# WHAT IS THE EFFECT OF AMPERAGE AND POLARITY IN THE GMAW (MIG-MAG) PROCESS? ## AMPERAGE (WIRE FEED SPEED) In the GMAW process, amperage is directly controlled by **wire feed speed (WFS)**. As wire feed speed increases, amperage increases proportionally. ### Effects of amperage variation: | Parameter | Low Amperage | High Amperage | |—|—|—| | Penetration | Shallow | Deep | | Deposition rate | Low | High | | Bead width | Narrow | Wide | | Heat input | Low | High | | Transfer mode | Short-circuit | Spray/Globular | **Low amperage** can cause: – Lack of fusion – Incomplete penetration – Unstable arc – Excessive spatter (in short-circuit transfer) **High amperage** can cause: – Burn-through on thin materials – Excessive reinforcement – Undercutting – Distortion due to high heat input — ## POLARITY ### DC

Knowledge and control of these variables are essential for producing consistent welds of satisfactory quality. The variables analyzed in this article are not completely independent — they are interconnected with other variables that will be covered in upcoming articles, since a change in the value of any one of them generally requires a change in one or more of the others in order to produce the expected results. Skill and sound experience are required to select the optimal values for each variable in a particular application. These optimal values are affected by the base metal type, electrode composition, welding position, and quality requirements; therefore, there is no single set of parameters that will provide optimal results in every case.

WELDING CURRENT OR AMPERAGE.

When all other variables are held constant, welding amperage varies with electrode feed speed or melt-off rate in a nonlinear relationship. When electrode feed speed is varied, welding amperage will vary in a similar manner when using a constant voltage power source. At low current levels for each electrode size, the curve is approximately linear; however, at higher welding currents — particularly with small electrode diameters — the curves become nonlinear, progressively increasing at a higher rate as welding amperage increases. This behavior is attributed to the resistive heating of the electrode extension beyond the contact tip. The relationship between electrode feed speed and welding current is also affected by the chemical composition of the electrode. The different positions and slopes of the curves are due to differences in melting temperatures and electrical resistivities of these metals. Electrode extension also affects these relationships. When all other variables are held constant, an increase in welding current (electrode feed speed) will result in the following:

• An increase in weld penetration (bead depth and width)

• An increase in deposition rate

• An increase in weld bead size.

POLARITY

The term polarity is used to describe the electrical connection of the welding gun relative to the terminals of a direct current power source. When the gun cable is connected to the positive terminal of the machine, the polarity is known as direct current electrode positive (DCEP), also referred to as reverse polarity. The vast majority of GMAW applications use reverse polarity (DCEP). This condition provides a stable arc, smooth transfer of weld metal, relatively low spatter, good weld bead characteristics, and greater penetration depth over a wide range of welding currents.

Straight polarity — that is, direct current electrode negative (DCEN) — is rarely used because axial spray transfer is not achievable. Straight polarity has the clear advantage of high melt-off rates, but these cannot be exploited because the transfer mode is globular. With steels, transfer can be improved by adding a minimum of 5% oxygen to the argon shielding gas (required in special alloys to compensate for oxidation losses) or by treating the wire to make it thermionic (which increases the cost of the filler metal). In both cases, deposition rates are reduced, eliminating the only real advantage of this polarity. Nevertheless, due to the high deposition rate and reduced penetration, negative polarity has found some application in surfacing and hardfacing operations.