Capacitor Discharge Welding

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Capacitor Discharge Welding Percussion
Welding Magnetic Force (AC) Stud Welding
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Capacitive Discharge Welding

• Learning Activities
• View Slides
• Read Notes,
• Listen to lecture
• Do on-line workbook

• Lesson Objectives
• When you finish this lesson you will understand
• The mechanism of percussion welding and magnetic
  force welding
• Applications of use of these methods

Keywords Percussion Weld, Capacitor Discharge,
Nib, Magnetic Force Weld
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Percussion Welding (PEW) A resistance welding
process which produces coalescence of the
abutting members using heat from an arc produced
by a rapid discharge of electrical energy.
Pressure is applied percussively during or
immediately following the electrical discharge.
Cueman, Process Model for Percussion
Welding, Welding Journal, Sept, 1989
Applications can be used to join like and
unlike metals that cannot usually be flash or
stud welded. It is used for fine wire leads to
filaments such as in lamps and electrical
components.
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AWS Welding Handbook
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• Variations of Percussion Welding
• Capacitor Discharge, Low V, Nib Start
• Capacitor Discharge, Low V, High Frequency Start
• Capacitor Discharge, High V
• Magnetic Force Method (AC)

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Metals Handbook, ASM, 1983
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Low Voltage Hand held bench type welders High
Voltage More uniform arc, No Nib start , More
control Transformer for Magnetic-force, weld
made during first 1/2 cycle
Metals Handbook, ASM, 1983
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Low Voltage Hand Held Unit
AWS Welding Handbook
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Metals Joined
Welded to Copper Gold Silver Copper-tungsten Silve
r-tungsten Silver-cadmium oxide Molybdenum
Like Metals Tantalum Alloys Copper
Alloys Aluminum Alloys Nickel Alloys Low-carbon
steel Medium-carbon steel Stainless Steels
Diffusion during prolonged high temperature
exposure may produce weak or embrittled
structures.Transition joints of a compatible
third material may help.
Other Dissimilar Wires Thermocouple
Wires Molybdenum Wires Nb-1Zr 85Zr-15Nb Tantalum
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Metals Handbook, ASM, 1983
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Welding Energy
E energy in watt-seconds (joules) C
capacitance in farads V voltage

• Amount of Energy Needed to Make Joint Depends on
• Cross-sectional area of joint
• Properties of work metal or metals
• Depth to which metal is melted on workpieces

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Metals Handbook, ASM, 1983
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Arc Time (Arc Duration)
Interval that begins when arc is initiated and
ends when workpieces touch
Generally, the shortest weld time allowing some
penetration into each part while
minimizing heating is recommended

• Factors Affecting Arc Time
• Work Metal or Combination
• Mass of Moving Workpiece
• Nib Dimensions
• Welding Voltage
• Welding Current
• Welding Force
• Synchronization of Machine Functions

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Mechanical Properties vs Weld Time
Weld Time
Bakshas, The quality of capacitor deischargelt
Automatic Welding, Mar., 1981
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Heat Affected Zone (Very Narrow)
Lower Melting Temp
Only a few Millionths Inch to 0.025
Higher Melting Temp

• Because HAZ is so small
• Heat treated metals can be welded without
  softening
• Heat sensitive components near weld unaffected

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Welding Current

• Polarity
• No Effect for Like materials
• Positive polarity for
• Large Cross Section
• Higher Melting Point
• Higher Thermal Conductivity

Peak Current Density 300,000 A/in2
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Welding Force

• Force Supplied By
• Electromagnet
• Gravity
• Cam-activated Direct Drive
• Spring
• Pneumatic

Force must be strong enough to accelerate moving
part over short gap. Peak loads 15 - 30
ksi Wire Size Impact Velocity 5 mils 10-60
in/s 0.01 80-150 in/s
Because the force applying unit may rebound and
put a tensile loading on welded part, a means of
damping must be provided.
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Gravity Unit
Thompson, Attachment of Thermocouple
Instrumetation, Welding Journal, June 1982
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Spring Loaded Unit
Thompson, Attachment of Thermocouple
Instrumetation, Welding Journal, June 1982
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Advantages of Percussion Welding

• Brevity of arc limits melting and heating.
  Heat-treated and cold worked materials can be
  welded without annealing.
• No Filler Metal required, No cast structure at
  interface.
• Charging rate is low and controlled, Line power
  line demand
• Can tolerate some contamination on faying
  surface.

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Limitations of Percussion Welding

• Limited to butt joints
• Total area limited
• Similar metals can usually be joined more
  economically by other processes.
• Usually confined to joining of dissimilar metals
  not normally considered weldable

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• Variations of Percussion Welding
• Capacitor Discharge, Low V, Nib Start
• Capacitor Discharge, Low V, High Frequency Start
• Capacitor Discharge, High V
• Magnetic Force Method (AC)

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Air Cylinder applies initial force to
bring contact to nib
Electromagnetic Force applied during Welding
Metals Handbook, ASM, 1983
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Schlegel, All About Percussion Welds, Welding
Design Fabrication, Oct. 1990
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AWS Welding Handbook
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AWS Welding Handbook
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AWS Welding Handbook
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• Arc Time is a Function of
• Magnitude of magnetic force
• Timing of the magnetic force with relation to
  welding current
• Inertia or mass of the moving parts in the force
  system
• Magnitude of the welding current and the
  diameter of the projection

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• Acceleration of the moveable head
• Directly proportional to the magnetic force
  applied
• Inversely proportional to the mass

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Section through a Silver Contact (Top) to a Brass
Terminal
AWS Welding Handbook