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Microstructure, permeability and rheological behavior of lost foam refractory coatings
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Surface & Coatings Technology 202 (2008) 4636 – 4643 www.elsevier.com/locate/surfcoat
Microstructure, permeability and rheological behavior of lost foam refractory coatings Yaser Akbarzadeha , Mostafa Rezaeia,⁎, Ali Akbar Babaluoa , Ali Charchia , Hamid Reza Azimia , Yahya Bahlulib a b
Research Center for Polymeric Materials, Sahand University of Technology, P.O. Box 51335-1996, Tabriz, Iran Metallurgical Research Center, Iran Tractor Foundry Company, Tabriz, Iran, P.O. Box 51845-338, Tabriz, Iran Received 29 September 2007; accepted in revised form 25 March 2008 Available online 10 April 2008
Abstract The effect of microstructure on rheology and permeability of three commercial lost foam refractory coatings available on the market called samples I, II, and III, respectively was investigated in this study. Thermal gravimetric/differential thermal analysis (TG/DTA) method was used for detailed analysis of the organic components and to determine the thermal stability of the coatings. Particle shape and size and particle size distribution (PSD) were obtained by optical microscopy and morphological studies were carried out by scanning electron microscopy (SEM). Also the scanning electron microscopy energy dispersive X-ray analysis (SEM/EDXA) technique was used for elemental analysis of refractory particles. To determine the crystalline structure of the samples, X-ray diffraction (XRD) analysis was carried out. Permeability measurements were conducted with a modified apparatus originally used for determining the permeability of casting sand. Finally the rheological behavior of the samples was investigated using a rotating coaxial rheometer to provide the flow curve for coating suspensions. The studies revealed that coating I has the highest permeability, which can be due to its large mean particle size and wide particle size distribution (PSD). Furthermore it was found that the investigated refractory coatings behave as non-Newtonian fluids with shear thinning behavior. All coatings exhibited yield stresses, indicate that they behave as Bingham-type pseudoplastic fluids. © 2008 Elsevier B.V. All rights reserved. PACS codes: SURFCOAT-D-07-01510 Keywords: Refractory coating; Lost foam; Microstructure; Permeability; Rheology
1. Introduction Lost foam casting is an economic and new method to produce complex metal parts. Although this technology has several advantages compared to conventional casting methods, it still suffers from some inherent disadvantages, including surface carbon defects, melt penetration, surface and bulk pinholes [1–6]. In this method the cast is made of expanded polystyrene (EPS) or styrene-methyl metacrylate (St-MMA) copolymer foam. Lost foam patterns are attached to a gating system and
⁎ Corresponding author. Tel.: +98 412 3459086; fax: +98 412 3224950. E-mail address: [email protected] (M. Rezaei). 0257-8972/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.03.036
then a thin layer of refractory coating material is applied to the entire assembly. After the coating has been completely dried, the foam pattern is entirely imbedded in unbounded sand in the vented container. During the sand pouring cycle, vibration is applied to the flask to compact the sand [7]. Lost foam casting is a much more complicated process in both physical and chemical aspects than traditional sand mold casting. Different phenomena, such as heat and mass transfer, fluid flow, chemical reaction, solidification, etc. are involved in this casting technique [8–11]. The most important factors in lost foam casting process are rheological, thermophysical and microstructural properties of the refractory coating materials. The coating is essentially composed of refractory particles, two binders, suspending media (especially water), surfactants, biocides, dispersing and thixotropic agents.
Y. Akbarzadeh et al. / Surface & Coatings Technology 202 (2008) 4636–4643
Silica, alumina, zirconia, chromite, and alumina-silicates such as mullite and pyrophyllite are used as refractory components. One binder provides adhesion and cohesion before drying, strength after drying and during pouring the molten metals, whereas another binder holds together the refractory particles. Surfactants and suspending agents are used to wet and coat the foam patterns and prevent from particles agglomeration and sedimentation [7]. The coatings used in lost foam casting are expected to play two key roles: limiting metal heat loss rate and facilitating a rapid foam pattern removal, both of which are critical to eliminate casting defects. The performance of coating heat and mass transfer often varies as casting shape, pattern quality, or alloy change. The difficulty in selecting proper coatings reflects the complexity of the casting variables (coating, foam, metal, part geometry and etc.) effects on metal fill and solidification. A thorough understanding of coating structure–property relationships would allow the coatings to be modified to reduce lost foam casting defects [12]. Many research studies have been reported on the structure– property relationships of various powder coatings. In powder coating methods, the voids that remain from high-temperature sintering processes are disconnected. In contrast, the coatings used in lost foam casting are made of numerous discrete solid particles and a void phase that forms continuous passage ways. Some research activities are intensively targeted on measuring void distributions using various liquid (miscible or immiscible)
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porosimetry techniques and describing the void distribution using different mathematical approaches [12]. Other studies focus directly on evaluating the heat and gas transfer properties of commercial lost foam casting coatings. Although the measured and coating transport properties have been compared [13], many coating structure–property correlations are still unknown. Dip-coating is used to prepare lost foam refractory coatings. The microstructure of the solid film is controlled by the relative rates of aggregation and evaporation in case of particulate systems. The rate of aggregation depends on the stability and dispersity of the coating. Aggregation and subsequent network formation tends to stiffen the structure. The rate of evaporation is determined by process parameters such as substrate dipping speed and drying conditions. The evaporation process leads to compaction of the structure [14]. It is, crucial to gain a detailed insight into the physico-chemical properties of the coating bath and their influence on microstructural evolution. Improvement of the lost foam casting products requires a detailed systematic study of the microstructure, permeability and rheological behavior of the lost foam coatings. This study investigates the microstructure, permeability and rheological properties of three commercial coatings. The relationship between coating microstructure, permeability and rheological properties has been derived from experimental observations.
Fig. 1. TG (%) and DTA results versus temperature for dried coating I.
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2. Materials and methods 2.1. Materials In this work, three commercial types of refractory coatings available on the market called samples I, II, and III, respectively have been investigated. These slurries are used in Iran tractor LFC foundries and are produced by three commercial suppliers.
Fig. 3. Particle size distribution of different refractory coatings.
These coatings were water base slurry which can be applied by dipping, flow coating, spraying and brushing. These coatings were used to examine the effects of alternative refractory materials, as well as the particle shape and size distribution on coating properties. The coatings were applied to the pattern by dipping. 2.2. Characterization 2.2.1. Thermal decomposition Primary investigation of evolved organic and refractory components of coatings was conducted by heating in a furnace from room temperature to 700 °C, preserving at this temperature for 5 h and weighing of the samples at different time intervals. The thermal gravimetric (TG) method was used for detailed analysis of organic components and thermal stability of the coatings. In this experiment, the samples were dried at 50 °C for 18h to ensure the complete vaporization of their solvent, and then 10–15mg of dried coating sample were heated at a heating rate of 20 °C/min from 30 °C to 550 °C under nitrogen purge.
Fig. 2. Optical micrographs of (a) coating I, (b) coating II and (c) coating III.
2.2.2. Morphological study Particles shape, mean size and size distribution play a key role in the permeability and rheological behavior of lost foam coatings. Particle size and particle size distribution (PSD) of coatings were studied with an optical microscope (Leica DMLS) under transmitted light. PSD was determined using Image Pro-Express software. In this experiment, coating samples were heated at 700 °C for 5 h to ensure that their binders degraded completely and only refractory materials remained. For morphological studies, scanning electron microscopy (SEM, LEO 440I, UK) were used. SEM micrographs were taken from samples which prepared at 700 °C as mentioned before. SEM energy dispersive X-ray analysis (SEM/EDXA) was also used for elemental analysis of refractory particles.
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2.2.3. XRD analysis The crystalline structure of the refractory particles was determined by X-ray diffraction (XRD) patterns. XRD was carried out on a D500 Siemens diffractometer using CUKα (λ = 1.54Å) radiation operating at 40 kV and 50 mA. 2.2.4. Permeability measurements An apparatus for measuring the gas permeability of casting sand (provided by George Fischer, Germany) was used to measure the permeability of refractory coating. The coating samples were obtained by dipping a disc of stainless steel mesh with 5cm diameter into the coating slurries, whose rheological properties were well controlled. The steel mesh has very high permeability, which will not affect the measuring results for the coating permeability. The samples were dried in an oven at 50 °C for 2 h. After drying, the thickness of each coating was measured by using a digital micrometer (provided by Mitutoya, Japan). Coated stainless steel support was held between two flanges and sealed. Air was applied to one side of the coating and the changes of the pressure were recorded in time intervals. By experience, it is known that these refractory coatings with densities lower than 1.3 g/cm3 could not form the required thickness on the foam pattern. On the other hand, if the density of suspension exceeds 1.4 g/cm3 a non-uniform layer of coating is formed on the pattern. So the densities of 1.3 to 1.4 were considered for permeability measurements and rheological studies. 2.2.5. Rheological measurements A Mettler RM180 digital coaxial cylinder rotary rheometer (Mettler-Toledo, Switzerland), was used to measure the rheological properties at ambient temperature (22 °C). The slurries were pre-sheared at constant shear rate (800s− 1) for 10 min before rheological experiments to ensure their uniformity.
Table 1 SEM/EDXA elemental analysis of different refractory particles Elements
Coatings Coating I
Fig. 4. SEM micrographs of particles in (a) coating I, (b) coating II and (c) coating III.
Al Si O Fe Cr Cl Mg C Na K Ca Cu Ti Total
Coating II
Coating III
%wt
% atomic
%wt
% atomic
%wt
% atomic
16.98 29.79 50.15 2.56 0.04 0.48 0 0 0 0 0 0 0 100
12.87 21.71 64.18 0.94 0.02 0.28 0 0 0 0 0 0 0 100
15.02 18.83 40.15 4.45 0 3.55 0 8.01 1.94 2.86 1.26 3.91 0 100
11.52 13.87 51.91 1.65 0 2.07 0 13.8 1.75 1.51 0.65 1.27 0 100
13.26 23.18 39.44 5.6 0 0.56 5.16 1.14 0 5.8 1.75 2.17 1.3 100
10.94 18.36 55.60 2.23 0 0.41 4.72 2.11 0 3.3 0.97 0.76 0.6 100
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3. Results and discussion 3.1. Thermal decomposition The results of thermal decomposition (weight fraction of the constituents) of dried refractory coatings indicate that coating I contains 91.3%wt and 8.7%wt of refractory materials and organic components (binders, surfactants, biocides, dispersing
and thixotropic agents) respectively. However, the weight percentage of refractory and organic materials respectively are 94.4 and 5.6%wt for coating II and for dried coating III are 85.2 and 14.8%wt. For detailed investigation, only coating I was selected as a sample for TG/DTA analysis. TG/DTA curve for this coating is shown in Fig. 1. As it is clear from this figure, the onset of thermal degradation is 246.2 °C and the weight loss up to that
Fig. 5. XRD pattern for (a) coating I, (b) coating II and (c) coating III between 2θ = 0–90°.
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Fig. 5 (continued).
temperature is less than 0.5%wt. There is a very sharp weight loss from 246.2 °C to 375.2 °C and almost 8%wt of organic compound is decomposed in this temperature range. At 375.2 °C the slope of the curve is changed and weight loss proceeds with a relatively slow rate up to 500 °C. TG results reveal that there are at least two organic components in this coating. DTA results indicate 3 peaks at the temperature range of 50 to 500 °C. Some of these peaks could be from phase transitions of organic components. 3.2. Morphological study The optical microscopy images of coatings are shown in Fig. 2. From these images, it is clear that all coatings have particles with irregular shape and uniformity and that the mean particle size of coating I is larger than the others. Existence of a high percentage of tiny particles in these coatings causes the pores between large particles to be closed and the permeability to be decreased. The particle size distribution (PSD) for three coatings was determined from Fig. 2 by Image Pro-Express software. The obtained results are shown in Fig. 3. PSD in coating I is the widest and average particle size is the largest compared to the other coatings, but in coating III, the PSD and average particle size are the narrowest and the smallest one, respectively. SEM images for three coatings are shown in Fig. 4. As expected, the particle size of coating I is coarser than other coatings, which is the major reason for high permeability of this
coating compared to coatings I and II. In fact, SEM images are consistent with the optical micrographs. SEM/EDXA elemental analysis results are presented in Table 1. The major elements in coating I refractory particles are Al, Si and oxygen. Also there is a trace of Fe in this coating. Fe element in hematite (Fe2O3) form is very useful to reduce lustrous carbon defects in lost foam iron casting. This component accelerates oxidization of lustrous carbon, which is produced from lost foam decomposition [4–6]. There is the same trend in other coatings. Some trace of Fe element could be observed in these coatings. There is an optimum content of hematite (2–3%wt) in refractory coating, because using more than this value reduces the mechanical properties of casted iron parts [4–6]. 3.3. XRD investigations XRD patterns for different coatings are shown in Fig. 5. XRD results show the presence of sillimanite (kyanite: Al2SiO5), hematite (Fe2O3), quartz, clay minerals and feldspar in coating I. As mentioned before, EDXA results confirm the presence of Al, Si, O and Fe elements. Furthermore mulite (Al6Si2O13) and feldspar were found in coating II. Also the results confirm the presence of sillimanite, mulite, trydymite and clay minerals in coating III. As it is clear from Fig. 5, more crystalline structures are observed in coating I and coating II than coating III. On the other hand, the presence of Fe element in hematite form (Fe2O3) in coating I can be the major reason for selecting
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of this coating in lost foam iron casting, because of its effect on elimination of lustrous carbon defects [4–6]. 3.4. Permeability measurements Fig. 6 shows the permeability of coatings in terms of millimeters of water scale (mm.w.s) in two different densities versus coating thickness. As it was expected, the permeability
Fig. 7. Permeability of coatings at density of 1.3 g/cm3.
of the samples decreases as their thickness increases. Also, samples with higher density show lower permeability. Comparison of the permeability of the coatings at density 1.3 g/cm3 is conducted in Fig. 7. It was found that coating I is more permeable than the others. On the other hand with increasing coating thickness, the effect of density is reduced, especially density effect in coatings I and II is negligible. As it was mentioned in the morphological study, coating I had a larger average particle size and wider particle size distribution than the other coatings. Larger average particle size and wider particle size distribution establish larger pore size in coating, which is the most important reason for the higher permeability of coating I compared to the others. With increasing of coating I thickness, some of these pores are isolated and considered as ineffective pores, thus the coating permeability are decreased with a higher rate than coatings I and II. Considering that in iron casting a high amount of gas is produced, it is advisable to use coating I in such applications and coatings II and III in aluminum casting. It should be noted that the selection of the optimum thickness of coating must be undertaken by considering all of the parameters affecting the quality of the product.
Fig. 6. Permeability of coatings at two densities (a) coating I, (b) coating II and (c) coating III.
Fig. 8. Shear stress versus shear rate for coatings at T = 22 °C.
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Fig. 9. Viscosity versus shear rate at T = 22 °C.
3.5. Rheological study Shear stress versus shear rate curves for different coatings is shown in Fig. 8. As seen from this figure, these refractory coatings have a non-linear flow curve and exhibit shear thinning behavior typical of non-Newtonian fluids. Also, a yield stress is found for these coatings indicating that these suspensions are Bingham pseudoplastic fluids. The apparent viscosities of the refractory coatings versus shear rate are shown in Fig. 9. At very low shear rates, viscosities significantly decrease with shear rate but this trend suddenly decelerates and these suspensions reach a constant viscosity. This behavior can be related to the presence of thixotropic agents in the suspension [7] and disagglomeration of refractory clusters with increasing shear stress. Among the samples, coating I has the lowest and coating III has the highest viscosity. The viscosity in coating I is more sensitive to shear rate. This behavior can be related to the wide PSD of coating I, as shown in Fig. 3. In such suspensions particles of different sizes are present and the smaller particles are interposed between larger particles, causing a reduction in the inter-particle impact, and then resulting in a considerable decrease in viscosity. In other words, the smaller particles act as lubricants to facilitate the rotation of larger particles, leading to a reduction in the viscosity [15]. The results of the rheological studies are consistent to the morphology of the materials: coating III with the narrowest PSD is the most viscous coating, whereas coating I has the lowest viscosity and coating II lies inbetween. 4. Conclusion A systematic study on the microstructure, permeability and rheological properties of three commercial refractory coatings was conducted in this article. Different methods were used for characterizing refractory coatings. A thermal decomposition investigation was conducted on the coatings and detailed analysis of the organic components was carried out by TG/DTA method on coating I. The obtained results revealed that there were at least two organic components in this coating. Morphological studies were carried out by optical microscopy and SEM. PSD in coating I, was the widest and average particle size was the largest compared to the other coatings.
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SEM/EDXA elemental analysis indicated that the major elements in refractory particles were Al, Si and oxygen. Also there was a trace of Fe in these coatings. The crystalline structure of the refractory particles was determined by XRD patterns. XRD results showed the presence of Fe element in hematite form in coating I. Permeability results indicated that the permeability of the samples decreased as their thickness and density increased. Also, it was found that coating I was more permeable than the other coatings. Larger average particle size and wider PSD establish larger pore size in coating which was the most important reason for higher permeability of coating I compared to coatings II and III. The rheological results indicated that refractory coatings had non-linear flow curve like Bingham pseudoplastic fluids. Among the samples, coating I had the lowest and coating III had the highest viscosity. According to the wide PSD in coating I and presence of particles with different sizes, the smaller particles were interdispersed between larger particles, causing a reduction in the inter-particle impact and resulting in a decrease in viscosity. However, shear thinning behavior of coating I was more pronounced than that of the other coatings. This trend could be related to the wide PSD of this coating. In other words, the smaller particles act as lubricants to facilitate the rotation of larger particles, leading to a considerable reduction in the viscosity. Acknowledgments The authors gratefully acknowledge the Sahand University of Technology and the Iran Tractor Foundry Company for the financial support. The authors would also like to gratefully acknowledge the research project team members of Research Center for Polymeric Materials and Metallurgical Research Center: Dr. Farhang Abbasi, Mr. Zeighami, Mr. Khalichi and Mr. Ferdosi for their involvement and encouragement. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
J.J. Green, C.W. Ramsay, D.R. Askeland, J. AFS Trans. 106 (1998) 339. F. Sonnenberg, K.M. Taristo, T.V. Johannes, US Pat 6,303,664 B1, (2001). F. Sonnenberg, US Pat., 0081379 A1, (2002). R.L. Naro, J. AFS Trans. 154 (2002) 1. R.L. Naro, D. Tenaglia, J. AFS Trans. 85 (1977) 65. R.L. Naro, J. Modern Casting 93 (2003) 32. S.L. Bakhtiyarov, R.A. Overfelt, J. Elast. Plast. 32 (2000) 73. Y. Liu, S.I. Bakhtiarov, R.A. Overfelt, J. Mater. Sci. 37 (2002) 2997. M. Sands, S. Shivkumar, J. Mater. Sci. 38 (2003) 667. M. Sands, S. Shivkumar, J. Mater. Sci. 41 (2006) 2373. M. Sands, S. Shivkumar, J. Mater. Sci. 38 (2003) 2233. X. Chen, D. Penumadu, J. Mater. Sci. 41 (2006) 3403. Q. Zhao, H. Wang, S. Biederman, D. Jason, J.S. Parish, AFS Trans. 11 (2005) 1. [14] L. Günther, W. Peukert, Colloids Surf. Physicochem. Eng. Aspect 225 (2003) 49. [15] A.V. Shenoy, Rheology of Filled Polymer Systems, Kluwer Academic Publishers, Dordrecht, 1999.
Surface & Coatings Technology 202 (2008) 4636 – 4643 www.elsevier.com/locate/surfcoat
Microstructure, permeability and rheological behavior of lost foam refractory coatings Yaser Akbarzadeha , Mostafa Rezaeia,⁎, Ali Akbar Babaluoa , Ali Charchia , Hamid Reza Azimia , Yahya Bahlulib a b
Research Center for Polymeric Materials, Sahand University of Technology, P.O. Box 51335-1996, Tabriz, Iran Metallurgical Research Center, Iran Tractor Foundry Company, Tabriz, Iran, P.O. Box 51845-338, Tabriz, Iran Received 29 September 2007; accepted in revised form 25 March 2008 Available online 10 April 2008
Abstract The effect of microstructure on rheology and permeability of three commercial lost foam refractory coatings available on the market called samples I, II, and III, respectively was investigated in this study. Thermal gravimetric/differential thermal analysis (TG/DTA) method was used for detailed analysis of the organic components and to determine the thermal stability of the coatings. Particle shape and size and particle size distribution (PSD) were obtained by optical microscopy and morphological studies were carried out by scanning electron microscopy (SEM). Also the scanning electron microscopy energy dispersive X-ray analysis (SEM/EDXA) technique was used for elemental analysis of refractory particles. To determine the crystalline structure of the samples, X-ray diffraction (XRD) analysis was carried out. Permeability measurements were conducted with a modified apparatus originally used for determining the permeability of casting sand. Finally the rheological behavior of the samples was investigated using a rotating coaxial rheometer to provide the flow curve for coating suspensions. The studies revealed that coating I has the highest permeability, which can be due to its large mean particle size and wide particle size distribution (PSD). Furthermore it was found that the investigated refractory coatings behave as non-Newtonian fluids with shear thinning behavior. All coatings exhibited yield stresses, indicate that they behave as Bingham-type pseudoplastic fluids. © 2008 Elsevier B.V. All rights reserved. PACS codes: SURFCOAT-D-07-01510 Keywords: Refractory coating; Lost foam; Microstructure; Permeability; Rheology
1. Introduction Lost foam casting is an economic and new method to produce complex metal parts. Although this technology has several advantages compared to conventional casting methods, it still suffers from some inherent disadvantages, including surface carbon defects, melt penetration, surface and bulk pinholes [1–6]. In this method the cast is made of expanded polystyrene (EPS) or styrene-methyl metacrylate (St-MMA) copolymer foam. Lost foam patterns are attached to a gating system and
⁎ Corresponding author. Tel.: +98 412 3459086; fax: +98 412 3224950. E-mail address: [email protected] (M. Rezaei). 0257-8972/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.03.036
then a thin layer of refractory coating material is applied to the entire assembly. After the coating has been completely dried, the foam pattern is entirely imbedded in unbounded sand in the vented container. During the sand pouring cycle, vibration is applied to the flask to compact the sand [7]. Lost foam casting is a much more complicated process in both physical and chemical aspects than traditional sand mold casting. Different phenomena, such as heat and mass transfer, fluid flow, chemical reaction, solidification, etc. are involved in this casting technique [8–11]. The most important factors in lost foam casting process are rheological, thermophysical and microstructural properties of the refractory coating materials. The coating is essentially composed of refractory particles, two binders, suspending media (especially water), surfactants, biocides, dispersing and thixotropic agents.
Y. Akbarzadeh et al. / Surface & Coatings Technology 202 (2008) 4636–4643
Silica, alumina, zirconia, chromite, and alumina-silicates such as mullite and pyrophyllite are used as refractory components. One binder provides adhesion and cohesion before drying, strength after drying and during pouring the molten metals, whereas another binder holds together the refractory particles. Surfactants and suspending agents are used to wet and coat the foam patterns and prevent from particles agglomeration and sedimentation [7]. The coatings used in lost foam casting are expected to play two key roles: limiting metal heat loss rate and facilitating a rapid foam pattern removal, both of which are critical to eliminate casting defects. The performance of coating heat and mass transfer often varies as casting shape, pattern quality, or alloy change. The difficulty in selecting proper coatings reflects the complexity of the casting variables (coating, foam, metal, part geometry and etc.) effects on metal fill and solidification. A thorough understanding of coating structure–property relationships would allow the coatings to be modified to reduce lost foam casting defects [12]. Many research studies have been reported on the structure– property relationships of various powder coatings. In powder coating methods, the voids that remain from high-temperature sintering processes are disconnected. In contrast, the coatings used in lost foam casting are made of numerous discrete solid particles and a void phase that forms continuous passage ways. Some research activities are intensively targeted on measuring void distributions using various liquid (miscible or immiscible)
4637
porosimetry techniques and describing the void distribution using different mathematical approaches [12]. Other studies focus directly on evaluating the heat and gas transfer properties of commercial lost foam casting coatings. Although the measured and coating transport properties have been compared [13], many coating structure–property correlations are still unknown. Dip-coating is used to prepare lost foam refractory coatings. The microstructure of the solid film is controlled by the relative rates of aggregation and evaporation in case of particulate systems. The rate of aggregation depends on the stability and dispersity of the coating. Aggregation and subsequent network formation tends to stiffen the structure. The rate of evaporation is determined by process parameters such as substrate dipping speed and drying conditions. The evaporation process leads to compaction of the structure [14]. It is, crucial to gain a detailed insight into the physico-chemical properties of the coating bath and their influence on microstructural evolution. Improvement of the lost foam casting products requires a detailed systematic study of the microstructure, permeability and rheological behavior of the lost foam coatings. This study investigates the microstructure, permeability and rheological properties of three commercial coatings. The relationship between coating microstructure, permeability and rheological properties has been derived from experimental observations.
Fig. 1. TG (%) and DTA results versus temperature for dried coating I.
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2. Materials and methods 2.1. Materials In this work, three commercial types of refractory coatings available on the market called samples I, II, and III, respectively have been investigated. These slurries are used in Iran tractor LFC foundries and are produced by three commercial suppliers.
Fig. 3. Particle size distribution of different refractory coatings.
These coatings were water base slurry which can be applied by dipping, flow coating, spraying and brushing. These coatings were used to examine the effects of alternative refractory materials, as well as the particle shape and size distribution on coating properties. The coatings were applied to the pattern by dipping. 2.2. Characterization 2.2.1. Thermal decomposition Primary investigation of evolved organic and refractory components of coatings was conducted by heating in a furnace from room temperature to 700 °C, preserving at this temperature for 5 h and weighing of the samples at different time intervals. The thermal gravimetric (TG) method was used for detailed analysis of organic components and thermal stability of the coatings. In this experiment, the samples were dried at 50 °C for 18h to ensure the complete vaporization of their solvent, and then 10–15mg of dried coating sample were heated at a heating rate of 20 °C/min from 30 °C to 550 °C under nitrogen purge.
Fig. 2. Optical micrographs of (a) coating I, (b) coating II and (c) coating III.
2.2.2. Morphological study Particles shape, mean size and size distribution play a key role in the permeability and rheological behavior of lost foam coatings. Particle size and particle size distribution (PSD) of coatings were studied with an optical microscope (Leica DMLS) under transmitted light. PSD was determined using Image Pro-Express software. In this experiment, coating samples were heated at 700 °C for 5 h to ensure that their binders degraded completely and only refractory materials remained. For morphological studies, scanning electron microscopy (SEM, LEO 440I, UK) were used. SEM micrographs were taken from samples which prepared at 700 °C as mentioned before. SEM energy dispersive X-ray analysis (SEM/EDXA) was also used for elemental analysis of refractory particles.
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2.2.3. XRD analysis The crystalline structure of the refractory particles was determined by X-ray diffraction (XRD) patterns. XRD was carried out on a D500 Siemens diffractometer using CUKα (λ = 1.54Å) radiation operating at 40 kV and 50 mA. 2.2.4. Permeability measurements An apparatus for measuring the gas permeability of casting sand (provided by George Fischer, Germany) was used to measure the permeability of refractory coating. The coating samples were obtained by dipping a disc of stainless steel mesh with 5cm diameter into the coating slurries, whose rheological properties were well controlled. The steel mesh has very high permeability, which will not affect the measuring results for the coating permeability. The samples were dried in an oven at 50 °C for 2 h. After drying, the thickness of each coating was measured by using a digital micrometer (provided by Mitutoya, Japan). Coated stainless steel support was held between two flanges and sealed. Air was applied to one side of the coating and the changes of the pressure were recorded in time intervals. By experience, it is known that these refractory coatings with densities lower than 1.3 g/cm3 could not form the required thickness on the foam pattern. On the other hand, if the density of suspension exceeds 1.4 g/cm3 a non-uniform layer of coating is formed on the pattern. So the densities of 1.3 to 1.4 were considered for permeability measurements and rheological studies. 2.2.5. Rheological measurements A Mettler RM180 digital coaxial cylinder rotary rheometer (Mettler-Toledo, Switzerland), was used to measure the rheological properties at ambient temperature (22 °C). The slurries were pre-sheared at constant shear rate (800s− 1) for 10 min before rheological experiments to ensure their uniformity.
Table 1 SEM/EDXA elemental analysis of different refractory particles Elements
Coatings Coating I
Fig. 4. SEM micrographs of particles in (a) coating I, (b) coating II and (c) coating III.
Al Si O Fe Cr Cl Mg C Na K Ca Cu Ti Total
Coating II
Coating III
%wt
% atomic
%wt
% atomic
%wt
% atomic
16.98 29.79 50.15 2.56 0.04 0.48 0 0 0 0 0 0 0 100
12.87 21.71 64.18 0.94 0.02 0.28 0 0 0 0 0 0 0 100
15.02 18.83 40.15 4.45 0 3.55 0 8.01 1.94 2.86 1.26 3.91 0 100
11.52 13.87 51.91 1.65 0 2.07 0 13.8 1.75 1.51 0.65 1.27 0 100
13.26 23.18 39.44 5.6 0 0.56 5.16 1.14 0 5.8 1.75 2.17 1.3 100
10.94 18.36 55.60 2.23 0 0.41 4.72 2.11 0 3.3 0.97 0.76 0.6 100
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3. Results and discussion 3.1. Thermal decomposition The results of thermal decomposition (weight fraction of the constituents) of dried refractory coatings indicate that coating I contains 91.3%wt and 8.7%wt of refractory materials and organic components (binders, surfactants, biocides, dispersing
and thixotropic agents) respectively. However, the weight percentage of refractory and organic materials respectively are 94.4 and 5.6%wt for coating II and for dried coating III are 85.2 and 14.8%wt. For detailed investigation, only coating I was selected as a sample for TG/DTA analysis. TG/DTA curve for this coating is shown in Fig. 1. As it is clear from this figure, the onset of thermal degradation is 246.2 °C and the weight loss up to that
Fig. 5. XRD pattern for (a) coating I, (b) coating II and (c) coating III between 2θ = 0–90°.
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Fig. 5 (continued).
temperature is less than 0.5%wt. There is a very sharp weight loss from 246.2 °C to 375.2 °C and almost 8%wt of organic compound is decomposed in this temperature range. At 375.2 °C the slope of the curve is changed and weight loss proceeds with a relatively slow rate up to 500 °C. TG results reveal that there are at least two organic components in this coating. DTA results indicate 3 peaks at the temperature range of 50 to 500 °C. Some of these peaks could be from phase transitions of organic components. 3.2. Morphological study The optical microscopy images of coatings are shown in Fig. 2. From these images, it is clear that all coatings have particles with irregular shape and uniformity and that the mean particle size of coating I is larger than the others. Existence of a high percentage of tiny particles in these coatings causes the pores between large particles to be closed and the permeability to be decreased. The particle size distribution (PSD) for three coatings was determined from Fig. 2 by Image Pro-Express software. The obtained results are shown in Fig. 3. PSD in coating I is the widest and average particle size is the largest compared to the other coatings, but in coating III, the PSD and average particle size are the narrowest and the smallest one, respectively. SEM images for three coatings are shown in Fig. 4. As expected, the particle size of coating I is coarser than other coatings, which is the major reason for high permeability of this
coating compared to coatings I and II. In fact, SEM images are consistent with the optical micrographs. SEM/EDXA elemental analysis results are presented in Table 1. The major elements in coating I refractory particles are Al, Si and oxygen. Also there is a trace of Fe in this coating. Fe element in hematite (Fe2O3) form is very useful to reduce lustrous carbon defects in lost foam iron casting. This component accelerates oxidization of lustrous carbon, which is produced from lost foam decomposition [4–6]. There is the same trend in other coatings. Some trace of Fe element could be observed in these coatings. There is an optimum content of hematite (2–3%wt) in refractory coating, because using more than this value reduces the mechanical properties of casted iron parts [4–6]. 3.3. XRD investigations XRD patterns for different coatings are shown in Fig. 5. XRD results show the presence of sillimanite (kyanite: Al2SiO5), hematite (Fe2O3), quartz, clay minerals and feldspar in coating I. As mentioned before, EDXA results confirm the presence of Al, Si, O and Fe elements. Furthermore mulite (Al6Si2O13) and feldspar were found in coating II. Also the results confirm the presence of sillimanite, mulite, trydymite and clay minerals in coating III. As it is clear from Fig. 5, more crystalline structures are observed in coating I and coating II than coating III. On the other hand, the presence of Fe element in hematite form (Fe2O3) in coating I can be the major reason for selecting
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of this coating in lost foam iron casting, because of its effect on elimination of lustrous carbon defects [4–6]. 3.4. Permeability measurements Fig. 6 shows the permeability of coatings in terms of millimeters of water scale (mm.w.s) in two different densities versus coating thickness. As it was expected, the permeability
Fig. 7. Permeability of coatings at density of 1.3 g/cm3.
of the samples decreases as their thickness increases. Also, samples with higher density show lower permeability. Comparison of the permeability of the coatings at density 1.3 g/cm3 is conducted in Fig. 7. It was found that coating I is more permeable than the others. On the other hand with increasing coating thickness, the effect of density is reduced, especially density effect in coatings I and II is negligible. As it was mentioned in the morphological study, coating I had a larger average particle size and wider particle size distribution than the other coatings. Larger average particle size and wider particle size distribution establish larger pore size in coating, which is the most important reason for the higher permeability of coating I compared to the others. With increasing of coating I thickness, some of these pores are isolated and considered as ineffective pores, thus the coating permeability are decreased with a higher rate than coatings I and II. Considering that in iron casting a high amount of gas is produced, it is advisable to use coating I in such applications and coatings II and III in aluminum casting. It should be noted that the selection of the optimum thickness of coating must be undertaken by considering all of the parameters affecting the quality of the product.
Fig. 6. Permeability of coatings at two densities (a) coating I, (b) coating II and (c) coating III.
Fig. 8. Shear stress versus shear rate for coatings at T = 22 °C.
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Fig. 9. Viscosity versus shear rate at T = 22 °C.
3.5. Rheological study Shear stress versus shear rate curves for different coatings is shown in Fig. 8. As seen from this figure, these refractory coatings have a non-linear flow curve and exhibit shear thinning behavior typical of non-Newtonian fluids. Also, a yield stress is found for these coatings indicating that these suspensions are Bingham pseudoplastic fluids. The apparent viscosities of the refractory coatings versus shear rate are shown in Fig. 9. At very low shear rates, viscosities significantly decrease with shear rate but this trend suddenly decelerates and these suspensions reach a constant viscosity. This behavior can be related to the presence of thixotropic agents in the suspension [7] and disagglomeration of refractory clusters with increasing shear stress. Among the samples, coating I has the lowest and coating III has the highest viscosity. The viscosity in coating I is more sensitive to shear rate. This behavior can be related to the wide PSD of coating I, as shown in Fig. 3. In such suspensions particles of different sizes are present and the smaller particles are interposed between larger particles, causing a reduction in the inter-particle impact, and then resulting in a considerable decrease in viscosity. In other words, the smaller particles act as lubricants to facilitate the rotation of larger particles, leading to a reduction in the viscosity [15]. The results of the rheological studies are consistent to the morphology of the materials: coating III with the narrowest PSD is the most viscous coating, whereas coating I has the lowest viscosity and coating II lies inbetween. 4. Conclusion A systematic study on the microstructure, permeability and rheological properties of three commercial refractory coatings was conducted in this article. Different methods were used for characterizing refractory coatings. A thermal decomposition investigation was conducted on the coatings and detailed analysis of the organic components was carried out by TG/DTA method on coating I. The obtained results revealed that there were at least two organic components in this coating. Morphological studies were carried out by optical microscopy and SEM. PSD in coating I, was the widest and average particle size was the largest compared to the other coatings.
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SEM/EDXA elemental analysis indicated that the major elements in refractory particles were Al, Si and oxygen. Also there was a trace of Fe in these coatings. The crystalline structure of the refractory particles was determined by XRD patterns. XRD results showed the presence of Fe element in hematite form in coating I. Permeability results indicated that the permeability of the samples decreased as their thickness and density increased. Also, it was found that coating I was more permeable than the other coatings. Larger average particle size and wider PSD establish larger pore size in coating which was the most important reason for higher permeability of coating I compared to coatings II and III. The rheological results indicated that refractory coatings had non-linear flow curve like Bingham pseudoplastic fluids. Among the samples, coating I had the lowest and coating III had the highest viscosity. According to the wide PSD in coating I and presence of particles with different sizes, the smaller particles were interdispersed between larger particles, causing a reduction in the inter-particle impact and resulting in a decrease in viscosity. However, shear thinning behavior of coating I was more pronounced than that of the other coatings. This trend could be related to the wide PSD of this coating. In other words, the smaller particles act as lubricants to facilitate the rotation of larger particles, leading to a considerable reduction in the viscosity. Acknowledgments The authors gratefully acknowledge the Sahand University of Technology and the Iran Tractor Foundry Company for the financial support. The authors would also like to gratefully acknowledge the research project team members of Research Center for Polymeric Materials and Metallurgical Research Center: Dr. Farhang Abbasi, Mr. Zeighami, Mr. Khalichi and Mr. Ferdosi for their involvement and encouragement. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
J.J. Green, C.W. Ramsay, D.R. Askeland, J. AFS Trans. 106 (1998) 339. F. Sonnenberg, K.M. Taristo, T.V. Johannes, US Pat 6,303,664 B1, (2001). F. Sonnenberg, US Pat., 0081379 A1, (2002). R.L. Naro, J. AFS Trans. 154 (2002) 1. R.L. Naro, D. Tenaglia, J. AFS Trans. 85 (1977) 65. R.L. Naro, J. Modern Casting 93 (2003) 32. S.L. Bakhtiyarov, R.A. Overfelt, J. Elast. Plast. 32 (2000) 73. Y. Liu, S.I. Bakhtiarov, R.A. Overfelt, J. Mater. Sci. 37 (2002) 2997. M. Sands, S. Shivkumar, J. Mater. Sci. 38 (2003) 667. M. Sands, S. Shivkumar, J. Mater. Sci. 41 (2006) 2373. M. Sands, S. Shivkumar, J. Mater. Sci. 38 (2003) 2233. X. Chen, D. Penumadu, J. Mater. Sci. 41 (2006) 3403. Q. Zhao, H. Wang, S. Biederman, D. Jason, J.S. Parish, AFS Trans. 11 (2005) 1. [14] L. Günther, W. Peukert, Colloids Surf. Physicochem. Eng. Aspect 225 (2003) 49. [15] A.V. Shenoy, Rheology of Filled Polymer Systems, Kluwer Academic Publishers, Dordrecht, 1999.