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Centrifugal casting of TiAl exhaust valves
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Intermetallics 16 (2008) 130e138 www.elsevier.com/locate/intermet
Centrifugal casting of TiAl exhaust valves P.X. Fu, X.H. Kang, Y.C. Ma, K. Liu, D.Z. Li*, Y.Y. Li Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China Received 16 April 2007; received in revised form 3 August 2007; accepted 21 August 2007 Available online 17 October 2007
Abstract The mould filling process and solidification of TiAl exhaust valves by centrifugal investment casting have been simulated. Two types of runner and gating systems are designed and analysed. In the preliminary design, a ‘‘tree-type’’ set up system is used and a significant amount of porosity is found in many valves of the simulation result. The fluid field simulations indicate that moulds are not filled well in the preliminary design, leading to the last hot spots deviating from the center line of castings. Casting defects deviated from the center line of the part, and the degree of deviation is affected by the mould filling process and temperature fields. Simulation results reveal that castings do not experience sequential solidification, so the design is not proper for the exhaust valve. Comparing the experimental and simulation results, the range of Niyama criterion in the centrifugal TiAl casting is defined, which is 0.14e0.20. Several key factors such as pouring temperature, mould temperature and rotation speed are studied in detail. An optimized design is developed in which valves are rearranged to reduce the neighboring heat radiation effect, and the gate size is enlarged to keep the feeding path open. Sound exhaust valves have been produced successfully using the optimized technique. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: A. Titanium aluminides, based on TiAl; C. Casting; C. Near-net-shape manufacturing; G. Automotive uses
1. Introduction One of the most important problems in the modern car industry is the pollution of the environment by exhaust fumes. Among automotive engine parts, the exhaust valve has been a particular focus of the drive for increasing efficiency. The substitution of light weight materials for steel [1e4] is an effective approach to reduce pollutant emissions. At present, TiAl alloy appears to be an ideal candidate material of the exhaust valve due to its low density, high specific strength, high stiffness and fatigue resistance at increased temperatures [5,6]. There are three traditional methods to shape TiAl alloy: forging, powder metallurgy (PM) and casting. It is difficult for the TiAl alloy to take shape by forging due to its inherent poordeformability and high machining losses [7,8]. The powder metallurgy route has been limited by the processing of TiAl
* Corresponding author. Tel.: þ86 24 23971281; fax: þ86 24 23971429. E-mail address: [email protected] (D.Z. Li). 0966-9795/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2007.08.007
powder [8,9]. In contrast, the works by Jones [10], Liu [6] and Sheng et al. [11] indicated that centrifugal casting process had significant advantages. The conventional trial and error method of designing casting process based on experience or manual craft often results in higher cost and longer cycle of preproduction. One typical case is the researches about the centrifugal casting. The centrifugal casting of investment moulds for the production of TiAl exhaust valves is restricted in use, because the quality of products is highly sensitive to damage as a result of inappropriate processing conditions and mould design. So the design based on computer simulation and real-time radiography technology is needed because it can improve quality, reduce cost and shorten preproduction time obviously [12e14]. Great promise and potential have been shown by using numerical simulation. In this paper, computer simulation and X-ray inspection technologies are adopted to investigate the centrifugal investment casting process for the production of TiAl exhaust valves.
P.X. Fu et al. / Intermetallics 16 (2008) 130e138
131
2. Design 1: a preliminary design
Table 2 Thermophysical properties of TiAl alloy varied with temperature
2.1. Numerical simulation
Temperature ( C)
Density (kg/m3)
Specific heat (J/kg K)
Thermal conductivity (W/m K)
25 200 400 600 800 1600 1800
3857
598 630 667 703 740 786 794
13.2 16.7 20.2 23.1 25.3 31 37
The mould filling and solidification processes of TiAl cast exhaust valves have been simulated using the ProCAST package. The finite element method (FEM) code which employs CarreaueYasuda viscosity model to describe the behavior of shear fluids in centrifugal casting has been chosen. The transient NaviereStokes equations for a Newtonian fluid are the basis of the ProCAST model. An enthalpy method is used to solve the phase transition problem during solidification. The thermophysical material properties of the casting and mould materials are summarized in Tables 1 and 2. The initial processing parameters used in the simulation are pouring temperature 1690 C, filling time 6 s, pre-heated mould temperature 900 C and rotation speed 450 rpm. 2.2. Experiments The TiAl alloy exhaust valve casting consists of the head with the dimension Ø 395.2 mm, and the stem with Ø 8.2 100 mm as shown in Fig. 1. In order to achieve a high metal yield and production efficiency, a ‘‘tree-type’’ wax assembly set up is used, as shown in Fig. 2. Due to heat transfer and solidification, 24 parts with some angles upward are placed on the tree in a regular formation to reduce part congestion. The three-dimensional (3D) model of exhaust valve based on the first design is shown in Fig. 2. The alloy used in this study has a nominal composition of Ti45Al8Nb1B (at.%). Cast parameters used in the experiments are the same as those in the simulation. The one-step melting and centrifugal casting process is adopted to reduce the cost of castings. It means that when the temperature of the molten alloy and rotation speed of the ceramic mould are adjusted to the pre-determined parameters, the liquid is directly poured into the turning Al2O3 mould. A sophisticated 3D X-ray inspection system is used to inspect defects in exhaust valves. 2.3. Results and discussion The mould filling process and solidification are calculated. Casting defect examination function RGL and ISOCHRONS in the ProCAST software are better than others. Function RGL is able to calculate the solidification rate ‘‘R’’, the
3676 3612
cooling rate ‘‘L’’ and the temperature gradients ‘‘G’’. The definition of G is as follows: G¼
vT vx
2 2 2 1=2 vT vT þ þ vy vz
ð1Þ
where T is the temperature. The cooling rate is calculated using a linear interpolation between two temperatures, and L is defined as follows: Tupper Tlower ð2Þ L ¼ tupper tlower where t is the time when the temperature reaches T. If the following parameters, i.e. a ¼ 1.0; b ¼ 0; c ¼ 1.0; d ¼ 0.5 are used in the equation M ¼ aRb Gc Ld , we can get the Niyama criterion (Nc) [15]: pffiffiffi ð3Þ M ¼ G= T_ > constant where the constant is dependent on the alloy being cast. The criterion function of Niyama is size-independent while it is alloy-dependent. Fig. 3(a) shows the experimental results, and Fig. 3(b) shows the simulation results. All the cast parameters used in the simulation are the same as those of the experiment. Seven Niyama criteria (0.12, 0.14, 0.16, 0.18, 0.20, 0.22, 0.24) are shown in the simulation results, as shown in Fig. 3(b). The results indicate that when the Niyama criterion is chosen from 0.14 to 0.20, the simulation results match well with the experimental results. When the Niyama criterion is chosen as 0.12 or 0.22, the simulation results deviate from the experimental results. It is concluded that the value of Niyama criterion has a great influence on simulation results. 5.2
Table 1 Thermophysical properties of casting and mould materials (constant) Thermophysical properties
TiAl alloy
Mould material (Al2O3)
Density (kg/m3) Specific heat (J/kg K) Thermal conductivity (W/m K) Liquidus temperature ( C) Solidus temperature ( C) Latent heat (J/kg K) Environment temperature ( C)
3857 598 13.2 1554 1478 435 22
3970 777 39 2323
100 Ø39
Ø8.2
120
22
Fig. 1. Schematic illustration of the cast valve.
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Fig. 2. 3D model of cast valves in the preliminary design.
Furthermore, according to the experimental results and simulation results, the critical value of the Nyiama criterion for this study is 0.18, as shown in Fig. 3(b). All the cast exhaust valves have porosities with different degrees, as shown in Fig. 3. The results indicate that casting defects are concentrated near the center of cast exhaust valves, and the porosity is serious. Casting defects are affected by the mould filling process and the solidification of the cast valve. Because of the centrifugal effects, mould filling is different from static mould filling. The filling process is divided into forward filling and backward filling regimes. Cast exhaust valves are filled along the inside of the shell surface, on the side dragging behind the direction of the rotation. At first the melt is filled along the shell surface, leaving the other part unfilled, as shown in Fig. 4. In the forward filling process, when cast exhaust valves are filled and the melt does not reach the casting shell bottom, the shell is half-filled and the solidification occurs in the interface between the forward filled field and the shell inner surface, as shown in Fig. 4. The thickness of the solidified material is affected by the length of the cavity and the filling velocity. The longer length of the cavity leads to a longer time when the backward filling returns, and resulting in a larger thickness of the solidified layer. When the backward wave finally reaches this section, the shell cavity will be filled completely. So the preliminary design of runner system results in surface turbulence. Splash and folding-over of the melt are observed when the liquid melt enters the mould cavity. Furthermore, the mould filling process has great influence on the temperature field of exhaust valves. The temperature of the parts that are filled by forward filling process is low, and the temperature of the parts filled by backward filling is high. So the temperature of exhaust valves does not experience sequential solidification, as shown in Fig. 5. The rapid return of the back wave ensures that any porosity is not greatly shifted from the center line. If the porosity in cast exhaust valves deviates significantly from the center line, cast exhaust valves unfortunately bend during hot isostatic pressing (HIP).
Fig. 3. Experimental and calculated porosities of cast valves in the preliminary design. (a) X-ray scanning image, (b) predicted porosity. A: Nc ¼ 0.12, B: Nc ¼ 0.14, C: Nc ¼ 0.16, D: Nc ¼ 0.18, E: Nc ¼ 0.20, F: Nc ¼ 0.22, G: Nc ¼ 0.24 (green areas represent porosity, dark areas represent the casting in the viewing direction). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The research results [16] indicated that the mechanical properties of the TiAl alloy casting were mainly related to cast features. The mechanical properties could not be easily optimized by any additional post-HIP heat treatment. Therefore, increasing the input molten metal and increasing filling velocity can decrease the deviation. The simulation results indicate that the cast valve does not experience sequential solidification, and temperature field has great influence on the potential porosity location, as shown in Fig. 5. The A areas in Fig. 5(b) and the B areas in Fig. 5(d) of cast exhaust valves tend to hold heat much longer than the rest of the part. Therefore, feeding paths back to the gate are cut
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Fig. 4. Mould filling simulation results of the cast exhaust valve in the preliminary design at different times. (a) t ¼ 3.37 s, (b) t ¼ 3.43 s, (c) t ¼ 3.48 s, (d) t ¼ 3.54 s.
off prematurely and then produce shrink defects. The experimental and simulation results indicate that porosities near the center of the stem are serious, as shown in Fig. 3. For the simulation results, the temperature field of cast exhaust valves is also affected by neighboring heat radiation effect besides the mould filling process. In the preliminary design, 24 parts with some angles upward are placed on the tree in a regular formation. As the distance of each part is limited, the neighboring heat radiation effect does have much influence on the temperature field of the exhaust valves. The neighboring radiation effects keep some exhaust valves with high temperature, so the solidification time of those exhaust valves is long and castings do not solidify well. Castings near the middle of the mould are influenced greatly in the preliminary design. Because of both the filling order and radiation effects, castings have not benefited from sequential solidification (i.e. directional solidification) towards a source of feed metal. The temperature of parts of the cast valve first filled by forward filling are lower than that of parts filled by the return wave. A higher temperature can be found from the parts filled
by the back wave. In addition, the feeding paths tend to be cut off prematurely because the distance required to be fed is long, as shown in Figs. 5 and 6. It is easy to produce defects in the poorly fed regime in the center of the long valve stems. It is concluded that enlarging the feeder to keep the feeding path open and increasing the temperature gradient are useful for reducing porosity. From simulation results, cast parameters are seen to be important. Accordingly, six sets of simulations are carried out in an effort to optimize the cast parameters. The cast parameters used in the simulation are shown in Table 3. Although it will be noted that the range of each of the parameters is not as large as would be desirable for an experiment targeted at optimizing parameters, the casting of TiAl is particularly constrained by experimental difficulties, so that the range of conditions listed here is probably as wide as can be achieved with current melting and casting technology. Potential porosity locations of casting simulation results of six projects used in the first design are shown in Fig. 7. The results indicate that all the exhaust valves have defects, and
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Fig. 5. Temperature field simulation results of the cast valve in the preliminary design at different times. (a) t ¼ 5.35 s, (b) t ¼ 5.50 s, (c) t ¼ 11.87 s, (d) t ¼ 19.45 s.
that the defects are caused by the filling order and temperature effects. Compared to other projects, project 4 has less potential porosity as shown in Fig. 7(d). The liquid superheat has a great effect on the complete filling time for the head of the valve, as shown in Fig. 8. The simulation results indicate that the higher the liquid superheat is, the longer is the solidification time. In project 6, the solidification time is about 43 s, while the solidification time of project 1 is 28 s, as shown in Fig. 8(a). When the pouring temperature is from 1650 C to 1740 C, the temperature gradient of exhaust valves has increased little, as shown in Fig. 8. When castings have large temperature gradient, exhaust valves will experience sequential solidification, and castings will be defect-free. From the results, it is no use for exhaust valves just to increase the metal superheat while the temperature gradient is small. So the higher pouring temperature induces longer solidification time of casting in the preliminary design.
The pouring rate and pouring temperature of the molten metal have apparent influences on the defects of exhaust valves. For a given pouring temperature, the faster the pouring rate is, the longer is the time interval between pouring the molten metal into the mould and completing the solidification and greater is the possibility of cast defect formation. In the case of the lower pouring temperature and pouring rate, the molten metal spreads forward under the action of centrifugal force. Because of the contact with the mould, the front-flowing molten metal loses a large amount of heat, which causes the temperature to drop suddenly. So it may no longer flow to the cold end of the mould. Therefore, control of the pouring temperature and pouring rate is clearly beneficial to the prevention of the formation of cast defects. Furthermore, the previous research results [6] demonstrated that the O increase vertically came from the interaction of the higher superheated liquid with CaO crucible. The research
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Fig. 6. Fraction of solid simulation results of the cast valve in the preliminary design at different times. (a) t ¼ 11.87 s, (b) t ¼ 14.52 s, (c) t ¼ 18.65 s (gray areas represent solid, red areas represent liquid).
results [17] indicated that superheating temperature and holding time played an important role in the interaction between melting crucibles with Y2O3 and the molten TiAl. With increase in the superheating temperature and holding time, the O contents were higher. From Fig. 7(d) and (e), project 5 with pouring temperature of 1730 C is not better than project 4 with pouring temperature of 1710 C. Thus, a pouring temperature range from 1690 C to 1720 C is adopted. Fig. 7(b) and (c) indicates that on increasing the centrifugal rotation speed from 400 to 450 rpm, at the same pouring temperature (1690 C) and mould temperature (900 C), the filling rate does not improve and the higher rotation speed has no significant effect on porosity. Additional practical considerations indicate that if the filling time is too long, the heat loss during filling will be too high. Of course, if the filling time is too short, the large filling system will allow room for turbulence under the centrifugal force. It is concluded that 5e7 s is better for the filling time. The simulated and experimental results show that there are porosity defects in many exhaust valves due to both mould filling and temperature field. Due to the thermal effect of neighboring valves, those parts kept hot by neighboring parts suffer larger defects near the center stems, as shown in Figs. 4 and 5.
It is concluded that the preliminary design is not proper for exhaust valves. 3. Design 2: an optimized design 3.1. Design of gating system From simulation results of the preliminary design, there are two primary solutions to improve the part quality. One is to reduce the number of parts on the tree, thereby eliminating radiation effects that kept some of the parts hotter than others depending on their positions on the tree. The other is to increase the gate size, thereby keeping the feeding path open longer. Obviously, reducing the number of parts in the set up drastically decreases the efficiency of metallic yield. Therefore, it is chosen to increase the gate size moderately, and not reduce the number of parts on the wax assembly but change positions of parts into a staggered arrangement, as shown in Fig. 9. Cast parameters used for the new design of the centrifugal investment casting mould are pouring temperature 1710 C, mould temperature 900 C and rotation speed 400 rpm. The 3D model of the new design is shown in Fig. 9.
Table 3 The parameters used in the simulation project Simulation project
Pouring temperature ( C) Filling time(s) Mould temperature ( C) Rotation speed (rpm)
Project 1
Project 2
Project 3
Project 4
Project 5
Project 6
1650 7 800 400
1690 6 900 450
1690 7 900 400
1710 7 950 400
1730 7 950 400
1740 7 950 420
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3.2. Results and discussion Because of the centrifugal effects and the enlarged filling gate, mould filling and temperature field are now modified. The melt flowing along the inner shell surface just leaves the center unfilled at the initial filling, as shown in Fig. 10. It indicates that the mould filling process in the optimized
Fig. 8. Temperature variation curves of cast valves during solidification in the preliminary design. (a) Temperature variation curve of A point, (b) temperature variation curve of B point.
design is much better than and completely different from that in the preliminary design. It should be attributed to the enlarging filling gate. In the preliminary design, the mould filling process is divided into forward filling and backward filling,
Fig. 7. Potential porosity locations of casting simulation results in the preliminary design. (a) Project 1, (b) project 2, (c) project 3, (d) project 4, (e) project 5, (f) project 6 (green areas represent porosity, dark areas represent the casting in the viewing direction). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. 3D model of cast valves in the modified design.
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Fig. 10. Mould filling simulation results of the cast exhaust valve in the modified design. (a) t ¼ 2.266 s, (b) t ¼ 2.322 s, (c) t ¼ 2.387 s, (d) t ¼ 2.406 s.
which have great influence on temperature field of valves. The castings do not experience sequential solidification for the bad mould filling. In the optimized design, the melt fills the shell along the inner surface and leaves exhaust valve’s center unfilled. All of those are affected by the enlarging filling gate. The molten TiAl alloy put into the shell is much more at the initial filling, and only the center is filled later. So the center field holds heat longer than the rest of the casting, and cast valves have a good temperature gradient. The castings benefit from sequential solidification for the feeding paths remain open longer. The simulation results indicate that only one or two exhaust valves have a slight defect, as shown in Fig. 11.
Fig. 11. Potential porosity locations in the modified design (green areas represent porosity, dark areas represent the casting in the viewing direction). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
It should be attributed to the random mould filling of cast exhaust valves. Under the centrifugal force, the molten TiAl alloy in the running and gating systems is also rotary, when the metals reach the shell. For the bottom layer, the parts on the ‘‘tree-type’’ shell are filled at the same time in initial filling. However, as the mould filling goes on, the parts are filled
Fig. 12. The modified design showing (a) X-ray scanning images, (b) simulation results (no porosity).
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randomly in different layers. Some exhaust valves in the lower layer are filled later than those in the upper layer. So the random mould filling results in some bad exhaust valves. The randomness in the modified design is higher than that in the preliminary design, because the filling gate is enlarged and the parts are changed in staggered arrangement. From the simulation results, only one or two exhaust valves are affected, so the influence is small. In the modified design the increased filling gate effectively keeps the feeding path open, and it can reduce the porosity. In addition, the harmful effects of radiation are reduced by changing the position of the parts on the tree. The parts benefit from sequential solidification, because the feeding paths are kept open. Using the modified design confirms that most of the valves have no porosity, as shown in Fig. 12. The modified design improves both the soundness and the metallic yield of the finished products. The feasibility for mass production and cost effectiveness of TiAl automotive valves are being assessed. 4. Conclusions The mould filling process and solidification of TiAl exhaust valves by centrifugal investment casting have been simulated. The Nc in the centrifugal investment casting of Ti45Al8Nb1B alloy is identified in the range of 0.14e0.20, and the optimized Nc is 0.18 in this study. Several key factors including the pouring temperature, the mould temperature and the rotation speed are studied in detail. The optimized cast parameters for the centrifugal investment casting process of one design of an investment mould of TiAl exhaust valves are found to be pouring temperature 1690e1720 C, mould temperature 900e950 C and rotation speed 400e450 rpm. In the preliminary design, the porosity deviation from the center line is dependent on the sequence of solidification, which is affected by forward and backward flows in the mould filling process. The whole filling process is divided into forward filling and backward filling in the preliminary design, which results in valves not experiencing sequential solidification. The potential porosity locations are affected by the mould filling process and temperature field. Keeping the feeding path open by enlarging the gate and reducing neighboring radiation effects encourage sequential solidification and can therefore reduce or eliminate porosity in the modified design.
Acknowledgements The authors are indebted to Professor John Campbell at the University of Birmingham, UK for valuable discussion and modification of English. References [1] Gebauer Klaus. Performance, tolerance and cost of TiAl passenger car valves. Intermetallics 2006;14:355e60. [2] Si-Young Sung, Young-Jig Kim. Economic net-shape forming of TiAl alloys for automotive parts. Intermetallics 2006;14:1163e7. [3] Noda T. Application of cast gamma TiAl for automobiles. Intermetallics 1998;6:709e13. [4] Loria EA. Gamma titanium aluminides as prospective structural materials. Intermetallics 2000;8:1339e45. [5] Xinhua Wu. Review of alloy and process development of TiAl alloys. Intermetallics 2006;14:1114e22. [6] Liu K, Ma YC, Gao M, Rao GB, Li YY, Wei K, et al. Single step centrifugal casting TiAl automotive valves. Intermetallics 2005;13:925e8. [7] Bartels A, Kestler H, Clemens H. Designed fully lamellar microstructures in a g-TiAl based alloy: adjustment and microstructural changes upon long-term isothermal exposure at 700 and 800 C. Mater Sci Eng A 2002;329e331:124e9. [8] Boyun Huang. TiAl intermetallics. Hunan: Central South University Press; 1998. [9] Gerling R, Leitgeb R, Schimansky FP. Porosity and argon concentration in gas atomized g-TiAl powder and hot isostatically pressed compacts. Mater Sci Eng A 1998;252:239e47. [10] Jones PE, Porter III WJ, Eylon D, Colvin G. Development of a low cost permanent mold casting process for TiAl automotive valves. In: Kim YW, Wagner R, Yamaguchi M, editors. Gamma titanium aluminides. Las Vegas, Nevada: TMS; 1995. p. 53e62. [11] Wenbin Sheng, Rui Yang, Yuyin Liu, Dongsheng Xu, Dong Li. Current status on low cost g-TiAl exhaust valve. Chin Foundry 2001;50:650e3. [12] Campbell J. Castings. 2nd ed. Oxford, UK: Butterworth-Heinemann; 2003. [13] Hu BH, Tong KK, Niu XP, Pinwill I. Design and optimisation of runner and gating systems for the die casting of thin-walled magnesium telecommunication parts through numerical simulation. J Mater Process Technol 2000;105:128e33. [14] Li DZ, Campbell J, Li YY. Filling system for investment cast Ni-base turbine blades. J Mater Process Technol 2004;148:310e6. [15] Hansen PN, Sahm PR, Flender E. How to select and use criterion functions in solidification simulation. AFS Trans 1993;101:443e6. [16] Thomas M, Raviart JL, Popoff F. Cast and PM processing development in gamma aluminides. Intermetallics 2005;13:944e51. [17] Barbosa J, Ribeiro C Silva, Caetano Monteiro A. Influence of superheating on casting of g-TiAl. Intermetallics 2007;15:945e55.
Intermetallics 16 (2008) 130e138 www.elsevier.com/locate/intermet
Centrifugal casting of TiAl exhaust valves P.X. Fu, X.H. Kang, Y.C. Ma, K. Liu, D.Z. Li*, Y.Y. Li Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China Received 16 April 2007; received in revised form 3 August 2007; accepted 21 August 2007 Available online 17 October 2007
Abstract The mould filling process and solidification of TiAl exhaust valves by centrifugal investment casting have been simulated. Two types of runner and gating systems are designed and analysed. In the preliminary design, a ‘‘tree-type’’ set up system is used and a significant amount of porosity is found in many valves of the simulation result. The fluid field simulations indicate that moulds are not filled well in the preliminary design, leading to the last hot spots deviating from the center line of castings. Casting defects deviated from the center line of the part, and the degree of deviation is affected by the mould filling process and temperature fields. Simulation results reveal that castings do not experience sequential solidification, so the design is not proper for the exhaust valve. Comparing the experimental and simulation results, the range of Niyama criterion in the centrifugal TiAl casting is defined, which is 0.14e0.20. Several key factors such as pouring temperature, mould temperature and rotation speed are studied in detail. An optimized design is developed in which valves are rearranged to reduce the neighboring heat radiation effect, and the gate size is enlarged to keep the feeding path open. Sound exhaust valves have been produced successfully using the optimized technique. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: A. Titanium aluminides, based on TiAl; C. Casting; C. Near-net-shape manufacturing; G. Automotive uses
1. Introduction One of the most important problems in the modern car industry is the pollution of the environment by exhaust fumes. Among automotive engine parts, the exhaust valve has been a particular focus of the drive for increasing efficiency. The substitution of light weight materials for steel [1e4] is an effective approach to reduce pollutant emissions. At present, TiAl alloy appears to be an ideal candidate material of the exhaust valve due to its low density, high specific strength, high stiffness and fatigue resistance at increased temperatures [5,6]. There are three traditional methods to shape TiAl alloy: forging, powder metallurgy (PM) and casting. It is difficult for the TiAl alloy to take shape by forging due to its inherent poordeformability and high machining losses [7,8]. The powder metallurgy route has been limited by the processing of TiAl
* Corresponding author. Tel.: þ86 24 23971281; fax: þ86 24 23971429. E-mail address: [email protected] (D.Z. Li). 0966-9795/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2007.08.007
powder [8,9]. In contrast, the works by Jones [10], Liu [6] and Sheng et al. [11] indicated that centrifugal casting process had significant advantages. The conventional trial and error method of designing casting process based on experience or manual craft often results in higher cost and longer cycle of preproduction. One typical case is the researches about the centrifugal casting. The centrifugal casting of investment moulds for the production of TiAl exhaust valves is restricted in use, because the quality of products is highly sensitive to damage as a result of inappropriate processing conditions and mould design. So the design based on computer simulation and real-time radiography technology is needed because it can improve quality, reduce cost and shorten preproduction time obviously [12e14]. Great promise and potential have been shown by using numerical simulation. In this paper, computer simulation and X-ray inspection technologies are adopted to investigate the centrifugal investment casting process for the production of TiAl exhaust valves.
P.X. Fu et al. / Intermetallics 16 (2008) 130e138
131
2. Design 1: a preliminary design
Table 2 Thermophysical properties of TiAl alloy varied with temperature
2.1. Numerical simulation
Temperature ( C)
Density (kg/m3)
Specific heat (J/kg K)
Thermal conductivity (W/m K)
25 200 400 600 800 1600 1800
3857
598 630 667 703 740 786 794
13.2 16.7 20.2 23.1 25.3 31 37
The mould filling and solidification processes of TiAl cast exhaust valves have been simulated using the ProCAST package. The finite element method (FEM) code which employs CarreaueYasuda viscosity model to describe the behavior of shear fluids in centrifugal casting has been chosen. The transient NaviereStokes equations for a Newtonian fluid are the basis of the ProCAST model. An enthalpy method is used to solve the phase transition problem during solidification. The thermophysical material properties of the casting and mould materials are summarized in Tables 1 and 2. The initial processing parameters used in the simulation are pouring temperature 1690 C, filling time 6 s, pre-heated mould temperature 900 C and rotation speed 450 rpm. 2.2. Experiments The TiAl alloy exhaust valve casting consists of the head with the dimension Ø 395.2 mm, and the stem with Ø 8.2 100 mm as shown in Fig. 1. In order to achieve a high metal yield and production efficiency, a ‘‘tree-type’’ wax assembly set up is used, as shown in Fig. 2. Due to heat transfer and solidification, 24 parts with some angles upward are placed on the tree in a regular formation to reduce part congestion. The three-dimensional (3D) model of exhaust valve based on the first design is shown in Fig. 2. The alloy used in this study has a nominal composition of Ti45Al8Nb1B (at.%). Cast parameters used in the experiments are the same as those in the simulation. The one-step melting and centrifugal casting process is adopted to reduce the cost of castings. It means that when the temperature of the molten alloy and rotation speed of the ceramic mould are adjusted to the pre-determined parameters, the liquid is directly poured into the turning Al2O3 mould. A sophisticated 3D X-ray inspection system is used to inspect defects in exhaust valves. 2.3. Results and discussion The mould filling process and solidification are calculated. Casting defect examination function RGL and ISOCHRONS in the ProCAST software are better than others. Function RGL is able to calculate the solidification rate ‘‘R’’, the
3676 3612
cooling rate ‘‘L’’ and the temperature gradients ‘‘G’’. The definition of G is as follows: G¼
vT vx
2 2 2 1=2 vT vT þ þ vy vz
ð1Þ
where T is the temperature. The cooling rate is calculated using a linear interpolation between two temperatures, and L is defined as follows: Tupper Tlower ð2Þ L ¼ tupper tlower where t is the time when the temperature reaches T. If the following parameters, i.e. a ¼ 1.0; b ¼ 0; c ¼ 1.0; d ¼ 0.5 are used in the equation M ¼ aRb Gc Ld , we can get the Niyama criterion (Nc) [15]: pffiffiffi ð3Þ M ¼ G= T_ > constant where the constant is dependent on the alloy being cast. The criterion function of Niyama is size-independent while it is alloy-dependent. Fig. 3(a) shows the experimental results, and Fig. 3(b) shows the simulation results. All the cast parameters used in the simulation are the same as those of the experiment. Seven Niyama criteria (0.12, 0.14, 0.16, 0.18, 0.20, 0.22, 0.24) are shown in the simulation results, as shown in Fig. 3(b). The results indicate that when the Niyama criterion is chosen from 0.14 to 0.20, the simulation results match well with the experimental results. When the Niyama criterion is chosen as 0.12 or 0.22, the simulation results deviate from the experimental results. It is concluded that the value of Niyama criterion has a great influence on simulation results. 5.2
Table 1 Thermophysical properties of casting and mould materials (constant) Thermophysical properties
TiAl alloy
Mould material (Al2O3)
Density (kg/m3) Specific heat (J/kg K) Thermal conductivity (W/m K) Liquidus temperature ( C) Solidus temperature ( C) Latent heat (J/kg K) Environment temperature ( C)
3857 598 13.2 1554 1478 435 22
3970 777 39 2323
100 Ø39
Ø8.2
120
22
Fig. 1. Schematic illustration of the cast valve.
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Fig. 2. 3D model of cast valves in the preliminary design.
Furthermore, according to the experimental results and simulation results, the critical value of the Nyiama criterion for this study is 0.18, as shown in Fig. 3(b). All the cast exhaust valves have porosities with different degrees, as shown in Fig. 3. The results indicate that casting defects are concentrated near the center of cast exhaust valves, and the porosity is serious. Casting defects are affected by the mould filling process and the solidification of the cast valve. Because of the centrifugal effects, mould filling is different from static mould filling. The filling process is divided into forward filling and backward filling regimes. Cast exhaust valves are filled along the inside of the shell surface, on the side dragging behind the direction of the rotation. At first the melt is filled along the shell surface, leaving the other part unfilled, as shown in Fig. 4. In the forward filling process, when cast exhaust valves are filled and the melt does not reach the casting shell bottom, the shell is half-filled and the solidification occurs in the interface between the forward filled field and the shell inner surface, as shown in Fig. 4. The thickness of the solidified material is affected by the length of the cavity and the filling velocity. The longer length of the cavity leads to a longer time when the backward filling returns, and resulting in a larger thickness of the solidified layer. When the backward wave finally reaches this section, the shell cavity will be filled completely. So the preliminary design of runner system results in surface turbulence. Splash and folding-over of the melt are observed when the liquid melt enters the mould cavity. Furthermore, the mould filling process has great influence on the temperature field of exhaust valves. The temperature of the parts that are filled by forward filling process is low, and the temperature of the parts filled by backward filling is high. So the temperature of exhaust valves does not experience sequential solidification, as shown in Fig. 5. The rapid return of the back wave ensures that any porosity is not greatly shifted from the center line. If the porosity in cast exhaust valves deviates significantly from the center line, cast exhaust valves unfortunately bend during hot isostatic pressing (HIP).
Fig. 3. Experimental and calculated porosities of cast valves in the preliminary design. (a) X-ray scanning image, (b) predicted porosity. A: Nc ¼ 0.12, B: Nc ¼ 0.14, C: Nc ¼ 0.16, D: Nc ¼ 0.18, E: Nc ¼ 0.20, F: Nc ¼ 0.22, G: Nc ¼ 0.24 (green areas represent porosity, dark areas represent the casting in the viewing direction). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The research results [16] indicated that the mechanical properties of the TiAl alloy casting were mainly related to cast features. The mechanical properties could not be easily optimized by any additional post-HIP heat treatment. Therefore, increasing the input molten metal and increasing filling velocity can decrease the deviation. The simulation results indicate that the cast valve does not experience sequential solidification, and temperature field has great influence on the potential porosity location, as shown in Fig. 5. The A areas in Fig. 5(b) and the B areas in Fig. 5(d) of cast exhaust valves tend to hold heat much longer than the rest of the part. Therefore, feeding paths back to the gate are cut
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Fig. 4. Mould filling simulation results of the cast exhaust valve in the preliminary design at different times. (a) t ¼ 3.37 s, (b) t ¼ 3.43 s, (c) t ¼ 3.48 s, (d) t ¼ 3.54 s.
off prematurely and then produce shrink defects. The experimental and simulation results indicate that porosities near the center of the stem are serious, as shown in Fig. 3. For the simulation results, the temperature field of cast exhaust valves is also affected by neighboring heat radiation effect besides the mould filling process. In the preliminary design, 24 parts with some angles upward are placed on the tree in a regular formation. As the distance of each part is limited, the neighboring heat radiation effect does have much influence on the temperature field of the exhaust valves. The neighboring radiation effects keep some exhaust valves with high temperature, so the solidification time of those exhaust valves is long and castings do not solidify well. Castings near the middle of the mould are influenced greatly in the preliminary design. Because of both the filling order and radiation effects, castings have not benefited from sequential solidification (i.e. directional solidification) towards a source of feed metal. The temperature of parts of the cast valve first filled by forward filling are lower than that of parts filled by the return wave. A higher temperature can be found from the parts filled
by the back wave. In addition, the feeding paths tend to be cut off prematurely because the distance required to be fed is long, as shown in Figs. 5 and 6. It is easy to produce defects in the poorly fed regime in the center of the long valve stems. It is concluded that enlarging the feeder to keep the feeding path open and increasing the temperature gradient are useful for reducing porosity. From simulation results, cast parameters are seen to be important. Accordingly, six sets of simulations are carried out in an effort to optimize the cast parameters. The cast parameters used in the simulation are shown in Table 3. Although it will be noted that the range of each of the parameters is not as large as would be desirable for an experiment targeted at optimizing parameters, the casting of TiAl is particularly constrained by experimental difficulties, so that the range of conditions listed here is probably as wide as can be achieved with current melting and casting technology. Potential porosity locations of casting simulation results of six projects used in the first design are shown in Fig. 7. The results indicate that all the exhaust valves have defects, and
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Fig. 5. Temperature field simulation results of the cast valve in the preliminary design at different times. (a) t ¼ 5.35 s, (b) t ¼ 5.50 s, (c) t ¼ 11.87 s, (d) t ¼ 19.45 s.
that the defects are caused by the filling order and temperature effects. Compared to other projects, project 4 has less potential porosity as shown in Fig. 7(d). The liquid superheat has a great effect on the complete filling time for the head of the valve, as shown in Fig. 8. The simulation results indicate that the higher the liquid superheat is, the longer is the solidification time. In project 6, the solidification time is about 43 s, while the solidification time of project 1 is 28 s, as shown in Fig. 8(a). When the pouring temperature is from 1650 C to 1740 C, the temperature gradient of exhaust valves has increased little, as shown in Fig. 8. When castings have large temperature gradient, exhaust valves will experience sequential solidification, and castings will be defect-free. From the results, it is no use for exhaust valves just to increase the metal superheat while the temperature gradient is small. So the higher pouring temperature induces longer solidification time of casting in the preliminary design.
The pouring rate and pouring temperature of the molten metal have apparent influences on the defects of exhaust valves. For a given pouring temperature, the faster the pouring rate is, the longer is the time interval between pouring the molten metal into the mould and completing the solidification and greater is the possibility of cast defect formation. In the case of the lower pouring temperature and pouring rate, the molten metal spreads forward under the action of centrifugal force. Because of the contact with the mould, the front-flowing molten metal loses a large amount of heat, which causes the temperature to drop suddenly. So it may no longer flow to the cold end of the mould. Therefore, control of the pouring temperature and pouring rate is clearly beneficial to the prevention of the formation of cast defects. Furthermore, the previous research results [6] demonstrated that the O increase vertically came from the interaction of the higher superheated liquid with CaO crucible. The research
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Fig. 6. Fraction of solid simulation results of the cast valve in the preliminary design at different times. (a) t ¼ 11.87 s, (b) t ¼ 14.52 s, (c) t ¼ 18.65 s (gray areas represent solid, red areas represent liquid).
results [17] indicated that superheating temperature and holding time played an important role in the interaction between melting crucibles with Y2O3 and the molten TiAl. With increase in the superheating temperature and holding time, the O contents were higher. From Fig. 7(d) and (e), project 5 with pouring temperature of 1730 C is not better than project 4 with pouring temperature of 1710 C. Thus, a pouring temperature range from 1690 C to 1720 C is adopted. Fig. 7(b) and (c) indicates that on increasing the centrifugal rotation speed from 400 to 450 rpm, at the same pouring temperature (1690 C) and mould temperature (900 C), the filling rate does not improve and the higher rotation speed has no significant effect on porosity. Additional practical considerations indicate that if the filling time is too long, the heat loss during filling will be too high. Of course, if the filling time is too short, the large filling system will allow room for turbulence under the centrifugal force. It is concluded that 5e7 s is better for the filling time. The simulated and experimental results show that there are porosity defects in many exhaust valves due to both mould filling and temperature field. Due to the thermal effect of neighboring valves, those parts kept hot by neighboring parts suffer larger defects near the center stems, as shown in Figs. 4 and 5.
It is concluded that the preliminary design is not proper for exhaust valves. 3. Design 2: an optimized design 3.1. Design of gating system From simulation results of the preliminary design, there are two primary solutions to improve the part quality. One is to reduce the number of parts on the tree, thereby eliminating radiation effects that kept some of the parts hotter than others depending on their positions on the tree. The other is to increase the gate size, thereby keeping the feeding path open longer. Obviously, reducing the number of parts in the set up drastically decreases the efficiency of metallic yield. Therefore, it is chosen to increase the gate size moderately, and not reduce the number of parts on the wax assembly but change positions of parts into a staggered arrangement, as shown in Fig. 9. Cast parameters used for the new design of the centrifugal investment casting mould are pouring temperature 1710 C, mould temperature 900 C and rotation speed 400 rpm. The 3D model of the new design is shown in Fig. 9.
Table 3 The parameters used in the simulation project Simulation project
Pouring temperature ( C) Filling time(s) Mould temperature ( C) Rotation speed (rpm)
Project 1
Project 2
Project 3
Project 4
Project 5
Project 6
1650 7 800 400
1690 6 900 450
1690 7 900 400
1710 7 950 400
1730 7 950 400
1740 7 950 420
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3.2. Results and discussion Because of the centrifugal effects and the enlarged filling gate, mould filling and temperature field are now modified. The melt flowing along the inner shell surface just leaves the center unfilled at the initial filling, as shown in Fig. 10. It indicates that the mould filling process in the optimized
Fig. 8. Temperature variation curves of cast valves during solidification in the preliminary design. (a) Temperature variation curve of A point, (b) temperature variation curve of B point.
design is much better than and completely different from that in the preliminary design. It should be attributed to the enlarging filling gate. In the preliminary design, the mould filling process is divided into forward filling and backward filling,
Fig. 7. Potential porosity locations of casting simulation results in the preliminary design. (a) Project 1, (b) project 2, (c) project 3, (d) project 4, (e) project 5, (f) project 6 (green areas represent porosity, dark areas represent the casting in the viewing direction). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. 3D model of cast valves in the modified design.
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Fig. 10. Mould filling simulation results of the cast exhaust valve in the modified design. (a) t ¼ 2.266 s, (b) t ¼ 2.322 s, (c) t ¼ 2.387 s, (d) t ¼ 2.406 s.
which have great influence on temperature field of valves. The castings do not experience sequential solidification for the bad mould filling. In the optimized design, the melt fills the shell along the inner surface and leaves exhaust valve’s center unfilled. All of those are affected by the enlarging filling gate. The molten TiAl alloy put into the shell is much more at the initial filling, and only the center is filled later. So the center field holds heat longer than the rest of the casting, and cast valves have a good temperature gradient. The castings benefit from sequential solidification for the feeding paths remain open longer. The simulation results indicate that only one or two exhaust valves have a slight defect, as shown in Fig. 11.
Fig. 11. Potential porosity locations in the modified design (green areas represent porosity, dark areas represent the casting in the viewing direction). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
It should be attributed to the random mould filling of cast exhaust valves. Under the centrifugal force, the molten TiAl alloy in the running and gating systems is also rotary, when the metals reach the shell. For the bottom layer, the parts on the ‘‘tree-type’’ shell are filled at the same time in initial filling. However, as the mould filling goes on, the parts are filled
Fig. 12. The modified design showing (a) X-ray scanning images, (b) simulation results (no porosity).
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randomly in different layers. Some exhaust valves in the lower layer are filled later than those in the upper layer. So the random mould filling results in some bad exhaust valves. The randomness in the modified design is higher than that in the preliminary design, because the filling gate is enlarged and the parts are changed in staggered arrangement. From the simulation results, only one or two exhaust valves are affected, so the influence is small. In the modified design the increased filling gate effectively keeps the feeding path open, and it can reduce the porosity. In addition, the harmful effects of radiation are reduced by changing the position of the parts on the tree. The parts benefit from sequential solidification, because the feeding paths are kept open. Using the modified design confirms that most of the valves have no porosity, as shown in Fig. 12. The modified design improves both the soundness and the metallic yield of the finished products. The feasibility for mass production and cost effectiveness of TiAl automotive valves are being assessed. 4. Conclusions The mould filling process and solidification of TiAl exhaust valves by centrifugal investment casting have been simulated. The Nc in the centrifugal investment casting of Ti45Al8Nb1B alloy is identified in the range of 0.14e0.20, and the optimized Nc is 0.18 in this study. Several key factors including the pouring temperature, the mould temperature and the rotation speed are studied in detail. The optimized cast parameters for the centrifugal investment casting process of one design of an investment mould of TiAl exhaust valves are found to be pouring temperature 1690e1720 C, mould temperature 900e950 C and rotation speed 400e450 rpm. In the preliminary design, the porosity deviation from the center line is dependent on the sequence of solidification, which is affected by forward and backward flows in the mould filling process. The whole filling process is divided into forward filling and backward filling in the preliminary design, which results in valves not experiencing sequential solidification. The potential porosity locations are affected by the mould filling process and temperature field. Keeping the feeding path open by enlarging the gate and reducing neighboring radiation effects encourage sequential solidification and can therefore reduce or eliminate porosity in the modified design.
Acknowledgements The authors are indebted to Professor John Campbell at the University of Birmingham, UK for valuable discussion and modification of English. References [1] Gebauer Klaus. Performance, tolerance and cost of TiAl passenger car valves. Intermetallics 2006;14:355e60. [2] Si-Young Sung, Young-Jig Kim. Economic net-shape forming of TiAl alloys for automotive parts. Intermetallics 2006;14:1163e7. [3] Noda T. Application of cast gamma TiAl for automobiles. Intermetallics 1998;6:709e13. [4] Loria EA. Gamma titanium aluminides as prospective structural materials. Intermetallics 2000;8:1339e45. [5] Xinhua Wu. Review of alloy and process development of TiAl alloys. Intermetallics 2006;14:1114e22. [6] Liu K, Ma YC, Gao M, Rao GB, Li YY, Wei K, et al. Single step centrifugal casting TiAl automotive valves. Intermetallics 2005;13:925e8. [7] Bartels A, Kestler H, Clemens H. Designed fully lamellar microstructures in a g-TiAl based alloy: adjustment and microstructural changes upon long-term isothermal exposure at 700 and 800 C. Mater Sci Eng A 2002;329e331:124e9. [8] Boyun Huang. TiAl intermetallics. Hunan: Central South University Press; 1998. [9] Gerling R, Leitgeb R, Schimansky FP. Porosity and argon concentration in gas atomized g-TiAl powder and hot isostatically pressed compacts. Mater Sci Eng A 1998;252:239e47. [10] Jones PE, Porter III WJ, Eylon D, Colvin G. Development of a low cost permanent mold casting process for TiAl automotive valves. In: Kim YW, Wagner R, Yamaguchi M, editors. Gamma titanium aluminides. Las Vegas, Nevada: TMS; 1995. p. 53e62. [11] Wenbin Sheng, Rui Yang, Yuyin Liu, Dongsheng Xu, Dong Li. Current status on low cost g-TiAl exhaust valve. Chin Foundry 2001;50:650e3. [12] Campbell J. Castings. 2nd ed. Oxford, UK: Butterworth-Heinemann; 2003. [13] Hu BH, Tong KK, Niu XP, Pinwill I. Design and optimisation of runner and gating systems for the die casting of thin-walled magnesium telecommunication parts through numerical simulation. J Mater Process Technol 2000;105:128e33. [14] Li DZ, Campbell J, Li YY. Filling system for investment cast Ni-base turbine blades. J Mater Process Technol 2004;148:310e6. [15] Hansen PN, Sahm PR, Flender E. How to select and use criterion functions in solidification simulation. AFS Trans 1993;101:443e6. [16] Thomas M, Raviart JL, Popoff F. Cast and PM processing development in gamma aluminides. Intermetallics 2005;13:944e51. [17] Barbosa J, Ribeiro C Silva, Caetano Monteiro A. Influence of superheating on casting of g-TiAl. Intermetallics 2007;15:945e55.