Civil Engineering Reference
In-Depth Information
the process for injection forging of solid billets can thus be extended to T
1.4 to 1.5 by using a
machined preform and to T
1.50 to 1.64 by using a preformed billet.
Forming Sequence Design and Die-stress Analysis for Pressure-assisted
Injection Forging of Thick-walled Tubes
FE simulation and experiments were conducted to investigate the development of material-flow defects
in pressure-assisted injection forging of thick-walled tubes to quantify the influence of individual process-
parameters on these flaws [5, 25] (
Fig. 4.14
)
. Based on these studies, the forming-sequence was optimized
with a view to achieving components with “uniform” wall-thickness. The forming sequence consists of
the simultaneous pressurization and injection of work material in three stages. Initially, a small pressure
was applied to the to ensure that the tube was supported uniformly; this was essential to prevent the
material from buckling at the initiation of injection. Simultaneous internal pressurization and injection
of the work material was then initiated to prevent the issuing material from folding while effecting die-
filling. Filling of the die corners and the ironing-out of the inside surfaces of the component was effected
at the final stage of injection by a substantial increase of pressure on the pressurizing material.
The simulation results, some examples of which are shown in
Fig. 4.20
,
suggest that the pressurization
at the second stage had an optimal value—too low a value would allow the development of a fold due
to the upsetting of the tube and complete die-filling may not be achieved. A high pressurization would
lead to thinning of the tube; this resulted as the tensile component of stresses in the work-material was
high while injection pressure was relatively small. Because of the matched radial expansion of the tube
under high pressurization, the material made early contact with the surface “A” of the die-cavity (refer
to
Fig. 4.20a
)
. It is difficult for material which is subsequently injected into die-cavity to flow along this
surface; the radial pressure exerted on the work-material is insufficient to effect the flow of the material
along this surface. As a result, the material accumulated at the exit of injection-chamber.
Two further factors which influence the quality of the component are the volume of material injected to
compensate for changes of tube thickness and the sequence of pressurization and injection.
Figure 4. 2
0
b
shows an example of material deformation due to insufficient injection. The thickness of the tube will
reduce with radial expansion, thus reduction may be compensated for by an increased rate of injection.
Should reduction of the wall of the tube occur, it may not be compensated for at a later stage of die-filling,
since the material injected at this stage did not contribute to more effective die-filling.
Simulation results showed that the quality of the component in pressure-assisted injection forging is
largely dependent on the forming sequence. The performance requirement of a tubular component may
demand uniformity of wall-thickness; this, in turn, would influence the prescription of forming sequence.
It would be difficult to form fine corner radii, especially for thick-walled components. Due to the difficulty
of causing material to flow along die-surfaces at later stage of the operation, the filling of the die-corners
has to be effected by drawing material which is already contained in the die-cavity. As a result, the material
at the section x-x (
Fig. 20(a-4)
)
was ironed out by increasing pressure on the pressurizing material, the
consequence being that a weakened section was produced in this area.
The forming sequence was “optimized” by comparing different combinations of pressurization and
injection—
Fig. 4.20c
shows the “optimized” sequence. The approach that was adopted for eliminating
the notch at the exit from injection-chamber and achieving “uniform” wall-thickness required that a low
value of pressurization and relative large volume of the injection of work-material were effected initially
to retain the wall-thickness as close to its original thickness as possible. The thickened wall prevented a
weak section from developing at the exit from injection-chamber during subsequent forming as the filling
of the corners could be effected by deforming the accumulated material into the corners of the die-cavity.
The reduction of the wall-thickness during the final stage was not significant.
The development of pressure-assisted injection forging was based on the premise that the applied force
would be reduced and, consequently, that the injection parameters would be of a lower magnitude. To
confirm this assumption, a solid billet was injected to fill the same die-cavity. Both, the experimental
and FE simulation results [26, 27] confirmed that the applied stress and energy requirements for the
