Laser welding principle
Laser welding can be achieved by continuous or pulsed laser beams. The principle 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 sunken into "cavities" by heating, forming deep penetration welding, which has It has the characteristics of fast welding speed and large aspect ratio.
The principle of heat conduction laser welding is: laser radiation heats the surface to be processed, and the surface heat diffuses to the inside through heat conduction. By controlling the laser pulse width, energy, peak power and repetition frequency and other laser parameters, the workpiece is melted to form a specific molten pool. .
The laser welding machine used for gear welding and metallurgical thin plate welding mainly involves laser deep penetration welding. The following focuses on the principle of laser deep penetration welding.
Laser deep penetration welding generally uses continuous laser beams to complete the connection of materials, and its metallurgical physical process is very similar to electron beam welding, that is, the energy conversion mechanism is completed through the "key-hole" structure. Under sufficiently high power density laser irradiation, the material evaporates and forms small pores. This small hole full of steam is like a black body, absorbing almost all the energy of the incident beam, and the equilibrium temperature in the cavity reaches about 2500 0C. The heat is transmitted from the outer wall of the high-temperature cavity to melt the metal surrounding the cavity. 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 (while 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 transmission). The liquid flow outside the pore wall and the surface tension of the wall layer maintain a dynamic balance with the continuously generated vapor pressure in the pore cavity. The beam continuously enters the small hole, and the material outside the small hole is continuously flowing. As the 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 leading beam, and the molten metal fills the gap left by the small hole and then condenses, so that the weld is formed. All this of the above process happens so quickly that welding speeds can easily reach several meters per minute.
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The main process parameters of laser deep penetration welding
1) Laser power. There is a threshold value of laser energy density 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. Plasma is generated only when the laser power density on the workpiece exceeds a threshold value (depending on the material), which marks the progress of stable deep penetration welding. If the laser power is below this threshold, only surface melting of the workpiece takes place, ie welding takes place with stable heat conduction. When the laser power density is near the critical condition for the formation of small holes, deep penetration welding and conduction welding are performed alternately, which becomes an unstable welding process, resulting in large fluctuations in penetration depth. During laser deep penetration welding, the laser power controls the penetration depth and welding speed at the same time. Welding penetration is directly related to beam power density and is a function of incident beam power and beam focal spot. In general, for a laser beam of a certain diameter, the depth of penetration increases as the beam power increases.
2) Beam focal spot. Beam spot size is one of the most important variables in laser welding because it determines power density. But for high-power lasers, its measurement is a difficult problem, although there are many indirect measurement techniques.
The diffraction-limited spot size of the beam focus can be calculated according to the light diffraction theory, but due to the existence of focusing lens aberration, the actual spot size is larger than the calculated value. The simplest practical method is the isothermal profiling method, which measures the focal spot and perforation diameter after charring and penetrating a polypropylene plate with thick paper. This method needs to master the laser power and the time of beam action through measurement practice.
3) Material absorption value. The absorption of laser light by materials depends on some important properties of materials, such as absorptivity, reflectivity, thermal conductivity, melting temperature, evaporation temperature, etc., the most important of which is absorptivity.
The factors that affect the absorption rate of the material to the laser beam include two aspects: the first is the resistivity of the material. After measuring the absorption rate of the polished surface of the material, it is found that the absorption rate of the material is proportional to the square root of the resistivity, and the resistivity varies with temperature. 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. The absorption rate of ceramics, glass, rubber, plastics and other non-metals is very high at room temperature, while the absorption rate of metal materials is very poor at room temperature, until the material is melted or even gas Its absorption increases dramatically. It is very effective to improve the material's absorption of light beams by using surface coating or surface oxide film formation.
4) Welding speed. The welding speed has a great influence on the penetration depth. Increasing the speed will make the penetration shallow, but if the speed is too low, the material will be over-melted and the workpiece will be welded through. Therefore, there is a suitable welding speed range for a specific material with a certain laser power and a certain thickness, and the maximum penetration depth can be obtained at the corresponding speed value. Figure 10-2 shows the relationship between welding speed and penetration depth of 1018 steel.
5) Protective gas. Inert gas is often used to protect the molten pool in the laser welding process. When some materials are welded regardless of surface oxidation, the protection may not be considered, but for most applications, helium, argon, nitrogen and other gases are often used as protection to make the workpiece Protected from oxidation during soldering.
Helium is not easily ionized (higher ionization energy), which allows the laser to pass through smoothly, and the beam energy reaches the surface of the workpiece without hindrance. This is the most effective shielding gas used in laser welding, but it is more expensive.
Argon gas is cheaper and denser, so the protection effect is better. However, it is susceptible to high-temperature metal plasma ionization, which shields part of the beam from hitting the workpiece, reduces the effective laser power for welding, and also damages the welding speed and penetration. The surface of the weldment protected by argon is smoother than that when protected by helium.
Nitrogen is the cheapest shielding gas, but it is not suitable for welding some types of stainless steel, mainly due to metallurgical problems, such as absorption, which sometimes produces porosity in the overlapping area.
The second function of using shielding gas is to protect the focusing lens from metal vapor contamination and sputtering of liquid droplets. Especially in high-power laser welding, because the ejection becomes very powerful, it is more necessary to protect the lens at this time.
The third function of the shielding gas is that it is very effective in dissipating the plasma shield produced by high-power laser welding. The metal vapor absorbs the laser beam and ionizes into a plasma cloud, and the protective gas around the metal vapor is also ionized due to heat. If too much plasma is present, the laser beam is somewhat consumed by the plasma. Plasma exists on the working surface as a second energy, which makes the penetration shallow and the surface of the weld pool widen. The recombination rate of electrons is increased by increasing the three-body collisions of electrons with ions and neutral atoms to reduce the electron density in the plasma. The lighter the neutral atoms, 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.
The size of the plasma cloud varies with the shielding gas used, with helium being the smallest, nitrogen being the second, and argon being the largest. The larger the plasma size, the shallower the penetration. The reason for this difference is firstly due to the different degree of ionization of gas molecules, and also due to the difference in the diffusion of metal vapor caused by the different densities of the shielding gas.
Helium is the least ionized and least dense gas, and it quickly drives off rising metal vapors generated from the molten metal bath. Therefore, using helium as a shielding gas can suppress the plasma to the greatest extent, thereby increasing the penetration depth and increasing the welding speed; due to its light weight, it can escape and is not easy to cause pores. Of course, from our actual welding effect, the effect of argon protection is not bad.
The effect of plasma cloud on penetration is most obvious in the low welding speed area. Its effect diminishes as the welding speed increases.
The shielding gas is injected at a certain pressure through the nozzle to reach the surface of the workpiece. The hydrodynamic shape of the nozzle and the diameter of the outlet are very important. It must be large enough to drive the sprayed shielding gas to cover the welding surface, but in order to effectively protect the lens and prevent metal vapor from contaminating or metal splashing from damaging the lens, the size of the nozzle should also be limited. The flow rate should also be controlled, otherwise the laminar flow of the shielding gas will become turbulent, and the atmosphere will be involved in the molten pool, eventually forming pores.
In order to improve the protective effect, an additional side blowing method can also be used, that is, through a nozzle with a smaller diameter, the protective gas is directly injected into the small hole of the deep penetration welding at a certain angle. The shielding gas not only suppresses the plasma cloud on the surface of the workpiece, but also exerts influence on the formation of the plasma and small holes in the hole, further increases the penetration depth, and obtains a weld with an ideal depth-width ratio. However, this method requires precise control of the size and direction of the air flow, otherwise turbulent flow is likely to occur and destroy the molten pool, making the welding process difficult to stabilize.
6) Lens focal length. Focusing method is usually used to condense the laser during welding, and a lens with a focal length of 63~254mm (2.5”~10”) is generally used. The focus spot size is proportional to the focal length, the shorter the focal length, the smaller the spot. But 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 of the small focal depth, the distance between the lens and the workpiece must be precisely maintained, and the penetration depth is not large. Due to the influence of spatter and laser mode generated in the welding process, the shortest focal depth used in actual welding is mostly the focal length of 126mm (5"). When the joint is large or the weld seam needs to be increased by increasing the spot size, 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 the laser power exceeds 2kW, especially for the 10.6μm CO2 laser beam, 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 reflective focusing method is often used, and a polished copper mirror is generally used as the reflector . It is often recommended for focusing high power laser beams due to effective cooling.
7) Focus position. When welding, the focus position is critical in order to maintain sufficient power density. Changes in the relative position of the focal point and the workpiece surface directly affect the width and depth of the weld. Figure 2-6 shows the effect of focus position on penetration depth and seam width of 1018 steel.
In most laser welding applications, the focal point is typically located approximately 1/4 of the desired depth of penetration below the surface of the workpiece.
8) Laser beam position. When laser welding dissimilar materials, the laser beam position controls the final quality of the weld, especially in the case of butt joints than lap joints. For example, when a hardened steel gear is welded to a mild steel drum, proper control of the laser beam position will help produce a weld with a predominantly low carbon component that is relatively resistant to cracking. In some applications, the geometry of the workpiece to be welded 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 absorption of laser energy by the workpiece will not be affected.
9) Gradual rise and fall control of the laser power at the start and end points of welding. During laser deep penetration welding, small holes always exist regardless of the depth of the weld. When the welding process is terminated and the power switch is turned off, a pit will appear at the end of the weld. In addition, when the laser welding layer covers the original weld seam, excessive absorption of the laser beam will occur, resulting in overheating of the weldment or generation of pores.
In order to prevent the above phenomenon from happening, the power start and stop points can be programmed to make the power start and end time adjustable, that is, the initial power is electronically increased from zero to the set power value in a short time, and the welding can be adjusted. Time, and finally the power is gradually reduced from the set power to zero when the welding is terminated.
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Features and advantages and disadvantages of laser deep penetration welding
Features of laser deep penetration welding
1) High aspect ratio. As the molten metal forms around the cylindrical cavity of hot steam and extends toward the workpiece, the weld becomes deep and narrow.
2) Minimum heat input. Because the temperature in the small hole is very high, the melting process occurs extremely fast, the heat input into the workpiece is very low, and the thermal deformation and heat-affected zone are small.
3) High density. Because the small pores filled with high-temperature steam are conducive to the agitation of the weld pool and the escape of gas, resulting in a penetration weld without pores. The high cooling rate after welding can easily make the weld structure finer.
4) Strong welds. Because of the blazing heat source and sufficient absorption of non-metallic components, the impurity content is reduced, and 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, so that the strength and toughness of the weld are at least equal to or even higher than that of the parent metal.
5) Precise control. Because the focused light spot is small, the weld seam can be positioned with high precision. The laser output has no "inertia", it can be stopped and restarted at high speed, and the complex workpiece can be welded with the numerical control beam movement technology.
6) Non-contact atmospheric welding process. Because the energy comes from the photon beam, there is no physical contact with the workpiece, so no external force is applied to the workpiece. In addition, magnetism and air have no effect on laser light.
Advantages of laser deep penetration welding
1) Since the focused laser has a much higher power density than conventional methods, the welding speed is fast, the heat-affected zone and deformation are small, and difficult-to-weld materials such as titanium can also be welded.
2) Because the beam is easy to transmit and control, and there is no need to replace the torch and nozzle frequently, and there is no vacuum required for electron beam welding, which significantly reduces the auxiliary time of downtime, so the load factor and production efficiency are high.
3) Due to the purification effect and high cooling rate, the weld strength, toughness and comprehensive performance are high.
4) Due to the low average heat input and high processing precision, reprocessing costs can be reduced; in addition, the operating cost of laser welding is also low, which can reduce workpiece processing costs.
5) It can effectively control the beam intensity and fine positioning, and it is easy to realize automatic operation.
Disadvantages of laser deep penetration welding
1) Welding depth is limited.
2) The assembly requirements of the workpiece are high.
3) The one-time investment of the laser system is relatively high
