BONDING OF THERMOPLASTIC AND THERMOSET COMPOSITE STRUCTURES TO METALLIC STRUCTURES

Abstract: Methods for bonding metallic structures to thermoplastic and thermoset composite structures are described. The methods include steps for preparing the metallic structure (14) for bonding by knurling a mating surface of the metallic structure and molding a layer of thermoplastic material (50) over the knurled mating surface to produce a bonding surface. To bond a metallic structure (14) to a thermoplastic composite structures (16), the bonding surface of the metallic fitting is consolidated with the thermoplastic composite structure (50). To bond a metallic structure to a thermoset composite structure, a bonding surface is formed on the thermoset composite structure by partially embedding a layer of dry fibre reinforcement in a thermoplastic material layer, coating the exposed dry fiber reinforcement with thermoset resin, and curing the coating dry fiber reinforcement into the thermoset composite structure. The bonding surfaces of the thermoset composite structure and the metallic structure are then fused together by applying heat sufficient to melt the thermoplastic material layers of the bonding surfaces. A resistance heating element (54) is disposed between the bonding surfaces of the thermoset composite structure (16) and the metallic structure to provide the heat required to melt the thermoplastic material layers of the bonding surfaces.

Description: Description BONDING OF THERMOPLASTIC AND THERMOSET COMPOSITE STRUCTURES TO METALLIC STRUCTURES ( Technical Field 5 This invention relates to methods of bonding thermoplastic and thermoset composite structures to metallic structures, and more particularly to a composite to metal bond that will transmit axial and torsional loads between the metallic structure and the 10 composite structure. Background of the Invention Thermoplastic and thermoset composite materials have received widespread use a structural elements. Such composite materials produce lightweight structures 15 having relatively high strength characteristics. In addition, thermoplastic composite materials have the characteristic of being reshapeable because they can be repeatedly heated to the melting temperature of the material. In the aircraft industry, the use of 20 thermoplastic and thermoset composite materials can produce significant weight savings and provide ease of replacement and repair. While the uses for structural elements made from thermoplastic and thermoset composite materials are 25 expanding, some applications still require metallic structural members. One such application is the splined connection of a rotor shaft to a gas turbine engine of a. helicopter. The splined connection fits within a complementary shaped recess and transfers «_ * J 30 rotational forces from the gas turbine engine to the shaft. Metallic splined connectors are preferred due to the difficulties in forming a composite splined shaft connectorhaving sufficient strength to withstand the engine loads. In such applications, however, the shaft itself could be made from a thermoplastic or thermoset composite material which has a metallic fitting affixed thereto. The metallic fitting would connect the shaft to the engine. A problem in mating thermoplastic or thermoset composite shafts with metallic fittings is the attachment between the fitting and the shaft. This attachment is required to transfer operational loads, both torsional and axial, between the two elements. Thermoplastic and thermoset composite materials do not fuse with metallic materials with sufficient strength to withstand operational loads. In addition, cured thermoset composite materials cannot be reheated to reverse the bond between the fitting and the shaft. Standard adhesives do not provide a bond of sufficient strength to withstand the torsional and axial loads that are transmitted from the fitting to the shaft. One solution in the prior art is to mechanically connect the metallic fitting to the composite shaft. A connection of this type may use pins and/or flanges adapted to be bolted together. The pinned connections and/or flanged portions add to the complexity and difficulty of fabricating the composite structure. In addition, the mechanical connectors increase the weight and size of the shaft and fitting relative to a simple bonded shaft and fitting. The above art notwithstanding, scientists and engineers under the direction of the owner of the instant invention are working to develop methods to bond metallic and thermoplastic and thermoset composite materials in a manner which will permit the transmission of axial and torsional loads through the bond. Disclosure of the Invention * According to the present invention, methods for < fe bonding a metallic structure to a thermoplastic or 5 thermoset composite structure include the steps of knurling the mating surface of the metallic structure, and molding a thermoplastic composite material over the knurled mating surface to produce a bonding surface. For thermoplastic composite structures, the bonding 10 method further includes the steps of laying up laminates of thermoplastic composite material over the bonding surface, and fusing the bonding surface to the laminates. For thermoset composite structures, the bonding 15 method further includes steps to prepare the thermoset composite structure with a thermoplastic bonding surface. Preparation of the thermoset composite structure includes the steps of partially embedding a layer of dry fiber reinforcement into a layer of 20 thermoplastic material, coating the exposed dry fiber with a thermoset resin, and co-curing the impregnated, exposed dry fiber of the thermoplastic material with the thermoset composite structure wherein the thermoplastic material forms the bonding surface for 25 the thermoset composite structure. The bonding surfaces of the metallic structure and the thermoset composite structure are consolidated by applying heat and pressure to cause melting and fusing of the thermoplastic material forming the bonding surfaces of 30 the thermoset composite structure and the metallic * structure. A principal feature of the present invention is * the knurled mating surface of the metallic structure. Another feature is the layer of thermoplastic material molded over the knurled surface to form a bonding surface. A specific feature of one embodiment of the thermoplastic bonding method is the circumferentially recessed portion of the knurled surface. Another feature of this embodiment is the high tension overwrap. A specific feature of the thermoset bonding method is the resin impregnated exposed portion of the partially embedded dry fiber reinforcement layer fused into both the thermoset composite structure and the thermoplastic material. Another feature of thermoset bonding method is the utilization of a resistance heating element sandwichedbetween the bonding surfaces of the thermoset composite structure and the metallic structure. The foregoing and other objects, features, and advantages of the present invention will become more apparent in light of the following detailed description of the exemplary embodiments thereof, as illustrated in the accompanying drawings. Brief Description of the Drawings Figure 1A is a perspective view of a drive assembly comprising a metallic fitting having a thermoplastic composite shaft bonded thereto. Figure IB is a partially sectioned perspective view of a drive assembly comprising a metallic fitting having a thermoset composite shaft bonded thereto. Figure 2A is a perspective view of the metallic fitting of Figure 1A illustrating the knurled mating surface thereof. Figure 2B is a perspective view of the metallic fitting of figure IB illustrating the knurled mating surface thereof. Figure 3A is a sectioned view taken along line 3A- 3A of Figure 1A showing the metallic fitting with a bonding surface formed by molding a layer of thermoplastic material to the knurled mating surface. Figure 3B is a sectioned view taken along line SB- SB of Figure IB showing the metallic fitting with a bonding surface formed by molding a layer of thermoplastic material to the knurled mating surface. Figure 4 is a perspective view of the metallic fitting of Figure 2B with a layer of thermoplastic material molded over the knurled mating surface thereof. Figure 5 is a perspective view of the prepared metallic fitting of Figure 2A with a mandrel attached thereto. Figure 6 is a perspective view of the metallic fitting of Figure 5 subsequent to the lay up of laminates of thermoplastic composite material over the bonding surface thereof and the mandrel. Figure 7 is a partially sectioned view of a thermoset composite structure showing a bonding surface formed of thermoplastic material and dry fiber reinforcement. Figure 8 is a perspective view of a thermoplastic cone having a layer of dry fiber reinforcement partially embedded therein. Figure 9 is a perspective view of a resistance heating element. Figure 10 is a perspective view of the prepared metallic fitting of Figure 4 with the resistance heating element of Figure 9 placed on the bonding surface thereof. Figure 11 is a perspective view of a drive assembly having a metallic fitting bonded to a thermoplastic composite shaft, wherein the drive assembly includes a high tension overwrap clamping the thermoplastic composite shaft to the mating surface. Figure 12 is a perspective view of the metallic fitting of Figure 11 which includes a knurled mating surface having a circumferentially recessed portion. Figure 13 is a sectioned view taken along line 13- 13 of Figure 11 showing the metallic fitting with a layer of thermoplastic material molded over the mating surface and the high tension overwrap. Modes for Carrying Out the Invention The present invention is directed to methods for bonding thermoplastic and thermoset composite structures to metallic structures or fittings. Figures 1A, IB illustrate a drive assembly 12 wherein a metallic fitting 14 is bonded to a hollow composite shaft 16. The metallic fitting 14 includes a splined portion 18 and a mating surface 22, as illustrated in Figures 2A, 2B. The splined portion 18 is configured to be engaged with a force transmitting element (not shown) , such as a gas turbine engine. During operation, the gas turbine engine will transmit rotational loads to the splined portion 18 of the fitting 14. The rotational loads are then transmitted to the composite shaft 16 through the bonded engagement between the mating surface 22 of the fitting 14 and the composite shaft 16, as described in further detail hereinbelow. Additionally, longitudinal loads may also be transmitted between the shaft 16 and the fitting 14. The bond between the metallic fitting 14 and the composite shaft 16 must be of sufficient strength to transmit both the rotational and longitudinal loads. The initial steps of the bonding methods according to the present invention involve preparation of the metallic structure, e.g., the metallic fitting 14. The mating surface 22 of the metallic fitting 14 is knurled by conventional techniques, as identified in Figures 2A, 2B by reference numeral 24, to prepare the metallic fitting 14 for bonding. The knurls 24 may be random, helical, or a combination of axial and circumferentially directed knurls. The precise pattern and depth d (see Figures 3A, 3B) of the knurls 24 depends on the forces to be transmitted between the metallic fitting 14 and the composite shaft 16 and the area of the mating surface 22. In general, the greater the area of the mating surface 22, the smaller the depth d of the knurls 24 may be. A random pattern of knurling is generally acceptable for typical operational loads. Although a randomly orientated knurling is discussed, an orientated configuration of knurls may be used to optimize the load transmission in specific applications. Next, a layer of thermoplastic material 26 is molded over the knurled mating surface 22/24 as illustrated in Figure 4 (see also Figures 3A, 3B) , the exposed surface of the thermoplastic material layer 26 forming a bonding surface 28 over the knurled mating surface 22/24 of the metallic fitting 14. The thermoplastic material utilized may be a short fiber reinforced thermoplastic material or an unreinforced thermoplastic material. Reinforced thermoplastic may be required in high load applications. During the molding step, the thermoplastic material 26 penetrates into and around the knurls 24 of the mating surface 22. The layer of thermoplastic material 26 should be of sufficient thickness t (see Figures 3A, 3B) to penetrate into and completely cover the knurls 24 of the mating surface 22. This ensures that a mechanical lock exists between the layer of thermoplastic material 26 and the knurled mating surface 22/24. To bond a thermoplastic composite structure such as the thermoplastic composite shaft 16 of Figure 1A to the metallic fitting 14 utilizing one method according to the present invention, a mandrel 32 is engaged with the preparedmetallic fitting 14, as shown in Figure 5. The mandrel 32 includes a support surface 34 for the laying up of laminates of thermoplastic composite material. Although a mandrel is shown in Figure 5, other means of supporting the laminates during lay-up may be used. In some applications, such as bonding a solid (non-hollow) thermoplastic composite structure to a metallic structure, a means of supporting the laminate lay-ups may not be necessary. The next step is to lay up laminates 36 of thermoplastic composite material over the bonding surface 28 of the metallic fitting 14 (see Figure 3A) and the support surface 34 of the mandrel 32. The lay up of laminates 36 is repeated until the desired thickness of the thermoplastic composite material is obtained, as shown in Figure 6. The desired thickness will depend upon the application and the specific characteristics of the thermoplastic composite material used. The final step is to consolidate the laminate lay- ups 36, in a manner suitable for the thermoplastic composite material, with the thermoplastic material layer 26 molded to the metallic fitting 14. As is well known, consolidation typically entails the application of heat and pressure to the laminate lay-ups 36 to form the thermoplastic composite shaft 16 and to bond the laminate lay-ups 36 to the thermoplastic material layer 26. After consolidation, the mandrel 32 may be removed and the hollow, thermoplastic composite shaft 16 is bonded to the thermoplastic layer 26 of the metallic fitting 14 to form the drive assembly 12 illustrated in Figure 1A. To bond a thermoset composite structure such as the thermoset composite shaft 16 of Figure IB to the metallic fitting 14 utilizing another bonding method according to the present invention, the thermoset composite shaft 16 must first be provided with a suitable bonding surface. As illustrated in Figure 2B, the mating surface 22 of the metallic fitting 14 has a frustoconical shape. An examination of Figure IB illustrates that the end of the thermoset composite shaft 16 to be bonded to the metallic fitting 14 has a mating surface that is complementary to the shape of the mating surface 22. With reference to Figure 7, the thermoset composite shaft 16 has a mating surface 40 that includes a short cylindrical portion 42 and a relatively longer frustoconical portion 44. The first step in producing the bonding surface for the thermoset composite shaft 16 requires that a layer of dry fiber reinforcement 48 be partially embedded into a thermoplastic cone 50 (see Figures 3B, 8) . The thermoplastic cone 50 has a configuration that is complementary to the mating surface 40 of the thermoset composite shaft 16. The dry fiber reinforcement layer 48 is embedded into the outer surface of the thermoplastic cone 50 in such manner as to produce an embedded portion and an exposed portion as exemplarily illustrated in Figure 8. Fabrication of the thermoplastic cone 50 having the dry fiber reinforcement layer 48 partially embedded therein may be achieved by various procedures. One procedure involves weaving commingled strands of thermoplastic material and dry fiber reinforcement with strands of dry fiber reinforcement. The commingled strands, in turn, are formed by intimately consolidating strands of dry fiber reinforcement with spun thermoplastic filaments. Strands of dry fiber reinforcement are then woven together with the commingled strands to form a woven material which is predominantly dry fiber reinforcement on one side and commingled strands on the other side. An alternate procedure is to weave strands of dry fiber reinforcement with spun thermoplastic filaments to produce a woven material that is predominantly dry fiber reinforcement on one side and thermoplastic filaments on the other side. A further alternative procedure for fabricating the thermoplastic cone 50 having the dry fiber reinforcement layer 48 partially embedded therein is as follows. First, a layer of semi-crystalline thermoplastic material and a layer of amorphous thermoplastic material are intimately fused together. The semi-crystalline thermoplastic material has a higher melting temperature than the amorphous thermoplastic material. The layer of amorphous thermoplastic material is thinner than the layer of dry fiber reinforcement. Next, the layer of dry fiber reinforcement is pressed onto the layer of amorphous thermoplastic material, and the fused layers of thermoplastic material are heated to melt the amorphous thermoplastic material, but not the semi-crystalline thermoplastic material. In this way, the fiber reinforcement layer is embedded in the amorphous thermoplastic layer, the layer of semi-crystalline material providing a barrier to prevent complete embedment of the layer of dry fiber reinforcement. Although the three procedures described above are suggested as possible ways of partially embedding the dry fiber reinforcement layer 48 into the thermoplastic cone 50, it should be understood that- various other procedures may also be used. The next step in the preparation of the bonding surface is to coat the exposed portion of the dry fiber reinforcement layer 48 with a thermoset resin that is compatible with the thermoset composite shaft 16. The coated exposed portion of the dry fiber reinforcement layer 48 is then engaged with the mating surface 40 of the uncured thermoset composite shaft 16. Subsequent to engagement, the thermoset composite shaft 16 and the thermoplastic cone 50 with the dry fiber reinforcement layer 48 partially embedded therein are co-cured. As a result of co-curing, the coated exposed portion of the dry fiber reinforcement layer 48 is bonded to the cured thermoset composite shaft 16. Since the dry fiber reinforcement layer 48 is partially embedded in the thermoplastic cone 50, the cone 50 forms an integral part of the thermoset composite shaft 16, as illustrated in Figure 7 (see also Figure 3B) , after co-curing. The inner surface of the thermoplastic cone 50 comprises the bonding surface 52 of the thermoset composite shaft 16, as illustrated in Figure 7. Once the thermoset composite shaft 16 has been provided with the bonding surface 52, a resistance heating element 54, as exemplarily illustrated in Figure 9, is placed over the bonding surface 28 of the metallic fitting 14 as illustrated in Figure 10. The resistance heating element 54 includes electrical leads 56 which extend out from the bond layer. The electrical leads 56 are configured to be connected to a source of electrical energy (not shown) . The bonding surface 52 of the thermoset composite shaft 16 is then engaged with the metallic fitting 14 such that v the resistance heating element 54 is sandwiched between the thermoplastic bonding surface 52 of the thermoset composite shaft 16 and the thermoplastic bonding surface 28 of the metallic fitting 14. An electrical current of sufficient strength to heat the resistance heating element 54 above the melting temperature of the thermoplastic material is passed through the heating element 54 concurrent with the application of pressure to the bond area. A suggested procedure for applying pressure to structures such as those shown in Figure IB is to apply an axial load to the thermoset composite shaft 16 and the metallic fitting 14. The conical shape of the bonding surfaces 28, 52 will provide a circumferential force component sufficient to provide adequate consolidation pressure. Other means to apply pressure may be used for structures having differently shaped bonding surfaces. The heat produced by the resistance heating element 54 and the applied pressure consolidates the thermoplastic material comprising the respective bonding surfaces 28, 52, thereby forming a thermoplastic bond layer between the metallic fitting 14 and the thermoset composite shaft 16 to complete the bonding process according to the present invention. It should be noted that the heat generated by the resistance heating element should be sufficient to melt the thermoplastic material, but not sufficient to cause mechanical degradation of the cured thermoset composite shaft. The resistance heating element may be left in place during use and may subsequently be used to reverse the bonding process. Reversing the bonding process will require passing an electrical current through the"resistance heating element to remelt the thermoplastic material within the bond layer. Once melted, the metallic fixture can be separated from the thermoset composite shaft. Reversing the bonding process permits replacement of either the metallic fitting or the thermoset composite shaft as necessary- The resistance heating element provides a means to heat the bond layer to effectuate the consolidation of the layers of thermoplastic material and to reverse the consolidation. The resistance heating element may be chosen from a variety of electrically conductive elements including pierced and stretched foils, etched foils, stamped elements, and wound wire serpentine. The resistance heating element selected should be functional to heat the bonding surfaces in a substantially uniform manner. Uniform heating will minimize the occurrence of hot spots and/or cold spots during the bonding process which may degrade the quality of the bond. Although an embedded resistance heating element is shown and suggested as a convenient means to heat the bonding surfaces, other means of heating, such as induction heating, may be used. Figure 11 illustrates a drive assembly 12 wherein a metallic fitting 14 of alternative configuration is bonded to a composite shaft 16 utilizing another embodiment of the bonding method according to the present invention. The metallic fitting 14 is illustrated in further detail in Figure 12 and includes the splined portion 18 and the mating surface 22 having knurls 24 formed therein as described hereinabove. In addition, the mating surface 22 of the fitting 14 of Figure 12 includes a circumferentially recessed portion 62 which is radially offset to provide an additional mechanical lock between the metallic fitting 14 and the composite shaft 16. The metallic fitting 14 having the circumferentially recessed portion 62 may be bonded to the thermoplastic composite shaft 16 utilizing the bonding method described hereinabove. Subsequent to the consolidation step, which bonds the thermoplastic composite shaft 16 to the layer of thermoplastic material 26 molded to the knurled mating surface 22/24 of the metallic fitting 14, a high tension overwrap 64 is placed around the thermoplastic composite shaft 16 in the vicinity of the recessed portion 62 of the metallic fitting 14. The high tension overwrap 64 functions to clamp the thermoplastic composite material into the recessed portion 62 and down into the knurled mating surface 22/24. The high tension overwrap may formed from a variety of high tensile strength materials. A suggested material is fiberglass. Although the invention has been shown and described with respect to exemplary embodiments thereof, it should be understood by those skilled in the art that various changes, omissions, and/or additions may be made therein and thereto, without departing from the spirit and scope of the invention. What is claimed is:

Claims: Claims 1. A method for bonding a metallic structure having a mating surface to a composite structure comprising the steps of: knurling the mating surface of the metallic structure; molding a layer of thermoplastic composite material over the knurled mating surface of the metallic structure to produce a bonding surface; and consolidating the composite structure with the bonding surface of the metallic structure. 2. The method according to claim 1 wherein the composite structure is a thermoplastic composite structure, and wherein said consolidating step comprises the steps of: engaging a mandrel having a support surface with the metallic structure; laying up laminates forming the thermoplastic composite structure over the bonding surface of the metallic structure and the support surface of the mandrel; and applying heat and pressure to fuse the layed-up laminates to the layer of thermoplastic composite material molded to the knurled mating surface of the metallic fitting and to form the thermoplastic composite structure. 3. The method of claim 1 further comprising the step of wrapping a high tension overwrap around the consolidated composite structure to clamp the composite structure to the metallic fitting. 4. The method according to claim 1 wherein the composite structure is an uncured thermoset composite structure, and wherein said consolidating step comprises the steps of: partially embedding a layer of dry fiber reinforcement into a thermoplastic structure having a configuration complementary to the knurled mating surface of the metallic structure to produce an embedded portion and an exposed portion of dry fiber reinforcement in the thermoplastic structure; coating the exposed portion of dry fiber reinforcement with a thermoset composite resin; engaging the resin-coated exposed portion of dry fiber reinforcement with a mating surface of the uncured thermoset composite structure; curing the uncured thermoset composite structure and engaged, resin-coated exposed portion of dry fiber reinforcement wherein the thermoplastic structure provides a bonding surface for the cured thermoset composite structure; engaging the bonding surface of the cured thermoset composite structure with the bonding surface of the metallic structure; and applying heat and pressure to fuse the engaged bonding surfaces of the metallic structure and the thermoset composite structure. 5. The method of claim 4 wherein the engaging step further comprises the step of placing a resistance heating element between the bonding surface of the cured thermoset composite structure and the bonding surface of the metallic structure; and wherein the applying step further comprises the step of passing an electrical current through the resistance heating element to generate heat to melt the thermoplastic material forming the bonding surfaces of the metallic structure and the cured thermoset composite structure wherein the bonding surfaces are fused together. 6. A drive assembly for transmitting forces from a force transmitting element to a driven element, the drive assembly including: a metallic fitting having means for engaging the force transmitting element and a knurled mating surface; a layer of thermoplastic material molded into the knurled mating surface of the metallic fitting, the molded layer of thermoplastic material defining a bonding surface for the metallic fitting; and a composite shaft having a bonding surface, the bonding surface of the composite shaft being consolidated with the bonding surface of the metallic fitting by application of heat and pressure. 7. The drive assembly of claim 6 wherein the knurled mating surface of the metallic fitting includes a circumferentially recessed portion. 8. The drive assembly of claim 7 further comprising a high tension overwrap placed around the composite shaft adjacent the circumferentially recessed portion of the metallic fitting to clamp the composite shaft to the knurled mating surface of the metallic fitting. 9. The drive assembly of claim 6 wherein the composite shaft is a thermoset composite shaft, and wherein the bonding surface of the thermoset composite shaft comprises a thermoplastic structure having a layer of dry fiber reinforcement partially embedded therein, an exposed portion of the dry fiber reinforcement layer being coated with a thermoset composite resin and cured in combination with the thermoset composite shaft such that the thermoplastic structure cured to the thermoset composite shaft defines the bonding surface thereof. 10. The drive assembly of claim 9 further comprising a resistance heating element interposed between the bonding surface of the thermoset composite shaft and the bonding surface of the metallic fitting.

Inventors: DAVIS GEOFFREY C, MCINTYRE ALLEN J, DAVIS, GEOFFREY, C., MCINTYRE, ALLEN, J.
Applicants: UNITED TECHNOLOGIES CORP, UNITED TECHNOLOGIES CORPORATION
Application year: 1993
Main classification: B29C65
IPC codes: B29C37, B29C65, F16C3