7xxx high-strength aluminium billet (7050, 7075): stepped homogenising and centre-to-surface variation in large DC-cast billets
Large-diameter Al-Zn-Mg-Cu (7xxx) billet · why 7050 and 7075 billets need stepped homogenising, what changes from centre to surface of a 700 mm billet, and how the cooling after the soak shows up later
7xxx alloys such as 7050 (AlZn6CuMgZr) and 7075 (AlZn5.5MgCu) are cast as large round billets for forging and for extrusion of high-strength sections. Much of the risk sits in the heat treatment that follows casting: the as-cast billet carries low-melting eutectic phases between the dendrites, and heating straight to a high soak temperature can melt them. The billet is also not uniform: its centre and surface solidify at different rates and with different composition, and a schedule that suits one radius may not suit the other. This page sets out two published homogenising schedules and what they are designed to avoid.
Direct-chill cast round billets and logs of high-strength 7xxx (Al-Zn-Mg-Cu) alloys such as 7050 and 7075, in large diameters for forging stock and hard-alloy extrusion, with their homogenising and post-soak cooling. Ordering 6xxx extrusion billet by diameter, length, straightness and casting practice is covered on the Aluminium alloy extrusion billet page; other metals are on the Magnesium alloy billet and Copper alloy continuous casting billet pages. Rolling slab, plate, forgings and finished extrusions are outside this page.
CNFX editorial research · Sources checked October 12, 2026 · 3 sources
Stepped homogenising: dissolve the low-melting phases before the hottest soak
Both studies below take the billet to its highest temperature only after a lower step has dissolved the phases that would otherwise melt. In the 7050 schedule, the T and V phases have to dissolve fully at 470 °C before the billet goes to 480 °C; in the 7075 study, a single 465 °C soak left low-melting phase behind. [1][2]
The Purdue authors warn that raising the temperature to 480 °C before the T phase has dissolved across the whole section may cause melting at the surface and mid-radius, leaving pores that degrade mechanical properties. [1] In the 7075 study, sheet rolled from the two-stage homogenised ingot gave the best T6 properties: 533.5 MPa tensile strength, 455 MPa yield strength and 16.5% elongation. [2]
Editorial calculation: the 7050 schedule holds the billet for at least 24 hours at temperature (10 + 4 + more than 10), and the 7075 two-stage schedule for 32 hours (24 + 8), before heating-up and cooling time are added.
Ask the billet supplier for the homogenising schedule actually used (each step's temperature and time), the furnace temperature uniformity, and evidence that the low-melting phases were dissolved before the top step, such as micrographs or DSC from centre and surface.
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Centre against surface in a 700 mm 7050 billet: segregation, dispersoids and recrystallisation
Priya, Fezi, Johnson and Krane at Purdue coupled a DC casting model of a 7050 billet, cast at a steady 90 mm/min, with a homogenising model, to see how a 0.35 m radius billet differs across its section. [1]
The authors link the lower dispersoid density at the periphery to the peripheral coarse-grained structure seen in industrial extrusions, and recommend sampling at mid-radius and surface, not only the centre, when setting a schedule. [1] They suggest adding 0.18–0.2% scandium to 7xxx alloys containing 0.1–0.2% zirconium, because scandium segregates the opposite way to zirconium and could even out the dispersoids. [1]
Editorial calculation: with the Aluminum Association's nominal density of 2.83 g/cm³ for 7050, a billet of 0.35 m radius weighs about π × 0.35² × 2,830 ≈ 1,090 kg per metre of length. [3][1]
Position
What the model predicts
Centreline
Leaner in Zn, Cu and Mg and richer in Zr; lower as-cast fraction of T + V; more and larger Al3Zr dispersoids [1]
Mid-radius and surface
Higher solute and more interdendritic phase, slower to dissolve at 470 °C; fewer dispersoids, so more prone to recrystallisation [1]
Surface after forced-air cooling
More and larger η and T precipitates; S platelets over 0.6 µm that may not dissolve in pre-heat and may affect surface finish; higher solute left in matrix, needing more extrusion pressure [1]
Centreline after cooling
More of the undesirable S phase than the surface [1]
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Cooling after the soak: forced air at about 148 °C an hour
The Purdue model takes industrial practice as forced air over the billet surface after the furnace is switched off, which it finds cools all three radial positions at about 148 °C an hour. [1] It uses a heat-transfer coefficient of 10 W/m²K for heating and 100 W/m²K for forced-air cooling, with an emissivity of 0.09. [1]
At that rate the model predicts S, η, T and θ precipitating in that order of falling temperature, unevenly across the section, and concludes that a faster cooling rate than this practice is needed if the coarse surface particles are to be avoided. [1] Editorial calculation: from a 480 °C soak to the model's 30 °C ambient at 148 °C an hour takes about three hours.
For a buyer this means the billet's condition depends on the cooling equipment as well as the furnace: ask whether billets are cooled by forced air, water mist or another method, and at what measured rate.
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Data for a 7xxx billet enquiry
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Alloy: [7050, 7075 or other; composition limits; Zr or Sc additions]
Size: [diameter; length; weight per piece]
Use: [open- or closed-die forging, or extrusion; product made]
Homogenising: [each step's temperature and time; furnace uniformity]
Cooling: [method and rate after soak]
Evidence: [micrographs or DSC at centre, mid-radius and surface; grain size]
Inspection: [ultrasonic class; macroetch; hydrogen]
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 13 October 2026
Buyer questions
Why is 7xxx billet homogenised in steps?
Low-melting phases must dissolve before the top soak. For a 0.35 m radius 7050 billet, Purdue proposes 10 h at 420 °C, 4 h at 470 °C, then more than 10 h at 480 °C. [1]
Is one soak enough for 7075?
In a Materials Reports study of a large 7075 ingot, 465 °C/24 h left low-melting phase; adding 478 °C/8 h eliminated the eutectic. [2]
Does a large billet differ from centre to surface?
Yes; the Purdue model finds the surface richer in Zn, Cu and Mg, with fewer Al3Zr dispersoids and a higher tendency to recrystallise. [1]
How heavy is a 700 mm 7050 billet?
About 1,090 kg per metre, using the Aluminum Association's 2.83 g/cm³ density (editorial calculation). [3][1]
Sources and what still needs a direct check
Sources checked 2026-10-12. Homogenising and radial-variation data come from a Purdue University modelling study of a DC-cast 7050 billet (arXiv preprint) and a Materials Reports study of a large 7075 ingot; alloy designations and densities from the Aluminum Association's registration record.
