A model based on conservation of energy for a moving heat source incident on a flat plate is used to predict the threshold laser beam irradiance required to initiate melting on a metal surface. With the use of a nondimensional variable and its solution, the threshold irradiance can be predicted with the use of a simple equation that is a function of the absorptivity of the surface, thermal conductivity of the metal, temperature increase for melting, diameter of the laser beam at the surface and the nondimensional variable value for the ratio of the thermal diffusivity of the metal to the product of the beam diameter and velocity used. Laser beam irradiances exceeding these predicted values are required for welding. The model predictions are validated with experimental results of beam irradiances required for welding different metals.

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