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
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.
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
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.
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