How to ensure the consistency of laser welding qua

2022-08-21
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How to ensure the consistency of laser welding quality

laser welding is a processing technology that permanently connects materials through the high-temperature melting produced by laser, and becomes a set functional structure. In recent years, almost all electronic products, such as computers, televisions, digital cameras and many electronic components, have applied laser welding technology to varying degrees in manufacturing. Laser welding has penetrated into all fields of manufacturing industry, which directly affects the quality, reliability and life of products, as well as the cost, efficiency and market reaction speed of production. In the process of mass laser welding production, how to ensure the consistency of laser welding quality of products is a very important topic in the process of production management

generally speaking, the factors affecting the effect of laser welding can be summarized as the following three choices: ① selection of welding materials ② selection of laser welding machine ③ selection of processing workbench

I. selection of welding materials:

the effect of laser welding is related to the optical, physical and mechanical properties of the processed materials. Optical properties: including reflectivity, absorptivity, transmittance, etc. of the material; Physical properties: including thermal conductivity, thermal diffusivity, density, specific heat, heat capacity, viscosity of molten liquid, vaporization temperature, etc; Mechanical characteristics: machining accuracy, stress intensity, material weldability, workpiece cleanliness, etc

Table 1 welding characteristics of different materials

first, when determining the processing materials, especially the welding processing of more than two different materials, it is necessary to refer to the relevant literature to determine whether the materials can be reliably welded (Table 1 lists the YAG laser welding characteristics of different materials, which can be used as a reference). In addition, different materials have different reflectivity to different laser wavelengths (see Figure 1). The higher the reflectivity, The more difficult laser welding is

Figure 1 laser reflection characteristics of several common materials

in addition, the physical and mechanical properties of different materials will also have a crucial impact on the welding effect. Generally speaking, in spot welding, the greater the heat conductivity, the better the welding effect; On the contrary, in seam welding, the smaller the thermal conductivity, the better the welding effect. Users need to make reasonable choices according to different application requirements

sometimes, even the same material, such as aluminum, may solve some of the problems that cause this weather. Due to the different purity of aluminum and the different doped elements, sometimes its material models are different, resulting in different welding effects, and even the same model of materials, different incoming batches, different welding effects; At this time, users need to properly adjust the power waveform of the laser welding machine according to different situations to improve

in addition, the physical and mechanical properties of different materials will also have a critical impact on the welding effect with a box level with an accuracy of not less than 0.1/1000. Generally speaking, in spot welding, the greater the heat conductivity, the better the welding effect; On the contrary, in seam welding, the smaller the thermal conductivity, the better the welding effect. Users need to make reasonable choices according to different application requirements

sometimes, even for the same material, such as aluminum, due to the different purity and doped elements of aluminum, sometimes its material models are different, resulting in different welding effects, and even the same model of materials, different incoming batches, different welding effects; At this time, users need to properly adjust the power waveform of the laser welding machine according to different situations to improve

II. Selection of laser welding machine

when selecting a welding machine, the following factors generally need to be considered:

1. Optical characteristics: spot size (laser rod diameter, fiber diameter and type, exit head parameters), focal plane height, depth of field, spot position, spot incidence angle

2. Control characteristics: selection of feedback control mode and power waveform

the optical characteristics of the welding machine can be determined at the time of proofing, and the user can intuitively determine the welding effect at the initial trial production. Therefore, it will not be repeated here, but focus on the laser control characteristics that affect the consistency of laser welding

at present, the laser welding machines sold in the market are mainly divided into two categories in terms of control characteristics: negative current feedback and negative laser power feedback

current negative feedback is a control method that controls the current of the xenon lamp pumped by the laser to keep the xenon lamp current constant every time. However, the laser output power is not linearly proportional to the current of the pumped xenon lamp, and with the extension of the service time of the pumped xenon lamp, the electro-optical conversion efficiency will be significantly reduced, which will reduce the output laser energy, thus affecting the consistency of the welding effect

laser power negative feedback is a control method that adds a photoelectric detector to the output end of the laser cavity to compare the detected laser power with the required laser power, and then control the pump current of the xenon lamp

laser power negative feedback is divided into real-time negative feedback and non real-time negative feedback. Real time negative feedback is a control method that compares the detected laser power with the required laser power in a laser power pulse, and then controls the pump current of xenon lamp. Non real time negative feedback is a control method that compares the laser power waveform detected by the previous pulse with the set waveform, and then determines the current of the xenon lamp pumped by the next pulse laser. The composition block diagram of the power negative feedback laser welding machine is shown in Figure 2

Figure 2 block diagram of power negative feedback laser welding unit

compared with current negative feedback control, the control accuracy of output laser energy can be greatly improved by using laser power negative feedback, which is especially suitable for mass production. Figure 3 shows the fluctuation of laser output energy of Lianying laser 15W laser under the control of negative current feedback and negative laser power feedback to meet the needs of agricultural operation environment and special equipment

Figure 3

of output laser energy under negative laser power feedback and negative current feedback. It can be seen from Figure 3 that under approximately the same output power, the energy instability under negative current feedback is greater than 8%, while the output energy instability under negative laser power feedback is less than 3%. Moreover, with the extension of the service time of the xenon lamp pumped by the laser welding machine, the electro-optical conversion efficiency of the xenon lamp pumped will be further reduced, and the output energy under the current negative feedback state will gradually decrease, which requires the quality control personnel to often detect the output laser energy, thereby increasing the production cost. When the negative feedback of laser power is adopted, the system automatically compares the output laser energy and adjusts the compensation xenon lamp current in real time, so it can ensure that the output laser energy is always in a relatively stable range

in addition, the laser welding machine with negative feedback control of laser power can easily realize the waveform control of laser power. In fact, for different materials, using different laser power waveforms can make the welding effect more exquisite. Sometimes even materials that cannot be welded in the traditional way can get better welding effect by changing the laser power waveform. Figure 4 shows the welding effect when square wave and programmed wave are used in butterfly laser welding in optical communication industry

in Figure 4, a is the schematic diagram of the butterfly laser to be welded. In the figure, the lower part of B is the welding effect obtained by using the laser power waveform shown in the upper part of B, and C is the welding effect obtained by using the laser power waveform shown in the upper part of C. The test shows that the strength of the weld spot in Figure C is much higher than that of the weld spot in Figure B when it is raised due to the concave weld spot. It can be seen that selecting the appropriate laser power waveform is very important to get a better welding effect that should be replaced

Figure 4 welding effect of butterfly laser under different laser power waveforms

when selecting laser power waveform, generally speaking, on the premise of outputting the same laser energy, the wider the pulse width, the larger the weld spot; The higher the peak power of laser power waveform, the deeper the weld spot. At present, there is no complete method to set the laser power waveform. Users can gradually explore the laser power waveform suitable for their products in the process of use

the selection of laser welding machine is very important to the yield of batch processing; Therefore, users can try their best to use laser power real-time negative feedback welding machine to improve the excellent rate of products on the premise that conditions permit

III. selection of processing workbench:

in mass production, automatic workbench is often needed to improve production efficiency, so the difference of workbench will also have a great impact on the welding effect. Theoretically speaking, the higher the accuracy, the better the specific selection of the workbench; However, the higher the accuracy, the higher the production cost of the workbench, so it is necessary to select the appropriate workbench. In addition, whether to use shielding gas or choose different shielding gas will also have a great impact on the actual welding effect. The characteristics related to the workbench mainly consider the following aspects:

mobile characteristics: Mobile accuracy, repeatability, and mobile speed

fixture characteristics: fixture accuracy, heat dissipation capacity, clamping force, convenience

auxiliary gas: shape, aperture, gas type, flow, pressure, angle, distance

IV. summary

in the actual welding production, there will be various problems. How to identify the causes of the problems and how to solve these problems are also very important in the process of production process management

generally speaking, if there is continuous poor welding at the same part of the welded product, it is likely that there is a problem with the workbench and fixture. At this time, it is necessary to solve the problem from the aspects of fixture and workbench; If the batch of incoming materials is different, resulting in poor welding processing, it may be the problem of incoming materials. It is necessary to check and replace the incoming materials or change the waveform setting process conditions of the laser welding machine to solve it; If there is welding penetration and faulty welding occasionally, you can check the energy stability of the welding machine or whether there is a problem with the workbench and fixture. Figure 5 is a general analysis flow of possible problems in the process of laser welding

Figure 5 Analysis of quality problems in laser welding process

in a word, the quality assurance of welding products is a very complex system engineering, and every small link will affect the excellent rate of products. This requires users to summarize according to different situations and find out the welding quality control methods suitable for their product characteristics

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