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Spot WeldabilityQuality and Performance
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Spot Weldability
Lesson ObjectivesWhen you finish this lesson you will understand:• visual identification of discontinuities• how to develop & use lobe curves• electrode life• peel test & failure modes
Learning Activities1. View Slides; 2. Read Notes, 3. Listen to lecture4. Do on-line
workbook5. Do Homework
KeywordsCurrent level Current Range Lobe Curve Electrode lifeNugget Dia Button Dia. Peel Test Tensile ShearCross tension Shunt current
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Process Requirements• Electrode Materials
• Electrode Geometry
• Welding Force
• Welding Cycle
– Squeeze time
– Welding time
– Hold time
• Welding Current
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W. Stanley, Resistance WeldingMcGraw-Hill, 1950
Visual Inspection of Quality
ab
cd
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W. Stanley, Resistance WeldingMcGraw-Hill, 1950
Visual Inspection of Quality
a
b
c
d
e
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Process Characteristics as a Measure of Weldability
• Current Level
• Current Range & Weldability Lobe
• Process Deterioration
– Characterization of the loss in weld size at constant welding conditions
– Characterization of the variation in process requirements for maintaining welds of an adequate quality
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Current Level
Material Welding Current (kA)
Uncoated Steel 9
Hot-Dipped Galvanized 13
Electro Galvanized 12
Galvannealed 10
Fe-Zn Electro Coated 10
Simple Current Levels for a Range of 0.8 mm Sheet Steels
(6.1 mm Electrodes & 12-14 Cycles of Welding Time)
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Current Range & Weldability Lobe
ExpulsionAcceptableNuggets
Small Nuggets
Minimum Nugget Diameter
Weld Current
Weld Current
ExpulsionLevel
TimeA Smaller
“Brittle” Nuggets
AcceptableNuggets
LobeCurve
Nu
gge
t Dia
met
er
We
ld T
ime
Time A
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Weld Current
Wel
d Ti
me
Effect of Weld Time on Current Range (Weld Lobe)
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Weld Current
Wel
d Ti
me
5 CycleHold Time
60 CycleHold Time
BrittleButtons
Effect of Hold Time on Lobe Curve
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Low Force
Higher Force
Weld Current
Wel
d Ti
me
Indentation, UnstableVery High Force
Lobe moves toHigher CurrentLonger Times
Effect of Electrode Force on Lobe Curve
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Effect of Electrode Misalignment on Lobe Curve
Karagoulis, “Process Control in Mfg”,AWS Sheet Metal Conf V, 1992
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Turn to the person sitting next to you and discuss (1 min.):•
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Pareto Charts Relate Importance of Process Parameters
• Current Density
• Weld Force
• Weld Time
• Current Density
• Weld Force
• Weld Time
• Weld Spacing
• Surface Conditions
Standard Approach Graeco-Latin Approach
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What is a Pareto Chart ?
• In our case, A Pareto Chart is a graphical comparison of process variables vs. weld quality. In other words it ranks the process variables in the order of their potency (amount they change weld quality).
A B C D E F G
Process Parameter
Pot
ency
of
a pr
oces
s pa
ram
eter
in
infl
uenc
ing
wel
d qu
alit
y
Scharfy & Kuhnash “Pareto Chart Development for Resistance Welding”Senior Capstone, OSU, 2000
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Defining Window Size• Develop weld windows for each process
parameter• Determine the affect of changing the parameter
has on the weld window
Weld
Nugget
Process parameter
Expulsion
Average
Initiation
Steep slope
Stable slope
Window Size = expulsion - initiation
Scharfy & Kuhnash “Pareto Chart Development for Resistance Welding”Senior Capstone, OSU, 2000
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Weld Force WindowsCYCLE TIME 11, FORCE 500 LBS
0
1
2
3
4
5
6
7
8
6.5 7 7.5 8 8.5 9 9.5 10
WELD CURRENT (kA)
BUTT
ON S
IZE (m
m)
CYCLE TIME 11, FORCE 800 LBS
0
1
2
3
4
5
6
7
8
6.5 7 7.5 8 8.5 9 9.5 10
WELD CURRENT (kA)
BUTT
ON S
IZE (m
m)
Scharfy & Kuhnash “Pareto Chart Development for Resistance Welding”Senior Capstone, OSU, 2000
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Weld Time Windows CYCLE TIME 8, FORCE 650 LBS
0
1
2
3
4
5
6
7
8
6.5 7 7.5 8 8.5 9 9.5 10
WELD CURRENT (kA)
BUTT
ON S
IZE
(mm
)
Scharfy & Kuhnash “Pareto Chart Development for Resistance Welding”Senior Capstone, OSU, 2000
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Window Size Pareto Chart
0
0.2
0.4
0.6
0.8
1
1.2
Increasingforce
Decreasingforce
Increasingcycle time
Loweringcycle time
% change window / % change parameter
Scharfy & Kuhnash “Pareto Chart Development for Resistance Welding”Senior Capstone, OSU, 2000
Ne g
a tiv
e %
Ch
ang e
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Current Median Pareto Chart
0
0.1
0.2
0.3
0.4
0.5
0.6
Decreasingforce
Increasingcycle time
Loweringcycle time
Increasingforce
% change of midpoint /% change in parameter
Scharfy & Kuhnash “Pareto Chart Development for Resistance Welding”Senior Capstone, OSU, 2000
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Turn to the person sitting next to you and discuss (1 min.):•
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Process Characteristics as a Measure of Weldability
• Current Level
• Current Range & Weldability Lobe
• Process Deterioration
– Characterization of the loss in weld size at constant welding conditions
– Characterization of the variation in process requirements for maintaining welds of an adequate quality
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Process Deterioration
MinimumAcceptableDiameter
Number of Welds
Wel
d D
iam
eter
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SurfaceExpulsion
Low Electrode Force
Hirsch, R & Leibovitz, R, “Improved Weld Quality and Electrode Life in Resistance Welding” Practical Welding Today, Nov-Dec, 1997
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Low Electrode Force
Hirsch, R & Leibovitz, R, “Improved Weld Quality and Electrode Life in Resistance Welding” Practical Welding Today, Nov-Dec, 1997
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Mechanical Properties as a Measure of Weldability
• Button Size as a Measure of Weld Quality
• Simulative Mechanical Tests– Tensile shear loads
– Peel loads
– Cross tension
• Modes of Failure in Spot Welds– Full button
– Irregular button
– Interfacial failure
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Button Size as a Measure of Weld Quality
ButtonDiameter
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Simulative Mechanical Tests
Tensile Shear Peel Cross Tension
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Introduction to Peel Testing Technique
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Geometric Effects on The Qualified Peel Test
W = 1”
W = 1.5”W = 2”
W
x
Load
450
350
250
1500 1/4 1/2 3/4 1 1-1/4
Distance x, in
Max
. Lo
ad,
lbs
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Modes of Failure in Spot Welds
Full Button Irregular Button
Interfacial Failure
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Microstructural Measures of Weldability
• Weld Geometry– Degree of weld penetration– Actual diameter of weld
• Weld Structural Integrity
• Weld Hardness
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Weld Geometry
Weld Diameter
Weld Diameter
Weld Penetration
Weld Penetration
Full Size Weld
Sub-Size Weld
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Weld Structural Integrity
Fine Weld Porosity
Residual DendriticStructure
Weld Nugget
Workpieces
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Weld Hardness
Ha
rdne
ss
Distance (mm)
(a) Weld Morphology (25X)
(B) Variation in Weld Hardness
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Turn to the person sitting next to you and discuss (1 min.):•
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Other Factors Effecting Weld Geometry
• Shunt Current• Electrode Radius
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Effect of Shunt Current on Spot Weld Quality
International Recommendations• Spot Spacing = 16 (sheet thickness)• Spot Spacing = 3 (recommended electrode face dia.)
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Shunting Currents
Howe, Spot Spacing Effect on Buton Size”AWS Sheet Metal Conf. VI, 1994
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International 16 t Formula
Conclusion: Above the internationally recommended spot spacing, there is little effect on button size
Howe, Spot Spacing Effect on Buton Size”AWS Sheet Metal Conf. VI, 1994
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Effect of Electrode Radius
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RSW Certification Training Class, Boeing
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RSW Certification Training Class, Boeing
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RSW Certification Training Class, Boeing