Thixoforming : Semi-solid Metal Processing

Table 8.8 Mechanical and phase properties of the deposited coatings.
Property
Monoclinic
ZrO 2
that with a decrease in the cathode voltage (increase in O2 partial pressure), the
hardness and Young s modulus increase. The amount of tetragonal phase also
increases.
The investigations showed that it is not possible to deposit only tetragonal
phase with this deposition parameter setup. Recent investigations showed that a
small grain size below 6 nm is needed to stabilize only the tetragonal phase [37].
By increasing the bias voltage to 40 V, the ion bombardment on the substrate also
increases, so it can be seen that only the tetragonal phase is growing on the
substrate. These deposition parameters are taken into account for the coating of
the forming dies.
8.4.3
Model and Near Application Tests
8.4 Multifunctional PVD Composites for Thixoforming Mouldsj265
Sample name
Tetragonal
ZrO 2 ID 650V ID 600V
Coating unit Z400 Z400 CC800/9 CC800/9
Phase type Monoclinic Tetragonal Monoclinic þ Monoclinic þ
tetragonal tetragonal
Young s
modulus (GPa)
146.76 13.62 292.7 33.2 213.3 22.6 294.6 23.8
Hardness,
HUniv (GPa)
5.72 1.09 21.35 3.44 8.06 1.5 13.9 1.9
To test the behaviour of the different coated samples, model tests were developed to
study by isolating single process loads, that is, wear, corrosion and thermal shock
resistance. The results obtained by these tests led to the identification of different
degradation and failure mechanisms that are related to the specific loads applied in
each test [5, 35, 36]. After assembling the data obtained in model tests for each coating
system, the prediction of material behaviour during thixoforming process is difficult.
Practical fast and cheap forming experiments and setups are needed. The hightemperature
compression tests combine mechanical, thermal and corrosive attack in
one test setup. For the investigation, both die materials 1.2999 and TZM and both
coating systems g-Al2O3 and t-ZrO2 were selected. The experiments were carried out
at the IBF (Institute of Metal Forming), conducted using a servo hydraulic testing
machine (Carl Schenck, Darmstadt, Germany), a maximum testing load of 640 kN
and a velocity of 600 mm s 1 . The testing machine was equipped with a tube furnace
for compression tools and an induction heater for steel billets. The compression tests
were carried out under an inert gas atmosphere. The testing tool plate was fixed on the
lower end of the top plunger. Billets were placed on a ceramic compression plate