The formation and development of keyholes:
Keyhole definition: When the radiation irradiance is greater than 10 ^ 6W/cm ^ 2, the surface of the material melts and evaporates under the action of laser. When the evaporation speed is large enough, the generated vapor recoil pressure is sufficient to overcome the surface tension and liquid gravity of the liquid metal, thereby displacing some of the liquid metal, causing the molten pool at the excitation zone to sink and form small pits; The beam of light directly acts on the bottom of the small pit, causing the metal to further melt and gasify. High pressure steam continues to force the liquid metal at the bottom of the pit to flow towards the periphery of the molten pool, further deepening the small hole. This process continues, ultimately forming a keyhole like hole in the liquid metal. When the metal vapor pressure generated by the laser beam in the small hole reaches equilibrium with the surface tension and gravity of the liquid metal, the small hole no longer deepens and forms a depth stable small hole, which is called the “small hole effect”.
As the laser beam moves relative to the workpiece, the small hole shows a slightly backward curved front and a clearly inclined inverted triangle at the back. The front edge of the small hole is the action area of the laser, with high temperature and high vapor pressure, while the temperature along the back edge is relatively low and the vapor pressure is small. Under this pressure and temperature difference, the molten liquid flows around the small hole from the front end to the back end, forming a vortex at the back end of the small hole, and finally solidifies at the back edge. The dynamic state of the keyhole obtained through laser simulation and actual welding is shown in the above figure, The morphology of small holes and the flow of surrounding molten liquid during travel at different speeds.
Due to the presence of small holes, the laser beam energy penetrates into the interior of the material, forming this deep and narrow weld seam. The typical cross-sectional morphology of the laser deep penetration weld seam is shown in the above figure. The penetration depth of the weld seam is close to the depth of the keyhole (to be precise, the metallographic layer is 60-100um deeper than the keyhole, one less liquid layer). The higher the laser energy density, the deeper the small hole, and the greater the penetration depth of the weld seam. In high-power laser welding, the maximum depth to width ratio of the weld seam can reach 12:1.
Analysis of absorption of laser energy by keyhole
Before the formation of small holes and plasma, the energy of the laser is mainly transmitted to the interior of the workpiece through thermal conduction. The welding process belongs to conductive welding (with a penetration depth of less than 0.5mm), and the material’s absorption rate of the laser is between 25-45%. Once the keyhole is formed, the energy of the laser is mainly absorbed by the interior of the workpiece through the keyhole effect, and the welding process becomes deep penetration welding (with a penetration depth of more than 0.5mm), The absorption rate can reach over 60-90%.
The keyhole effect plays an extremely important role in enhancing the absorption of laser during processing such as laser welding, cutting, and drilling. The laser beam entering the keyhole is almost completely absorbed through multiple reflections from the hole wall.
It is generally believed that the energy absorption mechanism of laser inside the keyhole includes two processes: reverse absorption and Fresnel absorption.
Pressure balance inside the keyhole
During laser deep penetration welding, the material undergoes severe vaporization, and the expansion pressure generated by high-temperature steam expels the liquid metal, forming small holes. In addition to the vapor pressure and ablation pressure (also known as evaporation reaction force or recoil pressure) of the material, there are also surface tension, liquid static pressure caused by gravity, and fluid dynamic pressure generated by the flow of molten material inside the small hole. Among these pressures, only steam pressure maintains the opening of the small hole, while the other three forces strive to close the small hole. To maintain the stability of the keyhole during the welding process, the vapor pressure must be sufficient to overcome other resistance and achieve equilibrium, maintaining the long-term stability of the keyhole. For simplicity, it is generally believed that the forces acting on the keyhole wall are mainly ablation pressure (metal vapor recoil pressure) and surface tension.
Instability of Keyhole
Background: Laser acts on the surface of materials, causing a large amount of metal to evaporate. The recoil pressure presses down on the molten pool, forming keyholes and plasma, resulting in an increase in melting depth. During the process of moving, the laser hits the front wall of the keyhole, and the position where the laser contacts the material will cause severe evaporation of the material. At the same time, the keyhole wall will experience mass loss, and the evaporation will form a recoil pressure that will press down on the liquid metal, causing the inner wall of the keyhole to fluctuate downward and move around the bottom of the keyhole towards the back of the molten pool. Due to the fluctuation of the liquid molten pool from the front wall to the back wall, the volume inside the keyhole is constantly changing, The internal pressure of the keyhole also changes accordingly, which leads to a change in the volume of the plasma sprayed out. The change in plasma volume leads to changes in shielding, refraction, and absorption of laser energy, resulting in changes in the energy of the laser reaching the material surface. The entire process is dynamic and periodic, ultimately resulting in a sawtooth shaped and wavy metal penetration, and there is no smooth equal penetration weld, The above figure is a cross-sectional view of the center of the weld obtained by longitudinal cutting parallel to the center of the weld, as well as a real-time measurement of the keyhole depth variation by IPG-LDD as evidence.
Improve the stability direction of the keyhole
During laser deep penetration welding, the stability of the small hole can only be ensured by the dynamic balance of various pressures inside the hole. However, the absorption of laser energy by the hole wall and the evaporation of materials, the ejection of metal vapor outside the small hole, and the forward movement of the small hole and molten pool are all very intense and rapid processes. Under certain process conditions, at certain moments during the welding process, there is a possibility that the stability of the small hole may be disrupted in local areas, leading to welding defects. The most typical and common ones are small pore type porosity defects and spatter caused by keyhole collapse;
So how to stabilize the keyhole?
The fluctuation of keyhole fluid is relatively complex and involves too many factors (temperature field, flow field, force field, optoelectronic physics), which can be simply summarized into two categories: the relationship between surface tension and metal vapor recoil pressure; The recoil pressure of metal vapor directly acts on the generation of keyholes, which is closely related to the depth and volume of the keyholes. At the same time, as the only upward moving substance of metal vapor in the welding process, it is also closely related to the occurrence of spatter; Surface tension affects the flow of the molten pool;
So stable laser welding process depends on maintaining the distribution gradient of surface tension in the molten pool, without too much fluctuation. Surface tension is related to temperature distribution, and temperature distribution is related to heat source. Therefore, composite heat source and swing welding are potential technical directions for stable welding process;
The metal vapor and keyhole volume need to pay attention to the plasma effect and the size of the keyhole opening. The larger the opening, the larger the keyhole, and the negligible fluctuations in the bottom point of the melt pool, which have a relatively small impact on the overall keyhole volume and internal pressure changes; So adjustable ring mode laser (annular spot), laser arc recombination, frequency modulation, etc. are all directions that can be expanded.
Post time: Dec-01-2023
