Transcript No Slide Title

Capacitor Discharge Welding
Percussion Welding
Magnetic Force (AC)
Stud Welding
Capacitive Discharge Welding
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
Learning Activities
1. View Slides;
2. Read Notes,
3. Listen to lecture
4. Do on-line
workbook
Keywords
Percussion Weld, Capacitor Discharge, Nib, Magnetic Force Weld
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.
Applications:
Cueman, “Process Model for
Percussion Welding”,
Welding Journal, Sept, 1989
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.
AWS Welding Handbook
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)
Metals Handbook, ASM, 1983
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
Low Voltage
Hand Held
Unit
AWS Welding Handbook
Metals Joined
Like Metals
Tantalum Alloys
Copper Alloys
Aluminum Alloys
Nickel Alloys
Low-carbon steel
Medium-carbon steel
Stainless Steels
Other Dissimilar Wires
Thermocouple Wires
Molybdenum Wires
Nb-1Zr
85Zr-15Nb
Tantalum
Welded to Copper
Gold
Silver
Copper-tungsten
Silver-tungsten
Silver-cadmium oxide
Molybdenum
Diffusion during prolonged
high temperature exposure
may produce weak or
embrittled
structures.Transition joints
of a compatible third
material may help.
Metals Handbook, ASM, 1983
Welding Energy
1
2
E  CV
2
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
Metals Handbook, ASM, 1983
Arc Time (Arc Duration)
Interval that begins when arc is initiated and ends when workpieces touch
Factors Affecting Arc Time
• Work Metal or Combination
• Mass of Moving Workpiece
• Nib Dimensions
• Welding Voltage
• Welding Current
• Welding Force
• Synchronization of Machine Functions
Generally, the
shortest weld time
allowing some
penetration
into each part
while minimizing
heating is
recommended
Mechanical Properties vs Weld Time
Weld Time
Bakshas, “The quality of capacitor deischarge…< Automatic Welding, Mar., 1981
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
Welding Current
Peak Current Density
=300,000 A/in2
Polarity:
No Effect for Like materials
Positive polarity for:
• Large Cross Section
• Higher Melting Point
• Higher Thermal Conductivity
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
5 mils
0.01
Impact Velocity
10-60 in/s
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.
Gravity Unit
Thompson, “Attachment of Thermocouple Instrumetation…”,
Welding Journal, June 1982
Spring Loaded Unit
Thompson, “Attachment of Thermocouple Instrumetation…”,
Welding Journal, June 1982
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.
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
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)
Air Cylinder applies
initial force to bring
contact to nib
Electromagnetic
Force applied during
Welding
Metals Handbook, ASM, 1983
Schlegel, “All About Percussion Welds”,
Welding Design & Fabrication, Oct. 1990
AWS Welding Handbook
AWS Welding Handbook
AWS Welding Handbook
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
Acceleration of the moveable head:
• Directly proportional to the magnetic force applied
• Inversely proportional to the mass
Section through a Silver Contact (Top) to a Brass Terminal
AWS Welding Handbook