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Numerical simulation and experimental study on tribological properties of stamping die with triangular texture
Accepted Manuscript Numerical simulation and experimental study on tribological properties of stamping die with triangular texture Ping Chen, Xiaojie Liu, Mingji Huang, Zhe Shi, Bin Shan PII:
S0301-679X(18)30604-2
DOI:
https://doi.org/10.1016/j.triboint.2018.12.025
Reference:
JTRI 5525
To appear in:
Tribology International
Received Date: 24 August 2018 Revised Date:
28 November 2018
Accepted Date: 19 December 2018
Please cite this article as: Chen P, Liu X, Huang M, Shi Z, Shan B, Numerical simulation and experimental study on tribological properties of stamping die with triangular texture, Tribology International (2019), doi: https://doi.org/10.1016/j.triboint.2018.12.025. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Numerical simulation and experimental study on tribological properties of stamping die with triangular texture Ping Chen*, Xiaojie Liu, Mingji Huang*, Zhe Shi, Bin Shan
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School of Mechanical Engineering, University of Science and Technology Beijing, Beijing 100083, China
Abstract: This paper discussed the effect of applying the LST to the stamping die on
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tribological properties under oil lubrication by simulation and experiments. And the triangular texture was induced at the fillet radius of the die. The simulation results showed that the molded parts processed by die with triangular texture exhibited lower
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thinning rate in the fillet area at the bottom than those processed by die without texture, and we also found that the friction coefficient has an effect on the thickness of molded parts. The experiment results indicated that the triangular texture makes the thickness variation of the molded parts smaller, which was in good agreement with the simulation results.
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Keywords: Triangular texture; Numerical simulation; Stamping experiment; Tribological properties
1. Introduction
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Stamping forming is one of the crucial plastic processing methods in sheet metal forming industry. The components produced with the die have many advantages of high precision, high complexity, high consistency, and high productivity, which are
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unmatched by other manufacturing methods and result in the metal processing method of stamping forming has been therefore widely used [1,2]. However, in the modern stamping process, the increase in demand of stamping parts materials such as high-strength steels or ultra-high-strength steels, the reduction in the use of lubricating oil for environmental protection, and so on [3,4], these factors require the die to withstand higher contact pressure, and result in unacceptable levels of friction and wear [5]. Therefore, the way to reduce wear of metal stamping die has become a critical
* Corresponding author. Tel: +86 01062332538; fax: +86 01062329145 E-mail address: [email protected] (P. Chen); [email protected] (M.J. Huang)
ACCEPTED MANUSCRIPT issue in stamping production research. Various aspects of stamping die for tribological applications have been studied by several researchers. Pereira et al. [6] firstly reported on the wear over the die radius caused by a combination of ploughing and galling mechanisms. Different angles on the die fillet radius had varying degrees of wear type and wear severity. In particular,
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use of lubricant during the stamping process can reduce the friction between the die and the sheet materials. Kim et al. [7,8] discussed the effect of the different lubricants during the stamping process, and obtained polymer based thin film lubricants with pressure additives performed better than other lubricants to improve tribological
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properties. With the aim of improving the tribological properties of the die-binder area under the oil lubrication condition, Chen et al. [9] carried out the pin-on-disk experiment and investigated the stamping die induced triangular texture expressed a
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good result in reducing friction to be compatible with the stamping die without texture. The laser surface texturing (LST) technology, performing micro-structure inducing on the metal surface, has become a significant surface engineering processing technology to improve the tribological properties. It was found that changing the surface topography of most materials by LST technology can achieve the
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effect of reducing the friction and wear of the material in different lubrication conditions [10]. An abrasion test on the steel plates with laser process induced two kinds of micro-hole shape and groove shape on the friction tester was performed by Wan et al. [11], and the authors found that the friction coefficient and wear amount of
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the textured samples were lower than those of the smooth samples under the condition of oil lubrication, while proper texture topography was beneficial to the formation of
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transfer film as well as the capture of abrasive debris to reduce abrasive wear under the dry friction condition. The results of the reciprocating test showed that the surface micro-structure significantly reduces the friction coefficient when the sample wears and the contact enters the mixed lubrication state. The remarkable result experimentally revealed, surface textured samples exhibit 70% lower friction compared to surface non-textured samples [12]. In order to study the impact of triangular texture of the die-binder area on the tribological properties during the stamping process, Chen et al. [13] performed simulation and experiments, and proposed that using LST technology to reduce the friction coefficient of the die-binder area can significantly improve the formability of the sheet material. Segu et al. [14,15] have also investigated that the multi-scale textured surface perform better in obtaining
ACCEPTED MANUSCRIPT lower and more stable friction coefficient, which can significantly reduce the friction and wear compared with the non-textured surface. Numerical simulations showed that during the stamping process, small changes at die fillet radius profile shape (of less than 15 µm) will cause a significant increase in local contact pressure, and result in increased wear and reduced life of the die [16].
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According to the research by Pereira et al.[6], in typical stamping processes (deep-drawing, plane strain stamping, cup forming, etc.), wear occurred at the die fillet radius, which usually had a cross-sectional profile that was a 90° circular arc (fillet) between the perpendicular surfaces at the die corner. Additionally, an amount
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of research have been developed to study the impact of the stamping process parameters: blank holder force, lubrication conditions, pressure holding time, etc. Indeed, these parameters affect the forming quality of the material [17,18]. At the
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same time, the surface quality of the die is important, and for the automotive sheet metal stampings, failure is typically characterized by the existence of micro-surface scratches of the die [19]. LST technology can generate various surface topography on the surface of metal die. The effect of textures in the form of circular, square, and equilateral triangular on the tribological performance through pin-on-disk tests was
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investigated, and Zhang et al. found that the triangular texture from a vertex angle to the side resulted in the lowest coefficient of friction [20,21]. Hence, in this paper, the effect of the triangular texture on the tribological properties of the stamping die under the oil lubrication is analyzed using a combination of numerical simulation and
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experiments. Based on the analysis of numerical simulation, that is, the triangular texture can improve the tribological properties of the die, stamping experiments
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applying LST to the die surfaces were then performed. It is worth noting that the texture used in our study is an equilateral triangle determined by side length. And the tribological properties of stamping dies were illustrated by observing the surface topography of die after a large number of stamping processes by a scanning electron microscope (SEM).
2. Experimental details 2.1. Stamping die The stamping die diagram is shown in Fig. 1. The stamping test configuration was based on the typical industrial automobile sheet metal stamping test [22] and
ACCEPTED MANUSCRIPT numerical simulation conducted based on the stamping test model. Removable die inserts were designed to achieve easy inspection and timely replacement of the wearing surfaces. One of the primary aims of the stamping test was to determine the tribological properties of textured and non-textured dies by changing the die inserts. The die insert and die were interference fits and were grinding before the assembly
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process to ensure that the surfaces of die insert and die were smooth. The entire sets of stamping die including the die inserts were made of Cr12MoV, and the entire experiment was carried out at room temperature (25 ℃) and relative humidity (40 %
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RH), which were kept constant.
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Fig. 1. Schematic diagram of the stamping die
The molded part is a cup-shaped part with a flange, the specific dimensions of the cup-shaped part are shown in Fig. 2. The molded parts not only meet the
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process.
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dimensional requirements, but also meet the processing requirements of the stamping
Fig. 2. The specific dimensions of the cup-shaped molded part.
2.2 Material The 6061 aluminium alloy in the form of sheet was used for the present study.
ACCEPTED MANUSCRIPT The key properties of 6061 aluminium alloy material are: 110 MPa yield strength, 205 MPa tensile strength. The blank sheet has been cut into circle with a diameter of 50 mm, and a thickness of 0.8 mm. The surface roughness (Ra) is about 0.1 µm. The stamping tests were performed by two different surface structures of removable die inserts (non-textured and triangular textured), respectively, to compare and analyze
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the influence of LST. The value of texture depth, area ratio, etc. will affect the tribological properties of the metal material [23,24]. In present study, the triangular textured die insert with the area ratio of 15% was laser ablated using a MFT-20 laser process system, and the triangular texture was distributed in the fillet radius area of
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the die insert. The side length of the triangular texture was 550 µm, which is optimized according to the research group [13]. The punching took place along the
Table 1. Table 1
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side to its diagonal of the triangular texture. The basic laser parameters are listed in
Laser parameters of producing triangular texture. Parameters
Marking times
Marking speed
Switch speed
Q rate
Power
Distance
Values
100
1000 mm/s
4000 mm/s
100 KHz
50%
0.01 mm
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Two sets of die inserts were smoothly grinding to get an average surface roughness of about 0.3 µm. All experimental die inserts were cleaned in acetone and alcohol ultrasonic cleaning for 10 min, and then dried in air to keep the surface condition as constant as possible before each stamping process. The choice of
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lubricant in the experiment was commercially available poly-alpha-olefin oil, which had a kinematic viscosity of 54cSt at 40 ℃.
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2.3. Simulation
Numerical simulation was performed using both SolidWorks and Dynaform.
SolidWorks was used to draw the stamping dies with non-textured and triangular textured die inserts according to the dimensions of the cup-shaped part in Fig. 2. Thus, Fig. 3 shows the 3D model of the deep drawing process, which is already imported into Dynaform. It can be seen that compared with the non-textured surface as shown in Fig. 3(a), the triangular texture represented by the triangle consistent with the actual laser process is drawn in the fillet radius area of the die as shown in Fig. 3(b). For the stamping of sheet metal, the theory of finite deformation is adopted. Elastic deformation and springback of materials must be considered in the process of stamping
ACCEPTED MANUSCRIPT process, and the blank material of stamping test in the model was a 6-series aluminum alloy sheet material (Al6061), adopting the elastoplastic material model. Its material density, Young’s modulus, and poisson’s ratio are 2700 Kg/m3, 70 Gpa, and 0.3 respectively. Based on obvious anisotropy in alloy sheet stamping, barlat yield criterion is used to analyze the friction problem. The quality of meshing affects the accuracy of
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CAE analysis and the speed of calculation. Especially, since the side length of the triangular texture is only 0.55 mm, the mesh size seting of the die includes a maximum size of 0.15 mm and a minimum size of 0.05 mm. Adaptive meshing method is adopted to automatically refine the mesh in areas with complex stress and strain and dramatic
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changes. Among them, the Belytschko-Tsay shell element (#2) is used, and plate element type is quadrilateral. Fig. 4 shows the mesh maps of the stamping process. Also, the dynamic explicit incremental algorithm is adopted without the problem of
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iterative convergence. As for the boundary conditions, since the type of double acting process is used in this simulation, the stamping process was realized by setting the action parameters of the binder and the punch. The penalty function was used to calculate the value of the contact force, and the forming problem of sheet metal was studied by setting different friction coefficients. Other boundary condition parameters
Table 2
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were shown in the Table 2.
The relevant parameters of simulation. Simulation parameters
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Die temperature (℃)
Value 25
25
Blank material
Al6061 (36#)
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Intial temperature of workpiece (℃)
Friction coefficient
0.03, 0.06, 0.09, 0.125, 0.15, 0.18, 0.21
The minimum mesh size of die (mm)
0.05
The maximum mesh size of die (mm)
0.15
Blank holder force (KN)
25
Stamping speed (mm/s)
20
Drawing depth (mm)
9
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b
Fig. 3. Deep drawing process 3D model schematics of non-textured (a), and triangular textured (b)
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die.
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a
b
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c
Fig. 4. The mesh maps of non-textured (a), triangular textured (b) and the partial magnified triangular textured (c) die.
2.4. Stamping test
The prepared non-textured and triangular textured die inserts are shown in Fig. 5. Two types of die inserts, except for the laser process, keep all other parameters consistency. It can be seen from Fig. 5(b) that the triangular texture is induced at the fillet radius area of the insert while the fillet radius of the insert shown in Fig. 5(a) is not laser processed.
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b
Fig. 5. The die inserts of non-textured (a), and triangular textured (b).
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The surface topography of the die inserts was measured before the experiment to ensure that the stamping experiments were carried out with high die surface quality. In particularly, it provided a guarantee that surface topography changes of die inserts
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were caused by the friction and wear between the blank sheet and the die during the stamping process. The 3D and 2D topography of triangular textured die insert are shown in Fig. 6. It can be seen from Fig. 6(a) that due to the ablating of laser process and the accumulation of molten metal, and Fig. 6(b) shows more clearly that the edge of the triangular texture produced iron filings and the interior was covered by burrs. Fig.
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6(c) shows that the average depth of the textured die surface was about 10 µm. The presence of metal oxide on the die insert surface indicated that some of the laser-removed material entered the air directly, and the other was oxidized then splashed onto the surface of the die insert. Burrs and metal oxide particles increase the
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surface roughness of the die insert and, if left untreated, can influent the reliability of the experimental results. Therefore, textured die insert was grinding to remove the burrs and metal oxide particles after the LST process, and keep the die inserts surface
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roughness around 0.3 µm during the whole test. a
b
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c
Fig. 6. The 3D topography (a), Enlarged 3D topography (b), and 2D topography (c) of triangular textured die inserts.
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Fig. 7 shows the surface topography of the die inserts after grinding. The surface topography of die insert was magnified 150 times by a scanning electron microscope,
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Such as SEM of non-textured die insert (Fig. 7(a)) and triangular textured die insert (Fig. 7(b)). The 3D topography of the inserts obtained by the white light interference three-dimensional shape meter is shown in the Fig. 7(c) and the Fig. 7(d), and it can be seen that the non-textured and the triangular textured die insert surfaces are smooth and flat, and defects such as bulges and burrs at the edge of the texture have been grinding without significant scratches. The roughness of the inserts surface can be
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seen in Fig. 7(e) and Fig. 7(f), and the surface topography fluctuates within a small
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range and the surface roughness is about 0.3 µm.
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c
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e
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f
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Fig. 7. SEM of non-textured (a), SEM of triangular textured (b), 3D topography of non-textured (c), 3D topography of triangular textured (d), 2D topography of non-textured (e), and 2D topography of triangular textured (f) die insert.
Through the comprehensive evaluating the quality of the molded parts under the
above different parameters conditions, the process parameters selected during the stamping process, the blank holder force was 25 KN, the appropriate amount of lubricating oil was set to form the fluid lubrication conditions and the stamping speed was 20 mm/s, which were consistent with the process parameters of the simulation. Before the experiment, two sets of rubber were placed diagonally to ensure the uniformity of the blank holder force. The entire set of stamping die was clamped and
ACCEPTED MANUSCRIPT fixed symmetrically on the press during the stamping process. Under the load of 15 MPa, 400 stamping experiments on 6061 aluminum alloy sheet material were performed using the non-textured die insert and the triangular textured die insert, respectively. After each stamping, the stamping press was held for 5s to prevent the spring back and other defects, the pressurizer's accumulator was pressurized every 5
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times for the accumulator a sufficiently stable pressure supply, approximately 15 MPa. The surface topography of the die inserts was measured by a HITACHI S-3400 scanning electron microscope after experiments.
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3. Results and discussion
3.1. Simulation of the effect of texture on die friction coefficient
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In this simulation, the friction coefficient was chosen to be 0.125. As shown in Fig. 8, the biggest thinning rate of molded part processed by the non-textured die and that of molded part processed by the triangular textured die were 14.58% (Fig. 8(a)) and 13.09% (Fig. 8(b)), respectively. Generally, whether the die surface was textured or not, the thinning rate of the molded part was relatively high, which might be due to the small diameter of the punch and extremely fast stamping speed. When the blank
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sheet was pressed inward, the molded part was squeezed and wrinkle defects were prone to occur. At the same time, a large profile curvature change occurs the fillet at the bottom of the cup-shaped molded parts and the blank sheet undergone a large resistance during the stamping process, which lead to the tensile stress was relatively
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high, so that the thinning rate was increased [25]. Compared with the molded part processed by the non-textured die, the thinning rate of the molded part processed by
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textured die is decreased by 1.5%. It shows that the texture can reduce the thinning rate of the molded part. This may be due to the triangular texture at the fillet radius area reduces the contact area during the stamping process, and improves the lubrication conditions. During the stamping process, when the direction of friction is along a vertex angle to the opposite side, the lubricant flowed from the wide space of triangle to the narrow vertex, which was beneficial to the generation of hydrodynamic action [21]. Zhang et al. [20] studied the tribological properties of different textures and the triangular texture had the lowest friction coefficient during the pin-on-disk tests. By analysis contour the maps of pressure distribution for the triangular texture, the authors evaluated the up-thrust/lift effect by relative magnitude of areas of pressure
ACCEPTED MANUSCRIPT peak and pressure trough permits, and found that the areas of pressure peak for tests of triangular are much larger than the areas of pressure trough, which suggesting the generation of significantly higher up-thrust to reduce contact and friction coefficient. On the other hand, introducing a surface texture can entrap the debris particles for prevent secondary wear, create a smoother interface and, consequently, reduce friction
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coefficient [26].As the blank sheet resistance was reduced, the deformation that occurred decreased during the stamping process, which proved that the triangular textured die did improve the quality of the molded part. a
b
13.09%
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14.58%
Fig. 8. Thinning rate of cup-shaped molded part under the action of the non-textured (a), and the triangular textured (b) die.
In order to study the influence of texture on the tribological properties of
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stamping die, the relationship between the friction coefficient and the thinning rate of the molded part during the stamping process was simulated. In the case where the molded part was free from defects such as damage and wrinkles, Fig.9 presented the
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thinning rate of cup-shaped molded parts processed by the non-textured and triangular textured die inserts with different friction coefficients at the same blank holder force condition. The results showed that there was a clear transition of thinning rate for both
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cup-shaped molded parts from a low value to a higher one with increasing the friction coefficients during the stamping process. However, thinning rate of molded parts processed by triangular textured die insert achieved a significantly decrease, compared with that of molded parts processed by non-textured die insert. Studies have shown that LST technology generated different microstructures by ablation, and these microstructures could retain lubricants, trap wear debris, and simultaneously formed hydrodynamic lubrication effects, thereby significantly improving the tribological properties of the material [27].The presence of texture can affect tribological properties of the metal, such as friction coefficient [12,28], Fig. 9 verified that the thickness of the molded parts and coefficient of friction had a definite relationship.
ACCEPTED MANUSCRIPT Therefore, it can be obtained that the change in the thickness of the molded parts is mainly due to the fact that the presence of the triangular texture changes the coefficient friction of the die. This indicated that triangular texture improves the tribological properties of stamping die by reducing the coefficient friction. 45
Parts processed by triangular textured die Parts processed by non-textured die
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40
30 25 20 15
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Thinning rate （ %）
35
10 5 0.10
0.15
0.20
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0.05
Friction coefficient
Fig. 9. Thinning rate of the fillet at the bottom of the cup-shaped molded parts with non-textured and triangular textured die at different friction coefficients.
Fig. 10 shows the forming limit diagram (FLD) of the molded parts. As can be
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seen from Fig. 10(a) and Fig. 10(b), the molded parts did not appear to be wrinkled or broken [25]. The nodes of molded parts processed by the non-textured or the triangular textured die inserts were located in the safety zone, and no node appears in the rupture zone or the rupture danger zone, even the thinnest area in the fillet at the
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bottom. It was worth noting that the molded part processed by triangular textured die insert had a larger area at the safe zone than the one processed by the non-textured die
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insert. Therefore, it can be concluded that by inducing the texture on the surface of the stamping die, the forming quality of the molded part had been improved. a
b
Fig. 10. Forming limit diagram (FLD) of non-textured (a), and triangular textured (b) molded parts.
ACCEPTED MANUSCRIPT 3.2. Surface topography of die inserts The SEM surface topography of the non-textured and triangular textured die inserts in the fillet radius area was magnified 400 times after experiments, as shown in Fig. 11. As can be seen from Fig. 11(a), there are some obvious scratches on the surface of the non-textured insert. The reason was that there were large contact area
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and contact stress between the sheet blank and die insert when the sheet blank was squeezed into the die insert, and stamping speed was relatively high, which resulted in a large range of friction on the die insert surface. At the same time, it can also be seen from Fig. 11(c) that due to the severe friction between the sheet blank and the die
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insert, the resulting abrasive particles had a larger volume and were subjected to ploughing and cutting by the pressing force of the contact surface, thereby forming
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obvious furrows. During the stamping process, a rising temperature was generated between the contact surface of the sheet blank and the die, which resulted in the destruction of the lubricating oil film [29]. And the punch continued to pull down deeply, the adhesion between the sheet and the die caused a surface topography similarly to the glue phenomenon (Fig. 11(e)). By observing the surface topography of the non-textured die inserts after experiments, the die surface had a large friction
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and wear, and the die insert surface was seriously damaged. However, comparing the surface wear of the non-textured die insert, although the surface of the triangular textured die insert was also worn, the surface wear was significantly lighter. The textured die insert was littered with fine abrasive particles Fig. 11(b) and produce
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shallow and sparse scratches Fig. 11(d). During the stamping process, triangular texture plays an active role. On the one hand, the abrasive particles produced during
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the relative sliding process are stored, and the three-body friction due to the presence of abrasive particles is reduced [21,30,31]. So, damage to the contact surface caused by the furrow effect is avoided. On the other hand, the lubricant oil stored in texture is squeezed by the relatively sliding contact surface, and provide sufficient bearing capacity for the contact surface. At the same time, secondary lubrication at the contact surface is formed, which creates a hydrodynamic lubrication effect on the contact surface [32]. At the edge of the texture, traces of exfoliation can be seen Fig. 11(f). The reason was that the contact stress is relatively large, and the stress of the edge of the texture concentrates, which caused the edge of the texture to break and the overall structure of the texture was destroyed.
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Fig. 11. Surface topography of three different areas of non-textured (a)(c)(e), and three different
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areas of triangular textured (b)(d)(f) die inserts after 400 stamping tests.
3.3. Thinning rate of molded parts It can be seen from the thinning rate of molded parts in Fig. 8 that the area where
the most significant change in thickness occurs was the fillet area at the bottom and the flange of the molded parts. After stamping tests, the thickness of each area of the cup-shaped part under two different die inserts processing conditions was measured. The thickness of the two areas with the most obvious changes was shown in Fig. 12. Fig. 12(a) shows the thickness of the flange area of the cup-shaped parts. It is generally seen that the average thickness of the flange portion of the cup-shaped parts
ACCEPTED MANUSCRIPT processed by non-textured die was larger than that of the cup-shaped parts processed by triangular textured die. This indicates that the stamping test using the triangular textured die insert can reduce the sharp change in the thickness of the flange area of the molded parts, so that the thickness variation of the molded parts is more uniform. Fig. 12(b) measured the average thickness of the 50 (the values of equally interval in
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400 molded parts) cup-shaped parts in the fillet area at the bottom under the conditions of the triangular textured die and the non-textured die, respectively. Although the degree of thickness variation of the fillet area is more pronounced, in general, the triangular textured die insert can alleviate the reduction in the thickness of
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the cup-shaped parts in the fillet area at the bottom, and avoid the problem of cracking of the molded parts. It can be noted that applying the triangular texture to the stamping die does improve the quality of the cup-shaped parts, which is consistent
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with the simulation results.
a 76.5
parts processed by triangular texture die parts processed by non-texture die
76.0 75.5
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74.5 74.0 73.5 73.0 72.5 72.0
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71.5
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Thickness(µm)
75.0
71.0
0
10
20
30
40
Cup-shaped molded parts
50
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parts processed by triangular texture die parts processed by non-texture die
88.0
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87.0 86.5 86.0 85.5 85.0 84.5 84.0 83.5 10
20
30
40
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Thickness (µm)
87.5
50
Cup-shaped molded parts
Fig. 12. Average thickness of molded parts in the bottom fillet area (a), and flange area (b) under the processing conditions of non-textured and triangular textured die insert.
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4. Conclusions
The study analyzed the effect of applying the triangular texture to the stamping die on the tribological properties by means of numerical simulation and experimental
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verification and the following conclusion could be reached.
1) Triangular texture was induced in the fillet radius area of the stamping die insert
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using the LST process. At the same time, according to the dimension parameters of the cup-shaped part, the study built a complete 3D simulation model of the die.
2) Numerical simulations have found that compared to the molded part processed by non-textured die inserts, thinning rate of the bottom fillet area of molded parts was reduced by 1.5% and the formability was improved after inducing the triangular texture in the die insert, which because the texture affects the tribological properties of the die during the stamping process and thus has an impact on the quality of molded parts. 3) The stamping study results demonstrated that applying LST technology to the die can significantly reduce the friction of the die surface. After 400 stampings, the
ACCEPTED MANUSCRIPT die induced triangular texture had fewer surface defects. At the same time, the thickness analysis of the molded parts found that the thinning rate of the molded parts processed by the triangular textured die was generally lower than that of the molded parts processed by the non-textured die at the thinnest area (in the fillet area at the bottom). The thickness of the molded parts processed by triangular
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textured die changed relatively small in the measured fillet area at the bottom and flange area.
Acknowledgements
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The authors would like to thank the National Key Research and Development Program of China (Grant No. 2018YFC0810500 and No. 2017YB0603500) and the Fundamental Research Funds for the Central Universities of China (Grant
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No.FRF-GF-17-B20) for the financial support. The authors also would like to express special thanks to Tianmin Shao (State Key Laboratory of Tribology, Tsinghua University) for scientific guidance. References
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ACCEPTED MANUSCRIPT Engineering 2015; 42:224-230. [9] Chen P, Shi Z, La Y, Xiang X. Stamping simulation analysis based on tribological properties of die-binder area. China Surface Engineering 2018; 31(1): 59-66. [10] Gachot C, Rosenkranz A, Hsu SM, Costa HL.A critical assessment of surface texturing for friction and wear improvement. Wear 2017; 372-373:21-41. [11] Wan Y, Li J. Effect of laser-texturing morphology on tribological performance of face seal
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pairs. Laser Technology, 2015; 39(04):506-509. [12] Vlădescu SC, Olver AV, Pegg IG, Reddyhoff T. Combined friction and wear reduction in a reciprocating contact through laser surface texturing. Wear 2016; 358-359:51-61.
[13] Chen P, Xiang X, Shao T, La Y, Li J. Effect of triangular texture on the tribological performance of die steel with TiN coatings under lubricated sliding condition. Appl Surf Sci
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[15] Segu DZ, Choi SG, Choi JH, Kim SS. The effect of multi-scale laser textured surface on lubrication regime. Appl Surf Sci 2013; 270:58-63.
[16] Pereira MP, Weiss M, Rolfe BF, Hilditch TB. The effect of the die radius profile accuracy on wear in sheet metal stamping. Int J of Mach Tools Manufact 2013; 66:44-53. [17] Ghosh M, Miroux A, Werkhoven RJ, Bolt RJ, Kestens LAI. Warm deep-drawing and post drawing analysis of two Al–Mg–Si alloys. J Mater Process Tech 2014; 214:756-766.
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[18] Li W, Zhan L, Zhao J. Effect of hot stamping process on forming quality of 6061 aluminum alloy U-shaped parts. The Chinese Journal of Nonferrous Metals 2016; 26(06):1159-1166. [19] Sandberg O, Bustad P.-Å., Carlsson B, Fällström, M, Johansson T. Characterization of tool wear in stamping of EHS and UHS steel sheets, in: Proceedings of the International
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Textures on the Performance of the Lubricated Point-Contacts. J.Tribol 2013; 135(2):021503.
[22] Pereira MP, Yan W, Rolfe BF. Sliding distance, contact pressure and wear in sheet metal stamping. Wear 2010; 268:1275-1284. [23] Scaraggi M, Mezzapesa FP, Carbone G, Ancona A, Sorgente D, Lugarà PM. Minimize friction of lubricated laser-microtextured-surfaces by tuning microholes depth. Tribol Int 2014; 75:123-127. [24] Sun Q, Hu T, Fan H, Zhang Y, Hu L. Dry sliding wear behavior of TC11 alloy at 500°C: Influence of laser surface texturing. Tribol Int 2015; 92:136-145.
ACCEPTED MANUSCRIPT [25] Trzepiecinski T, Malinowski T, Pieja T. Experimental and numerical analysis of industrial warm forming of stainless steel sheet. J MANUF PROCESS 2017; 30:532:540. [26] Joshi G, Putignano C, Gaudiuso C,Stark T, Kiedrowski T, Ancona A, Carbone G. Effects of the micro surface texturing in lubricated non-conformal point contacts. Tribol Int 2018; 127:296-301. [27] Braun D, Greiner C, Schneider J, Gumbsch P. Efficiency of laser surface texturing in the
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reduction of friction under mixed lubrication. Tribol Int 2014; 77:142-147. [28] Galda L, Sep J, Prucnal S. The effect of dimples geometry in the sliding surface on the tribological properties under starved lubrication conditions. Tribol Int 2016; 99:77-84.
[29] Xiong D, Qin Y, Li J, Tyagi R. Tribological properties of PTFE/laser surface textured stainless steel under starved oil lubrication. Tribol Int 2015; 82:305-310.
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[30] Kovalchenko A, Ajayi O, Erdemir A, Fenske G, Etsion I. The effect of laser surface texturing on transitions in lubrication regimes during unidirectional sliding contact*. Tribol Int 2005; 38:219-225.
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[31] Wang L. Use of structured surfaces for friction and wear control on bearing surfaces. Surf Topogr Metrol Prop 2014; 2 (2):043001.
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PTFE material under dry friction. Tribol Int 2018; 118:37-45.
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Highlights: 1. Triangular texture effectively improve the tribological properties of stamping die. 2. The thickness of molded part and friction coefficient have a certain relationship.
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3. The triangular textured die after stamping test has fewer friction defects.
S0301-679X(18)30604-2
DOI:
https://doi.org/10.1016/j.triboint.2018.12.025
Reference:
JTRI 5525
To appear in:
Tribology International
Received Date: 24 August 2018 Revised Date:
28 November 2018
Accepted Date: 19 December 2018
Please cite this article as: Chen P, Liu X, Huang M, Shi Z, Shan B, Numerical simulation and experimental study on tribological properties of stamping die with triangular texture, Tribology International (2019), doi: https://doi.org/10.1016/j.triboint.2018.12.025. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Numerical simulation and experimental study on tribological properties of stamping die with triangular texture Ping Chen*, Xiaojie Liu, Mingji Huang*, Zhe Shi, Bin Shan
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School of Mechanical Engineering, University of Science and Technology Beijing, Beijing 100083, China
Abstract: This paper discussed the effect of applying the LST to the stamping die on
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tribological properties under oil lubrication by simulation and experiments. And the triangular texture was induced at the fillet radius of the die. The simulation results showed that the molded parts processed by die with triangular texture exhibited lower
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thinning rate in the fillet area at the bottom than those processed by die without texture, and we also found that the friction coefficient has an effect on the thickness of molded parts. The experiment results indicated that the triangular texture makes the thickness variation of the molded parts smaller, which was in good agreement with the simulation results.
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Keywords: Triangular texture; Numerical simulation; Stamping experiment; Tribological properties
1. Introduction
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Stamping forming is one of the crucial plastic processing methods in sheet metal forming industry. The components produced with the die have many advantages of high precision, high complexity, high consistency, and high productivity, which are
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unmatched by other manufacturing methods and result in the metal processing method of stamping forming has been therefore widely used [1,2]. However, in the modern stamping process, the increase in demand of stamping parts materials such as high-strength steels or ultra-high-strength steels, the reduction in the use of lubricating oil for environmental protection, and so on [3,4], these factors require the die to withstand higher contact pressure, and result in unacceptable levels of friction and wear [5]. Therefore, the way to reduce wear of metal stamping die has become a critical
* Corresponding author. Tel: +86 01062332538; fax: +86 01062329145 E-mail address: [email protected] (P. Chen); [email protected] (M.J. Huang)
ACCEPTED MANUSCRIPT issue in stamping production research. Various aspects of stamping die for tribological applications have been studied by several researchers. Pereira et al. [6] firstly reported on the wear over the die radius caused by a combination of ploughing and galling mechanisms. Different angles on the die fillet radius had varying degrees of wear type and wear severity. In particular,
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use of lubricant during the stamping process can reduce the friction between the die and the sheet materials. Kim et al. [7,8] discussed the effect of the different lubricants during the stamping process, and obtained polymer based thin film lubricants with pressure additives performed better than other lubricants to improve tribological
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properties. With the aim of improving the tribological properties of the die-binder area under the oil lubrication condition, Chen et al. [9] carried out the pin-on-disk experiment and investigated the stamping die induced triangular texture expressed a
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good result in reducing friction to be compatible with the stamping die without texture. The laser surface texturing (LST) technology, performing micro-structure inducing on the metal surface, has become a significant surface engineering processing technology to improve the tribological properties. It was found that changing the surface topography of most materials by LST technology can achieve the
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effect of reducing the friction and wear of the material in different lubrication conditions [10]. An abrasion test on the steel plates with laser process induced two kinds of micro-hole shape and groove shape on the friction tester was performed by Wan et al. [11], and the authors found that the friction coefficient and wear amount of
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the textured samples were lower than those of the smooth samples under the condition of oil lubrication, while proper texture topography was beneficial to the formation of
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transfer film as well as the capture of abrasive debris to reduce abrasive wear under the dry friction condition. The results of the reciprocating test showed that the surface micro-structure significantly reduces the friction coefficient when the sample wears and the contact enters the mixed lubrication state. The remarkable result experimentally revealed, surface textured samples exhibit 70% lower friction compared to surface non-textured samples [12]. In order to study the impact of triangular texture of the die-binder area on the tribological properties during the stamping process, Chen et al. [13] performed simulation and experiments, and proposed that using LST technology to reduce the friction coefficient of the die-binder area can significantly improve the formability of the sheet material. Segu et al. [14,15] have also investigated that the multi-scale textured surface perform better in obtaining
ACCEPTED MANUSCRIPT lower and more stable friction coefficient, which can significantly reduce the friction and wear compared with the non-textured surface. Numerical simulations showed that during the stamping process, small changes at die fillet radius profile shape (of less than 15 µm) will cause a significant increase in local contact pressure, and result in increased wear and reduced life of the die [16].
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According to the research by Pereira et al.[6], in typical stamping processes (deep-drawing, plane strain stamping, cup forming, etc.), wear occurred at the die fillet radius, which usually had a cross-sectional profile that was a 90° circular arc (fillet) between the perpendicular surfaces at the die corner. Additionally, an amount
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of research have been developed to study the impact of the stamping process parameters: blank holder force, lubrication conditions, pressure holding time, etc. Indeed, these parameters affect the forming quality of the material [17,18]. At the
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same time, the surface quality of the die is important, and for the automotive sheet metal stampings, failure is typically characterized by the existence of micro-surface scratches of the die [19]. LST technology can generate various surface topography on the surface of metal die. The effect of textures in the form of circular, square, and equilateral triangular on the tribological performance through pin-on-disk tests was
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investigated, and Zhang et al. found that the triangular texture from a vertex angle to the side resulted in the lowest coefficient of friction [20,21]. Hence, in this paper, the effect of the triangular texture on the tribological properties of the stamping die under the oil lubrication is analyzed using a combination of numerical simulation and
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experiments. Based on the analysis of numerical simulation, that is, the triangular texture can improve the tribological properties of the die, stamping experiments
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applying LST to the die surfaces were then performed. It is worth noting that the texture used in our study is an equilateral triangle determined by side length. And the tribological properties of stamping dies were illustrated by observing the surface topography of die after a large number of stamping processes by a scanning electron microscope (SEM).
2. Experimental details 2.1. Stamping die The stamping die diagram is shown in Fig. 1. The stamping test configuration was based on the typical industrial automobile sheet metal stamping test [22] and
ACCEPTED MANUSCRIPT numerical simulation conducted based on the stamping test model. Removable die inserts were designed to achieve easy inspection and timely replacement of the wearing surfaces. One of the primary aims of the stamping test was to determine the tribological properties of textured and non-textured dies by changing the die inserts. The die insert and die were interference fits and were grinding before the assembly
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process to ensure that the surfaces of die insert and die were smooth. The entire sets of stamping die including the die inserts were made of Cr12MoV, and the entire experiment was carried out at room temperature (25 ℃) and relative humidity (40 %
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RH), which were kept constant.
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Fig. 1. Schematic diagram of the stamping die
The molded part is a cup-shaped part with a flange, the specific dimensions of the cup-shaped part are shown in Fig. 2. The molded parts not only meet the
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process.
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dimensional requirements, but also meet the processing requirements of the stamping
Fig. 2. The specific dimensions of the cup-shaped molded part.
2.2 Material The 6061 aluminium alloy in the form of sheet was used for the present study.
ACCEPTED MANUSCRIPT The key properties of 6061 aluminium alloy material are: 110 MPa yield strength, 205 MPa tensile strength. The blank sheet has been cut into circle with a diameter of 50 mm, and a thickness of 0.8 mm. The surface roughness (Ra) is about 0.1 µm. The stamping tests were performed by two different surface structures of removable die inserts (non-textured and triangular textured), respectively, to compare and analyze
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the influence of LST. The value of texture depth, area ratio, etc. will affect the tribological properties of the metal material [23,24]. In present study, the triangular textured die insert with the area ratio of 15% was laser ablated using a MFT-20 laser process system, and the triangular texture was distributed in the fillet radius area of
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the die insert. The side length of the triangular texture was 550 µm, which is optimized according to the research group [13]. The punching took place along the
Table 1. Table 1
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side to its diagonal of the triangular texture. The basic laser parameters are listed in
Laser parameters of producing triangular texture. Parameters
Marking times
Marking speed
Switch speed
Q rate
Power
Distance
Values
100
1000 mm/s
4000 mm/s
100 KHz
50%
0.01 mm
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Two sets of die inserts were smoothly grinding to get an average surface roughness of about 0.3 µm. All experimental die inserts were cleaned in acetone and alcohol ultrasonic cleaning for 10 min, and then dried in air to keep the surface condition as constant as possible before each stamping process. The choice of
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lubricant in the experiment was commercially available poly-alpha-olefin oil, which had a kinematic viscosity of 54cSt at 40 ℃.
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2.3. Simulation
Numerical simulation was performed using both SolidWorks and Dynaform.
SolidWorks was used to draw the stamping dies with non-textured and triangular textured die inserts according to the dimensions of the cup-shaped part in Fig. 2. Thus, Fig. 3 shows the 3D model of the deep drawing process, which is already imported into Dynaform. It can be seen that compared with the non-textured surface as shown in Fig. 3(a), the triangular texture represented by the triangle consistent with the actual laser process is drawn in the fillet radius area of the die as shown in Fig. 3(b). For the stamping of sheet metal, the theory of finite deformation is adopted. Elastic deformation and springback of materials must be considered in the process of stamping
ACCEPTED MANUSCRIPT process, and the blank material of stamping test in the model was a 6-series aluminum alloy sheet material (Al6061), adopting the elastoplastic material model. Its material density, Young’s modulus, and poisson’s ratio are 2700 Kg/m3, 70 Gpa, and 0.3 respectively. Based on obvious anisotropy in alloy sheet stamping, barlat yield criterion is used to analyze the friction problem. The quality of meshing affects the accuracy of
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CAE analysis and the speed of calculation. Especially, since the side length of the triangular texture is only 0.55 mm, the mesh size seting of the die includes a maximum size of 0.15 mm and a minimum size of 0.05 mm. Adaptive meshing method is adopted to automatically refine the mesh in areas with complex stress and strain and dramatic
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changes. Among them, the Belytschko-Tsay shell element (#2) is used, and plate element type is quadrilateral. Fig. 4 shows the mesh maps of the stamping process. Also, the dynamic explicit incremental algorithm is adopted without the problem of
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iterative convergence. As for the boundary conditions, since the type of double acting process is used in this simulation, the stamping process was realized by setting the action parameters of the binder and the punch. The penalty function was used to calculate the value of the contact force, and the forming problem of sheet metal was studied by setting different friction coefficients. Other boundary condition parameters
Table 2
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were shown in the Table 2.
The relevant parameters of simulation. Simulation parameters
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Die temperature (℃)
Value 25
25
Blank material
Al6061 (36#)
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Intial temperature of workpiece (℃)
Friction coefficient
0.03, 0.06, 0.09, 0.125, 0.15, 0.18, 0.21
The minimum mesh size of die (mm)
0.05
The maximum mesh size of die (mm)
0.15
Blank holder force (KN)
25
Stamping speed (mm/s)
20
Drawing depth (mm)
9
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b
Fig. 3. Deep drawing process 3D model schematics of non-textured (a), and triangular textured (b)
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die.
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a
b
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c
Fig. 4. The mesh maps of non-textured (a), triangular textured (b) and the partial magnified triangular textured (c) die.
2.4. Stamping test
The prepared non-textured and triangular textured die inserts are shown in Fig. 5. Two types of die inserts, except for the laser process, keep all other parameters consistency. It can be seen from Fig. 5(b) that the triangular texture is induced at the fillet radius area of the insert while the fillet radius of the insert shown in Fig. 5(a) is not laser processed.
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b
Fig. 5. The die inserts of non-textured (a), and triangular textured (b).
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The surface topography of the die inserts was measured before the experiment to ensure that the stamping experiments were carried out with high die surface quality. In particularly, it provided a guarantee that surface topography changes of die inserts
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were caused by the friction and wear between the blank sheet and the die during the stamping process. The 3D and 2D topography of triangular textured die insert are shown in Fig. 6. It can be seen from Fig. 6(a) that due to the ablating of laser process and the accumulation of molten metal, and Fig. 6(b) shows more clearly that the edge of the triangular texture produced iron filings and the interior was covered by burrs. Fig.
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6(c) shows that the average depth of the textured die surface was about 10 µm. The presence of metal oxide on the die insert surface indicated that some of the laser-removed material entered the air directly, and the other was oxidized then splashed onto the surface of the die insert. Burrs and metal oxide particles increase the
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surface roughness of the die insert and, if left untreated, can influent the reliability of the experimental results. Therefore, textured die insert was grinding to remove the burrs and metal oxide particles after the LST process, and keep the die inserts surface
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roughness around 0.3 µm during the whole test. a
b
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c
Fig. 6. The 3D topography (a), Enlarged 3D topography (b), and 2D topography (c) of triangular textured die inserts.
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Fig. 7 shows the surface topography of the die inserts after grinding. The surface topography of die insert was magnified 150 times by a scanning electron microscope,
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Such as SEM of non-textured die insert (Fig. 7(a)) and triangular textured die insert (Fig. 7(b)). The 3D topography of the inserts obtained by the white light interference three-dimensional shape meter is shown in the Fig. 7(c) and the Fig. 7(d), and it can be seen that the non-textured and the triangular textured die insert surfaces are smooth and flat, and defects such as bulges and burrs at the edge of the texture have been grinding without significant scratches. The roughness of the inserts surface can be
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seen in Fig. 7(e) and Fig. 7(f), and the surface topography fluctuates within a small
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range and the surface roughness is about 0.3 µm.
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c
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e
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f
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Fig. 7. SEM of non-textured (a), SEM of triangular textured (b), 3D topography of non-textured (c), 3D topography of triangular textured (d), 2D topography of non-textured (e), and 2D topography of triangular textured (f) die insert.
Through the comprehensive evaluating the quality of the molded parts under the
above different parameters conditions, the process parameters selected during the stamping process, the blank holder force was 25 KN, the appropriate amount of lubricating oil was set to form the fluid lubrication conditions and the stamping speed was 20 mm/s, which were consistent with the process parameters of the simulation. Before the experiment, two sets of rubber were placed diagonally to ensure the uniformity of the blank holder force. The entire set of stamping die was clamped and
ACCEPTED MANUSCRIPT fixed symmetrically on the press during the stamping process. Under the load of 15 MPa, 400 stamping experiments on 6061 aluminum alloy sheet material were performed using the non-textured die insert and the triangular textured die insert, respectively. After each stamping, the stamping press was held for 5s to prevent the spring back and other defects, the pressurizer's accumulator was pressurized every 5
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times for the accumulator a sufficiently stable pressure supply, approximately 15 MPa. The surface topography of the die inserts was measured by a HITACHI S-3400 scanning electron microscope after experiments.
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3. Results and discussion
3.1. Simulation of the effect of texture on die friction coefficient
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In this simulation, the friction coefficient was chosen to be 0.125. As shown in Fig. 8, the biggest thinning rate of molded part processed by the non-textured die and that of molded part processed by the triangular textured die were 14.58% (Fig. 8(a)) and 13.09% (Fig. 8(b)), respectively. Generally, whether the die surface was textured or not, the thinning rate of the molded part was relatively high, which might be due to the small diameter of the punch and extremely fast stamping speed. When the blank
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sheet was pressed inward, the molded part was squeezed and wrinkle defects were prone to occur. At the same time, a large profile curvature change occurs the fillet at the bottom of the cup-shaped molded parts and the blank sheet undergone a large resistance during the stamping process, which lead to the tensile stress was relatively
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high, so that the thinning rate was increased [25]. Compared with the molded part processed by the non-textured die, the thinning rate of the molded part processed by
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textured die is decreased by 1.5%. It shows that the texture can reduce the thinning rate of the molded part. This may be due to the triangular texture at the fillet radius area reduces the contact area during the stamping process, and improves the lubrication conditions. During the stamping process, when the direction of friction is along a vertex angle to the opposite side, the lubricant flowed from the wide space of triangle to the narrow vertex, which was beneficial to the generation of hydrodynamic action [21]. Zhang et al. [20] studied the tribological properties of different textures and the triangular texture had the lowest friction coefficient during the pin-on-disk tests. By analysis contour the maps of pressure distribution for the triangular texture, the authors evaluated the up-thrust/lift effect by relative magnitude of areas of pressure
ACCEPTED MANUSCRIPT peak and pressure trough permits, and found that the areas of pressure peak for tests of triangular are much larger than the areas of pressure trough, which suggesting the generation of significantly higher up-thrust to reduce contact and friction coefficient. On the other hand, introducing a surface texture can entrap the debris particles for prevent secondary wear, create a smoother interface and, consequently, reduce friction
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coefficient [26].As the blank sheet resistance was reduced, the deformation that occurred decreased during the stamping process, which proved that the triangular textured die did improve the quality of the molded part. a
b
13.09%
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14.58%
Fig. 8. Thinning rate of cup-shaped molded part under the action of the non-textured (a), and the triangular textured (b) die.
In order to study the influence of texture on the tribological properties of
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stamping die, the relationship between the friction coefficient and the thinning rate of the molded part during the stamping process was simulated. In the case where the molded part was free from defects such as damage and wrinkles, Fig.9 presented the
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thinning rate of cup-shaped molded parts processed by the non-textured and triangular textured die inserts with different friction coefficients at the same blank holder force condition. The results showed that there was a clear transition of thinning rate for both
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cup-shaped molded parts from a low value to a higher one with increasing the friction coefficients during the stamping process. However, thinning rate of molded parts processed by triangular textured die insert achieved a significantly decrease, compared with that of molded parts processed by non-textured die insert. Studies have shown that LST technology generated different microstructures by ablation, and these microstructures could retain lubricants, trap wear debris, and simultaneously formed hydrodynamic lubrication effects, thereby significantly improving the tribological properties of the material [27].The presence of texture can affect tribological properties of the metal, such as friction coefficient [12,28], Fig. 9 verified that the thickness of the molded parts and coefficient of friction had a definite relationship.
ACCEPTED MANUSCRIPT Therefore, it can be obtained that the change in the thickness of the molded parts is mainly due to the fact that the presence of the triangular texture changes the coefficient friction of the die. This indicated that triangular texture improves the tribological properties of stamping die by reducing the coefficient friction. 45
Parts processed by triangular textured die Parts processed by non-textured die
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40
30 25 20 15
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Thinning rate （ %）
35
10 5 0.10
0.15
0.20
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0.05
Friction coefficient
Fig. 9. Thinning rate of the fillet at the bottom of the cup-shaped molded parts with non-textured and triangular textured die at different friction coefficients.
Fig. 10 shows the forming limit diagram (FLD) of the molded parts. As can be
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seen from Fig. 10(a) and Fig. 10(b), the molded parts did not appear to be wrinkled or broken [25]. The nodes of molded parts processed by the non-textured or the triangular textured die inserts were located in the safety zone, and no node appears in the rupture zone or the rupture danger zone, even the thinnest area in the fillet at the
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bottom. It was worth noting that the molded part processed by triangular textured die insert had a larger area at the safe zone than the one processed by the non-textured die
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insert. Therefore, it can be concluded that by inducing the texture on the surface of the stamping die, the forming quality of the molded part had been improved. a
b
Fig. 10. Forming limit diagram (FLD) of non-textured (a), and triangular textured (b) molded parts.
ACCEPTED MANUSCRIPT 3.2. Surface topography of die inserts The SEM surface topography of the non-textured and triangular textured die inserts in the fillet radius area was magnified 400 times after experiments, as shown in Fig. 11. As can be seen from Fig. 11(a), there are some obvious scratches on the surface of the non-textured insert. The reason was that there were large contact area
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and contact stress between the sheet blank and die insert when the sheet blank was squeezed into the die insert, and stamping speed was relatively high, which resulted in a large range of friction on the die insert surface. At the same time, it can also be seen from Fig. 11(c) that due to the severe friction between the sheet blank and the die
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insert, the resulting abrasive particles had a larger volume and were subjected to ploughing and cutting by the pressing force of the contact surface, thereby forming
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obvious furrows. During the stamping process, a rising temperature was generated between the contact surface of the sheet blank and the die, which resulted in the destruction of the lubricating oil film [29]. And the punch continued to pull down deeply, the adhesion between the sheet and the die caused a surface topography similarly to the glue phenomenon (Fig. 11(e)). By observing the surface topography of the non-textured die inserts after experiments, the die surface had a large friction
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and wear, and the die insert surface was seriously damaged. However, comparing the surface wear of the non-textured die insert, although the surface of the triangular textured die insert was also worn, the surface wear was significantly lighter. The textured die insert was littered with fine abrasive particles Fig. 11(b) and produce
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shallow and sparse scratches Fig. 11(d). During the stamping process, triangular texture plays an active role. On the one hand, the abrasive particles produced during
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the relative sliding process are stored, and the three-body friction due to the presence of abrasive particles is reduced [21,30,31]. So, damage to the contact surface caused by the furrow effect is avoided. On the other hand, the lubricant oil stored in texture is squeezed by the relatively sliding contact surface, and provide sufficient bearing capacity for the contact surface. At the same time, secondary lubrication at the contact surface is formed, which creates a hydrodynamic lubrication effect on the contact surface [32]. At the edge of the texture, traces of exfoliation can be seen Fig. 11(f). The reason was that the contact stress is relatively large, and the stress of the edge of the texture concentrates, which caused the edge of the texture to break and the overall structure of the texture was destroyed.
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Fig. 11. Surface topography of three different areas of non-textured (a)(c)(e), and three different
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areas of triangular textured (b)(d)(f) die inserts after 400 stamping tests.
3.3. Thinning rate of molded parts It can be seen from the thinning rate of molded parts in Fig. 8 that the area where
the most significant change in thickness occurs was the fillet area at the bottom and the flange of the molded parts. After stamping tests, the thickness of each area of the cup-shaped part under two different die inserts processing conditions was measured. The thickness of the two areas with the most obvious changes was shown in Fig. 12. Fig. 12(a) shows the thickness of the flange area of the cup-shaped parts. It is generally seen that the average thickness of the flange portion of the cup-shaped parts
ACCEPTED MANUSCRIPT processed by non-textured die was larger than that of the cup-shaped parts processed by triangular textured die. This indicates that the stamping test using the triangular textured die insert can reduce the sharp change in the thickness of the flange area of the molded parts, so that the thickness variation of the molded parts is more uniform. Fig. 12(b) measured the average thickness of the 50 (the values of equally interval in
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400 molded parts) cup-shaped parts in the fillet area at the bottom under the conditions of the triangular textured die and the non-textured die, respectively. Although the degree of thickness variation of the fillet area is more pronounced, in general, the triangular textured die insert can alleviate the reduction in the thickness of
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the cup-shaped parts in the fillet area at the bottom, and avoid the problem of cracking of the molded parts. It can be noted that applying the triangular texture to the stamping die does improve the quality of the cup-shaped parts, which is consistent
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with the simulation results.
a 76.5
parts processed by triangular texture die parts processed by non-texture die
76.0 75.5
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74.5 74.0 73.5 73.0 72.5 72.0
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71.5
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Thickness(µm)
75.0
71.0
0
10
20
30
40
Cup-shaped molded parts
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parts processed by triangular texture die parts processed by non-texture die
88.0
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87.0 86.5 86.0 85.5 85.0 84.5 84.0 83.5 10
20
30
40
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0
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Thickness (µm)
87.5
50
Cup-shaped molded parts
Fig. 12. Average thickness of molded parts in the bottom fillet area (a), and flange area (b) under the processing conditions of non-textured and triangular textured die insert.
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4. Conclusions
The study analyzed the effect of applying the triangular texture to the stamping die on the tribological properties by means of numerical simulation and experimental
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verification and the following conclusion could be reached.
1) Triangular texture was induced in the fillet radius area of the stamping die insert
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using the LST process. At the same time, according to the dimension parameters of the cup-shaped part, the study built a complete 3D simulation model of the die.
2) Numerical simulations have found that compared to the molded part processed by non-textured die inserts, thinning rate of the bottom fillet area of molded parts was reduced by 1.5% and the formability was improved after inducing the triangular texture in the die insert, which because the texture affects the tribological properties of the die during the stamping process and thus has an impact on the quality of molded parts. 3) The stamping study results demonstrated that applying LST technology to the die can significantly reduce the friction of the die surface. After 400 stampings, the
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textured die changed relatively small in the measured fillet area at the bottom and flange area.
Acknowledgements
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The authors would like to thank the National Key Research and Development Program of China (Grant No. 2018YFC0810500 and No. 2017YB0603500) and the Fundamental Research Funds for the Central Universities of China (Grant
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No.FRF-GF-17-B20) for the financial support. The authors also would like to express special thanks to Tianmin Shao (State Key Laboratory of Tribology, Tsinghua University) for scientific guidance. References
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Highlights: 1. Triangular texture effectively improve the tribological properties of stamping die. 2. The thickness of molded part and friction coefficient have a certain relationship.
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3. The triangular textured die after stamping test has fewer friction defects.