How are residual stresses in cuttings generated

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Mechanism of generation: During the machining of parts, the machined surface undergoes severe non-uniform elastoplastic deformation due to the effects of cutting forces and cutting heat. Additionally, changes in the metallographic structure also contribute to the generation of residual cutting stresses. The main reasons for the generation of cutting residual stresses include the following three. (1) Effect of mechanical stress on plastic deformation: During the cutting process, the surface layer of metal, which was originally connected to the chip, undergoes considerable elastoplastic deformation in the same direction as the cutting direction. After the chip separates, residual tensile stress remains on the surface, while residual compressive stress persists in the core. Meanwhile, the surface layer metal also undergoes plastic deformation in the direction opposite to the cutting force. The extrusion of the tool on the machined surface causes tensile plastic deformation of the surface layer metal; however, due to the resistance from the base metal, residual compressive stress is generated on the surface of the workpiece. Furthermore, the cold plastic deformation of the surface metal causes the lattice to distort and become loose, resulting in a decrease in density and an increase in volume; this also generates residual compressive stress in the surface layer while leaving residual tensile stress in the core. (2) Thermal stress plastic deformation effect: During cutting, intense plastic deformation and friction cause the temperature of the processed surface layer to become very high, while the temperature in the core remains lower. When the thermal stress exceeds the yield strength of the material, the surface layer will elongate at high temperatures; however, due to the constraint imposed by the matrix material, this elongation is suppressed. During the cooling process after cutting, the elasticity of the metal gradually recovers. When cooled to room temperature, the surface layer of metal contracts; however, due to being restrained by the base metal, residual tensile stresses are generated in the surface layer of the workpiece. (3) Local change in the microstructure of the surface layer: The high temperatures generated during cutting can cause changes in the microstructure of the surface layer. Since different microstructures have different densities, the volume of the surface layer also changes. For example, the density of martensite is 7.75 g/cm3, the density of austenite is 7.968 g/cm3, the density of pearlite is 7.78 g/cm3, and the density of ferrite is 7.88 g/cm3. If the surface layer expands in volume, residual compressive stress will be generated ； Conversely, residual tensile stress is generated. Influencing factors: The nature and magnitude of residual stress after cutting are influenced by many factors. Understanding the effects of these factors and making appropriate choices is essential for reducing residual stress and optimizing the cutting process. (1) Influence of workpiece material: The inherent condition of the workpiece material and its physical and mechanical properties have a direct impact on the residual cutting stresses. Materials with good plasticity usually develop residual tensile stress after machining ； Materials with poor plasticity develop residual compressive stress. Depending on the specific initial stress state of the workpiece material, machining may increase or decrease the residual stress levels within the workpiece. (2) Influence of cutting parameters: The influence of the cutting speed is generally exerted through the “temperature factor”. When the cutting speed is low, residual tensile stress is likely to occur ； At higher cutting speeds, residual compressive stress is likely to occur due to the increase in cutting temperature. When the feed rate and cutting depth are increased, the cross-section and volume of the metal being cut increase, resulting in an enlarged plastic deformation zone and a greater degree of deformation in front of the cutting edge. If the cutting speed is relatively high at this time, the effect of temperature factors also becomes more pronounced; consequently, the residual tensile stress on the surface will increase. (3) Influence of tool parameters: When the rake angle and clearance angle of the tool are increased, and the radius of the tool tip arc and the radius of the bluntness circle of the cutting edge are decreased, the residual stress is reduced. The sharpness of the tool, the wear of the flank face, or the radius of the bluntness circle have a great influence on residual stress; followed by the rake angle of the tool.