Billet Quality That Shows Up at the Die
Most of what a die shop is asked to fix starts in the billet. The segregation layer, the gas content, the oxide inclusions and the state of the Mg2Si all arrive at the die before the first bearing is cut, and each of them has a signature: a streak that only appears after anodizing, a blister that only appears after ageing, a pick-up that no amount of polishing removes. This page sets out the billet properties the source treats as decisive, the numbers a foundry can actually hold, and what each one does at the press.
Why billet quality is a press variable, not a purchasing detail
The published position is blunt: billet quality is of prime importance to productivity, and the place to see it is the surface of large, thin sections. A billet that is clean extrudes faster and comes out with a better surface, because the same metal flow that produces the shape is also what drags the surface layer, the inclusions and the gas into the product. Improving the billet is therefore one of the few changes that raises speed and quality at the same time instead of trading one for the other.
The properties, the numbers, and the defect each one owns
| Property | Value the source gives | What it does at the press |
|---|---|---|
| Inverse segregation layer | 120 to 150 µm with conventional DC casting; 50 µm or less with hot top casting | A peritectic-rich skin that flows into the surface and shows as a streak after anodizing |
| Shell zone | Up to 15 mm deep | Feeds preferentially into the wider parts of the section, giving a band that etches and anodizes differently |
| Gas content | Below 0.12 cm³ per 100 litres of aluminium, measured with a Telegas analyser | Hydrogen porosity; blisters that open during solution treatment, worst on 7xxx and thin sections |
| Oxide inclusions | Removed with radiant-tube ceramic filters in the launder | Pick-up and die line initiation; the hard particle that notches the bearing entry |
| Mg2Si state | Coarse precipitates if the billet cools slowly from the homogenization temperature | A weak subcutaneous layer that shears at the die; the void left behind starts a surface crack |
| Homogenization completeness | Iron phase fully spheroidized, not partially | Partial spheroidization is enough to increase pick-up under conditions that would otherwise pass |
| Melt temperature control | Within plus or minus 5 °C, with a melt agitator | Composition uniformity along the log, which decides whether the last billet runs like the first |
| Charge composition | 50 to 60 percent scrap added to virgin ingot | Trace elements accumulate; above the limits they change flow stress and surface behaviour |
Values as published for commercial casting practice. The segregation figures are for the two casting routes named; the gas limit is the control point the source recommends, not a universal specification.
The segregation layer is a surface problem with a depth
Two different layers are involved and they behave differently. The surface segregation zone produced during the first moments of solidification is shallow: about 100 µm on a good 180 mm billet, up to about 1 mm on a poor one, and more where cold shuts or blebs are present. Because it is shallow, normal extrusion flow keeps it out of the product on a good billet. The shell zone is the other one, and it runs up to 15 mm deep, which is why it cannot be ignored: it is thick enough to be fed into the surface wherever the section is wide and the flow into that region is fast.
The practical consequence is that segregation streaks are not a die problem and not an anodizing problem. They are a casting and scalping problem, and the hot top route cuts the inverse segregation layer by more than half against conventional DC casting. A shop buying billet rather than casting it has one lever here: scalp the billet, or buy it scalped.
Gas is a heat treatment problem that starts in the melt
Hydrogen is the only gas appreciably soluble in aluminium, and it enters during melting, casting, cleaning and pickling. It precipitates at inclusions and grain boundaries as molecular hydrogen, which cannot diffuse back out, and it expands during solution treatment. That is why a billet that extrudes cleanly can still produce blisters after ageing, and why the defect is worst in thin sections and in the alloys with the highest magnesium content.
Degassing is therefore a billet specification, not a casting house courtesy. The published control point is below 0.12 cm³ per 100 litres of aluminium, measured in the trough or distributor before and after melt treatment.
What this changes for the die
Three of the defects above end up being blamed on the die, and none of them respond to die steel. A pick-up that survives a freshly polished and freshly nitrided die is usually inclusion-driven. A die line that returns after polishing can be the notch left by a hard particle at the bearing entry. And a surface that tears at a temperature inside the published window is often a subcutaneous layer full of coarse Mg2Si, which shears before the matrix does.
That is also the reason a cleaner steel does not fix everything. If the billet is delivering the defect, changing the grade moves the cost without moving the profile. The page on pick-up and die lines sets out how to tell those cases apart at the press.
Where this leaves a die steel order
None of this reduces what the die has to survive. It survives it better when the billet is clean, and the steel still has to hold hardness at the bearing and take a nitrided case. The 1.2344 ESR programme for extrusion tooling is on the 1.2344 ESR H13 extrusion supply page, the grade on the H13 tool steel page, and the case on the tool steel nitriding page. The defects this page explains are described at the press on the pick-up and die lines page, the temperature window on the extrusion temperature page, and the bearing that takes the damage on the die bearing length page.
Before you write these into a billet specification
These are published values from commercial practice, and several are control points rather than acceptance limits. The gas figure, the segregation depths and the charge ratio all depend on the casting route, the alloy and the foundry. Use them to ask the right question of a billet supplier, and confirm the actual limits against the delivered certificate and the first article.
Source, Extrusion of Aluminium Alloys, T. Sheppard, Kluwer Academic Publishers, 1999, sections 6.4, 6.7 and 7.9.2.