As a kind of brittle material, silicon will undergo brittle fracture at atmospheric pressure in conventional scale machining. Studies in the last two decades on hard and brittle materials including silicon, germanium, silicon nitride and silicon carbide have demonstrated ductile regime machining using single point diamond turning (SPDT) process. The mirror-like surface finish can be achieved in SPDT provided appropriate tool geometry and cutting parameters including feed rate, depth of cut and cutting speed are adopted.The research work in this thesis is based on combined experimental and numerical smoothed particle hydrodynamics (SPH) studies to provide an inclusive understanding of SPDT of silicon. A global perspective of tool and workpiece condition using experimental studies along with localized chip formation and stress distribution analysis using distinctive SPH approach offer a comprehensive insight of cutting mechanics of silicon and diamond tool wear. In SPH modelling of SPDT of silicon, the distribution of von Mises and hydrostatic stress at incipient and steady-state was found to provide the conditions pertinent to material failure, phase transformation, and ductile mode machining. The pressure-sensitive Drucker Prager (DP) material constitutive model was adopted to predict the machining response behaviour of silicon during SPDT. Inverse parametric analysis based on indentation test was carried out to determine the unknown DP parameters of silicon by analysing the loading-unloading curve for different DP parameters. A very first experimental study was conducted to determine Johnson-Cook (J-C) model constants for silicon.High strain rate compression tests using split Hopkinson pressure bar (SHPB) test as well as quasi-static tests using Instron fatigue testing machine were conducted to determine J-C model constants.The capability of diamond tools to maintain expedient conditions for high-pressure phase transformation (HPPT) as a function of rake angle and tool wear were investigated experimentally as well as using SPH approach. The proportional relationship of cutting forces magnitude and tool wear was found to differ owing to wear contour with different rake angles that influence the distribution of stresses and uniform hydrostatic pressure under the tool cutting edge. A new quantitative evaluation parameter for the tool wear resistance performance based on the cutting distance was also proposed. It was also found that the machinability of silicon could be improved by adopting novel surface defect machining (SDM) method.The ductile to brittle transition (DBT) with the progressive tool wear was found to initiate with the formation of lateral cracks at low tool wear volume which transform into brittle pitting damage at higher tool edge degradation. A significant variation in resistance to shear deformation as well as position shift of the maximum stress values was observed with the progressive tool wear. The magnitude and distribution of hydrostatic stress were also found to change significantly along the cutting edge of the new and worn diamond tools.
|Date of Award||28 Sep 2016|
- University Of Strathclyde
|Sponsors||University of Strathclyde|
|Supervisor||Xichun Luo (Supervisor) & Amir Siddiq (Supervisor)|