Civil Engineering Reference
In-Depth Information
Particular Modeling Technologies
Previous research in injection forging was largely experimental or dependent upon the use of simplified
analytical methods; FE simulation was only used in a few analyses. A preliminary attempt to use FE to
analyze injection forging was with a view to predicting the punch pressure for the injection of a flange
with a flange-compensation pressure [53]. Using a rigid plastic finite element code (PLADAN), combined
radial and forward extrusion of a can was simulated [15]; material flow patterns and tool stress were
analyzed for small aspect ratios of pdz (T, refer to
Fig. 4.1
)
. The research did not progress to the simulation
of the development of flow-dependent flaws. FE simulation was used to analyze material-flow during
injection forging of tubular material with mandrels [54]. Comparison between FE-predicted material-
flow patterns and that observed from experimental specimens showed differences which may have resulted
from the indiscriminate use of commercial FE software.
Only recently has large-scale FE simulation been applied to the design and analysis of injection forging
for both solid billets and tubular materials [5, 11, 12, 20, 21, 25-27, 52]. The approaches and techniques
have been developed to analyze the pressure losses in the injection chamber, the stability of the billet,
flow characteristics of material in the die-cavity, and to design preforms and the forming sequence to
prevent the development of flaws.
Modeling of Friction in Injection Chamber
The volume of material which can be injected into the die-cavity is dependent upon the friction character-
istics of the injection chamber. These characteristics also have a significant influence on tool-stress. The
research revealed that the pressure losses in injection chamber (pa-pb in
Fig. 4.1
)
for a long billet could be
substantial—40 to 60% of the injection pressure being expended to overcome friction in the injection-
chamber [9].
The modeling of friction in the injection-chamber is of fundamental and practical interest, and may
be effected by incorporating an appropriate friction model into FE analysis. There are two aspects which
have to be considered for friction modeling of a forming operation. First is the accuracy of the friction
model in replicating the prevalent interfacial friction conditions. Second is the numerical convergence
when incorporating a friction model into FE simulation. Currently, three friction models are popular
for the analyses of cold forging: Amonton's Law (Coulomb friction), Constant Friction Law, and inter-
facial pressure-dependent law (Bay and Wanheim's model [55]). The first two assumed either a constant
friction coefficient or a constant friction factor, while the latter considered real contact between work-
material and tool-surface (rough material-surface is considered).
The comparison between computed punch pressures and experimental results suggested that all three
friction models were unable to accurately predict the friction characteristics at the injection-chamber/billet
interface for high interfacial pressures. It was recognized that friction characteristics at the injection-
chamber/billet interface would have to be defined with reference to the interfacial pressure which is
much higher than these encountered in the application of three friction models. Since the existing
approaches for measuring friction and those for determining friction by measuring the deformation of
material (such as ring compression test or twist test) are limited to low working pressures, it is difficult
to define friction characteristics under high pressure (
5) levels using experimental approaches.
The measurement of friction would have to be conducted with conditions which match that prevailing
in the process under investigation. Injection forging is performed under high injection pressure; for a
low alloy material, the pressure in injection chamber could be 10 times the yield strength of the work-
material. It would be difficult to achieve this level of pressure using conventional friction tests. The
measurement of friction in the injection-chamber would, therefore, have to be based on the measurement
of the average pressure-loss in the injection chamber. As an approximate, equivalent friction coefficient,
which is pressure-dependent, was determined from the measurement of pressure loss in the injection-
chamber. These values were imported to an interface module of an FE software to define pressure-
dependent friction characteristics, which were to be used in the contact analysis for the simulation of
injection forging.
p
/
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