Welding in the Automotive Industry, D.W. Dickinson, p.125]
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
Current
Electrode
Pressure
Upslope/Downslope, Hold Time,
& Temper
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
Heat Dissipation
Water-Cooled Copper Alloy Electrode
Base Metal
Weld Nugget
Base Metal
Water-Cooled Copper Alloy Electrode
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 Electrode
Water
Distance
Weld
Nugget
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.
At the end of
welding time
After 20%
welding time
Workpiece
Temperature
distribution at
various
location
during
welding.
Electrode
Temperature Distribution
Temperature
Link to nugget growth demo
Nugget Solidification