Tungsten alloys are definitely on engineers' "nightmare materials" list. Their high density and hardness present significant processing challenges, making each step feel like a tough battle. How can this difficult material be transformed from raw material into a finished product with high efficiency?
Don't worry, this invaluable "secret weapon" is here! This article will analyze the process from the very beginning of raw material preparation, comparing the advantages and disadvantages of powder metallurgy and 3D printing, detailing the core techniques of cutting, grinding, and wire cutting, and introducing a cutting-edge technology that can help you achieve twice the results with half the effort-powder extrusion printing (PEP), providing a solid foundation for your design and manufacturing.
PART.01
Introduction
Tungsten alloys, as an alloy material with tungsten as the main component (tungsten content usually accounts for 85% to 99%) and added elements such as nickel, iron, copper, cobalt, molybdenum and chromium, play an indispensable role in many high-tech fields such as nuclear industry, military industry and medical care due to their extremely high density (16.5 to 19.0 g/cm3), high melting point and excellent mechanical strength[1]. In the medical field, tungsten alloys are used to manufacture key components of radiotherapy equipment. Due to their high density characteristics, they can accurately block and shape rays and are the main materials for making collimators and radiation shields[2]. These excellent properties also bring many difficulties to the processing and manufacturing of tungsten alloys. This article aims to systematically and deeply explore the blank preparation process and commonly used processing methods of tungsten alloy parts, and provide valuable reference for their precision manufacturing.
PART.02
Preparation process of tungsten alloy parts blanks
2.1 Powder metallurgy technology
Tungsten alloys are difficult to manufacture using conventional alloy smelting and preparation processes due to their high density, high melting point and high hardness. Powder metallurgy is a traditional and widely used method for preparing tungsten alloy blanks. Its key processes are shown in Figure 1, including tungsten powder preparation, mixing, forming and sintering [3]. In the tungsten powder preparation process, in order to ensure the purity of tungsten powder, processes such as hydrogen reduction and ammonia tungstate reduction are often used. At the same time, the oxygen content, particle size and shape of tungsten powder have a significant impact on the final performance and quality of the alloy and must be strictly controlled. Too high oxygen content will reduce the alloy performance, while particle size and shape will affect the effect of subsequent mixing and forming processes. For example, tungsten powder with uniform particle size helps to mix more evenly with other metal powders during mixing, ensuring the consistency of alloy composition. During the mixing process, it is necessary to ensure that the size of various metal powder particles is uniform and the proportion is accurate. Mechanical stirring, ball milling and other methods are often used for precise mixing. The forming methods include cold isostatic pressing, die pressing, powder extrusion forming and powder injection forming, etc. These methods can produce parts with complex shapes.
Figure 1 Key processes of powder metallurgy for tungsten alloy blanks
The sintering process is the key to ensuring that the tungsten alloy achieves the required density, strength and other properties. Two-step sintering [4] is widely used: the pre-sintering stage usually controls the temperature at 1000-1200℃. Within this temperature range, low-melting-point metals such as copper and iron will reach a liquid state and undergo solid-phase diffusion with the surrounding tungsten powder particles, fixing the position of the tungsten alloy powder particles and filling them evenly, thus achieving blank shaping. Next is the high-temperature sintering stage, where solid-phase and liquid-phase reactions occur between the powder particles, ultimately forming a dense tungsten alloy structure. The control of sintering temperature and time plays a decisive role in product performance. If the sintering time is too long, the tungsten crystal size will increase, affecting the density and overall performance of the alloy; if the temperature is too high, it will cause low-melting-point metals such as copper and iron to volatilize, reducing the density and mechanical strength of the alloy. The sintering temperature is generally controlled at around 1400℃, and pre-sintering and high-temperature sintering are usually carried out in a vacuum or inert gas environment to reduce oxidation and volatilization of low-melting-point alloys. A common process is to first pre-fire the formed tungsten powder blank at 1200℃ for 1 hour in a hydrogen atmosphere to give it certain strength and conductivity, and then perform self-resistance sintering using the heat generated by the blank's own resistance to further promote sintering.
2.2 Additive Manufacturing Process
Traditional powder metallurgy methods can only produce blanks with relatively regular shapes. For tungsten alloy parts with complex structures, especially those with complex cavities, complex processing is still required after blank forming to finally meet the part requirements. Currently, there are no effective processing and forming methods for complex closed internal cavities, which limits the design of tungsten alloy parts. Additive manufacturing technology provides a new solution for the design and manufacturing of tungsten alloy parts. Commonly used metal additive manufacturing technologies include selective laser melting (SLM), laser stereoforming (LSF), electric arc additive manufacturing (WAAM), powder bed selective laser melting (L-PBF), and laser directional energy deposition (L-DED) [5, 6]. A comparison of the advantages and disadvantages of different additive manufacturing processes for manufacturing tungsten alloy parts is shown in Table 1. SLM can manufacture parts with complex geometries, but it has problems such as rough surface, large interlayer residual stress and limited part size, and is suitable for manufacturing small parts; LSF can obtain fine, uniform and dense structure, improve the mechanical properties and corrosion resistance of materials, but when the process parameters are not matched, defects such as poor fusion in the deposited layer are easy to occur; WAAM is suitable for manufacturing large-scale, integrated aerospace structural parts, with high forming rate and high density, but the surface quality of the formed parts is poor. A typical tungsten alloy part manufactured by additive manufacturing technology is shown in Figure 2. Compared with the traditional powder metallurgy method, additive manufacturing has obvious advantages in manufacturing complex tungsten alloy parts. It can not only prepare parts with complex structures and cavities to meet special design requirements, but also realize the layer-by-layer deposition of materials, improve material utilization and reduce costs[7]. Table 1. Comparison of Advantages and Disadvantages of Tungsten Alloy Parts Manufactured by Different Additive Manufacturing Processes
a) Anti-scattering grid
b) Porous parts
Figure 2. Typical tungsten alloy parts manufactured using additive manufacturing technology
2.3 Process Comparison and Selection
Powder metallurgy and additive manufacturing are currently the two main methods for manufacturing tungsten alloy part blanks, each with its own advantages and disadvantages in blank preparation. A comparison of tungsten alloy blank preparation processes is shown in Table 2. Powder metallurgy is relatively more mature, yielding materials with better density and offering advantages in stable mass production. Additive manufacturing is a new process developed in recent years, with various derivative process routes, and it has advantages in manufacturing structural parts with complex cavities.
Table 2. Comparison of Tungsten Alloy Blank Preparation Processes
To pursue higher comprehensive performance, tungsten alloy parts in cutting-edge fields such as aerospace, defense, nuclear industry, medical equipment, and electronics tend to have more complex structural features, including thin walls, curved surfaces, and porous structures, which traditional powder metallurgy methods cannot handle. Producing high-density, defect-free pure tungsten alloy parts directly using additive manufacturing still faces several technical challenges. Powder extrusion printing (PEP), a metal indirect 3D printing technology combining 3D printing and powder metallurgy, uses tungsten alloy powder with a binder. The powder is shaped using 3D printing equipment and then post-processed through powder metallurgy debinding and sintering, ultimately yielding high-density, high-performance structural parts. This technology provides a novel solution to the difficulties of machining tungsten alloys and producing complex structures. Its low-temperature forming and high-temperature setting characteristics effectively solve problems such as deformation, cracking, and voids that are easily encountered in other 3D printing processes of tungsten alloys.
PART.03
Machining of Tungsten Alloy Parts
The high density of tungsten alloys results in enormous cutting forces on the cutting tool during machining, requiring the tool material to have high hardness and wear resistance. The tool design also needs to fully consider the distribution and balance of cutting forces. The high hardness of tungsten alloys (typically >40 HRC) leads to a significantly accelerated tool wear rate during machining, affecting machining accuracy and tool life. Tool life and machining efficiency are key factors to consider in machining. Tungsten alloys have relatively low thermal conductivity, and the heat generated during cutting is difficult to dissipate quickly. The cutting edge works at high temperature, which easily generates thermal stress, leading to tool deformation and increased wear [8]. Tungsten alloy chips are granular, making chip removal difficult. They tend to accumulate in the cutting area, forming chip edges, affecting the surface quality of the machined parts, and may also damage the tool and machine tool. A comparison of cutting tools and process parameters for tungsten alloy parts is shown in Table 3. In engineering practice, PCBN tools are one of the best choices. Typical tungsten alloy parts processed by cutting are shown in Figure 3. Table 3 Comparison of cutting tools and process parameters for tungsten alloy parts Figure 3 Typical tungsten alloy parts processed by cutting PART.04 Grinding of tungsten alloy parts Grinding, as a micro-cutting method, is suitable for processing high-hardness materials such as tungsten alloys. Grinding wheel grinding is shown in Figure 4. The grinding wheel is mainly composed of abrasive grains, pores and binders in a specific ratio. Its abrasive grains have obtuse angle characteristics, usually in the range of 90° to 120°. During the grinding process of tungsten alloys, the resulting grinding material is granular. Due to its relatively low viscosity and toughness, the chips are relatively easy to remove and do not easily clog the pores of the grinding wheel [9]. Therefore, when grinding tungsten alloys, grinding wheels with larger pores can be used to improve grinding efficiency and processing quality. In the grinding area, grinding heat is easy to accumulate, which may lead to grinding burns. In order to effectively deal with this problem, it is necessary to adopt high-pressure and high-flow forced cooling measures to remove the heat generated during the grinding process in time and reduce the thermal deformation and thermal stress in the grinding area. Usually, water-based emulsions are selected as cutting fluids to ensure that the cooling effect reaches the best state. The selection of grinding process parameters for tungsten alloy parts is shown in Table 4. Figure 4 Schematic diagram of grinding wheel Table 4 Selection of grinding process parameters for tungsten alloy parts Pure tungsten or tungsten-nickel-copper alloy parts have low magnetism. For thin-walled parts, it is difficult to reliably fix them with traditional electromagnetic chucks. Vacuum adsorption fixtures [10] can be considered for clamping. The vacuum adsorption positioning fixture for tungsten alloy parts is shown in Figure 5. Figure 5. Schematic diagram of vacuum adsorption positioning fixture for tungsten alloy parts
PART.05
Wire EDM Machining of Tungsten Alloy Parts
Tungsten alloys have high melting points and high hardness, and can be machined using high-hardness tools such as coated tools, PVD tools, and ceramic tools. However, these tools experience significant wear, making it difficult to machine features such as pores, narrow slits, and irregularly shaped holes using these traditional tools. Wire electrical discharge machining (EDM) is a special machining process. Its basic principle is to use a continuously moving fine metal electrode wire (usually copper or molybdenum wire) to generate pulsed spark discharges between the workpiece and the workpiece. The generated temperatures are typically as high as 8000–12000℃, sufficient to melt or even vaporize the surface material of the tungsten alloy, thereby achieving the cutting of the workpiece. The relative movement between the electrode wire and the workpiece allows the entire cutting process to form the desired shape on the workpiece surface. Figure 6 shows the machining of irregularly shaped holes on a tungsten alloy blank using wire EDM. Wire EDM is used to process tungsten alloy parts. The high temperature causes a change in the crystal phase of the tungsten alloy surface during cutting, resulting in a modified layer that degrades the unique properties of the tungsten alloy. A "cut-one-repair-three" method is employed during processing, gradually reducing the cutting depth and pulse power parameters to repair the modified layer.
Figure 6: Machining irregular holes on a tungsten alloy blank using wire EDM
PART.06
Conclusion This paper studies and summarizes the blank preparation, common processing methods, and processing difficulties of precision tungsten alloy parts. In blank preparation, powder metallurgy is relatively more mature, yielding materials with better density and offering advantages in stable mass production. Additive manufacturing, a relatively new process developed in recent years, has spawned various process routes and has advantages in manufacturing structural parts with complex cavities. In cutting, PCBN tools have advantages, achieving high tool life and reducing the impact of tool wear on machining accuracy. Grinding technology is advantageous for machining regular surfaces, achieving higher surface quality. For hole and groove features, wire EDM offers high processing efficiency. The aforementioned blank preparation process, as well as cutting, grinding, and wire cutting methods, can effectively improve the manufacturing quality and production efficiency of precision tungsten alloy parts, providing technical support for the development of related industries.
