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Improved tool design for fine blanking through the application of numerical modeling techniques
Journal of Materials Processing Technology 115 (2001) 70±75
Improved tool design for ®ne blanking through the application of numerical modeling techniques F. Klocke*, K. Sweeney, H.-W. Raedt Laboratory of Machine Tools and Production Engineering, Aachen University of Technology, Steinbachstraûe. 53, D-52074 Aachen, Germany
Abstract The economic and ef®cient design of tools for metal-forming processes is increasingly being supported by the application of modern techniques such as the ®nite element method (FEM). This is especially the case for bulk forming and deep drawing operations, where existing codes have proven to be quite valuable for the prediction of important process parameters, including punch force, tool stress distribution as well as the stress and strain distributions within the workpiece itself. In the case of blanking operations, where a true material separation takes place, standard codes are limited. However, with recent improvements in FEM codes, it is possible to make a prediction as to the initiation of cracks and thus part quality. In this paper, the results of investigations of both blanking and ®ne-blanking processes with the help of FEM are presented. This includes the identi®cation and elimination of critical locations of high tool stress, the determination of tribological conditions such as normal pressure, relative velocity between tool and part and temperature effects, and the prediction of crack initiation using various rupture criteria. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Fine blanking; FEM; Tribology; Tool design
1. Introduction The use of ®ne-blanking technology offers the ability to produce parts with close tolerances and a very high quality surface by eliminating any rough break in the part's sheared surface. These advantages over normal blanking are possible due to a very special tool design which differentiates itself from that of normal blanking in three ways [1,2]: (1) the clearance between punch and die plate is ca. 1% of the sheet thickness (on the diameter), in conventional tooling the clearance is ca. 5±10%; (2) the vee-ring, which follows the outer periphery of the part form on the blank holder, the use of which helps to prevent fracture in the part; (3) the use of a counter punch. These three aspects create a compressive state of stress in the shear zone which increases the formability of the material. This allows for a higher part quality which includes a very smooth sheared surface. The improved control of material ¯ow leads to close tolerances [3]. The extra tooling and machine costs that often accompany this relatively complicated tool design prohibit the economic use of ®ne blanking for a number of applications.
*
Corresponding author. Tel.: 49-241-807-401; fax: 49-241-8888-293. E-mail address: [email protected] (F. Klocke).
The economic and ef®cient design of tools is therefore even more critical for ®ne blanking. However, with recent improvements in the FEM codes and computer performance, the application of modern simulation techniques can provide a company with a signi®cant competitive advantage, giving important insights into the effects of tool design and process parameters, especially on the appearance of cracks or rupture zones. 2. Numerical modeling of the fine-blanking process In this paper, the results of investigations of both blanking and ®ne-blanking processes with the help of FEM are presented. This includes the identi®cation and elimination of critical locations of high tool stress, the determination of tribological conditions such as normal pressure, relative velocity between tool and part and temperature effects, and the prediction of crack initiation using various rupture criteria. 2.1. Simulation model The modeling of the ®ne-blanking process is made much easier if a few simpli®cations can be made. The most signi®cant being the ability to model the part as
0924-0136/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 0 7 7 1 - 3
F. Klocke et al. / Journal of Materials Processing Technology 115 (2001) 70±75
71
Fig. 1. FE-model for the simulation of the fine-blanking process.
axisymmetric. The ®ne-blanking part in discussion is a disk with a diameter of 21 mm and a height of 7.5 mm. In this case, only half of the part must be modeled, with the nodes of the workpiece on its axis of symmetry so de®ned that they have no movement in the x-direction. In Fig. 1, one such model is shown. One can see the concentration of elements in the area where the shear zone will build up. 3. Investigation into tool life
main punch. The material of the blank was de®ned to be 42 CrMo 4, with an initial yield stress of 840 N/mm2 and a tensile stress of 1110 N/mm2 [4]. In this case, the maximum equivalent stress in the main punch was calculated to be 2324 N/mm2. The ®rst principal stress (s1) is shown in Fig. 2 as well. This is the main factor leading to ductile fracture in the blank. For the tool, these obtained values have to be compared to values as obtained in fatigue tests in order to evaluate the feasibility of the planned process. 3.2. Micro-tribological aspects of tool life
3.1. Stresses in the tool An important ®eld of application for process modeling is the prediction of stresses in tool components which could lead to premature tool failure through rupture or plastic ¯ow of the tool materials. It is possible to predict such stresses, as seen in Fig. 2. Here the distribution of equivalent stress and the ®rst principal stress (s1) are shown in the part and in the
Fine blanking, in comparison to normal blanking, is characterized by the use of additional tool components in order to induce a compressive stress state in the shear zone. Consequently, the tribological load on the ®ne-blanking tool is higher, since not only the blanking forces but also the additional forces result in contact pressure on the tools. Coatings on the tool elements, which are increasingly used,
Fig. 2. Prediction of the equivalent stresses in part and tool components.
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F. Klocke et al. / Journal of Materials Processing Technology 115 (2001) 70±75
help in reducing the tool wear. In addition, they can help to reduce the amount of additives in the lubricants, which can lead to a more environmentally friendly production. The above-mentioned simulation of the tool load (Section 3.1) is a ®rst step in examining of the coating performance on the tool. Deformation of the substrate, even in only small amounts, leads to coating failure. The coating (with its ceramic material properties) cannot follow the deformation of the tool. The formation of micro-cracks occurs, which is the ®rst phase of coating failure. Another reason for coating failure can be excessive tensile stresses within the coating [5]. With its ceramic material behavior, it is much more prone to failure through cycling tensile stress. Consequently, avoiding tensile stresses in the coating through improved tool design can increase coating life. FE-modeling can help in this process as well. Fig. 3 shows the simulation of the edge of the coated ®ne-blanking tool. The punch edge is modeled with elastic material properties. The coating on top of the tool is a separate object in the simulation and has elastic properties as well. The workpiece has plastic material properties. The initial yield strength was set to 300 N/mm2 with a tensile strength of 560 N/mm2. The simulation results show the in¯uence of the micro-design of the edge on the resulting stresses in the coating. The standard edge radius of 20 mm leads to a maximum tensile stress (s1) of 438 N/mm2 (Geometry 1). A bevel on the cylindrical area of the punch increases the maximum tensile stress to 792 N/ mm2 (Geometry 2). The tensile stress ®eld grows in size as well. In contrast, a bevel on the front face of the punch
succeeds in minimizing the size of the tensile stress ®eld (Geometry 3). The maximum value is calculated to be 461 N/mm2, but this is suspected to be a numerical error because it occurs on the corner of the coating object. 4. Investigation into workpiece geometry 4.1. Crack prediction The ®ne-blanking process can be modeled without the use of a fracture criterion, since the ``ideal'' sheared surface of a ®ne-blanked part does not have a rupture area, i.e. the material ®rst separates at the end of the process. This requires, however, a good understanding of the process and a knowledge of the parameters necessary for a quality part. When modeling a process where it is known that no rupture occurs, ®ne blanking can be considered a continuous metal-forming operation and thus be solved using the theory of plasticity and standard FE packages. However, for the simulation of processes where rupture is to be considered, new methods must be applied using mechanical fracture models where a material separation can be predicted. There are three possible methods to realize a fracture in an FE-mesh, including element splitting, element separation and element deletion. The algorithm used by the software package applied in this research can be seen in Fig. 4. There are a number of existing fracture or damage models which have various degrees of applicability to the
Fig. 3. The effect of punch edge geometry on tensile stresses in the tool coating.
F. Klocke et al. / Journal of Materials Processing Technology 115 (2001) 70±75
73
Fig. 4. Method of modeling the ductile fracture.
®ne-blanking process. In the case of normal blanking, a number of researchers have been developing and testing these different damage criteria. The FEM program used in this research has 10 such models built in, of which four are given as examples in Fig. 5. Each model produces a slightly different crack development and propagation in the part. These models were tested and the results were compared to experiments.
part quality was investigated. In Fig. 6, we see that the increase in die-clearance brings an increase in the forming zone and in the roll-over-height. In addition, with an increase in counter punch force comes a decrease in the roll-over-height. These relationships matched those seen in the experiments.
4.2. Influence of the die-clearance on the roll-over-height
The material ¯ow during ®ne blanking is strongly controlled by the tool. Nevertheless, friction has a strong in¯uence on the resulting geometry. As a common practice in industry, in cases when roll-over formation tends to result in a part that does not meet the geometric speci®cations, the front sides of the main punch and die plate are de-coated with a grinding operation. This leads to higher friction forces
The numerical analysis of the ®ne-blanking process can also give additional insights as to the part quality. A series of experiments were performed with various die/punch clearances ranging from 1.3 to 20.0% and the effect of this on the
4.3. Influence of friction on roll-over
Fig. 5. Selected damage models for crack prediction.
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F. Klocke et al. / Journal of Materials Processing Technology 115 (2001) 70±75
Fig. 6. Influence of the die-clearance on the roll-over-height.
Fig. 7. Examination of part geometry with varying friction properties.
which leads to a better part geometry, even though wear resistance on the tool fronts is obviously worse. Fig. 7 shows an example of how this is being examined with FEA. The friction properties between the blank and the tooling are varied in order to in¯uence material ¯ow. The resulting geometry clearly shows differences: the roll-over in the remaining blank does not signi®cantly depend on the friction coef®cient between the blank holder, the main punch and the blank. It is, however, a function of the blank holder force, as can be seen in Fig. 6. The roll-over in the blanked part (disk) changes much more: the roll-over-height increases from 0.29 to 0.47 mm, while its length increases from 1.73 to 2.42 mm.
thus reducing unnecessary tool load. Different combination of tool element forces, clearance, friction properties and blanking material can be modeled and allow for an optimization of blanking parameters with the computer. This can hardly be done in practical tests due to the high effort that would be involved because of numerous variations. Further development in simulation codes and related topics (e.g. availability of physical data as a basis for simulation) will lead to improved precision of simulation results and will render the FE-simulation an even more valuable tool for process design. Acknowledgements
5. Conclusion Finite element simulations offer a wide variety of possibilities for investigating details of the blanking and ®neblanking processes. This holds true for the investigation into the tool load and coating-related topics, which helps in solving problems concerning tool life. Workpiece geometry can be calculated. The formation of cracks, which are to be avoided in ®ne blanking, can be forecasted. Therefore, optimal counter punch and blank holder force can be found,
Part of the work described in this paper was funded by the ``Deutsche Forschungsgemeinschaft'' within the Collaborate Research Center 442 ``Environmentally Improved Tribosystems''. References [1] W. KoÈnig, F. Klocke, Fertigungsverfahren Band 5, Blechbearbeitung, VDI-Verlag, DuÈsseldorf, 1995.
F. Klocke et al. / Journal of Materials Processing Technology 115 (2001) 70±75 [2] C. Rentsch, Feinschneiden mit beschichteten Werkzeugen, Dissertation RWTH Aachen, VDI-Verlag, DuÈsseldorf, 1997. [3] F. Klocke, K. Sweeney, Crack prediction and prevention in the fineblanking process: FEM simulations and experimental results, in: Proceedings of the Sixth International Conference on Sheet Metal, University of Twente, April 6±8, 1998, pp. 215±222.
75
[4] E. Doege, H. Meyer-Nolkemper, I. Saeed, Flieûkurvenatlas Metallischer Werkstoffe, Hanser Verlag, MuÈnchen, Wien, 1986. [5] K.-D. Bouzakis, K. Efstathiou, N. Vidakis, D. Kallindikis, S. Angos, T. Leyendecker, G. Erkens, H.-G. Fuss, R. Wenke, Experimental and FEM analysis of the fatigue behaviour of PVD coatings on HSS substrate in milling, Ann. CIRP 47 (1) (1998).
Improved tool design for ®ne blanking through the application of numerical modeling techniques F. Klocke*, K. Sweeney, H.-W. Raedt Laboratory of Machine Tools and Production Engineering, Aachen University of Technology, Steinbachstraûe. 53, D-52074 Aachen, Germany
Abstract The economic and ef®cient design of tools for metal-forming processes is increasingly being supported by the application of modern techniques such as the ®nite element method (FEM). This is especially the case for bulk forming and deep drawing operations, where existing codes have proven to be quite valuable for the prediction of important process parameters, including punch force, tool stress distribution as well as the stress and strain distributions within the workpiece itself. In the case of blanking operations, where a true material separation takes place, standard codes are limited. However, with recent improvements in FEM codes, it is possible to make a prediction as to the initiation of cracks and thus part quality. In this paper, the results of investigations of both blanking and ®ne-blanking processes with the help of FEM are presented. This includes the identi®cation and elimination of critical locations of high tool stress, the determination of tribological conditions such as normal pressure, relative velocity between tool and part and temperature effects, and the prediction of crack initiation using various rupture criteria. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Fine blanking; FEM; Tribology; Tool design
1. Introduction The use of ®ne-blanking technology offers the ability to produce parts with close tolerances and a very high quality surface by eliminating any rough break in the part's sheared surface. These advantages over normal blanking are possible due to a very special tool design which differentiates itself from that of normal blanking in three ways [1,2]: (1) the clearance between punch and die plate is ca. 1% of the sheet thickness (on the diameter), in conventional tooling the clearance is ca. 5±10%; (2) the vee-ring, which follows the outer periphery of the part form on the blank holder, the use of which helps to prevent fracture in the part; (3) the use of a counter punch. These three aspects create a compressive state of stress in the shear zone which increases the formability of the material. This allows for a higher part quality which includes a very smooth sheared surface. The improved control of material ¯ow leads to close tolerances [3]. The extra tooling and machine costs that often accompany this relatively complicated tool design prohibit the economic use of ®ne blanking for a number of applications.
*
Corresponding author. Tel.: 49-241-807-401; fax: 49-241-8888-293. E-mail address: [email protected] (F. Klocke).
The economic and ef®cient design of tools is therefore even more critical for ®ne blanking. However, with recent improvements in the FEM codes and computer performance, the application of modern simulation techniques can provide a company with a signi®cant competitive advantage, giving important insights into the effects of tool design and process parameters, especially on the appearance of cracks or rupture zones. 2. Numerical modeling of the fine-blanking process In this paper, the results of investigations of both blanking and ®ne-blanking processes with the help of FEM are presented. This includes the identi®cation and elimination of critical locations of high tool stress, the determination of tribological conditions such as normal pressure, relative velocity between tool and part and temperature effects, and the prediction of crack initiation using various rupture criteria. 2.1. Simulation model The modeling of the ®ne-blanking process is made much easier if a few simpli®cations can be made. The most signi®cant being the ability to model the part as
0924-0136/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 0 7 7 1 - 3
F. Klocke et al. / Journal of Materials Processing Technology 115 (2001) 70±75
71
Fig. 1. FE-model for the simulation of the fine-blanking process.
axisymmetric. The ®ne-blanking part in discussion is a disk with a diameter of 21 mm and a height of 7.5 mm. In this case, only half of the part must be modeled, with the nodes of the workpiece on its axis of symmetry so de®ned that they have no movement in the x-direction. In Fig. 1, one such model is shown. One can see the concentration of elements in the area where the shear zone will build up. 3. Investigation into tool life
main punch. The material of the blank was de®ned to be 42 CrMo 4, with an initial yield stress of 840 N/mm2 and a tensile stress of 1110 N/mm2 [4]. In this case, the maximum equivalent stress in the main punch was calculated to be 2324 N/mm2. The ®rst principal stress (s1) is shown in Fig. 2 as well. This is the main factor leading to ductile fracture in the blank. For the tool, these obtained values have to be compared to values as obtained in fatigue tests in order to evaluate the feasibility of the planned process. 3.2. Micro-tribological aspects of tool life
3.1. Stresses in the tool An important ®eld of application for process modeling is the prediction of stresses in tool components which could lead to premature tool failure through rupture or plastic ¯ow of the tool materials. It is possible to predict such stresses, as seen in Fig. 2. Here the distribution of equivalent stress and the ®rst principal stress (s1) are shown in the part and in the
Fine blanking, in comparison to normal blanking, is characterized by the use of additional tool components in order to induce a compressive stress state in the shear zone. Consequently, the tribological load on the ®ne-blanking tool is higher, since not only the blanking forces but also the additional forces result in contact pressure on the tools. Coatings on the tool elements, which are increasingly used,
Fig. 2. Prediction of the equivalent stresses in part and tool components.
72
F. Klocke et al. / Journal of Materials Processing Technology 115 (2001) 70±75
help in reducing the tool wear. In addition, they can help to reduce the amount of additives in the lubricants, which can lead to a more environmentally friendly production. The above-mentioned simulation of the tool load (Section 3.1) is a ®rst step in examining of the coating performance on the tool. Deformation of the substrate, even in only small amounts, leads to coating failure. The coating (with its ceramic material properties) cannot follow the deformation of the tool. The formation of micro-cracks occurs, which is the ®rst phase of coating failure. Another reason for coating failure can be excessive tensile stresses within the coating [5]. With its ceramic material behavior, it is much more prone to failure through cycling tensile stress. Consequently, avoiding tensile stresses in the coating through improved tool design can increase coating life. FE-modeling can help in this process as well. Fig. 3 shows the simulation of the edge of the coated ®ne-blanking tool. The punch edge is modeled with elastic material properties. The coating on top of the tool is a separate object in the simulation and has elastic properties as well. The workpiece has plastic material properties. The initial yield strength was set to 300 N/mm2 with a tensile strength of 560 N/mm2. The simulation results show the in¯uence of the micro-design of the edge on the resulting stresses in the coating. The standard edge radius of 20 mm leads to a maximum tensile stress (s1) of 438 N/mm2 (Geometry 1). A bevel on the cylindrical area of the punch increases the maximum tensile stress to 792 N/ mm2 (Geometry 2). The tensile stress ®eld grows in size as well. In contrast, a bevel on the front face of the punch
succeeds in minimizing the size of the tensile stress ®eld (Geometry 3). The maximum value is calculated to be 461 N/mm2, but this is suspected to be a numerical error because it occurs on the corner of the coating object. 4. Investigation into workpiece geometry 4.1. Crack prediction The ®ne-blanking process can be modeled without the use of a fracture criterion, since the ``ideal'' sheared surface of a ®ne-blanked part does not have a rupture area, i.e. the material ®rst separates at the end of the process. This requires, however, a good understanding of the process and a knowledge of the parameters necessary for a quality part. When modeling a process where it is known that no rupture occurs, ®ne blanking can be considered a continuous metal-forming operation and thus be solved using the theory of plasticity and standard FE packages. However, for the simulation of processes where rupture is to be considered, new methods must be applied using mechanical fracture models where a material separation can be predicted. There are three possible methods to realize a fracture in an FE-mesh, including element splitting, element separation and element deletion. The algorithm used by the software package applied in this research can be seen in Fig. 4. There are a number of existing fracture or damage models which have various degrees of applicability to the
Fig. 3. The effect of punch edge geometry on tensile stresses in the tool coating.
F. Klocke et al. / Journal of Materials Processing Technology 115 (2001) 70±75
73
Fig. 4. Method of modeling the ductile fracture.
®ne-blanking process. In the case of normal blanking, a number of researchers have been developing and testing these different damage criteria. The FEM program used in this research has 10 such models built in, of which four are given as examples in Fig. 5. Each model produces a slightly different crack development and propagation in the part. These models were tested and the results were compared to experiments.
part quality was investigated. In Fig. 6, we see that the increase in die-clearance brings an increase in the forming zone and in the roll-over-height. In addition, with an increase in counter punch force comes a decrease in the roll-over-height. These relationships matched those seen in the experiments.
4.2. Influence of the die-clearance on the roll-over-height
The material ¯ow during ®ne blanking is strongly controlled by the tool. Nevertheless, friction has a strong in¯uence on the resulting geometry. As a common practice in industry, in cases when roll-over formation tends to result in a part that does not meet the geometric speci®cations, the front sides of the main punch and die plate are de-coated with a grinding operation. This leads to higher friction forces
The numerical analysis of the ®ne-blanking process can also give additional insights as to the part quality. A series of experiments were performed with various die/punch clearances ranging from 1.3 to 20.0% and the effect of this on the
4.3. Influence of friction on roll-over
Fig. 5. Selected damage models for crack prediction.
74
F. Klocke et al. / Journal of Materials Processing Technology 115 (2001) 70±75
Fig. 6. Influence of the die-clearance on the roll-over-height.
Fig. 7. Examination of part geometry with varying friction properties.
which leads to a better part geometry, even though wear resistance on the tool fronts is obviously worse. Fig. 7 shows an example of how this is being examined with FEA. The friction properties between the blank and the tooling are varied in order to in¯uence material ¯ow. The resulting geometry clearly shows differences: the roll-over in the remaining blank does not signi®cantly depend on the friction coef®cient between the blank holder, the main punch and the blank. It is, however, a function of the blank holder force, as can be seen in Fig. 6. The roll-over in the blanked part (disk) changes much more: the roll-over-height increases from 0.29 to 0.47 mm, while its length increases from 1.73 to 2.42 mm.
thus reducing unnecessary tool load. Different combination of tool element forces, clearance, friction properties and blanking material can be modeled and allow for an optimization of blanking parameters with the computer. This can hardly be done in practical tests due to the high effort that would be involved because of numerous variations. Further development in simulation codes and related topics (e.g. availability of physical data as a basis for simulation) will lead to improved precision of simulation results and will render the FE-simulation an even more valuable tool for process design. Acknowledgements
5. Conclusion Finite element simulations offer a wide variety of possibilities for investigating details of the blanking and ®neblanking processes. This holds true for the investigation into the tool load and coating-related topics, which helps in solving problems concerning tool life. Workpiece geometry can be calculated. The formation of cracks, which are to be avoided in ®ne blanking, can be forecasted. Therefore, optimal counter punch and blank holder force can be found,
Part of the work described in this paper was funded by the ``Deutsche Forschungsgemeinschaft'' within the Collaborate Research Center 442 ``Environmentally Improved Tribosystems''. References [1] W. KoÈnig, F. Klocke, Fertigungsverfahren Band 5, Blechbearbeitung, VDI-Verlag, DuÈsseldorf, 1995.
F. Klocke et al. / Journal of Materials Processing Technology 115 (2001) 70±75 [2] C. Rentsch, Feinschneiden mit beschichteten Werkzeugen, Dissertation RWTH Aachen, VDI-Verlag, DuÈsseldorf, 1997. [3] F. Klocke, K. Sweeney, Crack prediction and prevention in the fineblanking process: FEM simulations and experimental results, in: Proceedings of the Sixth International Conference on Sheet Metal, University of Twente, April 6±8, 1998, pp. 215±222.
75
[4] E. Doege, H. Meyer-Nolkemper, I. Saeed, Flieûkurvenatlas Metallischer Werkstoffe, Hanser Verlag, MuÈnchen, Wien, 1986. [5] K.-D. Bouzakis, K. Efstathiou, N. Vidakis, D. Kallindikis, S. Angos, T. Leyendecker, G. Erkens, H.-G. Fuss, R. Wenke, Experimental and FEM analysis of the fatigue behaviour of PVD coatings on HSS substrate in milling, Ann. CIRP 47 (1) (1998).