JOINED STRUCTURE COMPRISING MEMBERS OF DIFFERENT COEFFICIENTS OF THERMAL EXPANSION AND JOINING METHOD THEREOF

Abstract: Joined structures are disclosed, which comprise first and second members (1,3) having different coefficients of thermal expansion and an intermediate member (2) joining the first and second members. The intermediate member (2) has yield stress smaller than those of the first and second members, and a tapered portion (4) is formed at an outer peripheral surface of the inter­ mediate member (2). The tapered portion (4) has a variable cross-sectional area orthogonal to an axis of the joined members, this area being larger at one end of the tapered portion near that one of the first and second members having a smaller coefficient of thermal expansion.

Description: The present invention relates to composite structures in which members are joined together through an intermediate member. More particularly, the invention relates to composite structures in which the joined members have different coefficients of thermal expansion, for instance, a ceramic member and a metallic member. The present invention also relates to a method of joining members having different coefficients of thermal expansion. In order to join members having different coefficients of thermal expansion through butting and bonding, it has formerly been known a technique to join these members while an intermediate member is interposed at an interface therebetween. For instance, Japanese utility model registration application Laid-open No. 59-160,533 discloses, as a joining structure between a ceramic member and a metallic member, a method of joining them while a clad made of a tungsten material or an Mo material at an interface therebetween. However, according to the above mentioned methods, sufficient joining strength cannot be obtained because residual stress is produced at the joining interface due to difference in thermal expansion between them. Now, explanation will be explained with reference to joining between a metallic member and a ceramic member shown in Fig. 4. Assume that the metallic member is joined to the ceramic member in a desired shape before cooling. Once the metallic member and the ceramic member having larger and smaller coefficients of thermal expansion, respectively, are cooled, the metallic member more shrinks. Consequently, as shown in Fig. 4, tensile stress is exerted upon each of the metallic member and the ceramic member at an edge A of the interface. When this state proceeds, they peel or crack at the interface edge A. When a clad made of a tungsten or molybdenum material is used as an intermediate layer, residual stress is removed to some extent as compared with the above-mentioned case where both the members are directly joined together. However, since this intermediate layer is not provided to remove the residual stress at the joining interface, it was not possible to prevent peeling or cracking at the joining interface. It is an object of the present invention to obviate the above-mentioned problems, and to provide a structure and a method for joining members having different coefficients of thermal expansion. The joining structure and method hardly develop peeling or cracking at an edge of an interface. According to the present invention, there is provided a composite structure as set out in claim 1. According to the present invention, there is further provided of a method of joining as set out in claim 6. Preferred optional features of the method are set out in claims 7 to 9. In the invention, when an yield stress of the intermediate member is made smaller than those of the first and second members having different coefficients of thermal expansion and if tensile stress which is produced due to difference in thermal expansion or thermal shrinkage acts upon the edge of the interface, the intermediate member is plastically deformed, that is, shear-deformed. Consequently, residual stress at the interface between both the members is mitigated so that peeling and cracking at the interface edge can effectively be prevented. When a tapered portion is formed at the outer periphery of the intermediate member such that the tapered portion include an edge of an interface between said intermediate member and one of said first and second members having a smaller coefficient of thermal expansion, and has a variable cross sectional area orthogonal to the axis of the members, the area being larger at one end of the tapered portion near one of the first and second members having a smaller coefficient of thermal expansion than the area at the other end of the tapered portion near the other one of first and second members having a larger coefficient of thermal expansion, the direction of the tensile stress upon the intermediate member having smaller yield stress (generally smaller hardness) is changed, so that the intermediate layer is likely to be plastically deformed or shear-deformed, and the effect to mitigating residual stress at the interface edge is promoted. In addition, preferably in the joining method of the present invention, the joined members are annealed during cooling after hot press joining to remove thermal stress produced due to difference in thermal expansion, and the tapered portion is then formed to effectively prevent peeling and cracking at the interface edge. For a better understanding of the invention, reference is made to the attached drawings, wherein: Figs. 1(a) through (e) are schematic views illustrating examples of tapered surfaces in joined structures according to the present invention; Figs. 2(a) and 2(b) are schematic views illustrating examples in which actual articles embody the present invention; Figs. 3(a) and 3(b) are schematic views illustrating a test piece produced according to the present invention; and Fig. 4 is a view illustrating a trouble encountered by a prior art joined structure. The present invention will be explained in more detail with reference to the attached drawings. Figs. 1(a) through (e) show various tapered shapes by way of examples. With respect to the tapered shape, any tapered shape will do in the present invention so long as a tapered portion is formed at an outer periphery of an intermediate member is formed such that the tapered portion has a variable cross sectional area orthogonal to an axis of joined members, the area being larger at one end of the tapered portion near one of first and second members having a smaller coefficient of thermal expansion than the area at the other end of the tapered portion near the other one of the first and second members having a larger coefficient of thermal expansion. That is, a tapered portion is provided at the intermediate member C from the joining member A (smaller coefficient of thermal expansion) to the member B (larger coefficient of thermal expansion). Further, the interface of the members A and C has only to be located at the tapered portion. In a wide meaning, the tapered portion may be formed as a curved surface as shown in Fig. 1(e). The definition of the tapered portion according to the present invention may include all such tapered portions that an area of a section of an intermediate member C cut at a flat plane passing through the tapered portion in parallel with the joining interface decreases from the member A to the member B. As shown in Figs. 1(a) through (e), the sectional edge of the tapered portion may be a straight line or a curved line. In addition, the shape of the joining face needs not be circular, but the present invention may be applicable to a joining face having a rectangular or polygonal shape. Furthermore, a tapered portion may be formed after or before the joining. A taper angle is used throughout the specification and claims to mean an angle defined between a center axis of the joined structure and a straight sectional edge of the tapered portion or a tangent of a curved sectional edge of the tapered portion at a joining interface between A and C. Now, the present invention will be explained in more detail below. Figs. 2(a) and 2(b) are schematic views illustrating examples in which the joining method according to the present invention are applied to actual articles. Fig. 2(a) is an example in which the invention method is applied to a piston. In this example, a piston cap 1 made of partially stabilized zirconia is bonded to a piston body 3 made of nodular graphite cast iron through an intermediate layer 2 made of copper. This example uses, as the intermediate layer 2, copper having yield stress of 100 MPa (10 kg/mm²) (hardness = H R F 40) smaller than 250 MPa (25 kg/mm²) of graphite cast iron. A tapered portion is provided in the intermediate layer such that the diameter of the tapered portion increases toward the piston cap 1. By so constructing, the intermediate layer 2 is plastically deformed to release residual stress at edges of interfaces. Fig. 2(b) is an example in which the invention method is applied to a rotary shaft of a turbocharger rotor or a gas turbine rotor. A rotary blade-shaft portion made of silicon nitride is joined to a metallic rotary shaft 9 through an intermediate layer 10 of copper provided with a tapered portion. The joining method according to the present invention may be applied to any combination composed of members having different coefficient of thermal expansion, but the joining method is particularly effective to the combination of ceramic members and metallic members as mentioned above. As ceramic members employed for this purpose, use may be made of silicon nitride, silicon carbide, sialon, zirconia, mullite, alumina, beryllia, etc. As metallic members, use may be made of stainless steel, nodular graphite cast iron, nickel-chrome-molybdenum steel, chrome-molybdenum steel, aluminum-chrome-molybdenum steel, maraging steel, precipitation hardenable type super alloy, etc. Metallic members as partially or entirely hardened by precipitation hardening, nitriding or high frequency hardening are preferably used. As intermediate members, use may be made of Cu, Al, Zn, Pb, Pt, Au, Ag and alloys thereof, having smaller yield stress. The joining may be effected by brazing, diffusion bonding, melting bonding, combination thereof, or the like. Next, in order to confirm effects of a tapered portion and physical properties of an intermediate member, a test sample 21 shaped as shown in Figs. 3(a) and 3(b) was prepared. In this test sample, a round disc 22 made of silicon nitride (Si₃N₄) in a diameter of 30 mm and a thickness of 10 mm was disposed between round rods 23 and 24 made of cast iron, while round thin discs 25 and 26 made of copper or a copper alloy were disposed as an intermediate member between the round disc 22 and the round rods 23 or 24, respectively. Each of the intermediate members 25 and 26 was tapered such that the outer diameter of the tapered portion increased toward the round disc 22 of silicon nitride. The test sample 21 was obtained by soldering the round rod 23, the thin disc 25, the round disc 22, the thin disc 26 and the round rod 24 with silver at 800°C. Various test samples were similarly prepared while the material of the thin disc 25 and 26, a taper angle of the tapered surface 27 provided at the thin disc, and the length, ℓ, of the tapered portion (see Fig. 3(b)) were varied. Each of these test samples was subjected to tensile test. Similar tests were carried out with respect to test samples in which thin discs 25 or 26 was not tapered during joining, annealing was carried out at 400°C for 30 minutes during the joining, and the thin discs 25 and 26 were tapered after the joining. For comparison purpose, test samples having not been annealed and/or tapered was subjected to similar tests. Results are shown in Table 1(a) and 1(b). As is clear from the above results, as compared with the test samples having no tapered portion, the tapered test samples were not cracked during cooling, and had high tensile strength. The taper angle ϑ is preferably 10° or more, and more preferably 30° or more. When the taper angle is in this range, preferred tensile strength can be obtained. Sufficient strength can be attained when the length of the tapered portion is not less than 0.1 mm. It is preferable to use, as a material of the intermediate layer, a metal having yield stress of not more than about 200 MPa (20 kg/mm²), for instance, 4/6 brass. It is more preferable to use copper or the like having yield strength of about 100 MPa (10 kg/mm²). Furthermore, a tapered portion may be formed prior to or posterior to joining. When the tapered portion is formed after the joining, it is effective to include an annealing step, because it can mitigate reduction in the joining stregth due to stress concentration. The present invention will not be limited to the above-mentioned examples only, but various modifications, variations, and changes could be made. For instance, although joining between ceramic members and metallic members has been explained in the above-mentioned examples, the joining methods according to the present invention may be applied to any joining between members so long as they have different coefficients of thermal expansion. As is clear from the foregoing description, according to the present invention, members having different coefficients of thermal expansion are joined together, while an intermediate member having a smaller yield stress than those of the former members is interposed therebetween and the intermediate member is tapered. Thereby, the intermediate member is plastically deformed to mitigate tensile stress caused due to difference in thermal expansion between the former members. In addition, the direction of the stress at the edge of the interface is changed due to an effect of the tapered portion to facilitate the plastic deformation of the intermediate member. Such effects serve to greatly decrease residual stress at edges of the interfaces. Furthermore, when members having different coefficients of thermal expansion are joined through an intermediate member and thermal stress is removed by annealing the joined members, thermal stress is removed. Thus, this is coupled with reduction in residual stress which is decreased by tapering the intermediate member prior to or posterior to the joining.

Claims: A composite structure comprising first and second members (A, B; 1, 3; 22, 23, 24) having different coefficients of thermal expansion and an intermediate member (C; 2; 25, 26), said first and second members being joined together through said intermediate member which is made of metal selected from Cu, Al, Zn, Pb, Pt, Au, Ag and alloys thereof, wherein said intermediate member has yield stress smaller than those of said first and second members, and a tapered portion is formed at an outer peripheral surface of the intermediate member such that said tapered portion includes an edge of an interface between said intermediate member and the one of said first and second members having a smaller coefficient of thermal expansion, and has a varying cross-sectional area orthogonal to an axis of the joined members, said area being larger at one end of the tapered portion at said one of said first and second members having a smaller coefficient of thermal expansion than said area at the other end of the tapered portion near the other one of said first and second members having a larger coefficient of thermal expansion. A structure according to claim 1, wherein the angle of the tapered portion relative to the axial direction of the joined structure is not less than 10°. A structure according to claim 1 or claim 2, wherein the yield stress of said intermediate member is not more than 200 MPa (20 kg/mm²). A structure according to any one of claims 1 to 3, wherein one of the first and second members is made of a ceramic material selected from silicon nitride, silicon carbide, sialon, zirconia, mullite, alumina and beryllia, and the other is made of a metallic material selected from stainless steel, nodular graphite case iron, nickel·chrome·molybdenum steel, chrome·molybdenum steel, aluminum·chrome·molybdenum steel, maraging steel and precipitation hardenable type super alloy. A joined structure according to claim 4, wherein said member made of a metallic material is partially or entirely hardened by a step selected from precipitation-hardening, nitriding, and high frequency hardening. A method of joining first and second members (A, B; 1, 3; 22, 23, 24) having different coefficients of thermal expansion through an intermediate member (C; 2; 25, 26) having yield stress smaller than those of said first and second members, said intermediate member being made of metal selected from Cu, Al, Zn, Pb, Pt, Au, Ag and alloys thereof, the method comprising the steps of butting and joining said first and second member to each other through said intermediate member, and before or after said joining step forming a tapered portion at an outer peripheral surface of the intermediate member, which said tapered portion includes an edge of one of interface between said intermediate member and one of said first and second members having a smaller coefficient of thermal expansion, such that said tapered portion has a varying cross-sectional area orthogonal to an axis of the joined members, said area being larger at one end of said tapered portion at said one of the first and second members having a smaller coefficient of thermal expansion than said area at the other end of the tapered portion near the other one of said first and second members having a larger coefficient of thermal expansion. A joining method according to claim 6, further including a step of annealing said joined first and second members and said intermediate member, said tapered portion-forming step being effected after the annealing step. A joining method according to claim 6, wherein the tapered portion is formed before said butting and joining steps, and said butting and joining are effected after the intermediate member is interposed between the first and second members such that the tapered portion has a varying cross-sectional area orthogonal to the axis of joined members, said area being larger at one end of the tapered portion near said one of said first and second members having a smaller coefficient of thermal expansion than said area at said other end of the tapered portion near other one of said first and second members having a larger coefficient of thermal expansion. A joining method according to claim 6, further including a step of annealing the joined members after the first and second members are joined through the intermediate member having said tapered portion at the outer peripheral surface. A joining method according to any one of claims 6 to 9, wherein one of the first and second members is made of a ceramic material selected from silicon nitride, silicon carbide, sialon, zirconia, mullite, alumina and beryllia, and the other is made of a metallic material selected from stainless steel, nodular graphite case iron, nickel-chrome-molybdenum steel, maraging steel and precipitation hardenable type super alloy.

Inventors: NAKASUJI, YOSHIZUMI, YOSHIDA, AKIHIKO
Applicants: NGK INSULATORS, LTD.
Application year: 1988
Main classification: C04B37
IPC codes: F16B4, B23P11, B23K1, F02F3, C04B37