Weld pool dynamics and joining mechanism in pulse wave laser beam welding of Ti-6Al-4V titanium alloy sheets assembled in butt joint with an air gap

• DOI: 10.1016/j.optlastec.2021.107558
• Publication Date: 2021-10-04T13:26:26Z
• Authors: Jicheng Chen; Zipeng Ouyang; Xinwei Du; Yanhong Wei
• Keywords: Keyhole; Weld pool; Titanium alloy; Butt joint; Butt welding; Free surface

Abstract A validated CFD model coupling the multiple phases and multiple physics was developed for the dynamic simulation of pulse wave laser welding (PWLBW) process of 1.2 mm-thick Ti-6Al-4V alloy sheets assembled in butt joint with a reserved air gap. A laser spot diameter of 700 μm was used to compensate for the energy loss with the clearance applied at 0.2 mm and therefore improve the gap bridging ability. The distributions and evolutions of temperature, velocity, phase interface and weld pool dimensions were characterized and discussed to reveal the joining mechanism and the heat and flow behaviors during welding. The results show that the liquid bridges firstly occur in the middle height of sheets as the welding begins, and rapidly extend to the whole thickness with the increasing heat input. The reduction on laser power causes the rebounding of keyhole surface and the backfilling of molten metal. The keyhole closes at root surface and rebounds completely in 2 microseconds, leading to the merging of liquid bridges, bottom-up moving of high temperature area, and temporarily enlargement on weld pool size at root surface. During the pulse interval, the weld pool volume shrinks significantly and the molten metal slows down from Marangoni vortexes to thermal buoyance flows. The proposed process parameters allow an air gap accounting for 16.67% of specimen thickness and result in a well-formed and defect-free weld bead.