Fundamental investigations on the development of a single-phase CO2-lubricant solution to support deep-hole drilling processes for difficult to cut materials by using a cryogenic CO2 snow-lubricant-jet

• Applicants: Professor Dr.-Ing. Dirk Biermann; Professor Dr.-Ing. Marcus Petermann
• Subject Area: Metal-Cutting and Abrasive Manufacturing Engineering ; Chemical and Thermal Process Engineering
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 452408713

Heat and chip removal from the cutting zone are very important for small diameter deep hole drilling in nickel-based alloys. The low thermal conductivity, high ductility, hardness and heat resistance make chip breaking difficult and result in high thermomechanical loads on tool and workpiece. Conventional cooling lubrication methods such as flood cooling and Minimum Quantity Lubrication provide insufficient results. A promising approach is the cryogenic Minimum Quantity Lubrication (cMQL) with a mixture of lubricant and liquid or supercritical carbon dioxide. As the carbon dioxide expands to dry ice when released from the drill's cooling channels to atmospheric pressure, heat is removed from the cutting zone and the dissolved oil is released to fulfil its lubricating function. The first project phase resulted in significant findings on the complex cMQL in deep hole drilling, making a substantial contribution to the overall understanding of the process. However, the central research hypothesis that a single-phase solution of lubricant and CO2 leads to superior results due to the complete transport of both media into the chip formation zone could not be confirmed. Oils with delayed dissolution showed excellent results in terms of tool life, but led to unstable feed conditions in combination with CO2. Furthermore, phase-splitting was observed for some oils in the presence of CO2, which has a negative effect on the cMQL process. In the continuation of the project, a high-performance lubricant that has already been identified is to be investigated in a more detailed manner and further developed. The aim is to gain a better understanding of the chemical composition and behavior of the CO2 lubricant mixture in the machine tool and the chip formation zone with the aid of a sensorized test setup and simulations in order to derive optimized variants with improved release properties, higher process stability, and consistently high tribological performance. Furthermore, the currently used setup does not allow CO2 pressure and mass flow to be controlled independently of each other. To ensure that the lubricant mixture is supplied as required, the tool holder is being redesigned so that CO2 pressure and mass flow can be controlled independently of each other in the future. This will minimize the risk of overcooling while ensuring a constant supply of lubricant. The combination of experimental analysis, variation of lubricant formulations, visual process diagnostics, and simulation provides a deep understanding of the key influencing factors within the cMQL system. This creates the basis for a robust, transferable, and sustainable process design, also with a view to industrial application.

• DFG Programme: Research Grants