WO2006056481A1

WO2006056481A1 - Aluminium alloy sheet for automotive applications

Info

Publication number: WO2006056481A1
Application number: PCT/EP2005/012811
Authority: WO
Inventors: Marc-Jan De Haas; Peter De Smet; Erik Smet
Original assignee: Corus Aluminium Nv
Priority date: 2004-11-25
Filing date: 2005-11-22
Publication date: 2006-06-01
Also published as: CH699832B1; FR2878255B1; CH699832B8; FR2878255A1

Abstract

An aluminium alloy sheet for automotive applications for improved pedestrian safety, having a chemical composition in weight percent: - 0.80 ≤ Si ≤ 1.20 - 0.10 ≤ Fe ≤ 0.30 - 0.05 ≤ Mn ≤0.20 - 0. 10 ≤ Mg ≤ 0. 30 - Cu ≤ 0.30 - Ti ≤ 0.15 - other elements up to 0.05 each, up to 0.15 in total Al balance, in T4 temper condition having a yield strength (Rp) of at least 50 MPa, a uniform elongation (Au) of at least 20% and a total elongation (A80) of at least 22%.

Description

ALUMINIUM ALLOY SHEET FOR AUTOMOTIVE APPLICATIONS

This invention relates to an aluminium alloy sheet for automotive applications. The invention also relates to a formed part from said aluminium alloy sheet and to a process for producing aluminium alloy sheet and part.

Aluminium alloys are enjoying growing use as automobile parts and are rolled into sheets which may be stamped into parts such as hoods, trunk lids, doors, and fenders, and the like from the aluminium alloy sheet. From the automotive industry a drive towards stronger materials is noticeable. Stronger materials allow for lighter constructions and a reduced consumption of material.

The safety of the pedestrians and other vulnerable traffic participants is a growing concern. A disadvantage of such materials is that when an automobile made from such products becomes involved in a collision with a vulnerable traffic participant such as a pedestrian, these strong materials will inflict considerable injury as a result of the high yield strength of such materials.

It is an object of the invention to provide an aluminium alloy sheet which is fit for application in an automobile and which is safer for the traffic participants outside the automobile in case they are involved in a collision with the automobile.

According to the invention, the object is reached by providing an aluminium alloy sheet for automotive applications for improved pedestrian safety, having a chemical composition in weight percent: 0.80 < Si < 1.20 0.10 < Fe < 0.30 0.05 < Mn < 0.20 0.10 < Mg < 0.30

Cu ≤ 0.30 Ti < 0.15 other elements up to 0.05 each, up to 0.15 in total

Al balance, in T4 temper condition having a yield strength (Rp) of at least 50 MPa, a uniform elongation (Au) of at least 20% and a total elongation (A80) of at least 22%.

This material has been found to possess a very low yield strength which makes the sheet very easy to form into a desired shape. The large uniform elongation and large total elongation as measured in a tensile test over an 80 mm gauge length according to (measured according to EN 10002) provides excellent forming properties. If the aluminium sheet according to the invention is subjected to a paint-baking cycle, the yield strength of the material increases as a result of the precipitation taking place during paint-baking to a level which is a compromise between sufficiently high values for it to be fit for safe application in the^; automobile, and sufficiently low values to reduce the risk for vulnerable traffic participants. It was also found by the inventors that the yield strength of the aluminium sheet according to the invention shows very stable values over the lifetime of the material in an automobile.

In order to obtain good formability in T4-condition a lower boundary for Si of 0.80% is set. On the other hand, in order to maintain a sufficiently low yield-strength in T4- or T4P-condition, the Si-content should not exceed 1.20%

A decrease in Fe-content was found to have a beneficial effect on formability. In order to obtain a high uniform (Au) and total (A80) tensile elongation in T4- or T4P- condition, an upper limit of 0.30 % needs to be set. However, when the Fe- concentration becomes too low, the grain size will reach too high a value and consequently a lower limit of 0.10 % is required. Large grain sizes are responsible for an effect called orange-peeling, which is a roughening effect occurring during forming. It deteriorates the appearance of the product.

Mn is added mainly for controlling the grain size of the material. When the concentration is too low, the grain size will become too large. A too high a Mn- concentration will negatively affect formability in T4 or T4P-condition. A Mn- concentration within the range of 0.05-0.20 % has been found to be a good compromise.

The Mg concentration needs to be sufficiently high in order to obtain a yield- strength in T4- or T4P-condition of at least 50 MPa. Yield-strength and tensile strength are found to show a decreasing trend with decreasing Mg-concentration. An Mg level of at least 0.10 % was found to be necessary. However, when the Mg content is above 0.30 %, the yield-strength after forming/paint-bake operations and during service life of the automotive vehicle becomes too high. Therefore, an upper limit of 0.30 % was found to be necessary.

A minimum amount of Cu is known to accelerate the yield-strength increase during forming- and paint-bake operations. However, when the Cu concentration becomes too high, the strength will increase too much during the life cycle of the vehicle. Furthermore, a slightly positive effect of Cu on the uniform and total tensile elongation was found. The Cu-concentration is therefore at most 0.30%.

A Ti-concentration of 0-0.15%, preferably of 0-0.10% was found to be required for controlling the grain size of the alloy sheet. The invented alloy is moreover a valuable alternative for complex design adjustments and composite material development in order to make automotive vehicles more pedestrian safe, yielding lower production costs. Further, by applying such a 6000-series alloy it is possible to improve pedestrian safety and maintain the automotive body sheet recycleability at the same time, particularly when compared with a 5000-series alloy. The uni-alloy concept within the automotive industry remains therefore applicable.

In an embodiment of the invention, the silicon content Si of the alloy is at least 0.90 % and/or at most 1.10%. It was found that this range provides an excellent combination of good formability in T4-condition and a low yield strength in 14- condition.

In an embodiment of the invention, the iron content Fe is at most 0.20%. In this embodiment the formability of the alloy sheet in T4-condition is improved.

In an embodiment of the invention, the magnesium content Mg is at least 0.12 % and preferably at most 0.20%. This compositional window provides an excellent compromise between low yield strength in T4-condition and low yield strength after paint baking or during the service life of the material in use in an automobile.

In an embodiment of the invention, the copper content Cu is at least 0.05% and preferably at most 0.20%. This compositional window provides an excellent compromise between accelerating the yield strength increase during paint baking and avoiding too large an increase in strength during the life cycle of the automotive vehicle.

In a preferred embodiment of the invention, the aluminium alloy sheet is provided in T4 temper condition having a yield strength (Rp) of between 50 and 100 MPa. It was found that this range is an optimal range for the material to be suitable for forming parts for automobiles by means of forming methods such as stamping or pressing. It is preferable that in T4 temper condition the uniform strain (Au) is at least 20 % and the total strain (A80) is at least 22% and more preferably the uniform strain (Au) is at least

23 % and the total strain (A80) is at least 25%. It is further preferable that the yield- strength after paint-baking the aluminium alloy as described hereinabove remains below 150 MPa and preferably remains between 105 MPa and 130 MPa. The yield strength during the life cycle of the material in use in an automobile is preferably kept below 180 MPa, more preferably below 160 MPa. The development of the properties during the life cycle of the material are simulated using a life-cycle test, which will be described in detail below. According to a second aspect of the invention, a formed part, such as a bonnet part, particularly an inner formed part, of an aluminium alloy according to the invention is provided, after forming and paint-baking having a yield strength of below 150 MPa, and preferably having a yield strength in the range of 105 to 130 MPa. The application of this formed part in an automobile is a good compromise between the requirements as to the mechanical properties and to the requirements as to the safety of vulnerable traffic participants.

In an embodiment of the invention a formed part having a yield strength of below

180 MPa, preferably below 160 MPa during the life cycle of the sheet is provided. This moderate increase in strength makes sure that the good compromise between the requirements as to the mechanical properties and to the requirements as to the safety of vulnerable traffic participants is maintained.

In an embodiment of the invention, a formed automobile part, such as a bonnet, is provided, the part comprising an inner formed part and an outer formed part wherein (i). both the formed inner and outer parts are produced from an alloy sheet as described above or wherein

(ii). the formed inner or the formed outer part is produced from an alloy sheet as described above and wherein the other part is produced from an alloy of the AA6000-series with a chemical composition outside the chemical composition as described above.

Preferably, all metal parts of the automobile part are produced from the alloy sheet as described above. Usually automobile parts, such as bonnets or doors, comprise an inner part partly providing the strength and stiffness of the part and an outer part which forms the outer structure of the automobile. Traditionally, the outer parts are substantially produced from an alloy of the AA6000-series, whereas the inner parts are substantially produced from an alloy of the AA5000-series. This combination of alloys causes problems during recycling of the part at the end of its life cycle. The part according to the invention allows both the inner part and the outer part of the automobile part to be made from an alloy of substantially the same composition or substantially the same alloy type. Consequently, the recyclability of the automobile part is greatly increased and the manufacturer of the automobile part is able to adopt a uni- alloy policy. The uni-alloy concept or policy maximises recyclability through the use of the same alloy series, in this case the AA6000-series. According to a third aspect of the invention a process for producing an aluminium alloy sheet is provided comprising the steps of: casting an aluminium alloy with a composition as described hereinabove homogenization and/or preheating hot-rolling cold rolling to an intermediate gauge optionally followed by inter-annealing - cold-rolling to final gauge solution heat-treating by continuous annealing quenching by forced air to produce a sheet in T4-condition. Using this method an aluminium alloy sheet is provided with a good compromise between the requirements as to the mechanical properties and to the requirements as to the safety of vulnerable traffic participants.

According to an embodiment of the invention the quenched sheet is subjected to a prebaking-treatment in which treatment the alloy is reheated and warm-coiled with subsequent cooling of the warm coil in an ambient atmosphere (i.e. at room temperature) to produce a sheet in T4P-condition thereby obtaining a material that is more stable towards the effects of storage time at room temperature, before the forming- and paint-baking operations take place.

In an embodiment of the invention, a process is provided as described above followed by a manufacturing process comprising the steps of forming a part, such as stamping and/or pressing a part from the sheet, - coating the part, baking the coated part.

The coating step may involve the coating of the part with a paint-layer, but it may also involve coating with a complex system of base coatings, paints and lacquers.

The composition has a relatively low yield-strength and high formability in T4 condition. It is preferable that in T4 temper condition the yield strength (Rp) is between

50 to 100 MPa, the uniform strain (Au) is at least 20 % and the total strain (A80) is at least 22% and more preferably the uniform strain (Au) is at least 23 % and the total strain (A80) is at least 25%. Furthermore a controlled increase in yield-strength during the forming and paint-bake operations is preferable after which the yield-strength stays below 150 MPa (preferably within the range 105 to 130 MPa). The subsequent increase in yield-strength during the life-cycle (90°C/500h) of the automotive vehicle is also controlled and kept below 180 MPa, preferably below 160 MPa.

It should be noted that where T4-condition is mentioned, T4- or T4P-condition is meant. It should be noted that where automobile is mentioned, luxury cars, transport vans, trucks and any other vehicle which participates in traffic in combination with vulnerable participants is deemed to be meant.

Examples

The invention will now be further explained by the following non-limitative examples given in Table 1.

Table 1 - Composition of alloys according to the invention and a reference alloy.

In Table 2, the mechanical properties of these compositions are given after having been produced by a process comprising the steps of casting, homogenization/preheating at 560 ⁰C for 5 hours, hot-rolling down to 4 mm, cold- rolling to 1 mm, finally solution heat treating by continuous annealing by heating at 11 °C/s to 540⁰C for 10 seconds followed by quenching at 25 °C/s using forced air. After this, the alloy is said to be in T4-state. A optional prebake-cycle, in which the alloy is reheated and warm-coiled may complete the cycle. The alloy is then said to be in T4P- state. Alloy 5 was solution heat treated at 56O⁰C (5a) and 540°C (5b)

Table 2 - Mechanical properties in T4-state, after paint baking and during life cycle.

The alloys according to the invention provide a combination of a yield strength in the range of 50 to 100 MPa with high formability in T4- or T4P-condition. Furthermore, the yield-strength after forming and paint baking is below 150 MPa₁ together with a controlled yield-strength increase during service-life simulation (Rp stays below 180

MPa).

The paint-baking process was simulated by a deformation of 5% and a thermal treatment for 20 minutes at 185 ⁰C in an oil-bath. The life cycle was simulated by a deformation of 5% and a thermal treatment for 20 minutes at 185 °C in an oil-bath, followed by 500 hours at 90 ⁰C in an air-furnace.

It is of course to be understood that the present invention is not in any way limited to the described embodiments and examples described above, but encompasses any and all embodiments within the scope of the description and the following claims.

Claims

1. An aluminium alloy sheet for automotive applications for improved pedestrian safety, having a chemical composition in weight percent: - 0.80 < Si < 1.20

0.10 < Fe ≤ 0.30 0.05 < Mn < 0.20 0.10 < Mg < 0.30 Cu < 0.30 - Ti < 0.15 other elements up to 0.05 each, up to 0.15 in total

2. An aluminium alloy according to claim 1 , wherein Si is at least 0.90 % and preferably at most 1.10%.

3. An aluminium alloy according to claim 1 or 2, wherein Fe < 0.20.

4. An aluminium alloy according to any one of the preceding claims wherein Mg is at least 0.12 % and preferably at most 0.20%.

5. An aluminium alloy according to any one of the preceding claims wherein Cu is at least 0.05% and preferably at most 0.20%.

6. An aluminium alloy according to any one of the preceding claims, in T4 temper condition having a yield strength (Rp) of between 50 and 100 MPa.

7. A formed part, such as a bonnet part, particularly an inner formed part, of an aluminium alloy according to any of the preceding claims, after forming and paint- baking having a yield strength of below 150 MPa, preferably a yield strength in the range of 105 to 130 MPa.

8. A formed part according to claim 7, having a yield strength of below 180 MPa, preferably below 160 MPa during the life cycle of the sheet.

9. A formed automobile part, such as a bonnet, comprising an inner formed part and an outer formed part wherein

(i). both the formed inner and outer parts are produced from an alloy sheet according to any one of claims 1 to 6, or wherein (ii). the formed inner or the formed outer part is produced from an alloy sheet according to any one of claims 1 to 6 and wherein the other part is produced from an alloy of the AA6000-series with a chemical composition outside the chemical composition of any one of claims 1 to 6, so as to increase recyclability of the formed automobile part.

10. Process for producing an aluminium alloy sheet according to any one of claims 1- 6 comprising the steps of: casting an aluminium alloy with a composition according to any one of claims 1 to 6 homogenization and/or preheating hot-rolling - cold-rolling to an intermediate gauge optionally followed by inter-annealing cold-rolling to final gauge solution heat treating by continuous annealing - quenching, preferably by forced air, to produce a sheet in T4-condition.

11. Process for producing an aluminium alloy sheet according to claim 10 wherein the quenched sheet is subjected to a prebaking-treatment in which treatment the alloy is reheated and warm-coiled to produce a sheet in T4P-condition.

12. Process according to claim 10 or 11 followed by a manufacturing process comprising the steps of forming a part, such as stamping and/or pressing a part from the sheet, coating the part, baking the coated part.