THE EFFECT OF HEAT TREATMENT PARAMETERS AND GRAIN REFINEMENT ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF A357 ALLOY

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International e-Journal For Technology And Research-2017
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THE EFFECT OF HEAT TREATMENT
PARAMETERS AND GRAIN
REFINEMENT ON MICROSTRUCTURE
AND MECHANICAL PROPERTIES OF
A357 ALLOY
Sandeep Hegade 1
, Shrishaila 2
, Sunil Kumar S3
, Prashanth M4
, Hemanth Kumar
N5
, Avinash L6
UG Student, Department of Mechanical Engineering, NMIT Bangalore, Karnataka, India1, 2, 3, 4
Assistant Professor, Department of Mechanical Engineering, NMIT Bangalore, Karnataka, India5, 6
Abstract: Aluminium A357 alloy is widely used
at automobile and aircraft industries in the form of
cast component with varying section size. This study
investigates how the microstructure and mechanical
properties of A357 alloy before and after heat
treatment processes. Solutionising at 5000
C-5h
followed quenching in water at room temperature
and Ageing at 1700
C-3h alloy. Aluminium ingot is
melted using a furnace and poured in to the mold
having mold cavities of varying dimensions. In
order to investigate the effect of heat treatment and
aging processes microstructure and mechanical
properties such as hardness, and tensile strength
were analyzed as-cast condition and after solution
treatment process. The aim of present study is to
evaluate the effect on the microstructure and
mechanical properties of Combined Grain Refined
and Modified aluminum alloy A357
Keywords: A357 alloy, grain refiner, grain
modifier, heat treatment, hardness,
microstructure.
1. INTRODUCTION
Aluminium-Silicon alloys are known for their
excellent combination of characteristics namely, low
density, excellent castability, formability, good
mechanical properties, cryogenic properties and
good machinability. Aluminium and its alloys have
wide range of applications particularly in
automobile, aerospace and marine sectors on
account of their light weight, good surface finish,
resistance to wear and corrosion high strength-to-
weight ratio. Since components with complex
geometries can be produced cost effectively, they
find enhanced utility particularly in Aerospace
sectors. Large aluminium extrusion components can
be produced with fewer joints, resulting in less
welding. Reduction in weight due to low density
leads to increased load capacity, increased mileage,
reduced pollution of environment and higher profits
to the manufacturers. The low melting temperature,
ease of handling, easy formability, easy recycling
has led to increased demand for aluminium alloy
components.
Further Aluminium alloys are classified into one of
the following three groups namely Wrought, non-
heat treatable alloys. Wrought, heat treatable alloys,
and Cast alloys. Primarily, cold working is
employed for heat treating non-heat treatable alloys
such as commercial pure aluminium series (1xxx)
which have good corrosion resistance, thermal
conductivity, exceptionally high formability, and
can be easily joined by soldering, welding and
brazing. The aluminium-manganese series (3xxx)
have high formability, corrosion resistance and can
be joined by all commercial procedures. The
aluminium-silicon series (4xxx) have good flow
characteristics, good tensile strength and can be
easily joined by conventional methods such as
Brazing and soldering.

The aluminium-magnesium series (5xxx) are easily
weldable, have superior corrosion resistance and
find application in automotive, cryogenic and
marine sectors. Few of the 4xxx alloys are hardened
by heat treatment, while others hardened by cold
working. Fine dispersion of precipitates for alloys
that respond to ageing have dominant effect in
inhibiting dislocation motion resulting in increased
yield and tensile strength. Dislocation produced by
cold working in substructure in case of wrought
alloys and the grain size of cast alloys are of prime
importance.
Precipitation hardening is adopted to develop high
strength levels in wrought heat treatable alloys such
as 2xxx series (Al-Cu and Al-Cu-Mg).They have
high strength at room and at elevated temperatures
and some of them are weldable. The heat treatable
Al-Mg-Si alloys (6xxx series) have excellent
extrudability, weldability and high corrosion
resistance. The 7xxx series (Al-Zn-Mg and Al-Zn-
Mg-Cu) are heat treatable, have very high strength
especially, high toughness. The heat treatable,
Aluminum-Lithium alloys (8xxx series) have high
conductivity, hardness and strength.
Cast alloys in comparison with wrought alloys
contain higher proportions of alloying elements such
as silicon and copper, which result in a largely
heterogeneous cast structure. These alloys produced
by various casting processes such as sand casting,
permanent mould casting etc., have low tensile
strength compared to wrought alloys but have wide
acceptability on account of its attractive combination
of physical properties and exceptional castability.
Wrought products are generally produced in the
form of round rods and rectangular sections
whereas; cast products can be produced with
complex geometries. Cast alloys have inferior
mechanical properties, particularly ductility when
compared to wrought alloys with similar chemical
composition. Porosity and relatively high shrinkage
are the other major problems with cast alloys.
 Material Selection and sample
preparation
Al357 is having good castability, high corrosion
resistance and low density and is having lot of
applications in automotive and aeronautical industry
is selected as a matrix material having a chemical
composition given in Table 1.1.
Table 1.1: Chemical composition of Al357 alloy (weight
percentage)
2. OBJECTIVES
Based on the problem definition the objectives of
this work has been listed below
 To fabricate A357 alloy using permanent
mould.
 Heat Treatment of the above alloy.
 Combined grain refinement and modification of
A357 alloy.
 Heat treatment of combined grain refinement
and modification of A357 alloy.
 Microstructural characterization of the heat
treated, combined grain refined and
modification alloy using Optical microscope
and scanning electron microscope.
 Evaluation of the mechanical properties of the
above alloy.
 Drawing Conclusion based on the obtained
result.
3. METHODOLOGY
Figure 3.1: Flow Chart of Experimental
Work

4. IMPLEMENTATION
 Automotive industrial applications.
 Aeronautical applications.
 Internal combustion engine parts.
 Crank case & Brake systems.
 Engine blocks.
 Transmission cases.
 Cylinder bodies of compressors &
pumps..
5. OUTCOMES
 Good Microstructure of As-cast due to heat
treatment is obtained.
 Further there is a good improvement in
mechanical properties.
 Minimization voids and porosity.
 Grain refinement resulted in reduction in size of
primary aluminium grains and fine grain
structure.
 Grain modification resulted in fibrous and
finely dispersed silicon and promotes the
formation of finer particles.
 The spheroidal morphology of Silicon particles
and fine precipitates is formed due to heat
treatment.
 Heat treatment also strengthens the matrix and
further stabilizes the subsurface.
 Optical Microscopic Studies
Figure.5.1: Optical Micrographs of A357 alloy (untreated,
heat treated, grain refined and modified)
Figure 5.1(a) shows the distribution of the primary
dendrite alpha phase (aluminum rich phase) in as-
cast alloy which is predominant in the matrix. Grey
needle shaped silicon particles seen in and around
the inter dendrite regions. Figure 5.1(b) shows the
microstructure of the Heat Treated Solutionising at
5000
C-5h followed quenching in water at room
temperature and Ageing at 1700
C-3h alloy, in which
more even distribution of spheroidised silicon
particles is observed Figure 5.1(c) shows the Optical
microstructure of A357.0 alloy with combined Grain
refinement (0.1% Grain refiner) and modification
(0.1% modifier). The microstructure consists of
partially spherical and well dispersed eutectic Si
particles at inter-dendritic regions. Few particles of
intermetallic compounds are also observed in the
microstructure. Figure 5.1(d) shows the Optical
microstructure of A357.0 alloy with combined Grain
refinement (0.1% Grain refiner) and modification
(0.1% modifier) heat treated (T6).Heat Treatment
leads to spheroidision of silicon of particles is
observed .
 Scanning Electron Microscopic
Studies
Fig.5.2: SEM images of of A357 alloy (untreated, heat
treated, grain refined and modified)

Fig.5.2 (a) and 5.2 (b) shows the SEM
microphotographs of A357 alloy in as-cast
condition, as well as heat treated condition.
The eutectic Al–Si alloys in an untreated condition
should consist only a eutectic mixture (a-Al+
eutectic Si) and spherodization takes place after heat
treatment.
Fig.5.2(c) & 5.2(d) shows the SEM
microphotographs of A357 alloy with combined
Grain refined and modified alloy in untreated and
heat treated condition.
The addition of master alloy to A357 alloy resulted
in fine equiaxed a-Al dendrites due to the presence
of Al-5Ti-1B particles present in the master alloy,
which acts as heterogeneous nucleating sites during
solidification. The toughness and strength of the
alloys increases with the presence of grain refiner
and or modifier due to change in microstructure,
which leads
to decrease in wear rate as compared to the absence
of grain refiner and or modifier.
 Hardness Test Result:
Table 5.1: Hardness of A357 alloy (untreated, heat treated,
grain refined and modified)
Fig.5.3: Hardness of A357 alloy (untreated, heat
Fig 5.3 shows the plot of hardness versus
solutionising temperature of as-cast and solution
treated alloys which is combined with grain
refinement and grain modification. From Figure
there is an increase in hardness of 5.45BHN of heat
treated A357 alloy.
Heat treated combined grain refined and modified
A357 alloy has highest hardness of
54.45BHN(~29.33%) compared combined grain
refined and modified alloy without heat treatment
which has an hardness of 51.56BHN (~22.47%) and
as-cast alloy has the least hardness of 12.94BHN .
 Tensile Test Result:
Table 5.2: Tensile Test Result of A357 alloy (untreated, heat
Fig.5.4: UTS Values of A357 alloy (untreated, Heat
Treated, Grain refined and Modified)
Fig.5.5: % Elongation Values of A357 alloy (untreated, heat
Figure 5.5 shows the plot of UTS values versus
solutionising temperature of as-cast and solution
treated alloys which is combined with grain
refinement and grain modification. From Figure
there is an increase in UTS of 10.9 Mpa (i.e. 7.71%)
of heat treated A357 alloy.

Heat treated combined grain refined and modified
A357 alloy has highest UTS of 171.6 Mpa (i.e.
21.52%) compared combined grain refined and
modified alloy without heat treatment which has an
UTS of 164.6 MPa (i.e. 16.57%) and as-cast alloy
has the least UTS of 141.2 MPa .
Figure 13 shows the plot of % Elongation (ductility)
values versus solutionising temperature of as-cast
and solution treated alloys which is combined with
grain refinement and grain modification. From
Figure there is an increase in ductility of about
4.29% of heat treated A357 alloy. Heat treated
combined grain refined and modified A357 alloy has
highest ductility of about 5.09 % compared
combined grain refined and modified alloy without
heat treatment which has an ductility of about
4.81% and as-cast alloy has the least ductility of
4.03% .
 Fractography Studies of A357 alloy
(untreated, Heat Treated, Grain
refined and Modified)
Fig.5.6: SEM of tensile fractured surface of A357 alloy
(untreated, heat treated, grain refined and modified)
Figure 5.6(a), 5.6 (b), 5.6 (c), 5.6 (d) shows the
Scanning Electron Micro photographs of tensile
fractured surface of as-cast A357 alloy, Heat
treated as-cast A357 alloy, combined Grain
refined and modified alloy and Heat treated
combined Grain refined and modified alloy
respectively.
Closer examination of the fractured surfaces shows
larger and uneven distribution of dimples left by
silicon particles during fracture in as-cast alloys in
untreated condition and smaller and more evenly
distributed dimples in peak Heat Treated alloys.
CONCLUSION
The key findings of the present investigation are:
a) It can be noted that the Mechanical properties
like hardness & tensile strength of the A357
alloy increased by heat treatment & adding
Grain Refiner & Grain Modifier and then
decreased in both untreated and heat treated
condition without added Grain refiner &
modifier compare to untreated as-cast A357
alloy. This may be due to more frequent
nucleation results in larger number of smaller
grains & Modifier produces a silicon phase that
is fibrous & finely dispersed.
b) The adding of Grain refiner, Grain Modifier &
Heat treatment (Solution treatment , Quenching
& ageing) had a significant increase in hardness
and tensile strength .
c) From the optical Micrographs & SEM images it
is observed that the distribution of Silicon
particles in A357 alloy is found to be uniform.
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THE EFFECT OF HEAT TREATMENT PARAMETERS AND GRAIN REFINEMENT ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF A357 ALLOY

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