Intelligent Virtual Design of Precision Forging Processes in Consideration of Microstructure Evolution

Transcription

1 Intelligent Virtual Design of Precision Forging Processes in Consideration of Microstructure Evolution Prof. Dr.-Ing. E. Doege, Dr.-Ing. J. Dittmann and Dipl.-Ing. C. Silbernagel Institute for Metal Forming and Metal Forming Machine Tools, University of Hanover, Hanover, Germany Prof. Dr.-Ing. E. Doege is head of the Institute for Metal Forming and Metal Forming Machine Tools (IFUM), University of Hanover, Welfengarten 1A, 3167 Hanover Dr.-Ing. J. Dittmann Project Manager BMW Group Munich Dipl.-Ing. C. Silbernagel is a research assistant of the IFUM Summary This paper represents the opportunity to establish the precision forging process as a suitable manufacturing method for helical gear wheels, using an innovative tool concept and an intelligent process design by FEM-simulation. Based on the simulation and implemented subroutines, this study shows a process design and optimisation as well as the prediction of the expected microstructure in the early stadium of process development. The represented precision forging process can be more than an alternative manufacturing method, especially in cost and time reducing respects, because of the use of lower-cost sheared billets, short cycle times and a reduced use of grinding operations. 1. Introduction Precision forging of gears is a near-net-shape technology, which is a distinguished manufacturing method characterised by high tolerance accuracy and end contour quality. The advantages of precision forging become especially obvious in high productivity. The most important criterion for the successful establishment of precision forging of gears is the economical aspect. This present paper shows the advantages of using an innovative tool system coupled with an intelligent virtual process design performed by the Finite Element Method (FEM) simulation. 2. An innovative tool concept and a new strategy of precision forging processes

2 Nowadays gears for passenger cars are manufactured with less exceptions by machining. The process of precision forging of gears represents more than an alternative, if the existing problems of still high die wear can be solved. At the institute (IFUM) intensive investigations were conducted to forge complex part geometry within an single-stage precision forging process in the last few years [1, 2]. The extreme process parameters, for instance the high pressure (12MPa), high work piece temperatures (125 C) as well as the small cycle times, are responsible for significant die wear. In particular at the end of the precision forging process the thermal and the mechanical loading rise up to the maximum, because of the closed dies and high process temperatures. For this reason on this point the tribological loading and die wear is reached the maximum. A promising strategy is to reduce the wear to a minimum especially at the final stadium of the forging process, where the teeth of the die are filled. Therefore the whole forging process is subdivided in three forging stages. To this an innovative tool concept was developed (figure 1). During the forging stage 1 (preforming) an axial material flow takes place. Due to the next stage the final top and bottom contours of the gear will be realised by an axial and radial material flow. As a result of the radial material flow during the last forging stage 3 the helical teeth of the gear will be shaped. A further advantage of the new tool system in respect of cost-saving is the possibility to use lower-cost sheared billets. Whereas in the case of the conventional one-stage precision forging processes only expensive precision tubes have been used. armouring mandril closing top bottom die gear die ejector Figure 1: Three-stage tool system of the precision forging process for gears A virtual process design should be performed by FEM-simulation, based on these requirements of the process. 3. Precision forging process design using FEM-simulation FEM-simulation (Finite Elements Method) has established for the last few years as an effective tool for design and planing in the field of metal forming. In particular for the design of complex multiple-stage forging processes the use of FEM-simulation is indispensable. Therefore the focal points in this present study are: Simulation of planned 3 stage precision forging process Parameter variation and optimisation of the tool geometry

3 Prediction of material flow, tool loading, material state etc. Simulation of the heating process and cooling process to predict the relevant microstructure evolution (austenizing, phase transformation) For forging processes and especially for precision forging processes a series of required working steps compose a complex process chain. Figure 2 shows the structure of the manufacturing chain of precision forged helical gear. Such a complex process chain starts usually with the heating processes (convective or inductive heating), than follow the multiple-stage forging process, the heat-treatment and some different grinding operations. The heating, forging and the cooling (heat-treatment) processes are considered in the FEM-simulation. The material properties are mostly defined within the first three working steps (heating, forging, cooling). The relevant microstructure mechanisms during these processes were described by user subroutines, which are based on macromechanical and empirical approaches. These subroutines are implemented in the used commercial FE-codes AUTOFORGE, ABAQUS and FORGE 3. The validation of the subroutines is published in [3, 4]. Figure 2: Manufacturing chain of precision forged helical gear wheel 3.1 Origin conceptual designed precision forging process Initial conditions for all FEM-simulations, in particular for the optimisation of the forging process, were the planned geometry of the 3 forging stages (figure 3) and a maximum of forming force up to 8-1 tons.

4 forging stage 1 forging stage 2 forging stage 3 forged gear upper die lower die work piece Figure 3: Planned geometry of the 3 forging stages of the precision forging process 3.2 Optimised precision forging process With the aim to develop an optimised multistage precision forging process some contours of tool geometry were varied. The variations of tool geometry performed by the FEMsimulations show an improved material flow and a lower needed force level for the forging stages. The optimum of the different tool geometry of the first stage (figure 4) is variation 3. variation 1a, b variation 2 variation 3 a b z a < z b z a z b Figure 4: Variations of tool geometry of the first forging stage Using the variation 1 b wrinkles has been predicted by the numerical simulation (figure 5). In fact this variation (1 b) was not considered for further investigations. The best results for this are performed by the variation 3.

5 . way of upper die in mm end of stage Figure 5: Prediction of wrinkles as a result of tool variation 1 b In order to design a multiple-stage precision forging process, where the needed forming force is below the allowed force limit of 8-1 tons, the forming force for the different tool variations during stage 1 and their effects for the further forging stages were determined. Figure 6 represents the needed forming force depending on the needed forming distance of the upper die. A comparison between the optimum tool variant 3 and the planned origin variant resulted in a significantly lower needed maximum of force up to 32%. Consequently the tool loading are lower in the upper and lower dies than in the case of the planned origin variant. At the forging stages 2 and 3 appear contours which are close by those of final predetermined of the component. For this reason the tool geometry was modified only in the first forging stage. forging stage 2 forging stage 3 1,2 8 8 legend 3 3 1, 3 2 origin,8 6 6 variation 1 1 1,6 4 4 variation 2 2, variation 3,2 1 MN = 1 t distance in % in % distance in %. Figure 6: Computed forming force versus the forming distance of the upper dies force in MN forging stage 1 force in MN An optimised material flow from the slug to the gear was predicted by the FEMsimulation for the case of tool variation 3 and here is only shown for the stage of teeth shaping in figure 7. The optimised tool geometry is responsible for a more homogeneous material flow and die filling during the forming stage 2. Most of the material was pushed force in MN

6 out of the piercing area during the forming stage 1. The final shape of upper and lower side of the gear was formed already after completion of the forging stage 2. For this reason the forging stage 2 is already a kind of precision forging process. The teeth of the gear start to develop not until 9% of the distance of upper die 3 is reached (figure 7). Furthermore the material thickness of the piercing area was reduced to 3 mm. At the end of the forging stage 3 the material of the workpiece (steel ) completely fill the die. FE-code: Forge 3 distance of upper die III: z 3 = 2.5 mm z 3 y z x z 3 = % z 3 = 15 % z 3 = 1 % z 3 = 3 % z 3 z 3 z 3 = 95 % z 3 = 4 % z z 3 =55 % 3 = 9 % z 3 = 7% Figure 7: Optimised material flow as a result of tool variation 3 of the final forging stage 3 The results of the FEM-simulations, which are represented in this present study, in respect of: an unwrinkled multiple-stage precision forging process, the dimensional accuracy of the final part shape, the needed forming force for the different tool geometry and the forging stages and the material flow for the different tool geometry and the forging stages, are the main aspects for a virtual precision forging process design and the process optimisation. The investigations and explanations mentioned before resulted finally in the following optimised sequence of forging steps (figure 8).

7 slug forging stage 1 forging stage 2 forging stage 3 massive bars scale removing & forming of a defined outside diameter preforming gear tooth forming Figure 8: Optimised sequence of forging stages Further positive effects of the optimised forging process are significantly lower die loading in particular for the forging stage 1 and 2. The reasons are the more homogeneous material flow and die filling due to the tool variation 3. For tool steel (hotwork steel ) of the FEM-model a pure elastic material description was assumed. A maximum of loading capacity of tool material using quenching and temper treatment could be realised up to 25 MPa. In the case of the origin conceptual designed tool geometry FEM-analyses considering meshed tools resulted in a maximum mechanical die loading of 3291 MPa of the lower die of stage 2. Wide areas of the dies have a stress state above the limit of yield stress. Plastic deformation would be the result. The simulation showed a significant improved situation of mechanical tool loading in the case of the optimised tool variant 3 of the stage 1. The maximum of the mechanical load (166 MPa) is on the half of the value of origin tool variant. 4. Microstructure evolution during multistage precision forging process In addition to the virtual process design the prediction of the microstructure during the process chain (figure 2 - considered processes in the FEM) and therewith the material properties performed by suitable FE-codes respectively subroutines have been more and more used in the last few years. Figure 9 shows the needed material mechanisms with the relevant mathematical, physical and empirical approaches for the numerical description and prediction of the microstructure evolution. The following exposition of this chapter includes the most relevant studies performed by other researchers in the field numerical description of thermo-mechanical analysis in consideration of microstructure changes during forming. During the first working step of the process chain (heating process) a phase transformation from Fe-α crystal lattice into the Fe-γ crystal lattice takes place. Depending on the heating parameter different states of austenizing and grain sizes could be developed. The state of the austenizing structure could be described by FEMsimulation with the aid of the implemented time temperature austenizing diagram within a subroutine. The numerical description of the grain growth during the heating and the further relevant working steps, transport steps and holding is based on the equations (3), where D is the grain size, t the time, Q KW the starting energy, R the general gas coefficient and T the temperature [5].

8 The phase transformations from the austenizing phase (Fe-γ) into the different structure constituents, depending on the current process and cooling conditions, can be described by the equations (1) and (2) [6]. In equation (1) ζ i is the volume fraction of the growing phase (ferrite, perlite, bainite) and t the time. The factor k represents the velocity of migration of the interface and certain time independent values to describe the nucleation. The factor n represents the kind of structural constituent growth. To consider the material dependent factors of a phase transformation within a FEM-simulation, it is necessary to identify the coefficients k and n in the equation (1) by experimental investigations. Results of these measurements performed using a dilatometer are continuous and isothermal TTT-diagrams. In equation (2) ζ M is the volume fraction of martensite, M S the martensite-start-temperature, T the occuring temperature and k and κ coefficients [7]. Hougardy [8] verified this formula and determined values for the coefficients k =.26 and κ =.93 for carbon-steels. These equations describe the phase growth of one structure constituent. The starting and endpoint of the phase growth are based upon the measurement data from the time temperature transformation diagram of the current steel alloy. Due to the forging process, in particular for fleshless precision forging processes, the material flow results in a typical continuous texture or fiber orientation. Main indicator for the texture are the flow lines or marking grids, which are described by the equation r (4) [9]. In equation (4) x( t + t) is a coordinate of a mesh point at the end of a time r r increment and u( ( t)) is the displacement vector. final part with desired material properties needed material mechanisms and processes for prediction accuracy of dimension and shape structural constituent volume fraction distribution of structural constituent grain size evolution degree of austenitizing fiber orientation (texture) mathematical and physical background FE-code / process design diffusion phase transformation: diffusionless phase transformation: ζ i, M D n D as a function of (x,y,z) Q kw n ( ) RT = A( t t ) e TTA-digrams r r r r x ( t + t) = x ( t) + u ( ( t)) Figure 9: Theoretical description of relevant microstructure mechanisms 4.1 FEM-simulation of the heating process Inductive heating processes are favorably applied for precision forging processes. Consequently the heating time is closed to the cycle time of the forging process. The (1) (2) (3) (4)

9 general FEM-model, the electric power profile (input parameter) as well as a result of the thermal computation are shown in figure 1. coils ceramics tube slug electrical power in kw temperature in C electric power profile 6 time in sec. time temperature profile surface C ceramics 3 plate core 6 time in sec. 18 Figure 1: Electric-magnetic-thermal coupled FEM-simulation On the basis of the assumed electric power profile as input parameter for the heating process a homogeneous temperature distribution in the small range of 125 C and 121 C was computed. Based on these thermal computations the effects on the state of austenizing and the grain growth were simulated (figure 11). temperature in C C / sec. 11 ASTM α +γ + Fe 3 C 8 α + Fe 3 C inductive heating rate convective heating rate 7 C /sec. 1.4 C /sec. material: carbon steel A C3 A C1 induction heating 7, ASTM 1 (>1µm) ASTM 4 time in sec. ASTM 9 (>15µm) (grain size 81µm) time temperature austenitizing diagram austenizing state austenizing state 18 convection heating Figure 11: Simulation of state of austenizing and grain size due to the heating processes

10 The simulations of the inductive and convective heating processes show a significantly different result for the state of austenizing. The heated slugs have a pretty homogeneous microstructure. Due to the longer heating duration with an average heating rate of 1.4 C/sec. (convective heating) a stronger grain growth (ASTM 4) took place. In the case of the inductive heating a grain size was computed to ASTM 9 in the most areas of the cylinder and ASTM 1 for a small zone of the slug. Regarding the small process times of the forging (.31 seconds) and the quenching step (some seconds), the final grain size and therefore the properties of strength are mostly defined due to the heating process. For this reason the inductive heated part has a more advantageous material behaviour. 4.2 FEM-simulation of the forging processes One characteristic result due to the precision forging process is the continuous texture in particular near the surface of the forged part. Figure 12: Start formations of the flow surfaces as indicator for the texture The schematic formations of the flow surfaces at the beginning of the forging simulation are represented in figure 12. The changes of the surface positions within the FE-grid during the forging stages are shown in the figures 13 and 14. horizontal flow markers slug end of stage 1 end of stage 2 end of stage 3 Figure 13: Simulated horizontal fiber orientation (texture) at the forging stages

11 vertical flow markers different fibers slug end of stage 1 end of stage 2 end of stage 3 Figure 14: Simulated vertical fiber orientation (texture) at the forging stages The compressed flow surfaces near the outer surface of the gear at the end of the forging process are very remarkable and typical for precision forging processes. This characteristic texture is the reason for the favorably material properties in particular in respect of the high fatigue strengths. In addition to this, state variables like temperature, deformation degree and equivalent stress characterise the forging process. Hence it is possible to design and control a safe precision forging process within the practicable process limits. As one example for the state variables the figure 15 shows the computed temperature distribution at the three forging stages. Figure 15: Computed temperature distribution after the forging stages 1, 2 and FEM-simulation of the forging integrated cooling process Due to the high current contact pressure and pressure dwell during the forging step the workpiece temperature sinks in the contact area until to 77 C. The time temperature transformation behaviour of the work piece depends on the process conditions of the forging and the cooling step [9]. The controlled cooling process takes place direct from forging temperature. It depends on the transport time, how strong a temperature equilibrium can occur. But a significant thermal gradient still exist before the cooling process occurs. The consequence of these aspects is a different microstructure evolution compared to single cooling processes without any precedent material history. Figure 16 represents the computed results of the phase transformation from Fe-γ into Fe-α of the integrated quenching process direct after the last forging stage.

12 Figure 16: FEM-simulation of the phase transformation process due to a quenching process This computed microstructure (figure 16) in particular the high rate of martensite near the outer surface of the teeth is very suitable for the use of the gear in practice, because of the high resistance to wear of the martensite structure. 5. Acknowledgement This present work has been developed within the project Do 19/161-3 of the German Research Group "Werkstoffbezogene numerische Simulation thermischer Prozesse in der Produktionstechnik" and the project Do 19/132-1 supported by the Deutsche Forschungsgemeinschaft. 6. References [1] Doege, E. et al. 1999, Precision Forging of helical Gear Wheel, Manufacturing, Heat-Treatment and Test, Final Report Do-19/92, University of Hanover, TP IV [2] Bohnsack, R., 1999, Investigations of precision forging of running gears, Dissertation, University of Hanover [3] Doege, E.; Dittmann, J., Neumaier, T., [4] Doege, E.; Dittmann, J.; Neumaier, T. 1999, FEM-Simulation of Phase Transformations for Steel in Metal Forming with integrated Heat-Treatment. Euromat 99, Munich, September of 1999, Proceeding, Vol. 7: FEM-Simulation of Phase Transformations for Steel in Metal Forming with integrated Heat-treatment. Advanced Engineering Materials 2, 2, No.7: [5] Sellars, C.M. The physical metallurgy of hot working. Hot working and forming processes, Eds. C.M. Sellars and G.J. Davies, Met. Soc. London, 198, 3-15 [6] Avrami, M. Kinetics of phase change. Journal of Chemical Physics, Volume 8, 194, [7] Koistinen, D.P.; Marburger, R.E. A general equation prescribing the extend of the austenitemartensite transformation and temperature evolution during quenching of steels Acta Metallurgica 195 No 7 pp 59 6

13 [8] Hougardy, H.P.; Yamazaki, R. quenching of steels, Acta Metallurgica, 195, No. 7, pp An improved calculation of the transformation of steels, Steel Research, 1986, No. 57, pp [9] Neubauer, I. Numerische Untersuchungen zur Auslegung von Präzisionsschmiedeprozessen am Beispiel schrägverzahnter Stirnräder. Dissertation, University of Hanover, 22 [1] Doege, E.; Dittmann, J. 21, Numerical Simulation of Microstructure Evolution During Forging Processes. 7 th International Conference on Production Engineering Design and Control, PEDAC 21, Alexandria, 13 th - 15 th February of 21, Proceeding, Vol. III: