![Schematic representation of the shear-cutting tool (a) and collar-forming tool (b). [13] Figure 3 illustrates the high accuracy of the shear-cutting tool based on the formation of the cutting surfaces of one-step sheared holes. The images show a homogeneous trend of the rollover, clean-shear and the fracture surface.](https://www.researchgate.net/publication/327812339/figure/fig1/AS:673484103512065@1537582531088/Schematic-representation-of-the-shear-cutting-tool-a-and-collar-forming-tool-b-13_Q320.jpg)
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IOP Conference Series: Materials Science and Engineering
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Maximizing the Expansion Ratio through Multi-stage Shear-cutting
Process during Collar Forming
To cite this article: M Feistle et al 2018 IOP Conf. Ser.: Mater. Sci. Eng. 418 012071
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1234567890‘’“”
International Deep Drawing Research Group 37th Annual Conference IOP Publishing
IOP Conf. Series: Materials Science and Engineering 418 (2018) 012071 doi:10.1088/1757-899X/418/1/012071
Maximizing the Expansion Ratio through Multi-stage Shear-
cutting Process during Collar Forming
M Feistle1, I Pätzold1, R Golle1, W Volk1, A Frehn2 and R Ilskens2
1 Chair of Metal Forming and Casting, Technical University of Munich,
Walther-Meißner-Straße 4, 85748 Garching, Germany
2 BENTELER Automobiltechnik GmbH, An der Talle 27-31, 33102 Paderborn,
Germany
Martin.Feistle@utg.de
Abstract. Previous investigations showed that a two-stage punching process reduced the edge
crack sensitivity on high-strength multi-phase steels significantly compared to a single-stage
process. This is caused by an alteration of the state of stress in the shear-affected zone during the
second stage, which results in not only higher expansion ratios, but also higher collars during the
collar forming test. Another preceding research project at the Chair of Metal Forming and
Casting, Technical University of Munich, investigated the optimized values for cutting offset,
and die clearance for both pre-cutting and re-cutting steps for a single-stage punching process.
The aim of this research project was to combine those previous findings by increasing the number
of stages in a punching process with each using the optimized parameters. The behavior of milled
edges served as a reference for optimum performance. The number of stages influenced material
properties, such as surface characteristics, material deformation in the shear-affected zone, and
roughness of the cutting surface. The quality of the cutting surface increases drastically due to
various factors, such as a changed surface hardness, which affects fatigue resistance and,
therefore, the component’s lifetime. The validation of this punching strategy not only achieves
an even higher expansion ratio in common sheet metals, but also makes highly edge-crack
sensitive materials available for industrial applications.
1. Introduction
High-strength steels are essential in the automobile industry. The microstructure of these multi-phase
steels causes high strength and good formability, which impacts lightweight construction and fuel
efficiency.
Not only the material selection, but also the cutting process contribute to the risk of edge fractures.
Shear cutting is one of the most common separation techniques, but it also induces work hardening and
thus reduced residual formability in the shear-affected zone. [1] The degree of work hardening is
influenced by the selected material, including its properties and thickness, and the selected shear cutting
parameters, such as die clearance, cutting speed, and shear-cut strategy. [2] The lower formability
increases the chance of edge cracks in the shear-affected zone during uniaxial load in subsequent
processing steps. [3]
Utilizing appropriate testing methods allow for determining both forming potential and a material’s
edge crack sensitivity based on material properties and cutting parameters. These testing methods
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International Deep Drawing Research Group 37th Annual Conference IOP Publishing
IOP Conf. Series: Materials Science and Engineering 418 (2018) 012071 doi:10.1088/1757-899X/418/1/012071
include the hole expansion test according to ISO 16630 [4], the collar-forming test [5], the open hole
tensile test [6], the Diabolo-Test [7], the sheared-edge tension test [8] and the Edge-Fracture-Tensile-
Test [9, 10].
The objective of this study was to determine the forming potential of high-strength complex-phase
steels using the collar-forming test. Several shear-cutting strategies were investigated to optimize
residual forming capacity of shear cut edges.
2. Collar-Forming Test
The collar-forming test is a more practically oriented testing method to determine edge crack sensitivity.
Since the sample geometry is not defined specifically, a more application-oriented one can be chosen.
Similar to the hole expansion test, the tests are performed in two steps. First, the initial hole is
subsequently shear cut with a constant hole diameter of 50 mm. In order to perform multi-stage shear-
cutting strategies, additional locating holes are required to ensure reproducibility and concentricity of
the holes with radially constant cutting offset. The specimen is oriented in the tool analogous to the hole
expansion test with the burr facing away from the drawing punch. A conical drawing punch allows the
specimen to be positioned in the collar-forming tool. The collar’s edge is subjected to tensile loads
during the collar-forming process. Figure 1 illustrates the experimental setup of the collar-forming test
according to [11].
Figure 1. Schematic illustration of the collar-forming test according to [11].
All final collar edges are visually analyzed for cracks or necking. Furthermore, the collar quality is
assessed by the expansion ratio λ, using the initial diameter D0 and diameter of the punch Dh. The ratio
describes the maximum expansion without complete cracks through the complete thickness of the sheet.
Drawbacks of this testing procedure are not only the incremental changes of λ, but also the friction
between the drawing punch and specimen, and the higher material consumption based on the specimen
dimensions compared to hole expansion test, the Diabolo-Test and Edge-Fracture-Tensile-Test.
3. Materials
The study was performed using a CP-W 800 sheet with 4 mm thickness. Its microstructure consists of a
ferrite phase with finely dispersed martensite and bainite phases. This microstructure is the reason for
its high yield strength of 680 MPa – 830 MPa, minimum tensile strength of 780 MPa, and minimum
elongation of 12% [12]. Table 1 lists the chemical composition of CP-W 800 [12]. The material with a
thickness of 4 mm is used in automobile industry, especially for chassis parts like control arms.
Table 1. Chemical composition in weight percentage of CP-W 800 sheet metal. [12]
Chemical
element
C
max.
Si
max
Mn
max
P
max
S
max
Al
max
Cr+Mo
max
Nb+Ti
max
V
max
B
max
Weight
percentage
0.14
1.00
2.20
0.08
0.015
0.015
1.00
0.25
0.20
0.005
Die
Blank holder
Drawing punch
Drawing clearance
Entry radius
Moving direction of the drawing punch
Blank holder clearance
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International Deep Drawing Research Group 37th Annual Conference IOP Publishing
IOP Conf. Series: Materials Science and Engineering 418 (2018) 012071 doi:10.1088/1757-899X/418/1/012071
The tool active elements were manufactured from high chromium, high-carbon tool steel
(X155CrVMo12-1). This common tool steel is usually hardened to 58 ± 1 HRC and contains type M7C3
carbides with 10 µm in size on average.
4. Tools
Both tools for shear cutting (figure 2 a) and collar forming (figure 2 b) have a very high tool stiffness in
order to prevent deflection or shifting of the active elements. Using the blank holder plate as a guide
provided maximum guiding accuracy. In addition, alignment pins are used for the initial positioning of
the blank holder plate and the die.
1 Punch
2 Punch extension
3 Head plate
4 Guide plate
5 Punch holder plate
6 Blank holder
7 Gas spring
8 Guide piller
9 Base plate
10 Die unit
11 Spring catch
12 Stopper
13 Guide ring
14 Spring catch
15 Drawing punch
16 Drawing ring
Figure 2. Schematic representation of the shear-cutting tool (a) and collar-forming tool (b). [13]
Figure 3 illustrates the high accuracy of the shear-cutting tool based on the formation of the cutting
surfaces of one-step sheared holes. The images show a homogeneous trend of the rollover, clean-shear
and the fracture surface.
Figure 3. Microscope images of cutting surfaces with variation of the die clearance, a) u = 10%,
r = 50 µm, b) u = 15%, r = 250 µm, c) u = 20%, r = 250 µm, d) u = 25%, r = 250 µm.
The die clearance on the shear-cutting tool can be adjusted using different cutting punches in
combination with a die with a constant diameter. The shear-cut holes are therefore smaller by twice the
die clearance. Furthermore, it is possible to implement a multi-step cutting process by adding locating
holes and locators in the experimental setup. This allows various cutting offsets in combination with
different die clearances.
The modular design of the shear-cutting tool makes it easy to exchange the active elements. When
collar forming, the die plate and the guide plate have to be changed and the drawing guide set, the
drawing punch and the drawing die can be integrated. This interchangeability is necessary in order to
adjust the tool for different collar-forming tests while maintaining a constant initial hole diameter. In
addition, the tool is provided with communicating gas pressure springs to prevent the blank holder from
tilting. Using stoppers combined with cylindrical drawing punches and a conical tip ensures a repeatable
positioning of the perforated specimens.
a) b) c) d)
a)
b)
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International Deep Drawing Research Group 37th Annual Conference IOP Publishing
IOP Conf. Series: Materials Science and Engineering 418 (2018) 012071 doi:10.1088/1757-899X/418/1/012071
5. Experimental Design
The process parameter for collar-forming test regarding edge-crack sensitivity are displayed in Table 2.
The unusual cutting clearances of 5%, 30% respectively, were chosen to reproduce a tool displacement
of the active elements on the shear-cutting tool. Not only the unusual die clearances of 5% and 30%
were investigated, but also the incremental steps with a step size of 5% in between.
Table 2. Process parameter for the collar-forming test.
Parameter
Value
Punch-cutting edge radius r
<10 µm, 50 µm
Die clearance u
5%, 10%, 15%, 20%, 25%, 30%
Cutting line
Closed
Cutting offset z1
2 mm, 3 mm, 4 mm (two-stage cutting process)
0.7 mm (three-stage cutting process)
Cutting offset z2
1.2 mm, 1.6 mm (three-stage cutting process)
In addition to the single-stage shear-cutting strategy, a two- and three-stage shear-cutting strategy
with different cutting offsets were tested. Each test with single- or two-stage shear-cut specimens was
performed using at least ten specimens. For the three-stage collar-forming tests, only five specimens
were used. All tests were carried out in accordance with chapter 2 irrespective of the number of shear-
cutting operations. First, the starting blank is made by a single or multi-stage shearing operation. In the
second step, the collar is formed in one stroke. The procedure is illustrated by schematic representations
of the process steps in Tables 3, 4 and 5.
6. Results
6.1. Collar forming combined with single-stage shear-cutting process.
Specimens after the collar-forming test were divided into two groups according to their edge quality.
While A specimens show no or only minor cracks extending over half the sheet thickness at maximum,
B specimens had major cracks or were completely torn. If a testing series revealed more than five
B specimens in a row, the testing was terminated. This evaluation scheme applies only to statically
loaded components. If components are subjected to oscillating loads, A specimens should not show any
cracks since the crack could grow due to the fatigue load. Consequently, the component could fail under
use.
Table 3. Results for single-stage shear-cutting collar-forming test.
λ
Die clearance (r = 50 µm)
u = 5%
u = 10%
u = 15%
u = 20%
u = 25%
u = 30%
A
B
A
B
A
B
A
B
A
B
A
B
5%
5
10
10
10
10
10
10%
5
5
4
6
5
5
6
4
15%
4
6
5
5
5
20%
5
5
5
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International Deep Drawing Research Group 37th Annual Conference IOP Publishing
IOP Conf. Series: Materials Science and Engineering 418 (2018) 012071 doi:10.1088/1757-899X/418/1/012071
Table 3 shows the testing results. All die clearances but 5% failed to produce A specimens with an
expansion ratio of 5%. Only the die clearances of 20% and 25% produce A and B specimens up to an
expansion ratio of 15%, which supported these settings as the choice for the single-stage shear-cutting
process.
As a reference, milled parts produced only A specimens to an expansion ratio up to 90%.
6.2. Collar-forming combined with a two-stage shear-cutting process.
Table 4 shows the comparison between a single-stage shear-cutting collar-forming test with
u1 =u = 15% die clearance and the two-stage shear-cutting collar-forming test with die clearance
u1 = 10% and u2 = u = 10% or u2 = u = 15%, with different variations in cutting offset. To minimize
work hardening during pre-cutting, die clearance u1 = 10% is selected for the first cutting process.
A maximum expansion ratio of 40% with only A specimens was achieved during the collar-forming
tests of the two-stage shear-cut specimens. Consequently, changing the cutting strategy to a two-stage
process resulted in an increase of good, reliable parts. The residual formability of a two-stage shear-cut
edge is increased because the stress condition of the second punching process of the final hole is
comparable to a shear-cutting process with an open cutting line. The reason for this effect is the reduced
stiffness of the piece punched out as compared with the pre-cutting process, which also changes the
properties of the final cutting surface. The shear-cutting zone does not harden as much with a suitable
choice of the re-cutting parameters. If that does occur, then the piece punched out is deformed more
plastically than the cutting surface to be tested. Consequently, the cutting surface has a higher residual
formability.
As a reference, milled parts produced only A specimens to an expansion ratio up to 90%.
Table 4. Results for single-stage and two-stage shear-cutting collar-forming test.
Cutting strategies
λ
(u1 = 15%
r = 50 µm)
(u1 = u2 = 10%,
r = 50 µm)
(u1 = 10%, u2 = 15%, r = 50 µm)
Single-stage
Two-stage
Two-stage
Two-stage
Two-stage
z1 = 0 mm
z1 = 1.2 mm
z1 = 2 mm
z1 = 3 mm
z1 = 4 mm
A
B
A
B
A
B
A
B
A
B
5%
10
10
10
10
10
10%
5
10
10
10
10
15%
20%
10
10
10
25%
10
10
30%
9
1
10
10
35%
10
10
40%
10
8
2
8
2
10
45%
8
2
3
7
50%
6
4
3
7
3
7
2
8
55%
5
5
60%
2
8
2
8
65%
5
5
6
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International Deep Drawing Research Group 37th Annual Conference IOP Publishing
IOP Conf. Series: Materials Science and Engineering 418 (2018) 012071 doi:10.1088/1757-899X/418/1/012071
6.3. Collar forming combined with an optimized multi-stage shear-cutting process.
Table 5 shows the results of a three-stage shear-cutting collar-forming test with die clearance
u1 = u2 = u3 = u = 10% utilizing all the variations in cutting offset in comparison to a single-stage collar-
forming test with the same die clearance. The cutting-edge radius of the punch is also varied.
The optimal shear-cutting process parameters are used to generate a shear-cutting surface with high
residual formability based on the scientific findings of the paper “Determination of the Minimum
Possible Damage Due to Shear-cutting Using a Multi-stage Shear-cutting Process,” which is also
presented at the 37th INTERNATIONAL DEEP-DRAWING RESEARCH GROUP CONFERENCE
2018, Waterloo, Canada.
With respect to the cutting offset z2, the achievable expansion ratio λ increases using a three-stage
shear-cutting process. The punch used in the final cut had an edge radius of 50 µm. A cutting offset
z2 = 1.2 mm results in an absolute expansion ratio of 65%. If a cutting offset z2 = 1.6 mm is used, an
absolute expansion ratio of 55% can be reached. The increase in the residual formability is based on the
explanation given in chapter 6.2.
As a reference, milled parts were used that produced A specimens to an expansion ratio λ up to 90%.
Table 5. Results for single-stage and three-stage shear-cutting collar-forming test.
λ
Cutting strategies (u1 = u2 = u3 = u = 10%)
Single-stage
Three-stage
Three-stage
Three-stage
Three-stage
z = 0 mm
z1 = 0.7 mm
z1 = 0.7 mm
z1 = 0.7 mm
z1 = 0.7 mm
z2 = 1.2 mm
z2 = 1.2 mm
z2 = 1.6 mm
z2 = 1.6 mm
r = 50 µm
r < 10 µm
r1 = r2 <10 µm
r3 = 50 µm
r < 10 µm
r1 = r2 <10 µm
r3 = 50 µm
A
B
A
A
A
B
A
B
A
B
5%
10
10%
5
15%
20%
25%
5
5
5
5
30%
35%
40%
45%
5
5
5
5
50%
5
55%
5
5
4
1
5
60%
3
2
4
1
4
1
65%
1
4
5
2
3
3
2
70%
5
3
2
2
3
3
2
75%
5
5
5
The influence of the cutting-edge radius on the expansion ratio could be demonstrated during the
investigations. The expansion ratio increases by 18% using a cutting offset of 1.2 mm and around 10%
for a cutting offset of 1.6 mm. The literature shows that if the punch-edge radius becomes too large or
the punch edge is worn, then the residual formability of the shear-cut edge, and hence the expansion
ratio, are reduced. [14]
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International Deep Drawing Research Group 37th Annual Conference IOP Publishing
IOP Conf. Series: Materials Science and Engineering 418 (2018) 012071 doi:10.1088/1757-899X/418/1/012071
Figure 4 shows two different cutting surfaces for the three-stage shear-cutting process (cutting offset
1.2 mm) with variation of the edge radius r3. Larger edge radii r3 lead to an increase of the rollover
height and clean-shear height. Consequently, the fracture height is reduced. The angle of the fracture
surface decreases from 84.4° to 82.9° using a punch with an edge radius of r3 = 50 µm as compared with
punch with a sharp edge. All shear-cut surfaces show burr heights less than 5 µm because all specimens
were shear cut using a die with a sharp cutting edge. The microscope images in figure 4 show that the
fracture zone is more homogeneous and smooth when utilizing a punch with an edge radius of 50 µm
as compared with sharp-edge radii.
Figure 4. Cutting surfaces analyzed according to VDI 2906-2 [15], a) with sharp cutting edges
(r < 10 µm), b) rounded cutting edge on the punch (r1 = r2 <10 µm, r3 = 50 µm) and a sharp die edge;
(u1 = u2 = u3 = u = 10%, z1 = 0.7 mm, z2 =1.2 mm).
Figure 5 visualizes the individual shear-cut surfaces on a three-stage shear-cutting process to generate
an initial hole that shows the highest residual formability according to table 5.
Figure 5. Microscope images of the three shear-cut surfaces for an initial hole with the following
shear-cutting parameters: die clearance u1 = u2 = u3 = u = 10%, cutting offset z1 = 0.7 mm, z2 =1.2 mm,
punch edge radius r1 = r2 <10 µm, r3 = 50 µm.
All three images in figure 5 reveal a homogeneous rollover and a constant clean-shear height across
the image section. Consequently, the shear cutting tool is manufactured with sufficient accuracy and
tool displacements can safely be excluded. In addition, the specimen is positioned exactly in the tool
during re-cutting. The cut surface from the first stage (figure 5 a) shows a higher clean-shear height and
an inhomogeneous fracture zone as compared with the final shear cut surface shown in figure 5 c). The
cut surface of the first re-cutting process (figure 5 b) shows the smallest rollover height and clean-shear
height when compared to the other shear cut surfaces. The upper part of the fracture zone shows a high
homogeneity but in the lower half, a secondary clean-shear is formed.
7. Conclusion
Adapting the shear cutting process revealed the necessity of modifying both the cutting clearance and
cutting strategy according to the material in order to reduce edge cracks. The residual forming capacity
of shear-cut edges was increased using a multi-stage shear-cutting process instead of the single-stage
process. Using an optimized multi-stage shear cutting process makes it possible to shift a bigger part of
the materials deformation into the piece that is punched out during the re-cutting process. Such edges
are characterized by high residual formability.
a) b)
0.033
0.531
3.436
0.105
0.624
3.271
c)a) b)
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International Deep Drawing Research Group 37th Annual Conference IOP Publishing
IOP Conf. Series: Materials Science and Engineering 418 (2018) 012071 doi:10.1088/1757-899X/418/1/012071
References
[1] Klocke F 2011 Manufacturing Processes 1 (Berlin, Heidelberg: Springer Berlin Heidelberg)
[2] Scheib S, Sartkulvanich P and Altan T 2008 Examining edge cracking in hole flanging of
AHSS: Part II: Modeling of blanking Stamping Journal pp 22–3
[3] Kardes, N, Altan, T 2008 Examining edge cracking in hole flanging of AHSS Stamping Journal
pp 18–9
[4] International Organization for Standardization Metallic materials - Sheet and strip - Hole
expanding test 77.040.10 (16630:2009) https://www.iso.org/standard/42813.html (accessed
14 May 2017)
[5] Verein Deutscher Ingenieure 2013 Flanged holes - Flange forming 25.120.10 (VDI 3359)
(Berlin: Beuth Verlag GmbH) https://www.vdi.de/uploads/tx_vdirili/pdf/2027498.pdf
(accessed 14 May 2017)
[6] D30 Committee 2011 Test Method for Open-Hole Tensile Strength of Polymer Matrix
Composite Laminates (West Conshohocken, PA: ASTM International)
[7] Liewald M and Gall M 2013 Experimental investigation of the influence of shear cutting
parameters on the edge crack sensitivity of dual phase steels IDDRG 2013 Conf. pp 219–24
[8] Ilinich A, Smith L and Golovashchenko S 2011 Analysis of Methods for Determining Sheared
Edge Formability SAE Technical Papers
[9] Feistle M., Krinninger M., Pätzold I., Volk W. 2015 Edge-Fracture-Tensile-Test 60 excellent
inventions in metal forming: Edge-Fracture-Tensile-Test ed A E Tekkaya et al 1st edn
(Berlin, Heidelberg: Springer Vieweg)pp 193–8
[10] Tekkaya A E, Homberg W and Brosius A (eds) 2015 60 excellent inventions in metal forming:
Edge-Fracture-Tensile-Test 1st edn (Berlin, Heidelberg: Springer Vieweg)
[11] Feistle M, Pätzold I, Golle R and Volk W 2016 Predicting Edge Cracks on Shear-Cut High-
Strength Steels by Modified Uniaxial Tensile Tests eKEM (Key Engineering Materials) 703
pp 49–55
[12] N N 2014 Complexphasen-Stähle CP-W® und CP-K® https://www.thyssenkrupp-
steel.com/de/produkte/feinblech-oberflaechenveredelte-
produkte/mehrphasenstahl/complexphasenstahl/ (accessed 15 Jan 2018)
[13] Gläsner T 2018 Reduzierung der Kantenrissempfindlichkeit von Mehrphasenstählen durch 2-
stufiges Scherschneiden (Chair of Metal Forming and Casting at Technical University of
Munich)
[14] Carlsson B and Bustard, P and Eriksson, D. Formability of High Strength Steel Dual Phase
Steels: Paper F2004F454 (Borlänge, Sweden: SSAB Tunnplåt AB)
[15] Verein Deutscher Ingenieure 1994 Schnittflächenqualität beim Schneiden, Beschneiden und
Lochen von Werkstücken aus Metall, Scherschneiden (VDI 2906-2)
