Resistance and Percussive Arc Welding - page 4

2.0 Percussive Welding

Percussive welding is a welding process in which heat is obtained from an arc produced by a rapid discharge of electrical energy across a gap, and an impact of the work piece which is applied during or immediately after the electrical discharge. A shallow layer of metal on the contact surfaces of the work piece is melted by the heat of the arc produced between them. One of the work pieces is impacted against the other, extinguishing the arc, expelling oxides and forging the weld.

Arc initiation, arc time, and welding impact are controlled and synchronized automatically. The weld power supply usually is of the capacitive discharge type. The welding impact (forging force) is applied by electromagnetic devices, electromechanical devices, cam-actuated direct drive, springs, or gravity.

The heat generated is intense but extremely brief in time, and is closely localized to the junction. It enables the percussive welding of a small component to a larger one, and of dissimilar metals that differ considerably in electrical resistivity and melting temperature. The electrical resistivity of the parts being welded does not noticeably affect the amount of heat generated at the junction. The arc supplies the heat for welding of the metals.

The work-holding clamp, jaws, or chuck of the welding head need not be a good electrical conductor, as in resistance welding, because the amount of current passed is comparatively small and the duration of current flow is extremely brief. The work-holding clamp material is usually selected primarily for strength and wear resistance. Hardened steel is commonly used.

2.1 Applicability of Percussive Welding

Percussive welding is used for welding fine wire leads to filaments in lamps and
to terminals of electrical and electronic components where a reliable joint is needed to withstand shock, vibration, and extended service at elevated temperature. It is used commonly in the welding of copper to molybdenum for use in the manufacture of power rectifiers, in making telephone and electrical devices, and for attaching large-area contacts to switch components.

Percussive welds can be made a few thousandths of an inch away from glass seals or other heat-sensitive materials without damage to these materials, because the total heat generated is small and can be localized. Percussive welding can be massive or it can be thin metal, as in a capacitor cathode can, or stranded wire, or solid wire. Flat work pieces of any shape can be percussively welded to mating flat surfaces with the aid of an arc-starting nib.

2.2 Design of Work Pieces

The work pieces must be separate objects. The ends of a continuous work piece cannot be joined to make a ring. One of the work pieces of an assembly must be able to be clamped into secure position in the welding head so that it can be impacted against a stationary work piece without slippage.

Capacitor-discharge percussive welding can be used to butt weld wires of similar diameters or of greatly different diameters. For some metals, wire diameter can be as small as .005 inches.

Heat treated, cold worked, or prefinished metals are unaffected by the heat of percussive welding, because the heat-affected zone is very shallow, usually only a few thousandths of an inch.

Cleaning is not critical to the production of sound percussive welds because at least a thin layer of metal is melted from each work piece and expelled from the junction.

Almost any pair of like or unlike metals or alloys can be joined by percussive welding. Work pieces of widely dissimilar composition, melting temperature, electrical conductivity, and thermal conductivity can be readily welded together.

Metals easily welded include copper alloys, aluminum alloys, nickel alloys,
low-carbon steels, medium-carbon steels, and stainless steels. Various combinations of these alloys also have been welded.

Copper may be percussive welded to molybdenum. Although true welds between these two metals at one time were considered impossible because of mutual insolubility, tests have shown copper penetration of .0004 inches into molybdenum at the weld junction.

2.3 Percussive Weld Power Supplies

Three types of power supplies are used for percussive welding. They are: low-voltage capacitor, high-voltage capacitor, and transformer. The low-voltage capacitor and high-voltage capacitor power supplies will be discussed.

Low-voltage capacitors that have high capacitance commonly are used in power supplies for capacitor-discharge percussive welding. The capacitor is charged by direct current from a rectifier or generator, and the welding energy is stored at 50 to 300 VDC and later discharged to make the weld.

The relatively low voltage makes this type of power supply appropriate for use with bench- mounted welding heads since the operator is not exposed to extremely high voltage.

The high-voltage capacitors that have low capacitance are also used to supply power for capacitor-discharge percussive welding. They function electrically the same as the low-voltage capacitors, but store the welding energy at 1000 to 6000 VDC.

High-voltage capacitors can produce a more uniform arc discharge, and the use of
this type of power supply is one way of avoiding the need for the arc-starting nib. The high voltage allows more latitude in controlling the operating variables of the welding process. However, it is more difficult and costly to provide operator protection against voltages that are often above 1000 volts.

A typical capacitive discharge welding circuit is shown in Figure 4 (below).

Capacitive Discharge Weldiong Circuit

Next: Resistance and Percussive Arc Welding - page 5

ROHS Compliant Precision Welding of Leadwires and Resistor Elements, Wireforms and Glass Tube Cutting for Lighting and Electronics

Outline

Contact CIT

Products & Services

CIT Innovations

Company Profile

Resistance and Percussive Arc Welding - page 4

2.0 Percussive Welding

2.1 Applicability of Percussive Welding

2.2 Design of Work Pieces

2.3 Percussive Weld Power Supplies

ROHS Compliant Precision Welding of Leadwires
and Resistor Elements, Wireforms and Glass Tube Cutting for Lighting and Electronics