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Effect of Microstructure on the Mirror-Like Surface Quality of FCC and BCC Metals

The microstructure of machined metals changes near the tool-affected zone. This paper presents some new results concerning mirror-like surface cutting of aluminum, copper and tungsten. The microstructure of aluminum and copper represents polycrystalline mild metals with face centered cubic (FCC) crystal structure. Examination of a mirror-like surface by optical microscopy, scanning electron microscopy (SEM), electron backscattered diffraction and atomic force microscopy revealed that grain boundaries and twin boundaries were present, which separates two domains for different crystal orientation. The Young's modulus that depends on orientation can change considerably on these boundaries and, consequently, the value of elastic deformation of the layer under the machined surface. This effect modified the roughness. Ultraprecision machining of tungsten, which is a body centered cubic (BCC) metal, proved useless using diamond and/or boron-nitride tools. Since tungsten is a very brittle metal at room temperature, its ductile to brittle transition temperature is much higher. Therefore, in contrast to normal cutting, the material that is incapable of plastic deformation will cause brittle fracture of the chip and bad surface quality.