Aluminium composite material and forming method

Abstract

The invention relates to a method for forming an aluminium composite material which has a core alloy made from an aluminium alloy of type AA5xxx or AA6xxx and at least one outer aluminium alloy layer provided on one or both sides, wherein the aluminium composite material is formed and the outer aluminium alloy layer provided on one or both sides has a yield strength R.sub.p0.2 of 25 MPa to 60 MPa in the soft or solution-annealed state. The method enables the production of large-surface, heavily formed aluminium alloy sheet metal parts, in particular also in outer skin quality.

Claims

1. Sheet metal part manufactured by forming an aluminium composite material which has a core alloy made from an aluminium alloy of type AA5xxx or AA6xxx and at least one outer aluminium alloy layer provided on one or both sides, wherein the aluminium composite material is formed in a forming tool and the outer aluminium alloy layer provided on one or both sides has a yield strength R.sub.p0.2 of 25 MPa to 60 MPa in the soft or solution-annealed state, characterised in that,
k.sub.f,outside/k.sub.f,core<0.5 for the flow stresses of the aluminium alloys of the core and of the at least one outer layer in the soft or solution-annealed state, the frictional shear stress ?.sub.R between the tool and the aluminium composite material in the contact surface reaches the shear flow stress k.sub.outside of the outer aluminium alloy layer at at least one local position in the forming tool during the formation of the aluminium composite material.

2. Sheet metal part according to claim 1, characterised in that, the forming comprises a deep drawing or stretch forming procedure.

3. Sheet metal part according to claim 1, characterised in that, at least one outer aluminium alloy layer having a thickness of 5% to 15% of the total aluminium composite material is formed.

4. Sheet metal part according to claim 1, characterised in that, the frictional shear stress ?.sub.R is enlarged during the formation by an increase of the surface pressure between the aluminium composite material and the hold-down clamp.

5. Sheet metal part according to claim 1, characterised in that, an aluminium core alloy of type AA6xxx or AA5xxx is formed which has a uniform strain A.sub.g of at least 20% in the solution-annealed or soft state.

6. Sheet metal part according to claim 1, characterised in that, an alloy of type AA6xxx is used as an aluminium core alloy and an aluminium alloy of type AA8xxx is used as at least one outer aluminium alloy layer or an alloy of type AA5xxx is used as an aluminium core alloy and an aluminium alloy of type AA8xxx, AA1xxx, AA5005, AA5005A is used as at least one outer aluminium alloy layer.

7. Sheet metal part according to claim 6, characterised in that, an aluminium core alloy of type AA6016 and at least one outer aluminium alloy layer of type AA8079 is used.

8. Sheet metal part according to claim 1, characterised in that, an aluminium core alloy of type AlMg6 and at least one outer aluminium alloy layer of type AA8079, AA1050 or AA5005 or AA5005A is used.

9. Sheet metal part according to claim 1, characterised in that, an aluminium composite material having an AA6xxx aluminium core alloy having a thickness of 0.5 mm to 2.0 mm or an aluminium composite material having an AA5xxx aluminium core alloy having a thickness of 0.5 mm to 3.5 mm is formed.

10. Sheet metal part according to claim 1, characterised in that, the sheet metal part is a structural part or an outer skin part of a motor vehicle.

11. A deep drawn or stretch formed structural part or outer skin part of a motor vehicle formed with a method for forming an aluminium composite material which has a core alloy made from an aluminium alloy of type AA5xxx or AA6xxx and at least one outer aluminium alloy layer provided on one or both sides, the method comprising forming the aluminium composite material in a forming tool wherein the outer aluminium alloy layer provided on one or both sides has a yield strength R.sub.p0.2 of 25 MPa to 60 MPa in the soft or solution-annealed state, wherein,
k.sub.f,outside/k.sub.f,core<0.5 applies for the flow stresses of the aluminium alloys of the core and of the at least one outer layer in the soft or solution-annealed state, the frictional shear stress ?.sub.R between the tool and the aluminium composite material in the contact surface reaches the shear flow stress k.sub.outside of the outer aluminium alloy layer at at least one local position in the forming tool during the formation of the aluminium composite material and the formation comprises a deep drawing and/or stretch forming procedure and wherein an alloy of type AA6xxx is used as an aluminium core alloy and an aluminium alloy of type AA8xxx is used as at least one outer aluminium alloy layer or an alloy of type AlMg6 is used as an aluminium core alloy and an aluminium alloy of type AA8079, AA1050, AA5005, AA5005A is used as at least one outer aluminium alloy layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Below the invention is explained in more detail by means of exemplary embodiments in connection with the drawing. In the drawing is shown:

[0038] FIG. 1 is a schematic, perspective sectional depiction, the cross tool for the implementation of the deep drawing test.

[0039] FIG. 2 is a simplified exploded depiction, the stamp, hold-down clamp and matrix of the cross tool from FIG. 1.

[0040] FIG. 3 is a diagram, the stamping force depending on the stamp displacement of the cross tool from FIG. 1 of a monolithic variant of type AA6016.

[0041] FIG. 4 is the diagram from FIG. 3 for implementation of an exemplary embodiment of the forming method according to the invention.

[0042] FIG. 5 is a diagram, flow stresses k.sub.f determined from a tensile test, depending on the strain for the materials AA6016, AA5005, AA6463A, AA8079, AA1050.

[0043] FIG. 6 shows the flow stresses of the aluminium materials AA1050, AA8079, AA6463A, AA5005 in relation to the flow stress of the aluminium alloy AA6016.

[0044] FIG. 7 is a diagram, the flow stresses of the material AA8079 depending on the strain in relation to the flow stress of different possible core alloys of type AA6xxx.

[0045] FIG. 8 is a diagram, flow stresses k.sub.f depending on the strain for the materials AlMg6, AA1050, AA5005.

[0046] FIG. 9 is a diagram, the flow stresses of the materials AA1050, AA5005 depending on the strain in relation to the flow stress of the aluminium material AlMg6.

[0047] FIGS. 10 and 11 are two exemplary embodiments of sheet metal parts according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0048] In FIG. 1, firstly the configuration of the cross tool is depicted in a perspective sectional view, wherein the cross tool comprises a stamp 1, a hold-down clamp 2 and a matrix 3. The sheet 4, which was cladded on both sides in the case of a clad variant, had, for example, a thickness of 1.5 mm. This applied both for the clad and for the unclad variant. The sheet metal provided as a circular blank is deep drawn by the stamping force F.sub.st, wherein the hold-down clamp 2 and the matrix 3 were pressed onto the sheet blank with the force F.sub.N.

[0049] The cross-shaped stamp 1 had a width of 126 mm along each of the axes of the cross, whereas the matrix had an opening width of 129.4 mm. The sheet blank made from the aluminium material had different diameters. A circular blank diameter of 195 mm was started with.

[0050] In FIG. 2, the stamp 1, the hold-down clamp 2 as well as the matrix 3 and the sheet 4 is depicted again in an exploded view from below. Stresses of the materials during industrial deep drawing tests can be copied with the cross tool and thus the forming performance of the material to be tested can be checked. Therein the stamp 1 was lowered in the direction of the metal sheet with a speed of 1.5 mm per second and the metal sheet 4 was deep-drawn according to the shape of the stamp. The stamping force and the stamp displacement until tearing of the sample were measured and recorded.

[0051] In FIG. 3, the force displacement diagram of the unclad variant having an aluminium alloy material of type AA6016 is depicted. It can be recognised that in the case of increase of the hold-down clamp forces from 30 to 300 kN, the stamping force increases to a value of more than 120 kN, provided at a 23 mm stamp displacement. The higher the hold-down clamp force, the earlier the stamping force increases. At a 26 mm stamp displacement and a hold-down clamp force of 300 kN, a tearing of the material occurs which limits the forming capability of the unclad variant. If the hold-down clamp force is reduced, the stamp displacement increases to approximately 35 mm at 75 kN hold-down clamp force until the material tears. No tearing of the material occurs at 30 kN.

[0052] The conventional method to adjust the hold-down clamp force and thus the friction in the deep drawing process can be seen by means of FIG. 3. The person skilled in the art will attempt to keep the hold-down clamp forces as low as possible such that a tearing of the material does not occur. On the other hand, the person skilled in the art will endeavour to adjust the hold-down clamp forces such that a wrinkling does not occur. The forming process is, however, limited by the occurrence of tears with the given hold-down clamp force.

[0053] FIG. 4 now shows the force displacement diagram for the stamping force according to one exemplary embodiment of the forming method according to the invention, wherein the sheet metal material is a variant of type A2-K1-A2 which is clad on both sides. The composition both of the unclad variant of type AA6016 depicted in FIG. 3 and the corresponding variant which is clad on both sides from the diagram in FIG. 4 are depicted in Table 1.

[0054] In FIG. 4 it is recognised that the stamping force F.sub.st is limited to a maximum of 100 kN, independently of the respective hold-down clamp force F.sub.N, which was varied from 300 to 450 kN. No tears were shown in the case of the forming method according to the invention. The material could also be deep drawn without tears at hold-down clamp forces of over 300 kN with a stamp displacement of more than 35 mm. This means that during the forming, the frictional shear stress of the material which acts against the stamping force F.sub.st is limited and indeed to the value of the shear flow stress of the outer aluminium alloy layer. Even at the maximum stamping force of 450 kN, no tearing of the aluminium composite material of the A2-K1-A2 variant results.

[0055] The tested aluminium composite materials were produced as follows:

[0056] A rolling ingot consisting of an aluminium alloy of type AA 6016 having the composition specified in Table 1 was cast, homogenised at 580? C. for more than 2 h and covered on both sides with a cladding material of alloys A1 A2, A3, A4 and subsequently roll cladded. Therein, the hot strip having a thickness of 12 mm and a hot rolling final temperature of at least 300? C. was produced. Subsequently the hot strip was annealed at a strip temperature of 350? C. for more than 2 h and cold rolled to 4 mm. To achieve an outer skin quality, i.e. to avoid the so-called roping, an intermediate annealing occurred at this thickness, wherein the strip had a temperature of approximately 350? C. for 2 h. Subsequently, the strip made from aluminium composite material was cold rolled to 1.5 mm final thickness and underwent a solution annealing at 500? C. to 570? C. with quenching such that the aluminium alloy strips having the core alloy K1 were present in the T4 state for the later tests after natural aging at room temperature for approx. 2 weeks.

[0057] The aluminium composite materials based on an AlMg6 aluminium core were produced as follows: homogenisation of an ingot made from an AlMg6 alloy at 500? C. to 550? C. for more than 2 h, construction of the cladding rolling ingot by coating of the cladding materials on both sides, subsequent roll cladding to 12 mm thickness, implementation of a hot strip annealing at 350? C. for more than 2 h, cold rolling to 4 mm thickness, intermediate annealing of the cold strip at 350? C. for more than 2 h and subsequent cold rolling to 1.5 mm final thickness. Instead of the solution annealing, a soft annealing is implemented in the chamber furnace at 350? C. for 2 hours at the end of the production process.

TABLE-US-00001 TABLE 1 Alloy Designation Si Fe Cu Mn Mg Ti AA A1 0.046 0.32 0.0034 0.0057 0.0036 0.0149 1050 AA A2 0.089 0.86 0.0019 0.021 0.0022 0.0061 8079 AA A3 0.25 0.21 0.0014 0.078 0.34 0.016 6463A AA A4 0.066 0.19 0.119 0.121 0.89 0.0039 5005 AA K1 1.31 0.18 0.016 0.078 0.32 0.0166 6016 AlMg6 K2 0.091 0.2 0.142 0.25 6.05 0.022

[0058] Table 1 shows the different alloy content of the substantial alloy components in percentage by weight. All six alloys have, besides aluminium and the specified alloy components Si, Fe, Cu, Mn, Mg and Ti, impurities which amount individually to a maximum of 0.05% by weight and in total to a maximum of 0.15% by weight. All information in Table 1 is, of course, understood to likewise be in % by weight.

[0059] In Table 2, the measured mechanical characteristic values of the used alloy types are documented. All information was determined in the soft or solution-annealed state according to DIN EN ISO 6892-1:2009.

TABLE-US-00002 TABLE 2 A.sub.g A.sub.80mm R.sub.P0.2 R.sub.m A.sub.g (smoothed) A.sub.80mm (hand) n.sub.4-6 r.sub.8-12 Alloy Reference MPa MPa % % % % value value AA A1 26 74 27.9 28.7 38.6 39.4 0.270 1.180 1050 AA A2 29 81 31.1 31.4 46.0 46.9 0.253 0.685 8079 AA A3 37 99 21.3 22.5 27.4 28.3 0.321 0.816 6463A AA A4 45 113 24.7 24.0 30.2 31.1 0.243 0.941 5005 AA K1 114 219 24 23.8 29.5 29.8 0.277 0.729 6016 AlMg6 K2 156 308 23 22.2 25.9 26.9 0.301 0.676

[0060] In a further test, the maximum hold-down clamp forces for different alloy combinations were determined with different circular blank diameters. It was shown that, in particular for the A2-K1-A2 variant, which represents an aluminium alloy of type AA6016 clad on both sides with an aluminium alloy layer of type AA8079, the circular blank diameter could be enlarged further and tears only occurred at a circular blank diameter of 205 mm and a maximum hold-down clamp force of more than 105 kN. At circular blank diameters of 195 mm or 200 mm, no tears could be generated even at the maximum possible hold-down clamp force of 600 kN in the forming test. As the monolithic variant already had tears at a circular blank diameter of 195 mm and a maximum hold-down clamp force of 50 kN, this proves the excellent formability of the clad variant in the forming method according to the invention. The results of the cross tool test are summarised in Table 3.

[0061] R.sub.p0.2 corresponds to the value of k.sub.f at 0.2% plastic strain and is measureable in the tensile test. In Table 3, additionally the ratio k.sub.f,outside/k.sub.f,core is entered for a true strain of approx. 0.025 which is gleaned from FIGS. 5 to 9 for the respective material combination.

TABLE-US-00003 TABLE 3 Ratio k.sub.f,outside/ Max. hold-down k.sub.f,core clamp force at circular (true blank diameter Ratio strain 195 200 205 R.sub.p0.2,outside/ approx. mm mm mm Variant R.sub.p0.2,core 0.025) F(kN) F(kN) F(kN) K1 unclad Comparison 50 A1-K1-A1 0.23 0.34 Invention 159 A2-K1-A2 0.25 0.38 Invention >600 >600 105.0 A3-K1-A3 0.32 0.46 Invention 130 A4-K1-A4 0.39 0.53 Comparison 60 K2 unclad Comparison 75 A1-K2-A1 0.17 0.27 Invention >600 A4-K2-A4 0.29 0.45 Invention >600

[0062] The clad aluminium alloy variants A1-K1-A1 and A3-K1-A3 likewise showed a clear increase with regard to the maximum hold-down clamp force at a circular blank diameter of 195 mm. In comparison to the unclad K1 variant, the maximum hold-down clamp force which amounted to 50 kN for the unclad K1 variant increased by a factor of 2.6 (A3-K1-A3 variant) or by a factor of 3.18 (A3-K1-A3 variant). On the other hand, the A4-K1-A4 variant enabled no significant increase of the maximum hold-down clamp force compared to the unclad K1 variant.

[0063] The measured values depicted in FIGS. 5 to 9 were determined by tensile test transversely to the roll direction according to DIN EN ISO 6892-1:2009. The flow stress k.sub.f is depicted depending on the true strain, wherein the true strain results as follows:

?=ln(1+?)

wherein ? indicates the true strain and ? the technical strain.

[0064] To explain, FIG. 5 shows stress-strain curves for different materials, wherein the flow stress k.sub.f is applied compared to the true stress ?. It can be recognised in FIG. 5 that the core material K1 has a considerably higher flow stress than the outer cladding layers A1, A2, A3 and A4.

[0065] In FIG. 6, the ratios of the flow stresses of the outer aluminium alloy layers are depicted in relation to the flow stress of the core alloy layer K1. All variants A1, A2 and A3 have a ratio of k.sub.f,outside/k.sub.f,core of less than 0.5. Only the variant A4, an outer aluminium alloy layer of type AA5005, has a flow stress ratio to an aluminium alloy of type AA6016 which is greater than 0.5.

[0066] The measured, maximum holding forces for a circular blank diameter of 195 mm show that the softer the outer aluminium alloy layer, the greater the maximum hold-down clamp forces for constant circular diameter. As the deep draw test is a plastic deformation, the solidification of the outer aluminium alloy layer also plays a role, however.

[0067] A distinctive effect with respect to the enlargement of the maximum hold-down clamp force can be observed in the comparison of the A1-K1-A1 and the A2-K1-A2 variants. It is known from the aluminium alloy of type AA8079 that this has a relatively low solidification in the case of plastic strain. This effect appears to favour the maximum achievable results in the cross tool tests. The combination of an aluminium alloy layer of type AA6016 with an aluminium alloy AA8079, so the A2-K1-A2 variant, showed an enormous increase of the hold-down clamp forces to over 600 kN even in the case of an enlargement of the circular blank diameter to 200 mm, despite a greater flow stress ratio in comparison to the A1-K1-A1 variant. At the time, the explanation of this result is seen in that the solidification of the outer aluminium alloy layer of type AA8079 is lower than that of the core material during the plastic deformation and that hereby the flow of the material is favoured during the forming procedure.

[0068] FIG. 7 shows the ratios of the flow stresses k.sub.f,outside of an outer aluminium alloy layer of type AA8079 in relation to the flow stresses of different conceivable core alloys of type AA6xxx. All variants have a ratio of k.sub.f,outside/k.sub.f,core of less than 0.5. Thus it can be expected that such combinations of AA6xxx core alloys established in the automobile industry having an outer aluminium alloy layer of type AA8079 also have the distinctive improvement of the formability referred to above.

[0069] Similar results could also be achieved for another core alloy of type AlMg6, the stress-strain curves of which are depicted in comparison to the A3 and A4 variants in FIG. 8 in a diagram. FIG. 9 in turn shows the ratio of the flow stresses of the outer aluminium alloy layers A3 and A4 in relation to the core aluminium alloy layer of type K2. Both aluminium composite materials showed a maximum hold-down clamp force of more than 600 kN at a circular blank diameter of 195 mm, whilst the unclad comparison material K2 already showed tears at a circular blank diameter of 195 mm and a maximum hold-down clamp force of 75 kN.

[0070] From these results it is clear that a considerably increase of the forming performance is enabled by skilful selection of the aluminium core alloy and the outer aluminium alloy layer. The enormous increases with regard to the forming performance which are accompanied by selection of the aluminium composite materials in connection with the forming method according to the invention enable large sheet metal parts, such as, for example, the side wall part of a motor vehicle depicted in FIG. 10 or also the floor pan of a motor vehicle depicted in FIG. 11 by way of example, to be able to be formed in one piece from a metal sheet consisting of an aluminium composite material formed using the method according to the invention.

[0071] Preferably, components produced according to the method according to the invention are, for example, visible outer skin parts of a body of a motor vehicle, in particular side wall parts, exterior door parts and exterior tailgate parts as well as bonnets, etc. of a motor vehicle which are produced with an aluminium core alloy of type AA6xxx. In addition, preferably all other structural and chassis parts, such as interior door parts, floor pans, etc., which are not visible and likewise require a very high degree of forming for an economic production, are produced with an aluminium core alloy of type A5xxx, for example with an AA5182 aluminium core alloy.