In recent years, notable progress has been made in the development of equipment components aimed at precision and miniaturization. These miniature components typically exhibit complex geometries. They are composed of diverse materials. Further miniaturization of these components has led to an increased demand for precision welding. Consequently, the assembly of small parts and the packaging of devices require increasingly high levels of connection accuracy and quality control. High-quality micro-welding technologies for metallic materials have important applications in aerospace, power batteries, biomedicine, and other fields. For instance, micro electro mechanical systems (MEMS), characterized by feature sizes ranging from 1 μm to 1 mm, are commonly packaged using micro-welding technology. Moreover, power battery electrode foils, with thicknesses as low as 6?12 μm, require precise connections for current export. Furthermore, the assembly of components and metal shell sealing in implantable biomedical devices rely heavily on micro-welding technology.
Common micro-welding techniques include resistance micro spot welding, ultrasonic micro-welding, micro tungsten inert gas(TIG) welding, and laser micro-welding. Compared with conventional micro-welding methods, laser micro-welding offers several advantages, including a small focusing spot size, precise heat input control capability, high welding speed, and compatibility with various weldable materials.
This study investigates the laser micro-welding technology of metal materials, providing a comprehensive analysis of its significance, microscale effects, welding modes, laser selection, and defect and quality control measures. It is difficult to reach a consensus on a precise definition of laser micro-welding. The connotations of laser micro-welding are comprehensively summarized based on previously reported studies. Strictly speaking, laser micro-welding pertains to a laser welding process where at least one feature size of the connected material or weld is less than 100 μm. Laser micro-welding involves two welding modes: conduction and penetration welding. In laser micro-welding, oxidation promotes fluctuations in the penetration-welding process, resulting in a transient phase. Subsequently, the influence of microscale effects is introduced. When workpiece dimensions are reduced to the micron scale, typical microscale effects occur. The physical characteristics observed during laser micro-welding, such as heat transfer and molten pool flow, differ from those observed during macro-welding (Fig.2). Based on microscale effects, the defects and quality control measures in laser micro-welding are summarized according to the process parameters. Welding defects such as lack of penetration, burn-through, spatter, humping, porosity, and cracking can occur during the laser micro-welding process, and optimization of the welding process parameters is an important means of controlling weld formation and welding defects. These parameters include the laser wavelength, laser power, spot diameter of the laser, pulse laser parameters, welding speed, and scanning path.
Furthermore, the applications of laser micro-welding to both similar and dissimilar metal materials are reviewed. Laser micro-welding is used to join precision components in the electronics, automotive, aerospace, and medical industries (Fig.12). Notable applications include pressure sensors, bipolar plates for fuel cells, aerospace engine blades, electronic component pins, copper-printed circuit boards, satellite collimator components, cardiac pacemakers, and lithium-ion battery tabs.
Finally, the challenges and future development directions of laser micro-welding technology for metallic materials are summarized, including the welding mechanism of metal and non-metallic materials, new process technology, and laser micro-welding systems.
The characteristics of laser micro-welding are complex owing to microscale effects. Although laser micro-welding has been widely used for connecting metal materials, some challenges remain. First, there is a burgeoning demand for the joining of dissimilar materials, including the micro-welding of dissimilar metals and metal/non-metallic materials. Dissimilar materials with different physical properties pose significant challenges in welding. Second, increasing the welding speed is important for improving the production rate. However, humping occurs at high welding speeds. To address this, process innovation and the recombination of multiple energy fields are required to further increase the critical speed of humping by controlling the flow characteristics of the molten pool and the solidification process during micro-welding. This is essential for improving the production rates and ensuring the product yield in high-speed welding. Finally, the development of intelligent laser micro-welding systems is a key future trend. The use of an intelligent laser micro-welding system has the potential to improve weld quality and welding efficiency.
