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Improvement in extrudability and mechanical properties of AZ91 alloy through extrusion with artificial cooling
Author’s Accepted Manuscript Improvement in extrudability and mechanical properties of AZ91 alloy through extrusion with artificial cooling Sang-Hoon Kim, Jong Un Lee, Ye Jin Kim, Byoung Gi Moon, Bong Sun You, Ha Sik Kim, Sung Hyuk Park www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(17)30942-5 http://dx.doi.org/10.1016/j.msea.2017.07.048 MSA35296
To appear in: Materials Science & Engineering A Received date: 4 April 2017 Revised date: 12 July 2017 Accepted date: 16 July 2017 Cite this article as: Sang-Hoon Kim, Jong Un Lee, Ye Jin Kim, Byoung Gi Moon, Bong Sun You, Ha Sik Kim and Sung Hyuk Park, Improvement in extrudability and mechanical properties of AZ91 alloy through extrusion with artificial cooling, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2017.07.048 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Improvement in extrudability and mechanical properties of AZ91 alloy through extrusion with artificial cooling Sang-Hoon Kim a, Jong Un Lee a, Ye Jin Kim a, Byoung Gi Moon b, Bong Sun You b, Ha Sik Kim b, Sung Hyuk Park a,*
a
School of Materials Science and Engineering, Kyungpook National University, Daegu 41566,
Republic of Korea b
Implementation Research Division, Korea Institute of Materials Science, Changwon 51508,
Republic of Korea
*Corresponding author. E-mail address: [email protected] (S.H. Park).
Abstract
This study demonstrates that the application of artificial water cooling during extrusion effectively increases the extrudability of the AZ91 alloy and significantly improves the mechanical properties of the extruded AZ91 alloy. The artificial cooling dramatically reduces the actual temperature of the deformation zone, which results in an increase in the maximum exit speed at which the alloy is extrudable without the occurrence of hot cracking from 4.5 m/min to 7.5 m/min. It also promotes dynamic recrystallization and precipitation behaviors during extrusion, which leads to a reduction in grain size and an increase in the amount of fine Mg17Al12 precipitates. As a result, for the AZ91 alloy extruded at an exit speed of 1.5 m/min, the tensile and compressive yield strengths improve significantly by 51 MPa and 114 MPa,
respectively, and its tension–compression yield asymmetry reduces from 0.73 to 1.02 owing to the refinement of the grain size by artificial cooling. In addition, the AZ91 alloy extruded at an exit speed of 7.5 m/min with artificial cooling exhibits a finer grain structure than and superior mechanical properties to the AZ91 alloy extruded at a slower exit speed of 4.5 m/min without artificial cooling. This result indicates that the application of artificial cooling can simultaneously improve the maximum extrusion speed and the tensile and compressive properties of Mg alloys.
Keywords: Magnesium alloy; Extrusion; Artificial cooling; Microstructure; Mechanical properties.
1. Introduction
The demand for Mg alloys in the transportation industry has been increasing continuously given that their low density and high specific strength help satisfy the requirement of weight reduction of automobile components with the aim of improving vehicle fuel efficiency and reducing carbon dioxide emissions [1]. In recent years, extruded Mg alloys have attracted attention on account of their much better mechanical properties than cast alloys and simpler manufacturing processes than rolled alloys. The extrusion speed in the extrusion process is a crucial parameter from an industrial viewpoint because it is directly related to the productivity of the fabricated products. In Mg-Al-Zn alloy systems such as AZ31, AZ61, AZ80, and AZ91—which are the most widely used Mg alloys—the maximum extrusion speed is known to decrease with increasing Al content [2,3]; AZ31 can be extruded at an exit speed of ~20 m/min, whereas AZ91 has an extrusion speed limit of ~5 m/min. This is because with an increase in the Al content of AZ alloys, more Mg17Al12 particles with a low melting temperature of ~432 °C are formed during extrusion, which, in turn, increases the susceptibility of the material to hot cracking and decreases its extrudability [4]. Recently, studies have been conducted for improving the extrudability of Mg alloys by decreasing the alloying content of AZ alloys. It has been reported that Mg-0.5Al-0.25Zn-0.1Mn (wt%) has a maximum extrusion speed equal to or higher than 30 m/min [5] and that Mg-0.3Al0.21Ca-0.47Mn (wt%) can be successfully extruded at an exit speed of 60 m/min [6,7], which is comparable to the extrudability of the AA6063 Al alloy. In addition, Mg-Zn-based alloys containing small amounts of alloying elements, such as Mg-1.58Zn-0.52Gd [8] and Mg-0.21Zn0.3Ca-0.14Mn [9] (wt%), have been developed; these alloys show excellent extrudability with maximum extrusion speeds of higher than 60 m/min. However, these dilute Mg alloys have a relatively low strength because of the insufficiency of alloying elements, which are responsible for inducing various strengthening effects (e.g., solid-solution, grain-refinement, and precipitate strengthening). For instance, the Mg-0.21Zn-0.3Ca-0.14Mn (wt%) alloy extruded at an exit speed of 6 m/min has a tensile yield strength (TYS) of only ~105 MPa [9]. Therefore, to ensure
both extrudability and strength of a material, efforts need to be made toward improving the maximum extrusion speed of high-strength Mg alloys with large amounts of alloying elements. When local temperatures in the die land area exceed the solidus temperature of the matrix or the melting temperature of the second phase during extrusion, incipient melting of the extrudate surface and subsequent cracking can occur [3]. Even if the extrusion begins at low temperatures, the actual temperature in the deformation zone increases owing to the deformation heating that occurs during extrusion. Furthermore, as the extrusion speed increases, the amount of heat generated increases and the actual temperature also increases; this deformation-induced temperature increase is known to be proportional to the logarithmic strain rate [10,11]. Therefore, reduction of the increased temperature in the deformation zone will cause suppression of hot cracking and an increase in the extrudability. Artificial cooling during extrusion, which is performed by directly spraying water onto the extruded rod at the die exit, is a highly effective method for lowering the actual extrusion temperature [12–14]. Kim et al. [12] reported indirect extrusion using artificial cooling of AZ31 and found that artificial cooling is effective in decreasing the grain size and improving the strength of the extruded alloy. Our previous investigations on the effects of artificial cooling in the Mg-7Sn-1Al-1Zn (wt%) alloy [13,14] also demonstrated that artificial cooling results in a decrease in the area fraction and size of dynamically recrystallized (DRXed) grains and promotion of Mg2Sn precipitation. However, in AZ31, only a small amount of the Mg17Al12 phase is formed or not formed at all during extrusion, and in Mg-Sn based alloys, a thermally stable Mg2Sn phase with a high melting temperature of ~773 °C is formed as the main second phase [4,15]. No in-depth studies have yet been conducted for examining the influence of artificial cooling on the extrudability and dynamic deformation behavior of high-Al-content AZ alloys, in which a large amount of thermally unstable Mg17Al12 phase is formed during hot extrusion. The present study therefore investigates the change in the maximum extrusion speed with artificial cooling of AZ91—which has the highest Al content among all the commercial AZ series alloys—by performing indirect extrusions at several exit speeds with and without artificial cooling. Variations in the microstructural characteristics and tensile and compressive properties of extruded AZ91 induced
by artificial cooling are also examined and discussed herein.
2. Experimental procedure
Cast billets of the Mg-9Al-1Zn-0.2Mn (wt%) (AZ91) alloy were prepared according to a previously described method [16]. The billets were homogenized at 420 °C for 24 h to fully dissolve any macrosegregated elements or intermetallic compounds formed during solidification, after which they were water-quenched to obtain a supersaturated solid solution. All billets (Ø80 mm × 200 mm) preheated to 350 °C were indirectly extruded at a temperature of 350 °C with an extrusion ratio of 25 by a previously described process [14]. To analyze the variation in the extrudability of AZ91 induced by the artificial cooling performed during extrusion, three billets were extruded at exit speeds of 3.0, 4.5, and 6.0 m/min without artificial cooling, and three other billets were extruded at exit speeds of 6.0, 7.5, and 9.0 m/min with artificial cooling at a water feed rate of 5 L/min. In addition, to investigate the effects of artificial cooling on the microstructure and mechanical properties of the extruded AZ91 alloy, two billets were extruded at the same exit speed of 1.5 m/min; one of these was subjected to artificial cooling during extrusion. The samples extruded without and with artificial cooling are hereafter referred to as NC-AZ91 and AC-AZ91, respectively. A schematic of the indirect extrusion process and its associated artificial cooling system can be found in the literature [13,14]. When the artificial cooling system is operated at the initial stage of extrusion, cold water moves through holes inside the stem to a die containing holes, and it is sprayed directly onto the extruded material that has just exited the die. In this study, the variation in die temperature during extrusion was measured via a thermocouple installed inside the die. The microstructural characteristics of the extruded samples were analyzed by optical microscopy (OM), field emission scanning electron microscopy (FESEM), and electron backscatter diffraction (EBSD). The preparation method of the samples used for each of these observations has been reported elsewhere [17]. The EBSD data were analyzed using the TexSEM Laboratories orientation imaging microscopy (TSL OIM) 7.0 software, and data with a
confidence index greater than 0.1 were used for grain size and texture analyses. The area fraction of DRXed grains in each extruded sample was determined by measuring the unDRXed regions in a relatively large area of ~54 mm2 in OM images. The tensile and compressive properties of the extruded samples were measured at room temperature by using an Instron 4206 universal testing machine with a strain rate of 1.0 × 10-3 s-1. Dog-bone-shaped (gage section: Ø6 mm × 25 mm) and cylindrical (Ø8 mm × 12 mm) samples were used for tension and compression tests, respectively; their axes corresponded to the extrusion direction (ED).
3. Results and discussion
3.1. Improvement in extrudability via artificial cooling
Fig. 1(a) shows images of the samples extruded with and without artificial cooling. When the billet is extruded at 4.5 m/min without artificial cooling, an extruded sample with high surface quality is obtained. On the other hand, when the billet is extruded at 6.0 m/min without artificial cooling, numerous hot cracks perpendicular to the ED are formed on the surface of the extruded sample. This means that when the indirect extrusion is performed at a temperature of 350 °C and an extrusion ratio of 25, the maximum exit speed at which the AZ91 alloy is extrudable is less than 6.0 m/min. Although the extrusion begins at the same temperature of 350 °C, as the extrusion speed increases, the actual temperature in the deformation zone near the die exit increases owing to the deformation induced and friction heat generated during extrusion. Fig. 1(b) shows changes in the measured die temperature during extrusion without artificial cooling, which reflects the evolution of the actual temperature applied to the alloy. As the billet begins to exit the die, the die temperature rises sharply at the beginning of the extrusion process. The maximum die temperature gradually increases with increasing extrusion speed: it is 388 °C at 3.0 m/min, 397 °C at 4.5 m/min, and 411 °C at 6.0 m/min. When extrusion is performed at 4.5 m/min, the actual deformation temperature during the extrusion is lower than the incipient melting temperature of AZ91 (~427 °C [18]), so it can
be extruded without the formation of hot cracks. However, in the case of extrusion at 6.0 m/min, the actual temperature applied to the material exceeds the incipient melting temperature, which causes the occurrence of hot cracking owing to local melting at the surface of the extruded bar. Application of artificial water cooling during extrusion results in lowering of the actual deformation temperature on account of a reduction in the deformation and friction heat generated during extrusion; this, in turn, suppresses the occurrence of hot cracking in highspeed extrusion. As shown in Fig. 1(a), although the billet is extruded at a relatively high speed of 7.5 m/min, an extruded sample free from cracks is successfully obtained by the application of artificial cooling; however, severe hot cracking occurs at an exit speed of 9.0 m/min. These results indicate that the extrudability of AZ91 alloy improves considerably from 4.5 to 7.5 m/min when it is subjected to artificial cooling during extrusion.
3.2. Microstructural evolution by artificial cooling
Optical micrographs of the AZ91 samples extruded at a speed of 1.5 m/min with and without artificial cooling are shown in Fig. 2. Although AC-AZ91 has a few large-sized unDRXed grains (their area fraction is only ~3.1%), both samples exhibit an almost completely DRXed structure. However, there is a drastic difference in the grain sizes of the two samples. In NC-AZ91, the grain size in the region with few precipitates is ~26.1 mm, whereas that in the region with precipitate bands is smaller, ~8.1 mm, owing to the grain boundary pinning effect of the precipitates; the average grain size of NC-AZ91 with bimodal DRXed grains is ~20.1 mm (Fig. 3(a)). On the other hand, the size of DRXed grains of AC-AZ91 is relatively uniform (~1– 4 mm), and the average size is rather small, ~2.6 mm (Fig. 3(b)); this indicates that the crystal grains of the extruded AZ91 alloy are significantly refined by artificial cooling. Both samples show similar crystallographic orientation distributions and maximum intensities of the inverse pole figure (Fig. 3), which means that application of artificial cooling has a negligible effect on the texture evolution of the extruded material under the extrusion conditions adopted in this study. It is noted, however, that under extrusion conditions where DRX does not occur
completely (e.g., at lower extrusion temperatures, speeds, or ratios [19,20]), the texture of the extruded material can change significantly with an increase in the amount of unDRXed grains through artificial cooling, because the DRX fraction of Mg alloys deformed at elevated temperatures generally decreases with a decrease in the actual deformation temperature [3,19] and unDRXed grains have a much higher intensive basal texture than DRXed grains [17,21,22]. The SEM micrographs in Fig. 4 show that Mg17Al12 particles precipitated dynamically during extrusion of the samples. In NC-AZ91, precipitate bands parallel to the ED (~40–100 µm in width) are visible (Figs. 2(a), 3(a), and 4(a)), in which a reasonable amount of fine Mg17Al12 precipitates (~0.5–1.5 µm in size) is distributed (Fig. 4(b)); however, a much smaller amount of precipitates exists in the region other than the precipitate bands (Fig. 4(c)). The formation of these precipitate bands can be attributed to the microsegregation of Al atoms dissolved in the matrix of the homogenized billet [3]. Inhomogeneous distribution of dissolved Al can result in a localized formation of dynamic Mg17Al12 precipitates in the region with high Al content. Furthermore, since the crystallographic orientation distribution of each grain of the homogenized billet is random, the shear stress imposed in each grain during extrusion also varies with the angle relationship between the external stress and the crystal orientation [23]. Accordingly, in the initial grains of the homogenized billet, in which a high resolved shear stress is applied during extrusion, Mg17Al12 precipitates can form preferentially owing to the enhanced dynamic precipitation. Since these initial grains are elongated along the ED during extrusion, the precipitate bands are also formed parallel to the ED. AC-AZ91 also exhibits a nonuniform distribution of Mg17Al12 precipitates (Fig. 4(d)); it consists of a region with a large amount of precipitates (Fig. 4(e)) and a region with a relatively smaller amount of precipitates (Fig. 4(f)). However, the total amount of precipitates is much larger than that in NC-AZ91 and their size (~300–700 nm) is also smaller than that in NC-AZ91 (~0.5–1.5 µm). This enhanced dynamic precipitation behavior of AC-AZ91 is attributed to the fact that the decrease in extrusion temperature due to artificial cooling promotes the formation of Mg17Al12 precipitates. In addition, artificial cooling applied during extrusion causes not only a decrease in the actual deformation temperature but also an increase in the cooling rate of the extruded bars. When the
alloy is extruded without artificial cooling, the extruded bar maintains a high temperature immediately after exiting the die, and thus, static precipitation of the Mg17Al12 phase can occur during cooling in air. Since the density of dislocations—which act as the nucleation sites of the precipitates—is low inside the recrystallized grains newly formed during hot deformation or heat treatment, the static precipitates are preferentially formed along the grain boundaries, where diffusion of solute atoms occurs more easily than in the grains owing to the high interface energy of the grain boundaries. In the case of NC-AZ91, static precipitation induced by a low cooling rate after extrusion is confirmed by the presence of Mg17Al12 precipitates formed along the DRXed grain boundaries (Fig. 4(c)). On the other hand, AC-AZ91 cooled rapidly by sprayed water during extrusion does not have sufficient heat energy and time for undergoing static precipitation after it exits the die. As a result, although AC-AZ91 has a larger amount of dynamically formed precipitates than NC-AZ91, it does not show any static precipitates at the grain boundaries, unlike NC-AZ91 (Fig. 4(f)). Fig. 5 shows the variation in the die temperature and extrusion load during extrusion with and without artificial cooling. When the billet is extruded without artificial cooling, the die temperature increases to 375 °C at the early stage of extrusion and then gradually decreases to 352 °C (Fig. 5(a)). On the other hand, when artificial cooling is performed, the die temperature rapidly decreases to 256 °C and then gradually decreases to 213 °C; therefore, at the end of extrusion, the die temperature is considerably lower (a reduction of 137 °C) on account of artificial cooling. This drastic reduction in the deformation temperature may accelerate the dynamic precipitation by decreasing the Al solubility in the a-Mg matrix. Fig. 6 shows the change in the Al solubility limit in the Mg-1Zn alloy and the phase fraction of Mg17Al12 in the Mg-9Al-1Zn alloy with increasing temperature for the equilibrium states, which are calculated using the PANDAT software. As the Al solubility at a homogenization temperature of 420 °C is 10.94 wt%, 9.0 wt% Al added to the AZ91 alloy dissolves completely in the matrix upon homogenization treatment; complete dissolution of the Mg17Al12 phase of cast AZ91 by homogenization has also been reported elsewhere [16,24,25]. Under the assumption that the die temperature is equal to the actual deformation temperature during extrusion, the sample without
artificial cooling is extruded at 352 °C and the Al solubility at this temperature is 7.82 wt%; therefore, 1.18 wt% of Al supersaturated in the matrix will precipitate as the Mg17Al12 phase. On the other hand, as the sample subjected to artificial cooling is extruded at a relatively low temperature of 213 °C, a much larger amount of Al (5.97 wt%) will precipitate as the Mg17Al12 phase on account of the greatly reduced Al solubility (3.03 wt%). With a decrease in the actual extrusion temperature from 352 °C to 213 °C, the calculated fraction of the precipitated Mg17Al12 phase increases significantly from 3.2% to 14.3%. In addition, the extrusion load increases from 183 ton to 209 ton during extrusion without artificial cooling, whereas it increases from 185 ton to 261 ton when artificial cooling is performed, as shown in Fig. 5(b). Accordingly, use of the cooling system increases the stress imposed on the material during extrusion, which, in turn, can promote the dynamic precipitation behavior owing to the formation of more dislocations that act as nucleation sites for precipitation [3,23,26]. Moreover, as the deformation temperature reduces, the dynamically formed precipitates become smaller in size because of the suppression of the Ostwald ripening phenomenon, wherein smaller particles disappear gradually while larger particles enlarge [27–30]. Therefore, AC-AZ91 has a much larger amount of finer precipitates than NC-AZ91, as shown in Fig. 4.
3.3. Improvement in strength and reduction in yield asymmetry by artificial cooling
The tensile and compressive stress–strain curves of NC-AZ91 and AC-AZ91 are shown in Fig. 7; the tensile and compressive properties are listed in Table 1. The TYS, ultimate tensile strength (UTS), and tensile elongation of NC-AZ91 are 211 MPa, 324 MPa, and 13.9%, respectively, whereas those of AC-AZ91 are 262 MPa, 364 MPa, and 13.6%, respectively. Although the same alloy (i.e., AZ91) is extruded under the same extrusion conditions (temperature, speed, and ratio), the TYS and UTS of the extruded material are considerably enhanced by 51 MPa and 40 MPa, respectively, without a loss of ductility, simply through application of artificial cooling. This significant improvement in tensile properties is attributed mainly to the decrease in grain size (from 20.1 µm to 2.6 µm) and to the increase in the amount
of Mg17Al12 precipitates (from 3.2% to 14.3%), each of which enhances the grain boundary hardening and precipitation hardening effects during tensile deformation. It is known that in extruded Mg alloys containing unDRXed grains, {10-11} contraction and {10-11}-{10-12} double twins, which act as microcracking sites during plastic deformation [31], can easily form in the unDRXed grains during tension along the ED owing to their sizes being larger than those of DRXed grains. This eventually causes deterioration of the tensile ductility of the extruded material [20,32,33]. However, under the extrusion conditions adopted in this study, the fraction of unDRXed grains formed by artificial cooling is so small (~3.1%) that such ductility deterioration does not occur. The compressive yield strength (CYS) also increases considerably, from 153 MPa to 267 MPa, by artificial cooling (Fig. 7(b) and Table 1); note that this increase in the CYS (114 MPa) is ~2.2 times that in the TYS (51 MPa). In general, when extruded Mg alloys are subjected to plastic deformation along the ED, dislocation slips and {10-12} twinning become the main deformation mechanisms in tension and compression, respectively. This is because of the combined effect of the basal texture of the materials, where the basal planes of most grains are oriented parallel to the ED, and the directional nature of twinning [34]. The activation stress for {10-12} twinning in compression along the ED is smaller than that for the dislocation slips in tension along the ED, owing to the high values of the Schmid factor for {1012} twinning [35–37]; this, in turn, results in a tension–compression yield asymmetry (i.e., CYS/TYS ratios lower than 1). NC-AZ91 also exhibits a low CYS/TYS ratio of 0.73 (Table 1). It is known that in Mg, the Hall–Petch coefficient for twinning is higher than that for slip [3,38], which indicates that the twinning stress is more sensitive to grain size than is the slip stress [39]. Accordingly, the increase in the CYS of the extruded AZ91 sample, which is caused by grain refinement through artificial cooling, becomes larger than that in the TYS. In addition, as twinning is known to be suppressed when the grain size is smaller than 1–2 mm [3,40], in ACAZ91 with a fine grain size of ~2.6 mm, the formation of {10-12} twins during compressive deformation is quite limited; this, in turn, results in almost the same TYS (262 MPa) and CYS (267 MPa). In other words, AC-AZ91 exhibits isotropic tension–compression yielding behavior with a CYS/TYS ratio of 1.02 (Table 1).
3.4. Simultaneous improvement in extrusion speed and mechanical properties
To investigate the effects of artificial cooling at high extrusion speeds, the microstructures and mechanical properties of NC-AZ91 and AC-AZ91 extruded at the highest speed at which hot cracking does not occur, i.e., of NC-AZ91 extruded at 4.5 m/min and of ACAZ91 extruded at 7.5 m/min, are compared. These samples are hereafter referred to as NC45AZ91 and AC75-AZ91, respectively. The optical and SEM micrographs of NC45-AZ91 and AC75-AZ91 are shown in Fig. 8; both samples are found to exhibit a fully DRXed structure. As the extrusion speed increases, the deformation and friction heat generated during extrusion become larger and the actual deformation temperature increases, which results in an increase in the area fraction and size of the DRXed grains [41–44]. However, although AC75-AZ91 is extruded ~1.67 times faster than NC45-AZ91, the grain size of the former is finer; the average grain size of AC75-AZ91 is 10.9 μm, which is ~0.45 times that of NC45-AZ91 (24.2 μm) (Fig. 9). This means that artificial cooling reduces the actual deformation temperature even at a high extrusion speed of 7.5 m/min, which results in grain refinement through suppression of the growth of DRXed grains. Unlike in the samples extruded at 1.5 m/min, nearly no fine dynamic precipitates are present in either NC45-AZ91 or AC75-AZ91; however, the latter two samples have static precipitates that were formed along the grain boundaries (Figs. 8(c) and (d)). When the billet is extruded at 4.5 m/min without artificial cooling, the Al solubility limit of the a-Mg matrix increases sufficiently to maintain the dissolved Al atoms in a solid state, and so, the Mg17Al12 phase does not precipitate dynamically. Artificial cooling reduces the deformation temperature even at high extrusion speeds. However, when the extrusion speed is as high as 7.5 m/min, the time for which the extruded bar is exposed to water sprayed through the die is shortened, and the cooling effect is weaker than that when the extrusion is performed at a relatively lower speed of 1.5 m/min. Therefore, although the grains of AC75-AZ91 become finer owing to a certain degree of cooling, the temperature drop that occurs is insufficient to cause dynamic precipitation. Moreover, the extruded bar passing through the sprayed water still
has a high temperature, because of which static precipitation occurs during natural cooling in air. As in the case of the samples extruded at 1.5 m/min, the texture of those extruded at higher speeds is not significantly influenced by artificial cooling (Fig. 9). Fig. 10 shows the tensile and compressive stress–strain curves of NC45-AZ91 and AC75-AZ91. For both the samples with and without artificial cooling, the TYS, UTS, and CYS decrease as the extrusion speed increases (Table 1); this is attributed mainly to the increased grain size and decreased amount of fine dynamic precipitates. However, it should be noted that AC75-AZ91 extruded at an approximately 1.67 times higher speed exhibits higher tensile and compressive strengths than NC45-AZ91, with almost no difference in their ductilities (Fig. 10); the TYS, UTS, and CYS of AC75-AZ91 are, respectively, 21 MPa, 15 MPa, and 25 MPa higher than those of NC45-AZ91 (Table 1). Since both samples have small amounts of dynamic precipitates and their textures are also almost the same, this higher strength of AC75-AZ91 can be attributed to a finer grain size (24.2 μm and 10.9 μm for NC45-AZ91 and AC75-AZ91, respectively). Furthermore, the tension–compression yield asymmetry is also smaller in AC75AZ91 than in NC45-AZ91; the CYS/TYS ratios for NC45-AZ91 and AC75-AZ91 are 0.75 and 0.80, respectively. These results show that the application of artificial cooling—wherein water is directly sprayed onto the extruded material during extrusion—not only increases the extrudability of commercial AZ91 alloy to up to 7.5 m/min but also improves the mechanical properties of AZ91 extruded at a relatively high speed of 7.5 m/min; these improved mechanical properties are superior to those of AZ91 extruded at a lower speed of 4.5 m/min without artificial cooling.
4. Conclusions
This study demonstrates that the extrudability and mechanical properties of the AZ91 alloy can be effectively improved by performing artificial water cooling during extrusion. The maximum extrusion speed increases from 4.5 m/min to 7.5 m/min through artificial cooling, owing to a reduction in the deformation and friction heat generated during extrusion. Owing to
artificial cooling, the TYS of the AZ91 sample extruded at 1.5 m/min improves significantly from 211 MPa to 262 MPa without a loss of ductility; this improvement is attributed mainly to the refined grain size and increased amount of fine Mg17Al12 precipitates. The tension– compression yield asymmetry also reduces greatly, i.e., the CYS/TYS ratio increases from 0.73 to 1.02, owing to the suppression of twinning behavior in compression by grain refinement. In addition, the AZ91 sample extruded at 7.5 m/min with artificial cooling exhibits a finer grain structure and higher strength than the AZ91 sample extruded at a lower speed of 4.5 m/min without artificial cooling. This clearly shows that the application of artificial cooling can simultaneously improve the maximum extrusion speed and the mechanical properties of Mg alloys. Application of artificial water cooling during extrusion will enhance the productivity of the extruded material by facilitating extrusion at higher speeds; this will result in a lowering of the cost of the final products and subsequently lead to an expansion of the application range of extruded Mg alloys by virtue of their improved mechanical properties. However, the abovementioned beneficial effects caused by artificial cooling can change depending on the compositions of the employed alloy and the processing parameters. Therefore, further studies are required to investigate the effects of artificial cooling in various Mg alloy systems and under various extrusion conditions in terms of variations in their dynamic recrystallization and precipitation behaviors, microstructural and textural characteristics, resultant mechanical properties, and deformation and fracture mechanisms.
Acknowledgments
This study was supported by the R&D Center for Valuable Recycling (Global-Top R&BD Program) of the Ministry of Environment of Korea (Project No. 2016002220003).
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Table and Figure Captions
Fig. 1.
(a) Images depicting surface quality of AZ91 samples extruded at 350 °C without and with artificial cooling. (b) Variation in die temperature during extrusion at 350 °C with exit speed. Vexit denotes the exit speed of the extruded bars.
Fig. 2.
Optical micrographs of AZ91 samples extruded at exit speed of 1.5 m/min (a) without and (b) with artificial cooling. fDRX denotes the area fraction of dynamically recrystallized grains.
Fig. 3.
Inverse pole figure map, ED inverse pole figure, and grain size distribution of AZ91 samples extruded at exit speed of 1.5 m/min (a) without and (b) with artificial cooling.
Fig. 4.
SEM micrographs of AZ91 samples extruded at exit speed of 1.5 m/min (a)–(c) without and (d)–(f) with artificial cooling. (b), (c) Enlarged images of rectangular areas A and B, respectively, in (a). (e), (f) Enlarged images of rectangular areas A and B, respectively, in (d).
Fig. 5.
Variation in (a) die temperature and (b) extrusion load during extrusion at exit speed of 1.5 m/min with and without artificial cooling.
Fig. 6.
Variations in Al solubility limit of Mg-1Zn alloy and phase fraction of Mg17Al12 of Mg-9Al-1Zn alloy for equilibrium states, as calculated with PANDAT software. Thomo. denotes the homogenization temperature (420 °C).
Fig. 7.
(a) Tensile and (b) compressive stress–strain curves of AZ91 samples extruded at exit speed of 1.5 m/min with and without artificial cooling.
Fig. 8.
(a), (b) Optical and (c), (d) SEM micrographs of AZ91 samples extruded (a), (c) without cooling at exit speed of 4.5 m/min and (b), (d) with artificial cooling at exit speed of 7.5 m/min.
Fig. 9.
Inverse pole figure map, ED inverse pole figure, and grain size distribution of AZ91 samples extruded (a) without cooling at exit speed of 4.5 m/min and (b) with artificial cooling at exit speed of 7.5 m/min.
Fig. 10.
(a) Tensile and (b) compressive stress–strain curves of AZ91 samples extruded without cooling at exit speed of 4.5 m/min and with artificial cooling at exit speed of 7.5 m/min.
Table 1. Microstructural characteristics and mechanical properties of AZ91 sample s extruded with and without artificial cooling.
Microstructural characte ristics Extruded AZ91
Without c ooling
With cool ing
Exit s peed (m/mi
fDR X
X
Im
n)
(%
(µ
ax
)
m)
1.5
10 0
20. 1
4.5
99. 8
1.5
7.5
Mechanical properties
*
dDR
EL
Al12
TY S (M
UT S (M
(%)
Pa)
Pa)
(% )
3. 3
3.2
211
324
13. 9
153
0.73
24. 2
3. 1
-
185
308
15. 4
139
0.75
96. 9
2.6
3. 0
14.3
262
364
13. 6
267
1.02
99.
10.
3.
2
9
0
-
206
323
164
0.80
*
fDRX, dDRX, Imax, and fMg
17Al12
fMg
17
14. 9
CY S (M
**
CYS/ TYS
Pa)
denote the area fraction of DRXed grains, average
size of DRXed grains, maximum intensity of the inverse pole figure, and calcul ated phase fraction of Mg17Al12 precipitates, respectively. **
YS, UTS, EL, CYS, and CYS/TYS denote the tensile yield strength, ultimate t ensile strength, tensile elongation, compressive yield strength, and tension–compre ssion yield asymmetry, respectively.
*QVETCEMKPI
8GZKV OOKP
*QVETCEMKPI
8GZKV OOKP
(b)
340
350
360
370
380
390
400
410
420
0
20
40
80 Ram distance (mm)
60
Exit speed 3.0 m/min 4.5 m/min 6.0 m/min
100
120
140
in die temperature during extrusion at 350 ¶C with exit speed. Vexit denotes the exit speed of the extruded bars.
Fig. 1. (a) Images depicting surface quality of AZ91 samples extruded at 350 ¶C without and with artificial cooling. (b) Variation
)QQF
8GZKV OOKP
With cooling
)QQF
8GZKV OOKP
(a) Without cooling
Die temperature (°C)
Precipitate band
100 μm
fDRX = 100%
(b) unDRXed region
100 μm
fDRX = 96.9%
Fig. 2. Optical micrographs of AZ91 samples extruded at exit speed of 1.5 m/min (a) without and (b) with artificial cooling. fDRX denotes the area fraction of dynamically recrystallized grains.
(a)
ED
Area fraction
20
30
40
50
60
70
80
Grain size (mm)
20 μm
0.00 10
0.00
0.10
0.15
0.20
0.25
0.30
0.05
0
20.1 μm
Average grain size:
(b)
0.05
0.10
0.15
0.20
0.25
0.30
Max. = 3.3
0
2
4
6
8
2.6 μm
Average grain size:
Grain size (mm)
Max. = 3.0
Fig. 3. Inverse pole figure map, ED inverse pole figure, and grain size distribution of AZ91 samples extruded at exit speed of 1.5 m/min (a) without and (b) with artificial cooling.
ED
Precipitate band region
Precipitate-free region
100 μm
(a)
Area fraction
10
ED
A
B
A
B
20 μm
20 μm
(e)
(b)
2 μm
2 μm
(f)
(c)
2 μm
5 μm
Fig. 4. SEM micrographs of AZ91 samples extruded at exit speed of 1.5 m/min (a)–(c) without and (d)–(f) with artificial cooling. (b), (c) Enlarged images of rectangular areas A and B, respectively, in (a). (e), (f) Enlarged images of rectangular areas A and B, respectively, in (d).
Precipitate-rich region
(d)
Precipitate band region
(a)
Die temperature ( C)
(a)
o
200
225
250
275
300
325
350
375
400
0
40
80
100
Ram distance (mm)
60
120
140
213 213°C C
Without cooling With cooling
160
160
180
200
220
240
260
280
0
20
40
60
80
100
Ram distance (mm)
Startingpoint pointof of Starting artificialcooling cooling artifical
Without cooling With cooling
120
140
209 ton
261 ton
Fig. 5. Variation in (a) die temperature and (b) extrusion load during extrusion at exit speed of 1.5 m/min with and without artificial cooling.
20
Starting Starting point point of of artifical artificialcooling cooling
350 °CC 352
(b)
Extrusion load (ton)
160
150
0
2
4
6
8
10
200
14.3%
300
400
7.82 wt.%
350
3.2%
THomo.
0%
10.94 wt.%
w/o cooling
Temperature (°C)
250
3.03 wt.%
with cooling
450
0.00
0.04
0.08
0.12
0.16
0.20
Fig. 6. Variations in Al solubility limit of Mg-1Zn alloy and phase fraction of Mg17Al12 of Mg-9Al-1Zn alloy for equilibrium states, as calculated with PANDAT software. Thomo. denotes the homogenization temperature (420 ¶C).
Al solubility in a-Mg (wt.%)
12
Phase fraction of Mg17Al12
Tensile stress (MPa)
0
50
100
150
200
250
300
350
400
0
2
4
6
10
Tensile strain (%)
8
12
14
AZ91 without cooling AZ91 with cooling
16
18
(b)
0
100
200
300
400
500
0
2
4
6
10
12
14
Compressive strain (%)
8
16
AZ91 without cooling AZ91 with cooling
18
20
Fig. 7. (a) Tensile and (b) compressive stress–strain curves of AZ91 samples extruded at exit speed of 1.5 m/min with and without artificial cooling.
(a) Compressive stress (MPa)
5 μm
100 μm
(d)
(b)
5 μm
100 μm
Fig. 8. (a), (b) Optical and (c), (d) SEM micrographs of AZ91 samples extruded (a), (c) without cooling at exit speed of 4.5 m/min and (b), (d) with artificial cooling at exit speed of 7.5 m/min.
(c)
(a)
ED
ED
Area fraction
20
30
40
50
60
70
80
Grain size (mm)
60 μm
0.00 10
0.00
0.10
0.15
0.20
0.25
0.30
0.05
0
24.2 μm
Average grain size:
(b)
0.05
0.10
0.15
0.20
0.25
0.30
Max. = 3.1
0
5
10
20
25
30
10.9 μm
35
Average grain size:
Grain size (mm)
15
Max. = 3.0
Fig. 9. Inverse pole figure map, ED inverse pole figure, and grain size distribution of AZ91 samples extruded (a) without cooling at exit speed of 4.5 m/min and (b) with artificial cooling at exit speed of 7.5 m/min.
100 μm
(a)
Area fraction
40
Tensile stress (MPa)
(a)
0
2
4
6
10
Tensile strain (%)
8
12
14
AZ91 without cooling (4.5 m/min) AZ91 with cooling (7.5 m/min)
16
18
(b)
0
100
200
300
400
500
0
5
15
Compressive strain (%)
10
20
AZ91 without cooling (4.5 m/min) AZ91 with cooling (7.5 m/min)
25
Fig. 10. (a) Tensile and (b) compressive stress–strain curves of AZ91 samples extruded without cooling at exit speed of 4.5 m/min and with artificial cooling at exit speed of 7.5 m/min.
0
50
100
150
200
250
300
350
Compressive stress (MPa)
PII: DOI: Reference:
S0921-5093(17)30942-5 http://dx.doi.org/10.1016/j.msea.2017.07.048 MSA35296
To appear in: Materials Science & Engineering A Received date: 4 April 2017 Revised date: 12 July 2017 Accepted date: 16 July 2017 Cite this article as: Sang-Hoon Kim, Jong Un Lee, Ye Jin Kim, Byoung Gi Moon, Bong Sun You, Ha Sik Kim and Sung Hyuk Park, Improvement in extrudability and mechanical properties of AZ91 alloy through extrusion with artificial cooling, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2017.07.048 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Improvement in extrudability and mechanical properties of AZ91 alloy through extrusion with artificial cooling Sang-Hoon Kim a, Jong Un Lee a, Ye Jin Kim a, Byoung Gi Moon b, Bong Sun You b, Ha Sik Kim b, Sung Hyuk Park a,*
a
School of Materials Science and Engineering, Kyungpook National University, Daegu 41566,
Republic of Korea b
Implementation Research Division, Korea Institute of Materials Science, Changwon 51508,
Republic of Korea
*Corresponding author. E-mail address: [email protected] (S.H. Park).
Abstract
This study demonstrates that the application of artificial water cooling during extrusion effectively increases the extrudability of the AZ91 alloy and significantly improves the mechanical properties of the extruded AZ91 alloy. The artificial cooling dramatically reduces the actual temperature of the deformation zone, which results in an increase in the maximum exit speed at which the alloy is extrudable without the occurrence of hot cracking from 4.5 m/min to 7.5 m/min. It also promotes dynamic recrystallization and precipitation behaviors during extrusion, which leads to a reduction in grain size and an increase in the amount of fine Mg17Al12 precipitates. As a result, for the AZ91 alloy extruded at an exit speed of 1.5 m/min, the tensile and compressive yield strengths improve significantly by 51 MPa and 114 MPa,
respectively, and its tension–compression yield asymmetry reduces from 0.73 to 1.02 owing to the refinement of the grain size by artificial cooling. In addition, the AZ91 alloy extruded at an exit speed of 7.5 m/min with artificial cooling exhibits a finer grain structure than and superior mechanical properties to the AZ91 alloy extruded at a slower exit speed of 4.5 m/min without artificial cooling. This result indicates that the application of artificial cooling can simultaneously improve the maximum extrusion speed and the tensile and compressive properties of Mg alloys.
Keywords: Magnesium alloy; Extrusion; Artificial cooling; Microstructure; Mechanical properties.
1. Introduction
The demand for Mg alloys in the transportation industry has been increasing continuously given that their low density and high specific strength help satisfy the requirement of weight reduction of automobile components with the aim of improving vehicle fuel efficiency and reducing carbon dioxide emissions [1]. In recent years, extruded Mg alloys have attracted attention on account of their much better mechanical properties than cast alloys and simpler manufacturing processes than rolled alloys. The extrusion speed in the extrusion process is a crucial parameter from an industrial viewpoint because it is directly related to the productivity of the fabricated products. In Mg-Al-Zn alloy systems such as AZ31, AZ61, AZ80, and AZ91—which are the most widely used Mg alloys—the maximum extrusion speed is known to decrease with increasing Al content [2,3]; AZ31 can be extruded at an exit speed of ~20 m/min, whereas AZ91 has an extrusion speed limit of ~5 m/min. This is because with an increase in the Al content of AZ alloys, more Mg17Al12 particles with a low melting temperature of ~432 °C are formed during extrusion, which, in turn, increases the susceptibility of the material to hot cracking and decreases its extrudability [4]. Recently, studies have been conducted for improving the extrudability of Mg alloys by decreasing the alloying content of AZ alloys. It has been reported that Mg-0.5Al-0.25Zn-0.1Mn (wt%) has a maximum extrusion speed equal to or higher than 30 m/min [5] and that Mg-0.3Al0.21Ca-0.47Mn (wt%) can be successfully extruded at an exit speed of 60 m/min [6,7], which is comparable to the extrudability of the AA6063 Al alloy. In addition, Mg-Zn-based alloys containing small amounts of alloying elements, such as Mg-1.58Zn-0.52Gd [8] and Mg-0.21Zn0.3Ca-0.14Mn [9] (wt%), have been developed; these alloys show excellent extrudability with maximum extrusion speeds of higher than 60 m/min. However, these dilute Mg alloys have a relatively low strength because of the insufficiency of alloying elements, which are responsible for inducing various strengthening effects (e.g., solid-solution, grain-refinement, and precipitate strengthening). For instance, the Mg-0.21Zn-0.3Ca-0.14Mn (wt%) alloy extruded at an exit speed of 6 m/min has a tensile yield strength (TYS) of only ~105 MPa [9]. Therefore, to ensure
both extrudability and strength of a material, efforts need to be made toward improving the maximum extrusion speed of high-strength Mg alloys with large amounts of alloying elements. When local temperatures in the die land area exceed the solidus temperature of the matrix or the melting temperature of the second phase during extrusion, incipient melting of the extrudate surface and subsequent cracking can occur [3]. Even if the extrusion begins at low temperatures, the actual temperature in the deformation zone increases owing to the deformation heating that occurs during extrusion. Furthermore, as the extrusion speed increases, the amount of heat generated increases and the actual temperature also increases; this deformation-induced temperature increase is known to be proportional to the logarithmic strain rate [10,11]. Therefore, reduction of the increased temperature in the deformation zone will cause suppression of hot cracking and an increase in the extrudability. Artificial cooling during extrusion, which is performed by directly spraying water onto the extruded rod at the die exit, is a highly effective method for lowering the actual extrusion temperature [12–14]. Kim et al. [12] reported indirect extrusion using artificial cooling of AZ31 and found that artificial cooling is effective in decreasing the grain size and improving the strength of the extruded alloy. Our previous investigations on the effects of artificial cooling in the Mg-7Sn-1Al-1Zn (wt%) alloy [13,14] also demonstrated that artificial cooling results in a decrease in the area fraction and size of dynamically recrystallized (DRXed) grains and promotion of Mg2Sn precipitation. However, in AZ31, only a small amount of the Mg17Al12 phase is formed or not formed at all during extrusion, and in Mg-Sn based alloys, a thermally stable Mg2Sn phase with a high melting temperature of ~773 °C is formed as the main second phase [4,15]. No in-depth studies have yet been conducted for examining the influence of artificial cooling on the extrudability and dynamic deformation behavior of high-Al-content AZ alloys, in which a large amount of thermally unstable Mg17Al12 phase is formed during hot extrusion. The present study therefore investigates the change in the maximum extrusion speed with artificial cooling of AZ91—which has the highest Al content among all the commercial AZ series alloys—by performing indirect extrusions at several exit speeds with and without artificial cooling. Variations in the microstructural characteristics and tensile and compressive properties of extruded AZ91 induced
by artificial cooling are also examined and discussed herein.
2. Experimental procedure
Cast billets of the Mg-9Al-1Zn-0.2Mn (wt%) (AZ91) alloy were prepared according to a previously described method [16]. The billets were homogenized at 420 °C for 24 h to fully dissolve any macrosegregated elements or intermetallic compounds formed during solidification, after which they were water-quenched to obtain a supersaturated solid solution. All billets (Ø80 mm × 200 mm) preheated to 350 °C were indirectly extruded at a temperature of 350 °C with an extrusion ratio of 25 by a previously described process [14]. To analyze the variation in the extrudability of AZ91 induced by the artificial cooling performed during extrusion, three billets were extruded at exit speeds of 3.0, 4.5, and 6.0 m/min without artificial cooling, and three other billets were extruded at exit speeds of 6.0, 7.5, and 9.0 m/min with artificial cooling at a water feed rate of 5 L/min. In addition, to investigate the effects of artificial cooling on the microstructure and mechanical properties of the extruded AZ91 alloy, two billets were extruded at the same exit speed of 1.5 m/min; one of these was subjected to artificial cooling during extrusion. The samples extruded without and with artificial cooling are hereafter referred to as NC-AZ91 and AC-AZ91, respectively. A schematic of the indirect extrusion process and its associated artificial cooling system can be found in the literature [13,14]. When the artificial cooling system is operated at the initial stage of extrusion, cold water moves through holes inside the stem to a die containing holes, and it is sprayed directly onto the extruded material that has just exited the die. In this study, the variation in die temperature during extrusion was measured via a thermocouple installed inside the die. The microstructural characteristics of the extruded samples were analyzed by optical microscopy (OM), field emission scanning electron microscopy (FESEM), and electron backscatter diffraction (EBSD). The preparation method of the samples used for each of these observations has been reported elsewhere [17]. The EBSD data were analyzed using the TexSEM Laboratories orientation imaging microscopy (TSL OIM) 7.0 software, and data with a
confidence index greater than 0.1 were used for grain size and texture analyses. The area fraction of DRXed grains in each extruded sample was determined by measuring the unDRXed regions in a relatively large area of ~54 mm2 in OM images. The tensile and compressive properties of the extruded samples were measured at room temperature by using an Instron 4206 universal testing machine with a strain rate of 1.0 × 10-3 s-1. Dog-bone-shaped (gage section: Ø6 mm × 25 mm) and cylindrical (Ø8 mm × 12 mm) samples were used for tension and compression tests, respectively; their axes corresponded to the extrusion direction (ED).
3. Results and discussion
3.1. Improvement in extrudability via artificial cooling
Fig. 1(a) shows images of the samples extruded with and without artificial cooling. When the billet is extruded at 4.5 m/min without artificial cooling, an extruded sample with high surface quality is obtained. On the other hand, when the billet is extruded at 6.0 m/min without artificial cooling, numerous hot cracks perpendicular to the ED are formed on the surface of the extruded sample. This means that when the indirect extrusion is performed at a temperature of 350 °C and an extrusion ratio of 25, the maximum exit speed at which the AZ91 alloy is extrudable is less than 6.0 m/min. Although the extrusion begins at the same temperature of 350 °C, as the extrusion speed increases, the actual temperature in the deformation zone near the die exit increases owing to the deformation induced and friction heat generated during extrusion. Fig. 1(b) shows changes in the measured die temperature during extrusion without artificial cooling, which reflects the evolution of the actual temperature applied to the alloy. As the billet begins to exit the die, the die temperature rises sharply at the beginning of the extrusion process. The maximum die temperature gradually increases with increasing extrusion speed: it is 388 °C at 3.0 m/min, 397 °C at 4.5 m/min, and 411 °C at 6.0 m/min. When extrusion is performed at 4.5 m/min, the actual deformation temperature during the extrusion is lower than the incipient melting temperature of AZ91 (~427 °C [18]), so it can
be extruded without the formation of hot cracks. However, in the case of extrusion at 6.0 m/min, the actual temperature applied to the material exceeds the incipient melting temperature, which causes the occurrence of hot cracking owing to local melting at the surface of the extruded bar. Application of artificial water cooling during extrusion results in lowering of the actual deformation temperature on account of a reduction in the deformation and friction heat generated during extrusion; this, in turn, suppresses the occurrence of hot cracking in highspeed extrusion. As shown in Fig. 1(a), although the billet is extruded at a relatively high speed of 7.5 m/min, an extruded sample free from cracks is successfully obtained by the application of artificial cooling; however, severe hot cracking occurs at an exit speed of 9.0 m/min. These results indicate that the extrudability of AZ91 alloy improves considerably from 4.5 to 7.5 m/min when it is subjected to artificial cooling during extrusion.
3.2. Microstructural evolution by artificial cooling
Optical micrographs of the AZ91 samples extruded at a speed of 1.5 m/min with and without artificial cooling are shown in Fig. 2. Although AC-AZ91 has a few large-sized unDRXed grains (their area fraction is only ~3.1%), both samples exhibit an almost completely DRXed structure. However, there is a drastic difference in the grain sizes of the two samples. In NC-AZ91, the grain size in the region with few precipitates is ~26.1 mm, whereas that in the region with precipitate bands is smaller, ~8.1 mm, owing to the grain boundary pinning effect of the precipitates; the average grain size of NC-AZ91 with bimodal DRXed grains is ~20.1 mm (Fig. 3(a)). On the other hand, the size of DRXed grains of AC-AZ91 is relatively uniform (~1– 4 mm), and the average size is rather small, ~2.6 mm (Fig. 3(b)); this indicates that the crystal grains of the extruded AZ91 alloy are significantly refined by artificial cooling. Both samples show similar crystallographic orientation distributions and maximum intensities of the inverse pole figure (Fig. 3), which means that application of artificial cooling has a negligible effect on the texture evolution of the extruded material under the extrusion conditions adopted in this study. It is noted, however, that under extrusion conditions where DRX does not occur
completely (e.g., at lower extrusion temperatures, speeds, or ratios [19,20]), the texture of the extruded material can change significantly with an increase in the amount of unDRXed grains through artificial cooling, because the DRX fraction of Mg alloys deformed at elevated temperatures generally decreases with a decrease in the actual deformation temperature [3,19] and unDRXed grains have a much higher intensive basal texture than DRXed grains [17,21,22]. The SEM micrographs in Fig. 4 show that Mg17Al12 particles precipitated dynamically during extrusion of the samples. In NC-AZ91, precipitate bands parallel to the ED (~40–100 µm in width) are visible (Figs. 2(a), 3(a), and 4(a)), in which a reasonable amount of fine Mg17Al12 precipitates (~0.5–1.5 µm in size) is distributed (Fig. 4(b)); however, a much smaller amount of precipitates exists in the region other than the precipitate bands (Fig. 4(c)). The formation of these precipitate bands can be attributed to the microsegregation of Al atoms dissolved in the matrix of the homogenized billet [3]. Inhomogeneous distribution of dissolved Al can result in a localized formation of dynamic Mg17Al12 precipitates in the region with high Al content. Furthermore, since the crystallographic orientation distribution of each grain of the homogenized billet is random, the shear stress imposed in each grain during extrusion also varies with the angle relationship between the external stress and the crystal orientation [23]. Accordingly, in the initial grains of the homogenized billet, in which a high resolved shear stress is applied during extrusion, Mg17Al12 precipitates can form preferentially owing to the enhanced dynamic precipitation. Since these initial grains are elongated along the ED during extrusion, the precipitate bands are also formed parallel to the ED. AC-AZ91 also exhibits a nonuniform distribution of Mg17Al12 precipitates (Fig. 4(d)); it consists of a region with a large amount of precipitates (Fig. 4(e)) and a region with a relatively smaller amount of precipitates (Fig. 4(f)). However, the total amount of precipitates is much larger than that in NC-AZ91 and their size (~300–700 nm) is also smaller than that in NC-AZ91 (~0.5–1.5 µm). This enhanced dynamic precipitation behavior of AC-AZ91 is attributed to the fact that the decrease in extrusion temperature due to artificial cooling promotes the formation of Mg17Al12 precipitates. In addition, artificial cooling applied during extrusion causes not only a decrease in the actual deformation temperature but also an increase in the cooling rate of the extruded bars. When the
alloy is extruded without artificial cooling, the extruded bar maintains a high temperature immediately after exiting the die, and thus, static precipitation of the Mg17Al12 phase can occur during cooling in air. Since the density of dislocations—which act as the nucleation sites of the precipitates—is low inside the recrystallized grains newly formed during hot deformation or heat treatment, the static precipitates are preferentially formed along the grain boundaries, where diffusion of solute atoms occurs more easily than in the grains owing to the high interface energy of the grain boundaries. In the case of NC-AZ91, static precipitation induced by a low cooling rate after extrusion is confirmed by the presence of Mg17Al12 precipitates formed along the DRXed grain boundaries (Fig. 4(c)). On the other hand, AC-AZ91 cooled rapidly by sprayed water during extrusion does not have sufficient heat energy and time for undergoing static precipitation after it exits the die. As a result, although AC-AZ91 has a larger amount of dynamically formed precipitates than NC-AZ91, it does not show any static precipitates at the grain boundaries, unlike NC-AZ91 (Fig. 4(f)). Fig. 5 shows the variation in the die temperature and extrusion load during extrusion with and without artificial cooling. When the billet is extruded without artificial cooling, the die temperature increases to 375 °C at the early stage of extrusion and then gradually decreases to 352 °C (Fig. 5(a)). On the other hand, when artificial cooling is performed, the die temperature rapidly decreases to 256 °C and then gradually decreases to 213 °C; therefore, at the end of extrusion, the die temperature is considerably lower (a reduction of 137 °C) on account of artificial cooling. This drastic reduction in the deformation temperature may accelerate the dynamic precipitation by decreasing the Al solubility in the a-Mg matrix. Fig. 6 shows the change in the Al solubility limit in the Mg-1Zn alloy and the phase fraction of Mg17Al12 in the Mg-9Al-1Zn alloy with increasing temperature for the equilibrium states, which are calculated using the PANDAT software. As the Al solubility at a homogenization temperature of 420 °C is 10.94 wt%, 9.0 wt% Al added to the AZ91 alloy dissolves completely in the matrix upon homogenization treatment; complete dissolution of the Mg17Al12 phase of cast AZ91 by homogenization has also been reported elsewhere [16,24,25]. Under the assumption that the die temperature is equal to the actual deformation temperature during extrusion, the sample without
artificial cooling is extruded at 352 °C and the Al solubility at this temperature is 7.82 wt%; therefore, 1.18 wt% of Al supersaturated in the matrix will precipitate as the Mg17Al12 phase. On the other hand, as the sample subjected to artificial cooling is extruded at a relatively low temperature of 213 °C, a much larger amount of Al (5.97 wt%) will precipitate as the Mg17Al12 phase on account of the greatly reduced Al solubility (3.03 wt%). With a decrease in the actual extrusion temperature from 352 °C to 213 °C, the calculated fraction of the precipitated Mg17Al12 phase increases significantly from 3.2% to 14.3%. In addition, the extrusion load increases from 183 ton to 209 ton during extrusion without artificial cooling, whereas it increases from 185 ton to 261 ton when artificial cooling is performed, as shown in Fig. 5(b). Accordingly, use of the cooling system increases the stress imposed on the material during extrusion, which, in turn, can promote the dynamic precipitation behavior owing to the formation of more dislocations that act as nucleation sites for precipitation [3,23,26]. Moreover, as the deformation temperature reduces, the dynamically formed precipitates become smaller in size because of the suppression of the Ostwald ripening phenomenon, wherein smaller particles disappear gradually while larger particles enlarge [27–30]. Therefore, AC-AZ91 has a much larger amount of finer precipitates than NC-AZ91, as shown in Fig. 4.
3.3. Improvement in strength and reduction in yield asymmetry by artificial cooling
The tensile and compressive stress–strain curves of NC-AZ91 and AC-AZ91 are shown in Fig. 7; the tensile and compressive properties are listed in Table 1. The TYS, ultimate tensile strength (UTS), and tensile elongation of NC-AZ91 are 211 MPa, 324 MPa, and 13.9%, respectively, whereas those of AC-AZ91 are 262 MPa, 364 MPa, and 13.6%, respectively. Although the same alloy (i.e., AZ91) is extruded under the same extrusion conditions (temperature, speed, and ratio), the TYS and UTS of the extruded material are considerably enhanced by 51 MPa and 40 MPa, respectively, without a loss of ductility, simply through application of artificial cooling. This significant improvement in tensile properties is attributed mainly to the decrease in grain size (from 20.1 µm to 2.6 µm) and to the increase in the amount
of Mg17Al12 precipitates (from 3.2% to 14.3%), each of which enhances the grain boundary hardening and precipitation hardening effects during tensile deformation. It is known that in extruded Mg alloys containing unDRXed grains, {10-11} contraction and {10-11}-{10-12} double twins, which act as microcracking sites during plastic deformation [31], can easily form in the unDRXed grains during tension along the ED owing to their sizes being larger than those of DRXed grains. This eventually causes deterioration of the tensile ductility of the extruded material [20,32,33]. However, under the extrusion conditions adopted in this study, the fraction of unDRXed grains formed by artificial cooling is so small (~3.1%) that such ductility deterioration does not occur. The compressive yield strength (CYS) also increases considerably, from 153 MPa to 267 MPa, by artificial cooling (Fig. 7(b) and Table 1); note that this increase in the CYS (114 MPa) is ~2.2 times that in the TYS (51 MPa). In general, when extruded Mg alloys are subjected to plastic deformation along the ED, dislocation slips and {10-12} twinning become the main deformation mechanisms in tension and compression, respectively. This is because of the combined effect of the basal texture of the materials, where the basal planes of most grains are oriented parallel to the ED, and the directional nature of twinning [34]. The activation stress for {10-12} twinning in compression along the ED is smaller than that for the dislocation slips in tension along the ED, owing to the high values of the Schmid factor for {1012} twinning [35–37]; this, in turn, results in a tension–compression yield asymmetry (i.e., CYS/TYS ratios lower than 1). NC-AZ91 also exhibits a low CYS/TYS ratio of 0.73 (Table 1). It is known that in Mg, the Hall–Petch coefficient for twinning is higher than that for slip [3,38], which indicates that the twinning stress is more sensitive to grain size than is the slip stress [39]. Accordingly, the increase in the CYS of the extruded AZ91 sample, which is caused by grain refinement through artificial cooling, becomes larger than that in the TYS. In addition, as twinning is known to be suppressed when the grain size is smaller than 1–2 mm [3,40], in ACAZ91 with a fine grain size of ~2.6 mm, the formation of {10-12} twins during compressive deformation is quite limited; this, in turn, results in almost the same TYS (262 MPa) and CYS (267 MPa). In other words, AC-AZ91 exhibits isotropic tension–compression yielding behavior with a CYS/TYS ratio of 1.02 (Table 1).
3.4. Simultaneous improvement in extrusion speed and mechanical properties
To investigate the effects of artificial cooling at high extrusion speeds, the microstructures and mechanical properties of NC-AZ91 and AC-AZ91 extruded at the highest speed at which hot cracking does not occur, i.e., of NC-AZ91 extruded at 4.5 m/min and of ACAZ91 extruded at 7.5 m/min, are compared. These samples are hereafter referred to as NC45AZ91 and AC75-AZ91, respectively. The optical and SEM micrographs of NC45-AZ91 and AC75-AZ91 are shown in Fig. 8; both samples are found to exhibit a fully DRXed structure. As the extrusion speed increases, the deformation and friction heat generated during extrusion become larger and the actual deformation temperature increases, which results in an increase in the area fraction and size of the DRXed grains [41–44]. However, although AC75-AZ91 is extruded ~1.67 times faster than NC45-AZ91, the grain size of the former is finer; the average grain size of AC75-AZ91 is 10.9 μm, which is ~0.45 times that of NC45-AZ91 (24.2 μm) (Fig. 9). This means that artificial cooling reduces the actual deformation temperature even at a high extrusion speed of 7.5 m/min, which results in grain refinement through suppression of the growth of DRXed grains. Unlike in the samples extruded at 1.5 m/min, nearly no fine dynamic precipitates are present in either NC45-AZ91 or AC75-AZ91; however, the latter two samples have static precipitates that were formed along the grain boundaries (Figs. 8(c) and (d)). When the billet is extruded at 4.5 m/min without artificial cooling, the Al solubility limit of the a-Mg matrix increases sufficiently to maintain the dissolved Al atoms in a solid state, and so, the Mg17Al12 phase does not precipitate dynamically. Artificial cooling reduces the deformation temperature even at high extrusion speeds. However, when the extrusion speed is as high as 7.5 m/min, the time for which the extruded bar is exposed to water sprayed through the die is shortened, and the cooling effect is weaker than that when the extrusion is performed at a relatively lower speed of 1.5 m/min. Therefore, although the grains of AC75-AZ91 become finer owing to a certain degree of cooling, the temperature drop that occurs is insufficient to cause dynamic precipitation. Moreover, the extruded bar passing through the sprayed water still
has a high temperature, because of which static precipitation occurs during natural cooling in air. As in the case of the samples extruded at 1.5 m/min, the texture of those extruded at higher speeds is not significantly influenced by artificial cooling (Fig. 9). Fig. 10 shows the tensile and compressive stress–strain curves of NC45-AZ91 and AC75-AZ91. For both the samples with and without artificial cooling, the TYS, UTS, and CYS decrease as the extrusion speed increases (Table 1); this is attributed mainly to the increased grain size and decreased amount of fine dynamic precipitates. However, it should be noted that AC75-AZ91 extruded at an approximately 1.67 times higher speed exhibits higher tensile and compressive strengths than NC45-AZ91, with almost no difference in their ductilities (Fig. 10); the TYS, UTS, and CYS of AC75-AZ91 are, respectively, 21 MPa, 15 MPa, and 25 MPa higher than those of NC45-AZ91 (Table 1). Since both samples have small amounts of dynamic precipitates and their textures are also almost the same, this higher strength of AC75-AZ91 can be attributed to a finer grain size (24.2 μm and 10.9 μm for NC45-AZ91 and AC75-AZ91, respectively). Furthermore, the tension–compression yield asymmetry is also smaller in AC75AZ91 than in NC45-AZ91; the CYS/TYS ratios for NC45-AZ91 and AC75-AZ91 are 0.75 and 0.80, respectively. These results show that the application of artificial cooling—wherein water is directly sprayed onto the extruded material during extrusion—not only increases the extrudability of commercial AZ91 alloy to up to 7.5 m/min but also improves the mechanical properties of AZ91 extruded at a relatively high speed of 7.5 m/min; these improved mechanical properties are superior to those of AZ91 extruded at a lower speed of 4.5 m/min without artificial cooling.
4. Conclusions
This study demonstrates that the extrudability and mechanical properties of the AZ91 alloy can be effectively improved by performing artificial water cooling during extrusion. The maximum extrusion speed increases from 4.5 m/min to 7.5 m/min through artificial cooling, owing to a reduction in the deformation and friction heat generated during extrusion. Owing to
artificial cooling, the TYS of the AZ91 sample extruded at 1.5 m/min improves significantly from 211 MPa to 262 MPa without a loss of ductility; this improvement is attributed mainly to the refined grain size and increased amount of fine Mg17Al12 precipitates. The tension– compression yield asymmetry also reduces greatly, i.e., the CYS/TYS ratio increases from 0.73 to 1.02, owing to the suppression of twinning behavior in compression by grain refinement. In addition, the AZ91 sample extruded at 7.5 m/min with artificial cooling exhibits a finer grain structure and higher strength than the AZ91 sample extruded at a lower speed of 4.5 m/min without artificial cooling. This clearly shows that the application of artificial cooling can simultaneously improve the maximum extrusion speed and the mechanical properties of Mg alloys. Application of artificial water cooling during extrusion will enhance the productivity of the extruded material by facilitating extrusion at higher speeds; this will result in a lowering of the cost of the final products and subsequently lead to an expansion of the application range of extruded Mg alloys by virtue of their improved mechanical properties. However, the abovementioned beneficial effects caused by artificial cooling can change depending on the compositions of the employed alloy and the processing parameters. Therefore, further studies are required to investigate the effects of artificial cooling in various Mg alloy systems and under various extrusion conditions in terms of variations in their dynamic recrystallization and precipitation behaviors, microstructural and textural characteristics, resultant mechanical properties, and deformation and fracture mechanisms.
Acknowledgments
This study was supported by the R&D Center for Valuable Recycling (Global-Top R&BD Program) of the Ministry of Environment of Korea (Project No. 2016002220003).
References
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Table and Figure Captions
Fig. 1.
(a) Images depicting surface quality of AZ91 samples extruded at 350 °C without and with artificial cooling. (b) Variation in die temperature during extrusion at 350 °C with exit speed. Vexit denotes the exit speed of the extruded bars.
Fig. 2.
Optical micrographs of AZ91 samples extruded at exit speed of 1.5 m/min (a) without and (b) with artificial cooling. fDRX denotes the area fraction of dynamically recrystallized grains.
Fig. 3.
Inverse pole figure map, ED inverse pole figure, and grain size distribution of AZ91 samples extruded at exit speed of 1.5 m/min (a) without and (b) with artificial cooling.
Fig. 4.
SEM micrographs of AZ91 samples extruded at exit speed of 1.5 m/min (a)–(c) without and (d)–(f) with artificial cooling. (b), (c) Enlarged images of rectangular areas A and B, respectively, in (a). (e), (f) Enlarged images of rectangular areas A and B, respectively, in (d).
Fig. 5.
Variation in (a) die temperature and (b) extrusion load during extrusion at exit speed of 1.5 m/min with and without artificial cooling.
Fig. 6.
Variations in Al solubility limit of Mg-1Zn alloy and phase fraction of Mg17Al12 of Mg-9Al-1Zn alloy for equilibrium states, as calculated with PANDAT software. Thomo. denotes the homogenization temperature (420 °C).
Fig. 7.
(a) Tensile and (b) compressive stress–strain curves of AZ91 samples extruded at exit speed of 1.5 m/min with and without artificial cooling.
Fig. 8.
(a), (b) Optical and (c), (d) SEM micrographs of AZ91 samples extruded (a), (c) without cooling at exit speed of 4.5 m/min and (b), (d) with artificial cooling at exit speed of 7.5 m/min.
Fig. 9.
Inverse pole figure map, ED inverse pole figure, and grain size distribution of AZ91 samples extruded (a) without cooling at exit speed of 4.5 m/min and (b) with artificial cooling at exit speed of 7.5 m/min.
Fig. 10.
(a) Tensile and (b) compressive stress–strain curves of AZ91 samples extruded without cooling at exit speed of 4.5 m/min and with artificial cooling at exit speed of 7.5 m/min.
Table 1. Microstructural characteristics and mechanical properties of AZ91 sample s extruded with and without artificial cooling.
Microstructural characte ristics Extruded AZ91
Without c ooling
With cool ing
Exit s peed (m/mi
fDR X
X
Im
n)
(%
(µ
ax
)
m)
1.5
10 0
20. 1
4.5
99. 8
1.5
7.5
Mechanical properties
*
dDR
EL
Al12
TY S (M
UT S (M
(%)
Pa)
Pa)
(% )
3. 3
3.2
211
324
13. 9
153
0.73
24. 2
3. 1
-
185
308
15. 4
139
0.75
96. 9
2.6
3. 0
14.3
262
364
13. 6
267
1.02
99.
10.
3.
2
9
0
-
206
323
164
0.80
*
fDRX, dDRX, Imax, and fMg
17Al12
fMg
17
14. 9
CY S (M
**
CYS/ TYS
Pa)
denote the area fraction of DRXed grains, average
size of DRXed grains, maximum intensity of the inverse pole figure, and calcul ated phase fraction of Mg17Al12 precipitates, respectively. **
YS, UTS, EL, CYS, and CYS/TYS denote the tensile yield strength, ultimate t ensile strength, tensile elongation, compressive yield strength, and tension–compre ssion yield asymmetry, respectively.
*QVETCEMKPI
8GZKV OOKP
*QVETCEMKPI
8GZKV OOKP
(b)
340
350
360
370
380
390
400
410
420
0
20
40
80 Ram distance (mm)
60
Exit speed 3.0 m/min 4.5 m/min 6.0 m/min
100
120
140
in die temperature during extrusion at 350 ¶C with exit speed. Vexit denotes the exit speed of the extruded bars.
Fig. 1. (a) Images depicting surface quality of AZ91 samples extruded at 350 ¶C without and with artificial cooling. (b) Variation
)QQF
8GZKV OOKP
With cooling
)QQF
8GZKV OOKP
(a) Without cooling
Die temperature (°C)
Precipitate band
100 μm
fDRX = 100%
(b) unDRXed region
100 μm
fDRX = 96.9%
Fig. 2. Optical micrographs of AZ91 samples extruded at exit speed of 1.5 m/min (a) without and (b) with artificial cooling. fDRX denotes the area fraction of dynamically recrystallized grains.
(a)
ED
Area fraction
20
30
40
50
60
70
80
Grain size (mm)
20 μm
0.00 10
0.00
0.10
0.15
0.20
0.25
0.30
0.05
0
20.1 μm
Average grain size:
(b)
0.05
0.10
0.15
0.20
0.25
0.30
Max. = 3.3
0
2
4
6
8
2.6 μm
Average grain size:
Grain size (mm)
Max. = 3.0
Fig. 3. Inverse pole figure map, ED inverse pole figure, and grain size distribution of AZ91 samples extruded at exit speed of 1.5 m/min (a) without and (b) with artificial cooling.
ED
Precipitate band region
Precipitate-free region
100 μm
(a)
Area fraction
10
ED
A
B
A
B
20 μm
20 μm
(e)
(b)
2 μm
2 μm
(f)
(c)
2 μm
5 μm
Fig. 4. SEM micrographs of AZ91 samples extruded at exit speed of 1.5 m/min (a)–(c) without and (d)–(f) with artificial cooling. (b), (c) Enlarged images of rectangular areas A and B, respectively, in (a). (e), (f) Enlarged images of rectangular areas A and B, respectively, in (d).
Precipitate-rich region
(d)
Precipitate band region
(a)
Die temperature ( C)
(a)
o
200
225
250
275
300
325
350
375
400
0
40
80
100
Ram distance (mm)
60
120
140
213 213°C C
Without cooling With cooling
160
160
180
200
220
240
260
280
0
20
40
60
80
100
Ram distance (mm)
Startingpoint pointof of Starting artificialcooling cooling artifical
Without cooling With cooling
120
140
209 ton
261 ton
Fig. 5. Variation in (a) die temperature and (b) extrusion load during extrusion at exit speed of 1.5 m/min with and without artificial cooling.
20
Starting Starting point point of of artifical artificialcooling cooling
350 °CC 352
(b)
Extrusion load (ton)
160
150
0
2
4
6
8
10
200
14.3%
300
400
7.82 wt.%
350
3.2%
THomo.
0%
10.94 wt.%
w/o cooling
Temperature (°C)
250
3.03 wt.%
with cooling
450
0.00
0.04
0.08
0.12
0.16
0.20
Fig. 6. Variations in Al solubility limit of Mg-1Zn alloy and phase fraction of Mg17Al12 of Mg-9Al-1Zn alloy for equilibrium states, as calculated with PANDAT software. Thomo. denotes the homogenization temperature (420 ¶C).
Al solubility in a-Mg (wt.%)
12
Phase fraction of Mg17Al12
Tensile stress (MPa)
0
50
100
150
200
250
300
350
400
0
2
4
6
10
Tensile strain (%)
8
12
14
AZ91 without cooling AZ91 with cooling
16
18
(b)
0
100
200
300
400
500
0
2
4
6
10
12
14
Compressive strain (%)
8
16
AZ91 without cooling AZ91 with cooling
18
20
Fig. 7. (a) Tensile and (b) compressive stress–strain curves of AZ91 samples extruded at exit speed of 1.5 m/min with and without artificial cooling.
(a) Compressive stress (MPa)
5 μm
100 μm
(d)
(b)
5 μm
100 μm
Fig. 8. (a), (b) Optical and (c), (d) SEM micrographs of AZ91 samples extruded (a), (c) without cooling at exit speed of 4.5 m/min and (b), (d) with artificial cooling at exit speed of 7.5 m/min.
(c)
(a)
ED
ED
Area fraction
20
30
40
50
60
70
80
Grain size (mm)
60 μm
0.00 10
0.00
0.10
0.15
0.20
0.25
0.30
0.05
0
24.2 μm
Average grain size:
(b)
0.05
0.10
0.15
0.20
0.25
0.30
Max. = 3.1
0
5
10
20
25
30
10.9 μm
35
Average grain size:
Grain size (mm)
15
Max. = 3.0
Fig. 9. Inverse pole figure map, ED inverse pole figure, and grain size distribution of AZ91 samples extruded (a) without cooling at exit speed of 4.5 m/min and (b) with artificial cooling at exit speed of 7.5 m/min.
100 μm
(a)
Area fraction
40
Tensile stress (MPa)
(a)
0
2
4
6
10
Tensile strain (%)
8
12
14
AZ91 without cooling (4.5 m/min) AZ91 with cooling (7.5 m/min)
16
18
(b)
0
100
200
300
400
500
0
5
15
Compressive strain (%)
10
20
AZ91 without cooling (4.5 m/min) AZ91 with cooling (7.5 m/min)
25
Fig. 10. (a) Tensile and (b) compressive stress–strain curves of AZ91 samples extruded without cooling at exit speed of 4.5 m/min and with artificial cooling at exit speed of 7.5 m/min.
0
50
100
150
200
250
300
350
Compressive stress (MPa)