On the effect of grain structure in micro-cutting of polycrystalline aluminate magnesium spinel (PAMS) crystals

Abstract

This paper studies the anisotropic effect induced by the grain structure of polycrystalline aluminate magnesium spinel (PAMS) on the mechanism of surface generation in the micro-cutting process. The critical depth of cut for ductile–brittle transition (DBT) of PAMS is estimated from 81 to 163 nm by plunge-cutting tests. In the cutting tests with constant cutting depths from 250 nm to 1.5 µm, an apparent regional distribution of crack features is clearly observed and it implies that the randomly oriented crystal grains with different slip systems play an important role in brittle-regime surface generation and fractures evolution. An effective way to obtain a ductile-regime machined surface is to decrease the cutting depth to 50 nm, and a flawless surface can be obtained when the cutting depth is as shallow as 30 nm. With such a low cutting depth, the material is mainly removed by the ploughing effect. Finite element method (FEM) model has been developed to simulate the cutting process crossing a grain boundary of two adjacent grains with different anisotropy material properties. The simulation result supports that the subsurface damage and boundary fractures can be evoked ahead of the actual material removal process by the cutting stress propagating from the precedingly machined grain. Different crack-patterns are shown on the finished surfaces of the two adjacent grains of different orientations.