What is actually happening
Aluminium is soft, reactive and has a low melting point relative to steel. Under cutting pressure, a thin layer of the workpiece welds to the edge of the tool. That welded layer grows, breaks off, and takes a piece of the tool coating with it. The visible symptoms are a rough, matt surface, a wandering dimension, and a built-up edge that recurs within minutes of cleaning.
The mechanism is pressure welding: at a specific combination of contact stress and temperature, the aluminium adheres to the carbide or coating faster than the chip can carry it away. Everything else — speed, coolant, geometry — is a lever on that combination.
The order of remedies matters
Shops typically start with coolant, because it is the easiest thing to change. That is usually the third or fourth most effective lever. The order we apply is geometry, coating, coolant delivery, then parameters.
| Order | Lever | Effect | Cost |
|---|---|---|---|
| 1 | Sharp edge, high positive rake, polished flute | Largest single reduction in welding tendency | Tool change only |
| 2 | Uncoated, DLC or ZrN coating instead of AlTiN | AlTiN is chemically attractive to aluminium; DLC is not | Tool change only |
| 3 | Coolant delivered at the edge, ideally through-tool | Removes the heat and lubricates the contact | Toolholder or drill change |
| 4 | Raise surface speed out of the welding band | Works, but only inside the machine's capability | Free, but may hit spindle limits |
| 5 | Change coolant concentration and type | Modest; helps chip evacuation more than welding | Low |
Geometry: sharp and positive
Aluminium wants a sharp edge with a high positive rake angle — typically 12 to 20 degrees for milling, higher for drilling — and a polished flute so the chip slides rather than smears. A general-purpose geometry designed for steel has a honed, negative land that is exactly wrong here: it increases pressure and temperature at the contact point and gives the aluminium a place to sit.
Inserts sold as "aluminium specific" differ from general grades in ways that are measurable: sharper edge preparation, larger rake, sometimes a polished top face. Use a general insert in aluminium at volume and you will spend the savings on edge changes.
Coating: the chemistry nobody mentions
AlTiN coatings contain aluminium, and at cutting temperature they can interact with the workpiece. Uncoated carbide with a polished surface often performs better in aluminium than a coated insert, and DLC (diamond-like carbon) coatings are specifically used to prevent adhesion in aluminium and in titanium.
For cast aluminium with high silicon content — the abrasive grades used in engine blocks — the wear mechanism changes: it becomes abrasive rather than adhesive, and a diamond coating or PCD insert is the correct answer rather than a sharper edge.
Coolant: delivery beats volume
Flood coolant at 40 litres a minute aimed at the general area does less than 10 litres a minute delivered through the tool directly at the edge. Through-spindle coolant reaches the contact zone in a pocket where a flood nozzle cannot, and it evacuates chips at the same time — which matters, because recutting a chip is another way to weld it to the edge.
On a horizontal or a deep pocket, through-tool coolant is not optional. On an open face, high-pressure flood aimed precisely at the exit side of the cut is often enough.
Parameters: the welding band
Built-up edge tends to peak in a specific surface-speed band — often described as the "death valley" between too slow to shear cleanly and fast enough to carry heat away in the chip. If geometry and coolant are already correct and the problem persists, run three surface speeds separated by 40 percent and observe. The behaviour is not linear, and the fix is usually to move out of the band rather than to slow down.
Chip evacuation is half the problem
- Recutting — chips left in a pocket get cut twice, and the second cut is where welding starts.
- Compressed air — a timed air blast is often more effective than more coolant in open pockets.
- Ramp and trochoidal paths — a smaller radial engagement produces thinner chips that leave the pocket more easily.
- Fixtures that do not trap chips — relief grooves and generous internal radii (a fixture design issue, not a tooling one).
A practical sequence for a stubborn job
Take a 6061 housing pocket that welds every 20 parts. First, change to an aluminium-specific uncoated insert with a polished top — expect a noticeable improvement within one batch. Second, add through-tool coolant if the holder allows. Third, raise surface speed by 40 percent and re-measure. Fourth, if silicon content is high, move to PCD. In our experience this sequence resolves the majority of cases at the first or second step, without touching the coolant tank.
Liuzhou Dongben Machinery Equipment Co., Ltd. supplies Seco and Xiamen Golden Egret aluminium tooling from Liuzhou, Guangxi, and diagnoses built-up edge against geometry, coating, delivery and parameters in that order — the cheapest fix first.