ALUMINIUM COMPOSITE SHEET MATERIAL

The invention relates to an aluminium composite sheet material in which a clad sheet is applied to at least one side of a core material. Composite materials of this type are known from the state of the art, such as from international application WO-01/02165. In such a composite material the core material (which determines by far the largest component of the composite material) mainly determines the bulk mechanical properties of the composite material, such as for example the strength thereof. The clad sheet however (which, consequently, only determines a small fraction of the composite material and thus will be of minor importance for the mechanical properties thereof) is in contact with the environment surrounding the composite material and thus will determine the chemical activity (for example to a very large extent the corrosion performance of the composite material).

As examples of aluminium composite materials according to the state of the art firstly brazing sheet (with an aluminium alloy from the AA3xxx-series as core material and an aluminium alloy from the AA4xxx-series as clad sheet), as well as sheets for aeronautical use (core material comprising an aluminium alloy from the AA2xxx -series and clad sheet typically comprising an aluminium alloy from the AA1000-series) may be mentioned. It is common for such known composite materials that they are developed for a specific use and for fulfilling specific demands.

It is noted that the above-mentioned document WO-01/02165, which represents the state of the art according to the preamble of the main claim, does not mention nor suggest specific aluminium alloys for use in a composite material, but only refers to a method for producing such an aluminium composite material (irrespective its composition).

As will be appreciated herein below, except as otherwise indicated, all aluminium alloy designations refer to the Aluminium Association designations in Aluminium Standards and Data and the Registration Records, as published by the Aluminium Association. It is an object of the present invention to provide an improved aluminium composite material of the type referred to above, which is especially designed for use in vehicles, and as Automotive Body Sheet in particular.

Thus in accordance with the present invention there is provided an aluminium composite material in which a clad sheet is applied to at least one side of a core material, characterized in that the core material comprises an aluminium alloy from the AA5XXX- or AAθxxx-series, whereas the clad sheet comprises an aluminium alloy from the AAIxxx-series, AA3xxx-series, or AA7xxx-series with less than 1.5 wt.% Zn.

To date panels for use in vehicles (also referred to as Automotive Body Sheets or ABS) did use only a single aluminium alloy (and thus define what is referred to as uni- alloy systems). Generally, aluminium alloys from the AAδxxx-series are used for structural and inner panel applications, whereas aluminium alloys from the AAθxxx- series are used for outer panel applications. Although by varying the chemical composition of the alloys a wide variation of mechanical properties can be achieved, not all possibilities are applied in practice. For applications in vehicles these possibilities are limited, among others, due to restrictions imposed by corrosion performance. For example, an increase of the Mg-level in AAδxxx-series aluminium alloys indeed enhances the formability as well as strength, but at the same time the corrosion performance (especially the so-called intergranular corrosion) is worsened. Thus, although one would like to increase the Mg-level for improving the mechanical properties, the reduction of the chemical properties does not allow it. Likewise, in AA6000-series aluminium alloys an increase of the Cu-level (which would be beneficial for the strength-performance) would adversely lead to an increased corrosion sensitivity (especially the so-called filiform corrosion) for painted surfaces.

According to the present invention the properties of the core material are disconnected from the demands imposed on the surface of the composite material. Thus, the core material can be chosen such as to meet the mechanical demands as good as possible (meaning, in the above-mentioned examples, an increase of the Mg and Cu levels), whereas the clad sheet can be chosen such as to meet the demands on interaction with the environment. It appears, that a combination of a core material comprising an aluminium alloy from the AAδxxx or AAθxxx-series with a clad sheet comprising an aluminium alloy selected from the group consisting of the AAIxxx- series, AA3xxx-series and AA7xxx-series with less than 1.5 wt.% Zn can meet such demands.

Further, in vehicles adhesive bonding of automotive body sheets occurs, regulated by specific requirements (although not as stringent as is the case in the aeronautical field). While a uni-alloy system needed an adequate pre-treatment of its surface, it appears that the composite sheet material according to the present invention does not need so to obtain a sufficient durability.

According to an embodiment the AA5xxx-series aluminium alloy of the core has the following chemical composition, in weight percent: Mg 3.50 - 5.00, preferably 4.0 - 4.70 Mn 0.05 - 0.80, preferably 0.20 - 0.55

Zn < 0.60, preferably < 0.30

Fe < 0.35, preferably < 0.25

Si < 0.25, preferably < 0.20

Cr < 0.30, preferably 0.05 - 0.25

Zr < 0.30, preferably 0.05 - 0.25 Cu < 0.20

Ti < 0.15, balance aluminium and inevitable impurities, each < 0.05, total <0.15.

This range encompasses the aluminium alloys AA5082, AA5182, AA5086, AA5186. Whereas as an example of aluminium alloy from the AAδxxx-series the aluminium alloy AA6016 may be mentioned. Other suitable AAθxxx-series alloys are those having a Cu-content up to 1.1 wt.%, and preferably up to 0.9 wt.%, for example the AA6111 -series alloy.

These alloys are extremely well-fitted for the intended use in the automobile field because of their specific properties, which per se are well known to the skilled person.

Further, it is preferred that the clad sheet comprises an aluminium alloy selected from the group of the AAIxxx-series, which includes essentially unalloyed aluminium or aluminium containing not more than 0.1 or 1 % of all other elements. Aluminium alloys herein designated AA1xxx-type series include all Aluminium Association (AA) alloys, including the sub-classes of the 1000-type, 1100-type, 1200-type and 1300- type. This the cladding on the core may be selected from various Aluminium

Association alloys such as 1050, 1060, 1045, 1100, 1200, 1230, 1135, 1235, 1435,

1145, 1345, 1250, 1350, 1170, 1175, 1180, 1185, 1385, 1188, or 1199. These alloys are preferred because of its advantageous chemical properties (i.e. its interaction with the surroundings) and its durability. Of these alloys the AA1050 alloy is most preferred.

In another embodiment the core is clad with an aluminium alloy from the AA7xxx- series with less than 1.5 wt.% of Zn, and which encompasses the AA7072 alloy containing 0.8 to 1.3% Zn. However, also AlZn-alloys with Zn-contents in the range

0.25 to 0.7 wt.%, and preferably 0.3 to 0.6 wt.%, provide adequate corrosion protection for the intended purpose of this invention.

Although, depending on its application, the aluminium composite material in accordance with the present invention may comprise only one clad sheet applied to only one side of the core material, preferably a clad sheet is applied to both sides of the core material. As a result, the composite material exhibits excellent balanced properties, viz. strength and formability versus corrosion performance.

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Although the dimensions of the aluminium composite material can be varied in many ways (mostly imposed by the specific use and concurrent demands), in an exemplary embodiment for use as automotive body sheet the core material has a thickness in the range of 0.5 to 2 mm, preferably 0.7 to 1.3 mm, and most preferably about 1 mm. For structural application of the composite sheet the thickness may range up to 3.5 mm for the core alloy. The clad layer or layers are usually much thinner than the core, each constituting 1 to 25% of the total composite thickness. A clad layer more typically constitutes around 1 to 12% of the total composite thickness.

In a further aspect the invention relates to a method for producing an aluminium composite material in which a clad sheet is applied to at least one side of a core material. In accordance with the present invention the core material comprises an aluminium alloy from the AA5xxx- or AA6xxx-series, whereas the clad sheet comprises an aluminium alloy from the AAIxxx-series, AA3xxx-series or AA7xxx-series with less than 1.5 wt.% of Zn, and wherein the clad sheet is attached to the core material by means of roll bonding.

Such a roll bonding process is very economic and results in a very effective composite material presenting the desired properties.

Of course, the roll bonding process may be accompanied by several additional processing steps (such as for example annealing) for determining the final properties of the composite material.

When carrying out such a roll-bonding process for producing the inventive composite material, it is preferred that both the core material and clad sheet experience a thickness reduction during the role bonding.

For example the core material initially can be a 80 mm thick block (for example from aluminium alloy AA5182 or AA6016) with at both sides a clad sheet initially 5 mm thick. After roll bonding the final width of the core material was about 1 mm, whereas the final thickness of the clad layer is, for example, 40 μm.

It is noted that the initial dimensions and final dimensions of the composite material will be determined both by the specifics of the roll-bonding process as well as the required properties of the final material.

The roll bonding process can be carried out in different manners. For example, it is possible that the roll-bonding includes both hot rolling and cold rolling.

Further, the roll-bonding process may be a one-step process or a multi-step process in which during successive rolling steps the material is gauged down. Separate rolling steps then may be separated by other processing steps (for example annealing steps, heating steps, cooling steps, etc).

As expressed above, the aluminium composite material in accordance with the present invention is extremely well-fit for use as automotive body sheet in vehicles. The choice of the core material from the AA5xxx- or AAδxxx-series leads to excellent mechanical properties, which are at least comparable with standard industrial substrate alloys. Tests conducted on specimens of the inventive aluminium composite material further showed, that there is no significant correlation between the thickness of the clad sheet and the mechanical properties, thus confirming what has been stated before with respect to disconnecting the bulk properties of the core material from the demands imposed on the clad sheet. Further, the choice of in particular an AAIxxx-series aluminium alloy as clad sheet enables to obtain an excellent performance of the composite material as regards to the environment. For example, its sensitivity for inter granular corrosion can be limited within acceptable limits; further its sensitivity for filiform corrosion, especially in combination with painted surfaces, can be reduced. The invention is not limited to the embodiments described before, which may be varied widely within the scope of the invention as defined by the appending claims.