Multi-cycles deformation modeling of hot forming tools under creep-fatigue regime

Hot forming processes (extrusion, die ca
sting and forging) allow the production
of a wide variety of products, both in terms
of worked material and achievable shapes.
However, critical working conditions are involved for the tools subjected to severe thermo-
mechanical loads, thus requiring an accurate design. Among the different classifications
proposed in literature for hot forming die failures, one focus on separating manufacturing
and in service failures. The latter category is additionally split, as proposed by the authors,
in static failure, damage and deformation/deflec
tion failures. Static failure appears after e
reduced number of extruded billets as a consequence of an overload or of a poor initial die
design. Damage and deflection failures are ind
eed induced by the synergic detrimental
action of creep and fatigue phenomenon. In disc
riminating the relative dominant role of
creep and fatigue in leading to the final die discard, the level of temperature and of the
applied load as well as of the dwell-time are d
ecisive. In extrusion, the latter is the time in
which a constant load acts on the die and represents the time required to extruded each
single billet function of both ram speed and billet length. Fatigue and creep can be seen as
limit cases with zero and infinite
dwell-time. A novel model is proposed for the prediction
of the deformation undergone by hot forming t
ools in the creep-fatigue regime after
multiple cycles. The model is presented as applied to extrusion dies and is based on a
modified version of a simple creep law already
implemented in all the FE codes. The model
is validated against small scale dies used
in physical experiments and then against
industrial extrusion dies.