Laser welding is one of the important aspects of the application of laser processing technology, but also the most eye-catching and promising welding technology in the 21st century. Compared with traditional welding methods, laser welding has many advantages, higher welding quality and faster efficiency. At present, laser welding technology has been widely used in manufacturing, powder metallurgy, automobile industry, electronics industry, biomedicine and other fields.
According to the formation mechanism of welding pool, laser welding has two basic welding mechanisms: heat conduction welding and deep penetration (small hole) welding. The heat generated by heat conduction welding is diffused to the workpiece through heat transfer, so that the surface of the weld is melted, basically no vaporization phenomenon, which is often used in the welding of low-speed thin-wall components. Deep fusion welding vaporizes the material and forms a large amount of plasma. Due to the large heat, there will be holes in the front end of the molten pool. Deep penetration welding can weld the workpiece thoroughly, and the input energy is large, the welding speed is fast, is the most widely used laser welding mode.
There are many process parameters affecting laser welding quality, such as power density, laser pulse waveform, defocus, welding speed and auxiliary blowing gas.
1. Laser power density Power density is one of the most critical parameters in laser processing. With a higher power density, the surface layer can be heated to boiling point within a microsecond time range, generating a large amount of vaporization. Therefore, the high power density is very advantageous for material removal processing, such as punching, cutting and engraving. For low power density, it takes several milliseconds for the surface temperature to reach the boiling point, and before the surface layer vaporizes, the bottom layer reaches the melting point, which is easy to form a good fusion welding. Therefore, in the heat conduction laser welding, the power density range is 104-106W/cm2.
2. Laser pulse waveform
Laser pulse waveform is not only an important parameter to distinguish material removal from material melting, but also a key parameter to determine the volume and cost of processing equipment. When the high intensity laser beam to the surface of the material, the material surface will have 60 ~ 90% of the laser energy reflection and loss, especially gold, silver, copper, aluminum, titanium and other materials strong reflection, fast heat transfer. The reflectance of a metal varies with time during a laser pulse signal. When the surface temperature of the material is raised to the melting point, the reflectivity decreases rapidly, and when the surface is in the melting state, the reflection stabilizes at a certain value.
3. Pulse width Pulse width is an important parameter of pulsed laser welding. The pulse width was determined by the depth of penetration and the heat affected zone. The longer the pulse width, the larger the heat affected zone, and the depth of penetration increased with the 1/2 power of the pulse width. However, the increase of pulse width will reduce the peak power, so the increase of pulse width is generally used for heat conduction welding, resulting in a wide and shallow weld size, especially suitable for lap welding of thin and thick plates. However, lower peak power results in excess heat input, and each material has an optimal pulse width that maximizes penetration.
4, defocus laser welding usually requires a certain amount of defocus, because the laser focus at the center of the spot power density is too high, easy to evaporate into holes. The distribution of power density is relatively uniform in each plane away from the laser focus. There are two defocusing methods: positive defocusing and negative defocusing. If the focal plane is located above the workpiece, it is positive defocusing; otherwise, it is negative defocusing. According to the geometrical optics theory, when the distance between the positive and negative defocusing planes and the welding plane is equal, the power density on the corresponding plane is approximately the same, but the actual obtained weld pool shape is different. In the case of negative defocusing, greater penetration can be obtained, which is related to the formation process of molten pool.
5, welding speed Welding speed determines the welding surface quality, penetration, heat affected zone, etc. The speed of welding will affect the heat input per unit time. If the welding speed is too slow, the heat input is too large, resulting in the workpiece burning through. If the welding speed is too fast, the heat input is too small, resulting in the workpiece welding opaque. Reducing welding speed is usually used to improve penetration.
6, auxiliary blowing protective gas auxiliary blowing protective gas is an essential process in high power laser welding. On the one hand, to prevent metal materials from sputtering and polluting the focusing mirror; On the other hand, it is to prevent the plasma generated in the welding process from focusing too much and prevent the laser from reaching the surface of the material. In the process of laser welding, helium, argon, nitrogen and other gases are often used to protect the molten pool, so that the workpiece is protected from oxidation in the welding engineering. Factors such as the type of protective gas, the size of air flow and blowing Angle have a great influence on the welding result. Different blowing methods also have a certain influence on the welding quality.
Helium does not ionize easily (it has a high ionizing energy), allowing the laser to pass smoothly and the beam energy to reach the workpiece surface unhindered. This is the most effective protective gas used in laser welding, but the price is relatively expensive. Argon is cheaper and denser, so it has better protection. However, it is easily ionized by high temperature metal plasma, thus shielding part of the beam from the workpiece, reducing the effective laser power of welding, but also damaging the welding speed and penetration. The surfaces of welds protected by argon are smoother than those protected by helium. Nitrogen is the cheapest as a protective gas, but it is not suitable for some types of stainless steel welding, mainly due to metallurgical problems, such as absorption, which sometimes creates pores in the lap zone.
As a new welding technology, laser welding has the characteristics of high energy density, high speed, high precision, deep penetration and strong adaptability. Its application is more and more extensive, which can not only improve the production efficiency, but also improve the welding quality. Laser welding technology will certainly play a more important role in the field of material processing.
Post time: Mar-28-2023