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Resistance spot welding


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Resistance spot welding

  1. 1. Resistance WeldingResistance Welding By: Majid Pouranvari Materials Science and Engineering Department, Sharif University of Technology Fall, 2014 A little about me… Education Authorship Reviewership Faculty PhD, Materials Engineering, Major in Welding, Sharif University of Technology, 2013 Sharif University of Technology 55 ISI-WoS Papers 22 National & Inter. Papers in Scientific Journals 40 Papers in conferences Reviewer for MSEA, STWJ, JALC, JMAD, JMPT, JMEP, …
  2. 2. Resistance Welding Table of ContentTable of Content (1) Resistance Spot Welding (2) Resistance Projection Welding (3) Resistance Seam Welding (4) Resistance Butt Welding
  3. 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. Resistance Welding  Heat generation is expressed as Q = I2R T, Q = Heat generated.  Resistance welding depends on three factors: Time of current flow (T). Resistance of the conductor (R) Amperage (I).
  4. 4. Principal Types of Resistance Welds Electrodes or Welding Tips Electrodes or Welding Wheels Electrodes or Dies Projection Welds Electrodes or Dies Spot Weld Seam Weld Projection Weld Upset Weld Flash Weld After Welding After Welding [Reference: Resistance Welding Manual, RWMA, p.1-3] The dominant process for welding of automotive sheets Resistance Spot Welding
  5. 5. Resistance Spot Welding Professor T. W. Eagar, MIT “ The more one studies the resistance welding process, the more one appreciates how complex the process is ”
  6. 6. RSW Equipments
  7. 7. Heat Generation in Resistance Spot Welding
  8. 8. Block Diagram of Single-Phase Spot Welder Spot Weld Main Power Line Contactor N = np /ns V s= V p /N Is = Ip N Typically, the turns ratio in resistance welding transformers is about 100 to 1. Therefore, if 480 V, 200 A power is available at the primary, 4.8 V at 20kA will be available at the secondary (neglecting losses).
  9. 9. Temperature Readings of A Spot Welding Process Workpiece This illustration was taken about 4/60th of a second after the welding current starts. (Note: Temp at Electrode Sheet Interface Higher than Bulk) After 20% welding time At the end of welding time Temperature ElectrodeElectrode Workpiece Temperature distribution at various location during welding. Temperature Distribution
  10. 10. Welding Cycle Upslope/Downslope, Hold Time, & Temper Weld Current Temper Current Electrode Pressure Current Squeeze Time Weld Time Off Time Hold Time Upslope Downslope Temper
  11. 11. Enhanced Welding Cycle Preheat Time Upslope Time CoolTime Weld Time CoolTime Preweld Interval Welding Cycle Weld Interval Postweld Interval Downslope Time Quench Time Temper Time Hold Time Pulse Impulse Tempering Current Welding Current Electrode Force Forge Delay Time Forge Force [Reference: Welding Handbook, Volume 2, AWS, p.539] Squeeze time Advantages of Resistance Spot Welding  Excellent for sheet metal applications, < ¼- inch  High speed, < 0.1 seconds in automotive spot welds  Adaptability for Automation in High-Rate Production of Sheet Metal Assemblies  No filler metal  Economical  Dimensional Accuracy
  12. 12. Process Disadvantages and Limitations  Higher equipment costs than arc welding  Power line demands  Nondestructive testing  Low tensile and fatigue strength  Not portable  Electrode wear  Lap joint requires additional metal  Difficulty for repair Macrostructural Features of a RSW
  13. 13. Weld Size Requirements Weld Penetration Requirements In general, a large penetration is acceptable if it does not create a large indentation. The requirements on penetration are often applied together with those on weld size.
  14. 14. Factors Affecting Heat Generation (Q):  Welding time  Welding Current Welding current is most effective in heat generation compared to welding time Q = I2Rt
  15. 15. Factors Affecting Heat Generation (Q):  Resistance Bulk Resistance vs. Contact Resistance R=f (Materials Properties and Pressure)
  16. 16. Factors Affecting Heat Generation (Q):  Resistance Surface Condition Steel Steel Steel Steel Oils/Dirt Oxide Oxide Oils/Dirt (a) Pickled Conditions (b) Rusted Conditions Rusty Pickled Polished Electrode Force Resistivity
  17. 17. Resistance Varies with Pressure Low Pressure Medium Pressure High Pressure (a) (b) (c)
  18. 18. Expulsion: A common Phenomena in Resistance Welding Expulsion: A common Phenomena in Resistance Welding
  19. 19. Zhang et al, “Expulsion Modeling in RSW of Steel and Al Alloys”, AWS Sheet Metal Conf VIII, 1998 Effect of Welding Parameters on Expulsion  Welding Current  Welding time  Electrode force Effect of Expulsion  Void  Excessive electrode indentation
  20. 20. Operating Window - Lobe Curve Current (1000’s of amperes) Time(cyclesofcurrent) ExpulsionNugget too small Acceptable nugget size Constant electrode force Effect of Process variable on Operating Window - Lobe Curve Materials properties
  21. 21. Some welding problems Some welding problems
  22. 22. Some welding problems Some welding problems
  23. 23. Some welding problems Some welding problems
  24. 24. Some welding problems Some welding problems
  25. 25. Heat Balance In RSW, heat balance can be defined as a condition in which the fusion zones in both pieces being joined undergo approximately the same degree of heating and applied pressure. It describes the ideal situation when a symmetric weld (with equal depth of nugget penetration) is made. Heat balance is influenced by the relative thermal and electrical conductivities of the materials to be joined, the geometry of the weldment, and the geometry of the electrodes. Dissimilar Thickness Welding In the case of dissimilar thickness, bulk resistance of thicker sheet is lager than that of the thinner sheet. This leads to an asymmetric weld nugget (i. e. penetration of the weld nugget into the thicker sheet is larger than that of the thinner sheet).
  26. 26. Dissimilar Thickness Welding Solution for Heat Unbalance in dissimilar Thickness Welding This can be overcome by using electrodes of two different diameters or by inserting a high-resistivity tip in one electrode. The smaller electrode or the one with high-resistivity insert should be placed against the thinner of the two sheets
  27. 27. Dissimilar Metal Welding Shunting Effect Previously made welds may affect the subsequent welding if the welds are spaced close to each other due to electric current shunting. The welding current may be diverted from the intended path by the previously made welds. As a result, the current or current density may not be sufficient to produce a quality weld.
  28. 28. Shunting Effect Shunting Effect
  29. 29. Shunting Effect Shunting Effect The shunting effect is a strong function of the bulk resistivity of the sheet material. A high conductive metal, such as aluminum, requires a large space between the welds. This should be taken into account when welds are designed into structures, as putting too many welds close to each other may not provide the intended strength.
  30. 30. electrode electrode Electrodes Electrode Functions  Conduct the welding current to the work  Transmit the proper electrode pressure or force to the work in order to produce a satisfactory weld  Help dissipate heat from the weld zone
  31. 31. Material Requirements for Electrode in Resistance Welding  High electrical conductivity  High Thermal conductivity  High temperature mechanical strenght Effect of Strengthening Mechanism on the Electrical Resistivity? RWMA Electrode Material Standards  Group A - Copper Base Alloys  RWMA Class 1  Zirconium Copper  Cadmium Copper  Chromium Copper  RWMA Class 2  Chromium-Zirconium Copper  Chromium Copper  RWMA Class 3  Cobalt-Beryllium Copper  Nickel-Beryllium Copper  Beryllium-Free Copper  RWMA Class 4  Beryllium Copper  RWMA Class 5  Aluminum Copper
  32. 32. RWMA Electrode Material Standards (CONT.)  Group B - Refractory Metals and Refractory Metal Composites  RWMA Class 10  Copper Tungsten  RWMA Class 11  Copper Tungsten  RWMA Class 12  Copper Tungsten  RWMA Class 13  Tungsten  RWMA Class 14  Molybdenum  Group C - Specialty Material  RWMA Class 20  Dispersion- Strengthened Copper Electrode Materials RWMA Class # Electrical Conductivity Composition (%) Ultimate Strength (ksi) Annealing Temperature (°C) Thermal Conductivity (Cal/cm-sec-°C) - 1 2 3 4 5 Cu 99-Cu, 1-Cd 99.2-Cu, 0.8-Cr 97-Cu, 2.5- Co, 0.5-Be Cu & Be Cu & Al 90 92 80 (C) 82 (F) 48 (C) 52 (F) 20 (C) 23 (F) 18 70 - 0.16 30 - - 60 (F) 30 (C) 62 (F) 95 (C) 105 (F) 110 (C) 170 (F) 660 0.82 710 0.77 (C) 0.75 (F) 0.43 (C) 0.45 (F) 0.18 (C) 0.19 (F) 930 (C) 900 (F) 1020 (C) 900 (F) * C = Cast, F = Forging
  33. 33. Typical Electrode Hardness-Temperature Curves [Reference: Resistance Welding Manual, p.18-2, RWMA] Electrode Geometry “A” “A” “A” “A” 6° 20° 45° 3 1 4 1 4 1 4 1 4 45°20° Pointed Dome Truncated Cone Truncated Cone [Reference: Welding in the Automotive Industry, p.135, D. W. Dickinson]
  34. 34. Electrode Size Net Electrode Force (lb x 102) Electrode Face Diameters (inch) Electrode Body Diameters (inch) [Reference: Resistance Welding Manual, p.18-14, RWMA] W. Stanley, Resistance Welding McGraw-Hill, 1950
  35. 35. Electrode Life Number of Welds Diameter Electrode Cap Diameter For Coated Steel Weld Button Diameter For Coated Steel Weld Button Diameter Uncoated Steel Electrode Degradation (1) Mechanical Degradation (2) Metallurgical Degradation
  36. 36. Electrode Life Steel Solid Zinc Coating Molten Zinc Copper Alloy Electrode brass MeltingPoint (F) Copper 1980 Brass Downto1710 Zinc 787 Measured ElectrodeFaceTemp(F) Bare 1000-1200 Galvanized 1500-1700 Cu Zn ~45% ~60% ~85%
  37. 37. Uncoated Hot Dipped Galvanized Spot Weld Mechanical Properties
  38. 38. Interfacial Mode Pullout Mode
  39. 39. Weld With Expulsion Failed in Pullout Mode Dickinson, “Welding in Auto Industry, AISI, 1981 Workshop Tests
  40. 40. AWS Spec D8.9-97, 1997 Advantages of Peel Test • Ease of Performance • Low Cost • Ability to use on Shop Floor as quality control test Disadvantages of Peel Test • A qualitative rather than a quantitative
  41. 41. Dickinson, “Welding in Auto Industry, AISI, 1981 Chisel Test 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?
  42. 42. Mechanical Properties of RSW Mechanical Properties of RSW Tensile-Shear Test
  43. 43. Mechanical Properties of RSW Coach-Peel Test Mechanical Properties of RSW Cross-Tension Test
  44. 44. Mechanical Properties of RSW Tensile-Shear Test Strength Requirements for Steels
  45. 45. Strength Requirement for Mg & Al
  46. 46. Heuschkel, “Expression of Spot Weld Properties”, Weldign Journal Oct 1952 FTS=Dt[-(C+0.05Mn)] BM
  47. 47. Failure Mode of Spot Welds
  48. 48. ‫اي‬ ‫ﻧﻘﻄﻪ‬ ‫ﻣﻘﺎوﻣﺘﯽ‬ ‫ﺟﻮﺷﻬﺎي‬ ‫ﺷﮑﺴﺖ‬ ‫ﻣﻮد‬ ‫در‬ ‫ﮐﻪ‬ ‫دارد‬ ‫وﺟﻮد‬ ‫ﺑﺤﺮاﻧﯽ‬ ‫دﮐﻤﻪ‬ ‫ﻗﻄﺮ‬ ‫ﯾﮏ‬ ‫ﻣﺸﺘﺮﮐﯽ‬ ‫ﻓﺼﻞ‬ ‫ﺷﮑﺴﺖ‬ ،‫آن‬ ‫از‬ ‫ﺗﺮ‬ ‫ﭘﺎﯾﯿﻦ‬ ‫ﻣﻘﺎدﯾﺮ‬ ‫اﺳﺖ‬ ‫ﺣﺎﮐﻢ‬. AWS: D=4t0.5 DVS/JIS: D=5t0.5 ‫درﺟﺎ‬ ‫ﺗﻤﭙﺮ‬ ‫ﻃﺮﯾﻖ‬ ‫از‬ ‫ﺷﮑﺴﺖ‬ ‫رﻓﺘﺎر‬ ‫ﺑﻬﺒﻮد‬
  49. 49. Introduction to Projection Welding (a) (b) (c) (d) [Reference: Welding Handbook, Volume 2, p.566, AWS] Examples of Various Projection Designs (c) (d) [Reference: Welding Handbook, Volume 2, p.562, AWS] (b) (a)
  50. 50. Examples of Various Projection Designs (CONT.) (e) (f) (g) [Reference: Welding Handbook, Volume 2, p.562, AWS] Projection Types for Sheet and Solid Applications [Reference: Metals Handbook, Volume 6 (Welding, Brazing and Soldering), p.503-524, ASM] Spherical Projections Elongated Projections
  51. 51. Cross-wire welding Projection in Nuts
  52. 52. Advantages of Projection Welding  Ease of obtaining satisfactory heat balance for welding difficult combinations  More uniform results in many applications  Increased output per machine because several welds are being made simultaneously  Longer electrode life
  53. 53. Advantages of Projection Welding (CONT.)  Welds may be placed more closely together  Finish, or surface appearance, is often improved  Parts may be projection welded that could not be otherwise resistance welded Limitations of Projection Welding  Requires an additional operation to form projections  Requires accurate control of projection height and precise alignment of the welding dies with multiple welds  Requires thickness limitation for sheet metals  Requires higher power capacity equipment than spot welding
  54. 54. Projection Weld Formation [Reference: Resistance Welding Manual, p.3-9, RWMA] Stage 1 Stage 3 Stage 2 Stage 4 Projection Weld Formation (CONT.) [Reference: Resistance Welding Manual, p.3-9, RWMA] Stage 5 Stage 7 Stage 6 Stage 8
  55. 55. Basic Projection Design in Steel Sheet [Reference: Welding Handbook, Volume 2, p.563, AWS] Spherical Radius Projection Wall Thickness Should Be at Least 70% of Sheet Thickness Punch Die Point Radius “R”Projection Should Blend into Stock Surface without Shouldering D T H A D B 45° 15° Effect of H? Recommended Projection Designs [Reference: Recommended practice for resistance welding, AWS Document C1.1-66, AWS, 1966] T(in.) D(in.) H(in.) L(in.) 0.010-0.014 0.055 0.015 1/8 0.016-0.020 0.067 0.017 5/32 0.025 0.081 0.020 3/16 0.031-0.034 0.094 0.022 7/32 0.044-0.050 0.119 0.028 9/32 0.062-0.070 0.156 0.035 3/8 0.078 0.187 0.041 7/16 0.094 0.218 0.048 1/2 0.109 0.250 0.054 5/8 0.125 0.281 0.060 11/16 0.140 0.312 0.066 3/4 0.156 0.343 0.072 13/16 0.171 0.375 0.078 7/8 0.187 0.406 0.085 15/16 0.203 0.437 0.091 1 0.250 0.531 0.110 1-1/4 H T D T: Thickness of thinnest outside piece D: Diameter of projection H: Height of projection L: Minimum contacting overlap L
  56. 56. [Reference: Resistance Welding Manual, p.3-13, RWMA] Projection Welding of Low Carbon Steel (Two Equal Thicknesses) Resistance Seam Welding
  57. 57. Resistance Seam Welding Upper Electrode Wheel Workpiece Lower Electrode Wheel Throat Knurl or Friction Drive Wheel Roll Spot Weld Overlapping Seam Weld Continuous Seam Weld [Reference: Welding Handbook, Volume 2, p.553, AWS] Lap Seam Weld Electrodes Overlapping Weld Nuggets Travel Front view Side View [Reference: Welding Handbook, Volume 2, p.554, AWS]
  58. 58. ASM Handbook Vol6, 1993 ASM Handbook Vol6, 1993 Schematic of seam weld. Length of electrode footprint holds 3 welds
  59. 59. RSW Certification Training Class, Boeing Effect of Cool Time (Heat %) on Nugget Properties Resistance Welder Manufacturers Association Bulletin # 23 @ 6 Cycle Heat
  60. 60. Welding Speed Resistance Welder Manufacturers Association Bulletin # 23 Tensile Tests Pillow Tests Seam Weld Quality Tests Resistance Welder Manufacturers Association Bulletin # 21
  61. 61. 6 x 6 inch Pillow 6 x 10 inch Pillow Typical data from Pillow Testing Resistance Upset Welding
  62. 62. Upset Welding Finished Upset Weld Heated Zone To Welding Transformer Clamping Die Upsetting Force Movable Part Clamping Die Stationary Part [Reference: Welding Handbook, Volume 2, p.598, AWS] Schematic of Typical Butt Weld Cycle Medar Technical Literature
  63. 63. Mechanism Stage 1: Resistance (Joule) Heating help ease of hot plastic deformation Stage 2: Application of extra upsetting pressure causes material extrudes out-wards forming an upset. Upset Resistance Welding
  64. 64. Upsetting  The main mechanism to remove the contaminants is the formation of the upset.  By changing the welding conditions to generate more upset the amount of contaminants will be reduced, although more material will be lost and more energy will be consumed. Wheel rim production