UDK 66.017:669.715:621.746	ISSN 1580-2949
Original scientific article/Izvirni znanstveni članek	MTAEC9, 45(1)13(2011)
SYNTHESIS OF ALUMINIUM FOAMS BY THE POWDER-METALLURGY PROCESS: COMPACTING OF
PRECURSORS
SINTEZA ALUMINIJEVIH PEN PO POSTOPKU METALURGIJE PRAHOV: STISKANJE PREKURZORJEV
Irena Paulin1,2, Borivoj Suštaršič2, Varužan Kevorkijan3, Srečo D. Skapin4,
Monika Jenko2
1TALUM, d. d., Kidričevo, Tovarniška cesta 10, SI-2325 Kidričevo, Slovenia 2Institute of Metals and Technology, Lepi pot 11, SI-1000 Ljubljana, Slovenia 3Independent Researcher, Betnavska cesta 6, SI-2000 Maribor, Slovenia 4Jozef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia irena.paulin@imt.si
Prejem rokopisa - received: 2010-12-20; sprejem za objavo - accepted for publication: 2011-01-19
Aluminium foams, produced by the powder-metallurgy route, have a good potential for use in weight-sensitive structural parts. The goal of this study was to evaluate the properties and to optimize the preparation of precursors by a powder-compacting process. Various compacting pressures, from 200 MPa to 900 MPa, were used in the double-axial powder-compacting process for two different aluminium powders: pure aluminium and an AlSi12 alloy with the addition of 1 % of TiH2 as a foaming agent. The green density of the precursors and the distribution of the foaming agent were examined. The powder particles was also characterised. The results of the effective preparation of precursors are shown as the effectiveness of the foaming of the precursors.
The relation between the powder characteristics, the aluminium-alloy properties and the preparation of precursors was studied by SEM/EDS analysis, powder-metallurgy standard testing of metallic powders, granulometry, etc. Different parameters were used for the precursor preparation and foaming. The foaming temperature varied between 680 °C and 770 °C, and the foaming time was from 6 min to 13 min. The relation between the properties and the applied production parameters was studied in detail and is described in this paper.
Keywords: aluminium foam, Al, AlSi12 alloy, powder-metallurgy, powder compacting, sintering
Aluminijeve pene, narejene po postopku metalurgije prahov, imajo velik potencial v uporabi lahkih konstrukcij. Cilj raziskave je ugotoviti lastnosti in parametre za optimizacijo priprave prekurzorja po postopku hladnega stiskanja. Za pripravo prekurzorjev smo uporabili obojestransko stiskanje s tlaki od 200 MPa do 900 MPa. Stiskali smo mešanico prahov tehnično čistega aluminija 99,7 % in zlitine AlSi12, v obeh primerih z dodatkom 1 % TiH2 kot penila. Določili smo zelene gostote in porazdelitev penila v prekurzorjih. Naredili smo tudi karakterizacijo vseh treh uporabljenih prahov kot vstopnega materiala. Rezultat uspešne priprave prekurzorjev se kaže v uspešnosti penjenja materiala.
Povezavo med lastnostmi prahov, lastnostmi aluminijevih zlitin in priprave prekurzorjev smo raziskovali s SEM/EDS-analizami, s preizkušanjem prahov s standardnimi metodami, uveljavljenimi v metalurgiji prahov, granulometrijo itd. Uporabljeni so bili različni parametri za pripravo in penjenje prekurzorjev. Temperature penjenja so bile med 680 °C in 770 °C, čas penjenja pa med 6 min in 13 min. Povezava med lastnostmi in uporabljenimi parametri za pripravo aluminijevih pen je bila raziskana in je podrobno opisana v tem članku.
Ključne besede: aluminijeve pene, aluminij, zlitina AlSi12, metalurgija prahov, stiskanje prahov, sintranje
1 INTRODUCTION	to the relatively high costs of the input materials. The
principle of PM is simple and the process consists of
Aluminium foams are metallic materials with various	three stages: 1) mixing the metallic powder and the
physical and cheemical properties that can be used for	powder of the foaming agent, usually TiH2, ZrH2, etc; 2)
several purposes1,2. Production methods can be classified	compacting the powder mixture and; 3) sintering as the into four groups: powder-metallurgy (PM), molten-metal
,, . ® /„• . . ' . 3	final stage of the process. All three steps are important foaming, metallic deposition, and sputter deposition3.
Each production method gives its own characteristic	fo/ !he quality of ;he final production and the properties
range of densities, cell sizes and shapes. In this study a	of the aluminium-foam products.
method that is adapted to produce complex shapes of	The first step of the PM process is mixing of the
metallic foams is discussed, i.e., the powder-metallurgy	metallic powder and the powder of the foaming agent in
process.	the proportions needed for the final properties of the
PM is one of the possible techniques to produce	product. The higher is the fraction of foaming agent,the
metallic foams. This production process is not as widely	greater and/or more numerous are the pores. The addi-
used as the less-expensive, molten-metal foaming pro-	tion of the powder ceramic compounds (SiC, CaO) is
cess, but it also has advantages 4,5. Its general application	desired for the stabilization of the foams, but this is not
in the metal-foaming industry is not very extensive due	essential ^-8. The important task of mixing is to achieve a
homogeneous distribution of foaming-agent particles and of the ceramic-powder particles if they are added.
The next important step of the foaming process is the compacting of the powder mixture. The density of the compacted material, the so-called green or theoretical density, is a significant property of precursors and it must be very high (up to 99 %). The shape and size of the metallic-powder particles plays an important role in achieving the highest possible green density of the precursors. However, depending on those properties of the powders, various angles of the repose and bulk densities are achieved. The green density of the precursors also depends on the possible plastic deformation of the metallic powder particles. The lower is the porosity of the precursors, the more of the liberated gas during the sintering is captured in the matrix material.
Sintering is the final stage of the PM production process. During the sintering process at a temperature that is above the temperature of the melting point, and which is specific for each alloy and depends on the foaming agent used, hydrogen is released from the hydride and forms pores in the material. A larger fraction of liberated gases is captured in the matrix material if the molten metal has a higher viscosity 9-11.
The sintering time varies from 3 min up to 10 min, depending on the alloy used for foaming, the foaming agent and the size of the sintered piece. In the decomposition of the TiHa used as a foaming agent, the Ti remains and hardens the matrix material while the hydrogen captured in the matrix forms foams with closed-cell structures.
In this study we have focused on the second stage of the PM process. The production of precursors by compacting powder mixtures can be performed in a variety of ways, e.g., by uni-axial, double-axial or isostatic pressing, extrusion, rolling, etc., and all the methods can be hot or cold. Furthermore, the compacting process can be performed in an inert atmosphere, in air or in vacuum 12. The most economical way is double-axial pressing in air, but the most efficient one is high-temperature extrusion.
2 EXPERIMENTAL WORK
The PM foaming process was applied in our research work. Air-atomized Al powder with a purity of 99.7 % and a D50 of 98 pm, air-atomized AlSi12 alloy powder with D50 of 42 pm, and TiH2 powder with a purity of 98
% and a D50 of 12.6 pm were used as the starting materials (the purity was specified by manufacturer). The metallic powders were prepared as Ecka granules (Non Ferrum Kranj, d. o. o.), while the TiH2 powder (325 mesh, 98 %) was purchased from the Sigma-Aldrich Company. In all cases, the metallic powder was mixed with mass fraction 1 % of TiH2 powder in a turbular mixer (TURBULA WAB Type T2C, 50 Hz, 180 W) for an hour. The characteristics of the powders are shown in Table 1. The size and the distribution of the powders were determined by laser granulometry (Alcatel CILAS HR 850-B, isopropanol as analysis medium).
The bulk density, tapped density and free-flowability of the powders were determined. The free-flowability and the bulk density of the powders were determined by the Hall flowmeter funnel that contains a standard flow-meter funnel for metallic powders 13. A dry test specimen must be carefully loaded into the flowmeter funnel and permitted to run into the density cup through the discharge orifice. In order to determine the free-flowability of the powders it is necessary to measure the time needed for 50 g of powder to pass through the orifice. When the bulk density is determined the powder must completely fill the density cup till it starts to overflow, then the powder is levelled with the top of the cup using a spatula with a blade held perpendicular to the top of cup. Afterwards, the powder is weighed. For a determination of the powder's tapped density a standard mechanical device is used that enables tapping of the graduated cylinder containing powder at a rate of 100 taps per minute 13. The analyzed powder must be weighed (50 g) and the volume of the powder is read after the tapping.
Powder mixtures were compacted cold, using a double-axial compaction, in a 24-mm-diameter, lubricated, tool-steel die with pressures in the range from 200 MPa to 900 MPa to achieve different green densities 14. The samples were compacted in INSTRON 1255 equipment with an INSTRON 8800 computer system and the Bluehill2 program. Different pressures were used for both metallic powder mixtures and the green densities were calculated by assuming that the density of the base metal is 2.7 kg/dm3 when aluminium is used, and 2.65 kg/dm3 for the AlSi12 alloy. The samples were later sintered at a pre-determined temperature in the air. After various times of sintering, samples were taken out of the retort furnace and quenched into water to solidify the foam structures.
The sintered samples were characterized with a light microscope (LM, Nikon microphot - FXA) and analyzed
Table 1: Characteristics of the used powders Preglednica 1: Lastnosti uporabljenih prahov
Powders	Manufacturer	Mean particle size d/pm	Purity w/%	Method of manufacturing
Al	Non ferrum, Kranj	106	99.7	Air atomization
AlSi12	Non ferrum, Kranj	47	composition w(Si) =11.29 %	Air atomization
TiH2	S^ma-Aldrich	14	98	Ball milling
with the AnalySIS PRO 3.1 program. The size and the distribution of the pores were determined by a standard metallographic method for determining the size and the distribution of the grains in the microstructures 15. Metallographic samples of the precursors and the foam samples were prepared using a standard metallographic procedure, examined with scanning electron microscope (FE-SEM JEOL JSM-6500F) and analyzed by energy-dispersive electron spectroscopy (EDS, INCA X-SGHT LN2 detector, INCA ENERGY 450 analyzing program) for the elemental analyses. SEM images of the powders were also made.
3 RESULTS AND DISCUSSION
Metallic powders produced in a gas atomizer, in our case in air, have an oblong, quasi-spherical shape with varying size distributions. Figure 1 shows SEM images of the used powders; (a) Al 99.7 %, (b) AlSi12 and (c) TiH2. The size and he characteristics of the powders are shown in Table 1. The aluminium powder particles (Figure 1 (a)) have a mean size of 106 pm, while the AlSi12 powder particles (Figure 1 (b)) have a mean size of 47 pm, which is half the size of the Al powder particles. However, the morphology of the powders is typical for gas-atomized powders from a liquid metallic melt, and it can be described as a bulk prolonged, semi-spherical shape. On the other hand, the morphology of the TiH2 powder particles is quite different from that of the metallic powders (Figure 1 (c)). The TiH2 powder was prepared by the ball milling of titanium in a hydrogen atmosphere under pressure 16-18 and it exhibits an irregular, sharp, polygonal morphology. The mean particle size was 14 pm, significantly smaller than that of the matrix-material powder particles 19.
Table 2: Influence of compacting pressures on the green densities of precursors
Preglednica 2: Vpliv pritiska stiskanja na zeleno gostoto prekurzorjev
Figure 1: SEM images of as-received powders at the same magnification; (a) Al powder of 99.7 % purity, (b) powder of the AlSi12 alloy, and (c) TiH2 powder
Slika 1: SEM-slike prahov, narejene pri enaki povečavi; (a) Al prah s cistoto 99,7 %, (b) prah zlitine AlSi12 in (c) prah TiH2
	Green density of precursor	
Compacting pressure p/MPa	w(Al + 1 % TiH2)/%	w(AlSi12 + 1 % TiH2)/%
200	94.8	75.6
300	97.9	82.4
400	98.1	86.1
500	98.5	89.9
700	98.8	93.2
750	/	94.8
800	/	95.8
900	/	96.2
The surface of all the powder particles was oxidized. The thickness of the oxide layer depended on the affinity of the basic material for oxygen, which was very high for both the aluminium and TiH2 20-24. This oxide layer was examined in our previous work 21,23 and found to be one of the influential parameters for the formation of metallic foams by the powder-metallurgy process. The oxide layer on TiH2 powders caused a delayed gas evolution, while the oxide layer on the aluminium powder particles increased the viscosity of the melt, which had a favourable influence on the foaming process 11,25.
The free flowability of the Al powder particles was 75 s per 50 g of powder, while the AlSi12 and TiH2 particles did not flow freely, since they did not have good flowing properties. The bulk density (cumulative volume fraction as a function of the particle size of powder/Al) of the Al powder was 1.23 kg/dm3, and 1.20 kg/dm3 for the AlSi12 powder. The TiH2 powder had a bulk density of 1.34 kg/dm3. Both powders, the Al and of the AlSi12, had a tapped density of 1.43 kg/dm3, while the TiH2 powder had a tapped density of 1.61 kg/dm3. There was no major difference in the densities of both metallic powders, but there was a difference in the flow properties of the two. Though produced in the same way, with similar tapped densities, the two metallic powders had different final green densities after the compacting process. The characteristics of the TiH2 powder did not influence
Figure 2: Green density of the compacted precursors Slika 2: Zelene gostote stisnjenih prekurzorjev
Figure 3: Precursors; double-axially compacted with pressure of 400 MPa, (a) Al precursor of 99.7 % pure Al, determined density was approximately 98.1 %, (b) AlSi12 precursor, determined density was approximately 86.1 %. BE image - bright spots are titanium particles and dark spots the porosity.
Slika 3: Prekurzorji, obojestransko stisnjeni s 400 MPa, (a) Al pre-kurzor iz Al prahu cistote 99,7 % z zeleno gostoto 98,1 %, (b) AlSi12 prekurzor z zeleno gostoto 86,1 %. BE slike - svetli predeli so titanovi delcki, temni predeli so poroznost.
the properties of the precursors, since a relatively small amount of TiHa powder was used in the mixture, but its properties did have an influence on the distribution of the TiH2 powder in the metallic powder matrix. In any case, it was preferable to have a homogenous distribution of TiH2 particles in the precursor material.
After one hour of mixing the powder mixtures were compacted with various pressures. The influence of the compaction pressures on the green densities of the precursors is presented in Table 2 and in Figure 2. The precursors were examined by SEM and analyzed by EDS. The aluminium precursor that was compacted by the double-axial process with a pressure of 400 MPa is shown in Figure 3 (a). The porosity of the precursor was approximately 2 %. TiH2, as the foaming agent, was homogenously distributed in the matrix material. On the other hand, the powder of the AlSi12 alloy was also double-axially compacted, with pressures in the range from 200 MPa to 900 MPa. With a pressure of 400 MPa the porosity of the precursors was approximately 14 %. When the compacting pressure was raised up to 900 MPa, the porosity was reduced to only 4 %. Different theoretical densities of the precursors depended on the different qualities of matrix powders, e.g., on the size and the shape of the particles, the bulk and the tapped density. Though the matrix powders had practically the
Figure 4: Cross-section of pure Al foams; (a) cold pressed with 200 MPa, sintered at 750 °C for 7 min, (b) cold pressed with 400 MPa, sintered at 750 °C for 6 min 30 s.
Slika 4: Prerez Al-pene iz tehnično čistega aluminija; (a) hladno obojestransko stisnjeno z 200 MPa, sintrano pri 750 °C, 7 min, (b) hladno obojestransko stisnjeno s 400 MPa, sintrano pri 750 °C, 6 min 30 s.
same shape, but different sizes, the compressibility of the powders differed. Compressibility also depended on the bulk and tapped densities of the powders. Double-axial compacting of both, the Al and the AlSi12 powders, at room temperature did not enable us to achieve the same green density through using higher pressures than for the alumosilicates. The limitation of the used compacting pressures was also related to the lubricated tool-steel die that started to stick due to the too high forces in compacting aluminium, while the AlSi12 alloy powder was harder and had a higher compacting resistance. In other words, a larger green density could be achieved with materials that had a better plastic deformability. Though high compacting pressures were applied, the porosity, especially in the AlSi12 precursors, still remained relatively high.
The EDS analysis determined the distribution of Si and Ti in the matrix material. The Si in the AlSi12 matrix was distributed uniformly throughout the whole
Figure 5: Histogram of pore-size distribution of pure Al foams Slika 5: Histogram velikosti in porazdelitve por v Al-penah
matrix material, which proved that the AlSi12 powder was manufactured from the AlSi12 aluminium alloy and was not a mixture of Al and Si powder in the demanded ratio. The titanium was randomly but uniformly distributed as individual particles that could be seen in Figure 3 (b) as bright phases in the BE image. On the other hand, only titanium particles were detected in the aluminium matrix, and they were similarly distributed as individual particles in the matrix material, as in the sample of the AlSi12 alloy (Figure 3 (a)). The distribution of TiHa, with the captured liberated gases around the particles of foaming agent, enabled a homogeneous distribution of pores in the aluminium foams.
The precursors were heat treated in a retort furnace at temperatures that were slightly higher than the melting temperature of the matrix material. All the samples were sintered at temperatures between 680 °C and 770 °C, for various sintering times. The temperatures of the retort in the furnace were controlled with CrNi thermocouple. After sintering at a pre-determined temperature and for a chosen time, the samples were taken out of the retort and quenched in air and/or water. The densities of the samples were then determined with the Archimedes' method.
The samples of pure aluminium that were compacted at lower pressures had fewer pores, and those were not spherical, while the samples that were pressed at 400 MPa or higher pressures contained more pores that were semi-spherical. Cross-sections of the obtained aluminium foams are shown in Figure 4.
Light microscopy was employed to determined the size and the distribution of the pores. A diagram of the pore-size distribution in our samples is presented in Figure 5. The same comparison was also made with samples of the AlSi12 alloy that were pressed with various forces and sintered at various temperatures and for various times. The cross-sections of the foams are shown in Figure 6 and a diagram of the pore size distribution is shown in Figure 7.
The Al precursors were foamed at 750 °C from 6 min to 10 min. A histogram of the Al foampore size distribution (Figure 5) gave evidence that the majority of the pore sizes varied from 1 mm to 3 mm, giving an average
Figure 6: Cross-section of AlSi12 foam, cold pressed with 900 MPa, sintered at 770 °C for 9 min
Slika 6: Prerez pene zlitine AlSi12, hladno obojestransko stisnjeno z 900 MPa, sintrano pri 770 °C, 9 min
Figure 7: Histogram of pore-size distribution of the AlSi12 foam Slika 7: Histogram velikosti in porazdelitve por v penah zlitine AlSi12
pore size of 2.6 mm. Only a smaller amount of pores larger than 5 mm was found. On the other hand, the AlSi12 foams were sintered at 770 °C and for times from 6 min to 13 min. A histogram of the pore-size distribution is presented in Figure 7. The distribution was very scattered and there was a larger amount of pores with sizes of more than 10 mm.
The difference between the size and the distribution of pores in both foamed materials was due to the green densities of the precursor materials. The compacted material with the higher green density was more easily and uniformly foamed. Therefore, the foaming effectiveness of the AlSi12 precursors was relatively low due to their low green densities (only up to 96 % in our case). The pores in this material were larger due to the prolonged foaming times that were needed to foam the material. This process causes further coarsening of the pores (the large pores were growing at the expense of the small ones).
The density of the foamed material, determined by the Archimedes' method, was from 0.58 kg/dm3 to 1.2 kg/dm3 for pure Al foams and from 0.9 kg/dm3 to 1.6 kg/dm3 for the AlSi12 foams. The reason for the difference in the densities was in foamability, which depends on the green density of the precursors.
Another difference in the foaming of both aluminium foams was caused by the Si. The AlSi12 alloy is a so-called casting alloy, and according to the Al-Si phase diagram, it is lying near the eutectic point, thus its melting-cooling properties are different from those of pure aluminium. The area between the liquidus and solidus lines is much larger for pure aluminium than for an alloy of the near-to-eutectic composition. Therefore, the temperature range between the liquidus and solidus state in which the pores can be formed is higher with pure aluminium.
4 CONCLUSIONS
It is very important for better foaming results to achieve a green density of at least 98 %. This can be easily achieved with pure aluminium powder. On the
other hand, it is practically impossible to achieve green densities higher than 96 % with AlSi12 powder using only a double-axial compression at pressures even up to 900 MPa at room temperature. For industrial applications it is also important to use as simple industrial machines as possible or to use techniques that are already used in the PM industry for the preparation of different types of complex products. Higher green densities can be achieved by increasing the compacting pressure and/or pre-heating the precursors to increase their compressibility 26'27.
The difference in size of our powder particles does no play such an important role in the difference in the green densities of both precursors, Al and AlSi12, though the bulk density is slightly higher with larger aluminium powder particles. The tapped density is approximately the same for both powders. The most important parameter for achieving higher green densities is the compressibility of powder particles, which depends on the properties of the metallic alloy.
A very important parameter in the foaming process is the foaming temperature. In our study, a temperature of 750 °C was chosen as the most suitable temperature for pure aluminium and 770 °C for the AlSi12 alloy. The results of our study and from the references confirm that the foaming temperature differs from the melting temperature of the matrix material and it cannot be correlated directly from the relation with the melting point of the material. However, while there is no direct correlation between the foaming and the melting temperature, so the foaming temperature should be determined for each matrix material separately, depending on the applied foaming agent. Also, the foaming time has to be chosen in a similar way. It should be determined separately for each alloy and for each foaming temperature.
The most efficient aluminium alloys used for foaming by the PM route are the alloys with higher contents of aluminium and technically pure Al powder. Therefore, so-called aluminium wrought alloys that have a lower content of impurities and of alloying elements (usually less than w = 1 %) are more often used for foaming than the casting alloys that have higher amounts of alloying elements. The reason for the selection of wrought alloys is also due to the fact that according to the Al-Si phase diagram the alloys closer to the aluminium corner of the diagram have a higher temperature interval between the liquidus and the solidus temperature in which pores can be formed. In any case, the viscosity of wrought alloys is higher than that of casting alloys, and therefore pores are formed due to the capture of liberated gases from the foaming agents.
Acknowledgements
This work has been supported by the "Physics and Chemistry of Porous aluminium for Al panels capable of highly efficiently energy absorption" project L2-2410
(D), funded by the Slovenian Research Agency. The views expressed in this publication reflect only the views of the authors and the SR Agency is not liable for any use that may be made of the information contained therein.
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