In the process of automobile manufacturing, it is necessary to connect parts.Especially in the light-weight requirements of automobile, the application of fiber-reinforced composite materials is increasing gradually. The connection of metal / composite materials is the problem that material and vehicle manufacturers must solve.Laser welding is a popular joining process of dissimilar materials in recent years, because it does not need additional additives and can avoid the damage to the structure of components caused by fastening connection.Laser welding is a kind of large-scale composite material / metal joining technology which has a bright future.

Preparation of roof reinforcement

The reinforcing rib is made of continuous glass fiber reinforced PA6 by hot pressing.The glass fiber reinforced composite semi-finished products are heated to the processing temperature by infrared heater, then molded and trimmed.Secondly, the metal bracket should be installed on the stiffener to realize the installation with the ceiling. The metal bracket is made of steel plate.

（1）The metal bracket is pretreated by laser in advance to change the metal surface structure.Then, the laser is used to heat the metal bracket, and the composite material in contact with the metal surface melts, and then the connection is realized by pressure.At the same time, in order to ensure the quality of the joint, the research group also carried out non-destructive defect test on the joint through thermal imaging technology.During welding, laser ablation occurs on the metal surface, forming a cavity structure.At the same time, the melt bead is formed by the melt of the composite near the processing direction. Repeat the process to form a special geometric structure on the joint surface, and a positive connection between the polymer and the metal.Therefore, the strength of the hybrid structure is affected by the welding arrangement, and the welding points can be arranged pertinently to better match the application requirements.In order to realize the on-line control of welding process, a micro structure tracking and processing system is developed, which can process the geometry of 2.5-D quickly. In addition, a high-speed infrared camera for process control is developed to realize the on-line control and verification of welding process.Induction heating technology is widely used in the connection of composite materials, but when it is used in the connection of composite materials, composite materials are mostly heated, while when it is used in the connection of mixed materials, metal elements are generally heated. The researchers used continuous induction heating technology in the project.When the high frequency AC current passes through the inductor, an electromagnetic alternating field is formed around the inductor, which generates heat energy in the metal through resistance and hysteresis loss.The metal is induction heated until it is above the melting temperature and below the degradation temperature of the used matrix polymer.In this region, the composite melts at the contact point with the metal and fills the cavity formed on the metal surface under pressure.

（2）Laser welding process

When the composite stiffener is welded to the metal bracket, the laser radiation is stacked by diodes and focused on the workpiece with appropriate optics. The metal absorbs the laser radiation and converts the energy into heat.

There are two main laser connection processes:

Laser transmission connection: through the selection of a specific wavelength of laser beam, the transmission is realized by the polymer material with better transparency, so that it impacts the heated metal surface.Due to the physical contact between polymer and metal, it will be heated and melted due to heat conduction. However, the transparency of the glass fiber reinforced composite material of this project is low, and the laser transmission connection is not applicable.

Laser heat conduction connection: as an alternative to laser transmission connection, the laser radiates on the metal surface and transmits the heat to the connection area through metal heat conduction.

The energy input depends on the laser power, irradiation duration and the absorption of laser radiation on the metal surface.The project team can introduce energy as evenly as possible by using elements that can adjust beam geometry and beam path.In addition, in order to achieve better connection effect, the parts are loaded with consolidation force through the clamping device, so as to realize the complete wetting and effective connection of the polymer on the metal surface.

（3）Thermal imaging quality inspection

In order to realize industrial batch application, the project team introduced thermal imaging technology to ensure the stability of product quality.With the help of optical heat source such as halogen lamp, the technology can adjust the detection depth by modulating the frequency and inspect the product quality.