Effect of bottom sheet thickness on weld properties during laser lap welding of aluminum and copper

Continuous laser lap welding for a dissimilar combination of aluminum (Al) and copper (Cu) was achieved successfully. The successful welding was achieved by varying laser power between 500 and 900 W with an interval of 100 W, whereas the variation in welding speed was maintained between 10 and 45 mm/min with an increment of 5 mm/min. A combination of various laser power and weld speed generated a broad range of volume energy inputs, among which acceptable welding with no macroscopic defect could only be generated between 1.81 and 7.64 kJ/mm3. The volume energy required to achieve successful welding increased with the increase in bottom Cu sheet thickness. The joint's mechanical properties were significantly enhanced for thicker bottom Cu sheets as the welding was carried out at a higher volume of energy achieving higher depth of penetration of 0.65 mm in case of 1.5-mm bottom Cu sheet thickness. Lap shear tensile tests and Vickers's microhardness testing confirmed the abovementioned results as the highest breaking load of 373.79 N, and Vicker's hardness of 350 Hv inside Al and 750 Hv inside Cu was obtained. Furthermore, SEM results on the weld cross sections showed that welds with thicker bottom Cu sheets exhibit a higher penetration depth and lower top weld bead. Detailed electron microscopy and spectroscopy revealed the formation of hard intermetallic compounds (IMCs) inside the weld zone due to the Cu diffusion inside the weld. They correlated well with the measured mechanical and electrical properties. Electron back-scattered diffraction (EBSD) revealed a sudden concentration of high-angle boundaries (HAB) inside the weld, confirming the presence of hard IMCs. Inside the weld, most of the grain boundaries transformed into HAB, which explains the joints' enhanced mechanical properties.