Stabilization Treatment of Alumnum alloys

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• STABILIZATION TREATMENT IN CAST ALUMINIUM ALLOYS

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STABILIZATION
The heat treatment of cast aluminium alloys is
carried out to increase their strength and
hardness and to change their physical, mechanical
and metallurgical properties.  There are lot of
stresses due to machining in cast Aluminum alloys
machined extensively
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STABILIZATION
In a cast Aluminum alloy used in an assembly, the
assembly was found to fail during leakage test
conducted after the temperature conditioning at
150 degC for 48 hours , though ok initially at
room temperature. Once failure was noticed
during the leakage test, even after cooling to
the room temperature still leakage was observed
from the joint when tested
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WORK DONE
1. Based on literature analysis ,since the
Aluminum and Silicon have different coefficients
of thermal expansion, as the molten mass
solidifies during cooling , besides the usual
macroscopic residual stress, additional residual
stresses develop at the microscopic level 2.
These residual stresses may get relieved during
subsequent process or any other test condition,
thus affecting the diemensional stability.
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WORK DONE
3. To confirm the same, one local casting was
subjected to the stabilization temperature of 200
DegC for 48 hours including heating, time by
which casting achieves the temperature, soaking
and then cooling as per the test arrangement
provided by the client. Only the temperature has
been kept 200 DegC. 4. The casting bore was
analyzed before the stabilization cycle for CMM
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WORK DONE
5. After the stabilization treatment again the
bore was checked for CMM and was found to
increase. 6. The microstructure was analyzed for
the local casting, imported casting, local
casting after stabilization and local casting
found OK and failed in test.
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WORK DONE
Microstructure Test Results Local casting
Mag 500X Remark Microstructure shows silicon
needles and few silicon particles in the matrix
of alpha aluminium.
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WORK DONE
Microstructure Test Results Imported casting

Mag 500X Remark Microstructure shows silicon
needles and few silicon particles in the matrix
of alpha aluminium. It is a fine structure
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WORK DONE
Microstructure Test Results Local casting
stabilized
Mag 500X Remark Microstructure shows silicon
needles and few silicon particles in the matrix
of alpha aluminium. It is a finer structure
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OBSERVATIONS
1. The microstructure of these casting alloys
contains a-Al dendrites as the main constituent,
which is present with eutectic Si particles and
many intermetallic phases such as Al2Cu, Mg2Si,
Fe-bearing phases etc. 2. The size, morphology
and distribution of microstructural features
govern the mechanical properties of these alloys
. It is well-known that a refined microstructure
results in improved tensile properties
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OBSERVATIONS
3. The microstructure of the Al-Si cast alloys
primarily consists of a primary phase (a-Al) and
eutectic mixture of Al-Si. The amount of eutectic
mixture in the microstructure depends on the
level Si. The eutectic mixture contains soft Al
as matrix containing Si particles. 4. Secondary
Dendrite Arm Spacing (SDAS) is one of the most
important microstructural features in dendritic
solidification of alloys (for e.g. Al-Si alloys)
during the casting process. SDAS has a
significant influence on the mechanical behavior
of the cast aluminum components
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OBSERVATIONS
5. As we can see microstructure, imported
components show a finer Alpha Alumnium or finer
Secondary dendritic arm spacing than local
components 6. The stabilization treatment makes
the structure finer for Alpha Aluminum . This is
close to the imported component.

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CONCLUSIONS
1Very little work is done on stabilization of
high pressure die cast Aluminum silicon alloys
which was attempted in the present work . This
was done in order to relieve the residual
stresses. 2. The stabilization treatment resulted
in increase in bore diameter which was studied
using CMM. This could be the reason for the
failure during leakage test after conditioning

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CONCLUSIONS
3. The microstructure shows refined primary
Alpha after the stabilization treatment which is
closer to the imported component. 4. The level
of residual stress in imported component could be
less than locally cast components . This can be
further verified using residual stress
measurements if required.

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RECOMMENDATIONS FURTHER WORK
1. The local cast components shall be subjected
to this stress relieving i.e stabilization
treatment at 200 degC for 48 hours before final
machining and bore grinding. 2. This exercise
shall be taken up at supplier end followed by the
testing for leakage and validation. 3. Once
validated , the same shall be incorporated in the
component drawing and supplier process documents.