Thixoforming : Semi-solid Metal Processing

The EDS graph clearly shows the zirconium reflex of the coating and the tungsten
reflex of the interlayer. Traces of iron can also be detected. The Al2O3-coated plates
show more iron build-up than the ZrO2-coated plates. However, both ceramic PVD
coatings show no visible signs of serve erosion or abrasion. These investigations
have shown that both coating candidates are suitable for industrial forming
operations.
8.4.4
Applicability of PVD-Coated Dies in Steel Thixoforming
8.4 Multifunctional PVD Composites for Thixoforming Mouldsj267
Thixoforging, -casting and -extrusion dies are coated with application of the parameters
determined on the above-mentioned samples. Prior to deposition, the tool
substrates are hardened to 54 HRC and subsequently cleaned ultrasonically with
industrial cleaning equipment containing surfactant solutions and rinsed with
deionized water. Finally, the dies are dried in hot air. During deposition, the dies
are moved in a planetary motion to ensure an even film distribution. Figure 8.23
shows (a) a coated thixoextrusion die and (b) three coated inserts for the thixocasting
process. For the bilayer coating system, the TiAlN interlayer is 5.2 and 2.3 mm for the
g-Al2O3 top coat.
The forming experiments on the different coated tools were carried out at the IBF
and GI (see also Chapter 9). The coated parts show good performance during the
thixoforming tests. The different coated parts were now analysed for phase stability
with EDS, SEM and XRD. For the extrusion dies, the coatings showed excellent
performance so far in the first experiments [38] (Figure 8.24).
The coated samples were also investigated in the thixoforging process. Figure 8.25
shows the results of the forming experiments carried out at the IBF with the hybrid
tool concept. A schematic drawing of the concept is shown in Figure 8.5. The upper
dies are coated with g-Al 2O 3 and t-ZrO 2 PVD coatings. For the examination, a g-Al 2O 3
coated insert was analysed after 33 forming cycles using X210CrW12 as forming
material.
The SEM and EDS investigations showed that there are several traces of iron in the
centre of the coated tool after the forming experiments. The coating is still present
here; this can be seen by the aluminium and oxygen peaks. Also, the coating remains
stable around the edges. Small pieces of iron can be detected here. No traces of iron
can be seen in the inner area.
Figure 8.23 PVD coated hot working steel and TZM dies with TiAlN/g-Al2O3.