Effect of liquid nitrogen jet impingement angle on machinability of titanium alloy in cryogenic cooling

In order to further research the effect of cryogenic cooling, the cooling capacity of LN2 jet was studied through the theoretical and experimental methods. Based on the boiling cooling mechanism of LN2, a set of predicted mathematical model considering the evaporating part before impacting, spreading adhesion, and impact spatter abilities was established. The effects of injection velocity and injection angle on the cooling speed, spreading area, and splashing state were analyzed. A series of milling experiments with LN2 impact cooling were executed. The results show that the smaller injection angle can cause a larger temperature difference, as well as achieving the easy nuclear boiling and improving the heat flux. At the same flow, the cooling effect is better at the condition of small milling depth following the smaller injection angle, as well as the excellent cutting quality. At large milling depth, the cooling effect at injection angle αn = 20° is not as good as that of 40°. Meanwhile, the relation between injection angle and feed is similar to that of milling depth. At small LN2 flow, the cooling rate will be less than the required speed at bigger cutting parameter; the processing area of the workpiece cannot be completely cooled, which causes the cooling effect, and cutting quality is not ideal. In addition, the cutting effect at angle αn = 40° is also better at large flow qf = 50 L/h. Furthermore, the smaller injection angle has the larger microhardness, and it is conducive to increase strain rate and rapidly reducing temperature, as well as the degree of high grain refinement. Ultimately, the small LN2 flow is followed small impact speed, the small injection angle 20° has the shortest cooling time with the strong cooling ability, and it is suitable for the cooling processing of small cutting parameters. Under the large flow, the injection angle of 40° can relatively increase the spreading time of LN2 and achieve full boiling heat transfer and can guarantee the complete cooling processing for large cutting parameters.