Bonding mechanism in the coal-flyash ceramic shells

TE TECHNICAL PAPER BONDING G MECHANISM IN THE FLYASH H CERAMIC SHEL LLS A. Chennakesava va Reddy*, V.S.R. Murti** and S. Sundara Ra ajan*** The characteristics dip-cas asting slurries judge the performance of investment cast asting process. The paper presents a hypothesis on the bo bonding mechanism in the ceramic shells developed from m coal flyash. Experiments have been carried out to study dy the effects of filler loading, aging and air-drying on the th strength of shells. The results indicate substantial red reduction in the sedimentation under operating conditions ns and enhancement in the shells strength. INTRODUCTION In the manufacture of ceramic ic shell moulds by the investment casting process, a mul ulti-layered shell is built up by repeatedly dipping a wa wax pattern cluster into a slurry containing liquid binde der and refractory filler and stuccoing with a coa oarse sand. Each individual coat is air-dried prior to aapplying the next coat. On achievement of the required ed thickness of the shell, the meltable pattern materiall iis removed from the setup and the shell is fired1-2. more effective in The dip-coating slurries are m controlling the quality of she hell because of characteristic problems associated wi with the method of handling of these slurries and the he nature bonding mechanism in the shells3-4. Sedimen entation, viscosity and strength are important characteris ristics by which the performance of shell for investmen ent casting can be evaluated. The most commonly used ed refractory fillers are silica, alumina and zircon powder ders. Especially the latter two fillers have high densitiess (4.0 and 4.6 g/cc respectively) and if the slurries are not maintained properly, the fillers settle down aand subsequently result non-uniform viscosity to the slurry. For practical use, slurries with the lowe west sedimentation are most advantageous. They also so insure a more constant strength of coats and shell5. The physico-chemical propert erties of coal flyash as refractory filler and its slurry rry and to compare the results with those of alum mina under the same moulding conditions. The bend nding strength of shells is also investigated with resp spect to the bonding mechanism developed in the shells. sh EXPERIMENTAL PROCE CEDURE Dip-coating slurries were prepared by adding the refractory filler to the liquid binder, b using sufficient agitation to break-up agglom merates and thoroughly wet and disperse the filler. Col olloidal silica binder was used for the preparation of both bo the slurries of coal flyash and alumina. The chemi mical composition of the binder is shown in table-1. The he particle size of fillers was 45 µm. the ambient temperature tem and relative humidity were respectively 30-35oC and 60-65. The measuring of sedimen entation was executed on the principle of of determina ination of the sediment height. This was performed in a glass tube of 25 mm in diameter, 400 mm high wit ith 1 mm scaling6. The viscosity of the slurry was measured me by a ford cup 7 with an orifice of 5 mm diamete eter . The objective of the present invest estigation is to study *Associate Professor, Dept. of Mechanica ical Engineering, MJ College of Engineering and Technolog logy, Banjara Hills, Hyderabad – 500 034, India. **Professor, Dept. of Mechanical Eng ngineering, Osmania University, Hyderabad – 500 007, India. ***Scientist G, Production Division, Def efence Research and Development Laboratories, Hyderabad -5500 058, India INDIAN FOUNDRY JOURNAL 21 Vol. l. 47, 47 No.4/April, 2001 T TECHNICAL PAPER The ceramic shells for bendingg sstrength test were made by dipping wax patterns into nto the slurry and stuccoing with coarse sand. Each coa oating was allowed to dry for 4 hours in the open air.. T The operations of dipping, stuccoing and drying were re repeated six times. The seventh coat was left unstucco coed to avoid the occurrence of loose particles on thee sshell surface. The first two coats were stuccoed with th a sand of AFS fineness number 120 and the next fou our coats with sand of AFS fineness number of 50. Aft fter all coats, the shells were air-dried for 24 hour urs. The bending strength test of shells was conducte ted on a universal sand-testing (hydraulic type) machine ne. RESULTS AND DISCUSSION Physico-Chemical Properties off C Coal Flyash Coal flyash was the residue of co coal combustion in coal fired power generations. When en coal was totally burnt, the constituents of coal viz. iz., principally the oxides of silica and alumina conve verted into flyash. The chemical composition of coal fl flyash is given in table-2 the data indicate that the bbulk of flyash is composed of mullet, quartz, ccristobalite and amorphous alumina-silicates. The sscanning electron microscopy of flyash as represented ted in fig.1 shows glassy spheres. The particle size vvaries from submicrometers to 100 µm. upto 75 µm pparticle diameters sized fractions of flyash have surfa face areas ranging 2 from 1.22 to o.45 m /g and densities es ranging from 2.0 to 2.9 g/cm3. Sedimentation of Slurry The behavior of dip-coating slu slurry was affected by the sedimentation of the refrac ractory filler. The extent of refractory filler particles les settling to the container bottom was determined inn tterms of sediment (hard pack) height. The results off sediment height measured (after one hour) due to aalumina and coal flyash sedimentation are shown in table-3. The sedimentation rate of alumina parti rticles was greater than that of coal flyash particles in the slurry. The formation of refractory particle hard rd pack was due to the density difference between the bin binder solution and the refractory filler. The densities of binder, alumina and flyash are respectively 1.23, 4.0 .00 and 2.13 g/m3. The difference between the downward ard gravity force of filler and the upward buoyancy fo force of colloidal silica binder is comparatively low inn fflyash slurries. INDIAN FOUNDRY JOURNAL Therefore, the particle ret etention along the slurry column is good in the flyas yash slurries. The slow sedimentation rate of flyash particles p is also die to their hollow structure. Viscosity of Slurry Table-4 shows the effectt of filler to binder ratio on the viscosity of slurry. Thee viscosity v of the slurry is directly proportional to the am mount of filler added to the binder. Viscosity initially ly tests excessively high because of air entertainment nt and lack of particle wetting; therefore, mixing is continued until the viscosity falls to its final level el before the slurry is put 22 Vol. l. 47, 47 No.4/April, 2001 T TECHNICAL PAPER into use. The flyash slurries exhibit it higher viscosity due to partial gelation caused by thee iimpurities present in the flyash. colloidal particles in the binder er and filler particles by mutual bonding. The coarsen ening action is fast in flyash slurries; a spontaneous us gellation occurs at 8 hours of aging. The evaporatio tion of water content of the binder from the surface of the slurry produces a pre-gelling condition into thee slurry (as the slurries continuously stirred to keep the he filler from settling out of suspension). Thus the whol ole volume of the slurry gradually comes into the pre-ge gelled state. BONDING MECHANISM V/S STR RENGTH OF SHELLS Silox binder consists of a colloidal dispersion of spherical silicon radicals in water. All the silicon radicals are negatively charge rged, they do not stick because of like charges repell as a shown in fig.1. The pH of silox binder is 9.5 to 10.5. When the filler particles re added to the liquid id binder, the pH of the binder falls due to the electrostatic ele bonds and subsequently the gels are form ormed in the slurries as shown in fig.2. Table-5 demonstrates the effect ct of aging time on the viscosity of slurries. With agin ging, the viscosity increases. The aging of slurry is coarsening of INDIAN FOUNDRY JOURNAL 23 When the filler particless are a added to the liquid binder, filler particles preferen entially absorb hydroxyl (OH-) ions from water present nt in the binder owing to the unsatisfied valence bondss at the surface of filler particle, and the filler-wat ater particle becomes Vol. l. 47, 47 No.4/April, 2001 T TECHNICAL PAPER negatively charged. As such, filler par particle hull attracts positive (H+) ions in the surrounding ng binder medium. The hydrogen counter ions and the ab absorbed hydroxyl ions about the filler particle comprise ise a double diffuse layer. The coal flyash consists of si silica and alumina along with some impurities like N Na, Ca and Mg. hydration of impurities may also ooccur in the coal flyash slurries. When the coarsee sand grains are stuccoed onto the dip-coated wax patt atterns, particles of stucco sand grains also form m micelles by the absorption of hydroxyl ions and hhydrogen counter ions. The bonding mechanism develo eloped between the dip-coat layer and stucco sand grainss is as follows: The negatively charged silica pparticles from the binder exhibit an attraction for positiv itive filler particles; positive stucco sand grains and tthe counter ions contained in the filler and stucco sand nd micelles in case of alumina shells (fig.3); whereas iin case of flyash shells there is also an attraction bet between negatively charged silica particles and positivee iimpurity particles and counter ions contained in im impurity micelles (fig.4). The final result is electrosta static bonds in the ceramic shells. In addition to electrostatic bond nds, there may be surface tension bonds and frictionall aand/or mechanical interlocking bonds in ceramic shells. lls. Each coating is allowed to set before the next one is applied. This is accomplished by air-drying. The he operations of coating, stuccoing and drying are re repeated a number of times until the required shell thick ckness is achieved. In the removal of water from each ch coating due to drying forces the particle togeth ther and thereby enhances the bonding mechanism iin ceramic shells (table-6). The effect of filler loading on the bending strength of shells is shown in table le-7. The bonding mechanism in the shells is due to el electrostatic bonds between colloidal silicon radicals in the finder, filler particles and stuccoing sand grain ains. Flyas shells exhibit high strength for filler to bin inder ratio of 0.65 whereas alumina shells high streng ength for filler to binder of 0.75. this is mainly due too tthe completion of electrostatic bond at these slurry com mpositions. Flyash shells have distinctly higher streng ngth over alumina shells, as the number of positive char arge carried by the cations are more in the flyash slurrie ries. This is mainly due to the presence of Na, Caa and Mg. these effectively introduce opposite charged ed particles, INDIAN FOUNDRY JOURNAL 24 Vol. l. 47, No.4/April, 2001 TECHNICAL PAPER which in turn linkup, the colloidal silica particles via a bridge and thereby complete gellation [complete neutralization of negatively charged silicon (binder particles]. 3. A. Chennakesava Reddy, K. M. Babu, P. M. Jebaraj and M. P. Chowdaiah, Accelerator for faster shell making and its effect on the properties of investment shells, Indian Foundry Journal, Vol. 41, No.10, 1995, P.3. CONCLUSION The bulk of coal flyash is composed of mullite, quartz, cristobalite and amorphous alumino-silicates. The scanning electron microscopy of flyash shows glassy spheres. Flyash results slow sedimentation and uniform slurry viscosity due to better retention of filer particles in the slurry column. The slurries are coarsened by aging. The coarsening action is fast in flyash slurries; spontaneous gellation occurs at 8 hours of aging. Flyash slurries have distinctly higher strength over alumina shells, as the number positive charges carried by the cations are more in the flyash slurries. The removal of water from coats by airdrying enhances the boding mechanism in the shells. 4. A. Chennakesava Reddy, V.S.R Murti and S. Sundara Rajaan, Some aspects of reducing sedimentation rate of filler in the Investment Casting Process, Engineering Advances, Vol.10, No.8, 1998, P.61. 5. J. Doskar and J. Gabriel, How dipcoat materials affect ceramic investment shells – Part 1, Vol. 166, April 1969, P294. 6. ASTM: Designation: D1200-88, Standard test method for viscosity by Ford Viscosity cup, p.143. 1. R.A. Horten, Investment casting, Metals Hand Book, 8th edition, Vol.5, 1975, P.253. 7. A. Chennakesava Reddy, H. B. Nirnajan and A. R. V. Murti, Optimization of investment shell mould using colloidal silica binder, Indian J of Engineering Materials Sciences, Vol.3, No.5, 1996, P.180. 2. A. Chennakesava Reddy and V.S.R Murti, Studies on the Lost-wax process using silox binder, Proceedings of Xth ISME Conference on Mechanical Engineering, 1996, P II-82. 8. J. Doelman, Standardization of methods of determining permeability and strength of ceramic shells, Foundry Trade Journal, Vol.121, December 1966, P.724. REFERENCES INDIAN FOUNDRY JOURNAL 25 Vol. 47, No.4/April,