“Carbon dioxide is a serious alternative to coolant for machining difficult-to-machine materials,” says Thomas Schaarschmidt, Director of Business & Application Development at Walter. And he can offer some good reasons to be confident about 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,” says Schaarschmidt. “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% without increasing the tool wear rate. If the metal removal rate remains constant, the tool life increases accordingly.”

The evidence provided by the experts at Walter is supported by various machine manufacturers and research institutes, including the Institute for Production Technology (IfP) at the West Saxon University of Applied Sciences of Zwickau (WHZ); 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% 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 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.

Titanium aluminide – lightweight, resistant, difficult to machine

Cooling with carbon dioxide is ideal for working with titanium structural components made from TiAl6V4, which are used in the aerospace industry. Here, the experts at Walter have managed to prove up to 35% longer tool life accompanied by a 50% 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 grams per cubic metre, compared to 8.5 grams per cubic metre. However, titanium aluminide can still 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, which is to say that 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.

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 degrees Celsius, 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

This is another reason why Schaarschmidt is convinced that cooling with CO2 has a golden future – in both senses of the word.

“We have now developed a new generation of cutting tool materials: Walter Tigertec Gold – a completely new technological platform based on titanium aluminium nitride,” he says. “In the future this will enable the use of completely new types of coating-substrate combinations. Tigertec Gold impresses partly due to its extremely high hot hardness, which makes it ideal for milling difficult materials with high cutting speeds. This also ensures reduced formation of hairline cracks in the indexable insert, which primarily occur with interrupted cutts and thermal stress variations, known as thermal shocks for short.”

This makes Tigertec Gold an ideal cutting tool material for use with cryogenic cooling.

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, Walter 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 -78.5 degrees Celsius (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 Walter’s facility 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 (ATS) 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,” explains Schaarschmidt. “However, we are also developing machining solutions for one-channel solutions. To do this, we have converted a machining centre in our Technology Center in Waukesha (USA).”

Tried and tested tools for cryogenic machining

Until now, Walter has been offering the tools required for cryogenic machining strategies as customer-specific special solutions. Walter customers can order all our tried and tested tooling systems with inserts, such as Walter BLAXX milling cutters, for cooling with CO2.

“This is still a semi-standard option,” says Schaarschmidt. “But we will develop it to become standard.”

The tool solutions in the area of solid carbide milling with two coolant channels for the separate supply of CO2 and minimum quantity lubrication (MQL) are new. The necessary tool adaptors were developed in cooperation with Haimer. According to Schaarschmidt, in addition to an efficient and reliable cooling and lubrication system, the correct machining strategy is also important.

“We have acquired a great deal of experience in this area over the past few years,” he says. “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.”

What’s more, 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, we can currently introduce a second channel up to a diameter of 63 mm,” says Schaarschmidt. “A maximum diameter of 25mm applies in the solid carbide area.”