A novel approach of composite turning for compacted graphite iron using minimum quantity lubrication and liquid nitrogen jetting by a developed setup

Compacted graphite iron (CGI) has high mechanical strength, and low thermal conductivity and content of sulfur, becoming a difficult-to-machine material. The tool life for machining grey cast iron (GCI) is 10–20 times of that of CGI, which greatly affecting the mass production in engine industry. To overcome this challenge, the influence of different spray positions on cooling and lubrication efficiency is analyzed based on the boundary layer theory, and a novel approach of composite turning with MQL lubricating flank face and liquid nitrogen (LN2) jetting cooling rake face is proposed. In virtue of boundary layer theory, negative graded pressure forms during the movement between the workpiece and cutter, resulting in that the MQL oil flows quickly to the contact area on flank face, which improves the lubrication effect. LN2 jetting is employed to cool rake face, effectively avoiding the accumulation of cutting heat on the tool and workpiece. The effects of four different cooling methods, including cutting fluid, MQL, LN2, and MQL + LN2, on the surface roughness, tool wear, and cutting force of cutting CGI and grey cast iron (GCI) were studied. According to the failure criterion of a cutting tool for the wear length of 200 μm on a flank face, the cutter life of cutting length is 2886 and 3884 m, correspondingly by cutting fluid and MQL + LN2, and the cutter life is increased by 34.6 %. In addition, the cutting force during turning CGI is 75.1 and 61.8 N by cutting fluid and MQL + LN2, respectively, and the cutting force is decreased by 17.7 %. The developed setup and findings provide a new turning method to cut a difficult-to-machine material for the use in mass production of the engine industry.