Study On Microscopic Mechanism Of Instability In The Primary Shear Zone During High Speed Machining Hardened Steel

The high speed machining has become the important component of the modern advanced manufacturing technology. During high speed machining, a serrated chip is most often generated which affects nearly every aspect of high speed machining process, such as cutting force, cutting temperature, cutting tool wear and life and machined surface quality. Therefore, it is necessary to research the formation mechanism of serrated chip. And the formation of a serrated chip can be seen as a result of material instability in the primary shear zone. In this paper, the microscopic mechanism of instability and damage-fracture in the primary shear zone is investigated during high speed machining AISI-1045 hardened steel by microstructural observation and theoretical analysis, the main research contents are as follows:The influence of material hardness and cutting conditions on the chip morphology is investigated by metallographic observation. By using optical microscope and SEM, the microstructure of deformed band and transformed band is observed. Based on the observations of the fracture surfaces with SEM micrographs, the microscopic mechanism of material instability with different cutting conditions is discussed. In this paper, two microcosmic models of material damage-fracture in the primary shear zone are proposed. One is a brittle damage-fracture process controlled by sprout, extension and connection of micro-cracks during serrated chip formation with deformed band. The other is a ductile damage-fracture process controlled by nucleation, growth and coalescence of micro-voids during serrated chip formation with transformed band. The cutting force is measured and recorded by a piezoelectric dynamometer. The influence of factors including cutting conditions, chip morphology and microscopic mechanism of material damage-fracture on cutting force is discussed.