Aluminum Extrusion Temperature: The Window, the Alloy Series, and What It Costs the Die

Every alloy series has an extrusion temperature range, and the range is narrow. Below it the press runs out of pressure; above it the surface fails or the metal starts to melt at the grain boundaries. The window is set by the solidus, adjusted down by the homogenization treatment, and it moves with the alloy series by more than 100 °C. This page sets out the published ranges, the step change that 6xxx alloys show at their solvus, and what poor temperature control actually costs in output.

The ranges, by alloy series

The three temperatures that matter are the solidus, the homogenization soak and the extrusion range. Homogenization for commercial alloys is normally 20 to 50 °C below the solidus and as close as practical to the initial hot working temperature, which is why the numbers below track each other.

Typical solidus, homogenization and extrusion temperatures by alloy series. The extrusion range is the billet temperature at the start of the stroke, not the temperature at the die.

Two things in that table decide how a press is run. The 7xxx range, 390 to 450 °C, sits roughly 60 °C below the 6xxx range and overlaps the region where 6xxx is still too stiff to extrude well. And the width of the window is not proportional to the temperature: 2xxx spans 80 °C, 6xxx spans 70 °C, and 7xxx spans 60 °C, but the 7xxx window is the one bounded by incipient melting at the top, because its solidus is only just above the working range.

6xxx has a step change at the solvus

On the 6xxx alloys the limit that matters is not the billet temperature but the temperature the profile reaches at the die exit. That limit moves depending on whether the Mg and Si are in solution when the billet enters the die. If the billet is heated straight into the working range and stays below the solvus, Mg2Si is still present as particles and the profile fails earlier. If the billet passes above the solvus, or is overheated and cooled quickly, the Mg and Si are in solution and the profile can run 20 to 30 °C hotter before incipient melting.

Profile surface temperature at which the surface fails, for billets heated below and above the solvus. The solvus is about 475 °C for 6060 and about 540 °C for 6082.

That is a press-practice point, not a metallurgical curiosity. A shop that preheats straight to 460 °C on 6060 is running the alloy with particles present and paying for it with a lower ceiling, even though 460 °C sits comfortably inside the published 430 to 500 °C range. The upper limit is set by what the metal carries into the die, not by the chart.

Temperature control is output, not tidiness

Because the window is narrow, the accuracy of the control sets the speed. The published figures are blunt: control to within plus or minus 20 °C, and the optimum constant press speed drops by about 10 percent against a perfectly isothermal run. Let the control drift to plus or minus 50 °C, and the figure is a 50 percent reduction. The die exit temperature is what matters, and it is not the same as the billet temperature in the furnace, because the work of deformation adds heat along the stroke.

This is why isothermal extrusion is worth the instrumentation. To hold the exit temperature constant while the billet cools against the container and heats under deformation, the ram has to slow down through the stroke, which is why speed ramps are programmed rather than set. A press run at one constant ram speed is, by definition, not isothermal.

The peak pressure takes part of the window

The other boundary is press capacity, and it is not the steady state load. A breakthrough or peak pressure is needed to start the billet moving, and it sits above the steady state figure that the usual calculations give. When the peak is included, the maximum extrusion ratio available at a given pressure and ram speed falls by almost 40 percent against the figure drawn on steady state pressure alone. A press that looks adequate on the steady state number can therefore be short of capacity at the start of every stroke, which shows up as a stall on a long billet or a hard alloy rather than as a clean pressure limit.

What the window does to the die

The die sits at the hot end of all of this. On an aluminum press the bearing runs at around 1050 °F (565 °C) without lubrication, against a metal that adheres to steel, and the working temperature is what forces the grade choice: H13 and its European equivalent 1.2344 are used worldwide because they hold hardness and temper resistance at that temperature, and because they respond to nitriding. A grade that would be adequate for a cold work die loses its hardness at the bearing and deforms, and the profile drifts before the die shows visible wear.

The link runs the other way too. Everything that raises the exit temperature shortens the die life: a higher billet temperature, a faster ram, a longer bearing, a colder die that has to be heated by the work. The surface page on this site sets out what happens when the bearing gets too hot; the wear page sets out what the bearing loses.

Where this leaves a die steel order

The extrusion grade is a chromium hot work steel, AISI H13 and its European equivalent 1.2344, supplied annealed so the die shop can machine it, harden and temper it, and nitride the bearing. 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. What comes off the surface when the die runs too hot is on the pick-up and die lines page, the bearing that controls the heat input on the die bearing length page, the grade by component on the extrusion die steel selection page, and the correction that follows a trial on the extrusion die correction page.

Before you set a press from these ranges

These are published ranges for commercial alloys, not settings for a particular press or profile. The extrusion temperature that works on a given job depends on the alloy, the extrusion ratio, the shape, the container and the die, and the published range is the envelope, not the target. The profile temperature limits are for the specific alloys and preheat conditions in the source. Final settings are established on the press.

Source, Extrusion of Aluminium Alloys, T. Sheppard, Kluwer Academic Publishers, 1999, Table 5.1 and sections 5.1 to 5.10, with the working temperature of the bearing from Aluminum Extrusion Technology, P. K. Saha, ASM International, 2000.