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 conduction type, that is, the surface of the workpiece is heated by laser radiation, and the surface heat diffuses to the interior through thermal 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 the precision welding of micro and small parts.
China’s laser welding is at the world’s advanced level, and it has the technology and ability to use laser to form complex titanium alloy components over 12 square meters, and has invested in the prototype and product manufacturing of many domestic aviation scientific research projects. In October 2013, Chinese welding experts won the Bruker Award, the highest academic award in the field of welding, and China’s laser welding level has been recognized by the world.
Technical principle
Laser welding can be realized by continuous or pulsed laser beam. 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 a thermal 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” under the action of heat, forming deep penetration welding. The welding speed is fast and the aspect ratio is large.
The principle of thermal conduction laser welding is: laser radiation heats the surface to be processed, the surface heat diffuses to the interior through thermal conduction, and the workpiece is melted by controlling laser parameters such as the width, energy, peak power and repetition frequency of the laser pulse to form a specific molten pool. .
Laser welding machines 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, and 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 sufficiently high power density laser irradiation, the material evaporates and small holes are formed. This small hole filled with steam is like a black body, which absorbs almost all the energy of the incident beam. The equilibrium temperature in the cavity is about 2500 ℃. The 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 the continuous evaporation of the wall material under the irradiation of the beam, the four walls of the small hole surround the molten metal, and the liquid metal surrounds the solid material (while 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 interior by transfer). The liquid flow outside the pore wall and the surface tension of the wall layer maintain a dynamic equilibrium with the steam pressure continuously generated in the pore cavity. 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, the hole and the molten metal surrounding the hole wall move forward with the advancing speed of the leading beam, the molten metal fills the gap left after the hole is removed and condenses, and the weld is formed. All of the above process happens so fast that the welding speed can easily reach several meters per minute.
Work equipment
It consists 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 in-phase light waves and reflects back and forth between the mirrors at both ends, forming a photoelectric tandem effect, amplifying the light waves, and obtaining enough energy to start emitting laser light.
Laser can also be interpreted as a device that converts primary energy such as electrical energy, chemical energy, thermal energy, light energy or nuclear energy into electromagnetic radiation beams of certain specific light frequencies (ultraviolet light, visible light or infrared light). Transformation of forms is easily carried out in certain solid, liquid or gaseous media. When these media are excited in atomic or molecular form, a laser beam with nearly the same phase and near single wavelength is generated. 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.
Laser classification
There are two main types of lasers used for welding, namely CO2 laser and Nd:YAG laser. Both CO2 laser and Nd: YAG laser are infrared light invisible to the naked eye. The light 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 absorption rate for this wavelength. For most metals, its reflectivity is 20% to 30%. The beam in the near-infrared band can be focused to a diameter of 0.25 mm as long as a standard optical mirror is used. The beam of CO2 laser is far-infrared light with a wavelength of 10. 6Lm. Most metals have a reflectivity of 80% to 90% for this light. Special optical mirrors are required to focus the beam into a diameter of 0. 75 – 0. 1mm . Nd: YAG laser power can generally reach about 4000~6000W, and now the maximum power has reached 10000W. The 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 irradiated by the light spot melts, a pinhole is formed. This vapor-filled pinhole is like a black body, almost absorbing the energy of the incident light. Equilibrium temperature reaches around 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 improving the maximum output power, but how to improve the beam quality and its focusing performance. In addition, in the high-power welding of CO2 lasers above 10 kW, if argon shielding gas is used, a strong plasma is often induced, which makes the penetration depth shallow. Therefore, helium gas, which does not generate plasma, is often used as a protective gas during high-power CO2 laser welding.
The application of diode-laser combinations for exciting high-power Nd:YAG crystals is an important development topic, which is bound to greatly improve the quality of laser beams and lead to more efficient laser processing. Using direct diode arrays 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 lifetime (10 000 h), which is beneficial to reduce the maintenance cost of the laser equipment. Development of a diode-pumped solid-state laser device (DPSSL).
Process parameters
(1) Power density. Power density is one of the most critical parameters in laser processing. With higher power densities, the surface layer can be heated to the boiling point in the microsecond time range, resulting in a large amount of vaporization. Therefore, high power density is beneficial for material removal processes such as punching, cutting, and engraving. For lower power density, it takes several milliseconds for the surface temperature to reach the boiling point. Before the surface vaporizes, the bottom layer reaches the melting point, which is easy to form a good fusion weld. Therefore, in conduction laser welding, the power density is in the range of 10^4~10^6W/CM^2.
(2) Laser pulse waveform. Laser pulse shape is an important issue in laser welding, especially for thin sheet welding. When the 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 varies with the surface temperature. During the action of a laser pulse, the reflectivity of metals varies greatly.
(3) Laser pulse width. Pulse width is one of the important parameters of pulse laser welding. It is not only an important parameter that is different from material removal and material melting, but also a key parameter that determines the cost and volume of processing equipment.
(4) The influence of defocus amount on welding quality. Laser welding usually requires a certain amount of defocus, because the power density in the center of the spot at the laser focus is too high, and it is easy to evaporate into a hole. The power density distribution is relatively uniform across the planes away from the laser focus. There are two defocusing methods: positive defocusing and negative defocusing. The focal plane above the workpiece is positive defocus, otherwise it is negative defocus. According to the theory of geometric optics, when the distance between the positive and negative defocus planes is equal to the welding plane, 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 larger penetration depth can be obtained, which is related to the formation process of the molten pool. Experiments show that when the laser is heated for 50~200us, the material begins to melt, forming a liquid phase metal and partially vaporizing, forming a high-pressure steam, and ejecting it at a very high speed, emitting a dazzling white light. At the same time, the high concentration of vapor moves the liquid metal 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 deeper into the material. Therefore, in practical applications, when the penetration depth is required to be large, negative defocusing is used; when welding thin materials, positive defocusing should be used.
(5) Welding speed. 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, causing the workpiece to be welded.
Welding characteristics
It belongs to fusion welding, which uses the laser beam as the energy source to impact on the joint of the weldment.
The laser beam can be guided by a flat optical element, such as a mirror, and then projected onto the weld seam by a reflective focusing element or mirror.
Laser Welding Equipment and Products
Laser welding equipment and products (17 photos)
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, and filler metal is occasionally used.
Laser welding can be combined with MIG welding to form laser MIG composite welding to achieve large penetration welding, and the heat input is greatly reduced compared to MIG welding.
Development process
The world’s first laser beam was generated in 1960 by using a flash bulb to excite ruby grains. Due to the limited thermal capacity of the crystal, only a very short-lived pulsed beam can be generated and the frequency is very low. Although the peak energy of the instantaneous pulse can be as high as 10^6 watts, it is still a low energy output.
Using neodymium (ND) as the excitation element yttrium aluminum garnet ingot (Nd:YAG) can generate a continuous single wavelength beam of 1-8KW. YAG laser, with a wavelength of 1.06uM, can be connected to the laser processing head through a flexible optical fiber, the equipment layout is flexible, and the welding thickness is 0.5-6mm.
Using CO2 laser (wavelength 10.6uM) with CO2 as the exciter, the output energy can reach 25KW, and it can make a single-pass full penetration welding with a thickness of 2mm. It has been widely used in metal processing in the industry.
In the mid-1980s, laser welding received extensive attention as a new technology in Europe, the United States, and Japan. In 1985, the German Thyssen Steel Company cooperated with the German Volkswagen AG to successfully use the world’s first tailor-made laser welded blank on the Audi100 body. In the 1990s, major automobile manufacturers in Europe, North America, and Japan began to use laser tailored blank technology on a large scale in body manufacturing. Both the laboratory and the practical experience of the automobile manufacturing plant have proved that tailor-welded blanks can be successfully used in the manufacture of automobile bodies.
Laser welding is to use laser energy to automatically assemble and weld several different materials, different thicknesses, and different coatings of steel, stainless steel, aluminum alloy, etc. to form a whole plate, profile, sandwich panel, etc. Components have different requirements for material properties, and use the lightest weight, optimal structure and optimal performance to achieve lightweight equipment. In developed countries such as Europe and the United States, laser tailor welding is not only used in the transportation equipment manufacturing industry, but also in the construction industry, bridges, home appliance plate welding production, rolling steel line steel plate welding (steel plate connection in continuous rolling) and other fields. use.
The world-famous laser welding companies include Swiss Soudonic Company, France Arcelor Steel Group, Germany ThyssenKrupp Group TWB Company, Canada Servo-Robot Company, Germany Precitec Company, etc.
The application of TWB technology in China has just started. On October 25, 2002, China’s first specialized commercial production line for TWB was officially put into operation. Introduced by Krupp Group TWB Company. Since then, Shanghai Baosteel Arcelor Tailored Laser Welding Co., Ltd. and FAW Baoyou Tailored Laser Welding Co., Ltd. have successively put into production.
In 2003, the A318 aluminum alloy lower wall structure double beam C02 laser wire filling welding and YAG laser wire filling welding were realized abroad. It replaced the traditional riveting structure and reduced the weight of the aircraft fuselage by 20%, and also saved 20% of the cost. . Gongshui believes that laser welding technology will be of great significance to the transformation and upgrading of my country’s traditional aviation manufacturing industry. Then he immediately applied for a number of related pre-research projects, organized a key research team, and took the lead in introducing the “dual-beam laser welding” technology into the project research in China, and planned to use this technology in aircraft manufacturing from the very beginning. A team of Chinese experts disclosed the preliminary technology to an aircraft design institute, and introduced the advantages and feasibility of dual-beam laser welding to them. The design institute has been verified and evaluated by many parties, and resolutely decided to use this technology in the manufacture of ribbed wall panels of an aircraft. Key technologies such as welding wire filling precision control, integrated and innovatively developed a dual beam laser wire filling composite welding device, established the first high-power dual beam laser wire filling welding platform in China, and realized the double beam double side of large thin-walled structure T-joint. Simultaneous welding has been successfully applied to the welding and manufacturing of key structural parts of aviation ribbed wall panels for the first time, and has played an important role in the development of new aircraft in my country.
In 2003, the first domestic complete set of large-scale strip on-line welding equipment provided by HGLASER passed off-line acceptance. The equipment integrates laser cutting, welding and heat treatment, making my country HGLASER the fourth company in the world capable of producing such equipment.
In 2004, HGLASER Farley’s “High-Power Laser Cutting, Welding and Cutting-Welding Combination Processing Technology and Equipment” project won the second prize of the National Science and Technology Progress Award, becoming the only domestic laser company with the ability to develop this technology and equipment.
With the rapid development of the industrial laser industry, the market has higher and higher requirements for laser processing technology. Laser technology has gradually shifted from single application to diversified application. Laser processing is no longer a single cutting or welding, and the market requires laser processing. The demand for the integration of cutting and welding is also increasing, and laser cutting and welding integrated laser processing equipment for laser cutting and laser welding has emerged as the times require.
HGLASER Farley researched and developed the Walc9030 integrated cutting and welding machine, 9×3 meters large format, which is currently the largest integrated laser cutting and welding equipment in the world. Walc9030 is a large-format cutting and welding equipment that integrates laser cutting and laser welding functions. The equipment has a professional cutting head and welding head. The two processing heads share a beam, and CNC technology is used to ensure that they will not interfere with each other. At the same time, two processes of cutting and welding are required. First cut and then welded, first welded and then cut, laser cutting and welding can be easily switched, one device, two functions, without adding new equipment, saving equipment costs for application manufacturers, improving processing efficiency and processing range, And because the cutting and welding are integrated, the machining accuracy is completely guaranteed, and the equipment performance is efficient and stable. In addition, it overcomes the difficulties that the plate is prone to thermal deformation and how to maintain the stability of the ultra-long flight optical path during the tailor-welding process of the super-large plate. It can weld two flat plates 6 meters long and 1.5 meters wide at one time. Flat, no other subsequent processing is required. At the same time, it can cut plates with a width of 3 meters and a length of more than 6 meters and less than 20mm, and it can be formed in one time without the need for secondary positions.
Shenyang Institute of Automation, Chinese Academy of Sciences cooperated with Japan Ishikawajima-Harima Heavy Industries Co., Ltd., following the national introduction and digestion and then innovating technology development strategy, conquered several key technologies of laser tailor welding, and developed the first domestic laser in September 2006. Tailored a complete set of production lines, and successfully developed a robot laser welding system to achieve laser welding of plane and space curves.
In October 2013, Chinese welding experts won the Bruker Award, the highest academic award in the field of welding. The Welding Institute (TWI) recommends nominations from more than 4,000 members from more than 120 countries each year, and finally awards the award to an expert in recognition of his outstanding contribution to the field of welding or joining science and technology and industrial applications . This award is not only a recognition of Gongshui Conservancy and its team, but also an affirmation of AVIC’s advancement of material connection technology.
Advantages and disadvantages
Advantage
(1) The required amount of incoming heat can be reduced to the minimum, the metallographic change range of the heat-affected zone is small, and the deformation caused by heat conduction is also the lowest;
(2) 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;
(3) There is no need to use electrodes, and there is no concern of electrode contamination or damage. And because it is not a contact welding process, the wear and deformation of the equipment 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 re-guided between tools or obstacles around the workpiece. Other welding rules are limited by the above space. and unable to perform;
(5) The workpiece can be placed in a closed space (evacuated or the internal gas environment is under control);
(6) The laser beam can be focused in a small area, and small and closely spaced parts can be welded;
(7) A wide range of weldable materials can be used, and various dissimilar materials can be joined to each other;
(8) It is easy to perform high-speed welding by automation, and it can also be controlled by digital or computer;
(9) When welding thin or thin wire, it will not be troubled by melting back like arc welding;
(10) It is not affected by the magnetic field (arc welding and electron beam welding are easy), and can accurately align the weldment;
(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 perforated 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.
Shortcoming
(1) The position of the weldment must be very precise and must be within the focusing range of the laser beam;
(2) When a fixture needs to be used for the weldment, it must be ensured that the final position of the weldment needs to be aligned with the welding point impacted by the laser beam;
(3) The maximum weldable thickness is limited. For workpieces with a penetration thickness far exceeding 19mm, laser welding is not suitable for the production line;
(4) For materials with high reflectivity and high thermal conductivity, such as aluminum, copper and their alloys, the weldability will be changed by the laser;
(5) When performing medium to high energy laser beam welding, a plasma controller should be used to drive out 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 pores and embrittlement;
(8) Equipment is expensive.
In order to eliminate or reduce the defects of laser welding and make better application of this excellent welding method, some composite welding processes using other heat sources and lasers are proposed, mainly including laser and arc, laser and plasma arc, laser and induction heat source. Welding, dual laser beam welding and multi-beam laser welding, etc. In addition, various auxiliary process measures have been proposed, such as laser wire filler 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 Wait.
Application field
Manufacturing
Tailored Bland Laser Welding technology has been widely used in foreign car manufacturing. According to statistics, in 2000, there were more than 100 laser tailor welding production lines for cutting blanks worldwide, with an annual output of 70 million tailor-welded blanks for car components, and continued grow at a higher rate. The domestically produced imported models Passat, Buick, Audi, etc. also adopted some cutting blank structures. In Japan, CO2 laser welding has replaced flash butt welding for the connection of rolled steel coils in the steel industry. In the research of ultra-thin plate welding, such as foils with a thickness of less than 100 microns, fusion welding cannot be performed, but through special output power waveforms The success of YAG laser welding shows the bright future of laser welding. Japan has also successfully developed, for the first time in the world, the use of YAG laser welding for the maintenance of thin tubes of steam generators in nuclear reactors. In China, Su Baorong and others have also carried out laser welding technology for gears.
Powder metallurgy
With the continuous development of science and technology, many industrial technologies have special requirements for materials, and materials manufactured by smelting methods can no longer meet the needs. Due to the special properties and manufacturing advantages of powder metallurgy materials, traditional smelting materials are being replaced in some fields such as automobiles, aircraft, and tool and cutting tools manufacturing. With the increasing development of powder metallurgy materials, it is connected with other parts. It is increasingly prominent, which limits the application of powder metallurgy materials. In the early 1980s, laser welding entered the field of powder metallurgy material processing with its unique advantages, opening up new prospects for the application of powder metallurgy materials, such as welding diamond by brazing, which is commonly used in powder metallurgy material connection. Low strength, wide heat-affected zone, especially incapable of adapting to high temperature and high strength requirements, causing the solder to melt and fall off. Laser welding can improve welding strength and high temperature resistance.
Auto industry
In the late 1980s, kilowatt-level lasers were successfully used in industrial production, and now 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. German Audi, Mercedes-Benz, Volkswagen, Sweden’s Volvo and other European car manufacturers took the lead in using laser welding of roof, body, side frame and other sheet metal welding as early as the 1980s. In the 1990s, GM, Ford and Chrysler in the United States The introduction of laser welding into automobile manufacturing, albeit a late start, has grown rapidly. Italian Fiat uses laser welding in the welding assembly of most steel plate components, and Japan’s Nissan, Honda and Toyota Motor Corporation all use laser welding and cutting processes in the manufacture of body panels, and high-strength steel laser welding assemblies are due to their excellent performance. It is used more and more in automobile body manufacturing. According to the statistics of the US metal market, by the end of 2002, the consumption of laser welded steel structure will reach 70,000t, an increase of 3 times compared with 1998. According to the characteristics of large batches and high degree of automation in the automobile industry, laser welding equipment is developing in the direction of high power and multi-channel. In terms of technology, the Sandia National Laboratory of the United States and PrattWitney have jointly conducted research on adding powder metal and metal wire in the laser welding process. The Institute of Applied Beam Technology in Bremen, Germany has carried out a lot of research on the use of laser welding of aluminum alloy body frames. It is believed that adding filler metal to the weld will help eliminate hot cracks, increase welding speed, and solve tolerance problems, and the developed production line has been put into production at Mercedes-Benz’s factory.
Electronics industry
Laser welding has been widely used in the electronics industry, especially in the microelectronics industry. Due to the small heat-affected zone, rapid heating concentration and low thermal stress of laser welding, it shows unique advantages in the packaging of integrated circuits and semiconductor device housings. In the development of vacuum devices, laser welding has also been applied, such as Molybdenum focusing electrode and stainless steel support ring, fast hot cathode filament assembly, etc. The thickness of the elastic thin-walled corrugated sheet in the sensor or thermostat is 0.05-0.1mm, which is difficult to solve by traditional welding methods. TIG welding is easy to weld through, the plasma stability is poor, and there are many influencing factors. Laser welding has a good effect and has been widely used. Applications.
In recent years, laser welding has been gradually applied to the assembly process of printed circuit boards. As circuits become more and more integrated, parts are smaller in size, and pin pitches become smaller, it is difficult for previous tools to operate in small spaces. Because the laser can achieve welding without touching the parts, it solves this problem very well, and is valued by circuit board manufacturers.
Biomedical Science
Laser welding of biological tissues began in the 1970s. Klink et al. and jain [13] successfully welded fallopian tubes and blood vessels with lasers and showed their superiority, which made more researchers try to weld various biological tissues, and extended them to Welding of other tissues. The research on laser welding nerves at home and abroad mainly focuses on the laser wavelength, dose and its effect on functional recovery and the selection of laser solder. Welding studies were carried out in the rat common bile duct. Compared with the traditional suture method, the laser welding method 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 future biomedicine. be more widely used.
Other
In other industries, laser welding has also gradually increased, especially in the welding of special materials. Many researches have been carried out in China, such as laser welding of BT20 titanium alloy, HEl30 alloy, Li-ion battery, etc., German glass machinery manufacturer GlamacoCoswig company and IFW joint A new technology for laser welding of flat glass has been developed in cooperation with the Institute of Technology and Materials Experimentation.
