Abstract
Mg and its alloys are the lightest structural metals available and are extremely attractive for applications as lightweight components, particularly in the automobile, electronic, and aerospace industries. The global market for wrought Mg alloys has steadily expanded over the past decade. And numerous studies have been carried out to meet this increasing demand of high-performance Mg alloys. However, Mg extrusion alloys have had a very limited usage so far. To overcome existing industrial challenges, one desirable approach is the development of low-cost rare earth (RE) free Mg extrusion alloys with superior mechanical properties. This review will introduce the recent research highlights in the extrusion of Mg alloys, specifically focusing on low-cost RE-free Mg alloy. The results from both the literature and our previous study are summarized and critically reviewed. Several aspects of RE-free Mg extrusion alloys are described in detail: (1) novel alloying designs including Mg–Al-, Mg–Zn-, Mg–Ca-, Mg–Sn-, and Mg–Bi-based alloys, (2) advanced extrusion techniques, and (3) extrusion-related severe plastic deformation (SPD) processing. Accordingly, considering the large gap in mechanical properties between the current RE-free Mg alloys and high-performance aluminum alloys, new alloy design, processing route control, and recommendations for future research on RE-free Mg extrusion alloys are also proposed. We hope this review will not only offer insightful information regarding the extrusion of RE-free Mg alloys but also inspire the development of new Mg extrusion technologies.
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1 Introduction
It is well known that Mg and its alloys, as the lightest metallic structural materials available now, have shown significant potential for applications in the automotive, aircraft, aerospace, and 3C industries because of their low density, high specific strength, good castability, high damping capacity, and other characteristics [1, 2]. To date, most structural parts made of Mg alloy are produced by high-pressure die casting [3, 4]. However, the cast Mg alloys have inherent shortcomings such as casting defects, which restricts the wide use of cast components [3, 5]. Recently, increasing effort has been put into the research and application of wrought Mg alloys with mechanical properties superior to those of cast Mg counterparts.
Among the various types of wrought Mg alloy processing, extrusion is one possibility for the production of long, shaped, or thin-walled components in just a single pass, which can extend the application range of Mg alloys. Aluminum (Al) extrusion alloy components have been widely adopted in infrastructure, automotive, and aerospace applications [6], accounting for ~ 25% of semifinished Al alloy products. In contrast, despite the fact that technology for the processing of Mg alloys has existed for over half a century, Mg extrusion alloy products, such as sacrificial anodes and newly designed 3C cases, have only a very minimal usage and industrial penetration at present, accounting for less than 1.5% of the annual output of Mg production in 2004 and less than 3% in 2013 [7, 8].
To date, the lower industrial uptake of Mg extrusion alloys than their competitive material, Al extrusion alloys, is predominantly attributed to their lower mechanical properties and higher cost, which results from lower extrudability and poor low-temperature formability of Mg alloys [9]. In recent years, developed Mg–RE alloys typically exhibit a remarkable age hardening response and promising mechanical properties. However, the high cost and scarcity of natural resources make the abundant use of RE elements as major solute additions unacceptable in commercial applications [10].
In this regard, the development of cost-effective RE-free Mg extrusion alloys with good extrudability and adequate mechanical properties for commercial usage, as well as advanced extrusion technology, is significant and strongly desired. Despite the aforementioned studies on Mg alloys, including a number of broad topic monographs and focused review papers [2, 9,10,11,12,13,14,15,16], almost none have specifically focused on the development of low-cost RE-free Mg extrusion alloys with advanced extrusion approaches.
In this paper, recent developments on low-cost RE-free Mg extrusion systems, including Mg–Al-, Mg–Zn-, Mg–Ca-, Mg–Sn-, and Mg–Bi-based alloys, and advanced processing technologies are critically reviewed from the viewpoint of alloying design and their experimental progress aiming to improve the mechanical properties and extrudability. In addition, to narrow the large gap in mechanical properties between the current RE-free Mg alloys and high-performance Al alloys, a discussion on extrusion-related severe plastic deformation (SPD) processing is also included. Accordingly, future research directions of the new alloy design and processing technologies for RE-free Mg extrusion alloys are also discussed.
2 Promising Low-Cost RE-Free Mg Extrusion Alloy Systems
2.1 Mg–Al-Based Alloys
Al has a large solubility in Mg of approximately 12.9 wt% at a eutectic temperature of 437 °C, which decreases to approximately 3.65 wt% at 200 °C, and Mg17Al12 precipitates typically form during aging treatment [17]. As one of the best-known solid-solution strengtheners for Mg, Al is the most commonly used alloying element at present, and the majority of commercial Mg extrusion alloys, such as AZ31, AZ61, and AZ80 alloys, are based on Mg–Al alloy system.
Mg–Al binary extrusion alloys are rarely used in engineering applications due to the inadequate mechanical properties and corrosion performance. Zn and Mn are commonly added to Mg–Al-based extrusion alloys, and the concentration of Zn and Mn is generally ~ 1 wt%. Zn is added as a solute strengthener, and Mn is applied to increase the corrosion resistance of the alloy by removing trace amounts of Fe [18]. The strengthening effect caused by precipitates in low Al containing Mg–Al-based alloys such as AZ31 is usually negligible, predominantly owing to the extremely low volume fraction of precipitates [19]. For more concentrated alloys such as AZ80, precipitation leads to an age-hardening response of approximately 30 MPa [20]. In recent years, a number of additional RE-free alloying elements, such as Ca, Sr, and Sn, have been selected to enhance the mechanical properties of Mg–Al-based alloys. The mechanical properties of the selected Mg–Al-based extrusion alloys are listed in Table 1 [21,22,23,24,25,26,27,28,29,30,31].
Ca has been added to AZ series alloys to increase their ignition temperature and oxidation resistance, suppress the formation of the β-Mg17Al12 phase during the solidification, and enhance the strength. For example, an AZ80–0.5Ca (wt%) alloy with a tensile yield strength (TYS) of 253 MPa and elongation (EL) of 7.8% was achieved through extrusion, followed by aging heat treatment [21]. Similar to Ca, Sr is another alkali earth element that can be added to Mg–Al alloys. For example, the effects of Sr addition on the precipitation, microstructure, and extrusion texture of AZ31 have been studied [32]. It was reported that 1 wt% Sr added to AZ31 weakened the extrusion texture at a high temperature (350 °C) as a result of particle-stimulated nucleation (PSN) of recrystallized grains near the Al4Sr particle stringers. In contrast, at a relatively lower temperature (250 °C), Sr strengthens the texture by reducing the solute-drag effect owing to the reduced Al solubility, and by increasing the dynamic recrystallization (DRX) through grain boundary bulging or twin induced recrystallization mechanisms [32]. However, the high amount of Sr added tends to deteriorate the ductility of the extruded material. For instance, the EL for Mg–4Al–2Sr–1Ca–0.3Mn and Mg–4Al–2Sr–0.3Mn alloys are 3.2% and 6.2%, respectively [22]. Therefore, Sr-containing Mg–Al-based extrusion alloys have yet to attract significant interest.
Sn has been added to improve the mechanical properties of extruded Mg–Al-based alloys. Through the addition of 3 wt% Sn to AM80 alloy [23], the bimodal structure of fine and coarse dynamically recrystallized (DRXed) grains in AM80 alloy is changed into a fine, homogeneous grain structure that produces a considerable improvement in strength without a significant loss of ductility. In addition, Mg–xAl–ySn–0.3Mn (x = 1, 3, 6, and y = 1, 3) alloys have been successfully fabricated using hot extrusion [24]. Their tensile YS increases with a decrease in Al content or extrusion temperature. Mg–1Al–ySn–0.3Mn (y = 1, 3) alloys exhibit a relatively higher TYS exceeding 250 MPa with an EL of approximately 20%, which is mainly attributed to the formation of fine DRXed grains and a high fraction of coarse un-recrystallized grains with strong (10–10) and (0001) textures. Recently, a new high-strength Mg–8Al–4Sn–2Zn–0.15Mn alloy was developed [25], which has a TYS of 371 MPa, an ultimate tensile strength (UTS) of 415 MPa, an EL of 9.1%, and a low tension–compression yield asymmetry of 1.02. These excellent mechanical properties are attributed to the combined effects of grain refinement, enhanced dispersion and precipitation strengthening, and improved solid solution strengthening. Furthermore, the influence of Sn addition on the microstructure and mechanical properties of extruded Mg–8Al–2Zn–0.15Mn alloy was investigated [26], the results of which showed that the TYS gradually increases with the Sn content mainly owing to a decrease in the size of recrystallized grains and an increased amount of fine Mg2Sn precipitates. It was also found that the UTS improves with Sn addition of up to 4 wt% but deteriorates beyond that point owing to premature fractures caused by crack initiation in large particles.
Recently, Xu et al. [27] fabricated an extraordinary high-strength RE-free Mg alloy, by hot extrusion of a DC cast Mg–3.6Al–3.3Ca–0.4Mn sample. The as-extruded bars exhibited a TYS of 410 MPa, an UTS of 420 MPa and an EL of 5.6%, which are in fact superior to those of the T6-treated heat-resistant 4032 aluminum alloy sample (UTS, TYS, and EL are 380 MPa, 315 MPa, and 9%, respectively [33]). And the high strength is attributed to the combined effect of fine dynamically recrystallized grains (~ 1.2 μm) pinned with fragmented Al–Ca intermetallic compounds, nanoscale plate-like Al–Ca precipitates, spherical Al–Mn–Ca precipitates, as shown in Fig. 1 [28], and a strong basal texture. Note that after extrusion, a direct chill cast Mg–3.6Al–3.3Ca–0.4Mn sample fabricated under a cooling rate of 100–110 °C/s is about 100 MPa stronger than a permanent mold cast sample with a lower cooling rate of 10–20 °C/s, because the higher cooling rate of direct chill casting results in the formation of much finer eutectic phases and a higher density of monolayer GP zones [28].
a, b TEM images of as-extruded Mg–3.6Al–3.3Ca–0.4Mn (wt%) sample, c high-angle annular dark field image of the unDRXed area, d 3DAP elemental mapping of Al, Ca and Mn for the precipitates in as-extruded sample. Each dot represents one atom [28]
In terms of extrudability, Al and Zn are known to decrease the alloy solidus temperature and/or lead to the formation of a eutectic Mg–Al–Zn phase with a low melting temperature of ~ 338 °C [34]. For example, by removing Zn from AZ31 alloy, the maximum extrusion speed can increase by ~ 20% [7, 34]. Furthermore, Mg–1.0Al–0.25Zn–0.3Mn diluted alloy can be extruded at 70 m/min [35], indicating that the poor hot extrudability of Mg alloys can be improved by reducing the Al and Zn amounts in AZ alloys and thus decreasing the amount of secondary phase. However, the TYS of a high-speed extruded Mg–1.0Al–0.25Zn–0.3Mn dilute alloy was only 180 MPa, which is much lower than that of commercial wrought Mg alloys, mainly due to the coarse grain size and little solute strengthening effect.
To improve the mechanical properties of high-speed extrudable Mg alloy, Nakata et al. [29, 30] developed a Mg–0.30Al–0.21Ca–0.47Mn alloy, which enables age hardening. The TYS of this alloy can be increased to 206 MPa through direct aging after the extrusion at a very high die exit speed of 60 m/min, which is higher than that of a commercial AZ31 alloy. Furthermore, by optimizing the alloying compositions and microstructures, a Mg–1.3Al–0.3Ca–0.4Mn dilute alloy can be extruded at a high exit speed of 24 m/min, with a TYS of 287 MPa and an EL of 20% [31]; these hot extrudability and strength obtained without the addition of expensive ingredients are comparable to those of the 6000 series Al alloys. Therefore, this type of alloys with an excellent combination of extrudability and mechanical properties may open up a new gate in application fields where a weight reduction is a priority issue.
2.2 Mg–Zn-Based Alloys
In addition to Al, Zn is another cost-effective alloying element and thus commonly used in Mg alloys. The maximum solubility of Zn in Mg is 6.2 wt% at a eutectic temperature of 340 °C, and the equilibrium solid solubility decreases substantially to ~ 2.6 wt% at 150 °C [17]. The precipitation sequence in Mg–Zn alloys during aging includes the sequential formation of a GP zone, [0001]α rod-shaped \(\beta_{1}^{\prime }\) phase, [0001]α plate-shaped \(\beta_{2}^{\prime }\) MgZn2 phase, and equilibrium β MgZn phase [36].
Table 2 [37,38,39,40,41,42,43,44,45,46,47,48,49,50,51] lists the mechanical properties of some reported Mg–Zn-based RE-free extrusion alloys. As-extruded Mg–Zn binary alloys have moderate ductility (15–25%) but low TYS of normally less than 170 MPa [37,38,39,40], which are inadequate for commercial applications; in addition, non-RE alloying elements, such as Zr, Ca, Mn, and Sn, are generally added to Mg–Zn series extrusion alloys.
Zr is often added to Mg–Zn-based alloys as a grain refinement element during the cast billet and ingot production. With regard to extrusion alloys, a recent study [40] revealed that when extruded at 350 °C, Mg–6Zn alloy exhibited a fully recrystallized microstructure, whereas ZK60 alloy was only partially recrystallized. Moreover, a large proportion of rod-shaped Mg(Zn, Zr) [0001]α precipitates with dimensions of approximately 48.9 nm in length and 11.6 nm in width were observed in ZK60 alloy, instead of Mg–Zn precipitates in Mg–6Zn alloy. These nanoscale rod-shaped Mg(Zn, Zr) [0001]α precipitates and the bimodal grain structure result in an increase in TYS and UTS of up to 275 MPa and 318 MPa, respectively. In addition, the extrudability can also be significantly enhanced by adding Zr, which is attributed to the increase in the solidus temperature of Mg–Zn alloy. It has been shown that the addition of 0.8 wt% Zr to Mg–6Zn alloy increases the maximum extrusion speed by 10 folds [7].
Mg–Zn–Ca-based alloys have attracted considerable interest because Ca is somewhat soluble in Mg (1 wt% at 500 °C) and has a much larger atomic radius than Mg. Moreover, Mg, Ca, and Zn are all biocompatible elements. Compared with an as-extruded Mg–6Zn–0.2Ca alloy exhibiting a low TYS of ~ 150 MPa [41], a study by Somekawa et al. [42] confirmed that Mg–4.69Zn–0.48Ca alloy exhibits comprehensive mechanical properties with a high TYS of ~ 291 MPa and great EL of ~ 15.8%. More importantly, it is possible to dilute Mg–Zn–Ca ternary alloys such as Mg–1Zn–0.5Ca alloy to achieve exceptionally high ELs of up to 44% [37]. The addition of Zr to Mg–Zn–Ca alloys can improve the strength of extruded alloys, i.e., by adding 0.8 wt% Zr to an Mg–6Zn–0.2Ca alloy, an extruded Mg–6Zn–0.2Ca–0.8Zr (ZKX600) alloy has been developed [41], which has excellent mechanical properties with a TYS of 310 MPa, UTS of 357 MPa, and EL to failure of 18%. The addition of Zr suppresses the recrystallization during extrusion, but leads to the formation of a very fine grain structure. Furthermore, Zr may work as a microalloying element, resulting in a fine and dense distribution of a MgZn2-based phase involving Ca and Zr in ZKX600 alloy.
Mn is also added to Mg–Zn-based alloys to improve both the strength and age-hardening response of an as-extruded alloy. By replacing an expensive Zr element with a cheaper Mn element in a Mg–Zn-based alloy, the cost of the alloy can be clearly reduced. Through extrusion and double aging treatment, Zhang et al. [43] developed an Mg–6Zn–1Mn alloy with an UTS of 366 MPa, owing to the precipitation from MgZn2 precipitates and Guinier–Preston (GP) zones. Furthermore, Qi et al. [44] studied varying Sn contents (0, 1, 2, 4, 6, 8, and 10 wt%) on the microstructure and mechanical properties of a hot-extruded Mg–6Zn–1Mn alloy. The results showed that in the studied alloys, a Mg–6Zn–1Mn–4Sn alloy subjected to double peak aging after solution treatment exhibits the highest TYS of 378 MPa and the highest UTS of 390 MPa, with an EL of 4.16%, which is mainly attributed to the synergistic effect of the precipitation strengthening of the MgZn2 and Mg2Sn precipitates. Although peak-aged Mg–Zn–Mn-based alloys show excellent strength levels, their ductility is inadequate for most applications. This has thus limited the use of this particular alloy class.
Considering the significant effect of Ca and Mn in Mg–Zn-based alloys, Mg–Zn–Ca–Mn alloys have attracted significant attention. For example, Tong et al. [45] designed a Mg–5.25Zn–0.6Ca–0.3Mn alloy, in which the addition of Mn led to a 52 MPa increase in TYS under the as-extruded condition owing to a grain refinement. Furthermore, by introducing Si, a Mg–6Zn–1Si–0.2Ca–0.5Mn alloy [46] was successfully fabricated, the YS of which was increased from 200 MPa under the as-extruded condition to 330 MPa at the peak-aged state, demonstrating significant post-extrusion aging strengthen effect.
Although the Mg–Zn-based concentrated alloys mentioned above can gain good mechanical properties through an aging treatment, their extrudability is poor at much less than 24 m/min (exit speed). For good extrudability, with the dilute additions of Zn, Ca, and Mn, three Mg–Zn–Ca–Mn alloys, namely Mg–0.21Zn–0.30Ca–0.14Mn, Mg–0.53Zn–0.24Ca–0.27Mn, and Mg–0.71Zn–0.36Ca–0.07Mn have been successfully extruded at 24 m/min, and a Mg–0.21Zn–0.30Ca–0.14Mn alloy can even be extruded at a high exit speed of 60 m/min without any surface defects [47]. Such outstanding extrudability is attributed to the formation of thermally stable Mg2Ca phase and the high solidus temperature (~ 620 °C). In addition, these three dilute alloys have shown nearly the same high uniform EL of ~ 23% owing to a similar RE texture. Therefore, if the grain growth can be further controlled during extrusion, dilute Mg–Zn–Ca–Mn alloys have significant potential as a high-speed extrudable wrought Mg alloy with improved ductility. When a Mg–0.21Zn–0.3Ca–0.14Mn alloy is extruded at an exit speed of 1.2 m/min, the extruded material shows a high TYS of 309 MPa, which is slightly higher than that of T6-treated ZK60 alloy, and interestingly, is also higher than more concentrated alloys of the same family [48]. In addition to these good strength levels, this particular alloy has a ductility of about 20% total EL, which is again better than that of more concentrated alloys [49]. Consequently, this low-cost, low-density, dilute Mg–Zn–Ca–Mn alloy with enhanced mechanical properties and extrudability is expected to be competitive in future industrial applications.
Furthermore, Sn was also introduced to improve the extrudability [50] or mechanical performance [51] of a Mg–Zn-based alloy. Recently, a new type of wrought Mg alloy Mg–3.0Zn–1.0Sn–0.3Mn–0.3Ca was developed with an extrusion exit speed of 60 mm/s [50]. The results indicate that the alloy presents the optimum comprehensive mechanical properties, with a TYS, an UTS, and an EL of 187 MPa, 301 MPa, and 19.6%, respectively, which are ascribed to the combined effect of a relatively small grain size, uniformly distributed discontinuous particles, and weak basal texture. Therefore, Mg–Zn–Ca–Sn–Mn alloys, which contain small amount of Zn and Sn, having good mechanical properties and extrudability can become good candidates for mass production.
2.3 Mg–Ca-Based Alloys
The equilibrium solid solubility of Ca in Mg is 1.35 wt% at a eutectic temperature of 516.5 °C and is approximately zero at 200 °C. The equilibrium intermetallic phase at the Mg-rich end of the Mg–Ca phase diagram is Mg2Ca, which has a crystal structure similar to that of the Mg matrix phase [17]. Because Ca has a low density of 1.55 g/cm3, Mg–Ca alloys have the advantage of preserving the low density of Mg, and the addition of Ca can also reduce the flammability of molten Mg metal. Moreover, the released Ca component is beneficial for bone regeneration. With these advantages, Mg–Ca binary alloys are expected to be one of the most promising candidates for high-performance biomaterials [52, 53]. In addition, Ca seems to be the only one that has a large atomic size equivalent to RE elements among all possible alloying elements in RE-free Mg alloys [54]. Thus, a significant segregation of Ca at the grain boundaries or dislocations can be expected, which is believed to play a critical role in enhancing the mechanical properties of the alloys. Therefore, efforts have been made to develop extrusion alloys based on the Mg–Ca system, as listed in Table 3 [55,56,57,58,59,60].
Through indirect extrusion, Pan et al. [55] reported a Mg–1Ca binary alloy possessing a TYS of 310 MPa and an UTS of 330 MPa. In contrast, the extruded Mg–1Ca binary alloy studied by Kim et al. [56] exhibits relatively poorer mechanical properties, with a TYS of 185 MPa and an UTS of 239 MPa. A careful comparison shows that the great strengthening effect in the extruded Mg–1Ca alloy fabricated by Pan [55] arises from the combined effects of nanoscale Mg2Ca heterogeneous particles and fine dynamically recrystallized grains (~ 1 μm), as shown in Fig. 2 [55], which is much finer than that of Kim (~ 4.6 µm) [56]; the exact composition seems to be different. In addition, the results from Kim et al. also showed that both the strength and ductility of the extruded Mg–xCa (x = 0.4, 1, 2, 3 wt%) alloys tended to increase as the Ca content increased, and a Mg–0.4Ca alloy shows a high EL of 34%. The extruded Mg–0.5Ca alloy demonstrated a limited EL of ~ 7.5% but a high tensile TYS of 420 MPa [57].
a, b Optical micrographs. c, d TEM images of the as-extruded samples. a, c are the sample extruded at 300 °C and b, d are the sample extruded at 350 °C. The enlarged morphology of typical nanoparticles in c and the corresponding FFT patterns are included in c1 and c2. Textures of the extruded samples are also shown in e. Extrusion direction and radial direction are marked as ED and RD, respectively [55]
By adding 0.5 wt% Sr to Mg–1.0Ca alloy, Henderson et al. [58] found that a Mg–1.0Ca–0.5Sr alloy extruded at 275 °C processes fine grains (1.6 µm) and a high tensile TYS of 304 MPa with an EL of 10%, making this alloy competitive against existing implant materials. By adding Zn and Zr to Mg–Ca alloy, an extruded Mg–1Ca–1Zn–0.6Zr alloy with a fine microstructure and a typical basal texture was obtained, which exhibited a TYS of 306 MPa along with a moderate EL of 11% [59]. The refined microstructure of this alloy is a result of the combination of both discontinuous and continuous dynamic recrystallization and the Zener pinning effect induced by the fine precipitates.
It is known that the grain refinement in Mg alloys contributes to the strengthening more significantly than the other alloy systems (such as Al alloys), according to their higher k values in Hall–Petch relationship [61]. According to this theory, the ultrafine grains of a-Mg matrix are thus considered to be crucial for development of the high-strength Mg alloys. Lately, Pan et al. [60] reported a low-alloyed Mg–2Sn–2Ca alloy using conventional extrusion process, which can exhibit an ultra-high absolute strength, a TYS of 443 MPa, or a good strength ductility balance, e.g., a tensile TYS of 358 MPa and an EL of ~ 8.9%. The characterization results revealed that the ultra-high strength is mainly associated with the submicron grains (~ 0.32 μm), Ca enrichment in most LAGBs, and residual Mg2Ca nanoprecipitates would in turn significantly contribute to the stunning high strength, as shown in Fig. 3 [60]. More importantly, there is little drop in mechanical properties when it is extruded at a speed several times higher than those used in the reported high-strength Mg alloys. Overall, the low-cost alloying elements and the one-step extrusion process are expected to open ever-bright prospects for fabricating high-performance RE-free Mg extrusion products for larger-scale industrial applications.
Typical TEM images of as-extruded samples, a, b TX22-220, c, d TX22-240, e, f TX22-280 alloys. LAGBs are marked by yellow arrows, and the dislocations are marked by red arrows. Nanoprecipitates are circled by the blue ellipses [60]
In general, the large difference in the microstructure, texture, and mechanical properties mentioned above indicates that further studies are required to identify the exact role of Ca in a Mg–Ca-based alloy, focusing on an understanding of the segregation formation mechanism and the influence of solute clusters in facilitating severe grain refinement for Mg alloys. Through these understanding, the strengthening mechanisms, and resultant mechanical properties of Mg–Ca-based alloys can be clearly figured out.
2.4 Mg–Sn-Based Alloys
The Mg–Sn system has also been shown to be aging hardenable because the solid solubility of Sn in Mg is as high as ~ 14.5 wt% at a eutectic temperature of 561 °C, and decreases to ~ 0.49 wt% at 200 °C [17]. Thus, a large amount of the Mg2Sn phase will precipitate during the aging treatment, and Mg–Sn alloy has therefore received significant attention with regard to the development of new high-strength RE-free Mg extrusion alloys. The mechanical properties of certain Mg–Sn-based extrusion alloys are shown in Table 4 [62,63,64,65,66,67,68,69,70].
Mg–Sn binary alloys demonstrate low mechanical properties, for example, extruded Mg–2Sn [62] and Mg–6Sn [63] binary alloys reveal a low TYS of 157 and 191 MPa, respectively. To enhance the strength of extruded Mg–Sn-based alloys, Zn and Al are often added. Sasaki et al. [64] developed a heat-treatable Mg–9.3Sn–2.45Zn–0.21Mn extrusion alloy, the mechanical properties of the as-extruded bars with a TYS of 163 MPa, an UTS of 274 MPa and an EL of 22%. In addition, a peak-aged sample exhibits an increase in TYS of 27 MPa as a result of T6 aging treatment.
Accordingly, Sasaki et al. [65] developed a new Mg–9.8Sn–1.2Zn–1.0Al extrusion alloy, reaching a TYS of 308 MPa, which results from second-phase hardening of a large number of nanosized Mg2Sn precipitates with a spherical shape and grain refinement (~ 2 μm). She et al. [63] systematically studied the properties of hot extrusion Mg–xAl–5Sn–0.3Mn alloys (x = 1, 3, 6, and 9), revealing that the UTS of as-extruded alloys increases with an increase in Al content, reaching 370 MPa for 9 wt% Al. However, the increase in Al content in a Mg–9.8Sn–1.2Zn-based alloy [66] did not show a clear enhancement when the YS of the as-extruded Mg–9.8Sn–3Al–1.2Zn (TAZ1031) alloy varies within the range of 214–319 MPa.
Considering the trace addition of Na enhanced precipitation kinetics and peak hardness substantially, sodium (Na) can be added to Mg–Sn–Al–Zn alloys to improve the age-hardening response [71], and a Na-doped TAZ1031–0.1Na alloy exhibiting a higher TYS of 336 MPa was developed through extrusion and T6 treatment by Elsayed et al. [66]. Moreover, a Na-doped Mg–5.4Sn–4.2Zn–2.0Al–0.2Mn–0.1Na alloy [67] exhibited a significant increase in TYS from 243 to 347 MPa by a T6 treatment due to the uniform dispersion of nanoscale precipitates by aging. And the characterization results indicate that the trace addition of Na causes the formation of Sn–Na co-clusters in the early stage of aging, which provides heterogeneous nucleation sites for Mg2Sn precipitates. However, the Na-doped T6-treated Mg–Sn-based extrusion alloys exhibited very limited ductility with the EL of ~ 3% because of the Na segregation at grain boundaries during aging. Hence, Mg–Sn–Al–Zn–Na alloys are unlikely to be widely used.
To overcome this problem, Sasaki et al. [67] developed a Na-free Mg–6.6Sn–5.9Zn–2.0Al–0.2Mn (TZAM6620) alloy, in which nanoscale MgZn2 precipitates are uniformly dispersed through double aging. The double-aged TZAM6620 sample exhibited a very high TYS of 370 MPa with a large EL of 14%. Similar to the case of Mg–Zn-based alloys, pre-aging treatment at 70 °C causes the formation of Zn-rich GP zones, which act as heterogeneous nucleation sites for MgZn2 precipitates, and thus refines the sizes of the precipitates, enhancing the precipitation hardening response, and eventually contributing to the higher strength.
In addition to Al, Zn, Na, Cu, and Ca have also been introduced to enhance the mechanical properties of Mg–Sn-based alloys. Research [68] has also shown that the addition of 0.5 Cu in extruded Mg–8Sn–2Zn alloys results in good mechanical properties, with a TYS of 365.9 MPa, an UTS of 388.4 MPa, and an EL of 5.8%, owing to the high number density of micron-sized broken MgZnCu and Mg2Sn particles, as well as nanosized Mg2Sn precipitates and refined grains. In addition, new extrusion alloys, Mg–2Sn–1Ca and Mg–2Sn–1Ca–2Zn (TXZ212), were studied [62], demonstrating high strength and ductility simultaneously. The TYS and UTS of TXZ212 alloy reached 218 and 285 MPa, and the EL reached as high as 23%; this high strength was expected owing to the high number density of nano-MgSnCa phases, GP zones, and an ultra-fine grain size (0.8 μm). The high EL of 23% in TXZ212 alloy is consistent with the high work-hardening rate, which is attributed to the larger grain size, higher angular grain boundaries, the presence of more nanoparticles, and a weaker texture.
However, all of these types of high-strength Mg–Sn-based alloys were fabricated using low-speed extrusion, which will increase the cost of extruded products owing to low productivity. By decreasing the Sn count in the alloy, several high-speed extrusion alloys were developed. A novel Mg–8Sn–1Al–1Zn alloy [69] developed for high-speed extrusion was successfully extruded at speeds within a range of 2–10 m/min at a temperature of 250 °C; additionally, Mg–7Sn–1Al–1Zn (TAZ711) alloy [70] has a smooth surface when extruded at 350 °C with an exit speed of 27 m/min, which is much different from severe surface cracking observed in AZ80 when extruded using a much lower exit speed of 6 m/min. The enhanced extrudability of Mg–Sn–Al–Zn alloys is mainly due to the formation of a thermally stable Mg2Sn phase. However, TAZ711 alloy extruded at high speeds has a relatively low TYS of 180 MPa. To further improve the strength of high-speed extruded Mg–Sn–Al–Zn alloys, advanced extrusion technics have been explored, which are described in Sect. 3.
2.5 Mg–Bi-Based Alloys
The maximum equilibrium solid solubility of Bi in Mg is 8.87 wt% at a eutectic temperature of 553 °C and decreases to approximately zero at 200 °C. The equilibrium intermetallic phase at the Mg-rich side in a Mg–Bi binary phase diagram is α-Mg3Bi2, which has a hexagonal structure, the maximum volume fraction of which is ~ 3.38% when a Mg–8.85Bi alloy is aged at 200 °C [17]. Similar to Sn, the Mg–Bi alloy system shows typical precipitation-type phase equilibrium. In addition, the Mg3Bi2 phase exhibits a high melting temperature of 821 °C, which is higher than that of Mg2Sn (Tm = 770 °C), and is likely to play an important role in the suppression of the grain growth during the hot extrusion process. The tensile properties of certain Mg–Bi-based extrusion alloys are shown in Table 5 [72,73,74,75].
We [72] recently investigated the microstructure and mechanical properties of extruded Mg–6Bi (B6) as compared with pure Mg. The results show that the as-extruded Mg–6Bi alloy represents a finer and more homogenous microstructure, and the average grain size decreases from 30 to 4 μm when 6% Bi is added. In addition, the B6 alloy contains strip-like fragmented Mg3Bi2 particles along the extrusion direction, as well as fine Mg3Bi2 precipitates, and demonstrates superior mechanical properties with a TYS of 189 MPa, an UTS of 228 MPa, an EL of 19.9%, and a low tension–compression yield asymmetry with a yield asymmetric ratio of 1.01. These significant improvements in the mechanical properties are mainly attributed to the combined effects of the grain refinement and a large number of coexisting micro-/nanosize Mg3Bi2 particles.
The addition of Zn is usually known to enhance the precipitation response of Mg–Bi alloy by promoting the refinement and homogeneous dispersion of the Mg3Bi2 phase [76]. Moreover, the addition of Al can provide extra strength of Mg–Bi alloys through the solution hardening effect in Mg alloys. Based on these effects of Zn and Al, we developed a high-strength Mg–8Bi–1Al–1Zn (BAZ811) alloy as a material for extrusion [73]. Compared with a commercial AZ31 alloy extruded under the same conditions, the BAZ811 alloy exhibits higher mechanical properties, with a TYS of 291 MPa, an UTS of 331 MPa, an EL of 14.6%, and a reduction in yield asymmetry; these good mechanical properties are mainly attributed to the synergetic effects of grain refinement and broken microscale Mg3Bi2 particles together with spherical nanoscale Mg3Bi2 precipitates.
In addition, by combing rapid solidification and extrusion processes, Remennik et al. [74] developed a biodegradable Mg–5Bi–1Ca/1Si alloy with high ductility (> 40%) at room temperature, which results from grain refinement, fine Mg2Bi2Ca and Mg2Si precipitates at the grain boundaries, and nanosized Mg3Bi2 precipitates within the grains.
Somekawa et al. [75] produced fine-grained (1–2 μm) Mg–2.5Bi dilute binary alloys through extrusion at various low temperatures between 105 and 210 °C. A Mg–2.5Bi alloy containing binary phase particles exhibited a remarkable tensile EL of 170% even at a strain rate of 1 × 10−3 s−1 at room temperature, owing to grain boundary sliding, which has not been observed in any other Mg alloys.
Therefore, it is believed that Mg–Bi-based alloys have significant potential to be developed as a new extrudable alloy with relatively high mechanical properties. However, to date, studies on Mg–Bi systems remain very limited, and more attention should be paid to the development of promising RE-free Mg–Bi-based alloys as an extrudable material with high strength and ductility.
3 Advanced Extrusion Technologies
3.1 Low-Temperature and Slow-Speed (LTSS) Extrusion
The microstructural and mechanical properties of extruded materials are greatly affected by not only the alloy composition and initial billet condition, but also the extrusion conditions such as the temperature, speed, and ratio. The size of the DRXed grains of an extruded material is strongly dependent on the deformation temperature. When the extrusion speed is reduced, the deformation-induced temperature increase at the deformation zone is restricted through a reduction in heat associated with friction and deformation during extrusion. In addition, the combination of a low temperature and slow speed generates a greater number of dynamic precipitates that help restrain the growth of DRXed grains, thereby increasing the strength of the alloy. From this point of view, Yu et al. [77] succeeded in developing an extraordinarily high-strength Mg alloy through low-temperature and slow-speed extrusion (LTSS extrusion).
Through LTSS extrusion, at a low temperature of 200 °C and a slow speed of 0.084 m/min, outstanding tensile properties, namely a TYS of 403 MPa, an UTS of 437 MPa, and an EL of 10.7%, were achieved in a commercial AZ80 alloy, along with an excellent compression/tension yield ratio of 0.97. These superior mechanical properties are attributed to the combined effect of ultra-fine recrystallized grains (~ 0.4 μm), numerous nanoscale Mg17Al12 (50–150 nm) precipitates, and a strong basal texture of 5.1, as shown in Fig. 4 [77].
a Optical micrograph, b dark field TEM micrographs, c inverse pole figure of LTSS-extruded AZ80 alloy [77]
Nominally extrusion AZ31 alloy has a low-to-moderate YS among Mg extrusion alloys, generally less than 250 MPa. A research by Zeng [78] confirmed the effect of LTSS extrusion. When extruded at 175 °C with a slow speed of 0.09 m/min, AZ31 alloy demonstrated exceptionally high strength with a TYS of 380 MPa in both tension and compression and a UTS of 430 MPa. The high-strength AZ31 had a uniform grain size of ~ 0.65 mm and contained dense particles with a size of < 10 nm.
This extrusion technology was also introduced in the development of Mg–Sn- and Mg–Zn-based alloy. For example, Sasaki et al. [63] developed an Mg–9.8Sn–1.2Zn–1.0Al alloy with a TYS of 308 MPa, an UTS of 354 MPa, and an EL of 12% through slow extrusion with the exit speed of 0.12 m/min at a low temperature of 250 °C. In addition, a TYS of over 300 MPa has been achieved in as-extruded Mg–Zn–Ag–Ca–Zr alloy through slow extrusion at a rate of 0.12 m/min [79].
3.2 Artificial Cooling-Assisted Extrusion (ACE)
To expand the application range of extruded Mg alloys, there is a clear need to improve the strength of Mg alloys extruded at high speeds and reduce their production cost. However, under high-speed extrusion conditions, the DRXed grains coarsen, and the resultant mechanical properties are usually deteriorated, because the actual temperature of the deformation zone increases with the extrusion speed owing to the increase in heat generated by an abrupt plastic deformation and friction during extrusion [80]. Therefore, the application of artificial cooling in an extrusion die is likely to dramatically reduce the size of the DRXed grains, which has been called artificial cooling-assisted extrusion (ACE) [81]. A schematic diagram of the indirect extrusion process and its associated artificial cooling system is shown in Fig. 5 [81]. Cold water is first fed through four inlets located on the surface of the stem, transferred through holes inside the stem to a specially designed die, and then sprayed directly onto the extruded bar at the die exit (Fig. 5a). Because a fast extrusion speed means that the extruded bar is only briefly exposed to the cooling water, a die with twelve holes located along its circumference is used to enhance the cooling effect over the entire surface of the extruded bar, as shown in Fig. 5b [81].
a Schematic diagram of indirect extrusion with an artificial cooling system. b Schematic diagrams and images of cooling die used [81]
Park et al. [81] reported that the application of artificial cooling to the high-speed extrusion (13.5 m/min) of TAZ711 alloy can dramatically reduce the size of the DRXed grains, and increase the number of fine Mg2Sn precipitates. This is attributed to a decrease in the temperature of the deformation zone and has the effect of improving the strength without a loss in ductility. As a result, high-strength TAZ711 Mg alloy (210 MPa TYS, 303 MPa UTS, and 10.2% EL) can be obtained at a high speed (13.5 m/min) using an ACE method.
Moreover, other studies [82, 83] have confirmed that the strength of extruded TAZ711 [82] and AZ31 [83] alloys can be improved through artificial cooling during indirect extrusion, even at relatively low extrusion speeds of less than 2 m/min.
3.3 Pre-aging Extrusion
As reviewed in Sect. 2, a number of studies have focused on post-aging and/or double aging after extrusion [30, 31, 40, 43, 44, 64, 67]; however, this largely overlooks the fact that pre-aging prior to extrusion can potentially be more effective in enhancing the mechanical properties of an extruded alloy, because the fine precipitates formed before extrusion can retard the growth of DRXed grains [84]. Indeed, it has already been demonstrated that undissolved eutectic Mg17Al12 phases are broken and distributed along DRXed grain boundaries during extrusion, which ultimately lead to a finer microstructure, and a resultant increase in strength and EL, demonstrating the feasibility of using preexisting precipitates to improve the mechanical properties of an extruded alloy [85].
Jung et al. [86] introduced commercial AZ80 alloy to confirm the effects of pre-aging extrusion (APE) and found that fine Mg17Al12 precipitates formed by aging prior to extrusion reduce the average size of the DRXed grains and increase the number of small Mg17Al12 particles present after extrusion, thereby improving the strength of the alloy, as well as enhancing its ductility by decreasing the number of large-band Mg17Al12 particles. Moreover, the effects of APE on the microstructure and mechanical properties of TAZ711 alloy [87] were also investigated, and the results showed that, at a high extrusion speed of 12 m/min, APE can bring about finer DRXed grains and a larger amount of precipitates, thereby simultaneously improving the strength and ductility, and showing superior strength and ductility compared to that aged after extrusion. This indicates that APE is a more effective method for improving the mechanical properties of an extruded alloy. However, a decrease in the ductility was observed at low extrusion speeds (1.5 and 0.15 m/min) owing to a reduction in the fraction of DRXed grains and an increased number of Mg2Sn precipitates.
3.4 Pre-twinning Extrusion
During hot deformation, preexisting twins, acting as preferential sites for DRX, can play an important role in the formation of DRXed grains, called twin-aided DRX (TDRX) [88,89,90]. Based on this concept, Park et al. [91] improved the mechanical properties of an extruded AZ31 Mg alloy using a novel cold pre-forging (CPF) process prior to extrusion, thereby introducing numerous initial twins into the material, which in turn provides nucleation sites for DRX during extrusion. As a result, the CPF process leads to an increase in the area fraction of DRXed grains of the extruded alloy, as well as an enhancement in the uniformity of the DRXed grain structure; this consequently improves the strength and ductility of the extruded alloy, by which the TYS, UTS, and EL are increased by 28 MPa, 32 MPa, and 1.9%, respectively, through the CPF process.
With regard to Mg–Sn-based alloy, CPF treatment also makes sense based on the microstructure and mechanical properties of extruded TAZ811 alloy by indirectly extruding cold-forged billets [92]. The results revealed that the recrystallized area fraction and tensile properties gradually increase with the amount of CPF up to 10% CPF, but then become nearly constant. The 10% CPF TAZ811 exhibited excellent tensile properties, demonstrating a TYS of 387 MPa, an UTS of 403 MPa, and an EL of 7.9%.
3.5 Asymmetric Extrusion
Inspired by the differential speed rolling process mentioned above, Yang et al. [93,94,95,96,97] developed a suitable constitutive model of differential speed extrusion, namely asymmetric extrusion (ASE), to fabricate extruded Mg sheets. Motivated by the extra asymmetric shear strain in the sheet thickness direction, different degrees of DRX appear during the ASE path, which makes the basal planes rotate approximately 12° toward the imposed shear direction, and the texture intensity is largely weakened owing to the asymmetry shear deformation.
Trapezoid extrusion (TE) is another way to introduce additional shear deformation [98]. Remarkable enhancements in the strength and ductility of Mg alloy sheets were obtained using the TE approach owing to the tilted weak basal texture and grain refinement.
As mentioned above, ASE and TE have been proven to be an effective candidate to gain homogenous and finer grains and to randomize the texture, thus improving the mechanical properties of Mg alloy sheets, which are ascribed to the introduction of shear deformation during severe plastic deformation. However, the design of an extrusion mold for the ASE process is complex and requires long-term maintenance.
3.6 Composite Extrusion (CE)
To overcome the shortcomings of the ASE process, an effective extrusion approach, called composite extrusion (CE), is carried out to improve the mechanical properties of the extruded Mg alloy sheets [99, 100]. A common extrusion mold is used to save money and improve the efficiency during the composite extrusion, which avoids the long-term maintenance of the mold after the ASE process.
In CE processing, round Mg alloy ingots are divided into several pieces to introduce specific interfaces before direct extrusion, as shown in Fig. 6 [99]. These intentionally introduced interfaces have been confirmed to play an important role in the microstructure evolution and texture modification of Mg alloys during hot extrusion.
Schematic of a conventional extrusion process, b–d designed CE process [99]
Pan et al. [100] applied CE to AZ31 alloy extrusion, the results of which indicated that the ductility of an extruded AZ31 sheet was increased from 18.5 to 22.5%, and the UTS was also improved through the CE process as compared with those using a conventional direct extrusion. These enhanced properties are mainly attributed to the more homogeneous and finer microstructure near the center, as well as to the weakened basal texture, which depend on the introduction of the shear deformation caused by fiction between the billets during the CE. Therefore, by introducing appropriate interfaces during extrusion, CE can be a simple but effective extrusion process for enhancing the comprehensive mechanical properties of Mg alloy sheets.
As reviewed above, CE is a simple but effective extrusion approach to improve the mechanical properties. It can effectively increase the total shear strain by introducing additional interfaces. However, the effective difference in strain for the sheet cannot occur during the CE process, which results in a lower degree of texture weakening.
To introduce an effective difference in strain during the CE process, asymmetric composite extrusion (ACE) was introduced [101]. The key aspect of ACE processing lies in splitting the billet into two asymmetric parts along the axis direction, as compared with CE processing. During the extrusion process, two asymmetric parts suffer from different extrusion forces, which bring about plastic deformation of different degrees. When the metal flows into the deformation zone, a difference in strain is formed between the top and bottom surfaces for the sheet processed using ACE throughout the sheet thickness direction, resulting in a tilted weak texture, thereby improving the sheet formability. For example, by splitting the homogenized billets into two parts with a ratio of 3 to 5, Wang et al. [101] demonstrated the effectiveness of ACE processing, as elaborated in Fig. 7 [101]. Compared with a CEed sheet, grains of an ACEed sheet were refined, and the anisotropy was improved in the tensile directions along the ED, 45° and TD, thus leading to a good formability of the ACEed sheet.
Schematic diagram of the fabrication of AZ31 alloy sheet using asymmetric composite extrusion (ACE) process [101]
Therefore, as a simple but effective extrusion approach, ACE can improve the mechanical properties of Mg alloy sheets.
3.7 Ultrasonic-Assisted Extrusion (UE)
Microforming technologies exhibit a significant advantage owing to their high production efficiency, decreased material removal treatment, enhanced material utilization, and appropriate mechanical properties. Exploring a new microformation process on Mg alloys is significant in promoting its commercial application. Ultrasonic vibrations possess high directivity and energy, and there is an ultrasonic softening effect that can be utilized for instantaneous softening when ultrasonic energy is applied during deformation [102]. Therefore, applying ultrasonic vibration in the formation process may be a feasible approach to achieve high formability during Mg alloy deformation.
Recently, Lou et al. [103] developed a new ZK60 micropin formed through ultrasonic-assisted extrusion at room temperature. Ultrasonic vibration-assisted micropin extrusion was conducted on an apparatus shown in Fig. 8 [103]. The billet was transmitted by an ultrasonic wave during the micropin-forming process. The results showed that ultrasonic-assisted microextrusion is beneficial for the fabrication of extruded ZK60 alloy. The extrusion load greatly decreases with the increase in amplitude, and the maximum reduction in load is approximately 80% because of the DRX and ultrasonic softening. In addition, the microhardness of the micropins increases at amplitudes of 39 and 42 μm, as compared with traditional extrusion. In addition, the compressibility of the micropins improves by 14–20% on average at room temperature as compared with that prepared using traditional extrusion.
Schematic of ultrasonic vibration-assisted microextrusion apparatus for ZK60 magnesium alloy [103]
3.8 Hydrostatic Extrusion (HE)
It is well known that the workability of Mg alloys is limited, or even impossible, at temperatures of below 200–225 °C [4, 104] in conventional extrusion processes owing to an insufficient number of activated slip systems. Unlike conventional direct or indirect extrusion, in the process of hydrostatic extrusion, a billet is forced through a die opening by means of a pressurized fluid, as shown in Fig. 9 [104]. There is no metallic contact between the ram and billet and thus only billet-tooling contact occurs with a die cone. Plastic deformation takes place under high hydrostatic pressure, and because the lubrication is nearly ideal, no significant friction is involved. However, in the case of conventional extrusion, considerable frictional forces and shearing within the billet add to the mechanical work. Thus, the thermal effects associated with the dissipation of such mechanical work are less during hydrostatic extrusion, which reduces the danger of incipient melting of the billet material and allows the processing to be conducted at higher speeds without the formation of surface hot tearing.
Hydrostatic extrusion of a solid, b hollow sections [104]
The behavior of Mg alloys during hydrostatic extrusion was investigated, and the results showed that the hydrostatic process offers the possibility to decrease the extrusion temperature to 100 °C for AZ31, AZ61, ZM21, ZK30, and ZE10 alloys, and 110 °C for AZ80 alloy with a relatively high extrusion rate of 8 m/min [105]. Moreover, the results revealed that the grain sizes of AZ31, ZK30, and AZ61 alloys obtained by these hydrostatic extrusion trails are comparable with that achieved through the ECAE process [106], and that their mechanical properties are higher than those of alloys obtained by conventional direct and indirect extrusions.
From the successful hydrostatic extrusion of AZ80 alloy at a low temperature and high speed, it can be concluded that the hydrostatic process will result in a significant advancement in Mg alloy extrudability. In addition, an extremely fine-grained microstructure, excellent ductility, and enhanced YS can be achieved using this hydrostatic extrusion method.
4 Extrusion-Related Severe Plastic Deformation Processing
Considerable interest has recently developed regarding the potential for the processing using severe plastic deformation (SPD) of Mg alloys. The principle of SPD is that it provides a large amount of strain in a sample, and the rearrangement of dislocations introduced by the imposed strain leads to a very substantial grain refinement down to the sub-micrometer or even the nanometer scale [107, 108]. The ultrafine grain structure produced from the SPD processing method is mostly associated with improvements in the mechanical properties, which is attributed to the formation of high-density internal interfaces such as twin, grain, and sub-grain boundaries [107]. SPD techniques are also expected to decrease the propensity for twinning as the grain size is reduced, thereby weakening the basal texture [109, 110]. Therefore, SPD offers the possibility of enhancing the mechanical properties of current RE-free Mg alloys.
Hereafter, extrusion-related SPD techniques including equal channel angular pressing (ECAP), an extrusion shear (ES), cyclic extrusion compression (CEC), repetitive upsetting extrusion (RUE), and accumulative back extrusion (ABE) are shown in Table 6 [111,112,113,114,115,116] and presented in the succeeding subsections. Each technique has its unique advantages over the others in terms of grain refinement and an improvement in the material properties.
4.1 Equal Channel Angular Pressing (ECAP)
Among the various SPD methods, equal channel angular pressing (ECAP), less appropriately referred to as equal channel angular extrusion (ECAE) in some publications, is currently the most highly developed SPD processing technique (Table 6a) [111]. ECAP is one of the simplest and most effective SPD techniques commonly used to produce bulk ultrafine Mg alloys. In ECAP, the bulk material is pressed through an angle die, and plastic strain is introduced without a change in the dimensions of the work piece. The die consists of two intersecting channels of equal cross section, and shear strain is introduced at the intersection of the channels; in addition, repeated pressing can result in strain accumulation.
The ECAP processing of Mg alloys at elevated temperatures results in significant grain refinement and activation of a non-basal slip in the crystal structure [117]. For example, Mabuchi et al. [118] applied ECAP on AZ91 alloy as early as 1996, and ultra-fine grains and a low-temperature super plasticity were observed in the ECAPed AZ91 alloy. Later, Yamashita et al. [119] improved the mechanical properties of pure Mg and Mg–0.9Al alloy using ECAP, with three- to fourfold increase in the UTS of the processed samples. Kim et al. [120] later found that the YS of ECAPed AZ61 alloy varies within a large range depending on the different processing routes. The authors proposed that the change in YS might be due primarily to the difference in texture.
However, a major concern associated with the ECAP of Mg alloys arises in the premature cracking of the alloys when deformed at low temperatures, which results from the absence of active slip systems at ambient temperature. Therefore, the ECAP of Mg alloys has often been constrained to processing at elevated temperatures, and refinement is limited due to grain growth [121]. Several attempts have been made to induce pressing at lower temperatures [122, 123]. Janeček et al. [122] studied the effects of ECAP at 200 °C on the evolution of the microstructure and mechanical properties of AZ31 alloy. The grain size of an ECAP sample can be severely refined to 0.5–0.8 μm, and even grains with a size of 100 nm appear. The low ECAP temperature of 200 °C has been believed to restrict grain growth and lead to fine grains, finally resulting in an increase in the YS of AZ31 alloy of up to ~ 210 MPa. Inspired by these innovative studies, Biswas et al. [123] conducted ECAP on pure Mg using a multi-pass process and successfully produced ultra-fine grains of 0.25 μm at room temperature. Both a lower average grain size and a favorable initial texture contribute to the availability in conducting ECAP at room temperature.
More interestingly, a higher strength would be possible when ECAP is conducted on Mg alloys with sufficient amounts of dispersive second phases. Based on these concepts, a previous study by Yan et al. [124] found that the UTS of ECAPed Mg–6Zn–2Al alloy reached 340 MPa. Wang et al. [125] conducted an ECAP on Mg–10Al and Mg–10Al–0.5Sb alloys and demonstrated superior strength at both room and elevated temperatures.
In addition to the Mg–Al–(Zn)-based alloys described above, other RE-free Mg alloys have been also attempted. For example, ECAP has been applied to ZK60 alloy at 250 °C, whereby its UTS increases up to 326 MPa after two ECAP passes, and its EL increases from an initial value of ~ 15 to ~ 30% after the four ECAP passes [126]. Mg–5.2Zn–0.6Ca alloy processed using ECAP shows high TYS and UTS of 246 and 332 MPa, respectively, in which the grain refinement plays a significant role [127]. In addition, the textures of ECAPed Mg alloys generally vary with the applied ECAP routes, and both the grain refinement and texture change strongly affect the final mechanical properties of the material.
4.2 Extrusion Shear (ES)
Combining direct extrusion and ECAE into a single process, a compound extrusion process has been explored to fabricate rods, which includes direct extrusion and subsequent ECAE, the processing technology of which is called an extrusion shear (ES) (Table 6b) [112, 113]. A larger amount of shear deformation in the materials can be introduced through the ES process than through direct extrusion with the same ratio. There are four deformation zones in an ES die. Deformation zones I and II are direct extrusion zones, which include an upsetting zone and a sizing zone, and deformation zones III and IV are the first and second shearing zones, respectively.
ZK60 [112] and AZ31 [113] alloys were processed using ES, and the experiment results showed that rods with good surface smoothness can be obtained using low-temperature and high-speed ES, and their grains are effectively refined through DRX.
4.3 Cyclic Extrusion Compression (CEC)
Cyclic extrusion compression (CEC) (Table 6c) [114], as a type of continuous SPD technique, is quite suitable for refining grains of hard-to-deform metals owing to the introduction of three-dimensional compression stresses during processing and thus can refine the grains, second-phase structures, and inclusions of Mg alloys [128, 129]. This processing involves pushing the materials from one cylindrical chamber of a known diameter to another cylindrical chamber with equal dimensions through a die with a diameter smaller than the cylindrical chambers. Thus, the processing induces an extrusion while the chambers provide compression, and thus, during one cycle, the material is subjected first to compression and then extrusion, and finally to compression again [130].
Chen et al. [128] reported that CEC was used to achieve grain sizes between the submicron and nanoscale levels in AZ31 alloy. In addition, the microtextures become weaker and the grain size is reduced as the number of passes increase, which are attributed to rotational DRX. Additionally, they established that the mechanical properties of AZ31 alloy depend on the grain size, dislocation density, texture, and grain boundary structure. Wang et al. [131] compared the efficiency of CEC processing for AZ31, AZ61, and AZ91 alloys. They reported that the first CEC pass is the most effective for all alloys and that a CEC critical pass exists for all three alloys beyond which their grain size and homogeneity remain almost constant. The homogeneity of the microstructures increases with an increase in the concentration of Mg17Al12 particles in the alloy structure.
4.4 Repetitive Upsetting Extrusion (RUE)
The processing of repetitive upsetting extrusion (RUE) is also an attractive method. Two main operations are involved in any individual cycle of the RUE process, i.e., a single upsetting operation and a single extrusion operation (Table 6d) [115]. First, the cylindrical sample is contained within a die chamber. Subsequently, the sample is subjected to one-dimensional upsetting and then to positive extrusion as the final deformation for a one-cycle RUE process. Consequently, the original shape of the sample will be reproduced at the end of any cycle, and intense plastic straining will be introduced after multiple cycle operations.
An as-cast AZ61 alloy was processed using RUE within a temperature range of 285–380 °C [132], the results of which indicate that a homogeneous fine-grained structure with an average grain size of 3.5 μm was obtained as the accumulated true strain in the axial direction increased to 4.28 after three RUE passes at 285 °C. The dominant reason for the grain refinement is considered to be from the promoted DRX induced by strain localization. The mechanical properties of RUE-processed AZ61 alloy were significantly improved owing to the grain refinement. In addition, the RUE process was employed to an AZ91D alloy at temperatures ranging from 285 to 380 °C, and the finite element method and a technical experiment were applied to reveal the non-uniform behavior during the RUE process [115]. The results from the finite element analysis showed that the effective strain was unevenly distributed in a one-cycle strained sample and that non-uniformity would permanently exist with an increase in the deformation cycles. A homogeneous structure with an average grain size of ~ 3.04 μm was obtained after the three-cycle RUE process at 285 °C, corresponding to an accumulated true strain of ~ 3.75. Furthermore, fragmentation and a dispersed distribution of the Mg17Al12 phase induced by shearing were confirmed.
As an innovative technique, the RUE process is easy to impose highly accumulated strains to a material and can be used to fabricate large material products with a fine grain structure while maintaining the shape and size for a deformed blank. If the productivity of the RUE process is enhanced, it will open up a new way for the preparation of fine-grained Mg alloys, which is of great significance in expanding the industrial application of Mg alloys.
4.5 Accumulative Back Extrusion (ABE)
Accumulative back extrusion (ABE) is another novel continuous SPD process, which combines the principle of back extrusion and two-dimensional constrained back pressing to achieve grain refinement (Table 6e) [116, 133], which is suitable for mass production. In comparison with conventional processes, ABE has brought about many advantages such as a lower required load, the production of nearly net-shaped products, and no need for inter-pass operations. Such operations are performed through a twin punch setup, and the punches are designed to slide past each other. In a one-pass process, the test material is first subjected to back extrusion to extrude the material into the gap between the inner punch and die, and the outer punch then presses the extruded material back into the inner punch until it reaches its initial cylindrical shape.
Fatemi-Varzaneh et al. [134] reported an ultrafine-grained AZ31 alloy subjected to ABE processing at relatively low temperatures. They also observed a significant change in the intensity of the basal texture as the number of passes increased, which was attributed to changes in the strain path during ABE processing. It was also revealed that the number of passes affected the orientation of the basal and prismatic planes developed during ABE processing.
In addition, ABE has been considered to modify the microstructural characteristics of a Mg–Al–Zn alloy with a higher Al content [135]. The results indicate that applying the ABE process up to five passes leads to simultaneous modifications of the Mg17Al12 phase morphology, grain structure (from 330 to 1 μm), and deformation texture. The occurrence of particle-stimulated nucleation, particle pinning, and shear banding phenomena has been suggested as the main reasons causing the weakened texture.
4.6 Other Related Extrusion Methods
Other SPD-involved extrusion techniques can be chosen for an enhancement of the mechanical properties of Mg alloys, such as a twist extrusion (TE) [136], shear assisted processing and extrusion (ShAPE) [137], friction stir extrusion (FSE) [138], simple shear extrusion (SSE) [139], continuous variable cross section direct extrusion (CVCDE) [140] and so on. These novel extrusion technologies can expand our knowledge regarding the modification of Mg extrusion processing. In addition, the use of a hybrid concept or architectural design [141] can also broaden our understanding regarding the development of high-performance RE-free Mg alloys.
5 Summary and Future Outlook
A brief review of the recent progress and development of low-cost RE-free Mg extrusion alloys to achieve higher mechanical properties, including for use in promising alloying systems and advanced processing techniques, was carefully presented.
The last two decades have witnessed considerable advances in the development of RE-free Mg extrusion alloys with high strength, good ductility, low tension–compression yield asymmetry, and improved extrudability. (1) To date, some RE-free Mg extrusion alloys exhibiting a good balance of strength and ductility have been developed, with TYS exceeding 200 MPa and EL of more than 20% [24, 29, 31, 45, 46, 48, 51, 63], which are competitive compared with 3xxx, 5xxx, and 6xxx Al extrusion alloys [33]. Moreover, several RE-free Mg extrusion alloys based on Mg–Al [25, 26], Mg–Sn [66, 67], and Mg–Bi [72, 73] alloy systems have demonstrated a nearly symmetric tension–compression yield characteristic. (2) Advanced extrusion technologies such as thermal and deformation treatments prior to extrusion have been developed to improve the mechanical properties. (3) Nascent processing routes have also been adopted for Mg alloys, which are significant in refining the size of the α-Mg matrix and the second phases, thereby improving the mechanical properties.
Despite the above-mentioned advances, to compete with Al extrusion alloys, a further enhancement of the extrudability and mechanical properties of RE-free Mg extrusion alloys is required. Thus, comprehensive experimental and theoretical studies should be conducted to understand the interplay among alloy composition, processing variables (e.g., temperature, speed, and extrusion ratio), metallurgical phenomena during extrusion (e.g., recrystallization and precipitation behaviors), microstructural characteristics (e.g., grain structure, precipitate, internal strain, and texture), and mechanical responses (e.g., strength, ductility, yield asymmetry, and toughness). The following viewpoints on future research directions for RE-free Mg extrusion alloys are proposed, based on the review herein.
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(1)
The highest extrusion speeds of Mg alloys reported have been 60–70 m/min for dilute Mg–Al [29, 30] and dilute Mg–Zn [47, 50] alloys, and 25–27 m/min for Mg–Sn alloy [63, 70], which are still lower than the extrusion speed used to process Al extrusion alloys (> 100 m/min) [6]. Alloying using dilute elements that can increase the solidus temperature of the alloy, such as Ca, Mn, Sn, and Bi, is a promising way for the development of high-speed extrudable Mg alloys. The concentration and ratio of the alloying elements should be carefully optimized to obtain high extrudability and mechanical properties simultaneously. Moreover, advanced extrusion technologies can be introduced to improve the mechanical properties. To develop new highly extrudable Mg alloys, much efforts are required to explore how microstructural factors such as the grain size, types of solute atoms and second-phase particles, and their distributions and individual features influence the thermal stability of the material and resultant extrudability.
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(2)
The highest TYS of RE-free Mg alloy is reportedly 420–440 MPa [57, 61], which is still lower compared with the high-strength Al extrusion alloys. Therefore, the development of ultra-high-strength RE-free Mg extrusion alloys is a promising and strongly required research area. As evidenced in the present review, several Mg alloys that have a microstructure containing submicron grains and dense and fine precipitates with a proper morphology and orientation tend to demonstrate ultra-high-strength characteristics. To date, newly developed LTSS extrusion technology and SPD processing have been consistently adopted to introduce this type of microstructure. However, the real challenge is how to produce submicron-grained Mg extrusion alloys at high extrusion speeds and at the moderate temperatures that are widely used in the existing extrusion industry. In this respect, hydrostatic extrusion may be a promising technology that needs to be given more attention in producing submicron-grained Mg extrusion alloys if the relevant equipment can be simplified with the technology development. In addition, to achieve this aim, not only discovery of new Mg alloy systems but only a comprehensive understanding regarding the precipitation behavior during extrusion, interaction between precipitates and DRX behavior associated with nucleation and growth of new grains, and strengthening mechanisms during plastic deformation are needed.
-
(3)
A number of RE-free Mg alloys show limited ductility at room temperature, and thus, the improvement in their ductility is another important issue. The typical basal texture of Mg extrusion alloys can be weakened through the appropriate addition of non-RE element, Ca, and advanced extrusion technologies such as AE, CE, and SPD, which consequently results in an improvement in ductility. However, some experimental results have shown no significant changes in the texture in Ca-containing Mg alloys. Thus, further studies are required for Ca-containing Mg alloys to identify the exact role of Ca in the RE texture formation. The SPD processes show significant potential in the fabrication of RE-free Mg alloys with excellent ductility. However, their usage at large scales and in mass production is restricted owing to the limitations of the size of products that can be produced and the difficulties of continuous production. In contrast, a super-formability at room temperature has already been achieved in pure Mg and Mg–2.5Bi extrusion alloy by virtue of the activation of grain boundary sliding mechanism, which can provide a new way to enhance the ductility of RE-free Mg extrusion alloys, and further studies are therefore strongly recommended.
In conclusion, it is clear that it will take a holistic approach, involving the alloying design, microstructure manipulation, innovative processing, and clever component design, to address these difficulties and provide a viable economic solution, probably on an application-specific basis. In addition, it is foreseeable that when all of the above-mentioned problems have been solved, the application of Mg extrusion alloys will become widespread in various industries.
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The authors acknowledge support from the National Natural Science Foundation of China (Nos. 51701060 and 51601181), the Natural Science Foundation of Hebei Province (Grant No. E2016202130) and Tianjin city (No. 18JCQNJC03900), the Graduate Student Outstanding Innovation Project of Hebei Province (Grant No. CXZZBS2018030), and the Joint Doctoral Training Foundation of HEBUT (Grant No. 2018HW0008).
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Meng, SJ., Yu, H., Fan, SD. et al. Recent Progress and Development in Extrusion of Rare Earth Free Mg Alloys: A Review. Acta Metall. Sin. (Engl. Lett.) 32, 145–168 (2019). https://doi.org/10.1007/s40195-018-00871-2
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DOI: https://doi.org/10.1007/s40195-018-00871-2