experimental and theoretical study of the influence of the addition of alumina powder to 7020...

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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976

6340(Print), ISSN 0976 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) IAEME

412

EXPERIMENTAL AND THEORETICAL STUDY OF THE INFLUENCE

OF THE ADDITION OF ALUMINA POWDER TO 7020 ALUMINUM

ALLOY FOAM ON THE MECHANICAL BEHAVIOR UNDER IMPACT

LOADING

ArkanJawdat Abassa

1

, DhaferSadeq Al-Fatal

2

1(Department of machines and equipment Engineering/ University of Technology, Baghdad,

Iraq, [email protected]).2(Department of machines and equipment Engineering/ University of Technology, Baghdad,

Iraq, [email protected]).

ABSTRACT

Aluminum foams are new materials mainly produced by expansion in proper chambers. A

relevant quantity of voids is generated in the metallic matrix during manufacturing, resulting

in a low material density. Aluminum foams are strongly affected by cells size, cells shape,

foam density, weight fraction and types of additives to aluminum foam. In this paper, theinfluence of particle size, and weight fraction of Al2O3 particles on impact behavior of 7020

aluminum alloy foam was investigated, and then modeled using ANSYS12 software. Three-

dimensional models are suggested to model aluminum foam structure under impact loading.

Experimental results showed that the increase of the weight fraction of alumina powder as

reinforcement raises the acceleration-time curves. The decrease of alumina particle size leads

to an increase in the acceleration-time response of aluminum foam. Theoretical results of the

models are in good agreement with the experimental acceleration-time, velocity-time and

displacement-time curves of 7020 aluminum alloy foam.

Keywords: Impact loads, Aluminum foam, Alumina particles, aluminum foam models,

Calcium carbonate.

I. INTRODUCTIONMetal foams are new, as yet imperfectly-characterized, class of materials with low densitiesand novel physical, mechanical, thermal, electrical and acoustic properties[1]. They offer

potential for light-weight structures, for energy absorption such as crash boxes in

automobiles, for thermal management such as heat insulation or heat exchangers according to

cells type, and for acoustic absorption such as sound insulation, and some of them, at least,

are cheap. Metal foam is a cellular solid, just like wood, coral, bone and bread, but with the

cells made out of metal. Usually the metal is an aluminum alloy, but it can also be made of

INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERINGAND TECHNOLOGY (IJMET)

ISSN 0976 6340 (Print)ISSN 0976 6359 (Online)

Volume 3, Issue 3, September - December (2012), pp. 412-428 IAEME: www.iaeme.com/ijmet.aspJournal Impact Factor (2012): 3.8071 (Calculated by GISI)

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IJMET I A E M E


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other metals, for example steel, nickel, titanium or gold [2]. In other words, metal foamsare

solids with high fraction of porosity.

Metallic foams are generally divided into two types, open and closed cell microstructures. In

the open-cell foam the cells are interconnected and the foam has the same appearance as a

scouring pad. The connections in open-cell foam are called struts, which meet in nodes. For

closed-cell foams the cells are closed by so-called faces, which meet in so-called cell edges,

which in turn meet in nodes. Generally, closed cell microstructures have higher mechanical

strength than open cell microstructures. The closed cell type of microstructure is particularly

attractive for applications in the field of light weight construction and energy absorption.

II. EXPERIMENTAL WORK

All foams made in this work were prepared by using 7020 aluminum alloy with the chemical

composition shown in table (1). A calcium carbonate CaCO3 powder, as a blowing agent,

with particle size of less than 10 m and weight fractions (2%) was used to generate pores

inside the aluminum alloy.

Many significant factors affect the use of CaCO3 powder instead of TiH2 powder as blowingagent. The main factors are

1- Calcium carbonate is cheap as compared with TiH2 as blowing agent[3].2- The density of CaCO3 is (2.71 to 2.83 gr/cm3), almost identical to the density of

molten aluminum. Moreover, its decomposition temperature is above the melting

point of aluminum, usually in the temperature interval between 660 C and 930 C.

Therefore, CaCO3 is particularly suitable for the melt-route settling-free production

of foamed aluminum based materials.

3- The use of CaCO3 was found to be potentially suitable as a foaming agent for direct(melt-route) foam manufacturing. Moreover, the decomposition products of CaCO3have a significant effect on foam stabilization [4].

In this work, 7020 aluminum alloy foams were produced with different weight fraction of

alumina particles (0%, 2% and 4%) as reinforcement. The alumina particles were prepared by

sieving them to multi sizes of 90-106 m, 106-125mand 125-150 m. An electrical furnace

was setto a temperature of 800C to melt the7020 aluminum alloy by using a graphite

crucible.

Alumina particles with specific particle size and weight fraction were added to the melt of

7020 aluminum alloy at a temperature of 800C. An electrical stirrer was used to mix the

alumina particles inside the melt of aluminum for about one minute. This melt was turned

back to the furnace to rise up its temperature, and this process was repeated three times until a

good distribution of alumina particles inside the melt of aluminum was achieved. Calciumcarbonate was added to the melt with 2% weight fraction, and again the stirrer was used to

get a good distribution of the blowing agent particles inside the melt of aluminum. This

mixture was turned back to the furnace but this time at a temperature of 740C for about 15

minutes to get a 7020 aluminum alloy foam reinforced with alumina powder. Aluminum

foam samples have a cylindrical shape with a diameter of 50.24mm, and a height of 44mm,

as shown in fig. 1.


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All aluminum foam samples are impacted by a drop weight, as shown in fig. 2. An aluminum

foam specimen was put on the solid base. A block which weighs 0.71 kg was let free to fall

onto the foam sample from a height of 1 m. This represents 6.965 J of kinetic energy at

impact. In the center of the block, a one axis accelerometer, type B&K 4370, was installed.

Electrical signals were generated by the accelerometer at the moment of impact. These

signals were sent to the charge conditioning amplifier, type B&K 2626 for amplification, and

were displayed by data storage oscilloscope, type ADS 1022C, 25MHz. This test involves a

dynamic load being applied to a cylindrical shape specimen. The impact load has a velocity

of 4.429 m/sec at impact moment. After impact, specimens deformation was measured, and

by using equation (10), the maximum force and the acceleration exerted by the impactor can

be calculated.

TABLEIChemical composition of 7020

Si % Fe % Cu % Mn

%

Mg

%

Cr % Ni % Zn % Ti % P % Pb % Sn % V % Zr % Al %

0.127 0.168 0.084 0.084 0.95 0.24 0.001 4.03 0.041 0.001 0.001 0.001 0.008 0.129 Bal

-A- -B- -C-Fig. 1: A- 7020 aluminum alloy foam samples, B-side view, C- Top view.

Fig. 2: Drop weight tester machine


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III. THEORETICAL ANALYSIS

A drop weight test of aluminum foam samples can be modeled, as shown in fig.3. A weight

W is released from rest at a height of h above an aluminum foam sample with cross sectional

area A, and a length L. The aluminum foam sample is modeled as a spring with stiffness k

and a mass Wb.

Fig. 3: Vertical beam (aluminum foam sample) under impact [5]

This model can be solved by the application of the law of conservation of energy to the

system.

The kinetic and potential energy are illustrated in equations at the start, when the block is at

rest above the spring at height h, and at the end, when the block is at its lowest point.

At the start,

Kinetic energy=0

Potential energy= W (h).

where W=mg, and h= height.

At the bottom,

Kinetic energy=

By equating the total energy at the start (at height h), and at the end (when contacting the

spring):

W (h) =

2 (1)Wherevh=velocity of weight W at contacting point.

First, it should be noted that the spring mass will contribute to the kinetic energy during

deflection of the spring, since each differential element of mass along the length will move

downward with some kinetic energy during the deflection. This will decrease the potential

energy of the system. The uppermost element of mass will move the same amount as the

block, since it is immediately below it.

Second, the block, upon falling through a height, and gaining speed, now impacts another

mass (the mass of the spring), and the change in speed upon impacting the spring needs to be

ascertained using the principle of conservation of momentum.

The next element will move slightly less, etc. The lowest element will not move at all,

since it is contiguous with the base of the spring.


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Term + is small as compared with

, so equation (9) can be written as,

(10)

Equation (10) is used to determine the magnitude of maximum acceleration exerted by the

impact load by dividing the force F by a mass of impactor.

VI. THEORETICAL WORK

The Finite Element Method (FEM) offers a way to solve complex continuum problems by

subdividing it into a series of simple interrelated problems. FEM is most commonly used in

numerical analysis for obtaining approximate solutions to wide variety of engineering

problems [6]. In the present study, a commercial general purpose finite element program

ANSYS 12.0 is used for numerical simulation of aluminum foam structure. ANSYS

program has many finite element analysis capabilities, ranging from simple, linear, static

analysis to a complex nonlinear, transient dynamic analysis [7]. The mechanical behavior of

aluminum foam under impact loading was investigated by finite element simulations. In this

study, many suggested structural models are investigated in impact mode.

V. ALUMINUM FOAM MODELS

Three types of 7020 aluminum alloy foams are modeled; the first is aluminum foam without

reinforcement, and its bulk density is 0.753 g/cm3.

The second type is aluminum foam reinforced with 2% weight fraction of alumina whose

particle size is 90-160m, and its bulk density is 0.565 g/cm3.

The third type is aluminum foam reinforced with 4% weight fraction of alumina whose

particle size is 90-160m, and its bulk density is 0.464 g/cm3.

The samples of aluminum foam are modeled in three dimensions using closed honeycomb

cells models with two types of arrangements, as shown in figures 4-Aand 4-B, and a cubic

model, as shown in figure 5. The behaviors of these models under impact loading are

compared with experimental results.

VI. CALCULATION OF CELLS WALL THICKNESS

Wall thickness of suggested models is calculated based on real values of aluminum foam and

aluminum solid densities. Aluminum foam density isfand aluminum density as solid iss.

Calculations of cell wall thickness are derived based on the rule of densities and volumes,


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(11)

A Three-dimensional honeycomb cell is shown in fig.6-A, and a cubic cell is shown in fig.6-

B, and the equations of wall thicknesses are:

1-

A three-dimensional honeycomb structure,

(12)

where: -

Aluminum foam density, Aluminum solid density, Side length of cell,

( 2 sin 30).=Wall thickness of cell,

=Cell length,=Radius of honeycomb cell (R=0.5 d).

2- A three-dimensional cubic cell,

=

(13)

In equations 11, 12, and 13, is 2.7 g/m3

for 7020 aluminum alloy, is foam density which

is calculated from measuring the weight and volume of aluminum foam.

VII. DIMENSIONS OF ALUMINUM FOAM MODELS

The honeycomb structure model-1 has a width of 34.641 mm, but honeycomb model-2 has a

width of 35 mm, and these differences are due to cell arrangement.

Honeycomb models 1, and 2 have height of h=44mm, cell length of tC=2mm (cell length

equal to sample thickness), and wall thickness of 0.1685mm which is calculated using

equation (12).

Cubic cell model has a width of 20 mm (equal to seven cells or more in each side) [8],

height of h=44mm which is equal to a real sample height, cell length of t C=2mm (cell length

equal to the sample thickness), and wall thickness of 0.186mm which is calculated using

equation (13).

-A- -B-Fig.4: A three-dimensional closed honeycomb cells model for 7020 aluminum alloy foam

under impact (A- model-1, B- model-2).


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Fig.5:A three-dimensional closed cubic cells model for 7020 aluminum alloy foam under

impact (model-3).

Fig.6: A-A honeycomb cell,

B-A cubic cell

.

VIII. ELEMENTS USED IN IMPACT LOADS.

A nonlinear transient (large displacement) solution is used for the analytical behavior of

aluminum foam under impact loads by using ANSYS Multi-physics. Elements used in

transient solution are of two types, the first is for aluminum foam structure which is SHELL

181,as shown in fig.7-A. The second is for drop weight which is SOLID 45, as shown in

fig.7-B.

Elements ofSHELL 181 are suitable for analyzing thin to moderately-thick shell structures.It is a 4-node element with six degrees of freedom at each node: translations in the x, y and z

directions, and rotations about the x, y and z-axes. SHELL 181 is well-suited for linear, large

rotation, and/or large strain nonlinear applications. Change in shell thickness is accounted for

in nonlinear analyses. In the element domain, both full and reduced integration schemes are

supported [7].


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SOLID 45 is used for the 3-D modeling of solid structures. The element is defined by eight

nodes having three degrees of freedom at each node: translations in the nodal x, y and z

directions. The element has plasticity, creep, swelling, stress stiffening, large deflection, and

large strain capabilities. A reduced integration option with hourglass control is available [7].

-A- -B-Fig.7:A-SHELL 181 geometry, B- A solid 45 geometry[7].

IX. EXPERIMENTAL RESULTS

Impact tests have been performed on 7020 aluminum alloy foam samples reinforced

with 2%, and 4% alumina with different weight fraction of particle size (d=90-106, d= 106-

125, and d=125-150 m). The impactor acceleration with respect to time from the moment of

impact is shown in figures (8to 11). These curves represent experimental results recorded by

data storage oscilloscope. The increase of weight fraction of alumina as reinforcement in

figures (9to 11) increases the acceleration of impactor. Figures (12and 13) show the influence

of particle size with 2%, and 4% weight fraction of alumina on the behavior of acceleration-

time curves. The decrease of alumina particle size increases the acceleration. The impactor

acceleration with respect to time is fitted to quadratic equations using a curve expert software

version 1.4. The integration of these curves gives the velocity and the double integration

gives the displacement of impactor with the boundary conditions: initial velocity of -4.429

m/sec and initial displacement of zero. The acceleration, velocity, and displacement curves

with respect to time are shown in figures (14 to 17). Figures (15 A, 16 A and 17 A) show an

increase in the impactor acceleration as the weight fraction of alumina particles increases and

the time needed to reach the maximum value of acceleration decreases. The increase of

acceleration means an increase in the impact force according to Newtons law (Force = mass

x acceleration) where mass=mass of impactor.

Increasing the weight fraction of alumina as reinforcement in aluminum foam increases the

strength of aluminum foam samples. The increase in strength of Al 7020 foam is due to the

distribution of alumina particles around boundaries of pores. This distribution decreases the


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pore size, and the number of pores is increased as illustrated by MarchoHaeche [9]. This

increase in the number of pores means an increase in the number of walls that carry the

compression load.

.

Fig.8:Impactor acceleration- Time curve for 7020 aluminum alloy foam.

Fig.9:Impactor acceleration-Time curve for 7020 aluminum alloy foam reinforced by (90-

106 m) alumina particles


125 m) alumina particles.


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150 m) alumina particles.

Fig.12:The influence of alumina particle size on impactor acceleration- Time curve for 7020

aluminum alloy foam reinforced by 2% weight fraction of alumina particles

Fig.13: The influence of alumina particle size on impactor acceleration-Time curve for 7020

aluminum alloy foam reinforced by 4% weight fraction of alumina particles.

The impactor velocity in figures (15B, 16B and 17B) at impact is -4.429 m/sec, and the time

needed to reach a zero velocity value decreases as the weight fraction of alumina particles is

increased in aluminum foam samples. The decrease of the time to reach zero value of velocity

is due to the increase of the reaction force generated by impact with the high strength

aluminum foam samples according to (F = m ) where V = V V.The negative values of velocity represent the downward movement of the impactor which

compresses the aluminum foam sample while the positive values represent the velocity of the

impactor after rebound. Impactor displacements in figures (15C, 16C and 17C) represent the

deflection of aluminum foam samples when displacements lie between zero, and maximum

negative values. Maximum deflection occurs when the velocity is zero. Displacements

between maximum negative values and to the end of the displacement curve represent the

rebound displacement of the impactor. The ability for deformation of aluminum foam


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samples is decreased when weight fraction of alumina particles is increased due to increased

strength.

-A- -B-

-C-Fig.14:A- AccelerationTime (sec) -B- VelocityTime -C- DisplacementTime for 7020

aluminum alloy foam without reinforcement.

-A- -B-

-C-Fig.15: A- AccelerationTime -B- VelocityTime -C- Displacement Time for 7020

aluminum alloy foam reinforced with alumina particles of (d=90-106 m).


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-A- -B-

-C-Fig.16:A- AccelerationTime -B- VelocityTime -C- Displacement Time for 7020


-A- -B-

-C-Fig.17: A- AccelerationTime -B- VelocityTime -C- DisplacementTime for 7020



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X. THEORETICAL RESULTS

Tensile tests were performed on 7020 aluminum alloy samples with, and without

reinforcement to determine the parameters of Youngs modulus, yield stress, and slope of

stress- strain diagram at the plastic region.

Mechanical properties of the as cast 7020 aluminum alloy is:

Modulus of elasticity is 4.2 GN/m2.

Yield stress is 120MPa.

Slope of plastic stress- strain region is 2GN/m2.

Mechanical properties of the as cast 7020 aluminum alloy reinforced with 2% of weight

fraction of alumina powder with particle size of 90-106 m is:


Yield stress is 112 MPa.

Slope of plastic stress- strain region is 1.5GN/m2.

Finally the mechanical properties of the as cast 7020 aluminum alloy reinforced with 4% of

weight fraction of alumina powder with particle size of 90-106 m is:


Yield stress is 116 MPa.

Slope of plastic stress strain region is 2 GN/m2.

The above parameters are used in ANSYS12 to make, and solve the models.

The comparison of accelerationtime, velocity-time, and displacementtime curves for the

suggested models under impact, and experimental curves for 7020 aluminum alloy foam

without and with reinforcement are shown in figures (18to 20).

A comparison of experimental, and theoretical results (transient solution) for many

types and densities of aluminum foam shows good agreement in displacement-time, and

velocity time response, especially in the region between zero, and maximum displacement

or velocity (i.e. impacting region). However, there is a slight difference in acceleration-time

response in the same region. The differences between experimental and theoretical results

increased in the region of rebound of impactor which is less important than the impact region.The best suggested model is a three-dimensional cubic model which shows a good

response in acceleration, velocity and displacement with time.

Fig. 21 shows acceleration-time, velocity-time and displacement-time for solid 7020

aluminum alloy subjected to impact.

It can be observed that the maximum acceleration for aluminum foam is less than one half

that for the solid aluminum alloy. For this reason, aluminum foam is widely used today to

absorb impact energy for many applications.


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-A- -B-

-C-Fig.18:A- AccelerationTime -B- Velocity Time-C- Displacement Time for 7020

aluminum alloy foam (without reinforcement).

-A- -B-

-C-Fig.19:A- AccelerationTime -B- VelocityTime (sec) -C- DisplacementTime for 7020

aluminum alloy foam reinforced with 2% alumina particles of (d=90-106 m).


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-A- -B-

-C-Fig.20:A- AccelerationTime -B- VelocityTime-C- Displacement Time for 7020

aluminum alloy foam reinforced with 4% alumina particles of (d=90-106 m).

-A- -B-

-C-Fig.21:A- AccelerationTime -B- VelocityTime -C- Displacement Time for solid 7020

aluminum alloy.


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XI. CONCLUSION

Aluminum alloy foams reinforced with Al2O3 particles have been successfully produced.

Impact tests results exhibit a decrease in the amount of deformation of aluminum foam

samples and the Impact tests results exhibit a decrease in the amount of deformation of

aluminum foam samples and an increase in the impactor acceleration with decreasing particle

size and increasing weight fraction of alumina powder. Maximum deformation of aluminum

foam samples at zero velocity of impactor and the time needed to reach zero velocity

decrease with decreasing particle size and increasing weight fraction of alumina powder.

Impacting force increases as the particle size decreases and the weight fraction of alumina

powder increases.The suggested models are in general in good agreement with experimental acceleration,velocity, deformation, and force response with time curves of 7020 aluminum alloy foam but

the best model is the three-dimensional cubic model.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the mechanical engineering department of Al-

Nahrain University, for the provision of laboratory facilities.

REFERENCES

[1]Michael F. Ashby and L.U. Tianjian, Metal foams: A survey, Science in China, seriesB, 46 (6), December 2003.

[2]E. Amsterdam, Structural performance and failure analysis of aluminum foams, Ph.D.thesis, Zernike Institute, 2008.

[3]VaruanKevorkijan, Lowcostaluminum foams made by CaCO3 particulates,Association of Metallurgical Engineers of Serbia, MJoM,16 (3), 205-219, (2010).

[4]J. D. Bryant, M. D. Clowley, M. D. Wilhelmy, J. A. Kallivayalil, W. Wang, InMetfoam 2007, DEStech Publications Inc., Lancaster, PA, pp. 27, 2008.

[5]Matthew Huang,Vehicle crash mechanics, CRC Press LLC, 2002.[6]Huebner, H.H., Dewhirst, D.L., Smith, D. E., and Byrom, T. G. The Finite ElementMethod for Engineers. 4th ed., New York: J. Wiley. 2001.

[7]ANSYS Release 12.0 Documentation, ANSYS Inc, 2009.[8]M.F. Ashby, A.G. Evans, N.A. Fleck, L.J. Gibson, J.W. Hutchinson and H.N. G.

Wadley, Metal foam: A design guide,Butterworth-Heinemann, 2000.

[9]MarchoHaeche, Jorg Weise, Francisco Garcia-Moreno and John Banhart, Influence ofparticle additions on the foaming behavior of AlSi11/TiH2 composites made by semi-

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experimental and theoretical study of the influence of the addition of alumina powder to 7020...

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