In 2007, the New England area experienced a scare after published reports suggested that tungsten had leached into various waterways. Certain compounds of the tungsten element show signs of toxicity.

Classification
Tungsten falls into the heavy metal category under the VIB group on the periodic table. Tungsten displays an extreme melting point as well as a high density. Tungsten has the ability to form a vast variety of compounds.

Warning
Various compounds of the metal tungsten have been proven to show signs of toxicity. Some of the most common toxic compounds of tungsten include nitrides, silicides, borides and oxides. Elemental tungsten often gets confused with the compounds of tungsten that display toxic characteristics. Finding elemental tungsten in a water system does not warrant an alarm as the toxicity of the elemental form of tungsten remains insignificant.

Leaching
Tungsten in its pure metallic form can not leach into water. Elemental tungsten has an insoluble composition when introduced to water. Tungsten also has a density 19 times greater than the density of water, making the transportation of tungsten by water nearly impossible.

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With excellent performance , the traditional W-2% ThO2 electrode is a widely used hot cathode material , swaging method is a widely used method of industrialization . Swaging method , also known as radial or rotary hammer forging method , it is actually a special form of forging . Swaging machine works by two or more blocks of one hand forging die forging billet axis surrounded by high-speed rotation , on the other hand on the billet for forging, forging a frequency of approximately 6800 to 12,000 times / min, thereby the billet deformation. Melting down the sintered article W , first through swaging , and then stretched to the required finished wire . Swaging and drawing , has the following advantages: ( 1 ) The stress state is good. In the rotary forging , the billet at a reasonable process conditions by three to compression stresses, swaging force changing periodically , but not outside the friction . More moderate deformation of the metal , so suitable for processing low plasticity billet ; ( 2 ) Can effectively improve the microstructure and properties of metals . With coarse and fine grain cross porous vertical melt distribution billet by swaging broken into fine grain , and with the mouth of the total increase in the degree of deformation , the messy arrangement of porous vertical melt coarse grain structure gradually transformed into dense aligned fine grain structure . Therefore, the metal density , plasticity and strength and so has been significantly improved, so as to produce qualified Rod and smoothly brushed created favorable conditions ; ( 3 ) Swaging blooming with other methods ( eg , rolling method and extrusion method ) , also has equipment compact and easy to operate , as well as the advantages of low consumption deformation . In the rotary forging process, due to the deformation of the blank is not reactive, and friction , and thus the required motor power is only about 1.5-2.0kW .

However , the material processed by swaging , W cathode of the rod-shaped or needle ThO2 , as coarse grains , serious deterioration of the cathode performance. Authors' by high energy ball milling , and then sintered , a new nano-composite W-2% ThO2 cathode material was developed to put success. The materials ThO2 particle size less than 100nm, showing better than the same grade commercial arcing electrodes and anti- arc ablation performance. However, as the process is less than the maximum compressive stress 50MPa, so the relative density of the material is only 94.5% , much lower than the conventional cathode ( fully dense ) , fracture toughness is also poor. The disadvantage of the method is the high cost , not suitable for industrial production. Thus , limiting the application of this new cathode material .

In order to make the performance of this material has been further improved , the authors were prepared using swaged nanocomposite W-2% ThO2 cathode material , and to explore the possibility of industrial production , it truly out of the lab .


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Raw Material

Experimental materials used for the business of W powder and ThO2 powder, W powder grades for the GW-06, a purity of 99.6%, an average particle size of about 0.6μm. The Th (NO3) 4 from the thermal decomposition, Fei average particle size of -300 mesh, 99.5% purity.

Equipment and Process Parameters

High-energy ball milling equipment for the German Simoloyer01-2LM type horizontal mill, like this:
this type of high-energy ball mill Simplified and stirring rod made ​​of special wear-resistant alloy, balls made ​​of bearing steel, spindle speed and the milling process are computer controlled milling time, milling can be used in inert gas or in a vacuum state. This type of high-energy ball milling with high efficiency, uniform particle size, and low impurity content. Ball Mill inner diameter of 146mm, ball diameter of 5mm. The aviation gasoline was chosen as the grinding media and through the circulating water cooling protection.

Milling time and the amount of energy is kinetic energy ball milling determinants made ​​of powder on the granularity of uniformity and impurities play a decisive impact. These factors have an impact on the high-energy ball milling process parameters as rotational speed, ball feed ratio, milling time, nature of raw materials, materials and so gentle atmosphere. In this experiment, the milling process of adding aviation gasoline role is to improve the grinding efficiency, improve the lubrication conditions and reduce equipment wear and improve the uniformity of the powder, the powder to reduce pollution. In the milling process, through the grind at different time intervals to take powder, the powder particle size test, uniformity and impurity content, etc., through process optimization.

Mixture Process

Nanoparticles due to the large specific surface area and surface atoms ligand insufficient, with a strong surface activity and strong adsorption, making them easy to join together to form a weak-link interface with a plurality of larger size aggregates .

Swaging Process

Preparation of nanocomposite carved forging W-2% ThO2 electrode material of this test in Taebaek tungsten alloy plant, the specific process is as follows:
(A) mixing (2) Compression (3) pre-sintered (4) vertical melt sintering (5) swaging (6) drawing (7) straightening polished.


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Process Points

Highly doped nano-composite W-ThO2 electrode materials Preparation is the key process mixed doped W powder. Added amount of ThO2, size, and other factors added directly affect the W powder, the dopant concentration and ultimately the final properties of the finished material.

Production of high-doped nano-composite W-ThO2 electrode wire craftsmanship and production of nano composite W-2% ThO2 electrode wire process is slightly different.

Doped nano-composite W-ThO2 electrode materials technology points are:

(1) Mixing powder is the key that can make high-doped oxide powder dispersed in the W are the factors that determine everything, so Mixture Process to be carefully controlled, the use of composite powder mixing process is more reasonable, such as the use of ultrasonic + high-energy ball milling method is more reasonable.

(2) Two kinds of fine powder in the production of blanks will appear low bulk density, easy to stick molds, compacted density, strength and low defects, resulting in process yield is very low. Therefore, the compression pressure is higher than the traditional process of molding pressure, dwell time longer than the traditional process.

(3) Pre-sintering, vertical melt, swaging and other processes to be slightly higher than the processing temperature of nano-composite W-2% ThO2 electrode.

ThO2 Content Determination: The nano-composite W-ThO2 electrode material terms. W is the base material, ThO2 as dopant.

Mixture Process

Particles in the matrix affect the uniformity of the distribution of particle reinforced / modified mechanical properties of composites and other features important factor is particularly important for nanocomposites. Particle distribution uniformity of the particles and the matrix itself including wetting properties, density, particle size, particle surface characteristics, material preparation method, particles in the matrix in an amount such factors are closely related, prepared by powder metallurgy particle reinforced / modified composite material , the particle distribution depends largely on the mixed powder method and process parameters. In this paper, an ultrasonic oscillation method + ball milling powder mixing method. First ultrasonic dispersion, drying the wet state, and then the ball mill mixing powder after drying, grinding.


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Prepared by swaging nanocomposite W-2% ThO2 electrode material of this test tungsten alloy plant in Taebaek, the specific process is as follows:

(1) Mixing: The After dispersing, mixing a good first composite powder sieved through a 100 mesh sieve manually three times and allowed to stand 30min. In sifted before adding binder, adhesive formulations as follows: ethanol + glycerol (volume ratio of 1: (1.5-2)). The proportion of binder added: Every 1000g sifted powder was added 3mL binder.

(2) Compression: W powder particle size of 0.6μm, very fine-grained, leading to increased friction, resulting in the actual sample pressure is still small. After the suppression of the larger specimens resilience, resulting in greater stress. Also causing uneven density inside the sample, the surface density, the center is less dense, prone to early stratification and micro-cracks, sintering is easy to burst. If the binder added more, since the surface density, gas difficult to discharge, it could easily lead to burst. If the mixed powder is uneven, the compacts are likely to cause an early break. Considering these factors, the pressing process point is: bonding agent added in moderation; powder mixed carefully controlled process to ensure uniform mixing powder; increase the pressure; increased dwell time. Specific process parameters: each billet quality powder 580g (normal 640g), pressure 70T (50T normal production pressure), a pressure of about 60MPa, dwell time 5min.

(3) Pre-Sintering: In the protection of hydrogen, at a temperature of 1150-1200 ℃ sintering 1h, aims to improve the density and strength of the green body in order to facilitate the subsequent sintering electrically heated melt down. Pre-sintered body size is about 400mm x 12mm X 12mm.

(4) Vertical melt sintering: current heating current is 90% of the fusing current, the temperature does not exceed 3000 ℃, time of l0min.

(5) The Swaging: the vertical melt sintered billet at a temperature of 1350-1450 ℃ swaging.

(6) Drawing: caustic deburring and oil after drawing wire.

(7) Straightening Polished.


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Milling Mixing Method

Generally the production of high-density tungsten alloy, mixture preparation materials are used mostly dry powder mixing method, namely mechanical mixing. This method is characterized by simple preparation process. Form a V-shaped or cone blenders. Since W, Ni, Cu metal powders of different densities, in order to achieve uniformity of mixture will generally be Ni, Cu, W powder, mixed 8-12h.

Secondary Pyrolysis Reduction Method

To obtain W-Ni-Cu highly uniform mixing of the components, and to mix with a high sintering activity, it developed a named "secondary pyrolysis Reduction" of new technology. The resulting coating has a certain performance W - Ni-Cu sintered mixture of different ingredients than normal milling mixture reduces 100-130 ℃, and the resulting aggregate is more excellent.

Secondary pyrolysis method is to first reduction of nickel nitrate dissolved in distilled water, then W powder and uniformly mixed solution of nickel nitrate, roasted solution and reduction, and the obtained composite powder of nickel tungsten package. Then the composite powder was mixed with copper chloride, drying, reducing, and finally obtain W, Ni, Cu composite powder.

Mechanical Alloying

Mechanical alloying process is the 1970s after the development of a special alloy materials Preparation mature technology. Mechanical alloying metallic raw material powder in proportion to the drum into a ball mill, adding the ball, in order to prevent dirty, the product of the same composition can also be a W alloy ball. Mechanical alloying time to reach the mixed powders reached after 60h alloying and composition distribution, then press forming and sintering processes. The sintering temperature can be considerably lower.

In addition, since the 1990s, foreign research developed yet another new job - RSU reactive spray deposition method, using this method time is short, quick, the production of tungsten alloy powder and mechanically mixed powder compared to greatly improve reactive sintering the alloy microstructure uniformity and mechanical properties of the alloy.




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