New progress in resistance welding technology for metal containers
Text / Yang Wenliang
In recent years, with the rapid development of the metal packaging industry, metal containers have been continuously updated and developed in the fields of materials, structures and applications. The quality requirements of users have been continuously improved, and the resistance welding technology as the main processing technology of metal containers has also been received. A big challenge.
Because the resistance welding process is quite complicated, it contains various influencing factors, such as: material to be welded, current, electrode pressure, energization time, electrode end face shape and size, shunt, distance of solder joint from edge, plate thickness, workpiece surface state, etc. And these factors are related to each other and have some interaction. At the same time, the invisibility of the nugget during the welding process and the transient nature of the welding process bring great difficulties to the welding quality control. In order to adapt to the needs of new materials, new processes and new products in the packaging industry, so that the resistance welding process and equipment can meet the requirements of modern production, in the past decade, the welding industry in various countries has done in the field of resistance welding process and equipment control. A lot of work.
I. Recent advances in computer simulation studies of resistance welding processes
Resistance welding is a complex process involving electrical, heat transfer, metallurgy and mechanics, including electromagnetics during welding, heat transfer processes, melting and solidification of metals, phase transitions during cooling, welding stresses and deformations. These factors must be controlled in order to obtain a high quality welding product. Traditional resistance welding processes and parameter development methods are obtained through a series of process tests and empirical data. However, from the perspective of development, with the development of computer technology, the method of numerical simulation will play an increasingly important role. For example, when metal materials change, there is not much experience to rely on. If it only takes a long time and money to accumulate data by experiment, and any attempt and failure will cause economic losses. At this point the numerical method will exert its unique capabilities and advantages. As long as a small number of verification tests prove the applicability of the numerical method in dealing with a certain problem, a large amount of screening work can be performed by a computer without having to carry out a large amount of experimental work in the workshop and the laboratory. This greatly saves manpower, material resources and time, and has great economic benefits. Once the various welding phenomena can be computer simulated, we can use a computer system to determine the best design, best process and welding parameters for welding.
Due to the invisibility of the formation of resistance welding nugget, it is quite difficult to test it. The establishment of theoretical model is of great value to its analysis and research.
With the in-depth application of finite element numerical simulation methods in the field of electric resistance welding research, international research in this field has focused on the following three aspects in recent years:
1. Electro-thermal-force coupled finite element simulation method
The electric resistance welding process is a complex process involving the interaction of electric, thermal, mechanical and metallurgical phenomena. This process includes electric field problems, heat conduction problems and thermal elastoplastic deformation problems. Therefore, the interaction and coupling effects of all these problems must be considered. Pressure-induced changes in the contact state between the workpiece and the electrode, as well as the interface between the workpiece and the workpiece, and thermal deformation play an important role in these interactions. Strictly speaking, solving such coupling problems should solve both the electric field and the thermal field. Force field.
Therefore, in recent years, the finite element analysis of resistance welding has evolved from the original isolated electric field, thermal field and force field analysis to electro-thermal-force coupling analysis. For example, American experts use electric-thermal-force coupled finite element simulation welding. The process nugget growth and heat distribution were studied. The electric resistance welding of the steel plate was studied. The simulation results were verified by experiments: the model can be used in the electrode selection stage to reduce the welding deformation and improve the welding quality. The British experts passed the electro-heating process of the electric resistance welding process. Force analysis results in residual stress after welding and welding parameters affecting residual stress; others have developed an integrated model to simulate the process and performance of resistance welding, which combines basic physical phenomena and load conditions in welding. This method consists of three parts: process model, microscopic model and structural model. It can comprehensively evaluate the welding performance under the action of electro-thermal-force; Japanese experts use electro-thermal-force coupling in the study of electric resistance welding. Finite element model predicts nugget size, penetration, electrode and electrode under different welding current, welding time and electrode force Contact area and the like, proven: this model is useful for off-line testing influence of welding parameters on the size of the joints.
2. Accuracy analysis of computer simulation
With the increasing application of computer simulation methods in the research of resistance welding process, in order to further develop it for metal packaging industry production, must we consider the error of this simulation method? How to improve the accuracy of numerical simulation, so that the results obtained are closer to the actual welding situation. Recently, some foreign experts have conducted special research on this aspect. American experts believe that an important question for the evaluation of computer simulation models is to judge whether it is accurate enough. Usually, Bayesian theorem statistical strategy is used to analyze the error range of the simulation calculation. However, the input quantity and the unknown parameters are large and the data volume is large. In the case of statistical analysis, it becomes quite difficult. Experts use the electric-thermal-force finite element model to analyze the resistance welding process, calculate the nugget size and surface indentation, use the main element method based on the uncertainty model method, and pass the linear mean square error on the nugget size and indentation statistics. The prediction accuracy of the finite element is obtained.
3. Industrial application of computer simulation
Computer numerical simulation has the advantages of low cost, flexible and convenient parameter change, but most of it is currently used for offline calculation and simulation. How to apply this method effectively to the industrial production of metal containers, online evaluation of welding quality and Control, this issue has also become a focus of research by welding scientists in recent years. Based on long-term engineering research and industrial cooperation, Denmark has developed a new welding software based on the finite element method for simulating the resistance welding process. In order for the software to be directly applied by engineers and technicians in the plant, all parameters in the resistance welding are considered and automatically implemented in software. The software supports a Windows-friendly interface, flexible operation, flexible geometry design for workpieces and electrodes, and parameter settings like a formal welder, which can be used in industry to support product development and process optimization. Some companies have started using this software. Scholars at the University of Washington in the United States have proposed a welding-quality assessment model based on the contact area. It uses a finite element analysis model to represent changes in the contact area. It is applied online based on the simulation results. Experiments: at different electrode sizes, electrode forces, This method is successful under soldering time and current and will provide important information for resistance welding monitoring and control.
Second, the latest developments in the study of solderability of new materials
With the rapid development of the industry, higher requirements have been placed on the performance of metal packaging products, and the upgrading of product materials has been promoted. For example, in order to improve the corrosion resistance of packaging containers and improve product adaptability, galvanized steel sheets are widely used in steel drum production instead of ordinary cold-rolled steel sheets; in order to reduce packaging weight and save energy consumption, the world's larger packaging companies are developing. Large capacity aluminum alloy or high strength steel plate container. Since the electric resistance welding method is widely used in the manufacture of the metal container sheet structure, in order to ensure the welding quality, it has become an urgent task to study the electric resistance welding performance of new materials such as aluminum alloy, galvanized steel sheet and high-strength steel. In recent years, welding workers in various countries have done a lot of theoretical and practical research work in this area, and have achieved certain results.
1. Research on resistance welding of aluminum alloy
Aluminum alloy has low melting point, low yield strength, good electrical and thermal conductivity and surface oxide film, which brings great difficulties to electric resistance welding. In recent years, welding experts in various countries have made the following researches:
In the study of aluminum alloy welding electrode life, the University of Waterloo in the study of aluminum alloy welding electrode life, using scanning electron microscopy, SEM / EDX, XRD and other methods, the research shows that from the electrode decay to the final failure mainly experienced aluminum stripping Aluminum, copper alloying, electrode surface plaque and electrode end face pits, since plaques and pits originate from the stripping and alloying of aluminum, researchers believe that regular cleaning of the electrode surface can increase electrode life. Conducive to the application of aluminum alloy in container production; University of California, USA studied the effect of alloy composition of copper electrode on electrode life, using Cu-Cr, Cu-Zr, Cu-Cr-Zr, Cu-Be, etc. Copper electrode materials; some scholars have proposed the use of composite electrodes to improve the life of welded aluminum alloys. The use of composite electrodes with tungsten embedded in the end of chrome-zirconium copper electrode to weld aluminum alloys, found that the electrode life can be increased by 1.5 to 2 times.
In the research of aluminum alloy welding process, some scholars have studied the relationship between the contact radius of the electrode and the contact resistance of the aluminum alloy during the welding process of the aluminum alloy, and the relationship between the contact radius of the electrode and the contact area between the workpiece and the workpiece; The University of Michigan adopted an experimental study method to compare the resistance welding process of aluminum alloy and steel. According to the test blade curve, the available welding current range and the tear width after weld failure were evaluated and evaluated by these two parameters. The welding quality of aluminum alloy and steel shows that the electrode size has a great influence on the tear width after the steel weld is broken, but there is no good correspondence between the aluminum weld and the tear width fluctuation after the aluminum weld is broken. Very big.
2. Resistance welding of high strength steel plate
Advanced high-strength steel has the advantages of high strength, good molding performance, high bake hardenability, high energy absorption rate and fatigue strength, and good anti-collision performance. Therefore, its application in metal container manufacturing is increasing. The research on the weldability of electric resistance welding of high-strength steel sheets has also emerged. At present, the research of high-strength steel plate resistance welding by national welding experts mainly focuses on the weldability of various high-strength steel plates, the influence of welding specification parameters on the microstructure of solder joints, the optimization of welding procedures and processes. For example, British scholars have studied the correction of high-strength steel plate welding procedures and the influence of material strength and weld quenching on weld performance. Japanese scholars have systematically studied the resistance welding characteristics of ultra-fine grain high-strength steel plates. The study found that: Because of the different resistivity and strength of high-strength steel sheets at high temperatures, unlike low-carbon steels, the same size of nugget size required for spot welding requires a larger welding current than low-carbon steel sheets, and the carbon equivalent of such steel sheets. Very low, although the main structure of the nugget after welding is martensite, because the low carbon component limits the hardening of the nugget, the spot welded joint of this material can obtain high tensile strength and vertical pull without tempering. Stretching strength; French scholars have proposed a method to improve the resistance spot welding performance of high-strength steel sheets and ultra-high-strength steel sheets through post-weld tempering process; Japanese scholars also conduct resistance welding weldability of high-strength galvanized steel sheets. Were studied.
3. Resistance spot welding of galvanized steel sheets
In order to improve the corrosion resistance of the products, various types of galvanized steel sheets are used more and more widely in the barrel industry. According to different galvanizing processes and galvanizing compositions, galvanized steel sheets are classified into: electrogalvanized sheets and hot-dip galvanized sheets. Plate, Zn-Ni alloy plate, Zn-Fe alloy plate, etc. Since the physical properties and electrical conductivity of the coated metal are different from those of the low carbon steel, the resistance welding performance of the galvanized steel sheet is greatly different from that of the same type of steel sheet which is not galvanized, and the welding quality is required to be higher from the viewpoint of the use performance. In the case of seam welding, it is necessary to ensure that the weld seam of sufficient strength is produced, and the plating layer should be reasonably protected.
Since galvanized steel sheets have certain difficulties in their weldability, in recent years, various countries have done a lot of research work on the welding properties of galvanized steel sheets around welding process specifications, numerical simulation of welding processes, and electrode life.
At present, the international research on the welding process of galvanized steel sheets is basically mature. The hotspots of further research mainly focus on how to improve the life of welding electrodes of galvanized steel sheets. For example, the use of dispersion strengthened copper alloy or low temperature treatment of electrodes can improve the electrode. Service life.
Third, the latest development of research on resistance welding quality monitoring methods
Due to the extensiveness, importance and representativeness of the resistance welding process, ensuring the welding quality has become the main goal of resistance welding research. Welding quality control has always been one of the important topics that domestic and foreign welding scholars are committed to. For example, the Japan Welding Association has set up a welding quality monitoring and testing research group, and the United States has also carried out research projects to solve the electric resistance welding in the manufacture of metal containers. At present, a major problem faced by welding process control engineers is to explore a reliable, low-cost, non-destructive technology to distinguish the quality of welding and the real-time prediction of welding strength.
In recent years, research on quality control of resistance welding has been on the rise, and its methods and methods are becoming more advanced, mainly focusing on the following aspects:
1. Real-time monitoring of direct variables in the welding process
The University of Michigan used high-speed digital cameras to monitor the formation process of nugget in real time, and studied the formation mechanism of resistance welding nugget and its influence on welding process parameters.
2. Real-time detection of indirect electrical parameters during spot welding
American scholars have proposed a new monitoring algorithm based on the form of energy balance integral. The system calculates the average temperature of the welding by calculating the welding voltage, current and total thickness, and predicts the diameter and spatter of the solder joint. Low-cost welding real-time monitoring method; some scholars have explored the monitoring method of welding strength and quality by changing the control parameters of the output signal. The research method is to create a process output and process input variable related to the quality of the solder joint. Relationship, and the input parameter selection directly affects the heat input percentage of the solder joint size and strength, and the output selects the electrode displacement, which can accurately reflect the growth and deformation of the nugget. This monitoring method has been applied in metal container steel plate welding.
In recent years, many scholars have engaged in the study of monitoring the quality of solder joints by dynamic resistance method. For example, American scholars have predicted, analyzed and classified dynamic resistance, and in practice, it proves that this method is effective for detecting bad solder joints. Chinese scholars have also made breakthroughs in experimental research. The dynamic resistance is obtained by the total temperature of the welding zone. This temperature is related to the resistance of the workpiece in the welding zone, the workpiece and the electrode, and the resistance between the workpiece and the workpiece. The measured dynamic resistance curve Can be divided into four stages: 1) rapid decline with the decrease of contact resistance; 2) mainly depends on the increase of the volume resistance of the workpiece, the dynamic resistance also increases accordingly; 3) the volume resistance of the workpiece increases, but the contact resistance decreases Small, and thus dynamic resistance is also increased; 4) Dynamic resistance is reduced due to nugget formation on the contact surface. The test found that the weld nugget was formed in the third and fourth stages; the University of Michigan adopted the measurement of the instantaneous current and voltage of the primary circuit, and used regression analysis to determine the relationship between these factors and the welding quality, overcoming the traditional measurement. The problem of loop dynamic resistance method in real-time monitoring.
3. Real-time monitoring of indirect force parameters of the welding process
The University of Warsaw established a welding spatter prediction model based on the interaction of mechanics and metallurgical processes in welding. The model is used to calculate the electrode force and liquid nugget force during welding. When the latter is larger than the former, splashing occurs, and the model is verified by experiments. The model is used to guide the selection of electrode pressure. American scholars The effects of the mechanical properties of the welder on the resistance welding process and the welding quality were studied. The results show that the rigidity and friction of the welder have a great influence on the welding process and the welding quality, while the moving inertia does not produce a large welding process and welding quality. Influence: Danish scholars believe that the mechanical dynamic reflection of resistance welding machine has a great influence on welding quality and electrode life. Therefore, it must be considered in welding production and simulation. Due to the complexity of the mechanical structure of the welding machine, some correlation coefficients in the simulation equation It is difficult to get, and the scholar has obtained these parameters through experiments and calculations.
4. Neural network prediction of solder joint quality
Scholars at the University of Michigan in the United States, in the system of dynamic welding method to evaluate the quality of welding, combined with neural network theory to predict the quality of solder joints, the results are in good agreement with the tensile strength of the actual welded specimens; The researchers used neural network methods to detect spot weld spatters from different materials. The results show that the electrode force signal is the most important sign of splash generation.
The research of welding scholars in various countries has made great contributions to the improvement of welding technology for metal packaging. With the continuous improvement of welding technology, the metal container manufacturing industry will surely usher in faster and better development.
(metal packaging)
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