FINITE ELEMENT MODELLING ON WELDING PROCESSES AND WELDING PHENOMENA B. Palotas ABSTRACT The Finite Element Modelling is a wide applicable method in the field of welding as well. The paper shows application examples of modelling of resistance welding processes, modelling of cold pressure welding and some possibilities of FEM application. The results of FEM can be used in the field of Computer Aided Process Planning applied in the field of welding processes. The application of Finite Element Modelling helps to understand the welding processes, the welding phenomena and it is an effective tool of Welding Specialists and Researchers in our time. KEYWORDS Welding, Computer Aided Welding, Modelling of Welding, Finite Element Modelling INTRODUCTION The Welding Engineer is often faced with the task of developing a welding technology, after he has first selected the welding procedure in accordance with the prescribed characteristics and the geometrical requirements of the weld. Then it is necessary to calculate the course of the welding process, the criteria for optimisation being the weld form, the quality stability and the weldability of the material under pre-established physical, technical and economical conditions. The physical processes should be described by natural laws, such as the conservation, transmission and conversation of energy. The technical and economic conditions, on the other hand, such as time, output and cost can be influenced. They therefore form the basis for the optimisation problem. This complicated problem can be solved theoretically or empirically. Less and less time available for project planning tasks makes it increasingly necessary to avoid empirical test methods and to search for more effective methods of studying and developing welding technologies. Automated preparation of the technological data increases the labour productivity of the engineer, but basically does not change the quality of the design work. This is due to the fact that the welding parameters are prescribed on the basis of empirical information. Associate Professor, Department of Materials Science and Engineering, Budapest University of Technology and Economics, Bertalan L. u. 7. Budapest, H-1111, Hungary, Tel: +36 (1) 463 1115, e-mail: palotasb@eik.bme.hu The quality of the design work can, however, be improved considerably using the theory of welding process in the form of certain mathematical models. These models should as far as possible contain complete information about the process to be calculated. It would be advantageous to create universal models for the form of the welds and the weld defects. The software for the mathematical model is only produced once and can be rendered more precise as knowledge of the process to be studied increases (Ref. 1.). MATHEMATICAL MODELLING OF WELDING PHENOMENA The welding phenomena should be written by differential equations given in the Table 1. on the basis of (Ref. 2.). The general solution of these equations can be given by Gauss-Ostrogradsky's theorem: √q ds = f div q dV = √(34 + да да да (1) where q is a flow through a Σ surface (flux of q) when its source is written with its divergent (div) in V volume bordered by Σ surface. The equation (1) can be used for analysis of phenomena occurring in solid, liquid, solid-liquid and gas agent during welding. Unfortunately, the analytical solution of equations (Tab.1.) is very difficult and it requires a lot of neglect and we have to know all the processes occurring during welding. Of course all the processes are not known that occur during welding and negligence lead to information losses. Due to these facts, we have to use numerical mathematical methods to solve the above-mentioned equations (Ref. 3.). One of these numerical methods is the Finite Element Method. FINITE ELEMENT MODELLING OF WELDING The processes occurring during welding are named coupled processes, because thermal, electrical, electro- magnetic, mechanical, metallurgical and surface processes are to be taken into consideration (Fig. 1.) (Ref. 4.). The heat source heats the material which heat causes thermal processes in the material with mechanical response and changes the microstructures. The first publications were about FEM modelling of stress and strain distribution of welded joints and many articles can be found about this question for example (Ref. 4.), (Ref. 5.). At the time of first FEM modelling the software and computer possibility were limited, but in our time many FEM Software exist and they are applicable in the field of welding successfully (for example, ANSYS, SYSWELD, MARC). In the following new results of FEM modelling will be shown, these acts of modelling were carried out at the Department of Materials Science and Engineering of Technical University of Budapest (new name is Budapest University of Technology and Economics). MODELLING OF PRESSURE WELDING PROCESSES Pressure welding processes are generally fully mechanized processes, so the human factors do not effect in the case of these processes. It means that, the Computer Aided Process Planning (CAPP) is well applicable for these processes. CAPP requires probable equations based on physical laws. The FEM modelling can give the basis of CAPP. The FEM modelling of pressure welding processes helps understanding physics of them as well. The simplest process is the cold pressure welding in the modelling. Cold Pressure Welding The cold pressure welding applies only force for jointing elements. When we use this welding process means we use a cold forming process. Cold forming processes are widely modelled by FEM. There are many software for FEM modelling of forming, for example MSC MARC AUTOFORGE 3.1. We used this software for modelling of cold pressure but welding as well. The plastic deformation processes and the condition of formation of welding joint can also be investigated by this modelling. The result of geometrical modelling of cold pressure welding of aluminium rods is given in the Fig. 2. The computer with a high speed can calculate the main welding parameters as pressure force and strain. This modelling should be become a theoretical basis of investigation of welded joint formation. If we can give a mathematical equation for conditions of welding joint formation then it will be a nice new technical result. We are working on this question nowadays. If we heat the materials during welding then the processes are more complicated, but we could model resistance welding processes by FEM. Another area of welding processes modelling is the FEM modelling of resistance welding processes at our department. Resistance Upset Welding The FEM modelling of resistance upset welding was carried out by MARC/AutoForge 2.2.0 software. The detailed conditions of modelling and results can be seen in Ref. 6. The strain distribution is given in the Fig.3. and the pressure force requirement can be seen in the Fig.4. The results of modelling can be used for welding parameter planning. The practice proved that the results as pressure force and length change, were applicable, correct welding parameters. The FEM modelling of resistance upset welding as pressure forming procedure is a new approach of welding parameter modelling and it has international interest. Resistance Spot Welding The Ref. 7. modelled the resistance spot welding, the result of temperature distribution in the moment of current turned off can be seen in the Fig. 5. The results of Ref. 7. are in accordance with international literature, the used software was the ANSYS 5.0. The three-dimensional geometrical model can be seen in Fig. 6. |