Laser welding is one of the important aspects of laser processing material processing technology. In the 1970s, it was mainly used for welding thin-walled materials and low-speed welding. The welding process was a heat conduction type. That is, laser radiation heats the surface of the workpiece. The surface heat diffuses to the inside through heat conduction, and the parameters such as width, energy, peak power and repetition frequency of the laser pulse are controlled. The workpiece is melted to form a specific bath. Laser welding as a high-quality, high-precision, low-deformation, high-efficiency and high-speed welding method, with the improvement of high-power CO2 and high-power YAG lasers and optical fiber transmission technology, metal molybdenum welding condenser objective lens, etc. The successful development makes it more and more widely used in the fields of machinery manufacturing, aerospace, automotive industry, powder metallurgy, and biomedical microelectronics.
Current research mainly focuses on the theory of C02 laser and YAG laser welding of various metal materials, including laser-induced plasma spectroscopy, absorption and scattering characteristics, and intelligent control of laser welding, composite welding, laser welding phenomena, and pinhole behavior. , welding defect occurrence mechanism and prevention methods, etc., and nickel-based heat-resistant alloys, aluminum alloys and magnesium alloys welding, welding phenomenon modeling and numerical simulation, steel materials, copper, aluminum alloy and dissimilar materials, laser joints Performance evaluation has done some research [1].
Laser welding principle:
Laser welding irradiates high-strength laser beam to the metal surface. Through the interaction between the laser and the metal, the metal absorbs the laser light and converts it into heat energy. After the metal melts, it cools and crystallizes to form a weld. There are two mechanisms of laser welding:
1, heat conduction welding When the laser irradiation on the surface of the material, a part of the laser is reflected, a part of the material is absorbed, the light energy is converted into heat and heat and melting, the material surface layer of heat continues to the material deep in the thermal conduction, and finally will The two weldments are welded together.
2. Deep penetration laser welding When a laser beam with a relatively high power density is irradiated to the surface of the material, the material absorbs light energy and converts it into heat energy. The material is heated and melted to vaporization, generating a large amount of metal vapor, and the reaction force generated when the steam exits the surface. Next, the molten metal liquid is pushed around to form pits. As the laser continues to irradiate, the pits penetrate deeper. When the laser stops irradiating, the melt around the pits returns, and the two weldments are welded after cooling and solidification. In - up.
These two kinds of welding mechanisms are selected according to the actual material properties and welding requirements. Different welding mechanisms are obtained by adjusting the welding process parameters of the laser. The basic difference between these two methods is that the former pool surface remains closed while the latter pool is penetrated by the laser beam. Conductive welding has less disturbance to the system because the radiation of the laser beam does not penetrate the material to be welded, so the weld seam is not easily invaded by gas during the process of conducting welding, while the continuous closing of the small hole during deep fusion welding can cause pores. Conduction welding and deep penetration welding methods can also be switched to each other during the same welding process. The transition from the conduction mode to the pinhole mode depends on the peak laser energy density and laser pulse duration applied to the workpiece. The time dependence of the laser pulse energy density enables the laser welding to change from one welding mode to another during the laser-material interaction, that is, during the interaction process, the weld seam can be formed in a conduction mode and then transformed. For the small hole.
The current expansion of laser welding applications is mainly applied to:
Manufacturing applications, powder metallurgy, automotive, electronics, biomedical, and other fields such as BT20 titanium alloy [22], HEl30 alloy [23], Li-ion battery [24] and other laser welding.
Laser welding is characterized by very small deformation of the welded workpiece, almost no connection gap, and a high welding depth/width ratio, so the welding quality is higher than the conventional welding method. However, to assure the quality of laser welding, that is laser welding process monitoring and quality control is an important area of ​​laser use, including the use of inductors, capacitors, sound waves, optoelectronics and other sensors, through the computer processing, for different welding Targets and requirements, such as weld seam tracking, defect detection, weld quality monitoring, and other items, through the feedback control to adjust the welding process parameters to achieve automated laser welding. In laser welding, the focus position of the beam is one of the most critical control process parameters. At a certain laser power and welding speed, only the focus is within the optimal position range to obtain maximum penetration and good weld shape. In actual laser welding, in order to avoid and reduce the factors that affect the stability of the focal position, special clamping and equipment techniques are needed. The accuracy of this device and the quality of the laser welding are complementary.
First, the main features of laser welding. Compared with other traditional welding techniques, the main advantages of laser welding are:
1, fast, deep, small deformation.
2. It can be welded at room temperature or under special conditions. The welding equipment is simple. For example, if the laser passes through an electromagnetic field, the beam will not be deflected; the laser can be welded in vacuum, air, and certain gas environments, and can be welded through glass or a material that is transparent to the beam.
3, can weld refractory materials such as titanium, quartz, etc., and can weld the opposite sex material, the effect is good.
4. After the laser is focused, the power density is high. When the high-power device is welded, the aspect ratio can reach 5:1 and the maximum can reach 10:1.
5, can be micro-welding. After the laser beam is focused, it can obtain a very small spot and can be accurately positioned. It can be applied to the mass welding of micro- and small-sized workpieces in large-scale automated production.
6. Can weld inaccessible parts, perform non-contact long-distance welding, with great flexibility. Especially in recent years, the optical fiber transmission technology has been adopted in the YAG laser processing technology, which has made laser welding technology more widely popularized and applied.
7. The laser beam is easy to realize beam splitting according to time and space. It can perform multi-beam simultaneous processing and multi-position processing, which provides the conditions for more precise welding.
However, laser welding also has some limitations:
1. It is required that the assembly precision of the weldment is high and the position of the beam on the workpiece must not be significantly shifted. This is due to the fact that the spot size of the spot after the laser is focused is small, and the weld seam is narrow, which is filled with metal materials. If the workpiece assembly accuracy or the beam positioning accuracy does not meet the requirements, it is very easy to cause welding defects.
2. The cost of lasers and related systems is high, and one-time investment is relatively large.
Second, laser welding heat conduction.
Laser welding radiates a high-strength laser beam onto a metal surface, and the metal is melted to form a weld by the interaction of the laser and the metal. In the laser-metal interaction process, metal melting is only one of the physical phenomena. Sometimes light energy is not primarily converted to metal melting, but is manifested in other forms, such as vaporization, plasma formation, and the like. However, to achieve good fusion welding, the metal must be melted to become the main form of energy conversion. To this end, it is necessary to understand the various physical phenomena generated in the interaction between laser and metal, and the relationship between these physical phenomena and laser parameters, so that by controlling the laser parameters, most of the laser energy is converted into the energy of metal melting to achieve welding. purpose.
Third, the laser welding process parameters
1, power density
Power density is one of the most critical parameters in laser processing. With 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 drilling, cutting, and engraving. For lower power densities, it takes several milliseconds for the surface temperature to reach boiling point. Before the surface vaporizes, the bottom layer reaches the melting point and it is easy to form a good fusion weld. Therefore, in the conduction laser welding, the power density is in the range of 104~106W/CM2.
2, laser pulse waveform.
Laser pulse waveform is an important issue in laser welding, especially for thin-film welding. When a high-intensity laser beam hits the surface of the material, the metal surface will reflect 60 to 98% of the laser energy and be lost, and the reflectivity will change with the surface temperature. During a laser pulse, the reflectivity of the metal varies greatly.
3, the laser pulse width.
Pulse width is one of the important parameters of pulsed laser welding. It is not only an important parameter that distinguishes material removal and material melting, but also a key parameter that determines the cost and volume of processing equipment.
4, the amount of defocus on the impact of welding quality.
Laser welding usually requires a certain amount of defocusing because the power density at the center of the spot at the laser spot is too high and it tends to evaporate into holes. The power density distribution is relatively uniform across the planes away from the laser focus.
There are two types of defocus: positive defocus and negative defocus.
The focal plane above the workpiece is positively defocused, whereas the focal plane is negatively defocused. According to the theory of geometrical optics, the power density in the corresponding plane is approximately the same when positive and negative are equal to each other. However, the shape of the molten pool actually obtained is different. In the case of negative defocusing, a greater penetration depth can be obtained, which is related to the formation process of the bath. Experiments show that the laser heating 50 ~ 200us material began to melt, the formation of liquid metal and the emergence of vaporization, the formation of market pressure steam, and spray at a very high speed, emitting dazzling white light. At the same time, the high concentration of vapor moves the liquid metal to the edge of the bath, forming a depression in the center of the bath. When negative defocusing, the internal power density of the material is higher than the surface, and it is easy to form a stronger melting, vaporization, so that light energy is transmitted deeper into the material. Therefore, in practical applications, negative defocusing is used when the required depth of penetration is greater, and positive defocusing is preferred when welding thin materials.
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