Lost foam casting(lfc)is a near-net shape forming

Transcription

1 CHINA FOUNDRY Vol.7 No.1 Influence of mechanical vibration on the solidification of a lost foam cast 356 alloy Zhao Zhong, *Fan Zitian, Dong Xuanpu, Tang Bo, Pan Di and Li Jiqiang (State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan , China) Abstract: Mechanical vibration was applied to the solidification of a lost foam cast (LFC) 356 aluminum alloy. Effects of mechanical vibration, with different peak acceleration, on the size and morphology of α-al phase, and also on the mechanical properties of the castings were studied. Results indicated that α-al dendrites gradually grow into equiaxed grains as the peak acceleration of vibration is increased. When the peak acceleration is between about 1 to 4 g,α-al phase distribution is uniform and is refined obviously. α-al dendrites are reduced and the mechanical properties of the castings are improved significantly when compared to those of the castings that are produced without vibration. However, when the peak acceleration is higher than 4 g, strong vibration will lead to defects formation, such as sand adhesion, while the amount and size of pores will be increased. And due to the turbulent flow that caused by strong vibration, the chance of forming large pores in the matrix has been increased significantly. The increase in defects will result in the deterioration of mechanical properties. Key words: aluminum alloy; microstructure; mechanical property; lost foam casting; mechanical vibration CLC number: TG249.5 Document code: A Article ID: (2010) Lost foam casting(lfc)is a near-net shape forming process. Compared to traditional sand casting, LFC has the advantages of high precision, three-dimensional shape controllability, repeatability and flexibility [1]. However, the poor heat transfer property of the dry sand used in LFC and the low cooling rate of the melt lead to coarse microstructure, especially for hypoeutectic 356 alloy, it is likely to form large dendrites. Applying vibration during metal solidification is an effective way for grain refinement. Literatures [2-6] show that vibration can modify the solidified microstructure by promoting nucleation and thus reduce the grain size and lead to a more homogenous microstructure. In this paper, mechanical vibration was applied in the solidification of a LFC 356 alloy. Effect of mechanical vibration, with different peak acceleration, on the microstructure and performance of the alloy was studied. 1 Experimental design and process Schematic of the experimental LFC setup is shown in Fig. 1. All parts and components were integrated into a single piece in order to achieve smooth vibration. In this study, vertical vibration mode was chosen because it can exert strong influence on solidification by fragmentizing the growing grains [5,6]. Plastic film Dry sand Flask EPS pattern Vacuum Vibration motor Eccentric block Angle 24 *Fan Zitian Male, born in 1962, professor. He is currently the advisor of doctoral candidates in Huazhong University of Science and Technology, and his research interests are mainly focused on green casting technology of sodium silicate bonded sand and precise forming technology of Mg/Al alloys. The first author, Zhao Zhong, is one of his doctoral students. fanzt@mail.hust.edu.cn Received: ; Accepted: Fig. 1: Schematic of the experimental setup Formula for calculating the amplitude is given by me 0 A m Where, m 0 is the weight of eccentric block, kg; e is effective eccentricity of the eccentric block, mm;m is the total weight of vibration table, flask, dry sand and EPS pattern, and the measured value is about 198 kg. The imum of m 0 e is (1)

2 February kg mm and according to equation (1), the imum amplitude can be obtained by: [ me 0 ] mm (2) m Relationship between the amplitude and angle of eccentric block is: = θ (3) A A Where, θ is the angle of the two eccentric blocks, rad, as shown in Fig. 1. Vibration peak acceleration a can be calculated by: = A (2 f) Where, f is vibration frequency, Hz. The peak exciting force F is: F α m 2 Research & Development In the experiment, θ was adjusted to 2/18,6/18, 10/18 a and 14/18. According to the formulas above, the values and mutual relationship of major mechanical vibration parameters can be obtained, and results are shown in Table 1. The peak acceleration error is ± 0.20 g. Where g is gravitational acceleration, g = 9.8 m s -2. (4) (5) Table 1: Relation between vibration parameters θ(rad) A(mm) f(hz) α (g) F(N) θ(rad) A(mm) f(hz) α (g) F((N) , ,400 14/ ,422 6/ , , , , , , , ,200 10/ ,845 2/ , , , , ,800 The recorded actual waveform of the mechanical vibration is shown in Fig. 2. In order to avoid the damage that caused by disorganized waveform, pouring could be carried out after the forced vibration was stable for fifteen seconds. At pouring temperature (750 ), the temperature changes at point 1 and point 2 in the pattern center were measured, as shown in Fig. 3. The cooling curve of alloy solidification shows that it took about 450 seconds for 356 alloy to cooled to eutectic temperature (557 ). Therefore, the vibration should last about 450 seconds, as shown in Fig. 4. Start-up vibration stage 5-15 s Forced vibration stage 450 s Stoping vibration stage 5-15 s Fig. 2: Actual vibration waveform of the vibrating table Fig. 3: Schematic of the pattern shape and tensile test sample Fig. 4: Cooling curves of LFC 356 alloy at a pouring temperature

3 CHINA FOUNDRY Vol.7 No.1 Samples were machined into standard tensile testing bars, as shown in Fig. 3, then tested at a tensile rate of 3 mm/min by using a universal material testing machine. Scanning electron microscope (SEM) was used to analysis the fracture surface. 2 Experimental results and discussion 2.1 Influence of peak acceleration on microstructure of 356 alloy Figure 5(a) shows the microstructure of LFC 356 alloy without vibration. Due to the slow cooling rate that caused by the dry sand with poor heat transfer property, the α-al phase formed is dendritic. It take about 10 minutes for the 356 alloy to cool (a) to 500, so there is enough time for the dendrite growth, as shown in Fig. 4. Microstructures of the alloy obtained under different peak accelerations are shown in Fig. 5(b)-(d). Figure 5(b) shows the microstructure of the sample under a peak acceleration of 0.36 g; the α-al grain size decreases slightly and part of the dendrite fractures. Figure 5(c) shows a more homogeneous microstructure of the alloy obtained under a peak acceleration of 3.27 g; primary α-al grains have been refined significantly, and secondary dendritic arms are broken. When the peak acceleration is 4.90 g, the grains and dendrites of α-al have been fractured under exciting force, but they are unevenly distributed, and large pores are formed, as shown in Fig. 5(d). (b) (a) No vibration 0 g (c) (b) 0.36 g (20 Hz, 0.23 mm) (d) (c) 3.27 g (60 Hz, 0.23 mm) (d) 4.90 g (60 Hz, 0.34 mm) Fig. 5: Microstructure of the solidified alloy with different peak accelerations Balandin [8] and Southin [9] showed experimentally that when alloy was in vibration, grain refinement was caused by the "showering" of dendrites [5,9,10]. Southin, Bower and Campbell [15,20-21] explained that the fracture of dendrites that caused by the impact of liquid will result in the generation of small crystals which will act as nuclei in melt, and this is the suggested refinement mechanism of bending stress. Jackson[11] et al. supported re-melting theory, they found that a washing of liquid on growing crystallites or dendrites caused re-melting at the dendrite neck and the detachment of crystals. As a result, it led to the fragmentation of more 26 dendrites as the peak acceleration increased, and caused grain refinement. In brief, a large number of fragmented dendrites were created by vibration, and they were carried to other parts of the melt to act as nuclei, and thus grain refinement was achieved [6]. 2.2 Influence of peak accelerations on the mechanical property of 356 alloy Tensile strength and elongation of samples under different peak acceleration are shown in Fig. 6. In Fig. 6(a), when the peak acceleration is between about 1 and 4 g, tensile strength

4 Research & Development February 2010 increases rapidly and reaches at a imum value, while elongation changes slightly at that particular peak acceleration values, as shown in Fig. 6(b). Hall-Petch relationship [12,13] indicates that by decreasing the average grain size the alloy strength can be improved. With the increase in peak acceleration, average α-al grain size decreases gradually, thus tensile strength and elongation of the alloy increase significantly. However, when the peak acceleration is higher than 4 g, tensile strength and elongation begin to drop, as shown in Fig. 6 and Table 2. Fig. 6: Tensile strength and elongation of the alloy obtained under g peak acceleration Table 2: Mechanical properties under peak acceleration above 4 g F (N) α (g) Tensile strength f A (MPa) (Hz) (mm) 8, , , Elongation (%) 1.8 When peak acceleration is higher than 4 g, due to strong vibration, the loose sands exerts strong impacts on the coating and the breakaway coatings will cause sand adhesion defect. Figure 7 shows sand adhesion defect at a vibration peak acceleration of 6.53g (0.45 mm, 60 Hz). Fig. 8: Porosity under different peak acceleration Fig. 7: Sand adhesions at 6.53 g The porosity of samples obtained under different peak acceleration is shown in Fig. 8. The porosity under a peak acceleration of < 4 g is lower than that without vibration. This may be caused by the breakage of dendrites and thus promotes an effective feeding. However, when the peak acceleration is above 4 g, the porosity increases rapidly, as shown in Fig. 8. Large pores can be seen in Fig. 5(d). Figure 9 shows a circular shaped pore that located in the Al-Si eutectic. Fig. 9: SEM micrograph of a pore 27

5 CHINA FOUNDRY Vol.7 No.1 Mechanism of forming large pore is schematically shown in Fig. 10, because of strong vibration, small bubbles and H atoms migrate and gather together at the big bubble surface near the solidifying grains, eventually forming a large spherical pore, as shown in Figs. 10(a)-(c). The migrating, (a) Migration gathering and growth of bubbles lead to the growth of pores. Shukla et al.[14] and Kocatepe et al.[6] believed that the size of pores will maintained at a large value if the peak acceleration was at a high level. (b) Adsorption (c) Growth Fig. 10: Mechanism of pore growth under vibration Figure 11 shows SEM micrograph of a fracture sample obtained under a peak acceleration of 6.53 g (60 Hz, 0.45 mm), a large hole near the edge of the fracture is shown in Fig. 11(a), and grains at the inner surface of the hole can be seen from Fig. 11(b). The hole is the crack source of fracture. Fig. 11: SEM of the fracture source at the cavity under a peak acceleration of 6.53 g Reynolds number [Re] of the melt is: Re = vdρ vl dρ vz dρ = + (6) In LFC process, the liquid metal vaporizes the EPS foam at contacted. If filling speed (v l ) of the melt is not considered,reynolds number under vibration is: Re = vz dρ 2 fadρ = (7) Where, d is diameter of sample (0.045 m) ρ is density of 356 alloy ( kg m-3), f is frequency (60 Hz), A is amplitude ( m), is viscosity of 356 alloy melt ( N s m-2 at 700 ). By substituting practical data into formula(7), Reynolds 28 number of 7630 was obtained. The value was beyond the critical value of Therefore, when peak acceleration was above 4 g,the strong vibration created a turbulent flow in the melt and significantly increased the chance of bubble forming, eventually, pores will be generated in the matrix and significantly reduced the mechanical property of 356 alloy. When the vibration frequency is 50 Hz, the amplitude is 0.34 mm, and the peak acceleration is 3.42 g, tensile strength of the sample reachs at 182 MPa and elongation reaches at 2.7%. SEM micrograph of the fracture surface is shown in Fig. 12. Typical cleavage fracture characteristic can be seen in Fig. 12(a) and a crack along the grain boundary can also be found. It clearly shows a small amount of cavities or dimples that caused by the pulling out of primary phases, meaning that the 356 alloy prepared under vibration has undergone plastic

6 February 2010 Research & Development Fig. 12: SEM of fracture surface; without vibration (a) and with vibration (b) deformation before fracture, but it is still a brittle fracture. So it is one of the reasons that the elongation of LFC 356 alloy is no more than 3%, no matter it is prepared with or without vibration, as shown in Fig. 6(b). 3 Conclusion Mechanical vibration with different peak acceleration is perfectly applied to the solidification of LFC 356 alloy. Without mechanical vibration, the coarse primary α-al dendrites are presented in the alloy, demonstrating a normal dendrite growth mode. However, if solidification is carried out with vibration, the grains will be refined gradually as the peak acceleration increases. When the peak acceleration is between 1 and 4 g, the primary α-al grains are refined significantly, secondary dendrite arms are broken, the average grain size is reduced and the alloy s mechanical properties increase notably. When the peak acceleration is higher than 4 g, strong vibration will lead to defects formation such as sand adhesion, and the migrating, gathering and growing of the bubbles, which will lead to the formation and growth of pores. The pores are most likely appeared at the grain boundary. In addition, because the turbulent flow that caused by strong vibration, the chance of forming large bubbles has been increased. These defects will decrease the mechanical properties of the LFC 356 alloy. References [1] Brown J R. The lost foam casting process. Metals Mater., 1992(8): [2] Numan A D, Marwan K, Kozo S. Silicon morphology modification in the eutectic Al Si alloy using mechanical mold vibration. Materials Science and Engineering A, 2005, 393: [3] Fisher T P. Effects of vibrational energy on the solidification of aluminium alloys. British Foundryman, 1973, 66: [4] Barbure R R, Hareesha I, Murthy K S S. Influence of low frequency vibrations on aluminium eutectics. British Foundryman, 1979, T2: [5] Campbell J. Effects of vibration during solidification. Int. Met. Rev., 1981( 2): [6] Kocatepe K. The effect of low frequency vibration on macro and micro structures of LM25 and LM6 alloys [Dissertation]. University of Strathclyde, UK, [7] Fan Z T, Ji S. Low pressure lost foam process for casting magnesium alloys. Mater. Sci. Tech., 2005, 21: [8] Balandin G F, and Yu Yakovlev P. Effect of vibration on the crystallization process in aluminium casting. Russ. Cast. Prod., 1963: [9] Southin R T. The influence of low-frequency vibration on the nucleation of solidifying metals. J. Inst. Met., 1966, 94: [10] Bower T F and Flemings M C. Formation of the Chill Zone in Ingot Solidification. Transactions of the Metallurgical Society of AlME, 1967, 239: [11] Jakson K A and Hunt J D. Lamellar and Rod Eutectic Growth. Transactions of the Metallurgical Society of AlME, 1966, 236: [12] Hall E O. The Deformation and Ageing of Mild Steel: III Discussion of Results. Proc. Phys. Soc. B, 1951, 64: [13] Petch N J. The Cleavage Strength of Polycrystals. J. Iron Steel Inst., 1953, 174: [14] Shukla D P, Goel D B, Pandey P C. Influence of vibration during solidification on ingot soundness and mechanical properties of aluminum alloy test casting. In: Proceedings of the All-India Seminar on Aluminum, New Delhi, India, October [15] Caulk D A. A foam melting model for lost foam casting of aluminum. International Journal of Heat and Mass Transfer., 2006, 49: This research was supported by the National High Technology Research and Development Program of China (Grant No. 2007AA03Z113), and The National Natural Science Foundation of China (Grant No ). 29