Laser welding can be achieved using continuous or pulsed laser beams. The principles of laser welding can be divided into heat conduction welding and laser deep penetration welding. When the power density is less than 104~105 W/cm2, it is heat conduction welding. At this time, the penetration depth is shallow and the welding speed is slow; when the power density is greater than 105~107 W/cm2, the metal surface is concave into “holes” due to heat, forming deep penetration welding, which has It has the characteristics of fast welding speed and large aspect ratio. The principle of thermal conduction laser welding is: laser radiation heats the surface to be processed, and the surface heat diffuses to the interior through thermal conduction. By controlling laser parameters such as laser pulse width, energy, peak power, and repetition frequency, the workpiece is melted to form a specific molten pool.
Laser deep penetration welding generally uses a continuous laser beam to complete the connection of materials. Its metallurgical physical process is very similar to that of electron beam welding, that is, the energy conversion mechanism is completed through a “key-hole” structure.
Under laser irradiation with a high enough power density, the material evaporates and small holes are formed. This small hole filled with vapor is like a black body, absorbing almost all the energy of the incident beam. The equilibrium temperature in the hole reaches about 2500°C. The heat is transferred from the outer wall of the high-temperature hole, causing the metal surrounding the hole to melt. The small hole is filled with high-temperature steam generated by the continuous evaporation of the wall material under the irradiation of the beam. The walls of the small hole are surrounded by molten metal, and the liquid metal is surrounded by solid materials (in most conventional welding processes and laser conduction welding, the energy first Deposited on the surface of the workpiece and then transported to the interior by transfer). The liquid flow outside the hole wall and the surface tension of the wall layer are in phase with the continuously generated steam pressure in the hole cavity and maintain a dynamic balance. The light beam continuously enters the small hole, and the material outside the small hole is continuously flowing. As the light beam moves, the small hole is always in a stable state of flow.
That is to say, the small hole and the molten metal surrounding the hole wall move forward with the forward speed of the pilot beam. The molten metal fills the gap left after the small hole is removed and condenses accordingly, and the weld is formed. All of this happens so quickly that welding speeds can easily reach several meters per minute.
After understanding the basic concepts of power density, thermal conductivity welding, and deep penetration welding, we will next conduct a comparative analysis of the power density and metallographic phases of different core diameters.
Comparison of welding experiments based on common laser core diameters on the market:
Power density of focal spot position of lasers with different core diameters
From the perspective of power density, under the same power, the smaller the core diameter, the higher the brightness of the laser and the more concentrated the energy. If the laser is compared to a sharp knife, the smaller the core diameter, the sharper the laser. The power density of the 14um core diameter laser is more than 50 times that of the 100um core diameter laser, and the processing capability is stronger. At the same time, the power density calculated here is just a simple average density. The actual energy distribution is an approximate Gaussian distribution, and the central energy will be several times the average power density.
Schematic diagram of laser energy distribution with different core diameters
The color of the energy distribution diagram is the energy distribution. The redder the color, the higher the energy. The red energy is the place where the energy is concentrated. Through the laser energy distribution of laser beams with different core diameters, it can be seen that the laser beam front is not sharp and the laser beam is sharp. The smaller, the more concentrated the energy is on one point, the sharper it is and the stronger its penetrating ability.
Comparison of welding effects of lasers with different core diameters
Comparison of lasers with different core diameters:
(1) The experiment uses a speed of 150mm/s, focus position welding, and the material is 1 series aluminum, 2mm thick;
(2) The larger the core diameter, the larger the melting width, the larger the heat-affected zone, and the smaller the unit power density. When the core diameter exceeds 200um, it is not easy to achieve a penetration depth on high-reaction alloys such as aluminum and copper, and a higher Deep penetration welding can be achieved only with high power;
(3) Small-core lasers have high power density and can quickly punch keyholes on the surface of materials with high energy and small heat-affected zones. However, at the same time, the surface of the weld is rough, and the keyhole collapse probability is high during low-speed welding, and the keyhole is closed during the welding cycle. The cycle is long, and defects such as defects and pores are prone to occur. It is suitable for high-speed processing or processing with a swing trajectory;
(4) Large core diameter lasers have larger light spots and more dispersed energy, making them more suitable for laser surface remelting, cladding, annealing and other processes.
Post time: Oct-06-2023
