Prediction of pore size in high power density beam welding

Abstract

This study is to investigate parameters responsible for the final pore size during high power density laser and electron beam welding processes. Dimensionless parameters include the surface tension parameter, Mach number and liquid pressure at the keyhole base, friction factor, melting temperature, ratio of specific heats at constant pressure and volume, and loss coefficient. The friction factor can also be treated as a combination of viscous shear stress and thermocapillary force. Although the formation of macro-porosity is recognized as an important problem that limits the widespread industrial application of laser and electron beam welding, the physics of the macroporosity formation is not well understood. In order to solve this problem, both the states at the times when the keyhole is about to be enclosed and the pore is completely formed are considered to be governed by the equation of state. The pore shape thus can be determined by the Young–Laplace equation. The gas pressure is determined by calculating a compressible flow of the two-phase, vapor–liquid dispersion in a vertical keyhole of varying cross-section, paying particular attention to transition between the annular and slug flows. It shows that regardless of a supersonic or subsonic flow at the base, the final pore size decreases as the dimensionless surface tension parameter, liquid pressure at the keyhole base, and melting temperature decrease. The pore size also decreases with increasing friction factor subject to a supersonic flow at the base. The effects of the ratio of specific heats at constant pressure and volume, friction factor and loss coefficient on the final pore size are insignificant for a subsonic mixture. This work provides a systematical understanding of the factors affecting the final pore size during keyhole mode welding.