Analysis of Molten Pool Characteristics in Laser Welding (Part 2)

01 Introduction
Laser welding technology plays a crucial role in modern manufacturing, with its core being the formation and rapid solidification of the melt pool. The characteristics of the melt pool not only determine welding quality but also directly affect the microstructure and mechanical properties of the metal. In recent years, with the development of laser technology and the expansion of its application fields, research on the characteristics of the laser welding melt pool has become increasingly in-depth. Researchers have conducted extensive and thorough investigations into the fluid dynamics of the melt pool, microstructure evolution, and the mechanisms of pore formation through experimental observation, numerical simulation, and theoretical analysis.

02 Melt Pool Dynamics
Melt pool dynamics is a key area of laser welding research, involving the formation, flow, and stability of the melt pool. Laser power and welding speed are the main parameters influencing melt pool dynamics. High laser power and low welding speed typically result in a deeper melt pool, but excessive power may cause overheating of the material and the formation of pores. The fluid flow within the melt pool is primarily driven by surface tension, evaporation recoil force, and buoyancy, as shown in Figure 1. These forces work together to affect the shape and stability of the melt pool.

Figure 1. Fluid flow in the melt pool

For example, the Marangoni convection driven by surface tension can create a temperature gradient on the surface of the melt pool, influencing the flow pattern and cooling rate. Marangoni convection causes radial outward flow on the melt pool surface, transferring heat from the center outward, thus making the melt pool shallower. This flow pattern is particularly noticeable in high-power laser welding, as it significantly alters the shape and size of the melt pool. Through numerical simulation and experimental studies, a better understanding of these complex processes can be gained to optimize welding parameters and improve welding quality.

Furthermore, the evaporation recoil force is also an important factor affecting melt pool dynamics. During laser welding, material evaporation generates a recoil force that pushes the molten metal in the melt pool outward. This recoil force is particularly significant in high-power laser welding, as it can lead to noticeable changes in the shape and size of the melt pool. For example, when the laser power is too high, the evaporation recoil force may cause the edges of the melt pool to bulge, affecting the quality of the weld joint.

Figure 2. Study of fluid dynamics in the melt pool of stainless steel laser deep penetration welding
a) Vertical view; b) Horizontal view

Buoyancy is another important factor influencing melt pool dynamics. Due to the temperature gradient within the melt pool, the hotter molten metal rises, while the cooler molten metal sinks, creating convection. This convection can improve the mixing and uniformity of the melt pool but also increases the instability of the melt pool. By properly controlling welding parameters, the buoyancy convection in the melt pool can be optimized to enhance welding quality.

03 Microstructure and Pore Formation
The microstructure and pore characteristics of the weld after laser welding significantly affect the performance of the welded joint. Rapid cooling typically leads to the formation of fine-grained structures, improving the hardness and strength of the weld joint. However, excessively rapid cooling can also lead to the formation of pores, reducing the density and mechanical properties of the weld joint. Pore formation is primarily related to the gas solubility, evaporation, and solidification processes within the melt pool. For example, when welding aluminum alloys, hydrogen has a much higher solubility in liquid aluminum alloys than in solid aluminum alloys. Therefore, during the welding process, hydrogen will precipitate from the liquid aluminum alloy, forming pores, as shown in Figure 3. Optimizing welding parameters, such as reducing welding speed and improving the quality of protective gases, can effectively reduce pore formation. Additionally, using protective gases with low dew points can also reduce the hydrogen content, thereby lowering the probability of pore formation.

Figure 3. Microstructure of laser welded aluminum

The evolution of microstructure is also closely related to the thermal cycle during the welding process. Proper thermal cycle control can achieve an ideal microstructure and improve the performance of the weld joint. For example, when welding austenitic stainless steel, rapid cooling leads to the formation of fine-grained martensitic structures, which enhance the hardness and strength of the weld joint. However, excessive cooling rates may lead to crack and pore formation, reducing the toughness of the weld joint, as shown in Figure 4. By properly controlling welding parameters, the thermal cycle can be optimized to achieve an ideal microstructure and improve the performance of the weld joint. Furthermore, the evolution of the microstructure is also influenced by the chemical composition of the material. Different chemical compositions result in different phase transformation behaviors and microstructural structures. For example, when welding low-carbon steel, adding appropriate amounts of manganese and silicon can promote the formation of ferrite, enhancing the toughness of the weld joint. By selecting the right chemical composition for the material, the microstructure can be optimized to improve the performance of the weld joint.

Figure 4. Simulation of crack and pore formation in stainless steel laser deep penetration welding

04 Conclusion
This paper summarizes the dynamic behavior of the laser welding melt pool, microstructure evolution, and pore formation mechanisms. By deeply understanding these characteristics, laser welding process parameters can be optimized to improve welding quality. Future research will continue to deepen the understanding of melt pool characteristics, combining multidisciplinary theories and methods to explore more efficient welding processes. The development of laser welding technology will bring more innovations and breakthroughs to modern manufacturing.

**--Cite the article published by 高能束加工技术 on January 31, 2025, in the WeChat public account "High-Energy Beam Processing Technology and Applications."