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Spot Welding

Spot Welding. Spot Welding. Learning Activities View Slides; Read Notes, Listen to lecture View Demo Do on-line workbook. Lesson Objectives When you finish this lesson you will understand: Basics of Resistance Welding Processes Heat Generation & Control

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Spot Welding

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  1. Spot Welding

  2. Spot Welding • Learning Activities • View Slides; • Read Notes, • Listen to lecture • View Demo • Do on-line workbook • Lesson Objectives • When you finish this lesson you will understand: • Basics of Resistance Welding Processes • Heat Generation & Control • Spot Welding Process and Applications Keywords: ResistanceSpot Welding, Heat Generation, Equipment Control, Contact Resistance, Upslope, Downslope, Hold Time, Temper, Squeeze Time, Electrode

  3. Definition of Resistance Welding • Resistance welding is a fusion welding process in which coalescence of metals is produced at the faying surfaces by the heat generated at the joint by the resistance of the work to the flow of electricity. • Force is applied before, during, and after the application of current to prevent arcing at the work piece. • Melting occurs at the faying surfaces during welding.

  4. Principal Types of Resistance Welds Electrodes or Welding Wheels Electrodes or Dies Electrodes or Welding Tips Projection Welds Spot Weld Seam Weld Projection Weld Electrodes or Dies Upset Weld Flash Weld After Welding After Welding [Reference: Resistance Welding Manual, RWMA, p.1-3]

  5. Typical Equipment of Resistance Spot Welding (a) (b) [Reference: Welding Process Slides, The Welding Institute]

  6. Advantages of Resistance Spot Welding • Adaptability for Automation in High-Rate Production of Sheet Metal Assemblies • High Speed • Economical • Dimensional Accuracy

  7. Limitations of Resistance Spot Welding • Difficulty for maintenance or repair • Adds weight and material cost to the product, compared with a butt joint • Generally have higher cost than most arc welding equipment • Produces unfavorable line power demands • Low tensile and fatigue strength • The full strength of the sheet cannot prevail across a spot welded joint • Eccentric loading condition

  8. Resistance Welding • Resistance weldingdepends on three factors: • Time of current flow (T). • Resistance of the conductor (R) • Amperage (I). • Heat generation is expressed as Q = I2R T, Q = Heat generated.

  9. Heating Value of Current = RMS Current Irms=0.707 Ipeak

  10. Block Diagram of Single-Phase Spot Welder Contactor Main Power Line Spot Weld

  11. Contact-Resistance Measurement Electrode Force Rec Small Current Rec = contact resistance between electrode and sheet surface Rsc = contact resistance at the faying surface Rv = volume resistance of the sheets Rec Rsc Rv Rtotal Rec Rv Contact Area Rec Electrode Force

  12. Factors Affecting Heat Generation (Q): • Welding pressure • as welding pressure increases both R and Q decrease. • Electrodes • deformation of electrodes increases contact area. As contact area increases, both R and Q decrease.

  13. Link to electrode force demo

  14. Surface Condition (a) Pickled Conditions (b) Rusted Conditions Steel Steel Resistivity Rusty Steel Steel Oils/Dirt Oxide Oxide Oils/Dirt Polished Pickled Electrode Force

  15. Resistance Varies with Pressure Low Pressure Medium Pressure High Pressure (a) (b) (c)

  16. Volume-Resistance Measurement Electrode Force Small Current Rec = contact resistance between electrode and sheet surface Rsc = contact resistance at the faying surface Rv = volume resistance of the sheets Rec Rsc Rv Rtotal Rec Rv Contact Area Rv Electrode Force

  17. Resistivity as a Function of Temperature 130 120 HSLA 110 100 90 80 70 Resistivity, mW-cm 60 Low Carbon 50 40 30 20 10 100 200 300 400 500 600 700 800 Temperature, °C [Reference: Welding in the Automotive Industry, D.W. Dickinson, p.125]

  18. Questions

  19. Heating Value of Current = RMS Current Irms=0.707 Ipeak

  20. Upslope/Downslope, Hold Time, & Temper Electrode Pressure Weld Current Current Temper Current Upslope Downslope Temper Squeeze Time Weld Time Off Time Hold Time

  21. Heat Dissipation Water-Cooled Copper Alloy Electrode Water-Cooled Copper Alloy Electrode Base Metal Base Metal Weld Nugget

  22. Questions

  23. Let’s put it all together

  24. Initial Resistance Through Weldment Top Electrode Water Weld Nugget Distance Resistance Bottom Electrode

  25. Temperature Readings of A Spot Welding Process (Note: Temp at Electrode Sheet Interface Higher than Bulk) Workpiece This illustration was taken about 4/60th of a second after the welding current starts.

  26. Temperature Distribution Temperature distribution at various location during welding. At the end of welding time Electrode After 20% welding time Workpiece Electrode Temperature

  27. Link to nugget growth demo

  28. Nugget Solidification

  29. Questions

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