List of Symbols
1.2 Motivation for the Present Research Work
5 workpiece due to the applied load data are then employed in the analytical model developed by Oliver and Pharr (1992, 2004) to obtain the mechanical properties. Lawn et al. (1994) observed ductile behavior of brittle materials during Hertzian indentation test. From then, researchers attempted a number of eminent works to find out phase transition, crack length and fracture toughness of brittle materials by using nanoindentation test.
Literature reports plunge cutting experiments to determine the critical depth of cut of brittle materials. In plunge cutting process, the cutting tool is moved along a path inclined to the workpiece surface. The depth of cut is varied from zero to few hundreds of nanometers, generally above the critical depth of cut of the material. The machined surface and cutting force values obtained from plunge cutting operation are then studied to understand the ductile to brittle transition. Reported research revealed that, damage free smooth surface was obtained up to few nanometers of depth of cut. When the depth of cut is increased further, surface roughness increases. This transition from smooth to rough surface provides useful information about ductile to brittle transition thickness of the material.
6
Understanding of the effect of process parameters of SPDT on its performance parameters such as machining forces and surface roughness is important for improving the product quality and process efficiency. However, the SPDT process is slow due to nanometric level material removal. SPDT machining of hard and brittle material results in rapid tool wear which affects the productivity and product quality. Machining of ferrous materials causes high tool wear because of the chemical affinity of diamond with the carbon. Silicon (Si) is extremely difficult to diamond machine, primarily due to its hardness. Because of high temperature and friction generated during machining, diamond reacts with carbon particles to form SiC bond and causes rapid tool wear. SiC is hard, but it is regarded as brittle because of its low fracture toughness. Therefore it is prone to fracture and subsurface damage during SPDT operation. On the other hand, machining of soft and ductile materials using diamond tool also quite difficult due to the formation of built up edge (BUE).
After an extensive literature review on various aspects of SPDT process such as analytical, experimental and numerical studies on various process parameters, tool geometry and material aspects, it was found that very scant literature is available on numerical simulation of silicon and silicon carbide. Also, none of the research has reported on the parametric study of the process to optimize the process conditions to improve the efficiency and product quality. A need thus was identified to develop a finite element model to understand the physics of SPDT process and to mimic the practical phenomena of chip formation during nanometric cutting of brittle and ductile material using a diamond tool.
Very few attempts have been reported on optimization of SPDT of brittle materials to improve the process productivity and product quality. There is hardly any comprehensive and systematic study reported in the literature on the influence of critical machining parameters such as speed, feed, depth of cut, tool geometry parameters viz. rake angle, cutting edge radius on the performance measures, i.e., cutting forces and surface roughness. Conducting experiments to understand the effects of various process parameters, tool geometry, workpiece material, working condition on process output and product quality is important;
however these are tedious; time consuming and costly. This provided the motivation to carry out the present research work on understanding the ductile regime machining phenomenon (DRM) of brittle material and ductile materials by using FEM based simulations of nanoindentation, plunge cutting, the SPDT operation of Si, SiC and Al; and to find out the optimum process conditions for desired productivity and product quality.
TH-2306_10610325
7 1.3 Scope of the Present Research Work
The present research work focuses upon understanding of DRM of brittle materials such as silicon, silicon carbide (difficult to machine) and machining of ductile material like Al6061-T6 (difficult to polish). Silicon (Si) is a vital substrate material used in the manufacture of refractive lenses, solar cells, electronic devices and infrared optics [Yan et al.
(2012)]. Silicon carbide (SiC) is now being used in the manufacture of space-based optical imaging systems [Robichaud et al. (2008)], critical parts of automobiles, biomedical implants, electronics systems, fiber optics communication systems [Pulliam et al. (2000)]. SiC is hard, chemically stable, wear and corrosion resistant material. It exhibits good electrical and thermal insulation characteristics. Aluminium 6061-T6 alloy is highly ductile and mostly used in the construction of aircraft structures, yacht, automotive parts, optical mold inserts for plastic lens and most importantly aluminum mirrors (with aspheric surfaces) for optical industries [Dashwood and Grimes (2010)].
The present work primarily focuses on the development of a two-dimensional (2D) numerical modeling of single point diamond turning process using FEM. Initially, finite element method based two-dimensional numerical model of nanoindentation and plunge cutting process is developed to determine the ductile to brittle transition (DBT) thickness and to understand the ductile regime machining (DRM) of silicon and silicon carbide. This thesis aims at understanding of the ductile regime machining by identifying the critical depth of transition and thereby predicting the machining force and surface roughness to optimize the process conditions for the improvement of process efficiency and product quality.
Initially, a two-dimensional numerical model of actual SPDT process of silicon and silicon carbide using FEM is developed and then the experimental validation of predicted machining force values has been carried out. During the study, the effect of employing two different material models viz. Johnson-Cook (JC) and Drucker-Prager (DP) on the chip formation was studied. Further, parametric studies were carried out by the developed numerical model to study the effect of process parameters viz. speed, rake angle, depth of cut, cutting edge radius on the performance parameters viz. components of machining force.
An integrated finite element method-image processing technique (FEM-IPT) based methodology has been developed for the prediction of surface roughness during SPDT of Al6061−T6. Due to limited availability of SPDT machine and related resources for experiments; in the present work experimental studies were only carried out on Al6061-T6.
The results were used to validate the developed FEM-IPT model. Further, the model was used
TH-2306_10610325
8
to investigate the influence of process parameters such as speed, feed and depth of cut on the surface roughness. Mixed level full factorial experiments were designed and carried out.
It is to be noted that the study on the effect of crystal orientation, tool wear, thermal and vibrations are out of the scope of this study. The work is primarily intended to determine the ductile to brittle transition zones and to obtain the required optical surface finish during the nanometric cutting of brittle as well as ductile materials. It is envisaged that the knowledge obtained from the present numerical and experimental studies will be useful to the researchers and industrial engineers to carry out efficient and quality SPDT operations.