Tungsten is a metal with very special properties. It has the highest melting point of all pure metals (over 3,000 degrees Celsius), making it particularly useful for producing heat-resistant metal alloys, used in applications such as rocket nozzles and turbines. Additionally, when combined with carbon, it forms tungsten carbide, a material with a hardness close to that of diamond. This hardness is essential for the production of cutting tools, drills, and other equipment used in metalworking and rock drilling.

Burton noted that although current processes may be working, upgrading to newer cutting tools with different coatings or grades can provide significant productivity gains and better surface finishes.

"Speed is the killer in all materials but very much so in titanium," said Burton. "If you go from uncoated to coated, then your speed can go up. Speed is productivity because your feed per revolution stays the same, but the speed goes up. This allows the feed per minute to increase, which is an advantage. Typically, this can be around a 20 per cent gain in productivity. If a shop goes to, say, a PCD insert for finish turning, there will be an even bigger productivity gain. With coated carbide you can expect 150 to180 SFM but up to 600 with PCD. The gain in productivity is huge."

Tungsten is a metal in which China has an undisputed position of strength. China not only holds the largest global reserves of tungsten ores but is also the leader in the production of all tungsten-based compounds. Consequently, the prices of various materials in the tungsten production cycle are primarily determined by the Chinese market, thus influencing prices in all other industrial areas worldwide.

The graph also shows the price of tungsten scrap, whose importance as a substitute for ores, as a source of tungsten, has increased in this century, reaching 35%. Tungsten has a relatively high recycling rate compared to other metals[5].

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China holds a less dominant position in tungsten oxides and hydroxides, which are the most important chemical compounds derived from tungstates. In this market, China competes with the United States and Vietnam for the top exporter position. Additionally, the international market sees significant presence from many other countries, such as South Korea and Japan in Asia, and the Netherlands and Germany in Europe. Even in this situation, the Chinese market still drives global prices, as shown in the graph below.

The experts agree that shops need to explore many options if they hope to improve surface finish when turning titanium.

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Tonne noted that there is a misconception that shops can use a roughing insert for finishing. Geometries and edge preparations are specifically designed for an application, so he recommends inquiring with the tooling vendor on the best insert, not a universal insert.

Given this market structure, Chinese export prices of tungstates influence price dynamics in the EU. The following graph shows the EU customs price of tungstates and the Chinese export prices of ammonium paratungstate and ammonium metatungstate[3].

The initial processing of tungsten ores produces sodium tungstate, which is then converted into ammonium paratungstate and ammonium metatungstate. These compounds are collectively known as tungstates. Global production of tungstates is highly concentrated, with China, Taiwan, and Vietnam representing over 80% of world trade, making this commodity one of the critical raw materials for the European Union[2].

A wiper insert is built up through a number of extra radii. This makes the engagement surface longer, which also affects the surface in a positive way. A long engagement surface also affects the cutting force and the vibration sensitivity.

Lindsay Luminoso, sr. editor/digital editor, contributes to both Canadian Metalworking and Canadian Fabricating & Welding. She worked as an associate editor/web editor, at Canadian Metalworking from 2014-2016 and was most recently an associate editor at Design Engineering.

"The rule of thumb is the surface finish will be twice as good when a normal feed rate is maintained; doubling the feed rate will maintain normal surface finish," said Burton.

"We noticed this many years ago when we introduced wipers," said Burton. "Because of the refraction of light, wipers produce a different-looking surface, showing a grey scale surface appearance. But the surface quality, when measured, is still better. That’s a challenge that many shops have to understand and overcome."

Certain applications, particularly critical components in aerospace, have stringent surface integrity requirements. Many shops have had tried-and-true certified processes in place for years. These processes may be working, but with the latest cutting tool advancements, there are often better or more production options available, particularly as they relate to finish turning.

Tonne added that it’s important for shops to inspect tooling centre height positions frequently. Incorrect cutting heights will result in reduced surface finish quality.

The analysis of the price dynamics highlights how the price of tungstates influences the prices of other products, including scrap. The effect of the Chinese market in determining global prices of tungstates and tungsten oxides and hydroxides is reflected in the prices of tungsten carbides in Europe.

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Tungsten is found in nature in the form of ore. China holds over half of the world's reserves and produces more than three-quarters of the globally extracted ore[1]. However, China is a net importer of ores to meet its high demand. Rwanda is the leading global exporter of tungsten ores, followed by Russia, Bolivia, and Australia. In Europe, Spain and Portugal are among the main producers and exporters.

Luminoso has a bachelor of arts from Carleton University, a bachelor of education from Ottawa University, and a graduate certificate in book, magazine, and digital publishing from Centennial College.

Published by Luca Sazzini. July 1, 2024 .

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Upgrading to newer cutting tools with different coatings or grades can provide significant productivity gains and better surface finishes. Photo courtesy of Sandvik Coromant.

Published by Luca Sazzini. July 15, 2024 .

"All titanium runs with the same general parameters is one misconception," said Tonne. "Usually the phase of the metallurgy is the best starting point. Alpha phase titanium alloys tend to run higher speeds and feeds. Alpha beta, which is the most prevalent, is well-documented, and finally beta alloys are the most difficult. The key is to research the material and understand the properties."

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The analysis of the graph clearly shows the substantial alignment of tungstate prices in the two global regions and how Chinese prices generally precede European prices by an average of two months.

For many years shops have been using uncoated inserts in titanium, and many still do. But now with so many different types of titanium, there are additional factors to consider for better surface finish when turning.

It’s no secret that titanium alloys are difficult to machine, particularly compared to general and stainless steels. It has poor thermal conductivity and generates high cutting forces and heat at the cutting edge. The demand this material places on cutting tools is significant and can lead to both premature tool wear and poor surface quality if not done properly.

Tungsten carbides are essential for providing hardness and abrasion resistance to metallurgical alloys. The global supply of carbides is relatively diversified. However, prices in the EU market are strongly linked to the supply and demand conditions in the Chinese markets for tungstates and tungsten oxides and hydroxides. In the EU market, the price of carbides reflects the costs of tungstates and oxides. Thus, the price formation in the EU market depends on what happens in China, not only for products where China has a strong leadership position but also for derived products.

The low modulus of elasticity causes greater workpiece springback in these materials. This modularity is one of the material’s strengths, particularly when it comes to structural aerospace components that require that movability quality. However, this makes it a challenge to machine, often resulting in tool vibration, chatter, and poor surface finish.

For example, Ti-5553 requires a very hard surface coating to basically get through it. This material can run at only around 50 per cent of the speed of Ti6Al4V.

Having the ability to direct the coolant to the cutting zone is particularly important with titanium. It’s also recommended that shops use both over- and under-coolant for finishing operations and depth of cuts below the nose radius. This allows for the full effect of the high-precision coolant for better chip control, surface finish, and tool life.

Published by Luca Sazzini. July 22, 2024 .

The third graph below shows how all prices along the tungsten supply chain have consistently moved in perfect alignment with one another.

Published by Luigi Bidoia. June 10, 2024 .

"One misconception is that turning almost no material on the last pass will improve surface finish," said Tonne. "In most cases, the finishing pass depth should be equal to the radius of the tool used for turning. Using zero-depth passes will often fail."

The experts recommend using a physical vapour deposition- (PVD-) coated insert because it is thinner. With titanium, it’s not advisable to use a thick coating because of the chemical reaction that can occur. Many of today’s coatings have titanium in them, like TiCN, which is used in chemical vapour deposition (CVD). This is why PVD is more commonly recommended.

Finish turning of titanium must be a wet process. A high-precision coolant offers significant advantages, particularly as it can be directed to the cutting zone.

"We have seen that many shops are unwilling to change from their current processes, even if it means better finishes," said Kevin Burton, turning product and application specialist, Sandvik Coromant, Mississauga, Ont. "If the certified process uses an uncoated carbide insert, shops may not want to change to a coated or diamond insert because of the extra cost both from a time and financial standpoint. It’s the same with grades. With the new generation of grades out, some look different with different colours even though they typically have the same elements, just different percentages. Yet shops tend to be concerned about the coating being left behind on the surface of the part."

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Finish turning of titanium must be a wet process. High-precision coolant offers significant advantages, particularly as it can be directed to the cutting zone. Photo courtesy of Sandvik Coromant.

China is the world's leading exporter of tungsten carbides. The global supply of carbides is relatively diversified, with significant contributions from European countries like Austria and the Czech Republic. The top five exporting countries account for just over 70% of the global trade in carbides, compared to 95% for tungstates. Tungsten carbides are particularly important as they represent 65% of global tungsten consumption. They are used for manufacturing metalworking and rock drilling tools, where their high abrasion resistance is crucial for all mechanical parts subject to wear[4].

"Titanium in general does not shear efficiently, so the material springs back and the size control of a component is more difficult," said Edwin Tonne, training and technical specialist, Horn USA Inc., Franklin, Tenn. "The finishing pass depth must be carefully maintained to account for this behaviour. Additionally, during finishing the forces and heat are concentrated over a small region of the cutting edge, which breaks down the edge more rapidly than in other materials."

"PVD coatings have high temperature resistance and lubricous surface, which reduces built-up edge and heat," said Tonne. "When it comes to inserts, the best shape is round, but it is often not practical due to part shape. C- or W-style inserts offer the best finish with clearance for part features. For negative-type inserts, make sure the top rake is positive. If tight tolerances are required, fully ground inserts can improve outcomes."

For finish turning, Burton suggests that polycrystalline diamond (PCD) inserts are the way to go because of the much higher surface speed, which of course reduces cycle time drastically. However, he noted that some shops have a difficult time accepting a visual difference of the surface.

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The graph shows the EU customs prices for tungsten oxides and hydroxides, compared with Chinese export prices of tungsten trioxide and other oxides and hydroxides. It is evident that the three series move similarly. Additionally, statistical analysis indicates a trend where Chinese prices precede EU price movements by two months.

Titanium generally is used to produce critical aerospace components and medical devices. It offers a high strength-to-weight ratio and excellent corrosion resistance, which enables it to be used for designs with thin walls.

"When we start finish-turning with PCD, it looks totally different than with other inserts," said Burton. "The titanium will look really nicely polished and almost have a hardened steel, superfinished look."

Similar to PCD inserts, wipers are the first choice in stable conditions and it’s recommended to use wipers whenever it is possible.

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Tonne added that titanium is much more sensitive to small changes in speed due to its heat-resistant nature. Speed should only be bumped up in 5-m/min. (15-SFM) steps. It’s also important not to run too slowly, because running below the recommended speed for a given tooling system will cause material to build up on the cutting edge, which effectively dulls the edge.

Wipers are considered primarily as high-feed-rate machining inserts. Using them shops can increase the feed rate, which translates into less time in the cut, effectively increasing tool life. But if wipers are used at a normal feed rate, shops can gain measurable improvements in surface quality.

"We recommend using tooling with internal coolant; this delivers coolant precisely to where it is needed," said Tonne.