Study on Pulsed Laser Shock Oscillation Welding of Dissimilar Aluminum Alloy and Copper Foils

1. Introduction

Aluminum alloys are widely used in batteries and energy storage devices due to their high electrical conductivity and low price. However, the conductive connections are typically copper foils. Welding of the aluminum alloys with copper is thus need but has been very challenging due to the large differences between their physical properties and formation of brittle intermetallic compound. Previous studies have been focused on resistance welding techniques [1,2], and continuous-wave (CW) laser welding [3,4]. The power of the CW laser is typically several hundred Watts to compensate the low light absorption coefficient of the metals. To control the thermal effect in laser welding, various spatial energy distributions have been proposed, such as top hat, ring, core-ring [5,6], etc., having side effects of increased equipment complexity. Few has carried out experimental studies on pulsed laser welding of the dissimilar couple [7,8,9].

Nanosecond pulsed lasers are typically used in marking and drilling applications [10]. The ablation effect is strong, resulting in obvious mechanical shock waves [11,12,13]. The thermal effect is reduced, compared to a CW laser, as the laser heating time is the pulse duration. The thermal effect in previous reports on nanosecond laser dissimilar welding is only modulated by the laser parameters and scanning velocities. Although there have been researches on oscillations of CW laser beam in structural material welding [14,15], the performance of pulsed laser oscillation during dissimilar welding has never been reported. Here, based on our research in laser shock modulation of molten pool [16,17,18,19,20,21,22], this paper firstly reported the pulsed laser shock oscillation welding of dissimilar aluminum alloy and copper foils with enhanced performance and with scale-up capability and low cost.

2. Material and Methods

2.1. Materials

The chemical compositions of materials of Al (1080 aluminum alloy) and Cu are shown in Table 1. The foil thicknesses of both foils are around 0.2 mm and that of Al can be increased to other dimensions without changing welding performance. The samples are polished by sandpapers to remove surface oxides and ultrasonically cleaned for 5 minutes. Acetone is then used to further clean the surface before welding.

2.2. Methods

A nanosecond pulsed laser with galvanometer scanners is used for laser beam delivery in this paper. The motions of the scanners are computer controlled. The experimental parameters of the pulsed laser are shown in Table 2. After welding, the cross sections are carefully cut to reveal the interfaces between two metals. Optical microscope and scanning electron microscope are used for morphology observations. Elemental analysis and phase analysis are done in EDS and XRD, respectively. Uniaxial tensile tests are done with 30 mm × 10 mm × 0.2 mm samples with multiple separated welding tracks in the center.

3. Results and Discussions

As the nanosecond laser irradiates the copper-aluminum dissimilar couple, significant localized heating effect is generated, the magnitude of which is dependent on the laser parameters (e.g., pulse energy, frequency, pulse width, etc.) and scan parameters (scanning path and velocity, diameter, etc.). In this paper, oscillation becomes a new parameter. The heating effect could be enlarged and the melt pool size is increased during oscillations. The experimental setup is schematically shown in Figure 1a. Along with the mechanical shock wave effect, the melt pool is disturbed and the interface between Cu and Al could be modulated to form interlocks (schematically shown in Figure 1b), similar to our previous reports on other dissimilar couples [19]. However, when the pulsed laser does not oscillate, the heating effect is not enough, and mechanical shock wave effect on a small melt pool could not be buffered, resulting in pores and ejections of melt pool.

Figure 2a shows the cross-section area morphology of the pulsed laser welding of the Al and Cu couple without oscillation. It can be seen the top surface of Cu is rough, and re-solidified metal exhibits ejection-like behavior around the scanning center of laser beam. Zoom-in observance of the Cu side shows the interface is three-dimensional. EDS analyses of five locations in Figure 2b are shown in Figure 2c. It shows that some intermetallic compounds form. This is also confirmed in the XRD analysis in Figure 2d. However, due to the pulsed laser impulsive heating effect, part of the metal in the scanning track is still in Cu single phase. EDS scanning line analysis in Figure 2e and Figure 2f confirmed the randomness in majority Cu or Al elemental distributions inside multiple tracks, contributing to interface interlocking.

Figure 3a shows the cross-section area morphology of the scanning paths for pulsed laser shock oscillation welding of the dissimilar couple. It has a curved feature, as shown by the dashed lines. This curve nature of the weldment in the cross section perpendicular to the main scanning path has not been reported previously. Typical cross-sectional re-solidified area in welding only has one slope. However, due to pulsed laser oscillation effect and multiple reflections inside the keyhole, the slope of the keyhole varies temporally, resulting in a curved line shown by the dashed lines. Zoom-in view of the cross section is shown in Figure 3b. The top surface of Cu becomes smoother than that in Figure 2a for samples processed by non-oscillation pulsed welding. The re-solidified metal on the top surface does not exhibit obvious ejection-like behavior anymore. Figure 3c and Figure 3d are demonstrations of slight increasement of the amount of intermetallic compound formation. Similar randomness of elemental distributions of the multiple tracks is shown in Figure 3e and Figure 3f.

The mechanical properties are tested by uniaxial tensile tests, and the facture area morphologies are shown in Figure 4. By comparing Figure 4a-c with Figure 4d-f, more dimples and ductile factures are observed in the case of pulsed laser shock oscillation dissimilar welding, agreeing with the more curved 3D interfaces formations in Figure 3. The ultimate shear strength is around 70 MPa for pulsed laser shock oscillation welding, and that for the non-oscillation case is 60 MPa. The ductility is also found to be better in the oscillation case.

4. Conclusions

This study employs pulsed laser shock oscillation welding of Al and Cu dissimilar couple and compares its performances with those from non-oscillation welding. It is found that welding interface become more curved and is three-dimensional in nature. The mechanical properties in terms of shear strength and ductility are found to be better than those in non-oscillation case. Facture area morphology in the oscillation case shows more small dimples, agreeing with the strength and ductility changes by beam oscillation. This study advances the welding technology of Al and Cu dissimilar couple, and has great potential applications in electrical vehicles and energy storage sectors.