MIG, MAG and TIG welding compared, and what to watch when welding aluminium

What they share: gas-shielded arc welding

In arc welding, an electric arc between the electrode and the workpiece melts the metal and forms a weld pool that joins the two parts. If molten metal comes into contact with oxygen and nitrogen in the air, the weld becomes porous and brittle. That is why the weld zone is covered by a shielding gas in MIG, MAG and TIG alike.

Two basic questions separate the processes: does the electrode melt into the weld, and does the shielding gas react with the weld pool?

MIG and MAG: continuously fed wire

MIG and MAG are two names for the same process: gas metal arc welding. A wire electrode is fed continuously through the torch; it carries the arc and melts to become the filler metal in the joint. Because the welder does not feed filler by hand, the process is fast and efficient on long seams.

The difference is the gas. MIG (Metal Inert Gas) uses inert gases such as argon or helium, which do not react with the weld pool; it is preferred for aluminium and other non-ferrous metals. MAG (Metal Active Gas) uses active gases such as carbon dioxide or an argon–carbon dioxide mix. The active gas affects arc behaviour and penetration; MAG is widely used on carbon and low-alloy steels.

• High travel speed and deposition
• Efficient on medium and thick sections and long seams
• Relatively easy for welders to learn
• Spatter and bead appearance may need more finishing work than TIG

TIG: a non-consumable tungsten electrode

In TIG welding (Tungsten Inert Gas) the arc burns between a non-consumable tungsten electrode and the part. Where filler metal is needed, the welder adds it by hand with a separate rod. Pure argon is the most common shielding gas.

Because heat input and filler are controlled separately, TIG is a very precise and clean process. It stands out on thin material, stainless steel, aluminium and visible welds. The price is speed: TIG is slower than MIG/MAG and demands more skill from the welder.

What do AC, DC and pulse mean?

TIG welding machines can work with direct current (DC) and alternating current (AC). Steel and stainless steel are usually welded with DC. Aluminium is welded with AC: in the electrode-positive (EP) half-cycle the arc breaks up and cleans the oxide layer on the surface, while in the electrode-negative (EN) half-cycle most of the heat goes into the workpiece and provides penetration. In pulse mode the current alternates periodically between a high and a low value, giving better control of heat input and reducing the risk of burn-through and distortion on thin parts.

Comparison at a glance

What to watch when welding aluminium

Aluminium is popular because it is light and corrosion-resistant, but welding it calls for more care than steel. The main points are:

The oxide layer

As soon as aluminium is exposed to air, its surface is covered by a thin oxide layer. This oxide melts at a much higher temperature than the aluminium itself. If it is not removed, it gets mixed into the pool and causes lack of fusion and inclusions. The surface is therefore degreased right before welding and the oxide is removed with a stainless steel brush reserved for aluminium only. The cleaning action of AC in TIG welding exists to deal with the same problem.

High thermal conductivity

Aluminium conducts heat much faster than steel. At the start of a weld the heat spreads quickly into the surroundings; as the seam progresses the part heats up and the pool grows. The welder has to adjust current and speed along the seam. On thick sections, preheating may be needed depending on the alloy and the job.

Porosity

Molten aluminium readily absorbs hydrogen, which remains in the weld as pores when it solidifies. The hydrogen usually comes from moisture, oil and dirt. Clean, dry filler wire, a clean surface and uninterrupted gas shielding are the basis for avoiding porosity. Draughts in open areas can blow the gas shield away.

Distortion and cracking

Aluminium expands more with heat than steel, which increases the tendency to distort after welding. Proper tack welding, welding sequence and fixtures keep distortion in check. Some aluminium alloys are prone to hot cracking during solidification, so it is important to choose a filler wire that matches the base material.

Heat-treated alloys

In aluminium alloys hardened by heat treatment, welding heat can reduce the strength of the material around the weld. This needs to be considered at the design stage; keeping welds away from critical load-bearing areas can be one solution.

The order of welding and machining

If a welded part has dimensionally precise surfaces, the order of operations directly affects the result. Welding heat creates internal stresses and distortion, so a surface machined to size before welding may move afterwards. The common approach is therefore to prepare the parts, weld them and then machine the critical faces, holes and mounting points.

For this sequence to work, the drawing needs enough machining allowance on the surfaces to be machined after welding. The design should also provide faces on which the welded body can be clamped on the machine. When welding and machining are planned together, the reference surface of the body should be agreed at the very start.

Notes for designers and buyers

• Use weld symbols on the drawing: joint type, size and side should be clear.
• Design the geometry so the torch can reach the joint.
• Mark surfaces that must be machined after welding; allowance for distortion is left on these surfaces.
• State the base material's alloy and temper; the filler wire choice depends on it.
• For visible welds, state your appearance expectations in advance.

Welded fabrication at Innotech

In our facility we carry out welded fabrication in aluminium and steel using MIG, MAG and TIG. For TIG welding we use a BLUWELD T-320 P with AC/DC and pulse. Because we also do CNC machining under the same roof, surfaces that must be machined after welding can be planned together with the welded part.

For a welded part or assembly, you can send your drawing and model through the quote form.