Surface temperature measurement in laser-assisted machining processes

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 heat the ceramic workpiece locally, prior to material removal by a single-point cutting tool, thereby reducing the yield strength of the ceramic below the fracture point and changing material deformation behavior from brittle to quasi-ductile. Temperature measurement during LAM is needed to verify thermomechanical models of the process, as wed as for implementation of process control algorithms. in this study a laser pyrometric technique, which measures surface temperature and emissivity concurrently, 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 +/-13 degrees C at 700 degrees C to +/-20 degrees C at 1,500 degrees C, 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 +/-36 degrees C 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.