Methods for making shape memory alloy products

[0001] This application claims priority to Japanese patent application number 2003-145971, filed May 23, 2003, the contents of which are hereby incorporated by reference as if fully set forth herein.

[0002] 1. Field of the Invention

[0003] The present invention relates to methods for making shape memory alloy products (e.g., clasp, biological implant, sensor, socket, clip, switch, eyeglass frame, etc.).

[0004] 2. Description of the Related Art

[0005] FIG. 5 shows a conventional method of manufacturing TiNi shape memory alloy product. As shown in FIG. 5, TiNi shape memory alloy is manufactured by the ordinary metallurgical processes of melting, hot forging, and rolling. Raw materials (e.g., sponge Ti and Ni pellets) are melted at 1,350 to 1,500° C. by a melting furnace (e.g., high-frequency vacuum induction melting furnace, arc melting furnace, or plasma-melting furnace). After the raw materials are melted, they undergo forging and rolling, and then is formed into a primary product (e.g., sheet or wire) through cold working. The primary product is machined and formed into the shape of the final product. Then, in order to get a shape memory effect or super-elasticity, a shape memory heat treatment is applied to the product that is formed into the shape of the final product. In the shape memory heat treatment, for example, the product is heated and held at a temperature of between 300 and 600° C. for several minutes to an hour, and is then cooled.

[0006] Further, Japanese Laid-open Patent Publication No. 11-106880 describes a method for manufacturing a shape memory alloy product. In this known manufacturing method, raw materials (e.g., sponge Ti and Ni pellets) for a shape memory alloy are melted and formed into an ingot. This ingot is then melted and cast using a casting mold. After the casting is finished, the cast alloy together with the casting mold is heated in a heating furnace (i.e., undergoes shape memory heat treatment). When the heat treatment is finished, the casting mold is removed. With this method, in order to get a super-elastic effect, the shape memory heat treatment must be applied while the cast alloy is constrained inside the casting mold.

[0007] However, in the known methods, gravity segregation occurs in the melting and/or casting process because of the big difference in the specific gravity between Ti and Ni. Consequently, it is difficult to control the phase transformation temperature (e.g., shape memory recovery temperature) accurately. Further, in the former method shown in FIG. 5, the primary product (e.g., sheet or wire) made of a shape memory alloy, which is a hard-to-process material, must be further machined in order to obtain the desired shape, thereby increasing manufacturing cost. On the other hand, in the latter method, because this method applies the heat treatment to the cast alloy constrained by the casting mold, a special casting mold that can be used in the heat treatment is required.

[0008] Accordingly, it is one object of the present teachings to provide improved manufacturing method capable of controlling the phase transformation temperature accurately and reducing manufacturing cost.

[0009] In one aspect of the present teachings, raw material powders (e.g., Ti and Ni powders) for shape memory alloy may be mixed and synthesized with the combustion synthesis. The combustion synthesized alloy (e.g., TiNi-based alloy) may be melted and then cast into a desired shape (e.g., a shape of the final product or a shape close to that of the final product). In the case where the combustion synthesized alloy is cast into the desired shape, gravity segregation is less likely to occur than in the case where a shape memory alloy manufactured by a conventional melting method is cast into the desired shape. Consequently, the final products, which is formed by casting the combustion synthesized alloy, has a precise phase transformation temperature (e.g., shape memory recovery temperature). Further, because a shape close to that of the final product can be obtained by casting method, manufacturing costs can be reduced.

[0010] Preferably, the mixed powder may include Ti and Ni powders as its main components. Further, the mixed powder may include additional elements (e.g., Cr, Fe, Co, V, Mn, Mo, B, Cu, Nb, etc.).

[0011] These features may be utilized singularly or, in combination, in order to make improved shape memory alloy products. In addition, other objects, features and advantages of the present teachings will be readily understood after reading the following detailed description together with the accompanying drawings and claims. Of course, the additional features and aspects disclosed herein also may be utilized singularly or, in combination with the above-described features.

[0017] A method of manufacturing TiNi shape memory alloy product according to a representative embodiment of the present teachings will be explained below. The manufacturing method in this representative embodiment may comprise a combustion synthesis step, a casting step, and a shape memory heat treatment step.

[Combustion Synthesis Step]

[0018] FIG. 1 shows a schematic illustration of the combustion synthesis process. First, raw material powders for shape memory alloy may be precisely mixed to produce the desired composition ratio. For example, a mixing ratio of Ti at 48–52% with Ni constituting the remainder can be used to set the desired phase transformation temperature. Next, a compound is synthesized from the raw material powder mixture using a combustion synthesis method. In combustion synthesis process, for example, the mixture of the raw material powder may be filled with a container (e.g., graphite crucible). The container may be placed in a combustion synthesis reactor. Then, the top of the raw material powder may be ignited by the electrical spark or the heat of electrical resistance of a heating wire (e.g., tungsten heating wire). Then, the exothermic chemical reaction occurs at the ignited portion and the heat of formation is released. This heat ( about 2200 to 2300° C.) is transmitted to the neighboring part of the mixture and ignites them. Again the exothermic chemical reaction occurs and the heat of formation is released. This chemical chain reaction is propagated to the end of the mixture and the whole mixture is transformed to the compound. During this combustion synthesis process, the raw materials do not melt macroscopically and are finished with an intermetallic compound with an extremely small number of impurities sintered on them. Detailed procedure of combustion synthesis is known (e.g., Japanese Patent publication No. 1816876) and thus, a detailed explanation will be omitted.

[Casting Step]

[0019] For the casting step, generally known methods, such as sand mold casting, lost wax, shell mold, and centrifugal casting, may be used. The desired method can be selected from these methods according to the shape and dimensions of the product and its mass-productivity. The combustion synthesized alloy is melted at about 1300° C. and cast. After the alloy has been cooled and solidified, it is released and removed from the casting mold. During this casting process, no special heat treatment is required.

[Shape Memory Heat Treatment Step]

[0020] The shape memory heat treatment may be applied to the product removed from the casting mold, within a temperature range of 300 to 600° C. for several minutes to several hours, resulting in super-elasticity or shape memory effect in the product. In this shape memory heat treatment, no restraining mold is required, and the shape memory heat treatment can be applied to the product in a free state. If it is necessary to alter or modify the shape of the product removed from the casting mold and to further apply a shape memory heat treatment, a heat treatment may be applied using a restraining mold.

[0021] The above illustrated representative manufacturing method can be applied to products in fields that have been known to use shape memory alloys. For example, the method can be applied to the clasps used for securing false teeth in dentistry. Other application examples include biological implants used for treating bone fractures and bonding staples used in surgeries. The manufacturing method according to the present teachings, which uses casting, is extremely effective in applications in which the product shape is complicated or machining is required in the middle of the manufacturing process. Applying the manufacturing method according to the present teachings to these products can reduce their manufacturing costs. Furthermore, the manufacturing method according to the present teachings can also be applied to the manufacture of various types of industrial components, such as sensors, sockets, and eyeglass frames.

[0022] Next, examples of the present teachings will be explained below. First, the composition ratio between the Ni and Ti powders was adjusted (to Ni at 50.1% and Ti at 49.9%) so that a shape memory effect could be obtained, and NiTi alloy was synthesized using the combustion synthesis method. This combustion synthesized NiTi alloy was then melted and formed into multiple pieces of straight material (with a diameter of φ1.9 mm and a length of 40 mm) using a precision casting method. One such piece is hereafter referred to as “Test Piece 1”.

[0023] Additionally, the composition ratio between Ni and Ti powders was adjusted (to Ni at 50.7% and Ti at 49.3%) so that super-elasticity could be obtained, and NiTi alloy was synthesized using the combustion synthesis method. This combustion synthesized NiTi alloy was then melted and formed into multiple pieces of straight material (with a diameter of φ1.9 mm and a length of 40 mm) using a precision casting method. One such piece is hereafter referred to as “Test Piece 2”.

[0024] Both Test Pieces 1 and 2 were straight when removed from the casting mold. Furthermore, Test Piece 1 exhibited a shape memory effect in the state in which it was removed from the casting mold (i.e., without application of any shape memory heat treatment).

[0025] Next, in order to provide the Test Pieces 1 and 2 with the required transformation points and bending strength, a shape memory heat treatment was applied. The specific heat treatment condition was as follows: After the Test Pieces was held at 460° C. for 60 minutes, they were water-cooled. The shape memory heat treatment was applied to both Test Pieces 1 and 2 in a free state (i.e., without any restraint). After the shape memory heat treatment, both Test Pieces 1 and 2 remained straight without any deformation.

[0026] After the shape memory heat treatment, both Test Pieces 1 and 2 were subjected to a metal material-bending test (JIS Z 2248) using the pressing bend method in an environment in which the room temperature was 20° C. Specifically a test piece was placed on two supports having a diameter of 10 mm (with a space of 26 mm between the two supports), a pressing bar having a radius of 10 mm was pressed against the center of the test piece, and a load was applied until the bending angle reached 90 degrees.

[0027] Test Piece 1 remained bent after the load from the pressing bar was removed. When this bent Test Piece 1 was heated, it returned to almost its original straight shape. This test confirmed that Test Piece 1 had been provided with a shape memory effect. On the other hand, Test Piece 2 returned to the straight shape after the load from the pressing bar was removed. This test confirmed that Test Piece 2 had been provided with a super-elastic effect. The above illustrated bending test was repeated 10 times on each test pieces respectively, and the above mentioned results were obtained in every case.

[0028] The shape memory effect was verified in each of the Test Pieces 1, and the super-elastic effect was verified in each of the Test Pieces 2. Therefore, it was verified that the manufacturing method according to the present teachings can stably provide a shape memory effect and a super-elastic effect.

[0029] Further, combustion synthesized NiTi alloy was formed into “Test piece 3” (straight material with a diameter of φ2.0 mm and a length of 45 mm as shown in FIG. 2) using a precision casting method. Additionally, NiTi alloy, which had been manufactured by the conventional method, was formed into “Test piece 4” (straight material with a diameter of 42.0 mm and a length of 45 mm) using a precision casting method. A shape memory heat treatment was applied to both Test Pieces 3 and 4. Specifically, the Test Pieces 3 and 4 were held at 460° C. for 60 minutes, and then they were water-cooled.

[0030] Then, the transformation temperature at each point A (i.e., one end of Test piece as shown in FIG. 2) and B (i.e., another end of Test piece as shown in FIG. 2) of Test pieces 3 and 4 were measured by the differential scanning calorimetry (DSC) method, respectively. FIGS. 3 and 4 show the DSC chart of each Test piece in which the peak of the transformation temperature of the Test piece 3 is clearly identified than that of the Test piece 4. This gives the proof that combustion synthesized NiTi alloy is homogeneous in chemical composition and the structure than the NiTi alloy manufactured by the conventional method.

[0031] Finally, although the preferred representative embodiment has been described in detail, the present embodiment is for illustrative purpose only and not restrictive. It is to be understood that various changes and modifications may be made without departing from the spirit or scope of the appended claims. In addition, the additional features and aspects disclosed herein also may be utilized singularly or in combination with the above aspects and features.