Abstract: From the aspects of computer simulation of resistance welding process, solderability research of new materials and quality control methods of resistance welding, the latest research trends in the field of resistance welding in recent years are discussed.

Keywords: resistance welding, new developments abroad

0 Preface

In recent years, with the rapid development of industries such as automobile and vehicle, aerospace, construction, transportation and light industrial appliances, the corresponding industrial products have been continuously updated and developed in the fields of materials, structures and applications, and the processing quality requirements for products have been continuously improved. As a widely used material processing technology in the manufacture of these industrial products, resistance welding has also been greatly challenged.

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 a certain interaction. At the same time, combined with the invisibility of the nugget during the welding process and the transient nature of the welding process, it brings great difficulties to the welding quality control. In order to adapt to the needs of new materials, new processes and new products in the industrial development and application, 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 a lot in the resistance welding process and equipment control. The work is mainly focused on the following aspects:

1) Computer simulation study of resistance welding process

2) Study on solderability of new materials

3) Research on quality monitoring method of resistance welding

1 Computer simulation study of resistance welding process

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 welded joint. Traditional resistance welding processes and parameter formulation 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, using new materials such as high-strength steel to make new engineering structures, especially for some important aerospace structures, there is not much experience to rely on. If it relies on experimental methods to accumulate data, it takes a long time and money, and any attempt and Failure will cause major 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 computer systems to determine the best design, best process and welding parameters for welding various structures and materials. In addition, numerical simulation is also widely used to analyze the strength and performance of spot weld joints.

Due to the invisibility of the formation of the resistance spot weld nugget, it is quite difficult to test it. The establishment of the theoretical model is of great value to its analysis and research. Since the initial finite element spot welding analysis model established by Nied [1] in 1984, many finite element models of spot welding have appeared and provided a theoretical basis for actual production [2-5].

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:

1.1 Using electric-thermal-force coupled finite element simulation method

The resistance spot welding process is a complex process involving the interaction of electricity, heat, mechanics and metallurgy. This process includes electric field problems, heat conduction problems and thermal elastoplastic deformation problems, so the interaction and coupling effects of all these problems must be considered. Pressure-induced workpieces—electrodes and workpieces—changes in contact state between workpiece interfaces and thermal deformation play an important role in these interactions. Strictly speaking, such coupling problems should be solved simultaneously to solve electric fields and heat. Field and force field. Therefore, in recent years, the finite element analysis of resistance spot welding has evolved from the original isolated electric field, thermal field and force field analysis to electro-thermal-force coupling analysis, for example: American scholar Sun, X. Electricity-heat-force Coupling finite element simulation of nugget growth and heat distribution in spot welding process [6], resistance spot welding of aluminum alloy and steel plate with intermediate transition material was studied. The simulation results were verified by experiments: the model can be used in the electrode selection stage to reduce Small welding deformation and improved welding quality; Korean scholar Cha, BW obtained the post-weld residual stress and spot welding parameters affecting residual stress by electro-thermal-force analysis of 304 stainless steel resistance spot welding process [7]; Feng, Z And other scholars have developed an integrated model to simulate the process and performance of resistance spot welding [8], which combines the basic physical phenomena and load conditions in spot welding. This method consists of process model, micro model and structural model. Three-part composition, which can comprehensively evaluate the performance of spot-welding head under electro-thermal-force; Japanese scholar De, A uses finite element of electro-thermal-force coupling in the study of aluminum alloy resistance spot welding Model [9] predicts the nugget diameter, penetration depth, electrode-to-plate contact diameter under different welding currents, welding time, electrode force, etc. It has been verified that this model is for offline inspection of welding parameters and solder joint size. The impact is very useful.

1.2 Accuracy analysis of computer simulation

With the increasing use of computer simulation methods in the research of resistance welding processes, in order to further develop it for industrial 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 scholars have made special research on this aspect, for example: American scholar Cavendish, James C. mentioned in the article [10]: An important question for the evaluation of computer simulation models is to judge whether it is accurate enough, usually used by people. The Bayesian theorem statistical strategy to analyze the error range of the simulation calculation, but in the case of a large amount of input and unknown parameters, the statistical analysis becomes quite difficult. Hasselman, Timothy and other scholars used the electric-thermal-force finite element model to analyze the resistance spot welding process of aluminum alloy. When calculating the nugget size and surface indentation, the main element method based on the uncertainty model method was adopted, and the nugget size and The linear mean square error of the indentation statistics gives the finite element prediction accuracy [11].

1.3 Industrial applications of computer simulation

Computer numerical simulation has the advantages of low cost, flexible and convenient parameter change, but most of them are currently used for offline calculation and simulation. How to apply this method effectively to industrial production for online evaluation and control of welding quality, this The problem has also become a focus of research by welding scientists in recent years. Danish scholar Zhang, Wenqi developed a new finite element method based welding software based on long-term engineering research and industrial cooperation [12]: SORPAS, used to simulate resistance projection welding and spot welding. 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. Companies such as Volkswangen, Volvo, Siemens and ABB are now using the software. Li, Wei of the University of Washington in the United States proposed a spot welding quality assessment model based on the contact area [13], which uses a finite element analysis model to represent changes in the contact area, and applies online based on the simulation results. This method is successful with electrode size, electrode force, soldering time and current, and it will provide important information for resistance welding monitoring and control.

2 Study on solderability of new materials

With the rapid development of the industry, higher requirements have been placed on the performance of industrial products (especially automotive), and the upgrading of product materials has been promoted. For example, in order to improve the corrosion resistance of the automobile casing and improve the service life of the automobile, a large number of galvanized steel sheets are used in the automobile body manufacturing instead of the ordinary cold-rolled steel sheets; in order to reduce the overall weight of the vehicle body and save energy consumption, the world's major automobile companies are developing. Aluminum or high-strength steel body car. Due to the large number of resistance spot welding methods used in the assembly and manufacture of thin-wall structures such as automobile bodies, it has become an urgent task to study the resistance spot welding performance of new materials such as aluminum alloy and galvanized steel high-strength steel in order to ensure the welding quality. 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.

2.1 Resistance spot 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 resistance spot welding. In recent years, welding scientists in various countries have mainly done the following research:

In the study of aluminum alloy spot welding electrode life, Lum, L of the University of Waterloo in the United States in the study of 5182 aluminum alloy spot welding electrode life, using scanning electron microscopy, SEM / EDX, XRD and other methods, research shows that [14]: from The electrode decays to the final failure mainly through the four stages of aluminum stripping, aluminum and copper alloying, electrode end face plaque and electrode end face pit. Since plaque and pit originate from the stripping and alloying of aluminum, the author believes that the periodicity Cleaning the surface of the electrode can increase the life of the electrode, which is beneficial to the application of aluminum alloy in automobile production. Fresz, B of the University of California, USA, studied the effect of the alloy composition of the copper electrode on the electrode life [15]. Cu-Cr was used in the test. Cu-Zr, Cu-Cr-Zr, Cu-Be and other copper electrode materials; Dorn, Lutz and other scholars have proposed a composite electrode to improve the life of spot-welded aluminum alloy [16], studied in chromium zirconium copper The aluminum alloy composite electrode is welded to the extreme part, and the electrode life is found to be 1.5 to 2 times higher.

In the research of aluminum alloy spot welding, Sari, H. and other scholars studied the relationship between electrode contact radius and contact resistance during spot welding of aluminum alloy through the spot welding process of aluminum alloy, as well as the contact radius of the electrode and the workpiece and workpiece. The relationship between contact areas [17]; Cho, Y of the University of Michigan, USA, used experimental research methods to compare the resistance spot welding process of aluminum alloy and steel [18], and determined the available welding current range according to the test blade curve. And the diameter of the button after the damage of the solder joint, and use these two parameters to evaluate the spot welding quality of aluminum alloy and steel. The test shows that the electrode size has a great influence on the diameter of the button after the steel joint is broken, but the aluminum welding There is no good correspondence between the points, and the diameter of the button after the aluminum solder joint is broken fluctuates greatly.

In addition, the finite element simulation of aluminum alloy spot welding process is also a research hotspot of scholars in various countries in recent years [9] [19].

2.2 Resistance Spot Welding of High Strength Steel

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 automotive lightweight construction is increasing. The research on the weldability of high-strength steel resistance welding has also emerged. At present, the research of high-strength steel resistance welding by national welding experts mainly focuses on the weldability of various high-strength steels, the influence of welding specification parameters on the microstructure of solder joints, the optimization of welding procedures and processes. For example, Shi, G and other scholars from the Global Center for TWI Material Bonding Technology in the UK studied the correction of high-strength steel spot welding procedures and the influence of base metal strength and weld quenching on the performance of solder joints [20]; Japanese scholars Otani, Tadayuki et al. The resistance spot welding characteristics of high-strength steel grains have been systematically studied. It is found that the high-strength steel has different resistivity and strength at high temperature than low-carbon steel, and the same size of nugget size is required for spot welding. The current is larger than that of the low carbon steel plate [21]. At the same time, the carbon equivalent of the steel plate is very low. Although the main structure of the nugget after welding is martensite, the material is limited by the low carbon component. The spot weld joint can obtain high tensile strength and vertical tensile strength without tempering [22]; French scholar Mimer, M proposed experimentally to improve high strength steel and ultra high strength steel through post weld tempering process. The method of resistance spot welding performance [23]; Japanese scholar Sakuma, Yasuharu also studied the spot weldability of high-strength galvanized steel sheets [24].

2.3 Resistance spot welding of galvanized steel sheets

In order to improve the corrosion resistance of products, various types of galvanized steel sheets are used more and more widely in the automotive and home appliance industries. Depending on the galvanizing process and galvanizing composition, galvanized steel sheets are classified into: electrogalvanized sheets and heat. Galvanized sheet, Zn-Ni alloy coated sheet, Zn-Fe alloy coated sheet, and the like. Since the physical properties and electrical conductivity of the coated metal are different from those of the low carbon steel, the resistance spot welding performance of the galvanized steel sheet is greatly different from that of the ungalvanized steel sheet, and the joint quality requirements are more important from the viewpoint of the use performance. High, that is, spot welding must ensure that joints of sufficient strength are produced, and the plating should be properly protected.

Because galvanized steel sheets have certain difficulties in spot welding and soldering, in recent years, welding workers in various countries have made a lot of problems on the welding properties of galvanized steel sheets around welding process specifications, numerical simulation of welding processes, and electrode life. research work.

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 spot welding electrodes of galvanized steel sheets, such as the use of dispersion strengthened copper alloy [25] or the low temperature treatment of the electrodes. Improve the life of the electrode [26].

3 Research on quality monitoring methods of resistance welding

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. Spot welding quality control has always been one of the important topics that the welding industry scholars at home and abroad are committed to. For example, the Japan Welding Association has set up a spot welding quality monitoring and testing research group [27], and the United States has also carried out a 2mm research project to solve resistance spot welding in the automotive industry [28]. 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 solder joints and predict the strength of solder joints in real time.

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.

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