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Guy Léon Kaza
Abstract
Contribution to the study of thermal contact resistance and its modelling in the case of the crushing of the sheet/tool interface in the hot forming of sheet steel
Guy Léon Kaza - 23 April 2010
An exploratory study has been conducted to estimate the thermal
contact resistance (TCR) in hot forming at the sheet/tool
interface, in order to understand its relationships with
pressure and surface roughness parameters. The importance of the
TCR in applications such as hot stamping comes from the fact
that it controls the heat exchanges directly and has an indirect
effect on the properties of the final parts.
The study is focused on the feasibility of measuring the TCR
between a tool made of X38CrMoV5 steel and a steel sheet made
especially for hot stamping. The grade of sheet used in the
study is 22MnB5 grade. Two facilities developed at the LETEE and
at ICA-Albi have helped to assess the influence of contact
pressure on TCR in constant thermal conditions for the LETEE and
transient conditions for the ICA-Albi respectively. A range of
pressures between 4 and 80 MPa has been explored on the
LETEE facility. A decrease in TCR from 2×10-4 to
6×10-5m2KW-1 in order of
magnitude was measured. The influence of surface roughness has
been investigated through two surface states on X38CrMoV5 tool
samples: one is a ground surface and the other is a polished
state. The results showed slightly larger values of TCR for
rectified samples.
The trend of decreasing TCR with an increase in pressure has
been confirmed on the ICA-Albi facility. Variations between
3.4×10-4m2KW-1 and
2.7×10-4m2KW-1 have been
measured for a range of pressures from 40 to 160 MPa. In
comparison, the values obtained are on average twice as high
than those measured at the LETEE and with lower measurement
variations.
The TCR has then been subject to a two steps modelling
process. The first step is focused on site crushing of
asperities taking into account their rheological behavior
(elastic, plastic or elastoplastic) to assess the contact
area. The second step consisted in defining a relationship
between the TCR and the contact pressure from a predefined
model, linking the contact area to the TCR. From the resulting
conductance model, a simulation of contact has been conducted
under ABAQUSTM. By simulation, the evolution of
temperatures according to contact pressure has been reproduced
with a maximum deviation of 5°C from experimental temperature
measurements.
Key words:
thermal contact resistance, contact area, surface roughness,
asperities.
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