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Textured surfaces for deep drawing tools by rolling
International Journal of Machine Tools & Manufacture 50 (2010) 969–976
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International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Textured surfaces for deep drawing tools by rolling V. Franzen n, J. Witulski n,1, A. Brosius, M. Trompeter, A.E. Tekkaya Institute of Forming Technology and Lightweight Construction, TU Dortmund University, Baroper Straße 301 D-44227, Dortmund, Germany
a r t i c l e in fo
abstract
Article history: Received 27 April 2010 Received in revised form 4 August 2010 Accepted 4 August 2010 Available online 12 August 2010
In this paper, macroscopic textured tool surfaces manufactured by rolling are investigated. Focus is on selective adjustment of friction by local texturing of tool areas to influence the material flow during deep drawing operations. Flat strip drawing tests were performed using friction elements with open textures. The texturing influences the friction conditions and affects the material properties of the stripes. The use of these surfaces results in a significant increase in friction, which allows an additional control of the material flow during sheet drawing operations. The main mechanisms for increased drawing forces are elastic deformation near the area of the texture and local plastic deformation on the sheet surface. Using strips made of mild steel, the texturing leads to an increased roughness of the sheet metal surface and, in the case of high surface pressure, to plastic deformations of the strips. Compared to conventional measures like draw beads, rolled-textured surfaces allow to retard the material flow during sheet drawing operation without excessive strain hardening in the sheet material. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Sheet metal forming Friction Textured tool surfaces Tooling Rolling
1. Introduction In metal forming, the process limit is influenced by the friction between the forming tool and workpiece. In comprehensive strip drawing and deep drawing experiments on different product shapes considering steel and aluminum sheets, Emmens [1] showed that the surface structure of sheet metal has a significant influence on friction between the forming tool and the workpiece. Different sheet surface structuring techniques and their influence on friction during drawing operations are described by Wagner and Siegert [2], who also introduced new ideas for describing 3D structured surfaces of sheets. In order to consider the contact situation of actual surfaces, Bay and Wanheim [3] defined a contact area ratio dividing the effective contact area by the nominal contact area. With rising contact stress the ratio converges to a value of 1. Sutcliffe [4] introduces a model to calculate increase in the contact area due to the flattening of surface asperities. A friction model considering Coulomb’s friction law, hydrodynamic effects and the local shearing of surface asperities is introduced by Frontzek and Mazilu [5]. Another promising approach to improve the tribological behavior in the forming process with regard to an extended process limit is the utilization of structured forming tool surfaces. Well-established
n
Corresponding authors. Tel.: +49 231 755 2680, 6917; fax: +49 231 755 2489. E-mail addresses: [email protected] (V. Franzen), [email protected] (J. Witulski), [email protected] (A. Brosius), [email protected] (M. Trompeter), [email protected] (A.E. Tekkaya). 1 Tel.: + 49 231 755 6926. 0890-6955/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2010.08.001
processes for structuring the surface of forming tools are laser-based techniques, electrical discharge machining (EDM), chemical etching techniques and high-speed milling. An overview is given by Bruzzone et al. [6]. Workpiece, tool and lubricant represent elements of a tribologic system. In the contact zone according to Czichos and Habig [7] five basic types of wear are possible: adhesion, abrasion, deformation, surface fatigue and tribo-chemical reactions. Broad strip drawing experiments focusing on the wear mechanisms adhesion and abrasion are described by Schmoeckel et al. [8]. Most significant parameters, which influence the friction between the workpiece and the tool in a sheet forming process, are the contact pressure, drawing velocity, properties of the lubricant, surface structures and type of material of both the tool and the sheet. However, the combination of all parameters has the strongest overall effect referring to Wagner and Siegert [2]. Neudecker et al. [9] analyzed the friction behavior of microtextured plates made of silicon nitride, which are used as inlets in friction elements for strip drawing tests. His results show that the drawing velocity, the contact pressure and the geometry (depth and length) of the textures have a significant influence on the friction coefficient. Neudecker et al. [10] also analyzed the friction behavior of micro-textured cast iron tool surfaces in strip drawing tests. He showed that there is a significant difference between the static and sliding friction coefficients, which is explained by undesired burrs generated during the laser-texturing of the cast iron surface. Once the sliding motion started, low sliding friction coefficients were measured compared to non-textured tool surfaces. The described effects are mainly accounted for the fluid dynamic lubrication theory. Kleiner et al. [11] investigated the friction behavior of sheet metal in strip drawing tests using tools with different milled
V. Franzen et al. / International Journal of Machine Tools & Manufacture 50 (2010) 969–976
surface textures. The textures in the tool were oriented along and across the drawing direction and in different zigzag directions. The friction coefficient was clearly increased by zigzag-shaped textures, while linear textures, parallel to the drawing direction, do not increase the friction coefficient compared to non-textured, grinded tool surfaces. The increased friction is explained by the flow of the sheet material ‘‘into’’ and ‘‘out of’’ the surface geometries. A similar effect was found by Balendra et al. [12] during compression tests with textured tools. In their studies, surface micro-geometries were incorporated into the compression tools by a conical diamond indenter. He found out that textured tool surfaces resulted in higher friction, depending particularly on the depth and density of the surface microgeometries. The increased friction results in a shortened lifetime of the forming tool, which is explained by the plastic deformation of the material, which flows through the impressions in the surface increasing the expenditure of energy. Wakuda et al. [13] observed a reduction in the friction comparing plane, lapped ceramic plates with textured ceramic plates, while mating them with hardened steel. He found out that the effect strongly depends on the size and density of the textures while the geometry of the dimple does not significantly affect the friction. After drawing experiments using tools with closed surface textures hydrostatic lubrication effects were described by Grahnert [14]. During the drawing process the lubricant was squeezed out of the surface dimples and penetrated the contact zone of the sliding partners. Popp and Engel [15] describe the prolongation of the tool lifetime due to textures in the tool surface, which were fabricated with an excimer laser. In compression tests they showed that the textures serve as a reservoir for the lubricant, while also picking up wear particles. In this paper, an innovative, alternative approach for the manufacture of textured tool surfaces is introduced—texturing by rolling with a hydrostatic ball-point-tool. The manufacture by rolling is faster compared to milling and provokes a strain hardening in the surfaces of the friction element. Additionally,
the rolling tool is more robust than a milling tool to produce fine channels. In this investigation, the friction behavior of the textured surface is characterized in flat strip drawing tests. This paper deals with first results of basic investigations of this alternative approach and investigates the potential of this technique in order to advance this process for industrial applications.
2. Texturing techniques and experimental set-up In general, roller burnishing with hydrostatic ball-point-tools is performed on lathes in order to achieve a smooth surface and strain hardening in the surface layer of rotationally symmetric parts. The strain hardening results in an increased resistance against microcracks due to external dynamic loads. R¨ottger [17] and N¨af [18] describe that roller burnishing more and more is used as an economic alternative finishing process to grinding. In case of more complex part geometries, roller burnishing can be performed in machining centers, e.g. as a subsequent finishing process to milling. The process control is similar to a finishing milling process. To manufacture the textures described in this article a hydrostatic ball-point rolling tool from ECOROLL was used, which was installed into a conventional 3-axesmilling-machine DMU 50 manufactured by deckle Maho, Fig. 1(a). A hydrostatic pressure pBall ¼15 MPa is applied to a ceramic ball with a diameter of 13 mm while rolling a channel into the surface of a ground, plane friction element made of tool steel C60, Fig. 1(b). The tool is moved along a defined tool path at a velocity of 1500 mm min 1 to form three different shapes of open-textured surfaces: longitudinal, transverse and zigzag oriented channels relating to the drawing direction. An open texture is defined by the possibility of the lubricant to discharge out rolled channel. Unlike, the lubricant is captured in closed texture. A typical measured roughness profile along a sectional view in the X-direction after the rolling process is shown in Fig. 1(c). The channels have a depth of approximately 6.5 mm and a width of 0.9 mm. The global dimensions of the three kinds of textured surfaces are given in Fig. 2. The percentage contact area is related to the nominal contact area of a non-textured, plane
Texture rolling
Rolling tool
CNC-3Axes-Milling machine
pBall
Rolling tool vRolling tool
z
z S∅13
Friction element
y
x x Friction element Roughness profile after rolling process
Base plate
Clamping device
Roughness profile in μm
970
10.0
0.0
-10.0 0
Fig. 1. Texture rolling process.
5 X-Direction in mm
10
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Longitudinal
Transverse
Zigzag
2 mm
45 mm
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3 mm Drawing direction
2 mm
40 mm
2 mm
Angl e 30°
x y
n = 19
Ac = 57%
n = 22
n: Number of edges of texture
n = 14
Ac = 55%
Ac = 68%
Ac: Calculated percentage contact area Fig. 2. Textured friction elements.
surface. The nominal contact area of the non-textured, plane surface is defined as 100% (size: 45 mm 40 mm). The textured friction elements are used in a plane strip drawing machine to measure the friction coefficient, Fig. 3(a). The uncoated sheet materials used are mild steel DC06 and a high-strength steel DP600. The specimens have dimensions of 280 mm 45 mm and a sheet thicknesses of t0,DC06 ¼0.8 mm and t0,DP600 ¼1.0 mm, respectively. The mineral oil based deep drawing lubricant Iloform PN226 is used, which has a dynamic viscosity of 66 mm2 s 1. The normal force is controlled by a closed-loop control system and the drawing force is measured continuously during the strip drawing test. The friction coefficient m is determined using the measured drawing force FDrawing force and the actual (measured) normal force FNormal force assuming Coulomb’s friction law [16]. After the drawing process, the average roughness Ra and the offset yield point Rp0.2 of the drawn sheets are measured. The roughness is recorded three times for each specimen by a portable roughness measuring station Perthometer M1 from the company Mahr. The offset yield point is determined by tensile tests in a universal testing machine SMZ250 from Zwick based on (norm) ISO 6892:1998. Tensile tests specimens are cut from of the drawn strips using a laser cutting machine. Four parameters are varied to investigate the influence of the textured friction elements on the friction conditions and material properties: contact pressure, drawing velocity, sheet material and type of friction element, Fig. 3(b). Each parameter combination is repeated three times for statistical reasons. The levels of contact pressure and drawing velocity represent upper and lower boundary conditions commonly used in the blank holder area in deep drawing processes.
material. The obtained results are subdivided into the three parts: friction coefficient, roughness and strain hardening.
3. Results and discussion
3.1. Friction coefficient
Structured surfaces and their influence on deep drawing processes can be characterized by the friction coefficient during the drawing operation and impacts to the sheet surface and
3.1.1. General fluid dynamic effects The results of the friction coefficients are given in Fig. 4. In general, a higher drawing velocity leads to a lower friction
Plane strip drawing Friction element FNormal force Clamp splits
µ FDrawing force Width: 40 mm
Strip
10 mm
Friction coefficient μ
Strip Drawing head
50 mm Hydraulic system
Parameters Contact pressure (Nominal)
2 MPa
10 MPa
10 mm s-1
100 mm s-1
DC06
DP600
Drawing velocity
Sheet material
Texture type
Longitudinal Transverse
Zigzag
Plane
Fig. 3. Experimental set-up and design for strip drawing tests.
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Friction coefficient
Sheet material: DC04 Contact pressure: 2 MPa 0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00
10 mm s-1
100 mm s-1 Drawing velocity
Longitudinal
Zigzag
Plane Transverse Texture type of friction element
Friction coefficient
Sheet material: DP600 Contact pressure: 2 MPa 0.20 10 mm s-1 0.18 0.16 0.14 0.12 0.10 100 mm s-1 0.08 Drawing velocity 0.06 0.04 0.02 0.00 Zigzag Longitudinal Transverse
Plane
Texture type of friction element
Lubrication: Iloform PN226 Contact pressure: 10 MPa 0.20 10 mm s-1 0.18 0.16 0.14 0.12 100 mm s-1 0.10 0.08 Drawing velocity 0.06 0.04 0.02 0.00 Zigzag Longitudinal Transverse
Plane
Texture type of friction element Lubrication: Iloform PN226 Contact pressure: 10 MPa 0.20 10 mm s-1 0.18 0.16 0.14 0.12 100 mm s-1 0.10 0.08 Drawing velocity 0.06 0.04 0.02 0.00 Zigzag Longitudinal Transverse
Plane
Texture type of friction element
Fig. 4. Friction coefficients depending on texture type of friction element, drawing velocity and contact pressure.
coefficient. In the state of mixed lubrication, this effect of decreasing friction at increased drawing velocities can be explained by the fluid dynamic behavior of the lubricant, which is drawn into the contact area between the tool and the workpiece during a forming process. Particularly high drawing velocities lead to low friction due to fluid dynamic effects. Similar results have been found in several sheet metal forming experiments using textured as well as plane tool and sheet surfaces (cp. [1]). It turned out that the dependency on the drawing velocity is lower at high contact pressures. At lower contact pressure more lubricant will be drawn between the tool and sheet surface. A high contact pressure on the other hand hinders the lubricant from penetrating into the contact zone between the tool and the sheet. Therefore the fluid-dynamic effect is stronger at low contact pressure (cp. [1,9]). According to the experimental results, these effects also occur in case of rolled-textured tool surfaces. Furthermore, the experiments show only a small influence of contact pressure on the friction coefficient. The experimental experience of decreasing friction coefficients in case of increasing contact pressures (cp. [1,9]) cannot be confirmed because of the low level of contact pressures used in this investigation.
3.1.2. Influence of rolled surface textures The results of the friction coefficient show a high dependence on the geometry of the textured surfaces. This dependence on the
texture type is almost independent of the choice of material. Highest values of the friction coefficient are reached by the use of a transverse texture type, and the lowest values by the use of nontextured plane surfaces. At low contact pressure the zigzag texture type results in lower values compared to values of longitudinal texture type. However, this effect appears not at high contact pressures. Here, the longitudinal and the zigzag texture types cause a comparable friction coefficient. The explanation for the increased friction coefficient in the case of rolled textures is based on the loading–unloading cycles under elastic compression due to the contact with the textured friction elements, Fig. 5. Because of this elastic compression, the resulting sheet thickness in the contact zone is lower compared to unloaded areas of the sheet. The resulting form-fit at the edge of each texture causes retarding effects on the drawing process. Since the elastic behavior of both DC06 (Fig. 4(a)) and DP600 (Fig. 4(b)) is nearly identical, there is a similar influence on the retarding effect for both materials. Furthermore, it can be assumed that the total retarding effect, and therefore the friction coefficient, depends directly on the number of edges oriented transverse to the drawing direction, because at each edge the strip is elastically compressed. In the case of the zigzag texture type, similar retarding effects occur. The zigzag texture leads also to loading–unloading cycles of the sheet metal material, whereas the projected retarding forces in drawing direction is equal to the transverse texture type for a single texture edge. A vectorial
V. Franzen et al. / International Journal of Machine Tools & Manufacture 50 (2010) 969–976
Initial state Friction element
Normal force applied
Elastic deformation
FNormal force
FNormal force
Sheet Strip drawing Form fit
FNormal force
FNormal force Drawing direction
Retarding effect
Fig. 5. Elastic loading and unloading cycle of the sheet material during contact with the friction elements.
Vectorial analysis of retarding effect (top view)
Zigzag texture y
Detail Le ng th ol
y x x
FR
Angle α
fe dg e
of te
xtu re
FR,Y
α qRetarding effect α
Drawing direction
Length lDetail of detail FR: Resulting retarding force FR,Y: Resulting retarding force in the drawing direction
Fig. 6. Vectorial analysis of retarding effect.
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Therefore, the retarding effect is independent from the angle of the texture. The Eq. (2) is valid for all values of the angle a except for a ¼901. The lower measured friction coefficients of the zigzag texture can be attributed to the lower number of textures (n ¼14, cp. Fig. 2) compared to the transverse texture type. Additionally, the zigzag textured surface hinders the deformation of the sheet compared to the transverse texture cause of the specific geometry. In the case of transverse texture the sheet is forced into a deformation state, which is compared to bending, while the zigzag structure causes a bulging state. The forming resistance to bulging is higher than to bending, therefore the flow into the texture is lower in the case of zigzag textures, which lowers friction. These experimental results agree with the investigation of milled textures described by Kleiner et al. [11]. Here, an increase in the friction coefficient for milled transverse and zigzag surface textures compared to non-textured friction elements particularly for mild steel was observed. This effect is explained by the deformation of the sheet, which slightly flows ‘into’ and ‘out of’ the surface textures. Compared to milled surface textures the retarding effect and the resulting friction coefficient of rolled transverse as well as zigzag textures is significantly increased due to their specific profile (cp. Fig. 1(c)). The rolled textures show piles of displaced material at the edges of the textures, which hinder hydrostatic effects and penetrate the drawing clearance given by the distance of the two friction elements. This leads to higher contact pressure in the contact area of the edges and increases the compression effects. The longitudinal textures also show a higher friction coefficient compared to non-textured plane friction elements. However, the increase of friction for the longitudinal oriented textures cannot be explained by the retarding effect, which occurs in the cases of transverse and zigzag textures. The longitudinal textures are aligned in the drawing direction and, consequently, the material flow of the strip is not hindered by loading–unloading cycles. Here, the cause of the increased friction is the penetration of the drawing clearance by the outer edge of the friction elements, which leads to the formation of gouges in the sheet metal material (concluded by small scratch marks on the strips after being drawn (cp. Fig. 7). Similar effects, which were caused by burrs on the textured friction elements, ploughing through the sheet surface, are described by Neudecker et al. [10]. In the case of the zigzag texture type, the formation of gouges is negligible low because the orientation of the edges of the textures is diagonal to the drawing direction, which prevents excessive stress concentration in the contact zone. The formation of the gouges correlates with the surface roughness of the strips, Fig. 7. 3.2. Roughness
analysis of the retarding effect is given in Fig. 6 to explain this characteristic. Here, a detail with the length lDetail of the zigzag texture type is shown. The length l of edge of texture can be determined by the length lDetail and the angle a:l ¼lDetail/cos(a). The retarding effect causes a constant load distribution along the edge of texture qRetarding effect. This distributed load can be summarized to a resulting retarding force FR FR ¼
lDetail q cos a Retarding effect
ð1Þ
The resulting retarding force in the drawing direction FR,Y can be determined by FR,Y ¼FRcos(a), which results finally into FR,Y ¼ lDetail qRetarding effect
ð2Þ
The entire results of the average roughness of the strip surface after the strip drawing process for the sheet material DC06 are given in Fig. 8. The average roughness of the strip surface increases during the strip drawing process using textured surfaces. The longitudinal texture type results in the highest values of roughness. In the case of the non-textured surface the roughness is the same as the initial roughness of the sheet. The contact pressure and the drawing velocity have a small influence on the roughness. Nevertheless, if a longitudinal texture type is used, the roughness differs at the different drawing velocities. In the case of the steel DP600, the roughness after the strip drawing process changes compared to the initial roughness only in the case of the longitudinal texture type, Fig. 8(b). The contact pressure and the drawing velocity have very little influence.
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Plane
Longitudinal
Transverse Drawn sheet metal strips
12.5 mm
Average roughness Ra
3.0
Drawing velocity 10 mm s
-1
Deep scratches
1.5 Initial value
0.0 2 MPa2
10 MPa
MPa2
10 MPa
MPa
10 MPa
Contact pressure
Fig. 7. Average roughness of selected drawn sheet metal strips (DC06) and images of their surfaces.
Sheet material: DC04 Contact pressure: 2 MPa 3.0
Lubrication: Iloform PN226 Contact pressure: 10 MPa 3.0
Average roughness Ra
Drawing velocity
Drawing velocity
2.5
2.5 10 mm s-1
2.0
100 mm s-1
1.5
10 mm s-1 2.0 1.5
1.0
1.0 Initial value
Initial value
0.5
0.5
0.0
0.0 Longitudinal Zigzag Transverse
Longitudinal Zigzag Transverse
Plane
Plane
Texture type of friction element
Texture type of friction element
Sheet material: DP600 Contact pressure: 2 MPa
Lubrication: Iloform PN226 Contact pressure: 10 MPa
3.0
3.0 Drawing velocity
Average roughness Ra
100 mm s-1
Drawing velocity
2.5
2.5 10 mm s-1
2.0 1.5
2.0 Initial value
1.5
1.0 0.5
100 mm s-1 Initial value
1.0 100 mm s-1
0.0 Longitudinal Zigzag Transverse
0.5
Plane
Texture type of friction element
10 mm s-1
0.0 Longitudinal Zigzag Transverse
Plane
Texture type of friction element
Fig. 8. Average roughness depending on texture type of friction element, drawing velocity and contact pressure.
Again, in the case of using a longitudinal texture type the drawing velocity has an effect on the roughness characteristics comparable to DC06. This behavior is based on the higher yield stress of the
material DP600 compare to DC06. Therefore, the DP600 provides a higher resistance against surface damage and features a lower surface roughness.
V. Franzen et al. / International Journal of Machine Tools & Manufacture 50 (2010) 969–976
3.3. Strain hardening The change in the offset yield point of the strip material after the strip drawing test is a suitable indicator of strain hardening caused by the strip drawing process, Fig. 9. The amount of strain hardening is significant to the springback behavior after the drawing process and influence the dent resistance of the sheet metal part. A minimum of strain hardening is desired. The strain hardening behavior of DC06 (Fig. 9(a)) and DP600 (Fig. 9(b)) differs, which depends on the different initial level of yield stress. Strain hardening occurs in the case of the longitudinal and transverse texture types. The effect is greater at higher contact pressures. Here, the zigzag texture type also causes a change in the offset yield point. However, the effect is marginal. The level of strain hardening is not affected by the drawing velocities. In the case of DC06 the observed hardening effect can be explained by local plastic deformation during the drawing experiments. Due the higher yield stress, the strip drawing process does not change the material properties of the DP600 except for longitudinal texture types. The measured values of the offset yield point are scattered around the reference value. The dependence of the strain hardening on the texture type is based on the different shapes of the contact area. The resistance to deformation is lower for the transverse and longitudinal textures due to the linear nature of these surface textures. The strain hardening effect is
enhanced by higher contact pressure. For DC06 the equivalent stress reaches values close to the yield stress. In the case of the DP600, the equivalent stress during the strip drawing experiments is far below the yield stress for both pressure levels.
4. Conclusions A new approach for manufacturing textured tool surfaces for deep drawing processes and their performance in plane strip drawing test have been presented in this research. Here, the tool surface textures have been manufactured by rolling, using a hydrostatic ball-point tool mounted to the spindle of a CNC-millingmachine. The textures can be used to increase the friction coefficients locally. The level of friction coefficient depends on the drawing velocity, the type of texture, and slightly on the initial yield stress of the stripes and the contact pressure. The reason for the increase by the use of textures can be found in local elastic deformations at the edges of textures which occur periodically during the strip drawing and result in a form fit. The effect depends directly on the elastic material characteristic of sheet and on the number of textures. Increase in friction by elastic deformation is independently to the direction of the textures relating to the drawing direction of the sheet apart from longitudinal textures. The orientation of the local elastic deformation at the edges of the
Lubrication: Iloform PN226 Contact pressure: 10 MPa
Sheet material: DC06 Contact pressure: 2 MPa 450
Rp0,2in MPa
400
450 Drawing velocity
400
350
350
300
300
100 mm s-1
250
100
Drawing velocity 10 mm s-1 100 mm s-1
250
200 150
975
200 Initial value
10 mm s
-1
50
150 100
Initial value
50
00 Longitudinal Zigzag Transverse
450 400
Rp0,2in MPa
250 200 150
Plane
Texture type of friction element
Texture type of friction element
Sheet material: DP600 Contact pressure: 2 MPa
Lubrication: Iloform PN226 Contact pressure: 10 MPa 450
100 mm s-1
100 mm s-1
400
350 300
Longitudinal Zigzag Transverse
Plane
350 Initial value
10 mm s-1
300 250 200
Drawing velocity
150
100
100
50
50
Initial value
10 mm s-1
Drawing velocity
00 Longitudinal Zigzag Transverse
Plane
Texture type of friction element
Longitudinal Zigzag Transverse
Plane
Texture type of friction element
Fig. 9. Offset yield point depending on texture type of friction element, drawing velocity and contact pressure.
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longitudinal textures is perpendicular to the drawing direction of the strips which prevents a form fit in drawing direction. Comparing the cross sectional profile of milled and rolled textures, the rolled textures show material displacements at the edges of the textures. This specific property increases on one hand the retarding effects by elastic deformations. However, in the case of longitudinal textures these material displacements increase the drawing forces by promoting the formation of gouges in the sheet material. Therefore, the use of longitudinal textured surfaces is not recommended. Unlike longitudinal textures the formation of gouges is negligible in the case of transverse and zigzag rolled textures. The use of both textures types is a suitable method to control the material flow during sheet metal forming process. Further steps will deal with the advancement of this technique for industrial applications. Here, the wear resistance of the textures is of particular importance. For this purpose, the tool steel C60 used in this basic investigation should be replaced by hardened or coated tool materials. By standardized Pin-on-disk-tests Tillman [19] compared the wear behavior of different thermal sprayed coatings to conventional steel (C45). First investigations show the transferability of the rolling process to thermally sprayed hard material coated surfaces.
Acknowledgement The cooperative research program ‘3D-Surface Engineering of Tools for Sheet Metal Forming—Manufacturing, Modelling, Machining (SFB708)’ is kindly supported by the German Research Foundation (DFG), the central public funding organization for academic research in Germany. References [1] W.C. Emmens, Tribology of flat contacts and its application in deep drawing, PhD Thesis, University of Twente, Enschede, The Netherlands, 1997. ¨ [2] S. Wagner, 3D-Beschreibung der Oberflachenstrukturen von Feinblechen, in: K. Siegert (Ed.), DGM Informationsgesellschaft mbH, 1996.
[3] N. Bay, T. Wanheim, Real area of contact and friction stress at high pressures sliding contact, Wear 38 (1976) 201–209. [4] M.P.F. Sutcliffe, Surface asperity deformation in metal forming processes, International Journal of Mechanical Sciences 30 (11) (1988) 847–868. [5] H. Frontzek, P. Mazilu, in: Tribologie der Blechumformung, Umformtechnisches Kolloquium, Darmstadt, 1988. [6] A.A.G. Bruzzone, H.L. Costa, P.M. Lonardo, D.A. Lucca, Advances in engineered surfaces for functional performance, Annals of the CIRP 57 (2008) 750–769. [7] H. Czichos, K.-H. Habig, in: Tribologie-Handbuch: Reibung und Verschleiß; ¨ Systemanalyse, Pruftechnik, Werkstoffe und Konstruktionselemente, Vieweg, Braunschweig, Wiesbaden, ISBN 3528163542, 2003. [8] D. Schmoeckel, H. Frontzek, E. von Finckenstein, Reduction of wear on sheet metal forming tools, Annals of the CIRP 35 (1986) 195–198. [9] Th. Neudecker, U. Popp, T. Schraml, U. Engel, M. Geiger, Towards optimized lubrication by micro texturing of tool surfaces, advanced technology of plasticity, 1, in: Proceedings of the 6th ICTP—International Conference on ¨ Technology of Plasticity, Nurnberg, Germany, 1999. [10] Th. Neudecker, U. Popp, U. Engel, M. Geiger, Micro texturing of tool surfaces and its influence on friction in deep drawing processes, in: Proceedings of the 8th SheMet—International Conference on Sheet Metal, Birmingham, UK, 2000. ¨ ¨ [11] M. Kleiner, K. Weinert, R. Krux, M. Kalveram, Oberflachenstrukturen fur Blechumformwerkzeuge, wt Werkstattstechnik online, Jahrgang vol. 93, 10, 2003, pp. 665–670. [12] R. Balendra, M. Rosochowska, K. Chondnikiewicz, R. Smith, Preliminary assessment of the influence of surface micro-geometries on friction, in: Proceedings of the 8th ICTP—International Conference on Technology of Plasticity, Verona, Italy, 2005. [13] M. Wakuda, Y. Yamauchi, S. Kanzaki, Y. Yasuda, Effect of surface texturing on friction reduction between ceramic and steel materials under lubricated sliding contact, Wear 254 (2003) 356–363. ¨ [14] R. Grahnert, Die Reibverhaltnisse im Flanschbereich beim Tiefziehen rechteckiger Teile, Fortschritt-Berichte VDI, Series 2, Betriebstechnik 105 (1985). [15] U. Popp, U. Engel, Excimer laser micro texturing of cold forging surfaces— influence on tool life, Annals of the CIRP 51 (2002) 231–234. [16] C.A. Coulomb, Memoires de Mathematique et de Physique de l’Academie Royale des Sciences (1785) 161. ¨ [17] K. Rottger, Festwalzen und Glattwalzen als wirtschaftliche Feinbearbeitungsverfahren. Konferenz-Einzelbericht: Moderne Schleiftechnologie und Feinstbearbeitung, 5 (2004), Villingen-Schwenningen: FH Furtwangen, 1–13. ¨ Die Alternative zum Schleifen. Glattwalzen auf Bearbeitungszentrum [18] R. Naf, und Drehmaschine, Schweizer Maschinenmarkt 31 (1999) 28–29. [19] W. Tillmann, E. Vogli, I. Baumann, J. Nebel, Verschleißminderung durch Wolframcarbid und Chromcarbid basierte Spritzschichten, in: W. Tillmann, ¨ (Hg.): SFB 708-2. offentliches Kolloquium, Dortmund, Praxiswissen (3D-Surface ¨ Werkzeugsysteme der Blechformteilefertigung, 2), 2008, Engineering fur pp. 97–109.
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International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Textured surfaces for deep drawing tools by rolling V. Franzen n, J. Witulski n,1, A. Brosius, M. Trompeter, A.E. Tekkaya Institute of Forming Technology and Lightweight Construction, TU Dortmund University, Baroper Straße 301 D-44227, Dortmund, Germany
a r t i c l e in fo
abstract
Article history: Received 27 April 2010 Received in revised form 4 August 2010 Accepted 4 August 2010 Available online 12 August 2010
In this paper, macroscopic textured tool surfaces manufactured by rolling are investigated. Focus is on selective adjustment of friction by local texturing of tool areas to influence the material flow during deep drawing operations. Flat strip drawing tests were performed using friction elements with open textures. The texturing influences the friction conditions and affects the material properties of the stripes. The use of these surfaces results in a significant increase in friction, which allows an additional control of the material flow during sheet drawing operations. The main mechanisms for increased drawing forces are elastic deformation near the area of the texture and local plastic deformation on the sheet surface. Using strips made of mild steel, the texturing leads to an increased roughness of the sheet metal surface and, in the case of high surface pressure, to plastic deformations of the strips. Compared to conventional measures like draw beads, rolled-textured surfaces allow to retard the material flow during sheet drawing operation without excessive strain hardening in the sheet material. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Sheet metal forming Friction Textured tool surfaces Tooling Rolling
1. Introduction In metal forming, the process limit is influenced by the friction between the forming tool and workpiece. In comprehensive strip drawing and deep drawing experiments on different product shapes considering steel and aluminum sheets, Emmens [1] showed that the surface structure of sheet metal has a significant influence on friction between the forming tool and the workpiece. Different sheet surface structuring techniques and their influence on friction during drawing operations are described by Wagner and Siegert [2], who also introduced new ideas for describing 3D structured surfaces of sheets. In order to consider the contact situation of actual surfaces, Bay and Wanheim [3] defined a contact area ratio dividing the effective contact area by the nominal contact area. With rising contact stress the ratio converges to a value of 1. Sutcliffe [4] introduces a model to calculate increase in the contact area due to the flattening of surface asperities. A friction model considering Coulomb’s friction law, hydrodynamic effects and the local shearing of surface asperities is introduced by Frontzek and Mazilu [5]. Another promising approach to improve the tribological behavior in the forming process with regard to an extended process limit is the utilization of structured forming tool surfaces. Well-established
n
Corresponding authors. Tel.: +49 231 755 2680, 6917; fax: +49 231 755 2489. E-mail addresses: [email protected] (V. Franzen), [email protected] (J. Witulski), [email protected] (A. Brosius), [email protected] (M. Trompeter), [email protected] (A.E. Tekkaya). 1 Tel.: + 49 231 755 6926. 0890-6955/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2010.08.001
processes for structuring the surface of forming tools are laser-based techniques, electrical discharge machining (EDM), chemical etching techniques and high-speed milling. An overview is given by Bruzzone et al. [6]. Workpiece, tool and lubricant represent elements of a tribologic system. In the contact zone according to Czichos and Habig [7] five basic types of wear are possible: adhesion, abrasion, deformation, surface fatigue and tribo-chemical reactions. Broad strip drawing experiments focusing on the wear mechanisms adhesion and abrasion are described by Schmoeckel et al. [8]. Most significant parameters, which influence the friction between the workpiece and the tool in a sheet forming process, are the contact pressure, drawing velocity, properties of the lubricant, surface structures and type of material of both the tool and the sheet. However, the combination of all parameters has the strongest overall effect referring to Wagner and Siegert [2]. Neudecker et al. [9] analyzed the friction behavior of microtextured plates made of silicon nitride, which are used as inlets in friction elements for strip drawing tests. His results show that the drawing velocity, the contact pressure and the geometry (depth and length) of the textures have a significant influence on the friction coefficient. Neudecker et al. [10] also analyzed the friction behavior of micro-textured cast iron tool surfaces in strip drawing tests. He showed that there is a significant difference between the static and sliding friction coefficients, which is explained by undesired burrs generated during the laser-texturing of the cast iron surface. Once the sliding motion started, low sliding friction coefficients were measured compared to non-textured tool surfaces. The described effects are mainly accounted for the fluid dynamic lubrication theory. Kleiner et al. [11] investigated the friction behavior of sheet metal in strip drawing tests using tools with different milled
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surface textures. The textures in the tool were oriented along and across the drawing direction and in different zigzag directions. The friction coefficient was clearly increased by zigzag-shaped textures, while linear textures, parallel to the drawing direction, do not increase the friction coefficient compared to non-textured, grinded tool surfaces. The increased friction is explained by the flow of the sheet material ‘‘into’’ and ‘‘out of’’ the surface geometries. A similar effect was found by Balendra et al. [12] during compression tests with textured tools. In their studies, surface micro-geometries were incorporated into the compression tools by a conical diamond indenter. He found out that textured tool surfaces resulted in higher friction, depending particularly on the depth and density of the surface microgeometries. The increased friction results in a shortened lifetime of the forming tool, which is explained by the plastic deformation of the material, which flows through the impressions in the surface increasing the expenditure of energy. Wakuda et al. [13] observed a reduction in the friction comparing plane, lapped ceramic plates with textured ceramic plates, while mating them with hardened steel. He found out that the effect strongly depends on the size and density of the textures while the geometry of the dimple does not significantly affect the friction. After drawing experiments using tools with closed surface textures hydrostatic lubrication effects were described by Grahnert [14]. During the drawing process the lubricant was squeezed out of the surface dimples and penetrated the contact zone of the sliding partners. Popp and Engel [15] describe the prolongation of the tool lifetime due to textures in the tool surface, which were fabricated with an excimer laser. In compression tests they showed that the textures serve as a reservoir for the lubricant, while also picking up wear particles. In this paper, an innovative, alternative approach for the manufacture of textured tool surfaces is introduced—texturing by rolling with a hydrostatic ball-point-tool. The manufacture by rolling is faster compared to milling and provokes a strain hardening in the surfaces of the friction element. Additionally,
the rolling tool is more robust than a milling tool to produce fine channels. In this investigation, the friction behavior of the textured surface is characterized in flat strip drawing tests. This paper deals with first results of basic investigations of this alternative approach and investigates the potential of this technique in order to advance this process for industrial applications.
2. Texturing techniques and experimental set-up In general, roller burnishing with hydrostatic ball-point-tools is performed on lathes in order to achieve a smooth surface and strain hardening in the surface layer of rotationally symmetric parts. The strain hardening results in an increased resistance against microcracks due to external dynamic loads. R¨ottger [17] and N¨af [18] describe that roller burnishing more and more is used as an economic alternative finishing process to grinding. In case of more complex part geometries, roller burnishing can be performed in machining centers, e.g. as a subsequent finishing process to milling. The process control is similar to a finishing milling process. To manufacture the textures described in this article a hydrostatic ball-point rolling tool from ECOROLL was used, which was installed into a conventional 3-axesmilling-machine DMU 50 manufactured by deckle Maho, Fig. 1(a). A hydrostatic pressure pBall ¼15 MPa is applied to a ceramic ball with a diameter of 13 mm while rolling a channel into the surface of a ground, plane friction element made of tool steel C60, Fig. 1(b). The tool is moved along a defined tool path at a velocity of 1500 mm min 1 to form three different shapes of open-textured surfaces: longitudinal, transverse and zigzag oriented channels relating to the drawing direction. An open texture is defined by the possibility of the lubricant to discharge out rolled channel. Unlike, the lubricant is captured in closed texture. A typical measured roughness profile along a sectional view in the X-direction after the rolling process is shown in Fig. 1(c). The channels have a depth of approximately 6.5 mm and a width of 0.9 mm. The global dimensions of the three kinds of textured surfaces are given in Fig. 2. The percentage contact area is related to the nominal contact area of a non-textured, plane
Texture rolling
Rolling tool
CNC-3Axes-Milling machine
pBall
Rolling tool vRolling tool
z
z S∅13
Friction element
y
x x Friction element Roughness profile after rolling process
Base plate
Clamping device
Roughness profile in μm
970
10.0
0.0
-10.0 0
Fig. 1. Texture rolling process.
5 X-Direction in mm
10
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Longitudinal
Transverse
Zigzag
2 mm
45 mm
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3 mm Drawing direction
2 mm
40 mm
2 mm
Angl e 30°
x y
n = 19
Ac = 57%
n = 22
n: Number of edges of texture
n = 14
Ac = 55%
Ac = 68%
Ac: Calculated percentage contact area Fig. 2. Textured friction elements.
surface. The nominal contact area of the non-textured, plane surface is defined as 100% (size: 45 mm 40 mm). The textured friction elements are used in a plane strip drawing machine to measure the friction coefficient, Fig. 3(a). The uncoated sheet materials used are mild steel DC06 and a high-strength steel DP600. The specimens have dimensions of 280 mm 45 mm and a sheet thicknesses of t0,DC06 ¼0.8 mm and t0,DP600 ¼1.0 mm, respectively. The mineral oil based deep drawing lubricant Iloform PN226 is used, which has a dynamic viscosity of 66 mm2 s 1. The normal force is controlled by a closed-loop control system and the drawing force is measured continuously during the strip drawing test. The friction coefficient m is determined using the measured drawing force FDrawing force and the actual (measured) normal force FNormal force assuming Coulomb’s friction law [16]. After the drawing process, the average roughness Ra and the offset yield point Rp0.2 of the drawn sheets are measured. The roughness is recorded three times for each specimen by a portable roughness measuring station Perthometer M1 from the company Mahr. The offset yield point is determined by tensile tests in a universal testing machine SMZ250 from Zwick based on (norm) ISO 6892:1998. Tensile tests specimens are cut from of the drawn strips using a laser cutting machine. Four parameters are varied to investigate the influence of the textured friction elements on the friction conditions and material properties: contact pressure, drawing velocity, sheet material and type of friction element, Fig. 3(b). Each parameter combination is repeated three times for statistical reasons. The levels of contact pressure and drawing velocity represent upper and lower boundary conditions commonly used in the blank holder area in deep drawing processes.
material. The obtained results are subdivided into the three parts: friction coefficient, roughness and strain hardening.
3. Results and discussion
3.1. Friction coefficient
Structured surfaces and their influence on deep drawing processes can be characterized by the friction coefficient during the drawing operation and impacts to the sheet surface and
3.1.1. General fluid dynamic effects The results of the friction coefficients are given in Fig. 4. In general, a higher drawing velocity leads to a lower friction
Plane strip drawing Friction element FNormal force Clamp splits
µ FDrawing force Width: 40 mm
Strip
10 mm
Friction coefficient μ
Strip Drawing head
50 mm Hydraulic system
Parameters Contact pressure (Nominal)
2 MPa
10 MPa
10 mm s-1
100 mm s-1
DC06
DP600
Drawing velocity
Sheet material
Texture type
Longitudinal Transverse
Zigzag
Plane
Fig. 3. Experimental set-up and design for strip drawing tests.
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Friction coefficient
Sheet material: DC04 Contact pressure: 2 MPa 0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00
10 mm s-1
100 mm s-1 Drawing velocity
Longitudinal
Zigzag
Plane Transverse Texture type of friction element
Friction coefficient
Sheet material: DP600 Contact pressure: 2 MPa 0.20 10 mm s-1 0.18 0.16 0.14 0.12 0.10 100 mm s-1 0.08 Drawing velocity 0.06 0.04 0.02 0.00 Zigzag Longitudinal Transverse
Plane
Texture type of friction element
Lubrication: Iloform PN226 Contact pressure: 10 MPa 0.20 10 mm s-1 0.18 0.16 0.14 0.12 100 mm s-1 0.10 0.08 Drawing velocity 0.06 0.04 0.02 0.00 Zigzag Longitudinal Transverse
Plane
Texture type of friction element Lubrication: Iloform PN226 Contact pressure: 10 MPa 0.20 10 mm s-1 0.18 0.16 0.14 0.12 100 mm s-1 0.10 0.08 Drawing velocity 0.06 0.04 0.02 0.00 Zigzag Longitudinal Transverse
Plane
Texture type of friction element
Fig. 4. Friction coefficients depending on texture type of friction element, drawing velocity and contact pressure.
coefficient. In the state of mixed lubrication, this effect of decreasing friction at increased drawing velocities can be explained by the fluid dynamic behavior of the lubricant, which is drawn into the contact area between the tool and the workpiece during a forming process. Particularly high drawing velocities lead to low friction due to fluid dynamic effects. Similar results have been found in several sheet metal forming experiments using textured as well as plane tool and sheet surfaces (cp. [1]). It turned out that the dependency on the drawing velocity is lower at high contact pressures. At lower contact pressure more lubricant will be drawn between the tool and sheet surface. A high contact pressure on the other hand hinders the lubricant from penetrating into the contact zone between the tool and the sheet. Therefore the fluid-dynamic effect is stronger at low contact pressure (cp. [1,9]). According to the experimental results, these effects also occur in case of rolled-textured tool surfaces. Furthermore, the experiments show only a small influence of contact pressure on the friction coefficient. The experimental experience of decreasing friction coefficients in case of increasing contact pressures (cp. [1,9]) cannot be confirmed because of the low level of contact pressures used in this investigation.
3.1.2. Influence of rolled surface textures The results of the friction coefficient show a high dependence on the geometry of the textured surfaces. This dependence on the
texture type is almost independent of the choice of material. Highest values of the friction coefficient are reached by the use of a transverse texture type, and the lowest values by the use of nontextured plane surfaces. At low contact pressure the zigzag texture type results in lower values compared to values of longitudinal texture type. However, this effect appears not at high contact pressures. Here, the longitudinal and the zigzag texture types cause a comparable friction coefficient. The explanation for the increased friction coefficient in the case of rolled textures is based on the loading–unloading cycles under elastic compression due to the contact with the textured friction elements, Fig. 5. Because of this elastic compression, the resulting sheet thickness in the contact zone is lower compared to unloaded areas of the sheet. The resulting form-fit at the edge of each texture causes retarding effects on the drawing process. Since the elastic behavior of both DC06 (Fig. 4(a)) and DP600 (Fig. 4(b)) is nearly identical, there is a similar influence on the retarding effect for both materials. Furthermore, it can be assumed that the total retarding effect, and therefore the friction coefficient, depends directly on the number of edges oriented transverse to the drawing direction, because at each edge the strip is elastically compressed. In the case of the zigzag texture type, similar retarding effects occur. The zigzag texture leads also to loading–unloading cycles of the sheet metal material, whereas the projected retarding forces in drawing direction is equal to the transverse texture type for a single texture edge. A vectorial
V. Franzen et al. / International Journal of Machine Tools & Manufacture 50 (2010) 969–976
Initial state Friction element
Normal force applied
Elastic deformation
FNormal force
FNormal force
Sheet Strip drawing Form fit
FNormal force
FNormal force Drawing direction
Retarding effect
Fig. 5. Elastic loading and unloading cycle of the sheet material during contact with the friction elements.
Vectorial analysis of retarding effect (top view)
Zigzag texture y
Detail Le ng th ol
y x x
FR
Angle α
fe dg e
of te
xtu re
FR,Y
α qRetarding effect α
Drawing direction
Length lDetail of detail FR: Resulting retarding force FR,Y: Resulting retarding force in the drawing direction
Fig. 6. Vectorial analysis of retarding effect.
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Therefore, the retarding effect is independent from the angle of the texture. The Eq. (2) is valid for all values of the angle a except for a ¼901. The lower measured friction coefficients of the zigzag texture can be attributed to the lower number of textures (n ¼14, cp. Fig. 2) compared to the transverse texture type. Additionally, the zigzag textured surface hinders the deformation of the sheet compared to the transverse texture cause of the specific geometry. In the case of transverse texture the sheet is forced into a deformation state, which is compared to bending, while the zigzag structure causes a bulging state. The forming resistance to bulging is higher than to bending, therefore the flow into the texture is lower in the case of zigzag textures, which lowers friction. These experimental results agree with the investigation of milled textures described by Kleiner et al. [11]. Here, an increase in the friction coefficient for milled transverse and zigzag surface textures compared to non-textured friction elements particularly for mild steel was observed. This effect is explained by the deformation of the sheet, which slightly flows ‘into’ and ‘out of’ the surface textures. Compared to milled surface textures the retarding effect and the resulting friction coefficient of rolled transverse as well as zigzag textures is significantly increased due to their specific profile (cp. Fig. 1(c)). The rolled textures show piles of displaced material at the edges of the textures, which hinder hydrostatic effects and penetrate the drawing clearance given by the distance of the two friction elements. This leads to higher contact pressure in the contact area of the edges and increases the compression effects. The longitudinal textures also show a higher friction coefficient compared to non-textured plane friction elements. However, the increase of friction for the longitudinal oriented textures cannot be explained by the retarding effect, which occurs in the cases of transverse and zigzag textures. The longitudinal textures are aligned in the drawing direction and, consequently, the material flow of the strip is not hindered by loading–unloading cycles. Here, the cause of the increased friction is the penetration of the drawing clearance by the outer edge of the friction elements, which leads to the formation of gouges in the sheet metal material (concluded by small scratch marks on the strips after being drawn (cp. Fig. 7). Similar effects, which were caused by burrs on the textured friction elements, ploughing through the sheet surface, are described by Neudecker et al. [10]. In the case of the zigzag texture type, the formation of gouges is negligible low because the orientation of the edges of the textures is diagonal to the drawing direction, which prevents excessive stress concentration in the contact zone. The formation of the gouges correlates with the surface roughness of the strips, Fig. 7. 3.2. Roughness
analysis of the retarding effect is given in Fig. 6 to explain this characteristic. Here, a detail with the length lDetail of the zigzag texture type is shown. The length l of edge of texture can be determined by the length lDetail and the angle a:l ¼lDetail/cos(a). The retarding effect causes a constant load distribution along the edge of texture qRetarding effect. This distributed load can be summarized to a resulting retarding force FR FR ¼
lDetail q cos a Retarding effect
ð1Þ
The resulting retarding force in the drawing direction FR,Y can be determined by FR,Y ¼FRcos(a), which results finally into FR,Y ¼ lDetail qRetarding effect
ð2Þ
The entire results of the average roughness of the strip surface after the strip drawing process for the sheet material DC06 are given in Fig. 8. The average roughness of the strip surface increases during the strip drawing process using textured surfaces. The longitudinal texture type results in the highest values of roughness. In the case of the non-textured surface the roughness is the same as the initial roughness of the sheet. The contact pressure and the drawing velocity have a small influence on the roughness. Nevertheless, if a longitudinal texture type is used, the roughness differs at the different drawing velocities. In the case of the steel DP600, the roughness after the strip drawing process changes compared to the initial roughness only in the case of the longitudinal texture type, Fig. 8(b). The contact pressure and the drawing velocity have very little influence.
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Plane
Longitudinal
Transverse Drawn sheet metal strips
12.5 mm
Average roughness Ra
3.0
Drawing velocity 10 mm s
-1
Deep scratches
1.5 Initial value
0.0 2 MPa2
10 MPa
MPa2
10 MPa
MPa
10 MPa
Contact pressure
Fig. 7. Average roughness of selected drawn sheet metal strips (DC06) and images of their surfaces.
Sheet material: DC04 Contact pressure: 2 MPa 3.0
Lubrication: Iloform PN226 Contact pressure: 10 MPa 3.0
Average roughness Ra
Drawing velocity
Drawing velocity
2.5
2.5 10 mm s-1
2.0
100 mm s-1
1.5
10 mm s-1 2.0 1.5
1.0
1.0 Initial value
Initial value
0.5
0.5
0.0
0.0 Longitudinal Zigzag Transverse
Longitudinal Zigzag Transverse
Plane
Plane
Texture type of friction element
Texture type of friction element
Sheet material: DP600 Contact pressure: 2 MPa
Lubrication: Iloform PN226 Contact pressure: 10 MPa
3.0
3.0 Drawing velocity
Average roughness Ra
100 mm s-1
Drawing velocity
2.5
2.5 10 mm s-1
2.0 1.5
2.0 Initial value
1.5
1.0 0.5
100 mm s-1 Initial value
1.0 100 mm s-1
0.0 Longitudinal Zigzag Transverse
0.5
Plane
Texture type of friction element
10 mm s-1
0.0 Longitudinal Zigzag Transverse
Plane
Texture type of friction element
Fig. 8. Average roughness depending on texture type of friction element, drawing velocity and contact pressure.
Again, in the case of using a longitudinal texture type the drawing velocity has an effect on the roughness characteristics comparable to DC06. This behavior is based on the higher yield stress of the
material DP600 compare to DC06. Therefore, the DP600 provides a higher resistance against surface damage and features a lower surface roughness.
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3.3. Strain hardening The change in the offset yield point of the strip material after the strip drawing test is a suitable indicator of strain hardening caused by the strip drawing process, Fig. 9. The amount of strain hardening is significant to the springback behavior after the drawing process and influence the dent resistance of the sheet metal part. A minimum of strain hardening is desired. The strain hardening behavior of DC06 (Fig. 9(a)) and DP600 (Fig. 9(b)) differs, which depends on the different initial level of yield stress. Strain hardening occurs in the case of the longitudinal and transverse texture types. The effect is greater at higher contact pressures. Here, the zigzag texture type also causes a change in the offset yield point. However, the effect is marginal. The level of strain hardening is not affected by the drawing velocities. In the case of DC06 the observed hardening effect can be explained by local plastic deformation during the drawing experiments. Due the higher yield stress, the strip drawing process does not change the material properties of the DP600 except for longitudinal texture types. The measured values of the offset yield point are scattered around the reference value. The dependence of the strain hardening on the texture type is based on the different shapes of the contact area. The resistance to deformation is lower for the transverse and longitudinal textures due to the linear nature of these surface textures. The strain hardening effect is
enhanced by higher contact pressure. For DC06 the equivalent stress reaches values close to the yield stress. In the case of the DP600, the equivalent stress during the strip drawing experiments is far below the yield stress for both pressure levels.
4. Conclusions A new approach for manufacturing textured tool surfaces for deep drawing processes and their performance in plane strip drawing test have been presented in this research. Here, the tool surface textures have been manufactured by rolling, using a hydrostatic ball-point tool mounted to the spindle of a CNC-millingmachine. The textures can be used to increase the friction coefficients locally. The level of friction coefficient depends on the drawing velocity, the type of texture, and slightly on the initial yield stress of the stripes and the contact pressure. The reason for the increase by the use of textures can be found in local elastic deformations at the edges of textures which occur periodically during the strip drawing and result in a form fit. The effect depends directly on the elastic material characteristic of sheet and on the number of textures. Increase in friction by elastic deformation is independently to the direction of the textures relating to the drawing direction of the sheet apart from longitudinal textures. The orientation of the local elastic deformation at the edges of the
Lubrication: Iloform PN226 Contact pressure: 10 MPa
Sheet material: DC06 Contact pressure: 2 MPa 450
Rp0,2in MPa
400
450 Drawing velocity
400
350
350
300
300
100 mm s-1
250
100
Drawing velocity 10 mm s-1 100 mm s-1
250
200 150
975
200 Initial value
10 mm s
-1
50
150 100
Initial value
50
00 Longitudinal Zigzag Transverse
450 400
Rp0,2in MPa
250 200 150
Plane
Texture type of friction element
Texture type of friction element
Sheet material: DP600 Contact pressure: 2 MPa
Lubrication: Iloform PN226 Contact pressure: 10 MPa 450
100 mm s-1
100 mm s-1
400
350 300
Longitudinal Zigzag Transverse
Plane
350 Initial value
10 mm s-1
300 250 200
Drawing velocity
150
100
100
50
50
Initial value
10 mm s-1
Drawing velocity
00 Longitudinal Zigzag Transverse
Plane
Texture type of friction element
Longitudinal Zigzag Transverse
Plane
Texture type of friction element
Fig. 9. Offset yield point depending on texture type of friction element, drawing velocity and contact pressure.
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longitudinal textures is perpendicular to the drawing direction of the strips which prevents a form fit in drawing direction. Comparing the cross sectional profile of milled and rolled textures, the rolled textures show material displacements at the edges of the textures. This specific property increases on one hand the retarding effects by elastic deformations. However, in the case of longitudinal textures these material displacements increase the drawing forces by promoting the formation of gouges in the sheet material. Therefore, the use of longitudinal textured surfaces is not recommended. Unlike longitudinal textures the formation of gouges is negligible in the case of transverse and zigzag rolled textures. The use of both textures types is a suitable method to control the material flow during sheet metal forming process. Further steps will deal with the advancement of this technique for industrial applications. Here, the wear resistance of the textures is of particular importance. For this purpose, the tool steel C60 used in this basic investigation should be replaced by hardened or coated tool materials. By standardized Pin-on-disk-tests Tillman [19] compared the wear behavior of different thermal sprayed coatings to conventional steel (C45). First investigations show the transferability of the rolling process to thermally sprayed hard material coated surfaces.
Acknowledgement The cooperative research program ‘3D-Surface Engineering of Tools for Sheet Metal Forming—Manufacturing, Modelling, Machining (SFB708)’ is kindly supported by the German Research Foundation (DFG), the central public funding organization for academic research in Germany. References [1] W.C. Emmens, Tribology of flat contacts and its application in deep drawing, PhD Thesis, University of Twente, Enschede, The Netherlands, 1997. ¨ [2] S. Wagner, 3D-Beschreibung der Oberflachenstrukturen von Feinblechen, in: K. Siegert (Ed.), DGM Informationsgesellschaft mbH, 1996.
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