The present invention relates to AA 6xxx series alloys and a method for producing extruded profiles or extruded solid bars, which are subjected to further processing to obtain products with good mechanical properties at reduced costs.

BACKGROUND

From WO 02/38821 A1 a process is described which involves thermal treatment of a billet or ingot before the extrusion process is initiated, as well as a subsequent thermal and mechanical treatment of the extruded blank. The aim of the thermal treatment before extrusion is to increase the extrusion speed while the subsequent treatment of the extruded blank involving thermal treatment and required forming processes gives a product that sustains good mechanical properties.

WO 02/38821 A1 specifies an alloy with Mg in the range of 0.5-2.0 wt % and Si in the range of 0.5-2.0 wt%, Fe in the range of 0-1 .0 wt%, Cu in the range of 0-1.0 wt%, Zn in the range of 0-3.0 wt% and other elements not specified below in the range 0-0.2 wt %. The alloy can contain dispersoid forming elements Mn in the range of 0-1.5 wt%, Or in the range of 0-1 .0 wt% and Zr in the range of 0-0.3 wt%. In the three examples described there are one AA6061 and two AA6082 alloys. The AA6061 alloy in example 1 contains 0.76 wt% Mg, 0.57 wt% Si, 0.15 wt% Cu, 0.07 wt% Cr, 0.18 wt% Fe and 0.02 wt% Mn. Alloys according to this patent does not provide the required ductility.

WO 02/38821 A1 recommend a cooling rate of 5-50°C per hour from the homogenizing temperature to the isothermal heat treatment temperature between 300 and 450°C. In example 1 of the prior art the extrusion billets of the 6061 alloy are homogenized at 575 °C for 3 hours before they are cooled by 25 °C/hour down to 400 °C where they are held for 8 hours before further cooling down to room temperature. This treatment gives a microstructure of the extrusion billets with a high number of Mg ₂ Si particles with diameters in the range 3-10 pm.

The cooling rate that is recommended in WO 02/38821 A1 is disadvantageous in two ways; a) the total cycle time for the soft annealing process is considerably increased; b) it takes much longer time to dissolve the Mg ₂ Si particles in the solutionizing step that takes place after extrusion.

There is therefore a need for a method for producing extruded profiles or extruded solid bars with good mechanical properties at reduced production costs.

SUMMARY OF THE INVENTION

The present invention describes a similar process as described in WO 02/38821 A1. However, the present invention provides a method resulting in reduced production cost and with more optimized alloys to increase both the strength and the ductility of the final product.

According to a first aspect, the present invention provides:

A method for processing extrusion billets produced from an aluminium 6xxx alloy containing Mg in the range 0.85-1.15 wt % and Si in the range 0.60-0.75 wt%, Fe in the range 0-0.5 wt%, Cu in the range 0-0.30 wt%, Cr in the range 0-0.10 wt%, Mn in the range 0-0.20 wt%, Zn in the range 0-0.5 wt%, Ti in the range 0-0.15 wt%, V in the range 0-0.15 wt% and other elements not specified below in the range 0-0.05 wt%, with balance Al and unavoidable impurities, wherein the Mg/Si _eff ratio of the alloy is above 1 .6, where Si _eff = Si - % wt%(Fe+Cr+Mn+Zr), the method comprises the steps:

(a) homogenizing the extrusion billet,

(b) soft annealing the extrusion ingot or billet at a temperature between 350 and 450 °C,

(c) preheating the extrusion billet,

(d) extruding the extrusion ingot or billet to form a profile or blank,

(e) cooling the profile or blank down to room temperature,

(f) exposing the profile or blank to a solutionizing and quenching operation,

(g) optionally stretching the extruded profile or blank,

(h) artificially ageing the profile, where the cooling rate from the homogenization temperature in step (a) to the soft annealing temperature in step (b) is at least 100 °C per hour.

The temperature of the soft annealing in step (b) may be between 380 °C and 420 °C.

The temperature of the homogenization step (a) may be between 540 to 590 °C.

The cooling rate from the homogenizing temperature to the soft annealing temperature lying between 350 and 450 °C, preferably 380-420 °C, is at least 100°C per hour, more preferably at least 200 °C per hour. Between 350 and 450 °C the maximum amount of Mg ₂ Si particles is precipitated and thereby the material is made as soft as possible. The most optimum temperature range for precipitation is between 380-420 °C.

The temperature of the preheating of the extrusion billet, step (c), may be from 350 to 450 °C, such as between 380 and 420 °C.

The minimum concentration of Mg is preferably 0.90 wt%, or more preferably the minimum concentration of Mg is 0.95 wt%. A higher Mg content is positive for ductility. This also allows to increase the Si content and still have a good Mg/Si ratio.

The concentration of Si is preferably 0.63-0.72, more preferably 0.65-0.70 wt% in order to maximize strength while being adapted to the preferred Mg content.

The minimum concentration of Ti is preferably 0.05 wt%, more preferably 0.07 wt% in order to increase the corrosion resistance. The content of Ti is limited to 0.15 wt% in order not to have a too negative effect on extrudability and avoid precipitation of primary AI ₃ Ti particles.

The maximum concentration of Cu is 0.20 wt%, or more preferably 0.15 wt%, or even more preferably 0.10 wt%. By limiting the copper content, a higher extrusion rate can be used since the required extrusion pressure diminishes.

The solutionizing temperature may be between 540 °C and 580 °C. It is preferred that the solutionising temperature is above 555°C, such as above 560°C, or above 565°C. A high solutionizing temperature enables dissolution of as much Mg ₂ Si particles as possible at a short time.

The present invention further provides, according to a second aspect: An extrusion billet produced with the method according to the invention where the average Mg ₂ Si particle diameter of the microstructure of the ingot is in the range 2-5 pm, or more preferably in the range 2-4 pm, and the average diameter is calculated based on the measured area in LOM (Light Optical Microscope) in a plane in a polished cross section converted to a circle. The extrusion billet is of an aluminium 6xxx alloy containing Mg in the range 0.85-1.15 wt % and Si in the range 0.60-0.75 wt%, Fe in the range 0-0.5 wt%, Cu in the range 0-0.30 wt%, Cr in the range 0-0.10 wt%, Mn in the range 0-0.20 wt%, Zn in the range 0-0.5 wt%, Ti in the range 0-0.15 wt%, in the range 0-0.15 wt% and other elements not specified below in the range 0-0.05 wt%, with balance Al, where the Mg/Si _eff ratio of the alloy is above 1 .6, where Sieff = Si - % wt%(Fe+Cr+Mn+Zr). According to a third aspect, the present invention also provides a semi-finished or finished product produced with the method according to the invention wherein the yield stress Rp0.2 is at least 300 MPa, preferably above 310 MPa, more preferably above 320 MPa. The semifinished or finished product comprises an aluminium 6xxx alloy containing Mg in the range 0.85-1.15 wt % and Si in the range 0.60-0.75 wt%, Fe in the range 0-0.5 wt%, Cu in the range 0-0.30 wt%, Cr in the range 0-0.10 wt%, Mn in the range 0-0.20 wt%, Zn in the range 0-0.5 wt%, Ti in the range 0-0.15 wt%, in the range 0-0.15 wt% and other elements not specified below in the range 0-0.05 wt%, with balance Al, where the Mg/Si _eff ratio of the alloy is above 1 .6, where Si _eff = Si - % wt%(Fe+Cr+Mn+Zr).

Unless otherwise stated the AA 6xxx series alloys as referred to herein refers to AIMgSi alloys as listed in the “International Alloy Designations and Chemical Composition Limits for Wrought Aluminum and Wrought Aluminum Alloys” published by The Aluminum Association.

Unless otherwise stated all alloy compositions are expressed as percentage by weight based on the total weight of the alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1. Schematic view of the process for manufacture of extruded Al products according to the invention.

Figure 2a. Microstructure of extrusion billets homogenized at 575°C for 3 hours and then cooled to 350°C using a cooling rate of 25°C per hour, and held there for 8 hours.

Figure 2b. Microstructure of extrusion billets homogenized at 575°C for 3 hours and then cooled to 350°C using a cooling rate of 250°C per hour.

Figure 3. Engineering yield stress, Rp0.2, the engineering ultimate stress, Rm and the true strain, sf, for the 5 alloys in Table 2.

Figure 4. Engineering yield stress, Rp0.2, the engineering ultimate stress, Rm, engineering total elongation, A and the true strain, sf are shown for the 4 alloys in Table 3. The left column for each alloy shows the engineering yield stress, Rp0.2, and the right column for each alloy show the ultimate tensile stress, Rm. DETAILED DESCRIPTION

The present disclosure describes a process with soft annealing of extrusion billets of an AA 6xxx alloy, preheating the billet, extrusion and cooling the profile, solutionising cut lengths of the profile, quenching the profile, performing any required forming of the profile, and ageing of the profile or formed part.

In the present disclosure the terms “extrusion ingot” and “extrusion billet” may be used interchangeably and should be understood to denote the same; a length of semi-finished casting products to be further processed into an extruded profile or blank, unless otherwise stated.

The term “solutionizing” corresponds to “solution heat treatment” in the present disclosure.

According to the first aspect, the present invention provides a method for processing extrusion billets produced from an aluminium 6xxx alloy containing Mg in the range 0.85-1.15 wt%

Si in the range 0.60-0.75 wt%

Fe in the range 0-0.5 wt%

Cu in the range 0-0.30 wt%

Cr in the range 0-0.10 wt%

Mn in the range 0-0.20 wt%

Zn in the range 0-0.5 wt%

Ti in the range 0-0.15 wt% in the range 0-0.15 wt% and other elements not specified in the range 0-0.05 wt%, with balance Al and unavoidable impurities, wherein the Mg/Si _eff ratio of the alloy is above 1.6, and

Sieff = Si - % wt%(Fe+Cr+Mn+Zr), and wherein the method comprises the steps:

(a) homogenizing the extrusion billet,

(b) soft annealing the extrusion billet at a temperature between 350 and 450 °C,

(c) preheating the extrusion billet,

(d) extruding the extrusion billet to form a profile or blank,

(e) cooling the profile or blank down to room temperature,

(f) exposing the profile or blank to a solutionizing and quenching operation,

(g) optionally stretching the extruded profile or blank,

(h) artificially ageing the profile, where the cooling rate from the homogenizing temperature in step (a) to the soft annealing temperature in step (b) is at least 100 °C per hour. In the homogenizing step the extrusion billets are heated to a temperature above the solvus temperature of the AA 6xxx alloy. A typical homogenization temperature according to the present method lies in the range 540-590 °C. A typical time of the homogenization step (a) may be up to 6 hours. In a standard, traditional process the extrusion billets are cooled from the homogenization temperature all the way down to room temperature at a cooling rate typically between 200 and 500°C/hour. According to the present method, a soft annealing step follows the homogenizing process, and the purpose of the soft annealing step is to reduce the deformation resistance of the extrusion billet. Reduction of the deformation resistance is achieved by precipitating a large volume fraction of Mg ₂ Si-particles. The amount of Mg tied up in the Mg ₂ Si-particles leads to a reduced amount of Mg in solid solution, and this especially contributes to the reduced deformation resistance. Si in solid solution has a much lower impact on the deformation resistance of the alloy.

The obtainable extrusion speed with an extrusion billet of an AA 6xxx alloy is not only affected by the deformation resistance of the material but also by the melting temperature of the material since melting reactions in most cases are the reason for getting tearing in an extruded profile. A highly alloyed AA 6xxx material will in most cases have some Mg ₂ Si- particles present in the material. During extrusion there will be a sharp increase in the temperature due to the deformation of the material when it is pushed through an extrusion die, and if the extrusion speed is too high there will be a melting reaction with tearing of the profile as the result. With a balanced content of Mg and Si to form Mg ₂ Si-particles the melting temperature is determined by the binary eutectic temperature between aluminium matrix plus Mg ₂ Si-particles, which is around 595°C (quasi-binary section according to H.W.L. Phillips in “Annotated Equilibrium Diagrams of Some Aluminium Alloy Systems”, Institute of Metals, 1959). If the aluminium matrix contains excess Si in solid solution the eutectic temperature will gradually decrease. If the excess Si is high enough to form Si-particles together with Mg ₂ Si-particles the ternary eutectic temperature between aluminium matrix plus Mg ₂ Si- particles plus Si-particles will drop to about 555°C. Thus, to maximize the melting temperature and thereby maximizing the extrusion speed it is an advantage to have composition close to the balanced content of Mg and Si to form Mg ₂ Si-particles.

The cooling down from the homogenizing temperature to a temperature in the range 350 to 450°C (the soft annealing temperature), and an isothermal hold temperature in that range to precipitate as much Mg ₂ Si-particles as possible, can be done at different cooling rates. However, as mentioned above, the slow cooling rate that is recommended in WO 02/38821 A1 is disadvantageous as the total cycle time for the soft annealing process is considerably increased; and it takes much longer time to dissolve the Mg ₂ Si particles in the solutionizing step that takes place after extrusion.

By performing a faster cooling of the extrusion billet after the homogenizing step the nucleation of Mg ₂ Si-particles is delayed and the matrix becomes more supersaturated by Mg and Si. This leads to precipitation of a significant higher number of Mg ₂ Si-particles which again results in a smaller average particle size of the Mg ₂ Si-particles. According to the present disclosure the cooling rate from the homogenization step to the soft annealing step is at least 100 °C per hour, and preferably at least 200 °C per hour. The effect of the soft annealing is determined by the amount of Mg left in solid solution, which again is affecting the deformation resistance. A high number of smaller Mg ₂ Si-particles is more effective in draining the aluminium matrix from Mg atoms than fewer and larger Mg ₂ Si-particles. This may reduce the necessary time at the final temperature between 350 and 450 °C. The holding time at the soft annealing step may typically be between 2 to 5 hours. The cooling rate from the soft annealing step is typically 200-500 °C per hour. Thus, by the relatively fast cooling from the homogenizing step and the following soft annealing step it is achieved an extrusion billet where the average Mg ₂ Si particle diameter of the microstructure of the ingot is in the range 2-5 pm, or more preferably in the range 2-4 pm. The average diameter may be calculated based on a measured area in LOM (Light Optical Microscope) in a plane in a polished cross section converted to a circle.

In order to extrude a billet of AA 6xxx alloy the billet needs to be preheated to a temperature typically in the range 400-520°C. The choice of billet temperature depends on the available pressure of the extrusion press and how hard it is to push the material through the die. A highly alloyed material will be harder to push (extrude) and a complex die for a hollow section would require a higher press force than a simple die for a solid section. In a standard traditional process where the extruded profiles are cooled and aged without any separate solutionizing afterwards the choice of billet temperature must consider that the Mg ₂ Si- particles formed during cooling after homogenizing need to be dissolved during the extrusion process to get the required strength of the material. This will in many cases restrict the lower temperature limit of the billet. With the soft annealed billets where cut lengths of the extruded profile are separately solutionized after the extrusion one does not need to account for the strength potential of the as extruded profile. Thus, only the press force and the type of extrusion die limit the choice of billet temperature. A low billet temperature is beneficial because this can allow for more deformation heating and a higher extrusion speed before tearing occurs in the profile. Due to the soft annealing step according to the present invention it is possible to have much lower billet temperature than with standard homogenized billets of the same alloy.

The highest extrusion speed is obtained if the preheating temperature before extrusion is the same or lower than the isothermal hold temperature in the soft annealing process. If a higher preheating temperature is needed due to lack of available press force the Mg ₂ Si-particles will start to dissolve and the effect of the soft annealing process is reduced.

After extrusion the profiles may be cut to shorter lengths and heated to a temperature above the solvus temperature to dissolve all the Mg ₂ Si-particles that were formed during the cooling and soft annealing process following the homogenizing process. The choice of temperature in the solutionizing process should be selected to minimize the necessary time at the temperature. If the solutionizing temperature is exactly at the solvus temperature dissolving all the Mg ₂ Si-particles requires that all Mg and Si atoms are evenly distributed in the material and the time to dissolve the particles completely would be very long. At a higher temperature there are two effects that reduces the solutionizing time: 1) the Mg and Si atoms do not have to be evenly distributed in the material and the average diffusion distance for the atoms is shorter; 2) the diffusion coefficient increases, and the atoms are travelling faster in the material.

As mentioned above, it is also important to reduce the size of the Mg ₂ Si-particles. According to the theory of dissolution of solid spherical particles in a metal (e.g., M.J. Whelan, Journal of Metal Science, 1969, Vol 3. P. 95) the time to dissolve a particle is proportional with the square of the particle size which implies that it takes 4 times longer to dissolve a particle that is twice as big.

To not limit the production capacity, a solutionizing furnace should be able to heat the cut profiles to a solutionizing temperature at a rate that keeps up with the subsequent production capacity. If the necessary solutionizing time is long due to large Mg ₂ Si-particles this will require a significant investment of solutionizing furnace capacity. Therefore, ensuring that the Mg ₂ Si-particles are relatively small is important. According to the present method, the solutionizing temperature is between 540 °C and 580 °C, preferably above 555 °C, or above 560 °C, or above 565 °C.

Example 1

An alloy with 0.90 wt% Mg, 0.65 wt% Si, 0.25 wt% Fe and 0.05 wt% Cr and the balance aluminium and impurity elements at levels below 0.02 wt% was cast to billets. The billets were homogenized at 575°C for 3 hours and then cooled at different rates down to the heat treatment temperature 350°C and held there for 8 hours. The picture to the left (Figure 2a) shows the microstructure of an extrusion ingot cooled at 25°C per hour whereas the right picture (Figure 2b) shows the microstructure of an extrusion ingot cooled at 250°C per hour (Mg ₂ Si particles are the ones with the darkest grey color). The reason for cooling fast to the soft annealing temperature is to nucleate many Mg ₂ Si particles. In this way the particle sizes will become smaller. This is beneficial for the solutionizing process because smaller particles will require shorter solutionizing times for total dissolution.

The Mg ₂ Si particles in the left picture of Figure 2 are typically in the range 5-15 pm whereas the Mg ₂ Si particles in the right picture are typically in the range 2-5 pm. In a dissolution process that is controlled by diffusion of the solute elements, the time to dissolve a particle is proportional with the square of the size of the particle. Thus, a particle with diameter 10 pm takes 11 times longer to dissolve than a particle of diameter 3 pm. Clearly the particle size will have a large effect when designing an industrial production line for separate solutionizing of the profiles produced according to the prior art. For the casthouse producing the extrusion billets a higher cooling rate from the homogenization treatment down to the temperature where the isothermal treatment (soft annealing) is conducted will reduce the cycle time and the production cost.

Example 2

Extrusion billets with diameter 95 mm of 5 different alloys with the chemical compositions shown in Table 1 were homogenized at 575°C for 2 hours, cooled with a cooling rate of 200°C per hour down to 400°C and held there for 4 hours before cooling down to room temperature at a rate of 200°C per hour. The extrusion billets were then preheated to temperatures between 400 and 420°C and extruded to a hollow rectangular section with outer dimensions 29 mm by 37 mm and with a wall thickness all around the cross section of 2.8 mm., whereafter the profiles were cooled in air to ambient temperature.

Table 1. Chemical compositions of alloys tested in example 2 in wt% including incidental impurities and with balance Al.

The profiles were subsequently separately solutionized at 550°C for about 30 minutes and quenched in water before being stretched by approximately 0.5% plastic strain, stored for 24 hours at room temperature and aged at 185°C for 8 hours.

In Figure 3 the engineering yield stress, Rp0.2 and the ultimate tensile stress, Rm are shown for the 5 alloys in Table 1 . For alloys 1-4 there is a marked increase in Ultimate tensile stress with increasing Si content. Alloy 5 has the same Si content as alloy 3 but with 0.13 wt% lower Mg content. Alloy 5 shows almost the same Rp0.2 value but slightly lower Rm value than alloy 3. These results indicate that a higher Si content is very important to maximize the strength of the material.

The preferred content of Si is 0.63-0.72 wt%, preferably 0.65-0.70 wt% in order to maximize strength while being adapted to the preferred Mg content.

Figure 3 also shows the true strain, c _f (e _f = Ln for the different alloys, where A _o is the initial cross-sectional area of the tensile sample and A _f is the area at fracture. The true strain, e _f is highest for the alloys with the lowest Si contents. For a constant Mg content (alloys 1-4), the true strain, e _f decreases with increasing Si contents. By comparing alloys 3 and 5 one can see that the true strain, also drops when the Mg/Si ratio is reduced.

Example 3

Example 3 of the present invention aims to increase the strength without too much reduction in the ductility. Extrusion billets were cast and processed in the same way as in example 2. Table 2 Chemical compositions of alloys tested in example 3 in wt%, including incidental impurities and with balance Al.

The extrusion billets were then preheated to temperatures between 400 and 420°C and extruded to a hollow rectangular section with outer dimensions 29 mm by 37 mm and with a wall thickness all around the cross section of 2.8 mm. The profiles were subsequently separately solutionized at 565°C for about 15 minutes and quenched in water before being stretched by approximately 0.5% plastic strain, stored for 10 minutes and aged at 195°C for 4 hours. Figure 4 shows the mechanical properties obtained from tensile testing of samples from profiles of the different alloys listed in Table 2. The left column for each alloy shows the engineering yield stress, Rp0.2, and the right column for each alloy show the ultimate tensile stress, Rm.

Alloys 7 and 8 show slightly higher strength levels than alloys 6 and 9. Regarding ductility, as measured by the true fracture strain e _f , especially alloy 8, but also alloy 7, show lower values than alloys 6 and 9. Alloy 8 has the lowest Mg/Si ratio, which is the main reason for the lower true fracture strain value.

It is thus clear that a composition according to the present invention, and as defined in claim 1 , with a Mg/Si ratio of above 1 .6 is a requirement for achieving the ductility required according to the invention.

Example 4

Example 4 investigates different preheating temperatures of the extrusion billet, and different extrusion parameters.

An alloy with a chemical composition in wt% is shown in table 3 was cast as 305 mm diameter billets. Table 3.

The billets were homogenized at 575 °C with a soaking time of 2 hours. Afterwards the billets were cooled at approximately 350-400 °C per hour down to 410 °C and held at this temperature for 4 hours. The cooling down from the soft annealing temperature to room temperature was done at 300-350 °C per hour.

The billets which were 1340 mm long, where extruded at an extrusion press with an available force of 7000 tons. A hollow one-chamber profile typically used for a 6082 alloy was selected for the trial. A typical extrusion speed for the profile with a 6082 alloy is 6.5 m/min. The first billet was heated to 450 °C in order to ease the extrusion of the first billet. Afterwards the billet temperature was reduced to 400-410 °C, which was at or slightly below the isothermal soak temperature in the soft annealing process. As can be seen from the results in table 4 below, the soft annealed billets could be extruded at 15-16 m/min before getting tearing in the profile. A 6082 alloy was not tested in the same trial, however the maximum extrusion speed for this alloy is not expected to be more than about 8 m/min.

Table 4.

With the present method and alloy a lower preheating temperature of the extrusion billet can be used, compared with the standard methods. This lower preheating temperature allows higher press force and higher extrusion speed before getting tearing in the profile. Moreover, present method results in reduced costs for the cast houses as the time spent on the soft annealing step is reduced due to the high cooling rate from the homogenization temperature to the soft annealing temperature. In addition, the smaller Mg ₂ Si particles obtained by the present method reduces the solutionizing time, which is also important in an industrial production line, as it will reduce the investment of furnace capacity.