Rapid solidification effects during laser welding

To a large extent, the integrity of a welded joint depends on the fusion zone (FZ) or weld metal microstructure. Solidification behavior controls the size and shape of grains, the microstructure, the distribution of inclusions, the extent of defects such as porosity and hotcracks and, ultimately, the properties of the weld metal. Depending on the process, weld metal solidification (cooling) rates may vary from 10 to 10³°C/s for conventional welding processes such as submerged arc (SA), gas tungsten arc (GTA), and electro slag welding to 10³ to 10⁶°C/s for modern, high-energy beam processes such as laser welding. In understanding the solidification behavior of welds, conventional solidification theories may be extended to welds. In doing so, one basic underlying assumption is that equilibrium is maintained at the solid/liquid interface. However, this assumption is not valid under rapid solidification conditions experienced by the weld metal during high-energy density processes such as laser welding. As a consequence of this, a number of important effects may arise. There are dramatic changes from the equilibrium partition coefficient K, formation of non-equilibrium phases and changes in the general microstructural features. Although some of these effects have been documented in a limited number of alloy systems during welding, they are commonly observed during surface glazing or splat quenching. With the extensive use of the laser welding process to weld a wide variety of alloys, these effects should be anticipated in a broader class of commercial alloy systems.