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Aluminum Spot Welding

Aluminum Spot Welding. General Considerations. Resistance Welding. Learning Activities View Slides; Read Notes, Listen to lecture Do on-line workbook. Lesson Objectives When you finish this lesson you will understand:. Keywords. Recommended Joint Designs Based Upon These Properties.

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Aluminum Spot Welding

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  1. Aluminum Spot Welding General Considerations

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

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

  4. 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. Material Parameters

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

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

  7. 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

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

  9. 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 • 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

  10. Overview of Thermal Considerations

  11. Case Studies of Common Electrode Heat Flow Conditions

  12. 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]

  13. 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]

  14. 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]

  15. 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]

  16. 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]

  17. 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 Material Parameters (CONT.)

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

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

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

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

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

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

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

  25. Factors Effecting Shear Load/Fatigue • Nugget Diameter • Metal Thickness • Metal UTS Select Nominal Diameter = 0.9524 t0.5 Shear Load = 0.659 t1.23 UTS (for weld with Nominal Diameter) Effect of Gage with weld made of Nominal Diameter Dewey, R, Mapes, R, “Observations on Spot Welding Aluminum for Automotive Applications” SAE Paper 770208, 1977

  26. 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 Surface-Related Problems

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

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

  29. Chemical Cleaning Solution Two to Six Minute Immersion • 15 oz/gal (120g/l) Nitric Acid [technical grade (68% HNO3 • 0.15 oz/gal (2g/l) Hydrofluoric Acid (48% HF) • 0.14 oz/gal (2g/l) Wetting Agent

  30. 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 H2CrO4, H3PO4, & F- • Coating weight 5 to 500 mg/sq ft • Colorless to emerald green

  31. Chromate Coatings • Emersion in H2CrO4 - HF - Other mineral Acids • Low contact resistance aids in Resistance Welding • Coating weight 15 to 30 mg/sq ft • 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

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

  33. 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

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

  35. 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

  36. Surface Abraded or Chemically Cleaned and Stearate Treated Untreated Al 1000 Weld Within 24 Hours if Untreated Monolayer of Stearate 100  Reoxidized + Stearate Surface Contact Resistance Al2O3 Aluminum 100 20  Reoxidized + Stearate 20 Time (Weeks)

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

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

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

  40. 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

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

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

  43. 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

  44. Caustic Cleaned Anodized Caustic Cleaned

  45. 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

  46. 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]

  47. Electrodes

  48. Required Properties of Ideal Electrodes (Because of Lower Resistance of Aluminum Than Steel More Current is Needed to Make the Weld) • 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

  49. Properties of Some Copper-Alloy Electrodes for Aluminum 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 Property Annealing Temperature Hardness (Rockwell) Electrical Conductivity (% I.A.C.S.) Availability

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