3-2 spot welding

8/2/2019 3-2 Spot Welding

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Spot Welding



Definition of Resistance

Welding

• Resistance welding is a fusion welding process in

which coalescence of metals is produced at the

faying surfaces by the heat generated at the joint by

the resistance of the work to the flow of electricity.

• Force is applied before, during, and after the

application of current to prevent arcing at the work

piece.

• Melting occurs at the faying surfaces during

welding.



Principal Types of Resistance Welds

Electrodesor WeldingTips

Electrodesor WeldingWheels

Electrodesor Dies

ProjectionWelds

Electrodes or Dies

Spot Weld Seam Weld Projection Weld

Upset Weld Flash Weld

After Welding After Welding[Reference: Resistance Welding Manual, RWMA, p.1-3]



Typical Equipment of Resistance Spot Welding

(a) (b)

[Reference: Welding Process Slides, The Welding Institute]



Advantages of Resistance Spot

Welding Adaptability for Automation in High-Rate

Production of Sheet Metal Assemblies

High Speed

Economical

Dimensional Accuracy



Limitations of Resistance Spot

Welding Difficulty for maintenance or repair

Adds weight and material cost to the product, compared witha butt joint

Generally have higher cost than most arc welding equipment

Produces unfavorable line power demands

Low tensile and fatigue strength

The full strength of the sheet cannot prevail across a spotwelded joint

Eccentric loading condition



Resistance Welding

• Resistance welding depends on threefactors:

– Time of current flow (T).

– Resistance of the conductor (R)

– Amperage (I).

•

Heat generation is expressed asQ = I2R T, Q = Heat generated.



Heat = I2 RTK

Where

Is a function of:

I = Current (Amps) Transformer Tap Setting

R = Resistance (Ohms) Material Prop., & Pressure

T = Time (Cycles 1/60

Second)

Control Setting

K = Heat Losses Conduction, Convection,Radiation



Heating Value of Current = RMS Current

Irms=0.707 Ipeak



Block Diagram of Single-Phase

Spot Welder

Spot Weld

Main Power Line

Contactor



N=np /ns

Vs= Vp /N

Is = Ip N



Heat = I2 RTK

Where

Is a function of:

I = Current (Amps) Transformer Tap SettingR = Resistance (Ohms) Material Prop., & Pressure

T = Time (Cycles 1/60

Second)

Control Setting

K = Heat Losses Conduction, Convection,Radiation



Contact-Resistance Measurement

ContactArea

Electrode Force

Electrode Force

Small Current

Rec

Rec

Rsc

Rv

Rv

Rec

Rec

Rtotal

Rec = contact resistancebetween electrode

and sheet surface

Rsc = contact resistanceat the faying surface

Rv = volume resistance ofthe sheets



Factors Affecting Heat

Generation (Q):

• Welding pressure

– as welding pressure increases both

R and Q decrease.

• Electrodes

– deformation of electrodes

increases contact area. As contactarea increases, both R and Q

decrease.



Surface ConditionSteel

Steel

Steel

Steel

Oils/DirtOxide

OxideOils/Dirt

(a) Pickled Conditions

(b) Rusted ConditionsRusty

Pickled

Polished

Electrode Force

R e s i s t i v i t y



Resistance Varies with Pressure

Low Pressure Medium Pressure High Pressure

(a) (b) (c)



Volume-Resistance Measurement

ContactArea

Electrode Force

Electrode Force

Small Current

Rec

Rec

Rsc

Rv

Rv

Rv

Rtotal

Rec = contact resistancebetween electrode

and sheet surface

Rsc = contact resistanceat the faying surface

Rv = volume resistance ofthe sheets



Resistivity as a Function of Temperature

100 200 300 400 500 600 700 800

10

2030

405060708090

100110120

130

HSLA

Low Carbon

Temperature, °C

R

e s i s t i v i t y , m W

- c m

[Reference: Welding in the Automotive Industry, D.W. Dickinson, p.125]



Heat Dissipation

Weld Nugget

Water-Cooled Copper Alloy Electrode

Water-Cooled Copper Alloy Electrode

Base Metal

Base Metal



Initial Resistance Through Weldment

Top ElectrodeWater

WeldNugget

Bottom Electrode

Resistance

D i s t a n c e



Temperature Readings of A Spot Welding Process

Workpiece

This illustration was takenabout 4/60th of a secondafter the welding currentstarts.

(Note: Temp at Electrode Sheet Interface Higher than Bulk)



After 20%

welding time

At the end of

welding time

Temperature

E l e c t r o d e

W o r k p i e c e

Temperature

distribution at

various

location

during

welding.

Temperature Distribution

3-2 spot welding

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