Spot Welding

Spot Welding

Lesson ObjectivesWhen you finish this lesson you will understand:• Basics of Resistance Welding Processes • Heat Generation & Control• Spot Welding Process and Applications

Learning Activities1. View Slides; 2. Read Notes, 3. Listen to lecture4. View Demo5. Do on-line

workbook

Keywords: Resistance Spot Welding, Heat Generation, Equipment Control, Contact Resistance, Upslope, Downslope, Hold Time, Temper, Squeeze Time, Electrode

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 DiesSpot 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 with

a 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 spot

welded joint

Eccentric loading condition

Resistance Welding

• Resistance welding depends on three factors:– Time of current flow (T).

– Resistance of the conductor (R)

– Amperage (I).

• Heat generation is expressed as

Q = I2R T, Q = Heat generated.

Heat = I2 RTK

WhereIs a function of:

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

WhereIs a function of:

I = Current (Amps) Transformer Tap SettingR = Resistance (Ohms) Material Prop., & PressureT = 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 resistance between electrode and sheet surface

Rsc = contact resistance at the faying surface

Rv = volume resistance of the 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 contact area increases, both R and Q decrease.

Link to electrode force demo

Surface ConditionSteel

Steel

Steel

Steel

Oils/DirtOxide

OxideOils/Dirt

(a) Pickled Conditions

(b) Rusted ConditionsRusty

Pickled

Polished

Electrode ForceR

esis

tivity

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 resistance between electrode and sheet surface

Rsc = contact resistance at the faying surface

Rv = volume resistance of the sheets

Resistivity as a Function of Temperature

100 200 300 400 500 600 700 800

102030405060708090

100110120130

HSLA

Low Carbon

Temperature, °C

Res

istiv

ity,

-cm

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

Heat = I2 RTK

WhereIs a function of:

I = Current (Amps) Transformer Tap SettingR = Resistance (Ohms) Material Prop., & PressureT = Time (Cycles 1/60

Second)Control Setting

K = Heat Losses Conduction, Convection,Radiation

Heating Value of Current = RMS Current

Irms=0.707 Ipeak

Upslope/Downslope, Hold Time, & Temper

Weld Current

Temper Current

Ele

ctro

de

Pre

ssu

reC

urr

ent

Squeeze Time Weld Time Off Time Hold TimeUpslope Downslope Temper

Heat = I2 RTK

WhereIs a function of:

I = Current (Amps) Transformer Tap SettingR = Resistance (Ohms) Material Prop., & PressureT = Time (Cycles 1/60

Second)Control Setting

K = Heat Losses Conduction, Convection,Radiation

Heat Dissipation

Weld Nugget

Water-Cooled Copper Alloy Electrode

Water-Cooled Copper Alloy Electrode

Base Metal

Base Metal

Heat = I2 RTK

WhereIs a function of:

I = Current (Amps) Transformer Tap SettingR = Resistance (Ohms) PressureT = Time (Cycles 1/60

Second)Control Setting

K = Heat Losses Conduction, Convection,Radiation

Let’s put it all together

Initial Resistance Through Weldment

Top ElectrodeWater

WeldNugget

Bottom Electrode

Resistance

Dis

tan

ce

Temperature Readings of A Spot Welding Process

Workpiece

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

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

After 20%welding time

At the end ofwelding time

Temperature

Ele

ctro

deE

lect

rode

Wor

kpie

ceTemperaturedistribution atvarious locationduring welding.

Temperature Distribution

Link to nugget growth demo

Nugget Solidification