A commercial Mg-6Al-2Sr (AJ62) alloy has been prepared by a semisolid rheo-diecasting (RDC) process. The microstructure of the RDC alloy exhibits typical semisolid solidification features, i.e., 8.4 vol% primary α-Mg globules (23 μm in diameter), formed in the slurry maker at the primary solidification stage, uniformly distributed in the matrix of fine α-Mg grain size (8.2 μm) and intergranular eutectic Al4Sr lamellae, which resulted from secondary solidification inside the die. A ternary Mg-Al-Sr phase was also observed. Heat treatment revealed the extreme thermal stability of the RDC AJ62 alloy. The hardness showed little change up to 12 hours at 450°C, whilst the Al4Sr eutectic lamellae were broken up, spheroidised and coarsened during the annealing. The RDC alloy offers superior mechanical properties, especially ductility, over the same alloy produced by high pressure die-casting

Fan, Z

Liu, G

Wang, F

Wang, Y

English

Brunel University Research Archive

600Microstructural characterisation and thermal stability of an Mg-Al-Sr alloy prepared by rheo-diecastingY. Wang, G. Liu, F. Wang and Z. FanBCAST (Brunel Centre for Advanced Solidiﬁcation Technology), Brunel University, Uxbridge, Middlesex UB8 3PH, UKAbstractA commercial Mg-6Al-2Sr (AJ62) alloy has been prepared by a semisolid rheo-diecasting (RDC) process. The microstructure of the RDC alloy exhibits typical semisolid solidiﬁcation features, i.e., 8.4 vol% primary α-Mg globules (23 µm in diameter), formed in the slurry maker at the primary solidiﬁcation stage, uniformly distributed in the matrix of ﬁne α-Mg grain size (8.2 µm) and intergranular eutectic Al4Sr lamellae, which resulted from secondary solidiﬁcation inside the die. A ternary Mg-Al-Sr phase was also observed. Heat treatment revealed the extreme thermal stability of the RDC AJ62 alloy. The hardness showed little change up to 12 hours at 450°C, whilst the Al4Sr eutectic lamellae were broken up, spheroidised and coarsened during the annealing. The RDC alloy offers superior mechanical properties, especially ductility, over the same alloy produced by high pressure die-casting.Keywords: Microstructural characterisation, thermal stability, Mg-Al-Sr (AJ62) alloy, rheo-diecasting.1. IntroductionIn recent years, Mg alloys face a challenge in meeting perfor-mance requirements for components used in automobiles at elevated temperature. Strength, creep resistance and corro-sion resistance as well as castability are the key issues. Much attention has been paid to research and development of cost competitive and high temperature creep resistant Mg based alloys. Such alloys would enable the automotive industry to use more Mg alloy components in order to produce light-weight and fuel efﬁcient vehicles. Among them, Mg-Al-Sr alloys have shown superior creep performance and tensile strength at temperatures as high as 175°C [1–7], with the particular AJ62 (Mg-6Al-2Sr) alloy giving an excellent combination of creep performance, tensile properties and castability [1,3,4,6,7].Semisolid metal (SSM) processing is a promising tech-nology capable of producing high integrity components [8–10]. Rather than a liquid metal, as used in conventional die casting technology, SSM processing uses a semisolid slurry with substantially increased viscosity, resulting in controlled die ﬁlling and close to zero porosity in the ﬁnal components. Compared with conventional die-casting, SSM processing has a number of advantages, such as low porosity, heat treat-ability, consistency and soundness of mechanical properties, the ability to make complex component shapes and longer die life. During the last two decades, SSM processing has been of considerable scientiﬁc and industrial interest, with a number of novel SSM technologies being developed. Among them, the rheo-diecasting process (RDC), devel-oped recently at BCAST, Brunel University, has been shown to have advantages over other SSM technologies in terms of technological ﬂexibility, component integrity, microstructural homogeneity and production cost efﬁciency [11–14]. In the present study, a commercial AJ62 alloy (Mg-6.3Al-2.5Sr) has been prepared by the semisolid RDC process. The microstructure and its evolution during heat treatment of the AJ62 alloy have been investigated. The tensile properties have also been evaluated and discussed in terms of the microstructure.2. ExperimentalThe composition of the AJ62 alloy used in the present study was Mg-6.3Al-2.5Sr-0.36Mn, which was provided by MEL (Magnesium Elecktron, Manchester, UK). The alloy was prepared using both RDC and HPDC processes with similar processing parameters for comparison. The RDC process is an innovative one-step SSM processing technique for manufacturing near net shape components of high integrity directly from liquid alloys. Detailed description of the RDC process can be found elsewhere [15–18]. The RDC equipment consists of two basic functional units; a twin-screw slurry maker and a standard cold chamber high pressure die casting (HPDC) machine. The twin-screw slurry maker has a pair of screws rotating inside a barrel. The specially designed screw proﬁles are fully intermeshing and self-wiping. The ﬂuid ﬂow inside the slurry maker is characterised by high shear rate and 601Proceedings of the 5th Decennial International Conference on Solidification Processing, Sheffield, July 2007high intensity of turbulence, with a cyclic variation of shear rate. In the RDC process, a predetermined dose of alloy melt is fed into the slurry maker, which is set to a desired temper-ature between the liquidus and solidus. The liquid alloy is cooled continuously to the processing temperature while being mechanically sheared by the pair of screws. Depending on the barrel temperature, a certain volume fraction of the alloy melt solidiﬁes inside the slurry maker, converting the liquid into semisolid slurry. After being sheared for a period of time, the slurry is then transferred to the shot sleeve of the HPDC machine for component shaping. In the present study, the AJ62 alloy ingot was melted at 715°C under the protec-tion of N2 + 0.5 vol%SF6 gas mixture and fed into the slurry maker. The slurry maker was operated at 612°C. The rotation speed of the twin-screw was 500 rpm, and the shearing time between 20 and 40 s. A 280-ton cold chamber HPDC machine was used to produce standard tensile test samples. The mechanical properties were evaluated by tensile tests using an Instron tensile machine.The microstructure of the samples was examined by optical microscopy (OM) with quantitative metallography. The specimens for OM were prepared by the standard tech-nique of grinding with SiC abrasive papers and polishing with an Al2O3 suspension solution. A Zeiss optical imaging system was utilised for the OM observations and the quantitative measurements. Scanning electron microscopy (SEM) was carried out with a FEG Zeiss Supera 35 machine, equipped with an energy dispersive spectroscopy (EDS) facility and operated at an accelerating voltage of 5 to 20 kV. EDS meas-urements were performed to obtain chemical information for the phases using a ﬁxed accelerating voltage of 15 kV, cali-brated with a standard sample at every session.3. Results and Discussion3.1 Solidiﬁcation microstructureFigure 1 shows the typical microstructure of AJ62 alloy produced by the RDC process, in comparison with the same alloy prepared by HPDC. The general view shows that the RDC alloy exhibits a non-dendritic and more uniform microstructure than the HPDC alloy. Apart from the rela-tively large α-Mg globules formed inside the slurry maker, the solidiﬁcation structure of the RDC alloy is composed of ﬁne α-Mg particles, contributed by solidiﬁcation of the remaining liquid inside the die, which is referred to as secondary solidiﬁcation. Detailed SEM observation revealed that the ﬁne α-Mg particles, formed inside the die, have equiaxed morphology and are delineated by the lamellar eutectic phase, as shown in Figure 2. The ﬁne α-Mg particles were measured to be 8.2 µm in size, compared to 23 µm for the primary globules. A dendritic morphology of the primary phase, typical of most conventional cast alloys, was hardly observed in the RDC alloy samples. In contrast, the AJ62 alloy prepared by HPDC exhibits a coarse and non-uniform a bdc   500µm    500µm   50µm 50µmFigure 1: Optical micrographs show the microstructure of the AJ62 Mg-6Al-2Sr alloy produced by (a) and (c) rheo-diecasting (RDC), and (b) and (d) high pressure die casting (HPDC).602Proceedings of the 5th Decennial International Conference on Solidification Processing, Sheffield, July 2007alloy is probably one of the main reasons for the superior creep resistance of the Mg-Al-Sr (AJ series) alloys [1]. In fact, Al is added to improve castability and room temper-ature tensile properties. The Sr content needs to be high enough to avoid any increase in Al supersaturation of the primary α-Mg phase and also to prevent formation of the β-Mg17Al12 phase [2]. The improvement in creep resistance of the Mg-Al-Sr alloy is due to strengthening by the ther-mally stable intermetallic phases [1].The solidiﬁcation behaviour of the RDC AJ62 alloy is characterised by two solidiﬁcation stages: primary solidiﬁca-tion inside the slurry and secondary solidiﬁcation inside the die cavity. Inside the slurry maker, a certain volume fraction of solid forms and takes the globular shape because of the intensive shearing [18]. The secondary solidiﬁcation, which has been extensively investigated [19] starts when the slurry leaves the twin-screw slurry maker and occurs under no forced convection.The remaining liquid in the slurry has uniform temperature and composition ﬁelds due to the prior inten-sive shearing. With the large cooling rate provided by the die, nucleation is expected to take place throughout the entire remaining liquid with a high nucleation rate giving numerous nuclei which grow and solidiﬁcation ﬁnishes microstructure with the primary α-Mg particles being dendritic, as shown in Figure 1. The dendrite arm spacing of the HPDC AJ62 alloy was measured to be 16.3 µm, double the size of the ﬁne α-Mg particles of the RDC alloy.The microstructure of Mg-Al-Sr alloys with different concentrations (varying Sr/Al ratios) of Sr and Al has been investigated before [1,4–6]. Two types of second phase were observed in the alloys with higher Sr contents. All these intermetallic phases are located at the grain boundaries of the α-Mg primary particles. The total volume fraction of second phase particles was measured to be 6.02 vol% for the Mg-6.3Al-2.5Sr alloy used in the present study. SEM/EDS analysis revealed the lamellar structure of the inter-granular Al4Sr eutectic intermetallic [1,4,6], while an Mg-Al-Sr ternary intermetallic phase was also observed occasionally with a brighter contrast, a coarser size and blocky-shaped structure, as shown in Figure 2. This phase has been tenta-tively identiﬁed as Al3Mg13Sr previously [1,6]. SEM/EDS and XRD analyses revealed no Mg17Al12 in the RDC alloy.Sr in the AJ series alloys combines with Al and makes the formation of the Mg17Al12 phase more difﬁcult, since the latter phase is thought to facilitate grain boundary migration and therefore decrease the creep resistance. The absence of Mg17Al12 phase in the microstructure of the AJ62 ba  10µm   5µmFigure 2: SEM backscattered electron images showing (a) ﬁne α-Mg particles formed by secondary solidiﬁcation and (b) lamellar structure for the eutectic Al4Sr intermetallic.a b2µm  10µmFigure 3: Microstructural evolution of the RDC AJ62 Mg alloy during heat treatment at 450°C for up to 12 h. (a) 450°C for 1 h, (b) 450°C for 12 h. It is seen that the eutectic intermetallic phase has spheroidised and coarsened with increasing treatment time.603Proceedings of the 5th Decennial International Conference on Solidification Processing, Sheffield, July 2007before growth instability even occurs, preventing them from developing dendritically. This produces the ﬁne non-dendritic α-Mg particles shown in Figure 2.3.2 Microstructural evolution during heat treatmentHeat treatment has been carried out to investigate the thermal stability of the RDC AJ62 alloy and its microstruc-tural evolution. Figure 3 shows the microstructure of the alloy after treatment at 450°C for 1 and 12 hours. The only microstructural modiﬁcation is breaking-up, spheroidisation and coarsening of the Al4Sr eutectic phase.Figure 4 shows hardness and particle size of the eutectic Al4Sr phase against time during treatment at 350 and 450°C. The alloy is thermally stable at temperatures as high as 450°C with little change in hardness or in percentage of the eutectic phase. However, the eutectic intermetallic does spheroidise and coarsen with increasing time, as shown in Figure 4b. A power law was used to ﬁt the experimental data and to extract the growth constants according to the well-established classic law, Dn – Dn0 = Kt. It was found that the value of n is 2.75, in between 2 and 3.Figure 5a shows a typical microstructure of the RDC alloy after treatment for 2 hours at 450°C. It can be seen that the majority of the eutectic lamellae have been broken-up and have started to coarsen. Particle size distribution of the modiﬁed eutectic Al4Sr phase after treatment at 450°C for 2 hours is shown in Figure 5b, with the average size being measured to be 510 nm in diameter.As mentioned above, two types of second phase compounds were observed in the RDC AJ62 alloy, with the major one being the Al4Sr intermetallic. Quantitative metallo-graphy indicated that the total volume fraction of the second 30405060700 2 4 6 8 10 12 14Annealing time (h)Hardness  Hv450C350C0.11101 10 100Annealing time (h)Particle size ( µm)D2.75=0.115tR2 = 0.984a bFigure 4: (a) Hardness of the RDC AJ62 alloy as a function of treatment time showing little change in hardness up to 12 h at 350 and 450°C. (b) Variation in particle size of the eutectic Al4Sr intermetallic as a function of treatment time at 450°C. The mean size of the eutectic intermetallic increased from 0.51 to 1.43 µm as the treatment time increased from 2 to 24 h.0510152025300.2 0.4 0.6 0.8 1 1.2 1.4 1.6Particle size, µmFrequency %ba  3µmFigure 5: (a) SEM (BEI) image showing the typical microstructure of RDC AJ62 Mg alloy after treatment at 450°C for 2 h. (b) Quantitative measurement of the modiﬁed eutectic particle size distribution. The morphology of the eutectic Al4Sr phase changes from lamellar to spherical.604Proceedings of the 5th Decennial International Conference on Solidification Processing, Sheffield, July 2007phase particles was 6.23 vol% for Mg-6.3Al-2.5Sr alloy used in the present study. Long time treatment  at elevated temper-atures up to 450°C gave little change in the volume fraction of the intermetallic phases present in the alloy. The Al4Sr intermetallic is extremely stable at these temperatures only with its morphology being spheroidised, similar to the modi-ﬁcation of  eutectic silicon in A356/357 Al alloys during T6 treatment [20,21]. It was proposed that the creep resistance of the AJ alloys is related to the low Al supersaturation of the primary α-Mg and the absence of the β-Mg17Al12 phase due to the precipitation of thermal stable Al- and Sr-rich phases.3.3 Mechanical propertiesThe mechanical properties have been evaluated by tensile testing for both RDC and HPDC AJ62 alloy under as-cast and heat treated conditions with the results given in Table 1. It is seen that the RDC alloy offers superior mechanical properties, i.e., higher ultimate tensile strength (UTS), higher yield strength, and especially signiﬁcant improve-ment in ductility, over the same alloy produced by tradi-tional HPDC. More importantly, the present study shows that specially designed heat treatment schedule, such as treatment at a temperature as high as 450°C, can be used to enhance the mechanical properties, particularly to signiﬁ-cantly improve the ductility, as shown in Table 1.The increase in elongation after heat treatment is attributed to the modiﬁcation of the eutectic morphology. Previous studies [13,14] have shown that, for RDC Mg alloy, Tx treatment modiﬁed the Mg17Al12 phase network into isolated particles located along the grain boundaries, and resulted an increase in the elongation. In the present study, similar treatment makes the Al4Sr eutectic morphology change from lamellar to spherical, which will reduce the interface between the eutectic and the α-Mg matrix, and then reduce the possibility of crack initiation and crack propagation along the interfaces.Sr content plays an important role in determining the mechanical properties of the AJ alloys. When Sr concentra-tion is low, the binary Mg17Al12 phase will form. This phase is thought to facilitate grain boundary migration and there-fore decreases the creep resistance. Therefore the Sr content needs to be high enough to avoid the formation of the Mg-Al binary compound. On the other hand, the elongation of the AJ alloys is sensitive to the Sr content. A modiﬁed AJ62 alloy, AJ62Lx (1.9 wt%Sr), has been proposed and it has elongation up to 9 to 11% [6], higher than for AJ62x (2.4 wt%Sr [6]). In the present study, however, the elonga-tion of the RDC AJ62 alloy with 2.5 wt%Sr is as high as 9.05% in the as-cast condition. Furthermore, the UTS and elongation can be signiﬁcantly improved by heat treatment with little sacriﬁce of yield strength.4. ConclusionsThe microstructure of the rheo-diecast Mg-6.3Al-2.5Sr (AJ62) alloy exhibits typical semisolid solidiﬁcation features, i.e., 8.4 vol% primary α-Mg globules (23 µm in diameter), formed in the slurry maker at the primary solidiﬁcation stage, uniformly distributed in the matrix of ﬁne α-Mg grain size (8.2 µm) and intergranular eutectic Al4Sr lamellae, which resulted from secondary solidiﬁcation inside the die. A ternary Mg-Al-Sr phase was also observed.Heat treatment revealed the extreme thermal stability of the rheo-diecast AJ62 alloy. The hardness showed little change up to 12 hours at 450°C, whilst the eutectic Al4Sr lamellae were broken up, spheroidised and coarsened during the heat treatment.The rheo-diecast AJ62 alloy offers superior tensile mechanical properties, especially ductility, over the same alloy produced by conventional high pressure die-casting. Moreover, specially designed heat treatment schedules can be used to enhance the mechanical properties, particularly elongation.References1. E. Baril, P. Labelle and M.O. Pekguleryuz, JOM, 55 (2003) 34.2. M.O. Pekguleryuz and E. Baril, in: J. Hryn (ed.), Magnesium Technology 2001, TMS, p. 119.3. M.O. Pekguleryuz and A.A. Kaya, in: K.U. Kainer (ed.), Magnesium, Wiley-vch 2003, p. 74.4. M.O. Pekguleryuz and A.A. Kaya, in: A.A. Luo (ed.), Magnesium Technology 2004, TMS, p. 281.5. A.A. Luo, Internat. Mater. Rev, 49 (2004) 13.6. M.O. Pekguleryuz and A.A. Kaya, Advanced Engineering Materials, 12 (2003) 866.7. M.O. Pekguleryuz, P. Labelle, D. Argo and E. Baril, in: H.I. Kaplan (ed.), Magnesium Technology 2003, TMS, p. 201.8. M.C. Flemings, Metall. Trans. A, 22A (1991) 957.9. D.H. Kirkwood, Internat. Mater. Rev., 39 (1994) 173.10. Z. Fan, Internat. Mater. Rev., 47 (2002) 49.11. Z. Fan, G. Liu, Y. Wang, J. Mater. Sci., 41 (2006) 3631.12. Z. Fan, Mater. Sci. Eng. A, 413–414 (2005) 72.13. Y. Wang, G. Liu, Z. Fan, Acta Mater., 54 (2006) 689.14. Y. Wang, G. Liu, Z. Fan, Scripta Mater., 54 (2006) 903.15. Z. Fan, S.J. Bevis, S. Ji., PCT Patent, WO 01/21343 A1, 1999.16. Z. Fan, S. Ji, G. Liu, Mater. Sci. Forum, 488–489 (2005) 405.17. Z. Fan, X. Fang, S. Ji, Mater. Sci. Eng. A, A412 (2005) 298.18. Z. Fan, G. Liu, Acta Mater., 53 (2005) 4345.19. M. Hitchcock, Y. Wang and Z. Fan, Acta Mater., 2007 (in press)(doi:10.1016/j.actamat.2006.10.018).20. Q.G. Wang, C.H. Caceres and J.R. Grifﬁths, Metall. Mater. Trans. A, 34A (2003) 2901.21. E. Ogris, A. Wahlen, H. Luchinger and P.J. Uggowitzer, J. Light Metals, 2 (2002) 263. ■Table 1: Tensile properties of the RDC AJ62 alloy compared to that from HPDC.ConditionsUltimate Tensile Strength (MPa)Yield Strength (MPa)Elongation (%) ReferenceHPDC (F condition) 227 138 7.0 [6]HPDC (F condition) 225.3 ± 11.3 123.5 ± 4.0 7.43 ± 1.49 This workRDC (F condition) 243.9 ± 10.4 129.8 ± 3.4 9.05 ± 1.01 This workRDC (Heat treated, 2 hrs@450°C)257.6 ± 7.7 121.4 ± 5.2 12.01 ± 1.06 This work

Microstructural characterisation and thermal stability of an Mg-Al-Sr alloy prepared by rheo-diecasting

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