Development Of New Cryogenic MQL Equipment And Research On The Cryogenic High Speed Cutting Of Difficult-to-machine Materials

Difficult-to-machine materials have been widely used in the aerospace, weapon, chemical, and electronic industry. However, rapid tool wear is the bottleneck that restricts the enhancement of machining efficiency of difficult-to-machine materials. As one of the effective approaches, cryogenic MQL may improve the machining efficiency of difficult-to-machine materials better than the current techniques. Aiming at some problems such as the key technologies of cryogenic MQL and its application in the machining of difficult-to-machine materials, the groping research was conducted in this dissertation. The main research works are included as follows:1. Composite refrigeration method was adopted to develop new cryogenic MQL equipment, and meanwhile experiments were carried out to test its performance. The results indicate that this equipment has many merits such as high refrigeration speed, adjustment capacity, credibility and control abilities of the temperature of refrigerated gas, which provides a new effective means for the research and application of cryogenic MQL cutting technology.2. By comparing the cutting temperature, cutting force and friction state at the rake face of cutting tool in the high-speed cutting of titanium alloys under different cooling/lubrication conditions, the cooling/lubrication effect of cryogenic MQL was investigated. The cooling/lubrication mechanism of cryogenic MQL in the high-speed cutting was disclosed as follows: the good cooling effect of cryogenic MQL reduces the evaporation of cutting oil, and benefits the maintenance of strength of lubrication film, thus improving the lubrication effect of cutting oil.3. High-speed turning experiments of nickel-based alloys were conducted to investigate the influence of cooling/lubrication conditions on tool wear, surface roughness and chip shape. The tool wear mechanism in the high-speed turning of nickel-based alloys was analyzed under cryogenic MQL condition. The effect mechanism of cryogenic MQL on chip shape was discussed in the high-speed turning of nickel-based alloys. 4. The tool wear mechanism and fracture mechanism of cemented carbide tools in the high-speed milling of titanium alloys were analyzed under cryogenic MQL condition. It was pointed out that the main wear mechanisms were adhesion and diffusion, and the main fracture mechanism was thermal cracking. The cryogenic MQL may improve friction and contact state, restrain the adhesion wear and diffusion wear. And its lubrication effect is the main reason of improvement of tool life in the high-speed milling of titanium alloys. Through the investigation of thermal cracking of cutting tool in the high-speed milling of titanium alloys, it was found that due to thermal fatigue, flood coolant resulted in the formation and propagation of thermal cracks at very fast rate, thus causing severe thermal cracking of cutting tool at the initial stage of milling. Although thermal cracks were also observed under cryogenic MQL condition, they did not generally induce the early fracture of cutting tool.