April 28, 2014

Challenges and Advances in Resistance Spot Welding Aluminum Sheet

Jerry E. GouldTechnology LeaderResistance and Solid State WeldingEdison Welding Instituteph: 001-614-688-5121e-mail: jgould@ewi.org

Scope the Current Presentation Background on spot welding

aluminum in the automotive industry

Differences in spot welding aluminum and steel

Technology as evolved from the aerospace industry

Influence of electrode design and topography

Effects of evolving equipment Spot welding aluminum using

third body elements Spot welding aluminum to steel

High Volume Automotive Manufacturing in 1980

Body-in-white of steel construction Unitized body designs

Technology challenge – galvanized steels Gauge reduction Corrosion resistance

Joining by resistance spot welding Increased welding

currents Electrode life Robotic implementation

But

ton

Siz

e (in

.)

Number of Welds (Thousands)

Development of Aluminum Intensive Vehicles

Mandates for improved fuel efficiency Governmental

regulations Market demands

Focus on aluminum demonstrators Ford AIV GM EV-1 Honda NSX

Assembly within existing manufacturing contexts

Sheet stamping and fabrication

Resistance spot welding as the primary assembly technology

Specific energy demands to form a resistance spot weld Theoretical

calculations of energies to form spot welds

Assumptions of: 6√t nugget size Energy for heating

up to Tm Nugget melting (Hf)

Calculations for different: Material types Sheet thicknesses

Energies range from a few hundred to a few thousand Joules

Page 5

10

100

1000

10000

0 0.5 1 1.5 2 2.5 3 3.5

Energy (J)

Sheet Metal Thickness (mm)

SteelAlTi

Material

Al 1100-0

Al 5052-H38

Al 7075-T6

Steel 1010

PlasticRange (°C)

607-643

593-649

477-643

927-1482

ElectricalConductivity(%) Cu

59

35

30

10

ThermalConductivity(CGS)

0.53

0.33

0.29

0.10

Mass(Units)

1

1

1

3

TensileStrength(MPa)

90

280

500

500

SpecificHeat(cal/g)

0.23

0.23

0.22

0.11

[Reference: Welding, p.11-6, Kaiser Aluminum & Chemical Sales, Inc.]

Comparison of Certain Properties of Aluminum and Steel

Thermal Electrical Response of Resistance Welds Resistance welding a

balance of Heat generation Heat extraction

Resistance heating sources Workpieces Surfaces Electrodes

Heat extraction Electrodes

Thermal conductivity Heat capacity Cooling water

Environment (minor) Page 7

Actual Energy Requirements for Steel Spot Welds made with Different Heating Times

Energy data taken from previous work

Mild and galvanized steels show similar results

Energy for 0.8-mm steel ~3.5kJ at 400-ms weld time

Energy required drops to 500-J at 50-ms

Energy variations due to heat loss to the electrodes

Short time energies compare to heat of fusion calculations for observed nugget sizes

Page 8

Energy Weldability Lobe for 0.8-mm Bare Steel Energy Weldability Lobe for 0.8-mm HDG Steel

Heat Generation Characteristics of Resistance Spot Welds

Simplified heat balance analysis of spot welds

Effects of process conditions Influence of material type and

geometry Assessments of process

efficiencyPage 9

Weld Nugget

Water-Cooled Copper Alloy Electrode

Water-Cooled Copper Alloy Electrode

Base Metal

Base Metal

0

5

10

15

20

25

30

35

0 50 100 150 200 250

Energy Retaine

d in th

e Weld (J)

Weld Time (ms)

0.5‐mm steel

1‐mm aluminum

122

2

tx

xAK

ITVC

E

p

Energy and Efficiency During Resistance Spot Welding

Energy demand curves match those seen experimentally

Linear behavior with time Material differences related:

Heat capacities Thermal diffusivities Melting points

Energy efficiency a strong function of weld time

Material thickness effect related to: Latent heat of workpiece Heat extraction capability

Page 10

0

100

200

300

400

500

600

700

0 50 100 150 200 250

Consum

ed Ene

rgy (J)

Weld Time (ms)

0.5‐mm steel

1‐mm aluminum

0

0.2

0.4

0.6

0.8

1

1.2

0 50 100 150 200 250

Energy

Efficien

cy

Weld Time (ms)

1‐mm aluminum

3‐mm aluminum

System Mechanical Dynamics

Stable weld forces critical to: Prevent inconsistent heat

patterns Avoid unstable expulsion Accomplish proper forging of the

projection Factors affecting mechanical

response requirements Weld force Weld head inertia Collapse distance Collapse time

Criteria for weld head inertia based on maintaining 95% of the applied force

Whead/Fapp typically less than 10%

Requirements for fast follow-up heads

x

ftgF

W

app

head

20

2

Relationship between projection collapse distance, projection collapse time, and the head weight-to-weld force ratio

Page 11

Challenges in Resistance Spot Welding Aluminum

Aluminum shows roughly 1/3 the density of steel (weight reduction!)

3 times the thermal conductivity of steel

4 times the electrical conductivity of steel

Implications of welding 3 times higher welding currents 1/3 of the welding time

Higher demands on welding equipment

Melting Point (oK)

Specific Heat

(J/kg-oK)

Density (g/cm3)

Thermal Conductivity (cal/cm3-s-oC)

Electrical Resistivity

(μΩ-cm)

Latent Heat of Fusion

(cal/g)Iron 1809 460 7.87 0.18 9.71 65.5Aluminum 933 900 2.7 0.53 2.65 95.5Al/Fe ratio 0.52 1.96 0.34 2.94 0.27 1.46

Material Thickness, mmW

eld

Cur

rent

, kA

Wel

d Ti

me,

cycl

esE

lect

rode

For

ce,

kN

Resistance Spot Welding of Aluminum – 1980 State of the Art

Welding practices based on MIL spec guidelines

Quality measures Metallurgical

integrity Surface finish

Radiused electrodes

Weld/forge practices

Approaches unsuitable for automotive use Equipment

expense Power demands System

maintenance

| 1-mm |

Resistance Spot Welding of Aluminum – 1980 State of the Art (cont.)

Automotive adaptation concerns High current demands Electrode life Use of pre-treatments

Lives of <1000 welds Lives on galvanized

steels 1000’s of welds Focus on cleaning

procedures Improvements in

electrode life Not considered viable

for automotive production

Understanding Electrode Life Based on Galvanized Steel Experience

Electrode life based on peel testing Simple destructive

test Coarse measure

of weld size Weld size stability Life defined by

loss in weld size Life corresponded

to electrode size variations

Weld size instability associated with end of life

Characterization of Electrode Life when Resistance Spot Welding Aluminum Alloys

Electrode life testing based on 100% peel testing

Nominally stable max weld size

Periodic drop-outs Drop-out vary in frequency

during the test Initially high Reduced at break-in Increased before failure

Taken from Spinella and Patrick, SMWC X, 2006.

0

2

4

6

8

10

0 500 1000 1500 2000

Number of Welds

But

ton

Size

(mm

)

0

2

4

6

8

10

0 500 1000 1500 2000

Number of Welds

But

ton

Size

(mm

)

Electrode Life Results for 2-mm 5754 Sheet

Electrode Life Results for 2-mm 6111 Sheet

Weld Consistency Variations During Life Testing – Metallurgical Interpretation

2-mm 5754 Al test Shallow penetration early in

wear cycle Centering of porosity with

increaseing penetration Expulsion and excessive

porosity with higher wear

0

2

4

6

8

10

0 500 1000 1500 2000

Number of Welds

But

ton

Size

(mm

)

Developments of Improved Welding Practices for Aluminum Sheet

Change in electrode geometry From 6√t down to 5

6√t Consistent with

steel practices No face radius

Focus from surface quality to electrode life

Immediate reduction in current requirements

Reduction in frequency of failures

Taken from Spinella and Patrick, SMWC X, 2006

Alternate Electrode Geometries and Materials Aluminum alloy

tested: 1.0mm Al 5754 Aluminum

Alternate materials and electrode designs CuZr, truncated cone CuZr, truncated cone

(w/o FF) CuCd, internal fins OFC C107, tuncated

cone

Frequency of Defects during Electrode Life Testing on Aluminum Sheet

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 250 500 750 1000 1250 1500 1750 2000

Number of Welds

Frac

tion

of D

ata

Gro

up 42 per. Mov. Avg. (Freq Sub Min BS)

42 per. Mov. Avg. (Freq Weld Quality Pbm)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 250 500 750 1000 1250 1500 1750 2000

Number of Welds

Frac

tion

of D

ata

Gro

up 42 per. Mov. Avg. (Freq Sub Min BS)

42 per. Mov. Avg. (Freq Weld Quality Pbms)

42 Weld Moving Average of Various Defects when using Cu-Zr Truncated Cone Electrodes and Fast Follow-up Heads

42 Weld Moving Average of Various Defects when using Cu-Zr Internally Finned Electrodes and Fast Follow-up Heads

Variations in Face Diameter during Life Testing for Different Electrode Configurations

Influence of Electrode Surface Texture on Resistance Spot Welding Aluminum Sheet

Surface modification by applying electrode coatings

Implication of surface roughness

Improvements in weld morphology

Corresponding reduction in weld quality variations

Resulting extended electrode life

0 Welds 1000 Welds 3000 Welds

Taken from Chan and Schotmer, SMWC XIII, 2008

Taken from Chan and Schotmer, SMWC XIII, 2008

Influence of Electrode Surface Texture on Resistance Spot Welding Aluminum Sheet

Surface textures introduced by shot blasting

Results indicate broader current ranges and improved process tolerances

Results related to improvements in weld morphology

Consistent with other surface texturing results

Consistent with previous studies on electrode wear

Standard Electrode Cap after Grit

Blasting Surface showing ~5-μm Texture

Sigler, et. al., SMWC XIV, 2010

Sigler, et. al., SMWC XIV, 2010

Advances in Resistance Welding Power Supplies

AC

MFDC

CD

I

I

I

t

t

t

AC

MFDC

CD

I

I

I

t

t

t

Alternative systems CD welding

Not cost effective MFDC welding

Low Power demands Small welding packages Technology of choice

AC systems Good electrode life Large system mass Large primary power requirements Not acceptable for automotive

prodcuction

Influence of Power Supply Type on Electrode Life

Resistance Spot Welding Electrode Set after 2000 welds using AC Current.Note the even wear on the opposing electrodes.

Resistance Spot Welding Electrode Set after 700 welds using MFDC Current.Note the un-even wear on the opposing electrodes and shortened life.

Role of Peltier Voltages on Differential Electrode Wear

Peltier voltage a thermo-electric effect

Peltier voltages can reach as high as 100-mV for copper in contact with aluminum

Peltier voltages either assist or resist current flow depending on polarity

Opposing voltages increase local heat generation and electrode wear

Assisting voltages reduce local heat generation

Heat generation terms as contact resistance measures

Differential wear promoted on DC systems

Effect normalized on AC systems

Automotive Resistance Welding Systems – State of the Art

Current best capability MFDC power supply Electric servo force system

Lightweight package Reduced loads for robotic application

“Air-free” operation Reduced manufacturing costs

Complex weld/forge capability

MFDC welding Short electrode lives Differential electrode wear

Electric-servo application Poor mechanical follow-up Implications on electrode life

Use of Tip Dressers for Resistance Spot Weld Quality Stabilization

New generations of tip dressing systems

Integral with robotic gun use

Optimisation of cutting times and forces

Dressing frequencies in the 10’s of welds

Mitigation of electrode wear from: Polarity based

issues (MFDC) Force stability issues

(electric servo-guns) Integral to resistance

welding aluminum in automotive production

Taken from Kusano, SMWC XIV, 2010

Tip Dressing for Improved Electrode Life on Aluminum Sheet (cont.)

Definition of desired electrode profiles

Determination of dressing schedules Frequency Force Time

Shown application – Dress every 20 welds

Typical material removal ~200-μm

Stability in weld quality throughout an electrode life test

Taken from Sigler, Gaarenstroom, and Militello, SMWC XII, 2006

Tip Dressing for Improved Electrode Life on Aluminum Sheet

Use of dressers to create surface topography

Definition of high initial contact resistance

Development of improved nugget penetrations

Reduction in interfacial failures

Process robustness similar to other roughing techniques

Combined surface roughening/ dressing for optimum weld consistency

Cap Dressed with a Ridged Tool Ridges Resulting from Dressing

Taken from Sigler, Schroth, Karagoulis, and Zuo, SMWC XIV, 2010

Conductive Heat Resistance Welding

Third body resistance heating process

Use of cover sheets over the area to be resistance welded

Heat generation In cover sheets Conducted into Al

Active pressures on the weld

Advantageous solidification path

Hourglass weld profile Welds free of porosity Application to single side

welding

Heat Flow

1

2

3Heat Flow

Steel cover Sheet

Steel cover Sheet

Experimental Procedures

Aluminum alloys studied 2024 – T3 7075 – T651

All materials 2-mm thick Sample cleaning

Etched in basic solution Stored in celophane Scotchbriting before use

Sample size 113-mm X 31-mm

Samples welded in the tensile shear configuration

Cover sheet material Bare steel Thicknesses 0.8-mm – 2.5-

mm

Experimental Procedures (cont.) Welding equipment

Push-pull welding configuration Conrac 120-kVA welding transformer Robotron 211 controller

Miyachi 326A current meter 12-mm flat face shunting electrode Range of workpiece electrodes Tensile shear testing - ASTM-E8 Metallographic inspections Post weld aging - B597-92 Aging times selected at 0, ¼, ½, ¾, and 1

times the recommended practice Aging temperatures at and 30oC above

the recommended practice

Developed Welding Practices

Iterative welding trials Target 6-mm button size Similar practices used for

both alloys Developed practice:

19-mm dia. electrode with a 100-mm face radius.

1.5-mm cover sheet 8-kN weld force 24-cyc on/4-cyc off pulsation 9 weld pulses 21-kA weld current 10-cyc downslope to ½ the weld

current >7-mm weld diameter at

faying surface

Macrostructure of Single Side Conductive Heat Resistance Spot Welds

Macrograph Showing the As-Welded CHRSW in 2024

Macrograph Showing the As-Welded CHRSW in 7075

Joining of Aluminum and Steel in the Automotive Industry

Mazda MX-5 aluminum trunk lid and steel bolt retainer joined by friction spot welding

2012 Audi A6 featuring aluminum and steel construction

Trends toward dissimilar metal joining driven by weight reduction efforts to drive toward new CAFE standards.

Renewed challenges for prevention of galvanic corrosion.

Issues with Joining Al to Steel Large difference in melting

points Formation of low melting

temperature constituents Difference in crystal structure

Al is FCC, Fe is BCC Multiple intermetallic phases

Process characteristics Short heating times Higher welding temperatures

require additionally reduced heating times

-500

0

500

1000

1500

2000

2500

3000

0 100 200 300 400 500

Time (ms)R

PM

Macrosection of an aluminum to steel inertia friction weld

Deceleration profile for an inertia weld between aluminum and steel

Knowledge Gained from Friction Welding Aluminum to Steel

Temperature profile of the interface of dissimilar FWed joint

0

50

100

150

200

250

300

350

400

450

0 5 10 15 20 25 30 35Time (s)

Tem

pera

ture

(C)

SteelAluminium

Typical thermal cycle for an Al-to-steel inertia weld

Intermittent Nature of Intermetallic Formation

Intermetallic across joint

Resistance Spot Welding Aluminum to Steel with Transition Materials Roll bonded transition

materials Aluminum Steel

Key aspects to the transition material Thickness Ratio of material

thicknesses Alloys

Positioning between the steel and aluminum

Spot welding with separate weld nuggets

Growth of the Aluminum and Steel Nuggets when using Transition Materials

Heating in the body of the steel

Heat soak to melt aluminum epitaxially

Growth of the aluminum nugget into the attached sheet

Steel nugget formation between the separated from the aluminum

Nugget distortion based on metal compliance and expulsion

Intermetallic Formation and Failure modes Process typically uses

extended cycle times Similar to steel welding

Observations of intermetallic formation

Failure modes may include Button pull-out Partial button pull out Interfacial failure

Strength supported by circumferential roll bonded structure

Use of Melting Interlayers to Facilitate Aluminum to Steel Spot Welding

Criteria Remain below aluminum solidus Compatible with both aluminum and

steel Potential for high joint strength Corrosion resistance Widely available

Zinc-5% Aluminum Standard aluminum soldering alloy

(Teut = 382°C) Bulk strength ~ 60% of 6061-T6 Galvanic corrosion protection

Coating Process

Ultrasonic bath immersion Ease of application No surface preparation

or fluxing Bath both preheats and

coats substrate Rapid Uniform

Resistance Joining

Widely accepted sheet joining method

Rapid Ease of achieving

heat balance Current and cycle

adjustments Electrode diameter

ratios adjustments

Joint Microstructure

Steel

Zn-5Al

6061 Aluminum

Steel

Zn-5Al

6061 Aluminum

Zinc (Galvanizing)Steel

6061 Aluminum

Steel

6061 Aluminum

Al-Fe intermetallic

Lap Shear Test Results

Weld Condition No.

RMS Current (kA)

Al : Fe Electrode Diameter

RatioNo. of Pulse

CyclesWeld Times

(Weld/Hold)Mean Ultimate Load

(kN)

Maximum Joint Efficiency

(%)

1 15.5 1:1 3 12/4 2.74 33

2 15.7 1:1 8 12/4 5.36 59

3 15.7 4:1 8 12/4 7.56 94

4 36.0 4:1 1 n/a 1.85 21

5 17.7 4:1 0.5 n/a 1.59 20

6 21.4 1:1 3 8/4 2.13 29

Weld Condition No.3

Challenges and Advances in Resistance Spot Welding Aluminum Sheet – Summary

Background on spot welding aluminum in the automotive industry Experience through the 1980’s

Differences in spot welding aluminum and steel Resistivity and conductivity Effects on spot welding

requirements Technology as evolved from the

aerospace industry Differing quality requirements Effects on necessary equipment Power demands and maintenance

Influence of electrode design and topography Limited impact of electrode

materials Effect of electrode diameters Role of surface roughness

Effects of evolving equipment Effect of DC welding Differential electrode wear Dressers mitigating wear effects Use of active surface profiling

Spot welding aluminum using third body elements Steel cover sheets Augmented surface heating Potential for single side welding

Spot welding aluminum to steel Metallurgical challenges welding

aluminum to steel Use of roll bonded transition

materials Use of active interlayers to

promote welding

Questions?Jerry E. GouldTechnology LeaderResistance and Solid State WeldingEdison Welding Instituteph: 001-614-688-5121e-mail: jgould@ewi.org