Experimental determination of temperature threshold for melt surface deformation during laser interaction on iron at atmospheric pressure

Recoil pressure is the principal driving force of molten metal in laser processing in the intensity range 10(-1)-10(2) MW cm(-2). It is thus essential to estimate the recoil pressure in order to describe physical processes or to carry out numerical simulations. However, there exists no quantitative estimation of the recoil pressure near the boiling temperature (T(v)), which is particularly important in the welding process. In this study we experimentally investigated the recoil pressure of pure iron around T(v). The main interest was to determine the threshold surface temperature to start deformation of melt surface. Using camera-based temperature measurement with accurate evaluation of emissivity from experiment, it was shown that the surface temperature has to reach T(v) to initiate the melt surface deformation. This result provides the first experimental evidence for the frequently used assumption that a deep keyhole welding requires surface temperature over T(v). It is indicated also that, in normal gas-assisted laser cutting process, the recoil pressure hardly contributes to material ejection when the surface temperature is lower than T(v), as opposed to the commonly believed presumption.