Laser welding process for thin-walled metal packaging materials
At present, the domestic three-piece cans mainly use traditional resistance welding, and there are not many laser welding applications. When the metal packaging container is welded by these conventional welding processes, since the energy density is not large, the heat source area is large, and thus the heat affected zone is large. After the tinplate is welded, due to the high temperature of the welding, an iron strip with no protective coating and coating will be formed on the inner and outer walls, which will be oxidized and blackened by the air, and is also highly susceptible to corrosion of the contents of the can. In order to improve the corrosion resistance of the welded part, the weld must be sprayed with nitrogen (high purity nitrogen) after welding, and the weld bead is applied. If the welding quality is not high, the welding surface will be uneven, making the recoating process difficult to control, and forming defects such as blisters or bubbles, which will inevitably cause problems such as rust or leakage of the weld. In addition, the current is too large during the welding, and burrs are easily generated. For example, the supplementary coating cannot completely cover the burrs, thereby causing corrosion, perforation, and leakage around the burrs. The current is too small, which may lead to poor soldering or soldering, that is, soldering, resulting in leakage. Laser welding technology can effectively solve these problems. The laser processing technology of small thin-walled metal packaging containers (such as three-piece cans) has become mature in foreign countries, and China is still in the exploration stage in this respect. China's laser welding system mainly relies on imports, and the welding process is not mature. Foreign laser welding system suppliers are likely to provide a specific set of process parameters, but because the wall thickness, composition and coating metal used in the can change, most of the process parameters provided by the system supplier cannot be used. . In this way, the application of the laser welding process in the manufacture of metal packaging containers is greatly restricted. Therefore, it is necessary to study the influence of process parameters, steel plate (skin) thickness, composition and coating metal on laser welding of metal packaging containers, and develop a suitable laser welding process for metal packaging container materials to achieve pre-selection and optimization of laser welding process parameters, reducing The number of process tests, judging the welding process parameters deep penetration welding penetration stability, preventing welding defects, and ensuring food hygiene and safety are of great significance.
In 1973, Swift-Hook first reported on laser welding heat transfer. The models established in the past 10 years were basically based on two-dimensional or semi-infinite plates, and the initial energy balance and pressure balance in the small holes were analyzed. However, because the model is too simple, the conclusions obtained are one-sided, and there is a big error with the actual results. L991 years N. Postaci091u established a three-dimensional quasi-steady-state model to simulate the flow of liquid metal in the laser non-penetrating weld pool, taking into account the buoyancy and surface tension gradient as the driving force of metal flow. In 2001, Xiong Jiangang studied the non-penetrating welding model by analytical method. The pore-change and molten pool morphology were obtained by solving the phase change interface of the “liquid-gas†and “solid-liquid†interfaces, and the Fresnel absorption mechanism was carried out. In-depth research. This model can be used to calculate the effect of laser welding process parameters on the shape of small holes and weld pools. In 2004, Du Hanbin established a three-dimensional quasi-steady-state laser penetration weld pool flow model, considering the influence of plasma and small hole effect on the welding process. Using this model, it is possible to predict the weld pool temperature distribution under different laser process parameters. And the shape of the welded joint.
The loss of gold elements in laser welding and stitching is mainly the evaporation and burning of volatile alloying elements. For the evaporation of alloying elements in the weld pool, there has been a lot of research work from both the experimental point of view and the theoretical analysis. The study on the loss of gold elements in laser welding and metallurgy of metal packaging materials has not been reported systematically. The first is that many scientists have studied the loss of alloying elements for other types of welding. For example, the evaporation loss of alloying elements Mg and zn during electron beam welding of Al alloy; according to the solution of the mass, momentum and energy conservation equation of a thin layer...Knudsen above the gas-liquid interface of the weld pool, Evaporation and condensation rate equation; using the results of predecessors to calculate the laser-induced evaporation rate of laser-welded Al alloy, Ti alloy and superalloy; in the calculation of the photo-evaporation rate of pure metal, the weld pool transfer phenomenon is considered comprehensively. Basic principles and principles of vapor phase dynamics. In summary, it can be summarized as follows: The mathematical model for calculating the evaporation rate of alloying elements must consider the boundary conditions; the temperature field can be treated as one-dimensional, and considering the conditions of steam condensation, the key calculation is net evaporation. Rate; comprehensively consider the effects of photo-evaporation rate, vapor condensation rate and plasma; by solving the Navier-StokeS equation and the energy conservation equation, the temperature field and velocity field of the weld pool can be obtained, combined with the velocity distribution function and quality of the vapor molecules. The momentum and energy conservation equations calculate the evaporation rate and condensation rate of the alloying elements to calculate the loss of alloying elements. In 2002, Hu Qiang used the research results of his predecessors and made some corrections to the component prediction model. He modeled the weld composition variation of laser deep penetration welding, and fully considered the various effects on the composition of the weld gold. The influence of the factors, the model prediction results and the experimental results agree well. Up to now, the application of high-power laser welding has become more and more extensive, and the existing models for studying the loss of gold elements in welding and sewing are mostly based on laser heat conduction welding. Therefore, the physical process of deep-melting welding of metal packaging materials is analyzed and analyzed. The variation of weld composition and microstructure properties has great economic benefits for guiding metal packaging industry production.
At present, there are many researches on the mechanism of weld microstructure formation in laser welding in China. The research focuses on the change of mechanism and performance of the structure, the corrosiveness of the inner package to the laser weld and the migration and packaging of the weld composition to the package. There is no systematic study on the compositional analysis of metal residues in the material. Wang Xiaohua et al. conducted a preliminary discussion on the safety hazards of food containers and packaging materials and their control measures. Li Hongmei applied computer technology to the analysis of corrosion resistance of metal packaging materials; Yang Wenliang applied new metal packaging material coated steel sheets and Development was introduced. The corrosion failure of the inner wall of canned asparagus container material was analyzed. More in-depth research has not been reported. There are many factors that affect the transformation of the organization. An important feature of laser welding is that the weld is cooled very quickly and easily forms unbalanced structure. The different microstructures have a great influence on the hot cracks in the weld. It is very important to fully understand the transformation mechanism of the weld structure to obtain the weld with the desired properties.
Laser processing characteristics of thin-walled metal packaging containers
(1) High-speed welding and simplified processes are possible. 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 high power density of the laser, small holes are formed in the metal material, and the laser energy is transmitted through the small holes to the deep part of the workpiece, and is less laterally diffused. Therefore, during one scanning of the laser beam, the material is fused to a deep depth and the welding speed is fast. The weld is deep and narrow, and the heat affected zone is small, which can achieve high speed welding. The surface of the butt welding of the black iron sheet and the tinplate is smooth by laser welding.
(2) The weld seam is smooth and beautiful, which can avoid deformation of the metal can. The weld is formed because the material (or filler) of the overlap between the two base metals is heated and then solidified. In laser welding, because the power density is large, the material of the joint portion can be melted rapidly, so the heat affected zone during welding is small, and the deformation of the container and the damage of the inner and outer wall coatings can be avoided. It also solidifies quickly due to the rapid removal of the laser beam. During the cooling process, since the spot is small, the heating time is short, and the temperature gradient around the weld is large, so the weld cooling speed is large, and fine grains are formed. When laser welding a thin plate, generally no filler is added, so the weld material is the base material. Due to the fine grain size, the weld bead performance can reach or exceed the base metal.
(3) For chrome-plated steel sheets, since soldering is not possible, it is more necessary to consider the application of laser welding. In addition, the metal container needs to be sealed after being canned. At present, most of the sealing glue is used. However, sometimes it is not tight enough after sealing and a leak occurs. After laser welding, because of the high strength of the weld, it can withstand higher stress and is not easily damaged or leaked during storage and transportation.
Can produce good social and economic benefits
The laser welding technology can be used in metal packaging containers to produce good economic benefits: the penetration depth is large, the steel plate can be penetrated, and the weld bead is fine and the performance is good, so that high-quality welding can be obtained, which is beneficial to avoid damage of the metal packaging container. The leakage of contents reduces the loss of enterprises and customers; the welding speed is large, the productivity can be significantly improved; the heat affected zone is small, the weldment deformation is small, and it is especially suitable for precision welding, and the surface of the weld is smooth and burr-free, and the post-weld orthopedics can be eliminated. Processing technology; the welding system is highly flexible and easy to automate; although the total efficiency of electro-optical energy conversion of the laser is only 10%, due to the small spot size and high power density, the energy required for laser welding is one order of magnitude smaller than conventional welding, so daily The operating cost is not higher than conventional welding, and the laser welding speed is large, which can reduce the unit cost.
It can also produce good social benefits: laser welding generally does not contain filler metal and does not pollute the environment. High-quality welding can effectively prevent the spoilage of the contents and protect the health of the consumer. The high-quality product packaging obtained by laser welding is conducive to enhancing the reputation of the company and promoting the export of the products. The change will mainly rely on the introduction of foreign advanced laser welding equipment, and form its own innovative technology through absorption and development.
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