Civil Engineering Reference
In-Depth Information
An upper-bound analysis was also conducted to define the force requirement to injection upset against
hydrostatic pressure [48]. The work extended previous formulations [46] in two aspects—calculation of
friction losses along the tool-workpiece interfaces, and assessment of the effect of internal surface discon-
tinuities with reference to a variety of geometrical shapes. In this analysis, friction at billet/injection-
chamber interface was described using a constant friction formulation and was assumed to occur at
all work-material interfaces. The thinning of the flange was also investigated, from which it was
concluded that this phenomenon could account for some of the discrepancy between upper-bound
models and experimental data.
Other applications of upper-bound techniques include the prediction of extrusion pressure for the
forming of tubular components and multi-branched solid components. A comparison of a simple parallel
velocity-field with a triangular velocity-field for upper-bound analysis was also conducted for injection
upsetting [49] from which it was concluded that the simple parallel velocity-field provided a better upper-
bound value of injection pressure than the triangular velocity-field. The upper-bound technique was also
used for predicting the load for forming a component with rotational-asymmetry as is the case of
segmented flanges (3D component) [33]. In this analysis, the effect of the segment angle on the load
characteristic was analyzed. The analysis gave higher load values than experimental results and this over-
estimate was 25% at the initial stage and 10% at final stage of injection.
The capability of process modeling using analytical approaches was assessed by referring to different
process conditions [50]. The results showed that previous upper-bound models over-estimated the punch
pressure to initiate injection and under-estimated that to produce the flange. The research also indicated
that the complexity of process parameters prevented the development of comprehensive models [51],
for example, the exit-geometry has a significant influence on the forming-pressure in injection forging.
It is, however, difficult to consider this parameter using analytical methods. To date, almost all upper-
bound analyses of injection upsetting disregarded geometric transitions in the die-configuration. In fact,
experimental evidence [47] suggests that a unique upper-bound field cannot cope with the entire range
of flange thicknesses and the distribution of shear surfaces is dependent on the extent to which the flange
has been formed. In addition, although some upper-bound analyses had been conducted by including
friction in the analyses, this was attended to using only the constant friction law. The calculation of
equivalent friction-coefficient from experimental data [51] showed that the friction in the injection
chamber would not be constant and increased with the interfacial pressure. Analytical approaches are
also unable to predict the development of flow-related flaws, such as the lateral sliding of the billet and
the folding of the free-surface of the work-material [5, 11, 12, 52].
Injection forging, both for solid and tubular material, is characterized by the injection of work-
material, which is contained in the injection-chamber, into a die-cavity or an annular space. The
process may be segmented into these components, each of which retains a quantifiable character: the
injection chamber, exit-geometry, and die-cavity (refer to
Figs. 1
and
2
)
. The modeling of the process
is required with reference to characteristics of work-material deformation and forming pressure in
these components. There would be no significant bulk-flow in the injection chamber; the pressure
in the chamber would, however, be several orders higher than the yield strength of the work-material.
Process modeling may be applied to the range of high pressures encountered in the chamber. Accurate
prediction of the pressure distribution in the injection chamber would require the use of friction
models which are pressure-dependent. Proposed models would have to be applicable to the analysis
of the influence of exit-geometry on material flow. The flow of work-material in the die-cavity largely
depends on die-cavity geometry. Analytical approaches can, however, only be capable of considering
simple die-cavity forms. The requirement to achieve the nett-form definition of complex component-
forms by injection forging suggests that the forming operations should be designed to achieve flawless
components with the required level of performance and characteristics. The capability, such as simu-
lating the development of material-flow flaws, quantifying the property of deformed work-material, and
analyzing component-form errors due to tool- and work-material-elasticity, is an essential requirement
for the modeling of nett-forming by injection forging. FEM is currently the most efficient tool for
achieving these.
