Civil Engineering Reference
In-Depth Information
Several FE simulation techniques have been used to design preforms with a view to excluding material-
flow defects in closed-die forgings [35]. Simulation of flow of material for a selected sequence of preform
geometries may be simulated with reference to specific material flaws to derive the “optimal” preform
geometries. The approach, also called “forward simulation,” requires several iterations with different
probable preform-geometires. An automatic backward tracing scheme using FEM was proposed [35], in
which reverse simulation of die-filling was conducted by tracing the loading path of the forming process
backward from a prescribed final configuration. Application of the approach included the preform design
for shell nosing operations, the design of edge profiles for the reduction of cropping losses in rolling,
and the preform design for the forging of H-shaped components and airfoil sections. Since the approach
relies on an assumed strain-distribution which formed the basis for the reverse loading path, the result
depended largely on the accuracy of this assumption. Further, for the forming of a complex component,
the “backward tracing” approach would result in too many intermediate stages with die-geometries which
may not be feasible from a manufacturing viewpoint. Recent research [63] has resulted in significant
improvements in the case of moving and stationary boundaries (tools). Further effort is required to refine
the approach with a view to replacing conventional preform-design approaches.
Although preform design using FE simulation has been applied to several forming processes [35],
these retain characteristics which differ from those of injection forging. The approaches and strategies
used in these applications may not be applicable to injection forging. In injection forging, preform design
has to be conducted with reference to three considerations: material-folding, billet stability, and discon-
tinuity in material-deformation in multi-stage forming. These were not addressed in previous research
on preform design using FEM.
FE simulation and experiments were conducted to identify approaches by which the initiation of
folds could be prevented, and hence improve the range of the process of injection forging. The exper-
imental investigation on the material flow for different aspect ratios of pdz enabled selection of two
approaches for the prevention of folds. Injection forging with a retractable anvil was proposed with a
view to gradually increase the aspect ratio of the pdz during the injection of material (
Fig. 4.12
)
as this
would reduce the extent of the rotation of material in the die-cavity. The alternative was to use a preform
with a view to reducing the expansion rate of the base of the billet during injection forging (
Fig. 4.11
)
.
FE simulation of injection forging with a retractable anvil indicates that the proposed configuration
was feasible for producing components with large pdz aspect ratios (T
1.3) without initiating flaws.
The simulation indicates that the initiation of injection with T
1.0 enabled the cylindrical surface of
the billet to roll on to the anvil without initiating folds (
Fig. 4.12(b)
)
; the base of the billet had not
expanded sufficiently to promote folding. The rotation of the material was small at the second stage of
injection due to the formation of a large base during the forming of the component (
Fig. 4.12(c)
)
.
Subsequent injection of the material allowed die-filling without significant rotation of the issuing mate-
rial; hence, a fold was not initiated (
Fig. 4.12(d)
)
. In addition, since the rotation of the work-material
was insignificant, die-filling was more symmetrical with reference to corners of the die. The corners of
the die-cavity would fill simultaneously. Asymmetrical die-filling required higher injection pressures to
effect filling of corners of the die. The simulation also showed that injection forging with a retractable
anvil would not always result in successful forming. An appropriate relative velocity between the punch
and the anvil must be maintained for effective forming; there is an optimal rate of injection to prevent
the initiation of folds.
The approach using preforming involves at least two stages of forming. The first of these would be
effected with a die-insert (
Fig. 4.11(a)
)
, which enables the forming of a frustum on the forward end of
the billet (
Fig. 4.11(b)
)
. The preforming tool was then dismantled (
Fig. 4.11(c)
)
and replaced either by
spacers for injection upsetting or by a closed-die for injection forging (
Fig. 4.11(d)
)
. The preformed
billet was not removed from the injection-chamber after preforming and was subjected to the second-
stage injection after re-assembly of tools. As an alternative, the frustum at the leading end of the billet
could be machined prior to forging; the preshaped billet was subjected to either injection upsetting or
injection forging to enable the comparison of the effectiveness of preventing folds. Both FE simulations
and experiments show that the frustrum-shaped preform may be used to prevent folds; the range of
