Laser welding

Laser welding of biological tissue began in the 1970s, Klink et al. and jain [13] used laser welding of the successful welding and display of fallopian tubes and blood vessels, enabling more researchers to try to weld various biological tissues and promote them to Welding of other organizations.
The laser device consists of an optical oscillator and a medium placed between the mirrors at the 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 two ends of the mirror to form a photoelectric series effect, amplifying the light waves, and obtaining sufficient energy to start emitting laser light. A laser can also be interpreted as a device that converts raw energy such as electrical energy, chemical energy, thermal energy, light energy, or nuclear energy into a specific optical frequency (ultraviolet, visible, or infrared electromagnetic radiation beam). In liquid or gaseous media, it is easy to carry out. When these media are excited in atomic or molecular form, they produce a beam with almost the same phase and nearly single wavelength—the laser has different angles due to the same phase and single wavelength. Very small, the distance that can be transmitted before being highly concentrated to provide functions such as welding, cutting and heat treatment is quite long.
The development process The world's first laser beam was generated in 1960 by using a flash bulb to excite the ruby ​​grains. Due to the heat capacity of the crystal, only a very short pulse beam can be generated with a low frequency. Although the instantaneous pulse peak energy can be as high as 10^6 watts, it is still a low energy output.

A bismuth aluminum garnet rod (Nd:YAG) using é’• (ND) as an excitation element produces a continuous single-wavelength beam of 1--8 KW. YAG laser, with a wavelength of 1.06uM, can be connected to the laser processing head through a flexible fiber. The device layout is flexible and suitable for welding thickness of 0.5-6mm. CO2 laser with CO2 as the excimer (wavelength 10.6uM), output energy up to 25KW, can make 2mm plate thickness single-channel full-infiltration welding, the industry has been widely used in metal processing.
Welding characteristics belong to the fusion welding, with the laser beam as the energy source, impact on the weldment joint.

The laser beam can be directed by a planar optical element, such as a mirror, and then the beam is projected onto the weld by a reflective focusing element or lens.

Laser welding is a non-contact welding. The process does not require pressurization, but an inert gas is required to prevent oxidation of the molten pool, and the filler metal is occasionally used.

Laser welding can be combined with MIG welding to form a laser MIG hybrid welding to achieve deep penetration welding, while the heat input is much smaller than MIG welding.
Main advantages of laser welding (1) The amount of heat input can be minimized. The metallographic range of the heat affected zone is small, and the deformation due to heat conduction is also the lowest.

(2) The welding process parameters of 32mm plate thickness single pass welding have been qualified, 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, no concerns about electrode contamination or damage. And because it is not a contact welding process, the wear and deformation of the machine can be minimized.

(4) The laser beam is easy to focus, align and guided by optical instruments, and can be placed at an appropriate distance from the workpiece and can be redirected between the implements or obstacles around the workpiece. Other welding methods are subject to the above space limitations. Can't play.

(5) The workpiece can be placed in a closed space (with vacuum or internal gas environment under control).

(6) The laser beam can be focused in a small area to weld small and closely spaced parts.

(7) The range of weldable materials is large, and various heterogeneous materials can be joined to each other.

(8) It is easy to automate high-speed welding, and it can also be controlled by digital or computer.

(9) When welding thin materials or thin-diameter wires, it is not as easy to melt back as arc welding.

(10) It is not affected by the magnetic field (easy for arc welding and electron beam welding), and can accurately align the weldment.

(11) Two metals that can weld different physical properties (such as different resistances)

(12) No vacuum is required and X-ray protection is not required.

(13) If the hole is welded, the width of the weld bead can be up to 10:1.

(14) The switching device can transmit the laser beam to a plurality of workstations.
Main disadvantages of laser welding (1) The position of the weldment needs to be very precise, and it must be within the focus range of the laser beam.

(2) When the fixture is to be used with a fixture, it must be ensured that the final position of the weldment is aligned with the weld point that the laser beam will impact.

(3) The maximum weldable thickness is limited to workpieces with a penetration thickness of more than 19 mm, and laser welding is not suitable for use on the production line.

(4) Highly reflective and highly thermally conductive materials such as aluminum, copper and alloys thereof, the weldability is changed by the laser.

(5) When performing medium-to-high-energy laser beam welding, a plasma controller is used to drive out the ionized gas around the molten pool to ensure re-emergence of the weld bead.

(6) The energy conversion efficiency is too low, usually less than 10%.

(7) The weld bead is rapidly solidified and may have pores and embrittlement concerns.

(8) The equipment is expensive.
Laser welding process enhancement technology In order to eliminate or reduce the defects of laser welding, better use of this excellent welding method, some processes of composite welding with other heat sources and lasers are proposed, mainly laser and arc, laser and plasma arc, Laser and induction heat source hybrid welding, double laser beam welding and multi-beam laser welding. In addition, various auxiliary technological measures such as laser filler welding (which can be subdivided into cold wire welding and hot wire welding), external magnetic field assisted laser welding, protective gas controlled molten pool deep laser welding, and laser assisted friction stir welding are also proposed. Wait.
Process parameters for laser welding (1) Power density. 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 in the microsecond time range, resulting in a large amount of vaporization. Therefore, high power density is advantageous for material removal processing such as punching, cutting, and engraving. For lower power density, the surface temperature reaches the boiling point and it takes several milliseconds. Before the surface layer 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 waveforms are an important issue in laser welding, especially for sheet welding. When a high-intensity laser beam is incident on the surface of the material, the metal surface will be reflected by 60 to 98% of the laser energy, and the reflectance changes with the surface temperature. The reflectance of the metal changes greatly during the action of one laser pulse.

(3) Laser pulse width. Pulse width is one of the important parameters of pulsed laser welding. It is an important parameter that is different from material removal and material melting. It is also a key parameter that determines the cost and volume of processing equipment.

(4) The effect of the amount of defocus on the quality of the weld. Laser welding usually requires a certain amount of disengagement because the power density at the center of the spot at the laser focus is too high and it is easy to evaporate into holes. The power density distribution is relatively uniform across the planes that exit the laser focus. There are two ways to defocus: positive defocusing and negative defocusing. The focal plane is located above the workpiece for positive defocusing, and vice versa for negative defocus. According to the theory of geometric optics, when the distance between the positive and negative defocus planes and the welding plane are equal, the power density on the corresponding plane is approximately the same, but the shape of the molten pool obtained is actually different. In the case of negative defocusing, a greater penetration can be obtained, which is related to the formation of the molten pool. Experiments show that the laser heating 50~200us material begins to melt, forming liquid phase metal and appearing vaporization, forming commercial pressure steam, and spraying at a very high speed, emitting dazzling white light. At the same time, the high concentration of vapor moves the liquid phase metal to the edge of the molten pool, forming a depression in the center of the molten pool. When negative defocusing, the internal power density of the material is higher than the surface, and it is easy to form a stronger melting and vaporization, so that the light energy is transmitted to the deeper part of 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.
Development of laser welding at home and abroad In the mid-1980s, laser welding as a new technology has received extensive attention in Europe, the United States, and Japan. In 1985, the German Thyssen Steel Company cooperated with Volkswagen AG of Germany to successfully adopt the world's first laser tailor welded blank on the Audi100 body. In the 1990s, major automobile manufacturers in Europe, North America, and Japan began to use laser tailor welded blanks in large-scale body manufacturing. At present, the practical experience of the laboratory and the automobile manufacturer proves that the tailor welded blank can be successfully applied to the manufacture of automobile bodies.

Laser tailor welding is the use of laser energy, a number of different materials, different thicknesses, different coatings of steel, stainless steel "target=_blank> stainless steel, aluminum alloy, etc. automatically flatten and weld to form a whole sheet, profiles, sandwiches Boards, etc., to meet the different requirements of parts for material properties, light weight with the lightest weight, optimal structure and best performance. In developed countries such as Europe and America, laser tailor welding is not only in the transportation equipment manufacturing industry. It is used in a large number of fields such as construction, bridges, home appliance sheet welding, steel wire and steel plate welding (steel plate connection in continuous rolling).

The world famous laser welding companies include Swiss Soudonic, France Arcelor Steel Group, Germany ThyssenKrupp Group TWB, Canada Servo-Robot, Germany Precitec.

China's application of laser tailor welded blanks has just started. On October 25, 2002, China's first professional laser tailored commercial production line was officially put into operation. It was laser welded from Wuhan ThyssenKrupp Middle German from Thyssen, Germany. Introduced by Krupp Group TWB. Since then, Shanghai Baosteel Arcelor Laser Tailoring Co., Ltd. and FAW Baoyou Laser Tailoring Co., Ltd. have been put into production.

In 2003, China's first large-scale strip online welding equipment provided by Huagong Laser passed the offline acceptance. The equipment combines laser cutting, welding and heat treatment, making China Huagong Laser the fourth company in the world capable of producing such equipment.

In 2004, Huagong Laser Farley's “High Power Laser Cutting, Welding and Shearing Combined Processing Technology and Equipment” project won the second prize of National Science and Technology Progress Award, and became the only laser enterprise in China with the technology and equipment development capability.

With the rapid development of the industrial laser industry, the market is increasingly demanding laser processing technology. Laser technology has gradually shifted from a single application to a diversified application. Laser processing is no longer a single cutting or welding. The market demands laser processing. There is an increasing demand for integration of cutting and welding, and laser cutting and laser welding integrated laser processing equipment has emerged.

Huagong Laser Farley Research and Development Walc9030 cutting and welding machine, 9 × 3 meters ultra-large format, is the world's largest format laser cutting and welding integrated equipment. Walc9030 is a large-format cutting welding equipment integrating laser cutting and laser welding. The equipment has a professional cutting head and welding head. The two processing heads share a beam and use CNC technology to ensure that they do not interfere with each other. At the same time, two processes of cutting and welding are required. First cutting and then welding, first welding and cutting, laser cutting, welding and easy switching, one device, two functions, without adding new equipment, saving equipment costs for application manufacturers, improving processing efficiency and processing range, Moreover, due to the integrated cutting and welding, the machining accuracy is fully guaranteed, and the performance of the equipment is efficient and stable. In addition, it overcomes the difficulty of thermal deformation of the plate during the over-welding process of the super-large plate and how to maintain the stability of the ultra-long flight path. Two flat plates of 6 meters in length and 1.5 meters in length can be welded at one time, and the surface after welding is smooth. Leveling, no additional processing is required. At the same time, it can cut the sheet of 20mm or less with a width of 3 meters and a length of 6 meters or more, and it can be formed at one time without secondary position. The Shenyang Institute of Automation of the Chinese Academy of Sciences and Japan Ishikawajima Heavy Industries Co., Ltd. have cooperated internationally, following the national technology of developing and digesting and innovating, and conquering several key technologies of laser tailor welding. In September 2006, the first domestic laser was developed. A complete set of tailor-welded production lines, and successfully developed a robotic laser welding system to achieve laser welding of plane and space curves.
Laser welding applications
1. The application of laser welding (TailoredBlandLaserWelding) technology has been widely used in the manufacture of foreign cars. According to statistics, in 2000, more than 100 laser tailoring production lines for cutting blanks were produced worldwide, and the annual output of tailor-welded blanks for car components was 7000. Ten thousand pieces and continue to grow at a higher rate. Domestically introduced models such as Passat, Buick, and Audi also use some cut blank structures. Japan used CO2 laser welding instead of flash butt welding to join the steel rolling coils in the steel industry. In the study of ultra-thin plate welding, such as foils with a thickness of less than 100 microns, it is impossible to weld, but through a special output power waveform. The success of YAG laser welding shows the broad future of laser welding. Japan has also successfully developed YAG laser welding for the maintenance of steam generator thin tubes in nuclear reactors for the first time in the world. In Japan, Su Baorong also carried out gear laser welding technology.

2. In the field of powder metallurgy With the continuous development of science and technology, many industrial technologies have special requirements for materials, and materials manufactured by the smelting method cannot 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 automobile, aircraft and tool cutting tools. With the development of powder metallurgy materials, its connection with other parts. It has become increasingly prominent, limiting 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 diamonds commonly used in the bonding of powder metallurgy materials. The strength is low, and the heat-affected zone width is particularly 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.

3. Automobile industry In the late 1980s, kilowatt-class lasers were successfully applied to industrial production. Today, laser welding production lines have appeared on the large scale in the automobile manufacturing industry and become one of the outstanding achievements of the automobile manufacturing industry. Germany's Audi, Mercedes-Benz, Volkswagen, Sweden's Volvo and other European car manufacturers took the lead in laser welding of roof, body, side frame and other sheet metal welding in the early 1980s. In the 1990s, GM, Ford and Chrysler actually The introduction of laser welding into automobile manufacturing, although starting late, has developed rapidly. Italy Fiat uses laser welding in the welding assembly of most steel plate components. Nissan, Honda and Toyota Motor Corporation of Japan use laser welding and cutting technology in the manufacture of body panels. High-strength steel laser welding assemblies have excellent performance. It is used more and more in the manufacture of automobile bodies. According to the statistics of the US metal market, by the end of 2002, the consumption of laser welded steel structures will reach 70,000 tons, which is three times higher than that in 1998. According to the characteristics of large batch size and high degree of automation in the automotive industry, laser welding equipment is developing in the direction of high power and multi-path. In the process of technology, the United States Sandia National Laboratory and PrattWitney jointly carried out research on the addition of powder metal and wire in the laser welding process. The Bremen Applied Beam Technology Institute in Germany has conducted a lot of research on the use of laser welded aluminum alloy body skeleton. It is believed that the addition of the filler to the weld will help eliminate hot cracks, increase the welding speed, and solve the tolerance problem. The developed production line has been put into production at the Mercedes-Benz plant.

4. Laser welding in the electronics industry has been widely used in the electronics industry, especially in the microelectronics industry. Due to the small heating concentration and low thermal stress in the heat-affected zone of laser welding, it is showing unique advantages in the packaging of integrated circuits and semiconductor device casings. In the development of vacuum devices, laser welding has also been applied, such as molybdenum. Focusing and stainless steel "target=_blank> stainless steel support ring, fast hot cathode filament assembly, etc. The thickness of the elastic thin-wall 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. Soldering, poor plasma stability, and many influencing factors, the laser welding effect is very good, and has been widely used.

5, laser welding of biomedical biological tissue began in the 1970s, Klink et al and jain [13] laser welding of the fallopian tube and blood vessels successfully welded and demonstrated the superiority, so that more researchers try to weld various biological tissues And promote welding to other organizations. At present, domestic and foreign research on laser welding is mainly focused on laser wavelength, dose and its function recovery and laser solder selection. Liu Tongjun carried out basic research on laser welding of small blood vessels and skin. A welding study was conducted on the common bile duct of rats. Compared with the traditional suturing method, the laser welding method has the advantages of fast anastomosis, no foreign matter reaction during the healing process, maintaining the mechanical properties of the welded part, and the growth of the repaired tissue according to its original biomechanical properties will be in the future biomedicine. Get a wider range of applications.

6. Other fields In other industries, laser welding has also been gradually increased, especially in the welding of special materials. For example, laser welding of BT20 titanium alloy, HEEl30 alloy, Li-ion battery, etc., German glass machinery manufacturer GlamacoCoswig The company has developed a new laser welding technology for flat glass in cooperation with the IFW Bonding Technology and Materials Experimental Institute.
Http://news.chinawj.com.cn Editor: (Hardware Business Network Information Center) http://news.chinawj.com.cn

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