At present, the domestic three-piece cans are mainly made of traditional resistance welding. There are not many laser welding applications. When these conventional welding processes are used to weld metal packaging containers, the heat-affected zone is large since the energy density is not so large and the heat source area is large. After the welding of tinplate, due to the influence of high temperature of welding, an iron strip with no protective coating and plating will be formed on the inner and outer walls, which will be oxidized by the air to become black, and it is also very vulnerable to the corrosion of the contents of the can. In order to improve the corrosion resistance of the welded part, the weld must be treated with nitrogen (high-purity nitrogen) after welding, and the weld bead must be applied. If the welding quality is not high, the welding surface will be uneven, which will make it difficult to control the coating process and create defects such as sand holes or bubbles, which will inevitably lead to rust or leakage of the weld. In addition, when the welding current is too large, it is easy to produce burrs, such as the coating can not completely cover the burr, which results in corrosion, perforation, leakage around the burrs. However, if the current is too small, it can easily lead to poor soldering or poor soldering, that is, soldering can cause leakage. Laser welding technology can effectively solve these problems. The laser processing technology of foreign small-sized thin-walled metal packaging containers (such as three-piece cans) has matured, and China is still at an exploratory stage in this regard. China's laser welding system mainly relies on imports, and the welding process is not mature. Laser welding system suppliers in foreign countries may provide a certain set of specific process parameters. However, due to changes in wall thickness, composition, and plating metal used in can making, the system suppliers may not use most of their process parameters. . In this way, the application of the laser welding process in the manufacture of metal packaging containers is greatly restricted. Therefore, we must study the process parameters, the thickness of the steel plate (skin), the composition and the influence of the coating metal on the laser welding of metal packaging containers, and develop a suitable laser welding process for the metal packaging container material to achieve the pre-selection and optimization of the laser welding process parameters, and reduce The number of process tests, the determination of the welding process parameters, the penetration stability of deep penetration welding, the prevention of welding defects, and the assurance of food hygiene and safety are of great significance.
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In 1973, Swift-Hook first studied the heat transfer of laser welding. The models established in the past 10 years were basically based on two-dimensional or semi-infinite slabs, and the internal energy balance and pressure balance of the pores were preliminarily analyzed. However, because the model is too simple, the conclusions obtained are one-sided, and there is a large error from the actual results. L991 N. Postaci091u established a three-dimensional quasi-steady-state model to simulate the liquid metal flow process in which the laser does not penetrate the weld pool, taking into account the buoyancy and surface tension gradients as the driving force of the metal flow. In 2001, Xiong Jiangang studied the non-penetrating welding model by analytical method. The pores and the shape of the molten pool were obtained by solving the phase transition interface between "Liquid-Vapour" and "Solid-Liquid" interfaces, and the Fresnel absorption mechanism was carried out. In-depth study. The model can be used to calculate the effect of laser welding process parameters on the shape of the small holes and the weld pool. In 2004 Du Hanbin established a three-dimensional quasi-steady-state laser penetration weld pool flow model, taking into account the effects of plasma and pinhole effects on the welding process. Using this model, it was possible to predict the temperature distribution of the weld pool under different laser process parameters. And the appearance of welded joints.
The loss of the alloy elements of laser welding welds is mainly the evaporation and burning of volatile alloy elements. For the evaporation of the alloying elements in the weld pool, a great deal of research has been done both from the experimental point of view and from the theoretical point of view. However, no systematic reports have been reported on the loss of the alloying elements in the laser welding of metal packaging materials. The first is that many scientists studied the loss of alloying elements in other types of welding. Such as electron beam welding Al alloy alloy elements Mg and zn evaporation loss; According to the solution exists in the welding pool gas-liquid interface above a thin layer of Knudsen (Knudsen) mass, momentum and energy conservation equation, proposed to understand the pure metal Evaporation and condensation rate equations; using previous results to calculate the evaporation rate of laser-induced materials when laser welding Al alloys, Ti alloys, and superalloys; integrated consideration of the welding bath transfer phenomenon when calculating the photoinduced evaporation rate of pure metals Basic principles and principles of vapor dynamics. In summary, the following can be summarized as follows: The mathematical model for calculating the evaporation rate of alloy elements must consider the boundary conditions; the temperature field can be treated as one-dimensional, and under the condition of steam condensation, the key calculation is the net evaporation. In consideration of the effects of photoinduced evaporation rate, steam condensation rate and plasma, the temperature field and velocity field of the weld pool can be obtained by solving the Navier-Stokes equations and the energy conservation equations, combined with the velocity distribution function, mass, and The momentum and energy conservation equations calculate the evaporation rate and the condensation rate of the alloying elements to calculate the loss of alloying elements. In 2002, Hu Qiang took advantage of previous research results and modified the composition prediction model to model the changes in the composition of the laser deep penetration welding welds, taking full account of the various effects on the alloy composition changes in the welds. With the influence of factors, the model prediction results are in good agreement with the experimental results. So far, the application of high-power laser welding has become more and more widespread, and most of the existing models for studying the loss of alloy elements in welds are based on laser heat conduction welding. Therefore, the physical process of deep-welded metal packaging materials is analyzed and analyzed. The law of weld composition and change of organization and performance has great economic benefits for guiding the metal packaging industry.
At present, there are many researches on the mechanism of weld formation in laser welding in China. The research focuses on the changes of the mechanism and performance of the microstructure transformation, the corrosiveness of the inner package to the laser weld, and the migration and packaging of the weld composition to the package. There is still no systematic study of the compositional analysis of metal residues. Wang Xiaohua and others conducted preliminary discussions on the safety hazards of food containers and packaging materials and their control measures; Li Hongmei et al. applied computer technology to the analysis of corrosion resistance of metal packaging materials; Yang Wenliang applied new metal packaging materials to coated steel sheets and Development was introduced. The corrosion failure of the inner wall of asparagus cans was analyzed. More in-depth studies have not yet been reported. There are many factors that affect the organizational transformation. An important feature for laser welding is the extremely fast cooling rate of welds and the formation of non-equilibrium structures. The different microstructures have a great influence on the thermal cracking in the weld. It is of great significance to fully understand the transformation mechanism of the weld microstructure and to obtain the ideal weld seam.
Thin-wall metal packaging laser processing characteristics
(1) High-speed welding can be realized and the process can be simplified. After the laser beam is focused, its spot size is below 0.1mm, but the power density can reach l06~108w/cm2, which is several orders of magnitude higher than the power density of the traditional welding process. Due to the large power density of the laser, small holes are formed in the metal material, laser energy is transmitted through the small holes to the deep part of the workpiece, and less lateral diffusion. Therefore, the depth of fusion of the material is large and the welding speed is high during one scanning of the laser beam. , The weld seam is deep and narrow, the heat affected zone is small, and high speed welding can be realized. The surface of butt welding with laser welding black iron sheet and tinplate is smooth.
(2) smooth and beautiful welds, to avoid deformation of metal cans. Welds are formed because the material (or filler) in the overlapping area between the two parent metals is heated and melted and then solidified. In laser welding, because the power density is large, the material at the joint can be rapidly melted, so the heat affected zone is very small during welding, which can avoid deformation of the container and damage of the inner and outer wall coatings. It also rapidly solidifies due to the rapid removal of the laser beam. In the cooling process, because the spot is small and the heating time is very short, the temperature gradient around the weld is large, so the weld cooling rate is very high, forming fine grains. When laser welding of thin plates, fillers are generally not added, so the weld material is the base material. Due to the fine grain, the weld performance can reach or exceed the parent material.
(3) For chrome-plated steel sheets, since soldering cannot be performed, it is necessary to consider the application of a laser welding process. In addition, metal containers can be sealed after they are canned. At present, most use of sealing glue. However, the tightness is sometimes not enough after sealing, resulting in a leak. After laser welding, because of the high weld strength, it can withstand high stress and is not easily damaged or leaked during storage and transportation.
Can produce good social and economic benefits
Laser welding technology can be used in metal packaging containers can produce good economic benefits: a deep penetration, can weld through the steel plate, and the weld microstructure is fine, the performance is good, so you can get high-quality welding, help to avoid damage to the metal packaging container, Leakage of contents will reduce the losses of enterprises and customers. Large welding speed can significantly increase productivity. Small heat-affected zone and small deformation of weldments are especially suitable for precision welding. The surface of weld is smooth without burrs, which can eliminate post-welding orthopedics. Processing technology; welding system has a high degree of flexibility, easy to achieve automation; Although the total efficiency of the laser's electro-optical energy conversion is only 10%, but because of the small spot, the power density, the energy required for laser welding is an order of magnitude smaller than the traditional welding, so daily The operating cost is not higher than that of conventional welding, and the laser welding speed is large, so that the unit cost can be reduced.
It can also produce good social benefits: Laser welding generally does not add filler metal and does not pollute the environment. High-quality welding can effectively prevent the spoilage of the contents and protect the health of consumers. The high-quality product packaging obtained by laser welding is beneficial to the improvement of corporate reputation and the promotion of product export. It will mainly rely on the introduction of foreign advanced laser welding equipment, and will form its own innovative technology through absorption and development.