Microstructural characterization of an ultra-precision-machined surface of a Zn-Al alloy

The surface of a furnace-cooled Zn-Al-based alloy was ultra-precision machined both with and without a coolant. The microstructure and phase decomposition of the ultra-precision-machined surface were studied using back-scattered electron microscopy and x-ray diffraction techniques while the characteristics of the cutting forces were analyzed using the data-dependent systems and power spectrum techniques. The present work reveals that a hardened layer was left on the machined surfaces. The increase in hardness is attributed by two quenched phases identified to be (1 1 1) α and (1 0 1 0) η′T respectively. These quenched structures could be found in samples machined both with and without coolants though the volume fraction of these quenched structures is found to be high in specimens machined with coolants. Heat generation in the course of machining is therefore expected to involve a heating and cooling cycle. The former is due to the cutting process while the latter is due to the heat sink effect of the workpiece and the application of coolants.