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1. constituents with respect to temperature. [2]

1.      
Heat
treatment analysis

This topic discuses about the variation of
mechanical properties of the alloys (Al-Si-Cu/Mg) with heat treatment. It is
found that properties such as hardness and tensile strength of the alloy is
influenced by heat treatment and this is studied to achieve optimal mechanical
properties of the alloy. Heat treatment improves the strength of aluminium
alloys through a process known as precipitation hardening which occurs during
the heating and cooling of Al alloys in which precipitates are formed in the
aluminium matrix. This improvement depends on the change in the solubility of
the alloying constituents with respect to temperature. 2

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1.1  
Solidification
process

The solidification rate under a real-time
condition will be greatly influenced by the nucleation efficiency and the atom
diffusion between phases. 2 The solidification rate determines the coarseness
of the microstructure including the fraction, size and distribution of
intermetallic phases and the segregation profiles of solute in the ?-Al phase. To
gain an optimum property of an alloy, the Dentrite arm spacing (DAS) must be
minimized and distributed homogeneously. 2

 

The table
below identifies the sequence of phase precipitation in hypoeutectic Al-Si
alloys. Al in the eutectic has the same crystallographic features, as the
primary ?-Al dendrites in unmodified alloys. 2

 

 

The
figure shows different sequence of phase precipitation in Al-Si alloys. Table 1

 

Copper
forms an intermetallic phase with Al that precipitates during solidification
either as

blocky
CuAl2 or as alternating lamellae of ?-Al + CuAl2. In the presence of nucleation
sites or high cooling rates during solidification can result in fine Cu-Al2 particles.
2

 

The
solidification of industrial alloys generally proceeds through the formation of
single phase primary crystals such as dentrites or polly phased structures such
as eutectics. Because eutectics alloys have a low melting point and excellent
casting behavior they are often used for casting. Directional solidification of
eutectic alloys is a self-assembling procedure that allows fabrication from
melt of fine homogenous microstructures controlled by temperature gradient and
growth rate. 5 The directional solidification technique by bridgeman- type equipment
is used for achieving crystal growth, which minimizes casting defects on
aluminium and its alloys this is known as controlled solidification technique.
When eutectic alloys are direc’tionally solidified using this technique, two or
phases that are aligned in the growth direction are formed and this
microstructural morphology effects the mechanical and thermophysical
properties. The value of micro hardness and tensile strength for the quaternary
Al-Cu-Si-Mg eutectic alloy increases with increase in growth rate and this is
due to the existence of inter metallic phase in these alloys which is not the
same with ternary and binary eutectic alloys. 5

 

The
material properties of cast aluminium alloys have not been adequate even when
high qualities Al alloys have been created, this is because the mechanical
properties of Al alloys are still low compared with those of conventional cast
iron materials. Twin role continuous casting is one of the casting technologies
to make cast components with high solidification speed resulting in fine
microstructure near the surface of the cast components. 3

 

 

Temperature vs cooling time curves
ofof the SGC and HMC processes 3 Figure.2

Al2024
alloy in Al-Cu-Mg alloy system is developed to use in aerospace applications
because it has low density and good damage tolerance. The high strength of this
alloy is primarily due to the precipitation and redistribution of fine Al2CuMg
particles.

 

In the journal of M.H.Ghoncheh, S.G.Shabestari
and M.H.Abbasi they concluded with the following observations:5

 

Ø   Solidification characteristics are influenced
by the cooling rate. The temperature of various phases reactions is shifted to
shorter time intervals with an increasing the cooling rate.

 

Ø   Increasing the cooling rate decreases
significantly both nucleation temperature and finish point of solidification.
Also, solidification time is decreased, and the range of solidification
temperature is increased by cooling rate enhancement.

 

Ø   By increasing the cooling rate, both liquidus
undercooling temperature and recalescence undercooling temperature initially
increase and after reaching the summit, go downward.

 

Ø   The plot of Dentrite arm spacing(DAS) as a
function of the cooling rate shows an exponential relationship. Increasing the cooling
rate from 0.4 to 17.5 C s-1 decreases DAS about 89 %. A numerical correlation
between the cooling rate and DAS is calculated for Al2024 alloy.

 

2.      
Effect on
mechanical properties with respect to temperature.

 

With heated mold continuous casting(HMC) the
microstructure of ADC12 alloys is oriented uniformly resulting in high strength
and ductility. With heating process the ductility is further increased. The
tensile strength decreases with increase of heating temperature to 400degree
Celsius in advance and then increases when heated further to around 520degree
Celsius and when heated beyond 540degree Celsius tensile strength drops. The
hardness and fatigue strength of these alloys are similarly altered. because of
the residual stress and precipitation characteristics of these alloys the
ductility increases non-linearly with increase in heating temperature. The
HMC-520degree Celsius samples may display excellent mechanical properties such
as high strength and ductility, in the present approach. 3

The aluminium alloys are heated continuously
for one hour immediately following heated mold continuous casting (HMC) and
sand gravity crafting (SGC) and the result is graphically represented. 3

 

 

 

                               

 

Vickers hardness for HMC and SGC ADC12 with and
without heating process. 3 Figure.3

Tensile properties of HMC and SGC
ADC12 with and without heating process. 3 Figure.4

 

The
mechanical properties of as-cast AL-Cu-Si alloys like the yield strength(YS),
Ultimate tensile strength and the elongation at 300degree Celsius reach 96 MPa,
117Mpa and 10.3% respectively.  

 

The alloy, with a chemical composition of
Al-27%Cu-5%Si and minor additions of Ni, was produced through a rapid solidification
casting method. The various phases during the solidification process of the
developed alloys was investigated with Thermo-Calc software. The results from
the thermodynamic calculations revealed that ?-Al2Cu intermetallic phase first
precipitates out of the Al-27%Cu-5%Si base alloy, followed by a binary eutectic
reaction and with the last melt solidifying in a ternary eutectic structure. It
is found that the addition of Ni to the base alloy alters its solidification profile,
and a Ni-rich phase (Al7Cu4Ni) is produced in the alloy which forms ahead of
the eutectic front at a temperature greater than the eutectic temperature. SEM-EDS
analysis of the cast alloys shows a bimodal eutectic composite microstructure
in the Al-27%Cu-5%Si base alloy, which consists of binary a eutectic cell
surrounded by a ternary eutectic matrix. The SEM characterization of the alloys
containing Ni addition reveals that Al7Cu4Ni intermetallic was formed in the
alloys in addition to the composite eutectic structure.

 The
presence of the Al7Cu4Ni inter-metallic contributes significantly to the elevated-temperature
tensile properties of the alloy. A significant increase in high-temperature
tensile strength of 272 MPa was achieved in cast Al-Cu-Si(-1.5%Ni) eutectic
alloy at 300 ?C compared to 91 MPa attained in the reference alloy A319 at the
same temperature. The yield strength and the ultimate tensile strength of the
alloy containing 1.5% Ni at 400 ?C were observed to be 220% and 309% higher,
respectively, than for conventional A319 reference alloy. 7

 

It is found that the mechanical properties of
as-cast alloys at elevated temperatures with and without Ni additions is found
to have improved. The bar graph given below summarizes the result of the
observations.

 

Bar graph showing comparison of yield strength
of the present and the conventional A319-Al alloys at elevated temperatures.
Figure.5 7

 

 

 

 

 

                                

Bar graph showing comparison of tensile
strength of the present and the conventional A319-Al alloys at elevated
temperatures. Figure.6 7

                               

Bar graph showing comparison of elongation of
the present and the conventional A319-Al alloys at elevated temperatures.
Figure.7 7

 

3.      
Structural
changes with respect to hot compression tests.

 

Hot
compression tests of 7150 aluminum alloy were per-formed on Gleeble-1500 system
in the temperature range from 300 °C to 450 °C and at strain rate range from
0.01 s?1 to 10 s?1, and the following structural changes are studied by analyzing
the observations of metallographic and TEM. To find that the true stress–true
strain curves exhibit a peak stress at a critical strain, after which the flow
stresses decrease monotonically until high strains, showing a dynamic flow
softening. It is noticed that the peak stress level decreases with an increase
in the deformation temperature and decreasing strain rate, which can be represented
by a Zener–Hollomon parameter in the hyperbolic sine equation with the hot
deformation activation energy of 229.75 kJ/mol.

 Another
observation was that the deformed structures exhibit elongated grains with
serrations developed in the grain boundaries. A decreasing Z value leads to
more extensive dynamic re-crystallization (DRX) and coarser recrystallized
grains.

It is found that as the Z value is increased the
subgrains is found to exhibit high-angle sub-boundaries with large number of
dynamic precipitation in subgrain interiors and a certain amount of
dislocations. The dynamic recovery and recrystallization are the main reasons
for the flow softening at low Z value,4 but the dynamic precipitation and
successive dynamic particles coarsening have been assumed to be responsible for
the flow softening at high Z value. 4

 

In hot deformation of metallic materials, the
relationship between peak stress or steady state stress, strain rate and
temperature can be expressed as. 11-13

 

 

where A1, A2, n1 and ? are constants, Z is the
Zener?Hollomon parameter, Q is an activation energy for hot deformation and R is
the gas constant.

 

Automotive cast Al-A319 alloys have been increasingly used in the
manufacture of engine blocks due to a combination of good fluidity properties
and mechanical strength.  Since engine
blocks operate over a wide range of temperatures and stress conditions,
alloying elements such as Cu and Mg are often added to improve the room and
high temperature strength of these alloys. The tensile properties of A319-Al
alloys are affected by the increasing tendency to develop porosity partly as a
result of Cu and Sr additions.

The tensile properties of an as-cast Al A319 were investigated as a
function of temperature and found that alloy Al-A319 is inherently brittle as
the alloy fractured prior to reaching the maximum defined by the Considere
criterion. In particular, the ?* = n condition was not reached and alloy brittleness
was found to be dominant in the temperature range of ?90 ?C

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