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Fabrication of aluminum alloy foams by using alternative thickening agents via melt route
Journal of Alloys and Compounds 698 (2017) 1009e1017
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Review
Fabrication of aluminum alloy foams by using alternative thickening agents via melt route lez Nava a, e, *, Alejandro Cruz-Ramírez a, e, Marlenne Gonza rrez-Pe rez c, Miguel Angel Suarez Rosales b, c, Víctor Hugo Gutie d , e lica Sa nchez-Martínez Ange Departamento de Ingeniería en Metalurgia y Materiales, Instituto Polit ecnico Nacional - Escuela Superior de Ingeniería Química e Industrias Extractivas (ESIQIE), UPALM, 07051, Ciudad de M exico, Mexico Instituto de Investigaciones en Materiales, UNAM, Circuito Exterior s/n. Cd. Universitaria, M exico D.F., 04510, Mexico c Departamento de Ingeniería Metalúrgica, UPIIZ- Instituto Polit ecnico Nacional. Blvd. del Bote S/N, Cerro del Gato, Ejido la Escondida, Col. Ciudad Administrativa, C.P. 98160, Zacatecas, Zac, Mexico d Departamento de Ingeniería en Metalurgia y Materiales, Instituto Polit ecnico Nacional - Escuela Superior de Ingeniería Química e Industrias Extractivas (ESIQIE) UPALM, 07738, M exico D.F., Mexico e Departamento de Ingeniería en Metalurgia y Materiales. ESIQIE-Instituto Polit ecnico Nacional, Ciudad de M exico 07051, Mexico a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 September 2016 Received in revised form 1 December 2016 Accepted 14 December 2016 Available online 18 December 2016
In our current work, closed-cell A356 aluminum alloy foams were successfully fabricated by using 0.8, 1.0 and 1.2 wt% of barite and wollastonite obtained from the primary minerals processing as thickening agents and calcium carbonate as a foaming agent. The microstructure and mechanical properties of the foams manufactured were analyzed and compared with foams produced with 0.8, 1.0 and 1.2 wt % of alumina and calcium carbonate as thickening and blowing agents, respectively. Foams produced with wollastonite additions showed the highest porosity percent values (86.9%) and cell sizes (0.438 mm) but the lowest energy absorption capacity of (1.58 MJm3). On the other hand, the foams produced with barite additions exhibit a good combination of structure and mechanical properties such as cell size of (0.312 mm), porosity percent (85%) and energy absorption capacity (3.81 MJm3). Foams obtained with alumina additions showed intermediate values of porosities and cell sizes between the foams obtained with barite and wollastonite and the presence of oscillations in the plateau region. The foam obtained with 1 wt % barite showed the highest mean ideality energy absorption efficiency and the longest sustained stage with the smallest fluctuation of the efficiency. The energy absorption efficiency of the foams produced decreases when the porosity and cell size increase. Therefore, the foam produced with barite addition show the best behavior of the energy absorption efficiency, which contributes as a potential thickening agent to the application in engineering field. © 2016 Elsevier B.V. All rights reserved.
Keywords: Metallic foams Thickening agent Barite Wollastonite Energy absorption efficiency
Contents 1. 2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1010 Experimental procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1010 2.1. Raw material characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1010 2.2. Fabrication of A356 aluminum alloy foams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1011 2.3. Foam characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1011 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1011 3.1. Characterization of thickening agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1011
cnico Nacional - Escuela Superior de Ingeniería Química e Industrias * Corresponding author. Departamento de Ingeniería en Metalurgia y Materiales, Instituto Polite xico, Mexico. Extractivas (ESIQIE), UPALM, 07051, Ciudad de Me Suarez Rosales), metalurgico2@ lez Nava), [email protected] (A. Cruz-Ramírez), [email protected] (M.A. E-mail addresses: [email protected] (M. Gonza rrez-Pe rez), [email protected] (A. Sa nchez-Martínez). hotmail.com (V.H. Gutie http://dx.doi.org/10.1016/j.jallcom.2016.12.170 0925-8388/© 2016 Elsevier B.V. All rights reserved.
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4.
3.1.1. X-ray diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. SEM characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3. Thermal gravimetrical analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Structure properties of the A356 aluminum alloys foams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Compressive behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Nowadays, the interest for light materials has been increasing due to the demand of the transportation industry, where lighteweight materials with good mechanical properties, comfortable and safe are needed. Therefore, new kinds of light-weight structural materials and functional materials, such as porous metals, are actively developed for these purposes. Metallic foams exhibit a low relative density combined with a high potential for absorbing impact energy, but its quality depend on the kind of production process, matrix metal, raw materials, density and morphology, size and distribution cell of the foams. Currently, there are two ways to directly foaming metallic melts by melt route. These are: the injection of gas into the molten metal and the addition of a foaming agent which decomposes releasing gas into the liquid metal. Liquid metallic foams are systems which go through a series of transient states and change their morphology constantly throughout their lifetime so its production is a quite a challenge to materials scientists due to their difficult manufacturing; they are hardly ever stable [1]. The foam stability depends mainly of the tendency for the liquid films to drain and become thinner, and their tendency to rupture as a result of random disturbances. Owing to their high interfacial area and surface free energy, all foams are unstable in the thermodynamic sense [2]. Deficiencies during production of metal foams can cause flow, drainage, rupture and coarsening and thus produce foams with low quality. In order to create a uniform distribution of cells, bubbles should be retained within a melt until the foam can be solidified, which generally means a decreasing of the bubbles rise velocity [3]. Aluminum alloys foaming via melt route by using thickening and foaming agents (CaCO3) is one of the few successfully commercialized techniques. Silicon carbide, aluminum oxide or other ceramic particles can be used to allow the foaming of aluminum alloys [1]. It has been reported for the Alporas process the addition of Ca or Mg in the range from 1 to 1.5 wt % to increase the melt viscosity [4,5]. Weigand [6] carried out a quantitative study varying the oxide content in the foaming powder used. It was found that an amount within the range between 0.2 and 0.8 wt % Al2O3 the foams expand to a maximum value, while oxide contents below and above this range lead to lower expansions. The effect of particles can be characterized by their reaction with the melt, their wetting behavior and their distribution in the melt (network formation, clustering and segregation). Aside from the particle concentration, recent investigations showed that the composition of the melt and of the particles also influence foam stability [7]. It has been shown that melts without a solid phase are not foamable [1]. Metal matrix composite melts also behave as colloids [8] and the factors which contribute most of the overall nature of a colloidal system are: Particle size, particle shape and flexibility, surface properties and particleesolvent interactions [2]. It is clear from fundamental considerations that a particle attaches to an
1011 1011 1012 1013 1013 1016 1016 1016
interface between a liquid and gas if it is partially wetted by liquid. Very small particles can become so tightly bound to interfaces that adsorption is irreversible. Alumina is stable in liquid aluminum matrix; however it becomes unstable in presence of magnesium and forms either Al2MgO4 (spinel) or a thin passive MgO layer that prohibits further reactions [8,9]. In addition, the Liquid Al-SiC system is reactive [9] and could form aluminum carbide which is detrimental to the foam properties due to its high hardness and brittleness. Mass foam production is still too expensive and there seems to be still some potential for an improvement of properties by optimizing foaming processes and materials selection. Therefore, the aim of this work, it is to evaluate the foamability of the A356 aluminum alloy by using alternative thickening agents, these are barite (BaSO4) and wollastonite (CaSiO3) compared with the widely used alumina (Al2O3). Foams were produced by adding CaCO3 as a foaming agent to 700 C. The morphology, structure and mechanical properties of the foams produced were determined by microscopy techniques and compression tests. 2. Experimental procedure 2.1. Raw material characterization Table 1 shows the composition, the mean size and purity of thickening and foaming agents used in this work. The thickening agents used were obtained from primary Mexican ore production xico has important barite and wollastonite ore deposits. where Me Barite is the mineralogical name for barium sulfate. However, in commerce, the mineral is sometimes referred to as baryte. Barite powders are chemically inert, easily dispersed, low abrasion and excellent resistance to heat and corrosion and have low oil absorption. Wollastonite is a natural origin mineral ecologically valuable, some properties are: low values of humidity and oil absorption and reduced volatiles content. Wollastonite is mainly used in ceramic products under friction (such as brakes and clutches), steels, plastics and paint fillers. In the metallurgical industry, wollastonite is added in a mixture of fluxes, in order to keep the surface defects prevent oxidation of the steel, lubricate the mold walls and absorb harmful inclusions. The size, morphology and a qualitative chemical analysis of the barite and wollastonite were determined with the SEM Jeol 6300 and with the energy dispersive spectra (EDS) analysis. An Au-Pd film was deposited on the surface of the powders to make them Table 1 Raw materials used in this work. Agents
Purity (%)
Particle mean size (mm)
Alumina (Al2O3) Barite (BaSO4) Wollastonite (CaSiO3) Calcite (CaCO3)
99 96 98 98.5
0.3 45 12 14
lez Nava et al. / Journal of Alloys and Compounds 698 (2017) 1009e1017 M. Gonza
conductive. Images were obtained at 20 and 500 X with backscattering electrons with 15 kV and 10 A. The barite and wollastonite were analyzed in an X-Ray Bruker D8 Focus with monochromatic Cu Ka radiation working in q/2q configuration. Data were collected in an angular range from 20 to 70 with a step size of 0.02 and a counting time of 2 min1. The thermal behavior of the thickening agents was determined in a thermal gravimetric analyzer Mettler Toledo TGA/DSC1 Instrument. A heating rate of 10 C min1 was used with an argon flow rate of 50 ml min1 in alumina crucibles. Data were collected for a temperature range from 150 to 1600 C.
2.2. Fabrication of A356 aluminum alloy foams A master A356 alloy was manufactured by conventional melting in a gas furnace at 750 C from pure metals. The following chemical composition was obtained by Atomic Emission Spectrometry for the A356 aluminum alloy (92.3 wt% Al, 7.12 wt% Si, 0.38 wt% Mg, and 0.2 wt% Cu). 500 g of the master alloy were set in a bipartite stainless steel crucible (24.89 cm in diameter and 38.1 cm in height). The alloy was melted and kept at 700 C in an electric furnace under atmospheric pressure. The heating system was an electrical furnace enabled with control of temperature to within ±3 C of the set values. The temperature was measured with a Ktype thermocouple. The experiments were carried out using a stircaster system with a stainless steel paddle axle. The viscosity of the melt was modified by adding 0.8, 1 and 1.2 wt % of the thickening agents (Al2O3, BaSO4, CaSiO3) of the mass charge at a constant stirring speed of 1600 rpm for 2 min. 1 wt% of CaCO3 of the mass charge was added as a foaming agent into the melt at a stirring speed of 1600 rpm for 100 s. After the foaming agent addition, the melt was kept in the furnace at the holding temperature of 700 C for 2 min to allow the foam formation. The cooling procedure was carried out as soon as the expansion process took place. The crucible containing the melt was removed from furnace and the crucible was cooled by sprayed water.
3. Results and discussion 3.1. Characterization of thickening agents 3.1.1. X-ray diffraction The proposed thickening agents were characterized by X-ray diffraction and the results are shown in Fig. 1. Fig. 1a shows that the main component of the barite sample is the Celestine-barian compound (JCPD file 00-039-1467), which is a mineral that belongs to the barite group. In this case, barite (BaSO4) and Celestine (SrSO4) form a solid solution with the following composition (Ba0.25Sr0.75SO4). From the same XRD pattern, the calcite (CaCO3) compound was also detected. Fig. 1b shows the XRD pattern of the wollastonite sample. The diffractogram is mainly formed by the CaSiO3 compound (JCPD file 00-043-1460), calcite (CaCO3) and small amounts of Al2O3, MgO and K2O compounds. 3.1.2. SEM characterization The morphology and the elemental mapping of elements of the barite and wollastonite particles used as thickening agents are shown in Figs. 2 and 3, respectively. From Fig. 2a, it can be observed that the particles of the barite agent show subhedral and anhedral morphologies. The elemental mapping analysis showed the presence of particles constituted by Ba, Sr and S and also particles constituted by Ca, Fig. 2 bee. According with the XRD results these particles may correspond to Celestine (Ba,Sr)SO4 and calcite (CaCO3) compounds. Fig. 3 aee shows the morphology and the elemental mapping of
2.3. Foam characterization The fabricated A356 aluminum foams were cut on the cross section in order to obtain samples to evaluate density, cell structure and compression resistance. Densities of the foams were deduced from weights and volumes of the foams and the relative density was defined by the ratio r/rs (where r and rs correspond to the foam and solid densities, respectively). The porosity (%) was obtained by using Eq. (1). Pr (%) ¼ 1 (rfoam/rA356
alloy)
100
(1)
where P is the porosity, rfoam is the foam density and rA356 ¼ 2.56 gcm3. The cell structure was observed by optical microscopy and the image analyzer Carnoy [10]. Cylindrical compression samples were obtained from metallic foams of 19 mm in diameter and 38 mm in height, according to the DIN 50134 Standard. Uniaxial compression tests were performed using a universal testing machine (Shimadzu 100 kN/10 ft capacity). All tests were applied under displacement control with a constant cross-head speed of 0.5 mm min1 (with strain rate of 2 102 s1). Stress-strain was deduced from the recorded loaddisplacement data which was recorded using a data acquisition unit and a computer. The stress-strain data are reported in terms of engineering stress and strain. The first peak of the deformation curve was used as yield strength (defined as high yield point stress).
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alloy
Fig. 1. XRD patterns of the a) barite and b) wollastonite ore samples.
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Fig. 2. SEM Image of morphology of barite (a) and SEM e microanalysis of Barium (b), Strontium (c), Sulfur (d) and Calcium (e) were detected.
Fig. 3. SEM Image of morphology of wollastonite (a) and SEM e microanalysis of Silicon (b), Calcium (c), Oxygen (d) and Magnesium (e) were detected.
elements of the wollastonite particles, respectively. The particles show typical tabular morphology constituted by Si, Ca and O, which are homogeneously distributed in the sample. Based on the XRD results, the main components in the sample correspond to Wollastonite (CaSiO3) and calcite (CaCO3). Small amounts of magnesium were also detected as impurities, which may correspond to magnesia (MgO). The analyzed ore samples showed that barite and wollastonite contain impurities, which may influence the thickening and foaming process. 3.1.3. Thermal gravimetrical analysis The thermal gravimetrical analysis of barite and wollastonite samples is shown in Fig. 4. A similar behavior for both samples in the mass loss is observed in the temperature range from 600 to 800 C. The TGA curve for barite shows three stages, the first stage
starts at 73.5 C and ends at 230 C; the second stage starts at 653 C and ends to 778 C, while the third stage occurs at 1359.4 C and ends at 1641 C. The mass loss estimated for the three stages is 6, 8 and 54%, respectively. The first stage is likely to be loss of water. The second stage is attributed to the calcium carbonate decomposition where CO2 gas is released based on reaction (2). CaCO3 ¼ CaO þ CO2
(2)
The third stage correspond to the thermal decomposition of the barite, specifically the celestine barian compound, where a high amount of SO2 gas is released, based on reactions (3) and (4). 2 BaSO4 ¼ 2 BaO þ 2 SO2 þ O2
(3)
lez Nava et al. / Journal of Alloys and Compounds 698 (2017) 1009e1017 M. Gonza
Fig. 4. Thermal gravimetrical curve of the barite and wollastonite.
2SrSO4 ¼ 2SrO þ 2SO2 þ O2
(4)
The TGA curve for wollastonite shows a mass loss of 6% in the temperature range from 692 to 761 C, which correspond to the calcium carbonate thermal decomposition. In this work, the foaming process was carried out at 700 C; therefore, reaction (2) will be expected to occur. The main objective of the thickening agents proposed was to increase the viscosity of the surface melt; however, from these results, it can be expected that the calcium carbonate contained in the thickening agents as an impurity may contribute to increase the foamability of the melt, where the barite sample showed a higher mass loss due to reaction (2) than the wollastonite sample.
3.2. Structure properties of the A356 aluminum alloys foams Fig. 5aec shows representative A356 aluminum alloy foams produced with CaCO3 (foaming agent) and alumina (Al2O3), barite (Ba0.25Sr0.75SO4) and wollastonite (CaSiO3) (thickening agents), respectively. As can be observed, the three thickening agents had a good response in the formation of the A356 alloys foams under the proposed experimental conditions. However, barite and wollastonite foams show a higher foam expansion than the alumina foam. The foam produced with barite shows some big bubbles located at the bottom of the foam due to vortices formed by agitation of the liquid metal. In the foam produced with alumina (Al2O3), the metal structure did not present big bubbles, nevertheless, high oxidation
1013
or segregation of alumina particles was observed on the top part of the foam. In general, the foams produced with wollastonite and barite showed a higher homogeneity in its metallic structure. The percentage of porosity Pr (%) of the foams was calculated using Eq. (1). Table 2 summarizes the results of density, relative density, porosity and the average cell size for the foams obtained with different amounts of thickening agents. It is observed that the higher values of porosity and average cell size were obtained for the additions of 0.8 and 1.0 wt% of barite and wollastonite, respectively. However, the differences of these parameters between the additions evaluated are minimal. When the amount of alumina was increased from 1.0 to 1.2 wt%, a decrease in the porosity and cell size was observed. From Table 2, it can be observed that the highest Pr (%) was obtained for the foam produced with the addition of 1.0 wt % of wollastonite (CaSiO3). On the other hand, the highest relative density was obtained in the foam produced with 1.2 wt% of barite (BaSO4) with the smallest pore size. The foams produced with alumina (Al2O3) showed intermediate values (e.g., cell size, Pr (%), relative density) between the thickening agents under study and are in accordance with the work reported by Gnyloskurenko et al. [11]. Fig. 6aec shows the cell structure of the A356 aluminum alloy foams manufactured with alumina and the proposed thickening agents. From these images, it can be observed that the foams cells are essentially spherical and partially closed. The foam produced with 1 wt % CaSiO3 showed the highest pore size (0.438 mm), while the smallest pore size (0.228 mm) was obtained for the foam produced with 1.2 wt % BaSO4. 3.3. Compressive behavior The experimental mechanical properties obtained from the compressive stress-strain curves are summarized in Table 3. It is observed that when the addition of barite and alumina was increased, the Yield Stress and Young's Modulus were increased while the wollastonite addition showed an opposite behavior. It is shown that the Yield Stress increases when the cell size and porosity decreases, similar behavior were reported in a previous study in Mg and Al cellular foams where the Yield Stress possess the tendency of increasing with decreasing porosity [12]. The energy absorption capacity and average Plateau were increased from 0.8 to 1.0 wt % BaSO4 and then decreased for the addition of 1.2 wt % BaSO4. On the other hand, when the wollastonite addition was increased from 0.8 to 1.2 wt % CaSiO3, the energy absorption capacity and average Plateau were decreased. From these results, the highest energy absorption capacities were obtained for the addition of 1.0 wt % of Al2O3 and BaSO4 and for the addition of 0.8 wt% CaSiO3. As expected, there is a close relationship between the foam structure properties (porosity, cell size and density) and their
Fig. 5. Structure of the A356 aluminum alloy foams manufactured to the thickening additions of a) Alumina, b) barite and c) wollastonite using CaCO3 as a foaming agent.
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Table 2 Experimental densities, porosity and cell size of the A356 alloy foams. Sample
Amount (wt %)
Density r* (g cm3)
Relative density r*/rA356
Alumina
0.8 1.0 1.2 0.8 1.0 1.2 0.8 1.0 1.2
0.364 0.383 0.397 0.379 0.415 0.548 0.359 0.335 0.384
0.142 0.151 0.155 0.148 0.178 0.214 0.140 0.137 0.150
Barite
Wollastonite
alloy
Pr (%)
Average cell size (mm)
85.77 85.02 84.48 85.18 83.77 78.57 85.97 86.90 84.98
0.415 0.380 0.343 0.351 0.312 0.228 0.336 0.438 0.412
Fig. 6. Optical-micrographs of A356 aluminum alloy foams manufactured to the thickening additions of a) alumina, b) barite and c) wollastonite using CaCO3 as a foaming agent.
Table 3 Mechanical properties and energy absorption capacity of the A356 aluminum alloy foams. Sample
Amount (wt %)
Yield stress sy* (MPa)
Young's modulus E (GPa)
Average plateau stress spl (MPa)
Energy absorption W (MJ m3)
Alumina
0.8 1.0 1.2 0.8 1.0 1.2 0.8 1.0 1.2
2.13 2.51 4.48 2.33 4.20 4.93 1.58 1.36 0.92
0.34 0.23 0.98 0.58 0.98 0.91 0.71 0.54 0.66
6.58 18.09 5.12 3.31 5.88 3.76 3.2 2.80 2.17
6.63 7.67 0.54 1.98 3.81 3.03 1.58 1.40 0.92
Barite
Wollastonite
mechanical properties. In spite of the similar structure properties of the foam produced, there is a remarkable difference on the mechanical behavior of the foams produced with an alumina addition. It has been reported [9] that the interaction of the alumina with the foaming agent and the melt produce compounds with high hardness and brittle behavior that increase the strength and brittleness of the foam. Fig. 7aec shows the stress e strain compression curves for A356 aluminum alloy foams manufactured with alumina and the proposed thickening agents. Typical stress e strain curves of closedcell metallic foams were obtained, which involves three distinct regimes under compressive loading i.e., elastic stage, followed by a plateau stage that occurs after yielding and finally a densification region where the stress increases drastically. Two mechanism of deformation has been identified for Al-based foams at plateau stage, these are cell wall buckling for ductile Albased foams and cell wall buckling/tearing for Al-based brittle alloys [12]. Fig. 7b and c shows a smooth behavior in the plateau regions for the foams obtained with the addition of barite and wollastonite, which is associated to a ductile mechanism and the failure of ductile foam, is controlled by bending of cell walls [13]. The opposite behavior was observed for the foam obtained with the addition of alumina (Fig. 7a); these curves show the presence of oscillations or serrations in the plateau region related to a brittle
mechanism of deformation. It has been identified that each oscillation is due to localization of deformation in a band and successive crushing of deformation bands leading to an unsmoothed plateau appearance [12]. Slight oscillations or serrations typically are associated with brittle failure of the cell walls [14] due to low ductile eutectic domains and brittle foreign particles attached to the cell wall caused by the addition of thickening and foaming agents. It is desirable in practical applications that foams show a uniform collapse with a wide plateau regime. From these results, it is evident that the deformation behavior of the foams produced with the additions of barite are more attractive than the foams obtained with additions of alumina or wollastonite. As an energy-absorbing material, aluminum foams have the characteristics that higher strain could be obtained at lower stress levels. Therefore, the energy absorption capacity (W), ideality energy absorption efficiency (I) and energy absorption efficiency (E) are three important aspects to evaluate in the metallic foams [12,15]. These parameters were evaluated as follows.
Zε W¼ 0
s dε
(5)
lez Nava et al. / Journal of Alloys and Compounds 698 (2017) 1009e1017 M. Gonza
E¼
1
sm
Zεm
s dε
1015
(6)
0
1 I¼ sm εm
Zεm
s dε
(7)
0
where W is the energy absorption capacity (also defined as the total kinetic energy absorbed by the foam during the compression test) prior to onset of densification, E is the energy absorption efficiency, I is the ideality energy absorption efficiency and sm is the stress where the strain is εm. Fig. 8 shows the energy absorption capacity of A356 aluminum alloy foams obtained for the addition of 1 wt % of the thickening agents evaluated, calculated according to Eq. (5). It is observed that the energy absorption capacity increases along with strain increasing for the three samples. The sample obtained with the alumina addition shows the highest energy absorption capacity while the sample obtained with the wollastonite addition shows the lowest. The energy absorption capacity for the foams fabricated in this work is 7.67, 3.81 and 1.4 MJm3, respectively when the strain is 0.6, 0.65 and 0.5. There is a remarkable difference of the energy absorption capacity for the foams obtained with different thickening agents. The explanation is attributed to the different thickness of the cell wall. If it is considered a similar relative density of the foams produced then, a larger cell diameter will result in thicker cell walls, and the loading capability of unit area is stronger, so energy absorbed is greater for a determined strain. The other important parameter of energy absorption of closedcell aluminum foam is energy absorption efficiency, which was determined according with Eq. (6). The results of the ideality energy absorption efficiency for the foams produced with 1 wt % of the thickening agents used are shown in Fig. 9. The curves are comprised of three main stages: (I) Fast rising stage, where the energy absorption efficiency increases until high efficiency point as strain increasing, (II) sustained stage; where the energy absorption efficiency mainly keep, but some fluctuations occurred as strain changing and (III) attenuating stage, where the energy absorption efficiency decreases with the strain increasing.
Fig. 7. Compressive stress-strain curves of A356 aluminum alloy foams to different additions of a) Al2O3, b) BaSO4 and c) CaSiO3.
Fig. 8. Energy absorption capacity of A356 aluminum alloy foams fabricated with different thickening agents.
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0.40, 0.46 and 0.36 which correspond to the strains of 0.46, 0.61 and 0.6 for the samples manufactured with the additions of alumina, barite and wollastonite, respectively, it can be concluded that the sample with addition of barite possess the highest energy absorption efficiency and according to the similar results reported in previous studies, generally the high energy absorption efficiency is obtained when the plateau region is horizontal and no peaks occurs [13,15], It can be seen that the energy absorption efficiency increases when the cell size average and porosity decreases. 4. Conclusions
Fig. 9. Ideality energy absorption efficiency of the A356 aluminum alloy foams fabricated with different thickening agents.
The mean ideality energy absorption efficiency in the sustained stage are 0.6, 0.75 and 0.74 for samples with addition of alumina, barite and wollastonite, respectively. All samples show fluctuations which are attributed to the compression process instability [15]. However this behavior is noticeable in the sample obtained with alumina addition [9]. The sample obtained with wollastonite addition show the highest efficiency point, and then a short sustained stage, followed by a fast decrease of the ideality energy absorption efficiency as the strain is increased in the sustained stage. The sample obtained with barite addition show the highest mean ideality energy absorption efficiency and the longest sustained stage with the smallest fluctuation of the efficiency. Therefore, the foam produced with barite addition show the best behavior of the energy absorption efficiency, which contributes as a potential thickening agent to the application in engineering field. The relation between energy absorption efficiency and strain is shown in Fig. 10 for 1 wt % of the thickening agents used. In all cases the energy absorption efficiency increases with increasing of strain and then decreases. The highest energy absorption efficiency are
Closed-cell A356 aluminum alloy foams were fabricated using 0.8, 1.0 and 1.2 wt % of alumina, barite and wollastonite as thickening agents and CaCO3 as a foaming agent. It was determined that barite sample correspond to the compound Celestine-barian (Ba0.25Sr0.75SO4). Barite and wollastonite samples were obtained by simple beneficiation methods of primary minerals where they contain calcium carbonate as an impurity that may contribute to the foaming process. A minimal difference on the microstructure and mechanical properties of the foams manufactured with barite and wollastonite was observed for the additions evaluated. The foam manufactured with 1 wt % barite shows a good combination of structure and mechanical properties such as cell size of (0.312 mm), porosity percent (83.7%) and energy absorption capacity (3.81 MJm3), while the foam produced with 1.0 wt % wollastonite shows the highest porosity percent (86.9%) and cell size (0.438 mm) but the lowest energy absorption capacity of (1.4 MJm3). The foam obtained with 1 wt% alumina showed intermediate values of porosity and cell size between the foams obtained for the same addition of barite and wollastonite. Both foams produced with the alternative thickening agents showed a smooth behavior in the Plateau region which contrast with the presence of oscillations or serrations showed by the foam obtained by alumina addition. The foam obtained with 1 wt % barite addition show the highest mean ideal energy absorption efficiency and the longest sustained stage with the smallest fluctuation of the efficiency. Therefore, the use of Barite shows an attractive potential as a thickening agent in the foaming process via melt route. Acknowledgements The authors wish to thank the Institutions CONACyT, SNI, COFAA cnico Nacional for their permanent assisand SIP-Instituto Polite tance to the Process Metallurgy Group at ESIQIE-Metallurgy and Materials Department. References
Fig. 10. Energy absorption efficiency of the A356 aluminum alloy foams fabricated with different thickening agents.
[1] John Banhart, Metal foams: production and stability, Adv. Eng. Mater. 8 (9) (2006) 781e794. [2] Duncan J. Shaw, Colloid & Surface Chemistry, fourth ed., Elsevier Science Ltd, Butterworth Heinemann, 1992, p. 276. [3] D.C. Curran, Aluminum Foam Using Calcium Carbonate as Foaming Agent, St. John’s Collage, Ph D. Thesis, University of Cambridge, Department of Materials Science of Metallurgy, 2003. rrez, J. Oron ~ o, Aluminium foams. Manufacture, properties and appli[4] J. Gutie cations, Rev. Metal. 5 (44) (2008) 457e476. [5] S. Gnyloskurenko, A. Byakova, A. Sirko, A. Dudnyk, Y. Milmund, T. Nakamura, Advance structure and deformation pattern of Al based alloys formed with calcium carbonate agent, Met Foam, Porous Metals Metal. Foams (2007) 399e402. [6] P. Weigand, Untersuchungder Einflubfaktoren auf die pulvermetallurgische Hertellung von Aluminiumschumen, Thesis RWTH Aachen, MIT-Verlag Bremen, 1999. [7] N. Babcs an, D. Leitlmeier, H.P. Degischer, J. Banhart, The role of oxidation in blowing particle-stabilized aluminum foams, Adv. Eng. Mater. 6 (6) (2001) 421e427. [8] N. Babcsan, Ceramic Particles Stabilized Aluminum Foams, PhD. Dissertation, University of Miskolc, 2003.
lez Nava et al. / Journal of Alloys and Compounds 698 (2017) 1009e1017 M. Gonza [9] V.M. Bermudez, Auger and electron energy-loss study of the Al/SiC interface, Appl. Phys. Lett. 42 (1983) 70e72. [10] Carnoy 2.0: Analysis Software for LM, SEM and TEM Images, Schools, P.&E. Smets Leuven, 2001. http://www.carnoy.org. [11] S. Gnyloskurenko, T. Koizumi, K. Kita, T. Nakamura, Aluminum metallic foams made by carbonate foaming agents, Resour. Process. 60 (2013) 5e12. [12] A. Heydari Astaraie, H.R. Shahverdi, S.H. Elahi, Compressive behavior of Zn22Al closed-cell foams under uniaxial quasi-static loading, Trans. NonFerrous Met. Soc. China 24 (2014) 162e169.
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[13] Jiaan Liu, Qingxiang Qu, Yan Liu, Rongguang Li, Bing Liu, Compressive properties of Al-Si-SiC composite foams at elevated temperatures, J. Alloys Compd. 676 (2016) 239e244. [14] Ashby MF. LJ, Cellular solids: structure and properties, in: D.R. Clarke, S. Suresh, I.M. Ward (Eds.), Cambridge Solid State Science Series, Cambridge University Press, Cambridge (UK), 1997. [15] X.C. Xia, X.W. Chen, Z. Zhang, X. Chen, W.M. Zhao, B. Liao, B. Hur, Effects of porosity and pore size on the compressive properties of closed-cell Mg alloy foam, J. Magnesium Alloys 1 (2013) 330e335.
Contents lists available at ScienceDirect
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Review
Fabrication of aluminum alloy foams by using alternative thickening agents via melt route lez Nava a, e, *, Alejandro Cruz-Ramírez a, e, Marlenne Gonza rrez-Pe rez c, Miguel Angel Suarez Rosales b, c, Víctor Hugo Gutie d , e lica Sa nchez-Martínez Ange Departamento de Ingeniería en Metalurgia y Materiales, Instituto Polit ecnico Nacional - Escuela Superior de Ingeniería Química e Industrias Extractivas (ESIQIE), UPALM, 07051, Ciudad de M exico, Mexico Instituto de Investigaciones en Materiales, UNAM, Circuito Exterior s/n. Cd. Universitaria, M exico D.F., 04510, Mexico c Departamento de Ingeniería Metalúrgica, UPIIZ- Instituto Polit ecnico Nacional. Blvd. del Bote S/N, Cerro del Gato, Ejido la Escondida, Col. Ciudad Administrativa, C.P. 98160, Zacatecas, Zac, Mexico d Departamento de Ingeniería en Metalurgia y Materiales, Instituto Polit ecnico Nacional - Escuela Superior de Ingeniería Química e Industrias Extractivas (ESIQIE) UPALM, 07738, M exico D.F., Mexico e Departamento de Ingeniería en Metalurgia y Materiales. ESIQIE-Instituto Polit ecnico Nacional, Ciudad de M exico 07051, Mexico a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 September 2016 Received in revised form 1 December 2016 Accepted 14 December 2016 Available online 18 December 2016
In our current work, closed-cell A356 aluminum alloy foams were successfully fabricated by using 0.8, 1.0 and 1.2 wt% of barite and wollastonite obtained from the primary minerals processing as thickening agents and calcium carbonate as a foaming agent. The microstructure and mechanical properties of the foams manufactured were analyzed and compared with foams produced with 0.8, 1.0 and 1.2 wt % of alumina and calcium carbonate as thickening and blowing agents, respectively. Foams produced with wollastonite additions showed the highest porosity percent values (86.9%) and cell sizes (0.438 mm) but the lowest energy absorption capacity of (1.58 MJm3). On the other hand, the foams produced with barite additions exhibit a good combination of structure and mechanical properties such as cell size of (0.312 mm), porosity percent (85%) and energy absorption capacity (3.81 MJm3). Foams obtained with alumina additions showed intermediate values of porosities and cell sizes between the foams obtained with barite and wollastonite and the presence of oscillations in the plateau region. The foam obtained with 1 wt % barite showed the highest mean ideality energy absorption efficiency and the longest sustained stage with the smallest fluctuation of the efficiency. The energy absorption efficiency of the foams produced decreases when the porosity and cell size increase. Therefore, the foam produced with barite addition show the best behavior of the energy absorption efficiency, which contributes as a potential thickening agent to the application in engineering field. © 2016 Elsevier B.V. All rights reserved.
Keywords: Metallic foams Thickening agent Barite Wollastonite Energy absorption efficiency
Contents 1. 2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1010 Experimental procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1010 2.1. Raw material characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1010 2.2. Fabrication of A356 aluminum alloy foams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1011 2.3. Foam characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1011 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1011 3.1. Characterization of thickening agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1011
cnico Nacional - Escuela Superior de Ingeniería Química e Industrias * Corresponding author. Departamento de Ingeniería en Metalurgia y Materiales, Instituto Polite xico, Mexico. Extractivas (ESIQIE), UPALM, 07051, Ciudad de Me Suarez Rosales), metalurgico2@ lez Nava), [email protected] (A. Cruz-Ramírez), [email protected] (M.A. E-mail addresses: [email protected] (M. Gonza rrez-Pe rez), [email protected] (A. Sa nchez-Martínez). hotmail.com (V.H. Gutie http://dx.doi.org/10.1016/j.jallcom.2016.12.170 0925-8388/© 2016 Elsevier B.V. All rights reserved.
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4.
3.1.1. X-ray diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. SEM characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3. Thermal gravimetrical analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Structure properties of the A356 aluminum alloys foams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Compressive behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Nowadays, the interest for light materials has been increasing due to the demand of the transportation industry, where lighteweight materials with good mechanical properties, comfortable and safe are needed. Therefore, new kinds of light-weight structural materials and functional materials, such as porous metals, are actively developed for these purposes. Metallic foams exhibit a low relative density combined with a high potential for absorbing impact energy, but its quality depend on the kind of production process, matrix metal, raw materials, density and morphology, size and distribution cell of the foams. Currently, there are two ways to directly foaming metallic melts by melt route. These are: the injection of gas into the molten metal and the addition of a foaming agent which decomposes releasing gas into the liquid metal. Liquid metallic foams are systems which go through a series of transient states and change their morphology constantly throughout their lifetime so its production is a quite a challenge to materials scientists due to their difficult manufacturing; they are hardly ever stable [1]. The foam stability depends mainly of the tendency for the liquid films to drain and become thinner, and their tendency to rupture as a result of random disturbances. Owing to their high interfacial area and surface free energy, all foams are unstable in the thermodynamic sense [2]. Deficiencies during production of metal foams can cause flow, drainage, rupture and coarsening and thus produce foams with low quality. In order to create a uniform distribution of cells, bubbles should be retained within a melt until the foam can be solidified, which generally means a decreasing of the bubbles rise velocity [3]. Aluminum alloys foaming via melt route by using thickening and foaming agents (CaCO3) is one of the few successfully commercialized techniques. Silicon carbide, aluminum oxide or other ceramic particles can be used to allow the foaming of aluminum alloys [1]. It has been reported for the Alporas process the addition of Ca or Mg in the range from 1 to 1.5 wt % to increase the melt viscosity [4,5]. Weigand [6] carried out a quantitative study varying the oxide content in the foaming powder used. It was found that an amount within the range between 0.2 and 0.8 wt % Al2O3 the foams expand to a maximum value, while oxide contents below and above this range lead to lower expansions. The effect of particles can be characterized by their reaction with the melt, their wetting behavior and their distribution in the melt (network formation, clustering and segregation). Aside from the particle concentration, recent investigations showed that the composition of the melt and of the particles also influence foam stability [7]. It has been shown that melts without a solid phase are not foamable [1]. Metal matrix composite melts also behave as colloids [8] and the factors which contribute most of the overall nature of a colloidal system are: Particle size, particle shape and flexibility, surface properties and particleesolvent interactions [2]. It is clear from fundamental considerations that a particle attaches to an
1011 1011 1012 1013 1013 1016 1016 1016
interface between a liquid and gas if it is partially wetted by liquid. Very small particles can become so tightly bound to interfaces that adsorption is irreversible. Alumina is stable in liquid aluminum matrix; however it becomes unstable in presence of magnesium and forms either Al2MgO4 (spinel) or a thin passive MgO layer that prohibits further reactions [8,9]. In addition, the Liquid Al-SiC system is reactive [9] and could form aluminum carbide which is detrimental to the foam properties due to its high hardness and brittleness. Mass foam production is still too expensive and there seems to be still some potential for an improvement of properties by optimizing foaming processes and materials selection. Therefore, the aim of this work, it is to evaluate the foamability of the A356 aluminum alloy by using alternative thickening agents, these are barite (BaSO4) and wollastonite (CaSiO3) compared with the widely used alumina (Al2O3). Foams were produced by adding CaCO3 as a foaming agent to 700 C. The morphology, structure and mechanical properties of the foams produced were determined by microscopy techniques and compression tests. 2. Experimental procedure 2.1. Raw material characterization Table 1 shows the composition, the mean size and purity of thickening and foaming agents used in this work. The thickening agents used were obtained from primary Mexican ore production xico has important barite and wollastonite ore deposits. where Me Barite is the mineralogical name for barium sulfate. However, in commerce, the mineral is sometimes referred to as baryte. Barite powders are chemically inert, easily dispersed, low abrasion and excellent resistance to heat and corrosion and have low oil absorption. Wollastonite is a natural origin mineral ecologically valuable, some properties are: low values of humidity and oil absorption and reduced volatiles content. Wollastonite is mainly used in ceramic products under friction (such as brakes and clutches), steels, plastics and paint fillers. In the metallurgical industry, wollastonite is added in a mixture of fluxes, in order to keep the surface defects prevent oxidation of the steel, lubricate the mold walls and absorb harmful inclusions. The size, morphology and a qualitative chemical analysis of the barite and wollastonite were determined with the SEM Jeol 6300 and with the energy dispersive spectra (EDS) analysis. An Au-Pd film was deposited on the surface of the powders to make them Table 1 Raw materials used in this work. Agents
Purity (%)
Particle mean size (mm)
Alumina (Al2O3) Barite (BaSO4) Wollastonite (CaSiO3) Calcite (CaCO3)
99 96 98 98.5
0.3 45 12 14
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conductive. Images were obtained at 20 and 500 X with backscattering electrons with 15 kV and 10 A. The barite and wollastonite were analyzed in an X-Ray Bruker D8 Focus with monochromatic Cu Ka radiation working in q/2q configuration. Data were collected in an angular range from 20 to 70 with a step size of 0.02 and a counting time of 2 min1. The thermal behavior of the thickening agents was determined in a thermal gravimetric analyzer Mettler Toledo TGA/DSC1 Instrument. A heating rate of 10 C min1 was used with an argon flow rate of 50 ml min1 in alumina crucibles. Data were collected for a temperature range from 150 to 1600 C.
2.2. Fabrication of A356 aluminum alloy foams A master A356 alloy was manufactured by conventional melting in a gas furnace at 750 C from pure metals. The following chemical composition was obtained by Atomic Emission Spectrometry for the A356 aluminum alloy (92.3 wt% Al, 7.12 wt% Si, 0.38 wt% Mg, and 0.2 wt% Cu). 500 g of the master alloy were set in a bipartite stainless steel crucible (24.89 cm in diameter and 38.1 cm in height). The alloy was melted and kept at 700 C in an electric furnace under atmospheric pressure. The heating system was an electrical furnace enabled with control of temperature to within ±3 C of the set values. The temperature was measured with a Ktype thermocouple. The experiments were carried out using a stircaster system with a stainless steel paddle axle. The viscosity of the melt was modified by adding 0.8, 1 and 1.2 wt % of the thickening agents (Al2O3, BaSO4, CaSiO3) of the mass charge at a constant stirring speed of 1600 rpm for 2 min. 1 wt% of CaCO3 of the mass charge was added as a foaming agent into the melt at a stirring speed of 1600 rpm for 100 s. After the foaming agent addition, the melt was kept in the furnace at the holding temperature of 700 C for 2 min to allow the foam formation. The cooling procedure was carried out as soon as the expansion process took place. The crucible containing the melt was removed from furnace and the crucible was cooled by sprayed water.
3. Results and discussion 3.1. Characterization of thickening agents 3.1.1. X-ray diffraction The proposed thickening agents were characterized by X-ray diffraction and the results are shown in Fig. 1. Fig. 1a shows that the main component of the barite sample is the Celestine-barian compound (JCPD file 00-039-1467), which is a mineral that belongs to the barite group. In this case, barite (BaSO4) and Celestine (SrSO4) form a solid solution with the following composition (Ba0.25Sr0.75SO4). From the same XRD pattern, the calcite (CaCO3) compound was also detected. Fig. 1b shows the XRD pattern of the wollastonite sample. The diffractogram is mainly formed by the CaSiO3 compound (JCPD file 00-043-1460), calcite (CaCO3) and small amounts of Al2O3, MgO and K2O compounds. 3.1.2. SEM characterization The morphology and the elemental mapping of elements of the barite and wollastonite particles used as thickening agents are shown in Figs. 2 and 3, respectively. From Fig. 2a, it can be observed that the particles of the barite agent show subhedral and anhedral morphologies. The elemental mapping analysis showed the presence of particles constituted by Ba, Sr and S and also particles constituted by Ca, Fig. 2 bee. According with the XRD results these particles may correspond to Celestine (Ba,Sr)SO4 and calcite (CaCO3) compounds. Fig. 3 aee shows the morphology and the elemental mapping of
2.3. Foam characterization The fabricated A356 aluminum foams were cut on the cross section in order to obtain samples to evaluate density, cell structure and compression resistance. Densities of the foams were deduced from weights and volumes of the foams and the relative density was defined by the ratio r/rs (where r and rs correspond to the foam and solid densities, respectively). The porosity (%) was obtained by using Eq. (1). Pr (%) ¼ 1 (rfoam/rA356
alloy)
100
(1)
where P is the porosity, rfoam is the foam density and rA356 ¼ 2.56 gcm3. The cell structure was observed by optical microscopy and the image analyzer Carnoy [10]. Cylindrical compression samples were obtained from metallic foams of 19 mm in diameter and 38 mm in height, according to the DIN 50134 Standard. Uniaxial compression tests were performed using a universal testing machine (Shimadzu 100 kN/10 ft capacity). All tests were applied under displacement control with a constant cross-head speed of 0.5 mm min1 (with strain rate of 2 102 s1). Stress-strain was deduced from the recorded loaddisplacement data which was recorded using a data acquisition unit and a computer. The stress-strain data are reported in terms of engineering stress and strain. The first peak of the deformation curve was used as yield strength (defined as high yield point stress).
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alloy
Fig. 1. XRD patterns of the a) barite and b) wollastonite ore samples.
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Fig. 2. SEM Image of morphology of barite (a) and SEM e microanalysis of Barium (b), Strontium (c), Sulfur (d) and Calcium (e) were detected.
Fig. 3. SEM Image of morphology of wollastonite (a) and SEM e microanalysis of Silicon (b), Calcium (c), Oxygen (d) and Magnesium (e) were detected.
elements of the wollastonite particles, respectively. The particles show typical tabular morphology constituted by Si, Ca and O, which are homogeneously distributed in the sample. Based on the XRD results, the main components in the sample correspond to Wollastonite (CaSiO3) and calcite (CaCO3). Small amounts of magnesium were also detected as impurities, which may correspond to magnesia (MgO). The analyzed ore samples showed that barite and wollastonite contain impurities, which may influence the thickening and foaming process. 3.1.3. Thermal gravimetrical analysis The thermal gravimetrical analysis of barite and wollastonite samples is shown in Fig. 4. A similar behavior for both samples in the mass loss is observed in the temperature range from 600 to 800 C. The TGA curve for barite shows three stages, the first stage
starts at 73.5 C and ends at 230 C; the second stage starts at 653 C and ends to 778 C, while the third stage occurs at 1359.4 C and ends at 1641 C. The mass loss estimated for the three stages is 6, 8 and 54%, respectively. The first stage is likely to be loss of water. The second stage is attributed to the calcium carbonate decomposition where CO2 gas is released based on reaction (2). CaCO3 ¼ CaO þ CO2
(2)
The third stage correspond to the thermal decomposition of the barite, specifically the celestine barian compound, where a high amount of SO2 gas is released, based on reactions (3) and (4). 2 BaSO4 ¼ 2 BaO þ 2 SO2 þ O2
(3)
lez Nava et al. / Journal of Alloys and Compounds 698 (2017) 1009e1017 M. Gonza
Fig. 4. Thermal gravimetrical curve of the barite and wollastonite.
2SrSO4 ¼ 2SrO þ 2SO2 þ O2
(4)
The TGA curve for wollastonite shows a mass loss of 6% in the temperature range from 692 to 761 C, which correspond to the calcium carbonate thermal decomposition. In this work, the foaming process was carried out at 700 C; therefore, reaction (2) will be expected to occur. The main objective of the thickening agents proposed was to increase the viscosity of the surface melt; however, from these results, it can be expected that the calcium carbonate contained in the thickening agents as an impurity may contribute to increase the foamability of the melt, where the barite sample showed a higher mass loss due to reaction (2) than the wollastonite sample.
3.2. Structure properties of the A356 aluminum alloys foams Fig. 5aec shows representative A356 aluminum alloy foams produced with CaCO3 (foaming agent) and alumina (Al2O3), barite (Ba0.25Sr0.75SO4) and wollastonite (CaSiO3) (thickening agents), respectively. As can be observed, the three thickening agents had a good response in the formation of the A356 alloys foams under the proposed experimental conditions. However, barite and wollastonite foams show a higher foam expansion than the alumina foam. The foam produced with barite shows some big bubbles located at the bottom of the foam due to vortices formed by agitation of the liquid metal. In the foam produced with alumina (Al2O3), the metal structure did not present big bubbles, nevertheless, high oxidation
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or segregation of alumina particles was observed on the top part of the foam. In general, the foams produced with wollastonite and barite showed a higher homogeneity in its metallic structure. The percentage of porosity Pr (%) of the foams was calculated using Eq. (1). Table 2 summarizes the results of density, relative density, porosity and the average cell size for the foams obtained with different amounts of thickening agents. It is observed that the higher values of porosity and average cell size were obtained for the additions of 0.8 and 1.0 wt% of barite and wollastonite, respectively. However, the differences of these parameters between the additions evaluated are minimal. When the amount of alumina was increased from 1.0 to 1.2 wt%, a decrease in the porosity and cell size was observed. From Table 2, it can be observed that the highest Pr (%) was obtained for the foam produced with the addition of 1.0 wt % of wollastonite (CaSiO3). On the other hand, the highest relative density was obtained in the foam produced with 1.2 wt% of barite (BaSO4) with the smallest pore size. The foams produced with alumina (Al2O3) showed intermediate values (e.g., cell size, Pr (%), relative density) between the thickening agents under study and are in accordance with the work reported by Gnyloskurenko et al. [11]. Fig. 6aec shows the cell structure of the A356 aluminum alloy foams manufactured with alumina and the proposed thickening agents. From these images, it can be observed that the foams cells are essentially spherical and partially closed. The foam produced with 1 wt % CaSiO3 showed the highest pore size (0.438 mm), while the smallest pore size (0.228 mm) was obtained for the foam produced with 1.2 wt % BaSO4. 3.3. Compressive behavior The experimental mechanical properties obtained from the compressive stress-strain curves are summarized in Table 3. It is observed that when the addition of barite and alumina was increased, the Yield Stress and Young's Modulus were increased while the wollastonite addition showed an opposite behavior. It is shown that the Yield Stress increases when the cell size and porosity decreases, similar behavior were reported in a previous study in Mg and Al cellular foams where the Yield Stress possess the tendency of increasing with decreasing porosity [12]. The energy absorption capacity and average Plateau were increased from 0.8 to 1.0 wt % BaSO4 and then decreased for the addition of 1.2 wt % BaSO4. On the other hand, when the wollastonite addition was increased from 0.8 to 1.2 wt % CaSiO3, the energy absorption capacity and average Plateau were decreased. From these results, the highest energy absorption capacities were obtained for the addition of 1.0 wt % of Al2O3 and BaSO4 and for the addition of 0.8 wt% CaSiO3. As expected, there is a close relationship between the foam structure properties (porosity, cell size and density) and their
Fig. 5. Structure of the A356 aluminum alloy foams manufactured to the thickening additions of a) Alumina, b) barite and c) wollastonite using CaCO3 as a foaming agent.
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Table 2 Experimental densities, porosity and cell size of the A356 alloy foams. Sample
Amount (wt %)
Density r* (g cm3)
Relative density r*/rA356
Alumina
0.8 1.0 1.2 0.8 1.0 1.2 0.8 1.0 1.2
0.364 0.383 0.397 0.379 0.415 0.548 0.359 0.335 0.384
0.142 0.151 0.155 0.148 0.178 0.214 0.140 0.137 0.150
Barite
Wollastonite
alloy
Pr (%)
Average cell size (mm)
85.77 85.02 84.48 85.18 83.77 78.57 85.97 86.90 84.98
0.415 0.380 0.343 0.351 0.312 0.228 0.336 0.438 0.412
Fig. 6. Optical-micrographs of A356 aluminum alloy foams manufactured to the thickening additions of a) alumina, b) barite and c) wollastonite using CaCO3 as a foaming agent.
Table 3 Mechanical properties and energy absorption capacity of the A356 aluminum alloy foams. Sample
Amount (wt %)
Yield stress sy* (MPa)
Young's modulus E (GPa)
Average plateau stress spl (MPa)
Energy absorption W (MJ m3)
Alumina
0.8 1.0 1.2 0.8 1.0 1.2 0.8 1.0 1.2
2.13 2.51 4.48 2.33 4.20 4.93 1.58 1.36 0.92
0.34 0.23 0.98 0.58 0.98 0.91 0.71 0.54 0.66
6.58 18.09 5.12 3.31 5.88 3.76 3.2 2.80 2.17
6.63 7.67 0.54 1.98 3.81 3.03 1.58 1.40 0.92
Barite
Wollastonite
mechanical properties. In spite of the similar structure properties of the foam produced, there is a remarkable difference on the mechanical behavior of the foams produced with an alumina addition. It has been reported [9] that the interaction of the alumina with the foaming agent and the melt produce compounds with high hardness and brittle behavior that increase the strength and brittleness of the foam. Fig. 7aec shows the stress e strain compression curves for A356 aluminum alloy foams manufactured with alumina and the proposed thickening agents. Typical stress e strain curves of closedcell metallic foams were obtained, which involves three distinct regimes under compressive loading i.e., elastic stage, followed by a plateau stage that occurs after yielding and finally a densification region where the stress increases drastically. Two mechanism of deformation has been identified for Al-based foams at plateau stage, these are cell wall buckling for ductile Albased foams and cell wall buckling/tearing for Al-based brittle alloys [12]. Fig. 7b and c shows a smooth behavior in the plateau regions for the foams obtained with the addition of barite and wollastonite, which is associated to a ductile mechanism and the failure of ductile foam, is controlled by bending of cell walls [13]. The opposite behavior was observed for the foam obtained with the addition of alumina (Fig. 7a); these curves show the presence of oscillations or serrations in the plateau region related to a brittle
mechanism of deformation. It has been identified that each oscillation is due to localization of deformation in a band and successive crushing of deformation bands leading to an unsmoothed plateau appearance [12]. Slight oscillations or serrations typically are associated with brittle failure of the cell walls [14] due to low ductile eutectic domains and brittle foreign particles attached to the cell wall caused by the addition of thickening and foaming agents. It is desirable in practical applications that foams show a uniform collapse with a wide plateau regime. From these results, it is evident that the deformation behavior of the foams produced with the additions of barite are more attractive than the foams obtained with additions of alumina or wollastonite. As an energy-absorbing material, aluminum foams have the characteristics that higher strain could be obtained at lower stress levels. Therefore, the energy absorption capacity (W), ideality energy absorption efficiency (I) and energy absorption efficiency (E) are three important aspects to evaluate in the metallic foams [12,15]. These parameters were evaluated as follows.
Zε W¼ 0
s dε
(5)
lez Nava et al. / Journal of Alloys and Compounds 698 (2017) 1009e1017 M. Gonza
E¼
1
sm
Zεm
s dε
1015
(6)
0
1 I¼ sm εm
Zεm
s dε
(7)
0
where W is the energy absorption capacity (also defined as the total kinetic energy absorbed by the foam during the compression test) prior to onset of densification, E is the energy absorption efficiency, I is the ideality energy absorption efficiency and sm is the stress where the strain is εm. Fig. 8 shows the energy absorption capacity of A356 aluminum alloy foams obtained for the addition of 1 wt % of the thickening agents evaluated, calculated according to Eq. (5). It is observed that the energy absorption capacity increases along with strain increasing for the three samples. The sample obtained with the alumina addition shows the highest energy absorption capacity while the sample obtained with the wollastonite addition shows the lowest. The energy absorption capacity for the foams fabricated in this work is 7.67, 3.81 and 1.4 MJm3, respectively when the strain is 0.6, 0.65 and 0.5. There is a remarkable difference of the energy absorption capacity for the foams obtained with different thickening agents. The explanation is attributed to the different thickness of the cell wall. If it is considered a similar relative density of the foams produced then, a larger cell diameter will result in thicker cell walls, and the loading capability of unit area is stronger, so energy absorbed is greater for a determined strain. The other important parameter of energy absorption of closedcell aluminum foam is energy absorption efficiency, which was determined according with Eq. (6). The results of the ideality energy absorption efficiency for the foams produced with 1 wt % of the thickening agents used are shown in Fig. 9. The curves are comprised of three main stages: (I) Fast rising stage, where the energy absorption efficiency increases until high efficiency point as strain increasing, (II) sustained stage; where the energy absorption efficiency mainly keep, but some fluctuations occurred as strain changing and (III) attenuating stage, where the energy absorption efficiency decreases with the strain increasing.
Fig. 7. Compressive stress-strain curves of A356 aluminum alloy foams to different additions of a) Al2O3, b) BaSO4 and c) CaSiO3.
Fig. 8. Energy absorption capacity of A356 aluminum alloy foams fabricated with different thickening agents.
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0.40, 0.46 and 0.36 which correspond to the strains of 0.46, 0.61 and 0.6 for the samples manufactured with the additions of alumina, barite and wollastonite, respectively, it can be concluded that the sample with addition of barite possess the highest energy absorption efficiency and according to the similar results reported in previous studies, generally the high energy absorption efficiency is obtained when the plateau region is horizontal and no peaks occurs [13,15], It can be seen that the energy absorption efficiency increases when the cell size average and porosity decreases. 4. Conclusions
Fig. 9. Ideality energy absorption efficiency of the A356 aluminum alloy foams fabricated with different thickening agents.
The mean ideality energy absorption efficiency in the sustained stage are 0.6, 0.75 and 0.74 for samples with addition of alumina, barite and wollastonite, respectively. All samples show fluctuations which are attributed to the compression process instability [15]. However this behavior is noticeable in the sample obtained with alumina addition [9]. The sample obtained with wollastonite addition show the highest efficiency point, and then a short sustained stage, followed by a fast decrease of the ideality energy absorption efficiency as the strain is increased in the sustained stage. The sample obtained with barite addition show the highest mean ideality energy absorption efficiency and the longest sustained stage with the smallest fluctuation of the efficiency. Therefore, the foam produced with barite addition show the best behavior of the energy absorption efficiency, which contributes as a potential thickening agent to the application in engineering field. The relation between energy absorption efficiency and strain is shown in Fig. 10 for 1 wt % of the thickening agents used. In all cases the energy absorption efficiency increases with increasing of strain and then decreases. The highest energy absorption efficiency are
Closed-cell A356 aluminum alloy foams were fabricated using 0.8, 1.0 and 1.2 wt % of alumina, barite and wollastonite as thickening agents and CaCO3 as a foaming agent. It was determined that barite sample correspond to the compound Celestine-barian (Ba0.25Sr0.75SO4). Barite and wollastonite samples were obtained by simple beneficiation methods of primary minerals where they contain calcium carbonate as an impurity that may contribute to the foaming process. A minimal difference on the microstructure and mechanical properties of the foams manufactured with barite and wollastonite was observed for the additions evaluated. The foam manufactured with 1 wt % barite shows a good combination of structure and mechanical properties such as cell size of (0.312 mm), porosity percent (83.7%) and energy absorption capacity (3.81 MJm3), while the foam produced with 1.0 wt % wollastonite shows the highest porosity percent (86.9%) and cell size (0.438 mm) but the lowest energy absorption capacity of (1.4 MJm3). The foam obtained with 1 wt% alumina showed intermediate values of porosity and cell size between the foams obtained for the same addition of barite and wollastonite. Both foams produced with the alternative thickening agents showed a smooth behavior in the Plateau region which contrast with the presence of oscillations or serrations showed by the foam obtained by alumina addition. The foam obtained with 1 wt % barite addition show the highest mean ideal energy absorption efficiency and the longest sustained stage with the smallest fluctuation of the efficiency. Therefore, the use of Barite shows an attractive potential as a thickening agent in the foaming process via melt route. Acknowledgements The authors wish to thank the Institutions CONACyT, SNI, COFAA cnico Nacional for their permanent assisand SIP-Instituto Polite tance to the Process Metallurgy Group at ESIQIE-Metallurgy and Materials Department. References
Fig. 10. Energy absorption efficiency of the A356 aluminum alloy foams fabricated with different thickening agents.
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