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Process optimisation for a squeeze cast magnesium alloy
Journal of Materials Processing Technology 145 (2004) 134–141
Process optimisation for a squeeze cast magnesium alloy M.S. Yong a,∗ , A.J. Clegg b a
Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore b Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK Accepted 28 July 2003
Abstract The paper reports the influence of key process variables on zirconium-free (RZ5DF) and zirconium-containing (RZ5) magnesium–zinc– rare earths alloys by examination of the microstructure and mechanical properties of specimens produced by squeeze casting. Applied pressures from 0.1 to 120 MPa were considered and it was established that an applied pressure greater than 40 MPa was required to suppress the formation of microporosity. Increasing the applied pressure from 0.1 to 60 MPa, produced a reduction in cell size from 127 to 21 m. The metal pouring and die temperatures considered in the investigation were within the range of 720–780 and 225–275 ◦ C, respectively. It was established that the intermediate die temperature of 250 ◦ C produced the highest tensile properties and that the presence of zirconium did not improve the as-cast properties of the squeeze cast alloy. The highest UTS value obtained for the zirconium-free RZ5DF alloy was 198 MPa compared to 195 MPa for the ZR5 alloy. These UTS value were approximately 50% higher than those for material cast under atmospheric pressure. © 2003 Elsevier B.V. All rights reserved. Keywords: Magnesium alloys; Squeeze casting; Mechanical properties; Grain refinement
1. Introduction Magnesium alloys have properties that make them attractive for certain applications. However, even complex alloys have limitations in respect of strength, stiffness and abrasion resistance. It is possible that these limitations can be overcome by using metal matrix composites (MMC) with a magnesium-based alloy. Although several manufacturing processes can be used to produce such composites, the casting route is especially attractive given its ability to produce complex shapes. However, in order to obtain the benefits of reinforcement, the casting process must deliver castings that are free of defects such as gas or shrinkage porosity. Squeeze casting is capable of delivering such castings and consequently has been used to produce cast MMC. Before advocating squeeze casting for MMC production, however it is necessary to understand the influence of process variables on the base alloy and that was the purpose of the work described in this paper. ∗ Corresponding author. Present address: Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore. E-mail address: [email protected] (M.S. Yong).
0924-0136/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2003.07.006
The majority of investigations to evaluate the effect of squeeze casting parameters have considered aluminium alloys and their composites. The most important parameters in squeeze casting have been identified as melt temperature, melt quality (i.e. the absence of oxide films and inclusions) and quantity, die temperature, applied pressure, and duration of applied pressure [1,2]. In the case of composites, infiltration velocity and preform temperature can be added to the list [3–5]. These variables are equally relevant to magnesium alloys and their composites was confirmed by several researchers [6–13]. A significant difference between magnesium and aluminium is the former’s lower volumetric heat of fusion, which means that solidification should occur at a faster rate [14]. An understanding of the effects of process variables is essential because the structure and properties of alloys can be optimised without recourse to expensive alloying elements or nucleating agents [6]. This paper reports an investigation of the influence of zirconium grain refinement, applied pressure, and pouring and die temperatures on the as-cast properties of specified magnesium-based alloys. This work was a prerequisite to that on composites that will be reported in a future paper.
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2. Experimental methodology Two magnesium alloys were used: commercially available RZ5, which contains zirconium, and a zirconium-free version (hereafter referred to as RZ5DF). Their compositions are presented in Table 1. The effect of applied pressure on the RZ5DF alloy was first evaluated whilst maintaining metal pouring temperature at 750 ◦ C, die temperature at 250 ◦ C, duration of applied pressure at 25 s, and delay before application of pressure at 4 s. Following establishment of the optimum applied pressure (at 60 MPa), the influence of metal pouring and die temperatures was investigated whilst the applied pressure, delay and duration were kept constant. The pouring temperatures investigated were 720, 750 and 780 ◦ C and the die temperatures were 225, 250 and 275 ◦ C. The selection of the experimental ranges for pouring and die temperatures was guided by the literature. Pure magnesium melts at 640 ◦ C and the recommended pouring temperatures for the alloys is 720–800 ◦ C [15]. A die temperature range of 200–300 ◦ C is advocated for commercial die-casting [1,16]. Test casting: The test casting was a rectangular plate with a length of 126 mm, a width of 75 mm and a depth of 16 mm. Melt processing: The alloys were melted in an electric resistance furnace using a steel crucible, the fluxless method and an argon gas cover. The die was coated with boron nitride suspended in water to protect it from excessive wear. The alloys were cast using the direct squeeze casting process. The squeeze casting process was described in previous papers [17,18]. Tensile testing: Tensile tests were conducted on a 50 kN Mayes testing machine using position control and the modulus was determined using a strain gauge attached to the gauge section parallel to the direction of tensile loading. Modified test specimens were machined according to BS18 (1987) and Magnesium Elektron Ltd. RB4 specifications [19]. The tensile properties were determined from as-cast material tested at ambient temperature. Hardness testing: Hardness measurements were conducted using the Rockwell B scale in preference to Vickers hardness testing as the former provided improved consistency. The locations of hardness measurements conducted on the castings are shown in Fig. 1. Metallography: Metallographic samples were prepared using standard techniques. Specimens were finally ground using 1000 grit silicon carbide paper, prior to polishing with six micron and finally 1 m diamond paste. A 5% Table 1 Specification of magnesium alloys used in the investigation
RZ5 alloy RZ5DF alloy
Zinc (%)
Rare earths (%)
Zirconium (%)
Magnesium
4.2 4.2
1 1
0.7 –
Balance Balance
Fig. 1. Locations of hardness measurements (each dot represents the position of a hardness measurement) taken in both longitudinal and transverse directions.
nital etchant was used for the RZ5 alloy but an aceticglycol etchant was found to be necessary for the RZ5DF alloy. Cell size: The cell size was established using the intersection method. Five areas were selected at random and twenty-one measurements of cell size were taken for each area. The average value for the 105 readings was determined. The results and observations from this experimental programme are presented in the following section, which also includes a comparison of tensile strength, material hardness and metallographic structures.
3. Results and observations The results are reported in the sequence that the experiments were conducted. As the first objective of the investigation was to establish an optimum applied pressure level, these experiments were the first to be conducted using the zirconium-free RZ5DF alloy. Once this level had been established, a second series of experiments was conducted using the RZ5DF alloy to evaluate the influences of metal pouring and die temperature. A third series of experiments was conducted using the zirconium-containing RZ5 alloy. By comparing the results of the second and third series of experiments, the influence of zirconium could be established. 3.1. Series 1 experiments: the influence of applied pressure 3.1.1. Tensile properties The effects of applied pressure on the tensile properties of squeeze cast RZ5DF alloy are presented in Fig. 2. It can be seen that the highest tensile properties were obtained with an applied pressure of 100 MPa and that the lowest were produced at atmospheric pressure (0.1 MPa), i.e. by gravity die-casting. The graph shows that there is a significant increase in tensile properties as the applied pressure is increased to 60 MPa but that after this point, a further increase in applied pressure produces little further improvement.
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3.2. Series 2 experiments: the evaluation of squeeze cast RZ5DF alloy 3.2.1. Tensile properties The relationships between pouring temperature, die temperature and tensile properties are shown in Figs. 6 and 7.
Fig. 2. Effects of applied pressure on the tensile properties of squeeze cast RZ5DF alloy.
3.1.2. Hardness The hardness values along the longitudinal and transverse directions of the RZ5DF alloy castings produced with different applied pressures are shown graphically in Fig. 3. The results show that applied pressure appears to have little effect on hardness, since the majority of values fall within the range 14–20 HRB. However, close inspection of the detailed hardness values of castings produced using low applied pressures (0.1, 20 and 40 MPa) revealed that these had the lowest hardness values. Low hardness was expected in these castings because of the presence of porosity.
25
-
15 10 5 0.1
20
40 60 80 Applied Pressure (MPa)
100
-
-
-
-
-
0
-
20
-
Rockwell Hardness (HRB)
3.1.3. Metallography Metallography was conducted to examine the influence of applied pressure on the cast structure. Examination of the RZ5DF alloy specimens produced at different applied pressures revealed that a critical pressure greater than 40 MPa was required to suppress microporosity. Selected microstructures that show the effect of applied pressure on structure are shown in Fig. 4. These are complemented by the graphical representation of the relationship between applied pressure and cell size presented in Fig. 5. These figures show a pronounced reduction in cell size for squeeze cast material.
120
Fig. 3. The average material hardness along the longitudinal and transverse directions of the squeeze cast RZ5DF alloy, cast with constant pouring temperature of 750 ◦ C and die temperature of 250 ◦ C.
3.2.2. Metallography Metallographic specimens were examined to evaluate the effects of pouring and die temperature on the RZ5DF microstructure. The examinations were conducted on specimens selected from those that had the highest, intermediate and lowest UTS values. The structure associated with the highest UTS value is shown in Fig. 8. 3.3. Series 3 experiments: the evaluation of squeeze cast RZ5 alloy 3.3.1. Tensile properties The relationships between pouring temperature, die temperature and tensile properties are shown in Figs. 9 and 10. 3.3.2. Metallography Metallographic examinations were conducted to evaluate the effects of pouring and die temperature on the squeeze cast RZ5 microstructure. These examinations were conducted on specimens selected from those that produced the highest, intermediate and lowest UTS values. Fig. 11 shows the structure associated with the highest UTS value.
4. Discussion As in all casting processes, the rate of solidification in squeeze casting is determined primarily by the rate at which heat is transferred by the metal to the die. In most casting processes, heat flow is controlled to a significant extent by resistance at the metal–mould interface. The thickness of the solid metal that forms is typically a parabolic function of time, being initially very rapid and then decreasing as the mould is heated [14]. However, in squeeze casting, the pressure applied through the punch promotes an intimate contact between the metal and die and this largely overcomes the resistance to heat flow. According to Campbell [20], the transfer of heat across the interface can be enhanced significantly in squeeze casting. For squeeze cast aluminium, the figure may be up to 60,000 W/m2 K compared to the more normal values of 100–1000 W/m2 K. The heat to be transferred consists of the volumetric heat of fusion (the major component) and the superheat. Higher pouring temperatures increase the superheat contribution. It should be noted that whilst the volumetric heat of fusion for magnesium is approximately 15% lower than that for aluminium, the specific heat for the liquid metal is approximately 15% higher for magnesium than aluminium. The temperature of the die influences its capacity to absorb heat. However, it is the die’s
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Fig. 4. Optical microstructures for squeeze cast RZ5DF produced with various pressure.
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120 100 80 60 40 20 -
12 10 8 9 8 7 6 5 4 -
80
100
120
Fig. 5. Influence of applied pressure on the cell size of the squeeze cast RZ5DF alloy.
3
7500C 7800C
2250C
-
-
-
60
7200C
-
40
Applied Pressure (MPa)
Pouring Temp (0C)
-
20
% Area Reduction
0.1
-
0
-
% Elongation
14
-
Grain Size (µm)
140
2500C
2750C
Fig. 7. The effects of pouring and die temperature on percentage elongation and percentage area reduction of RZ5DF alloy specimens tested at ambient temperature.
195 190 185 180 175 170 165 205 200
2250C
-
160
-
Pouring Temp (0C) 7200C 7500C 7800C
-
Ultimate Tensile Strength (MPa)
Die Temperature (degree C)
2500C
2750C
Die Temperature (degree C) Fig. 6. The effects of pouring and die temperatures on ambient temperature UTS (RZ5DF alloy).
thermal diffusivity that exerts the major influence on the rate of solidification [14]. From the results, it would appear that the optimum applied pressure range is from 50 to 100 MPa. The low UTS values produced by applied pressures below 40 MPa are primarily the consequence of porosity present in the castings because the pressure acting on the molten metal was insufficient for its elimination (Fig. 2). As the applied pressure was increased to 60 MPa, not only was porosity reduced, the rate of cooling was increased and the cell size reduced with a concomitant improvement in tensile properties. This is emphasised when the microstructures of castings produced by different applied pressures, shown in Fig. 4, are compared. An average cell size of 127 m was produced by gravity
Fig. 8. Optical microstructure of the squeeze cast RZX5DF alloy with the highest UTS of 198 MPa at ambient temperature, providing an average cell size of 18 m.
-
-
205 200 195 190 185 180 175 170 165 160
-
Ultimate Tensile Strength (MPa)
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2750C
2500C
2250C
Die Temperature (degree C) Fig. 9. The effects of pouring and die temperature on UTS at ambient temperature (RZ5 alloy).
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die-casting at atmospheric pressure (0.1 MPa) but the cell size was reduced to 21 m at an applied pressure of 60 MPa, a significant six-fold reduction in cell size. The reduction in cell size was attributed to the intimate contact between the melt and die wall that promoted rapid heat transfer, as applied pressure was increased. Solidification is a process of nucleation and growth and this process is influenced by the rate at which heat is transferred which in turn influences the structure and properties of the casting. In squeeze casting, we might expect solidification to commence as soon as the metal contacts the die, i.e. before pressure is applied. Once pressure is applied, heat transfer is promoted and concurrently the temperature of the metal increases, as predicted by the Clausius–Clapeyron equation and this might, in combination with the long
11 10 9 8 8 7 6 5 -
Pouring Temp (0C) 7200C 7500C 7800C
2250C
2500C
-
4
-
-
% Area Reduction
% Elongation
12
2750C
Die Temperature (degree C) Fig. 10. The effects of pouring and die temperature on percentage elongation and percentage area reduction of RZ5 alloy specimens tested at ambient temperature.
Fig. 11. Optical microstructure of the squeeze cast RZ5 alloy with the highest UTS of 195 MPa at ambient temperature, providing an average cell size of 21 m.
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freezing range of the alloys, be expected to promote constitutional undercooling. Such conditions are normally conducive to the formation of a dendritic structure but the copious nucleation promoted by the massive chilling effect of squeeze casting [7] produces an extremely fine casting structure. The results for the RZ5DF alloy suggest that the highest UTS was associated with the higher pouring temperature (780 ◦ C) and the intermediate die temperature of 225 ◦ C. A steep temperature gradient, a consequence of combining a high pouring temperature with a low die temperature, should yield a fine microstructure and produce higher mechanical properties [21–24]. Conversely, castings produced with a shallow temperature gradient are likely to have a large uniform cell structure, which will generally lead to lower mechanical properties. However, the results for the RZ5 alloy suggest that, although the intermediate die temperature was still as important, the highest UTS was obtained when this was combined with the intermediate pouring temperature (750 ◦ C). This suggests that a severe temperature gradient is less important because of the refining effect of the zirconium. 4.1. The role of zirconium By comparing the results generated by the series 2 and 3 experiments, it is possible to provide a view on the need for zirconium in a magnesium–zinc–rare earths alloy intended for processing by squeeze casting. Zirconium, in excess of its solubility limit, is used to produce a grain refining effect when the alloy is sand cast and consequently subjected to a relatively slow cooling rate. Zirconium in magnesium is the most effective grain refiner in commercial use [14]. However, its presence adds to the cost of the alloy and requires process controls and procedures that counteract the tendency for gravity segregation of the zirconium. It is possible that more than one mechanism is at work. However, the most likely is a peritectic reaction in which separating zirconium particles react with the liquid to acquire a layer of zirconium-enriched solid solution that serves as nuclei [24]. When the number of nuclei is large, crystallisation proceeds from a large number of points. The presence of zirconium in magnesium produces a fine equiaxed cell structure with typical cell sizes of 30–50 m in sand castings and this generally leads to higher mechanical properties [25]. However, the values of the cell size obtained in this investigation were in the range of 18–32 m, which is almost half that reported above. This may be due to the speed of solidification in a squeeze casting that is faster in comparison to that for sand castings. Differences in the cell shapes were observed between castings produced with and without the addition of zirconium. These differences can be seen by inspecting Fig. 11 (with zirconium, i.e. RZ5 alloy) and Fig. 8 (without zirconium, i.e. RZ5DF alloy). It can be seen from the figures that the zirconium addition caused the individual cells to assume a more regular and rounded form.
Contrary to the anticipated effects of a grain refinement addition, reported in the literature [25–28], the metallographic examinations did not show a significant difference in cell size. Castings produced with the addition of zirconium (RZ5 alloy) contained cell sizes which ranged from 21 to 26 m whereas those without (RZ5DF alloy) contained cell sizes ranging from 18 to 32 m. The addition of zirconium to the RZ5 alloy had little effect because the process of squeeze casting refined the cell structure. It can be concluded that the use of grain refinement in squeeze casting is unnecessary. This research has shown that the cell structure can be manipulated by such process variables as applied pressure, pouring and die temperature. This confirms Chadwick’s view that the microstructure can be controlled by controlling casting variables alone and without recourse to a nucleating agent [6].
5. Conclusions 1. The pressure applied in squeeze casting promotes rapid solidification and a refined cell structure. Increasing the applied pressure beyond 60 MPa provided little improvement in the tensile properties of squeeze cast RZ5DF alloy. An applied pressure of 60 MPa was sufficient to eliminate all traces of shrinkage and gas porosity within the casting. Metallographic examination of the castings revealed that the cell size reduced from 127 to 21 m when the applied pressure was increased from 0.1 to 60 MPa. 2. It is possible to achieve comparable tensile properties in the zirconium-free RZ5DF alloy to those in RZ5 alloy grain refined with a zirconium addition by selecting appropriate processing parameters. The highest UTS value obtained in the zirconium-free RZ5DF alloy was 198 MPa compared to 195 MPa for the RZ5 alloy. These values are significantly higher (approximately 50%) than those obtained when the alloys were cast at 0.1 MPa. Acknowledgements Dr. Yong gratefully acknowledges the receipt of an Overseas Research Students’ Award and a Loughborough University Research Studentship.
References [1] G.A. Chadwick, T.M. Yue, Principles and applications of squeeze casting, Met. Mater. 5 (1) (1989) 6–12. [2] G.A. Chadwick, C.A. Stubbington, High quality squeeze casting of monolithic and reinforced aluminium alloys, Foundryman 84 (12) (1991) 471–474. [3] H. Fukunaga, K. Goda, Fabrication of fiber reinforced metal by squeeze casting, Bull. JSME 27 (228) (1984) 1245–1250.
M.S. Yong, A.J. Clegg / Journal of Materials Processing Technology 145 (2004) 134–141 [4] H. Fukunaga, Processing aspects of squeeze casting for short fiber reinforced metal matrix composite castings, Adv. Mater. Manuf. Process. 3 (4) (1988) 669–687. [5] H. Fukunaga, Squeeze casting processes for fiber reinforced metals and their mechanical properties, in: Proceedings of the Advances in Cast Reinforced Metal Composites, Chicago, September, ASM International, Metals Park, OH, 1988, pp. 101–108. [6] G.A. Chadwick, Squeeze casting of magnesium alloys and magnesium-based metal matrix composites, in: Proceedings of the Magnesium Technology, London, November, The Institute of Metals, 1986, pp. 75–82. [7] T.U. Ha, Squeeze casting of magnesium-based alloys and their metal matrix composites, Ph.D Thesis, University of Southampton, 1988. [8] K. Purazrang, P. Abachi, K.U. Kainer, Investigation of the mechanical behaviour of magnesium composites, Composites 25 (4) (1994) 296– 302. [9] K. Purazrang, P. Abachi, K.U. Kainer, Mechanical behaviour of magnesium alloy MMCs produced by squeeze casting and powder metallurgical techniques, Comp. Eng. 3 (6) (1993) 489–505. [10] T. Wada, T. Shinkawa, S. Kamado, Y. Kojima, Effect of fiber volume fraction on microstructures and tensile properties of alumina short fiber reinforced by Mg–4 mass% Zn–RE alloy composites, J. Jpn. Inst. Light Met. 45 (9) (1995) 510–515. [11] S. Kamado, T. Shinkawa, T. Wada, Y. Kojima, Structure and mechanical properties of Mg–Zn–Ca alloy composites reinforced with aluminium borate whisker, J. Jpn. Inst. Light Met. 46 (2) (1996) 71–76. [12] O. Ottinger, G. Grau, R. Winter, R.F. Singer, The effect of aluminium additions on the interfacial microstructure and mechanical properties of C/Mg composites, in: K. Street, A. Poursatis (Eds.), Proceedings of the 10th International Conference on Composite Materials (ICCM10), vol. VI, Vancouver, Canada, August 1995, pp. 447–454. [13] D. Wurm, O. Ottinger, R.F. Singer, The influence of carbon fiber type and stress ratio on the fatigue behaviour of unidirectionally reinforced C/Mg composites, in: K. Street, A. Poursatis (Eds.), Proceedings of
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the 10th International Conference on Composite Materials (ICCM10), vol. I, Vancouver, Canada, August 1995, pp. 537–544. M.C. Flemings, Solidification Processing, McGraw-Hill, New York, 1974. A.W. Brace, F.A. Allen, Magnesium Casting Technology, Chapman & Hall, London, 1957. Working Group Report, Second Report of Institute Working Group T20 Casting Processes, The Foundryman 87 (11) (1994) 386–390. M.S. Yong, A.J. Clegg, Squeeze casting of magnesium-based alloys and composites, in: Proceedings of the Conference on Magnesium Alloys and Their Applications, Wolfsburg, Germany, April 1998, pp. 521–526. M.S. Yong, A.J. Clegg, Evaluation of squeeze cast magnesium alloy and composite, Foundryman 92 (3) (1999) 71–75. M.S. Yong, Process optimisation of squeeze cast magnesium–zinc– rare earth alloys and short fibre composites, Ph.D Thesis, Loughborough University, 1999. J. Campbell, Castings, Butterworths/Heinemann, Oxford, 1991. B. Chalmers, Principles of Solidification, Wiley, New York, 1964. E.F. Boultbee, G.A. Schofield, Typical Microstructures of Cast Metals, IBF Publications, Birmingham, 1981. R.E. Lewis, A. Joshi, H. Jones, Rapid solidified magnesium alloys for high-performance structural applications a review, in: Proceedings of the Processing of Structural Metals by Rapid Solidification, Florida, October, ASM International, Metals Park, OH, 1986, pp. 367–378. I.J. Polmear, Light Alloys, Edward Arnold, London, 1981. L.B. Duffy, Magnesium alloys: the light choice for aerospace, Mater. World 4 (3) (1996) 127–130. A. Luo, M.O. Pekguleryuz, Review of cast magnesium alloys for elevated temperature applications, J. Mater. Sci. 29 (1994) 5259– 5271. A.W. Brace, F.A. Allen, Magnesium Casting Technology, Chapman & Hall, London, 1957. E.F. Emley, Principle of Magnesium Technology, Pergamon Press, Oxford, 1966.
Process optimisation for a squeeze cast magnesium alloy M.S. Yong a,∗ , A.J. Clegg b a
Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore b Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK Accepted 28 July 2003
Abstract The paper reports the influence of key process variables on zirconium-free (RZ5DF) and zirconium-containing (RZ5) magnesium–zinc– rare earths alloys by examination of the microstructure and mechanical properties of specimens produced by squeeze casting. Applied pressures from 0.1 to 120 MPa were considered and it was established that an applied pressure greater than 40 MPa was required to suppress the formation of microporosity. Increasing the applied pressure from 0.1 to 60 MPa, produced a reduction in cell size from 127 to 21 m. The metal pouring and die temperatures considered in the investigation were within the range of 720–780 and 225–275 ◦ C, respectively. It was established that the intermediate die temperature of 250 ◦ C produced the highest tensile properties and that the presence of zirconium did not improve the as-cast properties of the squeeze cast alloy. The highest UTS value obtained for the zirconium-free RZ5DF alloy was 198 MPa compared to 195 MPa for the ZR5 alloy. These UTS value were approximately 50% higher than those for material cast under atmospheric pressure. © 2003 Elsevier B.V. All rights reserved. Keywords: Magnesium alloys; Squeeze casting; Mechanical properties; Grain refinement
1. Introduction Magnesium alloys have properties that make them attractive for certain applications. However, even complex alloys have limitations in respect of strength, stiffness and abrasion resistance. It is possible that these limitations can be overcome by using metal matrix composites (MMC) with a magnesium-based alloy. Although several manufacturing processes can be used to produce such composites, the casting route is especially attractive given its ability to produce complex shapes. However, in order to obtain the benefits of reinforcement, the casting process must deliver castings that are free of defects such as gas or shrinkage porosity. Squeeze casting is capable of delivering such castings and consequently has been used to produce cast MMC. Before advocating squeeze casting for MMC production, however it is necessary to understand the influence of process variables on the base alloy and that was the purpose of the work described in this paper. ∗ Corresponding author. Present address: Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore. E-mail address: [email protected] (M.S. Yong).
0924-0136/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2003.07.006
The majority of investigations to evaluate the effect of squeeze casting parameters have considered aluminium alloys and their composites. The most important parameters in squeeze casting have been identified as melt temperature, melt quality (i.e. the absence of oxide films and inclusions) and quantity, die temperature, applied pressure, and duration of applied pressure [1,2]. In the case of composites, infiltration velocity and preform temperature can be added to the list [3–5]. These variables are equally relevant to magnesium alloys and their composites was confirmed by several researchers [6–13]. A significant difference between magnesium and aluminium is the former’s lower volumetric heat of fusion, which means that solidification should occur at a faster rate [14]. An understanding of the effects of process variables is essential because the structure and properties of alloys can be optimised without recourse to expensive alloying elements or nucleating agents [6]. This paper reports an investigation of the influence of zirconium grain refinement, applied pressure, and pouring and die temperatures on the as-cast properties of specified magnesium-based alloys. This work was a prerequisite to that on composites that will be reported in a future paper.
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2. Experimental methodology Two magnesium alloys were used: commercially available RZ5, which contains zirconium, and a zirconium-free version (hereafter referred to as RZ5DF). Their compositions are presented in Table 1. The effect of applied pressure on the RZ5DF alloy was first evaluated whilst maintaining metal pouring temperature at 750 ◦ C, die temperature at 250 ◦ C, duration of applied pressure at 25 s, and delay before application of pressure at 4 s. Following establishment of the optimum applied pressure (at 60 MPa), the influence of metal pouring and die temperatures was investigated whilst the applied pressure, delay and duration were kept constant. The pouring temperatures investigated were 720, 750 and 780 ◦ C and the die temperatures were 225, 250 and 275 ◦ C. The selection of the experimental ranges for pouring and die temperatures was guided by the literature. Pure magnesium melts at 640 ◦ C and the recommended pouring temperatures for the alloys is 720–800 ◦ C [15]. A die temperature range of 200–300 ◦ C is advocated for commercial die-casting [1,16]. Test casting: The test casting was a rectangular plate with a length of 126 mm, a width of 75 mm and a depth of 16 mm. Melt processing: The alloys were melted in an electric resistance furnace using a steel crucible, the fluxless method and an argon gas cover. The die was coated with boron nitride suspended in water to protect it from excessive wear. The alloys were cast using the direct squeeze casting process. The squeeze casting process was described in previous papers [17,18]. Tensile testing: Tensile tests were conducted on a 50 kN Mayes testing machine using position control and the modulus was determined using a strain gauge attached to the gauge section parallel to the direction of tensile loading. Modified test specimens were machined according to BS18 (1987) and Magnesium Elektron Ltd. RB4 specifications [19]. The tensile properties were determined from as-cast material tested at ambient temperature. Hardness testing: Hardness measurements were conducted using the Rockwell B scale in preference to Vickers hardness testing as the former provided improved consistency. The locations of hardness measurements conducted on the castings are shown in Fig. 1. Metallography: Metallographic samples were prepared using standard techniques. Specimens were finally ground using 1000 grit silicon carbide paper, prior to polishing with six micron and finally 1 m diamond paste. A 5% Table 1 Specification of magnesium alloys used in the investigation
RZ5 alloy RZ5DF alloy
Zinc (%)
Rare earths (%)
Zirconium (%)
Magnesium
4.2 4.2
1 1
0.7 –
Balance Balance
Fig. 1. Locations of hardness measurements (each dot represents the position of a hardness measurement) taken in both longitudinal and transverse directions.
nital etchant was used for the RZ5 alloy but an aceticglycol etchant was found to be necessary for the RZ5DF alloy. Cell size: The cell size was established using the intersection method. Five areas were selected at random and twenty-one measurements of cell size were taken for each area. The average value for the 105 readings was determined. The results and observations from this experimental programme are presented in the following section, which also includes a comparison of tensile strength, material hardness and metallographic structures.
3. Results and observations The results are reported in the sequence that the experiments were conducted. As the first objective of the investigation was to establish an optimum applied pressure level, these experiments were the first to be conducted using the zirconium-free RZ5DF alloy. Once this level had been established, a second series of experiments was conducted using the RZ5DF alloy to evaluate the influences of metal pouring and die temperature. A third series of experiments was conducted using the zirconium-containing RZ5 alloy. By comparing the results of the second and third series of experiments, the influence of zirconium could be established. 3.1. Series 1 experiments: the influence of applied pressure 3.1.1. Tensile properties The effects of applied pressure on the tensile properties of squeeze cast RZ5DF alloy are presented in Fig. 2. It can be seen that the highest tensile properties were obtained with an applied pressure of 100 MPa and that the lowest were produced at atmospheric pressure (0.1 MPa), i.e. by gravity die-casting. The graph shows that there is a significant increase in tensile properties as the applied pressure is increased to 60 MPa but that after this point, a further increase in applied pressure produces little further improvement.
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3.2. Series 2 experiments: the evaluation of squeeze cast RZ5DF alloy 3.2.1. Tensile properties The relationships between pouring temperature, die temperature and tensile properties are shown in Figs. 6 and 7.
Fig. 2. Effects of applied pressure on the tensile properties of squeeze cast RZ5DF alloy.
3.1.2. Hardness The hardness values along the longitudinal and transverse directions of the RZ5DF alloy castings produced with different applied pressures are shown graphically in Fig. 3. The results show that applied pressure appears to have little effect on hardness, since the majority of values fall within the range 14–20 HRB. However, close inspection of the detailed hardness values of castings produced using low applied pressures (0.1, 20 and 40 MPa) revealed that these had the lowest hardness values. Low hardness was expected in these castings because of the presence of porosity.
25
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15 10 5 0.1
20
40 60 80 Applied Pressure (MPa)
100
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-
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0
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20
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Rockwell Hardness (HRB)
3.1.3. Metallography Metallography was conducted to examine the influence of applied pressure on the cast structure. Examination of the RZ5DF alloy specimens produced at different applied pressures revealed that a critical pressure greater than 40 MPa was required to suppress microporosity. Selected microstructures that show the effect of applied pressure on structure are shown in Fig. 4. These are complemented by the graphical representation of the relationship between applied pressure and cell size presented in Fig. 5. These figures show a pronounced reduction in cell size for squeeze cast material.
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Fig. 3. The average material hardness along the longitudinal and transverse directions of the squeeze cast RZ5DF alloy, cast with constant pouring temperature of 750 ◦ C and die temperature of 250 ◦ C.
3.2.2. Metallography Metallographic specimens were examined to evaluate the effects of pouring and die temperature on the RZ5DF microstructure. The examinations were conducted on specimens selected from those that had the highest, intermediate and lowest UTS values. The structure associated with the highest UTS value is shown in Fig. 8. 3.3. Series 3 experiments: the evaluation of squeeze cast RZ5 alloy 3.3.1. Tensile properties The relationships between pouring temperature, die temperature and tensile properties are shown in Figs. 9 and 10. 3.3.2. Metallography Metallographic examinations were conducted to evaluate the effects of pouring and die temperature on the squeeze cast RZ5 microstructure. These examinations were conducted on specimens selected from those that produced the highest, intermediate and lowest UTS values. Fig. 11 shows the structure associated with the highest UTS value.
4. Discussion As in all casting processes, the rate of solidification in squeeze casting is determined primarily by the rate at which heat is transferred by the metal to the die. In most casting processes, heat flow is controlled to a significant extent by resistance at the metal–mould interface. The thickness of the solid metal that forms is typically a parabolic function of time, being initially very rapid and then decreasing as the mould is heated [14]. However, in squeeze casting, the pressure applied through the punch promotes an intimate contact between the metal and die and this largely overcomes the resistance to heat flow. According to Campbell [20], the transfer of heat across the interface can be enhanced significantly in squeeze casting. For squeeze cast aluminium, the figure may be up to 60,000 W/m2 K compared to the more normal values of 100–1000 W/m2 K. The heat to be transferred consists of the volumetric heat of fusion (the major component) and the superheat. Higher pouring temperatures increase the superheat contribution. It should be noted that whilst the volumetric heat of fusion for magnesium is approximately 15% lower than that for aluminium, the specific heat for the liquid metal is approximately 15% higher for magnesium than aluminium. The temperature of the die influences its capacity to absorb heat. However, it is the die’s
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Fig. 4. Optical microstructures for squeeze cast RZ5DF produced with various pressure.
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120 100 80 60 40 20 -
12 10 8 9 8 7 6 5 4 -
80
100
120
Fig. 5. Influence of applied pressure on the cell size of the squeeze cast RZ5DF alloy.
3
7500C 7800C
2250C
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60
7200C
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40
Applied Pressure (MPa)
Pouring Temp (0C)
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20
% Area Reduction
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0
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% Elongation
14
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Grain Size (µm)
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2750C
Fig. 7. The effects of pouring and die temperature on percentage elongation and percentage area reduction of RZ5DF alloy specimens tested at ambient temperature.
195 190 185 180 175 170 165 205 200
2250C
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160
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Pouring Temp (0C) 7200C 7500C 7800C
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Ultimate Tensile Strength (MPa)
Die Temperature (degree C)
2500C
2750C
Die Temperature (degree C) Fig. 6. The effects of pouring and die temperatures on ambient temperature UTS (RZ5DF alloy).
thermal diffusivity that exerts the major influence on the rate of solidification [14]. From the results, it would appear that the optimum applied pressure range is from 50 to 100 MPa. The low UTS values produced by applied pressures below 40 MPa are primarily the consequence of porosity present in the castings because the pressure acting on the molten metal was insufficient for its elimination (Fig. 2). As the applied pressure was increased to 60 MPa, not only was porosity reduced, the rate of cooling was increased and the cell size reduced with a concomitant improvement in tensile properties. This is emphasised when the microstructures of castings produced by different applied pressures, shown in Fig. 4, are compared. An average cell size of 127 m was produced by gravity
Fig. 8. Optical microstructure of the squeeze cast RZX5DF alloy with the highest UTS of 198 MPa at ambient temperature, providing an average cell size of 18 m.
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205 200 195 190 185 180 175 170 165 160
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Ultimate Tensile Strength (MPa)
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2750C
2500C
2250C
Die Temperature (degree C) Fig. 9. The effects of pouring and die temperature on UTS at ambient temperature (RZ5 alloy).
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die-casting at atmospheric pressure (0.1 MPa) but the cell size was reduced to 21 m at an applied pressure of 60 MPa, a significant six-fold reduction in cell size. The reduction in cell size was attributed to the intimate contact between the melt and die wall that promoted rapid heat transfer, as applied pressure was increased. Solidification is a process of nucleation and growth and this process is influenced by the rate at which heat is transferred which in turn influences the structure and properties of the casting. In squeeze casting, we might expect solidification to commence as soon as the metal contacts the die, i.e. before pressure is applied. Once pressure is applied, heat transfer is promoted and concurrently the temperature of the metal increases, as predicted by the Clausius–Clapeyron equation and this might, in combination with the long
11 10 9 8 8 7 6 5 -
Pouring Temp (0C) 7200C 7500C 7800C
2250C
2500C
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4
-
-
% Area Reduction
% Elongation
12
2750C
Die Temperature (degree C) Fig. 10. The effects of pouring and die temperature on percentage elongation and percentage area reduction of RZ5 alloy specimens tested at ambient temperature.
Fig. 11. Optical microstructure of the squeeze cast RZ5 alloy with the highest UTS of 195 MPa at ambient temperature, providing an average cell size of 21 m.
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freezing range of the alloys, be expected to promote constitutional undercooling. Such conditions are normally conducive to the formation of a dendritic structure but the copious nucleation promoted by the massive chilling effect of squeeze casting [7] produces an extremely fine casting structure. The results for the RZ5DF alloy suggest that the highest UTS was associated with the higher pouring temperature (780 ◦ C) and the intermediate die temperature of 225 ◦ C. A steep temperature gradient, a consequence of combining a high pouring temperature with a low die temperature, should yield a fine microstructure and produce higher mechanical properties [21–24]. Conversely, castings produced with a shallow temperature gradient are likely to have a large uniform cell structure, which will generally lead to lower mechanical properties. However, the results for the RZ5 alloy suggest that, although the intermediate die temperature was still as important, the highest UTS was obtained when this was combined with the intermediate pouring temperature (750 ◦ C). This suggests that a severe temperature gradient is less important because of the refining effect of the zirconium. 4.1. The role of zirconium By comparing the results generated by the series 2 and 3 experiments, it is possible to provide a view on the need for zirconium in a magnesium–zinc–rare earths alloy intended for processing by squeeze casting. Zirconium, in excess of its solubility limit, is used to produce a grain refining effect when the alloy is sand cast and consequently subjected to a relatively slow cooling rate. Zirconium in magnesium is the most effective grain refiner in commercial use [14]. However, its presence adds to the cost of the alloy and requires process controls and procedures that counteract the tendency for gravity segregation of the zirconium. It is possible that more than one mechanism is at work. However, the most likely is a peritectic reaction in which separating zirconium particles react with the liquid to acquire a layer of zirconium-enriched solid solution that serves as nuclei [24]. When the number of nuclei is large, crystallisation proceeds from a large number of points. The presence of zirconium in magnesium produces a fine equiaxed cell structure with typical cell sizes of 30–50 m in sand castings and this generally leads to higher mechanical properties [25]. However, the values of the cell size obtained in this investigation were in the range of 18–32 m, which is almost half that reported above. This may be due to the speed of solidification in a squeeze casting that is faster in comparison to that for sand castings. Differences in the cell shapes were observed between castings produced with and without the addition of zirconium. These differences can be seen by inspecting Fig. 11 (with zirconium, i.e. RZ5 alloy) and Fig. 8 (without zirconium, i.e. RZ5DF alloy). It can be seen from the figures that the zirconium addition caused the individual cells to assume a more regular and rounded form.
Contrary to the anticipated effects of a grain refinement addition, reported in the literature [25–28], the metallographic examinations did not show a significant difference in cell size. Castings produced with the addition of zirconium (RZ5 alloy) contained cell sizes which ranged from 21 to 26 m whereas those without (RZ5DF alloy) contained cell sizes ranging from 18 to 32 m. The addition of zirconium to the RZ5 alloy had little effect because the process of squeeze casting refined the cell structure. It can be concluded that the use of grain refinement in squeeze casting is unnecessary. This research has shown that the cell structure can be manipulated by such process variables as applied pressure, pouring and die temperature. This confirms Chadwick’s view that the microstructure can be controlled by controlling casting variables alone and without recourse to a nucleating agent [6].
5. Conclusions 1. The pressure applied in squeeze casting promotes rapid solidification and a refined cell structure. Increasing the applied pressure beyond 60 MPa provided little improvement in the tensile properties of squeeze cast RZ5DF alloy. An applied pressure of 60 MPa was sufficient to eliminate all traces of shrinkage and gas porosity within the casting. Metallographic examination of the castings revealed that the cell size reduced from 127 to 21 m when the applied pressure was increased from 0.1 to 60 MPa. 2. It is possible to achieve comparable tensile properties in the zirconium-free RZ5DF alloy to those in RZ5 alloy grain refined with a zirconium addition by selecting appropriate processing parameters. The highest UTS value obtained in the zirconium-free RZ5DF alloy was 198 MPa compared to 195 MPa for the RZ5 alloy. These values are significantly higher (approximately 50%) than those obtained when the alloys were cast at 0.1 MPa. Acknowledgements Dr. Yong gratefully acknowledges the receipt of an Overseas Research Students’ Award and a Loughborough University Research Studentship.
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M.S. Yong, A.J. Clegg / Journal of Materials Processing Technology 145 (2004) 134–141 [4] H. Fukunaga, Processing aspects of squeeze casting for short fiber reinforced metal matrix composite castings, Adv. Mater. Manuf. Process. 3 (4) (1988) 669–687. [5] H. Fukunaga, Squeeze casting processes for fiber reinforced metals and their mechanical properties, in: Proceedings of the Advances in Cast Reinforced Metal Composites, Chicago, September, ASM International, Metals Park, OH, 1988, pp. 101–108. [6] G.A. Chadwick, Squeeze casting of magnesium alloys and magnesium-based metal matrix composites, in: Proceedings of the Magnesium Technology, London, November, The Institute of Metals, 1986, pp. 75–82. [7] T.U. Ha, Squeeze casting of magnesium-based alloys and their metal matrix composites, Ph.D Thesis, University of Southampton, 1988. [8] K. Purazrang, P. Abachi, K.U. Kainer, Investigation of the mechanical behaviour of magnesium composites, Composites 25 (4) (1994) 296– 302. [9] K. Purazrang, P. Abachi, K.U. Kainer, Mechanical behaviour of magnesium alloy MMCs produced by squeeze casting and powder metallurgical techniques, Comp. Eng. 3 (6) (1993) 489–505. [10] T. Wada, T. Shinkawa, S. Kamado, Y. Kojima, Effect of fiber volume fraction on microstructures and tensile properties of alumina short fiber reinforced by Mg–4 mass% Zn–RE alloy composites, J. Jpn. Inst. Light Met. 45 (9) (1995) 510–515. [11] S. Kamado, T. Shinkawa, T. Wada, Y. Kojima, Structure and mechanical properties of Mg–Zn–Ca alloy composites reinforced with aluminium borate whisker, J. Jpn. Inst. Light Met. 46 (2) (1996) 71–76. [12] O. Ottinger, G. Grau, R. Winter, R.F. Singer, The effect of aluminium additions on the interfacial microstructure and mechanical properties of C/Mg composites, in: K. Street, A. Poursatis (Eds.), Proceedings of the 10th International Conference on Composite Materials (ICCM10), vol. VI, Vancouver, Canada, August 1995, pp. 447–454. [13] D. Wurm, O. Ottinger, R.F. Singer, The influence of carbon fiber type and stress ratio on the fatigue behaviour of unidirectionally reinforced C/Mg composites, in: K. Street, A. Poursatis (Eds.), Proceedings of
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