A computational analysis of deep penetration laser welding.

Abstract

A model was devised and used as the basis of a computer simulation to predict the shape of and physical phenomena in the keyhole during deep penetration laser welding. The shape of the weld cavity was determined as a part of the solution, and a convection-dominated vaporization model was utilized. Deep penetration welding is characterized by the formation of the keyhole. Beyond a certain threshold laser power, the laser beam rapidly evaporates material creating a strong back pressure, which pushes the molten material sideways forming a cavity. Hence, the laser power is effectively transferred to the bottom of the cavity and penetrates into the material until an energy balance is achieved around the keyhole. Around the keyhole three different regions (solid, liquid, and vapor) are analyzed, each region with its most suitable method. The heat transfer within the solid region is solved by Boundary Element Method. A thin layer approximation is made to simplify the analysis in the liquid region. A scaling analysis shows that fluid dynamics in the liquid region does not contribute significantly to the heat transfer in the liquid region. In the vapor region, a one-dimensional gas dynamic model is adopted from the literature. The solutions in the three regions are matched to satisfy conservation of mass at the liquid-vapor interface and of energy at the solid-liquid interface. Specifically, the matching technique of energy at the solid-liquid interface is called the matching scheme, and with it the shape of the solid-liquid interface is calculated. Then the shape of the liquid-vapor interface can readily be obtained from the shape of the solid-liquid interface and the thin liquid layer approximation. The matching scheme and the use of modules combine to make a model which is capable of predicting the shape of the solid-liquid interface; depth of penetration; surface temperature of the keyhole; pressure acting on the keyhole; energy distribution, such as the energy of vaporization, fusion, and conduction; and the thickness of the liquid layer. As a model material, pure iron was analyzed in this study. The calculated penetration depths are compared to empirical data, in order to verify the current study, and good agreement was observed.