GB2285941A - Extrusion Method - Google Patents

Abstract

2000 Series Al alloys are hot extruded through an extrusion die having a die aperture including a bearing section and an upstream section wherein the upstream section has an axial length C of 12 to 240 mm and a positive taper T of 0.5 to 10 DEG . Peripheral coarse grain banding is avoided and extrusion speeds may be enhanced. <IMAGE>

Description

EXTRUSION METHOD This invention relates to a method of hot extruding aluminium alloys of the 2000 series (of the Aluminium Association Inc Register) through an extrusion die of unusual design. The invention addresses the problem of Peripheral Coarse Grain (PCG) Banding. Various extrusion dies according to the invention can reduce this problem and/or increase extrusion productivity. For some alloys and some extrusion sections, the onset of speed cracking (at increasing extrusion speeds) is also delayed.

The development of Peripheral Coarse Grain Banding in extruded high strength 2xxx aluminium alloys, is a common feature following solution treatment. Its origin lies in the high levels of stored strain energy near the product surface which is introduced during the working operation. The high levels of deformation at the surface are associated with the nature of metal flow during unlubricated direct aluminium extrusion wherein the surface of the product originates from a highly localised shear zone on the face of the dead metal zones. As extrusion proceeds the shear zones themselves are extruded which is associated with an increase in extrusion force and a rapid increase in the depth of PCG in the finished product as shown in Figure 1.

In order to contain PCG banding to within specified limits the billet temperature and extrusion speed have to be controlled. Typically this requires a relatively high billet temperature, for example 4000- 4400C for 2000 series alloys with average extrusion speeds of 2-3 m/min. In addition an extrusion discard of not less than 15k of the original billet length is required in order to prevent ingress of heavily sheared material and the excessive PCG banding at the rear of the extrusion.

This invention provides a method of forming an extruded section comprising the steps: providing a homogenised billet of a 2000 series extrudable aluminium alloy containing an effective concentration of a recrystallisation inhibitor; hot extruding the billet at a temperature chosen to avoid recrystallisation; through an extrusion die which has a die aperture including a bearing section and an upstream section, wherein the upstream section has an axial length of 12 to 240 mm and a positive taper of 0.50 to 100.

In order to explain the terminology being used, reference is directed to Figure 10 which is an axial cross section through an extrusion die according to the above definition. The die 10 has a flat upstream face 12. An aperture extending through the extrusion die has an axis 14, with the direction of flow of metal during extrusion being shown by arrows.

The die aperture has a bearing section 16 and an upstream section 18. The upstream section has an axial length C (choke). The upstream section has a positive taper, that is to say the cross section gets progressively smaller in direction of metal flow, and the taper angle is T. The bearing section is shown as having an axial length B.

The extrusion dies are readily made by standard wire spark erosion techniques, as the die aperture may consist simply of a straight cylindrical bearing section and a straight conical upstream section. However, other variations are also possible.

Instead of a sharp angle at the die entry point 11, the entrance may be rounded off. Instead of an angle between the upstream section and the bearing section at 13, the die may be rounded so as to be asymptotic to the die axis. Between the points 11 and 13, the tapered upstream section does not have to be flat (in an axial direction) although it is preferably flat. At the downstream end of the bearing section, the angle at 15 does not have to be sharp, but the die may be rounded off or there may be a downstream section of negative taper.

The upstream section has an axial length C of 12 to 240 mm, preferably 30 to 100 mm. It is only above about 12 mm that the benefits of the invention (described below) begin to be observed. Above about 30 mm, increased extrusion speeds become possible for appropriately shaped extruded sections. The upper limits are not critical, but require extrusion dies of increasing thickness, which in turn involve increasing the extrusion pressure.

The angle of positive taper is in the range 0.50 to 100, preferably 10 to 4.50. As the examples below show, the angle of taper is quite critical, with better results being obtained at 2.60 than at either 1.50 or 5 . The optimum angle may differ for differing alloys.

The bearing section in which the die lands are parallel, preferably has a length B of 0 to 10 mm.

If the bearing section is too long, then the surface finish of the extruded section may be poor. If the bearing section is too short, e.g. 0, then there may be loss of dimensional control resulting from wear of the extrusion die.

The die aperture is shown as having a transverse dimension (in the plane of the drawing) of D1 at the entry side and of D2 at the exit side. It is an advantage of the invention that there is no limit on the value of D2, i.e. the extrusion method is useful for making large extruded sections, e.g. those having a minimum transverse dimension of at least 5 mm. For reasons explained below, the difference between D1 and D2 may suitably be in the range of 1 to 5 mm, e.g.

about the thickness of the PCG layer that would have formed in a conventional extrusion die.

Preferably, when the length of the tapered upstream section is less than 30 mm, then the taper angle is not more than 4.50; and when the taper angle is more than 4.50, then the length is at least 30 mm.

The invention thus aims to provide extruded sections of which the surface is either free of PCG or has a PCG layer less than 1 mm deep, by controlling the extrusion flow by modifications to the extrusion die geometry. The basic concept is to iron the extrusion surface progressively as the metal passes through the die orifice. Preferred die design also utilises a long tapered die bearing to progressively iron the highly deformed material emanating from a shear zone in the extrusion container, rather than allowing it to flow directly into the section surface as would be the case with a normal flat-face die. The taper is set by fixing the die entry and exit dimensions such that the heavily sheared material is redistributed, forming either no PCG or only a thin film of PCG.

This die technology is in complete contrast to conical entry dies used previously for metal extrusion, where a conical die profile is used to replace the bulk of the dead metal zone. However, a dead metal zone, albeit reduced in size, does still form in the angle formed by the conical die surface and the container wall. This is particularly the case with high strength aluminium alloys due to the inherent high flow stress of the material, and resulting high container billet friction. Consequently a shear zone is often extruded along the conical face of the die i much the same way as that for a flat face die, resulting in similar PCG levels.

In the new die designs shallow die angles as described are necessary in order to only influence the subsurface regions of the metal being extruded. This in turn requires an increase in the length of the upstream section in order to influence the metal being extruded to normal PCG depths. The new dies also give the opportunity to increase the extrusion exit speed, for some extruded sections, by a factor of between 2 to 3 times while still maintaining PCG control. While this patent application is based on results and not on theories, a simple mechanism is proposed to explain this behaviour, related to the ability of the die to extract heat from the surface of the metal being extruded. As stated above, the surface regions of the metal being extruded originate from a highly worked shear zone on the face of a dead metal zone in the container.It is in this region that the largest adiabatic heat is generated, resulting in a temperature rise. With conventional dies this metal quickly passes over the normal short parallel bearing where it is subjected to a tensile stress caused by bearing friction while at high temperature. Surface break-up normally occurs when the temperature rises sufficient to allow tensile failure at this point. In our preferred extrusion dies, the distance between the shear zone and the die exit is considerably lengthened such that the surface temperature is reduced by conduction of heat both into the die and into the bulk of the metal being extruded, while simultaneously a hydrostatic stress is applied preventing any form of tensile failure. As a result, high speed extrusion is possible.Heat extraction from the die by providing a cooling medium such as air, liquid nitrogen or water may also be beneficial in this respect.

A simple schematic interpretation of the proposed mechanism by which such a die exerts PCG control, while allowing increased exit speed, is postulated in Figures 2a to 2c.

Figure 2a is a diagrammatic representation of part of an extrusion press, including part of a container having a side wall 20, and part of an extrusion die having an upstream surface 22 and a die aperture with a positively tapered upstream section having a surface 24 and a bearing section having a surface 26. The upstream section has a length of 65 mm and a positive taper of 2.60, giving a 3 mm reduction in transverse aperture dimensions, just enough to iron out a 3 mm thick potential surface PCG layer in the metal being extruded.

Figure 2b is a graph of metal temperature against position in the die, and contrasts the exit temperature for a normal die with that for a taper die used in this invention.

Figure 2c is a graph of stress state of the metal being extruded against position in the die, and contrasts the position of positive (tensile) stress in a normal die and in a taper die used in this invention.

As a further consequence of adiabatic heat extraction through the die, it is possible to utilise a higher billet temperature for a given exit speed (or to utilise a higher exit speed for a given billet temperature) such that the stored strain energy in the finished product is reduced, thus further controlling PCG formation. An interpretation of this effect is illustrated in the form of an extrusion limit diagram shown in Figure 3.

The invention is applicable to 2000 series alloys of the Aluminum Association designations among which may be mentioned:- 2014, 2017, 2024.

The extrusion method is particularly effective when other available techniques are used to minimise PCG formation. Thus: - The extrusion billet is preferably homogenised, e.g. at a temperature of 4500 - 5000C particularly 4700 - 4900C for a time of 1 to 48 hours particularly 4 to 8 hours, shorter times being appropriate at higher temperatures.

- It is advantageous to modify the composition of the alloy by including an effective concentration of a recrystallisation inhibitor. Mn, Zr, Cr and Fe are examples of recrystallisation inhibitors, which may typically be added to the alloy in proportions of 0.05 up to 1.5% by weight. For example, commercial 2014 alloy usually contains about 0.7W Mn, but the examples below show the benefit of increasing this proportion to 0.8 to 1.2%, e.g. 1.0% + 0.03%.

The billet is hot extruded at a temperature chosen to avoid recrystallisation, both on the surface and internally, which would reduce overall strength and introduce stress corrosion problems. Preferred billet temperatures at the start of extrusion are in the range 4000 - 4800C.

- The extruded section may be subjected to solution heat treatment prior to ageing in order to develop desired mechanical properties. The PCG surface layers are not present in the extruded section as it emerges from the press, but are only developed during solution heat treatment after extrusion. The extrusion method of this invention permit extruded sections to be solution heat treated without formation of damaging PCG layers.

A typical formal solution treatment involves the following steps. The section is extruded, allowed to cool. It is then loaded into a furnace and heated to the solution treatment temperature, held for a set time and quenched to room temperature or at least to a temperature below that at which significant amounts of precipitation occur. Quenching is usually into water.

Etw forms whilst the extrusion is abated in the furnace. Typial formal solution treatment times and temperatures for the 2000 series alloys are in the rnage 4900C to about 520"C for times between 2-12 hours.

The times at temperature are calculataed according to the diameter of the extrusion using the relationship 30 min at temperature for every 25 mm of diameter, plus the time required for the outside of the extrusion to reach temperature. For example a bar 100 mm in diameter would be held at temperature for 2 hours plus the time required for the outside of the bar to reach temperature. The 2-12 hours quoted above is very approximate and includes the time to reach temperature which depends on the bar size and on the type of furnace.

The following examples illustrate the invention. Examples 1 and 2 use conventional extrusion dies without any positively tapered upstream section, and such dies are also used for comparative purposes in Examples 3, 4 and 5.

The alloys have the following compositions.

AA No 2014A 2017 2024 Si 0.50 - 0.9 0.20 - 0.8 < 0.5 Fe < 0.50 < 0.7 < 0.5 Cu 3. 9 - 5.0 3. 5 - 4. 5 3. 8 - 4. 9 Mn 0.8 - 1.2 0.8 - 1.0 0.5 - 0.9 Mg 0.20 - 0.8 0.40 - 0.8 1.2 - 1.8 Cr < 0.10 < 0.10 < 0.10 Ni < 0.10 Zn < 0.25 c0.25 < 0.25 Zr < 0.25 Ti < 0.15 < 0.15 < 0.15 Zr + Ti < 0.25 0.05 - 0.2 Other < 0.05 each < 0.15 total Al remainder The following recrystallisation inhibitors were used in the Examples: 2014A 2017 2024 Mn 1.0 + 0.03 0.9 + 0.05 0.65 i 0.05 Zr + Ti < 0.25 < 0.25 0.13 + 0.1 EXAMPLE 1 Effect of Increased Manganese Content on PCG Formation The effects of increasing the manganese content of 2014A from 0.7 to 1 wt.% are shown in Figure 4. This data was produced under the following press and other conditions and was measured at the back end of the extruded length.

Billet temperature 420-4300C Exit speed 8 ft/min Billet diameter 300 mm Bar diameter 80-100 mm Homogenisation practice 4 hrs/4900C Solution heat treatment at 5050C.

The distribution of PCG depths was made tighter by the higher manganese content and the mean was decreased by a factor of two.

EXAMPLE 2 Effect of Homoqenisation Conditions on PCG Depth for 2014 Bar Two batches of 88.9 mm diameter 2014A bar were produced using conventional dies. The first was homogenised for 4 hrs/4900C the second was given an 8 hrs/4750C treatment. Both batches were produced from high manganese 2014A. The press conditions were as follows: Container temperature 4000C Container diameter 300 mm Billet temperature 400-4300C Exit speed 2-8.5 ft/min k Discard 15% Billet length 1160 mm After formal solution treatment of the extruded bar at 5050C, PCG depths were measured at 4 equidistant positions along the length, the results of which are presented in Figures 5a (homogenisation 4 hrs/4900C) and 5b (homogenisation 8 hrs/4750C).The use of the lower temperature homogenisation practice gave a maximum PCG depth of 3 mm whereas with higher temperature treatment depths up to 4 mm were recorded.

In Figures 5a and 5b (and Figures 7c and 7d): - Homo means Homogenised.

- F, M, B and B1 refer to samples taken from the front, middle, back and extreme back end of the extrusion.

EXAMPLE 3 PCG Depths Generated in 88.9 mm Diameter Bar Produced Through a Conventional Flat and a 2.60 Taper Die with 30 mm Bearings A batch of six billets of 2014A alloy were processed using a 2.60 taper die with a 30 mm taper region followed by a standard 5 mm bearing as shown in Figure 6a. The press and other conditions were as follows: Container temperature 4100C Container diameter 300 mm Billet temperature 430-4500C Exit speed 8 ft/min W Discard 15% Billet length 1140 mm Billet homogenisation 8 hrs/4750C Formal solution treatment at 5050C.

The maximum exit speed achieved prior to surface cracking was 8 ft/min i.e. the same speed achievable from conventional die design.

Figure 6b shows the PCG depth results for this die which can be directly compared with the results for the normal die in Figure 5b. PCG was eliminated at middle and back positions in most cases but was still present at the extreme back position to a slightly reduced depth compared to the normal die.

Therefore this die design offers some capability for reducing PCG levels but does not give any productivity advantages compared to conventional dies.

EXAMPLE 4 PCG Depths Generated for 88.9 mm Diameter Bar Produced Through Flat Dies and TaPer Dies with 1.5 and 2.6O Taper Angles and 65 mm TaDer Lengths Batches of six billets of 2014A alloy were extruded through two taper dies having 1.5 and 2.60 taper angles with a 65 mm taper length followed by a standard parallel 5 mm bearing. The die designs are shown in Figures 7a and 7b. The press and other conditions are as for Example 3 with the exception that an exit speed of 17 ft/min was achieved without surface break-up. Figures 7c and 7d show the PCG levels obtained which can be directly compared with the results for the conventional die in Figure 5b.With the 65 mm/1.50 taper die, (Figure 7c) the PCG levels were not significantly better than the conventional die, whereas with the 65 mm/2.60 taper die (Figure 7d) PCG was totally eliminated apart from on the first billet. Increased PCG levels were also encountered on the first billet of the 1.50 taper die.

Therefore the longer taper offers increased productivity and when combined with the increased angle also eliminates PCG. Comparison with Example 3 indicates that the increased taper angle is necessary for PCG control whilst the long tapered portion is necessary for increased productivity.

EXAMPLE 5 PCG Levels Generated for 57.15 mm Bar Musing 2.60/65 mm Taper Dies and Flat Face Dies A 22 billet batch of 2014A was extruded into a 57.15 mm diameter bar using a standard flat faced die and a 2.60 taper die having a 5mm parallel bearing as shown in Figure 7b. Six billets were extruded through the normal die followed by six through the taper die followed by a further ten billets through the normal die. Press and other conditions were as follows: Container temperature 3900C Container diameter 225 mm Billet temperature 430-4500C Exit speed 8 ft/min normal die 12 ft/min taper die Discard 15% Billet length 1140 mm Billet homogenisation 8 hrs/4750C Formal solution treatment at 5050C.

Figure 8 shows PCG results for this batch measured at the back end of the extruded length. The taper die gave zero PCG compared to 1.5-2 mm for the flat faced. Note that compared to the 88.9 mm diameter bar described in Example 3 the PCG depth is shallower for the smaller bar diameter. The maximum exit speed achieved before surface break-up was 12 ft/min for the taper die compared to 8 for the conventional die. Again the 2.60 taper die eliminated PCG and gave increased productivity.

EXAMPLE 6 PCG Levels Generated for an 80 mm Square Bar using a 1.50 Taper Die, 65 mm Lons An 80 mm square bar of 2014A alloy was produced using a 1.50 taper with a 65 mm bearing length as shown in Figure 9. This section had the same extrusion ratio as the 88.9 mm diameter bar described in Example 3. Press and other conditions were identical to those for Example 3 with the exception that the exit speed was limited to 8 ft/min due to surface break up at the corners above this speed. No PCG was formed on this material. In this case the 1.50 taper eliminated PCG but did not allow increased productivity.

EXAMPLE 7 PCG Control for 88.9mm diameter bar produced using a Taper Die with a 2.60 Taper Ankle and 65mm Taper length for 2017 and 2024 Allovs Two six billet batches of 2017 and 2024 containing controlled compositions of recrystallisation inhibitors, were extruded through a taper die having a 2.60 taper angle with a 65 mm taper length followed by a standard parallel 5 mm bearing. The die design is shown in Figure 7b. The press and homogenising conditions utilised were the same as in Example 3.

After formal solution treatment, at 5000C for 2017 or at 4950C for 2024, similar PCG levels to those observed for 2014 were obtained, e.g. zero to 1 mm over the billet temperature range of 420 - 4500C.

Claims (9)

1. A method of forming an extruded section comprising the steps: providing a homogenised billet of a 2000 series extrudable aluminium alloy containing an effective concentration of a recrystallisation inhibitor; hot extruding the billet at a temperature chosen to avoid recrystallisation; through an extrusion die which has a die aperture including a bearing section and an upstream section, wherein the upstream section has an axial length of 12 to 240 mm and a positive taper of 0.50 to 100.

2. A method as claimed in claim 1, wherein the length of the upstream section of the die aperture is 30 - 100 mm.

3. A method as claimed in claim 1 or claim 2, wherein the upstream section of the die aperture has a positive taper of 1 - 4.50.

4. A method as claimed in any one of claims 1 to 3, wherein the die aperture has a minimum transverse dimension of at least 5 mm.

5. A method as claimed in any one of claims 1 to 4, wherein the extruded section is subjected to solution heat treatment.

6. A method as claimed in any one of claims 1 to 5, wherein the extrudable Al alloy is 2014 containing 0.8 - 1.2% Mn.

7. A method as claimed in any one of claims 1 to 6, wherein the billet has been homogenised at 4500 5000C.

8. A method as claimed in any one of claims 1 to 7, wherein the billet is extruded at a temperature of 4000 - 4800C.

9. A method as claimed in any one of claims 1 to 8, wherein the extruded section is subjected to formal solution treatment at 4900 - 5200C for 2 - 12 hours.