Feb 26, 2020
1. Principle of laser welding
Laser welding can be achieved by continuous or pulsed laser beams. The principle of laser welding can be divided into thermal conduction welding and laser deep penetration welding. When the power density is less than 104 ~ 105W / cm2, it is heat conduction welding. At this time, the welding depth is shallow and the welding speed is slow. When the power density is more than 105 ~ 107W / cm2, the metal surface is recessed into a "cavity" under the action of heat, forming a deep fusion welding. Fast, wide aspect ratio.
The principle of thermal conduction laser welding is: the laser radiation heats the surface to be processed, and the surface heat is diffused to the inside by thermal conduction. By controlling the laser parameters such as the laser pulse width, energy, peak power, and repetition frequency, the workpiece is melted to form a specific molten pool.
Laser welding machines for gear welding and metallurgical sheet welding mainly involve laser deep penetration welding. The following focuses on the principle of laser deep penetration welding.
Laser deep penetration welding generally uses a continuous laser beam to complete the connection of materials. The metallurgical physical process is very similar to electron beam welding, that is, the energy conversion mechanism is completed through a "key-hole" structure. Under sufficiently high power density laser irradiation, the material evaporates and forms small holes. This steam-filled hole is like a black body, which absorbs almost all the energy of the incident beam. The equilibrium temperature in the cavity reaches about 2500 ° C. Heat is transferred from the outer wall of the high-temperature cavity, which melts the metal surrounding the cavity. The small hole is filled with high-temperature steam generated by continuous evaporation of the wall material under the beam. The four walls of the small hole surround the molten metal, and the liquid metal surrounds the solid material. (In most conventional welding processes and laser conduction welding, the energy is first (Deposited on the surface of the workpiece, and then transferred to the interior by transfer). The liquid flow and wall surface tension outside the pore wall are consistent with the steam pressure continuously generated in the pore cavity and maintain dynamic equilibrium. 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 steady state of flow. That is, the small hole and the molten metal surrounding the hole wall move forward with the forward speed of the leading beam. The molten metal fills the gap left after the small hole is removed and condenses with it, and a weld is formed. All of this happens so quickly that the welding speed can easily reach several meters per minute.
2. The main process parameters of laser deep penetration welding
(1) Laser power. There is a laser energy density threshold in laser welding. Below this value, the penetration depth is very shallow. Once it reaches or exceeds this value, the penetration depth will be greatly increased. Plasma is generated only when the laser power density on the workpiece exceeds a threshold (material-dependent), which signifies stable deep penetration welding. If the laser power is lower than this threshold, only the surface melting of the workpiece occurs, that is, the welding is performed in a stable heat conduction type. However, when the laser power density is near the critical condition for the formation of small holes, deep penetration welding, and conductive welding are alternately performed, which becomes an unstable welding process, resulting in large fluctuations in penetration depth. In laser deep-welding, the laser power controls both penetration depth and welding speed. The penetration depth of the weld is directly related to the beam power density and is a function of the incident beam power and the beam focal spot. Generally speaking, for a certain diameter laser beam, the penetration depth increases as the beam power increases.
(2) Beam focal spot. Beam spot size is one of the most important variables for laser welding because it determines power density. But for high-power lasers, its measurement is a difficult problem, although there are already many indirect measurement techniques.
The beam spot diffraction limit spot size can be calculated according to the theory of light diffraction, but due to the aberration of the focusing lens, the actual spot size is larger than the calculated value. The simplest measurement method is isothermal profiling, which measures the focal spot and perforation diameter after burning and penetrating a polypropylene board with thick paper. This method is to measure the laser power and the time of the beam through measurement practice.
(3) Material absorption value. The absorption of a laser 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 these is the absorptivity.
The factors that affect the absorption rate of a laser beam by material include two aspects: First, the material's resistivity. After measuring the absorbance of the polished surface of the material, it is found that the absorbance of the material is proportional to the square root of the resistivity, and the resistivity varies with temperature And changes; secondly, the surface state (or smoothness) of the material has a more important influence on the beam absorption rate, which has a significant effect on the welding effect.
The output wavelength of a CO2 laser is usually 10.6 μm. Non-metals such as ceramics, glass, rubber, and plastic have a high absorption rate at room temperature, and metal materials have a poor absorption at room temperature until the material is melted and even gas Its absorption has increased sharply.
It is very effective to improve the absorption of the light beam by the method of surface coating or the formation of an oxide film on the surface.
(4) Welding speed. Welding speed has a greater impact on penetration depth. Increasing the speed will make the penetration depth shallower, but too low speed will cause excessive melting of the material and welding of the workpiece. Therefore, there is a suitable welding speed range for a certain material with certain laser power and a certain thickness, and the maximum penetration depth can be obtained at the corresponding speed value.
(5) Protective gas. The laser welding process often uses inert gas to protect the molten pool. When some materials are welded, the surface oxidation can be ignored, but protection is not considered, but for most applications, helium, argon, nitrogen, and other gases are often used to protect the workpiece. Protected from oxidation during welding.
Helium is not easily ionized (higher ionization energy), which allows the laser to pass smoothly, and the beam energy reaches the surface of the workpiece unhindered. This is the most effective shielding gas used in laser welding, but it is more expensive.
Argon is cheaper and has a higher density, so the protective effect is better. However, it is susceptible to high-temperature metal plasma ionization. As a result, it shields part of the light beam from being radiated to the workpiece, reducing the effective laser power for welding, and damaging the welding speed and penetration. Surfaces of weldments protected with argon are smoother than those protected with helium.
Nitrogen is the cheapest gas as a shielding gas, but it is not suitable for welding certain types of stainless steel, mainly due to metallurgical issues, such as absorption, and sometimes pores are created in the overlap area.
The second role of using a protective gas is to protect the focusing lens from metal vapor contamination and the sputtering of liquid droplets. Especially during high-power laser welding, as the ejection becomes very powerful, it is more necessary to protect the lens at this time.
The third function of the shielding gas is to effectively dispel the plasma shield generated by high-power laser welding. The metal vapor absorbs the laser beam and ionizes into a plasma cloud. The protective 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 on the working surface as the second energy, which makes the penetration shallower and the surface of the welding pool wider. The electron recombination rate is increased by increasing the collision of electrons with ions and neutral atoms, so as 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 protective gas with high ionization energy will not increase the electron density due to the ionization of the gas itself.
Helium has the lowest ionization and the lowest density, and it can quickly remove the rising metal vapor generated from the molten metal pool. Therefore, using helium as a protective gas can suppress the plasma to the maximum extent, thereby increasing the penetration depth and the welding speed; it can escape due to its lightweight and is not easy to cause pores. Of course, from the effect of our actual welding, the effect of protecting with argon is not bad.
The effect of the plasma cloud on penetration is most obvious in the low welding speed region. As the welding speed increases, its effects diminish.
The protective gas is ejected to the surface of the workpiece through the nozzle with a certain pressure. The hydrodynamic shape of the nozzle and the diameter of the outlet is very important. It must be large enough to drive the sprayed protective gas to cover the welding surface, but in order to effectively protect the lens and prevent metal vapor pollution or metal spatter from damaging the lens, the nozzle size must also be limited. The flow rate must also be controlled, otherwise, the laminar flow of the protective gas becomes turbulent, the atmosphere is drawn into the molten pool, and eventually, pores are formed.
In order to improve the protection effect, an additional side blowing method can also be used, that is, a protective gas is directly injected into the small hole of deep penetration welding through a small diameter nozzle at a certain angle. The shielding gas not only suppresses the plasma cloud on the surface of the workpiece, but also exerts an influence on the plasma inside the holes and the formation of small holes, and the penetration depth is further increased to obtain an ideal weld with a depth-to-width comparison. However, this method requires precise control of the magnitude and direction of the gas flow, otherwise, turbulence is likely to occur and damage the molten pool, which makes the welding process difficult to stabilize.
(6) Lens focal length. When welding, focusing is usually used to converge the laser. Generally, a lens with a focal length of 63 ~ 254mm (2.5 "~ 10") is used. The focal spot size is directly proportional to the focal length. The shorter the focal length, the smaller the focal 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 effects of spatters and laser modes generated during welding, the shortest focal depth used in actual welding is mostly a focal length of 126mm (5 ”). When the seam is large or it is necessary to increase the weld by increasing the spot size, Choose a lens with a focal length of 254mm (10 ”). In this case, in order to achieve the deep-melt pinhole effect, a higher laser output power (power density) is required.
When the laser power exceeds 2kW, especially for the CO2 laser beam of 10.6μm, due to the use of special optical materials to form the optical system, in order to avoid the risk of optical damage to the focusing lens, the reflection focusing method is often used, and polished copper mirrors are generally used as mirrors. Due to its effective cooling, it is often recommended for high power laser beam focusing.
(7) Focus position. In order to maintain sufficient power density during welding, the focus position is critical. The change of the relative position of the focus and the workpiece surface directly affects the width and depth of the weld.
In most laser welding applications, the position of the focal point is usually set about 1/4 of the required penetration depth below the surface of the workpiece.
(8) Laser beam position. When laser welding different materials, the position of the laser beam controls the final quality of the weld, especially in the case of butt joints, which are more sensitive than the case of lap joints. For example, when hardened steel gears are welded to low-carbon steel drums, the correct control of the laser beam position will be beneficial to the production of welds mainly composed of low-carbon components, which have 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.
(9) The laser power at the start and end of welding is controlled gradually. In laser deep-welding, pinholes always exist regardless of the depth of the weld. When the welding process is terminated and the power switch is turned off, dimples will appear at the end of the weld. In addition, when the laser welding layer covers the original welding seam, excessive absorption of the laser beam may occur, resulting in overheating of the weldment or generation of porosity.
To prevent the above-mentioned phenomenon from occurring, a program can be made for the starting and ending points of the power, so that the starting and ending time of the power can be adjusted, that is, the starting power is increased from zero to the set power value in a short time by electronic methods, and the welding is adjusted Time, and finally the power is gradually reduced from the set power to zero when the welding is terminated.
3. Laser deep fusion welding features, advantages, and disadvantages
(1) Characteristics of laser deep penetration welding
①High aspect ratio. Because the molten metal is formed around the cylindrical high-temperature vapor cavity and extends toward the workpiece, the weld seam becomes deep and narrow.
②Minimum heat input. Because the temperature in the small holes is very high, the melting process occurs very quickly, the heat input to the workpiece is very low, and the heat distortion and heat-affected zone is small.
③High density. Because the small holes filled with high-temperature steam are conducive to the welding pool stirring and gas escape, resulting in the formation of pore-free penetration welds. The high cooling rate after welding makes it easy to miniaturize the weld structure.
④Strong welds. Because of the hot heat source and sufficient absorption of non-metallic components, the content of impurities is reduced, the size of the inclusions and their distribution in the molten pool are changed. The welding process does not require electrodes or filler wires, and the melting zone is less polluted, making the weld strength and toughness at least equivalent to or even greater than the parent metal.
⑤Precise control. Because the focal spot is small, the weld can be positioned with high accuracy. The laser output has no "inertia" and can be stopped and restarted at high speeds. The CNC beam moving technology can weld complex workpieces.
⑥Non-contact atmosphere welding process. Because the energy comes from the photon beam and there is no physical contact with the workpiece, no external force is applied to the workpiece. In addition, both magnetic and air have no effect on the laser.
(2) Advantages of laser deep welding
①Focused lasers have a much higher power density than conventional methods, resulting in faster welding speeds, less heat-affected zones and deformation, and welding of difficult-to-weld materials such as titanium.
②Because the beam is easy to transmit and control, there is no need to change the welding torch and nozzle frequently, and there is no vacuum required for electron beam welding, which significantly reduces the auxiliary time for shutdown, so the load factor and production efficiency are high.
③Due to the purification effect and high cooling rate, the weld has high strength, toughness, and comprehensive performance.
④Due to the low average heat input and high machining accuracy, reprocessing costs can be reduced; in addition, laser welding operation costs are also lower, which can reduce workpiece processing costs.
⑤It can effectively control the beam intensity and fine positioning, and it is easy to realize automatic operation.
(3) Disadvantages of laser deep welding
①Welding depth is Limited.
②Workpiece assembly requirements are high.
③One-time investment in laser systems is high.