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Knowledge-Based Systems and F.E. Simulations in Metal-Forming Processes Design An integrated Approach
Knowledge-Based Systems and F. E. Simulations in Metal-Forming Processes Design An integrated Approach N. Alberti ( l ) , L. Cannizzaro (2), F. Micari (2). Dipartimento di Tecnologia e Produzione Meccanica, Universita di Palermo Received on J a n u a r y 21, 1991 SLMMARY The automatic computer aided planning and design of cold forming processes includes several phases, among which the most important are the optimal choice of the forming operations sequence and, for each operation, the determination of the more suitable operating parameters. With this aim the Authors propose an integrated approach based on the preliminary choice of some feasible iorming sequences, carried out by means ot a knowledge-based system, and on the subsequent determination of the optimal one employing a finite element simulation of the process.
KEY WORDS :
Cold forming, CAPP, Expert systems.
1. INTRODUCTION Cold forming processes have assumed in the last years an always growing role in the manufacturing science, due to the recent advances in tools materials and design, which in turn allow significant improvements on the mechanical properties and narrow dimensional tolerances on the produced items. However it has to be observed that in cold forming processes the final shape of a mechanical part to be manufactured cannot be p r d u c e d generally by means of a single-stage operation, but several "preforming" operations must be performed to transform the initial simple billet geometry into a more complex flawless product. Moreover in the recent years cold forming evolves in the direction of net-shape manufacturing, in order to reduce the subsequent machining operations, thus allowing to minimize the total working cost. Consequently cold forming design represents a very important and complex task to be faced; in the past this aim has been pursued employing only the skill and the experience accumulated in the shop practice, but the necessity to produce a wide range of complex items requires the use of automatic procedures, using for this purpose the capabilities offered by modern computers. The computer aided planning and design of cold forming processes includes several phases, among which the most important are the optimal choice of the forming operations sequence (CAPP), the determination of the more suitable operating parameters for each operation (CAM) and the most suitable die design (CAD). In order to achieve this aim, in the literature two main approaches can be found; the former is based on knowledge-based expert systems, while the latter employes numerical techniques for the analysis of the plastic flow. Both the approaches present some drawbacks: the exclusive use of knowledge-based approaches, as proposed in a large number of papers on metal-forming 11-61, is not able to supply an enough accurate solution to the problem, since these systems, based on simple technological rules whose validity is not general, don't allow to forecast neither the material flow and the stress and plastic strain fields inside the workpiece, nor, consequently, the insurgence of defects, both due to an uncomplete die filling and to the occurrence of ductile fractures. Nevertheless, the exclusive use of advanced numerical algorithms, whose development has been largely affected by the growing capabilities supplied by the modem software and hardware systems, appears still today too expensive, both due to the large number of operating parameters whose influence should be investigated, and to
the numerical difficulties caused by the complexity of the mathematical models to be employed in order to describe the material behaviour in a realistic way [i-101. In the paper an integrated computer aided procedure able to assist the metal-forming process desizner in the choice of the elementary operations sequence and in the determination of the operating parameters is proposed; such an approach is based on the preliminary use of an expert system in order to select a limited class of feasible operation sequences and, for each of them, on a subsequent numerical analysis able to supply a complete knowledge of the plastic flow parameters and thus to determine the optimal sequence; in fact the numerical analysis, based on finite element techniques, allows a detailed comparison of the feasible sequences, concerning the omogeneity of accumulated plastic strain distribution, the integrity of the final product and the evaluation of the total plastic energy involved in the process. On the basis of the above considerations such a kind of procedure appears today to be the most suitable, since it is able to overcome the main drawbacks shown by the previous methods; in fact its use allows verifying the results supplied by an expert system, which, as above said, is generally based on a limited number of simple practical rules; on the other side in the proposed procedure the numerical analysis is employed according to its own nature, since surely the finite element method is a verification procedure, and thus its use as a direct tool for the process design represents an improper use. 2. THE DESIGN OF THE FORMING SEQUENCE A knowledge-based expert system has been setup able to select some feasible opera tion sequences concerning axisymmetrical products for which upsetting, forward and backward extrusion processes are required. Several rules derived from the specific literature and based on the practical experience have been taken into account, among which the main ones are shown in the following paragraphs. 2.1 Upsetting rules
Successful upsetting is strongly affected by two main factors: a) ductile fracture insurgence on the equatorial plane caused by the simultaneous presence in this zone of an high accumulated plastic strain and a positive mean stress; b) buckling occurrence due to an high value of the ratio of initial length to initial diameter (s=IO/dO). Concerning the first factor, the available rules are not
Annals of the CIRP, Vol. 40/1/1991
295
able to supply sufficient indications on a formability limit due to the large number of parameters involved in a fracture criterion, while more information are available about the second factor, even if the limit value of s is affected by the surface conditions (roughness), as well as by the lubricant used. For the above reasons in the developed expert system only this last factor has been taken into account, following the limit values proposed by Lange [1I]. In particular under the condition sc2.3 the workpiece fibers do not display any evidence of buckling and it is possible to carry out the operation in one blow; larger values of s require several deformation stages and, in this caw, the use of tapered heading preforms has been introduced in the code, since these dies allow higher upset ratios with =pea to the cylindrical ones. As proposed in (21 an equivalent diameter calculated as the fourth root of the arithmetic mean of the fourth power of the maximum and minimum diameters has been used in order to apply the general limiting rule. The shape of the tapered heading preform is decisive and should be drawn in such a way to guide the workpiece correctly and to avoid the formation of fold-type defects; in particular the values of the cone angle and of the lengths of the cylindrical and conical portions of the preheader, as proposed in [lo], has been introduced in the developed system. 2.2 Forward extrusion rules
Many mechanical parts are characterised by a geometrical shape with several decreasing diameters; in this case starting from an initial billet diameter, the lower sections are obtained by means of forward extrusion processes. It is important to observe that only the first operation could be a trapped-die extrusion, while the subsequent stages must be open-die extrusions. In the literature it is possible to find some limit values of the maximum possible reduction in area which depend on the maximum loadcarrying capacity of the machine tool, on the mechanical properties of the tool materials and on the tool design. Moreover, in the case of open-die extrusion, this rule must be integrated by means of a further more complicated rule, which has to ensure that the operation is carried out successfully without the occurrence of buckhg or upsetting of the unsupported part of the billet. Finally, a further rule linking the limit reduction area and the extrusion angle should be introduced in order to avoid any occurrence of central bursting defects. The analysis of the rules reported in the literature has shown that, in the case of open-die forward extrusion, the phenomenon which more limits the maximum possible reduction in area value is the insurgence of buckling or upsetting of the unsupported part of the billet; in the developed expert system the typical limits of reduction in area for open-die extrusion suggested in 131 have been employed, equal to 35 percent for low carbon steel, 25 percent for aluminium and 40 percent for AISI 4140 steel. However it should be observed that the above values have only an indicative meaning, since they do not take into account several parameters, like the lubricant type, the die finish and, above all, the geometrical ratios among the supported and the unsupported billet lengths and the initial and the final workpiece diameters. Thus further experimental and numerical investigations appear absolutely necessary, in order to define new more detailed rules. In the code some rules concerning the backward-extrusion process, derived by [31, have been introduced; in particular these rules limit the possible
296
reduction in area and the ratios of the maximum height of the cavity to the punch diameter and of the bottom thickness of the cavity to the extruded wall thickness. Moreover it must k considered that sometimes the production of items characterised by several decreasing diameters can be obtained carrying out two extrusion processes in one pass. Also for this complex operation it has been possible to find in the literature some indicative rules, which limit the maximum possible reduction in area and the axial distance between the extrusions. However, due to the complexity of this combined process, it is opinion of the Authors that the knowledge about the process mechanics should be furtherly increased, in such a way to define more detailed rules able to take into account the main operating parameters. 1.3. Expert-system architecture
At this point, on the basis of the above described rules, the developed code is able to suggest separately possible upsetting and extrusion sequences, starting from an assigned initial diameter. The choice of the billet diameter is carried out on the basis of two practical rules, which recommend that the number of extrusions over the same axial portion should be limited to a maximum of three, and that, in the case of multiple forward extrusions, it is better to try to have the highest reduction first. Thus the expert system developed examines and puts in a decreasing order the diameters to be produced; then, starting from the largest diameter, the code checks if it possible to extrude the smallest diameter with no more of three operations; in the affirmative case the largest diameter is assumed as the billet diameter and the forming sequence will consist only of extrusions operations; in the negative case, on the contrary, the same procedure will be repeated with the next diameter and so on. When the assumed billet diameter is not equal to the largest one, it will be necessary to carry out both upsetting and extrusion operations, and, consequently, a further rule concerning their sequencing should be introduced. In particular the choice of the best sequence depends both on technological and economical considerations: from the technological point of view, if the extrusion operations are camed out after the upsetting ones, open-die extrusions are necessary instead than trapped-die, with a lower value of the maximum possible reduction in area; otherwise, from the economical point of view, forward extrusion before upsetting causes the necessity of more complex (and consequently more expensive) upsetting dies, depending on the final item geometry. Thus in the developed expert system it is preferred to upset before forward extrusion, if the geometrical and technological considerations allow it. The above reported considerations concern axisymmetrical mechanical part, whose production requires upsetting, forward extrusion, doublereduction forward extrusion in one pass and backward extrusion. However in the practice other processes, combining cold-forming operations, are performed. It has to be observed that no general rules are available concerning the successful execution of these processes. Thus the Authors consider that in order to make more general the expert system developed, taking into account these more complex processes, further numerical and experimental analyses are necessary. The code is provided by utilities WO), to assist the process designer in the introduction of the geomehical data characterising the item to be produced and in the
visualisation of the technologically feasible sequences. 3. THE NUMERICAL SIMULATION INTEGRATED APPROACH
AND
'7.5
Jib THE
The generality and the limited number of the rules introduced in the expert system causes that several technologically admissible (able to satisfy the imposed rules) operations sequences are proposed for an assigned mechanical part. Moreover the employed rules are not able to indicate the operating parameters to be used in each operation and finally they are not sufficient to ensure the quality of the final manufactured item. Inner or surface defects, uncomplete die cavity fil!ing, excessive pressures at the tool-workpiece interface, excessive gradients in the accumulated plastic strain distribution and consequently of the induced mechanical properties, cannot be detected if the analysis is camed out by means of an exclusive use of the expert system. Thus the integrated approach proposed by the Authors, which includes the expert system use and the subsequent verification by means of a numerical analysis of the admissible sequences, appears to be the most suitable in order to choice the optimal operations sequence. The numerical analysis has been carried out by means of an advanced finite element code able to model the material behaviour in a realistic way. The numerical simulation of a complete cold forming operations sequence represents a very difficult task, since it is necessary to perform a detailed analysis of the process mechanics, characterised by large accumulated plastic strain gradients and by the presence of several tools, which, subsequently, determine the various stages of the complete process. In order to pursue this aim an Updated Lagrangian approach has been employed, in the hypothesis of an elastic-plastic strain-hardening material behaviour. Moreover proper algorithms model the complex contact and friction phenomena at the tool-workpiece interfaces, during the subsequent steps of the sequence. 4. APPLICATIONS The validity of the above described integrated approach has been tested by means of some applicative examplcs. In particular the optimal operations sequence has been determined for the simple mechanical part of low carbon steel shown in fig.1. The initial billet diameter is equal to 30 mm., as proposed by the expert system, thus requiring both upsetting and forward extrusion operations. Concerning the upsetting sequence, the high value of the ratio of initial unsupported length to initial diameter has imposed a two-stroke process and the code has suggested the use of the tapered heading preform as reported in fig.2; concerning the extrusion sequence, no double-reduction forward extrusions in one pass are possible, while, following the "highest reduction first" rule, the automatic procedure has supplied the sequence reported in fig.3a, as better with respect to the one shown in fig3b. Finally the expert system has suggested to carry out the upsetting sequence before the extrusion one. Following the proposed integrated approach, a finite element analysis has been carried out after the expert system run, with the main aims: - to verify the validity of the shape oi the tapered heading preform in order to ensure a complete filling of the die cavity thus obtaining the correct desired shape on the workpiece; - to compare the extrusion sequences reported in figg.3a, 3b.
m25 1
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---
&
025
a25 023
025 620
Fig. 3a
Fig. 3b
About the first task, the initial axisymmetrical billet has been discretized by means of 280 isoparametrical quadrilateral elements and proper rigid contact surfaces have been defined to model the tapered heading preform and the supporting die (fig.4). Frictional conditions a t the tool-workpiece interfaces has been taken into account using a constant tangential stress distribution, with a shear factor equal to 0.1. Figg.5 and 6 report the deformed shape of the workpiece after the fulfilment of the preforming operation and after the end of the upsetting sequence respectively, while the corresponding accumulated plastic strain distribution are shown in figg.7 and 8. The above figures highlight the complete filling of the die cavity and consequently prove the validity of the heading preform shape; moreover severe accumulated plastic strain gradients in the upsetted zone occur, with a maximum value equal to 1.79 . Otherwise further tests have shown that the heading preform cone angle has not a significant intluence on the plastic strain distribution omogeneity.
m7ffi V.P.
. o . m i a i mna aiar+oi
Fig. 4. Concerning the second tas'k, the finite element analysis has been performed employing the deformed mesh resulted by the previous upsetting analyses and the same frictional conditions; no rezoning was necessary since the extrusion operation concern billet parts which have been not interested to the previous deformation.
297
Fig. 9.
Fig. 6.
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t
d
TOTAL EWIV PLASTIC STRN WHO
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Fig. 7. The analysis has shown that the extrusion sequence reported in fig.3a is not feasible even if it satisfies the rules introduced in the expert system: in fact the open-die extrusion of the cylindrical zone with a diameter equal to 25 mm. directly from the billet diameter is not possible, since the upsetting of the unsupported part of the billet occurs. This phenomenon is evident in fig. 9 where nodal velocity vectors are reported. Thus only the extrusion sequence shown in fig3b can be practically employed. The above results highlight the validity of the proposed integrated approach which is able to join the design and synthesis capabilities of an expert system with the main features of a numerical analysis which is able to supply detailed information on the plastic flow.
5. REFTRENCES H. Kudo et al., (1980), "Cold forging of hollow cylindrical components having an intermediate flange - LBET analysis and experiment", Annals of CIRP, V01.29/1/1980, pp.129-133 M.I. Cokler, T.A. Dean, W.A. Knight, (1982), "Computer-aided sequence design for hot upset forgings", Proc. of 22nd MTDR Conference, pp.437-166 K. Sevenler, P.S. Raghupati, T. Altan, (19871, "Forming sequence design for multistage cold forging", Jnl. of Mech. Work. Tech., Vo1.14, pp.121-135 G. Eshel, M. Barash, W. Johnson, (19861, "Rule based modelling for planning axisymmetrical deep-drawing", Jnl. of Mech. Work. Tech., Vo1.14, pp.1-115 K. Osakada, T. Kado, G.B.Yang, (1988), "Application of AI-Technique to Process Planning of Cold Forging", Annals of the CIRP, Vo1.37/1/1988, pp.239-242 F'. Bariani, W. Knight, (1988), "Computer-aided cold forging process design: a knowledge based system approach to forming sequence generation", Annals of CIRP, V01.37/1/1988, pp.243-246 N. Alberti, L. Cannizzaro, L. DAcquisto, E. Lo Valvo, F. Micari (1989) "Computer aided simulation of die filling processes" Annals of CIRP, Vol. 38/1 pp.239-242. A. Bnrcellona, L. Cannizzaro, F. Micari, R. Riccobono (1939) "Die CAD in hot forging process" Proc. of the 5th International CAPE Conference. N.Alberti, L.Cannizzaro, F.Micari, R.Riccobono (1990) "Formability in closed-die processes - Computer aided design of the predeformed shape", Proc. of the Third International Conference on Technology of Plasticity, VOl.1, pp.183-189 NAberti, L.Cannizzaro, F.Micari (1990) "Coupled thermal-mechanical analysis of hot metal forming processes" Annals of CIRP, Vol. 39/1, pp.231-234 K. Lange, (1983), Handbook of metal forming, McGraw-Hill, N e w York ACWOLEM'DGEMENTS This paper has been supported by C.N.R. (Consiglio Nazionale delle Ricerche), contract n. 89.05018.CT07 .
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Fig. 8.
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KEY WORDS :
Cold forming, CAPP, Expert systems.
1. INTRODUCTION Cold forming processes have assumed in the last years an always growing role in the manufacturing science, due to the recent advances in tools materials and design, which in turn allow significant improvements on the mechanical properties and narrow dimensional tolerances on the produced items. However it has to be observed that in cold forming processes the final shape of a mechanical part to be manufactured cannot be p r d u c e d generally by means of a single-stage operation, but several "preforming" operations must be performed to transform the initial simple billet geometry into a more complex flawless product. Moreover in the recent years cold forming evolves in the direction of net-shape manufacturing, in order to reduce the subsequent machining operations, thus allowing to minimize the total working cost. Consequently cold forming design represents a very important and complex task to be faced; in the past this aim has been pursued employing only the skill and the experience accumulated in the shop practice, but the necessity to produce a wide range of complex items requires the use of automatic procedures, using for this purpose the capabilities offered by modern computers. The computer aided planning and design of cold forming processes includes several phases, among which the most important are the optimal choice of the forming operations sequence (CAPP), the determination of the more suitable operating parameters for each operation (CAM) and the most suitable die design (CAD). In order to achieve this aim, in the literature two main approaches can be found; the former is based on knowledge-based expert systems, while the latter employes numerical techniques for the analysis of the plastic flow. Both the approaches present some drawbacks: the exclusive use of knowledge-based approaches, as proposed in a large number of papers on metal-forming 11-61, is not able to supply an enough accurate solution to the problem, since these systems, based on simple technological rules whose validity is not general, don't allow to forecast neither the material flow and the stress and plastic strain fields inside the workpiece, nor, consequently, the insurgence of defects, both due to an uncomplete die filling and to the occurrence of ductile fractures. Nevertheless, the exclusive use of advanced numerical algorithms, whose development has been largely affected by the growing capabilities supplied by the modem software and hardware systems, appears still today too expensive, both due to the large number of operating parameters whose influence should be investigated, and to
the numerical difficulties caused by the complexity of the mathematical models to be employed in order to describe the material behaviour in a realistic way [i-101. In the paper an integrated computer aided procedure able to assist the metal-forming process desizner in the choice of the elementary operations sequence and in the determination of the operating parameters is proposed; such an approach is based on the preliminary use of an expert system in order to select a limited class of feasible operation sequences and, for each of them, on a subsequent numerical analysis able to supply a complete knowledge of the plastic flow parameters and thus to determine the optimal sequence; in fact the numerical analysis, based on finite element techniques, allows a detailed comparison of the feasible sequences, concerning the omogeneity of accumulated plastic strain distribution, the integrity of the final product and the evaluation of the total plastic energy involved in the process. On the basis of the above considerations such a kind of procedure appears today to be the most suitable, since it is able to overcome the main drawbacks shown by the previous methods; in fact its use allows verifying the results supplied by an expert system, which, as above said, is generally based on a limited number of simple practical rules; on the other side in the proposed procedure the numerical analysis is employed according to its own nature, since surely the finite element method is a verification procedure, and thus its use as a direct tool for the process design represents an improper use. 2. THE DESIGN OF THE FORMING SEQUENCE A knowledge-based expert system has been setup able to select some feasible opera tion sequences concerning axisymmetrical products for which upsetting, forward and backward extrusion processes are required. Several rules derived from the specific literature and based on the practical experience have been taken into account, among which the main ones are shown in the following paragraphs. 2.1 Upsetting rules
Successful upsetting is strongly affected by two main factors: a) ductile fracture insurgence on the equatorial plane caused by the simultaneous presence in this zone of an high accumulated plastic strain and a positive mean stress; b) buckling occurrence due to an high value of the ratio of initial length to initial diameter (s=IO/dO). Concerning the first factor, the available rules are not
Annals of the CIRP, Vol. 40/1/1991
295
able to supply sufficient indications on a formability limit due to the large number of parameters involved in a fracture criterion, while more information are available about the second factor, even if the limit value of s is affected by the surface conditions (roughness), as well as by the lubricant used. For the above reasons in the developed expert system only this last factor has been taken into account, following the limit values proposed by Lange [1I]. In particular under the condition sc2.3 the workpiece fibers do not display any evidence of buckling and it is possible to carry out the operation in one blow; larger values of s require several deformation stages and, in this caw, the use of tapered heading preforms has been introduced in the code, since these dies allow higher upset ratios with =pea to the cylindrical ones. As proposed in (21 an equivalent diameter calculated as the fourth root of the arithmetic mean of the fourth power of the maximum and minimum diameters has been used in order to apply the general limiting rule. The shape of the tapered heading preform is decisive and should be drawn in such a way to guide the workpiece correctly and to avoid the formation of fold-type defects; in particular the values of the cone angle and of the lengths of the cylindrical and conical portions of the preheader, as proposed in [lo], has been introduced in the developed system. 2.2 Forward extrusion rules
Many mechanical parts are characterised by a geometrical shape with several decreasing diameters; in this case starting from an initial billet diameter, the lower sections are obtained by means of forward extrusion processes. It is important to observe that only the first operation could be a trapped-die extrusion, while the subsequent stages must be open-die extrusions. In the literature it is possible to find some limit values of the maximum possible reduction in area which depend on the maximum loadcarrying capacity of the machine tool, on the mechanical properties of the tool materials and on the tool design. Moreover, in the case of open-die extrusion, this rule must be integrated by means of a further more complicated rule, which has to ensure that the operation is carried out successfully without the occurrence of buckhg or upsetting of the unsupported part of the billet. Finally, a further rule linking the limit reduction area and the extrusion angle should be introduced in order to avoid any occurrence of central bursting defects. The analysis of the rules reported in the literature has shown that, in the case of open-die forward extrusion, the phenomenon which more limits the maximum possible reduction in area value is the insurgence of buckling or upsetting of the unsupported part of the billet; in the developed expert system the typical limits of reduction in area for open-die extrusion suggested in 131 have been employed, equal to 35 percent for low carbon steel, 25 percent for aluminium and 40 percent for AISI 4140 steel. However it should be observed that the above values have only an indicative meaning, since they do not take into account several parameters, like the lubricant type, the die finish and, above all, the geometrical ratios among the supported and the unsupported billet lengths and the initial and the final workpiece diameters. Thus further experimental and numerical investigations appear absolutely necessary, in order to define new more detailed rules. In the code some rules concerning the backward-extrusion process, derived by [31, have been introduced; in particular these rules limit the possible
296
reduction in area and the ratios of the maximum height of the cavity to the punch diameter and of the bottom thickness of the cavity to the extruded wall thickness. Moreover it must k considered that sometimes the production of items characterised by several decreasing diameters can be obtained carrying out two extrusion processes in one pass. Also for this complex operation it has been possible to find in the literature some indicative rules, which limit the maximum possible reduction in area and the axial distance between the extrusions. However, due to the complexity of this combined process, it is opinion of the Authors that the knowledge about the process mechanics should be furtherly increased, in such a way to define more detailed rules able to take into account the main operating parameters. 1.3. Expert-system architecture
At this point, on the basis of the above described rules, the developed code is able to suggest separately possible upsetting and extrusion sequences, starting from an assigned initial diameter. The choice of the billet diameter is carried out on the basis of two practical rules, which recommend that the number of extrusions over the same axial portion should be limited to a maximum of three, and that, in the case of multiple forward extrusions, it is better to try to have the highest reduction first. Thus the expert system developed examines and puts in a decreasing order the diameters to be produced; then, starting from the largest diameter, the code checks if it possible to extrude the smallest diameter with no more of three operations; in the affirmative case the largest diameter is assumed as the billet diameter and the forming sequence will consist only of extrusions operations; in the negative case, on the contrary, the same procedure will be repeated with the next diameter and so on. When the assumed billet diameter is not equal to the largest one, it will be necessary to carry out both upsetting and extrusion operations, and, consequently, a further rule concerning their sequencing should be introduced. In particular the choice of the best sequence depends both on technological and economical considerations: from the technological point of view, if the extrusion operations are camed out after the upsetting ones, open-die extrusions are necessary instead than trapped-die, with a lower value of the maximum possible reduction in area; otherwise, from the economical point of view, forward extrusion before upsetting causes the necessity of more complex (and consequently more expensive) upsetting dies, depending on the final item geometry. Thus in the developed expert system it is preferred to upset before forward extrusion, if the geometrical and technological considerations allow it. The above reported considerations concern axisymmetrical mechanical part, whose production requires upsetting, forward extrusion, doublereduction forward extrusion in one pass and backward extrusion. However in the practice other processes, combining cold-forming operations, are performed. It has to be observed that no general rules are available concerning the successful execution of these processes. Thus the Authors consider that in order to make more general the expert system developed, taking into account these more complex processes, further numerical and experimental analyses are necessary. The code is provided by utilities WO), to assist the process designer in the introduction of the geomehical data characterising the item to be produced and in the
visualisation of the technologically feasible sequences. 3. THE NUMERICAL SIMULATION INTEGRATED APPROACH
AND
'7.5
Jib THE
The generality and the limited number of the rules introduced in the expert system causes that several technologically admissible (able to satisfy the imposed rules) operations sequences are proposed for an assigned mechanical part. Moreover the employed rules are not able to indicate the operating parameters to be used in each operation and finally they are not sufficient to ensure the quality of the final manufactured item. Inner or surface defects, uncomplete die cavity fil!ing, excessive pressures at the tool-workpiece interface, excessive gradients in the accumulated plastic strain distribution and consequently of the induced mechanical properties, cannot be detected if the analysis is camed out by means of an exclusive use of the expert system. Thus the integrated approach proposed by the Authors, which includes the expert system use and the subsequent verification by means of a numerical analysis of the admissible sequences, appears to be the most suitable in order to choice the optimal operations sequence. The numerical analysis has been carried out by means of an advanced finite element code able to model the material behaviour in a realistic way. The numerical simulation of a complete cold forming operations sequence represents a very difficult task, since it is necessary to perform a detailed analysis of the process mechanics, characterised by large accumulated plastic strain gradients and by the presence of several tools, which, subsequently, determine the various stages of the complete process. In order to pursue this aim an Updated Lagrangian approach has been employed, in the hypothesis of an elastic-plastic strain-hardening material behaviour. Moreover proper algorithms model the complex contact and friction phenomena at the tool-workpiece interfaces, during the subsequent steps of the sequence. 4. APPLICATIONS The validity of the above described integrated approach has been tested by means of some applicative examplcs. In particular the optimal operations sequence has been determined for the simple mechanical part of low carbon steel shown in fig.1. The initial billet diameter is equal to 30 mm., as proposed by the expert system, thus requiring both upsetting and forward extrusion operations. Concerning the upsetting sequence, the high value of the ratio of initial unsupported length to initial diameter has imposed a two-stroke process and the code has suggested the use of the tapered heading preform as reported in fig.2; concerning the extrusion sequence, no double-reduction forward extrusions in one pass are possible, while, following the "highest reduction first" rule, the automatic procedure has supplied the sequence reported in fig.3a, as better with respect to the one shown in fig3b. Finally the expert system has suggested to carry out the upsetting sequence before the extrusion one. Following the proposed integrated approach, a finite element analysis has been carried out after the expert system run, with the main aims: - to verify the validity of the shape oi the tapered heading preform in order to ensure a complete filling of the die cavity thus obtaining the correct desired shape on the workpiece; - to compare the extrusion sequences reported in figg.3a, 3b.
m25 1
U -A-
---
&
025
a25 023
025 620
Fig. 3a
Fig. 3b
About the first task, the initial axisymmetrical billet has been discretized by means of 280 isoparametrical quadrilateral elements and proper rigid contact surfaces have been defined to model the tapered heading preform and the supporting die (fig.4). Frictional conditions a t the tool-workpiece interfaces has been taken into account using a constant tangential stress distribution, with a shear factor equal to 0.1. Figg.5 and 6 report the deformed shape of the workpiece after the fulfilment of the preforming operation and after the end of the upsetting sequence respectively, while the corresponding accumulated plastic strain distribution are shown in figg.7 and 8. The above figures highlight the complete filling of the die cavity and consequently prove the validity of the heading preform shape; moreover severe accumulated plastic strain gradients in the upsetted zone occur, with a maximum value equal to 1.79 . Otherwise further tests have shown that the heading preform cone angle has not a significant intluence on the plastic strain distribution omogeneity.
m7ffi V.P.
. o . m i a i mna aiar+oi
Fig. 4. Concerning the second tas'k, the finite element analysis has been performed employing the deformed mesh resulted by the previous upsetting analyses and the same frictional conditions; no rezoning was necessary since the extrusion operation concern billet parts which have been not interested to the previous deformation.
297
Fig. 9.
Fig. 6.
!
d %
Y
t
d
TOTAL EWIV PLASTIC STRN WHO
V.P.
-O.IOCE+OIo.maO.IOOE+OI
Fig. 7. The analysis has shown that the extrusion sequence reported in fig.3a is not feasible even if it satisfies the rules introduced in the expert system: in fact the open-die extrusion of the cylindrical zone with a diameter equal to 25 mm. directly from the billet diameter is not possible, since the upsetting of the unsupported part of the billet occurs. This phenomenon is evident in fig. 9 where nodal velocity vectors are reported. Thus only the extrusion sequence shown in fig3b can be practically employed. The above results highlight the validity of the proposed integrated approach which is able to join the design and synthesis capabilities of an expert system with the main features of a numerical analysis which is able to supply detailed information on the plastic flow.
5. REFTRENCES H. Kudo et al., (1980), "Cold forging of hollow cylindrical components having an intermediate flange - LBET analysis and experiment", Annals of CIRP, V01.29/1/1980, pp.129-133 M.I. Cokler, T.A. Dean, W.A. Knight, (1982), "Computer-aided sequence design for hot upset forgings", Proc. of 22nd MTDR Conference, pp.437-166 K. Sevenler, P.S. Raghupati, T. Altan, (19871, "Forming sequence design for multistage cold forging", Jnl. of Mech. Work. Tech., Vo1.14, pp.121-135 G. Eshel, M. Barash, W. Johnson, (19861, "Rule based modelling for planning axisymmetrical deep-drawing", Jnl. of Mech. Work. Tech., Vo1.14, pp.1-115 K. Osakada, T. Kado, G.B.Yang, (1988), "Application of AI-Technique to Process Planning of Cold Forging", Annals of the CIRP, Vo1.37/1/1988, pp.239-242 F'. Bariani, W. Knight, (1988), "Computer-aided cold forging process design: a knowledge based system approach to forming sequence generation", Annals of CIRP, V01.37/1/1988, pp.243-246 N. Alberti, L. Cannizzaro, L. DAcquisto, E. Lo Valvo, F. Micari (1989) "Computer aided simulation of die filling processes" Annals of CIRP, Vol. 38/1 pp.239-242. A. Bnrcellona, L. Cannizzaro, F. Micari, R. Riccobono (1939) "Die CAD in hot forging process" Proc. of the 5th International CAPE Conference. N.Alberti, L.Cannizzaro, F.Micari, R.Riccobono (1990) "Formability in closed-die processes - Computer aided design of the predeformed shape", Proc. of the Third International Conference on Technology of Plasticity, VOl.1, pp.183-189 NAberti, L.Cannizzaro, F.Micari (1990) "Coupled thermal-mechanical analysis of hot metal forming processes" Annals of CIRP, Vol. 39/1, pp.231-234 K. Lange, (1983), Handbook of metal forming, McGraw-Hill, N e w York ACWOLEM'DGEMENTS This paper has been supported by C.N.R. (Consiglio Nazionale delle Ricerche), contract n. 89.05018.CT07 .
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TOTAL EWlV PLASilC STRN -O.lm+OlO . ~ * o O3.Im+Ol
CRTHO V.P.
Fig. 8.
298