Transcript Coated Steel Weldability

Coated Steel Weldability
Coated Steel Weldability
• Electrode Factors
• Coating Factors
• Welding Equipment Parameters
Electrode Life
•Electrode Material
• Electrode Design
Diameter
Electrode Cap Diameter
For Coated Steel
Weld Button Diameter
For Coated Steel
Weld Button Diameter
Uncoated Steel
Number of Welds
Electrode Life
brass
• Electrode Material
• Current History
• Electrode Design
Copper Alloy Electrode
Molten Zinc
Solid Zinc Coating
Steel
Bare
Galvanized
Measured
Electrode Face Temp (F)
1000-1200
1500-1700
Copper
Brass
Zinc
Melting Point (F)
1980
Down to 1710
787
~45%
Cu
~60%
~85%
Zn
Electrode Alloying during Resistance
Welding on Galvanized Steel
HV15
360
130
100
Cu
8
17
55
Zn Al Fe
59 33
44 32 7
45
10 mm
Uncoated
Hot Dipped Galvanized
Electrode Material
Class 1
Class 2
Zr-Cu
Dispersion
99 Cu - 1 Cd
99.2 Cu - 0.8 Cr
98.9 Cu - 1 Cr - 0.1 Zr
Cu- 2.68% Al2O3
Standard Electrodes
Brazed Tip Electrodes
Mo - called TZM Electrode
W - results scattered
Flame Sprayed or Coated Electrodes
Mo, W, MoC, WC, Ag, Au, Al2O3
Electrode Materials for Spot Welding Coated
Copper-Zirconium (Cu-Zr)
Traditional (Class 2)
Steel
Material
Copper-Chromium
(Cu-Cr) Material
• High hardness
• Moderate electrical
conductivity (about
80% IACS)
• Alloy addition: 0.8
wt% Cr
Copper-ChromiumZirconium (Cu-CrZr) Material
– Higher
hardness and
softening
temperature
– Its conductivity
is about the
same as Cu-Cr
material
– Alloy addition:
0.7 wt% Cr +
0.1 wt% Zr
– Lower hardness
than Cu-Cr material
– Higher electrical
conductivity (up to
93% IACS)
– Alloy addition: 0.15
wt% Zr
Dispersion-Strengthened
Copper (DSC) Material
•Higher electrical
conductivity (98%
IACS)
•A powder metallurgy
product
Electrode Material
Electrode Face Diameter
Class 1
Class 2 Cu-Cr
Cu-Zr
Al2O3 Flame Spray
Al2O3 Dispersion
(Variation in Results)
TZM
(Mo Alloy Brazed Cap)
Number of Welds
Comparison of Electrodes
Nugget Size (inch)
High Current on HDG Steel
Weld Number (Thousands)
Z-Trode
Cu-Zr
Cu-Cr
Al2O3 DSC Surprising since high
hot hardness
Excursions Above Expulsion Effect DSC
DSC AL-60 No Excursions
DSC AL-25
Cu-Cr
Cu-Zr
DSC - AL-60 Excursions Above Expulsion
Electrode Life
History of Current Excursions
Gugel, Comparison of Electrode Wear,
SMWC V, AWS, 1992
Deterioration
Model
History of Current
Excursions
In The Harder
DSC There Is No
Self Healing.
Excursions Above
Expulsion Where
Electrodes Are
Hotter Allow Some
Healing.
The Lower Hot
Hardness of
Cu-Cr & Cu-Zr
Allow Some
Healing of the
Pits
Gugel, Comparison of Electrode Wear,
SMWC V, AWS, 1992
Electrode Design
Electrode-Wear Pattern for Flat
Electrodes
Flat Electrode
Edges Broken Creating Natural Dome
Craters Forming
Self-Heating + Build-up along Sides
Large Central Cavity
[Reference: Welding in the Automotive Industry, p.174, D. W. Dickinson]
Electrode Design
Dome
Electrode
Flattening
r > 2 1/2 in.
Zinc Buildup
Reduced Current Density
Poor Welds
Electrode Design
Electrode Geometry for
Galvanized Steel
1/4” Flat Face x 45° 1/4” Flat Face x 20° 1” Radius Face
20°
45°
5/16”
1/32”
Wear
27/64”
1/32”
Wear
3” Radius Face
3”
1”
1/2”
1/32”
Wear
63% Increase
in Area
185% Increase
in Area
300% Increase
in Area
Recommended
for sheet up to
1/16” thick
Recommended
for sheet over
1/16” thick
Satisfactory
only when
alignment is a
serious problem
7/8”
1/32”
Wear
1100% Increase
in Area
Unsatisfactory
Electrode Design
Effect of Cone Angle
Heat Sink
Because of Higher Temp
More Copper From
Electrode Sticking To
Part
Ikeda et al, Effect of Electrode Configuration…,
Adv Tech & Proc, IBEC’94, 1995
Electrode Design
Effect of Cone Angle
Faster Face Enlargement
Lower Current Density
Ikeda et al, Effect of Electrode Configuration…,
Adv Tech & Proc, IBEC’94, 1995
Higher
Electrode
Temp
Best
Range
Electrode Design
Electrode Cooling
Poor Cooling
Good Cooling
Poor Cooling
(a)
(b)
(c)
Process Variables
Process Parameters:
– Weld Current
(Heat Generation)
– Weld Time
– Hold Time
– Electrode Force
Coating Parameters:
– Coating Thickness
– Coating Types
Uncoated
Coated
Distance
Zn has lower R
& is soft = good
Contact
Resistance
Weld Diameter vs. Current for Various Coatings
Weld Diameter, inches
0.25
Nominal Current
Level
0.20
Uncoated
Steel
0.15
0.10
27% Ni-Zn
30% Fe-Zn
20% Ni-Zn
Current Range
Uncoated
23%
Fe-Zn
0.05
6
8
9%
Fe-Zn
10
Current Range
Coated
Zinc Only
12
14
Weld Current, kA
[Reference: Welding in the Automotive Industry, p.179, D. W. Dickinson]
Comparison of Current Level
Simple Current Levels for 0.8 mm Sheet Steels
Material
Current Level (kA)
Uncoated Steel
9
Fe-Zn Electro Coated
10
Galvannealed
10
Electro Galvanized
12
Hot Dipped Galvanized
13
(6.1 mm Electrodes & 12-14 Cycles of Welding Time)
Lobe Curves
Hot-Dip
Galvanized
Weld Time
Weld Time
Uncoated
Zinc
Melting
Steel
Melting
Weld Current
Weld Current
Tensile-Shear Strength (lbs)
Weld Current vs. Tensile-Shear Strength
[Reference: Welding in the
Automotive Industry, p.203,
D. W. Dickinson]
Welding Current (Amps, x 103)
Electrode Sticking Test
Welding Current (kA)
28
24
Stick
20
Expulsion
16
Nominal
12
As the number
of welds &
Electrode
deterioration
increase the
current to get a
nominal size
weld (or
expulsion or
sticking)
increases
8
0
550
1100
1850
Number of Welds
2200
Shunting during Series Welding
The extra current required to compensate for the shunting causes
electrodes to run hotter and results in electrode wear.
AWS Welding Handbook
Process Variables
Process Parameters:
– Weld Current
(Heat Generation)
– Weld Time
– Hold Time
– Electrode Force
Coating Parameters:
– Coating Thickness
– Coating Types
Nugget Diameter, inches
Nugget Development during
Weld
Time
Interval
0.041” Bare & Galvanized Steel
[Reference: Welding in
the Automotive Industry,
p.175, D. W. Dickinson ]
Weld Time, cycles
Weld Time vs. Nugget Development
Nugget Diameter
Surface
Breakdown
Nugget Diameter
at Expulsion
Nugget
Growth
Uncoated
Coated
Steel
Surface
Nugget
Zinc Melts Breakdown Growth
Weld Time
Ave Button Size at Expulsion (inch)
Average Button Size at Expulsion as a
Function of Weld Time for Each Materials
Welding Time (Cycles)
[Reference: EWI Research Paper: MR8802, p.46, Gould & Peterson]
Nugget Dimension Vs. Weld Time
Nugget Width (mm)
Effect of Coating Type
Uncoated
Fe-Zn Electro
Electro-Galvanized
Galvannealed
Hot Dipped
Galvanized
Weld Time (Cycles)
[Reference: EWI Research Paper: MR8814, p.29, Gould & Peterson]
Mechanism of Heat Generation
2 Cycles
7 Cycles
4 Cycles
9 Cycles
5 Cycles
10 Cycles
6 Cycles
Step I
Step III
Step II
Step IV
Nugget Development of Hot-Dipped Galvanized
Steel
3 Cycles
7 Cycles
4 Cycles
9 Cycles
6 Cycles
12 Cycles
[Reference: EWI Research Paper: MR8814, p.21, Gould & Peterson]
Effect of Weld Time on Current Range
for Hot-Dip Galvanized Steel
Welding Time (Cycles)
Minimum
Nominal
Expulsion
Welding Current ( kA)
[Reference: EWI Research Paper: MR8802, p.19, Gould & Peterson]
Effect of Weld Time on Electrode Life
Electrode Life
Current
Short Time
High Current
and Overheating
Longer Time
Greater
Alloying
Weld Time
Electrode Life (Number of Welds)
Electrode Life Vs. Welding Time
Electrode Diameter: 0.19”
Electrode Diameter: 0.25”
Electrode Diameter: 0.28”
3200
2400
1600
800
0
5
9
13
17
21
Welding Time (Cycles)
25
29
Process Variables
Process Parameters:
– Weld Current
(Heat Generation)
– Weld Time
– Hold Time
– Electrode Force
Coating Parameters:
– Coating Thickness
– Coating Types
No Data Could be Found
On the Effect of Hold Time
Related to Coating other than
That already discussed for
Uncoated High Carbon Steels
This might be an area for research
Process Variables
Process Parameters:
– Weld Current
(Heat Generation)
– Weld Time
– Hold Time
– Electrode Force
Coating Parameters:
– Coating Thickness
– Coating Types
Effect of Electrode Force on
Electrode Deterioration
Steel
Diameter
Zinc Layer
Electrode Tip
Diameter
Nugget
Diameter
Number of Welds
Low
Electrode Force
High
Button Size After 2000 Welds
Current
Force
Dual Force Technique
Low Pressure
Electrodes Seat,
Zn Forced Out
Before High Pressure,
Less Mushrooming
Upslope Helps
Zn Flow
From Under
Electrode
Time
Process Variables
Process Parameters:
– Weld Current
(Heat Generation)
– Weld Time
– Hold Time
– Electrode Force
Coating Parameters:
– Coating Thickness
– Coating Types
Weldability Lobe Vs. Coating Weight
(Up To G90 Weight)
G60 Weldability Lobe
Weld Time (Cycles)
Coating Thickness
Only Minimal
Effect on Position
or Width
Weld Time (Cycles)
G40 Weldability Lobe
G90 Weldability Lobe
Weld Time (Cycles)
Current (kA)
Current (kA)
Thicker
Coating more
Erratic
Current (kA)
Coating Weight Above G90
G90
Improved
Electrode
Life
Welding Parameters Needed to get 0.20” Diameter Weld
Substrate/Coating Thickness Effects
Electrode Life
0.037 inch sheet thickness
0.020 inch thick
Electrode Face Closer to
Hot Weld Nugget
0.90
1.25
1.50
Coating Thickness oz/sq ft
Process Variables
Process Parameters:
– Weld Current
(Heat Generation)
– Weld Time
– Hold Time
– Electrode Force
Coating Parameters:
• Coating Thickness
• Coating Types
Incomplete
Galvannealed
hot
galvannealed
dipped
electrolytic
Electrode Life
Gamma
Zinc
Aluminum Oxide Passive Layer
Steel
High Si, Mn, Al in Steel
Ties up Oxygen
Also Causing Break
in Passive Layer
Incomplete Oxide Layer
Electrode
Life
0.1 0.3
% Al in Bath
Hot Dipped Galvanized
1.0
Possible Phases in Galvannealed
Delta
Alpha
Steel
Zeta
Gamma
Partially
Galvannealed
Unclean
Coating
Dross
Zinc
Gamma
Steel
Zeta
Delta
Gamma
Steel
Localized Hot Spots
Zinc Alloying in Electrode
Electrode Life Reduced
Zeta
Delta
Gamma
Steel
Pickett, Effect of Total Iron Content….
SMAW V, AWS, 1992
Seam Welding Galvanized Steel
Usually not recommended
• Zinc Contamination of Wheel Electrodes
• Some outer surface of sheet cracking
Wheels with continuous cleaning have helped
Foil Butt Welding has also been effective (see next Slide)
Foil Butt Welding of Galvanized Steel
AWS Welding Handbook
Projection Welding of Galvanized Steel
AWS Welding Handbook
Projection Welding of Galvanized Steel
• Heat Loss to Electrode (Flat Face) Higher than
Uncoated because Higher Thermal Conduction of
Coating
• Contact Resistance at Faying Surface Only Slightly
Lower than uncoated because soft zinc deformation
Therefore: Somewhat lower electrode forces recommended
Projection Welding of Galvanized Steel
•On sheet thinner than 0.09 in. projection welding
produces a forged bond rather than a fusion bond
• Increase in current causes burn off and expulsion of
projection.