01-08-2012, 02:29 PM
New concepts in die design Ð physical and computer modeling applications
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Abstract
The application of Computer Aided Engineering and physical modeling techniques in forging R & D continues to increase. In using tools
such as Finite Element Modeling and experiments with model materials, the forging tool designer can decrease costs by improving
achievable tolerances, increasing tool life, predicting and preventing ¯ow defects, and predicting part properties. At the Engineering
Research Center for Net Shape Manufacturing, Design Environment for Forming (DEFORM) and DEFORM 3D, and a multiple action
press for physical modeling are tools available for forging research and for educational purposes.
This paper summarizes the results of industrially relevant ``work-in-progress'' research with numerical and physical modeling systems.
Current projects include: a tool design for the forging of a cross groove inner race for a constant velocity joint, and the design of a tooling to
forge a connecting rod without ¯ash. # 2000 Elsevier Science S.A. All rights reserved.
Keywords: Physical modeling; Cold forging; Process modeling
1. Introduction
In research and development of forging processes the use
of process simulation programs and physical modeling
techniques is complementary. By using these tools the
forging tool designer could decrease costs by improving
achievable tolerances, increasing tool life, predicting and
preventing ¯ow defects, and predicting part properties.
In most cases numerical models provide more ¯exibility
in the analysis of the metal ¯ow than physical models
since they allow for quick changes in the tooling design
and its motion. On the other hand, physical modeling
helps the designer to visualize problems with the process
and the tooling that may arise during the tryout of the actual
tooling.
This paper summarizes some current research projects at
the engineering research center for net shape manufacturing
(ERC/NSM) which use both physical modeling and process
simulation. These include tool design of a cross groove inner
race for a constant velocity joint and a die for ¯ashless
forging of a connecting rod.
2. Design of forging process and tooling aided by
computer and physical modeling
In general forging entails the sequential deformation
of the workpiece material through a number of different
processes [1]. Furthermore, each forging operation comprises
all the input variables such as billet material,
dies, the conditions at the die-workpiece interface, the
mechanics of shape change in the workzone, and the
characteristics of the processing equipment, as illustrated
in Fig. 1 [2].
The main objectives of the physical and numerical modeling
for the design in forging processes are:
1. To develop adequate die design and establish process
parameters:
to assure die fill,
to prevent flow induced defects such as laps and cold
shuts,
to predict processing limits that should not be
exceeded so that internal and surface defects are
avoided,
to predict temperatures so that part properties, friction
conditions and die life can be controlled (only numerical
modeling).
Journal of Materials Processing Technology 98 (2000) 212±223
* Corresponding author. Tel.: 1-614-688-3461; fax: 1-292-5874.
0924-0136/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved.
PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 2 0 2 - 2
2. To improve part quality and complexity while reducing
manufacturing costs by:
predicting and improving microstructure and grain
flow (only numerical modeling),
reducing die try-out and lead times,
reducing rejects and improving material yield.
The steps involved in integrated product and process
design for forging are schematically illustrated in Fig. 2.
Based on functional requirements, the geometry (shape,
size, surface ®nish, tolerances) and the material are selected
for a part at the design stage. Then design rules and
experience are used to determine a preliminary design.
The modeling stage is needed to verify the design and make
appropriate modi®cations to the tooling and the process.
Once the process has been veri®ed several times the tooling
is manufactured and the tryout phase begins. It is expected
that through the use of modeling the number of changes after
tryout would be less than through the conventional design
procedures.
2.1. Physical modeling
In general physical modeling involves the use of nonmetallic
model materials like plasticine or wax, which can
be deformed easily and could be used to study and predict
the deformation of metals. To obtain a reliable material ¯ow
the friction conditions at the tool-workpiece interface for the
model must be very similar to the actual process. Thus, the
selection of the lubricant is critical for the success of the
modeling effort.
Physical modeling has several advantages like:
Material flow is very close to actual forging operations,
Forging loads could be estimated by dynamic similarity,
Several preform designs could be evaluated in short time
to obtain the optimum metal flow,
A physical model is available for demonstration and
discussion purposes,
The forming of very complex parts could be evaluated in
shorter time than with 3D finite element method (FEM)
simulations.
However, also physical modeling has the following limitations:
It is difficult to estimate the contact stresses on the
tooling,
Temperature effects, such as die chilling, cannot be taken
into account,
Production of layered billets (for better visualization of
metal flow) is time consuming,
Conventional measurement of the samples is difficult due
to the softness of the modeling materials,
Investment in tooling and forming equipment with
enough load capacity is required.
2.2. Computer modeling
Computer modeling is widely used in industry for the
simulation of forging processes. This is mainly due to the
Fig. 1. Variables of a bulk forming process [2].
Fig. 2. Product and process design for net shape manufacturing.
V. Vazquez, T. Altan / Journal of Materials Processing Technology 98 (2000) 212±223 213
low cost of the simulations compared to actual tryouts. The
main advantage is the ¯exibility of the systems to make
changes when a problem occurs or the load capacity is
exceeded. Although 2D computer simulations are faster than
3D simulations, the last ones are becoming more practical
with the use of new computers.
Several issues, such as material properties, geometry
representation, computing time, and remeshing capability,
must be considered in cost effective and reliable application
of numerical process modeling.
Several commercial codes are available for numerical
simulation of forging processes; such as DEFORM, and
DEFORM 3D. The accurate and ef®cient use of metal ¯ow
simulations require not only a reliable FE solver [3], but
also:
1. software packages for (a) interactive pre-processing to
provide the user with control over the initial geometry,
mesh generation and the input data, (b) automated
remeshing to allow the simulation to continue when the
distortion of the old mesh is excessive, and ©
interactive post-processing that provide more advanced
data analysis, such as point tracking and ¯ow line
calculation.
2. appropriate input data describing (a) thermal and
physical properties of die and billet material, (b) heat
transfer and friction at the die-workpiece interface under
the processing conditions being investigated, and ©
flow behavior of the deforming material at the relatively
large strains that occur in practical metal forming
operations.
3. analysis capabilities that are able to (a) perform the
process simulation with rigid dies to reduce calculation
time, and (b) use contact stresses and temperature
distribution from the process simulation with rigid dies
to perform elastic plastic die stress analysis.
The time required to run a simulation varies depending on
the computer used, its memory, and its workload. However,
with today computers it is possible to run a complex 2D
forging simulation in a few hours, while a 3D simulation
would take from several hours to several days to be completed
[4].
3. Process design to cold forge a cross groove inner race
for a constant velocity joint
Many complex automotive parts, like the cross groove
inner race (CGIR) for a constant velocity joint (see Fig. 3),
have asymmetric geometric features and undercuts. In the
case of the CGIR traditional cold forging methods are not
capable of producing such part. The cost to make the CGIR
is often relatively high because broaching is used to make
the grooves, this is a time consuming process. However, a
multi-action cold forging process could be used to reduce the
manufacturing cost of the CGIR.
3.1. Cross groove constant velocity joint
The cross groove CV joint is an improvement of the
Rzeppa CV joint, which consists of inner and outer races, six
balls, and a cage between the inner and outer races. The six
balls are maintained in the intermediate plane by means of
the cage (see Fig. 3).
The cross groove CV joint allows relative axial displacements
of the shafts (plunging: in some cases up to 48 mm).
The grooves of the outer and inner race cross at an angle of
32.48 to get this effect. This results in a reduction of
vibration and noise, a signi®cant reduction in joint size, and
an increase in the maximum speed and torque. This design
offers the most mass ef®cient CV joint, i.e. the joint has the
least mass for the same functionality compared to other
joints, and it has a wide application [5].
3.2. Manufacturing of cross groove inner race
The common steps to manufacture the inner race are:
peeling of bar to control the volume,
billet sawing from a cold drawn bar,
cold forging,
broaching of the grooves,
induction hardening or carbonizing of the grooves
depending on the type of steel that is used [6].
The inner races for other types of CV ball joints are
almost entirely made by cold forging [7±9], which is a very
Fig. 3. An example of a cross groove universal joint [6].
214 V. Vazquez, T. Altan / Journal of Materials Processing Technology 98 (2000) 212±223
cost-effective process. Therefore, by changing the process
from machining to cold forging several machining operations
could be reduced resulting in a lower manufacturing
cost.
The advantages of cold forging the CGIR are:
faster process, because the forming time takes only a few
seconds,
cheaper, because the high production rate of a forging
press eliminates the expensive tooling and time consuming
process required for machining.
The disadvantages are:
cold forging multi-action tools are expensive,
forging pressures are large in the die cavity and may
damage the tool.
A small number of advanced companies in Japan, including
Toyota [10] and more recently, Aikoku Alpha [9], have
begun to produce the inner race by cold forging with multiple-
action tooling.
The grooves must be very accurate for the proper operation
of the CV joint, and they must be produced to at least
near-net shape so that only one grinding operation is needed
to ®nish the part.