Introduction to laser welding process and characteristics
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1. Laser welding concept
Laser welding is an efficient and precise welding method that uses high-energy-density laser beams as heat sources. Laser beam 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 the heat conduction type, that is, the laser radiation heats the surface of the workpiece, and the surface heat diffuses to the interior through heat conduction. By controlling the width, energy, peak power and repetition frequency of the laser pulse and other parameters to melt the workpiece and form a specific molten pool. Due to its unique advantages, it has been successfully used in precision welding of micro and small parts.
2. Laser welding principle
Laser welding can be achieved using continuous or pulsed laser beams. The principles of laser beam 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 concave into “holes” due to heat, forming deep penetration welding, which has It has the characteristics of fast welding speed and large aspect ratio.
The principle of thermal conduction laser beam welding is: laser radiation heats the surface to be processed, and the surface heat diffuses to the interior through thermal conduction. By controlling laser parameters such as laser pulse width, energy, peak power, and repetition frequency, the workpiece is melted to form a specific molten pool. .
Laser beam welding machines used for gear welding and metallurgical sheet welding mainly involve laser deep penetration welding.
Laser deep penetration welding generally uses a continuous laser beam to complete the connection of materials. Its metallurgical physical process is very similar to that of electron beam welding, that is, the energy conversion mechanism is completed through a “key-hole” structure. Under laser irradiation with a high enough power density, the material evaporates and small holes are formed. This small hole filled with vapor is like a black body, absorbing almost all the energy of the incident beam. The equilibrium temperature in the hole reaches about 2500°C. The heat is transferred from the outer wall of the high-temperature hole, causing the metal surrounding the hole to melt. 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 (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 transfer). The liquid flow outside the hole wall and the surface tension of the wall layer are in phase with the continuously generated steam pressure in the hole cavity and maintain a dynamic balance. 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 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 pilot beam. The molten metal fills the gap left after the small hole is removed and condenses accordingly, and the weld is formed. All of this happens so quickly that welding speeds can easily reach several meters per minute.
3. Laser welding equipment
It consists of an optical oscillator and a medium placed between the mirrors at both ends of the oscillator cavity. When the medium is excited to a high-energy state, it begins to generate in-phase light waves and reflects back and forth between the mirrors at both ends, forming a photoelectric series effect, amplifying the light waves, and obtaining enough energy to start emitting laser light.
Laser can also be explained as a device that converts raw energy such as electrical energy, chemical energy, thermal energy, light energy or nuclear energy into electromagnetic radiation beams of certain specific light frequencies (ultraviolet, visible or infrared light). Conversions occur easily in certain solid, liquid, or gaseous media. When these media are excited in the form of atoms or molecules, a beam-laser with almost the same phase and almost a single wavelength is produced. Because they have the same phase and single wavelength, the difference angles are very small, and the distance they can transmit is quite long before being highly concentrated to provide functions such as welding, cutting and heat treatment.
4. Laser classification
There are two main types of lasers used for welding, namely CO2 laser and Nd:YAG laser. CO2 laser and Nd: YAG laser are both invisible infrared light to the naked eye. The beam generated by the Nd: YAG laser is mainly near-infrared light with a wavelength of 1. 06 Lm. The thermal conductor has a high light absorption rate of this wavelength. For most metals, its reflectivity is 20% ~ 30%. The beam in the near-infrared band can be focused to a diameter of 0. 25 mm using a standard optical mirror. The beam of CO2 laser is far-infrared light with a wavelength of 10. 6Lm. The reflectivity of most metals for this light reaches 80% ~ 90%. A special light mirror is required to focus the beam into a diameter of 0. 75 – 0. 1mm. . Nd: YAG laser power can generally reach about 4 000~ 6 000W, and now the maximum power has reached 10 000W. However, CO2 laser power can easily reach 20,000W or more.
The high-power CO2 laser solves the problem of high reflectivity through the pinhole effect. When the surface of the material illuminated by the light spot melts, a pinhole is formed. This vapor-filled pinhole is like a black body, absorbing almost all the energy of the incident light. The equilibrium temperature reaches 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 no longer about increasing the maximum output power, but about how to improve the beam quality and its focusing performance. In addition, when CO2 laser beam welding with high power of 10 kW or above is used, if argon shielding gas is used, strong plasma is often induced, making the penetration shallower. Therefore, during high-power CO2 laser beam welding, helium, which does not generate plasma, is often used as a protective gas.
The application of diode laser combinations for exciting high-power Nd:YAG crystals is an important development topic that will greatly improve the quality of the laser beam and result in more efficient laser processing. Direct diode arrays are used to excite lasers with output wavelengths in the near-infrared region. The 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 (DPSSL).
5. Laser welding process parameters
(1) Power density
Power density is one of the most critical parameters in laser processing. Using 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 processes such as drilling, 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, making it easy to form a good molten weld. Therefore, in conductive laser beam welding, the power density ranges from 10^4 to 10^6W/CM^2.
(2) Laser pulse waveform
Laser pulse waveform is an important issue in laser beam welding, especially for thin sheet welding. When a high-intensity laser beam hits the surface of a material, 60 to 98% of the laser energy will be reflected and lost on the metal surface, and the reflectivity changes with the surface temperature. During the action of a laser pulse, the reflectivity of metal changes greatly.
(3) Laser pulse width
Pulse width is one of the important parameters of pulse laser beam 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.
(4) The effect of defocus on welding quality
Laser welding usually requires a certain amount of defocus because the power density at the center of the spot at the laser focus is too high and can easily evaporate into holes. On each plane away from the laser focus, the power density distribution is relatively uniform. There are two defocus modes: positive defocus and negative defocus. When the focal plane is above the workpiece, it is positive defocus, and when it is above the workpiece, it is negative defocus. According to the theory of geometric optics, when the distance between the positive and negative defocus planes and the welding plane is equal, the power density on the corresponding planes is approximately the same, but in fact the shape of the molten pool obtained is 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 show that after laser heating for 50 to 200us, the material begins to melt, forming liquid metal and partially vaporizing, forming high-pressure steam, which is ejected at extremely high speeds and emits dazzling white light. At the same time, the high concentration of gas causes the liquid metal to move to the edge of the molten pool, forming a depression in the center of the molten pool. When negative defocusing occurs, the internal power density of the material is higher than the surface, which easily causes stronger melting and vaporization, allowing light energy to be transmitted deeper into the material. Therefore, in practical applications, when a larger penetration depth is required, negative defocus is used; when welding thin materials, positive defocus is appropriate.
(5)Welding speed
The speed of the welding speed will affect the amount of 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, resulting in incomplete welding of the workpiece.
6. Advantages of laser welding
(1) The amount of heat input can be reduced to the minimum, the range of metallographic changes in the heat-affected zone is small, and the deformation caused by heat conduction is also minimum;
(2) The welding process parameters for single-pass welding of 32mm plate thickness have been certified, which can reduce the time required for thick plate welding and even eliminate the use of filler metal;
(3) 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 machine tools can be minimized;
(4) 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 redirected between machines, tools or obstacles around the workpiece. Other welding laws are subject to the above-mentioned space restrictions. and unable to perform;
(5) The workpiece can be placed in a closed space (evacuated or the internal gas environment is controlled);
(6) The laser beam can be focused on a small area and can weld small and closely spaced components;
(7) There is a wide range of weldable materials, and various heterogeneous materials can also be joined to each other;
(8) It is easy to perform high-speed welding through automation and can also be controlled digitally or by computer;
(9) When welding thin materials or fine-diameter wires, there is no meltback problem like arc welding;
(10) Not affected by magnetic fields (arc welding and electron beam welding are easy), and can accurately align welding parts;
(11) Two metals with different physical properties (such as different resistances) can be welded;
(12) No vacuum or X-ray protection is required;
(13) If through-hole welding is used, the depth-to-width ratio of the weld bead can reach 10:1;
(14) The device can be switched to deliver the laser beam to multiple workstations.
7. Laser welding defects
(1) The position of the weldment must be very precise and must be within the focus range of the laser beam;
(2) When a fixture is required for the weldment, it must be ensured that the final position of the weldment is aligned with the welding point where the laser beam will impact;
(3) The maximum weldable thickness is limited and workpieces with a penetration thickness far exceeding 19mm are not suitable for laser beam welding on the production line;
(4) The weldability of highly reflective and high thermal conductive materials such as aluminum, copper and their alloys will be changed by laser;
(5) When performing medium to high energy laser beam welding, a plasma controller needs to be used to remove the ionized gas around the molten pool to ensure the reappearance of the weld bead;
(6) The energy conversion efficiency is too low, usually less than 10%;
(7) The weld bead solidifies rapidly, and there may be concerns about porosity and embrittlement;
(8) The equipment is expensive.
8. Laser welding application fields
(1) Manufacturing industry
Tailored Bland Laser Welding technology has been widely used in foreign car manufacturing. According to statistics, there were more than 100 tailor-welded blank plate laser beam welding production lines worldwide in 2000, with an annual output of 70 million pieces of tailor-welded blank plates for car components, and continues to do so. grow at a higher rate. The domestically produced imported models Passat, Buick, Audi, etc. also use some cut blank structures. Japan uses CO2 laser beam welding instead of flash butt welding to connect rolled steel coils in the steel industry. Research on ultra-thin plate welding, such as foils with a plate thickness of less than 100 microns, cannot be welded, but through a special output power waveform The success of YAG laser beam welding shows the broad future of laser beam welding. Japan has also successfully developed YAG laser beam welding for the first time in the world to repair thin tubes of steam generators in nuclear reactors. In China, Su Baorong and others have also developed laser beam welding technology for gears.
(2) Powder metallurgy
With the continuous development of science and technology, many industrial technologies have special requirements for materials, and materials manufactured by smelting and casting methods can no longer meet the needs. Due to the special properties and manufacturing advantages of powder metallurgy materials, they are replacing traditional smelting materials in certain fields such as automobiles, aircrafts, and tool and cutting tool manufacturing industries. With the increasing development of powder metallurgy materials, there are problems with their connection with other parts. It becomes increasingly prominent, which limits the application of powder metallurgy materials. In the early 1980s, laser beam welding entered the field of powder metallurgy material processing with its unique advantages, opening up new prospects for the application of powder metallurgy materials. For example, the brazing method commonly used in powder metallurgy material connection is used to weld diamond. The strength is low, the heat-affected zone is wide, and it cannot adapt to high temperatures and high strength requirements, causing the solder to melt and fall off. Laser welding can improve the welding strength and high temperature resistance.
(3) Automobile industry
In the late 1980s, kilowatt-class lasers were successfully used in industrial production. Nowadays, laser welding production lines have appeared on a large scale in the automobile manufacturing industry, becoming one of the outstanding achievements of the automobile manufacturing industry. European automobile manufacturers such as Germany’s Audi, Mercedes-Benz, Volkswagen, and Sweden’s Volvo took the lead in using laser beam welding of roofs, bodies, side frames and other sheet metal welding as early as the 1980s. In the 1990s, American General Motors, Ford and Chrysler followed suit. The introduction of laser beam welding into automobile manufacturing, although it started late, is developing rapidly. Italy’s Fiat uses laser beam welding in the welding and assembly of most steel plate components. Japan’s Nissan, Honda and Toyota Motor Corporation all use laser beam welding and cutting processes in the manufacture of body panels. High-strength steel laser beam welding assemblies have excellent performance It is increasingly used in automobile body manufacturing. According to U.S. metal market statistics, by the end of 2002, the consumption of laser welded steel structures will reach 70,000t, three times more than in 1998.According to the characteristics of large batches and high degree of automation in the automobile industry, laser beam welding equipment is developing in the direction of high power and multi-channel. In terms of technology, Sandia National Laboratory in the United States and Pratt Witney jointly conducted research on adding powder metal and wire during the laser beam welding process. The Institute of Applied Beam Technology in Bremen, Germany, has conducted a lot of research on the use of laser beam welding of aluminum alloy body frames. It is believed that adding filler metal to the weld can help eliminate hot cracks, increase welding speed, and solve tolerance problems. The developed production line has been put into production at the Mercedes-Benz factory.
(4) Electronic industry
Laser welding is widely used in the electronics industry, especially in the microelectronics industry. Due to its small heat-affected zone, rapid heating concentration, and low thermal stress, laser welding shows unique advantages in the packaging of integrated circuits and semiconductor device casings. In the development of vacuum devices, laser beam welding has also been applied, such as Molybdenum focusing electrode and stainless steel support ring, fast heating cathode filament assembly, etc. The thickness of elastic thin-walled corrugated sheets in sensors or thermostats is 0.05-0.1mm, which is difficult to solve using traditional welding methods. TIG welding is easy to weld through, plasma stability is poor, and there are many influencing factors. However, laser welding has a good effect and is widely used. Applications.
In recent years, laser welding has gradually been applied to the assembly process of printed circuit boards. As circuits become more and more integrated, parts become smaller and smaller, and pin spacing becomes smaller, making it difficult for previous tools to operate in small spaces. Since the laser does not need to touch the parts to achieve welding, it solves this problem very well and has attracted the attention of circuit board manufacturers.
(5) Biomedicine
Laser welding of biological tissues began in the 1970s. The successful welding of fallopian tubes and blood vessels by Klink et al. and Jain and the demonstrated superiority prompted more researchers to try to weld various biological tissues and extended it to Welding of other organizations. Domestic and foreign research on laser beam welding of nerves mainly focuses on laser wavelength, dose and its effect on functional recovery and the selection of laser solder. Liu Tongjun conducted basic research on laser welding of small blood vessels and skin and other aspects. Welding studies were conducted on the rat common bile duct. Compared with traditional suturing methods, laser beam welding has the advantages of fast anastomosis, no foreign body reaction during the healing process, maintaining the mechanical properties of the welded part, and the repaired tissue growing according to its original biomechanical properties. It will be used in biomedicine in the future. be more widely used.
(6) Others
In other industries, laser welding is also gradually increasing, especially in the welding of special materials. Many studies have been conducted in China, such as laser beam welding of BT20 titanium alloy, HEl30 alloy, Li-ion batteries, etc. The German glass machinery manufacturer GlamacoCoswig Company has joined forces with IFW The Institute of Technology and Materials Experiments has collaborated to develop a new laser welding technology for flat glass.
