CU-ALLOY MOLD FOR USE IN CENTRIFUGAL CASTING OF TI OR TI ALLOY AND CENTRIFUGAL-CASTING METHOD USING THE MOLD

Abstract: In a Cu-alloy mold for use in centrifugal casting of Ti or Ti-alloys, the mold body is made of a Cu alloy satisfying the following relationship: Ts + 0.3ρ ≧ 70 where Ts is the tensile strength (kg/mm²), and ρ is the electrical conductance (% IACS). A cavity disposed in the mold body has the volume which is at most 30% of the volume of the mold body. Also disclosed is a method for centrifugally casting Ti or Ti-alloy by the use of the above-described mold.

Description: BACKGROUND OF THE INVENTION The present invention relates to a Cu-alloy mold for use in centrifugal casting of Ti and Ti alloys superior in dimensional accuracy, to a centrifugal casting method using the mold and to a mold apparatus. Ti or Ti alloy has widely been used in many fields as Ti or Ti-alloy castings, because of their superior corrosion resistance and specific strength. Ti or Ti alloys cannot be cast in normal or usual foundry sand mold, because they are chemically active. For this reason, for casting Ti or Ti alloys, used are a graphite mold, a precision casting mold, utilizing specific ceramics, typically represented by a lost-wax process, and the like. Further, a water-cooled Cu mold or the like is also used for casting of the Ti or Ti alloy.. However, the above-described graphite mold is expensive, and lacks dimensional precision. Furthermore, since the precision casting mold represented by the lost-wax process must employ expensive ceramics, use of the precision casting mold increases the cost of production. On the other hand, the water-cooled mold is a mold which is most suitable for mass production as a general continuous casting mold. Casting of the Ti or Ti alloy by the use of the water-cooled mold is considered to enable Ti or Ti-alloy castings superior in dimensional precision, to be produced. However, the following problems arise. That is, if the water-cooled mold is to be used as a centrifugal casting mold, it is difficult to construct water-cooling mechanisms because the latter is rotated during the centrifugal casting. If molten Ti or molten Ti alloy is cast in the Cu mold without water cooling, the Cu mold is considered to be heated to temperatures in excess of the heat-resistant temperature. For this reason, melting loss or seizure will occur on a surface of a cavity in the Cu mold into which the Ti or Ti alloy is cast. Further, breakage or deformation will occur in the Cu mold body due to thermal stresses. Thus, it is impossible to produce Ti or Ti-alloy castings which are superior in dimensional accuracy. Apart from the above, where a precision product made of a titanium alloy is to be manufactured, it is general that, since the titanium alloy is high in melting point and is reactive, a mold made of ceramics is used to cast the product. Where relatively small components, for example, valve heads or the like used in engines are mass produced, of a titanium alloy, the following method has been taken. That is, in the method, a plurality of molds made of ceramics each having a plurality of molding cavities are stacked one upon the other vertically in a manner of a plurality of stages or steps, and the molds are rotated about a vertical central axis (trunk) to cause a centrifugal force to act upon molten metal within the cavities, thereby spreading the molten metal to every nooks and corners of the cavities(branches), to form the valve heads. By this method, a cast intermediate article is produced in which the plurality of valve heads are molded at forward-end portions of the branches. The reason why the aforesaid casting method can be carried into practice is that, after casting, the molds made of ceramics are disposable. The inventors of this application have considered that, in place of the throw away ceramic molds, copper alloy molds usable repeatedly should be used to cast titanium-alloy products. The copper alloy molds when used as described above, the following problems arise. That is, when the products are to be taken out after casting, even if each of the molds is made as a book mold, the mold halves will interfere with a cast article. Thus, the mold halves cannot be opened sufficiently to take out the products. In the state of the art it is already known to use a Cu alloy mold for centrifugally casting Ti or Ti alloys, comprising a split-type mold consisting of two blocks of beryllium-copper. The mold surfaces are not attacked or eroded by the titanium (Metal Progress. 63 (1953.03) 3,72; O.W. Simmons, A Method of Centrifugally Casting Titanium). SUMMARY OF THE INVENTION It is an object of the invention to provide a further Cu-alloy mold which can be used in centrifugal casting of Ti or Ti-alloy castings, which provides castings of high dimensional accuracy, in quantities larger than, and at a lower cost than the conventionally prepared castings. It is another object of the inventicn to provide a method of centrifugally casting Ti and Ti-alloys by the use of the above-mentioned Cu-alloy mold. It is still another object of the invention to provide a mold apparatus in which, even in a plurality of molds which are not discarded after use, it is possible to practice, a casting method in which the plurality of molds are stacked one upon the other vertically in stages or steps. According to the invention, there is provided a Cu-alloy mold for use in centrifugal casting of Ti and Ti-alloys, comprising a mold body having defined therein a cavity, wherein the mold body is made of a Cu alloy characterized in that said Cu alloy satisfies the following relationship: Ts + 0.3 ρ ≧ 70 where Ts is the tensile strength (kg/mm²) and ρ is the electrical conductance (% IACS), that the volume of said cavity is at most 30% of volume of the mold body, and that said Cu alloy is one selected from a group consisting of a Cu-0.2%Zr alloy, a Cu-0.9% Cr-0.2%Zr alloy, a Cu-2%Be-0.3%Co alloy, a Cu-1% Cr alloy, a Cu-1% Ag alloy and a Cu-0.5% Be-2,5%Co alloy. With the arrangement of the invention, in the Cu alloy for manufacturing the centrifugal casting mold, the higher the conductance, being related to the thermal conductivity, the smaller the increase in temperature of the surface of the cavity in the mold. Accordingly, since thermal stress is also small, a Cu alloy low in strength becomes difficult to be deformed. On the contrary, however, if a copper alloy low in conductance is used as a mold material, high strength is required because of high thermal stress. Thus the above-defined copper alloy satisfying the above empirical equation (1) is employed as a material of the centrifugal casting mold. These above-defined copper alloys are superior in tensile strength, and are relatively superior in conductance. Since the above copper alloys satisfy the above equation (1), when molten metal of the Ti or Ti alloy is centrifugally cast into the mold made of such an alloy, the surface of the cavity in the Cu-alloy mold, is heated instantaneously. Since, however, the mold is superior in heat conduction. heat is conducted away in a short period of time and temperature of the cavity surface is reduced quickly. Thus, erosion loss or seizure of the cavity surface does not occur. If the volume of the cavity in the Cu-alloy mold becomes larger than 30% of the volume of the Cu-alloy mold body, the heat can be accumulated at a portion of the Cu-alloy mold. Thus, there may be a case where the mold becomes overheated so that thermal deformation or thermal breakage occurs. Accordingly, it is required that the volume of the cavity should be equal to or less than 30% of the volume of the mold body. Thus, according to the invention, the use of the mold made of the above-mentioned Cu-alloys , permits centrifugal casting of Ti or Ti-alloys as defined in claim 2 with superior dimensional accuracy for a long period of time. Accordingly, it is possible to provide the Ti or Ti-alloy castings at cost lower than the conventional castings. According to the invention, there is also provided a mold apparatus comprising: at least two Cu-alloy molds for use in centrifugal casting of one of Ti and Ti-alloy, the molds being stacked vertically one upon the other, each of the molds being composed of a pair of upper and lower mold halves, each of the upper and lower mold halves being capable of being split into a plurality of mold sections, each of the molds comprising a mold body having defined therein at least one cavity, wherein the mold body is as defined above; at least one spacer means interposed between the molds, the spacer means being capable of being split into a plurality of spacer sections; and means for fixing the molds and the spacer means to each other under such a condition that the molds and the spacer means are stacked one upon the other. Prefered embodiments of the mold apparatus are defined in the subclaims In order to practice casting by the mold apparatus according to the invention, the molds and the spacer means are first stacked one upon the other vertically, and are fixed to each other by the fixing means. Molten metal is poured into the cavity in each of the molds, to mold desirable castings. In order to take out the products, the uppermost spacer means is divided to the right and left, and is drawn out of the mold apparatus. By doing so, a space corresponding to the spacer means is created at a location below the lower mold half of the uppermost mold. The lower mold half is moved vertically by the utilization of the space. Thus, it is possible to remove the lower mold half from casting. As there is initially nothing at a location above the upper mold half of the uppermost mold, the upper mold half can freely be moved vertically upwardly, making it possible to easily remove the upper mold half from the casting. The above-described operation is successively repeated, whereby it is possible to remove all the molds from the castings without breakage of the molds. Thus, castings are produced as in a tree similarly to the case of using the conventional ceramics molds. As described above, according to the invention, even for the molds which are not disposed of after casting, the casting method can be applied so that the molds are stacked one upon the other vertically. Thus, it is made possible to cast articles in the form of a tree. This keeps the door open to mass-production of titanium-alloy castings, which employs the molds made of, for example, the copper alloy. Accordingly, the advantages of the mold apparatus according to the invention are extremely high. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a longitudinal cross-sectional view of a mold apparatus according to an embodiment of the invention; Fig. 2 is a top plan view of one of a plurality of molds illustrated in Fig. 1; and Fig. 3 is a top plan view of one of a plurality of spacers illustrated in Fig. 1. DESCRIPTION OF THE EMBODIMENTS Various embodiments of the invention will be described in detail with reference to the drawings. In view of the problems discussed in the aforesaid background of the invention, the inventors of the present application have conducted studies and researches in order to centrifugally cast Ti or Ti-alloy castings superior in dimensional accuracy, in quantities larger than, and in cost lower than the conventional castings. As a result, the following knowledge has been obtained: (a) A centrifugal-casting mold is manufactured by the use of a Cu alloy which is superior in tensile strength and electrical conductance (mutually related to heat conduction), specifically, by the use of a Cu alloy which is one selected from the group of Cu alloys as defined above and which satisfies the the following relationship: Ts + 0.3ρ ≧ 70 where Ts is the tensile strength (kg/mm²), and ρ is the electrical conductance (% IACS), and (b) The centrifugal-casting mold has defined therein the cavity whose volume is equal to or less than 30% of the volume of the mold body, that is, the following relationship is satisfied: Volume of Cavity Volume of Mold Body x 100 ≦ 30% By simultaneously satisfying the above conditions (a) and (b), it is possible to reuse the mold many times without incurring any of erosion, seizure and deformation of the surface of the Cu-alloy mold. A centrifugal-casting mold was prepared which consisted of a Cu alloy whose component composition was illustrated in Examples 1 through 6 and Comparative Examples 1 through 7 in the table 1, and which had the indicated percentage of the cavity volume with respect to the volume of the mold body. The centrifugal-casting mold was set in a centrifugal casting apparatus which was located within a vacuum chamber. The centrifugal-casting mold was rotated at 500 r.p.m. On the other hand, a Ti alloy was prepared whose composition consisted of Ti-6%Al-4%V (% is wt%). The Ti alloy was plasma-arc melted at 2000 A. The Ti alloy melt at 1750°C, was centrifugally cast in the centrifugal-casting mold which was rotated at 500 r.p.m. The above-described centrifugal casting was repeated fifty (50) times. Subsequently, presence of erosion, seizure and deformation of the centrifugal-casting mold was observed. The results of the observation were shown in the below table 1. As will be clear from the results in the table 1, the centrifugal-casting mold made of the Cu alloy, which satisfies the condition according to the invention, does not cause melting loss, seizure and deformation to occur even after having centrifugally cast the Ti alloy fifty (50) times. However, at least one of the erosion, seizure and deformation problems occurs in the centrifugal casing mold which is made of a Cu alloy, which does not satisfy the equation (1) referred to in the aforementioned background of the invention, under the condition according to the invention. Further, as will be clear from the Comparative Examples 3 and 4, even in the mold made of the copper alloy satisfying the above-mentioned equation (1), if the volume of the cavity exceeds 30% of the volume of the mold body, deformation will occur in the mold. This is not desirable. Referring to Fig. 1, there is shown, in a longitudinal cross-sectional view, a mold apparatus according to an embodiment of the invention. In this connection, the mold apparatus comprises a plurality of molds each of which is similar in construction to the mold described previously. Thus, the description of the molds will be omitted. As shown in Fig. 1, a plurality of molds 1a through 1f is stacked one upon the other vertically. A spacer 2 is interposed between each pair of adjacent molds. A plurality of, four (4) in the illustrated embodiment, bolts 3 is passed through peripheries of the molds 1a through 1f and the spacers 2. By the bolts 3, the molds 1a through 1f and the spacers 2 are fixed together under such a condition that the molds and the spacers 2 are alternately stacked one upon the other vertically. Each of the molds 1a through 1f is composed of two mold halves 11 and 12 which are stacked one upon the other. As illustrated in Fig. 2, the mold is capable of being split into a plurality of, two in the illustrated embodiment, mold sections 21a and 21b which are substantially identical in dimension with each other. The mold has its disc-like configuration in plan, for example. The mold comprises a mold body 23 which has defined therein at least one, twelve (12) in the illustrated embodiment, cavity 13 for forming an engine valve head for a vehicle. That is, the mold body 23 of each of the molds 1a through 1f has defined therein the plurality of cavities 13 which extend radially and which are spaced radially from each other through a predetermined angle. In connection with the above, each of the intermediate molds 1a through 1f and the uppermost mold 1a are formed therein with respective through bores 14 for a sprue runner. The through bores 14 communicate with the cavities 13. The through bore for the sprue runner is not required for the lower mold half 12 of the lowermost mold 1f, and a normal or usual lower mold half is utilized for the lower mold half 12 of the lowermost mold 1f. Each of the spacers 2 is interposed between a corresponding pair of adjacent molds. The spacer 2 is identical in configuration in plan with the mold. The mold and the spacer 2 are substantially identical in diameter with each other. The spacer 2 is capable of being split into a plurality of, two (2) in the illustrated embodiment, spacer sections 2a and 2b which are substantially identical in dimension with each other, as illustrated in Fig. 3. The spacer 2 is formed with a through bore 22 for a sprue runner, which extends through a central portion of the spacer 2. The spacer 2 has its thickness slightly larger than that of the cavity 13. Specifically, the thickness t₀ of the spacer 2 is set to a value slightly larger than the thickness (t₁ + t₂) of the cavity 13 in the upper and lower mold halves 11 and 12 of the mold, such that, after the spacer 2 has been drawn out of the mold apparatus under the assembled condition to form a space, the upper mold half 11 or the lower mold half 12 of the mold is moved upwardly or downwardly by the utilization of the space so that the right and left mold sections 21a and 21b of the upper or lower mold half 11 or 12 can easily be drawn to the left and right. In connection with the above, the molds 1a through 1f and the spacers 2 are formed therein with a plurality of bolt inserting bores 15 and 25, respectively, as shown in Figs. 2 and 3, through which the connecting bolts 3 are inserted. The case where the valve head is cast by the aforementioned mold apparatus will next be described. First, the mold apparatus shown in Fig. 1 is manufactured. Specifically, the mold 1f, which is usually employed at the lowermost layer, is first installed. Subsequently, the spacer 2 is stacked upon the upper face of the lowermost mold 1f. The mold 1e having the through bore 14 for the sprue runner is stacked upon the upper face of the spacer 2. The procedure is repeated to alternately stack the molds 1d to 1a and the spacers 2 one upon the other. Lastly, the fastening bolts 3 are inserted through the stacked molds 1a through 1f and spacers 2 and are tightened by nuts so that the molds 1a through 1f and the spacers 2 are connected to each other and are united together. In the thus assembled mold apparatus, molten metal of the titanium alloy is poured into and flowed through the through bores 14 and 22 for the sprue runner, and the mold apparatus is rotated on a rotating table within a casting furnace, to centrifugally cast titanium-alloy products. After cooling, the mold apparatus is removed out of the casting furnace. The bolts 3 are removed from the stacked molds 1a through 1f and spacers 2. The upper mold half 11 of the uppermost mold 1a is moved upwardly and is removed from the lower mold half 12 of the uppermost mold 1a. Subsequently, the spacer sections 2a and 2b of the uppermost spacer 2 are moved to the left and right horizontally, and are drawn out of the remaining molds 1b through 1f and spacers 2. By doing so, a space corresponding to the drawn spacer 2 occurs at a location below the uppermost mold 1a. Thus, it is possible to move the lower mold half 12 of the uppermost mold 1a downwardly to remove the lower mold 12 from the casting. Subsequently, the pair of mold sections 21a and 21b of the lower mold 12 are also moved to the left and right, and are drawn horizontally from the casting. The above-described procedure is repeated, and the lowermost mold 1f is removed from the casting. Thus, similarly to the case of using the ceramics molds, it is possible to produce an intermediate casting article in the form of a tree. In connection with the above, in the above-mentioned embodiment, the number of the molds should not be limited to six (6) as in the illustrated embodiment, but any number of the molds can be used.

Claims: A Cu-alloy mold for use in centrifugal casting of one of Ti and Ti-alloy, comprising a mold body having defined therein a cavity, wherein said mold body is made of a Cu alloy, characterized in that: said Cu-alloys satisfy the following relationship: Ts + 0.3 ρ ≧ 70 where Ts is the tensile strength (kg/mm²), and ρ is the electrical conductance (% IACS); that the volume of said cavity is at most 30% of volume of said mold body, and that said Cu-alloy is one selected from a group consisting of a Cu-0.2%Zr alloy, a Cu-0.9% Cr-0.2%Zr alloy, a Cu-2%Be-0.3%Co alloy, a Cu-1%Cr alloy, a Cu-1%Ag alloy and a Cu-0.5% Be-2.5%Co alloy. A method of centrifugally casting one of Ti and Ti-alloy by the use of a Cu-alloy mold which comprises a mold body having defined therein a cavity, wherein said mold body is made of a Cu alloy, characterized in that: a Cu alloy is used as said mold body satisfying the following relationship: Ts + 0.3 ρ ≧ 70 where Ts is the tensile strength (Kg/mm²), and ρ is the electrical conductance (% IACS); that the volume of said cavity is at most 30% of volume of said mold body; and that said Cu alloy is one selected from a group consisting of a Cu-0.2%Zr alloy, a Cu-0.9%Cr-0.2%Zr alloy, a Cu-2%Be-0.3%Co alloy, a Cu-1%Cr alloy, a Cu-1%Ag alloy and a Cu-0.5% Be-2.5%Co alloy. A mold apparatus comprising: at least two Cu-alloy molds for use in centrifugal casting of one of Ti and Ti-alloy, said molds being stacked vertically one upon the other, each of said molds being composed of a pair of upper and lower mold halves, each of the upper and lower mold halves being capable of being split into a plurality of mold sections, each of said molds comprising a mold body having defined therein at least one cavity, wherein: said mold body is as defined in claim 1; at least one spacer means interposed between said molds, said spacer means being capable of being split into a plurality of spacer sections; and means for fixing said molds and said spacer means to each other under such a condition that said molds and said spacer means are stacked one upon the other. The mold apparatus according to claim 3 wherein each of the upper and lower mold halves of each of said molds is capable of being split into two mold sections substantially identical in dimension with each other, and said spacer means is capable of being split into two spacer sections substantially identical in dimension with each other. The mold apparatus according to claim 3 or 4 wherein the mold body of each of said molds has defined therein a plurality of cavities which extend radially and which are spaced radially from each other through a predetermined angle. The mold apparatus according to any one of the claims 3 to 5 wherein each of said molds has its disc-like configuration in plan, and said spacer means is identical in configuration in plan with the mold, and wherein the mold and said spacer means are substantially identical in diameter with each other. The mold apparatus according to any one of the claims 3 to 6, wherein said spacer means has its thickness which is set to a value slightly larger than that of said cavity.

Inventors: YOSHIHARU, MAE, YOSHIHARU, MAE, C/O CHUO-KENKYUSHO
Applicants: MITSUBISHI MATERIALS CORPORATION
Application year: 1991
Main classification: B22D13
IPC codes: B22D21, B22D13