Phase Transformation Die Casting Process for Manufacturing a Thin- Type Product and Its Mechanical Performance Assessment

Transcription

1 Key Engineering Materials Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Phase Transformation Die Casting Process for Manufacturing a Thin- Type Product and Its Mechanical Performance Assessment H. K. Jung 1, a, P. K. Seo 1, d, C. G. Kang 2, b and B. M. Kim 3, c 1 National Research Laboratory for Thixo-Rheo Forming, School of Mechanical Engineering, Pusan National University, Busan , Korea 2 School of Mechanical Engineering, Pusan National University, Busan , Korea 3 School of Mechanical Engineering, Pusan National University, Busan , Korea a hongkyuj@hanmir.com, b cgkang@pusan.ac.kr, c bmkim@pusan.ac.kr, d pkseo92@empal.com Keywords: Filling Limitation, Liquid Segregation, Mechanical Performance Assessment, Phase Transformation Die Casting Process, Thin-Type Product. Abstract. Thixo die casting of Al alloy is suitable for complicated large parts of the near net shape with less defect and excellent mechanical properties in comparison with conventional liquid metal die casting processes. To manufacture products with high performance, the reheating conditions, the casting plan, the die design, the injection conditions, the defect analysis, and the quantitative evaluation of mechanical properties are required. Al-Si binary alloys like A356 and A357 suitable for the thixo die casting process (phase transformation die casting process), which have outstanding fluidity, have been employed to completely fill the die cavity during forming. Thus, in the current work, the phase transformation die casting experiments using hypoeutectic A356 alloys were conducted for a variety of injection conditions and pressurization conditions. Finally, the ultimate tensile strength (UTS) and yield strength (YS) as well as the elongation of the thixo die cast component were evaluated in terms of both mechanical and metallurgical points of view. Introduction Recently, the weight of automobiles has been increasing due to the additional devices, which meet a variety of demands such as those of electronic systems, high-class quality and safety. In this trend, the requirements of fuel efficiency and restrictions on exhaust emissions have been reinforced in developed countries such as those of the U.S. and E.U. because of limited energy resources and environmental pollution problems, which have become gradually more serious. To keep pace with this situation, the development of automobiles for fuel efficiency and low emissions has been actively pursued in automobile industries both at home and abroad [1-9]. Fuel efficiency can be improved by an engine with high performance, the reduction of air resistance and the rolling resistance of a tire, the compact of a car body, as well and weight reduction; Thus, a large number of studies on the methods for reducing the weight of automobile parts have been actively conducted [3-9]. In order to reduce the weight of suspension components with high durability and strength, an optimal process design from conceptive points of view to manufacture the final product is required. Aluminum alloys with excellent mechanical properties from the point of view of lightweight, specific strength, corrosive resistance, and thermal conductivity have been employed for suspension parts connecting the front axle to the cross member. The most imperative factor regarding the weight reduction are an strength of parts for variations of shapes and dimensions, because the plastic deformation and fracture of the parts are caused by the reduction of strength resulting from the change of the shapes and dimensions when considering only weight reduction [3-7, 9]. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-04/03/16,21:37:48)

2 536 Advances in Engineering Plasticity and Its Applications Title of Publication (to be inserted by the publisher) Apart from the completion of the product design, the die structures and the applied pressure conditions for fabricating the component must be considered at the same time. Therefore, to produce the lower arm part, which meets the desired characteristics of automotive industries through the thixo die casting process, technical know-how for the die design is needed [3-7, 9]. This is because it is essential that the oxide film on the die surface, the defect due to an air entrapment, the impurities incorporated during the filling process, and the defect caused by solidification after the filling process are eliminated in the thixo die casting process [3-9]. Therefore, in the present work, the thixo die casting experiments were carried out for the variation of injection conditions and pressurization conditions. Finally, the ultimate tensile strength (UTS) and yield strength (YS) as well as the elongation of the thixo die cast component were investigated and the possibilities for practical use were estimated in terms of both mechanical and metallurgical points of view. Mechanical Properties of A356 for the Variation of Injection Conditions Experimental Set-Up. The starting material used for the fabrication of a thin-type component was an aluminum alloy, A356, fabricated by the electromagnetic stirring process and made by Pechiney (Voreppe, France). The chemical compositions of the A356 are shown in Table 1. Table 1. Chemical compositions of A356 Alloy Si Fe Cu Mn Mg Cr Zi Zn Ti Others A In order to manufacture the components by applying to the die casting process with phase transformation, a better understanding of quantitative mechanical properties is required. The mechanical properties strongly depend on the injection conditions and the specimens obtained from each position of the component. The injection conditions have an effect on the liquid segregation due to the separation between solid and liquid phases during the relatively slow die cavity filling. Since the liquid is of eutectic portion in nonferrous aluminum alloys, the liquid segregation will result in undesirable situation [6, 7, 9]. In the case of the casting process with phase transformation, the temperature of the billet is rapidly decreased during the feeding to the die casting machine after the reheating process, and the solid fraction of the billet increases. Therefore, the temperature decrement of the billet must be minimized during the transportation from the sleeve to the cavity in order to enhance the Table 2. Velocity of the plunger tip according to the injection speed and pressure Exp. No. V 1 (m/s) V 2 (m/s) V 3 (m/s) V 4 (m/s) Pressure(bar) formability. The high injection speed is needed until the reheated billet is filled completely with the sleeve. However, the low speed interval must be included due to the fact that the laminar flow is induced during the entrance into the runner or gate [7, 9]. To determine the optimal injection conditions at the high speed and low speed intervals is very critical, and to investigate the relationships among the injection conditions, the quantitative evaluation of the mechanical properties and resulting microstructures is essential in the phase transformation die casting process. Therefore, the thixo die casting experiments were conducted under various

3 Key Engineering Materials Vols injection conditions to prevent the liquid segregation and nonfilling phenomenon and in order to obtain the homogeneous mechanical properties and final microstructures over the cross sectional areas of the component. Fig. 1 shows the shapes of the sleeve and cavity, and the injection speed switching points of A, B, C and D. Point A is that filling completely the sleeve, and point B Fig. 1 Injection speed switching point

4 538 Advances in Engineering Plasticity and Its Applications Title of Publication (to be inserted by the publisher) is that flowed into the runner. Point C is that just before flowed into the component position, and point D is that for the final filling. The intervals of the speed variation from point A to D were divided into 4-steps, and they are assumed V1, V2, V3 and V4, respectively. Table 2 shows the injection speed and applied pressure used for this study. The speed of the plunger tip at point A was set to 1.2 m/s in order to prevent the rapid temperature decrement, and the speeds of the plunger tip passing through point A and B were changed to V2 and V3, respectively. By using the injection conditions of Exp. No. 6, the influence on the phase transformation die casting process for the variation of the applied pressure was also investigated. In the current work, the billet was reheated through 12-steps. After these steps, the thixo die casting experiments were performed using a real time controlled Buhler die casting machine with a capacity of 840 ton during forming. The die casting machine is equipped with a shot control system that provides real-time data [9]. Fig. 2 indicates the variation of the injection speed for a variety of the plunger stroke and the filling behavior of the (a) Exp. No. 1, 2, 3, 4 and 9 (b) Exp. No. 5, 6, 7, 8 and 9 Fig. 2 The variation of injection speed according to the stroke of plunger material into the sleeve and die cavity. Fig. 2 (a) denotes that the speed is changed to the 5-steps at point B after completely filling the sleeve with a speed of 1.2 m/s. Fig. 2 (b) shows that the speed in the component position is changed to the 5- steps after keeping the speed of 1.2 m/s to point C. After the final filling, the applied pressure was constantly set to 1250 bar. Fig. 3 shows the variation of the applied pressure after the final filling. In the phase transformation die casting process, if the transmission of the applied pressure is delayed after the final filling, due to solidification shrinkage, the gap between the die and the material is generated. This defect lowers the dimensional accuracy and surface roughness. To improve this and secure a denser microstructure, the transfer of the pressure must be completed as soon as the material is filled finally or an air entrapment in the die is eliminated completely through the air vent [3, 7, 9]. Fig. 4 shows positions for the observation of the resulting microstructures. The component for this investigation was a round type, and the die was designed for the thixo die casting of a thin part with the thickness of mm and a diameter of 105 mm. Fig. 5 indicates the positions and dimensions of the specimen to investigate the relationship between the injection conditions and the mechanical properties. The test specimens were machined on the basis of standard ASTM E 8M. Results and Discussions. Fig. 6 shows the final micro- structures of the fabricated component at positions as shown in Fig. 4. After the component was fabricated, it was quenched and then heat-

5 Key Engineering Materials Vols treated for the purpose of the improvement of the yield Fig. 3 The variation of pressure after strength. After thixo die casting, specimens were machined and heat-treated to T6 for a tensile test: solutionizing for 8 final filling hours at 540 -> water quenching -> ageing for 5 hours at 170. As shown in Fig. 6, the microstructures had few differences among them. The solid fraction at position (2) was relatively lower than the other positions due to the geometric shape of the component. Porosities were observed at position (5) where is the end position of the product. According to the increase of the applied Fig. 4 Positions of part to observe pressure, the degree of the globularization was the microstructures slightly improved. To investigate the relationship between the injection conditions and the mechanical properties, the tensile strength was measured in (1) to (4) of Fig. 5. The tensile tests were carried out at the condition of the tension velocity of 1 mm/min using an MTS machine with a maximum load of 25 ton. Fig. 7 indicates the ultimate tensile strength (UTS) for a variety of injection conditions. Fig. 7 (a) shows the UTS distribution for the variation of the speed in the runner. It was observed that the UTS distribution was not sensitive to the variation of the Fig. 5 Positions and dimensions of the speed in the runner. On the other hand, the tensile specimen to investigate the mechanical strength decreased gradually as the speed in the gate performance increased as shown in Fig. 7 (b). From the preceding results, it was concluded that the variation of the speed in the gate was more sensitive than that in Position (1) (2) (3) (4) (5) Exp.No Fig. 6 Microstructures of A356 alloy for the variation of injection velocity with T6 heat treatment the runner to

6 540 Advances in Engineering Plasticity and Its Applications Title of Publication (to be inserted by the publisher) the mechanical properties. Fig. 7 (c) shows the distribution of the tensile strength for the variation of the applied pressure after the final filling. In spite of the variation of the applied pressure, the variation of the UTS was not remarkable. The reason is that the solidification time is very short owing to the thin thickness (5 mm) of the specimen. This indicates the filling and solidification must be achieved simultaneously to ensure good mechanical performance without defects [3, 5-7, 9]. On the other hand, in the case of the product with a thick region, the ultimate strength increases owing to the formation of a dense microstructure as the applied pressure increases. The reason is that although the filling and solidification are achieved simultaneously, the solidification time is delayed. (a) Exp. No. 1, 2, 3, 4 and 9 (b) Exp. No. 5, 6, 7, 8 and 9 (c) Exp. No. 10, 11, 6, 12 and 13 Fig. 7 Ultimate tensile strength (UTS) after thixo die casting using phase transformation (T6 heat treatment) Fig. 8 shows the distribution of the yield strength for variations of injection conditions. In the case of varying the speed in the runner, as shown in Fig. 8 (a), the yield strengths under the conditions of Exp. No. 1 and 2 were slightly higher than the other injection conditions. This reason is that the flow pattern at the condition below the injection speed of 0.5 m/s in the runner is more laminar than at that over the injection speed of 0.7 m/s. Fig. 8 (b) shows the yield strengths of the component fabricated under various injection speeds in the gate. In the same way as shown in Fig. 8 (a), the yield strength of the product manufactured at the injection velocity below 0.5 m/s was higher than the other conditions. It was found that the applied pressure did not have a highly pronounced effect on the yield strength as the same on the tensile strength. (a) Exp. No. 1, 2, 3, 4 and 9 (b) Exp. No. 5, 6, 7, 8 and 9 Fig. 8 Yield strength (YS) after thixo die casting using phase transformation (T6 heat treatment) Fig. 9 represents the elongation distribution of the product fabricated at various injection conditions. In the case of varying the velocity in the runner as shown in Fig. 9 (a), the elongation distributions were not homogeneous. On the other hand, in the case of varying the velocity in the gate as shown in Fig. 9 (b), the elongation distributions were more uniform. The elongation of the part produced at the injection velocity below 0.5 m/s was also better than the other conditions. Fig. 9 (c) shows the elongation distribution for the variation of the applied pressure. It was observed that the applied pressure did not have a highly remarkable influence on the elongation as the same on the tensile strength and the yield strength.

7 Key Engineering Materials Vols By using thixo die casting process based on the phase transformation, the possibility of prediction of the optimal injection conditions and the quantitative assessment of the mechanical performance is expected. (a) Exp. No. 1, 2, 3, 4 and 9 (b) Exp. No. 5, 6, 7, 8 and 9 (c) Exp. No. 10, 11, 6, 12 and 13 Fig. 9 Elongation after thixo die casting using phase transformation (T6 heat treatment) Summary Through the mechanical performance assessment of nonferrous A356 alloys with the sample die for the fabrication of a thin-type component, the following particular results were obtained. (1) In the thixo die casting process with A356, it was found that the changes of the injection speed and applied pressure did not have a pronounced influence on the resulting microstructures and the tensile strength obtained from the experimental results for variations of the injection speed in the runner and gate, and the pressure after the final filling. (2) Excellent yielding strength and elongation were obtained at the injection velocity below 0.5 m/s in the runner and gate. This is because the laminar flow of the reheated material is caused at the injection velocity below 0.5 m/s. (3) On the basis of the experimental results for A356 materials for a variety of the injection conditions, a quantitative data base for the purpose of setting the optimal injection conditions like injection velocity and applied pressure was constructed. It will also be applied to the advanced thixo die casting process to manufacture components with excellent mechanical performance in the future. Acknowledgment This work has been conducted with funding from the National Research Laboratory for Thixo-Rheo Forming (NRL/TRF) of Pusan National University (PNU), which is a national research laboratory appointed by the Ministry of Science and Technology (MOST). References [1] M.C. Flemings: Metall. Trans. A Vol. 27A (1991), p [2] C.Y. Chen, J.A. Sekhar, D.G Backman and Mehrabian: Mater. Sci. Eng. A Vol. 40A (1979), p [3] H.K. Jung and C.G. Kang: Proc. 5th Asia-Pacific Symp. on Advances in Engineering Plasticity and Its Applications (AEPA 2000 ), 2000, 565. [4] P. Kapranos and H.V. Atkinson: Proc. 7th Int. Conf. on Semi-Solid Processing of Alloys and Composites, 2002, 167. [5] D. Liu, H.V. Atkinson, P. Kapranos and H. Jones: Proc. 7th Int. Conf. on Semi-Solid Processing of Alloys and Composites, 2002, 311. [6] C.G. Kang and H.K. Jung: Metall. Mater. Trans. B Vol. 32B (2001), p [7] C.G. Kang and H.K. Jung: Metall. Mater. Trans. B Vol. 32B (2001), p [8] L.A. Lalli: Metall. Trans. A Vol. 16A (1984), p [9] C.G. Kang and H.K. Jung: Metall. Mater. Trans. B Vol. 32B (2001), p. 363.

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