On the formation of the keyhole and its temperatures

A thermodynamically based model has been developed to calculate the size and temperature of the ‘keyhole’ through the thickness of the weldment. The model is based on calculating the power loss resulting from evaporation of an element in the chemistry of the plate and a minimum power density required to produce a keyhole. The evaporative power loss is set equal to the Gaussian power density, which is used to represent the laser beam. The result is a temperature distribution across the keyhole. Absorption of the laser power in the keyhole is calculated based on inverse Bremsstrahlung absorption, which can be calculated based on Sana's equation and the average Gaunt factor. The results indicate that peak powers below the minimum peak power density yield temperatures that are indicative of melting only; peak powers equal to a minimum power density yield temperatures that reach the evaporation temperature of the element; and that peak powers greater than the minimum power density yield temperatures greater than the evaporation temperature of the element. The results are used to explain the ‘nail head’ appearance of the fusion zone which is quite common in laser beam welds. The results also explain some visual observations of laser beam welds.