CPT International 02/2021

CASTING TECHNOLOGY
asafe and rewarding activity promising
minimal climate impact and great
growth opportunity.
Figure 3: MAXImolding operates in the semi-solid temperature range for the AZ91 alloy, which
is between 480 and 580 °C. The microstructure ofthe alloy has spherical, non-dendritic features
with 50 %solid content (Dr. Frank Czerwinski). This prevents the formation of defects.
Casting -Molding -Forging
Abasic distinction is made between two
processes for melting light alloy metals:
> Ingots are processed in amelting
furnace and then filled into the casting
chamber as aliquid melt: hot and cold
chamber die-casting machines dominate
this ecosystem.
> Processing and heating of the material
to asemi-solid mushy melt directly
in the machine, (cold to cold processing)
and without an external melting furnace:
rheo (Rheomolding) and thixo
(Thixomolding) processes dominate
here.
Die-casting is an industrial casting process
for series or mass production of
structural parts. Metallic materials or
light metal alloys with alow melting
point are generally used for this purpose.
Among the casting processes for
magnesium alloy parts, the die-casting
processes currently dominate.
Figure 4: Levitation preprocessor using argon to preheat and clean the chips. The use of argon
reduces costs by up to 80 %.
> and absorption of vibrations
makes magnesium an excellent candidate
to support human activities in
almost all aspects.
Combined with ease of processing
and very easy recycling, magnesium
alloys may be the material of the
future. And even unrecycled, MgO
becomes part of nature over time
without emitting pollutants. Compared
to plastics, which have seen tremendous
growth over the last 50 years, magnesium
has many qualities to offer. Magnesium
does not harm humans or animals
and is readily available. It can be
recyclable with only 4%ofthe original
energy of creation, thus contributing to
the achievement of climate goals.
If the materials are recycled at the
end oftheir life cycle, a„credit“ can be
issued for primary production avoided.
The credits are particularly high for aluminum
and magnesium in an assumed
closed recycling loop. As aresult, net
emissions change considerably, compared
with other materials. Magnesium
now has the lowest emissions over its
entire life cycle. Polyamide and polypropylene
have little credit potential, as
plastics are assumed to be thermally
recycled with relatively few credits.
However, inorder to take advantage of
the many benefits of magnesium, some
fundamental things in the foundry
industry need to be reconsidered. For a
long time, magnesium was regarded by
Foundrymen as being difficult to
handle, and the appropriate technology
for better utilization of magnesium was
lacking. Today, processing magnesium is
Cold and hot chamber die-casting
Cold chamber die-casting is frequently
used for components with large casting
weights. Machines with aclamping
force of between 2000 and 4000 tonns
are usually used for the production of
gearboxes or crankcases. The magnesium
is strongly superheated (> 680 °C)
in large crucibles (> 500 kg magnesium)
and supplied with alarge amount of
energy to melt and maintain temperature.
When the melt is injected into the
cold casting chamber, injection rate
must be very fast due to high thermal
conductivity of alloy. The turbulence of
the melt creates gas inclusions. The
inclusions are detrimental for structural
integrity of the part. Many discarded
parts are due to unacceptable inclusions.
Constant temperature variations
and long heat history casing formation
and growth of the dendritic structures
which can lead to premature casting
defects. The temperature loss between
melt in the crucible furnace and the
metal in the casting chamber can be up
to 150°C. For highly thermally conductive
alloy alot can happen in this temperature
range.
In hot-chamber die-casting, the casting
chamber is located directly in the
crucible and isthus more or less temperature-controlled.
Pre-solidification
occurs to amuch lesser extent in the
area of the nozzle and the gooseneck.
18