COMPOSITE JOINTED BODIES

Abstract: Composite joined bodies are disclosed which comprise constituent members (1,2) and an intermediate member (3) through which the constituent members are joined together. The intermediate member (3) has a honeycomb structure. This obviates stress caused by differences of thermal expansion and can provide thermal insulation.

Description: The present invention relates to composite joined bodies in which constituent members are joined together through an intermediate member. More particularly, the invention relates to composite joined bodies having a structure which is favorably used for joining constituent members having different coefficients of thermal expansion, for instance, ceramic members and metallic members. In order to obtain composite joined bodies by butting and joining together members having different thermal expansions, that is, different coefficients of thermal expansion, there are at present known a technique for joining them by direct pressure welding and a technique for joining them through a separate intermediate layer at an interface thereof. For instance, Japanese utility model registration application Laid-open No. 59-160,533 discloses a method of joining a ceramic member and a metallic member through a layer made of a tungsten material or a molybdenum material at an interface thereof as a joining structure. However, since a residual stress is retained at the joining interface due to difference in thermal expansion in the above-mentioned joining methods, sufficient joining strength cannot be obtained. Now, explanation is made on Fig. 9 by way of example in which a metallic member is joined to a ceramic member. Assume that the metallic member having a larger coefficient of thermal expansion is joined to the ceramic member having a smaller coefficient of thermal expansion in an intended joined shape before cooling. When the thus joined composite body is cooled, the metallic member shrinks more. Thus, as shown in Fig 9, tensile stress is exerted upon each of the metallic member and the ceramic member at an interface edge "A". If this phenomenon becomes more serious, they sometimes peel or crack at the interface edge "A". When a layer made of a tungsten material or a Mo material is used as an intermediate layer, as compared with the above joining of the metallic member and the ceramic member by direct pressure welding, residual stress is reduced to some extent. However, since the intermediate layer is provided not to remove the residual stress at the joining interface, it is not always possible to eliminate the peeling and the cracking there. Further, JP-A-62-182,170 discloses a structure of joining a ceramic member and a metallic member through an insert layer forming a part which is not bonded at least either between a ceramic member and an insert layer or between a metallic member and an insert layer. However, the residual stress is not effectively removed at the joining interface edge because the rigidity of the insert layer is not reduced near the edge portion. DE-A-2165696 discloses a ceramic member and a metal member, joined together by a foamed metal section. The foamed metal provides a "buffer" to compensate for the differential in coefficient of thermal expansion between the ceramic and metal members. This foamed metal has the disadvantage that it has isotropically weak properties, which would be particularly evident in bending stress. The present invention seeks to provide composite joined bodies each comprising members having different coefficients of thermal expansion, which obviate the above-mentioned problems and can effectively prevent an edge of an interface from peeling or cracking. In connection with the joining of members having the same coefficient of thermal expansion, mention is made of US-A-3 716 347, which shows metal sheets joined by honeycomb structures. The composite body according to the present invention is set out in claim 1. The honeycomb structure is used herein to mean not only honeycomb structures which are constituted by linear partition walls arranged in a honeycomb fashion but also those which are constituted by planar or curved strips or plates. In fact the term honeycomb here used includes any structure having hollow cells separated by walls. In the construction of the invention, rigidity of the intermediate member is reduced by making it as a honeycomb structural body constituted e.g. by strips or plates so that stress developed at the time of joining may be absorbed to remove residual stress without reducing the effect of the joint between the members. As a result, an interface edge can be prevented from peeling or cracking without damaging the effect of the joint. Another advantage of the invention can be to provide thermal insulation between the constituent members. In the body of the present invention preferably the rigidity of the honeycomb structure decreases towards the periphery of the joint interface. That is to say, for instance, when cylindrical members are to be joined together, the rigidity of the honeycomb structure is decreased from a central axis to the outer periphery. By so doing, the rigidity of a zone at which residual stress is more likely to remain can be lowered. The body of the present invention can also favorably be attained by setting the thickness of the honeycomb structure at 0.5 mm or more. Moreover, a circle, a polygon, an ellipse, and a torus may be employed as the sectional shape of the composite bodies according to the present invention. These and other optional features, and advantages of the present invention will be appreciated upon reading of the following description of the invention when taken in conjunction with the attached drawings, with the understanding that modifications, variations and changes of the same could be made by the skilled person in the art to which the invention pertains. Embodiments of the invention, are described below by way of example with reference to the attached drawings, wherein: Fig. 1 is a sectional view of an embodiment of the composite joined body according to the present invention; Figs. 2(a) and (b) are a perspective view in a developed state and a IIb-IIb sectional view of an intermediate layer used in the present invention, respectively; Figs. 3(a) through (e) and Figs. 4(a) through (d) are sectional views of other intermediate members used in the present invention; Fig. 5 is a front view of a composite joined body used in a tensile test; Figs. 6(a), (b) and (c) are a cross sectional view and a vertical sectional view of the composite joined body shown in Fig. 5, and a view illustrating a way of making a honeycomb structural body, respectively; Fig. 7 is a view illustrating a pipe-coupling joint structure embodying the present invention; Fig. 8 is a sectional view of a ceramic head plate embodying the present invention; and Fig. 9 is a view illustrating the problem possessed by the conventional technique. Fig. 1 is a sectional view of an embodiment of the composite joined bodies according to the present invention. In Fig. 1, a cylindrical ceramic member 1 and a cylindrical metallic member 2 having different coefficients of thermal expansion are pressure welded together through a honeycomb structural intermediate member 3 made of Ti, Aℓ, or the like. Figs. 2(a) and (b) show a developed perspective view and a IIb-IIb sectional view of the intermediate member 3, respectively. As seen in Figs. 2(a) and (b), the intermediate member 3 is constituted by assembling metal thin strips or plates 4 made of Ti, Aℓ or the like in a lattice fashion, and integrally adhering circular plates 5-1 and 5-2 made of the same metal as that of the metal thin strips or plates 4 to the opposite end faces thereof through welding, brazing or the like. The circular plates 5-1 and 5-2 are provided to increase the contact area and the joining strength between the intermediate member 3 and the ceramic member 1 and the metallic member 2 to be bonded thereto. If the adhering strength between the honeycomb structure and the ceramic member and the metallic member is enough, no circular plates need to be provided. Although the present invention is applicable to any composite bodies comprising members having different coefficients of thermal expansion, the invention is particularly effective to the composite joined bodies comprising metallic members and ceramic members as mentioned above. Ceramic material used for this purpose may include silicon nitride, silicon carbide, sialon, zirconia, mullite, alumina, beryllia, etc. As metallic materials, stainless steel, nodular graphite cast iron, nickel-chrome-molybdenum steel, chrome-molybdenum steel, aluminum-chrome-molybdenum steel ,maraging steel, precipitation-hardenable type alloy, etc. may be used. It is preferable that a part or the whole of the metallic member is hardened by precipitation-hardening, nitriding, high frequency hardening or the like. Further, stainless steel, Ti, Ni, W, Nb Cu or Ta, or an alloy thereof may be used as the intermediate member. The joining may be carried out by brazing, diffusion bonding, melting bonding, a combination thereof, or the like. Figs. 3 and 4 are sectional views showing other examples of intermediate members used in the present invention. In the intermediate members shown in Figs. 3(a) through (d), rigidity is made smaller toward the joint edge, that is, toward the outer periphery. Fig. 3(a) shows an example in which a space between adjacent lattice strips in the honeycomb structure is made greater toward the outer periphery. In Figs. 3(b), a radial space between adjacent concentrical partition walls is made greater toward the outer periphery. Fig. 3(c) shows an example in which rigidity in the radial direction is adjusted by forming the outermost peripheral portion in a concentrically petalous fashion. In Fig. 3(d), rigidity in the radial direction is adjusted by swirly partition walls. Fig. 3(e) shows an example in which rigidity is made smaller in the radial direction than that in the intermediate member of Figs. 2(a) and (b) by a twill-like pattern. Figs. 4(a) through (d) show honeycomb structural intermediate members having various annular sections. In any of these examples, residual stress can effectively be reduced. In the following, examples of the present invention will be given. These examples are merely given in illustration of the invention, and should not be interpreted to limit the scope thereof. Examples: As shown in Fig. 5, a ceramic member 1 made of Si₃N₄ having an outer diameter of 30 mm was joined to a metallic member 2 made of S45C through an intermediate member 3 made of Ti having a thickness of 5 mm by soldering them with silver solder. As shown in a cross sectional view and a vertical sectional view of Figs. 6(a) and (b), respectively, the intermediate member 3 is constituted by assembling a honeycomb structure at a pitch "t" from Ti strips or plates having a thickness of 0.2 mm through spot welding as shown in Fig. 6(c) and soldering circular plates made of Ti having a thickness of 0.2 mm to upper and lower end faces of the honeycomb structure. In the thus obtained composite bodies, Run Nos. 1-3 (Group I) of the present invention in which the pitch "t" was all set at 3 mm, Run Nos. 4-6 (group II) of the present invention in which the pitch was stepwise changed at 3 mm, 4 mm and 5 mm from the central portion to the peripheral portion, and Run Nos. 7 to 9 made of a 5 mm thick Ti plate with a non-honeycomb structure as an intermediate member were prepared. Tensile tests were carried out with respect to the composite bodies. Results are shown in Table 1. It is seen from the results in Table 1 that Run Nos. 4 to 6 of the present invention (Group II) in which rigidity was decreased toward the joint peripheral edge could withstand higher tensile loads as compared with the Comparative Examples. Figs. 7 and 8 show examples actually embodying the present invention. Fig. 7 shows a joint coupling for a pipe to be exposed to a gas, a liquid and a vacuum atmosphere inside and outside thereof. A ceramic pipe 7-1 is joined to a metallic pipe 7-3 through a annular honeycomb structural intermediate member 7-2. In this embodiment, the honeycomb structure mitigates stress due to difference in thermal expansion between the ceramic material and the metallic material and also seals the joint portion. Fig. 8 shows an example of a head plate embodying the present invention. A head plate 8-1 made of Si₃N₄ is joined to a cylinder head 8-2 through a honeycomb structural intermediate member 8-3 made of Ti. In this example, the honeycomb structure mitigates stress, effects heat insulation, and makes sealing between the head plate and the cylinder head at the inside of the valve holes. As is clear from the above-detailed description, according to the composite joined bodies in which constituting members are joined together through the intermediate member, rigidity of the joint portion is reduced by designing the intermediate member as the honeycomb structure. As a result, even the composite joined bodies are formed through the intermediate member, the interface can be prevented from peeling or cracking without damaging the joining strength. Further, also when members having the same coefficient of thermal expansion are to be joined together, they can thermally be isolated from each other without damaging the joining force between the members joined together. Thus, the present invention is effective for joining ceramic gas turbine parts such as ceramic gas turbine stationary and moving blades, ceramic diesel engine parts such as head plates, ceramic parts for electrically insulating NaS cells, etc.

Claims: A composite joined body comprising constituent members (1,2;7-1,7-3;8-1,8-2), rigidly joined by an intermediate member (3;7-2;8-3) which is interposed between faces of the constituent members, said intermediate member having a honeycomb structure of a plurality of hollow cells separated by walls, the cells extending longitudinally in a direction from one said constituent member to the other said constituent member, characterized in that said constituent members are a ceramic member (1) and a metallic member (2), having different coefficients of thermal expansion. A composite joined body according to claim 1, wherein said constituent members are thicker in the direction transverse to said faces thereof than said intermediate member. The composite joined body according to claim 1 or claim 2, wherein the rigidity of the intermediate member decreases towards the periphery of the joint interface between the constituent members. The composite joined body according to any one of claims 1 to 3, wherein the intermediate member (3) has a sealing structure between the constituent members. The composite joined body according to any one of the preceding claims, wherein the ceramic member is made from a ceramic material selected from silicon nitride, silicon carbide, sialon, zirconia, mullite, alumina and beryllia. The composite joined body according to any one of the preceding claims, wherein the metallic member is made from a metal selected from stainless steel, nodular graphite cast iron, nickel-chrome-molybdenum steel, chrome-molybdenum steel, aluminium-chrome-molybdenum steel, maraging steel and precipitation hardenable type alloy. The composite joined body according to claim 6, wherein at least a part of the metallic member is hardened by precipitation hardening, nitriding or high frequency wave hardening. The composite joined body according to any one of the preceding claims, wherein the intermediate member (8-3) is made from a material selected from stainless steel, Ti, Ni, W, Nb, Cu, Ta and an alloy thereof. The composite joined body according to any one of the preceding claims, wherein the constituent members are joined together through the intermediate member by brazing, diffusion bonding, melting bonding or a combination thereof.

Inventors: SOMA, TAKAO, YOSHIDA, AKIHIKO
Applicants: NGK INSULATORS, LTD.
Application year: 1988
Main classification: C04B37
IPC codes: C04B37, B23K35, B21D47, B32B3, B23K1