A new shell casting process based on expendable pattern with vacuum and low-pressure casting for aluminum and magnesium alloys

Transcription

1 Int J Adv Manuf Technol (2010) 51:25 34 DOI /s ORIGINAL ARTICLE A new shell casting process based on expendable pattern with vacuum and low-pressure casting for aluminum and magnesium alloys Wenming Jiang & Zitian Fan & Defeng Liao & Xuanpu Dong & Zhong Zhao Received: 22 October 2009 / Accepted: 22 February 2010 / Published online: 24 March 2010 # Springer-Verlag London Limited 2010 Abstract A new shell casting process, with the adoption of the foam pattern of lost foam casting (LFC) as prototype and the combination of the thin shell fabrication technology of investment casting and vacuum and low-pressure casting process, was proposed for manufacturing complicated and thin-walled aluminum and magnesium alloy precision castings. Loose-sand uniting vacuum was used in the new process to further reinforce the thin shell, and the new process proves to be a process with simple process, low cost, and high thin shell strength. Because the molten metal filling and solidification are completed under air pressure and vacuum level, the filling capability and feeding capacity of the molten metal are greatly improved, and the castings become denser. This paper mainly investigated the fabrication technology of thin shell based on foam pattern prototype, the removing foam and roasting shell process and vacuum and low-pressure casting process. The few-layer compound thin shell of silica sol sodium silicate was adopted for the new process. Removing foam pattern was carried out at 250 C for 30 min, and the shell was roasted at 800 C for 1 h. Combined with the vacuum and low-pressure casting process, this new shell casting process has successfully produced thin wall and complex aluminum and magnesium alloy parts with high quality. In addition, comparisons in terms of filling ability, microstructure, mechanical properties, porosity, and surface roughness among this new shell casting, gravity casting, and LFC were also made to show the characterization of this new shell casting process. W. Jiang : Z. Fan (*) : D. Liao : X. Dong : Z. Zhao State Key Laboratory of Material Processing and Die and Mould Technology, School of Material Science and Engineering, Huazhong University of Science and Technology, Wuhan , China fanzt@mail.hust.edu.cn Keywords Foam pattern. Vacuum and low-pressure casting. Thin shell. Complicated and thin-walled aluminum and magnesium alloy precision castings. Removing foam pattern 1 Introduction With the rapid development of aerospace and automotive industry, the demand for complicated and thin-walled aluminum and magnesium alloy precision castings are greatly increased due to their lightweight and high strength-to-weight ratio [1 4]. Unfortunately, it is very difficult to obtain these castings under gravity casting. It is well-known that the molten metal of aluminum and magnesium alloys is easily oxidized, and it is prone to misrun and oxide inclusion defects during the pouring. Meanwhile, the mechanical properties of castings are generally low. Currently, die casting, permanent mold casting, and sand casting are usually employed for manufacturing the aluminum and magnesium alloy components [5 7]. However, die casting method is costly and time-consuming because of the design and recurring modifications of dies, and the die castings cannot undergo heat treatment due to gas trapping [8, 9]. In addition, the permanent mold casting is usually difficult to produce complicated castings because of high solidification velocity of melt metal, while sand casting cannot meet the requirements including dimensional accuracy and surface roughness for aluminum and magnesium alloy precision castings. The conventional investment casting process, one of the oldest metal manufacturing techniques, is a precision casting process in which wax patterns are converted into metal parts through a multi-step process [10]. Despite the many advantages of investment casting, there are some

2 26 Int J Adv Manuf Technol (2010) 51:25 34 Fig. 1 Flow chart of the shell casting process based on expendable pattern with vacuum and low-pressure casting Preparing foam prototype Fabricating thin shell Removing foam Roasting shell Precision castings Vacuum and low pressure casting Boxing and modeling problems. Firstly, the wax patterns employed by investment casting are prone to deforming and softening due to low softening point of wax, and the transportation of wax pattern is inconvenient especially for large-scale parts. Secondly, investment casting generally adopts multi-layer shell to generate 6 8 mm thick shell, and most of the studies and applications with respect to shell preparation are mainly focused on ferrous metal. As a result, there exist some other problems including complex fabrication process, long fabrication time, and high cost. The aluminum and magnesium alloys are different from ferrous metal due to low density and low pouring temperature so that there is no necessity for excessivelayer shell to perform casting process. Furthermore, the productions of the large-scale complicated and thin-walled aluminum and magnesium alloy castings have become a challenge for investment casting process with gravity casting method. The lost foam casting (LFC) has been regarded as a costeffective and environment-friendly casting technology of near net shape method for manufacturing complicated aluminum and magnesium alloy precision castings without the need for cores [11, 12]. However, the decomposition of the foam pattern in the casting process could result in some defects such as pinhole, slag inclusion, etc. [13, 14], and the pouring temperature of the LFC is usually C higher than that of traditional cavity casting in order to overcome heat absorption from decomposition of foam pattern. As a result, the mechanical properties of LFC castings are low grade. Therefore, more improvements about LFC should be taken into account. Considering the above problems, the aim of this study is to develop a new shell casting process to improve the production of complicated and thin-walled aluminum and magnesium alloy precision castings. This new shell casting process combines foam pattern preparation of LFC, thin shell precision fabrication of investment casting, and vacuum and low-pressure casting technology. First, the foam pattern based on part shape is prepared as prototype, and then the thin shell with fewer layers is fabricated using shell fabrication technology of investment casting outside the foam prototype. Next, foam prototype is removed and shell is roasted. Lastly, the molten metal filling and solidification are completed under air pressure and vacuum level after boxing and modeling. The present work reports the progress in foam prototype and thin shell fabrication, removing foam and roasting shell process, and vacuum and low-pressure casting process. Moreover, the thin-wall and complex aluminum and magnesium alloy parts have been successfully manufactured with the use of this new shell casting process. In addition, some comparisons in terms of filling ability, microstructure, mechanical properties, porosity, and surface roughness among this new shell casting, gravity casting, and LFC are also made to prove the advantages of this new shell casting process for producing complicated and thin-walled aluminum and magnesium alloy precision castings. 2 Process of the new shell casting process based on foam prototype with vacuum and low-pressure casting The flow chart of this new shell casting process based on expendable pattern with vacuum and low-pressure casting is shown in Fig. 1, mainly including six steps: preparing Fig. 2 Foam prototypes

3 Int J Adv Manuf Technol (2010) 51: Table 1 Processing parameters of foaming molding Table 3 Bend strength of thin shell Pressure (MPa) Die preheating time(s) Molding time(s) Die cooling time(s) Room temperature strength/mpa High-temperature strength/mpa Retained strength/mpa foam prototype, fabricating thin shell, removing foam pattern, roasting shell, boxing and modeling, and vacuum and low-pressure casting. This shell casting process is characterized as follows: (1) The foam pattern prototypes employed by this shell casting process possess many merits, such as low cost compared to wax pattern, easy removing and bonding, convenient transportation, small shrinkage as well as better heat durability, and less deformation for complicated and thin-walled parts. In addition, because this shell casting process adopts the precision shell of investment casting, the castings have high dimensional accuracy and surface roughness compared to sand casting. (2) Because the foam prototypes are removed before pouring, the pinhole, shrinkage porosity, and slag inclusion defects due to decomposition of foam during casting process can be avoided. On the other hand, the molten metal filling capacity can be further improved, and the pouring temperature of this technology is usually lower than that of LFC. (3) Compared with die casting, the shell casting process is less costly, and castings can perform further heat treatment to enhance strength. (4) This compound technology has better filling capability and feeding capacity than that of traditional gravity casting because the molten metal filling and solidification are completed under air pressure and vacuum level. Consequently, the castings become denser. In a word, the shell casting process based on foam prototype with vacuum and low-pressure casting has many advantages, especially suitable for manufacturing complicated and thin-walled precision castings. The key technologies of this process, including foam prototype and thin shell fabrication, removing foam and roasting shell process as well as vacuum and low-pressure casting process, play important roles in fabrication of parts and will be investigated in the following sections. 3 Experimental 3.1 Foam prototype preparation It is obvious that the quality of castings is decided by the quality of foam patterns because the castings are faithful replica of the foam patterns. Since the foam patterns are removed before metal pouring during this shell casting process, the foam patterns with high density can be used to ensure better surface quality and high strength compared with LFC. Some foam patterns prepared with foaming molding technology are shown in Fig. 2 to illustrate the excellent surface finish and thin wall thickness as well as complex shape. The minimum wall thickness of thin-walled cylindrical parts is only 3.5 mm, and the overall size of parts is φ mm. It must also be emphasized that these foam patterns are structurally stronger than the low density Table 2 Preparation specifications of compound thin shell Coating Slurry Powder solution ratio Slurry viscosity (s) Slurry density (gcm 3 ) Stucco Primary Secondary 1 Secondary /60 Aluminosilicate Zircon and silica sol binder Aluminosilicate and silica sol binder Aluminosilicate and sodium silicate binder /100 Zircon /20 Aluminosilicate Fig. 3 The curve of removing foam and roasting process

4 28 Int J Adv Manuf Technol (2010) 51:25 34 Table 4 Bend strength of thin shell at different temperatures Temperatures ( C) Bend strength (MPa) Fig. 4 The TGA curve of foam patterns foam patterns. Thus large and thin-walled patterns fabricated with this foaming molding technology can be operated readily and are less prone to damage. The optimum processing parameters of foaming molding are shown in Table 1. The processing parameters vary according to the shape and size of different patterns. 3.2 Thin shell fabrication technology The shell plays an important role in this shell casting process, and it should possess not only high strength but also better surface finish. Currently, silica sol binder has been mainly used in shell fabrication process owe to its better surface quality and high strength [15 17]. In this paper, the three-layer compound thin shell was adopted with the silica sol binder as surface coating and sodium silicate binder as back-up coating. The specific preparation specifications of the compound thin shell are shown in Table 2. The thin shell was produced by coating the foam patterns with the ceramic slurry which mainly includes binder, refractory powder, antifoam agent, and suspending agent. Then, the stucco was formed on the coated patterns with refractory. This operation was repeated two or three times, depending on the size, shape, and complexity of the product. Furthermore, every layer must be dried before coating the next layer in order to prevent desquamation and dehiscence of ceramic shell. Eventually, the fabrication of thin shell was finished. The thickness of this compound thin shell is only 3 4 mm, far less than 6 8 mm shell thick of the traditional investment casting, and this thin shell has better surface quality, easy exuviation, short preparation time, and low cost because of the employment of fewer-layer shell and sodium silicate binder. Table 3 shows the bend strength of this compound thin shell, and its strength is high enough to prevent cracking during handling and pouring process. In addition, the loose-sand uniting vacuum was also employed in order to further reinforce the thin shell. 3.3 Removing foam and roasting process Generally speaking, there are two methods to remove foam pattern: burning and dissolving [18, 19]. In this paper, burning method was employed to remove foam pattern. The curve of removing foam and roasting process is shown in Fig. 5 Ceramic shell after removing foam. a 250 C. b 600 C a b

5 Int J Adv Manuf Technol (2010) 51: Fig. 6 Thin ceramic shell Fig. 3. The test showed the foam patterns could be completely melted into liquid state at 250 C. The thermogravimetric analysis (TGA) curve of the foam patterns is shown in Fig. 4. According to the TGA curve of the foam patterns, the foam patterns showed almost no weight loss below the temperature of 325 C. When the temperature was higher than 325 C, the weight of foam patterns rapidly decreased because of the decomposition and gasification of the foam material. When the foam patterns were heated to 388 C, the foam could almost be degraded entirely, with only 9.87% residue left. There are only 5.3% residue at 500 C, and 2.8% residue at 548 C, which does not influence the subsequent casting process and can be cleaned up. Only by controlling the furnace temperature, the removing foam and roasting process employed by this study could be completed together, and it is different from investment casting of which dewaxing and roasting are two independent processes. Therefore, the removing foam and roasting process is simple, mainly including three stages (Fig. 3). Firstly, foam pattern was removed at 250 C for 30 min in order to make the foam patterns flow out ceramic shell in liquid form, which is similar to the investment casting process, and it would prevent cracking of ceramic shell under expansion force of foam patterns gasification at high temperatures, as is shown in Fig. 5b. In this way, most of the foam was removed, and it is important that ceramic shell is not destroyed during removing foam process, as is shown in Fig. 5a. Then, the temperature rose to 500 C for 30 min to remove the residual portion of foam. Thirdly, the ceramic shell was roasted at 800 C for 1 h in order to enhance shell strength because the shell strength is lower at low temperature, as is shown in Table 4, and the thin shell roasted at low temperatures is easy to be destroyed when pouring. Finally, the ceramic shell with high strength and better surface quality was obtained through the removing foam and roasting process, as is shown in Fig. 6. Table 5 Chemical constitution of ZL101 aluminum alloy (mass fraction, %) Fig. 7 The principle schematic diagram of the vacuum and lowpressure casting process. 1 Loose sand. 2 Ceramic shell. 3 Pressurizing air. 4 Raising tube. 5 Crucible. 6 Sand box. 7 Vacuum supply Si Mg Fe Al Balance

6 30 Int J Adv Manuf Technol (2010) 51:25 34 Table 6 Chemical constitution of AZ91D magnesium alloy (mass fraction, %) Al Zn Mn Si Fe Cu Ni Be Mg Balance 3.4 Vacuum and low-pressure casting After the thin shell fabrication was finished, the next key step was the precision casting process with vacuum and low-pressure casting. The principle schematic diagram of the vacuum and low-pressure casting process is shown in Fig. 7. Firstly, the thin shell was placed into a sand box, and then the sand box was filled with 40/50 unbonded loosesand and vibrated compaction using a three-dimensional (3D) vibration table. Next, the sand was covered by plastic film, and then the sand box was covered with a plate cover. Furthermore, the sand box was pushed into low-pressure casting position, and cylinder device was operated in order to fasten the sand box. Lastly, the vacuum and air pressure were executed simultaneously to make the molten metal fill cavity. As a result, the filling and solidification of the molten metal were completed under air pressure and vacuum level [20, 21]. During the casting process, because the filling and solidification of the molten metal are completed under air pressure and vacuum level, the filling capability and feeding capacity of the molten metal are greatly improved. Accordingly, the misrun and cold shut defects are avoided, especially for the complicated and thin-walled aluminum and magnesium alloy precision castings. Moreover, the pinhole, slag inclusion, and shrinkage defects also decrease, and the castings become denser. Meanwhile, oxidation is also eliminated for the aluminum and magnesium alloys because the molten metal entering the mold is not exposed to the atmosphere during casting process. The vacuum level and air pressure are vital parameters for manufacturing parts. Generally, because of the rapid solidification and difficult filling of the molten metal, higher vacuum level and air pressure are required for both thin-walled castings and complicated castings to increase filling ability and obtain better castings. Therefore, the complexity and thickness of foam patterns exert great influence on casting ability. In order to prove the advantages of this new shell casting process for producing the complicated and thin-walled aluminum and magnesium alloys precision castings, comparisons in terms of filling ability, microstructure, mechanical properties, porosity, and surface roughness among this new shell casting, gravity casting, and LFC have been performed. The ZL101 aluminum alloy and AZ91D magnesium alloy materials were used by this study and their chemical constitutions are shown in Tables 5 and 6, respectively. Table 7 presents the experimental conditions under different processes for the comparison of their mechanical properties. In addition, the sheet specimens with length of 200 mm and thickness of 3 mm were adopted for comparison of filling ability among the new shell casting process, the shell casting under gravity casting and LFC. 4 Results and discussion 4.1 Comparison of filling ability among the new shell casting process, the shell casting under gravity casting, and LFC Figure 8 shows the filling length of sheet samples obtained by different processes. When the new shell casting process was employed, the sheet specimen with length of 200 mm and thickness of 3 mm was completely filled. However, gravity casting resulted in serious misrun and LFC in the shortest length of sheet sample. Therefore, it is obvious that the filling ability of the new shell casting process is improved more than that of gravity casting and LFC because of the employment of vacuum and low pressure. 4.2 Comparison of mechanical properties among the new shell casting process, the shell casting under gravity casting, and LFC Table 8 presents the comparisons of mechanical properties among the new shell casting process, the shell casting under gravity casting, and LFC. It can be obviously seen that the mechanical properties of the shell casting process with vacuum and low pressure has greatly improved compared to the shell casting under gravity casting and LFC. Firstly, the tensile strength, elongation, and hardness of castings produced by the new shell casting process are respectively Table 7 Experimental conditions of different processes Process The shell casting with vacuum and low pressure The shell casting under gravity casting Lost foam casting (LFC) Pouring temperature ( C) Air pressure (MPa) Vacuum level (MPa)

7 Int J Adv Manuf Technol (2010) 51: Fig. 8 Filling length of sheet samples obtained by a LFC, b shell casting under gravity casting, and c shell casting with vacuum and low pressure up to MPa, 8.1% and 93.1 HB, and are, respectively, 7%, 32%, and 8% higher than that of castings produced by the shell casting under gravity casting, and then are, respectively, 20%, 166%, and 18% higher than that of castings produced by LFC. As a result of these advantages, the filling capability and feeding capacity of the molten metal are improved, the structure of castings is denser, and the grains are finer. In addition, because of the adoption of ceramic shell and foam pattern with higher density, the castings made by the new shell casting process exhibit better surface roughness than that of LFC. Meanwhile, the fewer amount of air trapped in the casting results in the improvement of surface roughness due to the filling of the molten metal from bottom to top under vacuum and low pressure. Furthermore, the comparison of density among the new shell casting process, the shell casting under gravity casting, and LFC can also prove that the castings of the new shell casting process are denser than that of the other two casting processes, as is shown in Table 8. Figure 9 shows the low magnification photos of sample sections obtained by different processes. As is shown in Fig. 9, the sample section obtained by the shell casting process with vacuum and low pressure has the fewest pores, and its porosity is only 0.16%, far less than 1.97% of LFC in which many big pores can easily be seen, as is shown in Fig. 9a. The fracture morphology of tensile samples obtained by different processes is shown in Fig. 10. As is shown in Fig. 10a, many pores are seen in the tensile sample of LFC, which is in accordance with Fig. 9a. In Fig. 10b, the shrinkage porosity defect and coarse dendrite can be observed in the tensile sample of the shell casting under gravity casting due to poor feeding capacity of the molten metal, and it will lead to poor mechanical properties of castings. As can be seen in Fig. 10c, the morphology of dimple fracture of the new shell casting process is obvious, and it results in higher elongation of castings. Figure 11 illustrates the microstructures of castings obtained by different processes. The microstructure of castings obtained by LFC shows many coarse dendrites and large porosity presented in Fig. 11a, resulting in poorer mechanical properties of castings. Meanwhile, many coarse dendrites and shrinkage porosities can also be seen from the microstructure of castings obtained by shell casting under gravity casting in Fig. 11b. On the contrary, the microstructure of castings obtained by the shell casting with vacuum and low pressure has finer grains and is much denser compared to gravity casting and LFC, as is shown in Fig. 11c. The coarse dendrites are broken under vacuum level and air pressure, and the feeding capacity of the molten metal is also further improved. Therefore, the shell casting with vacuum and low-pressure process has great advantages in mechanical properties. 4.3 The thin-walled and complex parts fabrication The thin-walled cylindrical parts with 3.5 mm minimum wall thickness and complex cylinder head parts were used for the casting practice during the shell casting process with vacuum and low pressure. The specific process parameters used by the thin-walled cylindrical parts are shown in Fig. 12. Table 8 Comparisons of mechanical properties among the new shell casting process, the shell casting under gravity casting, and LFC Process Tensile strength (MPa) Elongation (%) Hardness (HB) Density (gcm 3 ) Porosity (%) Surface roughness (Ra, μm) The shell casting with vacuum and low pressure The shell casting under gravity casting Lost foam casting (LFC)

8 32 Int J Adv Manuf Technol (2010) 51:25 34 Fig. 9 Low magnification photos of sample section obtained by a LFC, b shell casting under gravity casting, and c shell casting with vacuum and low pressure Fig. 10 Fracture morphology of tensile samples obtained by a LFC, b shell casting under gravity casting, and c shell casting with vacuum and low pressure Fig. 11 Microstructures of castings obtained by a LFC, b shell casting under gravity casting, and c shell casting with vacuum and low pressure

9 Int J Adv Manuf Technol (2010) 51: Fig. 12 Curve graph of the vacuum and low-pressure casting process Figures 13 and 14 show magnesium and aluminum alloy castings manufactured by this new shell casting process, respectively. The thin shell can easily be cleaned up, and the castings have smooth surface and clear outline. In addition, the sectional photos of magnesium alloy castings obtained by the new shell casting and LFC are shown in Fig. 15. Figure 15a shows castings with dense section produced by the new shell casting process, while Fig. 15b presents the section of LFC castings with many porosity defects. The comparison proves the shell casting process based on expendable pattern with vacuum and lowpressure casting can excellently solve the pinhole and shrinkage porosity defects, and the castings become denser with high internal quality. 5 Conclusions 1. Theshellcastingprocessbasedonexpendablepattern with vacuum and low-pressure casting possesses some advantages, which are flexible design, low cost and shrinkage of foam, high shell precision of investment casting, as well as better formability under vacuum and low-pressure casting process. Furthermore, some pinhole, shrinkage porosity and slag inclusion defects, as well as high pouring temperature of LFC can be solved, and the misruns and cold shuts can also be avoided, especially for the complicated and thin-walled aluminum and magnesium alloy precision castings. 2. The foam patterns with smooth surface and high strength were fabricated using foaming molding technology, and the contraction rate of foam patterns is only 0.51%. The molding parameters should vary according to the shape and size of parts. 3. The three-layer compound thin shell of silica sol sodium silicate was adopted for the shell casting process. Removing foam pattern was carried out at 250 C for 30 min, and the thin shell was roasted at 800 C for 1 h. Finally, the thin shell with better surface quality and high strength was obtained. In addition, the loose-sand uniting vacuum was also employed in order to further reinforce the thin shell. 4. Under the shell casting process based on expendable pattern with vacuum and low-pressure casting, filling ability and mechanical properties of castings are greatly improved compared to the shell casting under gravity casting and LFC. After casting practice, it is confirmed that this new shell casting process can wholly meet the requirement of the fabrication of complicated and thin-walled aluminum and magnesium alloy precision castings, and the castings have higher internal quality. Therefore, this compound process will have great potentials and possess broad application prospects in aviation, aerospace, military, automotive, electronics, machinery, and other industries. Fig. 13 Pictures of magnesium alloy castings Fig. 14 Pictures of aluminum alloy castings

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