WE 602Resistance Welding Processes

Review&

Physics of Spot WeldingReference Web Site for Review:

www-iwse.eng.ohio-state.edu/we601

Resistance Welding Process Review

Lesson ObjectivesWhen you finish this lesson you should understand:• Heat generation & nugget growth in resistance spot welding• Basic, enhanced and pulsed weld cycles• Advantages and limitations of spot welding• A basic model of expulsion

Learning Activities1. View Slides; 2. Read Notes, 3. Listen to lecture4. View Demos5. Do on-line

workbook6. Do Homework

KeywordsSpot weld contact resistance expulsion heat flowWeld cycle bulk resistance pulsing electrode

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]

Introduction to Resistance Spot Welding

Top ElectrodeWater

WeldNugget

Bottom Electrode

Resistance

Dis

tan

ce

Typical Equipment of Resistance Spot Welding

(a) (b)[Reference: Welding Process Slides, The Welding Institute]

Process Operation of Resistance Spot Welding

[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 equipments

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

Basic Single Impulse Welding Cycle

Electrode Force

Welding Current

Welding Cycle

Squeeze Time Weld Time HoldTime

OffTime

[Reference: Welding Handbook, Volume 2, AWS, p.538]

Enhanced Welding Cycle

Pre

heat

Tim

e

Ups

lope

Tim

e

Coo

l Tim

e

WeldTime

Coo

l Tim

e

PreweldInterval

Welding Cycle

Weld Interval Postweld Interval

Dow

nslo

peT

ime

Que

nch

Tim

e

Tem

per

Tim

e

Hol

dT

ime

PulseImpulse

TemperingCurrent

Welding CurrentElectrodeForce

Forge Delay Time

Forge Force

[Reference: Welding Handbook, Volume 2,AWS, p.539]

Squeeze time

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

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]

Turn to the person sitting next to you and discuss (1 min.):• The total resistance across the spot weld for plain carbon steel usually starts high because of interfacial surface asperities and oxides and surface contaminants. But the total resistance changes as current passes. What would be the value of resistance as time increases (dynamic resistance)? Draw a curve.

Num ber of Cycles

Resis

tan

ce

Temperature Gradient of A Spot Weld

End of“Weld Time”

20% of “Weld Time”

Water Temperature

Water

Water

Electrode

Electrode

Work

Welding Temperature

[Reference: Resistance Welding Manual, RWMA, p.1-4]

Nugget Growth

(a) (b)

Point 1

Point 2

Point 3

Nugget Growth (CONT.)

(a) (b)

Expulsion at Interfacial Surface

Zhang et al, “Expulsion Modeling in RSW of Steel and Al Alloys”, AWS Sheet Metal Conf VIII, 1998

Temperature Readings of A Spot Welding Process

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

Zhang et al, “Expulsion Modeling in RSW of Steel and Al Alloys”, AWS Sheet Metal Conf VIII, 1998

Zhang et al, “Expulsion Modeling in RSW of Steel and Al Alloys”, AWS Sheet Metal Conf VIII, 1998

Zhang et al, “Expulsion Modeling in RSW of Steel and Al Alloys”, AWS Sheet Metal Conf VIII, 1998

Pulsing

Cool Time

Pulse 1 Pulse 2 Pulse 3

PulseTime

Without Pulsing With Pulsing

Turn to the person sitting next to you and discuss (1 min.):• We have seen the nugget growth with AC current to grow when the peak in current is reached and shrink slightly as the current goes to zero each half cycle. What might occur when DC current is used?