Coated Steel Weldability

Coated Steel Weldability

• Electrode Factors• Coating Factors• Welding Equipment Parameters

Electrode Life

Number of Welds

Dia

mete

r

Electrode Cap Diameter For Coated Steel

Weld Button Diameter For Coated Steel

Weld Button DiameterUncoated Steel

•Electrode Material• Electrode Design

Electrode Life • Electrode Material• Current History• Electrode Design

Steel

Solid Zinc Coating

Molten ZincCopper Alloy Electrode

brass

Melting Point (F)Copper 1980Brass Down to 1710Zinc 787

MeasuredElectrode Face Temp (F)

Bare 1000-1200Galvanized 1500-1700

Cu Zn

~45% ~60% ~85%

Electrode Alloying during Resistance Welding on Galvanized Steel

10 m

HV15

360

130

100

Cu Zn Al Fe8 59 3317 44 32 755 45

Uncoated

Hot Dipped Galvanized

Electrode Material

Class 1 99 Cu - 1 CdClass 2 99.2 Cu - 0.8 CrZr-Cu 98.9 Cu - 1 Cr - 0.1 ZrDispersion Cu- 2.68% Al2O3

Standard Electrodes

Brazed Tip Electrodes

Mo - called TZM ElectrodeW - results scattered

Flame Sprayed or Coated Electrodes

Mo, W, MoC, WC, Ag, Au, Al2O3

Electrode Materials for Spot Welding Coated Steel

Copper-Chromium-Zirconium (Cu-Cr-Zr) 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

Copper-Zirconium (Cu-Zr) Material

– Lower hardness than Cu-Cr material

– Higher electrical conductivity (up to 93% IACS)

– Alloy addition: 0.15 wt% Zr

Traditional (Class 2) Copper-Chromium (Cu-Cr) Material• High hardness• Moderate electrical conductivity (about 80% IACS)• Alloy addition: 0.8 wt% Cr

Dispersion-Strengthened Copper (DSC) Material

•Higher electrical conductivity (98% IACS)•A powder metallurgy product

Number of Welds

Ele

ctro

de F

ace

Dia

met

erClass 1

Class 2 Cu-Cr

Al2O3 Flame Spray

Al2O3 Dispersion (Variation in Results)

TZM(Mo Alloy Brazed Cap)

Electrode Material

Cu-Zr

Comparison of ElectrodesN

ug

get S

ize

(inc

h)

Weld Number (Thousands)Z-Trode Cu-Cr Al2O3 DSC

High Current on HDG Steel

Surprising since highhot hardnessCu-Zr

Elec

trod

e Li

fe

DS

C -

AL-

60 E

xcur

sion

s A

bove

Exp

ulsi

on

Cu-

Zr

Cu-

Cr

DSC

AL-

25

DS

C A

L-60

No

Exc

ursi

ons

Excursions Above Expulsion Effect DSC

Gugel, Comparison of Electrode Wear,SMWC V, AWS, 1992

History of Current Excursions

Gugel, Comparison of Electrode Wear,SMWC V, AWS, 1992

The Lower HotHardness of Cu-Cr & Cu-ZrAllow SomeHealing of thePits

In The HarderIn The HarderDSC There Is NoDSC There Is NoSelf Healing.Self Healing.

Excursions Above Excursions Above Expulsion WhereExpulsion WhereElectrodes AreElectrodes AreHotter Allow SomeHotter Allow SomeHealing.Healing.

Deterioration Model

History of Current Excursions

Electrode-Wear Pattern for Flat Electrodes

Flat Electrode

Large Central Cavity

Edges Broken Creating Natural Dome

Craters Forming

Self-Heating + Build-up along Sides

[Reference: Welding in the Automotive Industry, p.174, D. W. Dickinson]

Electrode Design

r > 2 1/2 in.

Zinc BuildupReduced Current DensityPoor Welds

Electrode Design

DomeElectrode

Flattening

Electrode Geometry for Galvanized Steel

1/4” Flat Face x 45° 1/4” Flat Face x 20° 1” Radius Face 3” Radius Face

45°

1/32”Wear

5/16” 27/64” 1/32”Wear

20°

1/2” 1/32”Wear

7/8” 1/32”Wear

1”3”

63% Increasein Area

185% Increasein Area

300% Increasein Area

1100% Increasein Area

Recommendedfor sheet up to

1/16” thick

Recommendedfor sheet over

1/16” thick

Satisfactoryonly when

alignment is aserious problem

Unsatisfactory

Electrode Design

Effect of Cone Angle

Heat Sink

Because of Higher TempMore Copper From Electrode Sticking ToPart

Ikeda et al, Effect of Electrode Configuration…,Adv Tech & Proc, IBEC’94, 1995

Electrode Design

Ikeda et al, Effect of Electrode Configuration…,Adv Tech & Proc, IBEC’94, 1995

Effect of Cone Angle

Faster Face EnlargementLower Current Density

HigherElectrode Temp

BestRange

Electrode Design

Electrode CoolingPoor Cooling Good Cooling Poor Cooling

(a) (b) (c)

Electrode Design

Process Variables

Process Parameters:

– Weld Current (Heat

Generation)

– Weld Time

– Hold Time

– Electrode Force

– Electrodes

Coating Parameters:

– Coating Thickness

– Coating Types

Resistance

Dis

tanc

eU

ncoa

ted

Coa

ted

Zn has lower R& is soft = goodContact

Weld Diameter vs. Current for Various Coatings

Weld Current, kA

Wel

d D

iam

eter

, in

ches

[Reference: Welding in the Automotive Industry, p.179, D. W. Dickinson]

Current RangeUncoated

UncoatedSteel

30% Fe-Zn27% Ni-Zn

20% Ni-Zn

Zinc Only 9% Fe-Zn

23%Fe-Zn

6 8 10 12 14

0.05

0.10

0.15

0.20

0.25

Current RangeCoated

Nominal CurrentLevel

Comparison of Current Level

Material Current Level (kA)

Uncoated Steel 9

Fe-Zn Electro Coated 10

Galvannealed 10

Electro Galvanized 12

Hot Dipped Galvanized 13

Simple Current Levels for 0.8 mm Sheet Steels

(6.1 mm Electrodes & 12-14 Cycles of Welding Time)

Lobe Curves

We

ld T

ime

We

ld T

ime

Weld Current Weld Current

UncoatedHot-DipGalvanized

ZincMelting

Steel Melting

Weld Current vs. Tensile-Shear Strength

Welding Current (Amps, x 103)

Te

nsile

-She

ar

Str

en

gth

(lb

s)

[Reference: Welding in theAutomotive Industry, p.203, D. W. Dickinson]

Electrode Sticking Test

0 550 1100 1850 2200

28

24

20

16

12

8

Number of Welds

We

ldin

g C

urr

ent (

kA) Stick

Expulsion

Nominal

As the number of welds & Electrode

deterioration increase the

current to get a nominal size

weld (or expulsion or

sticking) increases

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 Development during Weld Time Interval0.041” Bare & Galvanized Steel

Weld Time, cycles

Nu

gge

t Dia

met

er,

inch

es

[Reference: Welding inthe Automotive Industry,p.175, D. W. Dickinson ]

Weld Time vs. Nugget Development

NuggetGrowth

SteelSurface

BreakdownZinc Melts

SurfaceBreakdown

NuggetGrowth

Uncoated Coated

Weld Time

Nu

gge

t Dia

met

er

Nugget Diameterat Expulsion

Average Button Size at Expulsion as a Function of Weld Time for Each Materials

[Reference: EWI Research Paper: MR8802, p.46, Gould & Peterson]

Welding Time (Cycles)

Ave

Bu

tton

Siz

e a

t Exp

uls

ion

(in

ch)

Nugget Dimension Vs. Weld TimeN

ugge

t W

idth

(m

m)

Weld Time (Cycles)

[Reference: EWI Research Paper: MR8814, p.29, Gould & Peterson]

Uncoated

Fe-Zn Electro

Galvannealed

Electro-Galvanized

Hot Dipped Galvanized

Effect of Coating Type

Mechanism of Heat Generation

2 Cycles

4 Cycles

5 Cycles

6 Cycles

7 Cycles

9 Cycles

10 Cycles

Step I Step III

Step II Step IV

Nugget Development of Hot-Dipped Galvanized Steel

3 Cycles

4 Cycles

6 Cycles

7 Cycles

9 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 Current ( kA)

We

ldin

g T

ime

(Cyc

les)

[Reference: EWI Research Paper: MR8802, p.19, Gould & Peterson]

MinimumNominalExpulsion

Effect of Weld Time on Electrode Life

Weld Time

Ele

ctro

de

Life

Longer TimeGreaterAlloying

Short TimeHigh Currentand Overheating

Current

Electrode Life Vs. Welding TimeE

lect

rod

e L

ife (

Nu

mb

er

of W

eld

s) 3200

2400

1600

800

05 9 13 17 21 25 29

Welding Time (Cycles)

Electrode Diameter: 0.19”Electrode Diameter: 0.25”Electrode Diameter: 0.28”

Process Variables

Process Parameters:

– Weld Current

(Heat Generation)

– Weld Time

– Hold Time

– Electrode Force

Coating Parameters:

– Coating Thickness

– Coating Types

No Data Could be FoundOn 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

Zinc Layer Electrode TipDiameter

NuggetDiameter

Dia

me

ter

Number of Welds

Electrode ForceLow High

But

ton

Siz

e A

fter

200

0 W

elds

Dual Force Technique

Time

Cu

rren

tF

orc

e

Low Pressure Electrodes Seat, Zn Forced OutBefore High Pressure, Less Mushrooming

Upslope HelpsZn FlowFrom UnderElectrode

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)

Current (kA)

Current (kA)Current (kA)

G40 Weldability Lobe G60 Weldability Lobe

G90 Weldability Lobe

Wel

d T

ime

(C

ycle

s)

Wel

d T

ime

(C

ycle

s)

Wel

d T

ime

(C

ycle

s)

Coating Thickness Only Minimal

Effect on Position or Width

Thicker Coating more

Erratic

Coating Weight Above G90

G90

Welding Parameters Needed to get 0.20” Diameter Weld

Improved ElectrodeLife

Substrate/Coating Thickness Effects

0.90 1.25 1.50

Coating Thickness oz/sq ft

Ele

ctro

de L

ife0.037 inch sheet thickness

0.020 inch thickElectrode Face Closer to Hot Weld Nugget

Process Variables

Process Parameters:

– Weld Current

(Heat Generation)

– Weld Time

– Hold Time

– Electrode Force

Coating Parameters:

• Coating Thickness

• Coating Types

Elec

trode

Life

elec

troly

tic

hot

galv

anne

aled

dipp

ed

IncompleteIncompleteGalvannealedGalvannealed

SteelAluminum Oxide Passive Layer

Zinc

Gamma

0.1 0.3 1.0% Al in Bath

ElectrodeLife

Incomplete Oxide Layer

Hot Dipped GalvanizedHot Dipped Galvanized

High Si, Mn, Al in SteelTies up OxygenAlso Causing Break in Passive Layer

ZetaZeta

Delta

Gamma

Alpha Alpha SteelSteel

Possible Phases in Galvannealed

Steel

GammaDeltaZeta

Steel

GammaZinc

PartiallyPartiallyGalvannealedGalvannealed

Localized Hot SpotsZinc Alloying in ElectrodeElectrode Life Reduced

Dross

UncleanUncleanCoatingCoating

Steel

GammaDeltaZeta

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.