Effects of Welding Variables

The effects of the variables are somewhat dependent on the welding process being employed, but general trends apply to all the processes. It is important to distinguish the difference between constant current (CC) and constant voltage (CV) electrical welding systems. Shielded metal arc welding is always done with a CC system. Flux cored welding and gas metal arc welding generally are performed with CV systems. Submerged arc may utilize either.

Amperage is a measure of the amount of current flowing through the electrode and the work. It is a primary variable in determining heat input. Generally, an increase in amperage means higher deposition rates, deeper penetration, and more admixture. The amperage flowing through an electrical circuit is the same, regardless of where it is measured. It may be measured with a tong meter or with the use of an electrical shunt. The role of amperage is best understood in the context of heat input and current density considerations. For CV welding, an increase in wire feed speed will directly increase amperage. For SMAW on CC systems, the machine setting determines the basic amperage, although changes in the arc length (controlled by the welder) will further change amperage. Longer arc lengths reduce amperage.

Arc voltage is directly related to arc length. As the voltage increases, the arc length increases, as does the demand for arc shielding. For CV welding, the voltage is determined primarily by the machine setting, so the arc length is relatively fixed in CV welding. For SMAW on CC systems, the arc voltage is determined by the arc length, which is manipulated by the welder. As arc lengths are increased with SMAW, the arc voltage will increase, and the amperage will decrease. Arc voltage also controls the width of the weld bead, with higher voltages generating wider beads. Arc voltage has a direct effect on the heat input computation.

The voltage in a welding circuit is not constant, but is composed of a series of voltage drops. Consider the following example: assume the power source delivers a total system voltage of 40 volts. Between the power source and the welding head or gun, there is a voltage drop of perhaps 3 volts associated with the input cable resistance. From the point of attachment of the work lead to the power source work terminal, there is an additional voltage drop of, say, 7 volts. Subtracting the 3 volts and the 7 volts from the original 40, this leaves 30 volts for the arc. This example illustrates how important it is to ensure that the voltages used for monitoring welding procedures properly recognize any losses in the welding circuit. The most accurate way to determine arc voltage is to measure the voltage drop between the contact tip and the work piece. This may not be practical for semiautomatic welding, so voltage is typically read from a point on the wire feeder (where the gun and cable connection is made), to the workpiece. For SMAW welding, voltage is not usually monitored, since it is constantly changing and cannot be controlled except by the welder. Skilled welders hold short arc lengths to deliver the best weld quality.

Travel speed, measured in inches per minute, is the rate at which the electrode is moved relative to the joint. All other variables being equal, travel speed has an inverse effect on the size of the weld beads. As the travel speed increases, the weld size will decrease. Extremely low travel speeds may result in reduced penetration, as the arc impinges on a thick layer of molten metal and the weld puddle rolls ahead of the arc. Travel speed is a key variable used in computing heat input; reducing travel speed increases heat input.

Wire feed speed is a measure of the rate at which the electrode is passed through the welding gun and delivered to the arc. Typically measured in inches per minute (ipm) the wire feed speed is directly proportional to deposition rate, and directly related to amperage. When all other welding conditions are maintained constant (e.g., the same electrode type, diameter, electrode extension, arc voltage, and electrode extension), an increase in wire feed speed will directly lead to an increase in amperage. For slower wire feed speeds, the ratio of wire feed speed to amperage is relatively constant and linear.

For higher levels of wire feed speed, it is possible to increase the wire feed speed at a disproportionately high rate compared to the increase in amperage. When these conditions exist, the deposition rate per amp increases, but at the expense of penetration.

Wire feed speed is the preferred method of maintaining welding procedures for constant voltage wire feed processes. The wire feed speed can be independently adjusted, and measured directly, regardless of the other welding conditions. It is possible to utilize amperage as an alternative to wire feed speed although the resultant amperage for a given wire feed speed may vary, depending on the polarity, electrode diameter, electrode type, and electrode extension.

Although equipment has been available for twenty years that monitors wire feed speed, many codes such as AWS D1.1 continue to acknowledge amperage as the primary method for procedure documentation. D1.1 does permit the use of wire feed speed control instead of amperage, providing a wire feed speed amperage relationship chart is available for comparison. Specification sheets for various Lincoln electrodes provide data that report these relationships.

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Filed: Welding Knowledge
tags: Arc, electrode, SMAW, speed, Welding Procedure, welding technology, Wire feed speed