TECHNICAL FIELD

The present invention relates to an Al-Mg-Si aluminium alloy and extruded products thereof having excellent ductility and crush properties with good energy absorption and temperature stability, and which is particularly useful for structural components in crash exposed areas in vehicles. Extrusions according to the present invention also have high corrosion resistance.

BACKGROUND

W02007094686 discloses an Al-Mg-Si alloy where 0-0.4 wt Ti is added for improving the alloy's ductility. The Mg and Si ranges are wide, the preferred range of Si/Mg ratio is 1.4. The patent application also teaches that the best temperature stability is found for high Si/Mg ratios.

In EP 0 936 278 improved ductility is claimed for Al-Mg-Si alloys with additions of V in the range 0.05-0.20 wt.% in combination with addition of Mn in the range 0.15-0.4 wt.%. According to this patent application the preferred Mn/Fe ratio is 0.45-1.0, and more preferably 0.67-1.0. The role of Ti in EP 0 936 278 is explicitly stated to be as a grain refiner during casting or welding. The preferred range for Ti is not more than 0.1 wt.%.

In the prior art profiles are water-quenched after extrusion. However, water quenching may lead to distortions of the geometry of the extruded section, and the risk of distortion increases with increasing complexity of the profile.

SUMMARY

Extruded profiles of Al-Mg-Si (6xxx) alloys are used as structural components in crash- exposed areas of automobiles. Such components are required to absorb high amounts of energy in the event of a crash, and in order to do so they must deform without fracturing. One of the means of controlling that the extruded profile has the required properties is to test it by crushing. In this test a specimen of thin-walled extruded hollow profile with one or more chambers of pre-defined length is subjected to crushing in the axial direction at a controlled speed, which reduces the specimen length to typically 1/3 of the original length. Good deformation behaviour is characterised by regular folding of the specimen walls, little or no cracking of the specimens and a smooth surface of the deformed areas. Poor deformation behaviour is characterised by limited folding of the specimen walls, extensive cracking or fracturing of the specimens and a rough and uneven surface of the deformed areas. An alternative to crush testing is to test how much one of the walls in a profile can bend before one observes the first crack at the outside of the bend.

Some structural components in crash-exposed areas may also be exposed to elevated temperatures. Such exposure may have an influence on the mechanical properties of the alloy. For such applications it is important to select alloys that are less influenced by the thermal exposure. The term "thermal stability" refers to the ability of an alloy to retain mechanical properties after exposure to high temperatures. The present invention provides an alloy with high temperature stability that can be produced with use of lower cooling rates after extrusion with maintained corrosion and crush properties.

The invention is characterized by the features as defined in the independent claims 1 , 13 and 25 and dependent claims 2-12, 14-24, and 26.

According to a first aspect, it is provided a 6xxx aluminium alloy comprising in wt%:

Mg: 0.45-1.2;

Si: 0.40-1.0;

Ti: 0.05-0.20;

V: 0.05-0.15;

Cu: < 0.30;

Mn: < 0.30;

Cr: < 0.15;

Zr: < 0.15;

Fe: < 0.50;

Zn: < 0.50; the rest being aluminium and inevitable impurities, wherein the content of Ti + V is 0.10-0.30 wt%, and the Si _e ff/Mg ratio is <1.0.

Si _e ff= Si - (Fe+Mn+Cr)/3 [wt%]. When Zr is comprised in the alloys, Si _e ff= Si - (Fe+Mn+Cr+Zr)/3 [wt%].

In some embodiments, the content of Ti is 0.05-0.15 wt%, or 0.07-0.12 wt%.

In some embodiments, the content of V is 0.07-0.12 wt%.

In some embodiments, the content of Ti + V is 0.14-0.24 wt%, or 0.15-0.20 wt%. In some embodiments, the Sieff/Mg ratio is 0.50-0.96.

In some embodiments, the Sieff/Mg ratio is 0.60-0.85, or 0.65-0.75.

In some embodiments, the Sieff/Mg ratio is 0.80-0.96.

In some embodiments, the content of Si is 0.45-0.65 wt% and the content of Mg is 0.55-0.75 wt%.

In some embodiments, the content of Si is 0.45-0.55 wt% and the content of Mg is 0.55-0.65 wt%.

In some embodiments, the content of Mn is 0.10-0.20 wt%.

In some embodiments, the content of Cu is <0.20, or 0.08-0.15 wt%.

In some embodiments, the content of Cr is <0.08 wt.%, or <0.05 wt%.

In some embodiments, the content of Fe is < 0.35 wt%.

In some embodiments, the 6xxx aluminium alloy is an extrusion alloy.

According to a second aspect, there is provided a process for manufacturing extruded products from an alloy according to the first aspect or any of the embodiments thereof, the said process comprises the following steps; a. casting a billet from a 6xxx aluminium alloy comprising in wt%:

Mg: 0.45-1.2;

Si: 0.40-1.0;

Ti: 0.05-0.20;

V: 0.05-0.15;

Cu: < 0.30;

Mn: < 0.30;

Cr: < 0.15;

Zr: < 0.15;

Fe: < 0.50;

Zn: < 0.50; the rest being aluminium and inevitable impurities; where the content of Ti + V is 0.10-0.30, and the Sieff/Mg ratio is <1.0; b. homogenising the cast billet at a temperature between 480 and 600°C for 1-24 hours; c. cooling the homogenised billet; d. extruding said homogenised billet to form an extruded product; e. cooling the extruded product down to room temperature using a cooling rate of less than 80°C/second; f. optionally stretching of the profile; and g. ageing the extruded product

Si _e ff= Si - (Fe+Mn+Cr)/3 [wt%]. When Zr is comprised in the alloys, Si _e ff= Si - (Fe+Mn+Cr+Zr)/3 [wt%].

In some embodiments, the stretching in step f. is 1.5-4%, or 1.5-3%.

In some embodiments, the cooling rate in step e. is less than 40°C/second, or less than 20°C/second.

In some embodiments, the cooling rate in step e. is more than 5°C/second, or more than 7°C/second.

In some embodiments, the content of Ti + V is 0.14-0.24 wt%, or 0.15-0.20 wt%.

In some embodiments, the Sieff/Mg ratio is 0.50-0.96.

In some embodiments, the Sieff/Mg ratio is 0.60-0.85, or 0.65-0.75.

In some embodiments, the Sieff/Mg ratio is 0.80-0.96.

According to a third aspect, it is provided an extruded product comprising an alloy according to the first aspect or any of its embodiments, and manufactured according to the method of the second aspect or any of its embodiments, wherein the material of the final extruded product has a recrystallized grain structure.

In some embodiments, the extruded product is a structural component in crash exposed areas in vehicles. 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 DRAWINGS

The invention will be further described in the following by way of examples and with reference to the figures, where:

Fig. 1a is a diagram showing the yield strength values Rpo.2 for the 7 different alloy variants in Table 1 that were stretched 0.5%, 2% and 4% prior to ageing to T6.

Fig. 1b is a diagram showing post uniform elongation for the 7 different alloy variants in Table 1 that were stretched 0.5%, 2% and 4% prior to ageing to T6.

Fig. 2 shows how the bend tests of the samples are performed.

Fig. 3 shows how to measure the bending angle manually at appearance of the first crack in the sample.

Fig. 4 shows the bending angle in extrusions manufactured according to the disclosure from alloys 1 , 3, 5 and 7 in Table 1 and oil quenched after SHT with a cooling rate of 50-60°C per second.

Fig. 5 shows the bending angle in extrusions manufactured according to the disclosure from alloys 1 , 3, 5 and 7 in Table 1 and air cooled after SHT with a cooling rate of 6-7°C per second.

Fig. 6 (a) shows the yield stregth Rpo.2 for water quenched (WQ), oil quenched (OQ) and air cooled (AC) samples for alloy variants 1 , 3, 5 and 7 in Table 1 stretched 0.5%.

Fig. 6 (b) shows the post uniform elongation (A25mm-A _g ) for water quenched (WQ), oil quenched (OQ) and air cooled (AC) samples for alloy variants 1 , 3, 5 and 7 in Table 1 stretched 0.5 %.

Fig. 7 (a) shows the average of the 3 deepest IGC attacks of the seven alloys of Table 1 , tested after 0.5 %, 2 % and 4 % stretch. Fig. 7 (b) shows light optical microscope images of typical areas measured for the IGC-values presented in Figure 7 (a).

Fig. 8 shows the yield strength (Rpo.2) and ultimate tensile strength (Rm) after thermal exposure for 500h and 1000h at 150°C of an age hardened extruded profile made from alloy 3 in Table 1.

Fig. 9 shows a typical quench rate along a 10 cm long profile during air quench (AQ).

Fig. 10 shows yield strength (a) and ultimate tensile strength (b) after aging to T6 of the samples described in example 5.

Fig. 11 shows crush specimens from the three alloys of example 5, two parallels, from left to right (vertical columns) alloy 21-031 , 21-032 and 21-033.

DETAILED DESCRIPTION

The invention according to the present disclosure is related to aluminium alloys containing Mg and Si as the primary alloying elements in combination with additions of Ti and V. The alloy contains amounts of Ti in excess of the Ti amounts commonly added as grain refiner. The excess Ti contributes to improved ductility and improved corrosion resistance of the alloy. The aluminium alloy is a 6xxx aluminium alloy. The aluminium alloy may especially be a 6xxx aluminium extrusion alloy.

The present disclosure relates to a 6xxx aluminium alloy comprising in wt%:

Mg: 0.45-1.2

Si: 0.40-1.0

Ti: 0.05-0.20

V: 0.05-0.15

Cu: < 0.30

Mn: < 0.30

Cr < 0.15

Zr < 0.15

Fe < 0.50

Zn < 0.50, the rest being aluminium and inevitable impurities, wherein the content of Ti + V is 0.10-0.30, and the Si _e ff/Mg ratio being <1.0. Sieff = Si - (Fe+Mn+Cr)/3 [wt%]. If Zr is comprised in the alloys, Sieff = Si - (Fe+Mn+Cr+Zr)/3 [wt%].

The alloy may contain Cu for additional strength and temperature stability. The content of Cu should be <0.30 wt%. In some embodiments the content of Cu may be < 0.20 wt%. A Cu content between 0.08-0.15 wt% has in some embodiments shown good strength and temperature stability.

The alloy may contain Fe up to 0.50 wt%. Iron is typically an impurity element that comes from sources like the aluminium oxide, the production process and from scrap metal. Too high content of Fe may reduce the corrosion properties of the alloy. It will also reduce the strength of the material by binding up Si in AIFeSi containing primary particles. However, an increased maximum, or relatively high Fe content allows more use of post-consumer scrap, which is important for reducing the carbon footprint of aluminium.

The number density of dispersoids formed per weight percent element added is significantly higher for Cr than for Mn (O. Lohne and AL Dons: Scand. J. Metall. vol. 12 (1983) pp. 34-36), meaning that a lower Cr addition than Mn addition is required to achieve a specific number density of dispersoids. The dispersoids have three adverse effects on the extrusion process. The first adverse effect is that the hot deformation resistance of the material increases, leading to a decrease in the productivity potential. The second adverse effect is that increasing dispersoid number density imposes increasing demands on the cooling rate after extrusion to avoid loss of hardening potential of the alloy. The reason for this is that the dispersoids act as nucleation sites for non-hardening Mg-Si precipitates. The third adverse effect is linked to the grain size of the extruded profile. With a somewhat lower number of dispersoids than required for preventing recrystallization one may get the situation that only a few grains are able to grow, resulting in a very coarse grain structure in the extruded profile. Upon subsequent forming of the extruded profile the result can be extensive orange peeling. In the final product coarse grains are negative for crush and bending behaviour. Therefore, one seeks to avoid Mn and Cr additions in excess of what is necessary for benefiting from improved ductility. The optimal content of Mn and Cr depends strongly on processing conditions and profile geometry.

The final, extruded product manufactured according to the process disclosed herein is preferably recrystallized, thus having a recrystallized grain structure. A material with a high number of dispersoid particles, enough to prevent recrystallization after extrusion, will have a higher deformation resistance and therefore be harder to extrude, compared to a material with less dispersoid particles. Additionally, a high number of dispersoid particles will increase the quench sensitivity of the material and one may be forced to quench fast in order to meet the strength requirement of the product. In such cases it may be difficult to meet the dimensional tolerance requirements due to distortions of the extruded profile when quenching.

Therefore, the alloy according to the present disclosure may contain up to 0.30 wt% Mn. In some embodiments the amount of Mn may be between 0.10-0.20 wt%. Cr may be added up to 0.15 wt%. In some embodiments the amount of Cr is less than 0.08 wt.%, or even less than 0.05 wt%. The alloy according to the present disclosure may comprise 0.10-0.20 wt% Mn and up to 0.15 wt% Cr. If the two elements, Mn and Cr, are combined the amount of each element may have to be reduced in order to keep the total number of dispersoids at an acceptable level. Too much Mn and/or Cr could result in a mixed grain structure (recrystallized and nonrecrystallized) which results in unfavourable mechanical properties.

Addition of Zr is less common in 6xxx alloys but will form dispersoid particles in a similar way as with Mn or Cr additions. Zr may be added up to 0.15 wt%, but the same considerations as with Mn and Cr additions must be made, to keep the total number of dispersoids at an acceptable level.

The alloy may contain up to 0.50 wt% Zn in order to allow more post-consumer scrap additions to the melt. In an embodiment the alloy contains up to 0.20 wt% Zn. Some Zn does not significantly affect the extrudability, mechanical properties or the corrosion properties of the aluminium alloy. At higher Zn levels there will be a slight reduction in the extrudability and the corrosion properties, therefore the upper limit of Zn is 0.50 wt%.

The alloy is developed for extruded products where good crush behaviour is requested. The alloy is optimised for productivity and for obtaining a high ductility without requiring rapid quenching of the extruded profiles at the extrusion press. Hence, the alloy according to the present disclosure is especially suitable for extrusions having a complex profile. However, the alloy may also be used for other products such as forgings when an improved ductility or corrosion resistance are requested.

In order to find the optimal Si/Mg ratio of an alloy, one has to consider that some of the Si will be tied up in the Fe-bearing primary particles and other non-hardening particles that form during casting and homogenisation of the alloy. This Si may be considered as "lost" or without effect with respect to age hardening. One may introduce a term "effective Si content", Si _e ff, defined by Sieff = Si - (Fe+Mn+Cr)/3 [wt.%].

For alloys containing Zr the Sieff = Si - (Fe+Mn+Cr+Zr)/3 [wt.%]

The alloy according to the present disclosure has an amount of between 0.45 and 0.2 wt% Mg and between 0.40 and 1 .0 wt% Si. The Si _e ff/M g ratio should be less than 1 .0 in order to optimise the crush resistance of the of the extruded products made from the alloy according to the present disclosure. The Sieff/Mg ratio should preferably be kept within the range 0.50-0.96. An optimal Sieff/Mg ratio within the range 0.60-0.85, or 0.65-0.75 has been found to be advantageous, as will be substantiated by the examples. In some embodiments, however, the Sieff/Mg ratio may be between 0.80-0.96.

According to some embodiments, an optimal composition of Mg and Si for ductility and temperature stability is 0.45-0.65wt% Si and 0.55-0.75 wt% Mg, or in a more narrowly defined composition 0.45-0.55 wt% Si and 0.55-0.65 wt% Mg.

Titanium (Ti) is normally added to Al alloys together with boron (B) or carbon (C) for the purpose of refining the grain size of the alloy during casting. Extra Ti in the melt enhances the grain refining effect of the TiB2 particles. The Ti content needed for obtaining grain refinement in Al-Mg-Si alloys is typically in the range 0.005-0.03 wt.%. The amount of Ti in the alloy according to the present disclosure is within the range 0.05-0.20 wt.%, such as in the range within 0.05-0.15 wt.%, or 0.07-0.12 wt%. Furthermore, the amount of V in the alloy according to the present disclosure is within the range 0.05-0.15 wt%, or 0.07-0.12 wt%. Both Ti and V are peritectic elements that normally will segregate towards the centre of the grains during solidification. During extrusion, the Ti- and V-enriched areas in the grains will be stretched out into fine bands. Without wishing to be bound to the theory, it is thought that these Ti-/V-bands will slow down the diffusion speed of Mg and Si towards grain boundaries at elevated temperatures, thus, slower cooling rates after extrusion is possible, while not compromising the material properties such as bend angle, strength and crushability. Adding both Ti and V above 0.10 wt% in total, such as above 0.14 wt% in total, has shown an improved effect on the crush and corrosion resistance of the Al-Mg-Si alloys according to the present disclosure. This requires a Ti content in excess of what would typically be used for grain refinement. Therefore, according to the present disclosure the total amount of Ti and V (Ti + V) should be 0.10-0.30 wt%. In some embodiments, the content of Ti + V may be 0.14-0.24 wt%, such as 0.15-0.20 wt%. The total amount of Ti + V should not be too high as this would lead to precipitation of unwanted primary particles. Ti and V in solid solution leads to an increase in the deformation resistance and a reduction in the extrudability and the total amount of Ti and V should also for that reason kept within the indicated limits. The improvement in crush performance obtained by adding Ti and V to an Al-Mg-Si alloy is substantiated by the examples according to the invention.

The present disclosure further relates to a process for manufacturing extruded products. The process comprises the following steps:

Step a. Casting a billet from a 6xxx aluminium alloy comprising in wt%:

Mg: 0.45-1.2

Si: 0.40-1.0

Ti: 0.05-0.20

V: 0.05-0.15

Cu: < 0.30

Mn: < 0.30

Cr: < 0.15

Zr: < 0.15

Fe: < 0.50

Zn: < 0.50, the rest being aluminium and inevitable impurities, the content of Ti + V is 0.10-0.30, and the Sieff/Mg ratio is less than 1.0. The alloy may have a composition according to the above disclosure, or any of the above disclosed embodiments of the alloy further specifying alternative amounts of the alloying elements.

Sieff = Si - (Fe+Mn+Cr)/3 [wt.%], For alloys containing Zr the Sieff = Si - (Fe+Mn+Cr+Zr)/3 [wt.%]

Step b. Homogenising the cast billet at a temperature between 480 and 600°C for 1-24 hours. The homogenisation step homogenises the microstructure of the cast billet. A typical homogenisation temperature is above the solvus temperature of the relevant alloy to dissolve Mg and Si comprised in the alloy.

Step c. Cooling the homogenised billet from the homogenisation temperature to room temperature. The cooling rate from the homogenisation temperature may be more than 100°C per hour. Typically, the cooling rate from the homogenisation temperature may be more than 200°C per hour, or more than 300°C per hour.

Step d. Extruding said billet to form an extruded product. Before extrusion, the extrusion billet should be reheated to a suitable extrusion temperature, typically 450-510 °C. According to the present disclosure, the billet may be overheated before being cooled down to the desired extrusion temperature.

Step e. Cooling the extruded product down to room temperature. The cooling of the extruded product should use a cooling rate of less than 80°C/second. The cooling rate may be less than 40 °C/second, and even less than 20 °C/second. However, the cooling rate should be more than 5 °C/second, and in some embodiments more than 7 °C/second. Using too low cooling rate may lead to a loss in the potential strength of the final product as Mg and Si contained in the alloy might be precipitated as large Mg2Si that will not contribute to strengthening during ageing. This in turn leaves less Mg and Si available for precipitation of the strengthening nanosized, precipitate phases which form during artificial ageing. Usage of a low cooling rate for the extrusion enables production of complex extruded sharped having the desired strength and crush properties, without introducing geometrical distortions to the profiles. The advantages of using a lower cooling rate for cooling the extruded product is substantiated by the examples.

Step f. Optional stretching of the profile. The optional stretching step f. may involve stretching the cooled profiles up to 4%. A stretching between 1.5-4%, such as from 1.5 to 3% has shown to be beneficial to the ductility of the material, as will be substantiated by the examples. Stretching typically takes place within a short time after extrusion, such as within 10 to 30 minutes after extrusion, however, the indicated time is not critical, and the stretching may also be performed later.

Step g. Ageing the extruded product. The aging step g. comprises artificial aging of the product to a desired strength level. It should be understood that the aging step may also comprise natural aging as natural aging is practically unavoidable in industrial production. To achieve the desired crush performance the extruded product may be aged to a T6 temper. A profile that is stretched according to step f can be designated to be a T8 temper (cold work followed by aging), however, in the present disclosure the T6 designation is used for all aged profiles. The aging temperature is typically in the range of 160-210 °C for 1-24 hours, a typical aging temperature is within the range 175-205 °C. The artificial aging might be performed in one step or a stepwise manner.

The extruded product manufactured according to the process disclosed hereinabove comprises an alloy having the following composition (in wt%): Mg: 0.45-1.2 Si: 0.40-1.0 Ti: 0.05-0.15

V: 0.05-0.15

Cu: < 0.30

Mn: < 0.30

Cr < 0.15

Zr < 0.15

Fe < 0.50

Zn < 0.50, the rest being aluminium and inevitable impurities, wherein the content of Ti + V is 0.10-0.30, and the Si _e ff/Mg ratio is less than 1.0.

The alloy composition of the extruded product may have any composition as explained in the above disclosure.

Sieff = Si - (Fe+Mn+Cr)/3 [wt.%], For alloys containing Zr the Sieff = Si - (Fe+Mn+Cr+Zr)/3 [wt.%]

The extruded product manufactured by the present disclosed method should have a recrystallized grain structure. In some embodiments the extruded product has a recrystallized grain structure and having a yield strength, Rp0.2, of at least 240 MPa (C24 alloy requirement). Furthermore, an extruded product of the present disclosed alloy, and manufactured according to the present method has shown excellent bending angle properties, having an excellent combination of crush properties and corrosion resistance. As will be evident from the following examples, the extruded product also has good temperature stability.

EXAMPLES

Example 1

Seven alloys with the compositions shown in Table 1 were cast to 95 mm diameter billets by DC casting.

Table 1 Composition of the alloys in the examples below.

Homogenising was performed at a temperature of 575°C for 2 hours and 15 minutes before cooling at a rate of approximately 350°C per hour.

The billets were overheated to approximately 550°C for 8-10 minutes and then cooled down to around 490-500°C just before extrusion. A hollow rectangular profile with outer dimensions 29 x 37 mm and a wall thickness of 2.8 mm was extruded from the different alloys. The profiles were quenched in water at a distance of about 50 cm behind the die exit, at a quench rate that is estimated to be more than 300°C per second. After extrusion the profiles were stretched either 0.5%, 2% or 4%. About 24 hours after extrusion the profiles were aged to T6 using 200°C per hour heating rate to 150°C, 1.5 hours hold at 150°C, 15°C per hour heating rate to 195°C and 2 hours hold at 195°C for the profiles stretched 0.5%, 1 hour 40 minutes hold at 195°C for the profiles stretched 2%, and 1 hour 20 minutes hold at 195°C for the profiles stretched 4%. Figure 1 (a) shows the yield strength values Rp0.2 for the 7 different alloys in Table 1 that had been stretched 0.5%, 2% and 4% prior to ageing to T6. The strength levels for the different variants are very similar. This is as expected since vanadium and titanium both have a negligible effect on the strength in the T6 condition. There is a slight reduction in the strength for the profiles that were stretched 2 and 4% as compared to ones stretched 0.5% prior to T6 ageing. Tensile testing was performed according to ISO 6892-1 - Metallic Materials

- T ensile T esting - Part 1 : Method of T est at Room T emperature.

Uniform or total elongation is not a good measure of the ductility of a material subjected to deformation from a crash. The difference between the total elongation (A25mm) and the uniform elongation (A _g ) is known as the post uniform elongation which is a better measure of the materials ductility. Figure 1 (b) shows the post uniform elongation of the seven alloys that were tested. For the water quenched profiles there is only a small positive effect of adding either vanadium or titanium. Increasing the stretching from 0.5% to 2 or 4% prior to T6 ageing seems to have a more positive effect on the post uniform elongation.

Example 2

The bend testing was performed according to the standard VDA 238-100, with the exception that a 1 % load drop was used as a stop criterion as opposed to 60N. The measured bending angle based on this criterion typically corresponds to the angle where the first crack is observed in the sample. The test samples were taken from the widest side walls of the profile of example 1 . The samples were 30 mm wide and 60 mm long and 2.8 mm thick. The sample is bent along an axis 90° in relation to the extrusion direction (i.e. normal to the extrusion direction), see figure 2. Figure 3 shows how to measure the bending angle manually at appearance of the first crack in the sample.

In order to investigate the effect of different cooling rates from extrusion on the bending properties extruded samples that were stretched 0.5% after extrusion were subjected to separate solution treatment (SHT) at 530°C for 20 minutes (time measured from when the temperature reached 525°C) and then cooled in air, oil or water.

One set of samples (first set) were quenched in water that held a temperature of approximately 25°C. The cooling rate in the temperature interval between 450 and 250°C is estimated to be above 300°C per second. After quenching the samples were stretched 0.5 and 2% before ageing to T6 in the same way as in example 1.

Another set of samples (second set) were quenched in oil that held a temperature in the range 26-28°C. The cooling rate in the temperature interval between 450 and 250°C is measured to be in the range 50-60°C per second. After quenching the samples were stretched 0.5 and 2% before ageing to T6 by the same method as described in Example 1 . A third set of samples were cooled in forced air. Before the SHT blanks for bending samples being 30 mm wide, 150 mm long and 2.8 mm thick were made in order to get higher cooling rates than one could obtain with whole sample of the hollow profile. The cooling rate in the temperature interval between 450 and 250°C was measured to be in the range 6-7°C per second. After cooling to room temperature, the samples were stretched 0.5% and 2% before ageing to T6 by the same method as described in Example 1.

The bending angles for the water quenched samples all exceeded the limitation of the equipment and the results here are not representative.

Figure 4 shows the bending angle measured as described in example 2 for extrusions manufactured according to the invention from alloys 1 , 3, 5 and 7 and oil quenched after SHT with a cooling rate of 50-60°C per second. In this case one sees a clear effect of adding Ti or V, but the best effect is obtained according to the invention by adding Ti and V together. It is also a very clear effect of stretching 2% as compared to 0.5% before ageing to T6.

Figure 5 shows the bending angle in extrusions manufactured according to the invention from alloys 1 , 3, 5 and 7 in Table 1 and air cooled after SHT with a cooling rate of 6-7°C per second. Air cooling reduces the bending angles for all alloys, but also in this case one sees a clear effect of adding Ti or V, but the best effect is obtained according to the invention by adding Ti and V together. It is also a very clear effect of stretching 2% as compared to 0.5% before ageing to T6.

The bendability of the water and oil quenched samples is better than what is obtained for the air-cooled samples, but cooling at very high rates will impose limitations on deliverable profile geometries and geometric tolerances. By use of compositions according to the present invention it is possible obtain bending angles approaching that of the faster quenched samples of prior art with samples that are air-cooled after extrusion.

In Figure 6 (a) the yield stress Rpo.2 values are shown for some of the alloy variants that have been subjected to different cooling rates after SHT. There is not much reduction in Rpo.2 for the air-cooled samples as compared to the water cooled ones, and all variants fulfil a typical requirement of minimum Rpo.2 of 240 MPa.

In Figure 6 (b) the difference between the total elongation A25mm and uniform elongation A _g , which is referred to as the post uniform elongation, is plotted for the same alloy variants as in Figure 6 (a). The post uniform elongation is best for the highest cooling rates. Additions of Ti or V (variant 3 and 5) give an increase in the post uniform elongation as compared to the alloy variant with low Ti and V (variant 1). However, the largest increase in post uniform elongation is found according to the invention when both elements are added together (variant 7).

Example 3

The intergranular corrosion (IGC) testing was done according to the standard ISO11846 method B. The three parallels from each sample were immersed in the same beaker. IGC results from the seven alloys are given in Figure 7. Four areas were investigated for each sample, measuring the three deepest IGC attacks.

Figure 7 (a) shows the average of the three deepest IGC attacks of the seven alloys, tested after 0.5 %, 2 % and 4 % stretch. The alloys have been water quenched after extrusion, and stretched before ageing to T6. The alloy with no Ti or V (alloy 1) experience the deepest corrosion attacks, with maximum average depths measured in the range from 350-410 pm.

Figure 7 (b) shows light optical microscope images of typical areas measured for the average IGC-values presented in Figure 7 (a).

It is evident from Figure 7 (a) and the images in Figure 7 (b) how the IGC depth decreases significantly when increasing Ti and V in these alloys. Alloy 7 with high Ti and V clearly show the smallest IGC attacks followed by alloy 6 and 3. Stretching 2 % and 4 % before ageing seems to reduce the maximum IGC-depth compared to 0.5 % stretch.

Example 4

Figure 8 shows the decrease in yield strength (YS) and ultimate tensile strength (UTS) after thermal exposure for 500h and 1000h at 150°C of an age hardened extruded profile made from alloy 3 in Table 1. The result shows that the selected content of Mg and Si according to the invention produces a high thermal stability.

Since the thermal stability is believed to be independent of the Ti and V contents, the obtained results should be valid for all the alloys in this study.

Example 5

Three alloys with varying Mg/Si-ratios were cast. The nominal chemical compositions of three alloys are given in Table 2. The logs were GC cast in 095 mm. Table 2. Composition of the alloys of example 5.

Homogenising was performed at a temperature of 575 °C for 2 hours and cooling at a rate of approximately 350 °C/h to room temperature.

One chamber crash-box profiles with dimensions 28 mm x 37 mm x 2.8 mm, with seam-welds at the centre of the profile walls, were extruded. The billets were overheated to 560 °C and kept in an air circulating furnace at the same temperature, the billets were then water quenched to 500 °C before extrusion. The extrusion tool/container was kept at 430 °C. The profiles were quenched in water at a distance of about 50 cm behind the die exit.

A separate solution heat treatment (SHT) was performed before each cooling trial. Each sample was solution heat treated (SHT) at 530 °C for 20 minutes (time started when temperature reached 525 °C). Then different quench rates were applied: water quench (WQ), oil quench (OQ) and air quench (AQ). The three different quench lines are described in the following sections. All profiles were stretched 0.5% or 2% directly after quenching then naturally aged for about 24 hours before artificial ageing to T6. The aging to T6 used the temperature profiles as shown in Table 3. The results presented herein are after the final AA step to T6 if not otherwise stated.

Table 3. Temperature profile for artificial aging to T6.

Water Quench after SHT

35 cm long profile samples were subjected to SHT as described above. The profiles were immediately quenched in water kept at about 25 °C. The water was not stirred. The average quench rate was >300 °C/s. The profiles were stretched immediately after quenching, before being naturally aged (NA) at room temperature for approximately 24 hours before AA to T6. Oil Quench after SHT

35 cm long profiles were subjected to SHT as described above. The profiles were quenched in oil kept at 26-28 °C. The oil was stirred throughout the quench. The average quench rate was about 55 °C/s. The profiles were stretched immediately after quenching. The profiles were then naturally aged (NA) at room temperature for approximately 24 hours before AA to T6.

Air Cooling after SHT

The samples were subjected to SHT at 530 °C for 20 minutes (time after temperature reached 525 °C). The profiles were then air quenched by holding the profiles over a fan for 1 minute before quenching in water. The average quench rate was about 7 °C/s. The profiles were stretched immediately after quenching, then kept at room temperature for approximately 24 hours before AA to T6. The experimental set-up and sample geometry used during AQ with corresponding temperature logging is shown in Fig. 9, where T1 is the lower curve in the diagram, T2 is the upper curve.

The profiles were tested for mechanical properties. For tensile testing standard flat, 12 cm long, tensile specimens were machined from the narrowest profile walls of each alloy variant. Two parallels were tested for each condition. Tensile testing was performed according to ISO 6892-1 - Metallic Materials - Tensile Testing - Part 1 : Method of Test at Room Temperature.

Results from tensile testing are presented in Figure 10 a) and b), quench rates are as given above. Both yield- and ultimate tensile strength in Figure 10 a) and b) increases slightly with increasing Si _e ff/Mg-ratio. From Figure 10 b), R _m increases with increasing stretch from 0.5% to 2% for all quench rates. When it comes to the yield strengths in Figure 10 a) the effect of stretch after SHT seems less pronounced, except for the slowest quench rate (AQ), where Rpo.2 increases by 4-8 MPa from 0.5 % to 2 % stretch.

For crush testing, two parallels were cut for crush testing of each alloy after WQ and 0.5 % stretch. All crash boxes were crushed to 1/3 of their original height. The crush-specimens are given in Figure 11. The 21-031 alloy with lowest Sieff/Mg-ratio performed best, ref. Fig. 11 (a), receiving a perfect crush-score of 10, meaning the specimens showed no signs of cracking. The 21-032 alloy with a higher Sieff/Mg-ratio, ref. Fig. 11 (b), also performed well but received slightly lower crush grade. The 21-033 alloy having the highest Sieff/Mg-ratio performed worst and had at least one deep corner crack.