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THERMAL-RADIATIVE MODEL

A transient, three-dimensional thermal-radiative model of the workpiece undergoing laser-assisted turning has been developed. The model predicts temperatures in the workpiece at any time during the LAM process including the preheat phase (when no material removal is taking place). Some characteristics of the model include:

• The finite volume method (FVM) is used to discretize the energy equation.
• 500,000 control volumes are used to discretize the computational domain.
• The model accounts for:
  • energy leaving the computational domain with the chip
  • mixed convective cooling due to the rotating workpiece
  • radiative cooling of the workpiece
  • local energy absorption from the laser beam with a choice of laser energy distributions
  • forced convective cooling due to the high velocity air jet exiting the laser nozzle and impinging on the workpiece surface
  • energy generation in the primary shear zone due to plastic deformation
  • energy generation in the tertiary zone due to friction between the workpiece and the cutting tool
• Internal radiation is currently modeled by the diffusion approximation to the radiative transfer equation (RTE). This is valid for optically thick media and models internal radiative transfer by using an "effective" thermal conductivity which includes a radiation term.
• The discrete ordinates method (DOM) is being implemented to more accurately model internal radiative transfer in semi-transparent materials.
• The model predictions compared well with the measured surface temperatures for LAM of silicon nitride (opaque material). The model has not yet been validated for its predictions of temperatures in semi-transparent materials such as partially stabilized zirconia or mullite.

MACHINING MODEL

Machining modeling for LAM of ceramics has been performed using the material constitutive models developed for these materials. The commercial finite element method (FEM) code, Abaqus, is used to model LAM of ceramics and metals.   For modeling of machining of ceramics, cohesive models are used to simulate the inter grain cracks.

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