Study on vacuum die casting process of aluminum alloys. HU Bo*, XIONG Shoumei 1 *, Masayuki Murakami**, Yoshihide Matsumoto**, and Shingo Ikeda**.

Transcription

1 Study on vacuum die casting process of aluminum alloys HU Bo*, XIONG Shoumei 1 *, Masayuki Murakami**, Yoshihide Matsumoto**, and Shingo Ikeda**. * Key laboratory for advanced manufacturing by materials processing technology, Department of Mechanical Engineering, Tsinghua University, Beijing , P.R. China ** TOYO Machinery & Metal Co. Ltd, Japan Abstract The influences of process parameters and vacuum level in vacuum die casting technology on the density and mechanical properties of ADC12 alloy die castings were studied. Standard die casting tensile testing specimens were produced on a 650t cold chamber die casting machine with different vacuum pressures from 5kPa to 100kPa and different casting pressures from 66.7MPa to 22.7MPa. Other operating conditions were kept constant. Density, ultimate tensile strength, yield strength and elongation were measured. The relationship between operating conditions and casting properties was discussed and the conclusion was indicated that vacuum die casting is able to improve the property of castings and reduce the equipment tonnage while maintaining the property. Key words Vacuum Die Casting, Aluminum Alloy, Porosity, Mechanical Property. 179/1

2 Introduction High pressure die casting (HPDC) is a high efficiency, near net-shaped process, which has been widely used in modern industry. Aluminum alloys are widely used in die casting process because of their good combination of strength and light weight. Porosity is the main obstacle for the application of die castings in welding, heat treatment and pressure test. Without heat treatment, the strength of aluminum die castings is not good enough for the manufacturing of components with high importance. Studies on die casting process and methods to improve die casting properties have been carried on for many years [1,2]. Besides, particular attention was paid on the process of air venting during die casting to investigate the mechanism of porosity formation and reduce gas content [3-6]. However, gas entrapping is almost impossible to be eliminated in conventional die casting. With gas pores inside, casting strength is weakened and further machining or heat treating is impossible. Vacuum die casting is a kind of high integrity die casting process which is able to significantly reduce gas content of die castings. In vacuum die casting process, cavity gas is exhausted through a vacuum valve with a vacuum pump. A typical work cycle of vacuum die casting is shown in Figure 1 [7]. Along with the vast development of die casting application, attempts to improve and extend vacuum die casting never stopped. NISSAN Motor manufactured aluminum structural parts with vacuum die casting [8]. RYOBI LIMITED reported the MIG welding characteristics of aluminum vacuum die castings [9]. VAW Aluminum studied heat treatment properties of aluminum vacuum die castings [10]. In spit of the extensive application foreground, the process of vacuum die casting still requires improving. In this paper, the relationship between operating conditions and mechanical properties of vacuum die castings of ADC12 alloy is systematically investigated, and the influence of vacuum level on selecting traditional process parameters is discussed. Experimental Experimental Condition The experiment was carried on a TOYO 650t cold chamber die casting machine with a VCS vacuum system which has controlling linkage to a die casting machine. Vacuum pressure in the cavity was measured by a KEYENCE vacuum sensor, which has a minim precision of 0.1kPa. With a sealing plate on the back of the ejector die, a stable vacuum pressure lower than 5kPa was maintained during work cycle. Figure 2 is a layout of the test mold with the position of vacuum sensor. The test mold was made according to ASTM standard B a with H13 steel [11]. In order to realize the vacuum level inside the cavity during work cycle, two channels of vacuum system were connected to the cavity. The vacuum sensor was located at the terminal of the right channel as shown in Figure 3. A vacuum pump was connected to the left channel. In this way, the 179/2

3 vacuum sensor was able to show the vacuum pressure at the end of the cavity. Black arrows show the direction of air flow inside the cavity when the vacuum pump is working. Figure 4 gives a picture of the casting. The second bar from the left is round-sectioned and all of the mechanical properties were tested on this bar. Figure 5 gives detailed sizes of the specimen. Density measurements were performed according to Archimedes principle. Mechanical properties were measured on a CC-5510 electronic experimental machine at 1mm/min drawing speed. For each operating condition, five specimens were tested to get an average result. Experimental Procedure Standard process parameters were kept constant in this experiment as shown in Table I. Slow phase injection speed was much less than ordinary value because vacuum pump requires a minim slow phase injection time to exhaust cavity gas. If the slow phase injection speed is not low enough, melt metal will flow into the cavity before a satisfying vacuum level is reached. Vacuum pressures and casting pressures were set as shown in Table II. Results and Discussion Density Density of vacuum die castings increases with the increase of vacuum level as shown in Figure 6. As vacuum level increases from 100kPa to 5kPa, casting densities at all five casting pressures increase monotonously. At each vacuum level, casting density increases with the increase of casting pressure from 22.7MPa to 66.7MPa, which is similar with conventional die casting. It is know that porosity in die castings includes two parts: gas porosity and shrinkage porosity. Under vacuum conditions, the physically entrapped gas can be minimized with the increase of vacuum level. Therefore, the density of castings will increase with the increase of vacuum level at each of the five casting pressures. Shrinkage porosity could be reduced with the increase of casting pressure. Higher casting pressure influences the process of porosity formation, raises metal-mold interfacial heat transfer coefficient (IHTC), affects solidification phenomenon and reduces grain size. In this experiment, increasing casting pressure from 22.7MPa to 66.7MPa always contributes to the increase of casting density. Ultimate Tensile Strength Ultimate tensile strength of vacuum die castings increases with the increase of vacuum level at high casting pressure. At the casting pressure of 66.7MPa, ultimate tensile strength increases with vacuum level 179/3

4 monotonously as shown in Figure 7. However, the relationship is not clear at lower casting pressures. Casting pressure has great influences on the ultimate tensile strength of aluminum die castings. With a lower casting pressure, gas porosities and shrinkage porosities increase in the castings, therefore ultimate tensile strength of vacuum die castings does not show clear relationship with vacuum level at casting pressures of 55.7MPa to 22.7MPa. Yield Strength Yield strength of the vacuum die castings increases monotonously with the increase of both vacuum level and casting pressure as shown in Figure 8. At each casting pressure, yield strength increases obviously with the increase of vacuum level. At a particular vacuum level, yield strength increases with the increase of the casting pressure. The influence of vacuum level on yield strength is greater than that of the casting pressure. Elongation Elongation of vacuum die castings increases monotonously with the increase of vacuum level as shown in Figure 9. At each vacuum level, elongation increases with the increase of the casting pressure. The influence of vacuum level on elongation is also greater than that of the casting pressure. Machine Tonnage Requirement Since vacuum level is proved to be efficient in improving mechanical properties of die castings, it is reasonable to produce eligible components at a lower casting pressure in vacuum die casting process. Figure 10 shows the relationship between vacuum pressure and casting pressure under different yield strengths. Curves in Figure 10 show the same tendency from the bottom-left to the top-right with increasing yield strength. This indicates that increasing of either vacuum level or casting pressure has the same effect of improving casting yield strength. In conventional die casting process, if casting pressure is limited by machine tonnage, the application of vacuum condition will be capable to produce eligible castings at available casting pressure and to meet the requirements for the casting properties. Conclusions 1. Density of aluminum vacuum die castings increases with the increase of vacuum level at each of the five fixed casting pressures. Under a particular vacuum level, density increases with the increase of the casting pressure from 22.7MPa to 66.7MPa. 2. Ultimate tensile strength of aluminum vacuum die castings increases monotonously with the increase of vacuum level at the casting pressure of 66.7MPa. Yield strength and elongation increase with both vacuum level and casting pressure. The 179/4

5 influence of vacuum level on yield strength and elongation is greater than that of the casting pressure. 3. The application of vacuum condition on aluminum die casting process is able to reduce machine tonnage requirement for castings with the yield strength maintained. References 1. V D Tsoukalas, The effect of die casting machine parameters on porosity of aluminum die castings, Int J Cast Metals Res, 15, 2004, pp Haavard T Gjestland et al, Effects of casting temperature, section thickness and die filling sequence on microstructure and mechanical properties of high pressure die castings, Die Casting Engineer, 7, 2004, pp G Bar-Meir, E R G Eckert and R J Goldstein, Air venting in pressure die casting, J Fluids Eng, 1997, pp G Bar-Meir, E R G Eckert and R J Goldstein, A model of vacuum pumping, ASME J Manuf Sci Eng, 1996, pp A Nouri-Borujerdi and J A Goldak, Modeling of air venting in pressure die casting process, ASME J Manuf Sci Eng, 2004, pp J Hernandez, J Lopez and F Faura, Influence of unsteady effects on air venting in pressure die casting, ASME Trans 2001, pp X P NIU et al, Vacuum assisted high pressure die casting of aluminum alloys, J Materials Processing Technology, 105, 2000, pp Tetsuo Sakamoto, Keiji Kira, and Hiroshi Kambe, Development of automotive suspension part by high vacuum die casting, Journal of Japan Foundry Engineering Society, 76, 2004, pp Toru Komazaki, MIG welding characteristics of aluminum vacuum die castings, Journal of Japan Foundry Engineering Society, 76, 2004, pp Wolfgang Schneider and Franz Josef Feikus, Heat treatment of aluminum casting alloys for vacuum die casting, Light Metal Age, 1998, pp ASTM Standard, Standard test methods of tension testing wrought and cast aluminum- and magnesium-alloy products [Metric], ASTM B 557M-02a Acknowledgements The research was financially supported by Tsinghua-TOYO R&D center of magnesium and aluminum alloys processing technology jointly established between Tsinghua University and TOYO Machinery & Metal Co. Ltd. The study was also supported by National Natural Science Foundation of China ( ) and Program for New Century Excellent Talents in University. 179/5

6 Tables Parameter Table I: Die casting process parameters in the experiment Melting Temp. () Mold Temp. () Slow phase plunger velocity(m/s) Fast phase plunger velocity(m/s) Value Table II: Vacuum levels and casting pressures in the experiment Casting Pressure (MPa) Vacuum Pressure (kpa) /6

7 Figures Figure 1. Plunger position at vacuum pump on (a) and off (b) state in vacuum die casting process Figure 2. Layout of the vacuum system Figure 3. Vacuum sensor position and air flow direction in the cavity 179/7

8 Test Specimen Figure 4. Picture of a specimen casting Figure 5. Dimensions of the ASTM standard tensile specimen Figure 6. Density result of castings at different vacuum levels and casting pressures Figure 7. Ultimate tensile strength result of castings at different vacuum levels and casting pressure 66.7MPa 179/8

9 Figure 8. Three dimensional model of yield strength result at different vacuum levels and casting pressures Figure 9. Three dimensional model of elongation result at different vacuum levels and casting pressures Figure 10. Relationship between vacuum level and casting pressure of particular yield strength 179/9