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Archival Publications
Rozzi, J. C., Incropera, F. P. and Shin, Y. C., 2000, "Transient, Three-Dimensional Heat Transfer Model for the Laser Assisted Machining of Silicon Nitride: II-Assessment of Parametric Effects," International Journal of Heat and Mass Transfer, vol. 43, pp.1425-1437.
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Rozzi, J. C., Pfefferkorn, F. E., Incropera, F. P. and Shin, Y. C., 2000, "Transient, Three-Dimensional Heat Transfer Model for the Laser Assisted Machining of Silicon Nitride: I-Comparison of Predictions with Measured Surface Temperature Histories," International Journal of Heat and Mass Transfer, vol. 43, pp. 1409-1424.
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Lei, S., Shin, Y. C. and Incropera, F. P., 1999, "Material Constitutive Modeling under High Strain Rates and Temperatures through Orthogonal Machining Tests," ASME Journal of Manufacturing Sciences and Engineering, Vol. 121, pp. 577-585.
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Lei, S., Shin, Y. C. and Incropera, F. P., 1999, "Thermo-Mechanical Modeling of Orthogonal Machining Process by Finite Element Analysis," International Journal of Machine Tools and Manufacture, Vol. 39, pp. 731-750.
A plane strain finite element method is used with a new material constitutive equation for 1020 steel to simulate orthogonal machining with continuous chip formation. Deformation of the workpiece material is treated as elastic-viscoplastic with isotropic strain hardening, and the numerical solution accounts for coupling between plastic deformation and the temperature field, including treatment of temperature-dependent material properties. To avoid numerical errors associated with large deformation of elements, automatic remeshing is used, with at least 15 rezonings required to achieve a satisfactory solution. Effects of the uncertainty in the constitutive model on the distributions of strain, stress and temperature around the shear zone are presented, and the model is validated by comparing average values of the predicted stress, strain, strain rate and temperature at the shear zone with experimental results. Parametric effects associated with cutting speed and initial work temperature are considered in the simulations.
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Rozzi, J. C., Pfefferkorn, F. E., Incropera, F. P. and Shin, Y. C., 1998, "Transient Thermal Response of a Rotating Cylindrical Silicon Nitride Workpiece Subjected to a Translating Laser Heat Source: I-Comparison of Surface Temperature Measurements with Theoretical Results," ASME Journal of Heat Transfer, Vol. 120, pp. 899-906.
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Rozzi, J. C., Incropera, F. P. and Shin, Y. C., 1998, "Transient Thermal Response of a Rotating Cylindrical Silicon Nitride Workpiece Subjected to a Translating Laser Heat Source: II-Parametric Effects and Assessment of a Simplified Model," ASME Journal of Heat Transfer, Vol. 120, pp. 907-915.
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Pfefferkorn, F. E., Rozzi, J. C., Incropera, F. P. and Shin, Y. C., 1997, "Surface Temperature Measurement in Laser-Assisted Machining Processes," Experimental Heat Transfer, Vol. 10, pp. 291-313.
Laser assisted machining (LAM) is being investigated as a possible means of increasing the material removal rate, while minimizing surface flaws, for difficult-to-machine materials such as ceramics. The laser is used to locally heat the ceramic workpiece prior to material removal by a single point cutting tool, thereby reducing its yield strength below the fracture point and changing material deformation behavior from brittle to quasi-ductile. Temperature measurement during LAM is needed to verify thermo-mechanical models of the process, as well as for implementation of process control algorithms. In this study a laser pyrometric technique, which concurrently measures surface temperature and emissivity, was developed for use with a rotating silicon nitride workpiece heated by a translating CO2 laser. From a comprehensive uncertainty analysis, the 2-sigma uncertainty of the surface temperature was found to range from +/-13oC at 700oC to +/-20oC at 1500oC, while that of the emissivity was found to be +/-0.058 for values ranging between 0.8 and 0.9. However, scatter in the surface temperature measurements exceeded the calculated uncertainty limits, with the 2-sigma precision of data about a five-point moving average corresponding to +/-36oC over the detectable temperature range. An experimental study of the effect of process parameters indicated that the feed velocity and laser power have the greatest impact on the surface temperature field.
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Novak, J. W., Shin, Y. C. and Incropera, F. P., 1997, "Assessment of Plasma Enhanced Machining for Improved Machinability of Inconel 718," ASME Journal of Manufacturing Science & Engineering, Vol. 119, pp. 125-129.
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Conference Proceedings
Lei, S., Shin, Y. C. and Incropera, F. P., 1999, "Experimental Investigation of Thermo-Mechanical Characteristics in Laser-Assisted Machining of Silicon Nitride Ceramics," Proceedings, 1999 ASME International Mechanical Engineering Congress and Exposition, Nashville, Tenessee, MED-Vol 10, pp. 781-788.
Laser-assisted machining (LAM) of silicon nitride (Si3N4) is evaluated for its potential to become an economically viable process in fabricating precision ceramic parts. On-line measurements of cutting force and workpiece temperature are performed, and tool wear and surface integrity are examined. Tool wear characteristics are determined as a function of workpiece temperature, which is measured on-line using a laser pyrometer. Tool wear/failure mechanisms are characterized using optical microscopy, while application of scanning electron microscopy to heated and machined surfaces, as well as to chips, is used to infer material removal mechanisms and the extent of damage caused by LAM. The sub-surface damage of parts produced by LAM is compared with that of typical ground parts.
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Pfefferkorn, F. E., Incropera, F. P. and Shin, Y. C., 1999, "Transient, Three-Dimesnional Heat Transfer Model for Partially Stabilized Zirconia Undergoing Laser-Assisted Machining," Proceedings, 1999 ASME International Mechanical Engineering Congress and Exposition, Nashville, Tennessee, HTD-Vol. 364-3, pp. 197-209.
A three-dimensional, unsteady heat transfer model has been developed for predicting the temperature field in partially stabilized zirconia (PSZ) undergoing laser-assisted machining (LAM). PSZ is a semi-transparent ceramic which volumetrically absorbs, emits and scatters radiation across a spectral region extending from approximately 0.5 to 8 microns. As a first approximation, it is treated as optically thick within this spectral band, and the high density of scattering centers, as well as the random orientation of grain boundaries, permits the assumption of isotropic scattering. Accordingly, the Rosseland diffusion approximation is used to model internal radiative transfer. Since most of the CO2 laser radiation (wavelength = 10.6 microns) is absorbed in the first layer of control volumes adjacent to the surface, incident laser radiation is treated as a surface phenomenon.
The equivalent radiation conductivity of PSZ is strongly temperature dependent and enhances thermal energy transfer within regions of the workpiece which are close to the location of laser irradiation. However, the effective thermal conductivity of PSZ remains relatively low, even at the highest temperatures achieved during LAM, and is responsible for large temperature gradients near the irradiated surface of the workpiece. For representative operating conditions, comparative calculations are performed with and without the radiation model to assess the influence of volumetric radiation effects on the temperature field. Numerical simulations are also performed to consider the effect of operating conditions, such as the laser power, laser/tool feed and depth-of-cut, on thermal conditions in close proximity to the material removal zone. The results are contrasted with those for silicon nitride, which is an opaque ceramic that exhibits quasi-plastic deformation when its temperature is raised above a threshold value at the depth-of-cut and can therefore be machined with a cutting tool.
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Rozzi, J. C., Pfefferkorn, F. E., Incropera, F. P. and Shin, Y. C., 1998, "Experimental Evaluation of the Laser Assisted Machining of Silicon Nitride Ceramics," Proceedings, 1998 ASME International Mechanical Engineering Congress and Exposition, Anaheim, California, MED-Vol. 8, pp. 229-239.
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Rozzi, J. C., Incropera, F. P. and Shin, Y. C., 1997, "Transient, Three-Dimensional Heat Transfer Model for the Laser Assisted Machining of Ceramic Materials," Proceedings, 1997 ASME International Mechanical Engineering Congress and Exposition, Dallas, Texas, HTD-Vol. 351, pp. 75-85.
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Shin, Y. C. and Kim, J. N., 1996, "Plasma Enhanced Machining of Inconel 718," Proceedings, 1996 ASME International Mechanical Engineering Congress and Exposition, Atlanta, Georgia, MED-Vol. 4, pp. 243-249.
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Rozzi, J. C., Krane, M. J. M., Incropera, F. P. and Shin, Y. C., 1995, "Numerical Prediction of Three-Dimensional Unsteady Temperatures in a Rotating Cylindrical Workpiece Subjected to Localized Heating by a Translating Laser Source," Proceedings, 1995 ASME International Mechanical Engineering Congress and Exposition, San Francisco, California, HTD Vol.317-2, pp. 399-411.
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Theses
Lei, S., 1999, "Material Deformation and Characterization in Metal Cutting and Laser Assisted Machining of Silicon Nitride," Ph.D., School of Mechanical Engineering, Purdue University, West Lafayette, IN.
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Rozzi, J. C., 1997, "Experimental and Theoretical Evaluation of the Laser Assisted Machining of Ceramic Materials," Ph.D., School of Mechanical Engineering, Purdue University, West Lafayette, IN.
This study focused on the experimental and theoretical evaluation of the laser assisted machining (LAM) of silicon nitride ceramics. A laser assisted machining facility was constructed whose main components consist of a CO2 laser and a CNC lathe. Surface temperature histories were first measured and compared to a transient, three-dimensional numerical simulation for the translating laser heating of a rotating silicon nitride workpiece for ranges of the workpiece rotational and laser translation speeds, as well as the laser beam diameter and power. Excellent agreement was obtained between the experimental and predicted temperature histories.
Laser assisted machining experiments on silicon nitride ceramic workpieces were completed for a wide range of operating conditions. Data for cutting forces and surface temperature histories illustrated that the lower bound for the avoidance of cutting tool and/or workpiece fracture for LAM is defined by the YSiAlON glass transition temperature (920-970oC). As temperatures near the cutting tool increase to values above the glass transition temperature range the glassy phase softened, facilitating plastic deformation and, correspondingly, the production of semi-continuous or continuous chips. The specific cutting energy (uc=5.23 J/mm3) and silicon nitride machined workpiece surface roughness (Ra=0.39 micron) for LAM at the nominal operating condition was approximately an order of magnitude below and nearly equivalent to values associated with the grinding of silicon nitride using a diamond wheel (uc=40-100 J/mm3, Ra=0.2 micron), respectively. By examining the machined surfaces and chips, it was shown that LAM does not produce detectable sub-surface cracking or significant silicon nitride microstructure alteration, respectively.
A transient, three-dimensional numerical heat transfer model of laser assisted machining was constructed which includes a preheat phase and material removal, with the associated changes in the workpiece geometry. Excellent agreement was obtained between the measured and predicted temperature histories. The strong influence of the temperature distribution near the cutting tool location on the cutting forces, the specific cutting energy, and the tool wear demonstrated the usefulness of the theoretical heat transfer model for predicting successful operating conditions for the LAM of silicon nitride ceramics.
The near-chamfer surface temperature was found to adequately reflect the temperature distribution at the cutting tool location for a wide variety of operating conditions. A control scheme, based on adjustment of the laser/tool translational velocity or laser power, was proposed based on the measurement of the near-chamfer surface temperature after the conclusion of the preheat phase at a circumferential location where radial temperature gradients are small.
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Pfefferkorn, F. E., 1997, "Laser Pyrometry: Non-Intrusive Temperature Measurement for Laser Assisted Machining," M.S.M.E., School of Mechanical Engineering, Purdue University, West Lafayette, IN.
Advanced structural ceramics (e.g., silicon nitride, zirconia, alumina) have become very desirable as engineering materials due to their improved toughness, high strength at elevated temperatures and superior wear resistance, compared to metals. Presently, difficult and costly manufacturing processes preclude the use of ceramic materials in many products. Laser assisted machining (LAM) is being investigated as a possible means of increasing the material removal rate while minimizing surface flaws. The laser is used to locally heat the workpiece just prior to the material removal location. At high temperatures the yield strength of ceramics decreases below the fracture strength (changing the material deformation behavior from brittle to quasi-ductile), enabling the use of traditional metal machining techniques. Temperature measurement during LAM is very important for verification of a thermo-mechanical model under development and subsequent implementation of a process control algorithm, both of which are required to develop a LAM technology. After examining many temperature measurement methods, laser pyrometry was chosen as the non-intrusive temperature measurement technique because of its ability to measure the temperature and emissivity of a diffuse surface in-situ. The theory of operation, the calibration procedure and an uncertainty analysis is detailed. A 2-sigma uncertainty for the true surface temperature is calculated to range from 1.9% at 700oC to 1.3% at 1500oC. The experimental data displays a scatter that is greater than the calculated uncertainty limits, indicating an unknown effect. The 2-sigma precision for the experimental data about a five-point moving average is 5.5% at 700oC decreasing to 2.5% at 1500oC. Silicon nitride samples were heated by translating the laser along the rotating workpiece without material removal to validate the thermal model. Surface temperature measurement during the heating experiments is shown to be repeatable. The emissivity had a 2-sigma uncertainty of +/-0.022 with emissivity values ranging between 0.8 and 0.9 for different samples. The results of a parametric study of the process parameters is presented and suggests that the feed velocity and laser power have the greatest impact on the surface temperature field. Two hypothesis are presented to explain the scatter in the temperature measurement.
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