Near Net Shape Manufacturing of CuCr Vacuum Switching Contacts ...

Feist, Oberbreyer et al. 17th Plansee Seminar 2009, Vol. 3 WS 15/11
distribution it is decided to design both sunk-holes with identical diameter. In addition, for the analysis the
geometry is simplified by neglecting all fillets except for the base-fillet of the slot-punches. The FE-model
consisting of the fill body meshed with linear hexahedron elements and the tools modeled as rigid surfaces
is depicted in Fig. 4b. The model reproduces the fill-body assuming it in a state after a first powdertransfer
established by the inner punches forming the initial cavity of the upper sunk-hole (the powdertransfer
itself could be numerically resolved only with a high numerical effort using adaptive procedures
which is out of scope of the present work). A uniform fill-density ρf(x) = const is prescribed for the entire
fill-body. Contact interactions are imposed between the surfaces of the powder-body and the tools. The
upper edges of the dies are filleted or chamfered, respectively, in order to allow smooth ejection of the
green-compact. All simulations are carried out using the commercial finite element code Abaqus [7].
As a first step the inclined upper outer punch is brought into full contact with the powder leading to slightly
higher densities at the outer perimeter of the part. Afterwards both outer punches are driven against each
other ensuring most minimal effects related to friction against the die. Simultaneously, the inner punches
move against one another, however, with a reduced height of travel, thus preventing powder-transfer between
the inner and outer section and ensuring an almost uniform density-distribution. After full compaction
the upper punches are slightly released before the part is ejected and the punches are finally removed.
The model now represents the state of the unloaded green-compact and the initial state of the sinteringprocess
as shown in Fig. 5a. The latter can then be simulated employing the deformed mesh associated
to the end of the compaction-simulation. Besides, the green-density distribution ρg(x) serves as additional
input to the sintering-simulation. For the sintering-simulation only appropriate statically determine
and constraint-free support-conditions have to be provided, however, no additional parts or tools have to
be considered.
Results in terms of the obtained relative density measures (with respect to the theoretical density of the
alloy) are shown in Fig. 5 both for the green-state (a) as well for the fully sintered state (b). Deviations
from the simplified final nominal geometry ∆n are given in Fig. 5c in terms of oversize (+) and undersize
(-). It can be seen that the maximum geometrical deviations are about 0.8 mm taking place at the
finger-like acute-angled piece along the slot. This can be explained by the slight over-compaction taking
place in this region as a consequence of a dead-water-effect during compaction (i.e. the powder is not
allowed to flow in circumferential direction). However, for most of the part maximum deviations of app.
0.1 mm can be observed. The obtained geometrical deviations now serve as a basis for respective adjustments
of the relevant process-parameters in order to obtain the desired final geometry complying
to the required tolerances and exhibiting almost uniform density-distributions. To this end, an iterative
scheme is applied based on the numerical model until the geometrical requirements are met.
d g [-] d s [-] ¦n [mm]
1.0
0.8
(a)
1.0
0.9
+0.8
-0.2
(b) (c)
Figure 5: Selected results of PM analysis for a switching contact: (a) relative density after compaction, (b) relative density after
sintering, (b) geometrical deviation from simplified nominal geometry.