aluminum spot welding general considerations. resistance welding lesson objectives when you finish...

Aluminum Spot Welding

General Considerations

Resistance Welding

Lesson ObjectivesWhen you finish this lesson you will understand:•

Learning Activities1. View Slides; 2. Read Notes, 3. Listen to lecture4. Do on-line workbook

Keywords

Comparison of Physical Characteristic Which Effect Welding

Aluminum Copper Plain Carbon SteelMelting Points 890-1180F

480-640C1980F 2700-2800F

1480-1540CPlastic Temp

Range90C 540C

ThermalConductivity

High Highest Low

ElectricalResistivity

~5 Cm ~3 Cm ~15 Cm

SolidificationContraction

High Low

Surface Oxide Al2O3 (thick) CuO (thin) FeO, Fe2O3

Oxide MeltingPoint

3700F

Recommended Joint Designs Based Upon These Properties

AWS Welding Handbook

Material Parameters• Plastic Condition

Temperature Range– Aluminum reaches plastic range

at a lower temperature than steel due to lower melting point of aluminum (480 to 640 °C compared to 1480 to 1540 °C)

– Aluminum has a plastic temperature range of about 90 °C compared to steel, with wider range of about 540 °C

– Maintaining sufficient plastic material to constrain molten nugget is more difficult with aluminum alloys.

Temperature Range of Plastic Condition

Weld Cycles Weld Cycles

Tem

pera

ture

Tem

pera

ture

[Reference: Welding, p.11-6, Kaiser Aluminum & Chemical Sales, Inc.]

Aluminum Steel

Browne, D., Model to Predict,IBEC’95, Adv Tech & Processes, 1995

Pressure Distribution During Spot Welding

Only Slight Pressure Spreading & IncreasedElectrode and FayingSurface Contact During First Cycle

Thereafter, RapidIncrease in Electrode& Faying SurfaceContact and SheetSeparation

Then Nugget Growth

Zhang, Nugget Growth in RSW AluminumSMWC VII, AWS, 1996

Auhl, JR, SAE 940160,1994

Various Nugget Growth MorphologiesVarious Nugget Growth Morphologies

• Thermal Conductivity– Greater than that of

steel

– The rate of heat loss from the weld is much greater in aluminum

– Require short welding times and high current flows

Material Parameters• Electrical Resistivity

– The electrical resistivity of aluminum is quite low ( approx. 5 micro-ohm-cm compared to 15 micro-ohm-cm of steel)

– Require short welding times and high current levels

– Three to five times the current is required to weld an equivalent thickness of aluminum compared to steel

– Shunting losses in aluminum alloys are more of a problem than for steel

Overview of Thermal Considerations

Case Studies of Common Electrode Heat Flow Conditions

Effects of Electrode Tip Contour: Case A

• Equal thickness of like identical contours on each electrode. Wide nugget has equal penetration in each piece.

[Reference: Resistance WeldingManual, p.11-29, RWMA]

Effects of Electrode: Dissimilar Material: Case B

• Equal thickness of dissimilar materials but identical contours. Weld nugget has unequal penetration.


Effects of Electrode on Unequal Thickness: Case C

• Unequal thickness of similar materials with identical electrode contours. Note that the penetration is considerably greater in the thicker piece. Compared this with Case D.


Effects of Electrode Tip Contour: Case D

• Unequal thickness of similar materials with radiused electrode against the thin piece and flat electrode against the thick piece. Note that with this combination weld penetration is approximately even on both pieces. Compared this with Case E.


Effects of Electrode Tip Contour: Case E

• Unequal thickness of similar materials with radiused electrode against the thick piece and flat electrode against the thin piece. Note that in this case the electrode application is opposite to that recommended for good practice resulting in unequal weld penetration.


Material Parameters (CONT.)• Expansion and Contraction

– Undergo greater expansion and contraction during melting and solidification processes than does steel

– Dimensional changes are greatest in the weld zone and commonly result in nugget cracking

– Machines with Low Inertia Heads help electrodes “follow-up” nugget solidification

– Post weld forges are often used to prevent cracking

– Current Decay during the solidification process helps

Expansion and Contraction


With Current Decay Without Current Decay

Low IHigh F

High ILow F

Dewey, R, Mapes, R, “Observations on Spot Welding Aluminum for Automotive Applications” SAE Paper 770208, 1977

Nugget Expansion & Contraction Curves for Aluminum


ExpulsionExpulsion

No ExpulsionNo ExpulsionEffect of Current and Force on Expulsion

Design of a Low Inertia Welding Head

Air Inlet Relaxed Diaphragm

Welding-CurrentSwitch(Open)

Air Inlet

Piston

Cylinder

WeldingHead

OuterShaft

InnerShaft

Work pieces

Spring (Under InertiaCompression)

Upper Electrode

Lower Electrode

Compressed Diaphragm

Weld-CurrentSwitch(Closed)

Fully CompressedSpring

Work pieces

Partially CompressedDiaphragm

Welding-CurrentSwitch(Closed)

PartiallyCompressedSpring

Work pieces

(a) (b) (c)

Effect of Nugget Size & DefectsOn Tensile Shear Strength

OvalParallel or

Perpendicular

Aluminum AssociationT 10 Guideline

Porosity

Porosity & Cracking

Internal Cracking

Michie, KJ EtAl, SMWC VII, AWS 1996

Nugget Size Controls Strength BUTDefects Cause Scatter About Mean

Auhl, JR, SAE 940160,1994

Effect of Defects at a Constant Nugget Size

Michie, KJ EtAl, SMWC VII, AWS 1996

OvalParallel or

Perpendicular

Internal Cracking

Severe Cracking

Aluminum AssociationT 10 Guideline

Effect of Nugget SizeOn Fatigue Properties

Factors Effecting Shear Load/Fatigue• Nugget Diameter• Metal Thickness• Metal UTS Select Nominal

Diameter = 0.9524 t0.5

Effect of Gage with weld made of Nominal Diameter

Shear Load = 0.659 t1.23 UTS(for weld with Nominal Diameter)


Surface-Related Problems• Surface-Condition Issues

– Revolve around the aluminum oxide film which forms on the surface of the aluminum

– As this oxide grows, the effective contact resistance of the aluminum changes

– As the electrode comes in contact with the sheet surface, this oxide fractures non-uniformly and creates only small areas for the passage of current

– Results in Electrode Life deterioration

• Oxide Removal Methods– Chemical removal

– Abrasive removal

– Stabilization after oxide removal

– Arc cleaning the surface immediately before welding

– Conversion coatings

Surface Oxide

Aluminum T<400C

AlMg T<400C

Aluminum T>400C

AlMg T>400C

Patrick, EP et al, SAE 840291,1984

Chemical Cleaning SolutionTwo to Six Minute Immersion

• 15 oz/gal (120g/l) Nitric Acid [technical grade (68% HNO3

• 0.15 oz/gal (2g/l) Hydrofluoric Acid (48% HF)• 0.14 oz/gal (2g/l) Wetting Agent

Chromate Conversion CoatingsOccasionally a coating is added to increase Paint adhesion

Four Types• Alkaline Oxide• Chromium Phosphate

• ChromateChromate• No-rinse Process

Alkaline Oxide• Immerse in alkali chromate bath, 20 min, 95C• Coating weight 100-500 mg/sq ft• Color light to brownish green

Chromium Phosphate• Spray or Immerse in H2CrO4, H3PO4, & F-

• Coating weight 5 to 500 mg/sq ft• Colorless to emerald green

Chromate CoatingsChromate Coatings• Emersion in HEmersion in H22CrOCrO44 - HF - Other mineral Acids - HF - Other mineral Acids• Low contact resistance aids in Resistance WeldingLow contact resistance aids in Resistance Welding• Coating weight 15 to 30 mg/sq ftCoating weight 15 to 30 mg/sq ft• Color iridescent yellow to brownColor iridescent yellow to brown

No-rinse coating• Direct line application of composition containing Cr+6 & Cr+3

• Some formulations include organic compounds• Coating weight 5 to 25 mg/sq ft• Coating weight proportional to applied wet film

Contact-Resistance Measurement

AWS Welding Handbook,vol 3 1996

Low Currents Generally Used: Representative of Weld Currents?

ContactArea

Electrode Force

Electrode Force

Small Current

Rec

Rec

Rsc

Rv

Rv

Rec

Rec

Rtotal

Factors Effecting Contact Resistance

• Surface Roughness Electrode• Surface Roughness Sheet• Alloy or Oxide on Electrode• Alloy or Oxide on Sheet Outer Surface• Alloy or Oxide on Sheet Faying Surface• Differential Oxide on Sheet• Other Surface Coatings

Patrick, E, Spinella, D, “Surface Effects on Resistance Spot Weldability – Aluminum Body Sheet”, International Body Engineering Conference, 1995

Surface Roughness Studies(Low Oxide Surface)

MillFinish

20 %

5%

Ele

ctr

od

e M

ush

roo

min

g (

%)

ArcTextured

ElectrodeM ushroom

Arc Textured • Better Current Contact• Improved Electrode Life• Improved Lobe

ContactArea

Electrode Force

Electrode Force

Small Current

Rec

Rec

Rsc

Rv

Rv

Rec

Rec

Rtotal



General Practice: Clean Oxide Off

Surface Abraded or Chemically Cleaned and Stearate Treated

1000

100

20Su

rfa

ce C

on

tact

Res

ista

nce

Untreated Al

100 Reoxidized + Stearate

20 Reoxidized + Stearate

Time (Weeks)

Aluminum

Al2O3

Monolayer of Stearate

Weld Within 24 Hours if Untreated

Arc-Cleaning Method of Spot Welding Aluminum

Oxide

AluminumAluminum

AluminumAluminum

High Frequency Arc

High Frequency Arc

Initial Resistance 50 micro-ohms

Initial Resistance 1000 micro-ohms

Breakdown of Surface Oxide When Weld Current Applied(25kA Current)

Higher Initial ResistanceLonger Time

Alcan Internal Research, March 1994

Cleaned Surface

Cleaned and Held Surface

Initial Resistance 50 micro-ohms Initial Resistance 1000 micro-ohms

Effect of Surface Resistance on Nugget Growth & Expulsion

ExpulsionExpulsion

The more rapid and uncontrolled growth causes erratic expulsion


Slower Nugget GrowthNo Expulsion

Rapid GrowthRapid Growth

ExpulsionExpulsion

Effect of time after Cleaning on Mechanical Properties

Clean Surface =• Higher Strength• Wider Variation

Chihoski, R, “Variation in Aluminum Spot Welds”, Welding Journal, Dec 1970

.

Nugget

Perimetal Bond(Weld Heat = SS Bond)

Diffusion Bond (If Surface Clean)Diffusion Bond (If Surface Clean)

Effect of Surface Cleaning (Time After Cleaning) on Diffusion Bond Formation and Weld Strength

Chihoski, R, “Variation in Aluminum Spot Welds”, Welding Journal, Dec 1970

# of Welds

Nugg

et D

iamet

er

Arc Clean(If No Electrode DeteriorationFrom Arc)

Clean & Weld Immediately

Clean & Stearate Treatment

Clean & Left 1 Day

No Cleaning

Electrode Life Improvement of Various Surface Cleaning Technique

ContactArea

Electrode Force

Electrode Force

Small Current

Rec

Rec

Rsc

Rv

Rv

Rec

Rec

Rtotal



Anodized

Caustic Cleaned

Caustic Cleaned

Faying Resistance

Ele

ctr

od

e L

ife

ContactArea

Electrode Force

Electrode Force

Small Current

Rec

Rec

Rsc

Rv

Rv

Rec

Rec

Rtotal



Screening Tests of Various Treatments with Spherical Radiused Electrodes

Surface ConditionWater Stained

Mill Finish/Mill Finish

Abraded/Abraded

Abraded/Mill Finish

Arc Cleaned/Mill Finish

Conversion Coat/Conversion Coat

Abraded/ Conversion Coat

Arc Cleaned/Conversion Coat

[Reference: SAE 840291, Patrick, Auhl, and Sun]

Required Properties of Ideal Electrodes

• Maximum electrical and thermal conductivity

• Maximum hardness or resistance to deformation

• Tip Design Which Reduces Wear

• Minimum tendency to alloy with the material being welded

(Because of Lower Resistance of Aluminum Than SteelMore Current is Needed to Make the Weld)

Properties of Some Copper-Alloy Electrodes for Aluminum

Property

Annealing Temperature

Hardness (Rockwell)

Electrical Conductivity(% I.A.C.S.)

Availability

Class I

660 °C

70B

85

Bars, Forgings(not in cast form)

Class II

950 °C

65B Cast80B Wrought

80 Cast85 Wrought

Bars, ForgingsCastings

New Electrode Formulated to Give High Conductivity

STAR = Sumitomo Tough and Robust

Kumagai, M, High Performance Electrode Material…IBEC’95, Material & Body Testing, 1995

Higher Conductivity Lower Operating TempLonger Life

Kumagai, M, High Performance Electrode Material…IBEC’95, Material & Body Testing, 1995

Typical Electrode Configuration

0.625”Dia.

0.625”Dia.

0.37

5” M

ax.

0.25” Metal Contact

3” SP.R. 3” SP.R.

Radiused Electrode Truncated Cone Electrode

60 °

Both Electrodes Have Typical Dimensions for Welding 0.040 in. Thick Sheet to 0.040 in. Thick Sheet

[Reference: Guidelines toResistance Spot WeldingAluminum Automotive Sheet,p.8, The Aluminum Association]

Suggested Tip Radius Contours

[Reference: Resistance Welding Manual, p.11-32, RWMA]

An Electrode with a Radius Face is Usually Recommended(Sometimes one electrode may have a flat face)

Recommended Tip Radius as function of Al Thickness

Electrode Design

5/8” Dia.Electrode

1/4” to 3/8”Nose Length

Spherical

RadiusFace

Water-CooledElectrode

Spherical Radius Face

Offset Water-Cooled Electrode

5/8” Dia.Electrode

1/4” to 3/8”Nose Length

Water-CooledHolder

(a) (b)


Effects of Improper Tip Application: Case A

• This shows a satisfactory electrode application, but either too much welding force, excessive welding current or time, or a combination of these, resulting in excessive indentation and crater rim on topside and heat shrinkage on bottom. This also causes too much penetration on top and probable spitting or metal expulsion with voids or blowholes and excessive heat penetration.

[Reference: Resistance Welding Manual, p.11-30, RWMA]

Effects of Improper Tip Application: Case B

• A tip radius too small accentuates and aggravates the conditions described in Case A.


Effects of Improper Tip Dressing: Case C

• Tip misalignment causes a rough and bad-appearing electrode mark, misshapen weld nugget, and sheet separation. This condition, while exaggerated in the sketch, is caused by hand-filing the electrode face, electrode skidding, or a combination of the two.


Electrode Dressing Tool

A B C

A B C

Section A-A Section B-B Section C-C

Radius 1 Radius 2

0.05 in. 0.25 in. 0.05 in.

3 in. 3 in.

8 in.

1.25 in.

[Reference: Guidelines toResistance Spot WeldingAluminum Automotive Sheet, p.8, The Aluminum Association]

Emery Cloth or Sand paper wrapped around tool

On Welding Electrode Polisher

Beneteau, D et al, “Resistance Spot Welding of Metal, Particularly Aluminum”, US Patent 5,449,878, Sept 12, 1995

Expulsion Analysis


ExpulsionExpulsion


3 kN, 0.75 in. Radius

6 kn, 2 in. Radius

Effect of Electrode Radius on Expulsion

No ExpulsionNo Expulsion

Phase Diagrams of Al-Cu Binary Alloy Systems

Hansen, Binary AlloysMcGraw-Hill, 1958

Eutectic**

*

*

*

* = Intermetallics

Electrode Deterioration - Thick Oxide



Electrode Deterioration - Thin Oxide

Effect of Surface Roughness on Electrical Contact


Dynamic Resistance during First 1/4 Cycle of Welding

Mill finish/mill finish sheet

Weld Time (mill-second)

Re

sist

ance

(m

icro

-oh

ms)

Abraded/mill finish sheet

[Reference: SAE 840291, Patrick, Auhl, and Sun]

Dynamic Resistance Comparison

Auhl, JR, SAE 940160,1994

Effect of Electrode Cooling on Electrode Life

• Without Proper Cooling Alloying and Tip Wear Increases• Proper Cooling Tube Insertion is Beneficial• Lower Water Temperature is Beneficial• Higher Flow Rates are Beneficial

Hirsch R., Influence of Water Temp& Flow on Electrode LifeSMWC VII, AWS, 1996

Electrode Sticking

Wist, Electrode-Workpiece StickingSMWC VII, AWS 1996

Electrode Conditioning

Wist, Electrode-Workpiece StickingSMWC VII, AWS 1996

aluminum spot welding general considerations. resistance welding lesson objectives when you finish...

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