THE EFFECT OF SIZE AND MORPHOLOGY OF IRON POWDER ON SHELL DENSITY IN LOW CARBON STEEL HOLLOW SPHERES

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1 Powder Metallurgy Progress, Vol.11 (2011), No THE EFFECT OF SIZE AND MORPHOLOGY OF IRON POWDER ON SHELL DENSITY IN LOW CARBON STEEL HOLLOW SPHERES M. Behnam, A. S. Golezani, M. M. Lima Abstract Metallic Hollow Spheres (MHS) play an important role in production of hollow sphere structures and developed Composite Metal Foams. MHS are produced using advanced process based on powder metallurgy method, mostly. In this method, polystyrene spheres are coated in a fluidized bed process with a suspension of metal powder slurry and binder. The green spheres are heat-treated to remove the binder and then sintered to obtain high uniformity single MHS. Metal powders properties like size, morphology, density and others; affect final properties of produced hollow spheres. In this paper, some of the most important parameters in fabrication of MHS are investigated. MHS characteristic was carried out using optical microscope equipped with image analysis software, scanning electron microscopy and density measurement testing. The effect of size and morphology of metal powders on mechanical properties, porosity and surface quality of Low Carbon Steel Hollow Spheres were investigated. Keywords: metallic hollow spheres, composite metal foams, powder metallurgy INTRODUCTION Most commercially available cellular metals do not achieve the properties predicted from the scaling relations that connect the mechanical behaviour of the foam to the bulk material they are produced from [1,2]. This can be attributed to morphological defects in the structure such as missing cell walls, wiggles in the cell wall etc. [3,4]. Using pre-formed hollow spheres in place of irregularly shaped cells was found to overcome the problem of non-uniform deformation. As the hollow spheres possess uniform cell size, cell shape and wall thickness they will overcome the heterogeneity and anisotropy of the foam. Two such hollow sphere foams have been created and studied by Georgia Tech [5] and Fraunhofer Institute [6]. New closed cell Composite Metal Foam (CMF) is processed using casting and powder metallurgy (PM) techniques. The foam is comprised of steel hollow spheres packed into a random loose arrangement, with the interstitial spaces between spheres occupied with a solid metallic matrix [7]. CMF possess a uniform cellular structure and therefore could overcome the limitations caused by morphological defects [8-12]. Metallic Hollow Spheres (MHS) play an important role in production of hollow sphere structures and developed CMF. Several approaches have recently emerged for synthesizing MHS [13-17]. In an approach developed at Fraunhofer Institute (IFAM), Bremen, polystyrene spheres are coated in a fluidized bed process with a suspension Mohammad Behnam, Ali Salemi Golezani, Mahnaz Mehdizadeh Lima, Department of Metallurgy Engineering, Karaj-Islamic Azad University, Iran

2 Powder Metallurgy Progress, Vol.11 (2011), No of metal powder slurry and binder. The resulting green spheres are heat-treated to remove the binder and sintered to obtain high uniformity single metallic hollow spheres [18-20]. In this research, we fabricated Low Carbon Steel (LCS) Hollow Spheres based on powder metallurgy process developed at Fraunhofer Institute (IFAM). Expanded Poly Styrene, Ethyl Silicate binder, gas and water atomized iron powder are used as base materials. This paper reports the study of mechanical and micro structural properties of the LCS MHS using density tests, optical microscope (equipped with image analysis software) and scanning electron microscopy (SEM). It also proposes a relation between the particle size and morphology of iron powder on shell density and surface quality of hollow spheres. MATERIALS Expanded Polystyrene core: Expanded polystyrene cores (EPS) are used in production of the hollow spheres. Sieving the polystyrene spheres is necessary for a narrow size distribution of the final product. The average outer diameter of the EPS used in this investigation was ~4 mm. Coating: Water and Gas atomized iron powders (and the mixture of them) were used for coating the EPS. The chemical composition and properties of powders are listed in Table 1. In Figure 1, the shape and morphology of powders are shown. Binder: Prior to coating, the metal powders are suspended in a binder solvent mixture. Selecting ethyl silicate ((C 2 H 5 ) 4 SiO 4 ) as a binder system has advantages such as good pyrolysis, gluing ability and rheological properties. Tab.1. Physical and chemical properties of iron powders. Chemical Composition [%] Particle H Sample 2 Apparent size Type loss density N. C S Si Mn P Fe distribution [%] [g/cm ] [µm] 1 water atomized Base gas atomized Base water& gas atomized Base Fig.1. SEM Image of: (a) gas; (b) water atomized iron powder.

3 Powder Metallurgy Progress, Vol.11 (2011), No FABRICATION AND CHARACTERIZATION Coating apparatus for production of raw metal hollow spheres were designed and fabricated in Metallurgy Engineering Department of K. I. A. University. Metal powder, binder and solvent weighed and poured into a suspension-container, where stirred to forming a consistent suspension and avoiding any sedimentation. A certain amount of expanded polystyrene spheres are placed into the spray chamber of the coating apparatus where they are fluidized and rotated. An exact amount of suspension through a nozzle sprayed onto the rotating and fluidized polystyrene spheres. The constant air flow during the manufacturing process enables the polystyrene spheres (green-spheres) to dry and sufficiently harden after the coating process. Once sufficiently hardened they are collected from the bottom of the spray chamber. Now the green hollow spheres are prepared for the final heat (a) treatment, i.e. debinding and sintering. A further important stage in the production of metal hollow spheres is the heat treatment, where the green-spheres are transformed into solid hollow sphere shells by sintering. Sintering is done by agglomerate spheres transferring to the furnace tray but in order to prevent them from being sintered to each other, the hollow spheres are coated on the outside with an inert powder (corundum powder) which at the temperature employed will not undergo a chemical or physical reaction with the material of the hollow sphere. Heat treatment is carried out in two stages. During the first stage, the polystyrene core and the binding agent are removed by pyrolysis, in the next stage the aggregation of the metal powders to a solid metal takes place, i.e. the end product is sintered and hardened. Debinding and sintering process were carried out simultaneously under dissociated ammonia atmosphere in a walking beam sintering furnace. Schematic of metal hollow spheres fabrication is shown in Fig.2. (b) Fig.2. Schematic of metal hollow spheres fabrication. The hollow spheres shell, were analyzed by an optical microscope equipped with Material Plus 4.2 image analysis software to determine porosity. The whole and crosssectioned low carbon steel hollow spheres after sintering are shown in Fig.3.

4 Powder Metallurgy Progress, Vol.11 (2011), No Fig.3. 4 mm low carbon steel hollow spheres. Some of spheres were cut manually to show that they are hollow. RESULTS AND DISCUSSIONS Bulk Density The results indicate that there are partial differences between the spheres bulk densities. But in general, increasing in shell porosity of hollow spheres decreases the bulk density partially. Three types of hollow spheres density results can be seen in Fig.4. Fig.4. Bulk density results for hollow spheres produced by three types of powders. Shell Thickness The outer diameter of the hollow sphere is affected by powder properties. Shell thickness can be varied depending on particle size and grain size distribution. Outer shell characteristics such as porosity, density and grading are influenced by the coating and sintering process. Figure 5 illustrates the photo of hollow spheres shell with different size of iron powder. As shown in Fig.5, powder properties such as size and morphology affect the shell grading. In Figure 5 (a), increasing in size of iron powder lead to formation a graded shell and on the other hand increases the surface roughness. All specimens, were coated at the same time but shell thicknesses measured by image analysis software, for water atomized iron powder is about 200 µm while it is about µm for gas atomized and mixture of gas and water atomized powder. This is due to more pores in the spheres made of water atomized powder.

5 Powder Metallurgy Progress, Vol.11 (2011), No Fig.5. Shell dissections produced by: (a) water; (b) gas; (c) mixture of water and gas atomized iron powder. Surface Quality One of the most important parameter that has strong effects on surface quality of hollow spheres is pyrolysis-gas (pressure and volume) at debinding step. Extra amount of gas in green hollow spheres during debinding should exit from the space between powder particles. While using fine powders there isn t enough space for waste gas and so sudden explosion in hollow spheres shell occurs (Fig.6). Heating up during pyrolysis can affect the quality of surface but in this study for all hollow spheres the rate of heating was the same. (b) (c) (a) (b) Fig.6. Hollow spheres after sintering produced by: (a) gas; (b) water atomized iron powder. Fig.7. Iron powder types vs. wastage percent.

6 Powder Metallurgy Progress, Vol.11 (2011), No In Figure 7, the percent of exploded hollow spheres versus powder particle size is shown. As we can see, increasing in powder particle size is related to decreasing in sintering wastage. Shell Porosity Shell porosity after sintering of hollow spheres, depends on several parameters such as: iron powder properties (particle size, morphology and density), binder system, coating procedure, sintering time and temperature. Sintering in higher temperature or using double sintering cycle can decrease remained shell porosity and so improve the mechanical properties of hollow spheres, however for cost reasons, sintering process is preferably done for one time. Another method to optimization of remained shell porosity is to control the particle size and morphology of powder. Since there is no compaction in fabrication of hollow spheres, increasing the apparent density of powder, remained porosity decreases. Porosity results are shown in Fig.8. Porosity analysis reports about 10% porosity in hollow spheres shell produced by gas atomized iron powder (Fig.8 (a)). Due to very small particle size and high apparent density of gas atomized iron powder, remained porosity is supposed to be minimized after sintering. However experimental results indicate that decreasing in particle size causes increasing occupied surfaces by binder. On the other hand, removing more binder bond between iron particles led to formation of more porosity after sintering. In this case, as explained, due to more amount of binder bond and no sufficient space for waste gas exiting, there is a high possibility of hollow spheres explosion. (a) (b) (c) Fig.8. Porosity in HS shells produced by: (a) gas; (b) water; (c) Mixture of water and gas atomized iron powder. Comparing the morphology of gas atomized powder with that of water atomized powder, it can be concluded the morphology of water atomized powder is irregular and so there are large spaces between particles. These spaces help easy exit of waste gas during pyrolysis and sintering. That can be observed in Fig.8 (b) the average of porosity is about 17%. In order to optimization of remained shell porosity percent and wastage percent, mixture of gas and water atomized iron powder was used. For this purpose, the powders were mixed together in equal weight ratios. Mixing operation was performed in a laboratory double cone mixer for 30 minutes. SEM image of powder particles is shown in Fig.9. As can be seen in this figure, the empty spaces between larger particles of water atomized powder occupied by small gas atomized particles. This led to an increase in apparent density of powder. Density results for this type of powder are confirmed. In this case, surface contact between the particles is higher and thus after sintering process porosity percent decreases in hollow spheres shell below 7%.

7 Powder Metallurgy Progress, Vol.11 (2011), No Fig.9. SEM image of mixture of water and gas atomized iron powder size and morphology (3). The relationship between remained shell porosity percent versus powder apparent density is shown in the Fig.10. As can be seen from the results, increasing in apparent density of metal powders leads to decreasing remained shell porosity. Fig.10. Relationship between remained shell porosity percent and powder apparent density. Due to the use of reducing atmosphere (ammonia cracked) during sintering step and low thickness, it is predict that the carbon content in sintered hollow spheres somewhat reduces. The results of experiments are confirming. Carbon content for three types of sintered hollow spheres (produced by water, gas and mixture of water and gas atomized powder) is %, 0.57% and 0.23%, respectively. CONCLUSION Metallic Hollow Spheres as a base material in production of hollow sphere structures and developed Composite Metal Foams were produced using advanced process based on powder metallurgy method in Metallurgy Engineering Department of K. I. A. University. In order to achieve high performance of hollow spheres main focus was on optimization of base materials. Outer shell characteristics such as porosity, density and grading are influenced by base material, coating and sintering process. According to experimental results powder properties such as size and morphology affect the quality of surface. Increasing in particle size of iron powder leads to graded shell formation. Using

8 Powder Metallurgy Progress, Vol.11 (2011), No coarser powder during pyrolysis, waste gas exit from pores easily while using fine powders there isn t enough space for waste gas and so sudden explosion in hollow spheres shell occurs. Powder particle size also can influence remained porosity in sphere shells. Remained shell porosity in hollow spheres produced by gas atomized iron powder measured about 10%. For water atomized iron powder and mixture of water and gas atomized iron powder, remained shell porosity were measured about 17% and 7%, respectively. As can be seen from the results, increasing in apparent density of metal powders leads to decreasing remained shell porosity. Acknowledgements The authors acknowledge Iran Powder Metallurgy Complex for access to their sintering furnace and iron powders. Also thanks Soghra Yousefli, for her helpful as assistance in this study. REFERENCES [1] Sugimura, Y., Meyer, J., He, MY., Bart-Smith, H., Grenstedt, J., Evans, AG.: Acta Mater., vol. 45, 1997, no. 12, p [2] Sugimura, Y., Rabiei, A., Evans, AG., Harte, AM., Fleck, NA.: Mater. Sci. Eng. A, vol. 269, 1999, p. 38 [3] Ashby, MF., Evans, AG., Fleck, NA., Gibson, LJ., Hutchinson, JW., Wadley, HNG.: Metal Foams: A Design Guide, 2000 [4] Rabiei, A., O Neill, A., Neville, B. In: MRS Fall 2004 Proceedings 841, 2005, p. 517 [5] Rabiei, A., Neville, B., Reese, N., Vendra, L.: Mater. Sci. Forum, vol , 2007, p [6] Rabiei, A., O Neill, A.: Mater. Sci. Eng. A, vol. 404, 2005, p. 159 [7] Rabiei, A., Vendra, LJ.: Materials Letters, vol. 63, 2009, p. 533 [8] Rabiei, A., O Neill, A., Neville, B. In: MRS Fall 2004 Proceedings 841, 2005, p. 517 [9] Rabiei, A., Neville, B., Reese, N., Vendra, L.: Mater. Sci. Forum, vol , 2007, p [10] Rabiei, A., O Neill, A.: Mater. Sci. Eng. A, vol. 404, 2005, p. 159 [11] Rabiei, A., Vendra, L., Young, N., Neville, BP.: Mater Trans. JIM, 2006, p [12] Rabiei, A., O Neill, AT.: Materials Science and Engineering A, vol. 404, 2005, p. 159 [13] Ashby, MF., Evans, A., Fleck, NA., Gibson, LJ., Hutchinson, JW., Wadley, HNG.: Metal Foams: A Design Guide. Butterworth-Heinemann, Massachusetts, 2000 [14] Degischer, HP., Kriszt, B.: Handbook of Cellular Metals. Production, Processing, Applications. Weinheim : Wiley-VCH Verlag GMBH, 2002 [15] Gibson, Ashby: Cellular Solids: Structure and Properties. 2nd ed. Cambridge : Cambridge University Press, 1997 [16] Andersen, O., Stephani, G. In: Handbook of cellular metals. Production, Processing, Applications. Eds. HP. Degischer, B. Kriszt. Weinheim : Wiley-VCH Verlag GmbH, 2002 [17] Vendra, LJ.: Processing and Characterization of Aluminum-Steel Composite Metal Foams. Raleigh, North Carolina, 2008 [18] Cellular Metallic Materials. Frounhofer, IFAM [19] Jaeckel, M., Smigilski, H.: Coating of polymeric spheres with particles. European Patent DE , 1988 [20] Augustin, C., Hungerbach, W.: Materials Letters, vol. 63, 2009, p. 1109