Nightmare Material Tungsten Alloy CNC Machining Experience

Tungsten alloys are often considered a challenge for engineers due to their high density and hardness, which create significant processing difficulties. Transforming this tough material from its raw state into a finished product efficiently can feel like a struggle.

In this article, we will explore the entire process, starting with the preparation of raw materials. We will compare the advantages and disadvantages of powder metallurgy and 3D printing, outline key techniques such as cutting, grinding, and wire cutting, and introduce an innovative technology that can enhance efficiency—powder extrusion printing (PEP). This approach offers a solid foundation for your design and manufacturing needs.

 

PART.01 Introduction

Tungsten alloys, which primarily consist of tungsten (with tungsten content typically ranging from 85% to 99%), are enhanced by the addition of elements such as nickel, iron, copper, cobalt, molybdenum, and chromium. These alloys play a crucial role in many high-tech fields, including the nuclear industry, military applications, and medicine, due to their extremely high density (16.5–19.0 g/cm³), elevated melting point, and exceptional mechanical strength.

In the medical field, tungsten alloys are used to manufacture essential components of radiotherapy equipment. Their high density enables precise blocking and shaping of radiation, making them a preferred material for collimators and radiation shielding. However, these superior properties also present several challenges in the processing and manufacturing of tungsten alloys.

This article aims to provide a systematic and in-depth exploration of the blank preparation process and the commonly used processing methods for tungsten alloy parts, offering valuable insights for their precision manufacturing.

 

PART.02 Preparation Process of Tungsten Alloy Parts Blanks

2.1 Powder Metallurgy Technology

Due to the high density, high melting point, and exceptional hardness of tungsten alloys, conventional smelting and preparation processes are often ineffective. Powder metallurgy is a traditional and widely used method for preparing tungsten alloy blanks, and it involves key processes as illustrated in Figure 1. These processes include tungsten powder preparation, mixing, forming, and sintering.

In the tungsten powder preparation stage, methods such as hydrogen reduction and ammonia tungstate reduction are commonly employed to ensure the purity of the tungsten powder. Additionally, factors such as oxygen content, particle size, and particle shape significantly influence the final properties and quality of the alloy, making strict control essential. Excessive oxygen content can diminish the alloy’s properties, while the size and shape of the particles affect the effectiveness of subsequent mixing and forming processes. For instance, using uniformly sized tungsten powder facilitates more uniform mixing with other metal powders, ensuring consistent alloy composition.

During the mixing process, it is crucial to maintain uniform particle sizes among the various metal powders and to use accurate proportions. Mechanical stirring, ball milling, and other precise mixing methods are typically employed. The forming methods include cold isostatic pressing, die pressing, powder extrusion molding, and powder injection molding, which enable the production of parts with complex shapes.

 

The sintering process is essential for ensuring that tungsten alloys achieve the desired density, strength, and other critical properties. A commonly used method involves two key stages of sintering.

In the pre-sintering stage, the temperature is typically maintained between 1000 and 1200°C. At this temperature range, low-melting-point metals such as copper and iron reach a liquid state and engage in solid-phase diffusion with the surrounding tungsten powder particles. This process stabilizes the positions of the tungsten alloy particles and ensures uniform filling, shaping the blank effectively.

Following the pre-sintering stage, a high-temperature sintering stage occurs. During this phase, solid-phase and liquid-phase reactions take place among the powder particles, ultimately forming a dense tungsten alloy structure. Controlling the sintering temperature and time is critical for optimal product performance. If the sintering time is excessively long, the tungsten crystal size will increase, negatively impacting the alloy’s density and overall properties. Conversely, if the temperature is excessively high, low-melting-point metals like copper and iron may volatilize, leading to a reduction in the alloy’s density and mechanical strength.

Typically, the sintering temperature is controlled at approximately 1400°C. Both the pre-sintering and high-temperature sintering processes are usually conducted in a vacuum or inert gas environment to minimize oxidation and the volatilization of low-melting-point metals.

A common procedure starts with pre-firing the formed tungsten powder blank at 1200°C for one hour in a hydrogen atmosphere to impart certain strength and conductivity. After this, the blank undergoes resistance sintering, which utilizes the heat generated by the blank’s own resistance to further promote the sintering process.

 

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 intricate cavities, significant machining is required after blank formation to meet the necessary specifications. Currently, there are no effective machining techniques for complex closed internal cavities, which limits the design possibilities for tungsten alloy components.

Additive manufacturing technology presents a new solution for designing and producing tungsten alloy parts. Commonly used metal additive manufacturing technologies include selective laser melting (SLM), laser stereoforming (LSF), wire arc additive manufacturing (WAAM), powder bed selective laser melting (L-PBF), and laser-directed energy deposition (L-DED). A comparison of the advantages and disadvantages of these different additive manufacturing processes for tungsten alloy parts is summarized in Table 1.

SLM can manufacture parts with complex geometries; however, it faces challenges such as surface roughness, high interlayer residual stress, and size limitations, making it more suitable for small parts. LSF is capable of producing fine, uniform, and dense microstructures, which enhance the mechanical properties and corrosion resistance of the material. However, mismatched process parameters can lead to defects, such as poor fusion in the deposited layer. WAAM is suitable for creating large-scale, integrated aerospace structural components, offering high forming rates and density, but it typically results in lower surface quality.

An example of a tungsten alloy part manufactured using additive manufacturing technology is shown in Figure 2. Compared to traditional powder metallurgy methods, additive manufacturing offers significant advantages in producing complex tungsten alloy parts. It enables the fabrication of components with intricate structures and cavities that meet specific design requirements, while also allowing layer-by-layer material deposition, which improves material utilization and reduces costs.

 

 

2.3 Process Comparison and Selection

Powder metallurgy and additive manufacturing are currently the two primary methods for producing tungsten alloy part blanks, each with its own set of advantages and disadvantages in the preparation of these blanks. A comparison of the tungsten alloy blank preparation processes is presented in Table 2.

Powder metallurgy is a more established method that yields materials with superior density and excels in stable mass production. In contrast, additive manufacturing is a relatively new process that has emerged in recent years, offering various approaches. This method provides significant advantages in the production of structural parts with complex cavities.

 

Table 2 Comparison of Tungsten Alloy Blank Preparation Processes

To achieve higher overall performance, tungsten alloy parts in advanced fields such as aerospace, defense, nuclear energy, medical equipment, and electronics often have more complex structural features, including thin walls, curved surfaces, and porous designs. Traditional powder metallurgy methods struggle to accommodate these complexities. Additionally, directly using additive manufacturing to produce high-density, defect-free pure tungsten alloy parts presents several technical challenges.

Powder extrusion printing (PEP) is an indirect 3D printing technology that combines 3D printing and powder metallurgy. It utilizes tungsten alloy powder mixed with a binder. In this process, the powder is shaped using 3D printing equipment and subsequently undergoes powder metallurgy steps such as debinding and sintering. This method results in structural components that are dense and perform exceptionally well.

PEP offers an innovative solution to the challenges of machining tungsten alloys and fabricating complex structures. Its low-temperature forming and high-temperature setting capabilities effectively address issues like deformation, cracks, and voids, which are commonly encountered in other 3D printing processes for tungsten alloys.

 

PART.03 Machining of Tungsten Alloy Parts

The high density of tungsten alloys results in significant cutting forces acting on the cutting tool during metal CNC machining. This necessitates that the tool material has high hardness and wear resistance. Additionally, tool design must carefully consider the distribution and balance of these cutting forces. The high hardness of tungsten alloys (typically exceeding 40 HRC) leads to an accelerated wear rate of the cutting tool, which can negatively impact machining accuracy and tool lifespan.

Tool life and machining efficiency are crucial factors to keep in mind during the machining process. Tungsten alloys also have relatively low thermal conductivity, making it challenging to dissipate the heat generated during cutting quickly. As a result, the cutting edge can reach high temperatures, which generates thermal stress that may cause tool deformation and further accelerated wear.

Moreover, tungsten alloy chips are granular, which complicates chip removal and can lead to accumulation in the cutting area. This accumulation can create a chip edge that adversely affects the surface finish and may potentially harm both the tool and the machine.

A comparison of cutting tools and process parameters for machining tungsten alloy parts is presented in Table 3. In engineering applications, polycrystalline cubic boron nitride (PCBN) tools are considered one of the best options. An example of a typical tungsten alloy part is illustrated in Figure 3.

 

PART.04 Grinding of Tungsten Alloy Parts

Grinding, known as a micro-cutting method, is effective for machining high-hardness materials such as tungsten alloys. As illustrated in Figure 4, a grinding wheel consists mainly of abrasive grains, pores, and a binder, all combined in a specific ratio. The abrasive grains typically have an obtuse angle, varying between 90° and 120°. When grinding tungsten alloys, the resulting chips are granular. Due to their relatively low viscosity and toughness, these chips are easier to remove and are less likely to clog the wheel’s pores. Consequently, using grinding wheels with larger pores can enhance both grinding efficiency and machining quality.

In the grinding zone, heat tends to accumulate, which can lead to grinding burns. To effectively manage this issue, high-pressure, high-flow-rate cooling measures are necessary to quickly dissipate the heat generated during grinding. This helps to reduce thermal deformation and stress in the grinding zone. Water-based emulsions are typically chosen as cutting fluids to ensure optimal cooling. The selection of grinding process parameters for tungsten alloy parts is summarized in Table 4.

 

Pure tungsten and tungsten-nickel-copper alloy parts exhibit low magnetic properties, making it challenging to securely hold thin-walled components with traditional electromagnetic chucks. As an alternative, vacuum adsorption clamps can be used for effective clamping. An example of a vacuum adsorption positioning clamp designed for tungsten alloy parts is illustrated in Figure 5.

 

PART.05 Wire EDM Machining of Tungsten Alloy Parts

Tungsten alloys are known for their high melting points and exceptional hardness, which allow for machining with high-hardness tools such as coated tools, PVD (Physical Vapor Deposition) tools, and ceramic tools. However, these tools tend to wear down quickly, making it challenging to machine features like pores, narrow slits, and irregularly shaped holes using traditional methods.

Wire EDM (Electrical Discharge Machining) is a specialized machining process that addresses these challenges. It works by using a continuously moving fine metal electrode wire, usually made of copper or molybdenum, to create pulsed spark discharges between the wire and the workpiece. The temperatures generated during this process can reach up to 8000–12000℃, which is sufficient to melt or even vaporize the surface material of the tungsten alloy, thereby achieving precise cutting of the workpiece.

As the electrode wire moves in relation to the workpiece, it shapes the surface according to the desired design. For instance, Figure 6 illustrates the machining of irregularly shaped holes on a tungsten alloy blank using wire EDM.

However, when machining tungsten alloy parts with wire EDM, the high temperatures can alter the crystal structure of the tungsten alloy’s surface, resulting in a modified layer that can degrade its unique properties. To mitigate this issue, a “cut-one-repair-three” method is employed during the machining process. This technique involves gradually reducing the cutting depth and adjusting the pulse power parameters to repair the modified layer and preserve the material’s integrity.

 

PART.06 Conclusion

This paper studies and summarizes the preparation of blanks, common machining methods, and the challenges faced in machining precision tungsten alloy parts.

In terms of blank preparation, powder metallurgy is a well-established method that produces materials with superior density and has advantages in stable mass production. On the other hand, additive manufacturing, a relatively new process developed in recent years, has led to various methods that excel at fabricating structural parts with complex cavities.

When it comes to cutting, PCBN tools provide significant benefits, including extended tool life and reduced impacts of tool wear on machining accuracy. Additionally, grinding technology is effective for achieving high surface quality on regular surfaces. For features like holes and grooves, wire EDM is known for its high machining efficiency.

These blank preparation processes and machining methods—cutting, grinding, and wire EDM—can significantly enhance the manufacturing quality and production efficiency of precision tungsten alloy parts. This provides essential technical support for the advancement of related industries.

 

 

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