Optimization of Pore Walls Microstructure in Open Cell Aluminum Foams Utilizing Self-Propagating Reaction
2019 Volume 60 Issue 11 Pages 2292-2297
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2019 Volume 60 Issue 11 Pages 2292-2297
The Al(H2PO4)3 doped with thermite as the space holder is prepared for aluminum foams during powder metallurgy process. The pore walls of aluminum foams are covered with the in-situ synthesized residual phosphates produced from Al(H2PO4)3 after the self-propagating reaction in space holder. Influences of the thermite content dispersed in space holder on both phase transformation and the coating layers microstructure of pore walls are investigated. The results suggest that the added thermite could optimize the microstructure of pore walls coating layers even if the space holder sintered at the lower furnace temperature, due to the released larger energy from thermite reaction would increase the local reaction temperature for Al(H2PO4)3. The melted loose coating layers of the pore walls may shrink and lead to the higher porosity of aluminum foams can be obtained.
Fig. 5 (a) The formed local micropore structure in specimen 3#, and the coating layers microstructure of the pore wall in (b), (c), (d) corresponding to the 1#, 2#, 4#, respectively.
Aluminum phosphate (AlPO4) as the sealing coating not only can improve dry abrasion wear resistance, erosion wear resistance, corrosion resistance of the porous alumina coatings but also increase the micro-hardness of the coatings.1–4) The AlPO4 as inorganic binder has been widely applied in the thermal spray coating systems. The refractories bonded with AlPO4 will have high strength, high temperature stability.4) It is also reported that the AlPO4 sealing treatments result in a more uniform corrosion resistance and the enhanced long-term corrosion resistance for the stronger block capability.1–4) Therefore, researches and applications related to the AlPO4 have been gaining widespread attention.4–7)
Aluminum foams due to the splendid strength and stiffness to weight ratios, exceptional mechanical energy absorption to impact, good insulation and excellent sound absorption properties have been studied in recent years, which is ideal for the applications in the fields of military and commercial industries.8–19) The open-cell aluminum foams can be used for the heat exchangers because of the high thermal conductivity. They also usually were chosen as the light and stiff fillers in structural components.13,14) Simulation and experimental results have shown that excellent properties of aluminum foams are positive in correlation with the pore structure.11,12) To acquire the pore structure, the various methods of manufacturing aluminum foams include infiltration of liquid metal, liquid processes, and powder metallurgy have been given extensive attention. Powder metallurgy is an important method of manufacturing open cell aluminum foams, however, the incomplete dissolution of space holder may cause contamination or corrosion of the body material.20) Therefore, it is important to find the new space holder and improve the excellent pore structure for the better performance.
Dehydration reactions are known to be the mostly reactions for Al(H2PO4)3 during heating, which means structural water evaporates and new less water containing phases are formed.1,3) After the decomposition and dehydration of Al(H2PO4)3, and new condensed phases may provide a large volume for pore space, which will favor producing the porosity for aluminum. However, very few pay close attention to make aluminum foams using the Al(H2PO4)3 as the space holder. More importantly, the increase of setting temperature can accelerate the Al(H2PO4)3 reaction process.1,3) The thermite reaction is initiated from some type of detonator and it can burn at temperatures of thousands of degrees kelvins. The intense heat would promote the decomposion, dehydration and condension of Al(H2PO4)3, resulting in favoring further the formation of porosity in aluminum. The residual phosphate on the pore walls act as a modified surface layer may also improve the walls performance. To optimize the pore walls structure of residual phosphate layer, the Al(H2PO4)3 embedded with thermite is used as the space holder and employed the powder metallurgy method has been recently proposed.
Triple steps of mixing were carried out for preparing aluminum foams in present investigation. Firstly, the thermite powder included aluminum and MnO2 (the molar ratio Al/MnO2 is 4/3, 4Al + 3MnO2 → 2Al2O3 + 3Mn) was agitated until MnO2 attached to the aluminum particles uniformly. Then the mixture previously was doped in the Al(H2PO4)3 powder as the second agitating process for obtaining the space holder. Finally, the ready-prepared space holder and aluminum powder (purity of 99.5%, 300 mesh) were agitating milled until the space holder particles with tens to hundreds microns and attached to the aluminum powder uniformly. Then the powder mixed uniformly was uniaxially cold compacted in a cylindrical die cavity employed a hydraulic press (pressure 250 MPa, 10 min) for the cold compacted precursors with 8 mm high and 20 mm in diameter. The producing processes are shown schematically in Fig. 1. Initial components of the powder were mixed to prepare precursors with for all compositions (Al-34.13 vol% space holder with the weight ratios of thermite to Al(H2PO4)3 are 0:8, 1:8, 2:8 and 3:8, namely, 1#, 2#, 3# and 4#, respectively). The compacted pellets were dried at 200°C in an evacuated quartz tube for 1 h, followed by heating at 400°C for 1 h, and then subjected to a 2 h sintering at 620°C, finally allowed to cool to ambient temperature. During the sintering process, the contraction of the interconnected Al(H2PO4)3 in precursors would be more effective with the aid of self-propagating reaction, and a three-dimensional continuous through-holes formed in the aluminum matrix.
The flow chart of optimizing the pore wall of open aluminum foams.
The X-ray powder diffraction data of the sintered specimens was collected using an X-ray diffractometer (BRUKER, D8 Advance) with Cu-Kα radiation (λ = 1.54056 Å, 40 kV/30 mA) at room temperature. The system was set up with Bragg-Brentano optics in this investigation. Div Slit, Sct Slit and Receiving Slit were fixed as 0.50°, 0.50° and 0.15 mm, respectively. The 2θ scan range was from 10° to 90° with step size of 0.02°. The SEM analyses were characterized by a Hitachi SU8220 electron microscope (Japan) in conjunction with energy dispersive X-ray spectroscopy (EDS) to identify the pore structure of the foams. To improve the quality of the SEM pictures, a gold coating on the specimens were conducted. The porosity structure of section was captured using optical microscopy, and the outer region of this image was cut to evaluate the planar porosity using the ImageJ software.21)
Observed from the precursor SEM micrographs of sample 2# and 4# as shown in Fig. 2(a) and (b), respectively, the white region becomes bigger with the content of thermite increasing. The chemical composition of total region EDS of sample 4# in Fig. 2(c) is deduced to be Al:O:P = 74.6:19.2:5.2, which the aluminum atom content is more than that of chemical composition of Al(H2PO4)3. The results show that the region include the aluminum matrix and space holder though the phase interfaces between space holder and aluminum matrix is difficult to identify. From the distribution of elements as indicated in Fig. 2(d)–(h), it can be obtained that the white zones in Fig. 2(b) locate in the middle of elements both phosphorus and aluminum is the manganese oxide. The aluminum thermal reaction between manganese oxide and aluminum would be conducive to improve the decomposition and dehydration of Al(H2PO4)3. The more thermite, the better.
The precursors (a), (b) SEM micrographs for 2# and 4#, respectively; (c) EDS analysis of the total region for 4#; (d)–(h) distributions of element in 4#.
Typical foam structure of the open cell aluminum (1#) obtained from powder sintering process in an evacuated quartz tube is shown in Fig. 3. The pores obtained from the reactions of Al(H2PO4)3 are internal linked almost by the interconnecting pore channels. The sizes of pore range from tens to hundreds of microns and the morphologies depend on the distribution of Al(H2PO4)3 particles in compacted precursors. The volume fraction of Al(H2PO4)3 particles reach a certain level, most Al(H2PO4)3 particles would interconnect to form a three-dimensional continuous through-holes. On the contrary, some Al(H2PO4)3 particles are surrounded completely by the aluminum matrix as the Al(H2PO4)3 volume fraction is lower. The isolated Al(H2PO4)3 particles would form some closed pores in the foam. In addition, small amount of white residue originated from the Al(H2PO4)3 remains in the resultant foam due to the decomposition and dehydration of space holder is insufficient. The smaller the residue is, the greater the porosity is. In order to improve further the porosity, it is a better way to develop heat treatment technique to promote the decomposition and dehydration of Al(H2PO4)3.
The morphology of the pores formed from the decomposition of Al(H2PO4)3 after heat treatment.
The diffraction peaks of the synthesized aluminum foams are mainly reflected as Al (S.G. Fm-3m [No. 225]) are shown in Fig. 4(a) although the weight ratios of thermite to Al(H2PO4)3 increasing for the space holder. From the enlarged view of the small-angle regions in Fig. 4(a), the phase AlPO4 (S.G. P3112 [No. 151]), [Al(PO3)3]2 (S.G. P21/a [No. 14]) can be obtained when the space holder without thermite (1#) as indicated in Fig. 4(b). In addition, smaller amounts of Al2O3 phase also can be found in the samples. The Al(H2PO4)3 dehydrates and forms a long-chain polyphosphates [Al(PO3)3]2 during the heat treatment while there is no thermite in space holder. Whereas, the peak intensities of [Al(PO3)3]2 decrease remarkably and even disappear as the content of thermite rises. Strange that there are no significant increment in peaks of AlPO4 even if the peak intensities of [Al(PO3)3]2 decrease, which may be attributed to the [Al(PO3)3]2 on the pore walls generate into amorphous material, or part of them decompose and evaporate.1,3,22) Thus, the residual phase composition of the space holder coating on the pore walls depend on the content of thermite doped in the Al(H2PO4)3. No obvious diffraction peaks of the phase Mn after thermite reaction, it follows that the Mn exists in the phosphates as solid solution, or the content of Mn is too little to be detected obviously.
(a) XRD analysis of the synthesized Al foam with the increment of thermite in space holder; (b) the magnification image of the small-angle regions in (a).
The local morphology of tens of micron pores for the aluminum foams (3#) as indicated in the Fig. 5(a), and the micropore structure depend on the distribution of space holder in the compacted precursor. After the decomposition and dehydration of Al(H2PO4)3, the in-situ shrinked residues of Al(H2PO4)3 would coat on the walls of pore. The pore walls coating layers (1#) with a large number of micro-porous as shown in Fig. 5(b), and tend to be in a loose state when the space holder without adding thermite. With the content of thermite rising as shown in Fig. 5(c) and (d), it can be observed from the coating layers of pore walls that the formed liquid phase regions near the microhole edge increase gradually due to the self-propagating exothermic reaction in the space holder. The intense energy released from the thermite reaction would increase the local reaction temperature in Al(H2PO4)3. Then the pore walls coating layers with loose phosphate microstructure were melted and destroyed, and then some microholes formed in the partially melted phosphate layer. The melted and broken coating layers of the pore walls would shrink and become compact. The densified coating layer may benefit improving the walls performance.
(a) The formed local micropore structure in specimen 3#, and the coating layers microstructure of the pore wall in (b), (c), (d) corresponding to the 1#, 2#, 4#, respectively.
Figure 6 represents EDS analysis of the pore walls layers in the aluminum foams, indicating the existence of Al, O, P and Mn in a minor amount. With the increase of thermite content, the molar ratio of P to Al decreases from 2.56 to 1.03 are shown in Fig. 6(a) and (b), respectively, which also indicates that the increased thermite is beneficial to the formed components of residue tend to be AlPO4. The results of EDS of the pore walls layers in the aluminum foams are in agreement with that of XRD, indicated that the thermite doped in the Al(H2PO4)3 is beneficial to optimize microstructure of the pore walls due to the self-propagating exothermic reaction. The pore walls are covered with the more component AlPO4 would further benefit to the corrosion resistance, dry abrasion wear resistance, erosion wear resistance and increase the micro-hardness.1,2)
EDS analysis of pore wall layer with increasing the thermite in space holder.
The section topography of aluminum foams is shown in Fig. 7(a). Both the original image analysis and the binarization are performed by utilizing ImageJ software.21) To obtain the planar porosity, the image is cropped so that only the specimen interior remained. The appropriate threshold was chosen and the binarized micrograph picture composed of only black (referring as pores) and white (represents Al matrix) colors is shown in Fig. 7(b). Obviously, it can be contributed that the formed pores stem from the decomposition and dehydration of Al(H2PO4)3. The result evaluated from the binarized micrograph indicates that the proportion of the planar porosity in aluminum foam is 37.20%, agreeing with basically the 34.13 vol% of the space holder. The formed porosity and the structure of pores mainly rely on the arrangement of Al(H2PO4)3 particles during the preparation of sample. The more the content is, the higher the porosity is. The three-dimensional continuous through-holes would be formed as the volume fraction of Al(H2PO4)3 particles is beyond a certain level.
The pore structure of Al(H2PO4)3 (a) Original micrograph and (b) Binarized micrograph of 3#.
The [Al(PO3)3]2 reduces with the increase of thermite can be attributed from two effects. On the one hand, it was reasonable that the reaction between [Al(PO3)3]2 and the produced Al2O3 mainly occurred during thermite reaction and the phase AlPO4 formed.1,3) On the other hand, the [Al(PO3)3]2 decompose into AlPO4 directly,22) simultaneously, the generated P2O5 gas would evaporate at high temperature during the thermite reaction. The evaporated material produced from the self-propagating reaction would benefit improving the porosity of aluminum foams. The increment of thermite content in space holder helps the decrease of [Al(PO3)3]2, and the formed AlPO4 would improve the performance of surface coating.1,3)
With a higher weight fraction of thermite in the space holder, the excess heat will damage the metal matrix of pore. In contrast, it is difficult to provide sufficient energy to facilitate the reaction of [Al(PO3)3]2 when the weight fraction of thermite is lower in the perform. Therefore, to choose both the appropriate type and content of thermite are particular importance to optimize the coating layers of pore walls for the aluminum foams.
The pore walls of aluminum foams can be covered with the residual phosphate claddings via powder metallurgy. The microstructure and phase composition of pore walls coating layers are determined by the added thermite content in the Al(H2PO4)3. The results suggest that the self-propagating reaction in space holder could facilitate the decomposition and dehydration of Al(H2PO4)3 at the lower furnace temperature due to the released larger energy. The released intense energy also could melt and destroy the loose pore walls coating layers produced from the Al(H2PO4)3 reaction. The shrinked coating layers of pore walls would be conducive to the higher porosity. The coating layers with AlPO4 component would become compacted because of self-propagating exothermic reaction, leading to the microstructure of coating layer is optimized in aluminum foams.
This research work is supported by China’s Sichuan Science and Technology Program (2019YJ0441), Chengdu Normal University First-class Discipline Construction Major Scientific Research Projects (CS18ZDZ03), Chengdu Normal University Talent Introduction Scientific Research Special Project (YJRC2015-3) and The National Natural Science Foundation of China (NO. 11775228).
