Aluminium does not glow through a colour change the way steel and titanium do. There is no dull red warning before the metal turns liquid; it goes from solid to a bright, fluid, mirror-like puddle that reflects the arc straight back at the operator. The puddle has to be read directly, and that takes practice to trust.
It also moves heat away from the weld far faster than steel, and expands more as it heats. The result is a part that pulls energy out of the puddle quickly while cold, then runs away once the surrounding mass has warmed up. Welders describe aluminium as either stealing heat or losing it, rarely sitting still.
Aluminium carries an oxide skin that melts at a much higher temperature than the metal beneath it. Alternating current is used to break that skin up during welding: on the cleaning half of the cycle the oxide is lifted off and pushed to the edges of the puddle, where it can be carried out.
The setting matters as much as the technique. Too much cleaning action and the tungsten erodes and the bead turns black and sooty; too little and the puddle refuses to flow, sitting up in a craggy lump instead of wetting into the joint.
An intercooler end tank shows the problem clearly. The weld starts against a heavy casting or a thick plate that soaks up energy, so the operator pushes more current in to get the puddle moving at all. By the time the joint is halfway round, that same mass is hot, and the same current starts to blow through the edge.
Experienced aluminium welders taper the current as they travel, work in short runs, and let the part settle between them. On large tanks, preheating the mass or simply moving around the joint instead of running it end to end is often more effective than chasing the puddle with the pedal.
Filler is chosen to suit the parent alloy rather than for convenience. Some aluminium alloys are crack-sensitive when welded without the right filler, and mixing grades across a joint is a common cause of a weld that passes a leak check on the bench and cracks after a few heat cycles.
Joint design leans towards lap and flanged joints more than it does on steel. A flanged edge gives the puddle somewhere to sit and presents a consistent thickness to weld to, which matters when the two parts being joined are of different thickness or different alloys.
Tanks and intercoolers have to hold pressure, so the weld has to be continuous, and full penetration where the load is carried. A bead that merely sits on the surface can pass a visual check and still weep at temperature, because thermal cycling opens up small unfused edges over time.
That is why our TIG aluminum welding treats the root as part of the specification rather than a by-product of the bead. The reasoning is the same on every aluminium intercooler we build, where the core and the end tanks have to expand and contract together without opening a path for boost.
Aluminium is unforgiving about oil, coolant residue and swarf. A tank that has been machined, or one that has seen service, will pull contamination out of the surface as it heats, and the porosity shows up as a scatter of small holes across an otherwise sound bead.
Cleaning is a mechanical job first, using a brush kept for aluminium alone and never shared with steel, followed by a solvent wipe. Rags that have touched steel are a common source of the fine iron contamination that blackens an aluminium weld and leaves the part looking neglected.
