A novel method for making open-cell aluminum foams with soft ceramic balls.pdf

A novel method for making open-cell aluminum foams with soft ceramic balls Kan-Sen Chou *, Ming-An Song Department of Chemical Engineering, National Tsing Hua University, Kuang-Fu Road, Hsinchu, Taiwan 30013, ROC Received 30 August 2001; accepted 21 November 2001 Abstract Open-cell aluminum foams with porosities up to 90% were made from a novel process using soft ceramic balls that can be compressed easily. Compositions of the ceramic balls include alumina particle, polyvinyl alcohol, water and small amounts of bentonite and either carboxymethyl cellulose or hydroxypropyl-methyl cellulose. ? 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Casting; Open-cell aluminum foams; Soft ceramic balls 1. Introduction Metallic foams have received broad attention due to their potential applications in many dif- ferent fi elds, such as sound insulation, heat ex- changers, fi lters, and catalyst carriers. Although these materials have been studied for some years 1, there are still many recent publications on this subject 29, including both new methods of preparation and in-depth studies of microstructure and properties. With regard to the preparation methods, pow- der metallurgy and casting are two principal techniques adopted for making metallic foams 1. One of the earliest casting process utilized NaCl particle to form a mold that can be removed by simply dissolving in water. The porosity that can be achieved by this method is however limited to around 6070%. Also, the structure of the result- ing metallic foam is usually not very uniform due to the irregularity of starting NaCl particles. Many variations of this process have thus been proposed in the literature. For example, one can use poly- urethane foam and plaster mold 8, or polystyrene particles coupled with high-pressure infi ltration 9. Zhao and Sun tried the sintering and dissolu- tion process using Al and NaCl powders 10. In this article, we will report a novel method to make open-cell aluminum foams using soft ce- ramic balls to replace the hard, cubic-shaped NaCl particles. Since these ceramic balls are deformable, they can be compressed into dense packing. As a result, the products porosity after casting will reach around 90%. These ceramic balls can with- stand the casting temperature and pressure and are easily removed by running water. Scripta Materialia 46 (2002) 379382 www.actamat- *Corresponding author. Tel.: +886-035-715131; fax: +886- 035-715408. E-mail address: .tw (K.-S. Chou). 1359-6462/02/$ - see front matter ? 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(01)01255-6 2. Experimental procedure The soft ceramic balls were made of coarse a- Al2O3particles,polymericadditives,smallamounts of bentonite as the sintering aid and water. The quantity of bentonite was only 1.0% (weight) of alumina and was found suffi cient to provide the ceramic balls with necessary strength to withstand the casting pressure at the casting temperature. Polyvinyl alcohol (PVA) was chosen as the binder for the green strength. Two types of PVA were tested here, with one from Fluka (MW ?130,000; designated as C) and the other one from Sigma (MW ?30,00070,000; designated as D). First the alumina particles were thoroughly mixed with the sintering aid. Then an aqueous solution contain- ing PVA was added. The relative weight of alu- mina:water:PVA was maintained at 100:48:47 for the experiments reported here. This soft mix was then cut into cubes of the size 3, 4 or 5 mm and then put into an empty jar rolled on a ball mill for 120 240 s. Due to its softness, these cubes wouldquickly changeintosphericalballs.Thesphericalshapewas desirable to the regular packing of these ceramic balls in the mold. These ceramic balls were then storedinasealedbagtopreventanylossofmoisture content. In order to increase the compressibility of these ceramic balls, we added another polymer that might be considered as the water-retaining agent, i.e. either carboxymethyl cellulose (CMC; desig- nated as A) or hydroxypropyl-methyl cellulose (HPMC; designated as B). Their quantities were fi xed at 2 wt.% of alumina in this work. The com- pressibility of each ceramic ball was investigated by putting a fi xed weight on top of it and ob- serving its height change. The extent of change, i.e. u Dh=h0, where h0is the original height of the tested ceramic ball. The applied weights range from 5 to 70 g, depending on the softness of the balls. These ceramic balls were then put into a cylindrical mold made of stainless steel in a layer- by-layer manner. Between each layer, these balls were compacted to obtain high packing den- sity, thus decreasing the porosity of the packed bed. Since the ceramic balls are deformable, it is therefore possible to achieve a higher packing density than that using hard particle such as salt. Similar experiments were performed for compari- son using salt particles of various sizes obtained by sieving. To achieve uniform compaction, we placed a soft sponge on top of the ceramic balls to transmit the applied force. A stainless steel plate of fi xed weight was then put on top of the sponge. Another piece of stainless steel falling from a fi xed height of about 10 cm was applied to provide the necessary force for compaction. This action was repeated for 1040 times to continuously compress these ce- ramic balls. The polymeric additives help to keep these ceramic balls from cracking during the compaction. Pictures were taken during this pro- cedure and analyzed to understand the changes in porosity of this packed bed. Finally, the casting was carried out by putting a pre-determined weight of aluminum in circular shape on top of this bed. The whole mold was then put into the top-open furnace and its tempera- ture was raised to 740 ?C, held for about 1.5 h to completely melt the aluminum. Then, a small pressure was applied to push the molten aluminum fl ow through the bed. When this step was done, the mold was taken out of the furnace and cooled in the ambient for a short while. After removing the cast from the mold, it was then put in a water bath. The ceramic balls were removed with the aid of ultrasonic vibration. An open-cell aluminum foam was therefore obtained. 3. Results and discussion First shown in Fig. 1 is the compressibility of an individual ball having various combinations of polymeric additives and under diff erent loads. Here we can notice that ceramic balls containing additive B (i.e. HPMC) is in general softer than those with additive A (i.e. CMC). As a result, a smaller load was needed to compress this type of ceramic balls. Our experience indicated that it is necessary to keep suffi cient amount of water within the balls to avoid any cracking of balls during compaction, which would inevitably induce defects in the fi nal product. Additives B 2C were used throughout the rest of experiments reported here. 380K.-S. Chou, M.-A. Song / Scripta Materialia 46 (2002) 379382 In Fig. 2, the reduction in pore area (obtained from image analysis) of the bed as a function of knocking times was exhibited. Clearly, the eff ect of the falling weight is greater than that of the fi xed weight in aff ecting the porosity. It also seems that the porosity can be decreased continuously to a very low value when we increased the knocking weight or knocking times. This is totally due to the deformable nature of these ceramic balls. A rep- resentative picture of ceramic balls packed in the mold is shown in Fig. 3. The eff ect of ball size was shown next in Fig. 4. Here we notice that the po- rosity of the fi nal foam would increase slightly when the ball size was reduced from 5 to 3 mm. The porosity obtained from using salt particles was also exhibited for comparison. The increase in porosity due to the use of deformable ceramic balls is quite signifi cant. The porosity can be further increased to 88.5% when we pressed these balls with the help of a falling weight. The results are listed in Table 1. A picture of representative Fig. 1. Eff ect of load on the compressibility of an individual ceramic ball for various combinations of additives (B C additive B plus C; B 2C additive B plus two times of C). Fig. 2. Reduction in pore area as a function of knocking time for various combinations of loads (symbol 467/281 means that the fi xed weight is 467 g, while the knocking weight is 281 g). Fig. 3. A representative picture showing ceramic balls after random packing (original ball size 5 mm). Fig. 4. Eff ect of particle (ball) size on porosity of Al foams. No extra force was used to pack these (particles) balls here. Table 1 Product porosity and applied load Applied loadProduct porosity Brushing with hand82.3% 1600 g280 g (25 times)84.6% 1200 g400 g (15 times)87.5% 1200 g400 g (25 times)88.5% Ball size 5 mm; 1600 g/ 280 g (25 times)meaning fi xed weight of 1600 g, falling weight of 280 g and knock 25 times. K.-S. Chou, M.-A. Song / Scripta Materialia 46 (2002) 379382381 open-cell aluminum foam obtained from this work is shown in Fig. 5. 4. Conclusions A novel process was reported here to make open-cell aluminum foams. It utilizes soft ceramic balls to achieve high packing density and hence high porosity of the foams. The porosity of the fi nal product can be manipulated through either ball size and the load applied during packing. After casting with molten aluminum, these ceramic balls can be easily removed with ultrasonic vibra- tion in water. The porosity of th