▲ Figure 1. The top image is a schematic diagram of the laser beam oscillation welding technique, while the bottom image shows top and cross-sectional views of the weld bead under different laser power conditions. The parameters are as follows: welding speed = 4.0 m/min, focal position = 0, rotation diameter = 1.0 mm, rotation frequency = 100 Hz, and beam path = circular. The welding direction is from left to right.

　　It is well known that copper and its alloys are extremely challenging materials to weld, for the following reasons: Copper and its alloys have relatively high thermal conductivity, making it difficult to achieve melting. As a result, significant amounts of heat are required to counteract heat dissipation while simultaneously enabling localized, intense heating—leading to thermal stresses and, ultimately, defects such as distortion. Additionally, dissolved oxygen tends to form oxides and porosity during the process. Moreover, copper alloys exhibit low absorption rates for laser beams, particularly when the laser wavelength exceeds 700 nm, with absorption levels dropping below 3%. Recently, Auwal and colleagues conducted a comprehensive review of laser welding techniques applied to copper alloys, thoroughly addressing the challenges these materials pose in engineering applications.

　　▲ Figure 2. Top views and cross-sections of weld beads obtained at different welding speeds, with laser parameters set as follows: Laser power = 4000 W, spot focus position = 0, beam oscillation diameter = 1.0 mm, beam oscillation frequency = 100 Hz, and beam path = circular. The welding direction was from left to right.

　　Today, most research on laser welding of copper alloys focuses on the application of pulsed lasers for spot welding. Meanwhile, exploring the feasibility of using continuous lasers to weld thick copper plates could open up new possibilities for applications in heat exchangers, high-power wing converters, and equipment requiring excellent electrical and thermal conductivity.

　　▲ Figure 3. Schematic illustration of the interaction between the copper material as the beam moves (top view), with each color representing a single period T.

　　To overcome weldability challenges, different strategies are employed to produce continuous, penetration-type butt welds while enabling precise control over welding defects, microstructure, and residual stresses and distortions. Among these strategies, beam oscillation—or "swinging," as it’s often referred—is currently not yet applied to laser welding of copper. However, laser beam oscillation has already been successfully implemented in the laser welding of materials like aluminum alloys and titanium alloys, which are relatively easier to weld compared to copper alloys.

　　▲ Figure 4. Top views and cross-sections of the weld bead at different laser spot focusing positions, with welding parameters as follows: laser power = 4000 W, welding speed = 4.0 m/min, rotation diameter = 1.0 mm, optional frequency = 100 Hz, and beam trajectory set to circular. The welding direction progresses from left to right.

　　Beam oscillation (or modulation) expands the area of interaction between the laser beam and the material, increasing the weld width while simultaneously reducing the heat input required during the welding process. In highly reflective materials, such as copper alloys, beam oscillation helps elevate the local temperature of the material, enhancing its absorption rate of the laser energy. As a result, energy efficiency improves because reflection during welding is minimized. Another key advantage is that an appropriately designed oscillating laser head allows precise control over the thermal temperature gradient and ensures stable keyhole formation, leading to fewer welding defects and producing smoother, high-quality weld surfaces.

　　▲ Figure 5. Top and cross-sectional views obtained under different beam oscillation and rotation diameters: Laser parameters were set as follows—laser power = 4000 W, welding speed = 3.0 m/min, spot focus position = 0, rotation frequency = 100 Hz, and beam path = circular. The welding direction involved scanning from left to right.

　　The dynamic movement of the laser can take on various shapes, and its changing oscillation patterns help achieve superior temperature control during the welding process. This leads to more gradual heat input and cooling rates, as well as a smoother thermal temperature gradient. As a result, laser beam oscillation enhances the welding process by minimizing defects to the greatest extent possible—without affecting the post-weld microstructure. Consequently, this approach avoids the issues previously reported by Kraetzsch in Cu/Al dissimilar-material welding and those encountered by Wang et al. during Al welding.

　　▲ Figure 6. Top views and cross-sections of the weld bead obtained using different beam trajectories; laser parameters: laser power = 4000 W, welding speed = 4.0 m/min, spot focus position = 0, rotation diameter = 1.0 mm. The welding direction was from left to right.

　　Laser beam oscillation welding (beam oscillation technology) is now becoming increasingly feasible for a wider range of applications. Khodabakhshi and colleagues successfully demonstrated the use of beam oscillation to achieve dissimilar joining between aluminum and magnesium. They observed that crack sensitivity could be effectively controlled under optimized laser movement conditions. These findings were further validated by Shah, whose research confirmed that beam oscillation significantly enhances the quality of dissimilar material joints between aluminum and magnesium. Additionally, the formation of porosity was notably suppressed when using beam oscillation to weld Al-Mg alloys, providing further evidence of the innovative potential of this technique in the welding process. Moreover, studies by Jiang and Li revealed that grain refinement in the molten zone can be clearly observed during beam oscillation. The intricate laser beam movements employed in this process allow precise control over both the peak temperature and thermal gradients experienced by the materials throughout the welding cycle.

　　▲ Figure 7. Optical photographs of laser welding of a copper alloy: a) Macroscopic image of the weld joint; b) Base material; c) Heat-affected zone; d) Fusion zone

　　Since laser oscillation welding is still a relatively new technology, currently it remains primarily focused on welding steel and aluminum. There is currently a notable lack of understanding regarding its effective application to copper alloys. This study specifically investigates how beam oscillation parameters influence the weld bead morphology and microstructure when laser welding copper alloys. The goal is to gain insights into how beam oscillation affects molten pool dynamics, as well as the resulting weld bead geometry, cross-sectional features, and microstructural characteristics, enabling a comprehensive analysis and characterization of these aspects.

　　▲ Figure 8. a) and b) Schematic diagrams of the beam oscillation (wobble) welding process; c) Cross-sectional view of the molten zone; d) Comparison between the actual weld bead boundary and the boundary predicted by theoretical analysis

　　▲ Figure 9. Simulation results of velocity vector maps on the top surface of the molten pool at different beam oscillation parameters, with the keyhole remaining stationary at the same position (test unit: m/s): (a) 30 Hz; (b) 50 Hz; (c) 70 Hz; (d) 90 Hz; (e) 0.50 mm; (f) 0.75 mm; (g) 1.00 mm; (h) 1.25 mm. (Material: SUS304 stainless steel)

　　Key research findings

　　Welding of copper alloys using laser beam oscillation (swinging) technology has yielded the following positive results: When welding parameters were optimized, no welding defects—such as porosity or cracks—were observed. These are critical indicators for applications involving copper alloys.

　　When welding a 1.5-mm-thick copper alloy using multimode fiber laser, defect-free welds can be achieved by setting the welding parameters as follows: a welding speed ranging from 3.5 to 4 m/min, employing a circular welding pattern with a rotational diameter of 0.6–1 mm, and operating at a frequency of 100 Hz.

　　Beam oscillation has positively impacted the welding process by shifting the welding mode from the keyhole effect to conduction welding. Without beam oscillation, the samples exhibited numerous porosity defects, spatter, and voids—caused by the instability of the keyhole.

　　Higher rotational diameters and frequencies increase the overlap between weld passes, leading to shorter interaction times and, consequently, reduced penetration capability of the weld. However, the morphology of the weld surface shows fewer defects, such as spatter and surface porosity. Additionally, since the welding process lacks symmetry—due to the rotating beam—the bottom of the weld often remains underfilled at high rotation frequencies.

　　At high welding speeds, although surface porosity and spatter are reduced, the weld penetration depth significantly decreases. After testing various beam movement patterns, it was found that a circular trajectory yielded the best results.

　　Non-traditional microstructural features were observed in the molten zone of the weld, where circular bands formed due to the solidification front under beam-vibration conditions. These circular bands resemble the boundaries of interaction between the laser beam and the material. Thanks to the beam-sweeping effect, during the interaction interval Δt, the laser beam spends more time engaging with the molten zone—and thus with the molten pool—compared to a stationary beam. With each rotational cycle, partial melting occurs, giving rise to distinct circular melt lines that solidify sequentially, ultimately forming an epitaxial growth pattern. A similar phenomenon is also seen in the additive manufacturing of copper alloys; however, in this case, the formation is actually attributed to the presence of Cu₂O.