Effects of Mechanical Vibration on Cooling Rate and DAS of AC4C Aluminum Alloy Gravity Die Castings

Transcription

1 Materials Transactions, Vol. 50, No. 11 (2009) pp to 2608 #2009 The Japan Institute of Metals Effects of Mechanical Vibration on Cooling Rate and DAS of AC4C Aluminum Alloy Gravity Die Castings Naoki Omura 1, Yuichiro Murakami 1, Mingjun Li 1, Takuya Tamura 1, Kenji Miwa 1, Hideki Furukawa 2 and Masayuki Harada 2 1 National Institute of Advanced Industrial Science and Technology, Nagoya , Japan 2 Kotobuki Kinzoku Kogyo Co. LTD., Seki , Japan Gravity die casting of AC4C aluminum alloy with mechanical vibration (0 12) was conducted. Columnar rod specimens (25 mm L210 mm) were cast to investigate the effect of mechanical vibration on the cooling rate and the dendrite arm spacing of AC4C aluminum alloy castings. The cooling rate increased by imposition of the mechanical vibration, and increased with the increase of the vibration frequency. When the mechanical vibration imposed, the temperature increased quickly and reached higher temperature compared with no vibration condition. The dendrite arm spacing in outer region of the specimen decreased by mechanical vibration, and decreased with the increase of the vibration frequency. In the case without mechanical vibration, the specimen showed smooth surface. But the surface of specimen became rough by the imposition of the mechanical vibration. [doi: /matertrans.m ] (Received July 16, 2009; Accepted September 2, 2009; Published October 15, 2009) Keywords: mechanical vibration, aluminum, alloy, cooling rate, dendrite arm spacing, air gap, heat transfer, metallic 1. Introduction Various properties of castings strongly relate to its solidification structure. In general, it is known that many properties of castings such as yield point, ultimate tensile strength and elongation are improved by reduction of their grain size and their dendrite arm spacing (DAS). 1 5) The DAS of castings is strongly affected by the cooling rate 4) during solidification, and becomes fine by high cooling rate. Usually, the gravity die casting is applied to large and thick products, so it is difficult to increase the cooling rate due to the thermal extraction constraints. Thus, the DAS of these products becomes coarse, resulting in low properties. In term of the grain size, the grain refining agents are very effective to refine the grain size. Hence the grain refining agents are widely used in usual casting process. Titanium, boron, carbon, and mother alloys including these elements, are well known as the grain refining agents of aluminum alloy. 5 8) But, using of these grain refining agents gives rise to some problems in production and recycling. In production, both the grain refining agents and the addition process increase the cost of the final product. In recycling, on the other hand, grain refining agents are regarded as impurities. So separation and removal of grain refining agents is necessary to closed-recycling. When it is difficult to separate and remove the grain refining agents, the recycling becomes cascade recycling with high environmental load. It is well known that the vibration and the agitation of melt during solidification also can refine the grain size. 9) Hence, many vibration and agitation processes such as mechanical vibration process, 10,11) ultrasonic vibration process, 12,13) electromagnetic stirring process 14,15) and electromagnetic vibration process 16,17) have been developed and reported to refine the grain size of castings. However, the practical application of these processes is very few, because of the complex process and the expensive equipment such as ultrasonic horn, magnet coil and superconducting magnet. Furthermore, the shape and size of products are limited by these equipments. Our previous work 18) showed that reduction of grain size, casting defects and mechanical property scattering of AC4C aluminum alloys fabricated by gravity die casting was achieved by very simple process of mechanical vibration imposition. The object of the present work is to clarify the effect of mechanical vibration on cooling rate and DAS. The temperature measurement of AC4C aluminum alloy was carried out during the gravity die casting when the mechanical vibration was imposed at the frequency range of 012. And microstructures of specimens cast with and without vibration were observed. 2. Experimental Procedure 2.1 Experimental apparatus The schematic of the experimental apparatus of vibration casting is shown in Fig. 1. A vibrator (HKM154VS; EXEN, Tokyo, Japan) was placed at the fixed base. This vibrator generates vibration by the high speed rotation of eccentric pendulum. Thus, the vibration frequency and centrifugal force of the vibrator could be easily controlled by changing the frequency of the inverter current supplied (Fig. 2). A part of fixed (vibrating ; about 10 kg) was oscillated by the centrifugal force propagated through transfer rods and a transfer board by the same frequency as the vibrator. The commercially available AC4C aluminum alloy was used as a sample. The chemical composition of this material determined by emission spectrometric analysis is shown in Table 1. The cooling curve of this material cooled slowly in furnace is shown in Fig. 3. The liquidus temperature of this material was 888 K, and Al-Si binary eutectic was clearly seen in 851 K. Because of a little amount of magnesium, the peak of ternary (Al-Mg 2 Si pseudo-binary) eutectic was not observed.

2 Effects of Mechanical Vibration on Cooling Rate and DAS of AC4C Aluminum Alloy Gravity Die Castings 2605 Movable base Fixed base 1000 Movable Fixed Vibrator Liquidus temp. (888K) Al-Si eutectic temp. (851K) Vibration Fig. 1 Cavity Vibrating Vibration transfer board Vibration transfer rod Schematic diagram of the experimental apparatus (top-view). Fig. 3 Pouring cup Cooling curve of AC4C aluminum alloy. Raiser Thermocouples (T.C.) Top Vibrating Vibration frequency, f v /Hz Vibration frequency Centrifugal force Inverter current frequency, f i /Hz Fig. 2 Relation between inverter current frequency and vibration properties (frequency and centrifugal force) of the vibrator. Centrifugal force, F/kN Sprue Middle 100mm 100mm Bottom Gate Fixed T.C. 6.7mm 10mm Product (φ 25mm L210mm) Table 1 Chemical composition of AC4C aluminum alloy. (mass%) Si Mg Ti Fe Ni Cu Zn Al Bal. The schematic illustration of the fixed is shown in Fig. 4. Round bar samples (25 mm in diameter and 210 mm in length) were cast using bottom gate plan. The wash (HLP-806K; DIRECTSENBOU, Aichi, Japan) was applied to surface with the spray gun by the thickness of about 100 mm. The surface roughness (Rz) of wash was about 45 mm. Mechanical vibration casting was carried out at 1003 K (10 K) of the melt temperature and 633 K of the temperature. The imposition of mechanical vibration started before pouring, and stopped at 40 s after pouring. 2.2 Temperature measurement As shown in Fig. 4, three K-type sheathed thermocouples (1 mm in diameter and ungrounded) were inserted to the center of the cavity from raiser part to measure the melt temperature during mechanical vibration casting. The temperature measurement was carried out at three places (top, middle and bottom) with the intervals of 100 mm. Furthermore the temperature change of the vibrating during casting was also measured by another thermocouple inserted in the vibrating. These thermocouples were fixed by a bolt at non-vibration area of the to prevent moving thermocouple during vibration casting. 2.3 Measurement of dendrite arm spacing (DAS) Microstructure observation and DAS measurement were carried out at the outer region of the bottom area of cast specimen. To observe the microstructure, each specimen was ground following the standard procedure for metallographic preparation, and etched in a 0.2% hydrofluoric acid. Microstructure was observed using optical microscope. Dendrite arm spacing was measured according to the liner intercept method. Over 250 dendrite arms were counted at six fields of view. 3. Results Fig. 4 Schematic illustration of the fixed. Figure 5 shows the cooling curve of AC4C aluminum alloy during casting at middle area. In this figure, the time when the temperature increases rapidly, indicating the contact of melt and thermocouple, is shown as 0 s. The slope of cooling curves becomes gradual about 890 K and 850 K.

3 2606 N. Omura et al. Middle area Time, t /s Bottom area Fig. 5 Effect of vibration frequency on the cooling rate of the melt at middle area. Fig. 7 Effect of vibration frequency on the cooling rate of the melt at bottom area. Top area Fig. 6 Effect of vibration frequency on the cooling rate of the melt at top area. These temperatures correspond to the liquidus temperature and Al-Si binary eutectic temperature of this material, respectively. Thus, these slope changes of cooling curve indicate the start of solidification and eutectic reaction. The melt temperature drops soon after pouring, and reaches the liquidus temperature in a few seconds. The cooling rate just below liquidus temperature (cooling rate from K to K; afterwards simply written as cooling rate ) is relatively slow when the frequency of mechanical vibration is. By imposition of mechanical vibration, the cooling rate increases clearly. In term of the binary eutectic, the reaction time decreases significantly by imposition of the mechanical vibration. The cooling curve of AC4C aluminum alloy during casting at top and bottom area is shown in Fig. 6 and Fig. 7, respectively. In both areas, the cooling rate increases and the reaction time of binary eutectic decreases by imposition of the mechanical vibration, as well as the middle area. The cooling rate and the reaction time of binary eutectic of each condition is shown in Table 2 and Table 3, respectively. In all area, the cooling rate increases by imposition of the vibration, and slightly increases with the increase of the vibration frequency. In the case of vibration frequency, the cooling rate of the bottom area is the fastest and that of the middle area is the slowest. In the case of vibration frequency 7 and 12, on the other hand, the cooling rate of the bottom area is the fastest and that of the top area is the slowest. In term of binary eutectic, the reaction time decreases by the mechanical vibration, and decreases with Table 2 Cooling rate from K to K as a function of the vibration frequency at different area of specimen. (K/s) Vibration freq. (Hz) Top Area Middle Bottom Table 3 Reaction time of Al-Si binary eutectic as a function of the vibration frequency at different area of specimen. (s) Vibration freq. (Hz) Top Area Middle Bottom Fig s 38s Time, t /s the increase of the vibration frequency. In all case of vibration frequency, the bottom area shows the shortest reaction time. Figure 8 shows the temperature change of vibrating during mechanical vibration casting. In this figure, pouring end (melt temperature of top area reaches the maximum temperature) is assumed for 0 s. After pouring, the temperature of vibrating increases and reaches maximum temperature at about 35 s. When the vibration frequency is, temperature of vibrating increases about 30 K Mold temperature during mechanical vibration casting.

4 Effects of Mechanical Vibration on Cooling Rate and DAS of AC4C Aluminum Alloy Gravity Die Castings 2607 (a) (b) DAS, d/um (c) (d) Vibration frequency, f/hz Fig. 10 DAS in the outer region of specimen as a function of the vibration frequency. (from 633 K to 663 K). In the case of vibration frequency 7 and 12, on the other hand, the temperature of vibrating increases about 37 K and 39 K, respectively. Furthermore, the time to reach the maximum temperature decreases about 5 s (38 s to 33 s) by imposition of the vibration. These results of the melt and the temperature measurement indicate that the heat transfer efficiency between the melt and the is improved by imposition of the vibration, and much heat transfers from the melt to the in shorter time. So the cooling rate of melt and maximum temperature of vibrating temperature increase, when vibration frequency is 7 and 12. Typical microstructures in outer region of the bottom area of specimen cast with and without mechanical vibration are shown in Fig. 9. It is clearly seen that the microstructure becomes fine by imposition of the mechanical vibration. In addition, the surface of specimen has tendency to roughen with an increase in the vibration frequency. Figure 10 shows the DAS in outer region of specimen as a function of vibration frequency. In the case of vibration frequency, the DAS is about 33 mm. The DAS decreases by the mechanical vibration. Furthermore, the DAS decreases with the increase of the vibration frequency. When the vibration frequency is 12, maximum frequency value in this study, the DAS in outer region is about 26 mm. These results evince that the cooling rate increases by imposition of mechanical vibration, and increases with the increase of the vibration frequency. 4. Discussion (e) 100um Fig. 9 Microstructures of specimens cast under different vibration frequencies of (a), (b) 55 Hz, (c) 7, (d) 8, (e) 10 and (f) 12. By imposition of mechanical vibration on the during gravity die casting, the following results were observed; the (f) increase in cooling rate of melt, the decrease of eutectic reaction time, the increase in maximum temperature of vibrating and the decrease in DAS of specimen. These results indicate that the heat transfer efficiency between the melt and the is improved by imposition of the vibration, and much heat transfers from the melt to the within shorter time. Generally, it is known that a heat transfer efficiency between the melt and the is reduced by the air gap due to the solidification shrinkage of melt. 19,20) Additionally, it is considered that the contact of the melt and the ( wash) is partial because of the surface tension of the melt. Due to an existence of much air between the melt and the caused by the solidification shrinkage and the surface tension of the melt, the specimen cast without mechanical vibration shows slow cooling rate of melt and large DAS of specimen. In this study, only a part of on one side (vibrating ) was vibrated as shown in Fig. 1. This means that the diameter of cavity varies by the vibration of by the same amount as the amplitude of. Thus, it is considered that the air gap existence between the melt and the is pressed and narrowed by the change in the diameter of cavity (Fig. 11). Furthermore, it is also considered that the contact area of the melt and the ( wash) increases due to overcoming the surface tension of melt by pressing the vibrating. These may be the reason that high cooling rate, small DAS, and rough surface are observed when the mechanical vibration is imposed. 5. Conclusions Mechanical vibration was imposed on the gravity die casting of AC4C aluminum alloy, and the temperature change of the melt and the dendrite arm spacing of specimen were measured to investigate the effects of mechanical vibration on the cooling rate of the melt, and following results were obtained. (1) The cooling rate of melt increases by imposition of the mechanical vibration, and increases with the increase of the vibration frequency. (2) The maximum temperature of vibrating during casting increases and the time to the maximum temperature shortens by imposition of mechanical vibration.

5 2608 N. Omura et al. wash air surface (3) The dendrite arm spacing in outer region of specimens decreases by the mechanical vibration, and decreases with the increase of the vibration frequency. (4) The specimen cast without vibration has smooth surface. However, the surface of specimen cast with vibration becomes rough. vibration melt Amplitude of melt Fig. 11 Schematic presentation of the vibration effect on the interface between the melt and the. REFERENCES 1) N. J. Petch: J. Iron Steel Inst. 174 (1953) ) J. Hemanth: Mater. Design 21 (2000) ) G. Ran, J. Zhou and Q. G. Wang: J. Alloy. Compd. 421 (2006) ) C. H. Caceres, C. J. Davidson, J. R. Griffiths and C. L. Newton: Mater. Sci. Eng. A 325 (2002) ) S. A. Kori, B. S. Murty and M. Chakraborty: Mater. Sci. Eng. A 283 (2000) ) Y. C. Lee, A. K. Dahle, D. H. StJohn and J. E. C. Hutt: Mater. Sci. Eng. A 259 (1999) ) T. Sritharan and H. Li: J. Mater. Proc. Tech. 63 (1997) ) C. T. Lee and S. W. Chen: Mater. Sci. Eng. A 325 (2002) ) A. Ohno: Kinzoku Gyouko-gaku, (Chijin Shokan, Tokyo, 1973) pp [in Japanese] 10) T. P. Fisher: Brit. Foundryman 66 (1973) ) N. Abu-Dheir, M. Khraisheh, K. Saito and A. Male: Mater. Sci. Eng. A 393 (2005) ) Y. Osawa and A. Sato: J. JFS 72 (2000) [in Japanese] 13) O. V. Abramov: Ultrasonics 25 (1987) ) J. Dong, J. Cui, X. Zeng and W. Ding: Mater. Lett. 59 (2005) ) B. Zhang, J. Cui and G. Lu: Mater. Sci. Eng. A 355 (2003) ) Y. Mizutani, S. Kawai, K. Miwa, K. Yasue, T. Tamura and Y. Sakaguchi: Mater. Trans. 45 (2004) ) Y. Mizutani, Y. Ohura, K. Miwa, K. Yasue, T. Tamura and Y. Sakaguchi: Mater. Trans. 45 (2004) ) N. Omura, Y. Murakami, M. Li, T. Tamura, K. Miwa, H. Furukawa, M. Harada and Y. Yokoi: J. JFS 81 (2009) [in Japanese] 19) Y. Ito, Y. Tominaga and Y. Watanabe: J. Jpn. Inst. Light Met. 17 (1967) [in Japanese] 20) N. Fujii, S. Okada, S. Morimoto and M. Fujii: J. Jpn. Inst. Light Met. 33 (1983) [in Japanese]