Gas Metal Arc Welding (GMAW)

Introduction

Gas Metal Arc Welding or GMAW is an arc welding process that produces the coalescence of metals by heating them with an arc between continuous filler metal electrode and the workpiece.  Shielding is obtained entirely from an externally supplied gas or gas mixture. The weld area is that shielded by an effectively inert atmosphere of argon, helium, carbon dioxide or various other gas mixtures. The consumable wire is fed automatically through a nozzle into the weld arc. Originally developed to weld aluminum, it has become one of the most popular arc welding processes.

Equipment

 Gas metal arc welding equipment consists of a welding gun, a wire-drive system, a shielding gas supply, and a power supply which pulls the wire electrode from a spool and pushes it through a welding gun. A source of cooling water may be required for the welding gun. In passing through the gun, the wire becomes energized by contact with a copper contact tube, which transfers current from a power source to the arc. A system of accurate controls is employed to required the initiate and terminate the shielding gas and cooling water, operate the welding contactor, and control electrode feed speed. The basic features of MIG welding equipment are shown in figure below. The MIG process is used for semiautomatic, machine, and automatic welding. Semiautomatic MIG welding is often referred to as manual welding.

 

Principle of process

This system of welding was developed to weld metals thicker than ¹/₄" that Tungsten Inert Gas (TIG) welding could not weld.  Despite of this matter, MIG welding was limited to only thick metal applications.  However, some adjustments were made and the possibilities for welding various metals were gained. This welding procedure is very close to TIG welding in that an inert shielding gas is used to protect the weld and the arc is maintained between the electrode and the work piece. The difference comes from the fact that the electrode is consumed in MIG welding.  The electrode is actually a long wire that is fed through a hole in the tip of the handle that carries the current to the work piece and supplies a filler metal. The fact that the electrode is continuous makes the welds that are produced very well in quality. There is no need to start and stop to replace an electrode as in SMAW and long welds can be completed in one step.  The entire process can be automated and all welding positions can be used on any thickness of metals.  The process is easy to learn and there is little or no spatter and no slag is created in MIG welding which makes surface preparation for painting very easy.

 

Evaluation of Result

Two of the most prevalent quality problems in GMAW are dross and porosity. If not controlled, they can lead to weaker, less ductile welds. Dross is an especially common problem in aluminum GMAW welds, normally coming from particles of aluminum oxide or aluminum nitride present in the electrode or base materials. Electrodes and workpieces must be brushed with a wire brush or chemically treated to remove oxides on the surface. Any oxygen in contact with the weld pool, whether from the atmosphere or the shielding gas, causes dross as well. As a result, sufficient flow of inert shielding gases is necessary, and welding in volatile air should be avoided.

 

In GMAW the primary cause of porosity is gas entrapment in the weld pool, which occurs when the metal solidifies before the gas escapes. The gas can come from impurities in the shielding gas or on the workpiece, as well as from an excessively long or violent arc. Generally, the amount of gas entrapped is directly related to the cooling rate of the weld pool. Because of its higher thermal conductivity, aluminum welds are especially susceptible to greater cooling rates and thus additional porosity. To reduce it, the workpiece and electrode should be clean, the welding speed diminished and the current set high enough to provide sufficient heat input and stable metal transfer but low enough that the arc remains steady. Preheating can also help reduce the cooling rate in some cases by reducing the temperature gradient between the weld area and the base material.

 

Safety Aspect

Gas metal arc welding can be dangerous if proper precautions are not taken. Since GMAW employs an electric arc, welders wear protective clothing, including heavy leather gloves and protective long sleeve jackets, to avoid exposure to extreme heat and flames. In addition, the brightness of the electric arc can cause arc eye, in which ultraviolet light causes the inflammation of the cornea and can burn the retinas of the eyes. Helmets with dark face plates are worn to prevent this exposure, and in recent years, new helmet models have been produced that feature a liquid crystal-type face plate that self-darkens upon exposure to high amounts of UV light. Transparent welding curtains, made of a polyvinyl chloride plastic film, are often used to shield nearby workers and bystanders from exposure to the UV light from the electric arc.

Welders are also often exposed to dangerous gases and particulate matter. GMAW produces smoke containing particles of various types of oxides, and the size of the particles in question tends to influence the toxicity of the fumes, with smaller particles presenting a greater danger. Additionally, carbon dioxide and ozone gases can prove dangerous if ventilation is inadequate. Furthermore, because the use of compressed gases in GMAW pose an explosion and fire risk, some common precautions include limiting the amount of oxygen in the air and keeping combustible materials away from the workplace.