1. Aluminum Spot Welding General Considerations

2. Resistance Welding Lesson Objectives When you finish this lesson you will understand: Learning Activities 1.View Slides; 2.Read Notes, 3.Listen to lecture 4.Do on-line workbook Keywords

4. Recommended Joint Designs Based Upon These Properties AWS Welding Handbook

5. Material Parameters Plastic Condition Temperature Range –Aluminum reaches plastic range at a lower temperature than steel due to lower melting point of aluminum (480 to 640 °C compared to 1480 to 1540 °C) –Aluminum has a plastic temperature range of about 90 °C compared to steel, with wider range of about 540 °C –Maintaining sufficient plastic material to constrain molten nugget is more difficult with aluminum alloys.

6. Temperature Range of Plastic ConditionWeld Cycles Temperature [Reference: Welding, p.11-6, Kaiser Aluminum & Chemical Sales, Inc.] Aluminum Steel

7. Browne, D., Model to Predict, IBEC’95, Adv Tech & Processes, 1995

8. Pressure Distribution During Spot Welding Only Slight Pressure Spreading & Increased Electrode and Faying Surface Contact During First Cycle Thereafter, Rapid Increase in Electrode & Faying Surface Contact and Sheet Separation Then Nugget Growth Zhang, Nugget Growth in RSW Aluminum SMWC VII, AWS, 1996

9. Auhl, JR, SAE 940160, 1994 Various Nugget Growth Morphologies

10. Thermal Conductivity –Greater than that of steel –The rate of heat loss from the weld is much greater in aluminum –Require short welding times and high current flows Material Parameters Electrical Resistivity –The electrical resistivity of aluminum is quite low ( approx. 5 micro-ohm-cm compared to 15 micro-ohm-cm of steel) –Require short welding times and high current levels –Three to five times the current is required to weld an equivalent thickness of aluminum compared to steel –Shunting losses in aluminum alloys are more of a problem than for steel

11. Overview of Thermal Considerations

12. Case Studies of Common Electrode Heat Flow Conditions

13. Effects of Electrode Tip Contour: Case A Equal thickness of like identical contours on each electrode. Wide nugget has equal penetration in each piece. [Reference: Resistance Welding Manual, p.11-29, RWMA]

14. Effects of Electrode: Dissimilar Material: Case B Equal thickness of dissimilar materials but identical contours. Weld nugget has unequal penetration. [Reference: Resistance Welding Manual, p.11-29, RWMA]

15. Effects of Electrode on Unequal Thickness: Case C Unequal thickness of similar materials with identical electrode contours. Note that the penetration is considerably greater in the thicker piece. Compared this with Case D. [Reference: Resistance Welding Manual, p.11-29, RWMA]

16. Effects of Electrode Tip Contour: Case D Unequal thickness of similar materials with radiused electrode against the thin piece and flat electrode against the thick piece. Note that with this combination weld penetration is approximately even on both pieces. Compared this with Case E. [Reference: Resistance Welding Manual, p.11-29, RWMA]

17. Effects of Electrode Tip Contour: Case E Unequal thickness of similar materials with radiused electrode against the thick piece and flat electrode against the thin piece. Note that in this case the electrode application is opposite to that recommended for good practice resulting in unequal weld penetration. [Reference: Resistance Welding Manual, p.11-29, RWMA]

18. Material Parameters (CONT.) Expansion and Contraction –Undergo greater expansion and contraction during melting and solidification processes than does steel –Dimensional changes are greatest in the weld zone and commonly result in nugget cracking –Machines with Low Inertia Heads help electrodes “follow-up” nugget solidification –Post weld forges are often used to prevent cracking –Current Decay during the solidification process helps

19. Expansion and Contraction [Reference: Welding, p.11-7, Kaiser Aluminum & Chemical Sales, Inc.] With Current Decay Without Current Decay

20. Low I High F High I Low F Dewey, R, Mapes, R, “Observations on Spot Welding Aluminum for Automotive Applications” SAE Paper 770208, 1977 Nugget Expansion & Contraction Curves for Aluminum

21. Dewey, R, Mapes, R, “Observations on Spot Welding Aluminum for Automotive Applications” SAE Paper 770208, 1977 Expulsion No Expulsion Effect of Current and Force on Expulsion

22. Design of a Low Inertia Welding Head Air Inlet Relaxed Diaphragm Welding- Current Switch (Open) Air Inlet Piston Cylinder Welding Head Outer Shaft Inner Shaft Work pieces Spring (Under Inertia Compression) Upper Electrode Lower Electrode Compressed Diaphragm Weld-Current Switch (Closed) Fully Compressed Spring Work pieces Partially Compressed Diaphragm Welding- Current Switch (Closed) Partially Compressed Spring Work pieces (a) (b) (c)

23. Effect of Nugget Size & Defects On Tensile Shear Strength Oval Parallel or Perpendicular Aluminum Association T 10 Guideline Porosity Porosity & Cracking Internal Cracking Michie, KJ EtAl, SMWC VII, AWS 1996 Nugget Size Controls Strength BUT Defects Cause Scatter About Mean

24. Auhl, JR, SAE 940160, 1994 Effect of Defects at a Constant Nugget Size

25. Michie, KJ EtAl, SMWC VII, AWS 1996 Oval Parallel or Perpendicular Internal Cracking Severe Cracking Aluminum Association T 10 Guideline Effect of Nugget Size On Fatigue Properties

26. Factors Effecting Shear Load/Fatigue Nugget Diameter Metal Thickness Metal UTS Select Nominal Diameter = 0.9524 t 0.5 Effect of Gage with weld made of Nominal Diameter Shear Load = 0.659 t 1.23 UTS (for weld with Nominal Diameter) Dewey, R, Mapes, R, “Observations on Spot Welding Aluminum for Automotive Applications” SAE Paper 770208, 1977

27. Surface-Related Problems Surface-Condition Issues –Revolve around the aluminum oxide film which forms on the surface of the aluminum –As this oxide grows, the effective contact resistance of the aluminum changes –As the electrode comes in contact with the sheet surface, this oxide fractures non-uniformly and creates only small areas for the passage of current –Results in Electrode Life deterioration Oxide Removal Methods –Chemical removal –Abrasive removal –Stabilization after oxide removal –Arc cleaning the surface immediately before welding –Conversion coatings

28. Surface Oxide Aluminum T<400C AlMg T<400C Aluminum T>400C AlMg T>400C Patrick, EP et al, SAE 840291, 1984

29. Patrick, EP et al, SAE 840291, 1984

30. Chemical Cleaning Solution Two to Six Minute Immersion 15 oz/gal (120g/l) Nitric Acid [technical grade (68% HNO 3 0.15 oz/gal (2g/l) Hydrofluoric Acid (48% HF) 0.14 oz/gal (2g/l) Wetting Agent

31. Chromate Conversion Coatings Occasionally a coating is added to increase Paint adhesion Four Types Alkaline Oxide Chromium Phosphate Chromate No-rinse Process Alkaline Oxide Immerse in alkali chromate bath, 20 min, 95C Coating weight 100-500 mg/sq ft Color light to brownish green Chromium Phosphate Spray or Immerse in H 2 CrO 4, H 3 PO 4, & F - Coating weight 5 to 500 mg/sq ft Colorless to emerald green

32. Chromate Coatings Emersion in H 2 CrO 4 - HF - Other mineral Acids Emersion in H 2 CrO 4 - HF - Other mineral Acids Low contact resistance aids in Resistance Welding Low contact resistance aids in Resistance Welding Coating weight 15 to 30 mg/sq ft Coating weight 15 to 30 mg/sq ft Color iridescent yellow to brown Color iridescent yellow to brown No-rinse coating Direct line application of composition containing Cr +6 & Cr +3 Some formulations include organic compounds Coating weight 5 to 25 mg/sq ft Coating weight proportional to applied wet film

33. Contact-Resistance Measurement AWS Welding Handbook, vol 3 1996 Low Currents Generally Used: Representative of Weld Currents?

34. Factors Effecting Contact Resistance Surface Roughness Electrode Surface Roughness Sheet Alloy or Oxide on Electrode Alloy or Oxide on Sheet Outer Surface Alloy or Oxide on Sheet Faying Surface Differential Oxide on Sheet Other Surface Coatings

35. Patrick, E, Spinella, D, “Surface Effects on Resistance Spot Weldability – Aluminum Body Sheet”, International Body Engineering Conference, 1995 Surface Roughness Studies (Low Oxide Surface) Arc Textured Better Current Contact Improved Electrode Life Improved Lobe

36. Factors Effecting Contact Resistance Surface Roughness Electrode Surface Roughness Sheet Alloy or Oxide on Electrode Alloy or Oxide on Sheet Outer Surface Alloy or Oxide on Sheet Faying Surface Differential Oxide on Sheet Other Surface Coatings General Practice: Clean Oxide Off

37. Surface Abraded or Chemically Cleaned and Stearate Treated 1000 100 20 Surface Contact Resistance Untreated Al 100  Reoxidized + Stearate 20  Reoxidized + Stearate Time (Weeks) Aluminum Al 2 O 3 Monolayer of Stearate Weld Within 24 Hours if Untreated

38. Arc-Cleaning Method of Spot Welding Aluminum Oxide Aluminum High Frequency Arc

39. Initial Resistance 50 micro-ohms Initial Resistance 1000 micro-ohms Breakdown of Surface Oxide When Weld Current Applied (25kA Current) Higher Initial Resistance Longer Time Alcan Internal Research, March 1994 Cleaned Surface Cleaned and Held Surface

40. Initial Resistance 50 micro-ohms Initial Resistance 1000 micro-ohms Effect of Surface Resistance on Nugget Growth & Expulsion Expulsion The more rapid and uncontrolled growth causes erratic expulsion Alcan Internal Research, March 1994 Slower Nugget Growth No Expulsion Rapid Growth Expulsion

41. Effect of time after Cleaning on Mechanical Properties Clean Surface = Higher Strength Wider Variation Chihoski, R, “Variation in Aluminum Spot Welds”, Welding Journal, Dec 1970.

42. Nugget Perimetal Bond (Weld Heat = SS Bond) Diffusion Bond (If Surface Clean) Effect of Surface Cleaning (Time After Cleaning) on Diffusion Bond Formation and Weld Strength Chihoski, R, “Variation in Aluminum Spot Welds”, Welding Journal, Dec 1970

43. Arc Clean (If No Electrode Deterioration From Arc) Clean & Weld Immediately Clean & Stearate Treatment Clean & Left 1 Day No Cleaning Electrode Life Improvement of Various Surface Cleaning Technique

44. Factors Effecting Contact Resistance Surface Roughness Electrode Surface Roughness Sheet Alloy or Oxide on Electrode Alloy or Oxide on Sheet Outer Surface Alloy or Oxide on Sheet Faying Surface Differential Oxide on Sheet Other Surface Coatings

45. Anodized Caustic Cleaned

46. Factors Effecting Contact Resistance Surface Roughness Electrode Surface Roughness Sheet Alloy or Oxide on Electrode Alloy or Oxide on Sheet Outer Surface Alloy or Oxide on Sheet Faying Surface Differential Oxide on Sheet Other Surface Coatings

47. Screening Tests of Various Treatments with Spherical Radiused Electrodes Surface Condition Water Stained Mill Finish/Mill Finish Abraded/Abraded Abraded/Mill Finish Arc Cleaned/Mill Finish Conversion Coat/Conversion Coat Abraded/ Conversion Coat Arc Cleaned/Conversion Coat [Reference: SAE 840291, Patrick, Auhl, and Sun]

49. Required Properties of Ideal Electrodes Maximum electrical and thermal conductivity Maximum hardness or resistance to deformation Tip Design Which Reduces Wear Minimum tendency to alloy with the material being welded (Because of Lower Resistance of Aluminum Than Steel More Current is Needed to Make the Weld)

50. Properties of Some Copper-Alloy Electrodes for Aluminum Property Annealing Temperature Hardness (Rockwell) Electrical Conductivity (% I.A.C.S.) Availability Class I 660 °C 70B 85 Bars, Forgings (not in cast form) Class II 950 °C 65B Cast 80B Wrought 80 Cast 85 Wrought Bars, Forgings Castings

51. New Electrode Formulated to Give High Conductivity STAR = Sumitomo Tough and Robust Kumagai, M, High Performance Electrode Material… IBEC’95, Material & Body Testing, 1995

52. Higher Conductivity Lower Operating Temp Longer Life Kumagai, M, High Performance Electrode Material… IBEC’95, Material & Body Testing, 1995

53. How would the STAR Electrode work on: Uncoated Steel Coated Steel Formulate your answer to include: A theoretical argument of how they should behave Acquired Data from the Literature

54. Required Properties of Ideal Electrodes Maximum electrical and thermal conductivity Maximum hardness or resistance to deformation Tip Design Which Reduces Wear Minimum tendency to alloy with the material being welded (Because of Lower Resistance of Aluminum Than Steel More Current is Needed to Make the Weld)

55. Typical Electrode Configuration 0.625” Dia. 0.625” Dia. 0.375” Max. 0.25” Metal Contact 3” SP.R. Radiused ElectrodeTruncated Cone Electrode 60 ° Both Electrodes Have Typical Dimensions for Welding 0.040 in. Thick Sheet to 0.040 in. Thick Sheet [Reference: Guidelines to Resistance Spot Welding Aluminum Automotive Sheet, p.8, The Aluminum Association]

56. Suggested Tip Radius Contours [Reference: Resistance Welding Manual, p.11-32, RWMA] An Electrode with a Radius Face is Usually Recommended (Sometimes one electrode may have a flat face) Recommended Tip Radius as function of Al Thickness

57. Electrode Design 5/8” Dia. Electrode 1/4” to 3/8” Nose Length Spherical Radius Face Water-Cooled Electrode Spherical Radius Face Offset Water- Cooled Electrode 5/8” Dia. Electrode 1/4” to 3/8” Nose Length Water-Cooled Holder (a) (b) [Reference: Welding, p.11-22, Kaiser Aluminum & Chemical Sales, Inc.]

58. Effects of Improper Tip Application: Case A This shows a satisfactory electrode application, but either too much welding force, excessive welding current or time, or a combination of these, resulting in excessive indentation and crater rim on topside and heat shrinkage on bottom. This also causes too much penetration on top and probable spitting or metal expulsion with voids or blowholes and excessive heat penetration. [Reference: Resistance Welding Manual, p.11-30, RWMA]

59. Effects of Improper Tip Application: Case B A tip radius too small accentuates and aggravates the conditions described in Case A. [Reference: Resistance Welding Manual, p.11-30, RWMA]

60. Effects of Improper Tip Dressing: Case C Tip misalignment causes a rough and bad-appearing electrode mark, misshapen weld nugget, and sheet separation. This condition, while exaggerated in the sketch, is caused by hand- filing the electrode face, electrode skidding, or a combination of the two. [Reference: Resistance Welding Manual, p.11-30, RWMA]

61. Electrode Dressing Tool A BC Section A-A Section B-B Section C-C Radius 1Radius 2 0.05 in. 0.25 in.0.05 in. 3 in. 8 in. 1.25 in. [Reference: Guidelines to Resistance Spot Welding Aluminum Automotive Sheet, p.8, The Aluminum Association] Emery Cloth or Sand paper wrapped around tool

62. On Welding Electrode Polisher Beneteau, D et al, “Resistance Spot Welding of Metal, Particularly Aluminum”, US Patent 5,449,878, Sept 12, 1995

63. Expulsion Analysis Alcan Internal Research, March 1994

64. 3 kN, 0.75 in. Radius 6 kn, 2 in. Radius Effect of Electrode Radius on Expulsion No Expulsion

65. Required Properties of Ideal Electrodes Maximum electrical and thermal conductivity Maximum hardness or resistance to deformation Tip Design Which Reduces Wear Minimum tendency to alloy with the material being welded (Because of Lower Resistance of Aluminum Than Steel More Current is Needed to Make the Weld)

66. Phase Diagrams of Al-Cu Binary Alloy Systems Hansen, Binary Alloys McGraw-Hill, 1958 Eutectic * * * * * * = Intermetallics

67. Electrode Deterioration - Thick Oxide Patrick, EP et al, SAE 840291, 1984

68. Patrick, EP et al, SAE 840291, 1984 Electrode Deterioration - Thin Oxide

69. Effect of Surface Roughness on Electrical Contact Patrick, EP et al, SAE 840291, 1984

70. Dynamic Resistance during First 1/4 Cycle of Welding Mill finish/mill finish sheet Weld Time (mill-second) Resistance (micro-ohms) Abraded/ mill finish sheet [Reference: SAE 840291, Patrick, Auhl, and Sun]

71. Dynamic Resistance Comparison Auhl, JR, SAE 940160, 1994

72. Effect of Electrode Cooling on Electrode Life Without Proper Cooling Alloying and Tip Wear Increases Proper Cooling Tube Insertion is Beneficial Lower Water Temperature is Beneficial Higher Flow Rates are Beneficial

73. Hirsch R., Influence of Water Temp& Flow on Electrode Life SMWC VII, AWS, 1996

74. Electrode Sticking Wist, Electrode-Workpiece Sticking SMWC VII, AWS 1996

75. Electrode Conditioning Wist, Electrode-Workpiece Sticking SMWC VII, AWS 1996