Welding of Copper to Aluminium With Laser Beam Wavelength of 515 nm

In laser joining of copper (Cu) and aluminum (Al) sheets, the Al sheet is widely chosen as the top surface for laser irradiation because of increased absorption of laser beam and lower melting temperature of Al in contrast to Cu. This research focus on welding from Cu side to Al sheet. The main objective of irradiating the laser beam from the copper side (Cu on top) is to exploit higher solubility of Al in Cu. A significantly lower laser power can be used with 515 nm laser in comparison to 1030 nm. In addition to low laser power, a stable welding is obtained with 515 nm. Because of this advantage, 515 nm is selected for the current research. By fusion of Cu and Al the two sheet metals are welded, with presence of beneficial Cu solid solution phase and Al+Al2Cu in the joint with the brittle phases intermixed between the ductile phase. Therefore the mixed composition strengthens the joint. However excessive mixing leads to formation of more detrimental phases and less ductile phases. Therefore optimum mixing must be maintained. Energy dispersive X-ray spectroscopy (EDS) analysis indicate that large amount of beneficial Cu solid solution and Al rich phases is formed in the strong joint. From the tensile shear test for a strong joint, fracture is obtained on the heat-affected zone (HAZ) of Al. Therefore the key for welding from copper side is to have optimum melt with beneficial phases like Cu and Al+ Al2Cu and the detrimental phases intermixed between the ductile phases


Introduction
Laser welding of copper to aluminum sheet is challenging because of the poor absorption of laser radiation by these materials. Al sheet is widely selected as the top material to irradiate by the laser beam because of its relatively high absorption to laser radiation (1030 nm) and low meting point (660 o C) in comparison to Cu. To obtain a good joint with Al to Cu is to irradiate laser from Aluminum (Al) to control the weld depth and mixing of Cu in Al [1,2].
Irradiation of laser beam onto Al (660 • C) allows for braze welding of Al to Cu. This approach is useful for this combination because the detrimental intermetallic phases formed can be reduced [3,4]. Larger weld seam can be obtained with laser beam oscillation, by maintaining keyhole conditions (10 7 W/cm 2 ) required to weld highly reflective metals like Al and Cu [5]. Oscillated laser beam is beneficial in controlling the depth of penetration into the bottom sheet [6,7]. Welding from Cu sheet is challenging in terms of higher reflectivity, thermal conductivity and melting temperature (1080 • C) in contrast to Al. Therefore, to weld from copper side high intensity or good absorption of laser light is a fundamental requirement [8][9][10]. Very less investigations are conducted in the analysis of welding from Cu to Al [7,11,12]. The phase composition and the intermixing of Cu and Al in Cu-Al (Cu on top) configuration is not well studied. This paper investigates welding of Cu-Al (Cu on top) with green laser wavelength of 515 nm. Welding of Cu with green laser wavelength improves the absorption of laser light and increases the process stability [9,10,13], in comparison to near-IR laser wavelength of 1030 nm. This paper exploits the higher solubility of Al in Cu, which is about 18.5 at.%. With this, a higher amount of Al can be intermixed to Cu during welding process. Therefore, a higher depth of penetration and mixing of Cu and Al is feasible. The purpose of this paper is to investigate and show that a strong joint based on fusion welding of Cu to Al can be achieved with laser beam. The reminder of the paper is structured as follows. The laser equipment, oscillation strategy and microscope used are detailed in Materials and Methods section 2. The results are discussed in subsections starting from the benefit of using green laser wavelength of 515 nm in comparison to 1030 nm (section 3.1), the tensile shear test (section 3.2) and micro-structural analysis (section 3.3) is presented. Finally, conclusion is discussed in section 4.

Material
For laser welding experiment, thin copper (Oxide free-OF 99.95% pure) and aluminum (Al 1050) sheets each of thickness 0.4 mm is used wit the dimensions mentioned in Table 1.

Beam Oscillation
The laser welding was performed with wobbling in order to enlarge the seam width, as the beam diameter used is 89 µm. Laser beam oscillation in form of infinite shape with

Microstructural Characterization
The microstructural analysis is performed using optical microscope (Leica DM 4000 M) and Scanning Electron microscope combined with EDS (Energy dispersive X-ray spectroscopy with Apollo XV detector) for composition of elements in atomic % in the joint. For microstructural analysis, the joint cross-section is prepared by sectioning and moulding in the resin followed by grinding and polishing procedure to obtain a mirror polished surface. The moulding, grinding and polishing procedures are followed as outlined by [14].

Tensile shear test
The tensile shear test is carried out with Zwick Z010 machine at room temperature (25 o C). The feed rate of crosshead is 1.2 mm/min [5]. The tensile shear strength of the materials are reported in terms of N/mm 2 .

Result and discussion
3.1. Laser absorption issue for welding Cu-Al with 1030 nm in comparison to 515 nm laser In this section, the necessity of welding with green laser wavelength at 515 nm for Cu and Al is studied. For this reason, comparison of weld beads of Near IR laser wavelength (1030 nm) and green laser wavelength (515 nm) in continuous wave mode is performed. The experiment is conducted with the laser wavelengths of 515 nm and 1030 nm, using same optical parameters, resulting in focal spot diameter of 89 µm. The wobbling trajectory for both the laser wavelengths is fixed to wobbling speed of 50 mm/s, wobble width of 1.5 mm, wobble frequency of 100 Hz and thickness of Cu and Al is 0.4 mm each. Therefore, all the parameters are fixed and weld bead formation for green laser (515 nm) and Near IR laser (1030 nm) wavelength is compared for stability of bead formation. The figure 3 shows the bead on top of Cu sheet, welded with Al for green laser wavelength of 515 nm and near-infrared laser wavelength of 1030 nm.
The first difference is the power required to perform welding of copper and aluminium. For 515 nm, with improved absorption of the laser radiation, a weld is achieved with a minimum power of 650 W. The formed weld bead is stable and consistent with the trajectory of oscillation imprinted on the material. This in comparison to 1030 nm, higher power of 900 W is required. However, the bead suffers from inconsistency over the length direction ( figure 3).For 1030 nm (figure 3 marked in red box), the bead formed at low laser power is unstable, which is due to the poor absorption of Cu at 1030 nm. For 515 nm laser, the development of weld bead and intermixing is stable, and a good weld bead is obtained for a laser power of 700-750 W. This is the optimum melt condition and detailed analysis of this case is presented in section 3.2. From this condition, increasing the laser power promotes excessive inter-diffusion of Al and Cu. as shown in figure 3 for 515 nm at power 900 W. On the other hand for 1030 nm laser, good weld bead is obtained at laser power of 950 W and at 1000 W excessive Al and Cu is melted resulting is an over-weld. Therefore, with green laser a stable weld bead is obtained with low laser power, which is very important for welding Cu in dissimilar combination to Al. For this reason, in this research, the green laser is required for the study of welding from Cu to Al sheet. Detailed results of welding Cu-Al with the green laser is explained in the following sections 3.2 and 3.3

Tensile shear test
The weld strength is evaluated by tensile shear test with five sample per test (n=5). The relation of laser power to the tensile shear stress (N/mm 2 ) is shown in figure 4 and a schematic of the Cu-Al weld failure location, and the corresponding cross-section view of the failure after metallography is shown in figure 5. 5, case (b)) were obtained, which represents a good joint. For increased laser power (900 W) excessive penetration results in formation of over-mixing of Cu and Al and results in a joint with large amount of intermetallic phases. In this condition, the joint fails in the fusion zone (figure 5, case (c)). In conclusion, from tensile shear test, strong joint is obtained for case (b), despite intermixing of Cu and Al. A large depth into the bottom sheet (Al) is achieved. More detailed microstructural analysis and the elemental composition of the Al and Cu intermixing in fusion zone responsible for the strong joint is explained in section 3.3.

Microstructural analysis
The microstructural analysis is performed for the strong weld to understand the morphology and composition of Cu and Al in the joint that contribute to the weld strength.  with limited inter-diffusion (Blue arrow in figure 6). In the intermixed region and limited diffusion zone, interface is bonded metallurgically with a mix of chemical composition. The bonding is based on diffusion of Cu and Al in molten state. Detailed analysis of Al and Cu composition in these regions indicate that large amount of Cu solid solution phases are present, and the intermetallic compounds are thin and intermixed between beneficial Cu rich (> 80 at % of Cu) and Al rich phases. Energy Dispersive X-Ray Spectroscopy (EDS) is used to measure elemental composition of Cu and Al in the high intermixing and limited intermixing regions. The SEM (scanning electron microscope) image of the limited diffusion zone and the EDS measurement of Al composition (At %) is shown in 7. From the EDS analysis, Al+Al 2 Cu phase is mainly formed in the limited intermixed zone. The point 1 in figure 7 is Cu and point 2, 3 is Al+Al 2 Cu phase. The phase in point 2 is based on hypo-eutectic reaction. The point 3 is eutectic phase and point 4 is Al solid solution. All these phases are not detrimental intermetallic phases. The structure of the phase 3 at eutectic composition is Lamellar, which consist of many thin alternating layers. This phase is good for promoting both the strength and ductility of the joint. This region is mainly Al rich, which indicates metallurgical bonding based on the eutectic reaction. The diffusion in this region is mainly due to the heat conducted from highly intermixed zone. In the limited diffusion zone, formation of detrimental intermetallic phases are minimum or avoided.    To investigate the diffusion at the local interface, region marked in yellow line for a distance of 70 µm in figure 10 is selected. The EDS points are collected for 70 µm with a step size of 0.23 µm. The figure 11 shows the plot of Al and Cu composition (in at %) in Y-axis and the scan distance for EDS in X-axis. The first few microns of about 9 µm is Cu solid solution. The transition from the Cu solid solution to the Al 2 Cu is the detrimental intermetallic phases (50-80 at%) such as AlCu, Al 3 Cu 4 , Al 4 Cu 9 . The highly detrimental composition is in the range of 70-55 at. % of Cu and the transition is very quick and is thin layered of about 4-5 µm. The next phase is Al 2 Cu which is surrounded by Al+ Al 2 Cu phase based on hyper and hypo eutectic reaction. Because of activating the intermixing from copper, more ductile phase such as Cu solid solution and Al rich dendrite is formed at the interface. A combined presence of ductile and brittle phase increases the strength of the joint.

Conclusion
Laser welding with green wavelength of 515 nm is studied to join Cu to Al with Cu placed on top of Al. With green laser wavelength of 515 nm, the welding of Cu to Al is achieved with a low laser power of 700-750 W with stable weld bead formation. Welding with Cu on top and Al in bottom configuration allowed for a larger intermixing of Al into Cu, with formation of more beneficial phases like Al solid solution, Cu solid solution and Al+Al 2 Cu. With this approach, a strong joint with failure on the HAZ of Al is obtained with high depth of penetration and significant intermixing. Despite large Al and Cu melt involved in the joint, thickness of the detrimental intermetallic phase in the local interface is small (about 4-5 µm). The detrimental phases are intermixed in between the ductile phase compositions, i.e. Al rich and Cu solid solution. With this paper, fusion welding approach for joining Cu and Al system is demonstrated with a strong joint.
Funding: "This research was funded by European Regional Development Fund (FEDER)

HAZ Heat-affected zone EDS
Energy-dispersive X-ray spectroscopy SEM Scanning Electron Microscope Al Aluminium Cu Copper