Time scale effects in laser material removal : a review

In laser material removal using a continuous wave or long-pulsed laser, the primary material removal mechanism melts with molten metal often ejected by an assisting oxygen jet. Interactions between heat transfer, oxygen diffusion, and gas dynamics have been studied. In laser machining using a Q-switched solid state laser with pulse width on the order of nanoseconds, the primary material removal mechanism is ablation, but substantial melting is still present if a metallic material is concerned. Two mechanisms, ablation and melting, are modeled together and the property discontinuity and boundaryconditions at the liquid/vapor interface are provided. When the laser pulse width further reduces to the order of picoseconds or femtoseconds, as in a Ti:sapphire laser, the dominant material removal mechanism changes yet again, with little thermal damage and wavelength independence observed. Thermal aspects are examined in the context of the short timescales of these pulses compared with the timescale of photon-electron-lattice interactions. Research needs in laser material removal in each of the three time regimes are discussed.