Comparing Low Pressure Permanent Mold Casting of Magnesium AZ91E and Aluminum A356

Transcription

1 Paper pdf, Page 1 of 13 Comparing Low Pressure Permanent Mold Casting of Magnesium AZ91E and Aluminum A356 F. Chiesa, G. Morin Centre Intégré de Fonderie et de Métallurgie, Trois-Rivières, Québec B.Duchesne Collège de Trois-Rivières, Trois-Rivières, Québec J. Baril Technologie de l Aluminium et du Magnésium, Trois-Rivières, Québec Copyright 2009 American Foundry Society ABSTRACT While the low pressure permanent mold (LPPM) casting of aluminum parts is commonplace, this process is virtually not used with magnesium alloys for reasons very difficult to apprehend. In the present study, a step casting, 152mm wide, with plate thicknesses of 6, 13 and 25mm and an automotive bell housing, 460x380x170mm in overall dimensions, were poured in a low pressure permanent mold machine (LPPM), first in aluminum A356, then in magnesium AZ91E. The same molds and gating systems were used for both metals. The differences in behavior of the two alloys were highlighted and the metallurgical quality of the castings was assessed by radiographic analysis and by measuring metallographic properties such as grain size, secondary dendrite arm spacing (DAS) and level of micro voids at different locations in the castings. Samples were also excised from the castings and tested in tension in the T6 metallurgical temper for a wide range of solidification conditions. This study highlighted the challenges that would face a LPPM aluminum foundry which would wish to produce magnesium parts. This work also compared the in-situ tensile properties of the two most widely used aluminum and magnesium foundry alloys in a casting of variable wall thickness, rather than comparing textbook values obtained from separately cast specimens. INTRODUCTION Permanent mold casting of aluminum alloys is a mature process while this process is in its infancy for magnesium alloys. In North America, shipped magnesium castings (mainly poured in alloy AZ91) represent only 2% of their aluminum counterpart (mainly A356). Also, more than 90% of these magnesium castings are pressure die cast, a process normally not suitable to produce structural castings. These figures highlight two facts: the exiguity of the magnesium market when compared to aluminum, and the fact that the permanent mold process is very little used with magnesium. There is no rational explanation for this, but for the fact that magnesium alloys are rarely chosen for structural components, as was the case for aluminum, 30 years ago. The mechanical properties of magnesium and aluminum alloys are similar, and the increasing demand for magnesium structural castings should be expected to bring about the development of the permanent mold process. A review conducted for the Structural Cast Magnesium Development Program of the United States Automotive Material Partnership (USAMP) concluded that permanent mold could be a viable processing route to produce quality components of varying size and thickness 1. Some of the problems facing the foundry when pouring magnesium in permanent molds have been addressed 2. Over the past 5 years, breakthroughs have been made on bigger magnesium castings such as the BMW engine 3 containing 12 kg of AJ62 alloy and the GM Corvette engine cradle 4 with 11kg of AE44 alloy. Magnesium presents the obvious advantage of lightness; some shortcomings of magnesium have been solved (corrosion resistance), or are presently addressed (hot shortness via grain refining); however the present main obstacle is the absence of a critical mass of foundries with magnesium casting capabilities; this results in a lack of options for the potential user of magnesium permanent mold castings. Since most aluminum «high integrity» castings are poured in permanent mold, it is fair to assume that it should become the process of choice for the production of magnesium structural castings. 627

2 Paper pdf, Page 2 of 13 In our previous studies, the comparison of gravity permanent mold casting of magnesium and aluminum has been made on laboratory tensile test samples, and later, on an automotive torque ball housing 5. The present study consists in comparing the production of step castings and automotive bell housings poured by low pressure permanent mold process (LPPM), first in aluminum A356, then in magnesium AZ91E. GEOMETRY OF THE TWO CASTINGS The step casting and the automotive bell housing are shown in Figure 1 (left and right respectively). The step casting consists of three 152mm wide plates, with thicknesses of 25, 13 and 6mm, and a 2mm thick overflow. The aluminum trimmed castings weigh 1.35kg (step casting) and 6.5kg (bell housing), with casting weight to poured metal ratios of 76% and 87% respectively. Of course, the magnesium castings are one third lighter than their aluminum counterpart. 152mm wide, 25, 13 and 6mm thick step casting 460x380x170mm automotive bell housing Figure 1: Geometry of the two castings poured successively in aluminum A356 and magnesium AZ91E THE MOLDS AND THEIR PREPARATION The two steel mold halves (40kg+60kg) for the step casting are shown in Figure 2 as installed on the LLPM machine. The mold included four air cooling lines, 10mm in diameter, and two 3mm diameter type K thermocouples, the locations of which are indicated in the model of Figure 3. When artificial cooling was applied, the air flow rate was 18m 3 /s (SPTC), corresponding to a velocity of about 10m/s in the channels, and a surface heat transfer coefficient of 450 W.m -2. K -1 ; the outlet air temperature was 175 C. 2 air cooling lines Figure 2: Step casting mold with two air cooling lines installed on the LLPM machine platen 628

3 Paper pdf, Page 3 of 13 Figure 3: Location of air cooling lines (red) and thermocouples (green) in the step casting mold The equivalent pictures for the bell housing mold are shown in Figure 4 and Figure 5. Figure 4: Bell housing mold halves installed on the LLPM machine platen Figure 5: Location of the thermocouple (left)) and air cooling lines (right) in the bell housing mold 629

4 Paper pdf, Page 4 of 13 The mold coating used with AZ91E was a commercial one, recently developed specifically for magnesium; it is applied in two steps: a red primer coating and a sodium silicate free white top coating. The nature, spraying condition and purpose of this coating have been explained in details in the literature 6. The mold was preheated at C before the coating was applied as illustrated in Figure 6. Figure 6: Preheating the step casting mold at C (left) before spraying the coating (right) In the case of magnesium, the top coat had to be rebuilt by spraying a thin white coat every 4 cycles. The coating typical thickness was in the range µm, for aluminum and magnesium as well. POURING AND EJECTING THE CASTINGS Figure 7 shows typical pressure cycles applied to the melt surface for magnesium and aluminum (step casting). The difference in the pressure level accounts for the difference in density between magnesium and aluminum. The first ramp corresponds to the filling of the transfer tube and mold. An arrest in the pressure rise at 300mB for magnesium and 450mB for aluminum will minimize flashes at the parting line. The final pressure applied corresponds to a pressure head of 3 meters of liquid metal, which explains the exceptional feeding capability of the process, as long as directional solidification is ensured. Mg AZ91E Al A356 Figure 7: Typical LPPM pressure cycle applied for magnesium and aluminum (Step casting) 630

5 Paper pdf, Page 5 of 13 The pouring temperature was always maintained in the vicinity of 750 C (±5 C) for both alloys. No melt treatment was applied to the aluminum A356 melt from primary ingots, while a hexachlorethane grain refining treatment was performed on the magnesium AZ91E melt. The ejection, shown in Figure 8 for the two castings, occurred after a time varying between 3 and 5 minutes depending on the run. Figure 8: Ejection of a magnesium step casting (left) and of a magnesium bell housing (right) COMPARING THE ALUMINUM A356 AND AZ91E MAGNESIUM STEP CASTINGS THERMAL REGIME IN THE MOLD Two temperatures were recorded at the top and bottom of the step casting mold (see position in Figure 3). The thermocouples recordings are shown in Figure 9. The aluminum run reported started at a rate of one casting every 4 minutes; the cooling channels were activated after 20min, producing a drop of temperature of 40 C in the mold; after 65 min, the rate of casting was increased to one every 3 min, provoking an increase in mold temperature and an increased temperature gradient between the bottom and top of the mold. The air cooling channels were activated throughout the magnesium campaign, and the rate was one casting ejection every 3 minutes. The graphs in Figure 9 show that under similar casting conditions (one casting per 3 min with air cooling), the mold runs at a temperature C with aluminum and C with magnesium. temperature, C Aluminum A356, step casting cooling starts at 20min - speed up at 65min time. min temperature, C Mag AZ91E, step casting (with air cooling) time, min Figure 9: Temperature cycling at the top and bottom of the mold for the step casting 631

6 Paper pdf, Page 6 of 13 MICROSTRUCTURAL PROPERTIES IN THE A356 AND AZ91E LPPM STEP CASTINGS 25mm thick plate ALUMINIUM A356 MAGNESIUM AZ91E 25mm thick plate DAS: 40 m microporosity 0.38% grain size: 88 m microshrinkage:0.73% 13mm thick plate 13mm thick plate DAS : 27 m microporosity: 0.02% grain size: 80 m microshrinkage: 0.15% 6mm thick plate 6mm thick plate DAS= 25.0 m microporosity: 0.01% grain size: 77 m microshrinkage: 0.43% Figure 10: Micrographs in the three plates of the aluminum A356 and magnesium AZ91E step castings 632

7 Paper pdf, Page 7 of 13 Typical microstructures in the 3 steps of the aluminum A356 and magnesium AZ91E castings are shown in Figure 10. The secondary dendrite arm spacing (DAS) varied from 25 to 40µm in the A356 casting while the grain size did not vary much (from 77 to 88µm) in the AZ91E plates. In the aluminum alloy too, the grain size was not found to depend much on the solidification time; it varied between 800 and 900 m. It is not convenient to measure the DAS in grain refined AZ91E as only a few dendrite arms are contained in one grain. In magnesium alloys, grain size should be the indicator of metallurgical quality while DAS is a better choice to assess metallurgical quality in aluminum-silicon alloys. The aluminum step castings exhibited less % micro-voids (from 0.01 to 0.38%) than the magnesium ones (0.15 to 0.73%). Tests were performed on non grain refined AZ91E, so that dendrite arm spacing could be measured. These tests also allowed to assess the effectiveness of the grain refining treatment. It was found that the hexachlorethane refining of the magnesium melt had divided the grain size by 4 in the 25mm plate, and by 2 in the 6mm plate. Similarly to what takes place in Al-Si alloys, the secondary dendrite arm spacing (DAS) in AZ91E decreases with plate thickness (hence with solidification time). For equivalent solidification times, the DAS is smaller in magnesium: 17, 21 and 27 m in the 6, 13 and 25mm plates respectively, versus 25, 27 and 40 m for aluminum. MECHANICAL PROPERTIES IN THE 3 PLATES OF THE STEP CASTINGS (A356 and AZ91E) Flat tensile test bars were excised from two aluminum step castings as indicated on the sketch of Figure 11. Thus, 5 bars, 6mmx13mm in section, were cut from the 25mm thick plate, 4 bars, 13mmx13mm in section from the 13mm plate and 4 bars 6mmx13mm in section, from the 6mm plate. Similar tensile bars were cut from two magnesium step castings. Figure 11: Flat tensile test bars excised from the 3 plates of the step casting (red samples from 25mm plate) The tensile properties were measured after a standard T6 heat treatment was applied to the aluminum (ASTMB597) and magnesium (ASTM B661) castings; the results are listed in Table 1; the values are the average of at least 6 tensile results. Because the gage length was the same for all samples (25mm), the measured elongation on the bigger 13mx13mm test bars might be higher than they would have been on a 6mmx13mm bar. Table 1 Tensile Properties in the 25mm, 13mm and 6mm Plates of the A356 and AZ91E Step Castings A356-T6 versus AZ91E-T6 Average tensile properties (T6 temper) Plate thickness 25mm 13mm 6mm Alloy A356 AZ91E A356 AZ91E A356 AZ91E Yield strength 204 MPa 107 MPa 216 MPa 105 MPa 213 MPa 107 MPa Ultimate tensile stength, UTS 253 MPa 198 MPa 282 MPa 208 MPa 302 MPa 213 MPa Elongation in 25mm, E 3.2% 4.1% 5.5% 4.9% 10.5% 5.8% Quality Index UTS+150 Log E 329 MPa 290 MPa 393 MPa 312 MPa 455 MPa 328 MPa The results in Table 1 show that the yield strength does not vary much with the thickness of the plate for both aluminum A356 and magnesium AZ91E, while the elongation and ultimate tensile strength are higher when the solidification times are short (i.e. for the thinner plates). In order to lump the tensile properties into one convenient number, the notion of Quality index defined for aluminum 7 was used. 633

8 Paper pdf, Page 8 of 13 COMPARING THE ALUMINUM A356 AND AZ91E MAGNESIUM LPPM POURED BELL HOUSINGS THERMAL REGIME IN THE MOLD Artificial air cooling was activated in some of the bell housing runs. However, the results reported in this paper were obtained when no artificial cooling was applied. For both alloys, the time elapse between pours was maintained as close to 5 minutes as possible. With magnesium, ejection occurred 2 minutes after filling instead of 3 minutes with aluminum; however, a 0.5% SF 6 protective gas must be flushed prior to pouring magnesium. Two minutes were allowed for mold opening, ejection, mold cleaning and coating retouch when necessary; overall, the production rate was higher for magnesium. The temperature was recorded inside the mold, at a location indicated by a red dot on Figure 5 (left). The recordings for aluminum A356 and magnesium AZ91E are shown in Figure 12. Aluminum A356, bell housing Magnesium AZ91E, bell housing temperature, C time, min temperature, C time, min Figure 12: Temperature cycling in the mold for the bell housing Similarly to what was observed in the step casting runs, the equilibrium temperature is some 50 C higher when pouring aluminum. However, due to the bigger size of the bell housing, it takes about 6 pours to bring the mold to its equilibrium temperature, versus 4 for the step casting mold. STRUCTURAL AND MECHANICAL PROPERTIES IN THE A356 AND AZ91E BELL HOUSINGS Metallographic samples were excised from an aluminum A356 and a magnesium AZ91E bell housings at locations T, F and W shown in Figure 13. The predicted solidification times for the aluminum A356 bell housing are also indicated by the color code in the same figure. Figure 13: Predicted solidification times in aluminum A356 - Location of excised samples (F, T and W) 634

9 Paper pdf, Page 9 of 13 In the aluminum parts, the secondary dendrite arm spacing (DAS) and level of microporosity were measured by image analysis; similarly, the grain size and level of microshrinkage were measured at the same locations in the magnesium parts. The micrographs and the results are shown in Figure 14 for both alloys. Contrary to what was observed in the step castings, the microvoid level is less in magnesium. Quite expectedly, the DAS in aluminum A356 is increased at higher solidification times; however, in keeping with what was observed in the step casting, the grain size does not depend much on wall thickness in magnesium AZ91E. ALUMINIUM A356 MAGNESIUM AZ91E 25mm bracket (T) 25mm bracket (T) DAS: 37 m microporosity 0.52% grain size: 62 m microshrinkage:0.06% 13mm flange (F) 13mm flange (F) DAS : 29 m microporosity: 0.26% grain size: 77 m microshrinkage: 0.19% 6mm thick wall (W) 6mm thick wall (W) DAS= 27 m microporosity: 0.36% grain size: 66 m microshrinkage: 0.17% Figure 14: Micrographs at locations T, F and W of the A356 and AZ91E bell housings 635

10 Paper pdf, Page 10 of 13 As sketched in Figure 15, 6mm diameter tensile test bars were machined at locations F and T while 13mmx6mm flat specimens were cut at location W. Figure 15: Location of excised tensile bars in the aluminum and magnesium bell housings The results of the tensile tests in the T6 condition are listed in Table 2. In this table, the term fineness refers to the DAS for aluminum A356 and to the grain size for magnesium AZ91E. Table 2 Tensile Properties at Locations T, F and W of thea356 and AZ91E Bell Housings T6 condition YS UTS El fineness/microvoid Q T A MPa 260 MPa 3.9% 37um/0.52% 349 MPa T AZ91E 182 MPa 240 MPa 6.5% 62µm/0.06% 362 MPa F A MPa 270 MPa 5.1% 29um/0.26% 376 MPa F AZ91E 150 MPa 222 MPa 3.0% 77µm/0.19% 294 MPa W A MPa 279 MPa 5.7% 27um/0.36% 392 MPa W AZ91E 145 MPa 242 MPa 2.3% 66µm/0.17% 296 MPa Similarly to what had been observed in the step castings, the tensile properties are higher in aluminum A356. Also, the strength is higher in faster solidifying sections for aluminum A356; however, in the magnesium AZ91E castings, the highest tensile properties were observed in the thicker 25mm bracket (Sample T). In view of these unexpected results, tensile tests were repeated on 4 additional housings and it was confirmed that surprisingly high tensile strengths and elongations were obtained in this part of the AZ91E bell housing. It is noteworthy that this part of the casting is very well fed, resulting in an extremely low micro-shrinkage (0.06% versus 0.19% and 0.17% at locations F and W); however, it cannot totally explain the extremely high tensile properties observed in the 25mm thick section T. 636

11 Paper pdf, Page 11 of 13 DIFFERENCES IN CASTING CHARACTERISTICS OF ALUMINUM A356 AND MAGNESIUM AZ91E In spite of the fact that the mold runs at a lower temperature with magnesium, the capability of magnesium AZ91E in filling thin sections was always superior to that of aluminum A356; this is illustrated by the picture in Figure 16, where the 1mm thick vent of the step casting filled very easily when pouring magnesium, which was not the case when pouring aluminum. Figure 16: Step casting showing the good filling characteristics of magnesium AZ91E The original bell housing design (up to now cast in sand mold) exhibited hot spots when poured by the low pressure process, as indicated by the simulation at le left of Figure 17. These hot spots appeared as slumped surfaces in aluminum A356 and under the form of concentrated shrinkage cavity (piping) in the AZ91E castings. predicted hot spots aluminum A356 magnesium AZ91E Figure 17: Difference in the appearance of hot spot defects in the aluminum and magnesium housings This may be explained by the higher ability for AZ91E to provide interdendritic flow until the last stages of solidification, while in aluminum A356, the start of eutectic solidification hampers this process after 60% of melt has solidified. Also the higher thermal conductivity of aluminum widens the mushy zone with a similar consequence. In order to eliminate the hot spots identified in the modeling and observed on the castings, four modifications were made to the initial design. Excess metal masses were trimmed and padding was added at selected locations in order to ensure directional solidification. Contrary to the gravity or tilt pour process, risering goes against the nature of the LLPM process; consequently, for a given casting geometry, only local artificial chilling and mold coating can be used to ensure proper feeding of the casting. 637

12 Paper pdf, Page 12 of 13 Some hot cracking was observed in the early magnesium castings poured into a cold mold, which was not the case in the aluminum castings. Typical hot tears in a AZ91E bell housing are shown in Figure 18. CONCLUSIONS Figure 18: Appearance of hot tears in the early run in pours of the magnesium AZ91Ecampaign By pouring series of step castings and automotive housings in aluminum A356 and magnesium AZ91E by the low pressure permanent mold process, the following conclusions could be drawn: 1) For all the castings investigated, the level of micro void was always much lower than 1% in both aluminum A356 and magnesium AZ91E, insuring an excellent radiographic quality for this type of defect (Frame 3 or less per ASTM E155) 2) The tensile properties were higher in the aluminum A356 castings by about 20%. Tensile properties were surprisingly high in the thick (25mm) bracket of the magnesium AZ91E bell housings 3) When properly grain refined (grain size < 100µm), hot tearing was not a problem in AZ91E castings. 4) The measurement of the secondary dendrite arm spacing DAS is only practical in aluminum A356; DAS is mathematically tied to the local solidification time. On the contrary, in magnesium AZ91E, the grain size is a more convenient marker of the metallurgical quality than the DAS; 5) In spite of the lower mold temperature, magnesium AZ91E exhibited a better mold filling capability ( fluidity ) than aluminum A356 6) Shrinkage cavities are more concentrated in magnesium AZ91E parts; they are more spread out in aluminum A356 castings and often appear as depressed surfaces ACKNOWLEDGMENTS The authors wish to acknowledge the contribution of Ministère du Développement Économique, de l Innovation et de l Exportation from Québec and of the Canadian Foundation for Innovation to the CIFM infrastructure which made this project possible. Part of the operating expenses for this study were covered by the Québec government Programme d Aide à la Recherche Technologique. The authors are also indebted to TMA (Technologie de l Aluminium et du Magnésium) for making their facilities and personnel available for the casting runs. 638

13 Paper pdf, Page 13 of 13 REFERENCES 1. Emadi D., Davies K.G., Saddayapan M., Sahoo M.; A Review of Mechanical Properties and Casting Process Capabilities for Magnesium Alloys ; MTL Report (TR-R), Natural Resource Canada (2003) 2. Bergeron R., Chiesa F., Morin G., Experience in Casting AZ91E in Permanent Molds, Proceedings of the 13th Magnesium Automotive and end User Seminar, 11 p., September 22-23, 2005, Aalen, Germany, (2005) 3. Kluting M., Landerl C.; The new BMW Inline Six-cylinder Spark-ignition Engine, Concept and construction, MTZ Worldwide, 5p., (Nov. 2004) 4. Li N., Osborne R.J, Cox B.M., Penrod D.E.; Structural Cast Magnesium Engine Cradle, AFS Transactions, paper , 13p, (2006) 5. Chiesa F, Duchesne B., Morin G., Baril J.; Permanent Mold Casting of Magnesium versus Aluminum, Fonderie- Fondeur d aujourd hui, Vol. 266, 13p., (June-July 2007) 6. LaFay V.C., Robison S.; Selection and Application of Permanent Mold Coatings for Magnesium, AFS Transactions, Vol. 114, paper , 13p, (2007) 7. Jacob, S.; Quality Index in the Prediction of Properties of Aluminum Castings, AFS Transactions, Silver Anniversary paper, Vol. 107, pp (1999) 639

14