Welding Processes: A Closer Look at GMAWFebruary 12, 2015
One of the most important and versatile industrial welding processes is gas metal arc welding or GMAW. This crucial process allows for the welding of all commercially available alloys and metals as well as being compatible with all welding positions. Nevertheless, like every other welding process it has its own set of advantages and drawbacks.
What Is GMAW?Gas metal arc welding (GMAW) utilizes a continuously fed, consumable wire electrode and a power supply to create an electrical arc between the electrode and the workspace, which in turn heats the metals, allowing them to join. A shielding gas is also used to protect the weld from environmental contamination. If an inert gas is used then the welding process is often referred to as “MIG” welding for metal inert gas. If an active gas is used then the process is often called “MAG” welding for metal active gas. The process may also be referred to by its metal transfer mode. For example "GMAW-P" is pulsed gas metal arc welding. We’ll examine the metal transfer modes below. As stated above GMAW can utilize inert or active gas as the shielding gas and has several metal transfer modes. It usually uses a constant voltage, direct current power system, but it may also utilize alternating current and a range of different amps and volts as well as different diameter wire electrodes. Finally, it may be a semi-automatic process with a human operator or it can be fully automatic for greater productivity. This range of options means that when properly configured for its purpose GMAW can be used with just about any industrial metal or alloy and in any welding position.
Metal Transfer Modes for GMAWGMAW may use one of the following metal transfer modes: Globular - The globular metal transfer mode uses carbon dioxide as its shielding gas, which is advantageous since carbon dioxide is less expensive than argon, the other major shielding gas. Additionally the globular mode has a high deposition rate, allowing for faster weld speeds. However, globular also has a tendency to produce a great deal of heat compared to other modes, often creates irregular or uneven welding surfaces, is prone to spatter, requires thicker workpieces, and must be used on flat or horizontal weld positions. These drawbacks make it one of the least used GMAW variations for industrial welding. Short-Circuiting - The short-circuiting transfer mode is often known as SCT or short-arc GMAW. In this mode, the molten droplets of metal actually bridge the space between the electrode and the weld pool, thus extinguishing the arc. However, almost immediately the surface tension between the molten bead and the weld pool causes the bead to be pulled off of the electrode and the arc reignites. This process happens at a rate of about 100 times per second and is not visible to the human eye, causing the arc to appear constant instead. However, the process does require a slower wire feed rate. It also has the benefit of being able to be used on thinner pieces of work-metal than the globular method; however, it can still only be used on ferrous metals and when used on thicker metals may result in insufficient weld penetration and a lack of fusion. Spray - The spray transfer mode is the original transfer method for GMAW and was developed in the 1940s to allow for the welding of non-ferrous metals such as aluminum. In this transfer mode the welding electrode is rapidly passed along a stable electric arc to the workspace, resulting in a better weld finish with very little or no spatter. This is possible because at higher currents and voltages the molten droplets change from globules to smaller droplets and eventually to a vaporized steam. However, this requires more heat and a larger weld pool, which typically means that the workpiece must have a thickness of at least a quarter inch or more. The large weld pool also limits the weld positions possible. Pulsed-Spray - The pulsed-spray transfer mode, often called simply pulsed, pulsed gas metal arc welding, pulsed MIG, or GMAW-P, is a variation on the spray transfer mode. However, instead of using a stable current it instead uses a pulsing current. This allows one molten metal droplet to fall per pulse. The average current is also lower, thus reducing heat and allowing for a smaller weld pool. The lower heat and smaller weld pool allow for welding on thinner piece of metal and in all weld positions. This makes GMAW-P one of the most useful and popular industrial welding processes. We'll go into more detail about it in a subsequent article.
The Advantages of GMAW
- GMAW can be fully automatic, resulting in higher productivity.
- GMAW can be used for all metals and alloys.
- GMAW can be used in all weld positions.
- GMAW produces lower levels of fumes as compared to FCAW or SMAW.
- GMAW requires less operator skill than SMAW.
- GMAW utilizes a continuously-fed electrode which in turn minimizes defects since no restarts are required.
- GMAW doesn't use a slag, reducing post-weld clean up.
- GMAW has good weld penetration, allowing for good strength with smaller weld sizes.
The Disadvantages of GMAW
- GMAW uses relatively complicated, expensive equipment compared to other processes.
- GMAW is less portable than SMAW.
- GMAW cannot be used in areas with a draft or outdoors since this would dissipate the shielding gas.
- GMAW is less suited to smaller, constricted spaces due to the nature of the welding torch and need for the gas shielding to be relatively close to the weld area.
- GMAW requires very clean, rust-free base metals.
- GMAW has lower deposition rates than FCAW when welding out of position.
- GMAW requires a careful setting of process parameters to avoid fusion defects, especially on thicker base metals.