In modern manufacturing, laser welding technology is widely used in various fields, from aerospace to automotive manufacturing, from electronic equipment to medical devices, with its advantages of high efficiency, precision and adaptability. The core of this technology is the interaction of the laser with the material, forming a molten pool and rapidly solidifying, thus enabling the connection of metal parts. The weld pool is a key area in laser welding, and its characteristics directly determine the welding quality, microstructure and final performance. Therefore, in-depth understanding and precise control of molten pool characteristics are of vital significance to improve the level of laser welding technology and meet the needs of high quality welded joints in industrial production.
Molten pool geometry
The geometry of the weld pool is an important aspect in laser welding research, because it directly affects the heat transfer, material flow and the final welding quality during the welding process. The shape of a molten pool is usually described by its depth, width, aspect ratio, heat affected zone (HAZ) geometry, keyhole geometry, and molten metal zone (MMA) geometry. These parameters not only determine the size and shape of the welded joint, but also affect the thermal cycle, cooling rate and microstructure formation during the welding process.
Table 1. The influence of laser welding parameters on the geometrical parameters of each weld pool.
The research shows that laser power and welding speed are the two main process parameters that affect the geometry of the weld pool, as shown in Table 1. In general, as the laser power increases and the welding speed decreases, the depth of the weld pool increases, while the width changes relatively little. This is because higher laser power is able to provide more energy, allowing the material to melt and evaporate faster, resulting in deeper keyholes and pools, as shown in Figure 1. However, when the laser power is too high or the welding speed is too low, it may lead to overheating of the material, excessive evaporation, and even plasma shielding effect, which will reduce the welding quality. Therefore, in the actual welding process, it is necessary to reasonably select the laser power and welding speed according to the specific material characteristics and welding requirements in order to obtain the ideal weld pool geometry.
Figure 1. Different weld shapes formed by laser heat conduction welding and laser deep penetration welding.
In addition to laser power and welding speed, the thermal physical properties of the material, surface state, protective gas and other factors will also have an impact on the weld pool geometry. For example, the higher the thermal conductivity of the material, the faster the heat transfer through the material, and the faster the cooling rate of the molten pool, which may result in a relatively small size of the molten pool. The surface roughness and cleanliness of the material will affect the absorption rate of the laser, and then affect the formation and stability of the molten pool. In addition, the type and flow rate of the protective gas will also have a certain impact on the shape and quality of the molten pool, the appropriate protective gas can effectively prevent the molten pool from oxidation and pollution, but also can adjust the surface tension and flow characteristics of the molten pool, so as to improve the welding quality.
Figure 2. Shape of the molten pool when the laser is swinging.
By changing the trajectory of the laser beam, the laser wobble can significantly affect the shape and characteristics of the molten pool, as shown in Figure 2. As the laser beam wobbles, the shape of the molten pool becomes more uniform and stable. The oscillating laser beam creates a wider heated area on the surface of the pool, making the edges of the pool smoother and reducing sharp edges and irregular shapes. This uniform heating helps to improve the quality and mechanical properties of the welded joint and reduce welding defects such as cracks and pores. In addition, the laser swing can also increase the fluidity of the molten pool, promote the discharge of gases and impurities in the molten pool, and further improve the density and uniformity of the welded joint.
Molten pool dynamics
Molten pool thermodynamics is another key field in laser welding research, which involves the absorption, transfer and conversion of laser energy in the molten pool, as well as the temperature field distribution, cooling rate and phase transition behavior caused by it. The thermodynamic characteristics of the weld pool not only determine the shape and size of the weld pool, but also directly affect the microstructure and mechanical properties of the welded joint
In the process of laser welding, after the laser energy is absorbed by the material, it will produce a high temperature area in the melt pool, causing the material to melt and evaporate. At the same time, heat will be transferred from the high temperature region to the low temperature region through heat conduction, convection and radiation, so that the temperature of the material around the molten pool will increase, and then affect the microstructure and properties of the material. Because of the small size, large temperature gradient and fast cooling rate of the molten pool, it is very difficult to measure the temperature field and cooling rate directly. Therefore, most studies are conducted to study the thermodynamic properties of molten pools by establishing mathematical models and numerical simulation methods.
In the thermodynamic model of molten pool, the following key factors usually need to be considered: First, the absorption mechanism of laser energy, including the reflection, absorption and transmission characteristics of the surface of the material, and the scattering and absorption process of the laser inside the material. Different materials and laser parameters will lead to different absorption rates and energy distributions, which will affect the thermodynamic behavior of the molten pool. Secondly, the thermal physical properties of the material, such as specific heat capacity, thermal conductivity, density, etc., these parameters will change with the change of temperature, which has an important impact on the heat transfer process. In addition, it is also necessary to consider the fluid flow and phase change processes in the molten pool, such as melting, evaporation and solidification, which will change the shape and temperature field distribution of the molten pool, but also affect the microstructure and mechanical properties of the material.
Through numerical simulation and experimental study, the researchers found that the temperature field distribution in the molten pool usually presents a significant non-uniformity, the high temperature area is mainly concentrated in the laser action area and the keyhole, and the temperature gradually decreases to the edge of the molten pool and the heat affected zone. The cooling rate increases with the decrease of the size of the molten pool and the increase of the distance from the laser area. Generally, the cooling rate is lower in the center of the molten pool and the keyhole area, while the cooling rate is higher at the edge of the molten pool and the heat affected zone, as shown in Figure 2. This non-uniform temperature field and cooling rate distribution will lead to obvious gradient changes in the microstructure of the welded joint, such as grain size, phase composition and distribution, which will affect the mechanical properties and corrosion resistance of the welded joint.
Figure 3. Simulation results of keyhole and molten pool formation during laser deep penetration welding of stainless steel plate.
In order to improve the thermodynamic characteristics of molten pool, improve welding quality and reduce welding defects, a series of optimization methods and measures have been proposed. For example, by adjusting laser parameters, such as laser power, welding speed, spot diameter, etc., the input mode and distribution of laser energy can be changed to optimize the temperature field and cooling rate of the molten pool. In addition, the thermodynamic behavior and microstructure evolution of the molten pool can be adjusted by using preheating, post-heating, multi-pass welding and other process methods, as well as using different protective gases and welding atmospheres. At the same time, developing new welding materials and alloy systems to improve the thermal stability and welding performance of materials is also one of the important ways to improve the thermodynamic characteristics of molten pools.
The characteristics of the laser welding pool are the key factors affecting the welding quality, microstructure and mechanical properties. The in-depth study of the geometry and thermodynamic characteristics of the laser welding pool is of great significance for optimizing the laser welding process and improving the welding efficiency and quality. Through a large number of experimental research and numerical simulation analysis, researchers have achieved a series of important research results, which provide a strong theoretical support and technical guidance for the development and application of laser welding technology. However, there are still some shortcomings in the current research, such as the simplification of the model and too many assumptions, and the prediction of the melt pool characteristics under complex working conditions is not accurate enough. The systematic and comprehensive experimental research needs to be improved, and there is a lack of in-depth research on more materials and welding parameters.
Post time: Feb-28-2025
