Capacitor Discharge Welding

Percussion WeldingMagnetic Force (AC)Stud Welding

Capacitive Discharge WeldingLesson ObjectivesWhen you finish this lesson you will understand: The mechanism of percussion welding and magnetic force welding Applications of use of these methodsLearning ActivitiesView Slides; Read Notes, Listen to lectureDo on-line workbookKeywordsPercussion 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: 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.Cueman, Process Model for Percussion Welding,Welding Journal, Sept, 1989

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

Metals Handbook, ASM, 1983Low Voltage: Hand held & bench type weldersHigh Voltage: More uniform arc, No Nib start , More controlTransformer: for Magnetic-force, weld made during first 1/2 cycle

AWS Welding HandbookLow VoltageHand HeldUnit

Metals JoinedLike MetalsTantalum AlloysCopper AlloysAluminum AlloysNickel AlloysLow-carbon steelMedium-carbon steelStainless SteelsWelded to CopperGoldSilverCopper-tungstenSilver-tungstenSilver-cadmium oxideMolybdenumOther Dissimilar WiresThermocouple WiresMolybdenum WiresNb-1Zr85Zr-15NbTantalumDiffusion 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 EnergyE = energy in watt-seconds (joules)C = capacitance in faradsV = voltageAmount 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 touchFactors Affecting Arc Time Work Metal or Combination Mass of Moving Workpiece Nib Dimensions Welding Voltage Welding Current Welding Force Synchronization of Machine FunctionsGenerally, the shortest weld timeallowing some penetrationinto each part while minimizingheating is recommended

Mechanical Properties vs Weld TimeBakshas, The quality of capacitor deischarge< Automatic Welding, Mar., 1981Weld Time

Heat Affected Zone(Very Narrow)Higher Melting TempLower Melting TempOnly a fewMillionths Inch to 0.025Because HAZ is so small Heat treated metals can be welded without softening Heat sensitive components near weld unaffected

Welding CurrentPeak Current Density =300,000 A/in2

Polarity:No Effect for Like materialsPositive polarity for: Large Cross Section Higher Melting Point Higher Thermal Conductivity

Welding ForceForce Supplied By: Electromagnet Gravity Cam-activated Direct Drive Spring PneumaticForce must be strong enough to accelerate moving part over short gap.

Peak loads = 15 - 30 ksi

Wire Size Impact Velocity5 mils10-60 in/s0.0180-150 in/sBecause the force applying unit may rebound and put a tensile loading on welded part, a means of damping must be provided.

Gravity UnitThompson, Attachment of Thermocouple Instrumetation,Welding Journal, June 1982

Thompson, Attachment of Thermocouple Instrumetation,Welding Journal, June 1982Spring Loaded Unit

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 appliesinitial force to bringcontact to nibElectromagnetic Force applied duringWeldingMetals 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 TerminalAWS Welding Handbook

In this module, we will investigate capacitor discharge welding where the energy for welding is supplied by electrical charge stored up on a capacitor. There are several different variations of this process as listed above and in this module we will look at percussion welding and magnetic force welding.A definition for percussion welding is presented here. This process uses both heat and a force supplied in a rapid fashion. Because the process happens so rapidly, the heat is localized to a very small volume which is of benefit in many applications. The process can be used to join both like and unlike metals. Some typical applications are noted.This is an example of a typical percussive weld made on an electronic component being welded to a circuit board such as found in many microprocessor applications. The weld is made rapidly without damage to the terminal or board.There are several variations of this process as listed here. Three capacitive discharge with both high and low voltage variations and different weld starting conditions, and the forth is a process where an AC current is used to establish the percussive force.Let us look first at the low voltage variation with the nib start. As can be seen on this illustration, the part to be welded has a machined projection or nib on one of the pieces. This nib will get the current flowing and concentrate it to the nib portion for rapid heating as soon as the nib comes into contact with its mating surface. At first, the contactor switch is closed with the weld switch open while a DC current charges the capacitor. The the contactor switch is opened while the weld switch is closed and the part motion initiates in the percussive motion. When the nib touches, current rapidly flows and the weld is consummated. The duration of the current flow is timed through a variable resistor in the RC weld circuit.In the low voltage-high frequency start, the sample may or may not have a nib, but a high frequency generator is placed in line in the circuit and when the switch is closed to initiate current flow from the capacitor, a high frequency pulse to ionize the are gap between the parts is also initiated, thus starting a rapid arc simultaneously with the force impact. Care must be exercised when using the high frequency when welding on or near electronic circuitry.The high voltage variation has voltages sufficient to initiate the rapid arc without outside starting techniques.This is a listing of the process parameters comparing the various methods discussed. The low voltage machines tend to be hand held or bench type machines while the high voltage version tends to be attached to more automated machines. The higher voltage versions give a more uniform arc and then to be better controlled. The transformer magnetic force device to be described later uses a transformer to initiate force and the weld is usually made in the first 1/2 AC cycle.This is an example of the low voltage hand held unit.This table lists the various metals which can be joined with this process. Some like metal applications occur but most often the process is used with dissimilar metals and in particular with joint made to copper electronic components. In high temperature applications such as thermocouple wire, diffusion may occur forming brittle compounds which may weaken the joint. Often a third transition metal is applied to reduce this.This slide just lists a few of the weld parameters used for different wires with varying sizes.In order to make a good weld, the capacitor must be charged sufficiently. The amount of energy required is dependent on the cross sectional area of the joint, properties of the metals (electrical resistively and thermal conductivity in particular) and the depth top which the metal needs to be melted. This equation calculates the amount of energy in joules that the capacitor can store.This series of diagrams shows what happens during the process. The top curve is displacement. The parts are separated initially by some amount and as they are brought together through regions D1, D2, D3 and D4 up to the maximum displacement. The time is divided into three regions related to D1, D2 and D3. During the D1 time period the voltage is initially high when the parts do not touch. As soon as the nib touches, the voltage drops and current begins to flow rapidly (causing an arc), reaching a peak and then dropping as the capacitor is discharged. This arcing duration is the D1 time. When the parts fully contact, the arch is extinguished and the remainder of the current flow causes resistive heating, D2 time. Metal is expulsed during this time. Because of the percussive nature, the parts continue to move together during the forging duration, D3. The time for all this action is very short, perhaps just a few milliseconds. After the maximum displacement is reached, because of the recovery (recoil)from the impact loading, the displacement reverses and moves back to the final displacement. There may even be some harmonic damping occurring as the parts come to their final displacement. If the recoil is too great it can actually cause joint defects thus the proper selection of electrical and mechanical parameters is important for proper weld formation.Again, the arc time shown in red here can be controlled by a number of factors listed here. Generally the shortest weld time while still giving sufficient melt penetration is recommended.The weld mechanical properties as a function of this arc time are presented here. A beauty of this process is that this process is quite forgiving in arc time setting and there is a wide window as seen here. For too short to too long an arc time, however, properties are degraded.Here we see a representation of the heat affected zone in these welds. In generally they are extremely small, but somewhat dependent on the melting temperature of the various metals and their thermal conductivity. Because the HAZs are narrow, this process can be used for heat treated materials and heat sensitive components.The peak currents are quite high with this process as shown. Usually we dont have to worry about polarity with nib end being attached to either the positive or negative side of the capacitor. In some cases as noted the nib is made positive for the conditions noted.The welding force can be applied by a number of different mechanisms. Gravity is one of the most popular, but electromagnetic forces or other mechanical devices can be employed. The force must be strong enough and rapid enough to accelerate the parts to be welded over a short distance. Often damping devices are attached to minimize the rebounding.Here is an example of a gravity unit. The part to be welded is clamped into the end of the arm as illustrated here for the thermocouple.Here is an example of a spring loaded unit. This unit can be used in an out of position mode.The advantages of percussion welding are listed here.There are some limitations of this process as listed here.Let us now take at look at a variation of the process using a magnetic force to apply the acceleration and load and using an AC transformer to provide the current.Here is what the device looks like. Note that there is an air cylinder used to give the initial force to bring the workpieces into contact. When this happens. Current from the transformer begins to flow to both make the weld and to supply electromagnetic force to a ferromagnetic armature which causes the remainder of the acceleration and force application. Of course this needs to happen in just the first few cycles of the transformer in order to get the weld and force to occur. Thus, this action is extremely rapid.Here is a comparison of the current and force cycles applied in conventional resistance welding and the magnetic force method. With conventional resistance welding, the force is held constant and current cycles flow, but with the magnetic force method the percussive force pulsates as the current pulses occur.Here are some typical magnetic force percussion weldments. Electrical contacts are very common applications. These contaces might be silver or gold contacts welded to copper parts.This is a typical magnetic force percussion welding equipment.Again, here is an example of the weldment.The arc time for this magnetic percussion welding is a function of several facrots as listed here inclusing the magnitude of the magnetic force and timing, the mass of the machine and parts, and the current density through the projections or nibs.The acceleration of the movable head is directly proportional to the magnitude of the magnetic applied force and inversely proportional to the head mass. Often machine manufacturers try to make low inertia heads in order to increase the acceleration obtainable.This is a micrograph of a typical weld. Notice the small heat affected zone.