The Difflock Guide to MIG/MAG Welders

Introduction

Mig Welders (also known as MAG or MIG depending on gas use) offer considerable improvements both in ease of use and control over their Arc Welder counterparts. In fact, with the correct wire and gas combination (more on this later) they can be used for welding Mild Steels (even thin sheet), Aluminium and even Stainless Steel.

However, there is a small penalty to pay for this increased usefulness and it comes in the form of the machine’s extra complexity and higher cash cost of both initial purchase and ongoing use.

Nonetheless though, MIG Type Welders are now the machines of choice for millions of home users and professionals alike, especially where automotive work is concerned, since they are excellent for thin sheet metal work such as body panels.

Basic principles of operation

At the most basic level, MIG welders are similar in principle to ARC Welders in that they use an electric arc to melt and fuse metal surfaces and a sacrificial ‘electrode’ together. However whereas an ARC Welder uses a simple, short metal rod that has to be replaced frequently as it is consumed, MIG welders use a thin metal wire that is fed, by an electric motor, continuously into the melting weld pool. Because this wire is supplied in a very long coiled length, it lasts for many hours. The wire will be made of an alloy appropriate to the metal being welded and as we have already said, this could be steel, Aluminium or Stainless Steel.

But, just as in Arc Welding, something has to be done to keep Oxygen gas (from the atmosphere) reacting vigorously with the hot, molten weld pool and so spoiling the weld. Arc Welders use a special coating on the welding rod (caused flux) that melts and covers the weld pool to act as barrier between it and the surrounding oxygen. Unfortunately, this coating cools to form a hard ‘slag’ that retains heat (hence increasing distortion) and can also become trapped in the weld, seriously weakening it.

MIG Welders tackle the Oxygen problem in an entirely different way, by effectively blowing it away from the Weld pool with another gas! But this time it’s a gas carefully chosen for its inert properties and that means it will not react with the molten weld pool.

Take a look at the simple diagram on the right and you’ll see what we mean. As the electric arc melts the metal surfaces and wire to form the molten weld pool, the special protective gas is blown out of a surrounding Nozzle and so shields the weld pool from that pesky Oxygen.

There’s an important distinction to be made here. Conventional Oxy Acetylene (Ethyne) gas welders actually burn the highly flammable Acetylene gas with Oxygen to generate intense heat. With MIG welders the heat is generated by the electrical power from the welder NOT from the inert gas (which does not burn and is NOT flammable).

The Gas Shield and MIG vs MAG

However, there are some snags with this gas shield method.

One snag is that the shielding gas itself can easily be blown away and so successfully welding outdoors on breezy days, or even indoors in strong draughts, can be near impossible. This is where good old Arc Welders have a distinct advantage since they are largely unaffected by air turbulence, but as we will see later, MIG type Welders can be sneakily adapted to overcome air turbulence too.

Another snag is that highly inert, unreactive gases are relatively rare and so are comparatively expensive to isolate and comparatively expensive to buy. By way of example, one such gas used is Argon and it is in fact a pure element so cannot be ‘manufactured’. It does however occur naturally as a tiny percentage of the air we breathe and so it can be isolated industrially by cooling large quantities of air down until it reaches the one unique temperature where Argon alone condenses to form a liquid and so can be collected. By storing it in pressurised bottles the Argon can be prevented from expanding and forming a gas again at room temperatures. The pressure that builds up as the stored gas reaches room temperatures is harnessed and used to expel gas from the bottle and drive it through the Mig Welder and out onto the arc. However if uncontrolled the Argon would scream out of the pressure bottle like the exhaust from a rocket engine and so it first passes through a device called a Regulator that carefully controls the amount of escaping gas. See later for more details on Regulators.

We now know then that an inert gas is required to successfully MIG weld, but in addition to Argon, several other gases are also used depending on the type of metal being welded. In fact, some of these gases are combined in various mixtures to give the best balance between performance and economy. Whatsmore, these gases are supplied in either small disposable bottles (for occasional hobby use), or standard gas bottles (for prolonged and professional use). Whilst small disposable bottles are commonly available (see our online store), standard gas bottles have to be hired from specialist companies and are much bulkier, heavier and difficult to handle, but are far, far more economical for the serious, regular user or those with a large project to undertake.

Specialist gas companies often produce their own recommended mixtures of gases, depending on application. However, here’s a rough guide to help you select from the many available:

As can be seen from the table, it is essential that the correct gas mixture is selected depending on the metal to be joined. Try welding Aluminium or Stainless Steel with Carbon Dioxide, and the results will be as bad as if no gas was used at all.

In fact, although the name MIG stands for Metal Inert Gas, only Argon is truly inert and therefore only welds made with pure Argon are true Metal Inert Gas, or MIG welds. In the case of Carbon Dioxide and mixtures containing Oxygen, the gas itself (more correctly the Oxygen within the gas) actually undergoes a limited reaction with the molten weld pool and so is said to be active, hence welds of this type are said to be Metal Active Gas, or MAG welds. As can be seen, the term MIG or MAG simply denotes how the gas used reacts with the weld pool.

A good example of this occurs in the use of pure Carbon Dioxide gas when welding mild steels. Carbon Dioxide is actually used because it is very easy and cheap to produce, but it is not totally inert and so reacts with the mild steel weld pool to produce spatter (molten droplets expelled from the weld pool). The poorer the purity of Carbon Dioxide used, the more lively is this reaction. This is one of the reasons why so-called ‘pub gas’ (bottles of Carbon Dioxide borrowed from your local, friendly pub landlord) can produce inconsistent and untidy welds.

To get round the reactive properties of Carbon Dioxide, professional gas mixtures for welding mild steel also include up to 5% Argon, which ‘calms’ the otherwise lively nature of the weld pool.

Regulators and controlling the gas shield

As mentioned earlier, the Regulator is an essential part of all MIG welders so always check that any MIG welder you buy comes complete with one. There are several varieties of regulator too, so again here’s a quick guide to each:

Note that all three types of Regulator above are available for both small, disposable gas bottles and standard gas bottles, but they are not interchangeable between these bottle types. Typically, smaller MIG welders are supplied with a brass Regulator and contents gauge (a simple pressure gauge) to fit small, disposable gas bottles, whereas larger MIG welders (for serious home or professional use) are supplied with a brass Regulator and contents gauge to fit standard bottle types.

If you are welding frequently and want to achieve the best balance between high quality welds and economic gas usage, then we strongly recommend you invest in a Regulator with a built-in flow gauge in addition to a contents (pressure) gauge since only a true flow gauge (not a pressure gauge) will allow you to accurately measure, and therefore control, the amount of gas flowing out of the welding nozzle when actually welding. You can then use a little trial and error on test welds to achieve the minimum necessary gas flow whilst maintaining a high quality weld.

A flow gauge will also give an early indication of blockages within the gas tube or welding nozzle.

MIG Wires

Now that we understand how the gas is used and controlled we can turn our attention to the actual welding wire. As stated earlier, this wire is stored on coils and fed continuously to the actual weld pool by an electric motor. The wire can be mild steel, aluminium or a stainless steel alloy to match the metal being welded and also comes in varying diameters depending on the size and thickness of weld required.

There are some critical factors that affect the success of MIG weld and these relate to both wire and machine so lets look at these in more detail.

Firstly, the wire itself must be absolutely spotless and corrosion free. In MIG welding cleanliness is most definitely next to godliness and so the slightest trace of corrosion, paint, oil etc on the wire will affect the weld quality. This also goes for the metal actually being welded so make sure you thoroughly clean surfaces that you are MIG welding and be doubly sure your MIG wire is in tip top condition.

Better quality mild steel MIG wire comes with a very fine copper plating to both aid conduction and prevent surface corrosion. Even so, never store any type of MIG (even Aluminium) in damp environments such as garages, sheds or unheated workshops. It goes without saying that the MIG welder itself shouldn’t be stored in these conditions either but don’t leave MIG wire in the machine if you are laying it up for long periods between use, just in case. Corroded MIG wire will not only give poor quality welds, but also wear and contaminate the torch head and liner.

The actual diameter of the MIG wire is critical too because it needs to be pushed along between rollers by that electric motor. The rollers will be grooved to exactly fit a specific diameter of wire (typically 1.0mm, 0.8mm or 0.6mm) and it is extremely important that the wire and rollers are machined to be a precision fit or problems with inconsistent wire feed can result. Cheap rolls of wire may well not have a precise and consistent diameter so be careful what you buy!

When selecting a wire diameter, you should bear in mind the thickness of metal to be welded. As a rough rule of thumb, 0.6mm diameter wire should be selected for welding thinner sheet such as vehicle bodywork whereas 0.8mm wire should be used for thicker chassis sections and fabrication work. Note that 1.0mm thick wire is usually only compatible with high power professional machines. Again whatever diameter you choose make doubly sure your machine has the correct size of grooved rollers.

Lastly, as well as being supplied in different diameters, Wires are supplied on different size drums depending on the amount required. Not all weights of drum fit all MIG welders, especially if you are buying the large 15Kg sizes so you should double check before you part with your cash. Even if your welder can accommodate it we recommend that you don’t buy a large 15Kg drum of wire if you are not welding frequently because stored for long periods and forgotten you may find the dreaded corrosion sets in and ruins it.

Here’s a handy reference guide to the types and sizes of wire commonly available.

Note wires above 0.8mm diameter are generally only used on high power industrial machines and are therefore too large for the majority of home welders.

MIG Welder Output Power

Having considered the ancillary equipment, we can now think about the design and construction of the MIG welder itself.

MIG welders are ideal for welding thin sheet metals but for maximum flexibility you should choose a machine that will also weld reasonably thick plate. Although not a perfect rule of thumb, it’s generally true to say that the larger capacity machines also tend to be better designed and made.

To give a good balance between both heavier fabrication and thin panel welding, we recommend that you choose a machine with an output of at least 130 Amps (Note though that welders with outputs greater than this require a dedicated power supply as described in part 1 of this series). Just as with ARC welders, all this power generates heat in the welder’s transformer so if you are going to be using it for prolonged periods make sure it comes with a cooling fan (often called "Turbo" models). Related to this is the so called Duty Cycle which in simple terms is the percentage of time the welder can actually be used in any given period without it automatically switching off to cool down. For instance, a duty cycle of 40% at 120 Amps means that you could weld for up to 40 minutes in any 100 minute period at a 120 Amp output setting. In practise you won’t actually be welding continuously for 40 minutes (!!) but rather as you stop and start to complete different weld runs, the time the welder is ‘resting’ should average 60 percent of the time to prevent it switching off to cool. Copper wound transformers tend to produce less waste heat and so have better duty cycles than aluminium equivalents so these are a bonus.

The welder must be fitted with a thermal overload device to switch it off automatically if it gets too hot. Check too that it has a strong steel case and for smaller machines is balanced when lifted by its carrying handle.

Higher power welders should be wheel mounted otherwise they are a back injury waiting to happen.

Bottle and Wire Mounting

Larger welders usually have a rear platform and strong retaining chain so that standard gas bottles can be mounted on them. Remember what we said earlier about the pressure of the gas inside both standard and disposable bottles. If a bottle falls onto its side, either because it was poorly secured or the welder overbalances, the Regulator could be sheared off and the resulting rapid release of gas will be extremely dangerous. Never use a gas bottle that is not very, very firmly secured and make sure that your welder cannot be pulled or knocked over if it has a gas bottle mounted on it.

As we have said, larger welders are wheel mounted so look very, very carefully at these wheels and ask yourself if they look tough enough to stand the weight of the welder AND a gas bottle, especially the larger, standard sized bottles. A flimsy or poorly fixed wheel could break off during use and tip a bottle over with catastrophic consequences.

Open up the machine and check the ease with which drums of MIG wire can be mounted. Some manufacturers claim that horizontally mounted drums make for better feeding of ‘soft’ wires such as Aluminium but for most home users Aluminium welding will be occasional.

Look carefully too at the actual wire feed and rollers. The rollers should ideally both be of metal construction and stamped with the size of the precision-machined groove that will match the MIG wire diameter. The pressure exerted by these rollers has to be carefully set when setting up the welder prior to use so check that the machine allows the roller pressure to be varied and that this control is simple and straightforward (typically it’s a knurled knob).

The Trigger control and Torch

Take a look at a MIG welder and you will see that it has an earth clamp and cable rather like that found on a simple ARC welder. However, whereas an ARC welder has a simple holder and connecting cable for its welding electrodes, a MIG welder has a much more sophisticated trigger controlled ‘torch’ head and a flexible connecting tube down which shielding gas, welding wire and electrical current must flow.

The idea here is that the torch head is placed in close proximity (a few millimetres) to the metal being welded and the trigger is pressed. This pushes wire out of the Torch head’s nozzle and switches on the power so ‘igniting’ the welding arc. At the same time shielding gas is released out of the torch head and over the arc and weld pool. Note that on poorer quality welders, whilst the wire feed and gas release is trigger controlled, the actual power for igniting the arc is always on. This is not particularly desirable since it can lead to the inadvertent striking of surface damaging Arcs if the torch head gets too close to the work piece.

The nozzle within the torch head forms the final electrical connection to the wire and so it is important that it is kept clean and in good condition. Just like wire feed rollers, nozzles are precision matched to the diameter of welding wire being used so check that the stamped diameter on the nozzle exactly matches that of the wire you are using. All nozzles however, deteriorate with time, especially with novice operators and so will need to be replaced periodically. Again check that spare nozzles are readily available for any machine you buy.

MIG welder setting and the Golden Trinity!

Before we look at the other ‘critical’ components of the machine we have to spend a little more time considering how MIG welds are achieved.

With ARC welders the use of higher power settings and thicker rods enabled thicker metals to be welded. However with MIG welding the wire diameter is fixed and therefore to cope with higher power settings the wire speed is increased so that it feeds from the torch head faster.

Here then we find the Golden Trinity of successful MIG welding. This Trinity says that assuming you have a good quality welder with the correct wire and gas, there are three critical variables that are the key to success. These are:

Output Power – More correctly this is the actual voltage and current from the welder that strikes and sustains the intensely hot arc. Typically, MIG welders have a low power setting (for thin sheet work) and several intermediate power settings up to the highest setting (for thicker metalwork).

Wire Speed - These different output power setting are set using switches and as each is selected, the wire speed is also changed to suit. If the wire speed is too fast, the wire doesn’t fully melt into the weld pool and will instead fuse to it, extinguishing the arc and pushing the torch head away from the weld. If the wire speed is too slow, it simply melts too quickly and recedes back into the torch head, again extinguishing the arc or worse still burning the torch head.

Operator technique - This then leaves the actual skill of the operator to ensure that welds are strong and successful. This skill will only come with practise and we strongly recommend that novice operators seek formal tuition in welding techniques at a reputable training centre such as a local technical college.

Diagram 1 illustrates perfectly how getting any one of these three factors wrong will result in failed welds.

Diagram 1

Zone A
The correct output power is selected to give good penetration in the thickness of metal being welded. The operator is using an adequate technique but the wire speed is either too slow (so the wire melts too fast and the arc extinguishes) or too fast (so the Mig Wire is ejected from the Torch nozzle faster than the Arc can melt it hence the Mig wire will touch and fuse to the weld pool and the arc will again extinguish).

Zone B
The output power is correct for the thickness of metal being welded and the wire speed perfectly matches this. However the operator’s technique is poor and the resultant weld is not uniform, lacks penetration (torch moved too fast) or burns holes (torch moved too slow).

Zone C
The operator is using their best technique and the wire speed matches both this and the power setting. However the power setting is either too high or too low for the thickness of metal being welded. Too high a power setting and the resultant weld will burn holes in the work piece or produce very high spatter welds. Too low and the resultant weld will be ‘surface deep’ only and lack adequate penetration to give adequate strength.

Zone D
The sweet spot! The operator’s technique is adequate and the power setting and wire speeds perfectly match both the job at hand and the operator’s technique.

Now take a look at Diagram 2

Diagram 2

You can see that the area in Zone D is now much bigger. This means that for the same operator, producing good quality welds in Zone D is more easily achieved than before. But if the operator hasn’t got more skilled what has changed to make things easier? The answer is that Output Power and Wire Speed circles have moved closer together. In practical terms, this means that each now has greater flexibility than before, each now has a wider range of possible settings giving a greater chance matching these to the operator and the job at hand.

So, in Diagram 1 the MIG welding machine may only have had three power output setting and three possible wire speeds. This limits the choice of settings for the operator. However in Diagram 2 the MIG welding machine is a better model and has say six output settings with a wire speed that is infinitely variable so the operator can more easily set the machine to match the job at hand and their personal technique.

What this all adds up to say is that when you choose a MIG welder, make sure that both its output power and wire speed can be varied, and look for models that have a greater number of settings than others. Average models allow you to vary the output power and the wire speed is then automatically varied to suit that power. But better models also allow you to further vary wire speed by hand to fine tune its speed to the work piece and your technique. We strongly recommend you ONLY buy a MIG welder that allows you to vary the wire speed by hand IN ADDITION to any automatic selection that occurs.

Consumables

Unlike simple ARC welders, there are some parts of MIG Welders that should be considered consumables and these require regular inspection, maintenance and replacement if the machine is to produce consistently high quality welds.

Luckily, these for the most part are associated with the torch head and the tubular liner that carries wire and gas to this head.

The torch head and the precision nozzle within it should always be handled with care. Dropping it to the ground or laying it down where it can be tripped over or trod on are recipes for disaster. Over time too, the head, nozzle tip and gas outlet can become clogged with weld spatter, especially in the hands of a novice welder. Regularly inspect these components and remove any build up of spatter, a special tool is cheaply available to assist this. It may also help to apply an anti spatter spray to the torch head before each use.

The actual tube that carries the MIG Wire from the reel to the torch head also deteriorates over time as the passage of wire wears it or leads to a build up of deposits. Corroded or kinked wire can also lead to problems and the end result of all these factors is that the wire feed becomes erratic and so impinges upon weld quality. If this starts to happen the only solution is to replace the liner and perhaps the torch head too. On most home MIG welders this means buying spares specific to the machine and part dismantling the machine to effect replacement. However, higher quality welders, and certainly professional models, allow the torch and liner to be rapidly replaced since they are fitted with a rapid release ‘Eurotorch’ connector that allows standard torches and liners from any manufacturer to be used.

MIG welding outdoors

Remember we said that some MIG type Welders can be sneakily adapted to overcome air turbulence that would otherwise blow away the gas shield? Well they actually do this by pretending to be ArcWelders! Instead of a plain wire and gas combination, these welders use a special wire that carries a flux within itself. Just as in Arc welding, this flux melts onto the weld pool and shields it from atmospheric Oxygen.

Hence no inert gas shield is required and this gives rise to these types of welders being called ‘Gasless Mig’. Whilst this makes these welders much more versatile for welding outdoors, the penalty is the introduction of slag (which may become trapped in the weld pool) and a greater amount of spatter than with true MIG/MAG welding. Beware too since only machines designed and built to be ‘gasless’ can be used in this way. If you want the best of both worlds then it s possible to find dual purpose Gas/Gasless MIGS that are capable of both forms of welding.

Protect your vehicle from costly damage

In our introduction to welding earlier we talked about the need to protect yourself and your surroundings from the effects of welding. However, you also need to protect your vehicle since all and any electronic devices within it can be damaged during welding. Such devices include the alternator, car alarm, CD player, Radio, Air bag system, Engine management system and the multitude of sensors that feed them.

These are at risk not from the normal voltage and current given out by a welder during use, but instead from the very large voltage spike that occurs unavoidably when the arc is switched off

This spike occurs because the high current path through the welding cables, the weld pool and the arc itself will set up a significant electromagnetic field around itself. When this field collapses very suddenly as the Arc is switched off it will induce a voltage pulse in nearby electrical circuits even if they are electrically isolated from the actual welding current path.

The size of this pulse is determined by a whole host of factors but long connecting cables are particularly good at suffering from this induction effect, hence the need to disconnect them from sensitive circuits.

The welder's transformer itself also generates a big spike as its electromagnetic field collapses when the arc is switched off. This is directly carried into the component being welded and so distributed to anything connected to it.

Some mechanics disconnect the battery during welding in the mistaken belief this will protect those sensitive systems on the vehicle from voltage spikes. Others think that positioning the welding earth clamp near to the weld will offer protection.

In fact neither of these methods makes any difference to the voltage spikes ability to cause irreparable damage. The only way to guarantee protection is therefore one of three ways in order of effectiveness:

1. Remove the object to be welded from the vehicle and weld it before refitting it.
2. Fit a surge protection device such as those available from our online store.
3. Disconnect all sensitive equipment from long connecting cables.

Having said all this, welding a vehicle without taking such precautions doesn't mean it will always be damaged.

It does however mean you are risking damage and given the cost of replacing components such as the alternator, car alarm, CD player, Radio, Air bag system, Engine management system you have to ask yourself whether it’s a risk that is really worth taking.

By the way, if you are having someone else weld the vehicle for you, always check everything works before you give them the vehicle and mention this to them before they start work. If something doesn't work when you get the vehicle back you'll know whom to blame.

Try our Store for a great selection of welders and consumables

We hope you have now learned enough to decide what type of welder best suits your requirement. You should also now be armed with enough information to tell a good welder from a bad one. Those of you familiar with Difflock will know that we go to great lengths to carefully select quality products for our online store. If you would like to see Arc and MIG welders that we recommend, as well as a whole host of competitively priced consumables, then why not visit the welding section of our online store