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Springback Reduction in Draw-Bending Process of Sheet Metals
Springback Reduction in Draw-Bending Process of Sheet Metals D. Schmoeckel (I), M. Beth, Institute for Production Technology and Forming Machines, Technical University DarmstadtrGermany Received on January 15,1993
A model test has been developed in order t o examine the springback of sheet metals in draw bending under combined tensile stresses. The tensile stress components are of decisive influence concerning the springback behaviour. Therefore, a control of springback is possible with specially adapted blankholder pressure. Experimental results are discussed and compared with results of FE simulation.
KEY WORDS: sheet metal, springback, draw bending
All sheet metal forming processes with a certain amount of deformation by bending are characterized by a n inhomogeneous deformation distribution over the sheet thickness which due t o elastic-plastic material behaviour leads t o the occurrence of springback. Springback problems are known to occur in such processes as die bending and swing folding, roll rounding, roll forming and bulging as well as in deep drawing and stretch forming.
include in most cases straight bends or component edges and are commonly provided with projecting flanges for subsequent spot welding or joining operations. Due t o these straight bends no ideal force or drawing portions have t o be accounted for in forming for the flange area under the blank holder, while forming itself can be defined as a simple bend at the punch radius, with combined monoaxial tensile stress (blank holder pressure); continuous forward and backward bending with combined monoaxial tensile stress occurs a t the die radius.
For bending processes, basic information concerning springback problems has already been available for quite some time /1-3/. For deep drawing and stretch forming, appropriate studies have until now been made to a limited extent only 14-71. Such draw bending processes are frequently found in the production of vehicle body compo-
However, such a simple component geometry requires special action to be taken for studying the springback phenomena under complex deformation conditions with multi-axial draw bending. For this reason, it was necessary to create a possibility of including defined tensile stresses in the longitudinal component direction in addition t o the
nents, and it is precisely in this area that repeatable component properties play an increasingly important role considering the highly automated production and assembly concepts used. The reduction of masses automotive engineering attempts to achieve leads to growing substitution of the classic drawing grades by high strength steel sheet and aluminium materials. The well-known tendency of these materials for springback adds t o the existing springback problems.
vertical tensile stresses induced by bending or friction, in order t o create a biaxial draw bending process.
Problem Presentation
Since an accurate predefinition of the shape variations due to springback is not possible until this date, it is required t o conduct partially very time-absorbing preproduction tests and corrections on the drawing tools before the actual production of components can begin.
To ensure homogeneous combination of tensile stress, a process concept with the following functional principle was used for the model test (Fig. 1):
1: Sheet metal
2: Punch 3: Blank holder 4: Clamping tool
Fv
f
Against this background, a special test arrangement has been used t o study the springback behaviour of body sheets in a model test 181. The objective of the study was to cover and evaluate all process parameters with respect t o their effects on springback phenomena in draw bending processes. In addition, action for aimed reduction of springback was t o be developed and tested. Furthermore, the experimental data are used for examining the results of simulation.
Model Test "Draw bending with Combined Tensile Stress" The starting point in conceiving the test setup was the requirement for simulating defined draw bending relationships in a model test. For this purpose, the basis used was a hat-shaped section representing an appropriate idealization of a longitudinal runner of carriage body. Such components, often found in automotive construction,
Annals of the ClRP Vol. 42/1/1993
FV
Fig. 1: Model process "draw bending with combined longitudinal tensile stress"
339
Six clamping tools arranged in pairs opposite each other clamp both ends of a specially shaped blank. A powercontrolled counter-directional movement of the prestressing traverses accommodating these clamping tools permits t o induce a defined tensile stress in longitudinal direction of the component. While maintaining the prestressing force, subsequent forming of the longitudinally prestressed blank into a hat-shaped profile takes place. During the forming process, the outer clamping tools which are carried on low-friction linear bearings, move inward together with the pulled-in flange while the centrally located clamping tools move positively upward together with the punch. Together with the special blank geometry, this ensures a forming process free from any constraints due t o longitudinal forces, so that a homogeneous tensile stress combination can be assumed to occur in longitudinal direction of the component. The drawing tool, which is independent from the prestressing arrangement, is mounted in a specially conceived hydraulic double-action press along with the prestressing arrangement. Experimental Results In the experimental studies, the influences of the geometrical parameters - tool radii, drawing gap and sheet thickness - were considered. In addition, a study was made of those parameters which affect the tribological system in the area of the blankholder and the die radius and thus influence the tensile stresses which are combined in the bending processes. In the following, a report is given on the subject of these combined monoaxial tensile stresses induced by friction.
It has been successfully demonstrated that both high blankholder pressure as well as low lubricant viscosities result in an increase of the sidewall tensile stress which can be determined via the drawing force. As is shown in Fig. 2, an increase of combined tensile stress involves a decrease of all three springback characteristics covered. With respect t o material influences, there is a distinctly higher springback inclination of the high-strength steel material ZStE 340 and of thin-gauge aluminium AIMg5Mn. As compared with St 14, this results from a yield point which is more than twice as high (ZStE 340) or from an substantially lower modulus of elasticity (AIMgSMn).
However, increasing the blankholder pressure for improving shape accuracy also has its disadvantages, as with increasing blankholder pressure there is growing occurrence of surface damage in the sidewall and flange area or even rupture of the component. These problems occur in particular with the sheet material AIMg5Mn with its high susceptibility t o springback and surface damage.Therefore the question raised whether with relation t o springback and surface influences it would be possible t o achieve better results with a blankholder pressure which is variable over the drawing depth. The three blankholder pressure concepts studied and shown in Fig. 3 were based on the following deliberations: In the case of actual drawing tools with constant blank holder pressure the contact pressure ON increases with growing drawing depth and with progressing flange retreat, due t o the reduced contact area under the blankholder. The constant contact pressure from the model test can be adapted to these conditions by means of a linear blankholder pressure increase (blankholder pressure concept 1 ).
i Sheet metal
A AIMg5Mn
a ZSIE 340 0 Sl 14
Parameter
r=8mm WR = 0’ so = Imm
Both at the beginning and at the end of the forming process, specific forming conditions can be assumed to exist which influence in particular the respective component zone (bead or flange radius). Derivations from this are the blankholder pressure rates with high blankholder pressure at the beginning and at the end (blankholder pressure concept 2)or exclusively at the end of the forming process (blankholder pressure concept 3). The sudden rise of
idso = 1.2
v=1OOmrn2/s
vz
P
\
Sidewall tensile stress
02
= 30 mm/s
[Nlmm21
Fig. 2: Springback-related shape variations dependent on sidewall tensile stress
Drawing depth
Fig. 3: Blankholder pressure concepts studied
340
blankholder pressure at the end of the forming process can be considered t o be a transformation of the two-step Shapeset process known from literature /9/ into a flexible single-step forming process. For all three materials the tests showed that - related to the total component accuracy CAP) - the pressure portions becoming effective towards the end of the forming processes are of particular importance.
In simulating the tests with 8 mm radius tool, the mesh consisted of 73 elements (two-dimensional beam elements) which were subdivided into 25 layers in the material thickness direction. lnys 25
_.
In Fig. 4, the occurring springback is merely related to the contact pressure ON^ effective at the end of the forming process; there is no evaluation as to how this final value is reached over the drawing distance. For all three blankholder pressure concepts, the relationship proved t o be almost identical with the tests made at constant blankholder pressure. 16
.=
-a
'lank pressure
f
Parameter
Fig. 5: FE simulation model
12
1
In considering the stress distribution on individual elements
I
\ I
it has been shown among others that there is no complete
contact of the material with the total die radius contour. The back-bending process in the component sidewall thus takes place already non-tool-related, before the die radius end is reached.
o!
I
Fig. 4: Springback a t various blankholder pressure rates
This material back-bending located even before the actual end of the die radius is particularly distinct on small die radii and is in agreement with observations made during the drawing tests, but cannot be covered by elementary consideration.
For comprehensive evaluation of the blankholder pressure concepts, the springback results were related t o findings regarding material work hardening and influences on the material surface. With identical shape accuracies achieved, there is substantially less plastic deformation in the sidewall area when controlled blankholder pressure is used. With constant blankholder pressure, identical shape accuracies therefore involve increased material stress. If high shape accuracies are to be achieved, the failure limit is approached sooner when constant blankholder pressure is used.
For the calculated bending and residual stress distributions in the tool-related forming areas and in the completely bent-back sidewall area, however, there is good agreement between elementary calculation and FE simulation (Fig. 6). However, concerning the calculated springback, there are greater differences coming up again (Fig. 71, since the elastic conditions in the material zone spanning over the punch as well as the abovementioned material contact conditions at the die radius and in the drawing gap contribute to the springback-related shape variations t o an extent that cannot be neglected.
0
10
I
20
I
30
I Lo
K)
Final rate of contact pressure ' J N ~ [N/rnrn*]
Causally related t o this is the influence on the material surface due t o the forming process. While for the t w o steel materials smoothing effects are present up to high contact pressure values, AIMg5Mn material shows roughening tendencies which can be moved towards higher final contact pressure values and thus better shape accuracies by means of an appropriate blankholder pressure concept, based on a "softer" overall forming process.
Mathematical Springback Determination In order t o permit theoretical or mathematical verification
of the extensive experimental results, a Finite Element simulation of the forming process was made using a conventional FE program, besides elementary calculation of springback involving considerable simplifications. Taking benefit of symmetry, the simulation model shown in Fig. 5 was used. The influence of frictional force in the blankholder area was replaced by combined tensile stress; Coulomb friction is expected to occur in the radius area
= 0.06).
Bending- and residual-stress 'Jb , CS? [N/mrnz]
Fig. 6: Calculated bending and residual stress for backbending in the component sidewall, both elementary and FEM
341
15
1
J
Method IParameter W R = 0'
161 Bhattacharyya, D.; Thorpe, W. R.; Painter, M. J.: Residual Stress Prediction in Large Autobody Panels. Journal of Materials Shaping Technology 5 (1988) 4, S. 221 - 229 171 Duncan, J. L.; Bird, J. E.: Approximate Calculations for Draw Die Forming and their Application t o Aluminium Alloy Sheet. IDDRG-Tagung, Warwick 1978
5
0
100
0
200
181 Beth, M.: Untersuchungen zum Ruckfederungsverhalten von Feinblechen bei Tief- und Streckziehvorgangen. Dr.-lng. Dissertation, Technische Hochschule Darmstadt 1993
Sidewall tensile stress OZ [N/mm2]
Fig. 7: Comparison of experimentally and mathematically determined sidewall curl
Conclusion As is shown by the results presented here, the springback phenomena in the model test described are influenced t o a great extent by combined tensile stresses. It has been shown that in relation with springback and
material stress, a blankholder pressure control dependent on drawing depth is advantageous. For the typical component geometry of "hat-shaped section", a sudden increase of blankholder pressure at the end of the forming process proved t o be particularly beneficial. The FE simulations furnish important indications as t o the actual material contact behaviour at the tool contour under stress and are in good agreement with the experimental results. Literature References
I 1I
Schwark, H.-F.: Ruckfederung an bildsam gebogenen Blechen. Dr.-lng. Dissertation, Technische Hochschule Hannover 1952
I21
Proksa, F.: Zur Theorie des plastischen Blechbiegens bei groBen Formanderungen. Dr.-lng. Dissertation, Technische Hochschule Hannover 1958
131
Queener, C. A.; De Angelis R. J.: Elastic Springback and Residual Stresses in Sheet Metal Formed by Bending. Transactions of the ASM, 61 (19681, S. 757 -768
141 Reitzle, W.; Streidl, M.; Drecker, H.; Fischer, F.: MaRhaltigkeit von PreRteilen aus hoherfestem Stahlblech. Stahl und Eisen 103 (1983) 23, S. 61 - 66 I5I
342
Yu, T. X.; Johnson, W.: Influence of Axial Force on the Elastic-Plastic Bending and Springback of a Beam. Journal of Mechanical Working Technology, 6 (19821, S. 5 - 21
191 Ayres, R. A.: Shapeset: A Process t o Reduce Sidewall Curl Springback in High-Strength Steel Rails. Journal of Applied Metalworking 3 (1 984) 2, S. 127 - 134
A model test has been developed in order t o examine the springback of sheet metals in draw bending under combined tensile stresses. The tensile stress components are of decisive influence concerning the springback behaviour. Therefore, a control of springback is possible with specially adapted blankholder pressure. Experimental results are discussed and compared with results of FE simulation.
KEY WORDS: sheet metal, springback, draw bending
All sheet metal forming processes with a certain amount of deformation by bending are characterized by a n inhomogeneous deformation distribution over the sheet thickness which due t o elastic-plastic material behaviour leads t o the occurrence of springback. Springback problems are known to occur in such processes as die bending and swing folding, roll rounding, roll forming and bulging as well as in deep drawing and stretch forming.
include in most cases straight bends or component edges and are commonly provided with projecting flanges for subsequent spot welding or joining operations. Due t o these straight bends no ideal force or drawing portions have t o be accounted for in forming for the flange area under the blank holder, while forming itself can be defined as a simple bend at the punch radius, with combined monoaxial tensile stress (blank holder pressure); continuous forward and backward bending with combined monoaxial tensile stress occurs a t the die radius.
For bending processes, basic information concerning springback problems has already been available for quite some time /1-3/. For deep drawing and stretch forming, appropriate studies have until now been made to a limited extent only 14-71. Such draw bending processes are frequently found in the production of vehicle body compo-
However, such a simple component geometry requires special action to be taken for studying the springback phenomena under complex deformation conditions with multi-axial draw bending. For this reason, it was necessary to create a possibility of including defined tensile stresses in the longitudinal component direction in addition t o the
nents, and it is precisely in this area that repeatable component properties play an increasingly important role considering the highly automated production and assembly concepts used. The reduction of masses automotive engineering attempts to achieve leads to growing substitution of the classic drawing grades by high strength steel sheet and aluminium materials. The well-known tendency of these materials for springback adds t o the existing springback problems.
vertical tensile stresses induced by bending or friction, in order t o create a biaxial draw bending process.
Problem Presentation
Since an accurate predefinition of the shape variations due to springback is not possible until this date, it is required t o conduct partially very time-absorbing preproduction tests and corrections on the drawing tools before the actual production of components can begin.
To ensure homogeneous combination of tensile stress, a process concept with the following functional principle was used for the model test (Fig. 1):
1: Sheet metal
2: Punch 3: Blank holder 4: Clamping tool
Fv
f
Against this background, a special test arrangement has been used t o study the springback behaviour of body sheets in a model test 181. The objective of the study was to cover and evaluate all process parameters with respect t o their effects on springback phenomena in draw bending processes. In addition, action for aimed reduction of springback was t o be developed and tested. Furthermore, the experimental data are used for examining the results of simulation.
Model Test "Draw bending with Combined Tensile Stress" The starting point in conceiving the test setup was the requirement for simulating defined draw bending relationships in a model test. For this purpose, the basis used was a hat-shaped section representing an appropriate idealization of a longitudinal runner of carriage body. Such components, often found in automotive construction,
Annals of the ClRP Vol. 42/1/1993
FV
Fig. 1: Model process "draw bending with combined longitudinal tensile stress"
339
Six clamping tools arranged in pairs opposite each other clamp both ends of a specially shaped blank. A powercontrolled counter-directional movement of the prestressing traverses accommodating these clamping tools permits t o induce a defined tensile stress in longitudinal direction of the component. While maintaining the prestressing force, subsequent forming of the longitudinally prestressed blank into a hat-shaped profile takes place. During the forming process, the outer clamping tools which are carried on low-friction linear bearings, move inward together with the pulled-in flange while the centrally located clamping tools move positively upward together with the punch. Together with the special blank geometry, this ensures a forming process free from any constraints due t o longitudinal forces, so that a homogeneous tensile stress combination can be assumed to occur in longitudinal direction of the component. The drawing tool, which is independent from the prestressing arrangement, is mounted in a specially conceived hydraulic double-action press along with the prestressing arrangement. Experimental Results In the experimental studies, the influences of the geometrical parameters - tool radii, drawing gap and sheet thickness - were considered. In addition, a study was made of those parameters which affect the tribological system in the area of the blankholder and the die radius and thus influence the tensile stresses which are combined in the bending processes. In the following, a report is given on the subject of these combined monoaxial tensile stresses induced by friction.
It has been successfully demonstrated that both high blankholder pressure as well as low lubricant viscosities result in an increase of the sidewall tensile stress which can be determined via the drawing force. As is shown in Fig. 2, an increase of combined tensile stress involves a decrease of all three springback characteristics covered. With respect t o material influences, there is a distinctly higher springback inclination of the high-strength steel material ZStE 340 and of thin-gauge aluminium AIMg5Mn. As compared with St 14, this results from a yield point which is more than twice as high (ZStE 340) or from an substantially lower modulus of elasticity (AIMgSMn).
However, increasing the blankholder pressure for improving shape accuracy also has its disadvantages, as with increasing blankholder pressure there is growing occurrence of surface damage in the sidewall and flange area or even rupture of the component. These problems occur in particular with the sheet material AIMg5Mn with its high susceptibility t o springback and surface damage.Therefore the question raised whether with relation t o springback and surface influences it would be possible t o achieve better results with a blankholder pressure which is variable over the drawing depth. The three blankholder pressure concepts studied and shown in Fig. 3 were based on the following deliberations: In the case of actual drawing tools with constant blank holder pressure the contact pressure ON increases with growing drawing depth and with progressing flange retreat, due t o the reduced contact area under the blankholder. The constant contact pressure from the model test can be adapted to these conditions by means of a linear blankholder pressure increase (blankholder pressure concept 1 ).
i Sheet metal
A AIMg5Mn
a ZSIE 340 0 Sl 14
Parameter
r=8mm WR = 0’ so = Imm
Both at the beginning and at the end of the forming process, specific forming conditions can be assumed to exist which influence in particular the respective component zone (bead or flange radius). Derivations from this are the blankholder pressure rates with high blankholder pressure at the beginning and at the end (blankholder pressure concept 2)or exclusively at the end of the forming process (blankholder pressure concept 3). The sudden rise of
idso = 1.2
v=1OOmrn2/s
vz
P
\
Sidewall tensile stress
02
= 30 mm/s
[Nlmm21
Fig. 2: Springback-related shape variations dependent on sidewall tensile stress
Drawing depth
Fig. 3: Blankholder pressure concepts studied
340
blankholder pressure at the end of the forming process can be considered t o be a transformation of the two-step Shapeset process known from literature /9/ into a flexible single-step forming process. For all three materials the tests showed that - related to the total component accuracy CAP) - the pressure portions becoming effective towards the end of the forming processes are of particular importance.
In simulating the tests with 8 mm radius tool, the mesh consisted of 73 elements (two-dimensional beam elements) which were subdivided into 25 layers in the material thickness direction. lnys 25
_.
In Fig. 4, the occurring springback is merely related to the contact pressure ON^ effective at the end of the forming process; there is no evaluation as to how this final value is reached over the drawing distance. For all three blankholder pressure concepts, the relationship proved t o be almost identical with the tests made at constant blankholder pressure. 16
.=
-a
'lank pressure
f
Parameter
Fig. 5: FE simulation model
12
1
In considering the stress distribution on individual elements
I
\ I
it has been shown among others that there is no complete
contact of the material with the total die radius contour. The back-bending process in the component sidewall thus takes place already non-tool-related, before the die radius end is reached.
o!
I
Fig. 4: Springback a t various blankholder pressure rates
This material back-bending located even before the actual end of the die radius is particularly distinct on small die radii and is in agreement with observations made during the drawing tests, but cannot be covered by elementary consideration.
For comprehensive evaluation of the blankholder pressure concepts, the springback results were related t o findings regarding material work hardening and influences on the material surface. With identical shape accuracies achieved, there is substantially less plastic deformation in the sidewall area when controlled blankholder pressure is used. With constant blankholder pressure, identical shape accuracies therefore involve increased material stress. If high shape accuracies are to be achieved, the failure limit is approached sooner when constant blankholder pressure is used.
For the calculated bending and residual stress distributions in the tool-related forming areas and in the completely bent-back sidewall area, however, there is good agreement between elementary calculation and FE simulation (Fig. 6). However, concerning the calculated springback, there are greater differences coming up again (Fig. 71, since the elastic conditions in the material zone spanning over the punch as well as the abovementioned material contact conditions at the die radius and in the drawing gap contribute to the springback-related shape variations t o an extent that cannot be neglected.
0
10
I
20
I
30
I Lo
K)
Final rate of contact pressure ' J N ~ [N/rnrn*]
Causally related t o this is the influence on the material surface due t o the forming process. While for the t w o steel materials smoothing effects are present up to high contact pressure values, AIMg5Mn material shows roughening tendencies which can be moved towards higher final contact pressure values and thus better shape accuracies by means of an appropriate blankholder pressure concept, based on a "softer" overall forming process.
Mathematical Springback Determination In order t o permit theoretical or mathematical verification
of the extensive experimental results, a Finite Element simulation of the forming process was made using a conventional FE program, besides elementary calculation of springback involving considerable simplifications. Taking benefit of symmetry, the simulation model shown in Fig. 5 was used. The influence of frictional force in the blankholder area was replaced by combined tensile stress; Coulomb friction is expected to occur in the radius area
= 0.06).
Bending- and residual-stress 'Jb , CS? [N/mrnz]
Fig. 6: Calculated bending and residual stress for backbending in the component sidewall, both elementary and FEM
341
15
1
J
Method IParameter W R = 0'
161 Bhattacharyya, D.; Thorpe, W. R.; Painter, M. J.: Residual Stress Prediction in Large Autobody Panels. Journal of Materials Shaping Technology 5 (1988) 4, S. 221 - 229 171 Duncan, J. L.; Bird, J. E.: Approximate Calculations for Draw Die Forming and their Application t o Aluminium Alloy Sheet. IDDRG-Tagung, Warwick 1978
5
0
100
0
200
181 Beth, M.: Untersuchungen zum Ruckfederungsverhalten von Feinblechen bei Tief- und Streckziehvorgangen. Dr.-lng. Dissertation, Technische Hochschule Darmstadt 1993
Sidewall tensile stress OZ [N/mm2]
Fig. 7: Comparison of experimentally and mathematically determined sidewall curl
Conclusion As is shown by the results presented here, the springback phenomena in the model test described are influenced t o a great extent by combined tensile stresses. It has been shown that in relation with springback and
material stress, a blankholder pressure control dependent on drawing depth is advantageous. For the typical component geometry of "hat-shaped section", a sudden increase of blankholder pressure at the end of the forming process proved t o be particularly beneficial. The FE simulations furnish important indications as t o the actual material contact behaviour at the tool contour under stress and are in good agreement with the experimental results. Literature References
I 1I
Schwark, H.-F.: Ruckfederung an bildsam gebogenen Blechen. Dr.-lng. Dissertation, Technische Hochschule Hannover 1952
I21
Proksa, F.: Zur Theorie des plastischen Blechbiegens bei groBen Formanderungen. Dr.-lng. Dissertation, Technische Hochschule Hannover 1958
131
Queener, C. A.; De Angelis R. J.: Elastic Springback and Residual Stresses in Sheet Metal Formed by Bending. Transactions of the ASM, 61 (19681, S. 757 -768
141 Reitzle, W.; Streidl, M.; Drecker, H.; Fischer, F.: MaRhaltigkeit von PreRteilen aus hoherfestem Stahlblech. Stahl und Eisen 103 (1983) 23, S. 61 - 66 I5I
342
Yu, T. X.; Johnson, W.: Influence of Axial Force on the Elastic-Plastic Bending and Springback of a Beam. Journal of Mechanical Working Technology, 6 (19821, S. 5 - 21
191 Ayres, R. A.: Shapeset: A Process t o Reduce Sidewall Curl Springback in High-Strength Steel Rails. Journal of Applied Metalworking 3 (1 984) 2, S. 127 - 134