Capacitor Discharge Welding Percussion Welding Magnetic Force AC

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-1 Zr 85 Zr-15 Nb Tantalum Welded to Copper Gold Silver Copper-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 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 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/in 2 Polarity: No Effect for Like materials Positive polarity for: • Large Cross Section • Higher Melting Point • Higher Thermal Conductivity

Welding 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