DESCRIPTION

FIELD OF THE INVENTION

The present invention relates to a 6XXX series aluminium extrusion alloy, particularly useful for the automotive industry.

BACKGROUND OF THE INVENTION

Aluminium alloys are used in the form of extrusions for various applications including automotive usages. Among these alloys, Cu containing AA6xxx aluminium alloys series, such as AA6013 are known to combine interesting chemical and mechanical properties such as strength, crashabilty and even corrosion resistance. In addition to the requirements discussed above, another requirement is that the aluminium alloys can be processed easily and with high productivity, without surface defects such as those resulting from incipient melting or structural defects such as peripheral coarse grain (PCG) which may appear during extrusion or following thermal treatment step.

The patent application W02015086116 discloses a manufacturing process for obtaining extruded products made from a 6xxx aluminium alloy, comprising the following steps: a) homogenizing a billet cast from said aluminium alloy; b) heating the said homogenised cast billet; c) extruding the said billet through a die to form at least a solid or hollow extruded product; d) quenching the extruded product down to room temperature; e) optionally stretching the extruded product to obtain a plastic deformation typically between 0,5% and 5%; f) ageing the extruded product without applying on the extruded product any separate post-extrusion solution heat treatment between steps d) and f) characterised in that: i) the heating step b) is a solution heat treatment where: b1) the cast billet is heated to a temperature between Ts-15°C and Ts, wherein Ts is the solidus temperature of the said aluminium alloy; b2) the billet is cooled until billet mean temperature reaches a value between 400°C and 480 °C while ensuring billet surface never goes below a temperature substantially close to 400 °C; ii) the billet thus cooled is immediately extruded (step c)), i.e. a few tens seconds after the end of step b2).

The patent application US2011155291 discloses a high-strength aluminium alloy extruded product containing 0.6 to 1.2% of Si, 0.8 to 1.3% of Mg, and 1.3 to 2.1% of Cu while satisfying the following conditional expressions (1), (2), (3) and (4), 3%£iSi %+Mg %+Cu %£ϋ % (1) Mg %^1.7xSi % (2) Mg %+Si %^2.7% (3) Cu %/2^Mg %^(Cu %/2)+0.6% (4) and further containing 0.04 to 0.35% of Cr, and 0.05% or less of Mn as an impurity, with the balance being aluminium and unavoidable impurities. The cross section of the extruded product has a recrystallized microstructure with an average grain size of 500 pm or less.

The patent application WO2018073389 discloses extrusions for structural components, such as bumper, side impact beam, seat sill in vehicles and more particularly to a method for optimizing strength and energy absorption of 6XXX aluminium alloys extrusions by variations in thermomechanical ageing (TMA) consisting in i) an artificial preageing treatment with a duration t1 at a temperature T 1 selected to increase the yield strength of said extrusion between 5% and 20%, said temperature T1 being typically between 120°C and 180°C and said duration t1 being typically between 1 and 100 hours, to obtain an artificially preaged extrusion, ii) a plastic deformation of said artificially preaged extrusion between 1%> and 80%> to obtain a deformed extrusion, iii) a final artificial ageing treatment of said deformed extrusion with a duration t2 at a temperature T2, said temperature T2 being typically between 140°C and 200°C and said the duration t2 being typically between 1 and 100 hours.

The patent application FR2360684A1 discloses aluminium alloy products containing essentially by weight, 0.4 to 1.2% Si, 0.4 to 1.1% Mg, 0.2 to 0.8% Mn, 0.03 to 0.35% Fe, 0.1 to 0.6% Cu, the remainder being essentially aluminium. The alloy can be homogenized at a temperature of finally 480 and 595 degrees C and then is worked in sheets or profiles which are heat treated to form a solid solution, then which are cooled and aged to a T4 temper before their transformation into final products which can then be reinforced by heating or aging to the T6 temper.

The patent application US2004084119 discloses a method of manufacturing a high- strength aluminium alloy extruded product suitable for applications as structural materials for transportation equipment such as automobiles, railroad carriages, and aircrafts. The method includes extruding a billet of an aluminium alloy comprising 0.5% to 1.5% of Si, 0.9% to 1.6% of Mg, 0.8% to 2.5% of Cu, while satisfying the following equations (1), (2), (3), and (4), 3£Si%+Mg%+Cu%£4 (1) Mg%<1.7xSi% (2) Mg%+Si%£2.7 (3) Cu%/2£Mg%£(Cu%/2)+0.6 (4) and further comprising 0.5% to 1.2% of Mn, with the balance being Al and unavoidable impurities, into a solid product by using a solid die, or into a hollow product by using a porthole die or a bridge die, thereby obtaining the solid product or the hollow product in which a fibrous structure accounts for 60% or more in area-fraction of the cross-sectional structure of the product.

The patent application WO2016202810 discloses a manufacturing process for obtaining 6xxx-series aluminium alloy solid extruded products, comprising Si: 0.3-1.7 wt. %; Mg: 0.1-1.4 wt. %, Cu: 0.1-0.8 wt. %, Zn 0.005-0.7 wt. %, one or more dispersoid element, from the group consisting of Mn 0.15-1wt.%, Cr 0.05-0.4 wt.% and Zr 0.05-0.25 wt.%, Fe at most 0.5 wt.%, other elements at most 0.05 wt.% the rest being aluminium, having particularly high mechanical properties, typically an ultimate tensile strength higher than 400 MPa, preferably 430 MPa, and more preferably 450 MPa without the need fora post extrusion solution heat treatment operation. The invention also concerns a manufacturing process for obtaining a bumper system in which is integrated a towing eye, said towing eye being made with said high mechanical properties aluminium alloys. Patent application WO2019206826 discloses an extruded product made of 6xxx aluminium alloy comprising 0.40 - 0.80 wt. % Si, 0.40 - 0.80 wt. % Mg, 0.40 - 0.70 wt. % Cu, up to 0.4 wt. % Fe, up to 0.30 wt. % Mn, up to 0.2 wt. % Cr, up to 0.2 wt. % V, up to 0.14 wt. % Zr, up to 0.1 wt. % Ti, up to 0.05 wt. % each impurity and total 0.15 wt. %, remainder aluminium, wherein the ratio Mg/Sifree is between 0.8 and 1.2 where Si free is calculated according to the equation Si free = Si - 0.3*(Mn+Fe) where Si, Mg and Fe correspond to the content in weight % of Si, Mg and Fe of said 6xxx aluminium alloy and to the corresponding extruded product particularly suitable with a tensile yield strength higher than 280 MPa, and excellent crash properties.

CN 103131904 discloses an aluminum alloy material comprising the following components in percentage by mass: 0.8-1.3% of Si, 0.3-0.7% of Cu, 0.20-0.60% of Mn, 0.8-1.4% of Mg, 0.05-0.25% of Cr, 0.05-0.2% of Zr, at most 0.5% of Fe, at most 0.2% of Zn, at most 0.1% of Ti, and the balance of Al and impurities.

There is a need for an improved alloy to obtain AA6xxx alloy extrusions with high strength, typically having an ultimate tensile strength higher than 390 MPa, and high processability, in particular with a high productivity, as well as high surface quality and high corrosion resistance.

SUMMARY OF THE INVENTION

An object of the invention is an extruded profile comprising an Al-Mg-Si alloy containing, in wt.%,

Si 0.6 - 0.9,

Mg 0.55- 0.76,

Cu 0.65 - 0.9,

Mn 0.4 - 0.7,

Cr 0.05 - 0.2,

Zr 0.10 - 0.19,

Fe 0.05 - 0.5,

Zn £ 1.0, V £ 0.10,

Ti £ 0.10, other elements < 0.05 each and < 0.15 total, rest aluminium. Another object of the invention is a method to make an extruded profile according to the invention comprising the successive steps of

(a) Casting a billet containing, in wt.%,

Si 0.6 - 0.9,

Mg 0.55- 0.76, Cu 0.65 - 0.9,

Mn 0.4 - 0.7,

Cr 0.05 - 0.2,

Zr 0.10 - 0.19,

Fe 0.05 - 0.5, Zn £ 1.0,

V £ 0.10,

Ti < 0.10, other elements < 0.05 each and < 0.15 total, rest aluminium

(b) Homogenizing the billet, (c) Cooling the homogenized billet to room temperature,

(d) Solution heat treating the homogenized billet at a temperature from 500 °C to 560°C for a duration from 150 s to 500 s and quenching to a temperature from 300 °C to 500 °C or reheating the homogenized billet directly and without a solutionising step to a temperature from 300 °C to 500 °C, (e) Extruding at an extrusion rate from 5 m/mn to 15 m/mn said heat treated and quenched or reheated billet to obtain an extruded profile,

(f) Quenching, stretching and aging said extruded profile.

Yet another object of the invention is the use of an extruded profile according to the invention as an automotive component such as a crash box, a bumper, a side impact beam or a side sill. DESCRIPTION OF THE FIGURES

Figure 1: Microstructure of extruded profile made of alloy A4 (Figure 1a) or A6 (Figure 1b).

Figure 2: Calculated lines of same ultimate tensile strength in MPa in the flow stress / solidus temperature coordinate system.

DESCRIPTION OF THE INVENTION

All aluminium alloys referred to in the following are designated using the rules and designations defined by the Aluminium Association in Registration Record Series that it publishes regularly, unless mentioned otherwise.

Metallurgical tempers referred to are designated using the European standard EN-515. Static tensile mechanical characteristics, in other words, the ultimate tensile strength R _m (or UTS), the tensile yield strength at 0.2% plastic elongation R _p o,2 (or TYS), and elongation A% (or E%), are determined by a tensile test according to NF EN ISO 6892-1.

According to the invention, an improved Al-Mg-Si alloy allows to obtain high strength extrusions while having a high processability. By high processability it is meant in particular that the extrusion rate is high while maintaining a favourable microstructure wherein no incipient melting has occurred and the peripheral coarse grain layer has a limited thickness. This is obtained in particular through low flow stress and a high solidus temperature.

The composition of the extruded profile according to the invention is, in wt.%, Si 0.6 - 0.9, Mg 0.55 - 0.76, Cu 0.65 - 0.9, Mn 0.4 - 0.7, Cr 0.05 - 0.2, Zr 0.10 - 0.19, Fe 0.05 - 0.5, Zn £ 1.0, V £ 0.10, Ti £ 0.10, other elements < 0.05 each and < 0.15 total, rest aluminium.

Si, Cu and Mg content are carefully adjusted in order to obtain the desired properties of strength, flow stress and solidus temperature.

The Si content is preferably at least 0.70 wt.% and more preferably at least 0.75 wt.%. The Si content is preferably at most 0.90 wt.% and more preferably at most 0.85 wt.%. The Mg content is preferably at least 0.58 wt.% and more preferably at least 0.62 wt.%. The Mg content is preferably at most 0.75 wt.%, preferentially at most 0.74 wt.% and more preferably at most 0.72 wt.%. The Cu content is preferably at least 0.70 wt.% and more preferably at least 0.75 wt.%. The Cu content is preferably at most 0.90 wt.% and more preferably at most 0.85 wt.%. In a preferred embodiment the Cu content is from 0.75 to 0.85 wt.%. Preferably the sum of Si, Cu and Mg is also controlled. In an embodiment the sum Si + Mg + Cu is at least 2.0 wt.% and more preferably at least 2.15 wt.%. and/or at most 2.4 wt.% and more preferably at most 2.35 wt.% and even more preferably at most 2.33 wt.%.

Mn, Cr and Zr are added in particular to control the microstructure of the extruded profile. The microstructure of the extruded profile is preferably essentially unrecrystallized. By essentially unrecrystallized microstructure it is meant that the proportion of recrystallized grains is less than 35 %, preferentially less than 30 % and preferably less than 20% through the thickness of the walls the extruded profile. Advantageoulsy, the peripheral coarse grain is, per wall side, at most 400 pm thick, preferably at most 250 pm thick and most preferably at most 200 pm thick. The peripheral coarse grain (PCG) is a layer of recrystallized grains on the surface of the extruded profile. It is measured in appropriate location of the extruded profile, usually excluding angles and welding zones.

The Mn content is preferably at least 0.40 wt.% and more preferably at least 0.45 wt.%.

The Mn content is preferably at most 0.65 wt.% and more preferably at most 0.55 wt.%.

The Cr content is preferably at least 0.06 wt.% and more preferably at least 0.07 wt.%.

The Cr content is preferably at most 0.18 wt.% and more preferably at most 0.12 wt.%.

The Zr content is preferably at least 0.11 wt.% and more preferably at least 0.12 wt.%.

The Zr content is preferably at most 0.18 wt.% and more preferably at most 0.16 wt.%. The Zn content is at most 1.0 wt.%, preferably at most 0.8 wt.% or at most 0.7 wt.% or at most 0.6 wt.% or at most 0.5 wt.% or at most 0.4 wt.% or at most 0.3 wt.% or at most 0.2 wt.% or even at most 0.1 wt.%. The present inventors have found that the invention alloy can tolerate such content of Zn without adversely affecting the properties, which is beneficial for recycling purposes.

The V content is at most 0.10 wt.% and preferably at most 0.05 wt.% or even at most 0.03 wt.%.

Ti is preferably added to control the as-cast grain structure. In an embodiment the Ti content is from 0.01 wt.% to 0.07 wt.% and preferably from 0.01 wt.% to 0.05 wt.%.

Fe is at least 0.05 wt.% and at most 0.5 wt.%. The Fe content is preferably at least 0.10 wt.% and more preferably at least 0.15 wt.%. The Fe content is preferably at most 0.40 wt.% and more preferably at most 0.30 wt.%. The present inventors have found that the invention alloy can tolerate such content of Fe without adversely affecting the properties, which is beneficial for recycling purposes.

The content of other elements is less than 0.05 wt.% each and less than 0.15 wt.% total. The other elements are typically unavoidable impurities or incidental elements added in very small quantity such as boron which can be typically added together with Ti in the form of T1B2. Preferably, the composition is adjusted so that the calculated solidus temperature using standard thermodynamic database is from is from 580 °C to 610 °C, preferably from 585 °C to 600 °C and more preferably from 588 °C to 595 °C, and even more preferably from 589 °C to 594 °C. With this relatively high solidus temperature it is possible to increase extrusion rate without having the risk of incipient melting.

Preferably, the composition and the homogenization are adjusted so that the flow stress measured at 480 °C and a strain rate of 0.14 s-1 is at most 35 MPa and preferably at most 32 MPa in order to improve the processability. In particular, with the alloy of the invention is possible to obtain an extrusion rate from 5 to 15 m/mn and preferably from 6 to 12 m/mn without microscopic defects such as incipient melting or thick PCG. Preferably the extruded profiles according to the invention exhibit in a T6 temper an ultimate tensile strength in the longitudinal direction of at least 390 MPa and preferably of at least 400 MPa.

In particular, with the alloy of the invention, the present inventors found it possible to obtain simultaneously the preferred solidus temperature from 588 °C to 595 °C, the preferred flow stress measured at 480 °C and a strain rate of 0.14 s-1 of at most 35 MPa and an extruded profile ultimate tensile strength in a T6 temper in the longitudinal direction of at least 390 MPa, and preferably the more preferred solidus temperature from 589 °C to 594 °C, the more preferred flow stress measured at 480 °C and a strain rate of 0.14 s-1 of at most 32 MPa and an extruded profile ultimate tensile strength in a T6 temper in the longitudinal direction of at least 400 MPa.

A method to make an extruded profile according to the invention comprises the successive steps of casting a billet of an alloy according to the invention, homogenizing the billet, cooling the homogenized billet to room temperature, solution heat treating and quenching or simply reheating the homogenized billet, extruding, quenching, stretching and aging.

The homogenization temperature is preferably from 510 °C to 600 °C and more preferably from 530 °C to 590 °C, and even more preferably from 540 °C to 580 °C, and even more preferably from 560 °C to 580 °C.

There are two main embodiments to the process of the invention for the thermal treatment of the homogenized billet before extrusion. In a first embodiment, the homogenized billet is solution heat treated at a temperature from 500 °C to 560 °C for a duration from 150 s to 500 s and quenched to a temperature from 300 °C to 500 °C. Preferably in this first embodiment the homogenized billet is solution heat treated at a temperature from 530 °C to 560 °C for a duration from 150 s to 500 s and quenched to a temperature from 330 °C to 500 °C. Within the scope of the present invention, temperature referring to a billet is to be understood as billet average surface temperature. It is noted that the temperature after quenching referred to is the temperature obtained after stabilization: during and just after quenching the surface temperature of the billet may locally drop below 300 °C. In a second embodiment the homogenized billet is reheated directly and without a solutionising step to a temperature from 300 °C to 500 °C and preferably from 330 °C to 500 °C. The first embodiment usually enables to obtain extruded profiles having a higher strength than the second embodiment. Preferably the thermal treatment of the homogenized billet is carried out in a way to obtain before extrusion a temperature difference between one end, the head, and the other end, the foot, of the billet of at least 30 °C, preferably of at least 50 °C and even more preferably of at least 70 °C.

Extrusion is carried out at an extrusion rate from 5 m/mn to 15 m/mn with preferably an entry temperature of the head of the billet from 400 °C to 500°C and an entry temperature of the foot of the billet from 330°C to 450 °C to obtain an extruded profile. The head of the billet is the first part to be extruded and the foot of the billet is the last part to be extruded. Thanks to the invention, the processability is improved, in particular the extrusion rate is preferably at least 7 m/mn, preferentially at least 8 m/mn and more preferably at least 10 m/mn without having any defects such as incipient melting or excessive PCG.

In an embodiment, an extruded profile of the invention is made from a billet homogenized at a homogenization temperature from 530 °C to 580 °C and extruded at an extrusion rate of at least 7 m/mn.

After extrusion the extruded profile is quenched. Quenching can be realized a with strong air flow or preferably with a water spray and or more preferably through a standing wave. The extruded profile is then stretched which induces plastic deformation, preferably of at least 0.1 % and preferentially of at least 0.5 % and preferably of at most 4%, more preferably of at most 2% and even more preferably of at most 1%. Finally, the extruded profile is aged. Preferably, the extruded profile is aged to a T6 temper. In a preferred embodiment, the aging temperature is from 160 °C to 180°C for a duration from 5 to 20 hours. In another embodiment, the extruded profile is overaged to a T7 temper.

The extruded profiles of the invention can be used advantageously as an automotive component such as a crash box, a bumper, a side impact beam, battery enclosure or a side sill.

EXAMPLE

Billets having the composition disclosed in Table 1 have been cast. Alloys A2 and A4 are according to the invention.

Table 1. Composition in wt.%

The A1 billet was homogenized at 550 °C. The A6, A7 and A8 billet were homogenized at 555 °C. All the other billets were homogenized at 575 °C. After cooling to room temperature, the billets were subjected to solutionising and quenching of the billets prior extrusion. The billets were heated to 530 °C and held at this temperature for at least 2 min before being water quenched to a stabilized temperature close to 480 °C and not lower than 350 °C. The billets were then extruded and the extrusions exit temperature was 560 °C or above without generating surface defect. The extrusions were hollow profiles with a wall thickness of 1,8 mm.

Subsequently the extrusions were cooled down to room temperature preferably using water quench in order to assess full mechanical and ductility potential of the alloys. Extrusions were then stretched in order to induce 0.5% to 1.0% of plastic deformation and subsequently aged in order to reach their maximum strength. The extruded profiles were finally aged at 170 °C in order to reach peak strength (T6 temper).

Extruded profiles made of alloys A1, A2 and A4 had an essentially unrecristallized microstructure with a PCG about 150 pm thick per wall side, so that the proportion of recrystallized grains was less than 20 %. Figure 1a shows a cross-section of an extruded profile made of an extruded profile according to the invention with a PCG about 150 pm thick. Extruded profiles made of alloys A7 and A8 had an essentially unrecristallized microstructure with a PCG about 135 pm thick and 90 pm thick per wall side respectively. Extruded profiles made of alloys A3 and A5 had a recrystallized microstructure. Extruded profile made of alloy A6 had a microstructure with a PCG about 500 pm thick per wall side as illustrated by Figure 1b. In parallel flow stress of the raw material (i.e. billets) was measured for each alloy using a hot compression test at 480°C. Strain rate used in the compression trials was 0.14s ¹ . The flow stress is directly related to processability as the lower the flow stress, the less resistance to extrusion and the higher the possible extrusion rate for a given equipment. In parallel, the extrusion rate may be limited by the maximum temperature that the extrusion can undergo the mechanical properties and flow stress are provided in Table 2

Table 2 : mechanical properties and flow stress.

With the data in their possession, the present inventors established a model predicting the flow stress and the mechanical strength from the solidus temperature for alloys of the present invention transformed into extruded profiles with the method of this example. The result of this simulation is provided in Figure 2. With the alloy of the invention it is possible to obtain a very favorable balance between processability and strength, in particular for a calculated solidus temperature from 588 °C to 595 °C, it is possible to obtain simultaneously a flow stress measured at 480 °C and a strain rate of 0.14 s-1 of at most 35 MPa and an extruded profile in a T6 temper with an ultimate tensile strength in the longitudinal direction of at least 400 MPa without microscopic defects such as incipient melting or thick PCG.