| Research Article |
Open Access |
|
| FEM Analyses of Friction Coefficient in Open Die Coining Process of
Different Grain Sizes |
| Zdenka Keran*, Miljenko Math and Petar Piljek |
| Faculty of Mechanical Engineering and Naval Architecture, Department of Technology, Ivana Lucica, Croatia |
| *Corresponding author: |
Dr. Zdenka Keran
Faculty of Mechanical Engineering
and Naval Architecture
Department of Technology
Ivana Lucica 5HR-10000
Zagreb, Croatia
E-mail: zdenka.keran@fsb.hr |
|
| |
| Received January 19, 2012; Accepted January 23, 2012; Published January 26,
2012 |
| |
| Citation: Keran Z, Math M, Piljek P (2012) FEM Analyses of Friction Coefficient
in Open Die Coining Process of Different Grain Sizes. J Material Sci Engg 1:105.
doi:10.4172/2169-0022.1000105 |
| |
| Copyright: © 2012 Keran Z, et al. This is an open-access article distributed under
the terms of the Creative Commons Attribution License, which permits unrestricted
use, distribution, and reproduction in any medium, provided the original author and
source are credited. |
| |
| Abstract |
| |
| The study concerns the analysis of significant contact friction changes by changing the size of crystal grains in
the processes of free axisymmetric work pieces upsetting, initial height of 2 mm. The friction changes result with
significant changes in forming force. This phenomenon usually characterizes micro forming processes. Using hard
experimental data in the creation of numerical FE (finite element) model, a dependence of the changes of contact
friction in correlation to the change of the work piece crystal grain size for three different grain sizes: 39, 47 and
76 μm is presented. It is shown that the friction factor is increased by reducing the size of crystal grains. Physical
interpretation of the results is given by theory of Bowden and Tabor. |
| |
| Keywords |
| |
| Coining process; Numerical simulation; Friction
coefficient |
| |
| Introduction |
| |
| Coining is a deformation processing of metallic materials. In its
definition it includes production of coins, medals, and some other
products that demand very fine surface microgeometry. Although this
process seems very simple (the only condition is traceability of surface
microgeometry achieved by the deformation of surface layer of the
blank), some serious problems take a place [1]: |
| |
| - Surface damage, |
| |
| - Insufficient filling, |
| |
| - Excess lubricant, |
| |
| - Foreign substance, |
| |
| - Deformation during unloading caused by residual stresses in the
work piece and/or elastic springback of the material. |
| |
| Also, one of the major problems that occur in coining processes is a
large influence of grain size on amount of forming force. |
| |
| Experimental research work [3] verified solving of spring back
problems when coining is treated as a micro forming process. This
approach takes into consideration grain size influence on the process
parameters. Hypothesis of the research is that blank material, because
of the size influence, acts according to micro forming postulates [1].
Results confirmed this presumption and showed significantly different
forming force for different grain sizes. Aluminium blanks (Al 99.5),
initial high 2 mm, initial diameter 20 mm were deformed with different
reduction coefficient in open and closed die. Experimental tool is
designed with two parallel surfaces and deformation is accomplished
by their relative movement. Photography of the tool is presented at
figure 1. |
| |
| Previous annealing resulted with three different grain sizes: 76 μm,
47 μm and 39 μm – table 1. Measured parameters were: forming force,
total deformation, elastic spring back and die filling (Figure 1) |
| |
|
Figure 1: (A) photo of experimental tool. |
|
| |
|
Table 1: Grain sizes achieved by specific heat treatment regime. |
|
| |
| For the causes of further numerical investigation of friction
coefficient, only one pair of measured parameters is observed. That is
the relation between total forming force and total deformation. As it
was expected (according to hypothesis of the research), the opposite
results have been obtained in those two experimental conditions – open
and closed die. In open die experiments forming force raises up with
smaller grain size (Figure 2). On the opposite, in closed die forging,
forming force is decreased with decreased grain size (Figure 3). |
| |
|
Figure 2: A relation between forming force and total displacement - open die coining. |
|
| |
|
Figure 3: A relation between forming force and total displacement - closed die coining. |
|
| |
| Although, it would be interesting to make a numerical analyses in
both cases – open and closed die coining, in particular case it is not
possible because of very small geometry dimensions of gravure details
that cannot be digitalized and prepared for the numeric model. That
is why numerical simulation has been performed only for the case of
open die coining. |
| |
| Numerical Simulation |
| |
| As it is presented in the introduction, significant changes of contact
friction occur with changes in grain size. With intention to explain this phenomenon contact friction has been analyzed using finite element
method (FEM) and the purpose of the analyze is a description of
correlation between grain size and friction coefficient in case of open
die forging. |
| |
| Numerical simulation of an open die forging has been supported
by MSC Marc Mentat program package. Axysimmetric 2D FEM model
has been created. Material is defined as elasto-plastic isotropic. In
elastic field constants for Al99, 5 are: Young modulus E = 69000 N/
mm2, Poisson coefficient u = 0,33. Isotropic plasticity is modelled using
experimentally obtained results – the correlation between true strain
and maximal specific stress for three different grain sizes. |
| |
| Element type 10 is a four-node, isoparametric, arbitrary
quadrilateral is used. As this element uses bilinear interpolation
functions, the strains tend to be constant throughout the element.
This element is preferred over higher-order elements when used in
a contact analysis. The stiffness of this element is formed using fourpoint
Gaussian integration [2]. |
| |
| Coulomb friction, that is used, is a highly nonlinear phenomenon
dependent upon both the normal force and relative velocity. When the
stress based friction model is used, the following steps are taken [2]. |
| |
| 1. Extrapolate the physical stress, equivalent stress, and
temperature from the integration points to the nodes using the
conventional element shape functions. |
| |
| 2. Calculate the normal stress. |
| |
| 3. Calculate the relative sliding velocity. At the beginning of an
increment, the previously calculated relative sliding velocity
is used as the starting point. When a node first comes into
contact, it is assumed that it is first sticking, so the relative
sliding velocity is zero. |
| |
| 4. Numerically integrate the friction forces and the stiffness
contribution. |
| |
| Model has been created from 2000 elements and analyses open die
coining up to φ = 0,6 (true strain). Figures 4 and 5 present initial mesh
and deformed one with its maximum distortion. |
| |
|
Figure 4: 2D Axisymmetric FE model created of 2000 elements. |
|
| |
|
Figure 5: Distorted mesh at the end of coining process. |
|
| |
| Algorithm For Determination of Friction Coefficient |
| |
| By using well known constants for defining material properties
in elastic field, familiar geometry of axisymmetric 2D model,
experimentally defined curves (true strain – flow stress dependence for
different grain sizes in plastic area), simple boundary conditions and
contact definition, the initial base for algorithm developing is set. This
algorithm is presented in figure 6. |
| |
| Another relevant factor for its development is: experimentally
obtained maximal forming forces for different grain sizes. These forces
should be achieved using numerical simulation (in a range ±10%), so
that numerical model could be relevant, and friction coefficient valid. |
| |
|
Figure 6: Distorted mesh at the end of coining process. |
|
| |
| Results of Numerical Simulation |
| |
| By using a described algorithm a numerical simulation has been
performed. In calculation of forming forces, which have been achieved in numerical simulation, confidence interval of ±10% according
to experimental values was predicted. Maximal calculated forming
forces match different friction coefficients for different grain sizes.
Parallel overview of experimental and calculated results together with
associated friction coefficient is presented in table 2. |
| |
|
Table 2: Friction coefficients obtained by numerical simulation. |
|
| |
| Interpretation of Difference In Friction Coefficient |
| |
| For physical interpretation of difference in friction coefficient,
the Bowden and Tabor adhesion model or plastic junction model
is used. This model gives an access into friction nature and into
friction coefficient changes in relation to surface roughness and used
lubricants. The base of this model (as it is showed in figure 7) is that
the real area of contact is made up of a large number of small regions of
contact, in the literature called asperities or junctions of contact, where
atom-to-atom contact takes place. When rough surface slides against
a softer surface, in adhesive wear, asperity junctions plastically deform
above a critical shear strength, which depends on the adhesive forces
of the two surfaces in contact. Assuming during a frictional sliding
process a fully plastic flow situation of all asperities, friction is found
to change linearly with the applied load. This load regularly reaches
2000-2500 MPa distributed on convex spots. Under such point load
micro-welding occurs. In order to achieve sliding, micro welds must
be broken. Therefore according to Bowden and Tabor friction force
becomes a sum of all shear forces needed for this breaking. |
| |
|
Figure 7: Bowden and Tabor adhesion model [3]. |
|
| |
| According to described model it is possible to make an explanation
of friction coefficient increase by decrease of grain size of the material.
It can be assumed that smaller grain size makes larger number of critical
convex spots. In this way the number of micro-welds is increased, and
also a sum of all shear forces that need to be broken. Increased shear
forces indicate increased friction forces. |
| |
| Conclusion |
| |
| As it is presented, some significant changes of process parameters
take place when changes of grain size occur. These changes are referred
in a first place to the amount of maximal forming force that is needed
to achieve a proper deformation of the coin. In a case of open die
coining process maximal forming force raises with smaller grain size
for the same range of total deformation. Numerical analysis has been
performed to describe behaviour of friction (in cases of different grain
size) that can also affect the size of forming force. |
| |
| Analysis showed that calculated friction coefficient grows up when
grain size decreases. The physical explanation of this phenomenon is
given by Bowden and Tabor adhesion model. |
| |
| The conclusion leads to recommendation for achieving the optimal
process parameters. In order to reduce the level of friction it would be
necessary to enlarge the grain size of used material. By enlarging the
grain size, forming force will also be reduced. This recommendation is
valid only in a case of open die forging. |
|
| |
| Further research work should be based in analysis and description
of closed die forging and results comparison. |
| |
| Acknowledgements |
| |
| This study has been supported by The Ministry of Science, Education and
Sports of Republic of Croatia, within the project CAM technologies and modelling
in metal forming and micro forming. |
| |
|
| References |
| |
- U. Engel, R. Eckstein (2002) Microforming – from basic research to its realization. Journal of Materials Processing Technology 125-126: 35-44.
- Goup of authors (2005) MSC. Marc Volume A: Theory and User Information.
- Z. Keran (2010) Plitko gravurno kovanje s aspekta mikrooblikovanja, doktorska disertacija, Zagreb, Fakultet strojarstva i brodogradnje.
- Math M (1999) Uvod u tehnologiju oblikovanja deformiranjem, FSB, Zagreb.
- Justinger H, Hirt G (2009) Estimation of grain size and grain orientation influence in microforming processes by Taylor factor considerations. J Mater Process Technol 209: 2111-2121.
- Ike H (2003) Surface deformation vs. bulk plastic deformation – a key for microscopic control of surfaces in metal forming. J Mater Process Technol 138: 250-255.
- Carden WD, Geng LM, Matlock DK, Wagoner RH (2002) Measurement of springback. International Journal of Mechanical Sciences 44: 79-101.
- Li KP, Carden WP, Wagoner RH (2002) Simulation of springback. International Journal of Mechanical Sciences 44: 103-122.
- Vollertsen F, Schulze Niehoff H, Hu Z (2006) State of the art in micro forming. International Journal of Machine Tools & Manufacture 46: 1172-1179.
- Ike H, Plancak M (1998) Coining process as a means of controlling surface microgeometry. J Mater Process Technol 80-81, 101-107.
|
| |
| |
