Microstructural characteristics and mechanical properties of Al-2024 alloy processed via a rheocasting route
Behnam Rahimi1), Hamed Khosravi2), and Mohsen Haddad-Sabzevar1)
1) Department of Metallurgy and Materials Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad 9177948944, Iran
2) Faculty of Materials Science and Engineering, K.N. Toosi University of Technology, Tehran 1999143344, Iran
(Received: 13 April 2014; revised: 8 May 2014; accepted: 10 May 2014)
Abstract: This article reports the effects of stirring speed and T6 heat treatment on the microstructure and mechanical properties of Al-2024 alloy synthesized by a rheocasting process. There was a decrease in grain size of α-Al particles corresponding to an increase in stirring speed. By increasing the stirring speed, however, the globularity of matrix particles first increased and then declined. It was also found that the hardness, compressive strength, and compressive strain increased with the increase of stirring speed. Microstructural studies revealed the presence of nonsoluble Al15(CuFeMn)3Si2 phase in the vicinity of CuAl2 in the rheocast samples. The required time for the solution treatment stage was also influenced by stirring speed; the solution treatment time decreased with the increase in stirring speed. Furthermore, the rheo-cast samples required a longer homogenization period compared to conventionally wrought alloys. Improvements in hardness and compres-sive properties were observed after T6 heat treatment.
Rheocasting was developed at MIT in 1971 [1], and sig-nificant development has subsequently taken place with re-spect to stirrer design, control of process parameters, appli-cation areas, and so on [2−4]. In this technique, a molten al-loy is stirred continuously during solidification followed by pouring. The main benefit of the rheocasting process is that globular or nondendritic primary morphologies can be ob-tained in an as-cast condition [5−6]. Two main mechanisms have been proposed for the evolution of primary phases during semi-solid casting: the fragmentation–agglomeration mechanism [1,7] and the nucleation and separation from the wall mechanism [8−9]. Rheocasting has numerous advan-tages over conventional casting routes; these include grain refinement, reduced porosity, improved mechanical proper-ties, better die life in die-casting due to pouring at a com-paratively lower temperature, and near net shape [10−11].
The alloys currently used in rheocasting processes are mainly conventional Al–Si casting alloys [2,4,6,12−16].
Until now, however, few researchers have applied the rheo-casting route to wrought alloys [17−19]. Curle [17] investi-gated the semi-solid near-net shape rheocasting of heat-treatable wrought 2024, 6082, and 7075 Al alloys using induction stirring with simultaneous air cooling and high pressure die-casting. Mahathaninwong et al. [18] studied the rheocasting of Al-7075 alloy produced by a novel gas-induced semi-solid technique. In the study of Lü et al. [19], a semi-solid slurry of wrought Al-5052 alloy was pre-pared by indirect ultrasonic vibration and then shaped by direct squeeze casting. They demonstrated that the best ten-sile properties were achieved using a slurry with a solid fraction of 0.17 solidified under 100 MPa.
Al-2024 alloy is a wrought alloy with copper as a pri-mary alloying element. It is used in applications requiring high strength-to-weight ratio as well as good fatigue resis-tance. It is commonly used in the manufacture of truck wheels, aircraft structures, screw machine products, scien-tific instruments, veterinary and orthopedic braces, and equipment and rivets [20]. There are two common methods of forming Al-2024 alloy: the wrought manufacturing proc-
60 Int. J. Miner. Metall. Mater., Vol. 22, No. 1, Jan. 2015
ess [21−22] and casting. The cost of the wrought manufac-turing process is very high in comparison to casting routes. Conversely, casting routes produce structures with casting defects such as pores and shrinkage cavities. Thus, semi-solid metal processes including thixocasting and rheocasting routes may be the best methods to form this al-loy.
Heat treatments after the forming stage have critical ef-fect on the mechanical properties of Al alloys [15,18,23−24]. Al-2024 is an age-hardening alloy that responds to heat treatments, which accomplish the strengthening. There are typically two types of heat treatments for Al-2024 alloy: (1) T4, which is attained by heating at 647°C followed by quenching with cold water and aging at room temperature; (2) T6, which is attained by heating at 647°C and quenching followed by heating at 102°C for 10 h followed by air cool-ing [18,25].
The current study attempts to apply the rheocasting tech-nique to process Al-2024 alloy. The microstructure and mechanical properties of as-cast and T6-heat-treated sam-ples were examined.
2. Experimental procedure
The nominal chemical composition of Al-2024 alloy used in the present study is given in Table 1. Fig. 1 presents the differential thermal analysis (DTA) curve obtained at a heating rate of 10 K/min. The temperature range for the solid–liquid region was measured to be between 591 and 652°C. The relatively broad semi-solid interval (61°C) of Al-2024 alloy makes it suitable for semi-solid processing.
Table 1. Nominal chemical composition of Al-2024 alloy wt%
Al Cu Mg Si Mn Fe Ti Pb
93 4.05 1.43 0.43 0.38 0.32 0.017 0.031
Fig. 1. DTA curve of Al-2024 alloy.
The samples were prepared via the rheocasting technique. Fig. 2 shows a schematic representation of the rheocasting apparatus used in this study. In the first stage, 350 g of Al-2024 alloy was put in a ceramic crucible and melted by an electric resistance furnace. A bottom hole was present in both the crucible and the furnace; two calibrated thermo-couples were inserted into the melt and the furnace to meas-ure their temperatures. After reaching 720°C, the tempera-ture of the melt was decreased to 690°C (approximately 40°C above the liquidus temperature) by lowering the fur-nace temperature. In this stage, the melt was stirred at vari-ous speeds of 450, 500, 600, 700, and 820 r/min using a preheated graphite impeller. The schematic representation of the graphite impeller used in this study is shown in Fig. 3. In each experiment, the slurry was continuously stirred while cooling until its temperature reached 645°C, corresponding to the solid fraction of 0.3 (Fig. 4). As soon as the slurry temperature reached 645°C, the stopper was removed, and the sheared melt was cast into a preheated (90°C) cast-iron die with an internal diameter of 25 mm and a height of 180 mm.
Fig. 2. Schematic representation of the rheocasting appara-tus.
Fig. 3. Schematic representation of the graphite impeller (unit: mm).
B. Rahimi et al., Microstructural characteristics and mechanical properties of Al-2024 alloy processed via a rheocasting … 61
Fig. 4. Curve of solid fraction vs. temperature for Al-2024 al-loy determined by differential scanning calorimetry.
To enhance the mechanical properties of Al-2024 alloy, T4 and T6 heat treatments have been widely adopted in in-dustry. In this study, the rheocast 2024 samples were sub-jected to T6 heat treatment to assess its effects on the micro-structure and mechanical properties. The as-cast samples were solution treated at a 500°C, quenched in water at room temperature, and then aged at 190°C for 9 h.
The samples for metallographic investigations were cut from the geometric center of the produced samples. These samples were subjected to standard metallographic proce-dures, etched in Keller’s etching reagent (2 mL HF, 3 mL HCl, 5 mL HNO3, and 90 mL H2O) for 15 s, and examined with an Olympus-BX60M optical microscope. A Camscan MV2300 scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS) was also employed.
The equivalent diameter (De) and shape factor (SF) of globules were measured using the microstructural image processing software. For each sample, a total number of 100 randomly selected globules (N = 100) were analyzed. The De and SF of globules were calculated in each case using Eqs. (1) and (2), respectively [26]:
1e
4
π
N A
DN
=
(1)
1
4π
SF
N A
PN
=
(2)
where A and P are the area and perimeter of globules, re-spectively, and SF is a number indicating the degree of globule sphericity ranging from 0 to 1.
Macro-hardness and compressive strength measurements were used to determine the effects of T6 heat treatment on
the mechanical properties of the rheocast samples. Brinell hardness tests using a 3.25-mm ball and a 245 N force were performed in different regions of the processed samples. The mean values of at least five measurements conducted on different areas of each sample were considered. The samples for compression tests were machined into cylindrical shapes with 10-mm diameter and 16-mm height. Compression tests (1 × 104 kg) were performed using a universal electronic tensile testing machine. Compressive strength was measured at the moment of fracture. Compression tests were per-formed at a deformation rate of 0.001/s at room temperature.
3. Results and discussion
3.1. Microstructural investigations
The optical micrographs of Al-2024 semi-solid alloy with the solid fraction of 0.3 produced at different stirring speeds are presented in Fig. 5. Stirring speed plays an important role in the rheocasting process [3−4]. Fig 5(a) shows the microstructure of Al-2024 alloy produced without the ap-plication of shear force during cooling to the semisolid re-gion. It can be seen that the microstructure consists of den-dritic grains. With mechanical stirring of the molten metal, the microstructure is transformed from dendritic to globular, as indicated in Figs. 5(b)–5(f). The slurry formation was as-sumed to occur below Tliquidus [27]. The impeller was intro-duced into the molten aluminum alloy having a temperature slightly above Tliquidus. At this stage, the impeller was rotated at a defined speed, and stirring was continued to well below Tliquidus. The produced nuclei likely underwent spherical growth under forced convection, giving rise to fine, rela-tively globular α-grains surrounded by liquid. Finally, the semisolid slurry was rapidly quenched.
The effect of stirring speed on the size of primary α-grains is shown in Fig. 6. It can be observed that the size of the primary α-grains decreases with the increase in stir-ring speed. The rate of the change also decreases at higher stirring speeds. Increasing the stirring speed from 450 to 600 r/min (solid fraction = 0.3) leads to a decrease of 30% in grain size; however, the grain size is decreases by nearly 15% when the stirring speed is further increased from 600 to 820 r/min. A decrease in grain size with increased stirring speed has been reported by other authors [3−4], and can be attributed to the increased fragmentation of the initially formed dendrites because of enhanced shear forces. The rapid penetration of the liquid phase along dendrite arms is the main reason for the formation of primary particles. Note that grain boundaries are formed through the recovery of the geometrically necessary dislocations generated by the plas-
62 Int. J. Miner. Metall. Mater., Vol. 22, No. 1, Jan. 2015
tic bending of dendrite arms under the shear force. The dis-location density increases when the stirring speed is in-creased, and the bent arms subsequently detach due to the wetting of high angle grain boundaries by molten metal. Broken dendrite branches act as crystal nuclei, leading to re-
finement of the grains. Increasing the stirring speed beyond 600 r/min has an insignificant effect on the reduction of the primary particle size; the reduction beyond 600 r/min is only 7 µm. This can be attributed to the saturation of dendrite arm detachment at a stirring speed of 600 r/min.
Fig. 5. Rheocast microstructures at the solid fraction of 0.3 and different stirring speeds: (a) 0 r/min; (b) 450 r/min; (c) 500 r/min; (d) 600 r/min; (e) 700 r/min; (f) 820 r/min.
The effect of stirring speed on the globularity of primary α-Al particles, as indicated by SF, is shown in Fig. 7. The SF value first increases and then decreases with the increase in stirring speed. The maximum SF value was achieved at a
stirring speed of 600 r/min. According to the fragmentation mechanism, the increased stirring speed tends to improve the globularity of primary α-Al particles. The sample pro-duced at the lower stirring speed (450 r/min) had mostly
B. Rahimi et al., Microstructural characteristics and mechanical properties of Al-2024 alloy processed via a rheocasting … 63
Fig. 6. Variation in α-Al particle grain size as a function of stirring speed.
Fig. 7. Variation of SF as a function of stirring speed.
rosette-like primary particles that became spherical when the stirring speed was increased to 600 r/min. At even higher
stirring speeds, however, the globularity of the particles de-creased due to the agglomeration of the solid particles in the slurry, forming nonspherical clumps. Stirring at higher speeds causes more shear near the impeller and less turbu-lence away from the stirrer, resulting in globules that are less spherical. As the stirring speed is increased, the prob-ability of forming particles with irregular shapes becomes greater.
The SEM micrographs of the as-cast Al-2024 alloy ob-tained at the stirring speeds of 0 (nonstirred) and 820 r/min are shown in Fig. 8. The microstructure of the nonstirred al-loy consists of dendritic grains surrounded by a nonequilib-rium eutectic phase (CuAl2-white phase), while the rheocast sample has a nondendritic grain structure. The SEM micro-graph (Fig. 9) reveals two contrasts in the rheocast sample: gray and white. EDS patterns from the two observed regions are depicted in Fig. 10. The gray phase can be identified as Al15(CuFeMn)3Si2, while the white phase is CuAl2. Because of the low solubility of iron in aluminum, these elements combine with other alloying elements (Cu, Mn, and Si) seg-regated due to the applied shear stress. This leads to the formation of a nonsoluble phase (Al15(CuFeMn)3Si2) in the vicinity of the CuAl2 phase at the grain boundaries of the rheocast sample.
Fig. 8. SEM micrographs of the as-cast Al-2024 alloy obtained at the stirring speeds of 0 r/min (a) and 820 r/min (b). Arrays in (a) show the primary arms of dendrites.
Fig. 9. Presence of two different phases (white and gray) at the grain boundaries of the rheocast sample stirred at 820 r/min.
3.2. T6 heat treatment and mechanical properties
After rheocasting, T6 heat treatment was conducted. The aging treatment was performed at 190°C for 9 h in order to increase the hardness of the as-cast alloy. The complete dis-solution of the nonequilibrium phases at the first stage of T6 heat treatment (i.e., solution treatment) requires sufficient time (t), which can be roughly estimated from Eqs. (3) and (4) [28]:
2xt
D= (3)
e
2
Dx = (4)
64 Int. J. Miner. Metall. Mater., Vol. 22, No. 1, Jan. 2015
Fig. 10. EDS patterns of the two observed phases at the grain boundaries of the rheocast sample.
where x and D are the diffusion distance and diffusion coef-ficient for copper in aluminum, respectively. Given
14 2( 500 C) 4.8 10 cm /sTD −
= ° = × [28], the required solution treatment time was calculated for different experimental stirring speeds. The results are listed in Table 2. The solu-tion treatment time of the rheocast samples is greater than 2 h, which is the standard solution treatment time for conven-tionally wrought 2024 alloy. This means that the rheocast sample requires a longer period for homogenization than the conventionally wrought alloy. This result agrees with the results of Zoqui and Robert [29], who investigated the effect of solution and aging treatments on the structure and proper-ties of rheocast and conventionally cast Al-4.5wt%Cu alloys. They found that the rheocast material required a more time for the eutectic phase to completely dissolve compared to the cast structures. The solution treatment time decreases with the increase in stirring speed (Table 2) due to the de-creased diffusion difference.
Table 2. Required solution treatment time for different stir-ring speeds
Stirring speed / (r⋅min−1) De / µm x / µm t / h
450 78 40 8.8
500 70 35 7.1
600 62 31 5.6
700 58 29 4.8
820 54 27 4.2
Fig. 11(a) shows the SEM micrograph of the rheocast
Al-2024 alloy after the solution heat treatment. At the solution heat treatment stage, the nonequilibrium eutectic CuAl2 phase has been completely dissolved, while the Al15(CuFeMn)3Si2 phase remains unaffected. The presence of the nonsoluble Al15(CuFeMn)3Si2 phase after the solution heat treatment is shown at a higher magnification in Figs. 11(b) and 11(c).
Fig. 11. SEM micrograph of the rheocast Al-2024 alloy (stirring speed: 820 r/min) after the 4.2-h solution heat treatment (a) and the presence of the Al15(CuFeMn)3Si2 phase after the solution heat treatment (b, c).
The variation in hardness of the rheocast Al-2024 sam-ples as a function of stirring speed for the as-cast and T6
conditions is shown in Fig. 12. Compared to the nonstirred sample, the hardness values of rheocast samples stirred at
B. Rahimi et al., Microstructural characteristics and mechanical properties of Al-2024 alloy processed via a rheocasting … 65
450–800 r/min are increased. This can be attributed to the presence of the nonsoluble Al15(CuFeMn)3Si2 phase and the homogeneous distribution of the CuAl2 phase at the grain boundaries of these samples. It is also clear from Fig. 12 that the hardness value increases with the increase in stirring speed; an increase in stirring speed from 0 to 820 r/min leads to a 54% increase in hardness. The observed decrease in the grain size of α-Al globules with the increase of stir-ring speed explains this increase in hardness, which is con-sistent with the results shown in Fig. 6.
Fig. 12. Variation in hardness as a function of stirring speed.
Fig. 12 also demonstrates an improvement in hardness values after the post-forming T6 heat treatment. This in-crease in hardness is due to the cooperative precipitation of fine CuAl2 precipitates during the artificial aging stage. The precipitate particles act as obstacles to dislocation move-ment by forcing the dislocations to either cut through the
precipitated particles or go around them, thus strengthening the alloy.
The compressive properties of the rheocast samples pre-pared at different stirring speeds before and after T6 heat treatment are shown in Fig. 13 and Table 3. It should be noted that the compression tests were conducted at a defor-mation rate of 0.001/s at room temperature. The compres-sive strength increases with the increase in stirring speed. Increasing the stirring speed from 0 to 820 r/min resulted in a 45% increase in compressive strength of the as-cast alloy. This effect is attributed to the decreased grain size of α-Al particles that results from the increased stirring speed (Fig. 6). The increased compressive strength of the samples stirred at 450–820 r/min compared to the nonstirred sample can also be attributed to the presence of the (Al15(CuFeMn)3Si2) phase near the CuAl2 phase at the grain boundaries of the stirred samples (Fig. 10). By increasing the stirring speed, however, the compressive strain of the rheocast samples first increases and then declines, with a maximum compressive strain obtained at a stirring speed of 600 r/min. The compre-ssive strain of the sample increases with the increasing de-gree of globularity of α-Al particles and with the decreasing particle grain size. The compressive strain value is enhanced when the stirring speed is increased from 0 to 600 r/min, which is consistent with the results given in Figs. 6 and 7. As discussed earlier, stirring speeds beyond 600 r/min have only a minor effect on the reduction of the primary particle size. Thus, the globularity of α-Al particles is believed to be the main reason for the observed trend in this region. Hence, it can be deduced that increasing the stirring speed beyond 600 r/min results in decreased compressive strain due to the decrease in the degree of globularity of α-Al particles.
Fig. 13. Compression stress–strain curves of the rheocast samples: (a) in the as-cast condition; (b) after T6 heat treatment.
The compression test results also show that compressive properties are improved by the post-forming T6 heat treat-
ment. For example, the compressive strength and compres-sive strain of the sample stirred at 600 r/min increase by
66 Int. J. Miner. Metall. Mater., Vol. 22, No. 1, Jan. 2015
30% and 8.6%, respectively, because of T6 heat treatment. The improvement in compressive strength of the T6 heat-treated samples compared to the as-cast samples is due to the precipitation hardening effect because the precipitated particles act as obstacles to dislocation movement. The in-creased compressive strain can be also attributed to the col-lapse of the coarse, brittle and continuous nonequilibrium eutectic CuAl2 phase during T6 heat treatment.
Table 3. Compression properties of the rheocast samples with and without T6 heat treatment
Nonaged Aged Stirring speed /
(r⋅min−1) Compressive
strength / MPa Compressi-bility / %
Compressive strength / MPa
Compressi-bility / %
0 382 24.0 481 26.0
450 417 25.4 531 27.7
500 464 26.7 594 30.1
600 490 30.0 635 32.6
700 519 29.1 680 31.4
820 551 27.3 732 30.4
4. Conclusions
(1) By increasing the stirring speed of the rheocasting process, the grain size of α-Al particles was decreased. The globularity of these particles was increased with the stirring speed increasing up to an optimum level, after which the globularity declined.
(2) Rheocast samples require a longer period for homog-enization than conventionally wrought alloys.
(3) By increasing the stirring speed, the required time for the solution treatment stage (at a fixed temperature) was de-creased.
(4) The rheocast samples showed significant improve-ments in mechanical properties (hardness and compression properties) compared to the nonstirred sample.
(5) By increasing the stirring speed, the hardness and compressive strength of the alloys were increased; however, the maximum compressive strain was obtained at a stirring speed of 600 r/min.
(6) An improvement in hardness and compressive prop-erties of the alloy was observed after T6 heat treatment.
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