Recently metal forming process is preferred compared to other manufacturing
process such as casting and machining due to many advantages offered. The trade-off
is that, it required very high load to accomplish the process. A very high stress
will produce due to high loads involve in cold forging process and can cause
die failure to overloading. Xia et al. (2001) found that forming pressure
can greatly reduced by dividing the process into multi-steps. While Tong et
al. (2001) introduced three innovative methods to reduce working pressure
namely pre-chamfering, spread extrusion and relief axis, which all related to
the die geometry effect. Farrahi and Ghadbeigi (2006) and Wagner et al.
(2006) study the effect surface treatment to the die life of cold forging die.
Knoerr et al. (1994) found that significant increase of die life can
be achieved by reducing the stress in the highest loaded zone and suggest several
solution such as by increasing the transition radii. McCormack and Monaghan
(2001) conduct 2D and 3D FEA study to determine optimal corner fillet radii.
In the research, three parameters namely land width, petal angle and rake angle
are taken into consideration. Falk et al. (1998) utilized the FEM tools
to calculate the life of the die. They are considering numerous aspects such
as load cycles and bending stress. Koç and Arslan (2003) suspect that
catastrophic failure is due tensile stress especially at stress concentration
region of the die such as fillets and corner and to avoid this type of failure
two recommendations are suggested i.e., by changing the die profile or use pre-stress
elements. Vazquez et al. (2000) introduced three alternatives namely
double tapered insert, split insert design and change of insert material to
improve die life. The research also confirmed that changes in die geometry are
an effective method to improved die life as a result from their research. However,
the plastic flow characteristics of the workpiece require higher forging pressures.
This high forging pressure will deform the billet material into desired shape
and at the same time induce stress onto the die. As a result, this will reduce
the service life of the die and affect the quality of finished parts. Fortunately,
there are many researches show that this problem can be solved by applying appropriate
methods. There are either by improving the cold forging process or by improving
the die design. The purpose is the same, which to reduce the stress on die that
resulted during the forging operation. Process conditions and sequence of process
design are very important in improving the forging process. Process condition
such as the stress-strain relationship of the billet and die materials, the
friction between the billet and the die, etc. The resulted stress on die can
be reduced by improving the die design. For example:
||Improve the die material properties; Usually, die materials
must be hardened sufficiently to withstand severe service conditions, but
also need to have enough ductility to prevent their cracking and brittle
fracture, i.e., dies are designed for higher degrees of both hardness and
||Design the flash at appropriate location with proper size and shape; normally,
flash is presented in die to release the working pressure during forging
operation to complete the filling up of material into the die cavity.
||Select the optimal corner radius for the die.
In assisting the prediction of die life several works have been done such as Skov-Hansen et al. (1998) by considering the tool material, the pre-stressing condition and the radius of curvature in a critical tool corner, while Tong et al. (2003) estimate the life of the die based on Haigh Diagram from combination of FEA and experimental result. Ohasbi and Motomura (1996) developed a computer aided die life prediction method based on Fuzzy language risk analysis and Fuzzy interference. Main objective of this study is to investigate the effect of corner radius of the die and cavity orientation on resulted stress and number of cycles to failure (length of service life).
MATERIALS AND METHODS
Tool steel is selected as the dies material and the most suitable tool steel
used in cold forging is AISI type A2 tool steel. Moreover, the compressive strength
that are absolutely high also encourage this material served as tooling purpose
in cold forging process. Apply a scaling factor to the desired dimensions to
account for shrinkage during processing. A typical scaling factor to account
for shrinkage is 2% (default value set by the software). The parts will shrink
uniformly in all directions due to densification of the metal powder. An appropriate
factor of safety is chosen by using several considerations. Prime considerations
are the accuracy of load and wear estimates, the consequences of failure and
the cost of over-engineering the component to achieve that factor of safety
and for this case safety factor of two is used. In predicting die life, the
fatigue curve (S/N curve) of the die material is needed to be constructed. Before
the simulation, the forging load needs to be calculated. The forging load is
calculated using the Simplified Slab Method. An iterative method of analyzing
the stress levels in the dies using the static study and predicting the lifecycles
using the fatigue study with the finite element analysis software, CosmosWorks.
The methodology is summarized as shown in Fig. 1.
||The flow chart shows the procedures of the research
||Three different U-joint orientations
Die modeling: The closed-die forging of the universal joint (U-joint) was generated using three-dimensional CAD modeling software, Solid works. This CAD modeling software coupled with FEA tools, CosmosWorks, which can provide accurate, meaningful results while fitting tightly into the design cycle of component parts and assemblies. Rapid design and analysis of tooling components can be accomplished with the use of readily available and inexpensive CAD and FEA software.
Die configuration: Theoretically, different die configurations will
result in different stress pattern and provide different lifetime. This due
to the different load pattern is applied on the surface of the die. Therefore,
the solid model of the universal joint is used to generate die in different
configuration. One of the corner is selected to be one of the variable to be
studied and it varies between 0 to 3.5 mm for simulation. Therefore, there
are 21 different dies will be analyzed in this study (Fig. 2).
Material properties and boundary condition: Many cold-forging operations
are characterized by very high compressive tool/ workpiece interface stresses
and the cyclic loading of such tools during the repeated forming of workpiece
may result in fatigue crack formation and limited lifetime. Therefore, the dies
must be made of high-strength tool material that can withstand the high compressive
stresses (Brondsted and Skov-Hansen, 1997). The most suitable material used
in producing the dies is tool steel; the properties of the tool steel are listed
in Table 1. Load is applied from the top, where the upper
die is moving towards lower die and the distribution of load is assume to be
Forging load: By referring to the literature review, there is the method
in calculating forging load, so-called simplified slab method (Altan et
||Properties of AISI type A2 tool steel
||Front plane and top plane of the lower die
The method will be applied here to predict the forging load
for simulation purpose. In this analysis, it is assumed that the cavity has
a rectangular shape; therefore, the cavity is cut into five sections in rectangular
(4xarea A and 1xarea B). Then the load for each section is added together.
Area B is a square (Fig. 3); therefore the half width of
the cavity is denoted by r', while in area A, the length of Y is longer than
X. Half of width of the cavity is denoted by r, which equals to Y/2 (This will
cause the value of predicted load become larger, which is closer to the practical
value). Given that for the case of r = 16.32 mm, r' = 19.31 mm, H = 32.64 mm,
m = 1.0 and σc = 350 MPa, so the total pressure required is
pta = 31803 Pα of M = 0.29 ton.
S/N curve: In high-cycle fatigue situations, materials performance is
commonly characterized by an S-N curve, also known as a Wöhler curve, which
map the relationship between magnitude of a cyclical stress (S) against the
cycles to failure (N) (Weihsmann, 1980).
||S/N curve for AISI type A2 tool steel
S-N curves are derived from tests on
samples of the material to be characterized (often called coupons) where a regular
sinusoidal stress is applied by a testing machine which also counts the number
of cycles to failure. This process is sometimes known as coupon testing. Each
coupon test generates a point on the plot though in some cases there is a run
out where the time to failure exceeds that available for the test. For AISI
type A2 steel used in this study, the S/N curve obtained is as shown in Fig.
RESULTS AND DISCUSSION
In fatigue behavior study on the cold-forging die, a fatigue analysis is developed
using the CosmosWorks software. CosmosWorks is one of the design analysis software
that is fully integrated in Solid works. The full model and mesh model of the
horizontal die is as shown in Fig. 5.
In static analysis, the solver finds the displacements in the X, Y and Z directions at each node. Now that the displacements are known for every element, the program calculates the strains in various directions. Strain is the change in length divided by the original length. Finally, the program uses mathematical expressions to calculate stresses from the strains. Before starting the fatigue analysis, a static analysis of the die is needed to provide the stress data to accomplish the fatigue analysis.
Stress analysis: The minimum corner radius for die design is 1 mm. Results
in Fig. 6 shown that there is a range for the corner radii
to obtain lower stress on the die and the result shows that the optimal corner
radii is at 3 mm, where the von misses stress is the minimum. From the figure
also indicates that the stress is decreasing when the corner radius increasing
and the result shows that there is a trend, where the stress will continue reduced
when the corner radius is increased. The stress generated in die with corner
radii 3.5 mm is higher than stress generated in die with sharp corner.
||Full model and mesh model of the horizontal die
||Von Misses stress of the die for different die orientation
the corner radius continues to increase, the stress generated will continue
to be reduced until it become lower than the stress on sharp corner die. However, it is not practical to use corner radius that is too large, where
the final forging may need more finishing operations or may not accurate in
final dimension. The stress is decreasing with the increasing in corner radius.
However, there is no significant improvement in stress reduction if compare
to sharp corner die (original). It can be concluded that, the stress trend is
same with the stress trend obtained from vertically oriented die. From the result
obtained, it can be concluded that the optimal die orientation is horizontally
oriented, which resulted in minimum stress on the die with given range of corner
radius. Whereas the 45 degree oriented die design generated the highest stress
with the same range of corner radius.
||Number of cycles to failure for each corner radius, respectively
Fatigue life: From the result shown in Fig. 7 indicates
that the life cycles of the die seem to change gradually when the corner radius
is increasing. However, the dies with corner radius 3 mm still obtaining optimal
life cycles. In fatigue analysis, the load that calculated from Simplified Slab
Method no longer useful. It is because the endurance limit of the die material
is too high, until the simulation result will show the die can last forever.
Therefore, a load that larger than the endurance limit is selected for running
the fatigue simulation. As a result, it is clear that the number of cycles to
failure that predicted from the fatigue simulation is not accurate.
From the study carried out, it can be concluded that, the changing in corner radius can affect the stress level generated on the die. Where there is a range that the stress level will drop when the corner radius is increasing. The optimal design is the horizontally oriented die with corner radius 3 mm. This design not only induced in lowest stress level during the forging operation, but also causing minimum deformation on die, where the structure of the die can return to its original position after removing the load exerted. As a result, the number of cycles to failure is in the satisfactory region.
The authors would like to thank the School of Mechanical Engineering and University Sains Malaysia for their cooperation and fund provided (A/C 6035175).