resistance spot welding

Resistance WeldingResistance Welding

By:Majid Pouranvari

Materials Science and Engineering Department,

Sharif University of Technology

Fall, 2014

A little about me…Education

Authorship

Reviewership

Faculty

PhD, Materials Engineering, Major in Welding,Sharif University of Technology, 2013

Sharif University of Technology

55 ISI-WoS Papers

22 National & Inter. Papers in Scientific Journals

40 Papers in conferences

Reviewer for MSEA, STWJ, JALC, JMAD, JMPT,JMEP, …

Resistance Welding

Table of ContentTable of Content

(1) Resistance Spot Welding(2) Resistance Projection Welding(3) Resistance Seam Welding(4) Resistance Butt Welding

Definition of ResistanceWelding

Resistance welding is a fusionwelding process in whichcoalescence of metals isproduced at the faying surfacesby the heat generated at thejoint by the resistance of thework to the flow of electricity.

Force is applied before, during,and after the application ofcurrent to prevent arcing at thework piece.

Melting occurs at the fayingsurfaces during welding.

Resistance WeldingHeat generation is expressed as

Q = I2R T,Q = Heat generated.

Resistance welding depends onthree factors:Time of current flow (T).Resistance of the conductor (R)Amperage (I).

Principal Types of Resistance WeldsElectrodesor 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]

The dominant process for welding of automotivesheets

Resistance Spot Welding

Resistance Spot Welding

Professor T. W. Eagar, MIT“ The more one studies the resistance welding process, the more

one appreciates how complex the process is ”

RSWEquipments

Heat Generation in Resistance Spot Welding

Block Diagram of Single-PhaseSpot Welder

Spot Weld

Main Power Line

Contactor

N = n p /n s

V s= V p /N

I s = I p N

Typically, the turns ratio in resistance weldingtransformers is about 100 to 1. Therefore, if 480 V, 200A power is available at the primary, 4.8 V at 20kA willbe available at the secondary (neglecting losses).

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 ofwelding time

Temperature

Elec

trode

Elec

trode

Wor

kpie

ce

Temperaturedistribution atvariouslocationduringwelding.

Temperature Distribution

Welding Cycle

Upslope/Downslope, Hold Time,& Temper

Weld CurrentTemper Current

Elec

trode

Pres

sure

Cur

rent

Squeeze Time Weld Time Off Time Hold TimeUpslope Downslope Temper

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

peTi

me

Que

nch

Tim

e

Tem

per

Tim

e

Hol

dTi

me

PulseImpulse

TemperingCurrent

Welding CurrentElectrodeForce

Forge Delay Time

Forge Force

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

Squeeze time

Advantages of Resistance Spot Welding

Excellent for sheet metal applications, < ¼-inch

High speed, < 0.1 seconds in automotivespot welds

Adaptability for Automation in High-RateProduction of Sheet Metal Assemblies

No filler metal Economical Dimensional Accuracy

Process Disadvantages and Limitations

Higher equipment coststhan arc welding

Power line demands Nondestructive testing Low tensile and fatigue

strength Not portable Electrode wear Lap joint requires additional

metal Difficulty for repair

Macrostructural Features of a RSW

Weld Size Requirements

Weld Penetration Requirements

In general, a large penetrationis acceptable if it does notcreate a large indentation. Therequirements on penetrationare often applied together withthose on weld size.

Factors Affecting Heat Generation (Q):

Welding time

Welding Current

Welding current is most effective inheat generation compared to welding time

Q = I2Rt


Resistance

Bulk Resistance vs. Contact Resistance

R=f (Materials Properties and Pressure)


Resistance

Surface Condition

Steel

Steel

Steel

Steel

Oils/DirtOxide

OxideOils/Dirt

(a) Pickled Conditions

(b) Rusted Conditions Rusty

PickledPolished

Electrode Force

Res

istiv

ity

Resistance Varies withPressure

Low Pressure Medium Pressure High Pressure

(a) (b) (c)

Expulsion: A common Phenomena inResistance Welding

Expulsion: A common Phenomena inResistance Welding

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

Effect of Welding Parameters on Expulsion

Welding Current Welding time Electrode force

Effect of Expulsion

Void Excessive electrode indentation

Operating Window - Lobe Curve

Current (1000’s of amperes)

Tim

e (c

ycle

s of

cur

rent

)

ExpulsionNuggettoo small

Acceptablenugget

size

Constant electrodeforce

Effect of Process variable onOperating Window - Lobe Curve

Materials properties

Some welding problems

Some welding problems

Heat Balance

In RSW, heat balance can be defined as a conditionin which the fusion zones in both pieces being joinedundergo approximately the same degree of heatingand applied pressure. It describes the ideal situationwhen a symmetric weld (with equal depth of nuggetpenetration) is made.

Heat balance is influenced by the relative thermaland electrical conductivities of the materials to bejoined, the geometry of the weldment, and thegeometry of the electrodes.

Dissimilar Thickness Welding

In the case of dissimilar thickness, bulk resistance ofthicker sheet is lager than that of the thinner sheet. Thisleads to an asymmetric weld nugget (i. e. penetration of theweld nugget into the thicker sheet is larger than that of thethinner sheet).

Dissimilar Thickness Welding

Solution for Heat Unbalance indissimilar Thickness Welding

This can be overcome by using electrodes of two differentdiameters or by inserting a high-resistivity tip in one electrode.The smaller electrode or the one with high-resistivity insertshould be placed against the thinner of the two sheets

Dissimilar Metal Welding

Shunting Effect

Previously made welds may affect the subsequent welding if the weldsare spaced close to each other due to electric current shunting.

The welding current may be diverted from the intended path by thepreviously made welds. As a result, the current or current densitymay not be sufficient to produce a quality weld.

Shunting Effect

Shunting Effect

Shunting Effect

Shunting Effect

The shunting effect is a strong function of the bulk resistivity of thesheet material. A high conductive metal, such as aluminum, requires alarge space between the welds.

This should be taken into account when welds are designed intostructures, as putting too many welds close to each other may notprovide the intended strength.

electrode

electrode

Electrodes

Electrode Functions

Conduct the welding current to the work

Transmit the proper electrode pressure orforce to the work in order to produce asatisfactory weld

Help dissipate heat from the weld zone

Material Requirements forElectrode in Resistance Welding High electrical conductivity High Thermal conductivity High temperature mechanical strenght

Effect of Strengthening Mechanism on theElectrical Resistivity?

RWMA Electrode Material Standards

Group A - CopperBase Alloys RWMA Class 1

Zirconium Copper Cadmium Copper Chromium Copper

RWMA Class 2 Chromium-Zirconium

Copper Chromium Copper

RWMA Class 3 Cobalt-Beryllium

Copper Nickel-Beryllium

Copper Beryllium-Free Copper

RWMA Class 4 Beryllium Copper

RWMA Class 5 Aluminum Copper

RWMA Electrode MaterialStandards (CONT.) Group B - Refractory

Metals and RefractoryMetal Composites RWMA Class 10

Copper Tungsten

RWMA Class 11 Copper Tungsten

RWMA Class 12 Copper Tungsten

RWMA Class 13 Tungsten

RWMA Class 14 Molybdenum

Group C - SpecialtyMaterial RWMA Class 20

Dispersion-Strengthened Copper

Electrode Materials

RWMAClass #

ElectricalConductivity

Composition(%)

UltimateStrength

(ksi)

AnnealingTemperature

(°C)

ThermalConductivity

(Cal/cm-sec-°C)

-

1

2

3

4

5

Cu

99-Cu,1-Cd

99.2-Cu,0.8-Cr

97-Cu, 2.5-Co, 0.5-BeCu & Be

Cu & Al

90

92

80 (C)82 (F)48 (C)52 (F)20 (C)23 (F)

18 70 - 0.16

30 - -

60 (F)

30 (C)62 (F)95 (C)105 (F)110 (C)170 (F)

660 0.82

710

0.77 (C)0.75 (F)0.43 (C)0.45 (F)0.18 (C)0.19 (F)

930 (C)900 (F)1020 (C)900 (F)

* C = Cast, F = Forging

Typical Electrode Hardness-Temperature Curves

[Reference: Resistance WeldingManual, p.18-2, RWMA]

Electrode Geometry

“A” “A” “A” “A”6° 20°

45°

3

14

14

14

14

45°20°

Pointed

DomeTruncated

ConeTruncated

Cone

[Reference: Welding in the Automotive Industry, p.135, D. W. Dickinson]

Electrode SizeNet Electrode Force (lb x 102)

Electrode Face Diameters (inch)

Electrode Body Diameters (inch)

[Reference: Resistance Welding Manual, p.18-14, RWMA]

W. Stanley, Resistance WeldingMcGraw-Hill, 1950

Electrode Life

Number of Welds

Diame

terElectrode Cap DiameterFor Coated Steel

Weld Button DiameterFor Coated Steel

Weld Button DiameterUncoated Steel

Electrode Degradation

(1) Mechanical Degradation(2) Metallurgical Degradation

Electrode Life

Steel

Solid Zinc Coating

Molten ZincCopper Alloy Electrode

brass

Melting Point (F)Copper 1980Brass Down to 1710Zinc 787

MeasuredElectrode Face Temp (F)

Bare 1000-1200Galvanized 1500-1700

Cu Zn

~45% ~60% ~85%

Uncoated

Hot Dipped Galvanized

Spot Weld Mechanical Properties

Interfacial Mode

Pullout Mode

Weld With Expulsion Failed in Pullout Mode

Dickinson, “Welding in Auto Industry,AISI, 1981

Workshop Tests

AWS Spec D8.9-97, 1997

Advantages of Peel Test• Ease of Performance• Low Cost• Ability to use on Shop Floor as quality control test

Disadvantages of Peel Test• A qualitative rather than a quantitative

Dickinson, “Welding in Auto Industry,AISI, 1981

Chisel Test

Turn to the person sitting next to you and discuss (1 min.):• Why do automotive manufactures prefer the chisel test onthe production line over the peel test?

Mechanical Properties of RSW

Mechanical Properties of RSWTensile-Shear Test

Mechanical Properties of RSWCoach-Peel Test

Mechanical Properties of RSWCross-Tension Test

Mechanical Properties of RSWTensile-Shear Test

Strength Requirements for Steels

Strength Requirement for Mg & Al

Heuschkel, “Expression of Spot Weld Properties”,Weldign Journal Oct 1952

FTS=Dt[-(C+0.05Mn)] BM

Failure Mode of Spot Welds

مود شکست جوشهاي مقاومتی نقطه اي

یک قطر دکمه بحرانی وجود دارد که در مقادیر پایین تر از آن، شکست فصل مشترکی

. حاکم است

AWS: D=4t0.5

DVS/JIS: D=5t0.5

بهبود رفتار شکست از طریق تمپر درجا

Introduction to ProjectionWelding

(a) (b) (c) (d)

[Reference: Welding Handbook, Volume 2, p.566, AWS]

Examples of Various Projection Designs

(c) (d)


(b)(a)

Examples of Various Projection Designs(CONT.)

(e) (f) (g)


Projection Types for Sheet andSolid Applications

[Reference: Metals Handbook, Volume 6 (Welding, Brazing andSoldering), p.503-524, ASM]

Spherical Projections

Elongated Projections

Cross-wire welding

Projection in Nuts

Advantages of ProjectionWelding Ease of obtaining satisfactory heat balance for welding

difficult combinations

More uniform results in many applications

Increased output per machine because several weldsare being made simultaneously

Longer electrode life

Advantages of ProjectionWelding (CONT.) Welds may be placed more closely together

Finish, or surface appearance, is often improved

Parts may be projection welded that could not beotherwise resistance welded

Limitations of ProjectionWelding Requires an additional operation to form projections

Requires accurate control of projection height andprecise alignment of the welding dies with multiple welds

Requires thickness limitation for sheet metals

Requires higher power capacity equipment than spotwelding

Projection Weld Formation


Stage 1 Stage 3

Stage 2 Stage 4

Projection Weld Formation(CONT.)


Stage 5 Stage 7

Stage 6 Stage 8

Basic Projection Design in SteelSheet


SphericalRadius

ProjectionWallThickness ShouldBe at Least 70%of Sheet Thickness

Punch Die

Point Radius“R”Projection Should Blend

into Stock Surface withoutShouldering

D

T

H

AD

B

45°

15°

Effect of H?

Recommended ProjectionDesigns

[Reference: Recommended practice for resistance welding, AWSDocument C1.1-66, AWS, 1966]

T(in.) D(in.) H(in.) L(in.)

0.010-0.014 0.055 0.015 1/80.016-0.020 0.067 0.017 5/320.025 0.081 0.020 3/160.031-0.034 0.094 0.022 7/320.044-0.050 0.119 0.028 9/320.062-0.070 0.156 0.035 3/80.078 0.187 0.041 7/160.094 0.218 0.048 1/20.109 0.250 0.054 5/80.125 0.281 0.060 11/160.140 0.312 0.066 3/40.156 0.343 0.072 13/160.171 0.375 0.078 7/80.187 0.406 0.085 15/160.203 0.437 0.091 10.250 0.531 0.110 1-1/4

HT

D

T: Thickness of thinnest outside pieceD: Diameter of projectionH: Height of projectionL: Minimum contacting overlap

L


Projection Welding of Low Carbon Steel (Two Equal Thicknesses)

Resistance Seam Welding

Resistance Seam Welding

Upper Electrode Wheel

Workpiece

Lower Electrode Wheel

Throat

Knurl or FrictionDrive Wheel

Roll Spot Weld

Overlapping SeamWeld

Continuous SeamWeld

[Reference: Welding Handbook,Volume 2, p.553, AWS]

Lap Seam Weld

Electrodes OverlappingWeldNuggets

Travel

Front view Side View


ASM Handbook Vol6, 1993

ASM Handbook Vol6, 1993

Schematic of seam weld.Length of electrode footprint holds 3 welds

RSW Certification Training Class, Boeing

Effect of Cool Time (Heat %) on Nugget Properties

Resistance Welder Manufacturers Association Bulletin # 23

@ 6 Cycle Heat

Welding Speed

Resistance Welder Manufacturers Association Bulletin # 23

Tensile Tests

Pillow Tests

Seam WeldQuality Tests

Resistance Welder ManufacturersAssociation Bulletin # 21

6 x 6 inch Pillow6 x 10 inch Pillow

Typical data from Pillow Testing

Resistance Upset Welding

Upset Welding

Finished Upset Weld

Heated Zone

To Welding Transformer

Clamping Die

UpsettingForce

Movable Part

Clamping Die

Stationary Part


Schematic of Typical Butt Weld Cycle

Medar Technical Literature

Mechanism

Stage 1: Resistance (Joule) Heating help ease of hotplastic deformation

Stage 2: Application of extra upsetting pressure causesmaterial extrudes out-wards forming an upset.

Upset Resistance Welding

Upsetting

The main mechanism to remove thecontaminants is the formation of the upset.

By changing the welding conditions to generatemore upset the amount of contaminants will bereduced, although more material will be lost andmore energy will be consumed.

Wheel rim production