Effects of process settings on the porosity levels of AM60B magnesium die castings

Transcription

1 Published in Materials and Manufacturing Processes, Vol. 15, No. 1, 000, pp Effects of process settings on the porosity levels of AM0B magnesium die castings Y.J. Huang +, B.H. Hu*, I. Pinwill*, W. Zhou + & and D.M.R. Taplin + School of Mechanical and Production Engineering, Nanyang Technological University, Singapore * Gintic Institute of Manufacturing Technology, Singapore Faculty of Engineering, Central Queensland University, Australia 70 & WZhou@Cantab.Net Abstract The study examined effect of die casting process parameters on porosity in magnesium alloy AM0B. Porosity is unavoidable in die casting products, but it can be reduced by controlling the process parameters. It is shown in this work that die casting parameters such as plunger stroke, first phase speed, second phase speed and consolidation pressure all have a significant influence on the porosity level of AM0B magnesium die castings. The relationship between the porosity level and the processing parameters has been investigated. The paper also discusses the developing application of magnesium die castings in the microelectronics and automotive industries. 1. Introduction With the rapid growth of the microelectronics industry, magnesium has become a promising material for many components in handphones, CVD/DVD chassis, computer disk drives and camera casings due to its end of life properties (such as recyclability), lightweight and electro-magnetic radiation (EMR) shielding capability. With its density of only 1.8 magnesium is an extra-light structural metal, so it has been increasingly used in the automotive industry to increase fuel efficiency and reduce greenhouse gas emission [1]. Much of this is driven by more stringent environmental legislation [1-]. The AM0B alloy was selected for this study due to its high toughness and ductility compared with the more commonly used AZ91D alloy (Mg-9wt% Al 0.7wt%Zn 0.15wt% Mn) [3]. Porosity is a common problem in high pressure die casting (HPDC) products. Porosity in die castings can be caused by either entrapped gases or solidification shrinkage []. This project was designed to investigate the influence of the processing parameters, namely plunger stroke, first phase speed, second phase speed and consolidation pressure, on the porosity of AM0B magnesium die castings. 1

2 . Experimental Procedure The material studied was a high-purity magnesium alloy AM0B. Its chemical composition was analyzed and is shown in Table 1. Die casting test samples of the shape as shown in Fig. 1 were produced using a Frech DAM 80S hot chamber die casting machine [5]. The alloy was melted at 50 C and the die was pre-heated to a temperature of 10 ± 10 C. The protective gas used was a mixture of SF and dry air. The volume ratio of SF to dry air was in the range and SF flow rate was set at 7.03 l/min. As SF is a damaging greenhouse gas [], the alternative protective gases (e.g., SO ) are being separately evaluated. Various combinations of casting parameters were tried, as shown in Table. As can be seen from the table, the processing parameters studied are the first phase speed, second phase speed, stroke, and pressure. For the die casting machine used, plunger speed and pressure can only be controlled by controlling opening of the respective proportional valves. Therefore, the percentage shown in Table is the percentage of the valve opening. Table 1. Chemical composition of the AM0B alloy (in wt.%) Al Mn Ni Cu Zn, Ca Si K Fe Mg <0.01 < Bal. Table. Die casting settings Setting 1st Phase Speed (%) nd Phase Speed (%) Stroke (mm) Pressure (%) I II III IV V VI VII VIII It can be seen from Fig. 1 that each cast sample has four cylindrical rods with a length of 100 mm and a diameter of 10 mm (Fig. ). Metallographic samples were sectioned from different locations of rods, cold mounted, wet ground, polished and then etched using 10vol% HF (with 0vol% concentration) and 90vol% H O [3]. Three sections of each cast rod, namely tip, middle and gate sections (Fig. ), were microstructurally analyzed. Porosity levels were determined using an image analyzer Quantimet 50 [7]. 3. Experimental Results Typical microstructures of a setting VI (see Table 1) sample are shown in Fig. 3 and porosity levels for the various parameter settings are given in Table 3. Since the plunger speed and pressure were set using different valve openings, the actual speed

3 and pressure at each setting were measured using specially installed sensors. The measured values are shown in Table. Table 3. Results of the porosity level analysis Setting Porosity (%) Tip Middle Gate I II 0.8 III IV V VI VII 1 0. VIII 5.0 Table. Measured speed and pressure corresponding to different valve openings. Parameter Percentage of Measured Value Studied Valve Opening 1 st Phase Speed 30% 0.08 m/s (plunger speed) 0% 0.1 m/s (plunger speed) nd Phase Speed 30% 0. m/s (plunger speed) 0% 0.88 m/s (plunger speed) Pressure 75% 198 bar 90% 37 bar. Discussion From the above results, it can be seen that the middle sections of the samples have significantly higher porosity than the gate and the tip sections, and generally the gate sections have the lowest porosity levels compared with other sections (Fig. )..1. Effect of pressure In settings VII and VIII, the first phase speeds were the same, namely 0% (0.1 m/s) and second phase speeds were also the same 0% (0.88 m/s). Therefore, the effects of plunger stroke and consolidation pressure could be determined. It can be seen from Fig. 5 that the porosity level of setting VII is much less than that of setting VIII. This indicates that the increase of consolidation pressure can decrease the porosity level. Similar results were obtained for the gate and tip sections of settings I and II (see Table 3). The porosity level of setting I is less than that of setting II because of a higher consolidation pressure used. However, an exceptional phenomenon was observed that the middle section of setting I has higher more porosity level than that of setting II. This may be due to the effect of the stroke and requires further investigation. 3

4 .. Effect of first phase speed and second phase speed Settings III and V had the same stroke and pressure settings, namely 1 mm of stroke and 75% of pressure (198 bar). However, the first phase speed and the second phase speed were 0%, 30% for setting III and 30%, 0% for setting V respectively. Fig. shows that setting V has less porosity than setting III. It indicates that higher second phase speed and lower first speed resulted in a lower porosity level. It may be due to the fact that a lower first phase speed allows gas to easily vent out and a higher second phase speed decreases the filling time, which results in less tendency of cold shut and thus reduces the porosity level. The tip and gate sections of settings VI and IV exhibited a similar result..3. Effect of Stroke In settings V and VIII, the second phase speed and the consolidation pressure were the same, namely 0% (0.88 m/s) and 75% (198 bar) respectively. However the first phase speed and the plunger stroke were 30% (0.08 m/s), 1 mm for setting V and 0% (0.1 m/s), 5 mm for setting VIII respectively. The stroke was shorter and the first speed was lower in setting V than in setting VIII. The porosity level of setting V is much less than that of setting VIII, indicating that lower first phase speed with shorter stroke can result in less porosity (Fig. 7). A similar result can be obtained in the tip and gate sections of settings I and IV. It can be concluded from the above analysis that setting with 1 mm plunger stroke, 30% first phase speed (0.08 m/s), 0% second phase speed (0.88 m/s) and 90% consolidation pressure (37 bar) are the most promising setting in regard to the reduction of the porosity level. 5. Conclusions (1) The tip and middle sections of the cast tensile samples have higher porosity levels than the gate sections. () Higher pressure tends to reduce the porosity level. (3) A lower first phase speed in combination with a shorter plunger stroke generally results in less porosity. () A setting with 1 mm plunger stroke, 30% first phase speed (0.08 m/s), 0% second phase speed (0.88 m/s) and 90% consolidation pressure (37 bar) is the most promising setting in reducing the porosity level.. Acknowledgments The research work is part of a collaborative programme on "Precision Processing of Ultra-Light Alloys and Composites" between Nanyang Technological University and

5 Gintic Institute of Technology. The authors would like to thank Mr. W.C. Chan and Mr. K.B. Lim for preparing the samples and other staff in Gintic and technicians in the Materials Lab for their support and contributions. References [1] Cole, G. S., Can Magnesium Achieve its Potential for Large Volume Usage in Automotive Components, Ford, private communication, 1998 [] Allenby, B. R. and Richards, D. J., The Greening of Industrial Ecosystems, National Academic Press, Washington, DC, 199 [3] ASM Handbook, Vol., Properties and Selection - Nonferrous Alloys and Special-Purpose Materials, ASM International, Materials Park, OH, 198 [] Allsop, D. F. and Kennedy, D., Pressure Die Casting, Part, The Technology of the Casting and the Die, Pergamon Press, Oxford, pp & pp. 9-71, [5] Frech Technical Documentation, Oskar Frech GmbH, Germany, pp , 1997 [] Fjetsland, H. and Westengen, H., Use of SF in the magnesium industry: an environmental challenge, in Proceedings of the third International Magnesium Conference, Institute of Materials, London, 199 [7] Q50 Operation Manual, Issue 1, Leica Cambridge Ltd., Cambridge, England,

6 Fig. 1 AM0B die casting sample tip 10 middle 100 gate φ10 Fig. Tip (T), middle (M) and gate (G) for microstructural analysis

7 (a) (b) (c) Fig. 3 Microstructure for a setting VI sample. (a) gate section (b) middle section (c) tip section. 7

8 Porosity Levels (%) Setting I Setting II Setting III Setting IV Setting V Setting VI Setting VII Setting VIII Tip Middle Gate Fig. Porosity levels at different sections of a cast sample. Porosity Level (%) 0 Setting VIII Setting VII Middle Tip Gate Fig. 5 Effect of pressure on porosity levels at different sections. 8

9 Porosity Level (%) Setting V M T G Setting III (30,0) (0,30) (1st phase speed, nd phase speed) Fig. Speed vs. porosity levels Porosity levels (%) M Setting V T G (1, 30) (5, 0) (stroke, 1st phase speed) Setting VIII Fig. 7 Effect of stroke on porosity levels. 9