With a high mechanical strength (17.5-19.3 g/cm³), melting point (3422 g/cm³), and ultra-high density, tungsten alloy is recognized in the advanced military, aerospace, and nuclear engineering fields. But, it. It is still associated with high processing. difficulty. This difficulty stems from the low and high temperature oxidation, the processed materials' resistance to deformation, and the low temperature. As a result, the tungsten got brittle and lost some ductility, which makes working with it very challenging. As a solution, the industry has developed key technologies including powder metallurgy, positive deformation, and plastic forming. With accurate process optimization and sinter strengthening, it is possible to achieve high precision and tungsten surface performance. Below, we elaborate on tungsten alloy working methods.
1. Powder Metallurgy: The Basis of Producing High-Purity Billets. Including but not limited to the following critical activities, powder metallurgy is the first step in the processing of tungsten alloys.
1) Purification of the Raw Material and Its Subsequent Mixture
High purity tungsten powder is selected that is more than or equal to 99.95 percent and is then uniformly mixed with the alloying components Nickel, Iron, and Cobalt in the course of ball milling and screening. The formability of the powder is in the 5 to 10 micron range. For specific applications, like materials used in nuclear fusion, secondary phase particles like titanium carbide, TiC, and Yttrium oxide, Y2O3, are used to improve dispersion-strengthened radiation resistance.
2) Forming and Pre-Sintering
Bills are prepared using isostatic pressing (pressure ≥2500 MPa) or die-pressing techniques. Typical dimensions are 12×12×400 mm bars or plates. Pre-sintering is performed at 1200°C for one hour in a hydrogen atmosphere to initially enhance billet strength and conductivity.
2. Plastic Processing: The Key to Overcoming the Brittleness Bottleneck. Tungsten alloys' low ductility requires precision processing through high-temperature plastic forming:
1) Hot and Warm Rolling
Hot rolling begins at a billet temperature of 1350-1500°C. Through multiple rolling passes, the sheet thickness is reduced from 8 mm to 0.5 mm. The rolls must be preheated to 100-350°C to reduce deformation resistance. Warm rolling (1200°C) further refines the sheet to 0.2 mm. Graphite or molybdenum disulfide lubrication is sprayed throughout the process to prevent cracking.
2) Swaging and Wire Drawing
Swaging is performed in a hydrogen atmosphere at 1400-1600°C. This rotary forging transforms the billet into a uniform round bar (final diameter 3 mm) with a density of 18.8-19.2 g/cm³. Wire drawing utilizes a "warm drawing" process. After preheating to 100-350°C, the sheet is gradually drawn through a chain stretcher to a finer wire thickness of less than 0.06 mm, suitable for applications in electronics and lighting.
3. Sintering Process: Enhancements of Density and Performance. Sintering is important in increasing the density and mechanical characteristics of tungsten alloys. The important ones are:
(1) Vertical Melting (Self-Inhibited Sintering): A current is sent directly through the billet to create Joule heating. As the current is sintered from the current that melts. It controls the count of grains to about 10,000 to 20,000 grains per mm² and the density to 17.8 to 18.6 grams per cm³. It is ideal for wire and small parts.
(2) Spark Plasma Sintering (SPS): It combines a pulse of current together with some pressure, and achieves quick densification of below 2000 0 C, with the grain size controlling less than 300 nm and considerable improvement on creep resistance
3) Two-Step Pressureless Sintering: Temperatures are controlled in stages (2300-2700°C) in a vacuum or hydrogen atmosphere, achieving a theoretical density exceeding 98%. It is suitable for large-sized tubes and specially shaped parts.
4. Surface Treatment and Post-Processing: Functionalization and Precision
1) Electroplating and Coating
In response to the electroplating company's pressing need to reduce corrosion and wear on oilfield mechanisms, we have developed tungsten alloy electroplating technology. Tungsten alloys have better acid and alkali corrosion resistance, and wear resistance and hardness comparable to chrome electroplates. Hot-section components require spraying with oxidant-resistant coatings (like silicon-aluminide) to mitigate catastrophic oxidation above 1000°C.
2) Machining and Heat Treatment
During the cutting phase, when we are using carbide tools, it is essential to raise the Workpieces above 200-500°C, which is the ductile-brittle transition temperature, to eliminate the risk of cracking. The "aging" process means the workpiece has to go through a primary stage of modification, which is then followed by a secondary stage. For instance, if a W-Re alloy is heated to 1500°C, then we know that within it, we are bound to reach a temperature of 1650°C.
5. Innovative Processes: New Directions in Research
1) In-Situ Reaction Method
This approach conducts the in-situ reaction of tungsten powder with carbon and nitrogen to form the tungsten carbide (WC) and tungsten nitride (WN) reinforcing phases. This reaction lowers the cost of manufacturing composite materials.
2) Additive Manufacturing
This approach applies SLM (selective laser melting) technology, which directly fabricates intricate geometrical parts. In combination with other techniques, SLM, nanopowders, and gradient design resolve the spatial constraints of conventional methods.
Several advanced, high-performing materials are needed for nuclear fusion reactors and hypersonic vehicles, and this drives the development of tungsten alloy processing technology. Through collaborations in powder metallurgy, plastic forming, and sintering, and within every other batch of surface treatment, the DBTT (ductile-brittle transition temperature) of tungsten alloys was brought down from 400°C and below room temperature, improving their resistance to radiation and oxidation.
