Journal of Minerals & Materials Characterization & Engineering, Vol. 8, No.1, pp 1-14, 2009
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1
Effects of Variation of Some Process Variables on Recrystallization Rate of
Aluminium Alloy (6063)
O.E. Olorunniwo*, P.O. Atanda and K.J. Akinluwade
Department of Materials Science and Engineering
Obafemi Awolowo University, Ile-Ife. Nigeria.
*Correspondence author e-mail:[e-mail], Phone:[phone]
ABSTRACT
Effects of different annealing temperature, holding time and degree of deformation on
recrystallization rate of Aluminium alloy (6063) were studied. Fifteen suitably dimensioned
samples were prepared from Aluminium alloy (6063). Seven of these were subjected to 70% cold
plastic deformation, seven to 90% and one left undeformed. All the samples were then subjected
to annealing heat treatment to relief deformation-induced stresses. The average values of Yield
and Ultimate tensile loads were obtained from three preliminary tensile tests as 121.33 Kgf and
192.67 Kgf respectively. From these, 70 and 90% deformations were estimated. After a
metallographic test on the as-received, the samples were subjected to recrystallization annealing
under different conditions of temperature (380 oC and 450 oC) and holding time (20, 30 and 40
minutes). Photomicrographs of the heat treated samples were taken from which the number of
grains was counted, with the aid of a magnifying lens, from a 1cm2 area inscribed on their
surfaces. The results obtained showed that the higher the degree of cold work, the higher the rate
of recrystallization, the higher the nucleation rate and the finer the grains. The higher the
holding time at a given recrystallization temperature, the larger the grains due to a longer time
available for grain growth. It was also deduced that recrystallization is thermally activated and
its rate increa ses with increase in temperature.
Keywords: recrystallization, holding tim e , plastic deformation, nucleation rate

2 O.E. Olorunniwo, P.O. Atanda and K.J. Akinluwade Vol.8, No.1
1. INTRODUCTION
Strain hardening is the phenomenon whereby a ductile metal becomes harder and stronger as it is
plastically deformed. It is sometimes called work hardening or cold working because the
temperature at which deformation takes place is “cold” relative to the absolute melting
temperature of the metal [1]. Hot working on the other hand is a deformation carried out at
temperatures higher than 0.6Tm (Tm is the melting temperature in Kelvin) and relatively high
strain rates (10-1 to 103 s-1) [5].
Strain hardening or cold working is one of the well known strengthening mechanisms for single-
phase metals. It is often convenient to express the degree of plastic deformation as percent cold
work. With an increase in percent cold work, steel, brass and copper increase in yield and tensile
strength but the price for this enhancement of hardness and strength is in the ductility of the
metals [1]. In industries, strain hardening is often utilized on a commercial scale to enhance the
mechanical properties of materials during fabrication processes.
After a metal is plastically deformed its microstructure evidently undergoes some alterations.
However, the mechanical characteristics and microstructural features of a cold worked metal
may be restored to their pre-deformed condition by a suitable heat treatment during which
recovery, recrystallization and grain growth occur. The amount of recrystallization is a function
of time, temperature, and the degree of prior cold working [2]. The grain size depends upon the
temperature of recrystallization annealing, the holding time at this temperature, the degree of
previous deformation, the chemical composition of the alloy, the size of the initial grains, the
presence of insoluble impurities, etc [ 4].
Since a number of factors come into play during these processes, many possible outcomes are
obtainable. Gorelik (1981) observed that the structure of a deformed (strain hardened) material
can change on recrystallization within very wide limits: from a partially recrystallized structure
that retains a certain extent the original deformed structure, to a fully rec rystallized structure. [2]
Gorelik (op cit) noted that the large diversity of the still unsolved problems in the theory of
recrystallization is due to a number of reasons: first, fine details of the dislocation structure of
deformed metal, or in essence the initial conditions of recrystallization are still unclear, as are the
structures of various types of grain boundary. Without full clarity of this aspect, it is impossible
to understand the mechanisms of texture formation, the role of trace impurities, etc. Second, the
process of recrystallization, seemingly quite simple, is actually extremely complicated for the
mere reason that it is a structure-sensitive process that comes about owing to a non-equilibrium
state of the system. Third, recrystallization nuclei differ from the deformed matrix only in lattice
distortion, but have the same chemical composition.

Vol.8, No.1 Effects of Variation of Some Process Variables 3
In view of the aforementioned difficulties, Gorelik (op cit) advocated that, the experimental tools
for studying recrystallization be diversified and new indirect methods worked out to make
appropriate conclusions.[6,7] This study therefore looks at effects of some varied parameters on
the rate of recrystallization of Aluminium (6063) alloy. The varied parameters were annealing
temperature, holding time and degree of deformation.
2. MATERIALS AND METHODS
2.1. Sample Preparation
The sample of aluminium alloy 6063 was machined on a turret lathe machine to produce fifteen
standard tensile test pieces. All the samples were annealed at 350oC to homogenize the
microstructure and t o eliminate induced stresses due t o machining.
Three samples were subjected to preliminary tensile test (cold-deformation). The results obtained
are: Test I : Yield load = 120Kgf and Ultimate tensile load = 190Kgf. Test II: Yield load =
120Kgf and Ultimate tensile load = 188 Kgf. Test III: Yield load = 124 Kgf and Ultimate tensile
load = 200 Kgf.
From these results, the average Yield load and Ultimate tensile load were taken as 121.33 Kgf
and 192.67 Kgf respectively.
From the foregoing, a load of 121.33 Kgf corresponds to 100% cold plastic deformation. By the
same reasoning, loads of 134.87 and 173.40Kgf would produce 70 and 90% cold plastic
deformation respectively. Consequently, seven samples were given 70% cold plastic
deformation, seven 90% while one was left undeformed. To ensure adequate plastic deformation
in the samples, they were deformed with stresses which fall between the average Yield load and
the Ultimate tensile load.
2.2. Heat Treatment
All the samples were cut through their centres to expose that region of surface for metallographic
test. In handling the different groups of specimen which were subjected to recrystallization
annealing, a coding scheme was used. The fifteen samples were heat treated according to the
following procedure and coding scheme:
(1) One sample: As-received—AR
(2) Two samples: Cold worked only—CW ( 70%= CW-1, 90%= CW-2)
(3) Cold worked plus recrystalliza tio n a nnea lin g—A C
a. Two samples: Held at 380oC for 20 minutes
70%: AC1(20-1)
90% AC1(20-2)

4 O.E. Olorunniwo, P.O. Atanda and K.J. Akinluwade Vol.8, No.1
b. Two samples: Held at 380oC for 30 minutes
70%: AC1(30-1)
90% AC1(30-2)
c. Two samples: Held at 380oC for 40 minutes
70%: AC1(40-1)
90% AC1(40-2)
d. Two samples: Held at 450oC for 20 minutes
70%: AC2(20-1)
90% AC2(20-2)
e. Two samples: Held at 450oC for 30 minutes
70%: AC2(30-1)
90% AC2(30-2)
f. Two samples: Held at 450oC for 40 minutes
70%: AC2(40-1)
90% AC2(40-2)
After the heat treatment as described above, the samples were ground on 240, 320, 400 and 600
grit emery papers. T h e y w er e t h en po l i shed to mirror finish and etched with 1Kelle r’s r eagent.
2.3. Point Counting
This was done to count the n umber of grains per 1cm2 area. The formula is [8,9,10]:
and
Where K is number of grains per unit area, Z is total number of grains in the entire 1cm2, n is
number of grains intersected by the edges and not corners of the inscribed square, A equals 1cm2.
The area A was inscribed on each micrograph and the number of grains w ithin the area was
counted with the application of equations (1) and (2) above. The grains were made visible with
the use of a magnifying glass. In effect, the number of grains per unit area represents the rate of
nucleation.
3. RESULTS AND DISCUSSION
Results of counts obtained for all fifteen samples are as presented in Tables 1 and 2. The
micrographs obtaine d a t 10 0X ar e as presented in Plates 1— 4, while graphical variations of
recrystallization r a t e ( gr a i n count) with holding time are presented in Figures 1—4.
1 The composition is Hydrofluoric acid: 1cm3, Hydrochloric acid: 1.5 cm3, Nitric acid: 2.5 cm3 and water: 95 cm3

Vol.8, No.1 Effects of Variation of Some Process Variables 5
Table 1. Interpretation of codes and grain count.
code Explanation Grain count
CW-1 70% deformation, cold worked only 219
CW-2 90% deformation, cold worked only 232
AC1(20-1) 70% deformation, 380oC, 20 min 179
AC1(30-1) 70% deformation, 380oC, 30 min 171
AC1(40-1) 70% deformation, 380oC, 40 min 167
AC1(20-2) 90% deformation, 380oC, 20 min 200
AC1(30-2) 90% deformation, 380oC, 30 min 184
AC1(40-2) 90% deformation, 380oC, 40 min 180
AC2(20-1) 70% deformation, 450oC, 20 min 143
AC2(30-1) 70% deformation, 450oC, 30 min 135
AC2(40-1) 70% deformation, 450oC, 40 min 131
AC2(20-2) 90% deformation, 450oC, 20 min 161
AC2(30-2) 90% deformation, 450oC, 30 min 153
AC2(40-2) 90% deformation, 450oC, 40 min 149
Table 2. Recrystallization (grain count) as a function of % cold deformation,
annealing temperature and holding time.
Code % Deformation annealing temperature Holding Time Grain Count
CW-1 70 219
CW-2 90 232
AC1(20-1) 70 380 20 179
AC1(30-1) 70 380 30 171
AC1(40-1) 70 380 40 167
AC1(20-2) 90 380 20 200
AC1(30-2) 90 380 30 184
AC1(40-2) 90 380 40 180
AC2(20-1) 70 450 20 143
AC2(30-1) 70 450 30 135
AC2(40-1) 70 450 40 131
AC2(20-2) 90 450 20 161
AC2(30-2) 90 450 30 153
AC2(40-2) 90 450 40 149

6 O.E. Olorunniwo, P.O. Atanda and K.J. Akinluwade Vol.8, No.1
160
170
180
190
200
210
10 20 30 40 50
Holding Time, Minutes
recrystalliz at ion rate, no o f grai ns/ unit area
70% Col d Work90% Cold Work
Figure 1. Effect of holding time on recry stallization rate at
constant temperature (380oC).
100
110
120
130
140
150
160
170
10 15 20 25 30 35 40 45
Time, minutes
recrystallization rate, no of grai n / u nit area
70% Cold W ork90% Cold Work
Figure 2. Effect of holding time on recrystallization rate at
constant temperature (450oC).

Vol.8, No.1 Effects of Variation of Some Process Variables 7
100
120
140
160
180
200
10 20 30 40 50
Time, minutes
recrystallization rate, no of grai n / u nit area
380 oC450 oC
Figure 3. Effect of holding time on recrystallization rat e at c ons ta nt % co ld
deformation (70%)
50
70
90
110
130
150
170
190
210
0510 15 20 25 30 35 40 45
Time, minutes
recryst alli z at ion rate, n o of g r ai n/ u nit area
380 oC450 oC
Figure 4. Effect of holding time on recrystallization rat e at c ons ta nt % co ld
deformation (90%).

8 O.E. Olorunniwo, P.O. Atanda and K.J. Akinluwade Vol.8, No.1
Plate1. As-received
Plate2. 70% deformation, cold worke d only
Plate3. 90% deformation, cold worke d only

Vol.8, No.1 Effects of Variation of Some Process Variables 9
Plate 4. 70 % Deformation, 380oC, 20min
Plate 5. 70% Deformation, 380oC, 30min
Plate 6. 70% Deformation, 380oC, 40min

10 O.E. Olorunniwo, P.O. Atanda and K.J. Akinluwade Vol.8, No.1
Plate 7. 90% Deformation, 380oC, 20min
Plate8. 90% Deforma tion, 380oC, 30min
Plate9. 90% Deforma tion, 380oC, 40min

Vol.8, No.1 Effects of Variation of Some Process Variables 11
Plate10. 90% Deformation, 450oC, 20min
Plate11. 70% Deformation, 450oC, 30min
Plate12. 70% Deformation, 450oC, 40min

12 O.E. Olorunniwo, P.O. Atanda and K.J. Akinluwade Vol.8, No.1
Plate13. 90% Deformation, 450oC, 20min
Plate14. 90% Deformation, 450oC, 30min
Plate15. 90% Deformation, 450oC, 40min

Vol.8, No.1 Effects of Variation of Some Process Variables 13
3.1. Temperature and Recrystallization
From the analysis of the micrographs, it was found that rec rysta lliza tion te mperature affected the
rate of crystallization. The trend observed is: the higher the temperature, the higher the
recrystallization rate.
3.2. %Cold Deformation and Grain Size
The new grains formed during nucleation had undistorte d l a t ti c e w i t h e quiaxed grains w h e n
compared with that of samples which were only cold worked (CW-1 and CW-2). Mechanical
properties of a material are significantly affected by the size of its grains. In general, the finer the
grains, the better the mechanical properties.
3.3. Effect of Holding Time
It was observed from the experiment that an increase in holding time results in a decrease in
number of grains. However, the grains are larger with increased holding time. For insta nc e,
sample AC1(40-1) which was soaked at 380oC in the furnace for 40 minutes was observed to
have lesser grains count but bigger grains size than sample AC1(30-1) that was soaked at same
temperature b ut for 30 minutes.
3.4. Deformation, Grain Count and Grain Size
It was observed that the h i g h e r the degree of deformation, the finer the grain size. This accounts
for the observed increases in total counts per unit area for the samples having higher
degree/percent of deformation. For instance, sample AC2(40-1) which was given 70% cold
plastic deformation and annealed at 450oC had fewer number of grains per cm2 and larger grain
size than sample AC2(40-2) that was given 90% cold deformation, annealed at same temperature
and for same duration.
4. CONCLUSION
The following conclusion were drawn from the experiment
1. The higher the degree of cold work, the higher the rate of recrystallization
2. The higher the degree of cold work, the higher the nucleatio n r at e a nd the finer the grains.
3. The smaller the grain size, the higher the number of grain counts.
4. The higher the holding time at a given recrystallization temperature, the larger the grains
due to a longer time available for grain growth.

14 O.E. Olorunniwo, P.O. Atanda and K.J. Akinluwade Vol.8, No.1
5. Recrystallization is thermally activated and its rate increases with increase in
temperature.
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V.
[3] Linderberg, R. A. (1981). Processes and Materials of Manufacture. 2nd Edition. Allyn
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[4] Lakhtin, Y. (1979). Engineering Physical M etallurgy and Heat Treatment. MIR
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