ELECTRIC WELDING PART – 07 - PERCUSSION WELDING – ADVANTAGES & DISADVANTAGES

A percussion weld is similar to flash weld in which the required pressure is provided by a hammer like blow.

A short application of high intensity energy instantly heats the surfaces to be joined. This rapid heating is almost immediately followed by a quick blow to make the weld.

Generally ‘percussion’ welding is the term used in electronics industry for joining wires, leads and other items to a flat surface.

It is a high-speed welding process which can be used to produce either a fusion or forge (solid phase) weld. An arc is struck between a wire and component (a fuse body for example).

In this process, coalescence is produced simultaneously over the entire area of surfaces to be joined by the heat obtained from the arc. The arc is produced by a short pulse of electrical energy. Pressure is applied during or immediately following the electrical discharge.

There are two methods of percussion welding (i) the capacitor discharge and (ii) magnetic force methods.

Capacitor discharge method is again classified into two types (i) high voltage, low capacitance system and (ii) low voltage and high capacitance system.

Magnetic force percussion welding machine usually has an independent power source.

Metals which can be percussion welded include, copper alloys, aluminum alloys, nickel alloys, low and medium carbon steel and stainless steel.

Gold, silver, copper-tungsten, silver-tungsten and silver-cadmium oxide are percussion welded to copper alloys for commonly used electrical contacts. 

The equipment generally consists of press type resistance welding machine with specially designed transformer to obtain low voltage.

To produce the weld force electro-magnetic system is used. An air cylinder provides the initial force to bring the parts together.

In the capacitor discharge method, parts to be joined are connected to the capacitor and then forced together by a spring or by compressed air.

As the gap between the parts closes to a distance which the voltage can break down, an arc of high intensity is initiated and superficial melting takes place before the parts impact together. 

PRINCIPLE

Heat is generated by a high current arc between the two points to be joined. The high current melts a thin layer of the metal and the surfaces are brought together in percussion manner to get welded.

PROCESS

1. Remove the dirt, oil, paint or grease on the surfaces.

2. Work pieces are clamped into the fixture or machine.

3. Light pressure is applied on the parts and bring the faces together.

4. The arc developed due to high current strike the faces to be joined with high voltage to ionize the gap between the parts or melt and vaporize the projections on one part.

5. The arc established heats the faces of two parts to the welding temperature.

6. Move the parts together with the force, which may be spring, gravity, pneumatic or electromagnetic.

7. It extinguishes the arc while the parts are welded together.

8. Release the force and unclamp the welded parts.

ADVANTAGES

1. Parts with different thermal conductivities can be easily welded. 

2. Very quick process time, usually takes less than 0.1 second.

3. It causes very little damage to the material which is close to the weld.

4. Without any danger of annealing, hardened surfaces may be welded.

5. Heat balance is not a problem because the heat is concentrated at the ends of the work pieces.

DISADVANTAGES

1. The process is limited to butt welded joint only

2. The joint used in limited to about 1.5 to 3 cm² because of control of paths of arc is difficult.

3. The process cannot be used for welding heavy sections larger than 600 mm².

APPLICATIONS

1. This process joins similar or dissimilar metals.

2. This process is used for welding fine wire leads to filaments in lamps and terminals.

3. This welding is useful to make large contact assemblies for relays etc.

4. It can join wires and rods by butt joints.                                    

5. This process is used to attach metal tips to valve stems.