Approximate model for laser trepanning with microsecond Nd:YAG laser radiation

Trepanning of 200 µm cooling holes in 2 - 4 mm thick CMSX-4 turbine material is done by a lamp-pumped Nd:YAG slab laser with pulse durations of 100 - 500 µs and pulse energies up to 1 J determining the material removal mainly by melt expulsion. The high removal velocities achieved enable to describe the motion of the melt front - phase boundary solid/liquid material - approximately. Compared to the complete solution including heat conduction and phase transition (Stefan problem) the approximate model is asymptotically accurate and its errors are controllable. The Simulation is compared with experimental results of cross-sections of trepanning kerfs assuming that the melt is removed instantaneously while the melt is expelled by a coaxial gas stream during trepanning. The assumption of temperature independent material parameters is discussed. First results with an assumed Gaussian beam profile (M2 = 2) are compared with a refined simulation considering the measured spatial beam profile.