The hottest laser repair die welding technology

2022-08-14
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Laser repair mold welding technology

laser welding is one of the important aspects of the application 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 belongs to the heat conduction type, that is, the laser radiation heats the surface of the workpiece, and the surface heat is guided to the internal diffusion through heat transfer. By controlling the parameters such as the width, energy, peak power and repetition frequency of the laser pulse, the workpiece melts and forms a specific molten pool. 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, and the successful development of metal molybdenum welding bunched objective lens, laser welding is more and more widely used in machinery manufacturing, aerospace, automotive industry, powder metallurgy, biomedical microelectronics and other fields

the current research mainly focuses on the theory of CO2 laser and YAG laser welding of various metal materials, including the light splitting, absorption, scattering characteristics of laser-induced plasma, intelligent control of laser welding, composite welding, laser welding phenomenon and keyhole behavior, welding defect occurrence mechanism and prevention methods, etc., and the weldability, welding phenomenon modeling and numerical simulation of nickel based heat-resistant alloys, aluminum alloys and magnesium alloys, Some research has been done on the connection between steel materials, copper, aluminum alloys and dissimilar materials, and the performance evaluation of laser joints [1]

principle of laser welding:

laser welding is to radiate a high-intensity laser beam to the metal surface. Through the interaction between the laser and the metal, the metal absorbs the laser and converts it into heat energy, so that the metal melts and then cools and crystallizes to form welding. There are two mechanisms of laser welding:

1. Heat conduction welding when the laser irradiates the surface of the material, some of the laser is reflected and some of it is absorbed by the material, which converts the light energy into heat energy and heats it up and melts. The heat of the surface layer of the material continues to transfer to the depth of the material in the way of heat conduction, and finally the two weldments are fused together

2. Laser deep penetration welding when the laser beam with relatively high power density irradiates the material surface, the material absorbs light energy and converts it into heat energy. The material is heated and melted to vaporization, producing a large amount of metal vapor. Under the reaction force generated when the vapor exits the surface, the molten metal liquid is pushed around to form pits. With the continuous irradiation of the laser, Pit Jinmin new material is a scientific and technological company focusing on the research and operation of high-molecular new material industry. It penetrates deeper. When the laser stops irradiating, the molten liquid around the pit reflows, cools and solidifies, and then welds the two weldments together

these two welding mechanisms are selected according to the actual material properties and welding needs, and different welding mechanisms are obtained by adjusting the laser welding process parameters. The most basic difference between the two methods is that the former keeps the surface of the molten pool closed, while the latter is penetrated into holes by laser beams. The disturbance of conduction welding to the system is small, because the radiation of laser beam does not penetrate the welded material, so the weld is not easy to be invaded by gas in the process of conduction welding; During deep penetration welding, the continuous closing of small holes can lead to pores. Conduction welding and deep penetration welding can also be converted to each other in the same welding process. The conversion from conduction mode to keyhole mode depends on the peak laser energy density and laser pulse duration applied to the workpiece. The time dependence of laser pulse energy density can make laser welding change from one welding mode to another during the interaction between laser and material, that is, in the interaction process, the weld can be formed in the conduction mode first, and then change to the keyhole mode

at present, the expansion of the application field of laser welding is mainly used in:

manufacturing applications, powder metallurgy, automotive industry, electronic industry, biomedicine, and other fields, such as laser welding of BT20 titanium alloy [22], hel30 alloy [23], Li ion battery [24]

laser welding is characterized by minimal deformation of the workpiece to be welded, almost no connection gap, and high welding depth/width ratio, so the welding quality is higher than that of traditional welding methods. However, for example, ensuring the quality of laser welding, that is, monitoring and quality control of laser welding process is an important content in the field of laser utilization, including using various sensors such as inductance, capacitance, acoustic wave, photoelectric, etc., through electronic computer processing, aiming at different welding objects and requirements, realizing items such as weld tracking, defect detection, weld quality monitoring, etc., and adjusting welding process parameters through feedback control, So as to realize automatic laser welding. In the optical welding when the sensor is subjected to the effect of tension P, the beam focus position is one of the most critical control process parameters. Under a certain laser power and welding speed, the maximum penetration and good weld shape can be obtained only when the focus is within the optimal position range. In the actual laser welding, in order to avoid and reduce the factors that affect the stability of the focus position, special clamping and equipment technology are needed. The accuracy of this equipment and the quality of laser welding complement each other

I. main characteristics of laser welding. Compared with other traditional welding technologies, the main advantages of laser welding are:

1, fast speed, large depth and small deformation

2. It can be welded at room temperature or under special conditions, and the welding equipment is simple. For example, when the laser passes through the electromagnetic field, the beam will not shift; Laser can weld in vacuum, air and some gas environment, and can weld through glass or materials transparent to the beam

3. It can weld refractory materials such as titanium and quartz, and can weld dissimilar materials with good results

4. After laser focusing, the power density is high. When welding high-power devices, the depth width ratio can reach 5:1, up to 10:1

5. Micro welding can be carried out. After focusing, the laser beam can obtain a small spot and can be accurately positioned. It can be applied to the assembly and welding of micro and small workpieces produced in large quantities

6. The inaccessible parts can be welded, and the non-contact long-distance welding has great flexibility. Especially in recent years, optical fiber transmission technology has been used in YAG laser processing technology, which makes laser welding technology more widely promoted and applied

7. Laser beam is easy to realize beam splitting according to time and space, and can carry out multi beam simultaneous processing and multi station processing, which provides conditions for more precise welding

however, laser welding also has some limitations:

1. It is required that the assembly accuracy of the weldment is high, and the position of the beam on the workpiece cannot be significantly offset. This is because the spot size after laser focusing is small and the weld is narrow, which is filled with metal material. If the assembly accuracy of workpiece or beam positioning accuracy cannot meet the requirements, it is easy to cause welding defects

2. The cost of laser and its related systems is high, and the one-time investment is large

second, laser welding heat conduction

laser welding is to radiate high-intensity laser beam to the metal surface, and melt the metal to form welding through the interaction between laser and metal. Metal melting is only one of the physical phenomena in the interaction between laser and metal. Optical energy is not mainly converted into metal melting, but in other forms, such as vaporization, plasma formation, etc. However, in order to achieve good fusion welding, metal melting must become the main form of energy conversion. Therefore, it is necessary to understand the various physical phenomena produced in the interaction between laser and metal and the relationship between these physical phenomena and laser parameters, so that by controlling laser parameters, most of the laser energy can be transformed into the energy of metal melting, so as to achieve the purpose of welding

III. technological parameters of laser welding

1. Power density

power density is one of the most critical parameters in laser processing. With high power density, the surface layer can be heated to boiling point within microsecond time range, resulting in a large amount of vaporization. Therefore, high power density is beneficial to material removal processing, such as punching, cutting and engraving. For low power density, it takes several milliseconds for the surface temperature to reach the boiling point. Before the surface vaporizes, the bottom reaches the melting point, which is easy to form a good fusion welding. Therefore, in conductive laser welding, the power density is in the range of 104~106w/cm2

2. Laser pulse waveform

Ji light pulse waveform is an important problem in laser welding, especially for sheet welding. When a high-intensity laser beam strikes the material surface, 60~98% of the laser energy on the metal surface will be reflected and lost, 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 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. Influence of defocus on welding quality

laser welding usually requires a certain defocus, because the power density at 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 all planes away from the laser focus

there are two defocusing methods: positive defocusing and negative defocusing

if the focal plane is above the workpiece, it is positive defocusing, otherwise it is negative defocusing. According to the theory of geometric optics, when the positive and negative distances are equal, the power density on the corresponding plane is approximately the same, but in fact, the shape of the molten pool obtained is different. When defocusing is negative, a greater penetration can be obtained, which can protect the sensor, which is related to the formation process of the molten pool. Experiments show that the materials heated by laser for 50~200us begin to melt, form liquid phase metal and appear fractional vaporization, form municipal pressure steam, and spray at a very high speed. This time, Sony introduced the amorphous cathode materials developed for all solid batteries and the brilliant white light of their characteristic evaluation results. At the same time, the high concentration vapor makes the liquid 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 light energy can be transmitted deeper into the material. Therefore, in practical application, when the penetration is required to be large, negative defocusing is adopted; Positive defocusing should be used when welding thin materials

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