Spot Weld Al Equipment. Resistance Welding Lesson Objectives When you finish this lesson you will...

Spot Weld Al Equipment

Various Types of Equipment

• Single-Phase AC Machines– Without slope control

– With Slope control

• Polyphase Direct-Energy Machines– Frequency Converter

– Rectifier Type

• Stored-Energy Machines

Electromagnetic TypeElectrostatic TypeElectrochemical TypeHome-Polar Type

• Single-Phase DC Machines

- Rectified DC- Medium Frequency DC

Typical Current-Force Diagram for Single-Phase AC Type Machines

Weld Time

Weld Heat TimePostheat

Time Hold Time

ForgeForce

Forge-Delay TimeUpslope

Time

DownslopeTime

Weld ForcePostheatCurrentWelding Current

InitialCurrent

Time

SqueezeTime

[Reference: Resistance Welding Manual, p.11-21, RWMA]

Machine Settings for Spot Welding Aluminum Alloys on Single-Phase Machines


(Recommendations without Slope Control)

Recommended Weld Current with a Single-Phase AC Supply

1 2 3

Material Thickness, mm

70

60

50

40

30

20

We

ld C

urr

ent,

kA

Recommended Electrode Force with a Single-Phase AC Supply


Ele

ctro

de

Fo

rce,

kN

Effect of AC Conduction Angle on Electrode Life

Longer ConductionPeriods Allow moreUniform Heating &(Lower Peak Current)Without Long Cool Periods BetweenHalf-cyclesIncreasing Electrode Life

4.6 msec

4.6

4.6

DC

Lower PeakCurrents

LongerConductionPeriod

IncreaseElectrodeLife

Spinella, D, “Implications for Aluminum Resistance Spot Welding Using Alternating Current”, Materials & Body Testing, IBEC , 1995

Partial NuggetsWeld Cracking

No Weld Cracking





– Rectifier Type





Spot Welding Schedule of Single-Phase Direct Current Machines



Wel

d C

urr

en

t, k

AW

eld

Tim

e,

cyc

les

Ele

ctro

de

Fo

rce,

kN

Recommended Welding Conditions with a Single-Phase DC Supply

Effect of DC Current on Electrode Life

DC Results in Off Center WeldMore Wear on One Electrode

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

Electrode Face with HigherOperating Temperature





– Rectifier Type





Newton, et al, Fund of RW Aluminum,AWS, SMWC VI Oct 1994

Comparison of 50-Hz AC and MFDC Waveforms

MFDC

AC 50 HZ

Time (sec)

Cu

rre

nt (

kA)

AC Mid-Frequency DC

61114-T4 Aluminum

Michaud, E, A Comparison of AC & MFDCSMWC VII, AWS, 1996

ReducedExpulsion

Weld Current

Wel

d Ti

me

ACMF DC

Increase Force

As Increase Gage, Lobe Moves

Effect of Force and Gage on Lobe for MF DC

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


Effect of Electrode Tip Diameters on MFDC


Effect of Weld Spacing in MFDC


Effect of Deteriorated Tips with MFDC





– Rectifier Type





Typical Current-Force Diagram for Electrostatic Stored Energy Type Machines

SqueezeTime

WeldTime

HoldTime

Current

ForgeForce

Forge-Delay Time

Weld Force


Spot Welding Schedule of Electrostatic Stored Energy Machines






– Rectifier Type





Insert diagram of Frequency Converter waveform

Spot Welding Schedule of Typical Three-Phase Frequency Converter






– Rectifier Type





Typical Current-Force Diagram for Three-Phase Rectifier Type Machines


Final Force

HoldTime

Total WeldTime

PostheatCurrent

Initial Force

Welding Current

Weld Time

Forge-DelayTime

SqueezeTime

PostheatTime

Spot Welding Schedule of Typical Three-Phase Direct Current Rectifier


Process Parameters• Weld Current

– High current levels are required to break down the surface oxide and generate the heat necessary for developing an acceptable weld nugget

– Weld Time– Require short weld time due to

high current levels

• Upslope/Downslope– Generally not recommended for

welding aluminum alloys

• Electrode Design/Material– Due to the required higher current levels,

electrodes with high current capacity, such as Class 1 and Class 2, are commonly used

– Electrode designs have mainly been dome-shaped

– Electrode tips must also be dressed frequently

• Forging– Forging cycles are commonly used to

prevent weld cracking in aluminum alloys

Weld Defects• Surface Burning and Tip

Pickup

• Cracks

• Excessive Indentation

• Sheet Separation

• Porosity

• Weld Metal Expulsion

• Unsymmetrical Weld Nugget

• Inclusion of Unwelded

Surface Burning and Tip Pickup

[Cause]• Surface burning is caused by

excessive heat on the metal surface under the electrode and is indicated by burned, pitted and discolored welds.

• Excessive electrode pickup is caused primarily by the same factors.

[Remedy Those Conditions]• Improper surface conditions• Electrode skidding• Improper Electrode

Material - conductivity too low

• Dirty or improper cleaned electrodes

• Excessive “weld time”• Excessive welding current• Inadequate welding force

Cracks[Cause]

• Internal and external cracks in welds, generally caused by improper thermal and pressure conditions, are observed in the weld structure and surface, respectively.

[Remedy Those Conditions]

• Excessive penetration• Insufficient force• Improper rate of current rise• Improper electrode cooling• Improper electrode contour• Delayed application of

forging force• Electrode skidding

Excessive Indentation

[Cause]

• Excessive indentation, generally caused by improper force application, is indicated by depression on the weld surface.


• Excessive force• Weld metal expulsion• Improper electrode contour• Excessive surface heating• Improper forging cycle• Excessive Welding Current

Sheet Separation[Cause]

• Excessive sheet separation, generally caused by poor fitup of parts and excessive surface deformation, is indicated by a wide separation of the base metal adjacent to the weld.


• Excessive force

• Improper fitup of parts

• Weld metal expulsion

• Incorrect electrode contour

• Excessive welding current

• Tip misalignment

• Excessive “weld time”

Porosity[Cause]

• A porous weld structure, generally caused by improper application of heat and force, is observed by sectioning and etching the weld.


• Insufficient “weld time”• Improper rate of current rise• Improper electrode contour• Incorrect sequencing of weld

and forging force• Insufficient force

Weld Metal Expulsion

[Cause]

• Weld expulsion, generally caused by extreme heat and improper force, is indicated by expelled metal from the weld.


• Insufficient force• Tip misalignment• Erratic contact resistance• Foreign substance at faying

surface• Electrode skidding• Excessive welding current• Excessive “weld time”

Unsymmetrical Weld Nugget[Cause]

• Unsymmetrical welds, generally caused by unsymmetrical gauge combination, improper electrode contour, poor fitting workpiece or surface preparation, may be observed by sectioning the weld.


• Improper electrode contour• Inadequate surface preparation• Improper fitup of workpieces• Electrode misalignment• Electrode skidding

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