US 3,113,052 discloses a thermal treatment of an alloy that will sustain a considerable precipita- tion of Mg and Si to coarse Mg2Si particles. The treatment involves a slow cooling rate after homogenization together with isothermal treatment at a temperature between 370-400°C.

Following the extrusion the alloy is solutionised at a temperature between 482-496°C. The reason for this low temperature solution treatment is to avoid grain growth during the solutionis- ing step. It is stated that the mentioned thermal treatment process will render a uniform grain structure in the profile after the solutionising treatment and consequently more even mechanical properties along the profile. The alloy disclosed should comprise 0.43-1.40 % Mg and 0.24- 0.80 % Si. In addition the alloy should contain at least one of the following elements: B, Ti, Cr, Mn, Mo, W and Zr in the amount 0.01-0.30 %, but not exceeding 0.75 % in total. The alloy may contain 0.05-0.50 % Cu. The homogenization step is performed at 427-566°C for 3-20 hours, followed by slow cooling that will reduce the content of Mg and Si in solid solution and be present as coarse Mg2Si particles instead. The billet is then heated rapidly to 427-454°C and extruded. The extruded product is then solutionised at a temperature of 482-496°C and quenched in water followed by ageing at a temperature between 149-232°C for 1-24 hours.

The purpose of the above mentioned thermal treatment is stated to be the achievement of a uniform grain structure in the profile after solutionising.

Using the low temperature during the solution treatment (482-496°C) will as stated in the patent minimize the chances for grain growth during this operation. However, when using this low temperature, the full strength potential of the alloy will not be utilized if the alloy composition is high, as for a normal 6061 alloy (0.6% Si, 0.9% Mg). With a normal 6061 alloy the solutionis- ing temperature has to be above 530°C to utilize the full strength potential of the alloy.

US 3,990,922 discloses a thermal treatment of an extrusion ingot, following the normal homog- enization treatment, at temperatures below the solvus temperature for an aluminium alloy aiming to precipitate particles to reduce the deformation resistance. The effect is due to a reduc- tion of the Mg and Si in solid solution because of precipitation of coarse Mg2Si particles. In the process the billet is homogenized at 557-607 °C for 2 to 12 hours. Further homogenization of the alloy is performed at a temperature that is 11-56°C below the solvus temperature of the alloy for 2-12 hours. The billet is then cooled at a rate lower than 38°C per hour down to 427°C. The billet is then preheated to temperatures between 427-552°C before extrusion. The extruded profile is cooled and aged without separate solutionising. The objective of using this method is stated to be an improvement of the processability by extrusion.

The disclosed example relates to an alloy 6061 of 1.0 % Mg and 0.7 % Si extruded at tempera- tures between 482-510°C, which implies that much of the alloying elements are present as non hardening Mg2Si particles after extrusion. Because a separate solution treatment step is not run in this case the full strength potential of the alloy is not utilized. This means that almost the same results for extrudability and strength can be obtained with a lower alloy composition, just running a normal homogenization and extrusion process.

US 4,659,396 relates to precipitation of Mg and Si in coarse Mg2Si particles that are intended to initiate nucleation of grains by the following extrusion process. This will render a more even and more fine-grained structure in the extruded profile. Otherwise the grain structure will be

uneven having some parts where recrystallization has not occurred and some other parts comprising relatively large grains. With the presence of even sized grains along the extruded profile better mechanical properties are achieved, in particular with respect to subsequent forming operations. In addition to the thermal treatment for precipitating coarse Mg2Si particles the patent describes a maximum content of dispersoid forming elements at 0.15 %, preferably below 0.10 %.

The alloy is an Al-Mg-Si type having Mg2Si particles for initiating nucleation of a high number density of grains, where the material is deformed under circumstances giving recrystallization during the deformation step or after the deformation step in absence of a separate thermal treat- ment to effectuate recrystallization. The content of alloying elements such as Mn, Cr and Zr should be at such a level that recrystallization will occur. Homogenization is performed at 527- 582°C for 0.5 to 10 hours. The billet is further treated thermally at temperatures between 315- 427°C for 5-24 hours. The cooling speed between the homogenization step and the further thermal treatment should be 8-40°C per hour. The billet is preheated to 343-482°C before it is extruded. Following the extrusion, the profile is solutionised at a temperature between 524- 563°C. The main objective of the invention is to obtain a fine grained recrystallized micro struc- ture in the material.

In the examples described in US Patent no. 4,659,396 the positive effect of the particle stimul- ated nucleation (PSN) treatment on the extrudability has not been mentioned, probably because they mainly focus on the grain structure of the extruded product (this PSN treatment could also be called a soft annealing treatment because such annealing would make the deformation resis- tance of the extrusion ingot much lower than a standard homogenization treatment would do). Because the extrusion speed potential is not utilized, the process described in this patent will be more costly than a standard process because of the time consuming PSN treatment prior to extrusion. If the extruded profile can be formed also after a standard homogenization treatment, such PSN treatment of the extrusion ingots will not be necessary and will be avoided due to the increased costs related to it.

Also, none of the mentioned patents do not describe how to utilize the positive effect on mechanical properties of a short storage time between solutionising and the age hardening treat- ment.

The reference"H. Bichsel und A. Ried, Wärmebehandlung, Fachberichte zum symposium der Deutche Gesellshaft fur Metallkunde in Bad Neuheim, 1973, pp. 173-192"discloses an experi- mental work demonstrating the effect of storage time at room temperature between solutionising and ageing for 6xxx alloys. In figure 7 of this work the strength of artificially aged material obtained after 24 hours storage time at room temperature minus the strength after no storage time is shown for different 6xxx alloys. This figure discloses a negative effect of a long storage time at room temperature for 6xxx alloys with Mg and Si contents above approximately 0.5% each. This article does not consider how to extrude these alloys and obtain reasonable extrusion speeds for high alloy compositions.

The present invention describes a process line with soft annealing of the extrusion ingots, preheating of the ingot, extrusion and cooling the profile, stretching and possible forming, solutionising the profile, quenching the profile, performing any required forming of the profile, forming the profile and ageing the profile as soon as possible after solutionising. The described process line is improved with respect to prior art solutions in that based on metallurgical knowl- edge on 6xxx types of alloys, it optimizes the extrusion speed, increases the formability of the extruded sections and maximizes the mechanical properties of the final product. By performing an ageing treatment for instance within 10 minutes after a solution treatment, the strength of high composition 6xxx alloys can be increased by 20-50 MPa as compared to 4 hours or longer storage time at room temperature before ageing.

In the automotive industry there is an increasing demand for a reduction of the weight of the automobiles in order to reduce fuel consumption. This can for example be done by using aluminium solutions with thin walled extruded profiles. However, reducing the wall thickness' of the extruded profiles will at the same time require the use of aluminium alloys which give higher strength values. The problem related to this is that both the use of thinner walls of the

profiles and the use of alloys that give higher strength values will reduce the extrusion speed of the profile.

These and other advantages can be achieved in accordance with the present invention as defined in the accompanying claims.

Based on the work done by H. Bichsel and A. Ried one can wonder why the positive effect of a short storage time of high strength 6xxx alloys has not been utilized to larger extent before. The reason for this is most likely because of the extra costs related to doing the solutionising of the profiles after extrusion, which is required in order to control the storage time before ageing.

It is only in the US patent 3,990,922, that the positive effect of soft annealing on the extrusion speed is indicated, but in this case no separate solution treatment was described. The process route described in that reference can thus not take advantage of the positive effect of the short storage time between solutionising and ageing. The other patents described above do not describe how to utilize the advantage of the soft annealing process on the extrusion speed, and do not mention the effect of the storage time between solutionising and ageing.

It is only when these two positive effects are combined in an inventive manner that the real economical potential is discovered. The synergy effects by the inventive combination of these two processes are very strong. With the present invention the soft annealing process that gives considerably higher extrusion speeds, and which requires solutionising of the extruded sections anyway, is combined with a short storage time between solutionising and ageing to get extra strength out of the material. With this combination it is possible to meet the demands for lower weight and higher strength of the extruded profiles, and still be able to have a relatively high productivity and thus low costs for the products.

Due to the soft annealing treatment much of the Mg and Si will be out of solid solution and be present as large Mg2Si particles after extrusion. Thus, the material will need to be solutionised in order to utilize the strength potential of the material. Both the soft annealing treatment itself

and the solutionising afterwards will be process steps which will increase the cost of producing finished parts from extruded sections. The increased costs of these steps would have to be compensated for in one way or the other. Examples shown in the present invention show that it is possible to double the extrusion speed by applying a soft annealing treatment. This will more than compensate for the extra cost related to both the soft annealing treatment and the solution- ising treatment.

As the material have a lot of large Mg2Si particles in the as extruded condition, the material will be soft and have a good formability. Thus, some kind of preforming of the blank or profile can advantageously be performed in this condition, before the solutionising step.

A common way to produce finished parts from extruded sections today is to perform the extru- sion operation on normal homogenized billets and ensure that most of the Mg and Si is present in solid solution after extrusion. The extruded sections will then be cut into the desired lengths and the forming of the profiles would be performed in an as extruded condition. One of the problems with this is that the extruded section naturally age at room temperature and the yield strength of the section increases when the storage time at room temperature becomes longer.

Since extrusion of the sections normally is not in-line with the subsequent forming operation, different storage times before forming will result. For more complicated products this may lead to problems achieving the dimensional tolerances of the formed parts due to differences in springback after forming. Having a separate solution treatment in-line with the subsequent forming operation in accordance with the present invention, the time between these operations will be short and constant. The result will be much more consistent mechanical properties in addition to better ductility of the material prior to the forming operation than after longer storage times at room temperature. This will be an additional benefit of the proposed process line as compared to a standard way of producing finished parts from extruded sections.

Further, with the solution treatment in-line with the subsequent forming operation it will be possible to do the ageing operation in-line, thus utilizing the positive effect of short storage times at room temperature on the mechanical properties for high strength 6xxx alloys. Without a

separate solution treatment process it will not be possible to take advantage of this possibility, and the strength of the finished product will be lower.

By utilizing and optimizing all the process steps together, the present invention will have an advantage over the former solutions in that the final product can be produced at minimum costs, with better tolerances and with higher and more consistent mechanical properties than obtained with the process routes described in the prior art discussed above.

The present invention will be further described by examples, figures and tables where: Fig. 1 shows a schematic representation of the process steps in an embodiment of the invention, Fig. 2 shows a schematic temperature-time curve for the homogenization and the soft annealing process, Fig. 3 shows a schematic temperature-time curve for the preheating of the billet, extrusion and the cooling of the extruded profile, Fig. 4 shows a schematic temperature-time curve for the solutionising and quenching of the extruded profile or blank as well as the forming operation and the final ageing, Fig. 5 shows a picture of the micro structure in a soft annealed ingot of alloy 6061, Fig. 6 shows a picture of the micro structure in a soft annealed ingot of alloy 6082, Fig. 7 shows the effect of the storage time between solutionising and ageing on the resulting strength for a 6082 alloy,

Table 1 shows an alloy composition of a 6061 alloy, Table 2 shows results from extrusion trials with a 6061 alloy, Table 3 shows heat treatment and mechanical properties of a 6061 alloy, Table 4 shows the composition of the 6082 alloy used in example 2, Table 5 shows results from extrusion trials with the 6082 alloy in example 2, Table 6 shows heat treatment and mechanical properties of the 6082 alloy in example 2, Table 7 shows the composition of the 6082 alloy used in example 3.

In Figure 1 the billet is homogenized in a first process step 1 which for 6xxx alloys is typically performed at temperatures between 520-600°C for 1-12 hours, see also Fig. 2. The purpose of this treatment is to level out the segregations of alloying elements like Mg and Si. In addition, homogenization of 6xxx alloys modify the Fe-bearing particles both in shape and composition.

For 6060/6063 type of alloys it is important to get the Fe-bearing particles, formed during casting of the extrusion ingot, transformed into the alpha type which seems to give better surface quality of the extruded sections. However, when the extrusion ingots are subjected to a soft annealing treatment, it may be an option to skip the normal homogenization treatment.

In step 2 the billet is subjected to a soft annealing treatment which have the purpose of remov- ing as much Mg and Si as possible from solid solution and tie the elements up in the form of relatively coarse Mg2Si particles. The reason for doing this is to reduce the deformation resis- tance as much as possible during the subsequent extrusion operation. In order to achieve this the Mg2Si particles have to be large enough to be relatively stable during the preheating operation before extrusion. The best way to obtain such a particle structure will be to cool the extrusion ingots relatively slow (5-50°C per hour) from the homogenization temperature down to a

temperature between 300-450°C where an isothermal heat treatment is performed. The duration of this isothermal heat treatment will normally range from 2-24 hours, see Fig. 2. The cooling rate from this temperature of isothermal treatment down to room temperature is not considered to be important, but preferably slower than 200°C per hour. All temperature time combinations leading to the desired micro structure with relatively coarse Mg2Si particles in the extrusion ingot will give approximately the same result in the subsequent extrusion operation. This might be a process route not involving a homogenization treatment prior to the soft annealing treat- ment. In order to reduce costs, the soft annealing treatment should be optimized with regard to time.

The optimum chemical composition for an extrusion ingot subjected to the soft annealing process will depend on what properties the end product should have. However, if the objective is both to maximize the extrudability and maximize the mechanical properties the optimum composition will have the following characteristics: The Mg/Si ratio should preferably be high enough to avoid Si particles in the material together with Mg2Si particles. With both these types of particles present at the same time melting of these particles will occur when the ternary eutectic temperature (Al + MgzSi + Si -> Liquid) is reached in the profile during extrusion. When this happens tearing of the profile will occur. This ternary eutectic temperature is approximately 555°C, which is considerably lower than the binary eutectic temperature of 595°C (A1 + Mg2Si-> Liquid).

Due to this fact, the extrusion speed before tearing occurs will be higher without Si particles in the material before extrusion.

The extrusion ingots should preferably not have too high amounts of dispersoid forming elements because this will increase the deformation resistance during extrusion and thus take away some of the effect of the soft annealing treatment. Also, if the amount of dispersoid elements is on the limit to nearly prevent recrystallization to occur an undesirable micro structure with coarse grains can be the result.

'he soft annealing process will work best for high alloy compositions. This is due to the fact that the extrusion speed for normally processed extrusion ingots becomes lower as the Mg and Si content increases, whereas for soft annealed billets the extrusion speed will be almost independent of the Mg and Si content because this treatment will leave the same amount of Mg and Si in solid solution. For low alloy compositions, like 6060 alloys, the extrusion speeds may be lower for soft annealed billets than for normally processed billets because the latter ones will have so low amounts of Mg2Si particles that the critical temperature for tearing will be higher than for the soft annealed billets. Thus, for normal 6060 billets tearing will be limited by the solidus temperature of the alloy, which is higher than for a soft annealed alloy which will have the binary eutectic as the critical temperature. Since the alloy composition is so low for 6060 alloys there will not be very much to gain on the deformation resistance by precipitating Mg2Si particles. Another reason why the optimized process as described here will work best for high alloy compositions, is that the positive effect of a short storage time at room temperature on the mechanical properties is highest for high alloy compositions. For an alloy with 0.5 weight (wt) % Si and 0.5 weight (wt) % Mg there is almost no effect of room temperature storage between solutionising and ageing on the mechanical properties. For lower alloy compositions the effect of short storage times is negative and more negative as the alloy content is reduced.

In step 3, the billet is heated to a set temperature and loaded into the extrusion press, see also Fig. 3. The most important thing to consider during preheating is to avoid significant re-dissolution of the Mg2Si particles that have been formed during the soft annealing process. Both the temperature and the retention time at this temperature are important parameters. At temperatures above approximately 400°C the time should be as short as possible, and therefore induction heating of the billets seems to be the best solution. However, a gas furnace with fast heating from approximately 350°C in the last few zones will also work well in most cases. Since the billet is relatively soft and there is no requirements on dissolving the Mg2Si particles during the extrusion process, the billet temperatures prior to extrusion will be considerably lower than used in a normal process today.

Step 4 of the process line is the extrusion process including cooling and stretching after extru- sion. Since most of the Mg and Si is precipitated as coarse Mg2Si particles the deformation resistance and hence the heat generation will be much less than for billets homogenized in a normal way. The presence of such coarse Mg2Si particles in the material to be extruded has in tests shown to give an increase in the extrusion speed by as much as 100% compared with extrusion of the same alloy where such particles have not been allowed to precipitate in a soft annealing process. With the described process line the requirements on cooling after extrusion are not very strict, and the only thing that matters is to have the profiles cold enough to be stretched. Subsequent the extrusion, the profile or blank may advantageously undergo a preforming step while the deformation resistance is low as a consequence of the coarse Mg2Si particles in the material.

In step 5, blanks of the extruded section are solutionised to dissolve the Mg-Si particles formed in the soft annealing step, see also Fig. 4. In order to dissolve all the Mg2Si particles to utilize the full age hardening potential, the solutionising temperature has to be above the solvus temperature of the alloy. For a normal 6061 alloy with approximately 0.9 wt % Mg and 0.6 wt % Si, this temperature is between 530 and 540°C. To reduce the necessary time for the dissolu- tion of the Mg2Si particles a temperature of 10-20°C above this temperature will probably be the best practical choice of solutionising temperature. On the other hand, a too high solutionising temperature may lead to severe grain growth in the extruded sections and thus impair the properties of the finished product. After the solution heat treatment, the blanks of the extruded sections have to be cooled as quickly as possible down to room temperature in order to maintain a high age hardening potential. State of the art knowledge on profile quenching should be used to prevent too much geometrical distortions of the blanks.

The final forming and processing operation can be carried out in step 6. After quenching from the solutionising temperature the aluminum lattice contains a large amount of vacancies. These vacancies will tend to migrate towards heterogenities in the aluminium lattice and the concen- tration of vacancies will decrease with time at room temperature. If the material is deformed for example by bending, a lot of dislocations are generated in the aluminium lattice. These

dislocations will sweep through the material and may thus remove some of the vacancies which are present. Since these vacancies play an important role during the nucleation of the hardening precipitates, some of the hardening potential may be lost if a portion of the vacancies is removed. In case this effect is not very large, the forming operation can be performed right away. Otherwise, the profiles or the blanks can be subjected to a short annealing treatment at a temperature between 90 and 230°C for 1-120 minutes, preferably in an oil bath before the forming operation. The reason for doing this short annealing treatment before forming is thus to nucleate a high density of Mg-Si hardening particles which are stable enough to survive the forming operation without going into solid solution again.

The mechanical properties both directly after quenching or after a short ageing treatment prior to the forming operation will be consistent. Consistent mechanical properties in the extruded blanks before processing in forming step (s) will contribute to less geometric deviation of the measures in the finished product. The forming step (s) may involve processing such as bending, forging or hydroforming.

The formed profile or blank is finally treated in an ageing operation, step 7, to increase the mechanical properties of the product. In this step the profile is thermally treated at temperatures between 140-230°C for a period of time which may range from 1 to 24 hours depending on the temperature chosen for the ageing treatment. As mentioned earlier, literature data and tests have shown that high alloyed 6xxx alloys may gain a substantially higher mechanical strength by the ageing treatment if it is performed immediately after the solutionising step (an increase of 5-15 %).

When a precipitate hardening alloy like AlMgSi is quenched down to room temperature, reactions will take place at this temperature and the solute atoms Mg and Si will tend to cluster into what in the literature typically is called GP zones. The number of clusters will increase and the sizes of the clusters will increase with retention time at room temperature. When a material containing GP zones formed at room temperature is subjected to ageing at a higher temperature the resulting number density of Mg-Si hardening precipitates will depend on several factors. If

there is a reversion (dissolution) reaction before the final precipitation of the hardening phases this will result in a lower number density of Mg-Si hardening particles and thus lower strength. For short storage times at room temperature there will be none or only very small GP zones and there will probably be no reversion reaction before precipitation of the Mg-Si hardening precipi- tates. In addition, a high concentration of vacancies originating from quenching after solutionis- ing will support the nucleation reactions and probably give a higher density of Mg-Si hardening precipitates than without this excess in vacancy concentration. As the time increases the GP zones are becoming larger and there will probably be some reversion reactions before nuclea- tion of the Mg-Si hardening precipitates takes place. Also, the concentration of vacancies will be lower, resulting in a lower density of Mg-Si hardening precipitates and a corresponding lower strength. This may explain why high strength 6xxx alloys show this kind of behavior.

In the following there are shown test results of different alloys treated in accordance with the present invention. Each alloy is indicated by its commonly used alloy name and its contents of alloying elements.

Example 1 In this example a 6061 alloy with the composition given in table 1, was cast to 95 mm diameter extrusion ingots. Alloy Mg Si Mn Cr Fe Cu Others sum 6061 0, 76 0,57 0,02 0, 07 0, 18 0, 15 < 0. 03 Table 1. Alloy composition in weight % of the 6061 alloy The soft annealing treatment of the alloy was done as follows: Heating to 575°C within 3 hours, 3 hours hold at 575°C, cooling at 25°C per hour down to 400°C, 8 hours hold at 400°C followed by cooling in still air. Figure 5 shows a picture of the micro structure after this annealing treatment. As can be seen, the material has a lot of Mg2Si

particles with diameters in the range of 3-10 J, m. The lighter gray particles are primary particles containing originating from the casting.

The normally processed billets were homogenized at 575°C for 3 hours, and then cooled at a rate of approximately 350°C per hour down to 200°C, and at a rate of 150°C per hour from 200°C down to room temperature.

The extrusion tests were done at an 800 ton press equipped with an induction furnace for heating the billets. The heating rate to the final preheating temperature was approximately 80°C per minute.

In table 2 the extrusion speeds for similar billet temperatures are listed for both normal processed billets and soft annealed billets. Billet Breakthrough Alloy condition temperature Ram speed Force Profile quality temperature force 6061Normal 454°C 12mm/s 4314 kN OK 6061 Normal 450°C 13 mm/s 4377 kN OK 6061 Normal 454°C 15 mm/s 4380 kN Tearing 6061 Normal 449°C 14 mm/s 4354 kN Small tearing 6061 Soft 451°C 15 mm/s 3851 kN OK 6061 Soft 451°C 17 mm/s 3932 kN OK 6061 Soft 450°C 19 mm 3850 kN OK 6061 Soft 450°C 21 mm/s 3941 kN OK 6061 Soft 447°C 23 mm/s 3964 kN OK 6061 Soft 449°C 25 mm/s 3918 kN Tearing Table 2. Results from extrusion trials with 6061 alloy and 9 mm diameter round bar.

As can be seen from the table, the profiles from the soft annealed billets can be run approxi- mately twice as fast as the normal processed billets before tearing occurs. This is mainly due to the lower deformation resistance for these billets as indicated by the lower breakthrough pressure.

Given a 6061 alloy with higher Mg and Si content, the difference in extrusion speed will proba- bly be even higher, because normal processed billets extrude slower as the Mg and Si content increases. Also, if a lower temperature than 400°C for the isothermal treatment is applied a further reduction of the amount of Mg and Si in solid solution will result. This will reduce the deformation resistance and increase the effect of the soft annealing treatment, thus giving even better extrudability for the soft annealed billets.

The mechanical properties of extruded 1.9 x 25 mm'profiles are shown in table 3. After extru- sion the profiles were cut into pieces suitable for making tensile samples, and heat treated as shown in table 2 below. One of the profiles was made from soft annealed billets while the other one was made from a normally homogenized billet. In the latter case the billet was overheated: i. e. heated to 550°C to dissolve all Mg2Si particles and then quenched to 500°C just before extrusion. With this treatment the full strength potential of the alloy is utilized. Solution Storage time Yield Tensile Elongat treatment at room Ageing Strength Strength ion temperature [MPa] [MPa] [%] 6061 Normal-4 hours 10 hours at 175'C 285 314 12,8 6061 soft 20 min at 550°C 4 hours 10 hours at 175°C 285 312 14,8 6061 soft 20 min at 550°C 2 minutes 10 hours at 175°C 301 330 14, 2 Table 3. Heat treatment and mechanical properties of the 6061 alloy.

The table shows that the ultimate tensile strength has increased by 18 MPa by reducing the storage time from 4 hours to 2 minutes between solutionising and ageing, and the yield strength has increased by 16 MPa. With a higher alloy composition of the 6061 alloy the difference in mechanical properties in favor of the short storage time is expected to be higher. As will be seen from the table, there is no significant difference between the normally processed billet and the soft annealed billet in the situation where the soft annealed billet is stored as much as 4 hours at room temperature. Thus, the improvements in yield strength and tensile strength are directly related to the duration of the storage time before ageing.

Example 2 In this example a 6082 alloy with the composition given in table 4 was cast to 95 mm diameter extrusion ingots.

Alloy Mg Si Mn Cr Fe Cu Others sum 6082 0,86 1,02 0,55 0,15 0,19 0,00 < 0. 03 Table 4. Alloy composition in weight % of the 6082 alloy used in example 2.

The soft annealing treatment of the alloy was done as follows: Heating to 525°C within 3 hours, 4 hours hold at 525°C, cooling at 25°C per hour down to 400°C, 8 hours hold at 400°C followed by cooling in still air. Figure 6 shows a picture of the micro structure after this annealing treatment for the 6082 alloy. As for the 6061 alloy, the material has a lot of Mg2Si particles with diameters in the range of 3-10 Am. The lighter gray particles are primary particles originating from the casting.

The normal processed billets were homogenized at 525°C for 4 hours, and then cooled at a rate of approximately 350°C per hour down to 200°C, and at a rate of 150°C per hour from 200°C down to room temperature.

The extrusion tests were done at an 800 ton press equipped with an induction furnace for heating the billets. The heating rate to the final preheating temperature was approximately 80°C per minute.

In table 5 the extrusion speed for similar billet temperatures are listed for both normal processed billets and soft annealed billets of 6082. Billet Breakthrough Alloycondition temperature Ram speed Force Profile quality temperature force 6082 Normal 463°C 9 mm/s 4912 kN Tearing 6082 Normal 465°C 8.5 mm/s 4932 kN Tearing 6082 Normal 466°C 7.5 mm/s 4819 kN OK 6082 Normal 456°C 8 mm/s 4958 kN OK 6082 Normal 454°C 8.5 mm/s 4964 kN Tearing 6082 Soft 452°C 12 mm/s 4611 kN OK 6082 Soft 455°C 12 mm/s 4588 kN Tearing 6082 Soft 457°C 11 mm 4605 kN OK 6082 Soft 454°C 8 mm/s 4669 kN OK 6082 Soft 460°C 10 mm/s 4634 kN OK

Table 5 Results from extrusion trials with 6082 alloy and 9 mm diameter round bar.

As can be seen from the table, the profiles from the soft annealed billets runs approximately 50% faster than the normal processed billets. In this case with the 6082 alloy the difference between the soft annealed billets and the normal processed billets is less than for the 6061 alloy.

This is because of the high number of dispersoid particles in this particular 6082 alloy affecting the deformation resistance and thus the breakthrough pressure. Both the Mn and the Cr content and a rather low homogenization temperature contribute to this high number of dispersoid parti- cles. For this reason, lowering the amount of Mg and Si in solid solution will not relatively reduce the deformation resistance as much as it does for the 6061 alloy.

The mechanical properties of extruded 1.9 x 25 mm2 profiles are shown in table 3. After extru- sion the profiles were cut into pieces suitable for making tensile samples, and heat treated as shown in table 6 below. Solution Storage time Yield Tensile Elongat treatment at room Ageing Strength Strength ion temperature [MPa] [MPa] [%] 6082 soft 20 min at 550°C 4 hours 10 hours at 175'C 318 345 15, 0 6082 soft 20 min at 550°C 2 minutes 10 hours at 1750C 357 388 15,2 Table 6. Heat treatment and mechanical properties of the 6082 alloy.

The table shows that the ultimate tensile strength has increased by 43 MPa by reducing the storage time from 4 hours to 2 minutes between solutionising and ageing, and the yield strength has increased by 39 MPa.

Example 3 In this example a 6082 alloy with the composition given in table 7 was cast to 203 mm diameter extrusion ingots. Alloy Mg Si Mn Cr Fe Cu Others sum 6082 0,63 0,92 0,52 0,15 0,20 0,00 <0.03 Table 7. Alloy composition in weight % of the 6082 alloy used in example 3.

In this case the extrusion ingots were homogenized at 580°C for 3 hours and then cooled at a rate of approximately 350°C per hour down to room temperature. The extrusion ingots were then extruded in an industrial press to a hollow profile. Tensile samples where then made from the extruded profile. The samples were solutionised at 540°C for 30 minutes and quenched in water. After the wanted storage time the samples were put in an ageing oven preset at a temperature of 160°C. The ageing cycle was 8 hours at 160°C. Each point in Figure 5 represent the average of 3 tensile samples.

As can be seen from Figure 7, the mechanical strength is considerably higher for short storage times at room temperature (10 minutes and 1 hour) than after storage times of 4 hours or longer.

Also, the 10 minutes storage time has a considerably higher strength than the 1 hour storage time at room temperature before ageing. Thus, the curves indicate that the storage time should be as short as possible in order to obtain maximum strength. It is also quite evident from Figure 7 that there is very little difference between a storage time of 4 hours and longer storage times at room temperature before ageing.

These test results indicate the increase in extrusion speed as compared to that of commonly used methods, and also the resulting increase in mechanical strength in the final product as compared to that of prior art method.