METHOD FOR PRODUCING PLATINUM GROUP METAL OR PLATINUM GROUP-BASED ALLOY

Abstract

An object of the present invention is to provide a molten ingot of a platinum group metal or a platinum group-based alloy having a high material yield by suppressing a scattering phenomenon during heating and melting in a method for producing a platinum group metal or a platinum group-based alloy. The method for producing a platinum group metal or a platinum group-based alloy according to the present invention includes a preparing step of weighing a raw material that is partially or entirely of powder and, when the alloy is to be produced, mixing the weighed raw material to obtain a powder mixture, a molding step of molding and solidifying the prepared raw material to obtain molded bodies, a sintering step of sintering the molded bodies to obtain a sintered body, a melting step of melting the sintered body to produce a molten ingot, and a deformation processing step of processing the molten ingot. In the sintering step, the molded bodies are sintered in a stacked state to produce a sintered body as a joined body.

Claims

1. A method for producing a platinum group metal or a platinum group-based alloy, comprising: a preparing step of weighing a raw material that is partially or entirely of powder and, when the alloy is to be produced, mixing the weighed raw material to obtain a powder mixture; a molding step of molding and solidifying the prepared raw material to obtain molded bodies; a sintering step of sintering the molded bodies to obtain sintered bodies; a melting step of melting the sintered bodies using energy beam melting that uses a boat-shaped water-cooled copper crucible having a cavity formed therein to produce a molten ingot; and a deformation processing step of processing the molten ingot, wherein, in the sintering step, a shape and dimensions of the sintered bodies are determined so as to conform to the cavity, and wherein, in the melting step, the sintered bodies conforming to the cavity are densely arranged in the cavity of the boat-shaped water-cooled copper crucible to produce the molten ingot.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0037] FIG. 1 is an illustration of an exemplary sintered body.

[0038] FIG. 2 is an illustration of another exemplary sintered body.

DESCRIPTION OF EMBODIMENTS

[0039] A method for producing an electrode chip of a spark plug for an internal combustion engine is taken as an example and is described more in detail.

[0040] For an electrode chip of a spark plug, an iridium-base alloy or a platinum-base alloy is preferred for use. In this example, a whole quantity of a raw material is of powder, and Ir powder and Pt powder are used.

[0041] (Raw Material Preparing Step)

[0042] Predetermined amounts of the respective powders are weighed so as to obtain predetermined composition, and a V-mixer is used to mix the powders to obtain a uniform powder mixture.

[0043] (Molding Step)

[0044] The powder mixture is charged in a hopper of an automatic press forming machine (uniaxial pressing). A rectangular cavity having short sides of 20 mm and long sides of 50 mm is formed in a molding die, and four corners thereof have an R of 2 mm. The molded body is substantially in the shape of a rectangular parallelepiped having dimensions of 20 mm20 mm50 mm with corners thereof having an R of 2 mm (FIG. 1). A molding pressure is 200 MPa. The molding pressure can be appropriately set, but is preferred to be approximately 120 MPa or higher. When the molding pressure is 200 MPa or higher, a more highly dense molded body having a relative density of about 50% or more can be obtained. As the density of the molded body becomes higher, thermal energy necessary for the sintering can be reduced more, thereby being advantageous. However, an excessively high density may cause breakage of the molded body. Other than this, through charging the powder mixture in a rubber hose, hermetically sealing the rubber hose, and performing CIP mold, a round rod-like molded body can be obtained. Also in this case, a molding pressure is preferred to be 120 MPa or higher, and a molding pressure of about 300 MPa is suitable.

[0045] When the two exemplary molded bodies are molten as they are as in the related art, there can be visually recognized a state in which part of the heated powder and molten metal scatter in the melting furnace to cause sparkling. Further, the strength is to such an extent that a touch with a hand may cause the powder to attach to a finger, and that a corner of the molded body is broken when the molded body is dropped from a height of about 5 cm.

[0046] (Sintering Step)

[0047] Five molded bodies are vertically stacked with surfaces of 20 mm20 mm thereof being upper and lower surfaces, and these are counted as one unit (FIG. 2). Four units are arranged in a setter formed of carbon, and the molded bodies, together with the setter, are inserted into an atmospheric furnace including a carbon heater. Sintering is performed at 1,500 C. for 3 hours under argon airflow. The sintered body undergoes sintering shrinkage, with the result that a raw material rod having a relative density of 70% or more and dimensions of about 16 mm16 mm220 mm is obtained (FIG. 2).

[0048] (Melting Step)

[0049] The raw material rod is horizontally grasped by a raw material rod feeding mechanism of an atmospheric pressure plasma arc melting furnace (pull-down system) and is continuously molten and dripped in an argon atmosphere of from 0.9 atm (atmospheric pressure) to 1.2 atm, and a bottom portion of a water-cooled copper crucible is pulled down. With this, a cylindrical ingot having a diameter of 35 mm is obtained. The state of scattering is not observed during melting, and an effect of the sintering step can be confirmed. Further, although the raw material rod is in a cantilever state at this time, the raw material rod is not broken, and powder does not exfoliate during the melting step.

[0050] (Deformation Processing Step)

[0051] The molten ingot is formed into a square rod by hot forging, and then, into a wire having a substantially rectangular section by hot groove rolling. The molten ingot is further formed into a round wire having a predetermined outer diameter by hot drawing using a die.

[0052] (Cutting Step)

[0053] The round wire is cut into lengths suitable for a wire saw. A plurality of wires are arranged so as to be in parallel with one another, fixed with a resin, and cut by the wire saw, to thereby obtain electrode chips for a spark plug each having a predetermined length.

EXAMPLES

[0054] Further description is given using Examples.

[0055] In Table 1, there are shown results. Evaluation was made in accordance with the following criteria.

[0056] Reduction in mass represents reduction in mass of the molten ingot from the raw material powders at the time of the compounding, and is expressed in percentage. The reduction in mass which is more than 3% was denoted by x, and 3% or less was denoted by .

[0057] With regard to powder exfoliation, when a sintered body or a molded body before being molten was picked up with fingers, and attachment of powder to the fingers was observed, the powder exfoliation was denoted by x. When attachment was not observed at all, the powder exfoliation was denoted by .

[0058] With regard to a molten state, visual observation was made during the melting. When a sparkling-like scattering phenomenon was continually observed, the molten state was denoted by x. When the phenomenon was observed once in a while, the molten state was denoted by . When the phenomenon was hardly observed, the molten state was denoted by .

[0059] With regard to comprehensive judgment, these results were taken into consideration. When the effect of the present invention was not recognized, the comprehensive judgement was denoted by x. When the effect was recognized, the comprehensive judgement was denoted by . When the effect was more considerable, the comprehensive judgement was denoted by .

Example 1

[0060] Example 1 represents an example of the description above (best mode for carrying out the invention).

[0061] With regard to the density of the molded body calculated from the dimensions and the mass, the relative density was 52%. The density of the sintered body was 74%. Using the sintered body as the raw material rod, a molten ingot having a diameter of about 35 mma length of L 150 mm was manufactured.

[0062] When visual observation was conducted during the melting (under a pressure of 1.110.sup.5 Pa), the scattering phenomenon was not at all observed. The reduction in mass of the molten ingot from the raw material preparing step was 0.6% or less. Further, after the sintering until the melting was completed, the raw material rod was not broken or exfoliated.

[0063] Almost no scattered material was left in the furnace after the melting, and attachment of the scattered material to the water-cooled copper crucible was not recognized.

Example 2

[0064] In Example 2, the molded body was manufactured as in the case of Example 1. The molded body was substantially in the shape of a rectangular parallelepiped having dimensions of 20 mm20 mm50 mm with corners having an R of 2 mm. Such molded bodies were individually sintered without being stacked to manufacture sintered bodies of about 16 mm16 mm44 mm. The sintered bodies were placed on a boat-shaped water-cooled copper crucible and were molten by vacuum plasma melting to manufacture a molten ingot of about 15 mm30 mm100 mm. The pressure during the melting was adjusted to be 510.sup.1 Pa (Ar).

[0065] In visual observation during the melting, occasional scattering was observed. A small amount of the scattered material was left in the furnace after the melting, with part thereof being attached to the water-cooled copper crucible.

[0066] The reduction in mass of the molten ingot was 2.5%. Further, with regard to the shape of the molten ingot, the bottom portion was approximately smooth along the shape of the boat-shaped water-cooled copper crucible. However, there was a burr on a side surface, and the upper surface was solidified in a wavy state.

Comparative Example 1

[0067] In Comparative Example 1, after the raw material powder was mixed using a V-mixer, the CIP method was used to manufacture a cylindrical molded body having a diameter of 30 mm. The molding pressure was 300 MPa. With regard to the density of the molded body calculated from the dimensions and the mass, the relative density was 48%. The molded body was cut into lengths of about 30 mm, placed on a boat-shaped water-cooled copper crucible, and molten by arc melting to manufacture a molten ingot of about t 15 mmw 30 mmL 100 mm. The pressure during the melting was adjusted to 810.sup.4 Pa (Ar).

[0068] The strength of the molded body was not low to such an extent that a touch by a hand causes the molded body to be broken. However, when the molded body was taken out of a CIP die, powder attached to a finger, and powder attached to an inner wall of the CIP die could be confirmed.

[0069] In visual observation during the melting, continuous scattering from a molten portion until the molded body was entirely molten away was confirmed. The scattered material was left in the furnace after the melting, and attachment thereof to the water-cooled copper crucible was conspicuous. Further, the scattered material and powder that exfoliated from the molded body were left in a corner of the bottom portion of the boat-shaped water-cooled copper crucible. As described above, part of the compounded raw material powders was left unmolten and the reduction in mass of the molten ingot from the raw material preparing step was 3.2%.

[0070] With regard to the shape of the molten ingot, there were a burr and waviness as in the case of Example 2.

Comparative Example 2

[0071] In Comparative Example 2, a molded body manufactured as in the case of Comparative Example 1 was placed on a boat-shaped water-cooled copper crucible, and a molten ingot of about t 15 mmw 30 mmL 100 mm was manufactured by vacuum plasma melting. The pressure during the melting was adjusted to 510.sup.1 Pa (Ar).

[0072] In visual observation during the melting, continuous scattering from a molten portion until the molded body was entirely molten away was confirmed. More scattered material was left in the furnace after the melting, and attachment thereof to the water-cooled copper crucible was more conspicuous. Further, the scattered material and powder that exfoliated from the molded body were left in a corner of the bottom portion of the boat-shaped water-cooled copper crucible. As described above, part of the compounded raw material powders was left unmolten, and the reduction in mass of the molten ingot from the raw material preparing step was 4.5%.

[0073] From the results described above, it was confirmed that, in a method of directly melting a molded body without performing the sintering step, the reduction in mass was larger, and the material yield was lower, whereas, in melting the sintered body in the method according to the present invention, these were greatly improved.

TABLE-US-00001 TABLE 1 Experimental Results Reduction Powder Molten Comprehensive No. in Mass Exfoliation State Judgment Example 1 Example 2 Comparative x x x x Example 1 Comparative x x x x Example 2