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

Abstract

A process for producing a product from a platinum group metal or a platinum group-based alloy, comprising: a preparation step in which a starting material consisting partly or wholly of powder is weighed and, if the product is to be made from the alloy, the weighed starting material is mixed to obtain a powder mixture; a forming step in which the prepared starting material is shaped and solidified to obtain shaped bodies which are substantially in the shape of a rectangular parallelepiped; a sintering step in which the shaped bodies are sintered using a furnace to obtain a sintered body, wherein the sintering is carried out at a temperature of 1000°C or more and does not exceed the melting point of the starting material; a melting step in which the sintered body is melted to produce a melting ingot, wherein a pressure during melting is 1 Pa or more; and a deformation processing step in which the melt ingot is plastically processed, wherein the shaped bodies are sintered in a stacked state in the sintering step to produce the sintered body as a joined body, and where in the melting step a plasma arc melting furnace with a water-cooled copper crucible is used, which has a continuous cavity, in the cavity a molten bath of a metal melt of the sintered body is formed and a bottom portion of the molten pool is pulled down in the cavity to produce the melt ingot.

Description

Technical area

[0001] The invention relates to a method for producing a platinum group metal or a platinum group-based alloy, and more particularly to the production of a melting ingot in a method for producing a platinum group metal or a platinum group-based alloy. State of the art

[0002] A platinum group metal or platinum group-based alloy is designed by utilizing the heat resistance, oxidation resistance, and chemical resistance of the platinum group metal and is widely used as a high-temperature component or corrosion-resistant product. As used herein, the platinum group metal refers to Pt, Pd, Rh, Ir, Ru, and Os.

[0003] The processes for producing a platinum group metal or a platinum group-based alloy include a compounding step, a melting step, a deformation processing step, and the like for an alloy raw material. In the melting step, a melting method for producing a melting ingot can be divided into several types. A platinum group metal, as its main component, has a very high melting point (1500°C or higher), and therefore, an induction heat melting furnace or an energy beam melting furnace with a melting capacity of 2000°C or higher is used.

[0004] Energy beam melting includes non-consumable arc melting, vacuum plasma melting, atmospheric pressure plasma arc melting, electron beam melting, and the like. These melting processes have in common that they are carried out by irradiating an energy beam onto a feed material in a water-cooled copper crucible. The melt feed material is in the form of a plate, wire, powder, or the like, including an ingot and scrap, and is mixed in a predetermined amount to suit the application.

[0005] Energy beam melting is generally divided into two types according to the methods used with the water-cooled copper crucible. One method uses a boat-shaped water-cooled copper crucible. The boat-shaped water-cooled copper crucible is a water-cooled copper crucible that has a cavity (a hollow) in the shape of a circle, a rectangle, or the like, and the entire amount of a starting material placed in the cavity is melted to obtain a melt ingot (JP 2002 - 105631 A).

[0006] Another method is the use of a water-cooled copper crucible with a continuous cavity. In this method, a starting material rod is held horizontally as the starting material. One end of the starting material rod is exposed to an energy beam to melt it, and the molten metal continuously drips down, forming a molten pool in the cavity that receives the dripping molten metal. A bottom portion of the molten pool is continuously drawn down, yielding a rod-like melting ingot (JP 2009-93881A). The starting material rod is generally produced by first melting the starting material.

[0007] If the melt starting material consists partially or entirely of powder, the powdered starting material is melted as it is, with the starting material being thrown up or dispersed due to the energy beam flow. To prevent the powder from being thrown up, the powder is sometimes previously compression-molded using a molding process such as compression molding or cold isostatic pressing (JP 2004 - 137 580 A).

[0008] In the molded powder, the particles are in contact and entangled with each other, forming a uniform exterior. Therefore, even when an energy beam is irradiated, the particles are prevented from being blown away or thrown up. A molded body generally has a relative density of about 30% to about 60% and contains a significant amount of voids, with atmospheric gas or gas residue present in the voids.

[0009] In addition, the molded body only has a uniform exterior. This makes the molded body easily breakable due to impact caused by dropping or other factors, and also causes powder to detach from a surface of the molded body during transportation, thereby deteriorating the material yield. As used herein, material yield refers to the ratio of the mass of the melt ingot to the mass of the melt feedstock.

[0010] When an energy beam is radiated onto the mold, the mold is heated by heat transfer, radiant heat, and Joule heat, causing the mold temperature to rise suddenly, especially at the irradiated portion. The gas present in the gaps suddenly expands, and the particles that are only externally joined are flicked outward toward the water-cooled copper crucible. At the same time, the partially molten metal is also flicked outward. This correspondingly reduces the mass of the ingot. This means that the material yield deteriorates, causing a significant economic loss in the production of a very expensive platinum group metal.

[0011] If the starting material is a powder mixture, not only will the material yield deteriorate, but the composition may also change. If the mold breaks and a piece falls off, powder sheds on the surface of the mold, or the molten metal is scattered during melting, a component contained in that part will not be included in the ingot, and thus a precise alloy composition cannot be achieved. In addition, in energy beam melting, which uses the boat-shaped, water-cooled copper crucible, the energy beam is emitted from above for melting. However, the starting material is generally repeatedly rotated from top to bottom and also melted from the opposite side to obtain an ingot with a uniform composition.In the water-cooled copper crucible, a fallen piece or detached powder may remain unmelted in a corner of the boat-shaped cavity. This can also prevent the alloy composition from being precise.

[0012] J.-M. Oh et al., "Preparation and Purity Evaluation of 5N-Grade Ruthenium by Electron Beam Melting," Materials Transactions, Vol. 53, 2012, No. 9, pp. 1680-1684, proposes sintering a preform made of ruthenium powder prior to electron beam melting. The resulting product is a button-shaped ingot, which is further processed into sputtering targets.

[0013] US 3 416 919 A ​​proposes a process by which shaped bodies of different shapes and compositions can be sintered together to form an elongated electrode. Brief description of the inventionTechnical problem

[0014] The invention has been made in view of the above-described problems in the prior art, and it is an object of the invention to provide a method for producing a platinum group metal or a platinum group-based alloy which has a high material yield. Solution to the problem

[0015] According to the invention, a process for producing a platinum group metal or a platinum group-based alloy is provided, which comprises the features of the patent claim.

[0016] The preparation step listed in the patent claim is a step in which the starting material is weighed according to a desired amount of melting ingot. When the alloy is to be produced, each starting material is weighed to achieve a predetermined alloy composition. The starting material can have any desired shape. However, the starting material consists at least partially or entirely of powder.

[0017] The molding step is a step in which the raw material, which consists partly or entirely of powder in the total amount of the raw material, is shaped and solidified to obtain molded articles. A well-known dry molding method such as uniaxial pressing, tabletting, cold isostatic pressing, or rubber molding is suitable for this purpose. The shape can be appropriately selected from a disc / cylinder shape, a sharp-edged flat figure shape including a polygon / prism, a briquette with no regular shape, and the like. The number of molded articles can be determined depending on the shape and dimensions of the water-cooled copper crucible, and can be one or more.

[0018] The sintering step is a step in which sintering takes place to largely unite the shaped bodies. A well-known firing furnace such as a gas furnace or an electric furnace can be used for this purpose, and both batch and continuous firing are suitable. The firing temperature can be appropriately selected depending on the type of raw material, but for a platinum group metal or a platinum group-based alloy with a melting point above 1500°C, a range of 1000°C or more, which does not exceed the melting point of the raw material, is more suitable. An inert gas or a vacuum can be used as the firing atmosphere, and this is appropriately selected depending on the type of raw material. In the sintered body, individual particles are combined, which increases the strength and increases the density through sintering shrinkage.The relative density is not particularly limited, but it is preferred that the relative density be 60% or more, and it is even more preferred that the relative density be 70% or more.

[0019] With such a sintered body, the particles are united, increasing strength and preventing a piece of the molded body from falling off and the powder from separating. Furthermore, the occurrence of sputtering during melting can be suppressed, and therefore changes in the alloy composition can also be suppressed.

[0020] When the molded article is used as a raw material rod in a pull-down process in a melting step as described above, the molded article may break during melting due to insufficient strength. Furthermore, the molded article may collapse under low force. Therefore, it is difficult to grip the molded article with a raw material rod feeding mechanism and use the molded article as it is. According to the present invention, the particles are combined by sintering, and high strength is achieved. Therefore, the molded article can be used as a raw material rod without the risk of breakage and collapse.

[0021] In the sintering step, a plurality of molded bodies in a stacked state can be sintered as a joined body to form the sintered body. Specifically, when the molded bodies are sintered in a stacked state in the sintering step, the particles in the individual molded bodies and also the particles in contact with each other at an interface between the stacked molded bodies are sintered and joined. In this way, a rod-like sintered body (a joined body) can be obtained. This has the advantage that, by appropriately selecting the dimensions and the number of molded bodies to be stacked, changes can be made as needed from a very small and short raw material rod to a long raw material rod. The rod-like sintered body is particularly suitable for use as a raw material rod in a melting step of the pull-down system.

[0022] Regarding the known starting material rod, before the melting step, a melting ingot is prepared in an energy beam melting furnace using a boat-shaped, water-cooled copper crucible, and the melting ingot is used as a long starting material rod. The melting ingot thus produced has an irregular shape. Specifically, the shape of the boat-shaped, water-cooled copper crucible is transferred to a bottom portion of the melting ingot, so the bottom portion has a regular shape, but the side surface and a top surface of the melting ingot are in the form of the solidified molten metal as it is. During melting, when the latent heat at constant volume is as high as in the case of a platinum group metal or a platinum group-based alloy, the molten metal is likely to solidify immediately after the molten metal separates from the energy beam (heat source).Therefore, the appearance of a burr on one side surface and a waviness on the top of the ingot are obvious, and the starting material bar has an indeterminate cross-sectional area. The latent heat at constant volume (kJ / cm) 3 ) is, as used here, the latent heat required to melt a substance per unit volume, and it is determined by the heat of fusion (kJ / mol), the molar mass (g / mol) and the density (g / cm 3 ) is defined.

[0023] When drawing down melting is performed with such a starting material rod, it is difficult to drip the molten metal down at a constant rate. Therefore, in a section with a small cross-sectional area, the molten metal to be dropped down will be insufficient, which is likely to result in defects such as pores in the melting ingot. In a section with a large cross-sectional area, the molten metal to be dropped down will be excessive, which is likely to result in the problem of the molten metal dripping down from the cavity in the water-cooled copper crucible and solidifying.

[0024] According to the present invention, molded bodies with fixed dimensions can be sintered in the sintering step, and the molded bodies can be used as a raw material rod with fixed dimensions, thus eliminating such problems. Furthermore, the production of a raw material rod requires special melting equipment (melting furnace, crucible, etc.). However, according to the present invention, such equipment is not necessary, and a conventional electric furnace or the like can be used to very conveniently produce the raw material rod (sintered body).

[0025] The mold used for the starting material rod can have any suitable shape. However, if the mold is formed into a substantially rectangular parallelepiped by uniaxial pressing, molding is particularly easy. Furthermore, such a shape is very convenient for stacking the molds during the sintering step.

[0026] Furthermore, during energy beam melting, the furnace pressure varies depending on the melting method and the melting source material (from high vacuum to atmospheric pressure). In particular, an electron beam melting furnace requires a high vacuum range of 0.1 Pa or less. When the vacuum is as high as described above, the pressure difference for a gas component remaining in the voids in the sintered body can be large, resulting in almost imperceptible atomization. Therefore, the appropriate furnace pressure for melting is 1 Pa or more.

[0027] The melting step is a step in which the melting ingot is produced by using the sintered body as the starting material.

[0028] Furthermore, a plasma arc melting furnace with a water-cooled copper crucible having a continuous hollow space is used in the melting step. One end of the rod-like sintered body (the bonded body) as the starting material rod is exposed to an energy beam (plasma arc) to be melted, and the molten metal is continuously dripped down, thereby forming a molten pool in the cavity that receives the dripping molten metal. A bottom portion of the molten pool is continuously drawn down to obtain the rod-like melting ingot. Specifically, the drawing-down system is suitable for using the rod-like sintered body (the bonded body) as the starting material rod in the melting step.

[0029] The deformation processing step is a step in which the ingot is worked into a desired shape, such as a plate or wire, using a well-known method. The deformation processing of the ingot produced according to the invention can be performed in the same way as in the case of a known ingot obtained without the sintering step.

[0030] For example, when working into a plate, forging and rolling are performed. When working into a wire, forging, grooving, and wire drawing are performed. Depending on the extent of work hardening, in each of these cases, heat treatment is performed midway if necessary to achieve softening. After working into a plate or wire, processing such as cutting, bending, or welding can be performed depending on the intended use. Regarding the specific processing, both cold working and hot working, in which a material is heated during processing, are applicable. Advantageous effects of the invention

[0031] As described above, according to the manufacturing method of the present invention, compared with a known manufacturing method, atomization of a raw material during melting is effectively suppressed, and the material yield of an expensive platinum group metal or an expensive platinum group-based alloy can be improved.

[0032] In addition, the sintered body has higher strength than the molded body and is less likely to break, thus preventing powder from shedding during transportation. This feature is advantageous in that it prevents part of the raw material from falling or dissolving, thus preventing compositional changes. It also has the advantage that the sintered body, when used as a raw material rod, can be easily carried and gripped by an apparatus.

[0033] Another advantage is that the sintered body has a higher density compared to the molded body, i.e. a smaller volume for the same mass, and thus more starting material can be introduced into the water-cooled copper crucible, which contributes to improving productivity. Short description of the drawings Fig. 1 is a representation of an exemplary sintered body. Fig. Figure 2 is a representation of another exemplary sintered body. Description of implementation examples

[0034] As an example, a method for manufacturing an electrode pad of a spark plug for an internal combustion engine is taken and described in more detail.

[0035] For a spark plug electrode pad, an iridium-based alloy or a platinum-based alloy is preferred. In this example, the entire amount of a raw material is powder, and an Ir powder and a Pt powder are used. - Source material preparation step -

[0036] Predetermined amounts of each powder are weighed to achieve a predetermined composition, and a V-blender is used to mix the powders to achieve a uniform powder mixture. - Forming step -

[0037] The powder mixture is placed in the hopper of an automatic compression molding machine (uniaxial press). A rectangular cavity with 20 mm short sides and 50 mm long sides is formed in a mold, with its four corners having an R-value of 2 mm. The molded body essentially has the shape of a rectangular parallelepiped with dimensions of 20 mm × 20 mm × 50 mm, with its corners having an R-value of 2 mm ( Fig.1). The molding pressure is 200 MPa. The molding pressure can be adjusted as appropriate, but it is preferred that it be approximately 120 MPa or more. When the molding pressure is 200 MPa or more, an even denser molded article can be obtained, having a relative density of about 50% or more. As the density of the molded article becomes higher, the heat energy required for sintering can be further reduced, which is advantageous. However, an excessively high density may lead to breakage of the molded article. Apart from that, by placing the powder mixture in a rubber tube, hermetically sealing the rubber tube, and performing cold isostatic pressing, a round, rod-like molded article can be obtained. In this case, too, it is preferred that the molding pressure be 120 MPa or more, with a molding pressure of about 300 MPa being suitable.

[0038] When the two exemplary molded articles are melted as they are, as in the prior art, a state can be visually observed in which a portion of the heated powder and molten metal scatters in the melting furnace, causing sparking. Furthermore, the strength is such that a touch of the hand causes the powder to adhere to the finger, and a corner of the molded article breaks off when the molded article is dropped from a height of approximately 5 cm. - Sintering step -

[0039] Five molded bodies are stacked vertically with their 20 mm × 20 mm surfaces being their top and bottom, and these are counted as one unit ( Fig.2). Four units were arranged in a carbon shell, and the molded bodies were placed together with the shell in an atmospheric furnace containing a carbon heater. Sintering was carried out for 3 hours at 1500°C under an argon air stream. The sintered body underwent sintering shrinkage, resulting in a starting material rod with a relative density of 70% or more and dimensions of approximately 16 mm × 16 mm × 220 mm. Fig. 2). - Melting step -

[0040] The raw material rod is gripped horizontally by a raw material rod feeding mechanism of an atmospheric pressure plasma arc melting furnace (pulling-down system). It is continuously melted and dropped in an argon atmosphere of 0.9 atm (atmospheric pressure) to 1.2 atm, and pulled down a bottom portion of the water-cooled copper crucible. This yields a cylindrical ingot with a diameter of φ 35 mm. No atomization is detected during melting, and the effect of the sintering step can be confirmed. Although the raw material rod is freely supported at this time, it does not break, and no powder is shed during the melting step. - Deformation processing step -

[0041] The ingot is formed into a square bar by hot forging and then into a wire with a substantially rectangular cross-section by hot grooving. The ingot is further formed into a round wire with a predetermined outer diameter by hot drawing using a mold. - Cutting step -

[0042] The round wire is cut into lengths suitable for a wire saw. A plurality of wires are arranged parallel to each other, fixed with a resin, and cut with a wire saw to produce electrode pads for a spark plug, each of which has a predetermined length. Examples

[0043] The further description is based on examples.

[0044] The results are presented in Table 1. The assessment was carried out according to the following criteria.

[0045] The mass reduction indicates the mass reduction of the melt ingot compared to the starting material powders at the time of blending and is expressed as a percentage. A mass reduction of more than 3% was indicated by ×, and 3% or less was indicated by 0.

[0046] When the sintered body or molded body was picked up with the fingers before melting and powder adhesion was detected on the fingers, the powder detachment was indicated as ×. When no adhesion was detected at all, the powder detachment was indicated as 0.

[0047] Regarding the melting state, a visual assessment was performed during melting. If a spark-like atomization phenomenon was continuously observed, the melting state was indicated by ×. If the phenomenon was only occasionally observed, the melting state was indicated by Δ. If the phenomenon was hardly observed, the melting state was indicated by 0.

[0048] These results were considered for an overall assessment. If the effect of the invention was not recognized, the overall assessment was indicated as ×. If the effect was recognized, the overall assessment was indicated as 0. If the effect was more pronounced, the overall assessment was indicated as 00. - Example 1 -

[0049] Example 1 represents an example of the above description (best mode of the invention).

[0050] The relative density of the molded body, calculated from its dimensions and mass, was 52%. The density of the sintered body was 74%. Using the sintered body as the starting material rod, a melting ingot with approximately φ 35 mm diameter × L 150 mm length was prepared.

[0051] When during melting (under a pressure of 1.1 × 10 5When visual detection was performed at 0.5 Pa, the atomization phenomenon was not detected at all. The mass reduction of the melt ingot compared to the feedstock preparation step was 0.6% or less. Furthermore, after sintering until melting was completed, the feedstock rod did not break or peel.

[0052] After melting, almost no crushed material remained in the furnace, and no adhesion of the crushed material to the water-cooled copper crucible was detected. - Example 2 -

[0053] In Example 2, the melt body was prepared as in Example 1. The molded body was essentially in the shape of a rectangular parallelepiped, measuring 20 mm × 20 mm × 50 mm with corners having an R value of 2 mm. Such molded bodies were individually sintered without being stacked to prepare sintered bodies measuring approximately 16 mm × 16 mm × 44 mm. The sintered bodies were placed on a boat-shaped, water-cooled copper crucible and melted by vacuum plasma melting to produce a melt ingot measuring approximately 15 mm × 30 mm × 100 mm. The pressure during melting was set to 5 × 10 -1 Pa (Ar) is set.

[0054] Visual inspection during melting revealed occasional sputtering. After melting, a small amount of sputtered material remained in the furnace, some of which adhered to the water-cooled copper crucible.

[0055] The mass reduction of the ingot was 2.5%. Furthermore, regarding the shape of the ingot, the bottom section was almost smooth, following the shape of the boat-shaped, water-cooled copper crucible. However, there was a burr on one side surface, and the top surface had solidified in a wavy state. - Comparison example 1 -

[0056] In Comparative Example 1, after the raw material powder was mixed using a V-mixer, cold isostatic pressing was used to prepare a cylindrical molded article with a diameter of φ 30 mm. The molding pressure was 300 MPa. The relative density of the molded article, calculated from the dimensions and weight, was 48%. The molded article was cut into lengths of approximately 30 mm, placed on a boat-shaped, water-cooled copper crucible, and melted by arc melting to prepare a melting ingot measuring approximately D 15 mm × W 30 mm × L 100 mm. The pressure during melting was set to 8 × 10 4 Pa (Ar) is set.

[0057] The strength of the molded body was not so low that contact with the hand caused it to break. However, when the molded body was removed from the cold isostatic pressing mold, powder adhered to the finger and was observed adhering to an inner wall of the cold isostatic pressing mold.

[0058] Visual inspection during melting revealed continuous sputtering from one melt section until the ingot was completely melted. The sputtered material remained in the furnace after melting, and its adhesion to the water-cooled copper crucible was clearly visible. Furthermore, the sputtered material and powder that had separated from the ingot remained in a corner of the bottom section of the boat-shaped, water-cooled copper crucible. As described above, some of the mixed feedstock powders remained unmelted, resulting in a 3.2% mass reduction of the ingot compared to the feedstock preparation step.

[0059] As for the shape of the melting ingot, there was a burr and waviness as in the case of Example 2. - Comparison example 2 -

[0060] In Comparative Example 2, a molded body prepared as in Comparative Example 1 was placed on a boat-shaped, water-cooled copper crucible, and a melting ingot measuring approximately D 15 mm × W 30 mm × L 100 mm was prepared by vacuum plasma melting. The pressure during melting was set to 5 × 10 -1 Pa (Ar) is set.

[0061] Visual inspection during melting revealed continuous sputtering from one melt section until the ingot was completely melted. After melting, more sputtered material remained in the furnace, and its adhesion to the water-cooled copper crucible was even more noticeable. Furthermore, the sputtered material and powder that had separated from the ingot remained in a corner of the bottom section of the boat-shaped, water-cooled copper crucible. As described above, some of the mixed raw material powder remained unmelted, and the mass reduction of the melt ingot compared to the raw material preparation step was 4.5%.

[0062] From the results described above, it was confirmed that the mass reduction was greater and the material yield was lower in a process in which a molded body was directly melted without performing the sintering step, whereas these were significantly improved when the sintered body was melted in the process according to the invention. Table 1 - Test results Nr. Mass reduction Powder removal Melting state Overall assessment Example 1 ◯ ◯ ◯ ◯◯ Example 2 ◯ ◯ Δ ◯ Comparison example 1 × × × × Comparison example 2 × × × ×

Claims

[1] A process for producing a product from a platinum group metal or a platinum group-based alloy, comprising: a preparation step in which a starting material consisting partly or wholly of powder is weighed and, if the product is to be made from the alloy, the weighed starting material is mixed to obtain a powder mixture; a forming step in which the prepared starting material is shaped and solidified to obtain shaped bodies which are substantially in the shape of a rectangular parallelepiped; a sintering step in which the shaped bodies are sintered using a furnace to obtain a sintered body, wherein the sintering is carried out at a temperature of 1000°C or more and does not exceed the melting point of the starting material; a melting step in which the sintered body is melted to produce a melting ingot, wherein a pressure during melting is 1 Pa or more; and a deformation processing step in which the melt ingot is plastically processed, wherein the shaped bodies are sintered in a stacked state in the sintering step to produce the sintered body as a joined body, and where in the melting step a plasma arc melting furnace with a water-cooled copper crucible is used, which has a continuous cavity, in the cavity a molten bath of a metal melt of the sintered body is formed and a bottom portion of the molten pool is pulled down in the cavity to produce the melt ingot.

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