A silver tungsten carbide contact material and its preparation method

Abstract

The present invention belongs to the technical field of contact materials, and particularly relates to a silver tungsten carbide contact material and a preparation method thereof. The present invention innovatively proposes to first uniformly mix silver, tungsten, and graphite, and then press and form embryos. Since the embryos are formed by pressing silver, tungsten, and graphite, the difficulty of pressing embryo production is greatly reduced compared with that of silver and tungsten carbide, and it can be used for the preparation of silver tungsten carbide contact materials with a high tungsten carbide content. Then, the embryos of silver, tungsten, and graphite are carbonized in a hydrogen atmosphere, so that tungsten and graphite in the embryos react to form tungsten carbide, and then sintered and infiltrated with silver sheets to obtain silver tungsten carbide contact materials. Among them, the key is that through plasma ball milling, tungsten and graphite are first ball milled and mixed to make the powder surface, making diffusion, phase transformation, and chemical reactions extremely easy to carry out, so that a good reaction between tungsten and graphite in the embryo can be achieved, greatly increasing the content of tungsten carbide in the finished product and effectively improving the performance of the finished product.

Description

Technical Field

[0001] The present invention belongs to the technical field of contact materials, and particularly relates to a silver tungsten carbide contact material and a preparation method thereof. Background Art

[0002] The electrical contact is the core component of an electrical switch and a key factor affecting the on-off ability and reliability of the electrical switch. Its performance directly affects the reliability and stability of the electrical switch. The electrical switch requires the contact material to have good electrical and thermal conductivity, low and stable contact resistance, high erosion resistance, anti-welding property, etc. As the requirements for the reliability of electrical appliances at the power distribution end of the power grid are getting higher and higher, electrical appliance manufacturers are constantly introducing low-voltage distribution products with higher indicators.

[0003] Silver tungsten carbide and silver tungsten electrical contacts are commonly used contacts in circuit breakers. Under the same conditions, tungsten carbide is more difficult to oxidize than tungsten, so that the silver tungsten carbide contact has better oxidation resistance, anti-welding property, good on-off performance, and relatively stable contact resistance than the silver tungsten contact. Due to its superior performance, it has promoted the silver tungsten carbide and silver tungsten carbide graphite moving and static combined contacts to become the mainstream contacts used in current circuit breakers. In the silver tungsten carbide material, when the tungsten carbide content is <30wt%, the material hardness is not high and the processability is good, and a higher density can be obtained by solid-phase sintering and repressing or extrusion rolling; as the tungsten carbide content increases, the material hardness continuously increases. When the tungsten carbide content >50wt%, the high melting point, high hardness and good wear resistance of WC can withstand strong arc corrosion, have good anti-welding property and wear resistance, and at the same time achieve the effect of silver saving. However, the material hardness is relatively high and the processability is relatively poor, and only by liquid-phase sintering (infiltration) can a high-density material be obtained.

[0004] When the tungsten carbide content in the silver tungsten carbide powder is too high and the tungsten carbide particles are too small, when the mass percentage of tungsten carbide is increased to 50%-85%, the dispersibility and processing difficulty of the tungsten carbide particles are greatly improved. During the forming process, the tungsten carbide particles in the powder itself are the same as the matrix material; the matrix in the pressing area wears greatly, resulting in cracking after the lower punch of the product is ejected; due to the ultra-high hardness, when the tungsten carbide particles are in contact with the surface of the particles and are pressed into shape, almost no plastic deformation occurs, so the particles cannot form mutual meshing, resulting in cracks easily appearing in the green compact, and even the green compact cannot be formed; even if there are micro-cracks in the formed product, it is very difficult to find them during the forming stage, and they are only found when the finished product is made into an inlay sample; resulting in batch scrapping.

[0005] Therefore, the production of high-tungsten carbide materials has always been a difficult problem in the industry. This problem also restricts the further improvement of the breaking performance of electrical appliances to a large extent. Summary of the Invention

[0006] The object of the present invention is to overcome the disadvantages and deficiencies existing in the prior art, and to provide a silver tungsten carbide contact material and a preparation method thereof.

[0007] The technical solution adopted by the present invention is as follows: A preparation method of a silver tungsten carbide contact material, comprising the following steps:

[0008] S1. Add tungsten powder and graphite powder into a planetary ball mill for ball milling;

[0009] S2. Add silver powder and additives into the planetary ball mill in step S1, and continue ball milling to obtain planetary ball milled powder;

[0010] S3. Mix the planetary ball milled powder obtained in step S2 with a molding agent, make powder particles and dry them;

[0011] S4. Press the powder particles obtained in step S3 into a green compact;

[0012] S5. Place the green compact made in step S4 in a degreasing furnace protected by an ammonia decomposition atmosphere, and remove the molding agent in the green compact at a low temperature to obtain a skeleton;

[0013] S6. Carbonize the skeleton obtained in step S5 under a hydrogen atmosphere to generate tungsten carbide, and obtain a silver tungsten carbide green compact;

[0014] S7. Stack the silver tungsten carbide green compact obtained in step S6 with silver sheets, place them in a sintering furnace protected by an ammonia decomposition atmosphere for sintering infiltration, and obtain a silver tungsten carbide contact material.

[0015] Preferably, in step S1, the mass ratio of tungsten to graphite is required to be 15:1 to 15.4:1;

[0016] Preferably, in step S1, the planetary ball milling parameters are: the rotational speed of the vibration motor is 800 - 1200 rpm, the discharge voltage is 6 - 10 kv, the discharge frequency is 20 - 40 kHz, the ball milling time is 1 - 10 h, and the ball-to-material ratio is 1:1 to 10:1;

[0017] Preferably, in step S2, the planetary ball milling parameters are: the rotational speed of the vibration motor is 800 - 1200 rpm, the discharge voltage is 6 - 10 kv, the discharge frequency is 20 - 40 kHz, the ball milling time is 10 min - 1 h, and the ball-to-material ratio is 1:1 to 10:1;

[0018] Preferably, in step S3, the mass percentage of the molding agent relative to the powder particles is 0.03% - 0.1%, and the stirring time is 5 - 30 min;

[0019] Preferably, in step S4, the temperature for removing the molding agent is 200 - 600 °C, the removing atmosphere is hydrogen, and the heat preservation time is 1 - 5 h.

[0020] Preferably, in step S5, the carbonization temperature is 850°C to 950°C, the carbonization atmosphere is hydrogen, and the carbonization time is 6 - 15 h.

[0021] The silver-tungsten carbide contact material prepared by the preparation method as described above.

[0022] The beneficial effects of the present invention are as follows:

[0023] The present invention innovatively proposes to first uniformly mix silver, tungsten, and graphite, and then press and form embryos. Since it is to press and form embryos of silver, tungsten, and graphite, the difficulty of pressing embryo production is greatly reduced compared with that of silver and tungsten carbide, and it can be used for the preparation of silver-tungsten carbide contact materials with a high tungsten carbide content. Then, the silver, tungsten, and graphite pressed embryos are carbonized in a hydrogen atmosphere, so that tungsten and graphite in the pressed embryos react to form tungsten carbide, and then sintered and infiltrated with silver sheets to obtain the silver-tungsten carbide contact material.

[0024] Among them, the more crucial point is that the pressed embryos mixed by a conventional ball mill cannot undergo a good carbonization reaction, and the tungsten carbide content in the finished product is relatively low. Using plasma ball milling can promote the refinement of the powder structure, alloying, activation of activity, chemical reaction, and acceleration of in-situ gas-solid phase reaction, etc. The highly active particles (ions, electrons, excited atoms and molecules, free radicals, etc.) of the plasma are easy to adsorb with other substances and cause an increase in the surface activity of the material. The fresh surface and a large number of defects introduced by mechanical ball milling further enhance the activity of the ball-milled powder, making diffusion, phase transformation, and chemical reactions extremely easy to carry out. Therefore, a good reaction between tungsten and graphite in the pressed embryos can be achieved, greatly increasing the tungsten carbide content in the finished product and effectively improving the performance of the finished product. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, obtaining other drawings without creative efforts still belongs to the scope of the present invention.

[0026] Figure 1 It is the process route of the present invention;

[0027] Figure 2 It is the comparison of the metallographic structures of the AgWC50 contact materials prepared by the conventional method (a) and the preparation method of Example 1 (b);

[0028] Figure 3 It is the comparison of the metallographic structures of the AgWC65 contact materials prepared by the conventional method (a) and the preparation method of Example 2 (b). DETAILED DESCRIPTION OF THE INVENTION

[0029] The present invention will be further described below in conjunction with specific embodiments, but the present invention is not limited to these specific embodiments. Those skilled in the art should recognize that the present invention covers all alternative solutions, improvement solutions, and equivalent solutions that may be included within the scope of the claims.

[0030] Example 1

[0031] A preparation method of a silver tungsten carbide contact material includes the following steps:

[0032] a. Mix tungsten and graphite powder in a ratio of 15.2:1;

[0033] b. Add tungsten powder and graphite powder to a planetary ball mill for ball milling. Particle size of tungsten powder: 1um; Planetary ball milling parameters: vibration motor speed 1200rpm, discharge voltage 10kv, discharge frequency 40kHz, ball milling time 10h, ball-to-material ratio 1:1;

[0034] c. Continue to add silver powder and additives (trace additives, iron, cobalt, nickel, etc.) to the planetary ball mill. Silver powder: tungsten and graphite: additives are in a ratio of 30:69:1. Planetary ball milling parameters: vibration motor speed 1200rpm, discharge voltage 10kv, discharge frequency 40kHz, ball milling time 10h, ball-to-material ratio 1:1;

[0035] d. Mix the planetary ball milled powder with a forming agent (paraffin wax) to make powder particles and dry them; Amount of forming agent added per kilogram of powder: 0.1%, stirring time: 30min;

[0036] e. Press the powder particles with added glue into green compacts;

[0037] f. Place the green compacts in a degreasing furnace protected by an ammonia decomposition atmosphere to remove the forming agent in the green compacts at a low temperature; Temperature for removing the forming agent: 450°C, hydrogen gas for the removed atmosphere, time: 2h;

[0038] g. Carbonize the skeleton in a hydrogen atmosphere to generate tungsten carbide; Carbonization parameters: carbonization temperature 850°C, hydrogen gas for the carbonization atmosphere, carbonization time: 10h.

[0039] h. Stack the green compacts and silver sheets, place them in a sintering furnace protected by an ammonia decomposition atmosphere, sinter and infiltrate at 1100°C for 2 hours, cool and then take out of the furnace to obtain the silver tungsten carbide contact material AgWC50. As shown in Table 1, the flexural strength of AgWC50 prepared in this example is significantly improved compared with that prepared by the conventional process.

[0040] Table 1 Comparison of the properties of AgWC50 contact materials prepared by the conventional method and the method of the present invention

[0041] Material Name Process Density Flexural Strength AgWC50 Conventional 12.1、12.15 1000-1050 AgWC50 Process of the Present Invention 12.25、12.23 1190-1240

[0042] Example 2

[0043] A preparation method of a silver tungsten carbide contact material, comprising the following steps:

[0044] a. Weigh tungsten and graphite powder in a ratio of 15.3:1;

[0045] b. Add tungsten powder and graphite powder into a plasma ball mill for ball milling. The particle size of tungsten powder is 3um. Plasma ball milling parameters: rotational speed of the vibration motor is 800rpm, discharge voltage is 6kv, discharge frequency is 20kHz, ball milling time is 1h, and ball-to-material ratio is 10:1;

[0046] c. Continue to add silver powder and additives (trace additives, such as iron, cobalt, nickel, etc., the same as in Example 1) into the plasma ball mill. The ratio of silver powder: tungsten and graphite: additives is 20:79:1. Plasma ball milling parameters: rotational speed of the vibration motor is 800rpm, discharge voltage is 6kv, discharge frequency is 20kHz, ball milling time is 1h, and ball-to-material ratio is 10:1;

[0047] d. Mix the plasma ball milled powder with a forming agent (paraffin wax) to make powder particles and dry them. The dosage of the forming agent per kilogram of powder is 0.03%, and the stirring time is 10min;

[0048] e. Press the powder particles with added glue into green compacts;

[0049] f. Place the green compacts in a degreasing furnace protected by an ammonia decomposition atmosphere to remove the forming agent in the green compacts at a low temperature. The temperature for removing the forming agent is 550°C, the atmosphere for removal is hydrogen, and the time is 1.5h;

[0050] g. Carbonize the skeleton in a hydrogen atmosphere to generate tungsten carbide. Carbonization parameters: carbonization temperature is 950°C, carbonization atmosphere is hydrogen, and carbonization time is 15h.

[0051] h. Stack the green compacts and silver sheets, place them in a sintering furnace protected by an ammonia decomposition atmosphere, sinter and infiltrate at 1150°C for 2 hours, cool and then take out of the furnace to obtain the silver tungsten carbide contact material AgWC65. As shown in Table 1, compared with AgWC50 prepared by the conventional process, the flexural strength of this embodiment has been significantly improved.

[0052] Table 2 Comparison of the properties of AgWC65 contact materials prepared by the conventional method and the method of the present invention

[0053] Material Name Process Density Flexural Strength AgWC65 Conventional 12.99、13.01 950-1000 AgWC65 Process of the Present Invention 13.12、13.1 1050-1100

[0054] The above-disclosed are only the preferred embodiments of the present invention. Of course, the scope of the rights of the present invention cannot be limited thereby. Therefore, equivalent changes made according to the claims of the present invention still fall within the scope covered by the present invention.

Claims

1. A preparation method of a silver tungsten carbide contact material, characterized in that, It includes the following steps: S1. Add tungsten powder and graphite powder into a plasma ball mill for ball milling; S2. Add silver powder and additives into the plasma ball mill in step S1, and continue ball milling to obtain plasma ball milled powder; S3. Mix the plasma ball milled powder obtained in step S2 with a binder, make powder particles and dry them; S4. Press the powder particles obtained in step S3 into a green compact; S5. Place the green compact made in step S4 in a debinding furnace protected by an ammonia decomposition atmosphere, and remove the binder in the green compact at a low temperature to obtain a skeleton; S6. Carbonize the skeleton obtained in step S5 in a hydrogen atmosphere to generate tungsten carbide, and obtain a silver tungsten carbide green compact; S7. Stack the silver tungsten carbide green compact obtained in step S6 with silver sheets, place them in a sintering furnace protected by an ammonia decomposition atmosphere for sintering infiltration, and obtain a silver tungsten carbide contact material; In step S1, the mass ratio of tungsten to graphite is required to be 15:1 to 15.4:1; In step S1, the plasma ball milling parameters are: the rotational speed of the vibration motor is 800 - 1200 rpm, the discharge voltage is 6 - 10 kv, the discharge frequency is 20 - 40 kHz, the ball milling time is 1 - 10 h, and the ball-to-material ratio is 1:1 to 10:1; In step S2, the plasma ball milling parameters are: the rotational speed of the vibration motor is 800 - 1200 rpm, the discharge voltage is 6 - 10 kv, the discharge frequency is 20 - 40 kHz, the ball milling time is 10 min - 1 h, and the ball-to-material ratio is 1:1 to 10:1; In step S3, the mass percentage of the binder relative to the powder particles is 0.03% - 0.1%, and the stirring time is 5 - 30 min; In step S4, the temperature for removing the binder is 200 - 600 °C, the removing atmosphere is hydrogen, and the holding time is 1 - 5 h; In step S5, the carbonization temperature is 850 °C - 950 °C, the carbonization atmosphere is hydrogen, and the carbonization time is 6 - 15 h; The additive is iron, cobalt or nickel.

2. A silver tungsten carbide contact material prepared by the preparation method according to claim 1.

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