Introduction
The utilization of magnesium alloys as structural materials or automotive components is remarkably effective to reduce both the discharge of air pollutions (e.g.; CO2, SOX and NOX) and the energy consumption. The magnesium alloys, however, have poor formability, mechanical and physical properties, in particular Young’s modulus, corrosion and wear resistance, compared to the other industrial metals, e.g. steels or aluminum alloys. The previous works [1-4] indicated that Mg2Si bulky intermetallic, which was synthesized in solid-state from the elemental magnesium and silicon powder mixture, had a low density of 1.88 g/cm3 and good mechanical properties, such as 350~700 Hv micro-hardness, high elastic modulus of 120 GPa and a 1358 K high melting point. On the other hand, Hall-Petch relationship [5, 6], as well known, suggests the grain-refining process is employed to improve the mechanical properties. Equal-Channel-Angular-Extrusion (ECAE) with severely plastic deformation of materials in the die causes the high strength and toughness of magnesium alloys, because the refined grains effectively obstruct the basal plane slipping [7].
In this study, the characteristics of Mg2Si compounds are described first. The materials and processing designs by utilizing both the compounds and grains-refining methods via repeated plastic working to improve the mechanical and physical properties of magnesium alloys are introduced. As a viewpoint of the recycling process, the possibility to employ SiO2 glass powder to synthesize Mg2Si as input raw materials instead of silicon is discussed in this review. Furthermore, for establishing the scale-up oriented technology, the above processing design for the high-performance magnesium alloys is applied to produce products such as pipes, rods and plates production in industries.
Materials and Processing Designs of Magnesium Alloys
Characteristics of Mg2Si Intermetallics
Mg2Si bulky materials, which are formed in solid-state by consolidating Mg-33.33 mol%Si powder mixture via hot pressing process [3], have good mechanical properties, such as 350~700 Hv micro-hardness, high elastic modulus of 120 GPa and a 1358 K high melting point. They are much better than those of the conventional magnesium alloys. It suggests that the mechanical properties of the magnesium alloys are possible to be improved by the uniform distribution of fine Mg2Si particles in the matrix.
Current density of the anode corresponds to the corrosion reaction speed of the materials. That is, the lower current density means more noble corrosion resistance. Compared to the conventional AZ31 magnesium alloy, the current density of Mg2Si bulky materials is reduced to 1/40~1/60. The increase of the potential balanced between cathode and anode reaction also means that Mg2Si is superior potential to AZ31 alloys and improves the corrosion resistance in contacting with other metals. Furthermore, the salt spray test based on JIS Z 2731 (308 K temperature and 5% concentration) [8] reveals that the corrosion starts after 1 h in the case of AZ31 alloys. The corrosion phenomenon of the conventional magnesium alloys starts in about 1 h in salt spray test.
Mg2Si bulky material, however, has no damage after 2,000 h endurance evaluation. It means that the corrosion resistance of Mg2Si is much better than the conventional stainless steels.
Materials Design for High-Performance Magnesium Alloys
It is possible to supply some characteristics, which are required as structural components, to magnesium alloys by using Mg2Si intermetallics as shown in the materials design of Figure 2. Figure 2 (a) means metal matrix composites with Mg2Si dispersoids which are named “MgSiX®”, and (b) is surface modifications by Mg2Si layers coated on magnesium alloys. The former is useful in improving the mechanical properties by Mg2Si dispersion strengthening effect, and the latter is for the improvement of corrosion and wear resistance. In this paper, the characteristics of MgSiX® are described in details by the combination of grains-refining process and solid-state synthesis of Mg2Si.
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Figure 1. Schematic illustration of materials design for high-performance magnesium alloys, (a) metal matrix composite (MgSiX®) and (b) Surface modification.
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Grains Refining and Dispersion Strengthening Effects
Grains Refining Effect by Repeated Plastic Working
It is well known that the grains refinement improves the strength of materials. In this study, the repeated plastic working (RPW) process illustrated in Figure 3 is employed to produce the magnesium green compact with refined grains [9].
After feeding the raw powder mixture into the die installed in the press machine, it is consolidated by upper punch I. Upper punch II is put in the compact, and the plastic deformation of the green part by backward extrusion takes place in the die. After that, the compaction by punch I continues again.
One cycle in RPW process, which consists of compaction and backward extrusion by inserting two upper punches alternatively, requires about 6~10 s. The punch speed is about 0.5~1 m/s, and the energy consumed to mix and refine the raw material is controlled by the speed. Figure 4 shows the relationship between the ultimate tensile strength (UTS) and the mean grain size of the hot extruded AZ31 alloys via RPW process with various numbers of cycles. The UTS remarkably increases according to the reduction of the grain size. It corresponds to Hall-Petch equation well. For example, when the number of cycles in RPW (N) is 200, the hot extruded AZ31 alloy shows the mean grain size of 4.3 μm and UTS of about 350 MPa, which is superior to the conventional one with 265 MPa UTS. Accordingly, RPW process is effective to refine magnesium grains for the improvement of the tensile properties.
Magnesium Composites with Mg2Si Dispersoids
Figure 5 illustrates a schematic to fabricate magnesium composites with Mg2Si dispersoids, in employing the elemental mixture of magnesium alloy powder and silicon particles [10].
Mg2Si intermetallic can be formed by the reaction of magnesium and silicon with an exothermic heat when pre-heating the green compact before hot extrusion. Silica (SiO2) is also supplied as another raw material to synthesize Mg2Si and MgO via the deoxidization by magnesium. These occur based on the following equations.
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2Mg + Si → Mg2Si
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(1)
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4Mg + SiO2 → Mg2Si + 2MgO
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(2)
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For example, when using AZ31 alloy powder with a mean particle size of 145 μm and SiO2 powder with that of 22 μm, UTS of the composites with 8 mass% SiO2 increases in proportion to the SiO2 content, and is 362 MPa. The mean particle size of synthesized Mg2Si /MgO is about 20 μm. It is the same as that of SiO2 raw materials because the coarsening of the formed particles did not occur so much during solid-state reaction. Accordingly, from a viewpoint of the dispersion strengthening effect, the particle size control of Si and SiO2 powder in the green compact is very important to produce the magnesium composites with fine Mg2Si or Mg2Si/MgO dispersoids.
Materials Design of MgSiX®
The microstructures of high-performance magnesium alloys (MgSiXⓇ) is illustrated in Figure 6 by the combination of the grains refinement by RPW process and the dispersion strengthening by fine Mg2Si particles. The uniform distribution of in-situ formed Mg2Si contributes to not only the improvement of mechanical properties but also the corrosion and wear resistance of the magnesium composite alloys.
Figure 7 shows the dependence of UTS on SiO2 content via RPW process (N=200) compared to the conventional cold compaction process (N=1), when employing the elemental powder mixture of AZ31-5 mass%Si. In both composites, UTS proportionally increases in increasing the SiO2 content due to the dispersion strengthening effect of fine Mg2Si/MgO particles. The AZ31 composite via RPW, however, indicates higher tensile strength because of the matrix grains refinement by RPW. As shown in optical microstructures of Figure 7 (b), the RPWed composite alloy shows that finer Mg2Si and MgO particles distributed more uniformly in the matrix are effective on the improvement of UTS.
Concerning corrosion resistance of the composite, Figure 8 reveals the electrochemical polarization curve of AZ31 composite alloy with Mg2Si/MgO particles, when employing the elemental powder mixture of AZ31-2 mass%SiO2 as raw materials. The current density is obviously reduced to 1/2~1/3 by including Mg2Si, compared to the conventional AZ31 alloy. That is, the corrosion resistance of magnesium alloys can be improved by dispersing in-situ formed Mg2Si particles in the matrix. Furthermore, the tribological properties are also remarkably improved [11]. In particular, the friction coefficient under oil lubricant conditions is reduced by including MgO particles because of their “mild-offensive” properties to obstruct the abrasive phenomena on the counter materials [12]. The addition of graphite particles is also effective in reducing stabilizing the friction coefficient due to their self-lubrication.
Recycling of Wasted SiO2 Glass to Produce MgSiX®
Considering that the main ingredient of the glass products is SiO2, the glass scraps, in particular with a high purity, have a large possibility to be employed as input raw materials instead of silicon to form Mg2Si via the deoxidization and reaction process [13]. Figure 9 shows an appearance of small blocks of in-house scraps of the wrought optical SiO2 glass fiber with a purity of 99.99%. They are ground into fine powder, having a mean particle size of 16.8 μm. After mixing the SiO2 glass particles with AZ31 powder, the elemental powder mixture is consolidated by cold pressing under the applied pressure of 600 MPa.
SiO2 glass particle content of the powder mixture is 0, 2, 4, 6, and 8 mass%. After heating each green compact at 723 K for 300 s in nitrogen gas atmosphere to synthesize Mg2Si/MgO, it is immediately consolidated into a full density by hot extrusion with an extruding ratio of 37. Figure 10 shows XRD patterns of the green compacts after heating at 723 K for 300 s. The peaks of Mg2Si and MgO are definitely detected, that is, SiO2 glass particles are reacted with magnesium to form them. Furthermore, the Differential Scanning Calorimeter (DSC) curves of the green compacts heated at 723 K show no exothermic peak, which corresponds to the reaction between magnesium and SiO2, when heating up to 910 K. It means that no glass particle exists in the green compact after heating at 723 K, and completely reacts with magnesium alloy powder during pre-heating.
Figure 11 indicates the dependence of UTS, Yield Stress and elongation of hot extruded AZ31 composite alloys with Mg2Si/MgO dispersoids on the glass content. It shows the gradual increase of tensile strength with increase in the SiO2 content. This is because of the dispersion strengthening effect, when Mg2Si/MgO fine particles are uniformly distributed in the matrix. The yield stress also increases according to the increase of SiO2 content because of the extremely high Young’s modulus of the dispersoids compared to the magnesium matrix. Elongation decreases with increase in the SiO2 content, because in-situ formed Mg2Si and MgO dispersoids are brittle compared to the AZ31 matrix. Accordingly, wasted SiO2 glasses with a high purity are able to employ as raw materials to form Mg2Si via the reaction and synthesis process suggested in this study.
Practical Feasibility to Mass-Production Process
For the establishment of the scale-up oriented engineering on MgSiX® and its new process, it is necessary to evaluate the properties of the magnesium composite with Mg2Si dispersoids produced via mass-production process by using a large scale manufacturing equipment. As shown in Figure 12 (a), the green compacts, having a diameter of 140 mm and weight of about 3 kg, are produced on the route of RPW process by using 5000 kN screw-driven type press machine (ENOMOTO Machine 500ZES).
Figure 12 (b) shows an appearance of half-products of pipe by using the large scale manufacturing equipment. The MgSiX® rods also have superior mechanical properties, e.g.; UTS of 378 MPa and an elongation of 11%. From a viewpoint of the improvement of the productivity, the billet size and the number of cycles in RPW process must be optimized by using the manufacturing equipments installed in the industries.
Conclusions
The magnesium alloy with refined grains and fine magnesium silicides (Mg2Si) dispersoids (MgSiX®) was developed via repeated plastic working process in solid-state, and indicated a high strength. For the synthesis of Mg2Si, in-house wastes or scraps of SiO2 glasses with a high purity could be employed as input raw materials instead of silicon particles.
Acknowledgement
This study was financially supported by the project “Development of Environmentally Benign Process of High-Performance Magnesium Alloys” from Kanagawa Academy Science and Technology (KAST) and the matching foundation project to create university based businesses from the METI.
References
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