This research was
undertaken to investigate the effect of nickel macro-addition on the structure
and mechanical properties of aluminium bronze. Sand casting method was used in
the production of a dual-phase aluminium bronze alloy with pre-selected composition
of 10% Al-content. The properties studied were tensile strength, yield
strength, percentage elongation using universal tensile testing machine
(SRNO0723), impact strength using charpy machine (U1820) and hardness using
Brinell hardness tester model B 3000(H). The tests were conducted according to
BS 131-240 standards. The specimens were prepared by doping 1.0 -10wt% of
nickel into Cu-10%Al alloy at 1.0 percent interval. Microstructural analysis
was conducted using L2003A reflected light metallurgical bench microscope and
PHENOM ProX scanning electron microscope. Results obtained showed that
optimally improved mechanical properties were achieved at 4wt% nickel addition
with respect to ultimate tensile strength and %elongation. Hardness on the other hand, decreased with
increase in nickel content while impact strength increased with increase in
composition of nickel from 1-10wt%. Microstructural analysis revealed the
presence of primary α-phase,
-phase (intermetallic phases) and fine stable
reinforcing kappa phase and these phases gave rise to the enhanced mechanical
properties. This research have established that aluminium bronze doped with
nickel increased the tensile strength, ductility, and impact strength and
reduces hardness and is therefore recommended for applications in automobiles
and allied engineering industry.

1. Materials and Method
1.1 Materials and equipment
The under listed materials and equipment
were used for the research work; pure
copper scrap (99.9%), pure aluminium scrap, nickel granules, weighing balance,
crucible furnace, vernier caliper, bench vice, lathe machine, electric grinding
machine, hack-saw, stainless steel crucible pot, mixer, scooping spoon,
electric blower, rammer, moulding box, impact testing machine (U1820), hardness
testing machine (A 3000 H), universal tensile testing machine (model SRNO0723),
emery papers of different grits, air drying machine, metallurgical bench microscope (L 2003A) with
digital camera and PHENOM ProX scanning electron microscope.
1.2
Method
The methodology adopted to carry out
these research essentially involved alloy preparation by melting and casting
techniques. The alloying element (nickel) was added separately in concentration
of 1-10% by weight to molten Cu-10%Al alloy, stirred and sand cast.
Subsequently, specimens obtained from the casting were subjected to machining
and mechanical test such as ultimate tensile strength, impact strength, yield
strength, hardness and ductility. The microstructures of the samples were also
studied using, metallurgical microscope and scanning electron microscope.
1.2.1
Experimental
procedure
(a)
Alloy
preparation
The sequence of operations followed to
obtain the studied specimens and mechanical test samples include; the use of
calculated quantities of pure copper scrap, aluminium scrap, nickel granules.
The materials were weighed out in their appropriate proportions respectively
using a weighing balance.
Sand mould was prepared and used for the
casting of the specimens. Meanwhile, impurities such as metals, hard lumps,
stones etc. were removed from the moulding sand using 500μm and 400μm sieves to
obtained fine and uniformly distributed grain size. The sand was mixed well in
a sand mixing machine with the addition of a little quantity of water to ensure
uniform distribution of the ingredients. The foundry floor was cleared of dirty
and floor board was put in place. Some moulding sands were sprinkled on the
floorboard surface and then patterns were introduced. Sand was introduced and
rammed; the ingate runner and risers, plumbago (painting materials), rammers
etc. were used to prepare the mould. The patterns were removed and the cavities
created were repaired. The pattern removal was done slowly to prevent mould
damage. After the pattern was removed and mould repaired, ash was then
sprinkled on the cavities to enhance easy flow of the molten metal inside.
The furnace used for the melting
operation is a crucible furnace with a crucible steel pot of maximum controlled
temperature of about 1750oC. Prior to charging of metal into the
furnace, the crucible pot was removed and properly cleaned to avoid
contamination by other material inclusion.
(b)
Melting and
Casting of alloys
This operation was carried out to
produce eleven separate specimens for the research work. The bailout crucible
furnace with steel crucible pot was pre-heat for about 10minuties. For the
control sample, 163.44g of Cu and 17.18g of Al were measured out. Copper was
charged into the furnace pre-set at 1100oC and heated till it
melted. Aluminium was then allowed to dissolve in the molten copper for
6minutes and stirred properly to ensure homogeneity. The alloying element
(nickel) were then introduced separately into the melt (Cu-10%Al) based on the
compositions, after the control sample had been cast. The melt was manually
stirred intermittently in order to ensure homogeneity and facilitate uniform
distribution of the alloying element. Then molten metal was poured into the
mould cavities and allowed to solidify for about 3minutes before shakeout from
the mould.
(c)
Machining
The machining operation was carried out
using a three jaw chuck lathe machine. The samples to be machined were firmly
clamped on the machine and facing, turning and shaping operations were done on
the clamped samples with the aid of a cutting tool mounted on the post of lathe
machine. Eventually the required dimensions for impact, tensile and hardness
test samples as well as microstructural analysis were obtained.
(d)
Tensile test
The tensile test was conducted using
horizontal bench top Mansanto Tensometer machine (SRNO0723) and the test
carried out at room temperature. Specimens for this test were machined to a
dumbbell shape which is the standard specifications so as to fit the grips as
shown in Figure 2. The testing process
started with the specimen labelled 1 and continued on to 21. The specimens were
placed each between the two grips, these held the specimen in place, gradually
force was applied on the work piece till it fractured. Different values of
force and extension were obtained and reported.
Hence, the specimen were tested to determined their ultimate tensile
strength, ductility (%elongation) and yield strength. These properties determined
were tabulated in Table 1.
Figure
2: Tensile test specimens
(e)
Hardness Test
This test was conducted using a Brinell
testing machine model B3000 (H). The specimen each 20mm in diameter were
polished, placed on an adjusting table below the control panel separately, the
table was raised to the focus of the microscope which helped to determine the
exact spot for indentation. On pushing the start button on, the microscope
returned automatically to its resting position and the spherical indenter was
carefully placed on the specimen surface. A specified force was applied and
maintained for about 15seconds after which the indenter bounced back to its
former position. The indentation was clearly seen on the monitor of the Brinell
testing machine, the diameter of the indentation was obtained by placing four
metric lines on the edges of the indentation using hand control knob. The
diameter obtained and the force applied was used by the machine to calculate
the Brinell hardness of the work piece. Brinell hardness result was displayed
on the bottom left hand corner of the monitor. Three (3) indentations were
taken on each specimen and the mean was obtained.
(f)
Impact test
Impact test was carried out with charpy
impact test machine model (U1820). The specimens were machined to a dimension
of (10 x 10 x 55) mm with a V-notch of depth 2.5mm at its mid-point. The
samples to be tested were placed at the machine’s sample post with the notch
facing the hammer. The hammer was raised to an angle of 45oC and
released to swing through the positioned sample in order to break it. As the
sample was broken by the swing hammer, the impact energy absorbed was read from
the charpy impact energy scale calibrated in joules. Hence, the impact energy of all the samples
as well as the control sample was captured.
(g)
Microstructural
examination
The microstructure of the experimental
specimen was studied using optical metallurgical bench microscope and Scanning
electron microscope. In the process, a cubic sample was cut from each of the 11
cast samples. The samples were ground by the use of series of emery papers of
different grits with decreasing coarseness from 220, 340, 400, 600, 800, 1000
and 1200 grades and polished using fine α-alumina powder. The specimens were
washed thoroughly and dried using the oven dryer. After drying, the specimen
were inserted into dilute hydrofluoric acid which was the etching reagent for
about 10-15 seconds and layers of the specimens were attacked chemically until
the polished surface were slightly discoloured or dull in appearance. The
etched specimens were washed in water to stop the etching action. The specimens
were dried and viewed under a high power electron microscope with a
magnification of x400 and micrographs showing the different morphologies of the
cast alloy were taken. For SEM observation, the test sample was placed on the setup.
The setup was put in an ultrasonic
cleaning process. Both the sample and the setup were placed in front of an air
heater in order to make it dry before test. After the drying process, both the
sample and stup were placed in a special tube for pre-vacuum process. The
sample on the stup was put under scanning electron microscope machine for testing.
Results and Discussion
It was observed that the microstructure
in Plate 1 contains α-phase of the aluminium bronze in which the β-grains
appear to have absorbed the α-dendrites thereby preventing the precipitation of
other phases out of the solution. This must have been due to the absence of
alloying elements. Hence, the presence of alloying elements apart from
aluminium tends to stabilize β-phase and effectively permit slower cooling rate
[9]. From Plate 2-11, precipitation of fine lamellar form of kappa (κ) evolve
due to the presence of nickel as an alloying element in the Cu-10%Al alloys.
The presence of nickel aided the nucleation of a few fine lamellar kappa
precipitates. Plate 5, shows the effect of 4wt% nickel addition on the
aluminium bronze microstructure. The amount of fine lamellar kappa-phase
transformed within the matrix increased compared to 2 and 3wt% nickel addition.
These further explain that presence of more nickel in the alloy matrix within
the base metal provided an increase in nucleation sites for the precipitation
of kappa-precipitates from α-phase to occur. The sharp fall in values of UTS
and %elongation as seen in Table 1, could be as a result of casting defects
noticed on the microstructure of the casted samples. The preponderance of
nickel presence in the cast Cu-10%Al alloy effectively suppressed the formation
α + γ2 within the alloy matrix. This stands in agreement with the
work of Cook et al (1980). The addition of nickel to an alloy has a strong
influence in stabilization of β-phase. When nickel is added to an alloy, it
suppresses the formation of γ2 –phase and α-solid solution range is
extended towards higher aluminium contents. The combined effect produces a
kappa-phase which has the same structure as the β-aluminium bronze [11]. The
scanning electron micrographs of the control specimen (Cu-10%Al) shown on plate
12 revealed that the micrograph consists of α-phase (which is the grey region),
β-phase, and eutectoid α + γ2 phase (the twin dark light region).
The intermetallic phase Cu9Al4 existed in the form of
coarse plate-like precipitating from the β-phase through the grain boundaries.
Plate 13 showed micrograph of Cu-10%Al +10%Ni. It was observed that α-phase was
surrounded by little dark etching β-phase. The combined effect of Cu-10%Al and
nickel produces a kappa precipitate. The size and disposition of kappa phase
present in the structure caused reduction in hardness value. However, there was
good enhancement in their impact energy due to higher proportion of tough,
ductile and soft kappa precipitate present.
This article is published in peer review journal and open access journal, International journal of research in engineering and innovation (IJREI) which have a high impact factor journal for more details regarding this article, please go through our journal website.
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