Journal of Minerals & Materials Characterization & Engineering, Vol. 8, No.4, pp 303-315, 2009
jmmce.org Printed in the USA. All rights reserved
303
Investigation of the Quenching Properties of Selected Media on 6061
Aluminum Alloy
O. K. Abubakre*, U. P. Mamaki** and R. A Muriana*
*Department of Mechanical Engineering
Federal University of Technology, Minna
** Waziri Umaru Polytechnic,
Birnin Kebbi
*Corresponding Author, contact: [e-mail]
ABSTRACT
Specimens of 6061 Aluminum alloy were prepared and quenched in water, sheanut oil and palm
oil at temperature of 400
o
C, 450
o
C and 530
o
C to determine the effect of variation in temperature
and quenching media on some mechanical properties and the microstructure of the alloy.
Standard specimens from the rapidly quenched alloys were subject to various tests to determine
their ultimate tensile strength, hardness and impact strength. The results showed that the
specimen heat-treated to 530
o
C and quenched in water has the highest tensile strength of 109
N/mm
2
and yield strength of 70.89 N/mm
2
. The specimen heated at 530
o
C and quenched in water
gave the highest value of 35.50 in hardness (HRC). The toughness property of the alloy, as
indicated by Charpy impact values, is better at 530
o
C for specimen quenched in sheanut oil and
least impact strength is observed in specimen quenched in water at 400
o
C.
Key Words:
1. INTRODUCTION
Heat treatment is an important operation in the final fabrication process of any engineering
component. The objective of heat treatment however, is to make the metal better suited,
structurally and physically, for some specific application [1]. Solution heat treatment of
aluminum alloys allows the maximum concentration of a hardening solute to dissolve into
solution. This procedure is typically carried out by heating the alloy to a temperature at which

304 O. K. Abubakre, U. P. Mamaki, and R. A. Muriana Vol.8, No.4
one single, solid phase exists. By doing so, the solute atoms that were originally part of a two
phase solid solution dissolve into solution and create one single phase. Once the alloy has been
heated to the recommended solutionizing temperature, it is quenched at a rapid rate such that the
solute atoms do not have enough time to precipitate out of the solution. As a result of the quench,
a super saturated solution now exists between solute and aluminum matrix [2-3].
The cooling rate associated with the quench can be controlled through the variation of quenching
parameters such as bath temperature and degree of agitation. The variation of this parameter
allows the heat transfer, the ability to increase or decrease the cooling rate to achieve certain
mechanical properties as well as eliminate distortion and the possibility of cracking [4].
The primary objective of this work is to experimentally determine the effect sheanut oil and palm
kernel oil on the mechanical properties of 6061 aluminum alloys. The properties of interest
include: tensile strength, impact strength, hardness and the microstructural changes.
The results of tests conducted showed that the specimen heat-treated to 530
o
C and quenched in
water has the highest tensile strength of 109 N/mm
2
and yield strength of 70.89 N/mm
2
. The
specimen heated at 530
o
C and quenched in water gave the highest value of 35.50 in hardness
(HRC). The toughness property of the alloy, as indicated by Charpy impact values, is better at
530
o
C for specimen quenched in sheanut oil and least impact strength is observed in specimen
quenched in water at 400
o
C.
2. LITERATURE REVIEW
2.1. Solution Heat Treating of Aluminum Alloys
The purpose of solution heat treatment of aluminum is to obtain the maximum concentration of
hardening solute, such as zinc, magnesium and copper, in solution by heating the alloy to a
temperature in which single phase will be created. By doing so, the solute atoms that were
originally part of a two-phase solid solution dissolved back into solution and create one single
phase in equilibrium. Once the alloy has been held for a considerable amount of time to ensure
complete solutioning and homogenous phase, it is quenched rapidly such that the solute atoms do
not have enough time to precipitate out of solution. As a result, a super saturated solution now
exists between the solute and aluminum matrix. The heat – treating process is best understood by
studying a phase diagram and examining the temperature ranges and phase regions that are
involved. These diagrams do not show the actual structures formed during the heat treatment
processes, but they are a useful tool in predicting the solid state reactions that will take place at a
given temperature and composition [5].

Vol.8, No.4 Investigation of the Quenching Properties 305
2.2. Quenching
This is the process of rapid cooling of materials to room temperature to preserve the solute in
solution. The cooling rate needs to be fast enough to prevent solid – state diffusion and
precipitation of the phase. The rapid quenching creates a saturated solution and allows for
increased hardness and improved mechanical properties of the material. In addition, studies have
shown that the highest degrees of corrosion resistance have been obtained through the maximum
rates of quenching [3, 6].
In general terms, liquid quenching is performed in water, oil and more recently, in aqueous
polymer solutions. Water and oil quenching cover the extremes in terms of cooling rates, with
water being the fastest and oil being the slowest. Quenching takes place in three distinct stages,
namely: Vapour blanket stage, Boiling stage and Liquid cooling stage. The vapour blanket stage
begins when the hot part makes contact with the quenching medium. As the part is submerged,
an unbroken blanket surrounds the piece. This blanket exists between the specimen and
quenching media if the heat from the surface of the part exceeds the amount of heat needed to
form the maximum vapour per unit area on the piece. This stage is characterized by a relatively
slow cooling rate since the vapour of the quenching medium surrounds the part and acts as an
insulator. In this particular stage, heat is removed from the part by radiation and conduction
through the vapour layer. As the component cools, the vapour blanket cannot be maintained and
therefore, breaks down. After this breakdown, the boiling stage immediately begins. The surface
of the part now comes in direct contact with the fluid and result in violent boiling of the medium.
This stage is characterized by rapid heat transfer through the heat of vaporization. Size and shape
of the vapour bubbles are important in controlling the duration of this stage as well as its
corresponding rate. As the part continues to cool below the boiling point of the medium, the
boiling stage can no longer exist and it too break down giving way to liquid cooling stage. This
stage, much like the vapour blanket stage, is also characterized by slow rates of heat transfer.
Heat is dissipated from the part by movement of the quenching medium by conduction currents.
The difference in temperature between the boiling point of the medium and actual temperature of
the medium is the major factor influencing the rate of heat transfer in liquid quenchants.
Furthermore, viscosity of the medium at this point also affects the cooling rate since a less
viscous medium will dissipate heat faster than one of high viscosity. The final stage of quenching
is the most important in controlling and reducing distortion and cracking [7-8].
2.3. Quench Factor Analysis (QFA)
To successfully predict the metallurgical consequences of quenching, it is necessary to determine
the heat transfer properties produced by the various quenching medium during the cooling.
Cooling curve analysis has been considered to be the best method to obtain such information [9].

306 O. K. Abubakre, U. P. Mamaki, and R. A. Muriana Vol.8, No.4
Quench factor analysis (QFA) has several advantages over other methods. Quench factor
analysis provides a single value describing quench severity for the specific alloy being quenched.
In addition, the quench factor is directly related to the hardness of the quenched part and
intermediate manual interpretations are not required. So, it can be seen that QFA is highly
beneficial in the quenching process [10].
The determination of the Quench Factor, Q
1
begins with the calculation of a variable called the
incremental quench factor (τ). This variable is calculated for each time step in cooling process
using equation 1.0:
0.1−−−−−−−−−−−−−−−−−−−−−−−−−−−−
∆
=
T
C
t
T
T = Incremental quench factor
 t = Time step used in cooling curve data acquisition.
The C
T
function is defined below as well as the variables that help create it [9-11].
( )
0.2
5
2
4
2
43
21
−−−−−−−−−−−−−−−−−−=
















−
−
TT
R
K
TKR
KK
T
eeKKC
Where
C
T
= Critical time required to form a constant amount of a new phase or reduce the
hardness by a specific amount.
K
1
= Constant which equals the natural logarithm of fraction untransformed during
quenching (the fraction defined by the TTP curve).
K
2
= Constant related to the reciprocal of the number of nucleation sites.
K
3
= Constant related to the energy required to form a nucleus.
K
4
= Constant related to the Solvus temperature.
K
5
= Constant related to the activation energy for diffusion
R = 8.3143J/k mole
T = Absolute temperature (k)
Values for the above constant were found experimentally by Totten, Bates and Javis [7]. Table 1
illustrates these constants for four different aluminum alloys.
Table 1: Constant for various aluminum
Alloy K
1
K
2
K
3
K
4
K
5
Temperature
Range (
0
C)
7050-T76
7057-T6
2024-T851
7075-T73
-0.00501
-0.00501
-0.00501
-0.00501
2.20E-19
4.10E-13
1.72E-11
1.37E-13
5190
1050
45
1069
850
780
750
737
1.80E + 05
1.40E + 04
3.20E + 04
1.37E + 05
425 – 150
425 – 150
440 – 110
425 – 150

Vol.8, No.4 Investigation of the Quenching Properties 307
3. EXPERIMENTAL METHODOLOGY
3.1. Materials and Equipment
The materials and equipment used during the experiments include 6061 Aluminum alloy,
Quenching medium, Melting furnace, Heat treatment furnace, Impact testing machine, Tensile
testing machine, Hardness testing machine, Grinding machine, Rotary wheel polishing machine,
Etching reagent, and Binocular metallurgical microscope.
Three kilogram of 6061 Aluminum alloy was obtained from the Nigerian Aluminum Extrusion
Company (NIGALEX) Lagos. The chemical concentration analysis of the alloy was also
obtained from the company. Table 2 shows the concentration analysis of the 6061 Aluminum
alloy used.
Table 2: Chemical analysis of the 6061 aluminum
Run Si Fe Cu Mn Mg Zn Cr Ti Ca Sr Al%
Avg 4.4174 0.6269 0.1438 2.0626 2.5151 0.6489 0.02172 0.0105 -0.1332 -0.0002 89.6875
Source: Nigerian Aluminum Extrusion Company. 7/09/04
The quenching media used during the experiment were three (3): water, sheanut oil and palm
kernel oil. The two quenching media, Sheanut and Kernel oil were obtained locally from Birnin
Kebbi market.
Table 3: Quenching media physical properties
Quenching
Media
Temp
(
0
c)
Specific
Gravity
Specific
Heat
Capacity
(JKg
-10
k)
Density
(Kg/m
3
)
Viscosity
Ns/m
2
Tap water 26 1.002 4190.0 1000 0.8 x 10
-
3
Palm kernel oil 26 0.912 2020 970 36 x 10
-
3
Raw G/nut oil 26 0.904 1930 920 36 x 10
-
3
Engine oil [SAE
20 W-50
26 0.924 1900 890 050 x 10
-
3
3.2. Experimental Procedure
Three kilogram of 6061 Aluminum alloy was obtained from the Nigerian Aluminum Extrusion
Company (NIGALEX) Lagos. Sand moulds were prepared for casting rods of 30mm diameter
from the Aluminum alloy melted in an oil-fired crucible furnace. After solidification, the moulds
were broken to obtain the cast rods. The cast rods were taken to the machine shop for preparation

308 O. K. Abubakre, U. P. Mamaki, and R. A. Muriana Vol.8, No.4
into tensile, impact and hardness tests’ specimens. The specimens were prepared based on the
ASTM standard.
The prepared 6061 Aluminum alloy samples, was quenched in three (3) different quenching
medium, water, palm kernel, sheanut oil, using varying solutionizing temperatures of 530
0
C,
450
0
C and 400
0
C.
3.2.1. Heating Operation
A carbolite GLM3 model box furnace was used to heat the prepared samples to the selected
solutionizing temperature of 400
0
C, 450
0
C, and 530
0
C. The samples were arranged in the furnace
in such a way as to allow uniform circulation of hot air to all the specimens. Nine (9) samples
were heated at every heating operation. A total of three (3) heating operations were conducted in
each case.
3.2.2. Quenching Operation
After heating was completed, the various samples were quenched in water, shea-nut oil and palm
oil baths respectively. The baths were agitated in order to increase the rate of heat transfer
through the quenching process. After the completion of the quench, the samples were removed
from the various mediums and sanding was done after each quench to maintain a consistent
surface finish of the specimen throughout the experiments.
3.2.3. Testing Operation
Avery Denison T4CI/33470 model, 6705U/33458 model and 6408/33267 model machines were
used to test for the tensile strength, toughness and Rockwell hardness respectively.
3.2.4. Metallographic Examination
The specimens for micro examination were prepared by grinding on hand grinding deck of
abrasive papers of grades 240, 400, 600 and 1000. Polishing was done on a universal rotary
wheels using alumina as the polishing medium. The thoroughly polished specimen was washed
in warm water and swabbed with methylated spirit and dried using warm air. Etching was done
in a solution of one percent (1%) Sodium Hydroxide in water by swabbing the polished surface
of the specimen with the etching reagent. A Leitz model binocular metallurgical microscope (the
bench type) was used to view the micro – structural changes that took place in all the prepared
samples.

Vol.8, No.4 Investigation of the Quenching Properties 309
4. EXPERIMENTAL RESULTS AND DISCUSSIONS
4.1. Presentation of Result
4.1.1 Tensile test
The tensile test specimens were tested to fracture on a tensile testing machine. From the data
generated, ultimate tensile strength, yield strength, percentage elongation and percentage
reduction in area were calculated. The results are as presented in Table 4.
4.1.2 Rockwell hardness test
The results obtained from hardness test conducted using the Rockwell hardness test method and
three indentations are presented in Figure 1.
Table 4: Tensile test results of 6061 aluminum quenched in different media
Quenching
Media
Heat
Treatment
Temp. (0C)
Solution
Time (Hours)
UTS N/mm
2
Yield
Strength
N/mm
2
Percentage
Elongation %
Water
450 2 85.77 63.12 7.00
450 2 87.69 67.73 6.11
530 2 109.70 70.89 4.00
Sheanut oil
450 2 78.12 59.63 6.35
450 2 86.37 62.73 6.00
530 2 88.00 66.72 3.75
Palm Kernel
Oil
450 2 74.07 55.53 6.01
450 2 77.04 57.60 5.50
530 2 90.10 64.94 2.98
Control
sample
- - 83.80 64.44 5.0

310 O. K. Abubakre, U. P. Mamaki, and R. A. Muriana Vol.8, No.4
0
5
10
15
20
25
30
35
40
400450 530
Temperature (0C)
Rockwell Hardness (HRC)
w ater
sheanut oil
palm kernel oil
Figure 1: Effect of Temperature and Quenching Media on Rockwell Hardness of 6061
Aluminum Alloy.
4.1.3. Impact test
The results obtained from the V-notch Charpy test are shown in Table 5.
Table 5: Charpy impact test results for 6061 aluminum alloy quenched in different media
Quenching Media Temperature
0
C, Charpy Impact
Values(J)
Water
400
450
530
26
27
60
Sheanut Oil
400
450
530
49.6
50.1
65
Palm Kernel Oil
400
450
530
41.2
42.2
41
Control Sample - 26

Vol.8, No.4 Investigation of the Quenching Properties 311
4.1.4. Micro structural analysis
The microphotograph of specimens quenched in the various media at 530
0
C is presented in Plate
I. Plate II presented specimen quenched at 450
0
C and Plate III represent specimens quenched at
400
0
C. A,B and C in all cases represent microphotographs for specimens quenched in water,
Palm kernel and Sheanut oil respectively.
4.2 Discussion of Results
4.2.1 Tensile test
The results of the tensile strength test are shown in Table 4 when the samples were heated to
530
0
C, 450
0
C, and 400
0
C. The highest ultimate tensile strength values were obtained in samples
quenched in water, sheanut oil, and palm oil at 530
0
C. The corresponding ultimate tensile
strength was 109.0N/mm
2
, 88.0N/mm
2
and 90.1N/mm
2
respectively. Similarly the yield strength
obtained was 70.89N/mm
2
, 66.72N/mm
2
, 64.94N/mm
2
respectively. The values for the control
system are ultimate tensile strength 83.8 N/mm
2
and yield strength of 64.44 N/mm
2
. Samples
quenched in sheanut oil and palm kernel oil at 530
o
C showed a marginal increase in reduced
strengths (ultimate and yield strength) when compared to the values for the control sample.
However, the highest ultimate tensile strength of 109.7 N/mm
2
was obtained with sample heated
to 530
0
C and quenched in water. The values of tensile strength are influenced by the
microstructure of the aluminum alloy sample, which are controlled by the quenchant's cooling
rate. In contrast to the behaviour of the control sample specimen, specimens quenched in sheanut
and water exhibited high values tensile strength at 530
0
C at the same time maintaining low
ductility. This show that the high strength produced is due to the effectiveness of the quenchants
and their characteristics as shown.
4.2.2 Rockwell hardness
The results are presented also in graphical form (Figure. 1).The homogenous distribution of the
fine precipitate of copper, in Aluminum matrix is largely responsible for the hardness of the
quenched aluminum alloy. The hardness of the aluminum alloy specimens quenched in water,
sheanut and palm kernel at a solutionizing temperature of 530
0
C increased. This is as a result of
the fact that quenched alloy is no longer in "equilibrium" which precipitate silicon. The hardness
of the quenched aluminum alloy increased significantly as compared to the control sample.
4.2.3 Impact test (Charpy)
The results for the impact test are shown in Table 5. The treated samples in all cases show a
remarkable increase in the toughness when compared to a value of 20.50 Joules for the control

312 O. K. Abubakre, U. P. Mamaki, and R. A. Muriana Vol.8, No.4
sample. However, the sample heated at 530
0
C and quenched in palm kernel oil has the highest
value of 65 Joules.
4.2.4 Microstructure examination
The result of microstructural examination of the specimens shows that the specimens quenched
in water, Sheanut and palm kernel oil at 530
0
C show higher values of ultimate tensile strength as
shown in Table 4. This is associated with fine precipitates, which impact improved strength as
compared to other specimens, which have a little coarse dispersion of precipitates. The changes
in structure of the specimen are associated with the temperature of solutioning and the cooling
rate of the quenching medium.
The very high rate of heat extraction experienced by the specimen quenched in water after
solution treatment at 530
0
C could be said to be responsible for the fine microstructure observed
in plate 1A. The precipitates are also uniformly distributed in Aluminum matrix. Generally,
hardness and strength increases as grain size decreases. Both properties are proportional to the
reciprocal of the square root of the average grain diameter. The relatively slower cooling rate is
responsible for the formation of platelet-like precipitates in specimen quenched in sheanut oil. To
a lesser extent, (compared to water quenching) the presence of spherodised grains is observed in
specimen quenched in palm kernel oil.
5. CONCLUSION
The study on the effect of various quenching media (water, sheanut oil and palm kernel oil) on
rapidly quenched 6061 aluminum alloy was conducted and attempt were made to relate
mechanical properties to microstructure. As a result of this investigation, the following
conclusions were made regarding the effect of several quenching media on the alloy.
Increase solution treatment temperature was found in improved soluble precipitates in the alloy,
and the heat extraction capacity of the quenching medium also contributed to the formation of
fine precipitates. Water among the quenching media proved to have the highest cooling rate,
followed by Sheanut oil. Improvements in properties may be correlated with a more refined
metallurgical structure.
High values of strength, hardness and notch strength were typically associated with fine
precipitate distribution.
Careful choice of cooling media with appropriate choice of heat treatment procedure and
parameters were effective in producing desired mechanical properties.

Vol.8, No.4 Investigation of the Quenching Properties 313
A
Plate 1A: Aluminum 6061 quenched in
water at 530
0
C. Precipitates formed by Si
Mn and Mg spherodised in Al-matrix
(mag x 300)
B
Plate 1B: Aluminum 6061 quenched
in palm kernel oil at 530
0
C, plate lets
of precipitates sharing partial
spherodisation (mag x 300)
C
Plate 1C: Aluminum 6061 quenched
in Sheanut oil at 530
0
C showing
growth of precipitates leading to sub
grains of concentrated precipitates
(mag x 300)
A
Plate 2A: In 6061 quenched in water
at450
0
C less spherodisation of
precipitates (mag x 300)
B
Plate 2B: Aluminum 6061 quenched
in sheanut oil at 450
0
C more
spherodisation of precipitates as
compared to plate IIA (mag x 300)

314 O. K. Abubakre, U. P. Mamaki, and R. A. Muriana Vol.8, No.4
C
Plate 2C: Aluminum 6061 quenched in palm
Kernel oil at 430
0
C shows abnormal
Segregation of secondary phase (mag x 300)
A
Plate 3A: Aluminum 6061 quenched
in water at 400
0
C (mag x 300)
B
Plate 3
B: Aluminum quenched in
Sheanut oil at 400
0
C (mag x 300)
C
Plate 3C: Aluminum 6061 quenched
in Palm kernel oil at 400
0
C (mag 300)

Vol.8, No.4 Investigation of the Quenching Properties 315
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