Coolant For Difficult Materials: Carbon Dioxide Is The New Cool Featured

  • Wednesday, 29 March 2017 00:00

Recent research has shown that carbon dioxide for milling on exceptionally difficult materials with promising results. By Lim Gan Shu, Southeast Asia marketing manager, Walter AG


The F2334R copy mill for turbine blade machining with cryogenic
cooling by Walter. Other channel diameters and orientations
required a complete redesign.

Thomas Schaarschmidt is confident that carbon dioxide (CO2) presents a serious alternative to coolant for machining difficult-to-machine materials. And the director of business and application development at Walter AG provides some good reasons for this.

"During the development of cryogenic cooling with CO2, a large number of tests were carried out using various materials and under real production conditions. Our findings not only proved the general process suitability of CO2 cooling, but that it also increases the metal removal rate by up to 70 percent without increasing the tool wear rate. If the metal removal rate remains constant, the tool life increases accordingly," Mr Schaarschmidt said.

This evidence was provided by the company, as well as various machine manufacturers and research institutes such as the Institute for Production Technology at the West Saxon University of Applied Sciences of Zwickau, the Fraunhofer IPT and WZL at RWTH Aachen University and the Fraunhofer IWU at the Chemnitz University of Technology.

For example, when milling high-alloy steels containing nickel, a 70 percent longer tool life was achieved for these materials which are typically used in the manufacture of turbine blades and turbine housings.

Similar productivity gains were also demonstrated in the machining of turbocharger materials for the automotive industry, as well as of stainless steel casting and titanium alloys for the aviation industry.

Difficult-To-Machine Alloys


Using inserts specifically designed for machining materials with
difficult cutting properties along with CO2 will help to metal
removal rate without increasing the tool wear rate, reportedly
by up to 70 percent.

Cooling with carbon dioxide is ideal for working with titanium structural components made from TiAl6V4 which are used in the aerospace industry. Here, researchers have managed to prove up to 35 percent longer tool life accompanied by a 50 percent increase in the metal removal rate.

Materials such as gamma titanium aluminide are increasingly being used in the manufacture of turbine blades, in the aviation and aerospace industry in particular.

The reason is simple: The intermetallic compounds of titanium and aluminium are considerably lighter than the nickel-based alloys commonly used previously – with a density of 3.8 g per cubic cm compared to 8.5 g per cubic cm. However, titanium aluminide is still able to withstand the high temperatures found in the engines and is creep-resistant.

It goes without saying that the weight of the engines is extremely important in the production of aircraft or spacecraft. On the one hand, the increase in centrifugal force relative to weight is quadratic; if the weight is halved, the centrifugal forces will be reduced to a quarter of their starting value. On the other hand, a lighter aircraft consumes less fuel, flies better and is more environmentally friendly when in use.

The downside is that titanium aluminide is extremely difficult to machine. The result is extremely high tool wear and a tool life of just minutes.

High temperatures mitigate the machining problem, as the material starts to soften a little above 750 deg C, making it a little easier to machine. However, it is precisely this that causes maximum stress on the tools used and shortens their tool life. The solution is to cool the cutting edge as directly as possible.

"Golden“ Future For CO2?

Walter has developed a new cutting tool material based on titanium aluminium nitride (TiAlN) called Tiger Tec Gold. Mr Schaarschmidt said that that due to its extremely high hot hardness, the material makes it suitable for milling difficult materials with high cutting speeds.

He added that the coating material also reduces “formation of hairline cracks in the indexable insert, which primarily occur with interrupted cuts and thermal stress variations (known as thermal shocks for short),“ which also makes the material suited for use with cyrogenic cooling.

Two-Channel Supply System


The two-channel supply system delivers both CO2 and either
lubricant, compressed air or emulsion directly to the cutting
edges.

In terms of process reliability, it is crucial that the coolant is applied as precisely as possible to the cutting edge and at the desired temperature.

To do this, the company has worked with Starrag and various technology partners to develop a two-channel supply system—via the machine, spindle, toolholder and tool to the cutting edge. One channel delivers the CO2 and the other delivers the lubricant, compressed air or emulsion directly to the cutting edge without a drop in pressure and at room temperature.

Cooling to a maximum of negative 78.5 deg C (theoretically) first occurs at the nozzle when the CO2, which has been in liquid form until that point, expands. Unlike external supply systems, this barely cools the workpiece, which makes it relatively easy to machine. The separate supply system also ensures a very efficient lubricating effect.

At the company’s headquarters in Tübingen, Germany, an Aerosol Master 4000cryolub system from Rother Technologie is used for supplying the CO2 and cooling lubricant. It combines the aerosol dry lubrication technology developed by Rother with cryogenic cooling.

Depending on requirements, it can be used to adjust the supply of liquid CO2 or aerosol as required, meaning that the supply can be adapted according to the component and material.

"Controlling the amount of cooling lubricant supplied allows us to very precisely control the degree of cooling," explained Mr Schaarschmidt. “However, we are also developing machining solutions for one-channel solutions. To do this, we have converted a machining centre in our Technology Centre in Waukesha, USA.“

Tried And Tested Tools

Until now, Walter has been offering the tools required for cryogenic machining strategies as customer-specific special solutions. However, the tool solutions in the area of solid carbide milling with two coolant channels for the separate supply of CO2 and MQL (minimum quantity lubricant) are new. The necessary tool adaptors were developed in cooperation with Haimer.

In addition to an efficient and reliable cooling and lubrication system, the correct machining strategy is also important. "For example, long cuts are significantly more effective than interrupted cuts, so the machining strategy should be adapted to incorporate long, continuous cuts as far as possible," explained Mr Schaarschmidt.

Additionally,the two-channel supply system can be retrofitted without any problems, provided that the spindle and/or rotary feed-through used in the machine tool allows this. In the indexable insert area, a second channel up to a diameter of 63 mm can be introduced, and a maximum diameter of 25 mm applies in the solid carbide area.

Ready For Cost-Effective Series Production With CO2


Thomas Schaarschmidt, director of business
& application development at Walter AG,
thinks that carbon dioxide is a serious form
of coolant for cyrogenically machining
difficult-to-machine materials.

Thomas Schaarschmidt gives a rundown on cyrogenic machining’s feasibility and other considerations.

Q: Mr Schaarschmidt, what is the current status at Walter with regard to cutting tool materials development for cryogenic machining?

Thomas Schaarschmidt(TS): We have now carried out a large number of test runs with well-known customers from the automotive, energy and aviation industries, and they have similarly positive results. Based on these findings, we can tell our customers who want to work with CO2 cooling exactly which cutting tool materials are ideal and which is the best machining strategy. In short: From our point of view, this technology is ready to be used in large-scale production for the first time.

Q: What are the main challenges still currently facing cryogenic machining?

TS: The first is to find a technology partner who is willing to implement this technology in their production processes together with us and other partners. The second (which is linked to the first) is the lack of safety provisions to date. There have so far been no recommendations for action specifying the activities required by the user for the safe operation of a machine tool with CO2 cooling.

Over the course of the year, we have successfully solved the issues associated with this. Together with other tool manufacturers and system partners, the Research and Transfer Centre at the University of Applied Sciences of Zwickau started a working group on the topic of safety at work when using CO2 as a cooling medium.

As a result of the AMB, we were able to publish recommendations for the safe operation of a machine tool with CO2 cooling. The 38-page document contains all the essential legal framework conditions and explanations of the necessary safety measures when operating machine tools with CO2.

Q: Do you carry out development work together with cooling lubricant manufacturers as technology partners? Which ones? What synergy effects can be derived from this?

TS: We are part of a working group with a large number of partners in which a cooling lubricant manufacturer is also represented. Synergy effects include the fact that we use and test new cooling lubricants to determine whether, under certain conditions, these are more suitable than compressed air, MQL or even CO2. In the end, it is about offering the best solution for the customer.

Q: In your opinion, when will milling using CO2 as a cooling medium be established in the industry? What is the demand at present? Which applications have customers been using these tools for until now?

TS: Interest within the aerospace industry along with other areas is very high. We receive a great many inquiries about how CO2 cooling can be profitable under certain conditions. We have proven the potential savings associated with using CO2 in a large number of projects carried out together with end customers from various industries. There are currently at least three projects being undertaken at universities where end customers are working with us to test the potential of CO2 cooling.

This is in addition to projects from machine manufacturers which are raising similar issues. The extent to which CO2 will be established as a cooling medium depends on its suitability for use in series production. This must be tested together with the end customer in the next step. In light of this, the next question is "when".

Q: Are there development approaches other than fitting tried and tested standard tools with the option for cryogenic cooling?

TS: We know what the tool design needs to look like and how the interfaces and machining strategies need to be structured. In short, we have all the knowledge and the technology to start series production of our tools and assembly parts immediately. And we are, of course, open to any reasonable further development.

Cooling with CO2 works extremely well in defined application cases and is economically feasible but has not yet been tested in series production. One important starting point for further development is the standardisation of the interface between the coolant transfer in the tool adaptor and the rotary feed-through in the machine tool.

APMEN Cutting Tools, Mar 2017

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