Spot Weld Mechanical Properties

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Spot Weld Mechanical Properties
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Mechanical Properties

• Learning Activities
• View Slides
• Read Notes,
• Listen to lecture
• Do on-line workbook

• Lesson Objectives
• When you finish this lesson you will understand
• Peel Chisel and Tensile Mechanical testing
  techniques
• Factors which effect performance in these
  mechanical tests
• The importance of these test for part design

Keywords Peel Test, Chisel Test, Tensile Shear
Test, Multiple Nugget Test, Weld Bond Tensile
Test, Coupon Dimensions, Weld Spacing, Electrode
Conditioning, Button Shape, Plastic Hinge
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Mechanical Properties of Spot Welds

• Peel Tests
• Chisel Test
• Tensile Test
• Tensile Shear Ratios
• Torsion Test
• Impact Tests
• Fatigue Test

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AWS Spec D8.9-97, 1997
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AWS Spec D8.9-97, 1997
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AWS Spec D8.9-97, 1997
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AWS Spec D8.9-97, 1997
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AWS Spec D8.9-97, 1997
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AWS Spec D8.9-97, 1997
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Dickinson, Welding in Auto Industry, AISI, 1981
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AWS Spec D8.9-97, 1997
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AWS Spec D8.9-97, 1997
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• Advantages of Peel Test
• Ease of Performance
• Low Cost
• Ability to use on Shop Floor as quality control
  test

• Disadvantages of Peel Test
• Only in restricted cases are quantitative values
  relating to the strength of the weld nugget or
  its performance in service obtained
• Nugget fracture appearance is not a reliable
  predictor of load-carrying capacity or impact
  resistance

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Geometric Effects on the Peel Test
Distance x can cause a wide variation in load
results, thus peel test can NOT be used with
confidence to develop mechanical property data
Orts, Armco Steel, Private Communication, Nov 1980
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Instrumented Peel Test
Area Energy Static Toughness
Sawhill, Spot Weldability of High Strength Sheet
Steel, Welding Journal, Jan 1980
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Sawhill, Spot Weldability of High Strength Sheet
Steel, Welding Journal, Jan 1980
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Peel Tested When Cold
Peel Tested While Still Warm
Promotes Partial Nugget Failure
Promotes Full Nugget Pull-Out
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Mechanical Properties of Spot Welds

• Peel Tests
• Chisel Test
• Tensile Test
• Tensile Shear Ratios
• Torsion Test
• Impact Tests
• Fatigue Test

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Dickinson, Welding in Auto Industry, AISI, 1981
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Min. Nugget With Peel Test
Min. Nugget With Chisel Test
Chisel Test Tends to Expose Nugget Closer to
Fusion Line
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Questions?

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

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Mechanical Properties of Spot Welds

• Peel Tests
• Chisel Test
• Tensile Test - Tensile Shear
• Tensile Shear Ratios
• Torsion Test
• Impact Tests
• Fatigue Test

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Dickinson, Welding in Auto Industry, AISI, 1981
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AWS Spec D8.9-97, 1997
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Kraus, A Practical Design approach Welding
Journal, Oct 1961
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Dickinson, Welding in Auto Industry, AISI, 1981
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Vanden Bossche, Ultimate Strength and Failure
Mode.., SAE paper 770214, Feb 1977
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Dickinson, Republic Steel Corp Internal Document,
1978
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• Tensile Shear Test
• Single Nugget Test Tensile Shear
• Material Thickness
• Nugget Size (Current Time)
• BM Strength
• Carbon Content
• BM Structure
• Coatings
• Hold Time
• Post Weld Furnace Treatment
• Test Temperature
• Test Cross Head Speed
• Multiple Nugget Test Tensile Shear
• Weld Bonding (Nugget plus Adhesive)

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Material Thickness Tensile Shear
In the thinner gages, the lower C Steels tend to
have higher strength because the higher carbon
materials develop hard brittle interfacial
failures.
Heuschkel, Some metalurgical aspects of carbon
steel spot welding, Welding Journal, Oct 1947
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Nugget Diameter Tensile Shear

• Increased Tensile Shear
• Nugget Diameter
• Sheet Thickness
• BM Strength

Pollard, Spot Welding Characteristics of HSLA,
Welding Journal, Aug 1974
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Nugget Diameter (Current Time BM Strength)
Tensile Shear
Nugget Pullout
Increase BM Strength
Interfacial Failure

• Increased Weld Current
• Greatly Increased Nugget
• Greatly Increased Tensile Shear Strength

• Increased Weld Time
• Slightly Increased Nugget
• Greatly Increased Tensile Shear Strength

• Increased BM Strength
• Increased Tensile Shear

Pollard, Spot Welding Characteristics of HSLA,
Welding Journal, Aug 1974
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Is it Current, or Effect of Current on Nugget
Diameter?
Combinations of Current and Electrode Face
Diameter gives Constant Nugget Diameter
Relationship between Nugget Dia and Strength Not
Current/Strength
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Questions?

• Turn to the person sitting next to you and
  discuss (1 min.)
• What criteria do you suppose automotive and
  appliance designers use when selecting the type
  of weld joint they will recommend for their
  products? Think about the joint designs in cars
  and appliances.

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• Base Metal Strength
• Solid Solution Strengthening
• Grain Refinement Through Special Processing
• Precipitation Strengthening
• Cold-work Strengthening
• Heat Treatment Strengthening (Hardening)

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Effect of Carbon Content
Hard Brittle Martensite Interfacial Failures
Heuschkel, Some Metallurgical Aspects. Welding
Journal, Oct 1947
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Effect of Base Metal Strength
Grain Refinement Precipitation
10o
The Stronger Materials have less sample
rotation thus they carry greater tensile shear
load than expected
30o
Pollard, Spot Weldability of HSLA Steels,
ASM/ADDRG Seminar, Nov 1213, 1971
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Effect of Structure

• Tensile Shear Strength Increases when a material
  is giving the following
• Annealed
• Bainitic (Heat Treated)
• Martensitic (Heat Treated)
• Cold-rolled (Lower at higher currents due to HAZ
  softening)

Chandel, Mech Metallurgical Aspects Metals
Technology, Sept 1974
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Defourney, Spot Welding of HSS, IIW 646-80, 1980
Sawhill, Spot Weldability of HSS, Welding
Journal, Jan 1980
Heuschkel, Expression of Spot Weld
Properties, Weldign Journal Oct 1952
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Critical Diameter to Thickness Ratio
Thus, higher strength materials should have
larger nuggets (electrode)
d nugget diameter t sheet thickness ysbm
yield strength base metal yswm yield strength
weld metal
Vanden Bossche, Ultimate Strength and Failure
Modes SAE 770214, Feb 1977
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Effect of Coating
When only Zn-Zn Bonds lower strength than bare
seel.
When steel-steel bonds strength equal to or
greater
Armco Steel Fabrication Data, 1981
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Questions?

• Turn to the person sitting next to you and
  discuss (1 min.)
• We have looked at some factors which effect
  tensile shear strength. And some people have
  developed empirical equations to calculate
  tensile shear strength. How reliable are these
  equations? What factors might lead to
  inaccuracies?

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Hold Time Tensile Shear
Hold Time Sensitive Material in Peel Test
Not Hold Time Sensitive in Tensile Shear
Hold time appears to have no effect
Sawhill, Spot weldability for High Strength
Steels, SAE 810352, Feb 1981
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Effect of Post Weld Furnace Treatment gt0.2 C
Steels
Slight loss in strength No benefit Expensive
In Machine QT gt 0.2 C Steels
Up to 300 benefit Must be done correct
lt0.2 Carbon
No Benefit
Balasubramanian et al, Improvement of Resistance
Spot Weld Characteristics, Welding Research
Abroad, Nov 1978
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Effect of Test Temperature for Plain Carbon Steel
Heuschkel, The Expression of Spot Weld
Properties, Welding Journal, Oct 1952
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Effect of Cross Head Speed on Tensile Testing
Machine
Impact
Chandel, Mech Aspects pf Spot-WeldedJoints, Met
als Technology, Jan 1977
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• Tensile Test
• Single Nugget Test Tensile Shear
• Multiple Nugget Test Tensile Shear
• Total Spot Weld Area
• Sheet Thickness
• Nugget Rotation (spot pattern)
• Weld Bonding (Nugget plus Adhesive)

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Dickinson, Welding in Auto, AISI, Aug 1981
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Effect of Total Nugget Area on Double Transverse
Spot Welds
Capelli, et al, Fatigue Strength of Spot-welded
Joints.. Welding of HSLA Steels, ASM, 1978
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Effect of Increasing Number of Welds
Non-uniform loading on Outside welds
causes deviation
Hills, Effect of number of welds in spot
welded, Iron Steelmaking, Vol 23, No 2, 1996
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Effect of Sheet Thickness on Double Transverse
Spot Welds
Capelli, et al, Fatigue Strength of Spot-welded
Joints.. Welding of HSLA Steels, ASM, 1978
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Effect of Nugget Rotation Multiple Spot Array
Load f(load per weld, stiffness)

• Max load per weld reduced by
• Closely Spaced Welds
• In line with load
• Increased Nugget Size

• Stiffness Increased by
• Closely Spaced Welds inline
• Specimen Width

Orts, Fatigue Strength of Spot Weld Joints in
HSLA, SAR 810355, Feb 1981
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Questions?

• Turn to the person sitting next to you and
  discuss (1 min.)
• We have examined tensile shear strength on
  single welds and on combinations of multiple
  welds. Does anything surprise you about the
  multiple weld samples? Why are so few reports
  presented with multiple weld data?

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Tensile Test Single Nugget Test Tensile
Shear Multiple Nugget Test Tensile Shear Weld
Bonding (Nugget plus Adhesive)
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High Modulus Adhesive Only
Weld Bond, Relative Height of Peaks Depends on
Number of Welds
Low Modulus Adhesive Only
Spot Weld Only
Hills, Effect of number of welds in spot
welded, Iron Steelmaking, Vol 23, No 2, 1996
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Lower Modulus Adhesive
High Modulus Adhesive
Fewer Welds Adhesive Dominates
Hills, Effect of number of welds in spot
welded, Iron Steelmaking, Vol 23, No 2, 1996
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Questions?

• Turn to the person sitting next to you and
  discuss (1 min.)
• We will study more about weld bonding techniques
  later. Why is this technique so infrequently
  used?