How to Weld Aluminum with a MIG or TIG Machine

Aluminium has a reputation for being difficult to weld, and the reputation is half earned. The metal itself is not mysterious. What makes it feel impossible to people arriving from steel is that three of its properties — a stubborn oxide layer, very high thermal conductivity, and a total absence of colour warning before it melts — each invalidate a habit that steel rewarded. Fix those three things and aluminium becomes a normal metal that happens to have opinions.

The three problems aluminium actually presents

The oxide layer. Aluminium forms an oxide skin on contact with air, instantly and continuously. That oxide melts at a far higher temperature than the metal under it, so if you simply strike an arc on unprepared aluminium you end up melting the base metal beneath an unmelted skin. The puddle looks wrong, contamination is trapped, and the weld is compromised before it starts. Every aluminium procedure begins by dealing with this.

Thermal conductivity. Aluminium moves heat away from the arc far faster than steel does. In practice this means the work fights you at the start of a bead — the puddle is reluctant to form — and then, once the part reaches temperature, heat stops running away and the same settings that were barely working start blowing holes. Recognising that transition mid-weld and adapting to it is most of the skill.

No colour warning. Steel telegraphs its temperature by glowing. Aluminium does not. It looks essentially unchanged right up until the moment the puddle loses surface tension and drops through the joint. There is no amber warning stage to react to, so you learn to read the puddle's shine and movement instead of its colour.

Cleaning and preparation

This step is not optional and the order matters. Degrease first, then remove oxide. Reversing the order drives oil and cutting fluid into the surface with the brush and guarantees porosity.

1. Degrease. Use a solvent or degreaser suitable for aluminium and let it flash off completely. Consult the product's Safety Data Sheet — some common solvents produce hazardous decomposition products when heated by an arc, and chlorinated solvents in particular must be kept away from welding entirely.
2. Remove oxide mechanically. A stainless steel wire brush, dedicated to aluminium and never used on steel, is the standard tool. A brush that has touched steel will transfer iron particles into the joint.
3. Weld promptly. The oxide begins reforming immediately. Clean the joint as part of setting up the weld, not the day before.
4. Keep filler clean too. Filler rod handled with bare, oily hands or stored loose in a drawer brings its own contamination. Wipe rod before use and store it capped.

What your machine must be able to do

MIG: the feeding problem

Aluminium MIG is fast, and for thicker material and production work it is often the right process. The obstacle is mechanical. Aluminium wire is soft; it is easily crushed by drive rolls set for steel, and it buckles inside a long liner instead of pushing through — the classic bird's nest at the drive assembly.

• A spool gun carries a small spool at the torch, so the wire only travels a few inches. This is the standard solution for home and light-commercial work.
• Push-pull torches add a motor at the gun that works with the feeder, allowing longer reach. More capable, considerably more expensive.
• U-groove drive rolls cradle soft wire instead of biting into it. V-grooves are for steel.
• A nylon or graphite liner replaces the steel liner, which abrades aluminium and loads up with shavings.
• Reduced drive tension — enough to feed, not enough to deform the wire.

Technique-wise, aluminium MIG runs hotter and faster than steel. A push angle is generally preferred over a drag angle because it improves gas coverage ahead of the puddle. Travel speed is usually faster than instinct suggests, and once the part heats up you will need to move faster still or back the current down to avoid burning through.

TIG: AC, balance and frequency

TIG is where aluminium becomes genuinely satisfying, because the process gives you independent control over heat and filler. It requires AC, and the two AC parameters worth understanding are balance and frequency.

Balance sets the proportion of each cycle spent in the cleaning (electrode-positive) half versus the penetrating (electrode-negative) half. More cleaning helps on dirty or heavily oxidised material but puts more heat into the tungsten, causing it to ball and degrade. More penetration gives a narrower, deeper weld and a cooler electrode. Dirty material wants more cleaning; clean, well-prepared material wants more penetration.

Frequency changes the shape of the arc cone. Higher frequency narrows and stiffens the arc, which improves control in tight corners and on small fillets. Lower frequency broadens it, which is useful for wider beads and for spreading heat on flat work. Neither setting is correct in isolation — they are adjustments you make for the joint in front of you.

• Set gas flow to the machine and torch manufacturer's guidance, and use a gas lens where the cup and joint allow — aluminium is unforgiving about shielding coverage.
• Use a foot pedal or torch amperage control if you have one. Being able to back off the current as the part heats up is the single most useful aluminium skill.
• Add filler more frequently than on steel. Aluminium puddles want feeding.
• Fill the crater at the end of every bead. Unfilled craters are a classic aluminium crack initiation site.
• Expect to move faster once the part reaches temperature, or reduce current to compensate.

Filler selection and cracking

Filler alloy is not a matter of preference. Different aluminium base alloys respond very differently to different fillers, and some base alloys are crack-sensitive enough that the wrong filler produces a weld that fails. The two common general-purpose fillers are not interchangeable and each suits different base material and service conditions — one is generally more ductile and forgiving, the other offers higher strength and different corrosion and elevated-temperature behaviour.

The correct approach is to identify the base alloy, then consult the filler manufacturer's selection guidance or an aluminium association filler chart for that combination and that service condition. If the part is structural, load-bearing, or carries pressure, this stops being a shop decision and becomes an engineering one — get a qualified opinion rather than a forum consensus.

The last thing worth saying is that aluminium rewards practice on scrap far more than steel does. Settings that work on one thickness of one alloy in one position will not transfer cleanly. Run beads on offcuts of the actual material, at the actual thickness, in the actual position, before committing to the part — then bend and break those test coupons and look at what the fracture surface tells you.