Mechanical and Physical Properties of Micro Alumina ...

International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:20 No:03 32

201003-4747-IJMME-IJENS © June 2020 IJENS I J E N S

Mechanical and Physical Properties of Micro

Alumina Reinforced Direct Recycled AA6061

Chips Based Matrix by Hot Extrusion Process

H. M. Sabbar 1,a, Z. Leman1,2,b*, Mohammed H. Rady3,c, S. Shamsudin4,d,

Suraya Mohd Tahir1,e, C. N. Aiza Jaafar1,f , MA Azmah Hanim1,g, Nur Ismarrubie Zahari1,h and M. S.

Msebawi 1,i.

1Department of Mechanical and Manufacturing Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 Serdang, Selangor, MALAYSIA.

2Advanced Engineering Materials and Composites Research Centre, Faculty of Engineering, Universiti Putra Malaysia,

43400 Serdang, Selangor, MALAYSIA. 3College of Engineering, Wasit University, Iraq

4Faculty of Mechanical and Manufacturing Engineering, Universiti Tun Hussein Onn Malaysia, Batu Pahat, Johor,

Malaysia [email protected], [email protected], [email protected] [email protected],

[email protected], [email protected], [email protected], [email protected], [email protected].

Abstract-- Direct hot extrusion is an alternative process for

recycling aluminium without melting the scrap. It utilizes low

energy and is environmental friendly. This study shows the

effects of preheating temperature (PHT), preheating time

(PHti) and addition of volume fraction (VF) of micro alumina

on the microhardness (MH), density and microstructure of

the extruded profiles. Three values of PHT (450, 500, 550 °C),

PHti (1, 2, 3 hours) and VF (5, 10, 15 %) were considered

respectively. The full factorial design with center point

analysis was used to demonstrate the effect of process

variables on responses. A total of 19 experimental runs were

performed through the hot extrusion process. The results

show that the preheating temperature is the most important

factor to be controlled in order to obtain the optimum MH

and density, while preheating time and volume fraction

trailed behind the former. It can be concluded that

microhardness increases with the increase in PHT and

decrease in PHti and VF. On the contrary, an increase in

density was observed with a decrease in PHT, PHti and VF

apiece. The impact of hot extrusion parameters on the average

grain sizes and microstructural analysis of the recycled

samples were equally investigated and discussed. Index Term-- AA6061, Aluminum Alloy, Density, Hot

Extrusion, Micro Alumina, Microhardness, Metal Matrix

Composites, Microstructure.

1 INTRODUCTION

Recycling of aluminium alloy scraps is often done by conventional re-melting where part of the material is recovered and another part is lost to the processes of the production [1] in addition to the high energy demand and environmental pollution [2]. Because of these reasons, nowadays, there are delibrate efforts aimed at reducing energy consumption and environmental conservation by using economically viable methods instead of conventional recycling. This method is known as the direct treatment of alloy chips, called solid-state recycling [3,4]. It was suggested to be a good alternative to conventional recycling and offers a green process, hence

employing the plastic deformation technique. The solid-

state recycling converts the metal scraps into bulk material,

contrary to the conventional energy intensive technique [5-

7]. The hot extrusion process could not only conserve the

environment but equally eradicate the generation of new waste [8].

The physical, mechanical properties and

microstructure of products extruded using the solid-state

recycling of aluminum alloy chips are controlled by a

number of the hot extrusion parameters. Previous studies

had reported temperature related parameters, extrusion

ratio, die geometry, chip morphology and ram speed to be

among the relevant factors worthy of understudying in

order to obtain qualitative products from the direct

recycling process [9, 10]. This study focuses on the effects

of preheating temperature, preheating time and volume fraction on the resulting mechanical and physical

properties of micro alumina reinforced aluminum chips

based matrix composite. The influence of each factor was

analyzed using the Analysis of Variance (ANOVA) of full

factorial design. This work also investigates the analysis of

microstructure and the average grain sizes of the

extrudates.

2 WORKING PROCEDURES

2.1 Preparing the samples

The main raw material used in this study is a block

AA6061 aluminium alloy from which chips was prepared.

The high-speed milling technique was used in chips preperation. The choice of the high-speed milling was

owing to its ability to produce chips that have comparable

properties with the aluminium alloy. Previous studies

reported that this technique have no deformability on the

mechanical properties of recycled materials [8, 11]. The

dimensions of the examined chip particles were 3.40-3.70

mm length × 1.630 mm width × 0.094 mm thickness with

12.35 mm2 surface area per chip. The mechanical

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properties of the aluminum alloy AA6061 used in this study

are summarized in Table 1. Table I

Range of mechanical and physical properties of AA6061-T6 [12]

Properties Value (data obtained at temperature =25 °C)

Max. Min.

Density (g/cm3) 2.70 2.66

Tensile strength (MPa) 319 315

Yield strength (MPa) 292 291

Elongation (%) 13 12

In order to remove stains and impurities in the form of

drop of oil that might have stitch with the aluminium alloy

chips, the ASTM G131-96 was adopted in cleaning the

chips. The ultrasonic methods was used during the cleaning

operations. After cleaning the chips, the aluminuim was

mixed with micro alumina using the 3D mixer. The mixture

was compressed into a cylindrical die using cold press to produce billets of approximately 90 mm length x Ø30 mm.

The order used in conducting the experiment is presented

in Fig. 1. Additional conditions used during the extrusion

of the billets is presented in Table 2. Key among the

parameters varied are the preheating temperature, time and

volume fraction of alumina. The order of variation were

450 - 550 °C, 1 – 3 hours and 5 - 15%, apiece. In order to

avoid the formation of hot cracks, the highest preheating

temperature was limited to 550 °C. Previous study had

reported that the surface of extruded products at a

temperature higher than 550 °C had cracks [12]. A ceramic

heater was used in generating the required heat. The heater was fixed around the container. Graphite-based lubricant

was used on the inner surface of the die and container at

every extrusion cycle to prevent increase in the load of

extrusion due to friction.

Fig. 1. Process sequence of experimental work

Table II

Parameter used during the hot extrusion process of the billets

Parameter Value/type

Geometry/ shape of the die Round

Ratio used in extrusion, R 5.4

Diameter of billet, Ø (mm) 30

Speed of extrusion, s (mm/s) 1

Container temp, Tcont (°C ) 300

Die temp., Tdie (°C) 300

Preheating temp., Tph (°C) 450, 500, 550

Preheating time, tph (hr) 1, 2, 3

Volume fraction (Al2O3), Vf (%) 5%, 10%, 15%

2.2 Test methods

Microhardness test was done using a load of 0.9807 N

and 10 seconds as holding time for the test at room

temperature. The hardness test was carried out using the

Micro Vickers hardness machine, following the grinding of

the surfaces of each sample in order to obtain a balanced

indentation. A square base pyramid formed diamond was

utilized for testing in the Vickers scale. Three places (top,

center, and bottom) of the samples were indented, out of

which the average was taken as the hardness value for each

sample.

Sample preparation for density was done by obtaining circular pieces sectioned to a dimension of



approximately 1 mm in diameter and thickness. The HR-250AZ- Compact Analytical Balance density determination kit was used for this purpose. Density test was done using the Archimedes’ water immersion principle. The small pieces of specimens were weighted in air and distilled water to record the weight in different

environments. Each sample was immersed in distilled water during the density measurement. The room temperature was recorded to calculate the density of the

composite material based on the following formula:

𝐷𝑒𝑛𝑠𝑖𝑡𝑦 = 𝐴

│𝐵│ × 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑑𝑖𝑠𝑡𝑖𝑙𝑙𝑒𝑑 𝑤𝑎𝑡𝑒𝑟

(1) Where,

A = 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑛 𝑎𝑖𝑟

B = 𝑤𝑒𝑖𝑔ℎ𝑡 𝑖𝑛 𝑙𝑖𝑞𝑢𝑖𝑑

2.3 Experimental Design

A 23 full factorial with 2 replicates was adopted in

conducting the experiment. Parameters were varied in

accordance with each run. The choice of this 2 replicates

was to enable an analysis of the variation in parameters to

be conducted. The variation followed the three enumerated

process parameters. Three center points were included in

the factorial design to investigate the curvature effect. This

would further suggest whether the linear model was

sufficient to define the relationship between process factors

over response or vice versa. The interactions between factors were also investigated. The selected parameters

with their levels in factorial design are shown in Table 3.

Two runs were carried out in each corner, a combination of

which resulted in 19 experimental runs, herein referred to

as S1-S19. Table 4 is a representation of the full layout of

the planned experiment based on the design of experiment

(DOE). The focus of the study were represented by the

responses which are the microhardness and density of the

fabricated samples. In order to identify the key factor(s)

effecting on the properties measured, the Analysis of

variance (ANOVA) was deployed. The process was also

relevant in analysing the relationship between the input parameters. The results of ANOVA in the factorial design

can give a further insight into the experimental direction for

process optimization.

Table III

Outline of design and experimental process parameters, including levels

Parameter

symbol Parameter

Levels

Low (-1) Center (0) High (+1)

A Preheating temperature, (PHT) 450 500 550

B Preheating time (PHti) 1 2 3

C Volume fraction (VF) 5% 10% 15%

3 RESULTS AND DISCUSSION

The full factorial design with three center points were deployed as a guide for conducting the total nineteen

experimental runs. The results of the experiments are tabulated in Table 4. The result revealed that higher microhardness

were obtained from samples extruded using the maximum PHT and the minimum VF. On the other hand, a rise in density

was observed from samples extruded using minimum PHT, PHti and VF. Further discussions are made in the subsequent

sections to clarify the ANOVA findings of the factorial design.

Table IV

Results of microhardness and density, showing the effect of PHT, Phti and VF on both properties

Sample

Designation

PHT(A)

(°C )

PHti(B)

(hr)

VF(C)

(%)

Microhardness

(MH)

(HV)

Density

Analysis

(g/cm3)

S1 450 1 5 43 2.70

S2 550 1 5 53 2.57

S3 450 3 5 45 2.65

S4 550 3 5 55 2.53

S5 450 1 15 46 2.68

S6 550 1 15 57 2.56

S7 450 3 15 48 2.60

S8 550 3 15 50 2.50

S9 450 1 5 40 2.73

S10 550 1 5 61 2.57

S11 450 3 5 49 2.64

S12 550 3 5 56 2.51

S13 450 1 15 48 2.69

S14 550 1 15 52 2.55

S15 450 3 15 47 2.58



S16 550 3 15 50 2.49

S17 500 2 10 52 2.61

S18 500 2 10 61 2.59

S19 500 2 10 58 2.58

3.1 ANOVA Results of Microhardness

The result of microhardness revealed that samples

extruded using high preheating temperature had higher

hardness values as against their counterpart extruded at low

preheating temperature. This was a clear indication that the

microhardness was sensitive to the extrusion preheating

temperature. For clarifications, when billets are extruded at

higher preheating temperature and low volume fraction of

alumina, an efficient consolidate of the aluminium alloy

chips and alumina was better due to increase in the

bounding mechanism within the composite. The same reason was responsible for the increase in microhardness of

samples S10, S18 and S19. The implication was that at high

temperature, matters are diffused faster in accordance with

the transportation of natter principles, hence resulting to

better chips bonding. The same reason was responsible for

the voids free and compacted microstructure. The fine grain

sizes observed in these composites was in concurrent with

the hard surface texture observed in the samples extruded

using high preheating temperature.

Output of the ANOVA depicts that the relevant

factors effecting on the microhardness of the aluminium

matrix composite fabricated using the hot extrusion process

is only the preheating temperature. The p < 0.05 was a clear

indication of the effect of the factor as presented in Table 5

and further clarified by the Pareto Chart in Fig. 2. Other parameters like, preheating time (B), volume fraction (C),

the interaction of PHT and PHti (A*B) and the interaction

of PHT, PHti and VF (A*B*C) were of less significant

towards microhardness.

Table V

ANOVA results of factorial design for microhardness

Source DF Adj SS Adj MS F-Value P-Value

Model 7 390.898 55.843 2.74 0.066

Linear 3 288.372 96.124 4.72 0.024

PHT 1 287.252 287.252 14.10 0.003

PHti 1 0.345 0.345 0.02 0.899

VF 1 0.776 0.776 0.04 0.849

2-Way

Interactions 3 101.812 33.937 1.67 0.231

PHT*PHti 1 35.745 35.745 1.75 0.212

PHT*VF 1 50.719 50.719 2.49 0.143

PHti*VF 1 15.348 15.348 0.75 0.404

3-Way

Interactions 1 0.714 0.714 0.04 0.855

PHT*PHti*VF 1 0.714 0.714 0.04 0.855

Error 11 224.058 20.369

Pure Error 10 89.776 8.978

Total 18

Fig. 2. Pareto chart for microhardness

The implication of this result was that specimen

fabricated using higher preheating temperature may result

in higher microhardness. Increasing the temperature to 550

°C and decreasing the volume fraction to 5% will increase



the microhardness to the maximum level. Furthermore, the

samples extruded at 450 °C and 15% volume fraction of

alumina resulting in the least hardness performance. In

ANOVA analysis, the main effect plot as presented in Fig.

3 clearly shows that all the center points lie very close to

the linear lines of the average microhardness. In addition, they are connected from low to high setting of preheating

temperature, preheating time and volume fraction

parameters.

Apparently, the interaction plot as shown in Fig. 4

exhibits a similar trend to the main effect plot. This

indicates that all the observed data are accurately plotted

according to the ANOVA results. Besides, the curvature

effect was insignificant on the response. Mentioned here

was that the insignificancy of carvature was in agreement with the ANOVA report presented in Table 6. The

observed p > 0.05 showed that the linear model was

sufficient in fitting all the data. This p value was a

demonstration of the curvature term.

Fig. 3. Main effects plot for microhardness

Fig. 4. Interaction plot for microhardness

The response optimizer approach was used to further optimise the parameter settings. This allows the maximum

microhardness to be obtained with the least trials, which is an important consideration for the economic scale of

production. Fig. 5 shows the response optimizer analysis for microhardness which revealed that 58HV was the optimum

hardness obtainable at 550 °C, 1 hours and 5% volume fraction of alumina conditions.

Fig. 5. Optimization plot for microhardness



3.2 ANOVA Results for Density

The ANOVA conducted in respect of density

indicated that the value of density increased with decrease

in all the parameters used in extruding samples. Therefore,

it was noted the maximum value of density was at 450 Ċ, 1

hour and 5 % for PHT, PHti and VF respectively. The implication of this result was that the chips were compacted

to be outrageously dense which resulted in poor inter-chip

consolidation. It is therefore clear that stress was higher on

the samples when PHT, PHti and VF were low. By

extension, these conditions of extrusion were only suitable

at eliminating the voids, but incapable of improving the

chip bonding. Previous study had reported higher strength

in samples extruded using high temperature [13]. However,

such conditions resulted in lower density because the

extruded products encountered residual voids and cracks.

This indicates that the preheating temperature is the most

influential factors towards density in solid-state recycling method using hot extrusion process.

The ANOVA results indicate that all parameters

PHT, PHti and VF were significant terms contributing to

the density of hot extruded aluminium matrix composite.

However, the p < 0.05 for preheating temperature was a

more influential factor towards the density as against the

preheating time and volume fraction of alumina. The result presented in Table 6 and Pareto Chart in Fig. 6 clearly

indicated the aforementioned relationships. For

clarifications, the interaction between preheating

temperature and preheating time, preheating temperature

and volume fraction were not significant towards the

density of the samples herein reported. Furthermore, the

relationship between PHT, PHti and VF were equally not

significant towards density. Figs. 7 and 8 present the main

effect plot and interaction plot apiece. The main effect plot

indicates that PHT and PHti were the significant parameter

effecting on the response. It should be noted also that the

interaction plot followed a similar trend with the main effect plot and the curvature effect was unimportant.

Table VI

ANOVA results of factorial design for density

Fig. 6. Pareto chart for density

Source DF Adj SS Adj MS F-Value P-Value

Model 8 0.085725 0.010716 70.65 0.000

Linear 3 0.084069 0.028023 184.77 0.000

PHT 1 0.061256 0.061256 403.89 0.000

PHti 1 0.018906 0.018906 124.66 0.000

VF 1 0.003906 0.003906 25.76 0.000

2-Way Interactions 3 0.001569 0.000523 3.45 0.060

PHT*PHti 1 0.000756 0.000756 4.99 0.050

PHT*VF 1 0.000506 0.000506 3.34 0.098

PHti*VF 1 0.000306 0.000306 2.02 0.186

3-Way Interactions 1 0.000056 0.000056 0.37 0.556

PHT*PHti*VF 1 0.000056 0.000056 0.37 0.556

Curvature 1 0.000032 0.000032 0.21 0.657

Error 10 0.001517 0.000152

Total 18 0.087242



Fig. 7. Main effects plot for density

Fig. 8. Interaction plot for density

In order to independently investigate the effect of each parameter on the response individually, the response

optimizer method was adopted. The aim was to evaluate their impact on the extruded product in terms of density. The

response optimizer has the capability of revealing the optimum parameter required to obtain the maximum response. Figs.

9 represent the analysis of optimizer response from Design of Experiment for density.

Fig. 9. Optimization plot for density



3.3 Microstructure investigation

Microstructural investigation of mechanically

grinded, ploished and atched cupons were conducted on the

optical microscope (OM). SiC grits paper of 240, 600, and

1200 grades were used in grinding the samples for 3

miniutes each under the flow of water. This was followed by polishing using colloidal silica. The polished samples

were etched utilizing Barker’s reagent with a voltage of U

= 12 Volt DC for 2 miniutes. ASTM E112-13 standard was

adopted in measurement of grain sizes, in accodance with

the mean linear intercept procedure.

The microstructural analysis of the extrudates

processed at different preheating temperatures, time and

volume fraction of alumina revealed grains with differed

sizes as shown in Fig.10. For instance, fine grains can be

observed in the microstructure of the samples preheated at

450 °C, 15% Al2O3 . These grains were as a result of the

dynamic recrystallization during the hot extrusion. Further observed in the composites fabricated using the

aforementioned parameters were smaller grain sizes and

chip boundaries with no voids and visible cracks.

At the above condition, when preheating times

were 1 hour and 3 hours, the average grain size were 22.35

μm and 21.74 μm respectively. A decrease in the volume

fraction of alumina from 15 % to 5 %, while other

parameters were maintained as above resulted to an

increase in the average grain sizes of 34.82 μm and 32.44

μm. This indicates that particle sizes of alumina effected

on microstructure of extrudates. When preheating

temperature was increased to 500 °C and the volume

fraction of alumina was decreased to 10 %, the

unrecognized and non-uniform shapes were observed in

case of the new microstructure. Smooth boundary lines and

voids can be observed, while the composites contained a very dense microstructure. With this condition, the

measurement of grain size increased to 41.26 μm.

At 550 °C and 5% alumina, the shape of grains

become more equiaxed and recrystallized. That was an

indication that the grain coarsening had occurred at this

processing temperature. The same reason was responsible

for the increase in the average grain size which reached a

maximum value of 47.61 μm at 3 hours. When 550 °C

preheating temperature and 15 % volume fraction of

alumina was used, the grain size measurement decreased to

29.52 μm at 3 hours as shown in Table 7. These findings

are in agreement with the literature [14]. From this result, it is clear that increase in the

temperature increased the grain size of recycled chips.

However, the growth of the grain can be avoided by the

presence of the Al2O3 reinforcement particles which act as

obstacle to the grain boundary movements [15]. The

analysis of grain structure showed that fine and suitable

average grain sizes and smooth boundary lines were

beneficial to high tensile strength. This was in combination

with high preheating temperature and low volume fraction

of alumina. The duo could efficiently consolidate the

composite during the extrusion process.

Table VII

Grain size measurement

Sample No.

Parameter Condition

G No. Average diameter

(µm) PHT(A)

(°C )

PHti(B)

(hr)

VF(C)

(%)

S1 450 1 5 6.12 34.82

S2 550 1 5 4.70 45.27

S3 450 3 5 6.55 32.44

S4 550 3 5 4.46 47.61

S5 450 1 15 9.51 22.35

S6 550 1 15 6.68 31.83

S7 450 3 15 9.79 21.74

S8 550 3 15 7.21 29.52

S19 500 2 10 5.15 41.26



Fig. 7. Microstructure of extrudate produced at different PHT, PHti and VF: (a) 450 °C, 1 hour and 15%; (b) 450 °C, 1 hours and 5%; (c) 500 °C, 2

hours and 10%; (d) 550 °C, 3 hours and 15%; (e) 550 °C, 3 hours and 5%.

CONCLUSION

The focus of this study was to conduct a

comprehensive investigation and computational analysis of

the effects of preheating temperature (PHT), preheating

time (PHti) and volume fraction (VF) of alumina on the

microhardness, density and microstructure of AA6061

composite reinforced with micro alumina. It was clear from

the ANOVA that the main factor that was significant to the

microhardness was the PHT. On the other hand, PHti and

VF were not significant. Here, with PHT at 550 oC, PHti at

1 hour and VF at 5 %, the maximum hardness of 61 HV

and a density of 2.57 g/cm3 were obtained. On the other

hand, all the factors PHT, PHti and VF were considering as

significant factors effecting on the density of the

composite. The microhardness was sensitive to the

extrusion preheating temperature because the extrusion

process done at high billet temperature efficiently

consolidated the material. The highest density 2.73 g/cm3

was obtained at a PHT of 450 oC, PHti of 1 hour and VF of

5%, when hardness was 40 HV. Density was high at low

PHT, PHti and VF because the compression pressure and

extrusion stress produced at these conditions were only

sufficient at eliminating the voids which led to the increase

in density, but was incapable of improving chip welding.

ACKNOWLEDGMENTS

This research was financially supported by the Research

Management Centre, Universiti Putra Malaysia for support

of this research work. The first author thanks the School of

Graduate Studies, for financial support to study at



Universiti Putra Malaysia. We acknowledge the use of

facilities within the Centre for Graduate Studies, Universiti

Tun Hussein Onn Malaysia (UTHM) and Sustainable

Manufacturing and Recycling Technology, Advanced

Manufacturing and Materials Center (SMART-AMMC),

Universiti Tun Hussein Onn Malaysia (UTHM). We equally acknowledge the College of Engineering, Wasit

University, Iraq for research collaborations.

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