On the Metal Physical Considerations in the Machining of Metals

Experimental studies of plastic deformation produced during metal cutting have shown that a dynamic equilibrium is established between strain hardening and recovery during chip formation. Recrystallization studies on interrupted cut specimens show that the chip is formed by shear on a thin plane or surface which segments the chip into a lamella structure. Scanning and transmission electron microscopy studies on the lateral surfaces of prepolished interrupted cut specimens substantiate the evidence obtained from the recrystallization studies. The chip formation process has thus been found to be strongly sensitive to the metal physics and defect strticture of the material undergoing plastic deformation. The important variables involving dislocation interactions during chip formation are the number and orientation of operable slip systems, certain characteristic dislocation parameters such as stacking fault energy, the interaction of dislocations with vacancies and solute atoms or with second phase particles (both coherent and noncoherent types), the short and long range order of the material, and the temperature of the deformation, all of which affect the strain hardening behavior of the material. In addition, those factors which govern the kinetics of dynamic recovery such as outright collision of dislocation segments, cross slip, and climb induced by a supersaturation of point defects produced in the course of deformation must be considered.