WE 602 Resistance Welding Processes Review & Physics of Spot Welding

1. WE 602Resistance Welding Processes Review&Physics of Spot Welding Reference Web Site for Review: www-iwse.eng.ohio-state.edu/we601

2. Resistance Welding Process Review • Learning Activities • View Slides; • Read Notes, • Listen to lecture • View Demos • Do on-line workbook • Do Homework • Lesson Objectives • When you finish this lesson you should understand: • Heat generation & nugget growth in resistance spot welding • Basic, enhanced and pulsed weld cycles • Advantages and limitations of spot welding • A basic model of expulsion Keywords Spot weld contact resistance expulsion heat flow Weld cycle bulk resistance pulsing electrode

3. 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]

4. Introduction to Resistance Spot Welding Top Electrode Water Weld Nugget Distance Resistance Bottom Electrode

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

6. Process Operation of Resistance Spot Welding [Reference: Welding Process Slides, The Welding Institute]

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

8. 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 equipments • 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

9. Basic Single Impulse Welding Cycle Electrode Force Welding Current Squeeze Time Weld Time Hold Time Off Time Welding Cycle [Reference: Welding Handbook, Volume 2, AWS, p.538]

10. Enhanced Welding Cycle [Reference: Welding Handbook, Volume 2, AWS, p.539] Forge Force Forge Delay Time Pulse Welding Current Impulse Electrode Force Tempering Current Cool Time Cool Time Hold Time Upslope Time Temper Time Preheat Time Quench Time Downslope Time Weld Time Squeeze time Preweld Interval Weld Interval Postweld Interval Welding Cycle

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. 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]

13. Questions? • Turn to the person sitting next to you and discuss (1 min.): • The total resistance across the spot weld for plain carbon steel usually starts high because of interfacial surface asperities and oxides and surface contaminants. But the total resistance changes as current passes. What would be the value of resistance as time increases (dynamic resistance)? Draw a curve.

14. Temperature Gradient of A Spot Weld Water End of “Weld Time” Electrode 20% of “Weld Time” Work Water Temperature Electrode Welding Temperature Water [Reference: Resistance Welding Manual, RWMA, p.1-4]

15. Link to spot weld nugget growth video Link to nugget growth demonstration

16. Nugget Growth (a) (b) Point 1 Point 2 Point 3

17. Nugget Growth (CONT.) (a) (b)

18. Expulsion at Interfacial Surface Zhang et al, “Expulsion Modeling in RSW of Steel and Al Alloys”, AWS Sheet Metal Conf VIII, 1998

19. Temperature Readings of A Spot Welding Process This illustration was taken about 4/60th of a second after the welding current starts.

20. Zhang et al, “Expulsion Modeling in RSW of Steel and Al Alloys”, AWS Sheet Metal Conf VIII, 1998

21. Zhang et al, “Expulsion Modeling in RSW of Steel and Al Alloys”, AWS Sheet Metal Conf VIII, 1998

22. Zhang et al, “Expulsion Modeling in RSW of Steel and Al Alloys”, AWS Sheet Metal Conf VIII, 1998

23. Pulsing Pulse 1 Pulse 2 Pulse 3 Cool Time Pulse Time

24. Without Pulsing With Pulsing

25. Questions? • Turn to the person sitting next to you and discuss (1 min.): • We have seen the nugget growth with AC current to grow when the peak in current is reached and shrink slightly as the current goes to zero each half cycle. What might occur when DC current is used?