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Extrudability of aluminium alloys

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Aluminium extrusions

The types of aluminum extrusions include:

Extrudability

Extrudability is the material formability under conditions of the extrusion process. This term does not have a precise physical definition. The relative extrudability of aluminum alloys can be roughly ranked as measured by extrusion exit speed.

Some facts on extrudability:

Evaluating extrudability of aluminium alloys

To evaluate the extrudability of various aluminum profiles, the following approaches are used:

Flow Stress

In the range 350 to 550 ⁰C, the flow stress of aluminum alloys is very dependent on the temperature and the composition [x]. The increase in the flow stress with increasing content of the most common alloying elements is shown in Fig. 1 [1].

Fig. 1 – Increase in the flow stress of aluminum in hot working with different alloying additions [1 (Ake)]

The range of extrusion speeds that can be practically obtained with aluminum alloys is shown in Fig. 2. The range plotted against the flow stress at the extrusion temperatures used in practice [1].

Fig. 2 – Flow stress and extrudability [1 (Ake)]

Figure 3 shows the product strength of the most common aluminum alloys as a  function of the flow stress at the extrusion temperature [1].

Fig. 3 – 0.2% proof stress of extruded aluminum alloys in
the standard as-supplied condition as a function
of the flow stress at the usual extrusion temperatures [1 (Ake)]

The extrusion load depends mainly on the flow stress kf. Thus the wrought aluminum alloys are classified according to the flow stress into the groups [1]:

The flow stress of the most important extrusion alloys is shown in Table 1. The groups of aluminium alloys that differ in exrudability is shown in Table 2.

Table 1 – Experimental material: homogenized cast billets. Test: torsion flow curves; test temperature 450 ⁰C [1]

Table 2 – The groups of aluminium alloys: Easy to extrude, Moderately difficult to extrude and Hard to extrude [1]

The relative extrudability of aluminium alloys

The relative extrudability of AI-alloys are shown in Table 3. Here all alloys are compared with 6063 whose extrudability index is 100. Increasing number indicates greater difficulty.

Table 3 – Comparison of the relative extrudability of Al-alloys [2]

Soft, Medium and Hard aluminium alloys

R. Saha classified extrudable aluminium alloys into three different groups according to their extrudability in following way [3]:

The relative extrudability ratings of some soft and medium-strength alloys are given in Table 4. The extrudability ratings of hard alloys are given in Table 5.

Table 4 – Extrudability ratings of soft- and medium-grade alloys [3]

Table 5 – Extrudability ratings of soft- and medium-grade alloys [3]

 Complexity of extruded shape

The section shape is the most important factor in the extrudability considerations (Table 6). Thus, the complex shapes are extruded from 6ххх aluminium alloys. The most complex shapes are extruded from 6060 and 6063 alloys.

Two accepted methods are available for defining the complexity of an extruded shape. One method involves the use of the sha  as follows [3, 4]:

This factor is a measure of the amout of surface generated per unit weight of metal citruded. The shape factor affects the production rate as well as the cost of manufacturing and maintaining the dies. It is used as a basis for pricing and provides the designer with a means of comparing the relative complexities of alternate designs [4].

The other measure of shape complexity is the classification of extruded shapes into different groups, based on the dificulty of extrusion (Table 6).

Table 6 – Classification of aluminum extruded section [3]

Limit diagram

These approaches are based on plotting the relationship between maximum extrusion exit speeds and preheat billet temperature (Fig. 1) for a particular profile or industrial shape. The maximum exit speed (Vmax in Fig. 2), which ensures obtaining a product without surface tearing, has been frequently suggested.

An additional advantage of selecting maximum exit speed as a measure of extrudability is the fact that this parameter is both a metallurgical and productivity measurement. Flow stress and extrudability (expressed as exit speed) are plotted in Fig. 3 for several types of aluminum alloys.

Fig. 4 – Extrusion exit speed as a function of temperature [5]

Fig. 5 Limit diagram of extrusion speed, V, versus temperature
for a given extrusion load and the alloy limit for surface cracking (hot shortness) [5]

Fig. 6 Extrusion rate versus flow stress
for some aluminum аlloys [5]. (Сompare with Fig. 2)

Minimal wall thickness

Table 7 –  Relationship between minimum wall thickness, circumscribing circle
diameter (CCD), and press size for different soft- and medium-grade alloys [6]


Fig. 7 – Relation between material strength, minimum wall thickness and extrudability/press velocity.
To be applied only to a profiles with a specific geometry.
Similar curves may be obtained for all profiles [6]

Fig. 8 – SAPA’s recommended wall thickness. Amongst the factors having an effect on wall thickness are
extrusion force and speed, the choice of alloy, the shape of the profile,
desired surface finish and tolerance specifications [7]

Tight tolerance capability of alloys

Tolerance European Standard

European standards that set tolerances for extruded aluminum products are the followings:

These standards distribute the alloys into two groups which correspond to varying degrees of difficulty when manufacturing the products:

 Table 8 – Two groups of aluminium alloys with various degrees of
difficulty to extrusion  according to EN 755-9

Fig. 9 – Dimension H tolerances of profiles
(The fragments of Table 2 and 3 in EN 755-9)

 Table 9 – Two groups of aluminium alloys with various degrees of
difficulty to extrude bars and rod according to EN 755-3


Fig. 10 – Diameter tolerances of round bars
(The fragment of Table 2 in EN 755-3)

Sources:

  1. Extrusion / Eds M. Bauser, G. Sauer, K. Siegert – 2nd edition, ASM International, 2006
  2. Extrusion of aluminium alloys / T. Sheppard
  3. Aluminum Extrusion Technology / P. Saha
  4. Aluminum and Aluminum Alloys – ASM International Handbook – 1992
  5. Extrusion of Aluminum Alloys /W. Misiolek, and R.M. Kelly // ASM Handbook, Volume 14A: Metalworking: Bulk Forming – 2005
  6. Extrusion /S. Stvren and P.Th. Moe //Encyclopedia of Aluminum and Its Alloys – Edited by G. Totten, M. Tiryakioğlu, and O. Kessler – 2019
  7. Sapa: Design Manual – 2007
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