Numerical simulation and simulation analysis techn

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Numerical simulation and simulation analysis technology of resistance spot welding process

Abstract This paper introduces the key technologies used in the self-developed visual simulation and analysis software of resistance spot welding process, such as contact surface, contact resistance, object-oriented technology, visualization technology. In the pre-processing, through the interactive graphic input interface, the user can easily and quickly establish the solution model. The user inputs the shape parameters, material performance parameters, welding specifications, etc. of the spot welding workpiece and electrode in the form of graphical interface, and freely selects the density of the mesh division to realize the automatic mesh division; The established model data can not only be stored, but also be well embedded with the finite element simulation calculation; In post-processing, the results of finite element analysis and calculation can be displayed visually in a graphical manner

key words: resistance spot welding numerical analysis visualization technology finite element

0 introduction

resistance spot welding technology, as an efficient thin plate structure connection method, is widely used in all walks of life of the national economy, especially the automobile manufacturing industry. With inaccurate statistics, the shell of a car needs a welding spot. However, the completion time of spot welding is very short, about 0.2S. Therefore, it is very difficult to study its process behavior by experimental methods. With the development of computer technology, numerical simulation method has become an important means to analyze the mechanism of resistance spot welding process. Many scholars at home and abroad have established their own numerical models, and widely used the finite element analysis method to simulate the spot welding process, and made great progress []. However, the deficiency of this kind of work is the pre-processing and post-processing part of developing the software, which needs to collect, sort out and input a large amount of original data. These data are cumbersome and error prone, and because of the lack of a fully functional post processor, it also brings great difficulties to promote and use

scientific computing visualization involves many fields such as computer graphics, image processing, computer-aided design, computer vision and human-computer interaction, and is one of the hotspots of new computer application technologies [10]. With the progress of science and technology, the development and research work in the engineering field is increasingly developing in the direction of quantification and refinement. Engineers need to understand the distribution of each field quantity inside the structure in more detail. With the graphic ability of modern computing technology, scientific computing visualization technology turns the data generated in the calculation process into an intuitive, easy to understand, static or dynamic picture expressed in the form of graphics or images, so as to help people effectively understand the calculation data and liberate them from the cumbersome data post-processing

this paper introduces the key technologies used in the self-developed visual simulation and analysis software system visual ssrsw for resistance spot welding process, such as contact surface, contact resistance, nugget formation and growth, object-oriented technology, visualization technology, etc

1 system design and implementation technology

according to the basic requirements of the structural design of visualization software, this paper describes the structural process of this software from the perspective of user system interaction, as shown in Figure 1. There are two ways for users to operate the system: one is to establish a numerical model by inputting data parameters, the system performs numerical calculation, and then processes the results and feeds them back to users in the form of graphic display; Another way is to use graphic operations to directly obtain visual information. Part a in the figure constitutes the pre-processing module of the system, and part B is the post-processing module of the system. Part a, Part B and data processing together constitute the visual processing module of the system, and the numerical calculation module is set separately. The work to be completed by each module in this system is simple and clear, and can be realized independently by using different programming languages

Figure 1 the structural framework of the system

vc++ has very powerful functions in drawing and image processing. It specially defines a group of graphic objects and components to draw graphics or complete some basic image functions. Using these objects and components, we can easily draw various commonly used graphics. By setting their attributes, we can also change the different styles of graphics. For the required functions in the pre-processing and post-processing modules, the reusable DC class of vc++ device context can be realized. DC class encapsulates all drawing methods and most GDI functions, and generally does not need to directly call windows API functions

in visual processing, in addition to the implementation method of drawing, it also involves the mapping relationship between equipment coordinates (such as display screen) and actual coordinates, and the conversion of equipment coordinate information into output data information

2 finite element numerical calculation technology

resistance spot welding process is a highly nonlinear multi factor coupling process, involving electrical, thermal, mechanical, magnetic, metallurgical and many other aspects. At present, the two-dimensional axisymmetric model is mostly used in numerical analysis to study the interaction of electrical, thermal and mechanical behaviors during spot welding, and the influence of contact resistance and other factors on the formation process of spot welding nugget is also considered [10]

in the design of finite element analysis program for spot welding, the key technology is to deal with the contact between electrode and workpiece, and between workpiece and workpiece. In this procedure, the spring unit is used to deal with the contact problem. When the contact force of the spring is less than or equal to zero, it is judged as separation, and when the spring force is greater than zero, it is judged as contact. This determines whether the interface is in contact or not. The treatment of interface resistance is to simulate interface resistance by using the change of resistivity of the nearest Gauss point of the unit close to the interface. Coupling calculation is carried out in the program design, considering the influence of contact area on current intensity, the change of material constant with temperature and the influence of temperature change on the contact surface []

FORTRAN language is used for program design. Because of its high-precision data structure and calculation structure, this language has become a classic programming language for analysis and calculation software. For example, the famous large-scale finite element analysis software ANSYS adopts fortran77 language. However, FORTRAN language is a process oriented language after all, and its human-computer interaction and graphics processing capabilities are not ideal, while the object-oriented programming language vc++ can make up for the deficiencies of FORTRAN language in these two aspects. Therefore, vc++ language is used to develop the overall framework and pre-processing and post-processing systems of finite element analysis software, FORTRAN language is used to independently develop numerical calculation programs, and then certain methods are used to connect them into an organic whole. Among them, interprocess communication is an effective connection method among these methods

process is a concept involved in Windows operating system. It is a running instance of an application and a dynamic execution of an application. You can create a new process through Win32 API function:: createprocess(). The process calling this function is called "parent process", and the process created by "parent process" is called "child process". A program group of closely related programs can be developed by using the method of communication between different processes. As shown in Figure 2, this analysis software 3 Multifunctional formaldehyde tester: to detect the content and release of formaldehyde, the main program of the framework of visual ssrsw system (including pre-processing and post-processing) is taken as the parent process, and the two-dimensional axisymmetric thermoelastoplastic large deformation spot welding process simulation program SPFEM is taken as the sub process. The parent process uses the menu message mapping function to start the sub process, so as to realize the pre and post processing of the finite element analysis system of spot welding process Integration of post module and analog calculation module. This method has simple programming, clear thinking and fast running speed

Figure 2 Interprocess communication model

3 model building technology -- preprocessing module

visual modeling, as the main content of the preprocessing stage of finite element analysis, is very important to ensure the performance of the simulation computing system. The basic requirement of modeling process is simple and fast, and interactive technology is essential. The quality of interactive interface design is related to the efficiency of the whole system and the satisfaction of users. Excellent interface can enable users to focus on their own goals and tasks without caring about the details of the software itself. Visual ssrsw system is an interactive graphical interface system, which allows users to use interactive graphical methods to input raw data and observe graphic entities

The main function of the pre-processing module is to establish the model of the solution object, define the corresponding material properties, determine the spatial position and shape parameters of each sub domain in the field, and automatically perform lattice division, so as to prepare for simulation calculation. The initialization parameters of visual ssrsw system pre-processing include performance parameters of spot welding workpiece, performance parameters of electrode material, electrode shape parameters, welding specifications, lattice data, etc. In the system, the four node quadrilateral isoparametric element is used for automatic lattice division of the field, and the main part of the calculation area (the contact area between the electrode head and the workpiece) is divided more closely, while other areas are divided more sparsely. In order to deal with the mixed field of workpiece and electrode, the elastic surface element is used to simulate the contact state between workpiece and workpiece, workpiece and electrode. Finally, the pre-processing file that interfaces with the simulation calculation module is generated in the lattice division part. The graphics operation sub module can translate, zoom, print preview and save the graphics displayed in the window. In order to prevent misoperation, the pre-processing module has a fallback function, which can return to the first interface of parameter input, so as to improve the safety of the software that only selecting the appropriate experimental machine can ensure to meet the measurement requirements. Figure 3 shows the parameterization of the electrode, which is convenient for the automatic division of the lattice

Figure 3 parametric design of typical electrodes

4 result processing technology -- post processing module

visualization technology of finite element analysis results includes two categories: vector field and scalar field. The vector field contains fluid and other substances, and the visualization mainly uses the arrow segment representation; The scalar field has a certain direction of temperature, stress and strain, etc.

this paper processes the finite element calculation results, including graphic display, result output, etc., which is the main content of the post-processing module. Visualization mainly adopts isoline, color cloud map, line frame surface map and slice map, among which isoline map and color cloud map are most commonly used. The system provides many post-processing methods for result analysis, such as spot weld nugget growth curve, structural deformation diagram, isoline diagram and color cloud diagram

(1) nugget size and electrode displacement diagram

in order to facilitate the observation of the growth process of spot welding nugget, the system provides the change curve of nugget diameter and thickness displayed according to the welding cycle, and it can also display the change curve of electrode displacement. In the system, two forms are used to draw the deformation diagram or displacement diagram of the structure, one is to draw the deformation diagram of the whole lattice points, and the other is to only draw the deformation diagram of the outer frame of the structure. These two methods have their own advantages and disadvantages. The former method can reflect the deformation of each part of the structure because it draws the deformation of each point, but when the lattice is dense, the graphic display will be not clear enough; The latter method only draws the deformation of the external frame, and the figure is very clear, but the deformation inside the structure is not clear. Therefore, the system provides the above two deformation diagrams to comprehensively reflect the deformation of the structure. Figures 4 and 5 show the influence of welding current on the formation and growth of nugget and electrode displacement of LY12CZ aluminum alloy sheet with a thickness of 1.0mm under the condition of electrode pressure of 2.5kn. The calculation results well simulate the process of actual nugget formation and growth and electrode displacement

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