welding process that produces coalescence with an arc resulting from a rapid discharge of electrical energy. Pressure is applied percussively during or immediately following the electrical discharge.
The electrical energy is stored in a capacitor or group of capacitors at a relatively high voltage and discharged directly, or through a transformer, to the part to be welded. Discharge is initiated by closing a mechanical or electronic switch.
Percussion welding is the process used in the electronics industry for joining wires, contacts, leads, and similar items to a flat surface. However, if the item is a
metal stud that is welded to a structure for attachment purposes, it is called capacitor discharge stud welding.
In applying the process, the two parts are initially separated by a small projection on one part, or one part is moved toward the other. At the proper time, an arc is initiated between them. This arc heats the faying surfaces of both parts to welding temperature. Then, an impact force drives the parts together to produce a welded joint. There are basically two variations of the percussion process: capacitor discharge and magnetic
force.
Although the steps may differ in certain applications because of process variations, the essential sequence of events in making a percussion weld is as
follows:
(1) Load and clamp the parts into the machine.
(2) Apply a low force on the parts or release the driving system.
(3) Establish an arc between the faying surfaces
(a) with high voltage to ionize the gas between the parts or
(b) with high current to melt and vaporize a projection on one part.
(4) Move the parts together percussively with an applied force to extinguish the arc and complete the weld.
(5)Turn off the current.
(6) Release the force.
(7) Unclamp the welded assembly.
(8) Unload the machine.
Operation. Welding heat is generated by an arc between the two parts to be joined. The current density is very high, and this melts a thin layer of metal on the faying surfaces in a few milliseconds. Then the molten surfaces are brought together in a percussive manner
to complete the weld.
Capacitor Discharge
With the capacitor discharge method, power is furnished by a capacitor storage bank. The arc is initiated by the voltage across the terminals of the capacitor bank (charging voltage) or a superimposed high-voltage pulse. Motion may be imparted to the movable part by mechanical or pneumatic means.
Magnetic Force
For magnetic force welding, power is supplied by a welding transformer. The arc is initiated by vaporizing a small projection on one part with high current from the transformer. The vaporized metal provides an arc path. The percussive force is applied to the joint by an electromagnet that is synchronized with the welding current. Magnetic force percussion welds are made in less than one-half cycle of 60 Hz. Consequently, the
timing between the initiation of the arc and the application of magnetic force is critical.
Advantages of Percussion Welding
The extreme brevity of the arc in both versions of percussion welding limits melting to a very thin layer on the faying surfaces. Consequently, there is very little upset or flash on the periphery of the welded joint, only enough to remove impurities from the joint.
Heat-treated or cold-worked metals can be welded without annealing them. Filler metal is not used and there is no cast metal at the weld interface. A percussion welded joint usually has higher strength and electrical conductivity than a brazed joint. Unlike brazing,
no special flux or atmosphere is required.
A particular advantage of the capacitor discharge method is that the capacitor charging rate is easily controlled and low compared to the discharge rate. The line power factor is better than with a single-phase a-c machine. Both these factors contribute to good operating efficiency and low power line demand.
Percussion welding can tolerate a slight amount of contamination on the faying surfaces because expulsion of the thin molten layer tends to carry any contaminants out of the joint. Figure P-3 shows several electrical contact designs joined by magnetic force
percussion welding.
Limitations
The percussion welding process is limited to butt joints between two like sections, and to flat pads or contacts joined to flat surfaces. In addition, the total area that can be joined is limited, since control of an arc path between two large surfaces is difficult.
Joints between two like sections can usually be accomplished more economically by other processes. Percussion welding is usually confined to the joining of dissimilar metals not normally considered weldable by other processes, and to the production of joints where avoidance of upset is imperative. Another limitation of this process is that two separate pieces must be. joined. It cannot be used to weld a ring from one piece.
Safety
Mechanical. The welding machine should be equipped with appropriate safety devices to prevent injury to the operator’s hand or other parts of the body. Initiating devices, such as push buttons or foot switches, should be arranged and guarded to prevent them from being actuated inadvertently. Machine guards, fixtures, or operating controls should prevent the hands of the operator from entering between the work-holding clamps or the
parts to be welded. Dual hand controls, latches, presence-sensing devices, or any similar device may be employed to prevent operation in an unsafe manner.
Electrical. All doors and access panels on machines and controls should be kept locked or interlocked to prevent access by unauthorized personnel. When the equipment utilizes capacitors for energy storage, the interlocks should interrupt the power and discharge all
the capacitors through a suitable resistive load when the panel door is open. A manually operated switch or other positive device should also be provided in addition to the mechanical interlock or contacts. Use of this device will assure complete discharge of the
capacitors.
A lock-out procedure should be followed prior to working with the electrical or hydraulic systems. Personal Safety Equipment. Eye protection with suitable shaded lenses should be worn by the operator.
When the welding operations produce high noise levels, operating personnel should be provided with ear protection. Metal fumes produced during welding operations should be removed by local ventilating systems.