What is Percussion Welding?

MicroWelding Percussion welding systems provide a means for rapid, accurate butt welding of many materials that would be impossible to join by any other process.  The basic percussion welding phenomenon overcomes many incompatibility problems formerly presented by materials of divergent size, composition, melting point, and columnar strength. The system controls permit the handling of a wide range of material types and sizes.

The Percussion Welding process uses a brief but intense arc temperature in performing a weld.  It is safe for temperature-sensitive electronic components because it uses an extremely short duration energy discharge at low voltage levels.  Mechanical shock is kept to a minimum as only a small force is required to bring the wire in contact with the mating material. Thermal shock is also minimized because of the brief pulse duration and its intense concentration in a small area.

Since weld integrity is independent of contact resistance and the melting point of the materials, the percussion welding technique can be used to weld many combinations of dissimilar metals with widely varying melting points.

Weld schedule development can occur rapidly as a general understanding of the parameters develops.  The range of control present in the equipment and the ease of operation establishes direct control over the process and guarantees a high degree of repeatability between welds, even by relatively untrained operators.

As with any joining process, percussion welding involves certain process variables;  these variables are handled quite simply once understood.  They fall into two basic categories:  materials and process controls.

MATERIALS

Since the materials to be welded are heated by the intense arc temperature rather than resistance to current flow, the opposing faces of materials with different melting points or thermal conductivity may be successfully welded. For example, copper can be as easily welded to steel as it is to aluminum. Furthermore, the relatively low impact force permits the direct attachment of stranded wire to solid or other stranded materials.

Generally, thin flashes, platings, or surface finishes have little effect on the weld. Conversely, the materials should be as clean as practical of oil films and non-conductive oxides. Good electrical contact to the parts is necessary for consistent power delivery.

Odd alloys pose no problem except when they may contain high percentages of lead, zinc, or other very low melting temperature metals that tend to vaporize at the temperatures induced by the arc.

The use of inert shielding gases during welding improves weld strength when employing materials sensitive to oxygen or hydrogen enrichment (e.g. tantalum, thermocouples, some clad materials) and improves surface melt appearance.

The welding arc is drawn between the two opposing points.  Useful material size range is limited by the smaller of the two materials (the opposed piece may be the same size or hundreds of times larger).  The process is essentially a butt-welding one.

The percussion phase of the process requires the free motion of one of the workpiece materials.  Constraining it inconsistently produces varying results.  Parts positioning and gripping should be carefully controlled.  While special preparation of wire ends is not necessary, the ends should be consistent and a flush cut is desired.  When presenting wire to a flat surface or to another wire, always avoid an angle, as this could cause skidding during the weld resulting in a poor bond.  Slippage of one (or both) parts is frequently used, but it should be consistent to prevent too much movement or bounce during cooling and to maintain good initial electrical contact.

Special fixturing is frequently employed to handle oddly shaped parts and increase production rates. In addition to meeting the above requirements, it should permit a concentric flow of weld energy to the impact point so that the arc shape remains even. Usually, such fixtures are made of materials chosen for conductivity and wear resistance. While copper is often used, beryllium copper or stainless steel are sometimes used where the ability of the wire holder to resist wear is important.

The actual fixture members should provide low resistance contact to the workpiece.

PROCESS CONTROLS

ACTUATOR: The actuator setting controls the output voltage to the electromagnetic actuator.  The voltage is continuously variable from 0 to 150.  Increasing the actuator velocity decreases the “burn” time and increases the impact force.  Excessive velocity can cause “splashing” of the moltenized metal at the time of impact.

DELAY:   The delay is 0.5 to 5.0 milliseconds between actuation and discharge of weld energy.  Initiating the arc at an earlier point in the cycle provides more heating time on the weldments. The optimum setting for timing is determined by developing a welding schedule. The DELAY and ACTUATOR settings work in concert to determine the arc burn time.

WELD:  The weld energy voltage control determines the amount of voltage applied to the weld capacitors. The available voltages are 0-150. The optimum setting for any particular type and size of the material is determined while developing a weld schedule. WELD voltage determines the peak current and the total energy delivered to the weld.

CAPACITANCE: There are three capacitance ranges labeled 1, 2, and 3.  The ranges may be set up for regular or heavy-duty storage.  See the specifications on page 14 of this manual for the bank values. Increasing the capacitance increases the total energy delivered to the weld.

POLARITY: Polarity may or may not affect the weld. In most cases where the polarity has an obvious effect on the operation, incorrect polarity will produce either an excessively explosive weld or no weld at all. In some cases, the difference may be less obvious; therefore, it is recommended that welds be made at each polarity and tested for strength. An example of this is that when copper is being welded to another metal, the copper is usually set as the “+” polarity.

PULSE WIDTH:  The long/short pulse width switch mounted on the front panel provides the operator with pulse width control. A long pulse delivers less peak energy for a longer time than a short pulse. This feature increases the range of Model 858’s capability in joining a wider variety of material combinations and sizes.

SLIPPAGE: (hardware adjustment on the wire holder) When the arc is struck, the ends of the workpiece become molten. One of the pieces is then accelerated into the other, fusing the molten ends. If one of the workpieces is allowed to slip on impact, the impact force can be slightly reduced and the oxides and inclusions can more easily be expelled from the inter surface.  If there is no slippage, the actuator velocity setting will be more critical, or some of the molten metal may be splattered from the inter surface, reducing the weld bead and causing a weaker weld joint. The exact amount of slippage is not as important as ensuring that the same amount of slippage occurs with each weld.

GAP SETTING:  (hardware adjustment on the bench fixture)  The correct gap (air space separating the workpieces) varies with the type and size of the materials. Therefore, it is quite difficult to establish a fixed rule. Generally speaking, the most useful gap is between .010″ and .040″, however, the gap normally doesn’t exceed the diameter of the smaller workpiece.  The gap setting, along with the actuator velocity, determines the amount of force on impact.

 

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