CPT International 02/2021
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CASTING TECHNOLOGY<br />
asafe and rewarding activity promising<br />
minimal climate impact and great<br />
growth opportunity.<br />
Figure 3: MAXImolding operates in the semi-solid temperature range for the AZ91 alloy, which<br />
is between 480 and 580 °C. The microstructure ofthe alloy has spherical, non-dendritic features<br />
with 50 %solid content (Dr. Frank Czerwinski). This prevents the formation of defects.<br />
Casting -Molding -Forging<br />
Abasic distinction is made between two<br />
processes for melting light alloy metals:<br />
> Ingots are processed in amelting<br />
furnace and then filled into the casting<br />
chamber as aliquid melt: hot and cold<br />
chamber die-casting machines dominate<br />
this ecosystem.<br />
> Processing and heating of the material<br />
to asemi-solid mushy melt directly<br />
in the machine, (cold to cold processing)<br />
and without an external melting furnace:<br />
rheo (Rheomolding) and thixo<br />
(Thixomolding) processes dominate<br />
here.<br />
Die-casting is an industrial casting process<br />
for series or mass production of<br />
structural parts. Metallic materials or<br />
light metal alloys with alow melting<br />
point are generally used for this purpose.<br />
Among the casting processes for<br />
magnesium alloy parts, the die-casting<br />
processes currently dominate.<br />
Figure 4: Levitation preprocessor using argon to preheat and clean the chips. The use of argon<br />
reduces costs by up to 80 %.<br />
> and absorption of vibrations<br />
makes magnesium an excellent candidate<br />
to support human activities in<br />
almost all aspects.<br />
Combined with ease of processing<br />
and very easy recycling, magnesium<br />
alloys may be the material of the<br />
future. And even unrecycled, MgO<br />
becomes part of nature over time<br />
without emitting pollutants. Compared<br />
to plastics, which have seen tremendous<br />
growth over the last 50 years, magnesium<br />
has many qualities to offer. Magnesium<br />
does not harm humans or animals<br />
and is readily available. It can be<br />
recyclable with only 4%ofthe original<br />
energy of creation, thus contributing to<br />
the achievement of climate goals.<br />
If the materials are recycled at the<br />
end oftheir life cycle, a„credit“ can be<br />
issued for primary production avoided.<br />
The credits are particularly high for aluminum<br />
and magnesium in an assumed<br />
closed recycling loop. As aresult, net<br />
emissions change considerably, compared<br />
with other materials. Magnesium<br />
now has the lowest emissions over its<br />
entire life cycle. Polyamide and polypropylene<br />
have little credit potential, as<br />
plastics are assumed to be thermally<br />
recycled with relatively few credits.<br />
However, inorder to take advantage of<br />
the many benefits of magnesium, some<br />
fundamental things in the foundry<br />
industry need to be reconsidered. For a<br />
long time, magnesium was regarded by<br />
Foundrymen as being difficult to<br />
handle, and the appropriate technology<br />
for better utilization of magnesium was<br />
lacking. Today, processing magnesium is<br />
Cold and hot chamber die-casting<br />
Cold chamber die-casting is frequently<br />
used for components with large casting<br />
weights. Machines with aclamping<br />
force of between 2000 and 4000 tonns<br />
are usually used for the production of<br />
gearboxes or crankcases. The magnesium<br />
is strongly superheated (> 680 °C)<br />
in large crucibles (> 500 kg magnesium)<br />
and supplied with alarge amount of<br />
energy to melt and maintain temperature.<br />
When the melt is injected into the<br />
cold casting chamber, injection rate<br />
must be very fast due to high thermal<br />
conductivity of alloy. The turbulence of<br />
the melt creates gas inclusions. The<br />
inclusions are detrimental for structural<br />
integrity of the part. Many discarded<br />
parts are due to unacceptable inclusions.<br />
Constant temperature variations<br />
and long heat history casing formation<br />
and growth of the dendritic structures<br />
which can lead to premature casting<br />
defects. The temperature loss between<br />
melt in the crucible furnace and the<br />
metal in the casting chamber can be up<br />
to 150°C. For highly thermally conductive<br />
alloy alot can happen in this temperature<br />
range.<br />
In hot-chamber die-casting, the casting<br />
chamber is located directly in the<br />
crucible and isthus more or less temperature-controlled.<br />
Pre-solidification<br />
occurs to amuch lesser extent in the<br />
area of the nozzle and the gooseneck.<br />
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