The role of an assist gas during laser-metal interaction

An experimental and theoretical investigation has been performed to study the effect of change in absorptivity and other thermophysical properties associated with oxide formation during laser-metal interaction.The study was divided into three parts. In the first part the change in directional-hemispherical spectral reflectance of metallic targets (Al6061, Cu, 304 stainless steel, and Low C Steel) during Nd-TAG laser pulse irradiation was measured using an integrating sphere under controlled environments of both oxygen and argon gas. The spectral transmittance of the plume formed over the targets was also measured using a He-Ne probe laser. Results showed that once a plume is formed, absorption by the plume significantly limits the amount of laser energy available for absorption at the target surface. The relative magnitude of the absorptivity enhancement observed for Al6061 and Cu due to oxide formation, prior to plume formation, depends on the respective absorptivities of the oxide and metal.In the second part the minimum pulse width (drilling time) required to drill a hole through a given thickness of metal sample, using argon and oxygen assist gas was determined. The results showed that the oxide formed during laser drilling with oxygen affected the drilling time by changing the absorptivity of the surface and by changing the temperature required to expel the molten material (due to the difference in melting point of the metal and oxide). The competing effect of these two factors determined whether the oxygen assist was helpful to the drilling process. The effect of incident laser power, sample thickness, and gas pressure on drilling time were also studied.And in the third part a mathematical model for gas assisted molten expulsion (GAME) mechanism was developed. The experimentally observed effects of a change in absorptivity and temperature required to expel molten material associated with the oxide formation were incorporated in the model. The model was compared with experimental values of the drilling time obtained and reasonable quantitative and qualitative agreement was found. The effect of incident laser flux on melting front velocity, liquid layer thickness, surface temperature, and drilling time under argon and oxygen assist gas cases was also discussed.U of I OnlyETDs are only available to UIUC Users without author permissio