The limited solid solubility between aluminum (Al) and copper (Cu) leads to the formation of massive brittle intermetallic compounds in the resultant Al/Cu dissimilar joints, increasing crack sensitivity and decreasing mechanical properties. High-quality aluminum/copper joining in power batteries is a key process to ensure the battery module works efficiently. However, owing to the high reflectivity of a regular laser on a nonferrous metal surface, the laser power threshold is high, and it is difficult to control the formation of brittle intermetallic compounds. In this study, a blue-red composite laser is used to produce an aluminum/copper hybrid structure, and the effects of the welding speed on the microstructural, tensile, and electrical properties of the joint are studied.
A 1050 aluminum plate with 0.5 mm thickness and a T2 copper plate with 1 mm thickness are used in this experiment. The surfaces of the test pieces are roughened and cleaned with ethanol before welding. The blue-red composite laser is modulated by the superposition of two circular uniform laser beams with different spot diameters. Welding is conducted using high-purity argon gas at a flow rate of 15 L/min. The aluminum plate is lapped on the copper, and the lap width is maintained constant at 20 mm. An electrical discharge wire-cutting machine is used to manufacture metallurgical and tensile specimens. The metallographic specimens are inlaid with an epoxy resin, sandpapered to smooth surfaces, and polished for metallographic observation.
The microstructural morphology and chemical composition of the feature phases are examined by scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). Tensile tests are performed under different welding speeds (V) using a universal testing machine. Three specimens are tested, and the average fracture loads are calculated. The fracture paths and surface morphologies are examined. The contact electric resistance is measured using a direct current power supply and a digital multimeter.
With increasing welding speed, the melting widths of 1050 aluminum and T2 copper decrease gradually. The melting width of 1050 aluminum is larger than that of T2 copper. At V=100 mm/s, the metal at the copper side is only fused slightly with a depth of 52 μm. At V=60 mm/s, the weld is in good shape, and the edges and center of the aluminum side are mainly composed of α-Al solid solution, and no obvious continuously distributed brittle phases are formed. The overall morphologies of the weld are similar under different welding speeds. The Al-side weld seam mainly consists of Al solid solution interspersed with a portion of the Al-Cu subeutectic phase mesh structure (Fig. 4). The weld fracture load increases first and then decreases steadily with an increase in the welding speed, and the maximum fracture load of the specimen reaches 571.5 N under the condition of V=60 mm/s. At different welding speeds, the Al/Cu composite laser-welded joints fracture at the Al/Cu interface, and the primary fracture is cleavage fracture. At V=60 mm/s, resistance values can be as small as 89 μΩ. At a low welding speed, the interface is prone to cracking, leading to deterioration of the joint conductivity. However, at higher welding speeds, the melt pool exists for a short period, and the melt width and joining area decrease, resulting in an increase in the electrical resistance.
In this study, the lap welding of 1050 Al and T2 Cu is achieved using a blue-red composite laser. A welding speed in the range of 60?100 mm/s can yield aluminum/copper joints with flat surfaces and good connection quality. When the welding speed is less than 40 mm/s, the spatter and melt-through holes are formed on the weld surface, which deteriorates the welding quality. From top to bottom, the weld microstructures consists of the Al solid solution, Al solid solution + Al-Cu eutectic, and Al2Cu phases. Owing to the increase in the welding speed, the existence time of the molten pool is shortened, the diffusion of Cu is limited, and many Al-Cu eutectic compounds accumulate at the Al/Cu interface. The maximum fracture load of the Al/Cu joint is 571.5 N at the welding speed of 60 mm/s, and the fracture is cleavage fracture. The aluminum/copper contact resistance reaches a minimum of 89 μΩ at the welding speed of 60 mm/s. Welding speeds that are too high or too low tend to aggregate compounds and reduce conductivity.
