How Aluminum Extrusion Dies Wear Out
An aluminum extrusion die does not fail in one way. It wears, it softens, and it cracks, and on a normal die all three run at once. The wear is concentrated on the bearing, because that is the only surface in sliding contact with hot aluminum. The damage is measurable, it is progressive, and it changes the profile the die produces long before the die is finished. This page sets out the mechanisms that consume an extrusion die, the conditions that speed them up, and the shape of the wear that a die shop will actually see on a mandrel and a cap.
Three mechanisms consume a die
Aluminum extrusion is a hot working process. The die runs at around 1050 °F (565 °C) at the bearing, without lubrication, and against a metal that sticks to steel. The source names three failure mechanisms as the common ones, hot wear, plastic deformation and cracking. Hot wear is the one that takes the shape out of tolerance first, and it comes in two forms, adhesive and abrasive. Adhesive wear is the more damaging of the two on the bearing, because the aluminum that sticks and breaks away removes die steel with it and leaves the crater that shows up as pick up on the extrusion.
| Mechanism | What it is |
|---|---|
| Hot wear, adhesive | Aluminum has a strong tendency to stick to steel, so a layer of aluminum builds on the die bearing and then breaks off. The break off takes die steel with it and leaves the aluminum in the extrusion |
| Hot wear, abrasive | Die wash. The moving metal grinds the bearing away. Abrasive wear is gradual at low temperature and much faster once the bearing is hot, and adhesive wear makes it worse |
| Plastic deformation | The die loses hardness at temperature. On a long run the bearing reaches the softening temperature, the tongues deform, the opening grows and the dimensions are lost |
| Cracking | Repeated heating and cooling plus the load of the press crack the die, on the bearing and at the mandrel bridges |
The mechanisms that consume an aluminum extrusion die. The source names hot wear, plastic deformation and cracking as the most common failure mechanisms, and hot wear is split here into the adhesive and abrasive forms the chapter describes.
Wear is measured at the bearing, and it has a shape
The way to measure die wear is to measure the maximum depth of the wear spots on the bearing surface of the mandrel or the cap. On a hollow die for a 6063 square tube, wear spots appear as separate patches with gaps between them. They start at the leading edge of the bearing, the edge the metal reaches first, and they spread toward the middle of the bearing as the run goes on. The mandrel bearing, which sees more heat, wears more than the cap. The table below gives the extrusion the source measured, a thin wall square tube with screw bosses run from a 6 in. billet on a hollow die at an extrusion ratio of 90.
| Parameter | Value |
|---|---|
| Billet diameter | 6 in. (152 mm) |
| Billet length | 19 to 21 in. (482 to 533 mm) |
| Billet temperature, front | 860 °F (460 °C) |
| Billet temperature, middle | 801 °F (427 °C) |
| Billet temperature, back | 759 °F (404 °C) |
| Container bore | 6.38 in. (162 mm) |
| Container temperature | 806 °F (430 °C) |
| Die | Hollow square, 2 in.2 (50.8 mm2) |
| Die bearing length | 0.08 to 0.14 in. (2.0 to 3.5 mm), blended |
| Die temperature at the start | 824 °F (440 °C) |
| Extrusion ratio | 90 |
Extrusion parameters of the die the wear and temperature work was measured on. The die is a tapered seal hollow die with a nitrided bearing, and the bearing length is given as a blended range rather than one figure.
What makes the wear go faster
Every variable that raises the temperature at the bearing raises the wear. That is the single idea behind the results below. A slower die runs cooler and lasts longer. A die that has been run hard is hot, and a hot bearing sticks, so the wear that has already happened makes the next wear happen faster. The table lists the variables the source measured and the direction each one moves the wear depth.
| Variable | Effect on wear depth | Reason |
|---|---|---|
| Press cycles, the number of billets run | Wear depth rises with every billet | The bearing gets hotter as the run goes on, and the hotter it is the faster the adhesive layer builds and breaks off |
| Billet length | Longer billets wear the bearing more | A longer billet lengthens the extrusion cycle, so the bearing spends longer at its peak temperature |
| Ram speed | Higher ram speed gives deeper wear | A higher ram speed raises the exit temperature, and the exit temperature raises the bearing temperature |
| Bearing surface condition | A new nitrided bearing wears more slowly than an old one | A hard nitrided surface sticks to the aluminum less than a softened old surface, so less friction heat is generated |
| Position on the die | The mandrel wears more than the cap, and wear starts at the leading edge | More heat is generated inside the mandrel, where the metal is deformed and rubbed again before it leaves the die |
The variables that control wear depth on the die bearing. Wear depth is the maximum depth of the wear spot, averaged over three identical tests for each point. The source ran each die with the same die steel, heat treatment and nitriding, from one die maker.
Wear changes the flow, and the flow changes the shape
Wear removes metal from the bearing, so the area of the bearing that is in contact with the aluminum becomes smaller, not larger. That loss of contact area changes the friction on each side of the profile, and because the friction on the inside and the outside of a hollow shape is no longer the same, the two surfaces move at different speeds. On the square tube the source studied, the walls that started as flat sides came out convex once the bearing had worn. A die that still passes a gauge check at the start of a run may be out of shape by the end of it, which is why extrusion dies are run to a number of billets and then corrected.
Temperature and speed are the two variables behind all of it. The exit temperature of a 6063 extrusion rises with ram speed and with billet length, and a new nitrided bearing surface runs cooler than an old, softened one because it sticks less. That is why the surface treatment of the bearing is treated as part of the die, not as a finish. A harder, cleaner bearing holds its shape for more billets before the profile drifts.
Where this leaves a die steel order
Wear resistance at temperature is a property of the steel, and it is why the extrusion die grade is a chromium hot work steel rather than a cold work grade. The grade is AISI H13 and its European equivalent 1.2344, covered on the H13 tool steel page, with the extrusion supply programme on the 1.2344 ESR H13 extrusion supply page. Surface hardening of the bearing is on the tool steel nitriding page, and the coatings and treatments tried on extrusion dies are on the TD coating page. The bearing that wears is described on the die bearing length page, the stack around it on the die types and tooling page, and the wider failure picture on the die wear and failure mechanisms page.
Which grade to put behind that wear, by component, is set out on the aluminum extrusion die steel selection page.
Before you read these figures as a die life
This page is a reference summary of published practice and it is not an Aobo Steel specification. The extrusion parameters and the wear behaviour are the values published for the specific die, alloy and press in the source, and the die life in another shop will differ with the press, the shape, the alloy and the profile of the run. Wear depth is a measured quantity on a specific bearing, not a guaranteed service life. Final die life is confirmed on the job.
Source, Aluminum Extrusion Technology, P. K. Saha, ASM International, 2000.