Principles of laser micro sintering

Purpose - The purpose of the paper is the elucidation of certain mechanisms of laser material processing in general and laser micro sintering in particular. One major intention is to emphasize the synergism of the various effects of q-switched laser pulses upon metal and ceramic powder material and to point out the non-equilibrium character of reaction steps. Design/methodology/approach - Recent results and observations, obtained in development of laser micro sintering, are surveyed and analyzed. By breaking down the overall process into relevant steps and considering their possible kinetics, an approach is made towards interpreting specific phenomena of laser micro sintering. Thermodynamics upon heating of the material as well as its photo-electronic response to the incident radiation are considered. Findings - The findings corroborate a model whereby short pulses of high intensity provide non-equilibrium pressure conditions at the location of incidence, that allow for the melting of metal powder with an almost immediate expansion of a plasma and/or vapor bulb. Thereby the molten material is condensed and propelled towards the substrate. A final boiling eruption after each pulse is the reason for the morphology of the laser micro-sintered surfaces and can prevent oxidation when the process is conducted under normal atmosphere. In sintering of ceramics, the short pulsed and intensive radiation increases the chance to excite the material even with photon energies below the bandgap value and it lowers the risk of running into a destructive avalanche. Research limitations/implications - Owing to the stochastic character of the respective sintering event, that is initiated by each individual pulse, the gathered data are not suitable yet for the formulation of an exact quantitative function between sintering behavior and laser parameters. Practical implications - The qualitative findings yield a good rule of thumb for the choice of parameters in laser sintering on a micrometer scale and the model is conducive for advanced interpretation of other phenomena in laser material processing besides sintering. Originality/value - The kinetics and thermodynamics of laser sintering with q-switched pulses are approached by a qualitative explanation. The heterogeneous and non-equilibrium character of the processes is taken into account, this character is often neglected by researchers in the area.