Thixoforming : Semi-solid Metal Processing

inherent to steel thixoforming and the necessity for evaluating process parameters
and designing specific tools for each intended work piece geometry make die
construction a challenging task. In this respect, the latter concept of high-temperature
dies is advantageous in terms of process stability owing to the comparatively
stable defined operation parameters.
8.7
Conclusions and Perspectives
8.7 Conclusions and Perspectivesj301
The work on PVD process technology focused on the improvement of tool life in
semi-solid metal forming by developing an adapted deposition process for oxide
coatings on hot working steel and TZM substrates. Two different candidates were
identified as suitable coating materials, g-Al2O3 and t-ZrO2, using tailored model
tests, owing to the high hardness of alumina in the relevant temperature range and
less adhesion of the semi-solid steel forming material on zirconium oxide. The PVD
coatings g-Al2O3 and t-ZrO2 were confirmed as suitable coating materials. In order to
improve adhesion to the substrate, a TiAlN interlayer was provided to increase the
coating performance. Regarding thin-film synthesis, the objectives of the study were
to reduce the deposition temperature to values below the annealing temperatures of
the substrate materials and to determine parameters for reliable and reproducible
thin-film properties. This was achieved by a pulsed PVD process with tight and fast
process control to prevent target poisoning. Coating experiments showed that
substrates with surface features up to an aspect ratio of 1:2 can be coated successfully
with a constant coating thickness. The applicability of PVD-coated hot working steel
and TZM dies was demonstrated in small-scale steel thixoforming (thixoforging, -
casting and -extrusion) series. The long-term stability of g-Al2O3 and t-ZrO2 coatings
has to be evaluated in industrial forming series. Future work will focus on enhancement
of the PVD process on the target for coating substrates of complex surface
morphology.
The aim of the PECVD process technology development was to ensure a
reduction in the deposition temperatures for a-Al2O3 at substrate temperatures
allowing deposition on steel dies, namely at 550 C. To achieve that goal, a bipolar
DC PECVD method was developed. The constitution and mechanical properties of
the films were found to be strongly dependent on the deposition conditions such
as substrate temperature, gas mixture and peak voltage. The elastic properties of
the films were strongly affected by the formation of the pores in the structure
and were intimately linked to the normalized ion flux. Performance-related
tests such as adhesion, high-temperature tribological and thermal shock evaluations
were performed to evaluate the behaviour of coatings in a close to real
environment. The results underline that alumina thin films deposited by PECVD
are very promising candidates for die protection during semi-solid processing of
steel.
The excellent high-temperature properties inherent to ceramics make them
candidates for die materials for high-temperature metal forming, for example,