High density tungsten alloy has become an essential material for a wide variety of applications. It has the highest melting point of any metal, and its high density is equaled only by gold, platinum, and a few other rare and expensive metals. Pure tungsten is quite brittle and difficult to be machined. The addition of elements such as nickel, copper, and iron, creates a family of materials with extremely useful properties. High density tungsten alloys are economically fabricated by powder metallurgy techniques and are routinely machined by conventional methods. Mechanical properties add tensile strength, ductility, hardness-compare favorably with common engineering materials such as steel. Twice as heavy as steel and more than 50% heavier than lead, high density tungsten alloys have found wide acceptance where a concentrated mass is required in a limited space for such applications as radiation shielding, and a large variety of other uses.

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The silver tungsten points’ preparation methods: infiltration method and re-pressuring method.

1. Infiltration method. Infiltration method is a method of producing refractory metal and. low melting point metal prosthesis alloy Firs of all , to form a porous skeleton, you should press and pre-sinter the metal powder which possess high melting point, secondly, you should control the temperature until its initial temperature is higher than the metal that owns low melting point, then place the metal which has low melting point under/above the porous skeleton, and make the metal melt and immerge porous skeleton’s pores, to get the final product- silver tungsten points which own high density and other good properties.

2. Mechanical alloying. Mechanical alloying, as a new material preparation process, is gradually stepping into people's vision in recent years. And its concept was first put forward in the 1960's. Mechanical alloying has gradually been developed and is widely used nowadays. Mechanical alloying refers to: adopting high-energy ball milling, mixing two or more kinds of elements powder for a long time. Mixing powder will bear attrition force, impact, shear force, compression force and other forces and absorb mechanical energy in this process. In other words, under the role of milling media, after repeatedly pressing, welding, and crushing and fracture process and through solid state reaction or diffusion, the mixing powder will combined with each other to get the result-the product- silver tungsten points.



 

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Crystallization kinetics of calcium tungstate from unstirred supersaturated solutions in sodium tungstate melts was studied by continuous cooling from initial crystallization temperature TO=1000° to 800°C to room temperature at cooling rates RT=0.67° to 3.3°C min−1. Crystals were examined by optical microscopy and analyzed by various methods. The main crystal growth was diffusion-rate-controlled. The final crystal growth was rate-controlled by the development rate of excess solute concentration. Diffusion rate constant (kD) values were estimated. These values increased with cooling rate and initial crystallization temperatures but they are very much smaller than the rate constants for diffusion-controlled growth of calcium tungstate from lithium chloride melts.

 

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Silver tungsten points to produce electric locomotives regulating switch:

Firstly, batching and mixing tungsten powder, silver powder and a small amount of nickel powder;
Secondly, to form porous skeleton, you should press the mixing powder;
Thirdly, you should stack the porous skeleton and the infiltration silver pieces, and place them in tungsten boat;
Finally, to make the molten silver infiltrated into porous skeleton pores completely, you should carry out infiltration sintering; and you can get the product-silver tungsten points after cooling. Sintering conditions: under the protection atmosphere, you should putt them in the furnace of molybdenum wire, and the infiltration sintering temperature is higher than the melting point of silver.

Using silver tungsten as raw material to produce electric locomotives regulating switch by infiltration processes:

Batching (tungsten powder (70%), silver powder, nickel powder) → Mixing powder(silver powder, tungsten powder(70%),nickelpowder) → Pressing → Semi-inspection,infiltration+hydrogen → sintering → Checking → Cleaning → Packing → Silver tungsten points



 

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