Thixoforming : Semi-solid Metal Processing

some places can be observed as small non-metallic inclusions. The analytical analysis
with respect to form factor, particle size and area and also the share of the globulitic
phase confirms the phenomena. The distribution of the solid phase and the fine
particles at the ingate area of the knife geometry shows clearly the strong segregation
of the phases which is caused by the distinctive channel flow. A considerably more
homogeneous structure formation can be seen at the knife tip, and a considerably
higher content of fine particles which leads to a decrease of the form factor appears.
The mechanical quality of the components was checked by measuring the Rockwell
hardness (HRC) of the knife in the application-relevant areas. The parts were
extracted after a solidification time of 5 s and cooled in air. The matrix of the hardness
distribution for a knife in the as-cast condition shows a decrease in the hardness
values for the knife point from about 47 to 39 HRC as a result of the long flow distance
and the steady increase in the metal speed.
By a post-connected annealing treatment of the knifes for 2 h at 550 C, a hardness
increase appears around þ 20 HRC. Furthermore, easy homogenization of the
hardness distribution is achieved by heat treatment in the area of the knife point
which is caused by the transformation of the metastable austenite into bainite. On
average, the hardness values are 62–66 HRC, which corresponds to the quality of a
forged knife. Considering the process- and material-specific boundary conditions,
high-quality steel components have been produced by the thixocasting process.
Because a more complex real component was sought, a diving knife with an
integral handle was selected and provided by B€oker Baumwerk (Figure 9.29). This
knife fitted the needs of a thixo-steel component also regarding economic issues,
because of a high added value (the price of the forged knife is more than D160) and a
low charge quantity of 1000 knives.
It was the aim to produce a diving knife which only needs to be freed from gating
and overflows to be able to obtain a finish-worked article in the process of sharpening.
As opposed to the kitchen knife, the greatest thickness of 6 mm is the same in the
handle and blade areas. The handle has round openings of 22 mm diameter for
weight reduction. The blade has the full height of 6 mm up to 30 mm in front of the tip
because of the arched blade. To produce a knife ready for the sharpening process, the
smallest wall thickness of 0.8 mm must be achieved at the cutting edges of the knife
and hatchet.
Due to the component weight of 282 g a double mould lend itself. Two blades
were arranged vertically in the die cast frame of the step sample.
For the ingate the height was 0.8H part ¼ Hingate. For the positioning of the ingate at
the knife and the area of the ingate, two in principle different variants were possible.
Figure 9.29 Diving knife as-cast (overflows and gating removed).
9.3 Thixocasting of Steel Alloysj339