Find out how stretching aluminium extrusions can improve the dimensional stability and minimize distortions in your profiles.

Stretching and straightening

Aluminium extrusions coming off the press are usually in need of some corrective processes:

• stretching/strain hardening – to apply a permanent set to the metal by passing the yield point;
• straightening—to overcome bows, bends, camber and twist.

The stretching process is done at nearly room temperature, not higher of 40 degrees Celcius.

The aluminium extrusions stretching process can be defined as the stress relieving process applied to extruded rod, bar, profiles and tubes after cooling from an elevated temperature extrusion. This is achieved by applying pulling force to the sections, resulting in a specified percent of permanent length increase, measured within straight product.

Actual extruded profiles have very significant distribution of stress after extrusion and quench. The stress can be high enough to cause notable distortion of the profile on the runout table. The primary objective of the stretch is therefore to relieve these stresses to the level that meets the product’s final application requirement. Extruded profiles with relieved or significantly reduced stress are very likely to stay straight and true, even after subsequent machining or forming [1].

Stretchers

Stretchers are usually sized to provide stretching force for the largest profile produced, based on the cross sectional area multiplied by the yield stress of the alloy. Some machines use a laser device to measure actual distance between head and tailstock, then use an encoder device on the stretch cylinder to calculate and control the percentage of stretch [2].

Figure 1 – A stretching machine as the part of the handling system
of an aluminium extrusion press [2]

Stretching principle

The aluminium profiles are transferred from the runout and a cooling table to the stretcher bed to be straightened by stretching 0,5 to 3%. Separate shaped grips are used in both ends of the stretcher to stretch critical shapes without further distortion during stretching. Both heads are fitted on the same straight bed as shown in Fig. 2. The fixed head, also called the tailstock, can be moved and locked on the bed at various positions to adjust the starting position of the moving head to match the extrusion length. The clamping jaws are operated by either hydraulic or pneumatic means.

The stretcher capacity has to be greater than the required stretching force, which is a function of the following factors [3]:

• Cross section of the extrusion
• Yield stress of the extruded alloy.

 
Figure 2 – Stretching principle [3]

The stretching principle is shown in Fig. 2. The amount of stretching is determined by the permanent strain in the longitudinal direction:

Stretching = (L_S – L_E)/L_E ×100 %

where

L_S is the length of extrusion after stretching, and
L_E is the length before stretching.

The amount of stretching may be adjusted to suit a particular product, depending on a number of factors, including [3]:

• product shape and size;
• critical dimensions, such as gap or tongue;
• close tolerance; and
• surface finish.

The following factors can affect stretching [3]:

• Design of the jaws to accommodate different shapes
• Stretching force for a particular alloy and shape
• Stretching speed.

Yeld strength of aluminium alloy

The fresh extruded profile is quite soft:

• Heat-treatable aluminium alloys are in a state of solid solution (Temper W). The yield strength of the material in this state depends on the degree of alloying of the alloy. The moderately alloyed 6061 alloy has a yield strength of about 60 MPa, and the heavily alloyed 2014 alloy has a yield strength of about 140 MPa (Figure 3).
• Freshly extruded non-heat-treatable aluminum alloys are practically in the annealed state. The yield strength of the alloy at this point depends on the degree of alloying, for example, magnesium (Figure 4).

Therefore, it seems that in order to estimate the maximum stretching force, it is necessary to take the maximum yield strength of heat-treatable aluminum alloys in a freshly extruded temper W, and not in heat-strengthened tempers T4 and T6, as suggested in the authoritative source [2].

(a)

(b)

Figure 3 – Change in elapsed time after quenching
of tensile yeld strength of some heat-treeatable aluminium alloys:
(a) – 6061 alloy and (b) – 2014 alloy) [4]

Figure 4 – Tensile yield strength of aluminium-magnesium alloys
in annealing condition [4]

Mechanics of the stretching

Stretching process and tensile test

Mechanics of the stretching process are very similar to the mechanics of the tensile test. Figure 3 outlines a typical stress-strain curve obtained from a tension test, showing various features. Tensile test data are important in calculating forces and predicting the behavior of material during stretching. The stretching process is stopped a long time before any necking is visible within the extruded profile and therefore is not be taken under consideration. On the other hand, the moment of transition between elastic and plastic deformation conditions is of the greatest importance for the stretching process of the extruded profiles [1].

Figure 5 –Typical engineering (nominal) stress-strain curve for aluminum alloys [5]

As the load is increased, the specimen begins to undergo permanent plastic deformation at some level of stress. Beyond this stress level, the stress and strain are no longer proportional, as they were in the elastic region. The stress at which this phenomenon occurs is known as the yield stress Y of the material. The ratio of stress to strain in the elastic region is known as the modulus of elasticity (E), or Young’s modulus.

Elastic limit for most metallic materials has a range of values, and it may not be easy to determine the exact position on the stress-strain curve where yielding occurs. Therefore, we usually define Y as the point on the stress-strain curve that is offset by a strain of 0.002 or 0,2 percent of elongation. A good way of looking at offset yield strength is that after a specimen has been loaded to its 0,2 percent offset yield strength and then unloaded, it will be 0,2 percent longer than before the test.

The model of aluminium extrusions stretching

The engineering (nominal) stress-strain response of aluminum alloys is characterized by a continuous rounded curve with an absence of a sharply defined yield point, as shown in Fig. 6. More specifically, the curve features an initial linear-elastic region up to the proportional stress fp, which is generally taken as the 0.01% proof stress, followed by a nonlinear “knee” region up to the conventionally defined yield strength fy (i.e. the 0,2% proof stress) and strain hardening, the extent of which varies between grades, before reaching the ultimate tensile strength fu and corresponding ultimate strainεu. The initial slope of the stress-strain curve and 80 the tangent slope at the 0,2% proof stress are denoted E and E0,2, respectively [5].

A number of material models have been developed to describe the nonlinear stress-strain behavior of aluminum alloys, with the simplest being piecewise linear models. The piecewise linear models defined in EN 1999-1-1 [6] consist of two or three straight lines (corresponding to a bi-linear or a tri-linear material model, respectively) with each line representing a certain region of the stress-strain curve, with or without allowance for strain hardening. The piecewise linear models, particularly the idealized bi-linear model (Fig. 6), fail to capture the roundedness of the stress-strain response that is characteristic of aluminum alloys [5].

Figure 6 – Bi-linear model without hardening [5]

The temperature of stretch

• The stretching of the aluminium extruded profiles must be cooled to below 50 ℃, or may be to 40 ℃,  before it can be moved to the stretcher for stretching work.
• When the temperature is too high, it causes bending, twisting, and poor performance during aging because the internal stress of the profile cannot be completely eliminated [7].

The amount of stretch

• The stretching amount of aluminium profiles should be controlled at about 1%, but it must not exceed 2% [7].
• Source [1] considers the optimal amount of residual stretch to be 0,5%.
• If the stretching amount is too high, the head, middle and tail size will be deviated.  This can lead to profile locations with low elongation and high hardness [7].

The calculation of stretch

Stretchers are usually sized to provide stretching force for the largest profile produced, based on the cross sectional area multiplied by the yield stress of the alloy. The amount of stretch may be controlled in several ways [2]:

• by sight (the operator’s judgment);
• by force;
• by length of stretch; or
• by the percentage of total strand length (Figure 7).

Figure 7 – Automated stretching process – sequence of operations [1]

Figure 8 shows the explanation of the procedure for determining the stretching parameters for two aluminum alloys with different yield strengths. It can be seen that alloys with higher yield strength require more stretch to achieve a set strain of 1,0% than alloys with lower yield strength.

Figure 8 – The the procedure for determining the stretching parameters with prescribed 1,0% set strain
for two aluminum alloys with different yield strengths.

T tempers that include stretching operations

The most extruded products are made in T4, T5 or T6 tempers. The stretching operations significantly alters the characteristics of extruded product with respect to the basic treatments T4, T5 or T6. To indicate a variation in treatment additional digits are added to designations T4, T5 and T6. These digits may relate the following [8]:

• Tx510 or Txx510 applies to extruded rods, bar, profiles or tubes, when stretched to 1 % to 3 % permanent set after solution heat-treatment or after cooling from an elevated temperature extrusion. The products receive no further straightening after stretching.
• Tx511 or Txx511 applies to extruded rods, bars, profiles and tubes when stretched to 1 % to 3 % permanent set after solution heat-treatment or after cooling from an elevated temperature extrusion. These products may receive minor straightening to comply standard tolerances.

The variations in stretching treatment with the amount of permanent set less 1 % are considered to be those that do not alter the characteristics of the product. For those treatment additional digits to the basic tempers are not assigned.

Sources:

1. Evaluation of Process Mechanism and Parameters for Automated Stretching Line / P. Kazanowski and R. Dickson, Aluminum Extrusion Technology Seminar, Chicago, 2012.
2. The Extrusion Press Maintenance Manual / Al Kennedy
3. Aluminum Extrusion Technology / Pradip K. Saha – ASM International, 2000.
4. Aluminum and Aluminum Alloys /ASM Speciality Handbook, 1993
5. Full-range stress-strain curves for aluminum alloys / X. Yuna, Zh. Wang, L. Gardnera – Journal of Structural Engineering 147(6) – 2021
6. EN 1999-1-1
7. https://www.otalum.com 
8. EN 515