A R C H I V E S
o f
F O U N D R Y E N G I N E E R I N G
Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences
ISSN (1897-3310)
Volume 8
Issue 4/2008
149 – 152
27/4
A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 8 , I s s u e 4 / 2 0 0 8 , 1 4 9 - 1 5 2 149
Mechanical
properties of investment casting
moulds reinforced with ceramic fibre
M. Nadolski, Z. Konopka, M. Łągiewka, A. Zyska*
Department of Foundry, Technical University of Cz
ę
stochowa, ul. Armii Krajowej 19, 42-200 Cz
ę
stochowa, Poland
*Corresponding author. E-mail address: zyska@mim.pcz.czest.pl
Received 15.07.2008; accepted in revised form 22.07.2008
Abstract
This work deals with mechanical properties of thin-walled ceramic moulds for investment casting at the dewaxing stage. The main purpose of this work has been increasing their bending strength, expressed by the modulus of rupture (MOR), and determining their corresponding work of fracture, characterised by the fracture index (FI). An essence of the concept is applying aluminosilicatematerials in the fibrous form along with grain silica materials as the reinforcing material for the thin-walled mould matrix.
Keywords: Investment casting, Ceramic mould, Modulus of rupture
1. Introduction
The quality improvement of ceramic shell moulds takes recently two main courses, because of the recommended restricted use of ethyl silicate as a mould binder. One of them is improving binders, the second consist in matrix strengthening. Improving binders based on the colloidal silica is aimed to:
− overcome poor wettability of the low-melting pattern sets – by addition of surfactant and antifoamers,
− overcome decreased strength at the dewaxing stage – by applying additions of liquid polymers based on the polyvinyl alcohols and latexes (acrylic, vinyl acetate, butadiene-styrene, and others),
− shorten the drying time between subsequent layer applying and the final drying time – by addition of high molecular weight silica sols with SiO2 particles greater by 3 to 4 times
and the 40% concentration of these particles in the sol. Gelation rate can be increased physically – by selecting the temperature, the humidity and the speed of air motion, or chemically – by changing the pH of a binder.
Matrix strengthening is achieved by introducing components stopping crack propagation, e. g. fibres. Glass, polymer, ceramic
and other fibres are used. Their weight fraction does not exceed 12%, and usually it ranges from 0.5 to 5 wt%. The ceramic slurry within this range exhibits rheologic properties which enable safe dipping pattern sets in it. If the fibre content exceeds 12 wt%, a change of the method of applying the slurry to the pattern sets is necessary, e.g. applying the spraying method. Also other solutions are employed, namely fibre, mat and cloth braids.
Liquid self-hardening slurries for precision castings are applied in technologies intended for achieving the castings of high dimensional accuracy and clean and smooth surface.
Due to the diversity of materials applied for mould construction, the final mould of full value used for investment casting technology can vary in thickness from about 3 to about 12 mm. Such great differences in thickness result however not only from the diversity of used materials, but also from various number of ceramic coats; usually 5 to 7 coats is applied [10].
The ceramic moulds, however various with regard to their construction and methods used to build them up, should satisfy multiple requirements. So, the following demands are formulated as to the ceramic matrix material:
− high refractoriness,
− lack of polymorphous transitions which would result in volume change,
− proper grain composition.
The binder, on the other hand, should exhibit:
− good interaction with matrix material (including lack of reaction deteriorating properties of the achieved slurry),
− lack of reaction with molten casting alloy,
− good wettability of the pattern sets.
And the final mould made of the matrix and the binder should of course exhibit:
− suitable strength at the stage of dewaxing (MOR 1),
− suitable strength at the stage of filling with molten metal (MOR3),
− proper permeability,
− high chemical stability,
− dimensional stability,
− good thermal shock resistance,
− easy way of being removed from the casting surface [7]. The following parameters are checked at the stage of the mould construction in order to ensure adequateproperties of the mould:
− for the ceramic slurry: its pH, viscosity, and density,
− for conditions of applying coats: humidity, temperature, time, air motion [5, 6].
The strength properties of moulds for investment casting technology can be determined in various ways [8]. There can be determined e. g. critical fracture load, deflection, fracture index based on the measured work of fracture, but most often the moulds are mechanically examined at three stages for which the modulus of rupture is determined: the stage of partial ceramization (MOR 1), the stage after burning and cooling the mould to a room temperature (MOR 2), and the stage under the elevated temperature conditions (MOR 3) [9]. The MOR index is usually determined from the three-point (3P) bending test, which can be extended to the four-point (4P) bending test if more data describing the mould material behaviour are needed [7, 8, 9].
The essence of the undertaken problem is the significant replacement of grain silica materials with the fibrous aluminosilicate materials. The investigations have been carried out to find the answer to the question if it is advantageous to introduce such a type of fibrous material for improving the mechanical properties of the mould at the stage of partial ceramization and how this addition would influence other technological properties of the material, such as permeability, deformation susceptibility, thermal expansion [1, 2, 3, 4].
2. Methodics of examination
Experiments have been performed according to the composition design of the second order with four variable factors. (Values and ranges for the individual variables are shown in Table 1, where the appropriate factors designate as follows: X1 – TC Kaowool HP E08, X2 – silica flour MK 75, X3 – Sizol 030, X4 – LBS 3030 latex.
Table 1.
Mass percentages of components of the examined slurries
– 0 + ΔXn –α +α
Factor g,
(%) g, (%) g, (%) g g, (%) g, (%) X1 57 (16.0) 150 (28.6) 243 (35.0) 93 0 (0.0) 300 (37.5) X2 297 (84.0) 375 (71.4) 453 (65.0) 78 250 (100.0) 500 (62.5) X3 645 (95.8) 750 (90.9) 855 (87.5) 105 581 (100.0) 919 (86.0) X4 28 (4.2) 75 (9.1) 122 (12.5) 47 0.00 (0.0) 150 (14.0)
Designations “–“, “0”, “+”, “ΔXn”, “–α” and “+α” in Table 1 denote the lower, the basic, and the upper levels, the width of the range of variable changes, and the lower and upper values for star arm in the design, respectively. Weights of individual components used for preparing the determined quantities of slurry are given in grams in the Table. The quantity of each slurry has been calculated in such a way that it would be possible to produce a multi-layer shell for a given simple pattern by spraying method. These quantities have been different and have depended on the slurry composition, due to various densities of slurries. Percentage values in brackets refer to the fraction of a given component in a group of solid or liquid components of a given slurry.
The weight percentage of solid TC Kaowool HP E08 fibre in the matrices of slurries ranges within 0÷37.5%, and the weight percentage of the second matrix component, the silica flour, falls within 62.5÷100%. The liquid slurry components have been: Sizol 030 colloidal silica as a binder and LBS 3030 butadiene-styrene latex. The weight fraction of the Sizol binder has been changed within the range of 86÷100%, and the LBS 3030 butadiene-styrene latex within 0÷14%. Eventually, the weight fraction of solid and liquid phases in the examined slurries ranged from 30 to 42 wt% and from 58 to 70 wt%, respectively, where a fraction of colloidal silica has not been taken into account.
Slurries have been produced by mechanical mixing with ribbon stirrer and subsequent ultrasonic mixing, the latter for 2 minutes.
The sample moulds have been built by spraying these slurries onto the plastic patterns of dimensions 250×300 mm. A pneumatic spraying gun with nozzle diameter of 6 mm, working periodically at the pressure of 2 bars, has been used for preparing samples. The obtained ceramic material has been cut with diamond disk to prepare cuboidal specimens of dimensions about
10×20×200 mm, which have been tested to determine their bending strength by means of the balance deflectometer, recording simultaneously the bending force P and the corresponding specimen deflection f. Obtained deflection curve
δ-ε (stress-strain) has allowed for calculating the fracture energy determined by the area beneath this curve.
The dependence between the bending strength of the material expressed by the MOR1 index and the fractions of its four basic components is represented by the following adequate regression equation:
ŷMOR1 =3,71+0,38x1+0,13x3 -0,14x4-0,58x1x2+0,65x1²+0,15x3 2
(1)
It has been found, according to expectations, that the addition of fibre results in an increase of the material strength MOR1, what is proved by the high positive values of coefficients for linear and squared terms in the Equation 1. It also results from the equation that an increase of binder quantity involves an increase in MOR1 index. Also the negative, though weak, influence of latex on the bending strength of the material (however other advantages make its addition necessary). The presumed reinforcing effect of applying fibre materials to strengthen the mould has been doubtlessly confirmed by the achieved results. For better understanding of the strengthening effect it would be helpful to analyse the regression equation ŷFI calculated on the basis of the
test results. This regression equation concerning the dependence between the work of fracture FI and the material composition takes the following form:
3. Description of the obtained results
The results of measuring the mechanical properties of thin-walled moulds reinforced with ceramic fibre are gathered in Table 2.
Table 2.
Results of measuring the mechanical properties of the examined materials
MOR1 SMOR1 fmax Sf FI SFI
Slurry No.
MPa MPa mm mm kJ/m3 kJ/m3
1 5.21 0.31 1.99 0.55 9.02 0.70
2 4.20 0.39 1.32 0.19 3.18 0.99
3 5.66 0.21 1.77 0.17 5.26 0.31
4 3.79 0.57 1.63 0.12 3.34 0.90
5 4.55 0.13 1.69 0.14 8.18 1.18
6 3.95 0.71 1.49 0.45 2.86 0.55
7 4.53 0.12 1.87 0.27 6.02 0.66
8 4.06 0.50 1.58 0.12 2.90 0.52
9 5.71 0.29 2.04 0.11 7.10 0.70
10 4.36 0.16 1.25 0.17 4.56 1.65
11 5.42 0.18 1.41 0.15 5.46 1.25
12 4.77 0.25 1.32 0.11 7.26 1.71
13 4.79 0.17 1.82 0.05 9.66 1.81
14 4.28 0.46 1.67 0.14 6.06 0.66
15 5.53 0.40 1.40 0.21 10.40 0.53
16 3.85 0.50 1.11 0.18 2.98 0.74
17 5.04 0.06 0.75 0.11 1.34 1.12
18 4.93 0.48 1.52 0.20 4.78 1.07
19 3.19 0.34 1.44 0.05 4.14 0.63
20 3.50 0.17 1.16 0.07 5.28 0.51
21 3.97 0.31 1.26 0.01 5.34 0.16
22 3.45 0.22 1.46 0.05 5.66 0.68
23 3.40 0.34 1.36 0.04 5.22 0.41
24 3.33 0.32 1.53 0.13 4.52 0.72
25 4.17 0.09 1.64 0.04 5.50 0.60
26 4.02 0.54 1.77 0.34 5.14 0.61
27 3.95 0.32 1.55 0.11 4.84 1.18
28 3.81 0.54 1.63 0.12 5.32 0.66
ŷFI =4,67+1,54x1+0,42x2-0,65x4+0,41x1x2+0,68x1x3+
0,34x3x4-0,33x1 2
+0,31x2 2
+0.62x3 2
+0,37x4
2 (2)
This mathematical model describing the fracture resistance of the material points to the fact that additions of fibre and silica flour increase the FI value, and an addition of latex diminishes it. But it is worth noticing that the regression coefficient at the variable corresponding to the fibre fraction in the material is extremely high. This confirm a large contribution of the applied fibre to the strengthening of the matrix of examined materials. Interactions of components x1x2, x1x3, and x3x4 also increase the work of
fracture. In general, all components influence advantageously the fracture resistance of the obtained material. For comparison, materials from slurries No. 1, 5, 13 containing the largest quantities of fibre and flour have exhibited fracture resistance at the level of 9 kJ/m3, and the grain material (No. 17) has the FI value equal to 1.34 kJ/m3. This means an increase of fracture resistance by nearly eight times for the fibre-reinforced material.
It should be emphasized that all the examined grain-fibre materials have been characterised by high bending strength MOR1 and all of them have reached the level of minimum material strength during dewaxing of 2.4 MPa, as demanded by American regulations. However, in order to avoid the shell cracking during pattern burning off process, a trend occurs to achieve higher values of MOR1 index, over 4.5 MPa if possible. This condition has been satisfied by a majority of slurries produced according to the experimental design, what confirms that optimum levels of component fractions have been taken into account.
The regression equation describing the change of deflection
ŷfmax depending on the material composition clearly shows the
strong influence of the fibre fraction, indicated by the high value of regression coefficient for the x1 variable.
ŷfmax =1,50+0,19x1+0,08x4 (3)
The rather weak influence of latex fraction is here also revealed. The materials reinforced with aluminosilica fibre exhibit deflections at the level of 1.5÷2.0 mm, which are greater than the deflection value for grain material equal to 1.34 mm.
4. Conclusions
1. The spraying method provides for building of multiple layers of a mould exhibiting uniform structure and controlled thickness at minimum slurry expense and without the risk of its lifetime expiration, what compare favourably with the conventional dipping and stuccoing method.
2. Addition of the chopped aluminosilica fibre to the newly developed moulding material with silica sol matrix results in an improvement of mechanical properties of the material as compared with these properties for grain moulding materials, commonly used for investment casting. This improvement of mechanical properties employs the mechanism of reinforcing action of the chopped fibre which create light and rigid skeleton structure cemented with a binder and transferring stresses under the load. Fibre enlarges the fracture surface due to their pulling out of the matrix, what increases the dimension of a critical crack and rises the work of fracture for these moulds.
3. The apparent enhancement of mechanical and technological properties of the recently developed materials with addition of aluminosilicate fibre has allowed for building the thin-walled self-supported casting mould, optional for the so far used plaster moulds, what has influenced beneficially the economics of the production process by eliminating a large amounts of plaster waste material.
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