Thermal-mechanical modeling of laser ablation hybrid machining
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Hard, brittle and wear-resistant materials like (ceramics pose a problem when being machined using conventional machining processes. Machining ceramics even with a diamond cutting tool is very difficult and costly. Near net-shape processes, like laser evaporation, produce micro-cracks that require extra finishing. Thus it is anticipated that ceramic machining will have to continue to be explored with new-sprung techniques before ceramic materials become commonplace. This numerical investigation results from the numerical simulations of the thermal and mechanical modeling of simultaneous material removal from hard-to-machine materials using both laser ablation and conventional tool cutting utilizing the finite element method. The model is formulated using a two dimensional, planar, computational domain. The process acronymed, LAHM (Laser Ablation Hybrid Machining), uses laser energy for two purposes. The first purpose is to remove the material by ablation. The second purpose is to heat the unremoved material that lies below the ablated material in order to "soften" it. The softened material is then simultaneously removed by conventional machining processes. The complete solution determines the temperature distribution and stress contours within the material and tracks the moving boundary that occurs due to material ablation. The temperature distribution is used to determine the distance below the phase change surface where sufficient "softening" has occurred, so that a cutting tool may be used to remove additional material. The model used for tracking the ablative surface does not assume an isothermal melt phase (e.g., Stefan problem) tor laser ablation. Both surface absorption and volume absorption of laser energy as a function of depth have been considered in the models. LAHM, from the thermal and mechanical point of view, is a (complex machining process involving large deformations at high strain rates, thermal effects of the laser, removal of materials and contact between workplace and tool. The theoretical formulation associated with LAHM for solving the thermal-mechanical problem using the finite element method is presented. The thermal formulation is incorporated in the user defined subroutines called by ABAQUS/Standard. The mechanical portion is modeled using ABAQUS/Explicit’s general capabilities of modeling interactions involving contact and separation. The results obtained from the FEA simulations showed that the cutting force decreases considerably in both the LAHM Surface Absorption (LAHM-SA) and LAHM volume absorption (LAHM-VA) models relative to the LAM model. It is observed that the HAZ can be expanded or narrowed depending on the laser speed and power. The cutting force is minimal at the last extent of the HAZ (heat affected zone). In both the models the laser ablates material thus reducing material stiffness as well as relaxing the thermal stress. The stress values obtained showed compressive yield stresses just below the ablated surface and chip. The failure occurs by conventional cutting where tensile stress exceeds the tensile strength of the material at that temperature. In this hybrid machining process, the advantages of both the conventional and laser machining processes were realized.