Research progress and prospects of grain refinement in cast magnesium-rare earth alloys

1.Background and Significance

Cast magnesium rare earth alloys have the characteristics of low density, high specific strength and specific stiffness, and good creep resistance. They are often used to form important aerospace components such as engine casings and carrier cabins, which can reduce aircraft weight, improve load and maneuverability, and have broad application prospects in the fields of aerospace, defense, and military. However, compared with deformed alloys, the structure of cast magnesium rare earth alloys is relatively coarse, resulting in insufficient strength and toughness, and easy to produce casting defects such as hot cracking and shrinkage, which seriously restricts the development and application of high-performance cast magnesium rare earth alloys. Grain refinement treatment can improve the casting process performance and mechanical properties of magnesium rare earth alloys at the same time, and is a key link in determining the preparation quality and service performance of cast magnesium rare earth alloys. However, the casting of magnesium rare earth alloys involves many process links, and different process links have an important influence on grain refinement.

Recently, Professor Wu, Assistant Researcher Tong, and Associate Professor Wang from Shanghai Jiao Tong University took the whole casting process of magnesium rare earth alloy as the main line and systematically summarized the effects of alloying, melt refinement, and purification treatment, external energy field, and different casting processes on the grain refinement of magnesium rare earth alloy (as shown in Figure 1). They discussed the control methods of grain size of magnesium rare earth alloy from different angles and focused on the latest progress in the refinement mechanism of different refiners, the interaction between melt purification and refinement, and the composite treatment of external energy field and refiners. They pointed out the current problems and looked forward to the development trend of grain refinement treatment of magnesium rare earth alloy, providing new ideas for the efficient refinement treatment of magnesium alloy in the future.

Figure 1: Effects of different processes on grain refinement during magnesium rare earth alloy casting

2.Graphic tour

First, the effects of rare earth solute elements and heterogeneous nucleation particles on the grain refinement of cast magnesium rare earth alloys are reviewed. Common rare earth alloying elements in magnesium alloys include Gd, Y, Nd, La, Ce, Sm, Yb, etc, which will segregate at the front of the solid-liquid interface to form a supercooled composition, promote the formation of new nuclei, and inhibit the continued growth of the original nuclei. However, it is difficult to achieve efficient grain refinement of magnesium alloys by supercooling the composition of rare earth elements alone. Therefore, magnesium rare earth alloys are often treated with Zr for grain refinement and are often added in the form of Mg-Zr master alloys. Studies have found that the existence form of Zr in Mg-Zr master alloys is the key to its grain refinement effect. Mg-Zr master alloys with fine and uniform Zr particles have higher grain refinement efficiency. As shown in Table 1, pretreatments, such as extrusion, rolling, stir friction processing, and equal channel extrusion, can optimize the existence form of Zr in Mg-Zr master alloys, break up Zr particle agglomeration, and refine Zr particle size. The latest research also shows that nano-scale Zr particles can be prepared in Mg-Zr master alloys through high-frequency pulse remelting pretreatment, which promotes the dissolution of Zr and increases the number of effective Zr nuclei in the melt, and improves the grain refinement efficiency by more than 50%, as shown in Figure 2. Using Zr-containing molten salt instead of Mg-Zr master alloy can in-situ generate 0.5~3μm Zr particles with a good grain refinement effect. However, some molten salt inclusions are difficult to effectively separate from the melt, reducing the purity of the melt.

Table 1: Commonly used pretreatment modification methods for Mg-Zr master alloys

Fig.2: Microstructure and refinement effect of Mg-Zr master alloy after high-frequency pulse remelting pretreatment

In addition to Zr, Al2RE (including Al2Gd, Al2Y, Al2Sm, etc.) is also often used as heterogeneous nucleation particles in magnesium rare earth alloys. Al2RE particles are mainly formed by the in-situ reaction of Al and rare earth elements, which is different from the Zr alloying method. Due to its the high melting point, Al-RE has a pinning effect on the grain boundary and the microstructure stability of Mg-RE-Al alloy is higher than that of Mg-RE-Zr alloy. However, the formation of Al2RE will consume a certain amount of rare earth elements, so excessive addition of Al will lead to a decrease in the yield strength of the alloy and a weakened aging hardening response, which has been verified in alloys such as Mg-Gd-Y and Mg-Sm. In addition, Al2RE is formed during the solidification of the alloy, and its size and distribution characteristics are determined by the Al/RE concentration ratio and cooling rate. Under casting process conditions with a slow cooling rate (such as sand casting), the grain refining effect of Al2RE will be reduced. In summary, to obtain the best grain refining effect, the chemical composition and actual solidification conditions of the magnesium rare earth alloy must be considered at the same time.

Secondly, the physical refinement methods of magnesium-rare earth alloys are reviewed, including pulse, pulse magnetic field, and ultrasonic treatment. These external physical fields can induce strong convection in the melt, and the cavitation effect produced will break the dendrite arm and increase the nucleation rate; the external energy fields are also conducive to satisfying the nucleation work required for nucleation and increasing the number of embryos in the melt. In particular, when an external physical field is combined with Zr or Al2RE fining treatment, even better grain refinement effects can be achieved. For example, after ultrasonic treatment is applied in the Mg-5Sm-Al alloy, the cavitation effect and acoustic streaming effect of the ultrasonic field effectively refine the in-situ generated Al2Sm particles and improve their distribution homogeneity. The heterogeneous nucleation potency of the Al2Sm particles is enhanced, and the grain refinement effect is more remarkable. The study also found that the temperature range of the physical field also has an important influence on its grain refinement effect. For example, B.Nagasivamuni et al. found that when ultrasonic treatment is applied above the wire temperature of Mg-Zr alloy, the precipitation of Zr particles can be effectively adsorbed and the dissolution of Zr in magnesium liquid can be accelerated; if the ultrasonic treatment continues to act on the alloy solution, the precipitation and adsorption of Zr particles will be further reduced. Thus, the use of Zr/Al2RE and physical field composite treatment is expected to achieve grain refinement of magnesium rare earth alloys, which will be one of the important directions for the development of high-performance cast magnesium rare earth alloys.

After refining, magnesium rare earth alloys often need to be purified. Since both refining and purification involve complex high-temperature metallurgical reaction processes, there is a certain interaction between the two, and the result of the interaction will directly affect the final preparation quality of the melt. The interaction between magnesium rare earth alloy refinement and purification is mainly reflected in three aspects, including: (1) certain inclusions or impurity elements may bring a certain refinement effect; (2) certain refinement media may bring a certain purification effect; (3) certain refinement media and purification media will interact with each other. For example, the common oxide inclusions MgO and impurity elements Fe in magnesium alloy melts have a certain effect on the grain refinement of magnesium alloys mainly because MgO particles and some Fe-containing phases can serve as heterogeneous nucleation sites for α-Mg; and the refinement elements or particles such as Zr and RE in the melt will react with impurity elements such as Fe and Ni in the melt and combine to form insoluble compounds, thereby improving the purity of the melt. The interaction between the refining medium and the purification medium is mainly manifested in two points: first, during the standing process after melt refining, high-density components (such as Zr particles, RE elements) will agglomerate with the refining agent-inclusions and settle at the same time. As the standing time increases, the melt purification effect is improved, but the refinement effect begins to decline; second, during the refining process, the flux will adsorb or react with RE and Zr elements, causing the loss of RE and Zr elements and reducing the grain refinement effect, as shown in Figure 3. Hence, there is a very complex interaction between the refinement and purification treatment of magnesium rare earth alloy melts. Understanding and making good use of the above interaction will help improve the preparation quality of existing magnesium rare earth alloy melts.

Fig.3: Structure and sedimentation behavior of agglomerates formed by refining medium and purification medium in magnesium rare earth alloy melt

Finally, various casting methods of magnesium rare earth alloys are reviewed, including high-pressure casting, squeeze casting, semi-continuous casting, twin roll casting, semi-solid forming, etc., and the effects of different casting methods on the grain refinement of magnesium rare earth alloys are discussed. The essence of different casting methods is different solidification conditions, including different temperature fields, solute fields, flow fields, etc. These factors have an important influence on the solidification behavior of magnesium rare earth alloys, resulting in different grain refinement effects. For example, the wall thickness of die castings is small and the cooling rate is extremely fast. The grain size of die-cast magnesium rare earth alloys is generally the smallest, only 3~10μm, but due to the pre-crystallization effect in the barrel, die-cast magnesium rare earth alloys usually present a bimodal grain structure; in the squeeze casting process, the melt flows in a stable laminar flow, and the internal quality of the casting is good. Therefore, squeeze-casting magnesium alloys can be heat treated to give full play to the advantages of strong aging hardening of magnesium rare earth alloys; the semi-continuous casting ingot size is large, and the cooling rate of the surface and core of the magnesium rare earth alloy ingot is quite different, resulting in uneven grain size distribution. Electromagnetic-assisted casting at the crystallizer can effectively improve the uniformity of the microstructure of semi-continuously cast magnesium rare earth alloys; twin-roll casting provides a short-process magnesium rare earth alloy sheet forming method. Under the coupling of rapid cooling and deformation force, the grain size of magnesium rare earth alloys is small; the semi-solid formed magnesium rare earth alloy contains primary grains and secondary primary grains, so the semi-solid magnesium rare earth alloy also presents a typical bimodal grain structure. It can be seen that both composition and casting process conditions have an important influence on the grain size of magnesium rare earth alloys.

3.Conclusion and Outlook

Grain refinement is the key to determining the quality and performance of cast magnesium rare earth alloys. Zr alloying through Mg-Zr master alloys is still one of the simplest and most effective grain refinement treatment methods in the actual production process of magnesium rare earth alloys, but the Zr alloying temperature is high, the Zr yield is low, and the refinement effect needs to be further improved. Gaochuang Rare Earth directly alloys Mg and Zr to develop a new type of Mg-Zr master alloy. Compared with traditional Mg-Zr master alloys, the Zr content is evenly distributed and the grain size is small, which greatly improves the Zr yield and provides a good solution for the grain refinement of rare earth magnesium alloys.