ALUMINIUM ALLOY SHEET FOR AUTOMOTIVE APPLICATIONS AND STRUCTURAL AUTOMOBILE BODY MEMBER PROVIDED WITH SAID ALUMINIUM ALLOY SHEET

The invention relates to an aluminium alloy sheet for automotive applications.

More in particular to an aluminium alloy sheet for an automobile body part as such or applied as outer part on a structural member, such as for instance the larger automobile structural members, like roof structural members.

The application of an aluminium alloy sheet on larger steel structural members may give rise to manufacturing problems because of the different thermal expansion coefficients of the aluminium alloy sheet and the steel frame of the structural member. This is of particular interest with paint-bake processes as generally applied in the automobile industry and typically involving heating at a temp of approximately 170 to 205°C for a time of 15 to 30 minutes. Due to the difference in thermal expansion coefficients of an aluminium alloy sheet and that of a steel structural member it may occur that the aluminium alloy sheet undergoes plastic deformation in the paint-bake process finally resulting in an irregular outer appearance of the specific part. The invention relates also to the use of the aluminium alloy sheet as an automobile body part and to a structural automobile body member provided with a body part made from said aluminium alloy sheet.

An aluminium alloy sheet with the specific properties for these kind of automotive applications, such as among others good formability and dent resistance, should at the same time also have properties to overcome problems because of said different thermal expansion coefficients. Such an aluminium alloy sheet should preferably have the following specific properties: sufficiently low yield-strength and high elongation in as-delivered condition for better formability. The yield-strength should however not be too low in order to maintain an acceptable yield-strength during the first stage of the paint-bake process; a minimal decrease in yield-strength during the first stage of the paint-bake hardening process of the aluminium sheet. This is of crucial importance in order to avoid plastic deformation of the aluminium alloy sheet attached to a steel structural member. Therefore, the yield-strength has to stay above a certain critical value, and

a yield- strength as high as possible after completion of the paint-bake process. Furthermore, with automotive applications it is also of utmost importance that after completion of the paint-bake cycle the final product exhibits excellent corrosion resistance. Therefore, an objective of the invention is to provide an aluminium alloy sheet with the above yield strength properties.

Another objective of the invention is to provide an aluminium alloy that after completion of the paint-bake cycle has a high filiform corrosion resistance.

A further objective is to provide a structural automobile body member comprising a steel frame provide with said aluminium alloy sheet.

Still a further objective is to provide a method to produce said aluminium alloy sheet.

In order to meet one or more of the above objectives an aluminium alloy sheet for automotive applications is provided consisting essentially of, in wt.%: Si : 0.50 - < 0.70

Cu: 0.40 - 1.20

Fe: 0.20 - 0.4

Mn: > 0.1 - 0.60

Mg: 0.60 - 1.40 Zn: < 0.5

Ti: < 0.2

Cr: < 0.15, impurities and incidental elements up to 0.05 each and up to 0.15 in total, and balance aluminium. It was found that with a relative low Si-content the drop in yield-strength during the first stages of the paint-bake process is decreased considerably. However, when the Si-content becomes too low, the yield-strength in as-delivered and paint- baked condition will decrease too much. Therefore, the lower limit should be at least 0.50%, and the range for the Si-content is 0.50 to < 0.70%, and preferably 0.55 to <0.70% and even more preferably 0.60 to < 0.70%. According to a further preferred embodiment the upper limit of the Si-content is set at 0.65%. Because of the low drop in yield-strength a yield-strength in as-delivered condition can be tuned towards

a value, that is closer to the critical level during the first stage of the paint-bake process. An advantage is that the formability of the aluminium alloy sheet in the as- delivered state is improved, while preventing plastic deformation during the paint bake cycle of the aluminium alloy sheet attached to a steel structure of a structural automobile body member. Another advantage is the improved resistance to filiform corrosion, in particular in combination with a significant Cu-content. Furthermore, when Si is > 0.70% a too high yield-strength is obtained in as-delivered condition and therewith a too low formability. It has been found also that when the Si-content is above 0.7% the resistance against filifom corrosion adversely decreases. Cu is known to accelerate the yield-strength increase during forming- and paint-bake operations. Furthermore, a positive effect of Cu on the uniform and total tensile elongation was found. However, when the Cu content is too high, resistance to filiform corrosion and to intergranular corrosion will become too low and the yield-strength in as-delivered condition will become too high and formability will decrease. The Cu-content is 0.40 to 1.20%, and preferably 0.40 to 1.10%. In a more preferred embodiment there is a low Cu content in the range of 0.40 to 0.65% and preferably 0.40 to 0.60%. And in another embodiment the alloy has a high Cu content in the range of 0.90 to 1.10%.

Fe is desirable for control of the grain structure. Fe refines the recrystallized grains and reduces the alloy's susceptibility to surface roughening phenomena like orange-peel. Therefore, a lower limit of Fe is set at 0.20%. However, a too high Fe- content was found to have a detrimental effect on formability. hi order to obtain sufficient uniform (Au) and total (A80) tensile elongation, measured according to ENl 0002, in as-delivered condition, T4 or T4P, an upper limit of 0.4 %, preferably 0.3 %, is set. The "T4P" temper stands for preaging of aluminum sheet prior to sheet forming to enhance the response during paint baking while maintaining the desired sheet formability.

Mn is added for grain size control. When the Mn-content is too low, the grain size will become too large during solution heat treatment. This will cause orange peel during subsequent deformation. It was also found that an increase of the Mn-content decreases the drop in yield strength during the first stage of the paint-bake process. Therefore, a lower limit of 0.1%, preferably 0.20%, is set. A too high Mn-content

will however negatively affect formability in T4 or T4P condition. Therefore, a Mn- content of maximum 0.60%, preferably maximum 0.50%, is set.

The Mg-content needs to be sufficiently high in order to obtain a yield-strength in T4 or T4P condition in the target range. Yield-strength and tensile strength are found to increase with increasing Mg-content. Also, in order to obtain a sufficiently large yield-strength after the paint-bake process, the Mg-content should be sufficiently high. A minimum Mg content of at least 0.60% and preferably 0.80% is therefore necessary. However, when the Mg content is above 1.40%, the yield- strength in T4 or T4P condition becomes too high and consequently formability will decrease too much. Therefore, the maximum upper limit is 1.40%, however a more preferable upper limit is 1.20% and even more preferably 1.10%.

Zn may be added up to 0.5% for strength increase of the artificially aged state of the aluminium alloy sheet after the paint bake cycle. However, Zn increases also the yield strength in the as-delivered condition. Therefore, a more preferable upper limit of the Zn content is set at 0.4%, more preferably at 0.3% and even more preferably at 0.08%.

Furthermore, a Ti-content of maximum 0.2% is set for grain size control and in particular as grain refiner during casting. Preferably the Ti-content is < 0.1%.

Furthermore, a Cr-content of maximum 0.15% is set to improve the resistance to intergranular corrosion. This effect is obtained in combination with Ti and preferably Cr + Ti ≤ 0.25% or more preferably Cr + Ti < 0.2%. Furthermore, these elements provide an alternative for Mn in recrystallisation inhibition and grain refinement.

Within the range of the composition according to the invention the aluminium alloy sheet as delivered prior to forming into a shaped part for an automobile, that is in a T4 temper or in a T4P temper, has a yield strength in the range of 130 to 170 MPa, and preferably at least 140 MPa, which is favourable for good formability.

The aluminium alloy sheet according to the invention is used as an automobile body part, and more in particular as an automobile body part attached to a structural automobile body member, in which at least the automobile body part is subsequently subjected to a paint-bake process. Such a structural automobile body

member forms for instance at least partly a roof of an automobile. During the first stage of the paint-bake process the yield-strength will drop to a lower value which has to stay above a certain critical lower value in order to avoid plastic deformation during the paint-bake process. Such plastic deformation may occur during the paint- bake process as a result of the different thermal expansion coefficients of the aluminium alloy sheet and the structural automobile body member, for instance a steel frame, to which the aluminium alloy sheet is attached by means of for example welding or adhesion.

The range of the composition of the aluminium alloy sheet according to the invention is such that during the paint-bake process the minimum yield strength of the aluminium alloy sheet will remain above 130 MPa. If expressed as difference between the yield-strength before the paint-bake process and the lowest yield- strength during the paint-bake process, the maximum drop in yield-strength during the paint bake process will be less than 10 MPa, and preferably less than 7 MPa. Furthermore, within the composition range according to the invention the yield-strength of the aluminium alloy sheet after the paint-bake process will be at least 240 MPa, and preferably at least 250 MPa.

Another favourable property of the aluminium alloy sheet according to the invention is the weldability thereof, more in particular the weldability of the aluminium alloy sheet to a steel frame, such as a steel structural automobile body part.

The invention also relates to a method of producing an aluminium alloy sheet according to the invention comprising the steps of: casting an aluminium alloy according to the composition as described - homogenization / preheating, hot rolling, cold rolling, optionally intermediate annealing during cold rolling, solution heat treatment by continuous annealing, - quenching in water followed by natural ageing for at least one day to produce an alloy in T4 condition.

According to a further embodiment intermediate annealing is achieved by a continuous process at a temperature in the range of 350 to 500 ⁰ C. According to still a further embodiment intermediate annealing is achieved by a batch process at a temperature in the range of 300 to 45O ⁰ C.

According to another embodiment after quenching the sheet is subjected to a pre-bake treatment in which the sheet is reheated and warm-coiled in the range of 50 to 120 ⁰ C within 1 hour after quenching to produce a sheet in T4P condition.

With the final cold rolling step the thickness of the sheet is preferably reduced to a final thickness in the range of 0.8 to 1.2 mm.

Example 1.

Table 1 shows 4 examples within the range of the composition of the aluminium alloy sheet according to the invention and 4 reference examples.

Table 1. Compositions of alloys according to the invention compared to reference allo s in wt% balance aluminium.

In Table 2, the mechanical properties of the examples within the range of the composition of the aluminium alloy sheet according to the invention are given, as well as those of the reference examples. The reference examples were produced in the same way as the examples according to the invention.

Table 2. Mechanical properties parallel to the rollin direction.

The values of the properties given in bold, italic characters are outside the desired range of properties according to the invention. The term "Hot" and "Cold" in Table 2 refer to the temperature of the specimen during the tensile test. "Hot" tensile tests were performed at a temperature of 180°C which is in concordance with the temperature during a typical paint-bake process. The heating-rate to 180°C is about 1.3°C/s. Soak times of 1.5, 3 and 5 minutes were chosen to evaluate the development of yield-strength during the first stages of the paintbake procedure. "Cold" tensile test were performed at room temperature (first three columns) or after cooling down to room temperature after the paint-bake process (last column). The mechanical property tests in cold-state were done according to the ASTM norm EN10002 for tensile tests. From the mechanical properties as shown it will be clear that the drop in yield- strength of the examples according to the invention during the initial stage in the paint-bake process is significantly smaller than that of the reference examples. In this way, a lower yield-strength and consequently better formability in as-delivered condition can be combined with a minimum yield-strength during the first stage of the paint-bake process that is still sufficiently high to prevent plastic deformation of the aluminium alloy sheet attached to a steel frame.

Furthermore, the examples according to the invention combine a relatively low yield-strength in T4P condition with a high yield-strength after 20 minutes at 190°C, that is after completing a paint-bake process. hi Table 3 the resistance against filiform corrosion is given. Filiform corrosion is a thread-like form of corrosion that occurs under organic coatings. The source of initiation is usually a defect or mechanical scratch in the coating.

Table 3. Resistance to filiform corrosion.

The filiform corrosion resistance of the examples according to the invention with respect to that of the reference alloys was investigated according to the European norm EN3665. The lower the value in Table 3, the higher the resistance to filiform corrosion.

By combining the test results given in Table 2 and Table 3 it follows that the examples according to the invention provide the best combination of filiform corrosion resistance and mechanical properties, that is formability in T4(P) condition, yield-strength development during the first stages of artificial ageing in the paint-bake process and the final yield-strength. The good filiform corrosion resistance of Invention 1 can be explained by the low Cu content of that example. The better filiform corrosion resistance of Invention 2 with a high Cu-content in comparison with Reference 1 is believed to be the result of the lower Si-content of Invention 2 and/or the fact that the alloy according to Invention 2 does not contain any Zn.

Example 2.

Four alloys have been manufactured and tested in the same manner as those in

Example 1. Two reference alloys (Reference 1 and 2) can be compared to the alloy according to the invention and illustrates in particular the effect of Si content on various engineering properties. Reference 3 is a typical alloy of the AA6111 -series. The chemical composition are listed in Table 4, and the mechanical properties as tested are given in Table 5.

Table 4. Compositions of an alloy according to the invention compared to three reference alloys, all percentages are in wt.%, balance aluminium.

Table 5. Mechanical ro erties arallel to the rolling direction.

From the results of Table 5 it can be seen that Reference 1 has a too low yield strength after the paint bake process. Whereas Reference 2 and 3 both have a too large yield drop.