Resistance and Percussive Arc Welding - page 5

2.4 Arc Time and Heat Affected Zone

Arc time is the time interval which begins when the arc is initiated and ends when one work piece strikes the other and the arc is quenched.

Factors affecting arc time include the work metal or combination of work metals, the mass of the moving work piece and moving parts of the machine, nib dimensions, welding voltage and current, welding force and synchronization of arc initiation with the application of welding force.

The shortest arc time that will permit the formation of a sound metallurgical bond with some penetration of the work piece is generally used to minimize heating effects on adjacent areas of the work pieces. Typical arc times in percussive welding are from .5 to 1.5 milliseconds.

Because of the short arc time, the heat-affected zone is very shallow. For capacitor- discharge welding, it is often only about .0015 to .005 inches. In percussive welds between metals that have widely different melting temperatures, the heat-affected zone may be only a few millionths of an inch in the higher-melting temperature metal and .015 to .025 inches in the lower-melting temperature metal.

2.5 Welding Energy

The charge on the capacitor(s) and the voltage give an approximate measure of the welding energy expended at the joint in the arc discharge. This energy can be calculated by the following equation: W=1/2CE2, where W is energy in watt-seconds or joules, C is capacitance in farads, and E is voltage in volts.

The amount of energy used in making a percussive weld depends on the cross-sectional area of the junction, the properties of the work metal or metals and the depth to which metal is melted into the work pieces.

Welding current, or the arc-discharge pattern, in percussive welding varies with the application and is not usually measured. However, current peaks of 400 amps are equivalent to nearly 1/2 million amps per square inch on a .032 inch diameter wire.

Polarity is of no consequence in making percussive welds between work pieces made of the same material and having the same cross-sectional area, but can drastically affect the welding of dissimilar metals or materials of different cross-sectional area. In the welding of metals of different melting points, the metal having the highest melting point or greatest cross-section area is normally given the POSITIVE polarity.

The selection of polarity is of special importance in the percussive welding of unlike metals that differ greatly in melting temperature and is used to minimize the depth of heat-affected zones in the lower melting point metal.

The temperature difference of the two work pieces relative to polarity is explained by the effect of the electron bombardment of the anode during the arc discharge. This electron bombardment of the anode causes the anode to become intensely hot, attaining a temperature of approximately 3600° Kelvin (3326° centigrade). Although the temperature of the cathode is much lower than this, it will still be hot enough to melt most metals.

2.6 Welding Force

The force used in percussive welding is difficult to measure, because it is dynamic rather than static and depends on the velocity and mass of the moving work piece and the moving parts of the machine.

To produce good welds, the welding force must be adjusted empirically until the proper weld quality is achieved. Welding force can be supplied by an electromagnet, gravity, a cam-actuated direct drive, or a spring, depending on the type of welding machine and parts being joined.

2.7 Arc Starting

Three methods of starting the arc are used in percussive welding.

In high-voltage start, the arc is started by applying to the work pieces a direct current voltage, which is high enough to overcome the resistance of the air in the gap between the work pieces as one moves toward the other. The air is ionized and the flow of welding current is started.

In the RF-START method, the process involves superimposing a high-frequency, high- voltage alternating current on low-voltage direct current across the gap between the work pieces. The high-frequency field ionizes the air in the gap producing an arc, and the low-voltage direct current from the capacitors maintains the are. This arc-starting method is used on some low-voltage capacitor discharge percussive welders. It eliminates the need for preparing a nib on one of the work pieces

In the third method, the STARTER NIB is prepared, as shown in Figure 5A, on one of the work pieces by cutting it at an angle, or in the shape of a chisel tip. The low-voltage direct current supplied by the capacitors, when the two work pieces come together, will create enough heat to melt the nib, which is heated so rapidly that an explosion of molten particles occurs. This explosion helps to further form the electric arc, which then spreads progressively over the junction.

2.8 Progress of the Percussive Weld

As the work pieces approach contact and the capacitors discharge, melting the nib, the intense heat of the arc raises the work surface interface to melting temperature in a fraction of a millisecond. When one work piece impacts against the other at high velocity, molten metal is expelled from the work surface interface, and the work pieces are forged together to complete the weld. The sequence of steps is shown graphically at the left of Figure 5.

Percussive Weld

fig. 5

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