Journal of Minerals & Materials Characterization & Engineering, Vol. 11, No.2 pp.117-131, 2012 Printed in the USA. All rights reserved
The Effect of Time, Percen t of Copper and Nickel on Naturally Aged Al-Cu-
Ni Cast Alloys
Mohammad M. Hamasha1*, Ahmad T. Mayyas2, Adel M. Hassan3,
Mohammed T. Hayajneh3
1 Center for Autonomous Solar Power, State University of New York at Binghamton,
Binghamton, NY 13850, USA
2 Clemson University International Center for Automotive Research (CU-ICAR), 4 Research
Drive, Greenville, SC 29607, USA
3 Industrial Engineering Department, Jordan University of Science and Technology, P.O. Box
3030, Irbid 22110 Jordan
Corresponding Author:
In this paper, the hardness property during natural age hardening phenomenon for aluminum
based alloy has been studied. Different factors play role in aging hardening of aluminum. In this
study, the chosen factors were percentages of copper and nickel in aluminum alloys. The
specimens were manufactured using casting process, and then heat treatment was carried out for
all produced samples together at 550 °C for 3 h before quenching in water. Finally, the
specimens were left at room temperature for 936 hours (39days) to allow solute atoms to defuse
and form coherent phases to allow the age hardening to take place. The results show that the
hardness increased with time in the first 300 hour after the quenching time, and then it remained
constant for the rest of the 936 hours. Furthermore, the hardness did not drop until the end of
936 hours which means the over-aging status was not achieved. To get full analysis of the
118 M.M. Hamasha, A.T. Mayyas, A.M. Hassa n, M.T. Hayajneh, Vol.11, No.2
natural aging, design of experiment technique was used to study the effect of %Cu, %Ni and
aging time
Keywords: Al; Powder Metallurgy; solution treatment; Natural Precipitation hardening.
Age hardening (also known as precipitation hardening) is a process by which steel and aluminum
based alloys are heated and solution treated. The aim of this process is to improve the strength
and hardness of these alloys [1]. Age hardening treatment procedure consists of three major
steps: solution treatment, rapid cooling (quenching), and aging. Solution treatment involves
keeping the alloy on temperature higher than solvus temperature but less than liquidus
temperature for a period of time to get homogeneous one phase of the solid solution, where the
solute atoms are dissolved completely in the solution. Rapid cooling or quenching of the solid
solution is carried out in a medium like water, oil or even air. During this step, the solute atoms
are not allowed to move and form α-phase, so they stay dissolved in a supersaturated phase.
Finally, during the aging stage, the alloy hardness starts to increase when the solute atoms start to
diffuse as clusters that distort and strengthen the material.
The age hardening process differs widely in composit ion and in the microstructures formed. Age
hardening is often used in aluminum based alloys to improve the physical properties, such as
strength and hardness [1]. The changing in allo y physical properties is due to formation of fine-
dispersed second phase particles in the lattice. These particles strain the lattice internally and
restrict the dislocation movement. Then the alloy has to be kept at room temperature or reh eated
to a temperature below the solvus line and held on that temperature for enough time to ensure
full transformation. This temperature is known as the age hardening temperature. The purpose of
reheating the alloy is to increase the diffusion rate of the solute atoms by increasing the internal
energy of the atoms and then reducing the time needed to age the alloy.
Vol.11, No.2 The Effect of Time, Percent of Copper and Nickel 1 19
Heat-treated aluminum based alloys are widely used for high performance light weight parts,
such as aircraft and space shuttle parts. There are several groups of aluminum based alloys that
can be aged,the 2xxx alloys group is one of the heat treatable aluminum based alloys in which
the princaple alloying element is copper [2]. Magnesium and/or small amounts of other elements
could also be added to enhance the strength. In this alloy, Al2CuMg is the main hardening
precipitate [2]. A normal use of the 2xxx allo ys, such as alloy 2024, is for aircraft structures. The
6xxx group of allo ys contains magnesium and silicon as primary alloying elements [2]. Mg2Si is
the hardening precipitate for these alloy [2]. Alloy 6061 is one of the common examples about
this group; this alloy is ver y popular in the structural and construction application. The princapal
alloying elements for the 7xxx group are zinc, magnesium and copper. The intermetallic
compound, that serves as a hardening precipitate is MgZn 2 [2-4]. Si nce Zinc and magnesium are
very soluble, a high density precipitates could be achieved and then produce extremely stronger
and hard er al loys. These al l o ys su ch as alloy 7075 are gen eral ly used fo r aircra ft s tru ctu r es [2-4].
Silicon improves the fluidity of the molten metal while copper and magnesium increase strength
However, the copper is primary alloying element in 2xxx alloys which mainly increases the
hardness [4-5].Furthermore, addition of copper in association with nickel improves the strength
and hardness stability at high temperature [6].The tensile strength of aluminum alloys varies
from one alloy to the other, for example, 2024 has a tensile strength of about 64 ksi, while 6061
has a tensile strength of about 42 ksi, however, 7075 has higher tensile strength that reaches 73
ksi [2]. When the alloy is selected in the design stage, the mechanical parameters, such as alloy
strength, castability, machinability, as well as prices of materials are important factors to be
Solution treatment plus aging for the 2618 aluminum alloy (Si: 0.10-0.25, Fe: 0.9-1.3, Cu: 1.9-
2.7, Mg: 1.3-1.8, Ni: 0.9-1.2, Zn: 0.10) wt%, produces a machineable material to a fine detail [7].
When using magnesium and copper as primary allo ying elements for aluminum based alloys, age
hardening is associated with the precipitation of Guinnier-Preston (Cu, Mg) zones and the semi-
coherent S phase, which is closely related to the equilibrium S-phase (Al2CuMg) [7].
120 M.M. Hamasha, A.T. Mayyas, A.M. Hassa n, M.T. Hayajneh, Vol.11, No.2
The age h ardening p rocesses in Cu–Mg alloys which falls within the (α+S) region in the ternary
phase diagram have been subjected to numerous investigations due to their fundamental concept
and commercial interest since the development of the alloy duralumin in 1906. Since then, they
have been investigated by different mechanical testing, calorimetric measurements and
microscopy anal ysis. Two hardness rise stages are noticed during aging at normal temperatures,
which are the rapid initial rise (about 50–70% of the total hardness increase), and the second rise,
which is slower than the first one. In semi-logarithmic representation, they are separated by a
sort of plateau whose duration depends strongly on temperature and alloy composition. In normal
scale, a plateau does not always appear [8].
In the literature many researchers have studied the aging of aluminum alloys. For example, Ou
[9] studied the influence of pre-aging at low temperature on the thermal age hardening for
aluminum alloy 6063 (Al–0.72 Mg–0.42Si). He found that any increase in pre-aging temperature
and/or holding period time will significantly decrease the strength of the alloys during and after
thermal aging.
Age hardening can change the dimensions of the material due to internal stresses, alloy
concentrations, and the aging process. It is hard to predict these dimensional changes because of
the number of the intervening factors. Oneda et al [10] examined the effect of pre-aging on the
artificial age hardening of 1.32wt% Mg 2Si alumi num based alloy at 170 °C. They found that the
change in hardness was strongly dependents on the pre-aging temperature. Özbek [11] studied
the effects of re-solving treatment of AA2618 aluminum based alloy, at solution heat treatment
temperature between 520–640 °C and holding time of 14–24 h, followed by artificial aging. He
found that further solution treatment temperature leads to increase both the grain and the
precipitate size, and then reduces the hardness significantly. Al 9FeNi-type intermetallics are not
completely dissolved by these solution treatments.
Desmukh et al [12] studied aluminum 7010 alloy (Zn 6.3, Mg 2.3, Cu 1.55, Zr 0.14) fati gue life,
the fatigue life was corresponding to 106 cycles and the results revealed that the over aged alloy
has a fairly high valu e of fati gue stren gth as com pared to th e peak a ged all oy. Thes e resul ts were
discussed with the aid of microscope images to show th e di fferen ces bet w e en t he m i cros tru ctu res
Vol.11, No.2 The Effect of Time, Percent of Copper and Nickel 1 21
developed in the materials that subjected to different aging treatments. Zeren [13] studied the
effect of heat-treatment on aluminum piston alloys, AlCu4MgNi alloys and AlSiCuMgNi alloys
with 10.5%, 12%, 18% and 24% Si are widely using in automotive industry for this purpose. It
was found that the mechanical properties of aluminum based piston alloys are highly dependent
on the h eat t reat men t t em p eratu r e and tre at i ng ti m e. Thu s, char act e ris t ics of h eat t re atment play a
significant role for excellent microstructure and mechanical properties.
Ouellet et al [14] studied the aging phenomenon of 356 and 319 aluminum alloys. They found
that the main parameters that control the mechanical properties are the iron and copper
intermetallics, the eutectic silicon particle characteristics, the porosity size and distribution, and
the supersaturation level of Mg and Cu in the α-Al matrix after solution heat treatment.
Gonzalez-Martınez et al [15] studied the age hardening of magnesium wrought alloys
(magnesium- aluminum zinc series) by damping measurements and hardness. They found that
the age hardening temperature significantly accelerates the damping and the hardening of the
specim ens.
The Experiment was designed totally using of design of experiment (DOE) with an assistance of
MINITAB software. The selected study factors were: aging time, %Cu and % Ni with eleven
levels for aging time factor and 4 levels for each of %Cu and %Ni. Table 1 shows the considered
factors an d level s in thi s stud y. The hardness in ter m of Rockwel l scal e B (HRB) w as measu red
at different combination of study factors. After selecting factors and their levels, the
experimental were conducted based on a random sequence to ensure reproducibility of the
apparatus and maintain high level of accuracy.
Two replicates were measured for each factors combi nation, so the total number of requir ed test
specimens was 352. In order to reduce the measurement error, the hardness measurement was
taken five times at five different locations along the specimen for each sample, and then the
averages of these five measurements were considered for further analysis. Actual hardness
measurement would be represented by equation (1) according to design of experiment model.
122 M.M. Hamasha, A.T. Mayyas, A.M. Hassa n, M.T. Hayajneh, Vol.11, No.2
Table 1: Factors and levels
Time (Hours)
Cu (%)
Ni (%)
Where is the actual measure of hardness, is the overall mean, is the effect of the ith
level of time, is the effect of the jth level of copper percentage, is the effect of the kth
level of nickel percentage, is the effect of interaction between the time and copper
percentage, is th e effect o f in ter acti on bet ween t h e t im e and ni ckel percen tag e, is
the effect of interaction between the copper percentage and nickel percentage, is the
effect of interaction between the time, copper percentage and nickel percentage, and is
randomized error.
Analysis of variance (ANOVA) was used to test if the factors and the interactions affect the
hardness significantly or not. Main effect plots for all factors were used to show the trend
(decreasing or increasing and concavity) of the relation curve between the factors and response
(hardness values), and tw o factor interaction plots were used to check if the int eractions between
the factors significantly affect the response. However, ANOVA is very sensitive to normality in
the distribution of errors, to make sure that normality assumption is valid; the normality
validation was conducted before any further anal ysis. Furthermore, to avoid any trend that arises
Vol.11, No.2 The Effect of Time, Percent of Copper and Nickel 1 23
in the sequence of experimental runs, validation for randomization in the experimental run was
done as well.
The specimens were produced using casting technique. The aluminum ingot blocks were melted
at temperature of 750 °C in graphite crucibles. Copper and nickel were then added to the molten
metal and agitated vigorously to ensure good distribution of copper and nickel. The melt poured
into the dried cylindrical permanent metallic molds (30 mm in diameter and 175 mm height) as
shown in Figure 1. Then the mold was left in air to cool down to room temperature.
The next step was heat treatment of the samples which was carried out at 550 ±5 °C in an electric
arc furnace for 3 h to turn the material into one homogenous solid solution phase. The specimens
were then quenched in water at room temperature immediately after being taken out of the
furnace. Surface polishing takes place to remove the oxide layer and get smoother surfaces.
Then the hardness change due to natural age hardening was measured continuously for 936 h (39
days) starting from the time of quenching. The hardness was measured using Rockwell hardness
test scale B (1/16-inch hardened steel ball with minor load 10 kgf and major load 90 kgf). In
order to reduce the reading errors and get more representative hardness value, the hardness
readings were t aken at five differ ent lo cati on s on the surface for each sp ecim en.
Figure 1: Permanent mold which contains two splits. a) Opened, b) Closed.
124 M.M. Hamasha, A.T. Mayyas, A.M. Hassa n, M.T. Hayajneh, Vol.11, No.2
Figure 2 shows the aging curves that represent the relationship between natural age hardness and
the aging time for all %Cu - %Ni combinations. It is clearly shown that the difference in the
initial hardness between the specimens depending on the copper and nickel percentages. The
higher copper and/or nickel percentage the higher hardness value is, and vice versa. The
replacement of aluminum atoms with other alloying atoms leads to deform the lattice, so at a
higher alloying percentage, the lattice gets more deformation with more solute atoms and hence
the hardness gets high. These deformations inhibit the movement of the dislocations. Rockwell
hardness number (HRB) increases rapidly for all samples over about 300 hours, and then it keeps
constant until the end of the 936 hours. HRB increasing at the early stage is due to the diffusion
of the solute atoms as coherent phases in different areas within the lattice of the aluminum based
Figure 2: Effect of copper and nickel percentage on the natural precipitation hardness over 936 h
The percentages of copper and nickel not only influence the initial hardness, but also influence
the range of hardness increase over the time, where the high percentage of alloying element has
high impact on the hardness incremental range and vice versa. For example, the maximum
Vol.11, No.2 The Effect of Time, Percent of Copper and Nickel 1 25
increasing range was for the combination of 4 % copper and 2 % nickel with 35.5 HRB and the
minimum increasing range was for the combination of 1 % copper and 0 % nickel with 3.5 HRB.
This is partially due to the increased amount of precipitated coherent phase inside the lattice.
No over-aging status had been noticed in this study. However, it seems that the time of 936 hours
was too short for the considered alloys to allow the complete separation of the coherent phases,
which cause the phenomenon of over- aging to occur.
Design of experiment (DOE) tool was used in this study to analyze data and draw the
conclusions. The normal distribution of the randomized error is the right distribution for
validating the DOE model. So the proof of normality in the error distribution was made before
start analyzing the data. Figure 3 shows the normal probability plot. The linearity of the relation
refers to the normality of the standardized error. As shown in this figure, almost all points are
clust ered strai ght alon g the lin e ex cept ver y few poin ts at ends of the l inear line. Hence, it can b e
concluded that the standardized error is very close to normal distribution and the model is
statistically valid for further analysis. In order to avoid any unintentional trend in the run order,
the observation order versus residual was plotted as shown in figure 4; however, the standardized
residual points are distributed around the zero and there is no sign of non randomized patterns
within the distribution.
Normal Probability Plot
(response is Hardness)
Figure 3: Normal probability plot for the standardized residuals.
126 M.M. Hamasha, A.T. Mayyas, A.M. Hassa n, M.T. Hayajneh, Vol.11, No.2
Figure 4: Standardized residuals versus experimental run order.
After validating the normality of the randomized error distribution and randomization of the
experimental run, the significance of the factors and their interactions on the response was
conducted using analysis of variance (ANOVA). ANOVA is basically a collection of statistical
procedures, in which the variance is divided into many components depending on the cause of
variation. Two statistical hypotheses, as seen in equation 3, were developed in the ANOVA
model. If the means are equal, null hypothesis is failed to be rejected and hence the term is not
significant. If at least two means are different, null hypothesis is rejected and instead alternative
hypothesis is accepted and then the term is significant. The statistical test that used in ANOVA is
F-test whe re F0 is calculat ed and com par ed to st and ard F-critical, and then the conclusion can be
drawn based on that. A detailed description of F test is in Montgomery et al [16].
Also the comparison between P-value and confidence level α (usually 0.05) can draw similar
conclusion whether to accept or reject the null hypothesis. If P-value is less than 0.05, then the
null hypothesis has to be rejected and the term is significant, otherwise the term is not
significant. In the last column of the ANOVA table, P-value for each factor and interaction is
Observation Order
Versus Order
(response is Hardness)
Vol.11, No.2 The Effect of Time, Percent of Copper and Nickel 1 27
Table 2 lists the factors and the interactions between them, degree of freedom (DF), sum of
square (SS), Mean sum of square (MS), F 0 value, and P-value. From Table 2 it can be said that
all terms (factors and the interactions between them) are significantly affecting the hardness of
the alloy, where all P-values are less than α=0.05. The fitting level of the data represented by R2
and R2adjus tab le, where both have values more than 95%, which means that the data fits the model
very well. The main effect plot of all factors is shown in Figure 5 to check the trend of the
factors and interaction between them. It can be concluded that hardness is directly proportional
to the level of each factor. The curvature of the main effect on hardness of both copper and
nickel percentages is not clear. However, the main effect on hardness of aging time is concave,
in other words the hardness has a negative acceleration rate.
Table 2: ANOVA table
Source DF SS MS F P
% Cu
% Ni
Time (h)*% Cu
Time (h)*% Ni
% Ni*% Cu
Time *% Cu* % Ni
S = 0.699025 R-Sq = 99.86% R-Sq(adj) = 99.72%
The plot of two factor interactions is shown in Figure 6. Also the surface response plot is shown
Figure 7. Form both figures, there is a great interaction between all of the factors. For example,
the response of one factor highly depends on the level (high or low) of the other factor. The
interactions between factors are important when the optimization for the hardness is the aim of
the study.
128 M.M. Hamasha, A.T. Mayyas, A.M. Hassa n, M.T. Hayajneh, Vol.11, No.2
Figure 5: Main effect plot
Figure 6: Interaction plot
Time (h)
Main Effects Plot for Hardness
Data Means
Time (h)
Time (h)
Interaction Plot for Hardness
Data Means
Vol.11, No.2 The Effect of Time, Percent of Copper and Nickel 1 29
Figure 7: Surface plots of the significant interactions of a) % Cu – time, b) %Cu - %Ni, and c) %
Ni –time.
The hardness increased for all %Cu - %Ni combinations sharply starting from the beginning of
the aging time, but it was slowing down with the time until about 300 hours where it kept
approximately constant till the end of aging time (936 hours). This indicates that the solute atoms
start to diffuse in a short time producing severe lattice distortion in different areas to form the
coherent phase, causing an increase in hardness of the studied alloys. The hardness was higher
for a higher copper and nickel percentages at the initial time. The reason behind that is due to
higher precipitated coherent and then higher internal strain at higher alloying element
percentage. Also the hardness increasing was more at higher copper and nickel percent ages. Th is
is due to higher amount of alloying atoms available for diffusing to form more coherent phases
20 500
Time (h)
Surface Plot of Hardness vs Time (h), %Cu
20 1
Surface Plot of Hardness vs %Ni, %Cu
20 1
250 500 0
Time (h)
Surface Plot of Hardness vs %Ni, Time (h)
130 M.M. Hamasha, A.T. Mayyas, A.M. Hassa n, M.T. Hayajneh, Vol.11, No.2
and add more internal strain to the lattice. Over-aging was not observed in this study regardless
of the used composition of the different alloying elements. It seems that the time was not long
enough to separate the non-coherent phases and remove the internal strain from the lattice.
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