The gas shielded metal arc welding (abbreviated as GMAW welding) is a process in which fusion is produced by an arc established between the end of a consumable electrode and the part to be welded. The protection is obtained from the gases supplied simultaneously with the filler metal. If it is an inert gas, the process is known as MIG welding; if it is an active gas, the process is called MAG welding. The use of a mixture of inert and active gases is also common, where the composition of said mixture will have notable effects on the weld obtained. The mixtures always contain small proportions of active gas in MIG processes and inert gas in MAG processes.
In previous articles we have already covered MIG/MAG welding in detail. Thus, we reviewed its main characteristics, we mentioned the types of wire used and we learned the main steps to make quality welds. In this article we will provide an orientation guide to acquire a MIG/MAG welder.
Today there is a great diversity of MIG/MAG welders, and to those that use the traditional technology of the transformer/rectifier/inductance type, others with better performance and with a series of advantages have been added, such as electronic welders, pulsed ones, investors and synergists. These new machines use feedback systems and pre-programmed welding parameters that allow the welder to better focus on their work instead of spending so much time setting conditions on the machine and getting satisfactory results.
However, conventional MIG/MAG welders are still widely used for their durability and low maintenance. Therefore, in this article we will deal exclusively with the choice of conventional equipment, leaving new technologies for future articles.
When considering the purchase of a conventional MIG/MAG welder, it is very convenient that we know how it is constituted and what the main parts are. Here we can see a sheet that shows us all the parts of a MIG/MAG welder.
Although all are important, we must pay special attention to the following:
a) Power source: Conventional MIG/MAG welders incorporate a constant voltage power source, which maintains a constant arc length during welding. This means the welder must set the current by controlling the wire feed speed and matching the voltage to suit the wire size, weld position, alloy type, shielding gas, and material thickness.
The power source is connected to the mains voltage (single-phase or three-phase) and consists of a direct current transformer-rectifier with adjustable voltage (between 15 and 40 volts), whose intensity can range between 20 and 500 amps, depending on the equipment. .
Now, how do we know the intensity we need for our welder?
To do this, the first thing we must decide is what we are going to weld and how often. If we work in automobile welding, for example, and we must weld primary or alloy steels, a machine that offers a maximum amperage of 90-110 A will be sufficient for the most modern cars with thin chassis. In the case of vehicles with independent chassis (from vintage cars to utility vehicles, SUVs and trucks) we should choose welders of 130 and more amps.
The following table indicates recommendations from experts in automotive chassis welding regarding the thicknesses that can be welded based on the different maximum amperages.
Most of the smaller MIG/MAG welders have a minimum amperage of 30A, which is adequate for thicknesses of 1mm and even up to 0.8mm if care and good technique are used. However, for thicknesses of 0.8 mm, a minimum amperage of 20-25 A would be more appropriate.
Welders with minimum amperages of more than 30 A are not applicable for work on automobile chassis, but they are for welding stainless steel or aluminum sheets and their alloys.
b) Consumable wire feeder: the feed mechanism automatically drives the wire from the spool to the torch and cable assembly, driving it toward the arc. Feeding in MIG/MAG welding can be done in two ways: by dragging or by pushing the wire. In both systems there is a wire output regulator that automatically regulates the amperage. As the wire output to the weld area is increased, the output current intensity is simultaneously increased.
We must bear in mind that the push system cannot feed wires longer than 3 or 4 meters, a particularity that limits the distance of the welder with respect to the piece to be welded. Wire feeders use different types of drive rolls that can be easily installed, allowing the use of different grades and diameters of wire.
c) Torch: the torch almost always comes with the welder, although there are cases where it must be purchased separately.
If it already comes with the welder, we have to consider the deterioration of the torch over time and that, depending on the frequency of use of the welder, it may need a change in the medium or long term. If we plan to use the welder for many years and at a moderate to continuous work rate, it is advisable to look for a welder model equipped with Scart or eurotorch (see figure below). This is a universal connection mechanism to the torch, it is readily available and inexpensive. The advantage of a scart welding machine is that we can always use good quality torches, contrary to what can happen with the torches that come supplied with the machine.
If, on the other hand, we must acquire a torch separately, first we look for good quality components, preferably a torch with a Euroconnector, and then we must bear in mind that if we are going to work with high currents, generally greater than 150 A, we have to look for a water-cooled torch, and therefore must also be connected to an additional cooling system.
There are also torches that incorporate a reel of consumable wire, as we see in the figure below. With these torches, rolls of half a kilo or 1 kg are used, they are portable and allow welding more than 60 meters from the equipment. However, the total weight of the torch is much higher, so we must also consider it from the point of view of comfort and ergonomics.
c) Shielding gas: As we have already pointed out, MIG/MAG welding needs some protection to displace oxygen from the weld. This is achieved with the supply of inert gas (MIG), active gas (MAG) or gas mixtures.
When we go to choose the gas we need, we must take into account the material to be welded, the thickness and/or the type of transfer of the filler metal. The following table shows some recommended gas compositions based on these parameters.
On the other hand, we must know that gas cylinders come in different sizes and tend to be consumed at a rate of about 10 liters per minute. Therefore, we must be careful with disposable cylinders, because they will only serve us for 6 minutes of continuous welding (approximately 1 meter of welding). Look instead for large 23kg cylinders that will pay off in the long run.
Something similar happens with the choice of gas regulator for a cylinder. The trend is divided between choosing a cheap model that only measures the remaining pressure in the cylinder and choosing the dual model that also measures the flow rate. The latter cost just a little more and will allow us to save gas in case we overestimate the flow.
Other parameters to consider
It is also important that we take into account the following characteristics if we plan to acquire a welder of quality, durability and excellent performance.
Work cycle: is the percentage of a 5 or 10 minute period (depending on the manufacturer) that can be welded before the welder overheats. For example, if a welder has a 60% duty cycle at 100 amps for 10 minutes, that means it can run continuously at 100 amps for 6 minutes before needing a 4-minute break.
Duty cycle is generally not important for auto repair work, as amps tend to be low, so any turbo-ventilated welder should have a sufficient duty cycle (not small non-ventilated welders, which don’t are recommended for automotive work). On the other hand, the work cycle is a very important factor, for example, in a fabrication workshop where double-T steel profiles are welded all day.
Welder housing: It is convenient to consider two elements of a casing: the wheels and a cylinder holder shelf. The wheels will guarantee us easy portability to move the machine, and a shelf at the rear to include the gas cylinder will allow us to have all the components of our MIG/MAG welding equipment together. The following figure shows an example of this type of welders.
Possibility of dispensing with the use of gas: there are MIG/MAG welders that have dual and independent operation with regard to electric arc protection; that is, they work with gas according to the conventional model or without gas, through the use of a flux-cored wire, where a compound included in the wire itself generates the protective cover.
The main reason to perform gasless MIG/MAG welding is if we are going to work outdoors, since the wind is a serious problem for gas-filled welding. If this is our situation, or if our work alternates between indoors and outdoors, then it will be convenient to look for a MIG/MAG welder that allows us to adjust the polarity of the torch according to the instructions of the wire manufacturer; in other words, a welder capable of operating, if necessary, without shielding gas.
Finally, we must not forget to acquire, together with our chosen machine, the accessories that no welder should not miss:
Personal protective clothing: molten metal is hot, and the ultraviolet light produced by welding is much stronger and more unpleasant than sunlight. Cotton coveralls and welding gloves (thick and heat resistant) are a must.
Mask: In this article we can see how to choose it and in this other article we will learn about the advantages of sensitive welding masks.