April 28, 2014 Challenges and Advances in Resistance Spot Welding Aluminum Sheet Jerry E. Gould Technology Leader Resistance and Solid State Welding Edison Welding Institute ph: 001-614-688-5121 e-mail: [email protected]
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: [email protected]
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: [email protected]