1. Spot Weld Mechanical Properties

2. 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

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

4. AWS Spec D8.9-97, 1997

10. Dickinson, “Welding in Auto Industry, AISI, 1981

11. AWS Spec D8.9-97, 1997

13. 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

14. Geometric Effects on the Peel Test Orts, Armco Steel, Private Communication, Nov 1980 Distance “x” can cause a wide variation in load results, thus peel test can NOT be used with confidence to develop mechanical property data

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

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

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

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

19. Dickinson, “Welding in Auto Industry, AISI, 1981

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

21. 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?

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

23. Dickinson, “Welding in Auto Industry, AISI, 1981

24. AWS Spec D8.9-97, 1997

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

26. Dickinson, “Welding in Auto Industry, AISI, 1981

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

28. Dickinson, Republic Steel Corp Internal Document, 1978

29. 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)

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

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

32. Nugget Diameter (Current & Time & BM Strength) – Tensile Shear Increase BM Strength Increased Weld Current Greatly Increased Nugget Greatly Increased Tensile Shear Strength Pollard, Spot Welding Characteristics of HSLA”, Welding Journal, Aug 1974 Increased Weld Time Slightly Increased Nugget Greatly Increased Tensile Shear Strength Interfacial Failure Nugget Pullout Increased BM Strength Increased Tensile Shear

33. 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

34. 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.

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

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

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

38. Effect of Structure Chandel, “Mech & Metallurgical Aspects…” Metals Technology, Sept 1974 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)

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

40. Critical Diameter to Thickness Ratio Vanden Bossche, “Ultimate Strength and Failure Modes…” SAE 770214, Feb 1977 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)

41. Effect of Coating Armco Steel Fabrication Data, 1981 When only Zn-Zn Bonds lower strength than bare seel. When steel-steel bonds strength equal to or greater

42. 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?

43. Hold Time – Tensile Shear Hold time appears to have no effect Sawhill, “Spot weldability for High Strength Steels”, SAE 810352, Feb 1981 Hold Time Sensitive Material in Peel Test Not Hold Time Sensitive in Tensile Shear

44. Effect of Post Weld Furnace Treatment >0.2% C Steels Balasubramanian et al, “Improvement of Resistance Spot Weld Characteristics”, Welding Research Abroad, Nov 1978 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

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

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

47. 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)

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

49. 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

50. Effect of Increasing Number of Welds Hills, “Effect of number of welds in spot welded…”, Iron & Steelmaking, Vol 23, No 2, 1996 Non-uniform loading on Outside welds causes deviation

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

52. Effect of Nugget Rotation – Multiple Spot Array Orts, “Fatigue Strength of Spot Weld Joints in HSLA”, SAR 810355, Feb 1981 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

53. 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?

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

55. High Modulus Adhesive Only Low Modulus Adhesive Only Spot Weld Only Weld Bond, Relative Height of Peaks Depends on Number of Welds Hills, “Effect of number of welds in spot welded…”, Iron & Steelmaking, Vol 23, No 2, 1996

56. 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

57. 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?