EFFECT OF INHIBITOR GAS ON MOULD-MAGNESIUM REACTIONS IN INVESTMENT CASTING
Zhan Zhang1*, Guy Morin2
1Intermag-Modelex Inc., 820 Ch. Olivier St-Nicolas , (Quebec), G7A 2N1, Canada
2Centre Intégréde Fonderie et de Métallurgie, 3247, rue Foucher, Trois-Rivières,(Quebec), G8Z 1M6, Canada * Now with Alcan International Ltée, Arvida Research and Development Center, 1955Mellon Blvd., Jonquière ,G7S 4K8, Canada
Keywords: mould-magnesium reactions, inhibitor gas, investment casting.
Abstract
In order to assess the behaviour of mould-magnesium reactions, the ceramic shell moulds with different binders and refractory particles were prepared for pouring AZ91 magnesium alloy. To restrict mould-magnesium reactions, inhibitor gas was guided into shell moulds for removal of oxygen in shell moulds and formation of barrage between mould and magnesium. The results of experiments show that a mixture of CO2 and proper concentration of SF6 used as inhibitor gas can effectively limit mould-magnesium reactions. A surface analysis with AES (auger electron spectroscopy)and ESCA (electron spectroscopy for chemical analysis) has been performed on the surface of magnesium parts cast under the inhibitor gas. It was discovered that a special layer appeared on the part surface, in which the elements such as magnesium, oxygen, fluorine, aluminum, sulphur, silicon, etc. were detected. The mechanisms of mould-magnesium reactions and their prevention are discussed in this paper.
Introduction
Ceramic shell moulds are made by applying ceramic coatings, which consist of ceramic slurry and coarse ceramic particles, to wax patterns. The main components of slurry are binder and filler, fine refractory particles.
According to the thermodynamic data of refractory materials commonly used in shell moulds, magnesium would react with silica, zirconia,and alumina at the temperature from 20 to 1000°C [1]. The rank of the free energy of the materials is in the order with increasing in the following sequence: MgO,Al2O3, ZrO2, and SiO2.
To restrict mould–magnesium reactions, inhibitors such as, sulphur, boric acid, potassium fluoborate, and borofluoride compounds have been used for sand casting and plaster mould casting processes successfully[2].
Since shell moulds are preheated to high temperature before pouring magnesium melt into them, shell moulds are moisture-free. Consequently, magnesium melt would react with air in cavity and porous shell wall, as well as shell materials as melt enters and takes the form of the mould cavities.
________________________________
*To whom the correspondence should be address.Since the inhibitor such as potassium borofluoride would decompose when shell moulds are fired at high temperature, this type of inhibitor can not be used to prevent from the reactions between magnesium melt and shell moulds [3]. It was discovered that a mixture of CO2 and 1-2%SF6 can restrict magnesium-shell mould reactions [3-4]. But, when the mould temperature reaches 600°C, serious reactions occur between magnesium melt and the conventional silica binder, which bonds alumina and zircon, under the atmosphere of CO2 / 1%SF6 [4].
(a)
(b)
Fig. 1 Reaction layers: silica binder shell (a), and zirconia binder shell(b)[5].
Shell
Shell
120μm
Reaction layer
AZ91E
Reaction layer
Magnesium Technology 2004Edited by Alan A. Luo TMS (The Minerals, Metals & Materials Society), 2004
In order to assess the behaviours of moulds for magnesium investment casting, two types of moulds bonded by silica binder or zirconia binder were built to pour AZ91E magnesium alloy[5]. It was found that the magnesium castings stuck on the shell moulds if the magnesium melt was poured into moulds without inhibitor gas. The image analysis demonstrated that there was a reaction layer between parts and moulds (see Figure 1), in which magnesium and oxygen were detected. AZ91E magnesium melt reacts with oxygen in cavity and within shell wall. And, the binders, which enclose the refractory particles, would react with the melt also [5]. It was found that the concentration of SF6 in the gas mixture as well as the atmosphere enclosing the moulds affect the magnesium-mould reactions [5]. However, it is not clear on the mechanisms of restricting shell mould – magnesium reactions by inhibitor gas.
To produce extra thin wall magnesium castings, ceramic shell moulds have to be preheated to high temperature. It is a great challenge for the prevention from mould– magnesium reactions at the situation. In this paper, the behaves of mixture of CO2 and SF6 to limit the mould-magnesium reactions have been discussed.
Experimental procedure
Shell mould fabrication
Two types of shell moulds, shell A, and B, which have different primary coats and same back up coats, were prepared. The components of the coats for the shell moulds are on Table 1.
Table 1. Components of primary and back up coats for shell moulds.
Primary coat
Shell A Shell B
Slurry Slurry
Binder:Filler Binder Filler
SiO2
suspension + solvents ZrSiO4
ZrO2
suspension +
solvents
ZrO2
Stucco Stucco
ZrSiO4ZrO2
Back up coats
Slurry
Binder Filler
SiO2suspension + solvents +
additives nAl2O3 mSiO2
Stucco
nAl2O
3 mSiO2
Shell moulds are fabricated by dipping wax pattern in slurry then
stuccoing and drying for several hours. These operations are
repeated for several times until the wall thickness of shell moulds
reaches 5-7 mm. After dewaxing and firing,shell moulds are
preheated at 650°C in a furnace over 3 hours. Before pouring
magnesium melt into shell moulds, inhibitor gas is introduced into
mould cavities.The temperature on the wall of shell moulds is
about 460°C when magnesium melt is poured into the moulds.
Magnesium melt preparation
AZ91E magnesium alloy was used for the experiments.The
chemical composition of this alloy is on Table 2. A electric
resistance furnace was used for melting operation under CO2/SF6
protective atmosphere. After grain refinement treatment,
magnesium melt of 700°C was poured into shell moulds.
Table 2. Chemical composition (wt.%) of AZ91E.
Al Zn Mn Si Fe
8.580.570.210.00670.0038
Cu Ni Be Mg
0.00160.000630.0000bal.
Examination of Mould– magnesium reactions
A part with a wall thickness of1.272 mm was selected for this
experiment. An electron spectroscopy for chemical analysis
(ESCA), and an auger electron spectroscopy (AES) were used to
examine the chemical composition and the element distribution
profiles in reaction layer on magnesium castings.
Results and discussion
Mould – magnesium reactions under a mixture of 1%SF6 in CO2
Shell
Figure 2 shows the shell moulds used for experiments after
dewaxing and firing.
Fig. 2 Shell mould.
Before pouring, preheated shell moulds were installed in a container with an atmosphere of CO 2 / SF 6. To remove oxygen in a cavity and porous shell wall, a mixture of 1% SF 6 in CO 2, was guided into a cavity for 40 seconds with 190 ml/second. Then, magnesium alloy AZ91E at 700°
C was poured into shell moulds.
(a)
(b)
Fig. 3. Magnesium part cast by shell B (a), inside surface of shell B (b).
.
(a)
(b)
Fig. 4. Magnesium part cast by shell A (a),and inside surface of shell A (b).
After cooling down, magnesium parts can be shaken out easily from shell moulds A and B. The surface of the part cast by shell B was clean (Fig. 3a ). No obvious reaction products were observed on the inside surface of shell B (Fig. 3b). However, for shall A, a black layer appeared on the inside shell surface. Figure 4 shows the part and the inside surface of shell A. The results indict that the mixture of CO 2 + 1%SF 6 can limit magnesium-mould reactions in shell B. For shell A, the mould-melt reactions can not be stopped completely by the protective gas (1%SF 6 in CO 2).Mould – magnesium reactions under a mixture of over 1% SF 6 in CO 2
For controlling the mould – magnesium reactions, a mixture of CO 2 and over 1 % SF 6 was guided into shell A before pouring AZ91 melt into the moulds. The inside surface of a shell A is showed in Fig. 5.
Basically, the black layer disappeared (Fig. 5), and clean part surface was observed. So, increasing the concentration of SF 6
in the gas mixture is benefit to limit the mould-magnesium reactions in shell A.
Fig. 5. Inside surface of shell A.
Surface analysis
In order to better understand the behaves of the mixture of CO 2and SF 6on the prevention from mould – magnesium reactions,a surface analysis on the magnesium cast parts with shells A or B was carried out.
1. Parts cast with shell A (inhibitor gas: over 1%SF 6 in CO 2)Table 3 is the results of the surface analysis on the surface of the parts cast with shall A by ESCA ( around 5 nanometer depth).Mg, O, F, Si, S, C, N were detected, but Al was not found on the surface. Figure 6 shows the depth profiles of element distribution detected by AES. The noisy profiles would be contributed by the rough of the surface. A special layer (thickness ～ 3 micrometer)on the surface of the magnesium part was formed, in which the main elements are Mg, O, F, S, Si, and Al. Fluorine is concentrated on the most heavily oxidized layer of a 0.75micrometer depth (Figure 6 (a)), and the highest electron intensity of fluorine is around 240000 electrons, and at the same point, that of oxygen is about 250000 electrons.The ratio of electron intensity of fluorine to oxygen is around one to one. Interestingly,the distribution profile of fluorine is similar to that of oxygen.However, the profile of sulphur is not same as that of fluorine although both of the elements are from SF 6. So, it indicts that SF 6decomposes after it entered into mould cavity.
The concentration of magnesium and aluminium in surface layer increases with an increase of the depth of the sample. And their profiles are very similar. Aluminium and magnesium, both of the elements, would participate the reactions. Carbon was detected by ESCA (Table 2). But, carbon intensity reaches to zero after a very short sputtering time (Figure 6e). So, it should be from the surface contamination [6]. The detected nitrogen would be from magnesium nitride, which is the products of magnesium with air at high temperature [2]. The silicon concentration near surface is higher (Fig. 6d).This would be due to the silicon in the binder.Table 3. Apparent concentration of detected elements (at. %)
on the sample surface cast with shell A (The standard deviation is given in parenthesis).Mg O F Si Al S C N 18.7(0.3)
29.1(0.2)
2.0(0.1)
3.4(0.3)
-0.6(0.1)
45.2(0.3)
0.3(0.1)
(a)
(b)
(c)
(d)
(e)
Fig. 6. Depth profiles: oxygen,and fluorine (a), magnesium, and aluminium (b), sulphur (c),silicon (d), and carbon (e).
2. Parts cast by shell B (inhibitor gas: 1%SF 6 in CO 2)
The surface analysis results by ESCA are listed on Table 3.Magnesium, oxygen, fluorine, sulphur, carbon, and nitrogen are detected. Silicon and aluminium are not found. It would be related to the binder silica-free. Figures 7 is auger depth profiles of oxygen, magnesium, aluminium, fluorine, sulphur, and silicon.There exists also a special layer on the surface of the part.Fluorine concentrates in a 0.3 micrometer depth of the most heavily oxidised layer, which is smaller than that of over 1%SF 6(Figure 6a), and the ration of electron intensity of fluorine to oxygen is around one to six. It is much lower than that of over 1%SF 6(one to one). The distributions of magnesium and aluminium are similar those in shell A. On the surface, carbon is also detected, however, the examination by AES shows that the carbon only focuses on the surface. So, it should come from surface contamination.Sulphur was detected (Table 3 and Fig. 7b), but near the surface (0 ～ 0.25 micrometer) the sulphur can not be detected by auger depth profile. This conformed further that SF 6 was decomposed . Besides, silicon has been detected in the zone (0.1～3 micrometer), probably, it is from the alloy.So, based on the experiment data, there exist a special layer on the surface of the magnesium castings. In the layer,the composition is not same as that the bulk. The distributive depth and intensity of fluorine in the layer is increasing as the increase of fluorine concentration in the mixture gas. However, the distribution of sulphur in the layer is not same as that of fluorine. Sulphur concentrates in the interior of the layer, but, most of fluorine is in exterior of the layer. This phenomenon should be due to the decomposition of SF 6. The concentration of silicon in the layer would be related to the composition of the binder and refractory materials in the primary layer as well as the alloy composition.Table 4. Apparent concentration of detected elements (at. %)
on the sample surface cast with shell A (The standard deviation is given in parenthesis).Mg O F Si Al S C N 19.6(0.4)
57.7(0.3)
1.4(0.1)
--0.4(0.1)
20.2(0.1)
0.3(0.1)
(a)
(b)
(c)
Fig. 7 Depth profiles: oxygen,magnesium, and aluminium, and fluorine (a); sulphur (b); silicon (c).Models of mould-magnesium reactions
Since the oxide film that formed by the reactions between magnesium and air is porous, magnesium or magnesium vapour will go through the oxide layer to react further with shell materials. Forming an effective barrier to magnesium access is critical to prevent from the excessive oxidising reaction. Based on the surface analysis on the magnesium parts and the examination on the shell section, when no inhibitor gas guides into cavity,magnesium reacts with the oxygen in mould cavity. In addition,passing through the porous magnesium oxide film, magnesium or its vapour reacts with shell materials and oxygen in shell wall.
(a)
(b)
Fig. 9. Models of mould-magnesium reaction: without inhibit gas (a); under protection of inhibitor gas (b).
Figure 9a is the sketch of the model. In this case, magnesium parts stuck on shell wall and can not be separated [5]. However, when the inhibitor gas guides into cavity, fluorine and sulphur from SF 6react with magnesium, or magnesium oxide (Fig. 9b). These reactions result in the improvement of the quality of porous oxide film (less pores). Aluminium would participate the reactions also because it appears in the special layer. But, it is not clear that the role of aluminium in the reaction prevention. The improved layer make the atomics such as magnesium and aluminium passing through the film more difficulty. Consequently,the content of magnesium and aluminium in the reaction layer decrease as the increase of the thickness of the reaction layer. Besides, as the increase of concentration of SF 6 in the inhibitor mixture,the intensity ratio of fluorine to oxygen in the special layer increases.This would be of benefit to make a greater improvement of the quality of the special layer.
When the mixture of 1%SF 6 in CO 2 was purged into shell moulds, the formed special layer can prevent from the reactions between magnesium and zirconia binder moulds, however, it is not able to completely stop the magnesium– silica binder mould reactions.The magnesium-silica binder mould reactions will be avoided,only when the concentration of SF 6 is more than 1% in CO 2.
Conclusions
1. Ceramic shell moulds with silica or zirconia binders can be used to cast AZ91 magnesium alloy under an inhibitor gas with a proper concentration of SF 6 in CO
2.
2. A special layer is formed on the surface of magnesium parts cast in the atmosphere of the mixture of SF 6 and CO 2. The layer is mainly composed of magnesium, oxygen, fluorine, sulphur, and aluminium. The intensity ratio of fluorine to oxygen as well as the distributive depth of fluorine in the special layer increases with an increasing of the SF 6concentration in the mixture.
3. The distribution profile of sulphur is not the same as that of fluorine in the special layer although both of them are from SF 6.Sulphur concentrates in the interior of the layer, but, most of fluorine is in exterior of the layer.
Special layer
Acknowledgement
The authors wish to acknowledge the Natural Science and Engineering Research Council of Canada for his financial supports. It is a pleasure to thank Dr. A. Adnot in Laval University for his help in surface analysis and valuable discussion.The authors would like to thank Mrs. C. Turcotte, Mr. R. Castonguay,F. Rhéaume, and other staff from Intermag-Modelex for their helps in the project.
References
1. G. Geirnaert, Céramiques – métaux liquids. Compatibilités et angles de mouillages, Bulletin de la société fran?aise de céramique, No.106, janvier – mars 1975.
2. E. F. Emley,Principles of Magnesium Technology, Pergamon Press Ltd. 1966.
3. M. H. Idris, A. J. Clegg, AFS Transaction, (1996) 237.
4. S. K. Kim, J. I. Youn, and Y. J. Kim, Materials Science and Technology, 16 (2000) 769 .
5. Z. Zhang, C. Turcotte, and G. Morin, Proc. Materials Week 2002, Munich,Germany, September 30 – 2 October.
6. A. Adnot, report: Surface analysis of magnesium alloy samples by ESCA, Auger electron spectroscopy and auger depth profiling,CERPIC’s surface analysis laboratory, Laval University, Quebec,Canada, (2002).