Heat transfer, fluid flow and mass transfer in laser welding of stainless steel with small length scale.

摘要

Nd: YAG Laser welding with hundreds of micrometers in laser beam diameter is widely used for assembly and closure of high reliability electrical and electronic packages for the telecommunications, aerospace and medical industries. However, certain concerns have to be addressed to obtain defect-free and structurally sound welds. During laser welding, Because of the high power density used, the pressures at the weld pool surface can be greater than the ambient pressure. This excess pressure provides a driving force for the vaporization to take place. As a result of vaporization for different elements, the composition in the weld pool may differ from that of base metal, which can result in changes in the microstructure and degradation of mechanical properties of weldments. When the weld pool temperatures are very high, the escaping vapor exerts a large recoil force on the weld pool surface, and as a consequence, tiny liquid metal particles may be expelled from the weld pool. Vaporization of alloying elements and liquid metal expulsion are the two main mechanisms of material loss. Besides, for laser welds with small length scale, heat transfer and fluid flow are different from those for arc welds with much larger length scale. Because of small weld pool size, rapid changes of temperature and very short duration of the laser welding process, physical measurements of important parameters such as temperature and velocity fields, weld thermal cycles, solidification and cooling rates are very difficult. The objective of the research is to quantitatively understand the influences of various factors on the heat transfer, fluid flow, vaporization of alloying elements and liquid metal expulsion in Nd:YAG laser welding with small length scale of 304 stainless steel.; In this study, a comprehensive three dimensional heat transfer and fluid flow model based on the mass, momentum and energy conservation equations is relied upon to calculate temperature and velocity fields in the weld pool, weld thermal cycle, weld pool geometry and solidification parameters. Surface tension and buoyancy forces were considered for the calculation of transient weld pool convection. Very fine grids and small time steps were used to achieve accuracy in the calculations. The calculated weld pool dimensions were compared with the corresponding measured values to validate the model. (Abstract shortened by UMI.)