Aluminium alloy sheet optimised for forming

Abstract

The invention relates to a strip or sheet consisting of an aluminium alloy having a unilateral or bilateral surface structure prepared for a forming process, in particular it relates to a strip or sheet for formed motor vehicle components. The object of providing an aluminium alloy strip or sheet having a surface structure prepared for a forming process, which is easy to produce and has improved tribological characteristics in respect of a subsequent forming process, is achieved for a strip or sheet consisting of an aluminium alloy in that the strip or sheet has on one side or on both sides a surface with depressions as lubricant pockets which are produced using an electrochemical graining process.

Claims

1. Strip or sheet consisting of an aluminium alloy having a unilateral or bilateral surface structure which is provided at least in some regions and is prepared for a forming process, wherein the strip or sheet has on one side or on both sides a surface with depressions as lubricant pockets which are produced using an electrochemical graining method, the at least one surface of a strip or sheet having a reduced depression depth S.sub.vk of 1.0 μm to 6.0 μm and a closed empty volume V.sub.vcl amounts to at least 450 mm.sup.3/m.sup.2, wherein a depression density N.sub.clm amounts to 80 to 180 depressions/mm.sup.2.

2. Strip or sheet according to claim 1, wherein the strip or sheet at least partially consists of an aluminium alloy of a type AA7xxx, AA6xxx, AA5xxx or AA3xxx.

3. Strip or sheet according to claim 1, wherein at least one surface of the strip or sheet has a reduced depression depth S.sub.vk of 1.5 μm to 4.0 μm.

4. Strip or sheet according to claim 1, wherein the sheet or strip is in an annealed state (“O”), a solution-annealed and quenched state (“T4”) or a 3/4 hard state (“H19”) or 4/4 hard lacquered and burned in state (“H48”).

5. Strip or sheet according to claim 1, wherein the strip or sheet has a passivation layer which is applied after electrochemical graining.

6. Strip or sheet according to claim 1, wherein a lubricant is provided at least in some regions on the surface of the strip or sheet.

7. Strip or sheet according to claim 1, wherein a mean roughness of the surface S.sub.a is 0.7 μm to 1.5 μm.

8. Method for producing a strip or sheet according to claim 1 having a unilateral or bilateral surface structure, prepared for a forming process characterised in that a hot-rolled and/or a cold-rolled strip or sheet is subjected to an electrochemical graining process after rolling, wherein said electrochemical graining process introduces homogeneously distributed depressions as lubricant pockets at least into some regions of the strip or sheet, depressions being introduced into the surface of the strip or sheet by electromechanical graining having the reduced depression depth S.sub.vk of 1.0 μm to 6.0 μm and a closed empty volume V.sub.vcl amounts to at least 450 mm.sup.3/m.sup.2, wherein a depression density amounts to 80 to 180 depressions/mm.sup.2.

9. Method for producing a strip according to claim 8, wherein depressions having the reduced depression depth S.sub.vk of 1.5 μm to 4.0 μm are introduced into the surface of the strip or sheet by electrochemical graining.

10. Method for producing a strip according to claim 8, wherein prior to electrochemical graining, the strip is subjected to a cleaning step in which the surface is cleaned and material is removed homogeneously by alkaline or acidic pickling.

11. Method for producing a strip according to claim 8, wherein electrochemical graining is carried out using HNO.sub.3 in a concentration of 2.5 to 20 g/l with an introduction of charge carriers of at least 200 C/dm.sup.2.

12. Method for producing a strip according to claim 8, wherein after electrochemical graining, the surface is passivated, preferably by applying a conversion layer, and/or a protective layer having a meltable forming aid is applied to the surface of the strip.

13. Method for producing a strip according to claim 8, wherein a strip is grained electrochemically after an annealing procedure (state “O”), after a solution heat treating and quenching procedure (state “T4”), or rolled in state H19.

14. Method for producing a strip according to claim 8, wherein the method steps are carried out inline in a production line: unwinding the strip from a reel, cleaning and pickling the strip, electrochemically graining the strip and applying, at least in some regions, a forming aid and/or a conversion layer or alternatively a protective oil.

15. Method for producing a strip according to claim 14, wherein after applying the conversion layer, a protective layer having a meltable forming aid is subsequently applied.

16. Use of the sheet according to claim 1 of a formed sheet for a motor vehicle.

17. Strip or sheet according to claim 1, wherein the strip or sheet at least partially consists of an aluminium alloy of type AA7020, AA7021, AA7108, AA6111, AA6060, AA6014, AA6016, AA6005C, AA6451, AA5454, AA5754, AA5182, AA5251, AlMg6, AA3104 and AA3103.

18. Strip or sheet according to claim 1, wherein at least one surface of the strip or sheet has a reduced well depth S.sub.vk of 2.2 μm to 4.0 μm.

19. Strip or sheet according to claim 1, wherein a mean roughness of the surface S.sub.a is 0.7 μm to 1.3 μm.

20. Strip or sheet according to claim 1, wherein a mean roughness of the surface S.sub.a is 0.8 μm to 1.2 μm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be described below in more detail with reference to embodiments in conjunction with the drawings, in which:

(2) FIG. 1 schematically shows the determination of the parameters S.sub.k, S.sub.pk and S.sub.vk using an Abbott curve;

(3) FIG. 2 shows a microscopic image of an embodiment not according to the invention;

(4) FIG. 3 shows a microscopically enhanced image of an embodiment of a strip surface according to the invention;

(5) FIG. 4 schematically shows an embodiment of a production line for implementation of the method according to the invention;

(6) FIG. 5 is a schematic sectional view of an embodiment of a strip or sheet according to the invention;

(7) FIG. 6a), 6b) are schematic, perspective sectional views of the test arrangement of the drawing test using a cross tool for determining the forming behaviour;

(8) FIG. 7 is a diagram showing the maximum sheet holding force in kN during the drawing test using a cross tool as a function of the round blank diameter of the sheet;

(9) FIG. 8 shows the maximum sheet holding force for different round blank diameters with normal and very high use of lubricant; and

(10) FIG. 9 is a diagram showing the maximum sheet holding force in kN as a function of the quantity applied in g/m.sup.2 of a lubricant.

DETAILED DESCRIPTION OF THE INVENTION

(11) FIG. 1 shows how the parameter values for the core roughness depth S.sub.k, the reduced well depth S.sub.vk and the reduced peak height S.sub.pk can be calculated from an Abbott curve. According to DIN-EN-ISO 25178, the determination is carried out for a standardized measurement area. Optical measuring methods, for example confocal microscopy, are usually used to calculate a height profile of a measurement area. By the height profile of the measurement area, it is possible to calculate the area portion of the profile which intersects an area parallel to the measurement area in height c or which runs above the area. Presenting the height c of the intersecting area as a function of the area portion of the intersecting area to the total area, the Abbott curve is obtained which shows the typical S-shaped course for rolled surfaces.

(12) In order to determine the core roughness depth S.sub.k, the reduced well depth S.sub.vk and the reduced peak height S.sub.pk, respectively, a secant D with 40% length is displaced on the determined Abbott curve so that the amount of increase of the secant D is minimal. The core roughness depth S.sub.k of the surface results from the difference of the abscissa values of the intersection points of the secant D with the abscissa at 0% area portion and at 100% area portion. The reduced peak height S.sub.pk and the reduced well depth S.sub.vk correspond to the height of a triangle which is coextensive with the peak area A1 or the groove area A2 of the Abbott curve. The triangle of the peak area A1 has as base area the value Smr1 which results from the intersection point of a parallel to the X-axis with the Abbott curve, the parallel to the X-axis running through the intersection point of the secant D with the abscissa at 0% area portion. The triangle of the groove area or well area A2 has as base area the value 100%-Smr2, where Smr2 results from the intersection point of a parallel to the X-axis with the Abbott curve and the parallel to the X-axis runs through the intersection point of the secant D with the abscissa at 100% area portion.

(13) These characteristic values can be used to characterise the measurement profile. It can be determined whether it is a plateau-like height profile with depressions, or, for example, whether the peaks predominate in the height profile of the measurement area. In the former case, the value for S.sub.vk increases, and in the latter case, the value for S.sub.pk increases.

(14) As a further parameter of the surface, the well density of the texture n.sub.clm ican be calculated from the optical measurement of the surfaces via the maximum number of closed empty volumes n.sub.clm, i.e. of the depressions or wells as a function of the measuring height c in percent per mm.sup.2. This generates the number of closed empty areas per unit area (1/mm.sup.2) at a given measuring height c (%). The maximum n.sub.clm is determined from n.sub.cl(c). The greater n.sub.clm, the finer the surface structure.

(15) Furthermore, the closed empty volume V.sub.vcl can also be calculated by the optical measurement by integrating the closed empty areas A.sub.vcl(c) via the measuring height c. The closed empty volume is also a characteristic surface feature of the strips and sheets according to the invention.

(16) As already stated, the roughness of the surface is measured optically, as in this way, sampling can be carried out substantially faster compared to a tactile measurement. Optical detection is carried out, for example, by interferometry or confocal microscopy, as was done with the present measured data. According to EN ISO 25178-2, the size of the measuring areas is also established. The measured data were calculated via quadratic measuring areas of a side length of respectively 2 mm.

(17) To illustrate the differences between the conventional strips roughened with EDT-structured rolls for example, and the strips structured according to the invention, FIG. 2 shows a 250-fold enlarged view of a conventional strip surface. On the other hand, FIG. 3 shows an embodiment of a strip surface according to the invention which was produced by an electrochemical graining method, also in 250-fold magnification. It can be clearly seen that, on the one hand, the structures in the electrochemical graining are finer and consist of depressions in a plateau-like surface. In contrast to the conventional roll-embossing shown in FIG. 2, in electrochemical graining according to the invention no peaks are introduced into the material, but the rolled surface, here a mill finish surface, is only altered or modulated by the introduction of depressions. It is presently assumed that the depressions produced by electrochemical graining can provide more lubricant for the forming process due to the greater closed empty volumes, and therefore improved forming properties are achieved. It has also been found that the greater well depth S.sub.vk can obviously provide lubricant during forming even with a great surface stress and thus improves the forming behaviour.

(18) FIG. 4 shows a first embodiment of a method using a schematic of a production line for producing a strip B according to the invention. In the embodiment shown, the strip B which preferably at least partially consists of an aluminium alloy of type AA7xxx, type AA6xxx or type AA5xxx or type AA3xxx, in particular AA7020, AA7021, AA7108, AA6111, AA6060, AA6014, AA6016, AA6106, AA6005C, AA6451, AA5454, AA5754, AA5182, AA5251, AA3104, AA3103 or AlMg6 is unwound from a reel 1. The thickness of the strip is preferably at least 0.8 mm but at most 3 mm and preferably between 1.0 mm and 1.5 mm, for example for use in the automotive sector. In principle, the thickness can also be 0.1 mm to 0.5 mm, for example in the case of strips for beverage can production. The improved forming behaviour during beverage can production requiring maximum forming degrees is apparent in the case of these thin strips as well.

(19) According to the present embodiment, the strip unwound from the reel 1 is preferably in the annealed state “O”, if it is an aluminium alloy of type AASxxx, AlMg6 or AA3xxx, or in the solution heat treated and quenched state “T4” state, in the case of an aluminium alloy of type AA6xxx or AA7xxx. The strip is thus already in a particularly effectively formable state. However, it is also conceivable to carry out the heat treatment after the surface processing or after the introduction of the depressions and, in so doing, to process the surface of rolled strips. In addition, strips and sheets of type AASxxx or AA3xxx for beverage can production are also in state H19 or lacquered in state H48 before they are formed.

(20) According to the embodiment, the unwound aluminium alloy strip B is delivered to an optional trimming procedure to trim the side edges 2. Thereafter, the strip also optionally passes through a straightening device to remove deformations from the strip. In the device 4, the strip is subjected to a cleaning and a pickling step. As etchant, it is possible to use mineral acids, but also bases, for example based on caustic soda. This can improve the response of the strip to electrochemical graining. The pickling step 4 is also optional. After rinsing, the aluminium strip undergoes an electrochemical graining process in step 5, in which depressions are introduced into the surface. During electrochemical graining, depressions are introduced into the strip and aluminium is dissolved out in the corresponding spots as a result of the reaction of the electrolyte with the aluminium alloy strip. Electrochemical graining is preferably adjusted such that a well depth S.sub.vk of 1.0 μm-6.0 μm, preferably 1.5 μm-4.0 μm, more preferably 2.2 μm-4.0 μm is achieved. It has been found that with these characteristic values, the forming behaviour of the aluminium alloy strip is very good in a subsequent forming process.

(21) Electrochemical graining is preferably carried out using HNO.sub.3 (nitric acid) in a concentration of 2.5-20 g/l, preferably with 2.5 to 15 g/l with alternating current of a frequency of 50 Hz. The introduction of charge carriers is preferably at least 200 C/dm.sup.2, preferably at least 500 C/dm.sup.2, to achieve a satisfactory surface covering with electrochemically introduced depressions. For this purpose, at least 1 A/dm.sup.2, preferably up to 100 A/dm.sup.2 and more are used as peak current densities. The choice of current densities and the concentration of the electrolyte depend on the production rate and can be adjusted accordingly. In particular, the reactivity and thus the production rate can also be influenced by the temperature of the electrolyte. The electrolyte can preferably have a maximum temperature of 75° C. When nitric acid is used as electrolyte, a preferred working range is between room temperature and approximately 40° C., at most 50° C. In addition to nitric acid, hydrochloric acid is also suitable as electrolyte.

(22) The surface of strip B is preferably subjected to electrochemical graining on both sides in step 6. However, it is also conceivable for a corresponding surface structure to be introduced only on one side. Thereafter in working step 6, according to the embodiment shown in FIG. 5, either a protective oil can be applied or the surface of the aluminium alloy strip can be passivated, for example by applying a conversion layer. These processing steps are also optional.

(23) A drying procedure is preferably carried out in step 7, before an optional layer having a forming aid is applied to the strip, preferably on both sides thereof, in step 8 according to the embodiment shown. The forming aid is preferably a lubricant, in particular a meltable dry lubricant, for example a hotmelt. A meltable dry lubricant as a protective layer and lubricant can simplify the handling of the aluminium alloy strips or sheets according to the invention and, at the same time, further improve the forming properties. Wool wax, for example, can also be used as a dry lubricant from renewable raw materials.

(24) As an alternative to winding up the strip B with the reel 11, the belt shears 10 can cut the strip into sheets. In step 9, the strip is examined visually for defects so that surface defects can be detected early.

(25) As already stated, the embodiment from FIG. 4 shows several optional working steps which are carried out inline directly one after another in the same production line. Therefore, the embodiment from FIG. 4 is a particularly economical variant of the method according to the invention. However, it is also conceivable to merely combine the unreeling of a strip according to step 1 and the electrochemical graining according to step 5 with an operation for winding-up or cutting into sheet metal blanks. In principle, an electrochemical graining of sheet metal blanks is also conceivable.

(26) FIG. 5 is a schematic sectional view of an embodiment of a strip B according to the invention which has depressions 12 introduced into both sides of the surface and also an applied layer of a meltable dry lubricant 13. A corresponding strip B has maximum forming properties and furthermore can be easily stored as the surface is protected. Corresponding strips B, also with a unilaterally grained surface, can also be used as external skin parts of a motor vehicle since the surface is maximally protected against the forming process and assists the forming procedure to a significant extent. Sheets produced from a strip B have a very good handling ability in the forming process due to the surface protection.

(27) To test the forming properties of the sheets which have electrochemically grained surfaces in the forming process, drawing tests using a cross tool were carried out. FIG. 6a is a perspective sectional view of the configuration of the cross tool. The cross tool comprises a punch 21, a hold-down device 22 and a die 23. The sheet 24 to be tested was roughened either by a conventional method, for example only by EDT rolling, or only by the electrochemical graining according to the invention, but also in addition to EDT rolling.

(28) During the drawing test in the cross tool, the sheet 24 formed as a round blank is deep-drawn by the punching force FsT, the hold-down device 22 and the die 23 being pressed onto the round blank with force FN. The cross-shaped punch 21 respectively has a width of 126 mm along the axes of the cross, while the die has an aperture width of 129.4 mm. The round sheet blank 24 was produced from different aluminium alloys and had different diameters. The round sheet blanks were also provided with different surface topographies to examine the forming behaviour.

(29) The surface topographies of the comparative examples were produced by conventional methods by roll-embossing using an EDT-textured roll or by rolling using a roll with a “mill finish” surface. The surfaces imprinted by EDT rolls as well as the “mill finish”-prepared surfaces were roughened electrochemically by the method according to the invention in order to show the technical effect of the roughening procedure.

(30) In the tests, the punch 21 was lowered at a rate of 1.5 mm/s in the direction of the sheet and the sheet 4 was deep-drawn according to the form of the punch. The punch force and the punch path were measured up until the sample cracked and were recorded. The greater the diameter of the round blank which could be formed without cracking, the better the forming properties of the sheet.

(31) Finally, sheets having different surface topographies were produced from an aluminium alloy of type AA5xxx and also of type AA6xxx and were measured in respect of their surface parameters using a confocal microscope. The strips of aluminium alloy of type AA5xxx were in state “O” and the strips of aluminium alloy of type AA6xxx were in state “T4”. An aluminium alloy of type AA 5182 was used as AA5xxx. The aluminium alloy of AA6xxx alloy corresponded to an aluminium alloy of type AA6005C. Tests V1 to V4 were carried out using an identical aluminium alloy of type AA6005C and tests V5 to V8 were carried out using an identical aluminium alloy of type AA5182 to rule out influences of different compositions within the types of alloys.

(32) The sheets roughened by an EDT-textured roll as well as the sheets provided with a “mill finish” surface were additionally subjected to electrochemical graining and were designated as tests V3 and V4. During electrochemical graining, charge carriers of 500 C/dm.sup.2 were introduced at an HNO.sub.3 concentration of 2.5 g/l to 15 g/l, so that sheets having homogeneously distributed depressions were produced for tests V3 and V4. The well depth S.sub.vk of the surface of the electrochemically grained sheets was between 1.0 μm and 6.0 μm. All the surfaces were coated with a lubricant of the AVILUB Metapress type. The layer thickness was 1 g/m.sup.2. The following Table shows the four different surface variants and the associated sheet thicknesses:

(33) TABLE-US-00001 TABLE 1 Strip No. Alloy Surface EC graining thickness V1 Comparison 6005C Mill finish No 1.15 mm V2 Comparison 6005C EDT No 1.10 mm V3 Invention 6005C Mill finish Yes 1.15 mm V4 Invention 6005C EDT Yes 1.10 mm V5 Comparison 5182 Mill finish No 1.15 mm V6 Comparison 5182 EDT No 1.10 mm V7 Invention 5182 Mill finish Yes 1.15 mm V8 Invention 5182 EDT Yes 1.10 mm

(34) The samples were then tested in the cross tool in respect of their forming behaviour. All tests were carried out in state T4, i.e. in the solution heat-treated and quenched state. In the drawing test, using a cross tool, the sheet holding force at which the sheet cracks during the drawing procedure is determined. It was found that with the round sheet blanks which have a “mill finish” surface according to V1, holding forces of 45 kN could be achieved with a round blank diameter of 185 mm. The roll-embossed round sheet blanks achieved 55 kN holding forces with the same round blank diameter. It was found that an additional roughening of the EDT-roll-embossed surface according to test V4 produced identical results. The combination of “mill finish” surface and subsequent electrochemical graining according to V3 showed cracks only at sheet holding forces of more than 65 kN. This is a significant improvement in the forming behaviour compared to the EDT variants V2 and V4.

(35) The four test variants V1 to V4 were also subjected to further drawing tests using a cross tool, in which a drawing film was additionally used on both sides. A conventional PTFE deep-drawing film of a thickness of 45 μm was used as the drawing film. In a third variant, the sheets were coated before the drawing test with a very large amount of lubricant (8 g/m.sup.2) and, using a drawing film, the drawing tests were carried out in the cross tool. As a result, the effect of the different surfaces should be suppressed.

(36) FIG. 8 shows the test results. It can be seen that using a drawing film in the case of the surfaces of sheets V3 and V4 also roughened by electrochemical graining, the sheet holding forces could be significantly increased compared to the non-roughened surfaces of sheets V1 and V3. Here, it is found that variant V4 with 520 kN and a round blank diameter of 185 mm achieved the highest values, followed by variant V3 with 490 kN. Significantly lower values were achieved with 410 kN for variant V2 and 385 kN for variant V1. Without a drawing film, the sheet holding forces are almost identical for all four test variants.

(37) In the tests with a 195 mm round blank diameter, with a bilateral drawing film using a large amount of lubricant coating of 8 g/m.sup.2, it was found, as expected, that the sheets according to V1 and V3, provided with a greater wall thickness, achieve higher values than the roll-embossed sheets of tests V2 and V4 provided with a smaller wall thickness. As expected, while disregarding the effects of the different surface topographies of tests V1 to V4 due to the use of a high proportion of lubricant (8 g/m.sup.2), the forming properties of the sheets in the drawing test using a cross tool depend only on the wall thickness of the sheets.

(38) In FIG. 9, it was examined how the addition of lubricant improves the formability of the different surface topographies. It was found that the electrochemically grained variants showed a significantly stronger effect upon the addition of lubricants, so that it can be assumed that a greater quantity of lubricants can be applied and a greater lubricant effect can be achieved. In the cross tool test, the sheet holding force in the case of the electrochemically grained “mill finish” surface according to V3 could be increased to approximately 85 kN. The electrochemically grained EDT-textured surface according to V4 allowed 80 kN and the conventional EDT-textured surface according to V2 allowed 70 kN. In contrast, the conventional “mill finish” surface according to V1 achieved a maximum of only approximately 55 kN in this test.

(39) Finally, sheets having the different topographies were produced from an aluminium alloy of type AASxxx as well as of type AA6xxx, and they were measured in respect of their surface parameters using a confocal microscope. The strips of aluminium alloy of type AASxxx were in state “O” and the strips of aluminium alloy of type AA6xxx were in state “T4”. As AASxxx, an aluminium alloy of type AA 5182 was used. The aluminium alloy of the AA6xxx alloy corresponded to an aluminium alloy of type AA6005C.

(40) Tests V2, V6 were textured conventionally using EDT rolls. Tests V1 and V5 had conventional “mill finish” surfaces. As can be seen from Table 2, the EDT-textured surfaces were subjected to an electrochemical graining process and were evaluated as tests V4 and V8. The same was carried out for the sheets with “mill finish” surfaces of both aluminium alloys. The electrochemically grained sheets were evaluated as tests V3 and V7. During electrochemical graining, an HNO.sub.3 concentration of 4 g/l was used with a charge carrier introduction of 500 C/dm.sup.2 in tests V3 and V4, and an HNO.sub.3 concentration of 5 g/l with a charge carrier introduction of 900 C/dm.sup.2 in tests V7 and V8. The electrolyte temperature was between 30° C. and 40° C. for all the variants.

(41) In the visual measurement of the surfaces of the test sheets, it is noted, as expected, that sheets V2, V6 produced conventionally by EDT-textured rolls have significantly higher values in respect of the arithmetic mean roughness value S.sub.a and the reduced peak height S.sub.pk than the strips of tests V1 and V5 which have “mill finish” surfaces. The electrochemically grained embodiments V3, V4, V7 and V8, however, showed a mean roughness S.sub.a approximately on the level of the EDT surface texture of tests V2 and V6. The measured values are recorded in Table 2.

(42) However, in contrast to the conventional texture, in electrochemical graining, the value for the reduced well depth S.sub.vk increases by more than factor 4, here at least by factor 5. This clearly indicates the differences in the textures.

(43) The closed empty volume V.sub.vcl, which represents the volume for the provision of lubricant in lubricant pockets, is greater for the strips textured conventionally by EDT rolling with 362 or 477 mm.sup.3/m.sup.2 compared to 151 mm.sup.3/m.sup.2 or 87 mm.sup.3/m.sup.2 of the “mill finish” variants V1 and V5.

(44) However, the electrochemically grained embodiments V3, V4 as well as V7 and V8 according to the invention exhibit a closed empty volume V.sub.vcl of a least 500 mm.sup.3/m.sup.2. In the case of the strips according to the invention which have passed through an electrochemical graining step, the closed empty volume which is important for receiving lubricant can be increased by significantly more than 10%.

(45) The well density of the structure with values of the variants V3, V4, V7 and V8 according to the invention of more than 80 per mm.sup.2, preferably between 100 per mm.sup.2 and 150 per mm.sup.2, is greater by significantly more than 25% than in the case of conventionally EDT-textured strip surfaces of comparative tests V2 and V6.

(46) The different topography of the embodiments according to the invention, which is characterised using the different values of the reduced well depth S.sub.vk, the closed empty volume V.sub.vcl and the well density of the surface, is responsible for the improvement of the forming behaviour.

(47) As a result, also a formed sheet, for example a door inner sheet or an external skin part of a motor vehicle, can thereby be provided which passes through high forming degrees until it is produced to the final form. With the method according to the invention and with the strip or sheet according to the invention, it is thus possible to provide an even broader field of application for aluminium alloys in the motor vehicle sector since the greater forming degrees allow further possibilities of use.

(48) TABLE-US-00002 TABLE 2 S.sub.a S.sub.pk S.sub.k S.sub.vk N.sub.clm V.sub.vcl No. Alloy μm μm μm μm Ssk 1/mm.sup.2 mm.sup.3/m.sup.2 V1 Comp. 6005C 0.38 1.21 0.98 0.57 2.72 75 151 V2 Comp. 6005C 0.83 1.56 2.79 0.40 0.79 66 362 V3 Invent. 6005C 0.93 0.47 1.33 3.34 −1.32 123 555 V4 Invent. 6005C 1.13 1.50 3.21 2.08 −0.18 94 566 V5 Comp. 5182 0.37 0.51 1.21 0.37 0.32 56 87 V6 Comp. 5182 1.13 2.66 2.54 0.34 1.35 67 477 V7 Invent. 5182 0.93 0.55 1.84 3.13 −2.15 135 605 V8 Invent. 5182 1.19 2.42 2.87 2.03 0.56 83 542 V13 (litho sheet Comp. AA1xxx 0.3-0.6 0.2-0.55 0.9-1.5 0.44-1.1 −0.85-0.32 200-240 <360 after EC graining)

(49) Since electrochemical graining is also used in the production of printing plate carriers, a plurality of EC-grained litho sheets of alloy A1xxx were measured and the measured results were summarised as test V13. Although litho sheets are roughened electrochemically, the roughening procedure serves a different purpose. Furthermore, litho strips and sheets are not delivered to a forming procedure, but after electrochemical roughening, they are coated with a photosensitive layer. The roughening is to allow the most uniform printing result possible. Thus, litho sheets and strips are not prepared for forming within the sense of the present invention.

(50) Therefore, the surfaces optimised according to the invention in respect of forming exhibit clear differences in topography compared to litho sheets, as demonstrated by the summarised measuring results of different measured litho sheets, shown in comparative example V13. Litho sheets usually not only have significantly lower mean roughness values S.sub.vk, but also have a significantly less reduced well depth S.sub.vk. The mean well density n.sub.clm, however, is slightly above the electrochemically grained, forming-optimised surfaces of sheets V4, V3, V7 and V8 according to the invention.

(51) In addition, electrochemically grained surfaces of an embodiment according to the invention were examined during differently strong forming procedures in the cross tool compared to surfaces of conventional sheets of an alloy of type AA6xxx textured by EDT rolling. It was found that the surfaces differ significantly in regions of slightly formed areas, as is also shown in FIGS. 2 and 3.

(52) However, after the forming process, the surfaces exhibited almost identical formations, for example in the hold-down region and in the die radius of the cross tool, i.e. in strongly formed regions. In spite of providing an improved forming behaviour, it is therefore expected that the different starting topography will not have any effects on the surface impression. Therefore, aluminium alloy strips and sheets according to the invention are very well suited for the provision of external skin parts of a body of a motor vehicle, for example.

(53) All references, including publications, patent applications, and patents cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.

(54) The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

(55) Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.