In the construction of structures and equipment, various components are placed next to each other and connected to form the structure or equipment. The connection between components can be made using bolts, pins, rivets, welding, and other methods. Among these methods, welding is a metallurgical joining method used in various industries. To date, welding has been utilized in the industry using various methods such as manual arc welding, TIG, MIG, friction, electron beam, laser, and so on.
Based on studies and experience, it has been shown that creating turbulence in the molten pool remarkably improves the properties of the weld zone. To this end, many welders oscillate the electrode during welding to create turbulence in the molten pool. Since laser welding is widely used in the automotive, electronics, heat exchanger, and other industries, in recent years, efforts have been made to create turbulence in the weld zone to improve the weld properties. Therefore, at first, it was attempted to create turbulence by moving the laser head in the molten pool. However, this was very difficult for small parts or complex weld geometries. Therefore, a new type of laser welding called laser wobble welding was invented.
Mechanism of laser wobble welding
The conventional laser beam is stationary at a point while exiting the head. Therefore, by moving the head, the beam moves in a linear manner. However, in laser wobble welding, the beam is not stationary at a point while exiting the head and is constantly oscillating. Consequently, during laser wobble welding, the beam has both an oscillating motion and a linear motion (by moving the head). The oscillating motion of the laser wobble beam is due to the presence of a galvanometer mirror in the head. The galvanometer mirror is a mirror attached to a galvanometer. By changing the current, the galvanometer and the attached mirror move. Then, the laser beam hits the moving mirror and exits the head in an oscillating manner. Figure 1 shows a schematic of the galvanometer mirror in a laser wobble head.
Today, laser wobble machines have the ability to oscillate in different shapes. The motion patterns of the laser beam, called laser modes, can oscillate in circular, linear, figure-eight, infinity, and other shapes. Figure 2 shows a schematic of the laser beam motion along with a sample of its weld in different wobble modes.
Metallurgical Properties:
The oscillating motion of the laser beam during welding creates a turbulent molten pool. This increases the cooling rate and refines the grain structure. On the other hand, during grain growth, if a columnar grain is formed, it breaks down due to beam oscillation and molten pool turbulence and forms an equiaxed grain. Figure 3 shows the analysis of the microstructure of conventional laser welding and laser wobble welding at different wobble amplitudes. According to Figure 3, the microstructure obtained from laser wobble welding has equiaxed and fine grains.
Weld Defects:
The turbulence of the molten pool helps to increase the speed at which gases and bubbles rise to the surface of the pool. Therefore, all bubbles are removed before the weld zone solidifies. Consequently, there are no porosities and cavities in the microstructure of the weld zone. Figure 4 shows a schematic of the bubble removal from the weld zone.
Mechanical Properties:
The improvement of the weld zone microstructure, including grain refinement, the presence of equiaxed grains, and so on, leads to an increase in mechanical properties such as strength and hardness. On the other hand, the absence of defects in the weld zone increases the resistance to fracture and contributes to the improvement of the mechanical properties of the weld. Figure 5 shows the tensile strength and elongation in the no-wobble, and different wobble modes. This figure indicates the superiority of the mechanical properties of the weld zone in laser wobble welding compared to conventional laser welding.
Conclusion
As mentioned, creating turbulence in the molten pool improves the properties of the weld zone. Laser wobble welding, as a technology, helps to improve the properties of laser welds. By creating turbulence caused by the oscillation of the laser beam, the weld microstructure is improved. On the other hand, the reduction of defects such as porosity is observed in the laser wobble welding method. Consequently, with the improvement of metallurgical properties and the reduction of weld zone defects, we witness an increase in the mechanical properties of the weld.
Reference
- Sanati, S., Nabavi, S.F., Esmaili, R. et al. A Comprehensive Review of Laser Wobble Welding Processes in Metal Materials: Processing Parameters and Practical Applications. Lasers Manuf. Mater. Process. (2024). https://doi.org/10.1007/s40516-024-00245-w
- https://www.metalworkingworldmagazine.com/welding-with-wobbling-technique/
- https://www.laserfocusworld.com/industrial-laser-solutions/article/14215944/fiber-laser-welding-technique-joins-challenging-metals
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