Laminate for bent structure as well as method for its production

Abstract: To improve forming properties of laminated structures comprising two sheets of a non-ferrous metal alloy connected by an adhesive, which hardens at heating and between which an adhesive layer is bonded to the metal sheets, a new sequence of heat treatments is proposed. The alloy of the sheets is in soft condition before the sheets are bonded to each other. The bonding temperature and time are selected such that it remains substantially in soft condition and only after forming further heating is applied to obtain hardening of the alloy sheets.

Description: This invention relates to hybrid laminates built up from two or more alternating layers of non-ferrous alloy sheet and fibre layers which are impregnated with resin, bonded together during a time/temperature cycle under pressure. The metal sheets are on both the outer faces. A version of such a laminate is ARALL which is typically built up from aluminium alloy sheets thinner than one millimeter and intermediate layers of impregnated aramid fibres. The built-up of the ARALL laminate can be coded according to the ALCOA ARALL material specification. A commercially introduced 5/4 ARALL laminate by Alcoa for example, consists of five sheets of an aluminium alloy, each sheet having a thickness of 0.3 mm, and four intermediate layers of unidirectional fibre/resin, each having a thickness of 0.2 mm. A digit added to the code indicates the quality of the metal sheet used. For example, ARALL 3 refers to sheets of aluminium alloy 7475 - T761 according to Aerospace Material Specification SAE AMS 4085B. The code T761 indicates the heat treatment of the sheet used which is a solution heat treatment, quenching, double stage precipitation heat treatment and stabilisation. By using metal sheets of different alloys and tempers, or different adhesive/fibre layers (fibre type and resin type), or different fibre orientations, or by applying more or less layers, a wide range of hybrid laminates with different characteristics can be obtained. Hybrid laminates are used instead of monolithic material because of the improved characteristics with regard to strength, damage tolerance, lower density and good damping caracteristics. Referring to the durability properties, the fatigue strength is considerably better than that of monolithic material. The sigma 0.2, the strengt and fatigue resistance of hybrid metal laminates in general are substantially higher than of monolithic AL alloys. Moreover, the specific weight is between 8-15% lower. In the case of ARALL the elongation at failure in fibre direction is relatively low because of the small elongation of the aramide fibres. Forming of the laminate perpendicular to the fibres is limited due to the precipitated metal used. The hybrid laminates can be used in the form of sheets, doublers and stiffeners. Aircraft structures for example comprise a combination of all three. An indication of the use of hybrid laminates for stiffeners (strengthening profiles) is disclosed in the magazine "Aircraft Production" of November 1953 and in "Aero Research Technical Notes" No. 114 of June 1952. In "Aircraft Production" a method is disclosed for producing profiles for wing structures. A metal laminate is described, having a non reinforced adhesive layer. The edges of the laminate were bent to an angle profile. Another method for obtaining a laminated profile (stiffeners) is described by R.J. Schliekelmann in "Aero Research Technical Notes" indicated above. Several sheets are formed individually before bonding after which they are bonded together. More recent examples of stiffeners of a metal laminate can be found in the proceedings AGARD-LS-102 of April 1979, "Bonded Joints and Preparation for Bonding", page 1-14; in the proceedings of the 14th ICAF Aircraft Symposium in Ottawa, June 10-12, 1987, the lecture 3.1 by L.H. van Veggel c.s.; the SAMPE Conference in Mineapolis, Autumn 1988, the lecture of R.N. Bently et al and also the article of J.W. Gunnink in the Journal of Aircraft of Novemer 1988. The latter three publications are related to ARALL reinformed metal laminates and conform to the patents EP 56288 and 56289. Bentley indicates a method for forming a ARALL laminate into a profile. Here a pad of elastomeric rubber material, having a hardness of about 95 shore A, is placed between the laminate to be formed and the open side of a U-profile. The U-profile cannot be distorted and has at least the same length as the stiffener to be formed. Bending of the rubber pad in combination with the laminate can be compared with a three point bending test. By the use of rubber material even distribution of forces over the laminate is guaranteed. Although the use of hybrid laminates such as ARALL for bonded structures consisting of skins stiffened by profiles is highly desirable, up to now several problems have occurred during the forming of flat hybrid laminates into profiles with relatively small radii. This is caused by the fact that the aluminum alloys sheets of the laminate have been subjected to a solution heat treatment, quenching and precipitation. It has been shown that in the after-precipitation condition the ARALL-laminate can not be bend to relatively small radii. Such a small radius is desirable because it increases the structural efficiency of panels (e.g. local buckling). Furthermore the structural efficiency of panels requires small radius stiffeners because of rib/panel coupling. If a laminate, e.g. a laminate as shown in fig 14 at page 1-14 of above mentioned AGARD report, or an ARALL-laminate is subjected to bending beyond a predetermined point, delamination can occur due to the considerable shear forces or the metal sheets can crack. Even using the method described above, with a rubber pad and U-profile, has not given satisfactory results although the bending forces were evenly distributed over the laminate. If the material of the stiffener was not a laminate but a monolithic layer of an aluminium alloy, it would be possible to heat-treat it before bending to about 495.C, which is the solution heat treatment temperature. After bending, the material is heat treated to its final condition. However, it is not possible to do so with metal laminates because such high temperatures will deteriorate the adhesive and/or fibre properties and the adhesion between adhesive and metal. To avoid too high temperatures it might be possible to first shape the individual sheets (in the end condition T3 or T6) which is usual for aluminium alloy and then bond them together to a thicker profile. Such a method is indicated in "Aero Research Technical Notes", as mentioned above. This can also be done with thin Arall laminates later bonded together to a thick profile. This method however, is quite complicated and expensive. According to the invention, a hybrid laminate is provided having improved forming properties. Generally according to the invention the metal sheets used for the laminates are in the soft condition. Only after shaping the laminates to its desired configuration, it is heat treated to obtain the finally desired mechanical properties. More particularly according to the present invention a hybrid laminate comprises at least two sheets of a solution heat treated and quenched non-ferrous metal alloy and one or more adhesive layers bonding said metal metal sheets together to a laminate; the alloy is in a soft condition and is capable of being hardened to a final temper, the adhesive is selected to be curable under temperature conditions which are not effective to harden said soft metal alloy and the alloy has a hardening temperature not affecting the cured adhesive layer. A main advantage of the invention is that these laminates can be bent to stiffeners with smaller radii than obtained when the prior art laminates were used, without deteriation of the mechanical properties. According to a preferred embodiment in the adhesive a fiber reinforcing material is introduced. This material can comprise aramide fiber, glas fiber, carbon fibers or boron fibers. The metal sheet material can comprise any material having the property that it hardens at heating, such as aluminium alloys, copper alloyers and nickel alloys. Examples of aluminium alloys are alluminium copper alloys such as those of the AA(USA) no. 2024 type or aluminium-copper-lithium alloys, such as those of the AA(USA) no. 2090022091, or aluminium magnesium-silicium alloys, such as those of the AA(USA) No. 6013 or 6061 type or aluminium-zinc alloys such as those of the AA(USA) No. 7075 or 7074, 7075 type or aluminium-lithium alloys, such as those of the AA(USA) No. 8090 or 8091 type. According to a preferred embodiment the adhesive layer consists of a synthetic material, such as epoxy resin or thermoplastic resin or phenolic resin or bismalimide resin. The invention also relates to a method for producing a bent structure made from a laminate, comprising provision of the laminate by bonding together at least two non-ferrous alloy sheets with an adhesive layer, curing of said adhesive layer and bending the laminate to the shape of the product; wherein the non-ferrous alloy of the sheets is in a soft condition at the moment of bonding and is capable of being hardened at an elevated temperature to a final temper, the adhesive is cured at such a temperature that hardening of the alloy will substantially not occur and, after curing, the laminate is bended to the desired shape of the product. After bending the laminate is heated at elevated temperature to the final temper under temperature conditions not affecting the adhesive layer. Before or after hardening by heating post stretching can be realized. Bonding is realized before heating by such a temperature/time/pressure cycling that hardening of the metal sheets is avoided. To promote bonding the metal sheets can be anodized before hand. The adhesive material used is preferably a high temperature curing epoxy resin or thermoplastic resins able to obtain overaged temperconditions. Figure 1 shows partially exploded an example of the laminate according to the invention; Figures 2 and 3 show schematically methods for forming metal sheet, prior art metal laminates or a laminate according to the invention; Figure 4 shows a diagram of the heat treatment and forming of a laminate according to the prior art; and Figure 5 shows a diagram of the heat treatment and forming of a laminate according to the subject invention. Hybrid laminates are built-up from two or more alternating layers of non ferrous alloy sheet and fibre layers wich are inpregnated with resin, bonded together during a time temperature cycle under pressure. In Figure 1 a 3/2 laminate is shown comprising metallic sheets 1, 2 and 3 and intermediate fibre reinforced resin layers 4. The metallic sheets can be of an aluminum alloy from the 2000, 6000 and 7000 series as specified by the aluminium Association. The intermediate layers can comprise a structural adhesive, strengthened by fibres e.g. carbon, glass or polyaramide. In Figure 2 a device for bending a laminate 5 is shown. On a rigid U-shaped profile 6 a pad of rubber material 7 is placed, above which laminate 5 is arranged. From the flat condition it is formed into a curved shape by lowering of die 8. Because of the rounded tip of die 8 and the presence of rubber pad 7, pressure distribution is as even as possible. In figure 3 another device for the bending of hybrid laminates is shown. Instead of using a U-shaped profile wherein a rubber pad is bent, a metal support 10 is used having an opening wherein rubber block 11 with opening 12 is introduced. As indicated above, in the prior art, problems occurred during bending of laminates such as the above mentioned 3/2 ARALL-1 laminate to very small radii. In Figure 4 a diagram is shown schematically indicating the heat treatment according to the prior art, e.g. for ARALL-1. The metal sheets used for ARALL 1 laminates are solution heat treated (495 C) and quenched. A 24 hours precipitation heat treatment at 120°C to the T6 condition follows. This procedure is indicated in Figure 4 with block 1 and 5 respectively. Finally the surfaces of the sheets are prepared for bonding by anodising and priming. This process, described for example by the British specification DEF STAN 03-24, issue 1, is indicated with block 2. In block 3 the sheets are put together with the interposition of the fibre reinforced adhesive layer and cured under pressure at 120 C for one hour. After cooling down the laminate is ready to be supplied to the purchaser. Metal forming by the purchaser is indicated by block 4. In Figure 5 heat treatment and forming of the laminate according to the invention is shown diagrammatically. As an example for Arall-1, after solution heat treatment the metal sheets are quenched as is indicated in block 1. The sheets are now in the AQ-condition. This condition is not stable. The material will transform into the W-condition. Then the surfaces are pretreated for bonding, indicated with block 2, by a process as noted above with block 2 of Figure 4. After that, the sheets are bonded with aramid reinforced adhesive layers. The curing process is indicated by block 3. An attendant effect of the heating of the metal sheets during the bonding process during one hour at 120°C is, that a start is given to the precipitation. Because a complete treatment takes 24 hours, no substantial precipitation will occur and only an undefined TX-condition is created. Such a condition is far from a T6 condition. That means that the laminate is much softer than a laminate in the T6-condition and consequently it's forming properties are improved. In this condition the laminate is delivered to the purchaser. Block 4 represents forming of the laminate by the purchaser. To obtain, after forming, the desired mechanical properties, in case the T6-condition, it is necessary to complete above mentioned heat treatment for about 23 hours at 120°C. This step is expressed by block 5. After this treatment the mechanical properties substantially correspond to the material as indicated in Figure 4. However, the forming step, indicated by block 4, is much improved as can be seen from the comparative bending test results. The results are conducted with a device as shown in figure 3 and are summarised in table I. In this table R is the radius of the tip of die 8 in figure 3. H is the extention of the rubber block 11 above support 10 in millimeters. Vb is the speed of die 8. Die 8 was moved downward until an angle of 110° of the deformed laminates was realised. In table I 3/2 ARALL 1 produced according to the prior art is compared with 3/2 laminate produced according to the invention being substantailly the same as 3/2 ARALL 1 except for the heat treatment as explained above. From this table it is clear that with the laminate produced according to the invention much improved bending properties are obtained. After bending the laminate produced according to the invention, it is subjected to a further heat treatment to obtain its final strength. In table II the minimum bending radius in millimeters is given for different kinds of ARALL materials produced according to the prior art and according to the invention. Minimum bending radii of different Arall laminates and laminates according to the invention. radius (mm) 3/2 ARALL 1 6 3/2 ARALL 3 5 3/2 laminate invention 4 4/3 ARALL 1 10 4/3 ARALL 3 8 4/3 laminate invention 6 5/4 ARALL 1 11 5/4 ARALL 3 10 5/4 laminate invention 8 Although the invention as described above referring to a preferred example it is clear that many amendments can be made without leaving the scope of protection of the subject application. First of all it is possible to use the laminate without bending. Of course any other laminate combination can be used as long as the metal sheets do possess a precipitation hardening property and the temperature of precipitation does not affect the properties of the adhesive layer being sandwiched between the sheets. Also the adhesive layer can comprise all kinds of layers known in the prior art both thermosetting and thermoplastic resin material which can incorporate all kind of fibres. It is also possible to subject the laminate to a poststressing treatment after which the internal stress distribution is favourable for fatigue (compression in the metal sheets). This post stressing can be adjusted either after curing of the laminate or after the precipitation treatment.

Claims: A hybrid laminate comprising at least two sheets of a solution heat treated and quenched non-ferrous metal alloy, one or more adhesive layers bonding said metal sheets together to a laminate, characterized in that the alloy is in a soft condition and capable of being hardened at an elevated temperature to a final temper, in that the adhesive being selected to be curable under temperature conditions which are not effective to harden said soft metal alloy and wherein the alloy has a hardening temperature not affecting the cured adhesive layer. A laminate according to claim 1, wherein the adhesive layer comprises a reinforcing material resin layer. A laminate according to claim 1 or 2, wherein the metal sheets comprise an aluminium alloy. A laminate according to claim 1 or 2, wherein the metal sheets comprise a copper alloy. A laminate according to claim 1 or 2, wherein the metal sheets comprise a nickel alloy. A laminate according to one of the preceding claims, wherein the adhesive layer consists of a synthetic material. A laminate according to one of the claims 2-6, wherein the fiber reinforcement comprises aramid fiber. A laminate according to one of the claims 2-6. wherein the fiber reinforcement comprises glass fibers. A laminate according to one of the preceding claims wherein the non-ferrous metal is 7075 or 7475 material according to AA(US), which is in a W-condition before bonding. Method for producing a bent structure made from a laminate, comprising provision of the laminate by bonding together at least two non-ferrous alloy sheets with an adhesive layer, curing of said adhesive layer and bending the laminate to the shape of the product, characterized in that the non-ferrous alloy of the sheets in a soft condition being a W-condition at the moment of bonding and is capable of being hardened at an elevated temperature to a final temper, wherein the adhesive is cured at such a temperature that hardening of the alloy will substantially not occur, in that after curing the laminate is bended to the desired shape of the product, and in that after bending the laminate is heated at elevated temperature to the final temper under temperature conditions not affecting the adhesive layer. A laminate according to claim 10 wherein high temperature curing epoxy resins (e.g. 180°C) or thermoplastic resins are used to be able to obtain overage temper conditions (e.g. T671). Method according to claim 10 or 11 wherein the alloy is 7075 or 7475 material according to AA(US) and the adhesive layer consist of a 120°C curing epoxy resin, reinforced with aramid or glass fibres, wherein the laminate after bonding (and/or forming) is precipitation hardened to the T6-condition. Method according to claims 10-12, wherein bending is realized by insertion of the laminate between a rubber pad with a circular hole and a die and moving the die and rubber pad towards each other.

Inventors: BOTTERMAN RENE, LEFEBER PAUL, VAN TILBORGH CORNELIS, VAN VEGGEL LOEK, BOTTERMAN, RENE, LEFEBER, PAUL, VAN TILBORGH, CORNELIS, VAN VEGGEL, LOEK, Botterman, René
Applicants: FOKKER AIRCRAFT, FOKKER AIRCRAFT B.V.
Application year: 1990
Main classification: B32B15
IPC codes: B32B15, B64C1