Laser welding is an efficient and precise welding method using high energy density laser beam as heat source

Laser welding is one of the important aspects in the application of laser material processing technology.

China's laser welding is at the world's advanced level. It has the technology and ability to use laser to form complex titanium alloy components of more than 12 square meters, and has invested in prototype and product manufacturing of many domestic aviation research projects.

Technical principles

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 heat conduction welding. At this time, the penetration is shallow and the welding speed is slow; When the power density is greater than 105~107 W/cm2, the metal surface will be concave into "holes" under the effect of heat, forming deep penetration welding, which is characterized by fast welding speed and large depth width ratio.

Among them, the principle of heat conduction laser welding is: laser radiation heats the surface to be processed, the surface heat diffuses through the heat transfer, and the workpiece is melted to form a specific molten pool by controlling laser parameters such as the width, energy, peak power and repetition frequency of the laser pulse.

The laser welding machine used for gear welding and metallurgical thin plate welding mainly relates to laser deep penetration welding.

Laser deep penetration welding generally uses 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 the "Key hole" structure. Under the irradiation of laser with high enough power density, the material will evaporate and form holes. 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 hole cavity reaches about 25000C. The heat is transferred from the outer wall of this high-temperature hole cavity, melting the metal around the hole cavity. The small hole is filled with high-temperature steam generated by continuous evaporation of wall materials under the irradiation of light beam. The four 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 conductive welding, energy is deposited on the surface of the workpiece first, and then transmitted to the interior by transmission). The liquid flow outside the hole wall and the wall surface tension are consistent with the steam pressure continuously generated in the hole cavity and maintain a dynamic balance. The light beam continuously enters the hole, and the material outside the hole is flowing continuously. With the movement of the light beam, the hole is always in a stable state of flow. That is to say, the hole and the molten metal surrounding the hole wall move forward with the advance speed of the leading beam, and the molten metal fills the gap left after the hole is removed and condenses, thus forming the weld. All of the above processes happen so fast that the welding speed can easily reach several meters per minute.

Laser classification

There are mainly two kinds of lasers used for welding, namely CO2 laser and Nd: YAG laser. CO2 laser and Nd: YAG laser are invisible infrared light to the naked eye. The beam generated by 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%. As long as a standard optical lens is used, the near-infrared beam can be focused to a diameter of 0.25 mm. The beam of CO2 laser is far-infrared light with a wavelength of 10.6 Lm. The reflectivity of most metals to this light reaches 80%~90%. A special optical lens is required to focus the beam into a diameter of 0.75 - 0.1 mm. The Nd: YAG laser power can generally reach about 4000~6000 W, and now the maximum power has reached 10000 W. However, CO2 laser power can easily reach 20000 W or more. The high-power CO2 laser solves the problem of high reflectivity through the hole effect. When the material surface irradiated by the light spot melts, a hole is formed. This small hole filled with steam is like a black body, which absorbs almost all the energy of the incident light. The equilibrium temperature in the hole cavity reaches about 25 000 e. In a few microseconds, the reflectivity decreases rapidly. Although the development focus of CO2 laser is still on the development of equipment, it is not to improve the maximum output power, but to improve the beam quality and focusing performance. In addition, when CO2 laser is used for high-power welding above 10 kW, if argon shielding gas is used, strong plasma is often induced, making the penetration shallower. Therefore, helium, which does not generate plasma, is often used as the shielding gas in CO2 laser high-power welding. The application of diode laser combination used to excite high-power Nd: YAG crystal is an important development topic, which will greatly improve the quality of laser beam and form more effective laser processing. The direct diode array is used to excite the laser with the output wavelength 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 (10000 h), which is conducive to reducing the maintenance cost of the laser equipment. Development of diode pumped solid-state laser equipment (DPSSL).

process parameters 

(1) Power density. Power density is one of the most important parameters in laser processing. With higher power density, the surface layer can be heated to the boiling point within microseconds, resulting in a large amount of vaporization. Therefore, high power density is beneficial to material removal processing, such as drilling, cutting and engraving. For lower power density, it takes several milliseconds for the surface layer temperature to reach the boiling point. Before the surface layer vaporizes, the bottom layer reaches the melting point, which is easy to form good fusion welding. Therefore, in conductive laser welding, the power density is in the range of 10 ^ 4~10 ^ 6W/CM ^ 2. (2) Laser pulse waveform. Laser pulse waveform is an important problem in laser welding, especially for sheet welding. When the high intensity laser beam hits the material surface, the metal surface will have 60~98% of the laser energy reflected and lost, and the reflectivity changes with the surface temperature. During the action of a laser pulse, the reflectivity of metal varies greatly. (3) 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 determining the cost and volume of processing equipment. (4) The influence of defocusing amount on welding quality. Laser welding usually requires a certain amount of defocusing, because the power density in the center of the spot at the laser focus is too high, which is easy to evaporate into holes. The power density distribution is relatively uniform on the planes away from the laser focus. There are two defocusing modes: positive defocusing and negative defocusing. The focal plane above the workpiece is positive defocusing, otherwise it is negative defocusing. According to the geometrical optics theory, when the distance between the positive and negative defocusing planes and the welding plane is equal, the power density on the corresponding plane is approximately the same, but the shape of the weld pool obtained is actually different. When negative defocusing occurs, greater penetration can be obtained, which is related to the formation process of the molten pool. The experiment shows that the laser heated 50~200us materials begin to melt, forming liquid phase metal and partial vaporization, forming high-pressure steam, and spraying at a very high speed, emitting dazzling white light. At the same time, the high concentration of vapor makes the liquid phase metal 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 that of the surface, which 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 application, negative defocusing is adopted when large penetration is required; When welding thin materials, positive defocusing should be used. (5) Welding speed. The speed of welding 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 weld through.

developing process 

The first laser beam in the world was generated by using a flash bulb to excite ruby crystals in 1960. Due to the thermal capacity of the crystal, it can only produce a very short pulse beam with a very low frequency. Although the instantaneous pulse peak energy can be as high as 10 ^ 6 W, it is still a low energy output. The yttrium aluminum garnet crystal rod (Nd: YAG) using neodymium (ND) as the excitation element 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 flexible optical fiber. The equipment layout is flexible, and the applicable welding thickness is 0.5-6mm. Using CO2 laser (wavelength 10.6uM) as the exciter, the output energy can reach 25KW, which can make single pass full penetration welding of 2mm plate thickness. The industry has been widely used in metal processing. In the mid-1980s, laser welding, as a new technology, received extensive attention in Europe, the United States and Japan. In 1985, German Thyssen Steel Company cooperated with German Volkswagen to successfully use the world's first laser tailor welded blanks on Audi100 body. In the 1990s, major automobile manufacturers in Europe, North America and Japan began to use laser tailor welded blanks (TWBs) technology on a large scale in body manufacturing. The practical experience of both laboratories and automobile manufacturers has proved that tailor welded blanks can be successfully applied to the manufacturing of automobile bodies. Laser butt welding uses laser energy to automatically splice and weld several steels, stainless steels, aluminum alloys, etc. with different materials, thicknesses, and coatings to form an integral plate, profile, sandwich plate, etc., to meet the different requirements of parts on material properties, and to achieve equipment lightweight with the lightest weight, the best structure, and the best performance. 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 widely used in the construction industry, bridge, household appliance plate welding production, steel rolling line plate welding (steel plate connection in continuous rolling) and other fields. The world famous laser welding enterprises include Soudonic of Switzerland, Arcelor of France, TWB of ThyssenKrupp of Germany, Servo Robot of Canada, Precitec of Germany, etc. The application of laser tailor welded blanks technology in China has just started. On October 25, 2002, China's first professional commercial production line of laser tailor welded blanks was officially put into operation, which was introduced by Wuhan ThyssenKrupp Chinese Laser tailor welded blanks from TWB Company of ThyssenKrupp Group in Germany. Since then, Shanghai Baosteel Arcelor Laser Tailor Welding Co., Ltd. and FAW Baoyou Laser Tailor Welding Co., Ltd. have been put into production successively. In 2003, the A318 aluminum alloy lower panel structure was realized with double beam C02 laser filler wire welding and YAG laser filler wire welding abroad. It replaced the traditional riveting structure to reduce the weight of the aircraft fuselage by 20%, while saving 20% of the cost. Gong Shui recognized that laser welding technology will be of great significance to the transformation and upgrading of China's traditional aviation manufacturing industry. Later, he immediately applied for a number of related pre research projects, organized a research team, took the lead in introducing the "double beam laser welding" technology into the research project in China, and planned to use this technology in aircraft manufacturing from the very beginning. The Chinese expert team disclosed the preliminary technology to an aircraft design institute, and introduced the advantages and feasibility of dual beam laser welding to them. After multiple verifications and evaluations, the design institute resolutely decided to apply this technology to the manufacturing of a stiffened panel of an aircraft, realizing the initial goal of applying the "double beam laser welding" technology to aircraft manufacturing, breaking through key technologies such as the precision control of light alloy laser welding and wire filling, integrated and innovatively developed a double beam laser wire filling composite welding device, and established the first high-power double beam laser wire filling welding platform in China, The double beam synchronous welding of T-joint of large thin-walled structure on both sides was realized, and it was successfully applied to the welding and manufacturing of key structural parts of aircraft stiffened wallboard for the first time, playing an important role in the development of new aircraft in China. In 2003, the first large-scale online strip welding equipment in China provided by Huagong Laser passed the offline acceptance. This equipment integrates laser cutting, welding and heat treatment, making China Huagong Laser the fourth enterprise in the world capable of producing such equipment. In 2004, the "High power laser cutting, welding and cutting welding combined processing technology and equipment" project of Huagong Laser Farley won the second prize of the National Science and Technology Progress Award, becoming the only laser enterprise in China that has the ability to develop this technology and equipment. With the rapid development of industrial laser industry, the market has higher and higher requirements for laser processing technology. Laser technology has gradually shifted from a single application to multiple applications. Laser processing is no longer a single cutting or welding. The market has more and more demands for laser processing that requires cutting and welding to be integrated. The cutting and welding integrated laser processing equipment for laser cutting and laser welding has emerged as the times require. Huagong Laser Farley researched and developed Walc9030 all-in-one cutting and welding machine, 9 × The 3-meter large format is the world's largest integrated laser cutting and welding equipment. The Walc9030 is a large format cutting and welding equipment that integrates laser cutting and laser welding functions. The equipment has professional cutting heads and welding heads. Two processing heads share a beam. The numerical control technology is used to ensure that they will not interfere with each other. The equipment can complete two processes that require cutting and welding at the same time. Cutting before welding, welding before cutting, laser cutting and welding can be switched easily. One equipment has two functions without adding new equipment, which saves equipment costs for application manufacturers, improves processing efficiency and processing range. Moreover, due to the integration of cutting and welding, processing accuracy is fully guaranteed, and equipment performance is efficient and stable. In addition, it overcame the difficulty of thermal deformation of plates and how to maintain the stability of ultra long flight light path in the process of splicing and welding of super large plates. Two flat plates with a length of 6 meters and a width of 1.5 meters can be welded at one time. After welding, the surface is smooth and flat, without any subsequent processing. At the same time, plates with width of 3m and length of 6m and less than 20mm can be cut, forming at one time without secondary position. The Shenyang Institute of Automation of the Chinese Academy of Sciences has conducted international cooperation with Ishikawa Shimo Heavy Industry Co., Ltd. of Japan. Following the national scientific and technological development strategy of introducing, digesting and re innovating, it has overcome several key technologies of laser tailor welding. In September 2006, it developed the first complete set of laser tailor welding production line in China, and successfully developed a robotic laser welding system, which has realized laser welding of plane and space curves. In October 2013, Chinese welding experts won the Brooke Prize, the highest academic award in the field of welding. The British Welding Research Institute (TWI) recommends nominations from more than 4000 member units from more than 120 countries every year, and finally awards the award to an expert to recognize his outstanding contributions in the field of welding or connection science and technology and industrial applications. This award is not only recognition of Gong Water Resources and its team, but also recognition of AVIC's promotion of material connection technology progress.

Advantages and disadvantages

advantage

(1) The heat input can be reduced to the minimum required amount, the metallographic variation 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 thick single pass welding have been verified as 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 about electrode pollution 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 be guided by optical instruments, can be placed at a proper distance from the workpiece, and can be guided again between the machines and tools or obstacles around the workpiece. Other welding rules cannot be used because of the above space restrictions; (5) The workpiece can be placed in a closed space (under the control of vacuum pumping or internal gas environment); (6) The laser beam can be focused on a very small area, and can be used to weld small and closely spaced parts; (7) The range of weldable materials is large, and various heterogeneous materials can also be joined with each other; (8) It is easy to carry out high-speed welding automatically, and can also be controlled by digital or computer