Spot Weld Mechanical Properties Mechanical Properties Lesson Objectives

Spot Weld Mechanical Properties

Mechanical Properties 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 Learning Activities 1. View Slides; 2. Read Notes, 3. Listen to lecture 4. Do on-line workbook 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

Mechanical Properties of Spot Welds • Peel Tests • Chisel Test • Tensile Shear Ratios • Torsion Test • Impact Tests • Fatigue Test

AWS Spec D 8. 9 -97, 1997

Dickinson, “Welding in Auto Industry, AISI, 1981

AWS Spec D 8. 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 • 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

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

Instrumented Peel Test Area = Energy “Static Toughness” Sawhill, Spot Weldability of High Strength Sheet Steel”, Welding Journal, Jan 1980

Sawhill, Spot Weldability of High Strength Sheet Steel”, Welding Journal, Jan 1980

Peel Tested While Still Warm Promotes Full Nugget Pull-Out Peel Tested When Cold Promotes Partial Nugget Failure

Mechanical Properties of Spot Welds • Peel Tests • Chisel Test • Tensile Shear Ratios • Torsion Test • Impact Tests • Fatigue Test

Dickinson, “Welding in Auto Industry, AISI, 1981

Min. Nugget With Peel Test Min. Nugget With Chisel Test Tends to Expose Nugget Closer to Fusion Line

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?

Mechanical Properties of Spot Welds • Peel Tests • Chisel Test • Tensile Test - Tensile Shear • Tensile Shear Ratios • Torsion Test • Impact Tests • Fatigue Test

Dickinson, “Welding in Auto Industry, AISI, 1981

AWS Spec D 8. 9 -97, 1997

Kraus, “A Practical Design approach…” Welding Journal, Oct 1961

Dickinson, “Welding in Auto Industry, AISI, 1981

Vanden Bossche, “Ultimate Strength and Failure Mode. . ”, SAE paper 770214, Feb 1977

Dickinson, Republic Steel Corp Internal Document, 1978

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)

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

Nugget Diameter – Tensile Shear Increased Tensile Shear • Nugget Diameter • Sheet Thickness • BM Strength Pollard, Spot Welding Characteristics of HSLA”, Welding Journal, Aug 1974

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

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

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.

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

Effect of Carbon Content Hard Brittle Martensite Interfacial Failures Heuschkel, “Some Metallurgical Aspects…. ” Welding Journal, Oct 1947

Effect of Base Metal Strength Grain Refinement Precipitation 10 o The Stronger Materials have less sample rotation thus they carry greater tensile shear load than expected 30 o Pollard, “Spot Weldability of HSLA Steels”, ASM/ADDRG Seminar, Nov 12&13, 1971

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

Defourney, “Spot Welding of HSS, IIW 646 -80, 1980 Heuschkel, “Expression of Spot Weld Properties”, Weldign Journal Oct 1952 Sawhill, “Spot Weldability of HSS”, Welding Journal, Jan 1980

Critical Diameter to Thickness Ratio d = nugget diameter t = sheet thickness ysbm= yield strength base metal yswm = yield strength weld metal Thus, higher strength materials should have larger nuggets (electrode) Vanden Bossche, “Ultimate Strength and Failure Modes…” SAE 770214, Feb 1977

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

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?

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

Effect of Post Weld Furnace Treatment >0. 2% C Steels Slight loss in strength No benefit + Expensive In Machine Q&T > 0. 2 % C Steels Up to 300% benefit Must be done correct <0. 2% Carbon No Benefit Balasubramanian et al, “Improvement of Resistance Spot Weld Characteristics”, Welding Research Abroad, Nov 1978

Effect of Test Temperature for Plain Carbon Steel Heuschkel, “The Expression of Spot Weld Properties”, Welding Journal, Oct 1952

Effect of Cross Head Speed on Tensile Testing Machine Impact Chandel, “Mech Aspects pf Spot-Welded. Joints…”, Metals Technology, Jan 1977

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)

Dickinson, “Welding in Auto…”, AISI, Aug 1981

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

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

Effect of Sheet Thickness on Double Transverse Spot Welds Capelli, et al, “Fatigue Strength of Spot-welded Joints. . ” Welding of HSLA Steels, ASM, 1978

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

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?

Tensile Test Single Nugget Test – Tensile Shear Multiple Nugget Test – Tensile Shear Weld Bonding (Nugget plus Adhesive)

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

High Modulus Adhesive Lower Modulus Adhesive Fewer Welds Adhesive Dominates Hills, “Effect of number of welds in spot welded…”, Iron & Steelmaking, Vol 23, No 2, 1996

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?