TWI427158B - Magnesium alloy and method for making the same - Google Patents

Description

Magnesium alloy and preparation method thereof

The invention relates to a magnesium alloy, in particular to a magnesium alloy for high strength casting and a preparation method thereof.

Magnesium alloy is the lightest structural metal in industrial applications. The density of pure magnesium is 1.738g/cm3, which is 2/3 of aluminum density and 1/4 of steel. Magnesium has a large reserve in the earth's crust, accounting for 2.7%, second only to aluminum and iron. Compared with other metal materials, magnesium alloys have high specific strength, specific stiffness, strong electromagnetic shielding and radiation resistance, easy cutting, easy recycling, etc., and are extremely valuable in the automotive, electronics, aerospace and defense industries. Important application value and broad application prospects.

Due to the close-packed hexagonal crystal structure of magnesium, it has only a single slip system at room temperature, which makes the magnesium alloy have poor plasticity at normal temperature, and the deformation process is difficult. Therefore, most of the magnesium alloys are produced by casting. Die-cast magnesium alloys such as AZ91D and AM60B are commonly used in the industry for excellent room temperature strength and good casting properties, and are low in cost. However, the strength and toughness of these magnesium alloys are difficult to further increase, which greatly limits the application range of the magnesium alloys.

In view of this, it is necessary to provide a magnesium alloy having high strength and toughness and a method of preparing the same.

a magnesium alloy containing 10.8% to 11.8% aluminum by weight; 0.1.54% to 1.93% zinc; 0.19% to 0.24% manganese; 0.9% to 0.94% rare earth metal, and the balance being magnesium and Inevitable impurities.

The method for preparing the magnesium alloy comprises the steps of: melting a magnesium alloy raw material to obtain the above magnesium alloy; casting the magnesium alloy; heating the magnesium alloy to 330 degrees Celsius to 420 degrees Celsius, and heating the time period from 30 minutes to 180 minutes; The temperature was kept for 0 to 60 minutes; the magnesium alloy was cooled to room temperature.

The higher content of aluminum and zinc in the magnesium alloy of the invention is the main strengthening element, and the manganese can improve the corrosion resistance of the magnesium alloy, and the introduction of the rare earth metal is beneficial to improve the casting performance of the magnesium alloy and on the other hand to improve the magnesium. The thermal stability of the alloy grain interface hinders the increase of the size of the grains during the heat treatment. The mechanical properties test results show that the tensile strength and elongation of the magnesium alloy are much higher than those of the existing magnesium alloy, and have higher strength and toughness.

Figure 1 is a metallographic diagram of a magnesium alloy of the AZ91D.

2 is a metallographic diagram of a magnesium alloy prepared in accordance with an embodiment of the present invention.

The magnesium alloy of the present invention and its preparation method will be further described in detail below with reference to the accompanying drawings.

The invention provides a magnesium alloy containing 8.7% to 11.8% aluminum (Al); 0.63% to 1.93% zinc (Zn); 0.1% to 0.5% manganese (Mn); 0.51% to 1.5 by weight. % of rare earth metal (RE), the remainder being magnesium (Mg) and unavoidable impurities. Wherein RE is preferably one of Ce, La, Pr, Nd, Y or a combination thereof.

One of the main strengthening elements of Al-based magnesium alloys. When the alloy solution is slowly cooled to room temperature, Al and Mg undergo eutectic reaction to form α-Mg solid solution strengthening phase and β-Mg ₁₇ Al ₁₂ precipitation strengthening phase, which can improve the room temperature strength and hardness of the magnesium alloy. In addition, the addition of Al can also improve the casting properties of the magnesium alloy. The experiment proves that if the content of Al is less than 8.7%, the magnesium alloy cannot show good fluidity and castability; and when the content of Al is higher than 11.8%, the brittleness of the magnesium alloy is improved. A more preferred range of Al is between 8.8% and 10.8%.

Zn is also a strengthening element in magnesium alloys. Zn is mainly present in a solid solution state in the α-Mg phase and the β-Mg ₁₇ Al ₁₂ phase in the Mg-Al alloy, which can improve the room temperature strength of the magnesium alloy and improve the plasticity of the magnesium alloy. Another effect of Zn is to positively shift the corrosion potential of the magnesium alloy, thereby reducing the corrosion rate of the magnesium alloy. The experiment proves that the content of Zn is generally controlled below 1.93%. When the content exceeds 1.93%, the magnesium alloy is prone to cracking during quenching.

Although Mn has little effect on the strength of the magnesium alloy, it can increase the elongation of the magnesium alloy. An important function of adding Mn during the smelting process is to separate some of the harmful metals. For example, when smelting, Fe combines with Mn to form a precipitate into the slag. In addition, Mn can also react with Fe in the alloy to precipitate a compound phase ((Fe,Mn)Al ₃ ) which has little influence on corrosion resistance, and effectively improve the corrosion resistance of the magnesium alloy. In addition, Mn can also serve to improve the weldability of magnesium alloys. As the Mn content increases, a brittle d-Mn phase will appear in the structure, which reduces the ductility of the magnesium alloy, so the content of Mn is generally controlled to be less than 0.5%.

The RE added in the magnesium alloy mainly has the effect of refining the crystal grains, and transforms the β-Mg ₁₇ Al ₁₂ phase which is originally distributed along the grain boundary network into a short rod shape or granular shape which is intermittently and dispersedly distributed, thereby improving magnesium. The strength and elongation of the alloy. The experiment proves that when the RE content is less than 0.51%, the grain refining effect is not obvious; when the RE content exceeds 1.5%, the further refinement of the crystal grains is not obvious, but the excessive Al ₄ RE phase will appear, so that the magnesium alloy The castability is lowered and the defects of the molded casting are increased. A more preferred range of RE is between 0.51% and 1.23%.

The addition of RE in the magnesium alloy also has the effect of purifying the alloy. In the smelting process, impurities mainly composed of MgO are often formed in the magnesium alloy. Since the binding force of RE and oxygen is greater than the binding force of Mg and oxygen, RE is added to the magnesium alloy liquid to form rare earth oxide with oxygen, thereby functioning to remove impurities. In addition, due to the reaction between Mg and water vapor, the magnesium alloy has a strong hydrogen absorption tendency, and the hydrogen dissolved in the magnesium alloy melt is caused by casting defects such as pores, pinholes and shrinkage. When RE is added to the magnesium alloy melt, RE can react with hydrogen in the water and magnesium to form a rare earth hydride, thereby achieving the purpose of hydrogen removal.

The invention also provides a method for preparing a magnesium alloy, comprising the steps of:

Step 1. Melting the magnesium alloy raw material to obtain a magnesium alloy having a characteristic of 8.7% to 11.8% by weight of Al; 0.63% to 1.93% of Zn; 0.1% to 0.5% of Mn; 0.51%~ 1.5% of RE, the rest is Mg and unavoidable impurities.

In step 2, the magnesium alloy is cast molded. The casting method may be die casting, casting, and thixmolding, and the casting method is preferably a thixoforming type.

In step 3, the magnesium alloy is heated to 330 degrees Celsius to 420 degrees Celsius, and the temperature rise time is 30 minutes to 180 minutes. When the heating temperature is lower than 330 degrees Celsius, the solid solution strengthening effect is not so obvious, and when the heating temperature is higher than 420 degrees Celsius, since the temperature is already close to the melting point of the second phase, the solid solution strengthening effect starts to weaken. More preferably, the heating temperature is between 350 and 400 degrees Celsius. The extension of the heating time is beneficial to release the residual stress of the cast-formed magnesium alloy, but the heating time is too long, which leads to a decrease in production efficiency. More preferably, the temperature rise time is from 60 minutes to 120 minutes.

Step 4, keep warm for 0 to 60 minutes. The prolongation of the holding time is beneficial to improve the solid solution effect, but the long time will cause the grain to grow and the strength of the magnesium alloy will decrease. More preferably, the incubation time is from 0 to 30 minutes.

In step 5, the magnesium alloy is cooled to room temperature. The cooling method can be water cooling or air cooling.

Please refer to FIG. 1 and FIG. 2 together, and the metallographic diagram of the magnesium alloy of the grade AZ91D and the metallographic diagram of the magnesium alloy prepared by the embodiment of the invention are shown. It can be seen from the figure that after heat treatment, the β-Mg ₁₇ Al ₁₂ phase originally formed on the grain boundary due to non-equilibrium solidification is gradually decomposed due to decomposition and dissolution into the α-Mg matrix, and the original mesh is formed. The distribution of β-Mg ₁₇ Al ₁₂ phase is transformed into a short rod or granular shape with intermittent and diffuse distribution. The surface-active RE will preferentially precipitate a high melting point, high heat stable Al ₄ RE phase during the solidification of the alloy. These preferentially precipitated Al ₄ RE phases are concentrated around the grain boundaries, hindering the formation of secondary phases and refining the crystal grains. Due to the existence of the heat stable phase and its pinning effect, even if the magnesium alloy is kept at the solution temperature, the grain size will not increase significantly. In addition, the α-Mg matrix can form a solid solution with elements such as Ce, Nd and La in RE, and the magnesium-rich region is a simple eutectic with a low melting point, and forms a network at the grain boundary to suppress the formation of shrinkage cavities.

The invention is further illustrated by the following examples.

The magnesium alloy of the present invention is obtained by melting in a 100 kg stainless steel crucible resistance furnace. A mixed gas of N ₂ + 0.3% SF ₆ was used as a protective atmosphere. The raw materials used are as follows: Mg: primary magnesium, magnesium content 99.8%

Mn: When the melting temperature is 710 ° C ~ 730 ° C, Al-15% Mn master alloy is added to the solution, stirred vigorously for 20 ~ 30min until completely melted.

Al: pure aluminum, aluminum content of 99.7%, added at a melting temperature of 650 ° C ~ 680 ° C, Stir for 3 to 5 minutes.

Zn: pure zinc, zinc content 99.995%, added at a melting temperature of 650 ° C ~ 680 ° C, stirred for 3 to 5 minutes.

RE: rich rare earth, added at a melting temperature of 690 ° C ~ 710 ° C, stirred for 10 to 15 minutes.

13 examples of the invention and 1 comparative example were prepared according to the formulations listed in Table 1. After the preparation is completed, the casting temperature is maintained at 660 ° C ~ 670 ° C when cast into a 7 kg rectangular ingot, which is protected by blowing protection gas during the casting process. The surface of the ingot obtained in all tests was free from burning and oxidation. The ingots of different compositions were sampled and analyzed by ICP-AES.

The obtained ingot was cut into a magnesium pellet used for the thixotropic type by using a pelletizing machine. Each ingot was processed into a standard tensile test bar using a JLM280MGIIe type thixoforming device.

Example 2, Example 6, Example 8 and Example 10 were heat-treated according to the preparation method of the present invention, and the remaining examples and comparative examples were not subjected to any treatment.

In order to further verify that the magnesium alloy of the present invention has high strength and toughness, the tensile properties of the above 13 examples and comparative examples were tested in accordance with ASTM-B557-02. The test data obtained are shown in Table 2.

Table 1 Chemical examples of each example and comparative examples (% by weight, remaining Mg and unavoidable impurities)

Table 2 Results of each example and comparative performance test

Conclusion: Compared with the existing magnesium alloy, the magnesium alloy of the embodiment of the invention has a certain improvement in tensile strength and elongation, and the mechanical properties of the magnesium alloy prepared according to the preparation method of the invention are more obviously improved. Higher strength and toughness. In addition, compared with the conventional T4 (solution treatment), T6 (solution treatment + artificial aging) treatment methods, the preparation method of the invention has the advantages of less time consuming, higher production efficiency and the like.

In summary, the present invention has indeed met the requirements of the invention patent, and has filed a patent application according to law. However, the above description is only a preferred embodiment of the present invention, and it is not possible to limit the scope of the patent application of the present invention. Equivalent modifications or variations made by persons skilled in the art in light of the spirit of the invention are intended to be included within the scope of the following claims.

Claims (6)

A magnesium alloy containing 10.8% to 11.8% aluminum by weight; 1.54% to 1.93% zinc; 0.19% to 0.24% manganese; 0.9% to 0.94% rare earth metal, the balance being magnesium and inevitable Impurities. A method for preparing a magnesium alloy, comprising the steps of: (1) melting a magnesium alloy raw material to obtain a magnesium alloy having a characteristic of 10.8% to 11.8% by weight of aluminum; 1.54% to 1.93% of zinc; 0.19 %~0.24% manganese; 0.9%~0.94% rare earth metal, the rest is magnesium and unavoidable impurities; (2) casting the magnesium alloy; (3) heating the magnesium alloy to 330 degrees Celsius to 420 degrees Celsius, The heating time is from 30 minutes to 180 minutes; (4) the holding time is from 0 to 60 minutes; (5) the magnesium alloy is cooled to room temperature. The method for preparing a magnesium alloy according to claim 2, wherein the heating temperature of the magnesium alloy in the step (3) is from 350 degrees Celsius to 400 degrees Celsius. The method for preparing a magnesium alloy according to claim 2, wherein the magnesium alloy has a heating time of 60 minutes to 120 minutes in the step (3). The method for preparing a magnesium alloy according to claim 2, wherein the magnesium alloy has a holding time of 0 to 30 minutes in the step (4). The method for preparing a magnesium alloy according to claim 2, wherein the casting method of the magnesium alloy in the step (2) is a thixoforming type.