Hybrid lubricating/cooling strategies to reduce the tool wear in finishing turning of difficult-to-cut alloys

The increasing use of Difficult-To-Cut (DTC) alloys for high performance components has recently pushed the implementation of different lubricating/cooling strategies during machining operations in order to improve the machinability of these alloys as well as increase the tool life. Whereas one lubricating/cooling strategy at a time is commonly implemented, the use of two strategies simultaneously, one devoted mainly to lubrication and the other mainly to cooling, hasn't been applied yet to finishing machining operations. To this aim, the objective of the paper is to evaluate the effect of hybrid lubricating/cooling strategies to reduce the tool wear when finishing machining the wrought Ti6AI4V titanium alloy, commonly regarded as a DTC alloy. A commercial Minimum Quantity Lubrication (MQL) system was implemented together with Liquid Nitrogen (LN2) and Carbon Dioxide (CO2) distribution systems designing the position of the nozzles to optimize the lubrication and cooling effects. The tool wear mechanisms were identified and quantified by means of Scanning Electron Microscope (SEM) and optical profiler, respectively. The experimental results showed that the crater wear was predominant when the sole MQL technique was implemented, whereas the use of LN2 and CO2 reduced it, with the most evident advantages highlighted when adopting CO2 that drastically reduced the thermally activated tool wear mechanisms, but still preserving the lubricating effect of the MQL. It was proved that the adoption of hybrid lubricating/cooling strategies is a viable alternative to the conventional ones, leading to a drastic reduction of the crater wear as well as good surface integrity given an optimized positioning of the lubricant/coolant nozzles. (C) 2017 Elsevier B.V. All rights reserved.