HEAT TRANSFER, FLUID FLOW AND MASS TRANSFER IN LASER WELDING OF STAINLESS STEEL WITH SMALL LENGTH SCALE
Open Access
- Author:
- He, Xiuli
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 26, 2005
- Committee Members:
- Tarasankar Debroy, Committee Chair/Co-Chair
Long Qing Chen, Committee Member
Christopher Muhlstein, Committee Member
Stephen M Copley, Committee Member - Keywords:
- Heat transfer and fluid flow
stainless steel
Laser welding - Abstract:
- During Nd: YAG 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 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, because of 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. Using the computed temperature fields, the mass loss and composition change due to vaporization of various alloying elements and vapor composition during laser spot welding were calculated. The computed vapor loss was found to be lower than the measured weight loss. Therefore, the liquid metal expulsion was examined by both calculations and experiments. The liquid metal expulsion can be predicted by balancing the vapor recoil force with the surface tension force at the periphery of the liquid pool. The results in this thesis contribute to the growing quantitative knowledge base in laser welding with small length scale.