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

Feist, Oberbreyer et al. 17th Plansee Seminar 2009, Vol. 3 WS 15/9
To this end, it is assumed that the density is homogeneous across the entire domain at any process-time.
Since any friction taking place between the tools and the powder will preclude this assumption to hold, a
frictionless state µ = 0 is assumed. Furthermore, it is assumed that the considered part will not change
its topology during the entire process – however, it can change its size and its proportions (e.g. a cylindrical
part will remain cylindrical, however, with changing diameter d and height h and changing ratio d/h).
Under this assumption one can easily derive a step-back procedure as outlined below for geometric primitives
as cubes, cones or cylinders. More complex geometries can be assembled from these primitives
allowing the determination of initial guesses for process-parameters also for industrial components.
For the sake of simplicity, the further outline is done by means of a single cylindrical part. The procedure
starts by defining the nominal geometry of the part at the end of sintering given by its diameter ds and
height hs, hence determining its volume Vs. In addition an approximate fill density ρf and a desired green
density ρg have to be assumed – on the basis of the chosen green density the required axial compactionload
Fz can be determined later and compared to the specifications of the press available for production.
Hence, the chosen green density will become an adaptable parameter for the design. On basis of the selected
green density the attainable sinter density ρs can be computed together with the radial and axial
sinter strains εr and εz (cf. Fig. 2). Using the computed sinter density ρs and the given volume Vs one can
determine the total mass m of the part. By reversely applying the sinter strains to the final geometry the
geometrical parameters at the start of the sintering phase (i.e. the green state) dg and hg can be found.
For the given green density ρg and the associated volumetric plastic strain εv (5) all constitutive parameters
of the cap-plasticity model can be computed employing their evolution equations (8), (9) and (10).
Hence, the constitutive state at the end of the compaction phase is fully given and the associated stress
state in terms of the axial and radial stress σz and σr can be computed. Using the diameter of the green
compact dg the maximum axial load Fz required for compaction can be found and compared to the load
available with the press used within the later production.
Using this stress state the radial and axial elastic deformations during the ejection-phase can be esti-
mated in terms of the radial and axial elastic strains ε el
r and ε el
z using the density-dependent Young’s modulus
(10). By reversely applying these strains on the green-compact geometry one arrives at the height hp
and diameter dp of the powder-domain in the compacted state associated to the maximum compactionload.
Diameter dp now must equal the diameter of the used die and consequently the diameter of the fillbody
df .
Postulating conservation of powder-mass m the fill volume Vf can be computed using fill-body diameter
df and consequently its fill-height hf .
Summarizing the outlined procedure it allows to determine appropriate initial estimates for the relevant
process-parameters in the following principle
given : ds, hs
estimate : ρf, ρg
determine : ρs, m, dg, hg, df, hf, Fz
One of the most critical phases of the process-chain is certainly the ejection stage, where the compact radially
constrained by the die is gradually turned to an unconstrained state. It is a common procedure to either
keep a fixed residual axial load on the punches during ejection (load-control) or to keep the distance
between the lower and upper punches fixed (displacement-control). Selecting an appropriate level of the
associated process-parameters is a crucial feature for successful ejection: the application of a residual
(16)