Views: 75 Author: Site Editor Publish Time: 08-05-2021 Origin: Site
With the continuous development of laser technology, laser welding technology is constantly being updated and updated in traditional technology, making laser welding machines gradually play a better advantage. In the laser welding process, if you have a good grasp of the changing laws of some process parameters, you can adjust the parameters according to different requirements, and then obtain better welding quality by controlling the process parameters. It can be seen that the process parameters have a great influence on the quality of laser welding. Let's analyze the Leapion laser below. Which specific process parameters will affect the quality of welding?
1) There is a laser energy density threshold in laser welding. Below this value, the penetration depth is very shallow. Once this value is reached or exceeded, the penetration depth will be greatly increased.
2) Only when the laser power density on the workpiece exceeds the threshold (related to the material), plasma will be generated, which marks the progress of stable deep penetration welding.
3) If the laser power is lower than this threshold, only the surface of the workpiece will melt, that is, the welding will be performed in a stable thermal conductivity type. When the laser power density is near the critical condition for the formation of small holes, deep penetration welding and conduction welding alternately become an unstable welding process, resulting in large fluctuations in penetration.
4) During laser deep penetration welding, the laser power controls the penetration depth and welding speed at the same time. The welding penetration is directly related to the beam power density, and is a function of the incident beam power and the beam focal spot.
5) Generally speaking, for a laser beam with a certain diameter, the penetration depth increases as the beam power increases.
The beam spot size is one of the most important variables in laser welding, because it determines the power density.
The diffraction-limited spot size of the beam focus can be calculated according to the theory of light diffraction, but due to the aberration of the focusing lens, the actual spot is larger than the calculated value. The simplest actual measurement method is the isothermal profile method, which is to measure the focal spot and perforation diameter after scorching thick paper and penetrating the polypropylene plate. This method should be practiced through measurement to master the size of the laser power and the time of beam action.
The absorption of laser light by a material depends on some important properties of the material, such as absorptivity, reflectivity, thermal conductivity, melting temperature, evaporation temperature, etc. The most important of which is absorptivity.
The factors that affect the material's absorption rate of the laser beam include two aspects:
1) The first is the resistivity of the material. After measuring the absorptivity of the polished surface of the material, it is found that the absorptivity of the material is proportional to the square root of the resistivity, and the resistivity changes with temperature;
2) Secondly, the surface state (or finish) of the material has a more important influence on the beam absorption rate, which has a significant effect on the welding effect.
The welding speed has a great influence on the penetration depth. Increasing the speed will make the penetration shallower, but if the speed is too low, it will cause excessive melting of the material and weld penetration of the workpiece. Therefore, there is a suitable welding speed range for a certain laser power and a certain thickness of a certain material, and the maximum penetration depth can be obtained at the corresponding speed value.
1) In the laser welding process, inert gas is often used to protect the molten pool. When some materials are welded, the surface oxidation can also be ignored. However, for most applications, helium, argon, nitrogen and other gases are often used for protection. The workpiece is protected from oxidation during the welding process.
2) The second function of using protective gas is to protect the focusing lens from metal vapor contamination and liquid droplet sputtering. Especially in high-power laser welding, because the ejection becomes very powerful, it is more necessary to protect the lens at this time.
3) The third function of the shielding gas is to dissipate the plasma shielding produced by high-power laser welding. The metal vapor absorbs the laser beam and ionizes into a plasma cloud, and the shielding gas surrounding the metal vapor is also ionized by heating. If there is too much plasma, the laser beam is consumed by the plasma to some extent. Plasma exists as the second kind of energy on the working surface, which makes the penetration depth become shallower and the weld pool surface becomes wider. Increase the electron recombination rate by increasing the three-body collision of electrons with ions and neutral atoms to reduce the electron density in the plasma. The lighter the neutral atom, the higher the collision frequency, and the higher the recombination rate. On the other hand, only the shielding gas with high ionization energy will not increase the electron density due to the ionization of the gas itself.
When welding, a focusing method is usually used to converge the laser, and a lens with a focal length of 63~254mm (2.5”~10”) is generally used. The focal spot size is proportional to the focal length, the shorter the focal length, the smaller the spot. However, the focal length also affects the focal depth, that is, the focal depth increases synchronously with the focal length, so a short focal length can increase the power density, but because the focal depth is small, the distance between the lens and the workpiece must be accurately maintained, and the penetration depth is not large. Due to the influence of spatter and laser mode during the welding process, the shortest focal depth used in actual welding is mostly 126mm (5").
When the seam is large or the spot size needs to be increased to increase the weld seam, you can Choose a lens with a focal length of 254mm (10"). In this case, in order to achieve the deep penetration pinhole effect, a higher laser output power (power density) is required.
When welding, in order to maintain sufficient power density, the focal position is very important. The change of the relative position of the focus and the surface of the workpiece directly affects the width and depth of the weld.
In most laser welding applications, the focal point is usually set at approximately 1/4 of the required penetration depth below the surface of the workpiece.
When performing laser welding on different materials, the position of the laser beam controls the final quality of the weld, especially the case of butt joints is more sensitive than the case of lap joints. For example, when a hardened steel gear is welded to a low-carbon steel drum, correct control of the laser beam position will help to produce a weld mainly composed of low-carbon components, which has better crack resistance.
In some applications, the geometry of the welded workpiece requires the laser beam to be deflected by an angle. When the deflection angle between the beam axis and the joint plane is within 100 degrees, the workpiece's absorption of laser energy will not be affected.
In laser deep penetration welding, no matter the depth of the weld, the pinhole phenomenon always exists. When the welding process is terminated and the power switch is turned off, pits will appear at the end of the weld. In addition, when the laser welding layer covers the original weld, excessive absorption of the laser beam will occur, causing the weldment to overheat or produce pores.
In order to prevent the occurrence of the above phenomenon, the power start and end points can be programmed to make the power start and end time adjustable, that is, the start power is electronically raised from zero to the set power value in a short period of time, and the welding is adjusted. Time, and finally the power is gradually reduced from the set power to zero when the welding is terminated.
Due to the interaction of the above 9 process parameters, it has formed an impact on the quality of laser welding. When using the laser welding machine, we must set the process parameters reasonably to achieve the perfect welding quality and effect. For more professional questions, please contact Leapion Laser.
