Thixoforming : Semi-solid Metal Processing

248j 8 Tool Technologies for Forming of Semi-solid Metals
Figure 8.7 CuCrZr based alloy segment (a) and the surface (b) (after 10 produced parts).
soldering effects on the semi-solid steel. To avoid this, different tool materials have
been tested in recent years regarding their applicability as die materials for steel
thixoforming moulds. The processing of steel results in high surface loads, such as
thermal, mechanical and tribological stresses. There are different concepts to address
these challenges. One is to employ tool materials exhibiting high strength at working
temperatures and low thermal conductivity (e.g. hot working steel).
Alternatively, die materials showing high thermal conductivity, such as copper- and
molybdenum-based materials, may be applied. From this group of materials, the
CuCrZr alloy 2.1293 with a high thermal conductivity (320 W mK 1 ) was selected for
forming experiments. After the production of 10 HS 6-5-2 steel parts using CuCrZr
die material, the tool surface showed strong abrasion and erosion of the uncoated
copper material (Figure 8.7). This degradation of the surface indicated that copperbased
materials are not suitable for steel thixoforming.
Further investigations were concentrated on alloys exhibiting higher strength at
elevated temperatures. Seeking metallic die material with low thermal conductivity,
the hot working steel 1.2343 and the new developed 1.2999 from Deutsche Edelstahlwerke
(Witten, Germany) were investigated. Also, a molybdenum-based alloy
(TZM) from Plansee Holding (Reutte, Germany) with very high thermal conductivity
was used. The mechanical and thermal properties are reported in Table 1.2. Preinvestigations
such as corrosion tests in air and exposure to liquid steel showed that
there is a need for tool protection for the uncoated metallic dies to withstand the
chemical and mechanical wear associated with the semi-solid forming of steel [11].
Different nitride- and boride-based coatings were investigated for semi-solid
processing of aluminium A356 and AA6082. Not only different coating systems
were evaluated, but also sundries depositing technologies, such as magnetron
sputter ion plating (MSIP)–PVD, arc ion plating (AIP)–PVD, PECVD and chemical
vapour deposition (CVD) processes. Additionally, different interlayers were
applied, for enhancing the adhesion to the top coat and also to support the top
coating (Figure 8.8).
Starting with the material screening from standard titanium-based coatings such
as TiN, TiAlN and TiB 2, the chromium nitride family offered very good durability in
the forming tests. Especial the PVD-Cr1 xAlxN showed less adhesion to the semisolid
aluminium and high mechanical strength compared with all other coating
types [11]. During the forming process in the contact zone between the semi-solid
material and the coating, a small, dense Al2O3 and Cr2O3 layer is formed. Further
investigations on a suitable coating material for the semi-solid forming of steel like