1. Laser Machining of Structural Ceramics: An Integrated Experimental & Numerical Approach for Surface Finish Hitesh D. Vora and Narendra B. Dahotre Laboratory for Laser Materials Processing & Synthesis, Materials Science and Engineering, University of North Texas, Denton, Texas, USA Background Motivation Numerical Study Funding: National Science Foundation ( CMMI 1010494 ) Schematic of laser-material interaction Conventional machining techniques (grinding) Unacceptable tool wear & insufficient accuracy Mechanical or/and thermal damage Lower material removal rate or machining time Higher operating costs Potential solution: Laser Machining Innovative and potential tool for bulk material removal and shaping of structural ceramics Non-contact process - eliminates tool wear Efficient, reliable, cost effective solution to fabricate complex structures at large scales Laser-Material Interaction Current research aims at presenting the state of the art in the field of laser machining of alumina and emphasizes on experimental and computational approaches in understanding physical nature of the complex phenomena. Need : Obtaining desired surface finish at much higher material removal rate Solutio n: Better understanding of various physical phenomena (heat transfer & fluid flow) and its influence on the evolution of surface finish during laser machining of ceramics Objective All explained physical phenomena happened within the small interval of time very difficult to observed physically Solution → finite element method Experimental Study Laser power density in Gaussian distribution Governing Equations A COMSOL ® multiphysics based two-step numerical model coupled with heat transfer and fluid flow was developed Recoil pressure (P r ) at the evaporating surface depends on the incident laser energy density and is given by the following equation Navier-Stokes equations was used to model the movement of the liquid under the action of the recoil pressure Results & Discussions Heat conduction Surface melting Surface Vaporization Plume formation Recoil pressure Liquid pile-up Rapid Solidification Ongoing efforts include the extension of one- dimensional numerical model into two- and three- dimensional with inclusion of effects of multiple laser pulses on the resulting surface morphology during laser machining of alumina Defense and space exploration Thermal protection systems in exhaust cones, insulating tiles for space shuttle, ceramic coatings: engine components, and windshield glass of many airplanes Refractory High Temperature Strength, high wear resistance, high hardness and durability. Used in refractory products such as furnace liners, crucibles, structural insulation Machining and fabrication Excellent hardness & heat resistance properties ideal for drilling, shaping, grinding and forming metal work pieces Reference: Google images a)prediction of solid, liquid & vapor interface by Level-set method, b)prediction of crater and melt pool dimensions, c)flow of molten material due to various boundary conditions, d)prediction of surface profile after solidification Test Average Energy Density Laser pulses Numerically calculated R t Experimentally measured R t  standard deviation Difference #(J/m 2 )(1/s) (  m) (%) 13.5x10 6 1170.3 174.1  2.81 3.49 23.5x10 7 10465.8 490  5.92 5.19 37.1x10 7 20559.6 604  3.80 7.93 410.6x10 7 30967.5 898  5.16 7.18 514.1x10 7 401030.96 1010  4.84 2.03 617.7x10 7 501870.061920  3.892.67 Experimentally measured and numerically predicted and surface roughness Material lost due to evaporation causes an increase in crater depth Liquid expulsion created by the recoil pressure increase the pile-up height After a critical crater depth (  260  m), recoil pressure was insufficient to eject the liquid material out of the crater Hence, liquid material solidified inside the crater wall leading to formation of typical tear drop shape topography Surface roughness increased with increasing pulse rate (1, 10, 20, 30, 40, and 50) or the increase in average laser energy density Process model can be used as a handy tool for the process engineers to configure the process variables to obtain the specified quality characteristics (surface finish & machining rates) Acknowledgement Path to multidimensional model