Quality of Simulation Packages for Flashless Hot Forging Operations

Transcription

1 363 Simulation of Materials Processing: Theory, Methods and Applications, Mori (ed.) 2001 Swets & Zeitlinger; Lisse, ISBN Quality of Simulation Packages for Flashless Hot Forging Operations Hans Christoph Altmann Institute for Integrated Production Hanover, Ltd, Hanover, Germany Wim J. Slagter MSC.Software (E.D.C.) B.V., Gouda, The Netherlands ABSTRACT: The development of new forming processes in the field of hot forging technology includes many difficulties due to the large number of parameters constituting the process. By developing a process the design engineer has to consider both technical and economical limits in order to obtain competitive forgings. Preferred process parameters include small number of forging steps, less tool abrasion, less contribution of flash material, and the stability of the forming process with the minimum of rejects. Experimental testing is one method of forging process development. It shows directly to what extent the new process meets the requirements of the market. Experimental testing usually needs much time and money, especially during development of new processes. Time and costs of developing a forging process can be reduced with the help of forging simulation packages. For this aim the forging practice has to be taken into account. Most important for meaningful forging simulation is the determination of correct process parameters. In addition a check and a compensation of the data base after the comparison between experiments and the computation of the developed process is necessary. The existence of a systematic process parameter data bank for special kinds of forming process in combination with forging specific simulation software lifts the value of the simulations, and more important, the process development speed. The IPH - Institute for Integrated Production Hanover develops hot flashless precision forging technology based on near-net shape technology. In the past it succeeded to form connecting rods in both warm and hot temperature range. Today this process is applied to other products as well. For information transfer a digital camera, a camera for heat radiation, wire strain gauges, a 3D measuring machine and a thermometer have been used. Several precision forging processes have been simulated with two commercial software packages: MSC.SuperForge and MSC.AutoForge. The handling of the machines and programs as well as simulation results are described. 1 INTRODUCTION The Institute for Integrated Production Hanover (IPH) advises companies and realizes development projects. Most important fields are the production engineering, the logistics, and the information systems. One focal point of business in the division of production engineering is forming processes. Development projects deals with the problems of precision forging in theory and practice test. The realization of the precision forging process of connecting rods is an important milestone in the history of the institute. Precision forging is a near-net shape way of forging without a flash. It reduces the number of single forming-operations. Other subjects of the Institute for Integrated Production are the forming with multidirectional punch movement and the forming with segmented dies. The development of new precision forging processes focuses on complex geometries that are suited to be used by industrial partners. For developing new forging processes the IPH uses numerical calculation codes. These codes make it possible to vary a relative great number of process parameters in an easy way. This method is faster and cheaper than constructing, manufacturing, mounting, and testing of tools during all periods of development. In the beginning when the initial tool geometry has to be found, the only way is to conduct different tests. During the second half of development the optimization can effectively be realized by making use of simulation. Here many small modifications can virtually be tried out. Theoretic examinations result in process knowledge which never would appear in such an evident form.

2 364 lay in the workpiece close the tools forming with the dies table small hole Figure 1. Principle character of the hot precision forging shaft big hole lower punch upper punch In the workshop such investigations are usually expensive and time-consuming. The IPH makes use of both MSC.AutoForge and MSC.SuperForge because of its complementary strengths. MSC.AutoForge is very suited for the prediction of tool temperature, as well as elastic and thermal tool deformation. MSC.SuperForge is suited for complex 3-D forging simulation in which high resolution of material flow details is desired with reasonable CPU processing times. During the precision forging development the IPH needs both complementary qualities. This paper shows examples. plates coupled with four bearings and four springs. The springs connecting both plates are pre-stretched. The outer plate is the main plate and fixed at the forming machine. The inner plate is the second one and carries the impression. Punches are fixed on the inner side of the outer plate. They go through holes in the inner plate and they end in the inside of the impression. All punches on the same side are fixed on one main plate. 2 FLASHLESS HOT FORGING 2.1 The Forging Process Flashless hot forging is the deformation of metal above the temperature zone of warm forming in closed dies towards near-net shape or net shape geometries. The flashless process is reached with the separation of the outer geometry tools from the forming tools. The outer dies determine the outer geometry. The inner punches provide the forming energy. First of all, the dies for the geometry are closed around the work piece without any deformation. Secondly, the punches get into the impression and cause the forming process up to the complete filling of the impression. Figure 1 shows the sequence. 2.2 The Precision Forging Tool As shown in figure 2 typical precision forging tools consist of two groups of parts: the upper group and the lower group. Each group consists of two parallel Figure 2. Typical precision forging tool for near net shape forging with horizontal and vertical gaps. 2.3 Moving Characteristics of the Different Tools The Tool is used with a screw press. During the first phase of moving the upper tools, die and punches move down with the same speed. Then the upper die gets in contact with the lower die. The impression is closed and the moving speed and the balance of forces changes. When the upper and lower die have contact with each other the pre-stretched springs are compressed and hold the dies closed. During this phase of process the machine force will increase

3 365 rapidly. The dies move down half the speed of the upper punches. And the upper and lower punches move into the impression, get in contact with the work piece and cause the deformation of the work piece. During the compression of springs and the deformation of work piece the speed of the tools decreases because of loss of kinetic energy. After the reducing of the speed to zero the lowest position of the tools is reached. After that point of time the springs and the elasticity of the machine cause lifting of the tools in opposite order as described before. 2.4 Different Types of Gaps The typical grouping of the different tools causes horizontal and vertical gaps. Vertical gaps develop between the different tools coming from the same direction. Horizontal gaps develop between the different dies coming from different directions. Every gap contains the danger of material flowing in it, which makes the forging useless. Horizontal gaps are closed and opened during the precision forging process. The danger of material in the gap is influenced by the time of closing the gap and the force of holding the gap closed. If closing of the gap occurs too late, the material will have reached the critical area. If the holding force is too small, the gap will be opened again during the forming operation. A typical example is shown below in figure 3. Simulation of Materials Processing: Theory, Methods and Applications, Mori (ed.) 2001 Swets & Zeitlinger; Lisse, ISBN The precision forging process of the connecting rod consists of several single forming processes. First a metal cylinder is cut. Secondly, this cylinder is warmed. Thirdly, the hot cylinder is cross rolled to an axi-symmetric geometry. Furthermore, this geometry is preformed and finally it is precision forged. The latter is shown in figure 4. Figure 3. Connecting rod with a forging mistake because of problems with the horizontal gap Vertical gaps occur between the different tools working from the same side. These gaps are never closed. The width of the gap differs with temperature and the elastic deformation of the material under forging loads. 3 THE CONNECTING ROD The connecting rod geometry is suited for the precision forging process. The geometry consists of several convex bodies and three concave areas. The concave areas are suited for the using of punches to push the work piece material into the convex zones. Figure 4. Connecting rod with a forging mistake because of problems with the horizontal gap 4 THE SIMULATION OF HOT FLASHLESS FORGING 4.1 Characteristics of Flashless Hot Forging As previously mentioned, the simulation focuses on the last precision forging process. Most qualities of the precision forged connecting rod occur during the last step and depend on the filling of the impression as well as the possible existence of folds.

4 4.2 Forging Temperature During the warming process the work piece temperature increases from 20 C up to 1200 C. This warming process takes place in a furnace. The optimum result is a homogeneous temperature of the complete work piece. In reality this state cannot be reached. First the temperature of the metal increase with high speed. At the end the warming speed decreases as it is shown in figure 5. T [ C] Time [s] Figure 5. Temperature increases during the warming During and after the warming the temperature is inhomogeneous as shown in figure 7. The reason is the non-uniform thickness of the work piece. This inhomogeneous temperature never disappears because of the asymptotic character of the heat-exchange. The thermo-photograph (figure 6) of the connecting rod shows the temperature field. To synchronize the measured and the calculated result (figure 7.) the heat transfer coefficient used in the simulation has to be optimized. 1550,7 C Figure 7. Calculated temperature distribution after heating 4.3 Punch Forces The punch forces document the momentary balance of the forces inside the impression. The magnitude of these forces determines the size of both tools and machines, as well as the extent of tool wear. The correct prediction of these forces decides the success of a tool construction. Using the described method, a good agreement in punch force is obtained (figure 8). The experimental force is sensitive to the change of initial temperature, and the accuracy is considered to be adequate for precise tool construction. Force of the Die [kn] Forging (1150 C) Forging (850 C) SuperForge (980 C) Forces in Comparison 0 0,00 0,03 0,05 0,08 0,10 0,13 Time of process [s] Figure 8. Calculated and measured force of the punches during the precision forging 60,2 Figure 6. Thermo-photograph of the heated material The correct temperature field at the beginning of the numerical calculation is an important condition for accurately predicting the range of yield stress. The second important connection is between yield stress and strain rate of the material. In both tests and calculations a screw press is used. The speed of this machine is determined by the mechanical energy that passes during the process to the work piece. The balance of the forces reduces the speed of the screw press in accordance with the yield stress and the momentary work piece geometry. Figure 9. Sectional view of the calculated and measured geometry of the connecting rod during the process Figure 9 shows the excellent agreement between the calculated and measured contours near the big eye of the connecting rod. 366

5 Simulation of Materials Processing: Theory, Methods and Applications, Mori (ed.) 2001 Swets & Zeitlinger; Lisse, ISBN TOOL GAPS 5.1 Vertical Gaps The vertical width of the gap between the die and punch does not depend on the springs or the toolkinematics. The width of the vertical gap depends on the construction of the tools, the elastic deformation, and the thermal expansion. To discover the relation between the width of the gap and material flow into that gap a special experiment is necessary. Figure 10 shows the finite element calculation of the test geometry. Ten cycles of forging are shown in the result. Because of the heating during the period of contact between work piece and tool the temperature increases and the material expands. During the time between two cycles the tool is cooled by water and the temperature decreases again. The material contracts again. In figure 11 it is shown that the thermal effect behaves asymptotically. After the first ten cycles of warming, the process appears to be quasi-static. The maximum change of gap width due to this thermal effect is r = 0.12mm. 5.3 Elasticity Caused Changes The precision forging causes relatively high pressure in the impression. This pressure may result in large strain and deformation in the tools. The deformation can also influence the width of the gap between the punch and the die. To reproduce the elastic deformation of the tools during the forging process the tool are generated in the simulation model, and the deformation is calculated. Figure 10. Gap experiment and numerical calculation 5.2 Thermally Caused Changes In this model the thermal expansion of the tool is shown. Figure 11 shows the changes of the width of the gap. Figure 12. Elasticity caused change of the gap width During the forming operation there is pressure inside the die resulting in an increase of diameter. The punch is pushed down and because of that loading the diameter of the punch also increases. Figure 12 shows the increasing diameter of both die and punch. The green-colored line is for the die and the red one is for the punch. The diameter of the die increases more than the diameter of the punch. The gap width changes with r = 0.02mm. Figure 11. Thermally caused change of the gap width

6 Work Piece Contact with the Gap At the end of the process the work piece should completely fill the impression. The forging material should have contact with the tool gap only at the end of the precision forging process. Figure 13. Contact at the gap at the end of the process Otherwise there is the danger of material moving in the gap. Figure 13 shows the material in the experimental tools reaching the gap just at the end of the process. 6 FUTURE TRENDS 6.1 Crankshaft The new knowledge and the more precise parameters for the precision forging process are suited for development of the precision forging of more complex geometries (figure 14). The most important issues during the development of the precision forging of crankshafts are the gap, the possible existence of folds, the contact pressure and the extent of filling of the impression. During design of geometrically complicated precision forging processes it is most important to compare the mechanical and thermal parameters of the numerical model with reality. This paper describes some measurements and tests required. These investment in time and energy is considered to be valuable. More precise and optimized results are now feasible. Particular aspects like detailed die loads and material flow appear to be well predictable in the simulation. The further technical development of precision forging processes for more complex geometries like the precision forging of crankshafts is modeled by making use of simulation packages. With the help of MSC.AutoForge and MSC.SuperForge and the described procedure for fixing process parameters, it is possible to obtain the most important design information before the manufacturing of tools takes place. 8 REFERENCES 1. Slagter W, Florie C, Venis A; Advances in Three-Dimensional Forging Process Modelling. Proceedings of the 15th National Conference on Manufacturing Research, pp73-78, 1999, UK. 2. Lange K; Umformtechnik, Handbuch für Industrie und Wissenschaft, Band 2: Massivumformung, pp42, 563,1988, D. 3. Lange K; Umformtechnik, Handbuch für Industrie und Wissenschaft, Band 1: Gundlagen, pp98ff.,1984, D. 4. Schmidt B. C., Fluess A., Kohlstette J.; Gratloses Präzisionsschmieden von Langteilen, Umformtechnik, Nr.3, 2000, D. 5. Muessig B., Boromandi F., Numerische Simulation optimiert Werkzeuge und Fertigungsprozesse, Maschinenmarkt, Das IndustrieMagazin, Nr. 41, pp 43-46, 2000, D. Figure 14. Precision forging of crankshafts 7 CONCLUSIONS