Due to the dependence of absorption on wavelength and temperature as well as the high thermal conductivity of copper, the laser welding of copper with a 1 μm infrared (IR) laser beam generally shows clear instability, splashes, and poor surface morphology. Thus, blue and green lasers were used to weld copper and excellent welding quality were obtained for the high absorption of copper in short wavelength. Currently, the cost of the red and green lasers is significantly high. Hence, considering the cost and welding quality, we employed an IR nanosecond MOPA fiber laser with a power and wavelength of 120 W and 1.07 μm, respectively, to weld the copper sheets using a fiber-coupled blue diode laser with a power and wavelength of 100 W and 450 nm, respectively. The function of the blue laser is to preheat the surface of the copper sample to increase the effective absorption of the IR laser and slow the cooling process to help the expulsion of bubbles. As expected, surface morphology and tensile force of welded copper sheets are improved, and almost no splashes and microholes can be observed inside the pool in the welding area.

A dual-wavelength coaxial beam-welding system was designed. Two collimated beams from an IR MOPA fiber laser and a fiber-coupled blue diode laser were coaxially combined using a dichroic mirror, and further were focused on the workpiece using a galvanometer scanner and a dedispersive telecentric f-θ lens. The weld position on the workpiece was controlled by scanner 2. Another galvanometer scanner, scanner 1, was inserted between the collimator of the IR MOPA beam and the dichroic mirror to control the movement of the MOPA beam inside the blue spot to achieve different welding modes, such as swing and spiral. In the experiments related to the tensile test, each workpiece sample was welded to four spots in a square-like pattern with a 2 mm pitch. The diameter of each welding spot is about 350 μm. Copper sheets with a thickness of 100 and 200 μm were welded onto a copper sheet with a thickness of 250 μm, which were marked as samples 1# and 2#, respectively. The welding process is as follows. (1) Turn on the blue laser to start the preheating process. (2) After the preheating time (T₁), turn on the IR MOPA laser to start the welding process. The MOPA beam was scanned along a spiral line to form a 350 μm welding spot, which was then turned off. (3) Keep the blue laser on a warm-holding time, T_2,to maintain the molten pool at a relatively high temperature, and cool it down naturally further. The preheating and warm-holding time dependencies and the blue and IR laser power dependencies of the joint tension were measured. The IR power dependency of the joint tension with and without blue-assistant welding was also compared. Finally, the surface quality of the weld spots and the microstructure of the molten pools were observed, compared, and explained.

The optimal preheat time, T_1,is approximately 20 and 30 ms for samples 1# and 2# at the blue laser power of 50 and 60 W, respectively; the optimal warm-holding time, T_2,for both samples 1# and 2# is 50 ms. A proper warm-holding time is essential for expelling bubbles from the molten pool, which increases tension. However, an excessive preheating or holding time will slightly decrease the welding tension due to the coarsening of grains in the molten pool. A 30 W blue laser-assisted 70 W IR MOPA laser welding process achieves the best tensile force (76.5 N), whereas a 110 W MOPA laser welding process achieves a tensile force of only 58.6 N. The blue-assisted MOPA laser welding shows higher welding strength and laser efficiency than the only IR MOPA laser welding. Moreover, the blue-assisted welding significantly improved the splash and surface quality (Fig. 6). From the observation of the cross-section of the blue-assistant welding molten pool using metallographic and scanning electron microscopy (Figs. 7 and 8), we can obtain the following results: (1) the zigzag shape of the molten pool of the blue-assisted welding is not obvious; (2) the internal holes of molten pool are significantly reduced; and (3) the grains in the molten pool are slightly coarsened.

The blue laser assists the IR MOPA fiber laser to weld pure copper sheets through the preheating and warm-holding processes. The welding quality is significantly improved compared with the only IR MOPA fiber-laser welding. The welding strength is increased to 1.31 times. There are no splashes and obvious surface voids on the weld spot surface, and the surface granularity is considerably improved, which benefits from the effective preheating using blue light. The multi-sawtooth molten pools tend to fuse, and the widespread and numerous micropores in the interior are eliminated, which is attributed to the exhaust effect during the warm-holding process using the blue light. The blue laser-assisted IR MOPA laser welding can significantly improve the welding quality of copper sheets.