Mechanical Properties of Aluminum Foam Fabricated by Aluminum Powders with Na Or Carbamide Replica

AASCIT Journal of Materials 2015; 1(2): 22-30 Published online June 20, 2015 (http://www.aascit.org/journal/materials)

Mechanical Properties of Aluminum Foam Fabricated by Aluminum Powders with Na or Carbamide Replica

G. Kavei

Semi Conductor Division, Material and Energy Research Centre, Tehran, Iran Email address

[email protected], [email protected] Citation G. Kavei. Mechanical Properties of Aluminum Foam Fabricated by Aluminum Powders with Na Keywords or Carbamide Replica. AASCIT Journal of Materials. Vol. 1, No. 2, 2015, pp. 22-30. Foaming, Amorphous Metals, Abstract Mechanical Properties, Aluminum foams with different porosities were produced by a powder metallurgical Powder Metallurgy, processing, so-called space-holding sintering method. The fabricating method consists of Crashworthiness, the following four steps: mixing, compacting, sintering and leaching. The mixture Carbamide, consisted of aluminum powder and an auxiliary space-holding component here was NaCl Replication Process or carbamide. The mixture was then poured into a mold and compacted into green compacts. The green compacts were subsequently sintered into composites. Thereafter, the composites were placed into a hot running water bath to leach out the embedded space-holding particles. The mechanical, physical and significant properties of produced Received: May 13, 2015 aluminum foam were characterized using test systems and theoretical evaluation methods. Revised: May 29, 2015 Attractive applications of the foam where crashworthiness, acoustic damping and weight Accepted: May 30, 2015 are crucial, e.g. automotive industry and construction materials. Structures, phase formation and micrographs were examined by X-ray diffraction and scanning electron microscope, respectively. Replica particles act as micro-model and Al particles take their shape.

1. Introduction Aluminum foams are lightweight components and show excellent impact energy absorption. In particular, they have a unique combination of physical, mechanical, thermal, electrical and acoustic properties. This makes them attractive in applications where crashworthiness and weight are crucial, e.g. automotive industry and construction materials [1-2]. The applications of the foams have limited the fabrication method, so architectures of the foam and bonding the Al powder have an important role. Many efforts for the improvement in quality of metal foams have been made over the recent years. However, some deficits such as nonuniformities and imperfect mechanical properties in aluminum foams were observed [3-4]. Fabrication method, architecture signifies the foam mechanical properties that, also improved by matrix reinforcement, e.g. adding fine ceramic particles or alloying elements in the base powder in the powder metallurgical route [5]. Among current foam-production industry, the replica technique is a method for uniform, pre-designed and fine shape of close or open-cell of lower melting-point metal foam such as aluminum [6]. Normally, the cells in the foam occupy more than 70% of the total volume. Mechanical and physical properties depend strongly on porosity, which gives the density lies typically in the range of 0.4 -1.2 g.cm -3 for Al foams [7]. Considering the mechanical response of the under load foam by developing micromechanical models, most of the proposed simulations are carried out by analytically [3], [8-10] or numerical 23 G. Kavei: Mechanical Properties of Aluminum Foam Fabricated by Aluminum Powders with Na or Carbamide Replica

models [11]. Finite element based model approach ideal controlled relative weights. Compression was exerted on the representations of the repetitive cubic or hexagonal array of cross section 5x20mm. So the force direction (longitudinal the cell structure. However, most metallic foams show little direction) was along 10mm length in all samples using a periodicity [12]. For industrial applications, the combination displacement-controlled hydraulic load frame. Longitudinal of concurrent properties of the foams, e.g. size of powdered and transverse directions refer to those directions are parallel elements and foam porosity affect on the strength of and perpendicular to a direction to which a sample is pressed, compressibility, tension and bending should be taken into the respectively. In this manner, the hot-pressing was carried out account [13]. at 550 °C under a pressure of 500 MPa in argon atmosphere The present work studies fabrication, testing and for 1 minute and this compression were repeated several theoretical evaluation of the mechanical and physical times. properties of Al foam. The internal connectivity and binding Second stage, devoted to fabricate Al foam taking the strength of the Al particles in aluminum foams were examined. blend of Al and carbamide tablets. The density of Carbamide It is concluded that the applications of the foams have limited (1.32 g.cm-³ [17]) was taken into the account to define the fabrication method; so careful design of the foam for relative weights of Al/Carbamide. Carbamide used in particular applications is essential. However, Al foam was experiments has the melting point of 132.7 - 135 °C, fabricated via blended Al powder and NaCl or carbamide the solubility in water 107.9 g/100 ml (20 °C). The cold-pressing bonding condition carefully investigated under hot or cold was carried out for Al/Carbamide at room temperature at a pressing. pressure of 300 MPa in argon atmosphere. However, the property depends on the time of sample was kept under 2. Experimental Details compression or load, otherwise cannot be retained on normal state. Malfunctioning of the system prevents checking the Foam samples were prepared by blended aluminum and time effects. salt powder or carbamide tablets. The former is as metal Foaming process was followed by simple removing of the matrix and the latter as the replica. The commercial pure sample from the mold. In the carbamide case the samples aluminum powder was supplied from (MERCK Art. No. were sintered up to 200°C to improve bonding Al particles 101056 aluminumpowdery 250 meshes, 50–70 µm powder and change solid state carbamide to gas phase. The pressed particle size, and 99% purity); salt and carbamide as replica sample was boiled in water for 2 hours to remove NaCl and agent are domestic elements. Carbamide is a solid organic carbamide from the raw foam. As mentioned above, some compound with the chemical formula (NH2) 2CO in the form residual carbamide also removed by a sintering process; in of tablets and granule, highly soluble in polar solvents (such this method, pure Al foam is attainable. Archimedes's test as water, ethanol, liquid ammonia). Carbamide or urea was carried out for a predetermined foam relative density of (CH4N2O) being heated higher than 150°C changes into 35, 30 and 25%. The Little higher value of relative density NH3, CO2, as gases, [14]. It is in the form of tablets with indicates that about 5-9% residuals remained into the foam ~2mm size, domestic salt (NaCl) (150–212µm powder this could be further reduced by exposing the sample on a particle size, 99.0% purity). At first stage, sodium chloride stream of pressurized water or air [6]. The skin was removed and Al powder were ball-milled in a planetary ball-mill with from these foams, and the specimen was ready at (20 x 10 x 5 the stainless steel cup for 20 h. Ball-milling was performed at mm) for both compression tests and electrical conductivity cup speed of 270 rpm. The balls of 17mm diameter and the measurements. Such a dimension was chosen to avoid edge ratio of balls to powder weights are 10:1. The milled powder effects, which may reduce the measured values of Young’s particle sizes were measured by “Fritsch GmbH analyst 22” modulus and compressive strength [18]. system. Particle size diagrams show, more than 30% particles size: a) for Al within 25-40 µm and, b) for NaCl this was 3. Result and Discussions within 4-8µm, [15]. The foam formed by milled Al and NaCl powders blend 3.1. Structural Characterizations of final relative porosity (65, 70 and 75%). Not only, one can change the relative weights of Al/NaCl but it may be possible The structural properties of the foam with different to change the relative porosity by changing the particle size porosities was examined by X-ray diffraction using a Philips of elementary material (Al) and replica material NaCl or (30 and 25 mA) diffractometer with Cu Kα radiation (λ = Carbamide [16]. The densities of NaCl, (2.17 gr.cm-3) and Al 1.5405 Å). X-ray diffraction patterns of pure Al foams were (2.7 g.cm-3) [17] were taken into the account to define recorded at longitudinal and transverse directions. There was relative weights of Al/NaCl. In the designated experimental no difference in the peak position of Al powder. Moreover, a settings, foams with 65, 70 and 75% of porosity’s selecting few peaks with low density were detected and assigned to relative weights of NaCl/Al powder were produced. A residual replicas (NaCl or carbamide). The structure of the mixture of the constituents was charged into a mold (high samples was also affected by the pressing process. In addition, speed steel) with a dimension of 5×20×10mm. Foaming was this is true for a raw foam (NaCl+Al or carbamide+Al foam as made possible by compression of mixed constituents with taken off from the press). Fig. 1 shows XRD patterns for pure AASCIT Journal of Materials 2015; 1(2): 22-30 24

Al and raw foams. These are selected patterns taken from; a) perpendicular to press direction. Pure Al foam and b) raw foam, both from a surface

Fig 1. X-ray diffraction spectra of, a) Pure Al foam recorded from a surface at the transverse direction and b) raw foam (NaCl+Al) taken from the hot pressing.

Micrograph and phase formation of Al foams with relative porosity for the samples shown in Fig. 4a and b is 70%. Fig. 4a porosities, and replica were characterized by (Philips XL30 and b has different scales of 500 and 100 µm respectively. scanning electron microscope (SEM) operating at 25 kV). The The relative porosity for the samples shown in Fig. 4c-f is SEM images taken from the longitudinal and transverse 65%. Fig. 4a illustrates larger pores while the size of the pores directions. Selected images are shown in Figs. 2 to 4. Fig. 2 is smaller in Fig. 4c. In order to illustrate more details of NaCl shows SEM images of the Al foam with 65% of porosity reside in the pores, Fig. 4c is enlarged in Fig. 4d, e, and f. The prepared by cold press and carbamide replica at different figures and Archimedes's test confirmed salt free aluminum scales of 50 µm and 2 µm respectively. Fig. 3 is the surface foam. More detailed analysis have been reported for NaCl morphology of pure Al foam with 75% of porosity prepared reside in the pores, the result shows absence of NaCl particles by cold pressing and carbamide replica. The figure illustrates and the configures of Al particles bonds, [14]. The interface of that the wall is thinner regarding to the foam with 65% of pores on both replicas is not spherical but wide and stretched. porosity which results in lower compression strength. These images show a densely compacted shell and a porous These figures illustrate the structures of the samples were core of Al. The surface within the pores has no porosity as very similar to those of Al foams fabricated with the other shown clearly in Fig. 4e. The images show a significant methods; (see, [19-20]). Fig. 4 shows SEM images of the increment in porosity and fewer connections between Al foams fabricated by hot pressing with the NaCl replica at particles that result inferior strength of structural bonding. relative porosities of 70% and 65%. The percentage of

Fig 2. SEM images of the Al foam with a porosity of 65% prepared by cold pressing and carbamide replica, at different scales, of 50µm and 2µm respectively. 25 G. Kavei: Mechanical Properties of Aluminum Foam Fabricated by Aluminum Powders with Na or Carbamide Replica

Fig 3. Morphology of Al foam with 75% of porosity fabricated by cold pressing and carbamide replica. The micrographs are recorded at different scales of 50 µm and 2 µm respectively. The wall of the cavities is thinner than at 65% porosity.

Fig 4. SEM images of the foams fabricated by hot pressing with NaCl replica; at relative porosities. a) and b) at relative porosity of 70%, with 500 and 100 µm scales respectively. The relative porosity for the samples shown in Fig. 4c-f is 65%. In order to illustrate more details of NaCl reside in the pores, Fig. 4c is enlarged in Fig. 4d, e, and f, the pores are larger in (a) and smaller in (c).

data-acquisition system. All stress and strain data used here is 3.2. Compression Test the nominal deduced from the recorded load-displacement The uniaxial compression test was performed on the data. Nominal stress is the average force per unit area on a specimens using Semiram Co. p30 machine. The load and surface within a deformed body while nominal strain was displacement were monitored by a computer equipped taken as the ratio of displacement increment to the initial AASCIT Journal of Materials 2015; 1(2): 22-30 26

height of the specimen. In measurement process, the were derived partly from modeling most of them extensively thickness of the sample is automatically measured by the tested on foams and somewhat from empirical fits to system, and next is preconditioned by compressing it three experimental data. The relation between mechanical and times to a percentage of original thickness. The strain was physical parameters has been discussed. The foam with small determined after correction for loads using a calibration of amount of metal (<40%) the thermal conductivity was pre and post test data the load is then applied followed by a reduced significantly, but the electrical conductivity remains crosshead displacement measurement. In this work, the load unchanged, in this case the structure has good damage was applied at a constant displacement speed of 0.02 mm.s -1 tolerance and impact energy-absorbing capability. The and the specimens were compressed between parallel platen relations are particularly useful in the early stages of the steel plates to ensure perfectly axial loading. Compression design when approximate analysis of components and was stopped when 85 % strain was reached. In Table 1, a structures is needed to decide whether metal foam is a number of useful theoretical formulas for physical and potential candidate for example to sound damping mechanical parameters calculation of the foams and their application. relations with base element parameters are listed, [21]. They Table 1. Summery of useful formula to calculate the mechanical properties of foams, [18].

Mechanical properties Closed-cell foam Open-cell foam E= (0.1 ÷ (up to) 1.0) E s 2 d  Young's modulus E, (GPa). d2 d  E=(0.1 ÷ 4) E 0.5( )+ 0.3( ) s     ds  ds d s  3 3 Shear modulus G ، (GPa) G≈ E G≈ E 8 8 σ= ÷ σ c(0.1 1.0) cs 2 2    3 3 d Compressive strengthσc ، (MPa) d  d σ= ÷ σ   c(0.1 1.0) cs   0.5+ 0.3  d  d  d  s s s  σ= ÷ σ σ= ÷ σ Tensile strengthσt ، (MPa) t(1.1 1.4) c t(1.1 1.4) c d d  d d  ε ε = ÷ × − + 3 ε =(0.9 ÷ 1.0) × − + 3 Densification strain D D (0.9 1.0) 1 1.4 0.4( )  D 1 1.4 0.4( )  ds d s  ds d s  Electrical Resistivity R (Ω) (/)dd-1.6

The internal connectivity and bonding strength in the and relative elasticity of the experiment evaluated using foams were examined by applying the compressive tests on equations 1 and 2 respectively, [25]. In the first order, there is the specimens that were conducted on a compression test an agreement between experiment and the values predicted machine (Semiram Co. p30). Fig. 5 illustrates uni-axial by these equations, the discrepancy assigned to the impurity compressive tests on the foams replicated using NaCl with entered to the foam by replica method. different microstructures and porosities of 65, 70 and 75%. Fig. 6 shows compression test curves on a foam the replica was carbamide with a porosity of 65% made by cold pressing followed sintering; defined sizes shown as the inset in the figure. The compression force was exerted on transverse direction against the samples. Both Figs. 5&6 reveal a three stage compressive behavior, namely an initial elastic regime, nearly linear region was partially reversible cell wall bending occurs. Second, plastic plateau regime appeared, in which successive bands of cell collapse, buckle yield. Third, fracture and a densification regime, i.e. deformation plateau densification take place is then stressed rises sharply as complete compaction commences, which is in agreement with [22]. Engineering stress–strain curve of the aluminum foam is similar to the other metal foam as shown in Fig. 7, [23]. In this respect, Ashby et al., [21] and Simon et al., [24] analyzed the deformation of metal foams where, governed by two mechanisms: a) Bending of cell wall has a quadratic Fig 5. Uniaxial compressive tests on the aluminum foams replicated using dependency on the relative density, b) stretching of cell faces NaCl with different microstructures and porosity. The inset shows the linear is linearly dependent on relative density. Compressive stress variation of σ versus ε. 27 G. Kavei: Mechanical Properties of Aluminum Foam Fabricated by Aluminum Powders with Na or Carbamide Replica

Fig 6. Compression test curves of samples made by cold pressing followed sintering for foams that replica was carbamide and porosity 65%.

Fig 7. Tentative stress–strain curve of the compressive test to study Fig. 2 in detail.

2   modulus, d is foam density, and ds is density of solid material. 3  σ= ÷ σ d + d  Table 2 lists experiment results of the Young Modulus c(0.1 1.0) cs 0.5 0.3  (1) d  d  s s  obtained from inset in Fig. 5 and calculated values from Eq. (2) taking Es=7.4 – 9.5 GPa, [26]. For additional studies, = σ ε some bending tests were also performed as compression tests. E c / The outcomes for different porosities are listed in Table 3. Tensile test may also be performed, because the available d2 d  E= (0.1 ÷ (up to) 1.0)Es 0.5( )+ 0.3( )  (2) testing system had restriction on a specific sample shape and d d  s s size it is not included.

σc and σcs are the foam and solid compressive stress, ε is strain , E is Young modulus of the foam, Es is solid Young Table 2. The experimental results of the Young Modulus obtained from inset in Fig. 5 and calculated values from Eq. (2).

Porosity Young's Modulus E, (Pa). 65% 70% 75% Spacer NaCl (Spacer)

Experiment 3.10E5 2.25E5 1.62E5

Theory 9.54E6 8.4E6 7.44E6 AASCIT Journal of Materials 2015; 1(2): 22-30 28

Table 3. Bending strength comparison of two fabricated samples via NaCl and carbamide spacers with different porosities.

65% 70% 75% Porosity Spacer Spacer Bending strength (MPa) (MPa)strength(MPa) Carbamide 15 11 6 NaCl 17 12 5

Fig 8. Electrical conductivity variations versus porosity sizes; experimental values and values from theory formula; (r=-1.85) for NaCl and carbamide replica.

Fig 9. The electrical conductivity variation of the porous samples fabricated via different size of carbamaide replica the porosity was 65%, σs =3.5×107(Ω.m) -1.

3.3. Electrical Conductivity −r ≈ d  R R s   (4) The electrical conductivity of the foams was measured ds  using two-probe resistivity measurement method. The probes ρ to be fixed on the specimen they were glued by silver paste Where, ,σ , R, V and I are the electrical resistivity and on the surface at accurate distances, (20mm). Electrical conductivity, resistance, potential difference, electrical current current and voltage measured across the points. Electrical ρ σ across the foam, respectively. s , s Rs, Vs and is electrical conductivity was then computed using the equations: resistivity and conductivity, resistance, potential difference V= IR.ρ = RAL . / and electrical current across the standard precursor material, respectively. A is cross-sectional area, L is distance between V= IR.ρ = RAL . / two probes on the specimen, i.e. the distance between two s ss s s measuring probes. Tentatively, the parameter r varies from ρ= σ = ρ (3) (RAs /)(/) L V V s 1/ 1.60 to 1.85 in Eq. 4 based on the variations in porosity [21]. Summaries of the Eqs.1-4 yield to a tentative relation between Finally, tentative formulas were introduced [21]. physical and mechanical properties of the foam. The equation proves a close dependency between the thermal, electrical 29 G. Kavei: Mechanical Properties of Aluminum Foam Fabricated by Aluminum Powders with Na or Carbamide Replica

parameters and porosity. Fig. 8 is an illustration of the Open-Pore Microcellular aluminum by Replication Processing. experimental and calculated values of σ for the foams Advanced Structural and Functional Materials Design produced by NaCl and carbamide replica with different Proceedings, 512, (2006), 281–288. porosities of 65, 70 and 75%. The electrical conductivity of [8] Simančík F, Jerz J, Kováčik J and Minár P. Aluminum foam – a solid Al is 3.5×10 7(Ω.m) -1. The experiments and calculations new lightweight structural material. Kovovémateriály, Roč.35, by taking into the account of r =-1.85 [21] reveal that σ is č. 4, (1997), 65–277. obtained by experiment 7 order lower than theory. The shift [9] Nieh T G, Higashi Kand Wadsworth J, Effect of cell between experimental values ascribed to impurities and morphology on the compressive properties of open-cell oxidization of the Al powder, which drastically reduce aluminum foams. Mater Sci. Eng. A, 283, no.(1-2), (2000), electrical conductivity. Fig. 9 illustrates the electrical 105–110. conductivity variation of the 65% porosity samples fabricated via different size of Carbamaide tablets. [10] Kenesei P, Kadar Cs, RajkovitsZs and Lendvai J The influence of cell-size distribution on the plastic deformation in metal foams. Scr. Mater., 50, (2004), 295–300. 4. Conclusion [11] Hall I W, Guden M and Yu C J Crusing of aluminum closed cell Powders mixture of NaCl/Al or carbamide/Al was selected foams: density and strain rate effects. Scr. Mater., 43, (2000), 515–521. to fabricate Al foam. The produced Al foams have been studied over the range of relative densities of 0.35 to 0.25 i.e. [12] Hodge A M and Dunand D, Measurement and modeling of 65% to 75% of porosity. The impact energy absorption, creep in open-cell NiAl foams. Metall. Mater. Trans. A, 34, increase on the peak stress and elastic modulus of aluminum (2003), 2353–2363. foams were studied. In particular, among the foams made [13] Markaki A E and Clyne T W The Effect of Cell Wall using two kinds of replica particle, the NaCl powder showed Microstructure on the Deformation and Fracture of significant effect on the energy absorption properties while; aluminum-Based Foams. Acta Mater., 49, (2001), 1677–1686. the carbamide/Al showed a moderate effect; alternatively it [14] Ma X, Peyton A J and Zhao Y Y Measurement of the electrical showed a minor impact energy absorption enhancement. The conductivity of open-celled aluminum foam using non-contact aluminum foam showed obvious strain rate and sensitivity and eddy current techniques. NDT&E International, 38, (2005), exhibited much higher stress and elastic modulus. Therefore, 359–367. it demonstrated much higher energy absorption during [15] Meessen, J H "Urea" Ullmann's Encyclopedia of Industrial compression. Computational work based on electrical Chemistry (2010) doi: 10.1002/14356007.a27_333 conductivity measurement has been proposed for evaluating the mechanical properties of the foams. The method is [16] Kavei G and Ahmadi K. Fabrication of thermoelectric foam and evaluating associated properties. Special Topics & effective for property considerations, in particular of Al Reviews in Porous Media – An International Journal, 3 no. 2, foams. (2012), 105–113.

[17] Brothers A H, Prine D W and Dunand D C. Acoustic emissions References analysis of damage in amorphous and crystalline metal foams. Intermetallics, 14, (2006), 857–865. [1] Sun D and Zhao Y. Static and dynamic energy absorption of Al foams produced by the sintering and dissolution process. Metal. [18] Lide D R, CRC Handbook of Chemistry and Physics, 88th Ed., Mater. Trans. B, Vol. 34, (2003), 69–74. Boca Raton, FL: CRC, (2007). [2] Goodall R and Despois JF, Marmottant A, Salvo L, Mortensen [19] Andrew E, Sanders W and Gibson L J. Compressive and A. The effect of preform Tensile Behavior of aluminum Foams. Mat. Sci. and Engineering, 270, (1999), 113-124. [3] processing on replicated aluminum foam structure and mechanical properties. Scripta Mater. Vol. 54, (2006), [20] 19.Lemlich R. Foam flotation, theory, and applications. Journal 2069–2073. of Colloid and Interface Science, 1, no. 98, (1984), 291-291.

[4] Goodall R, Marmottant A, Salvo L and Mortensen A. Spherical [21] Jiang B, Zhao N Q, Shi C S and Li J J. Processing of open cell pore replicated microcellular aluminum: Processing and aluminum foams with tailored porous morphology, Scripta influence on properties. Mater. Sci. Eng. A, vol. 465, (2007), Materialia, 53, (2005), 781–785. 124–135. [22] Ashby M F, Evans A G, Fleck N A, Gibson L J, Hutchinson J W [5] Cantat I, Cohen-Addad S, Elias F, Graner F, Hohler R, Pitois O, and Wadley H N G, Metal Foams: A Design Guide, Rouyer F, Saint-Jalmes A, Flatman R, Cox S, Foams: Structure Butterworth Hinemann, An Imprint of Elsevier, Oxford, and Dynamics, Oxford University Press, (2013) (2000). [6] Byakova A, Bezim’yanny Y, Gnyloskurenko S, Nakamura T, [23] Koza E, Leonowicz M, Wojciechowski S and Simancik F Fabrication Method for Closed-cell Aluminum Foam with Compressive Strength of aluminum Foams. Materials Letters, Improved Sound Absorption Ability, Procedia Materials 58, (2003), 32-135. Science Vol.4, (2014), 13–18. [24] Yamada Y, Shimojima K, Sakaguchi Y, Mabuchi M, Nakamura [7] Despois J F, Marmottant A, Conde Y, Goodall R, Salvo L, San M, Asahina T. Processing of cellular magnesium materials. Marchi C and Mortensen A. Microstructural Tailoring of Adv. Eng. Mater., 2, (2000), 184-187. AASCIT Journal of Materials 2015; 1(2): 22-30 30

[25] Simone A E and Gibson L J. The Effects of Cell Face [27] http://catalog.isotechinc.com/Asset/foamed-aluminum.pdf Curvature and Corrugations on the Stiffness and Strength of Metallic Foams. Acta Mater., 46, (1997), 3929-3935. [26] Yi F, Zheng H, Zhu Z and F Zu F The Microstructure and Electrical Conductivity of aluminum Alloy Foams. Materials Chemistry and Physics, 78, (2002), 196-201.