What Is GMAW?
GMAW is a commonly used welding process. It has become one of the most widely used processes due to its relatively low cost of purchasing standard equipment, fast welding time, and ease of use. It is a flexible welding process and can be modified to suit the weld requirements. It is also suitable for many different steel and alloy materials, adding to its benefits.
There are many variables involved in GMAW, so it is recommended to use GMAW specific Welding Procedure Specifications to ensure the weld is to standard.
GMAW was developed in the 1940s for welding aluminium and other non-ferrous materials but was used on steels soon after. It was found to provide faster welding compared to other processes.
The high cost of inert gas limited its use in steels until active gases like carbon dioxide and argon/carbon dioxide mixes became more common. In the current day, GMAW is the most common industrial welding process. It is favoured for its versatility, speed, and ability to adapt to robotic automation.
GMAW is commonly known as Metal Inert Gas welding (MIG) or (MAG) Metal Active Gas welding. An electric arc generates a continuous feed of electrodes that are shielded by gas, which is applied separately.
This process requires an electric power supply, an electrode feed-wire unit, and the shielding gas. The wire and gas are fed through the welding torch. The arc length is regulated by maintaining a constant voltage power supply in conjunction with a wire feed speed unit.
This process may be semi-automatic or mechanised using a constant voltage, direct current power source. Alternatively, constant current or alternating current systems can be used. GMAW’s electrode-based system is one of the most efficient welding procedures.
When to use GMAW
The four primary methods of metal transfer in GMAW are globular, short-circuiting, spray, and pulsed-spray. These all have distinct properties and of course, have their advantages and limitations.
GMAW is mainly used for metal parts where it is imperative to reinforce the strength of the joint. It lends itself to a range of solid and metal cored electrodes for a range of parent materials. The process can be modified from semi-automatic to mechanized, to fully automatic robotic systems.
- Carbon steel
- Copper alloys
- Nickel alloys
- Stainless steel
- Silicon bronze
- Tubular metal-cored surfacing alloys
There are many advantages to choosing the Gas Metal Arc Welding method. It has the ability to produce great quality welds, at a reasonable price. Generally, the cost per weld metal length is lower than other open arc weld processes. Additionally, it’s applicable to a range of both non-ferrous and ferrous alloys of varying thickness.
GMAW produces high-quality welds at high-speeds. Although there are both positives and negatives to bare wire GMAW, the initial outlay for solid welding wire is relatively low compared to FCAW or MCAW wires. The variety of welding gas mixes suitable for use with GMAW provides flexibility across areas where, for example, certain gases like argon are expensive. In a busy shop-floor with multiple welders and high output, these costs can add up very quickly!
Welders can weld as fast as they desire, as there is no stopping and starting due to the continuous feed of electrodes. The ability to easily mechanize the process also minimises operator fatigue and increases arc on time which can have a positive effect on profit.
There is no job too big or too small for GMAW and it can replace other joining methods such as riveting, brazing, silver-soldering, or resistance welding.
The operating system for GMAW is much more efficient for the welder than other methods.
Equipment is simple to set up and control; the Gas Metal Arc Welding equipment has been designed with controllability in mind. The welder must only watch the angle of the torch on the material, electrode stick out, control the travel speed, and weld pool shape.
A lesser amount of distortion is one of the main benefits for the welder. There is minimal weld slag and spatter when solid wire or metal-cored wires are used, making for quick and easy cleanup. The lack of slag and spatter generally just makes for a smoother weld surface.
These characteristics provide substantial cost savings since metal finishing is an expensive production exercise. Another safety point is that it creates less fumes and produces a lower heat input in comparison to other forms of welding.
The weld is not restricted to position or by material. It can be facilitated through any position on light or heavy gauge materials. For a heavier material, a narrower groove angle with a thicker root face can be used. A narrow groove reduces the amount of filler metal deposit required and therefore cuts preparation time and welding time.
Current density is the amperage per square inch of the cross-sectional area of the electrode. With GMAW, the current concentration is higher than it is for stick electrode welding. This allows for higher deposition rates than there would be in manual welding. High current density concentrates more energy at one point than one with low density.
GMAW is less portable than other welding methods. Due to the requirement of a gaseous shield, GMAW is usually carried out in enclosed or partially enclosed areas to prevent the shielding gas being blown away. Some GMAW wire feeders are detachable from the power source and are able to be taken into small spaces, although the guns can be bulky for accessing tight weld areas.
GMAW uses materials that can become contaminated easily by seemingly innocuous pollutants such as rust, dirt, paint, and dust particles. Proper cleanliness and maintenance measures must be implemented in the shop-floor to prevent contamination of GMAW/MIG materials.
Components of GMAW
There are 3 main components to a Gas Metal Arc Weld, a wire feeder, a gas shield, and a power circuit.
The quality of the wire is one of the most important elements of the process. A poor quality wire has a larger impact on diameter tolerance and therefore the wire is not as consistent. As the welding machine delivers relatively constant amps and volts, if the ‘power to weight ratio’ is constantly fluctuating around the arc, the result is an erratic arc.
Gas Shielding System
The gas shielding system is not that complex. Gas is delivered via a regulator and flow control meter on the cylinder. It then goes through an on/off solenoid which is turned on and off with the trigger so it coincides with the wire feed, thus, the gas is delivered down to the torch. A common mistake by welders is to turn the gas up excessively.
Having the gas too high can create just as many problems as having it too low. Too low means that the weld will not be adequately shielded. Gas flow that is too high results in turbulence whereby the gas will hit the plate and vortex.
This will drag some of the atmosphere into the shielding gas which can cause porosity in the weld. This also means you are consuming too much gas.
Poor connections within the power circuit can create issues with the weld. The power source can cause serious damage to the machinery and affects the overall quality of the weld. If your electric circuit isn’t maintained properly or set up correctly, this can also be hazardous to welders. Having a welding inspector oversee the process is the best way to ensure that all procedures are running to standard.
There are many variables that need to be adjusted when welding with the GMAW process
that all have different effects on the finished weld profile. These include:
- Arc length
- Characteristics of the shielding gas
- Electrode extension
- Gas flow
- Joint preparation
- Size and type of filler wire
- Speed of travel
- Work and travel angle
- Wire-feed speed (the current)
If the travel speed and welding current are not providing adequate weld metal to fill in a particular joint, they must be adjusted accordingly. Arc voltage is very important to create a good weld. Ultimately, arc voltage controls the surface heating, bead contour and some defects such as undercutting, porosity, and discontinuities.
To complete the weld bead, the arc needs to be manipulated to prevent what is called a ‘weld crater.’ This is achieved by welding to the end of the required weld length, and then reversing the direction of travel for approximately an inch before releasing the gun trigger or terminating the arc.
GMAW Welding Procedure Specifications
A well developed GMAW WPS will guide welders through the trusted and accepted procedures they need to create a weld to standard. It is developed for the specific materials and welding techniques being used. With so many variables in the GMAW process, it is always best to follow a specific GMAW procedure, written by an expert welder.
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