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Laser welding is an efficient and precise welding method that uses a high-energy-density laser beam as a heat source. Laser welding is one of the important aspects of the application of laser material processing technology. In the 1970s, it was mainly used for welding thin-walled materials and low-speed welding. The welding process is of thermal conductivity type, that is, the surface of the workpiece is heated by laser radiation, and the surface heat is diffused to the inside through thermal conduction. By controlling the width, energy, peak power, and repetition frequency of the laser pulse With other parameters, the workpiece is melted to form a specific molten pool. Because of its unique advantages, it has been successfully applied to the precision welding of micro and small parts.
China’s laser welding is at the world’s advanced level. It has the technology and ability to use a laser to form complex titanium alloy components of more than 12 square meters and has invested in the prototype and product manufacturing of multiple domestic aviation scientific research projects. In October 2013, Chinese welding experts won the Brooke Prize, the highest academic award in the field of welding, and China’s laser welding level has been recognized by the world.
Laser welding can be realized by a continuous or pulsed laser beam. The principle of laser welding can be divided into heat conduction welding and laser deep penetration welding. The power density is less than 104~105 W/cm2 for 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 recessed into “holes” under the action of heat to form deep penetration welding. Features 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 into the interior through thermal 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 sheet welding mainly involves laser deep penetration welding.
Laser deep penetration welding generally uses continuous laser beams 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, absorbing almost all of the incident beam energy.
The equilibrium temperature in the cavity reaches about 2500°C. The heat is transferred 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 four walls of the small hole are surrounded by molten metal, and the liquid metal is surrounded by solid materials (and in most conventional welding processes and laser conduction welding, the energy is first deposited on the surface of the workpiece, and then transported to the inside by transfer).
The liquid flow outside the pore wall and the surface tension of the wall layer are maintained in a dynamic balance with the continuously generated steam pressure in the cavity. The light 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. In other words, the small hole and the molten metal surrounding the hole wall move forward with the forward speed of the leading light beam, and the molten metal fills the gap left after the small hole is moved away and condenses, and the weld is formed. All of the above processes happen so fast that the welding speed can easily reach a few meters per minute.
It is composed of an optical oscillator and a medium placed between the mirrors at both ends of the cavity of the oscillator. When the medium is excited to a high-energy state, it begins to generate light waves in the same phase and reflect back and forth between the mirrors at both ends, forming a photoelectric string junction effect, amplifying the light waves, and obtaining sufficient energy to start emitting laser light.
The laser can also be interpreted as a device that converts raw energy such as electric energy, chemical energy, thermal energy, light energy, or nuclear energy into a beam of electromagnetic radiation of certain specific light frequencies (ultraviolet light, visible light, or infrared light). The conversion form is easy to carry out in some solid, liquid, or gaseous media. When these media are excited in the form of atoms or molecules, they produce light beams-lasers with almost the same phase and nearly a single wavelength. Due to the same phase and single wavelength, the difference angle is very small, and the distance that can be transmitted is quite long before being highly concentrated to provide functions such as welding, cutting, and heat treatment.
Sensor sealing welding methods are resistance welding, argon arc welding, electron beam welding, plasma welding, and so on.
It is used to weld thin metal parts. The workpiece is clamped between two electrodes and the surface of the electrode contacted by a large current is melted by a large current, that is, welding is performed by the resistance heating of the workpiece. The workpiece is easy to deform. Resistance welding is welded on both sides of the joint, while laser welding is only performed on one side. The electrodes used in resistance welding need to be maintained frequently to remove oxides and metal adhering from the workpiece. Laser welding of thin metal lap joints is not In contact with the workpiece, the beam can also enter the area that is difficult to be welded by conventional welding, and the welding speed is fast.
Argon arc welding: using non-consumable electrodes and shielding gas, it is often used to weld thin workpieces, but the welding speed is slower, and the heat input is much larger than laser welding, which is prone to deformation.
Argon Arc Welding Machine
Plasma arc welding: similar to argon arc, but its welding torch will produce a compressed arc to increase the arc temperature and energy density. It is faster than argon arc welding and has a greater penetration depth, but is inferior to laser welding.
Electron beam welding: It relies on a beam of accelerated high-energy-density electrons to hit the workpiece, and generate huge heat in a small dense area on the surface of the workpiece, forming a “small hole” effect, thereby implementing deep penetration welding. The main disadvantage of electron beam welding is that it requires a high vacuum environment to prevent electron scattering. The equipment is complicated. The size and shape of the weldment are restricted by the vacuum chamber. The assembly quality of the weldment is strictly required.
Non-vacuum electron beam welding can also be implemented. Scattering and poor focusing will affect the effect. Electron beam welding also has magnetic offset and X-ray problems. Because the electrons are charged, they will be affected by the deflection of the magnetic field. Therefore, electron beam welding workpieces are required to be demagnetized before welding. X-rays are particularly strong under high pressure, and operators need to be protected. Laser welding does not require a vacuum chamber and demagnetization treatment before welding the workpiece. It can be carried out in the atmosphere and there is no problem with X-ray protection, so it can be operated online in the production line and can also weld magnetic materials.
Laser Welding Classification
There are two main types of lasers used for welding, namely CO2 lasers and Nd: YAG lasers. Both CO2 laser and Nd: YAG laser is infrared light invisible to the naked eye. The beam produced by the Nd: YAG laser is mainly near-infrared light with a wavelength of 1.06 Lm. The heat conductor has a high light absorption rate of this wavelength. For most metals, its reflectivity is 20% ~ 30%. As long as a standard light mirror is used, the near-infrared beam can be focused to a diameter of 0.25 mm. The beam of the CO2 laser is far-infrared light with a wavelength of 10.6Lm.
The reflectivity of most metals to this light reaches 80% ~ 90%, and a special light mirror is required to focus the beam into a diameter of 0.75-0.1mm. The power of Nd: YAG laser can generally reach about 4000~6000W, and the maximum power has now reached 10 000W. The CO2 laser power can easily reach 20000W or more.
The high-power CO2 laser solves the problem of high reflectivity through the pinhole effect. When the surface of the material irradiated by the light spot melts, a pinhole is formed. This vapor-filled pinhole is like a black body, which almost completely absorbs the energy of the incident light. The equilibrium temperature is about 25 000 e, and the reflectivity drops rapidly within a few microseconds.
Although the development focus of CO2 lasers is still focused on the development of equipment, it is not about increasing the maximum output power, but how to improve the beam quality and focusing performance. In addition, when using high power welding with CO2 laser above 10 kW, if argon shielding gas is used, a strong plasma is often induced and the penetration depth becomes shallower. Therefore, when CO2 laser high-power welding, helium gas that does not generate plasma is often used as a shielding gas.
The application of diode laser combination for exciting high-power Nd: YAG crystals is an important development topic, which will greatly improve the quality of laser beams and form more effective laser processing. Using a direct diode array to excite the laser whose output wavelength is in the near-infrared region, its average power has reached 1 kW, and the photoelectric conversion efficiency is close to 50%. The diode also has a longer service life (10 000 h), which helps reduce the maintenance cost of laser equipment. Development of diode-pumped solid-state laser equipment.
Power density is one of the most critical parameters in laser processing. With a higher power density, the surface layer can be heated to the boiling point within a microsecond time range, resulting in a large amount of vaporization. Therefore, high power density is beneficial for material removal processing, such as punching, cutting, and engraving. For lower power densities, it takes several milliseconds for the surface temperature to reach the boiling point. Before the surface layer vaporizes, the bottom layer reaches the melting point, which makes it easy to form a good fusion weld.
Laser Pulse Waveform
The laser pulse waveform is an important issue in laser welding, especially for sheet welding. When a high-intensity laser beam hits the surface of the material, 60~98% of the laser energy will be reflected and lost on the metal surface, and the reflectivity changes with the surface temperature. During a laser pulse, the reflectivity of the metal changes greatly.
Laser Pulse Width
Pulse width is one of the important parameters of pulsed laser welding. It is not only an important parameter different from material removal and material melting but also a key parameter that determines the cost and volume of processing equipment.
The Effect of Defocusing Amount on Welding Machine Quality
Laser welding usually requires a certain amount of defocus, because the power density in the center of the spot at the laser focal point is too high and it is easy to evaporate into a hole. On each plane away from the laser focus, the power density distribution is relatively uniform. There are two defocusing methods: positive defocus and negative defocus. If the focal plane is above the workpiece, it is a positive defocus, otherwise, it is a negative defocus. According to geometrical optics theory, when the distance between the positive and negative defocus plane and the welding plane is equal, the power density on the corresponding plane is approximately the same, but the shape of the molten pool obtained is actually different. When the defocus is negative, a greater penetration depth can be obtained, which is related to the formation process of the molten pool.
Experiments have shown that the laser heating 50~200us of the material begins to melt, forming liquid metal and partially vaporizing, forming high-pressure steam, and spraying at a very high speed, emitting dazzling white light. At the same time, the high concentration of vapor causes the liquid metal to move to the edge of the molten pool, forming a depression in the center of the molten pool. When the defocus is negative, the internal power density of the material is higher than that of the surface, and it is easy to form stronger melting and vaporization so that the light energy can be transmitted to the deeper part of the material. Therefore, in practical applications, when the penetration depth is required to be large, the negative defocus is used; when the thin material is welded, the positive defocus should be used.
The speed of the welding speed will affect the heat input per unit time. If the welding speed is too slow, the heat input will be too large, causing the workpiece to burn through. If the welding speed is too fast, the heat input will be too small and the workpiece will not be welded through.
Laser Welding Characteristics
It belongs to fusion welding, which uses a laser beam as an energy source to impinge on the weldment joint.
The laser beam can be guided by a flat optical element (such as a mirror), and then a reflective focusing element or lens is used to project the beam on the weld.
Laser welding is non-contact welding. No pressure is required during the operation, but inert gas is required to prevent oxidation of the molten pool. Filler metal is occasionally used.
Laser welding can be combined with MIG welding to form laser MIG hybrid welding to achieve large penetration welding, and at the same time, the heat input is greatly reduced compared with MIG welding.
Advantage And Shortcoming
- The amount of heat input can be reduced to the minimum required, the metallographic change range of the heat-affected zone is small, and the deformation caused by heat conduction is also the lowest;
- The welding process parameters of 32mm plate thickness single pass welding have been verified and qualified, which can reduce the time required for thick plate welding and even save the use of filler metal;
- There is no need to use electrodes, and there is no concern about electrode contamination or damage. And because it is not a contact welding process, the wear and deformation of the equipment can be minimized;
- The laser beam is easy to focus, align and be guided by optical instruments. It can be placed at an appropriate distance from the workpiece, and can be re-guided between tools or obstacles around the workpiece. Other welding rules are subject to the above-mentioned space limitations. Unable to play
- The workpiece can be placed in a closed space (after vacuuming or the internal gas environment is under control);
- The laser beam can be focused on a small area and can weld small and closely spaced parts;
- A wide range of materials that can be welded, and various heterogeneous materials can also be joined to each other;
- It is easy to automate high-speed welding, and it can also be controlled by digital or computer;
- When welding thin materials or thin-diameter wires, it will not be as easy to be troublesome as arc welding;
- It is not affected by the magnetic field (arc welding and electron beam welding are easy), and can accurately align the weldment;
- Two metals with different physical properties (such as different resistances) can be welded;
- No vacuum or X-ray protection is required;
- If the through-hole welding is adopted, the depth-to-width ratio of the weld bead can reach 10:1;
- The device can be switched to transmit the laser beam to multiple workstations.
- The position of the weldment must be very precise and must be within the focus range of the laser beam;
- When the weldment needs to use a jig, it must be ensured that the final position of the weldment is aligned with the welding spot impacted by the laser beam;
- The maximum weldable thickness is restricted and the penetration thickness of the workpiece is far more than 19mm, and laser welding is not suitable for the production line;
- For materials with high reflectivity and high thermal conductivity, such as aluminum, copper and their alloys, the weldability will be changed by laser;
- When performing medium to high energy laser beam welding, a plasma controller needs to be used to drive out the ionized gas around the molten pool to ensure the reappearance of the weld bead;
- The energy conversion efficiency is too low, usually less than 10%;
- The weld bead solidifies rapidly, and there may be concerns about porosity and embrittlement;
- The equipment is expensive.
To eliminate or reduce the defects of laser welding and make better use of this excellent welding method, some other heat source and laser hybrid welding processes have been proposed, mainly laser and arc, laser and plasma arc, laser and induction heat source composite Welding, dual laser beam welding and multi-beam laser welding, etc. In addition, various auxiliary process measures have been proposed, such as laser filler wire welding (which can be subdivided into cold wire welding and hot wire welding), external magnetic field-assisted enhanced laser welding, shielding gas controlled molten pool depth laser welding, and laser assisted friction stir welding.