Harish P. Cherukuri

NUMERICAL SIMULATIONS OF DUCTILE MACHINING OF SILICON NITRIDE WITH A CUTTING TOOL OF DEFINED GEOMETRY

Recent experiments involving machining of silicon nitride at the micrometer scale has shown that silicon nitride behaves in a ductile fashion under high pressures and when the depth of cut is small. In this paper, a preliminary study of ductile machining of Silicon Nitride (Si3N4) using numerical simulations is reported. The commercial software package ADVANTEDGE is used to model the cutting process. The numerical simulations involve a parametric study carried out to understand the effects of cutting speed, feed, rake angle and tooltip radius to evaluate conditions that are conducive to brittle-to-ductile phase transition. The feed, tooltip radius and depth of cut are of the order of tens of micrometers and the cutting speeds range from 0.5 m/min to 300 m/min. The results indicate that machining of silicon nitride may be carried out in a ductile fashion for small tooltip radii, high negative rake angles and small depths of cut.

The impact of linear deformations on stationary hydrostatic thrust bearings

In this work, the operating sensitivity of the hydrostatic thrust bearing with respect to pressure-induced deformations will be studied in a stationary setting. Using the classical lubrication equations for low Reynolds number flow, closed-form expressions are generated for describing the pressure distribution, the flow rate, and the load carrying capacity of the bearing. These expressions are developed to consider deformations of the bearing that result in either concave or convex shapes relative to a flat thrust surface. The impact of both shapes is compared, and the sensitivity of the flow rate and the load carrying capacity of the bearing with respect to the magnitude of the deformation is discussed. In , it is shown that all deformations increase the flow rate of the bearing and that concave deformations increase the load carrying capacity while convex deformations decrease this same quantity relative to a non-deformed bearing condition.

Experimental and Numerical Investigation of Ductile Regime Machining of Silicon Nitride

Recent research has shown that many semiconductor and ceramic materials can be machined in ductile fashion under very high pressures and at low depths of cut. In a previous study, the authors reported results from numerical simulations of orthogonal machining of Silicon Nitride using a pressure‐independent material model with von Mises yield criterion. Experimental evidence strongly suggests that the brittle‐to‐ductile transition of Silicon Nitride is pressure‐dependent. Specifically, experiments indicate that the ductile regime machining is possible when the hydrostatic pressure within the workpiece is of the same order of magnitude as the hardness. In the present work, the ductile behavior of Silicon Nitride is studied by carrying out single point diamond turning operation on Silicon Nitride samples at depths of cut ranging from 250nm to 10μm. Force and surface roughness data collected from machining tests are presented. Chip morphology is also investigated to determine the depth of cut at which brittle...

Modelling vertical continuous casting with temperature-dependent material properties

An extension of the analytical model for vertical continuous casting of bars previously developed by the authors (Johnson and Cherukuri. Proc. Roy. Soc. London A 455 (1999) 227) is presented. The new features include temperature dependence of thermal conductivity and the constitutive equations. The model can be used to readily predict the effect of various process parameters such as cooling conditions, liquid pool pressure, casting speed on the solidification process. Consequently, it can be readily implemented in a control system to adjust the process parameters of a typical solidification process. In addition, it can also be used as a guide to gain insight into the nature of the solution prior to conducting more general and expensive finite element simulations.

Vertical continuous casting of bars

The problem of vertical continuous casting of bars is considered in this paper. An asymptotic solution is constructed using the ratio of radius of the bar to the solidification length as the small perturbation parameter. The key feature is that the deformation is gradually varied in the processing direction. The goals are to predict the distance to reach complete solidification and to predict the dimensional change (bulging) in the bar due to the deformation from the internal pool pressure. These are both believed to play an important role in the metallurgical properties of the solidified material. The solution is semi–analytical in the sense that the governing partial differential equations describing the deformation of the solid shell are reduced to two coupled nonlinear ordinary differential equations involving only the inner and outer radii of the shell which are solved numerically. Solutions for the shell growth both inside and outside the mold are provided for several cooling conditions. A closed–form solution is also provided in one limiting case. The effect of several factors including the pool pressure, casting speed, cooling conditions, material behavior of the solidifying shell, deposition rate and thermal gradient across the shell thickness are investigated. For an ingot being produced at a rate ŵ0, complete solidification is found to occur at an axial position CρLŴ0R2/KΔT where L is the latent heat and C is a constant between 0.2 and 1.0 that depends on the details of the cooling strategy and material behaviour.

Prediction of microstructure evolution during multi-stand shape rolling of nickel-base superalloys

In this paper, a comprehensive numerical approach to predict the microstructure of nickel-base superalloys during multi-stand shape rolling is presented. This approach takes into account the severe deformation that occurs during each pass and also the possible reheating between passes. In predicting the grain size at the end of the rolling process, microstructural events such as dynamic recrystallization (DRX), metadynamic recrystallization (MDRX), and static grain growth are captured at every deformation step for superalloys. Empirical relationships between the average grain size from various microstructural processes and the macroscopic variables such as temperature (T) and effective strain (ε̄) and strain rate (ε̄̇) form the basis for the current work. These empirical relationships are based on Avrami equations. The macroscopic variables are calculated using a finite element analysis package wherein the material being rolled is modeled as a non-Newtonian fluid with viscosity that depends on the effective strain rate, strain, and temperature. A two-dimensional transient thermal analysis is carried out between passes that can capture the MDRX and/or static grain growth during the microstructural evolution. The presented microstructure prediction algorithm continuously updates two families of grains, namely, the recrystallized family and strained family at the start of deformation in any given pass. In addition, the algorithm calculates various subgroups within these two families at every deformation step within a pass. As the material undergoes deformation between the rolls, recrystallization equations are invoked depending on critical strain and strain rate conditions that are characteristics of superalloys. This approach predicts the microstructural evolution based on recrystallization kinetics and static grain growth only. The methodology was successfully applied to predict the microstructure evolution during the multi-pass rolling of nickel-base superalloys. The predicted results for Alloy 718 for a 4-stand rolling followed by air cooling and for a 16-stand rolling followed by a combination of air and water cooling are also compared with experimental observations.

Evaluation of fiber-based tools for glass polishing using experimental and computational approaches

Polymeric pad or pitch-based tools combined with loose abrasive slurries are typically used in the polishing of optical materials. In this paper, the potential of fiber-based tools to both remove material and provide high quality surface finishes on BK7 glass is explored. The potential advantage of fiber-based tools over traditional tools is their inherent compliance, which could accommodate varying workpiece surface curvatures as found in aspheres and freeforms. To evaluate the new tools, both experimental and finite element (FE) modeling approaches were taken. A FE model consisting of a single fiber engaged with the workpiece surface was used to estimate the shape and magnitude of the pressure distribution exerted by the fiber on the workpiece surface. Two different tool configurations, yielding two different Fes, predicted pressure distributions, were used to polish BK7 samples, and the material removal profiles were interferometrically measured. The resulting profiles and the predicted pressure distributions share the same v-shape. While differences in scale exist between the experimental and FE-predicted profiles, the tool generating higher material removal had the greater predicted pressure distribution, thus demonstrating the ability of the FE model to provide insights into tool design. Additional testing was conducted to determine if the tools removal rate can be predicted by Prestons equation. Initial results indicate the equation is valid within the range of parameters tested. The surface roughness of BK7 samples processed by this tool was measured and some deterioration on the Sq value was noted; the surface roughness increased from 1.89 to 3.66 nm Sq. Over several hours of continuous use, the load applied by the fibers decays in a repeatable manner, and little wear was observed on the fibers after 5.33 h of polishing.

Noninvasive laser coagulation of the human vas deferens: Optical and thermal simulations

Successful noninvasive laser coagulation of the canine vas deferens, in vivo, has been previously reported. However, there is a significant difference between the optical properties of canine and human skin. In this study, Monte Carlo (MC) simulations of light transport through tissue and heat transfer simulations are performed to determine the feasibility of noninvasive laser vasectomy in humans.

A rate-dependent model for hot-rolling

A rate-dependent model for the plane-strain sheet-rolling problem is proposed. The governing equations are solved using an asymptotic scheme that assumes that the ratio δ of thickness of the sheet material at the entry to the roll-bite length is small. Both the relative-slip and no-slip sheet-roll interface conditions are considered. Depending on the magnitude of the friction, different regimes that correspond to different levels of shear deformation have been identified and asymptotic solutions are provided for each of these regimes. The effect of the reduction, the strain-rate hardening parameter and the magnitude of the friction on the field variables and the roll-speed is also studied. Further, it is shown that in the limit as the strain-rate hardening index n → ∞, the asymptotic solutions for the rate-dependent model are shown to approach those predicted by rigid perfectly-plastic theory. The theoretical predictions are compared with experimental results for a commercial purity aluminum. The comparisons indicate a reasonable agreement between theory and experiment.

RELATIONSHIP BETWEEN MACHINING FORCES AND DIAMOND TOOL WEAR DURING DUCTILE REGIME MACHINING OF SILICON NITRIDE

Silicon nitride by nature is hard and brittle. In previous works, it has been demonstrated that silicon nitride can be machined by single-edge diamond tool as an alternative to the traditional finishing process. But commercial viability of such diamond turning processes is limited by tool wear. In this paper, an attempt has been made to edge machine silicon nitride disks with diamond tools of different geometries and operating conditions. Amount of tool wear under these operating conditions is studied and characterized. Our studies show that radius tools perform better than straight-edged tools but with a cost trade-off.