Tungsten is one of the strongest metals available, making it ideal for a variety of applications.
But is it possible to machine tungsten? What are the challenges it poses?
Machining tungsten can be extremely challenging, owing to its high strength and brittleness. Generally, tungsten machining is performed by heating the workpiece to around 400˚C to enhance its ductility. However, good process control and appropriate cutting tools can enable to machine tungsten at room temperature at the cost of tool-life.
This article discusses tungsten machining in detail such as the challenges of machining it and provides insights on overcoming those challenges.
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Machining Tungsten: Why is it so Challenging?
Machinability defines the ease with which a material can be cut or shaped while providing a fine surface finish.
Tungsten is a material with poor machinability because it wears the cutting tool very quickly, and machining it usually results in a poor surface finish.
Although machining tungsten is possible, its high hardness and high ductile-to-brittle transition temperature make it difficult to machine.
High Hardness of Tungsten
| Metals | Brinell Hardness | Rockwell A Hardness | Vickers Hardness |
|---|---|---|---|
| Aluminum alloys | 97.1 | 45.2 | 116 |
| Brass | 65.1 | - | 118 |
| Copper | 89 | 35 | 100 |
| Stainless steel | 251 | 54 | 294 |
| Iron | 217 | - | 235 |
| Tungsten | 294 | 66 | 310 |
The hardness value of a material primarily determines its machinability.
When compared to other metals, tungsten has high hardness values, which leads to poor machinability.
Higher hardness values of tungsten increase the required cutting forces to remove the material, and subsequently deteriorates the tool life by increasing the wear rate.
Furthermore, tungsten is a refractory metal, which means that it shows exceptional resistance to heat and wear, making it ideal for various applications, but also reduces its machinability and increases tool wear.
As a result, machining tungsten leads to an extremely short tool life.
High ductile-to-brittle Transition Temperature (DBTT)
DBTT is the temperature above which tungsten transits into ductile behavior from an inherent brittle behavior.
Tungsten generally tends to be brittle at room temperature, making it difficult to machine, and heating it to DBTT (200˚C to 400˚C) enhances its ductile nature, improving its machinability.
Therefore, tungsten machining is generally performed by heating it up to its DBTT, which requires specialized heating systems.
In contrast to tungsten, metals like aluminum and copper do not have high DBTT and tend to remain ductile even at room temperatures.
As a result, machining tungsten can be challenging, especially for DIY or hobbyist projects.
However, it can be performed by using the right tools and maintaining good process control throughout the machining process.
How to Machine Tungsten: Practical Advice
Machining tungsten at room temperature requires a rigid machine that can deliver strong cutting force to the workpiece without failing.
Machining tungsten leads to microchipping and flaking which act as abrasive, degrading the tool life and surface finish of the workpiece.
A C-4 tungsten carbide tool with an indexable carbide tip is recommended for machining tungsten.
As machining tungsten will drastically reduce tool life, it is economical to use an indexable end mill that alloys you to replace the cutting tip without replacing the entire tool.
Generally, machining tungsten leads to high tool wear and a poor surface finish when compared to other metals like steel.
However, setting the optimal parameters for your application can minimize tool wear and enhance the machining quality of tungsten.
Turning Tungsten
The two major problems associated with turning tungsten are short tool life and chipping of the surface.
Turning operation generally uses a single-point cutting tool that is susceptible to quick wear and can often lead to chipping of the surface, if not performed under optimal parameters.
Optimal Parameters for Turning Tungsten
| Type of Tungsten | Depth of Cut (inch) | Speed (ipm) | Feed rate (inches/rev) |
|---|---|---|---|
| 180HB Tungsten | 0.2 | 1000 | 0.01 |
| 290HB Tungsten | 0.2 | 1050 | 0.01 |
| 320HB Tungsten | 0.04 | 1500 | 0.007 |
A single-point uncoated brazed carbide tool with a negative rake angle is recommended for turning tungsten at room temperature.
This is because a negative rake provides high strength to the cutting tool, enabling it to withstand the strong cutting forces required for machining tungsten.
However, a positive rake is preferable when machining tungsten at its ductile-to-brittle transition temperature of around 400˚C.
Using a positive rake angle is preferable in this condition because heating the workpiece enhances its ductility and a positive rake facilitates smooth gliding of the cutting tool with less cutting force requirement.
Although tool wear is unavoidable during the machining of tungsten, selecting the optimal machining parameters will help in controlling the wear, thereby enhancing tool life and increasing surface finish.
Generally, when machining a tungsten workpiece with a hardness of around 180HB, a shallow depth of cut of 0.2 inches (4mm) with a cutting speed of around 1000 ipm (30 m/min), and a feed rate of 0.01 inches per revolution (0.25 mm/rev) is recommended.
Drilling Tungsten
Drilling tungsten is comparatively more challenging than turning.
This is because drilling tungsten at room temperature leads to chipping of edges along the hole, radial cracking that initiates near the hole, and drill fracture due to chip interference.
Therefore, the tungsten workpiece is usually heated to 200˚C to 450˚C to enhance its ductility before drilling.
However, satisfactory results can be achieved by following a proper procedure to drill tungsten at room temperature.
Optimal Parameters for Drilling Tungsten
| Type of Tungsten | Hole Diameter | Speed (ipm) | Feed rate (inches/rev) |
|---|---|---|---|
| 180HB Tungsten | 5mm to 20mm | 2000 | 0.002 |
| 290HB Tungsten | 5mm to 15mm | 1500 | 0.002 |
For drilling tungsten, it is recommended to use properly grounded solid carbide drill bits, with a drill tip runout of less than 0.025 mm.
While drilling tungsten, several factors must be taken into account; for instance, holes must be drilled by maintaining a gap of around one drill diameter from the edge of the workpiece.
This minimizes cracks and produces a smooth drill surface.
Generally, a solid carbide drill bit can drill around 14 to 15 holes in tungsten before losing its sharpness.
However, using a heavy stream of cutting fluid to flush away chips might increase tool life by reducing friction and heat.
For drilling tungsten with minimal wear and a smooth surface finish, it is advised to use moderate speed with a slow feed rate.
For example, when drilling a 5mm hole in a tungsten workpiece, a speed of around 2000 ipm with a feed rate of under 0.002 inches/rev is recommended.
Milling Tungsten
In the case of face milling operations, the composition and the percentage of tungsten play a crucial role.
However, in end milling, the percentage of tungsten doesn’t play a crucial role, but heavy feed during the process wears out the tool very fast.
Climb milling (also called down milling) should preferably be used in the case of end milling to reduce chipping by minimizing chip load on the cutting edge.
End milling under normal circumstances is difficult and usually requires specialized tools like a Monocrystalline diamond tool (MCD) or Polycrystalline diamond tool (PCD).
Optimal Parameters for Milling Tungsten
| Parameters | Optimal Value |
|---|---|
| Depth of Cut | 0.04 inches |
| Speed | 1200 inches/min |
| Feed rate | 0.008 inches per tooth |
If your application demands milling tungsten at room temperature, it is advised to use 85% Tungsten with a Brinell hardness number (HB) of 180 to 200.
Generally, a carbide tool with a negative rake and around 15˚ of relief angles is recommended for milling tungsten.
Furthermore, a sharp tool with a moderate depth of cut ensures smooth cuts with a burr-free finish.
However, maintaining low feed rates and shallow depth of cuts usually leads to the accumulation of chips and results in the clogging of the cutter.
Therefore, it is recommended to use an oil-based cutting tool to provide lubrication and ensure proper chip clearance.
Grinding Tungsten
Grinding involves glazing the surface of the workpiece which can often result in high thermal stress and lead to surface cracking.
A soft grinding wheel is preferred to minimize glazing and thereby reduce the possibility of cracking the tungsten workpiece.
Optimal Parameters for Grinding Tungsten
| Machining Parameter | Optimal Value |
|---|---|
| Abrasive type | Diamond Abrasive |
| Wheel Speed | 1200 m/min |
| Down Feed | 0.015 mm/pass |
Industrial applications of tungsten grinding use diamond abrasives to achieve a high-quality surface finish and minimize the risk of cracking the workpiece.
However, these abrasives are extremely expensive and are not suitable for small-scale applications.
Using a C46N5V silicon carbide-based abrasive wheel with a speed of around 1200 m/min and a down feed of 0.015 mm/pass can also produce good results at the cost of tool life.
Although similar results can also be achieved by an aluminum oxide-based abrasive wheel, the higher hardness of silicon carbide abrasives provides better material removal with comparatively less wear.
Tungsten is vulnerable to flaking around the corners even under the slightest grinding pressure due to its brittleness. Therefore corner grinding of tungsten workpieces is not recommended.
Generally, a constant flow of a soluble oil coolant, heated to around 40˚C to 50˚C is recommended to minimize the risk of surface cracking during grinding tungsten workpieces.
Cutting Fluid for Machining Tungsten
Soluble oil (1:20) emulsions are recommended for most machining operations of tungsten except drilling.
This is because soluble oil coolants provide high lubrication with comparatively less heat dissipation than water-based coolants.
Increasing the lubrication ensures smooth gliding of the cutting tool, resulting in a smoother cut with a high surface finish.
On the other hand, oils containing chlorine and sulfur are preferred for drilling operations for better chip flow.
Optimal Tool Path for Machining Tungsten
Adopting a traditional tool path strategy (pocketing and slotting) for CNC machining of tungsten can lead to excessive vibration and premature wearing of the tool.
Instead, the trochoidal performance cutting (TPC) approach must be attempted.
TPC tool path planning involves a selection of irregular helix angles along with a varying length of front cutting edge to obtain a better surface finish with minimal vibrations.
Moreover, trochoidal slot milling is inherited with intermittent cutting, which reduces the negative effect of the heat on the finished surface by altering the tool's contact with the workpiece.
Selecting the Right Tungsten Alloy for Machining
Adding other materials to tungsten improves its machinability and facilitates its multifunctional attributes.
Tungsten-based Heavy Metal Alloys
Tungsten-based heavy metal can be bifurcated into magnetic and non-magnetic types, wherein the tungsten content lies between 90% and 98%.
The composition of magnetic heavy metals consists of W-Ni-Fe-(Co, Mo), whereas that of non-magnetic is W-Ni-Cu.
Depending on the tungsten percentage, these materials' densities vary from 17-18.8 gm/cm3.
Generally, tungsten-based heavy metal alloys provide enhanced ductility, increased elongation, and better machinability.
Owing to their capability of absorbing γ and X-rays, they are suited for applications where radiation shielding is desired.
Tungsten Composites of Copper and Silver (W-Cu and W-Ag)
W-Cu and W-Ag, also known as pseudo alloys, exhibit outstanding thermal and electrical conductivity and excellent wear resistance.
The composites usually consist of 10 and 40 weight percent of copper, whereas the percentage of silver lies in the range of 20-50 percent.
Taking into account their superior electrical and mechanical properties, they are used for applications such as circuit breakers, heat sinks, and welding electrodes.
Cemented Carbides (WC-Co)
Wc-Co products represent the most considerable tungsten utilization (60%-70% approximately) across many industries.
The material provides an outstanding combination of properties, including excellent wear resistance, high hardness, and excellent toughness properties at high temperatures.
WC-Co material is mainly used for manufacturing cutting tools (milling, drilling, turning) suitable for machining a variety of materials.
Cemented carbides are also used as abrasives for making grinding wheels.
Tungsten in Steel
Tungsten in weight percentage of 1-18% is also added in several grades of steel.
Steels with the added tungsten demonstrate better yield strength and increased ultimate tensile strength.
One typical example is high-speed steel (HSS) tools alloyed with tungsten, allowing them to exhibit better resistance and high hardness at high temperatures of around 600˚C.
Final Thoughts
Although adopting the proper tool (tungsten carbide) and ensuring good process control, can enable you to machine tungsten at room temperature, it drastically reduces tool life and results in poor surface finish.
For industrial applications, tungsten machining is performed by heating the workpiece to increase its ductility, thereby reducing the need for strong cutting forces and enhancing its machinability.
As a result, it is difficult to machine tungsten for small-scale applications.
Therefore, it is advised to opt for an alternative tungsten alloy such as W-Cu or W-Ni-Cu to improve its machinability while maintaining its major mechanical properties.
Frequently Asked Questions (FAQ)
What tools must be opted for reaming tungsten?
For reaming, tungsten carbide tools are recommended. However, HSS reamers can also be used at reduced machining speed and feed compared to carbide tools.
How to perform tapping on tungsten?
Tapping of tungsten is quite challenging at room temperatures, and the workpiece must be heated up to 425˚C. Plug tap made up of nitrided M10, or HSS (T15 grade) with four flute stubs must be preferred with a speed of 1.5 m/min.
Why is W-Cu called a pseudo alloy?
Tungsten and copper are not soluble in each other; consequently, copper tends to remain dispersed in the tungsten matrix. Therefore when observed under the microscope, the material appears more like a composite than an alloy.
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