High-rare-earth VW91M magnesium alloy large-size cast ingot and preparation method thereof

Abstract

The invention is suitable for the technical field of magnesium alloy cast ingots, and provides a high-rare-earth VW91M magnesium alloy large-size cast ingot and a preparation method thereof.The method comprises the steps of precise burdening, crucible preheating, smelting agent adding, melting temperature control, gradual intermediate alloy adding, melt protection, filtering impurity removal, heat preservation refining and component fine adjustment; and finally, semi-continuous casting is conducted through a crystallizer, parameters such as the casting temperature, the temperature of the liquid level of the crystallizer, the casting speed and the flow speed of cooling water are strictly adjusted and controlled, and the flat cast ingot with the size of 450 mm * 900 mm * 3500 mm is successfully prepared. According to the characteristics of VW91M magnesium alloy, the efficient and stable preparation method suitable for the ingot with the section size of 450 * 900 mm is developed, the problem that in the prior art, a high-rare-earth magnesium alloy flat ingot is prone to crack defects is solved, and ingot cracking is effectively prevented. The method has important theoretical significance and can promote wide application of the high rare earth magnesium alloy in various fields.

Description

Technical Field

[0001] The invention belongs to the technical field of magnesium alloy ingots, and in particular relates to a high-rare earth VW91M magnesium alloy large-size ingot and a preparation method thereof. Background Art

[0002] Magnesium alloys are widely used in aerospace, automobile, electronics and other fields due to their excellent mechanical properties, low density and good corrosion resistance. In recent years, with the growing demand for lightweight materials, magnesium alloys, especially high rare earth magnesium alloys, have become a new focus of research and application. As a new type of material, high rare earth magnesium alloys have become a hot topic of research due to their excellent strength, toughness and high temperature resistance. VW91M magnesium alloy is a magnesium alloy with added high rare earth elements Gd and Y. It has good mechanical properties in high temperature environments and is suitable for industrial applications in complex service environments.

[0003] However, when producing large-sized ingots, traditional melting and casting processes often face problems such as internal defects, uneven organization, and component segregation in the ingots, which seriously affect the performance and application effect of the alloy. In addition, compared with round ingots, flat ingots have complex flow states during the casting process due to their uneven thickness and uneven solidification rate, which makes it easier to produce problems such as uneven cooling and excessive temperature gradients during the solidification process, leading to stress concentration and cracks inside the ingot. According to the current literature search results, there is no record of successfully preparing a high-rare earth magnesium alloy flat ingot with a cross-sectional size of 450×900mm. In order to further broaden the application scenarios of high-rare earth VW91M magnesium alloy, based on the characteristics of VW91M magnesium alloy, the present invention proposes a large-size ingot of high-rare earth VW91M magnesium alloy and a preparation method thereof. Summary of the invention

[0004] The object of the present invention is to provide a high rare earth VW91M magnesium alloy large-size ingot and a preparation method thereof, aiming to solve the problems raised in the above-mentioned background technology.

[0005] The purpose of the present invention is achieved through the following technical solutions:

[0006] A method for preparing a large-size ingot of a high-rare earth VW91M magnesium alloy, wherein the composition and mass percentage of the high-rare earth VW91M magnesium alloy are as follows:

[0007] Gd: 8.7-10.2wt%;

[0008] Y: 0.02-2wt%;

[0009] Zr: 0.3-0.6wt%;

[0010] The balance is magnesium and unavoidable impurities;

[0011] The preparation method comprises the following steps:

[0012] Step 1: Ingredients;

[0013] The amount of ingredients is calculated according to the chemical composition requirements of the high rare earth VW91M magnesium alloy, using a remelting magnesium ingot with a purity of more than 99.95%, and alloying elements Gd, Y, and Zr are added in the form of Mg-30Gd, Mg-20Y, and Mg-30Zr master alloys;

[0014] Step 2: Addition of smelting agent;

[0015] Preheat the crucible to dark red, and evenly sprinkle a layer of melting agent on the crucible wall and bottom; preheat the charge to above 150℃, then add the magnesium ingot at a time, and sprinkle a layer of melting agent on the charge;

[0016] Step 3: Melting temperature control;

[0017] After the magnesium ingot is added once, the temperature is raised and smelting begins. As the temperature rises, Mg-30Gd, Mg-20Y and Mg-30Zr master alloys are added;

[0018] Step 4: Melt protection and filtration to remove impurities;

[0019] During the smelting and pouring process, the air is isolated by introducing mixed gas;

[0020] Step 5: heat preservation and refining;

[0021] Select the refining agent, set the refining temperature and time, and after refining, analyze the melt again before furnace and fine-tune the composition, then heat up and let it stand, and cool down before casting;

[0022] Step 6: Casting;

[0023] Semi-continuous casting is carried out using a crystallizer, and a flat ingot with a size of 450×900×3500 mm is obtained by controlling the casting temperature, the crystallizer liquid surface temperature, the casting speed and the cooling water flow rate.

[0024] Furthermore, the specific process of step 2 is: preheating the crucible to a dark red color at a temperature of 400-500°C, and evenly sprinkling a layer of RJ-2 flux on the crucible wall and bottom; preheating the charge to above 150°C, then adding the magnesium ingot at one time, and sprinkling a layer of RJ-2 flux on the charge, the total amount of flux accounting for 1-2% of the weight of the charge.

[0025] Furthermore, in step 2, during the charging and smelting process, if the solution in the crucible is exposed and burns, RJ-2 flux needs to be added immediately.

[0026] Furthermore, the specific process of step 3 is as follows: after the magnesium ingot is added once, the temperature is raised for smelting, and when the temperature reaches 700-720°C, Mg-30Gd and Mg-20Y intermediate alloys are added, and then the temperature is continued to be raised to 835-855°C, and then Mg-30Zr intermediate alloy is added.

[0027] Furthermore, in step 4, the mixed gas is SF6+N2 mixed gas.

[0028] Furthermore, the specific process of step 5 is: RJ-2 flux and magnesium fluoride are selected as refining agents, the refining temperature is 775-785°C, the refining time is 20-30 minutes, after refining, the melt is analyzed before furnace again and the composition is fine-tuned, then the temperature is raised to 800°C and allowed to stand for 30 minutes, and finally the temperature is lowered to 750-770°C for casting.

[0029] Furthermore, in step 6, the crystallizer adopts a three-stage cooling method: the first-stage cooling is internal circulation cooling of the crystallizer; the second-stage cooling is flushing cooling of the bottom of the crystallizer; the third-stage cooling is external flushing cooling of the lower part of the crystallizer; in the semi-continuous casting process, the lowest point of the molten liquid hole of the solid-liquid solidification line in the crystallizer is located between the second and third-stage cooling.

[0030] Furthermore, in step 6, the parameters of semi-continuous casting include: melt temperature before casting is 760-770°C, crystallizer liquid surface temperature is 660-670°C, casting speed is 10-25 mm / min, cooling water temperature is ≤20°C, primary cooling water flow rate is 3-5 m / min, 3 / min, the secondary cooling water flow rate is 0.5~1m 3 / min, the third-stage cooling water flow rate is 1~2m 3 / min.

[0031] A large-size ingot of high rare earth VW91M magnesium alloy produced according to the above-mentioned production method.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] In view of the characteristics of VW91M magnesium alloy, the present invention has developed an efficient and stable preparation method suitable for ingots with a cross-sectional size of 450×900mm. The method aims to solve the problem that high rare earth magnesium alloy flat ingots are prone to crack defects in the existing process and effectively prevent the ingot from cracking. This method not only has important theoretical significance, but also can promote the wide application of high rare earth magnesium alloys in various fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1Vertical cross-sectional views of Mg-xGd-(1wt%, 2wt%, 3wt%, 4wt%)Y alloys; wherein (a) is a vertical cross-sectional view of a Mg-xGd-1Y alloy, (b) is a vertical cross-sectional view of a Mg-xGd-2Y alloy, (c) is a vertical cross-sectional view of a Mg-xGd-3Y alloy, and (d) is a vertical cross-sectional view of a Mg-xGd-4Y alloy.

[0035] Figure 2 It is a vertical cross-sectional view of the Mg-rich region of the Mg-Mg-xGd-1Y alloy.

[0036] Figure 3 It is the actual object and schematic diagram of the 450×900mm crystallizer; among them: a represents the primary cooling water, b represents the secondary cooling water, c represents the tertiary cooling water, d represents the schematic cross-sectional structure of the 450×900mm flat ingot crystallizer, e represents the flat ingot, and f represents the solid-liquid solidification line in the crystallizer.

[0037] Figure 4 This is a physical picture of the VW91M flat ingot with dimensions of 450×900×3500 mm prepared in Example 2.

[0038] Figure 5 This is a cracking diagram of a high rare earth magnesium alloy flat ingot with a size of 450×900 mm prepared in Comparative Example 1. DETAILED DESCRIPTION

[0039] In order to have a clearer understanding of the technical features, purposes and beneficial effects of the present invention, the technical solution of the present invention is now described in detail below, but it should not be construed as limiting the applicable scope of the present invention.

[0040] The specific implementation of the present invention is described in detail below in conjunction with specific embodiments.

[0041] An embodiment of the present invention provides a high rare earth VW91M magnesium alloy, the composition and mass percentage of the high rare earth VW91M magnesium alloy are as follows:

[0042] Gd: 8.7-10.2wt%;

[0043] Y: 0.02-2wt%;

[0044] Zr: 0.3-0.6wt%;

[0045] The balance is magnesium and unavoidable impurities.

[0046] In the embodiment of the present invention, the vertical cross-sectional view of the Mg-(0-40wt%)Gd-(1wt%, 2wt%, 3wt%, 4wt%)Y alloy calculated by the phase equilibrium thermodynamic calculation software is shown in FIG. Figure 1As shown in (a) to (d). The coordination effect of Gd and Y was analyzed and it was found that: with the increase of Y content, the maximum solid solubility of Gd in α-Mg gradually decreased from 19.38wt% to 17.04wt%; the area of ​​α-Mg+Mg5Gd two-phase region gradually decreased, while the area of ​​α-Mg+Mg24Y5 two-phase region and α-Mg+Mg24Y5+Mg5Gd region gradually increased. The aging temperature range of the alloy gradually increased, the eutectic point composition shifted to the left, and the eutectic temperature decreased. The crystallization temperature interval of the alloy increased. The increase in Y content is beneficial to improve the aging hardening effect of the alloy, but too much Y content increases the crystallization temperature interval, reduces the fluidity of the alloy, and affects the casting performance of the alloy. Therefore, during the ingot preparation process, the Y content needs to be strictly controlled to reduce the risk of cracking of the flat ingot. Therefore, as a preferred option, the Y element is selected to be ≤2wt%.

[0047] According to the calculation results of the above phase diagram, Mg-xGd-1Y with a Y content of 1wt% was selected to study the effect of Gd content on the solidification process of the alloy. Figure 2 It is a vertical cross-section of the Mg-rich area of ​​the Mg-Mg-xGd-1Y alloy, where the Gd content varies from 0 to 40%. It can be seen that the vertical cross-section of the Mg-Mg-xGd-1Y alloy includes: two single-phase regions: liquid phase (L), a (Mg) solid solution based on magnesium; four two-phase regions: L+a (Mg), L+Mg5Gd, L+Mg24Y5, Mg5Gd+a (Mg), a (Mg) + Mg24Y5; two three-phase regions: L+Mg5Gd+a (Mg), Mg5Gd+a (Mg) + Mg24Y5. With the decrease of temperature, the solid solubility of Gd element in α (Mg) decreases sharply, so Mg5Gd is the most important aging strengthening phase in Mg-Gd-Y alloy. Combined with the vertical cross-section of Mg-(0-40wt%)Gd-(1wt%, 2wt%, 3wt%, 4wt%)Y alloy, it can be analyzed that in order to obtain sufficient Mg5Gd second phase and avoid the precipitation of Y element in the form of Mg24Y5 to reduce the casting performance, the Gd content should be ≥4.3wt%, so the effective heat treatment strengthening phase region of this series of alloys is Gd content of 4.3-12.3wt%. The alloy elements Gd and Y achieve the effect of improving mechanical properties through atomic dislocation and the difference in elastic modulus between the matrix and Gd and Y elements. When the rare earth element content is too low, a sufficient number of second phase particles cannot be formed, and the strengthening effect is not obvious. Therefore, the preferred Gd content is 8.7-10.2wt%.

[0048] The Zr element acts as a nucleation core during crystallization, refines grains, and reduces the tendency of hot cracking of ingots. The amount of Zr added in the present invention is 0.3-0.6wt%, within which the combination of casting performance and mechanical properties can be achieved.

[0049] One embodiment of the present invention provides a method for preparing a large-size high-rare earth VW91M magnesium alloy ingot, comprising the following steps:

[0050] Step 1: Ingredients;

[0051] The amount of ingredients is calculated according to the chemical composition requirements of the above VW91M magnesium alloy, using a remelting magnesium ingot with a purity of more than 99.95%, and alloying elements Gd, Y, and Zr are added in the form of Mg-30Gd, Mg-20Y, and Mg-30Zr master alloys.

[0052] Step 2: Addition of smelting agent;

[0053] Since the raw materials may bring in impurities, which increases the difficulty of subsequent refining, degassing and impurity removal, flux protection smelting technology and appropriate rare earth element addition process are used in the smelting process. In order to optimize this process, we have conducted in-depth research and investigated the effects of factors such as the amount of impurity removal flux added, the addition time, and the melt standing time on the rare earth element loss and the impurity floating and precipitation rate. Based on these research results, we have developed the best impurity removal flux addition plan: first preheat the crucible to dark red (400-500℃), and then evenly sprinkle a layer of RJ-2 flux on the crucible wall and bottom. Preheat the charge to above 150℃, then add the magnesium ingot at one time, and sprinkle a layer of RJ-2 flux on the charge, with the total amount of flux accounting for about 1-2% of the weight of the charge. During the charging and smelting process, once the solution in the crucible is exposed and burns, RJ-2 flux should be added immediately to reduce the burning loss of magnesium alloy.

[0054] Step 3: Melting temperature control;

[0055] Non-metallic inclusions will seriously reduce the strength, toughness and fatigue resistance of the alloy, and metal inclusions will reduce the corrosion resistance of magnesium, deteriorate the quality and mechanical properties of castings. The active chemical properties of magnesium alloys make it very easy to produce metal and non-metallic oxide inclusions during the smelting process. Therefore, by reasonably adjusting the addition time and smelting time of each element in the smelting process and controlling the melting temperature, on the one hand, the oxide inclusions of the melt can be reduced, the burning loss of the alloy, especially the rare earth elements, can be reduced, and the accuracy of the alloy composition can be controlled. At the same time, a reasonable temperature can also prevent iron and other impurities in the low-carbon steel crucible from entering the magnesium alloy melt at high temperature, and help to extend the service life of the crucible.

[0056] The specific implementation process is: after the magnesium ingot is added once, the temperature is raised and smelting begins. When the temperature reaches 700-720°C, Mg-30Gd and Mg-20Y intermediate alloys are added. Then the temperature is continued to rise to 835-855°C, and then Mg-30Zr intermediate alloy is added.

[0057] Step 4: Melt protection and filtration to remove impurities;

[0058] In the molten state, magnesium liquid easily reacts with oxygen, water vapor, and nitrogen in the atmosphere to produce MgO and MgO which are insoluble in magnesium liquid and difficult to dissolve. 3 N 2 Therefore, during the smelting and pouring process, it is necessary to adopt a comprehensive process of gas protection and ceramic filtration to purify the alloy melt. During smelting and pouring, the contact between air and melt is isolated by introducing SF6+N2 mixed gas into the time and position where the molten alloy contacts the air.

[0059] Step 5: heat preservation and refining;

[0060] The gas content in the magnesium alloy melt is high, especially the hydrogen content is high, which can easily lead to defects such as looseness in the ingot. At the same time, the melt is also prone to oxidation and combustion, producing a large number of oxide inclusions and non-metallic slag. Therefore, heat preservation refining is very necessary in magnesium alloy casting. RJ-2 flux and magnesium fluoride are selected as refining agents, the refining temperature is 775-785℃, the refining time is 20-30min, the melt is analyzed before the furnace again and the composition is fine-tuned, and the temperature is raised to 800℃ and left to stand for 30min, and finally the temperature is lowered to 750-770℃ for casting.

[0061] Step 6: Casting;

[0062] The casting method adopts a semi-continuous casting process of transferring metal with compressed inert gas at low pressure. This process can avoid oxidation and burning of the melt during the casting process, and improve the purification degree and purification efficiency of the magnesium alloy. In the process of transferring the molten metal from the crucible to the crystallizer, the highly purified magnesium alloy melt is obtained by isolating the contact with air. The size of the crystallizer used is 450×900mm. The actual crystallizer and schematic diagram are shown in the figure. Figure 3 As shown. The crystallizer adopts a three-stage cooling method. The first-stage cooling is the internal circulation cooling of the crystallizer; the second-stage cooling is the flushing cooling of the bottom of the crystallizer; and the third-stage cooling is the external flushing cooling of the lower part of the crystallizer. During the semi-continuous casting process, it is necessary to strictly control the position of the lowest point of the molten liquid hole of the solid-liquid solidification line in the crystallizer to be between the second and third-stage cooling. The parameters of semi-continuous casting are: the melt temperature before casting is 760-770°C, the crystallizer liquid surface temperature is 660-670°C, the casting speed is 10-25mm / min, the cooling water temperature is ≤20°C, and the first-stage cooling water flow rate is 3-5m 3 / min, the secondary cooling water flow rate is 0.5~1m 3 / min, the third-stage cooling water flow rate is 1~2m 3 / min.

[0063] Example 1: A flat ingot with a chemical composition of Mg-8.7wt%Gd-1.0wt%Y-0.35wt%Zr was prepared. The melting and casting parameters were shown in Table 1.

[0064] Example 2: A flat ingot with a chemical composition of Mg-9.5wt%Gd-1.5wt%Y-0.55wt%Zr was prepared. The melting and casting parameters were as shown in Table 1.

[0065] Example 3: A flat ingot with a chemical composition of Mg-10.5wt% Gd-1.8wt% Y-0.45wt% Zr was prepared. The melting and casting parameters were as shown in Table 1.

[0066] Table 1 Melting parameters of Examples 1 to 3

[0067] Serial number Example 1 Example 2 Example 3 Mg-30Gd, Mg-20Y master alloy addition temperature 715℃ 715℃ 715℃ Mg-30Zr master alloy addition temperature 840℃ 844℃ 850℃ Refining temperature and time 780℃ / 25min 778℃ / 25min 780℃ / 25min Static temperature and time 800℃ / 30min 800℃ / 30min 800℃ / 30min Melt temperature before casting 765℃ 770℃ 770℃ Crystallizer liquid surface temperature 667℃ 665℃ 670℃ Casting speed 10~25mm / min 10~25mm / min 10~25mm / min Cooling water temperature 19℃ 19℃ 19℃ First stage cooling water flow rate <![CDATA[4m 3 / min]]> <![CDATA[4m 3 / min]]> <![CDATA[4m 3 / min]]> Secondary cooling water flow rate <![CDATA[0.8m 3 / min]]> <![CDATA[0.8m 3 / min]]> <![CDATA[0.5m 3 / min]]> Three-stage cooling water flow rate <![CDATA[1.8m 3 / min]]> <![CDATA[1.8m 3 / min]]> <![CDATA[1m 3 / min]]>

[0068] The casting speed in Table 1 changes dynamically. For example, the initial speed is 10 mm / min, then gradually increases to 12 mm / min, 15 mm / min, 20 mm / min, and finally stabilizes at about 25 mm / min.

[0069] Figure 4 This is a real picture of a VW91M flat ingot with a size of 450×900×3500 mm prepared in Example 2, where 450×900 mm is the cross-sectional size and 3500 mm is the length of the ingot.

[0070] Comparative Example 1: In the early test process, under the premise of the same smelting and casting process, a two-stage cooling crystallizer was used to prepare VW91M flat ingots with a cross-sectional size of 450×900mm. Due to the uneven stress distribution, thermal cracking of varying degrees occurred during the semi-continuous casting process, and it was impossible to prepare a complete 450×900mm high rare earth magnesium alloy flat ingot. The cracked ingots were Figure 5 shown.

[0071] The above are only preferred embodiments of the present invention. It should be pointed out that, for those skilled in the art, several modifications and improvements can be made without departing from the concept of the present invention. These should also be regarded as the protection scope of the present invention. These will not affect the effect of the implementation of the present invention and the practicality of the patent.

Claims

1. A method for preparing a large-size ingot of a high-rare earth VW91M magnesium alloy, characterized in that the composition and mass percentage of the high-rare earth VW91M magnesium alloy are as follows: Gd: 8.7-10.2wt%; Y: 0.02-2wt%; Zr: 0.3-0.6wt%; The balance is magnesium and unavoidable impurities; The preparation method comprises the following steps: Step 1: Ingredients; The amount of ingredients is calculated according to the chemical composition requirements of the high rare earth VW91M magnesium alloy, using a remelting magnesium ingot with a purity of more than 99.95%, and alloying elements Gd, Y, and Zr are added in the form of Mg-30Gd, Mg-20Y, and Mg-30Zr master alloys; Step 2: Addition of smelting agent; Preheat the crucible to dark red, and evenly sprinkle a layer of melting agent on the crucible wall and bottom; preheat the charge to above 150℃, then add the magnesium ingot at a time, and sprinkle a layer of melting agent on the charge; Step 3: Melting temperature control; After the magnesium ingot is added once, the temperature is raised and smelting begins. As the temperature rises, Mg-30Gd, Mg-20Y and Mg-30Zr master alloys are added; Step 4: Melt protection and filtration to remove impurities; During the smelting and pouring process, the air is isolated by introducing mixed gas; Step 5: heat preservation and refining; Select the refining agent, set the refining temperature and time, and after refining, analyze the melt again before furnace and fine-tune the composition, then heat up and let it stand, and cool down before casting; Step 6: Casting; Semi-continuous casting is carried out using a crystallizer, and a flat ingot with a size of 450×900×3500 mm is obtained by controlling the casting temperature, the crystallizer liquid surface temperature, the casting speed and the cooling water flow rate.

2. The preparation method according to claim 1, characterized in that: The specific process of step 2 is: preheating the crucible to dark red, the temperature is 400-500°C, and evenly spreading a layer of RJ-2 flux on the crucible wall and bottom; preheating the charge to above 150°C, then adding the magnesium ingot at one time, and sprinkling a layer of RJ-2 flux on the charge, the total amount of the flux accounting for 1-2% of the weight of the charge.

3. The preparation method according to claim 1, characterized in that: In step 2, during the charging and smelting process, if the solution in the crucible is exposed and burns, RJ-2 flux must be added immediately.

4. The preparation method according to claim 1, characterized in that: The specific process of step 3 is as follows: after the magnesium ingot is added once, the temperature is raised and smelting is started. When the temperature reaches 700-720°C, Mg-30Gd and Mg-20Y intermediate alloys are added, and then the temperature is continued to be raised to 835-855°C, and then Mg-30Zr intermediate alloy is added.

5. The preparation method according to claim 1, characterized in that: In step 4, the mixed gas is SF6+N2 mixed gas.

6. The preparation method according to claim 1, characterized in that: The specific process of step 5 is: RJ-2 flux and magnesium fluoride are selected as refining agents, the refining temperature is 775-785°C, the refining time is 20-30 minutes, after refining, the melt is analyzed again before the furnace and the composition is fine-tuned, then the temperature is raised to 800°C and allowed to stand for 30 minutes, and finally the temperature is lowered to 750-770°C for casting.

7. The preparation method according to claim 1, characterized in that: In step 6, the crystallizer adopts a three-stage cooling method: the first-stage cooling is internal circulation cooling of the crystallizer; the second-stage cooling is flushing cooling of the bottom of the crystallizer; the third-stage cooling is external flushing cooling of the lower part of the crystallizer; in the semi-continuous casting process, the lowest point of the molten liquid hole of the solid-liquid solidification line in the crystallizer is located between the second and third-stage cooling.

8. The preparation method according to claim 7, characterized in that: In step 6, the parameters of semi-continuous casting include: melt temperature before casting is 760-770°C, crystallizer liquid surface temperature is 660-670°C, casting speed is 10-25 mm / min, cooling water temperature is ≤20°C, primary cooling water flow rate is 3-5 m / min, 3 / min, the secondary cooling water flow rate is 0.5~1m 3 / min, the third-stage cooling water flow rate is 1~2m 3 / min.

9. A large-size ingot of a high-rare earth VW91M magnesium alloy produced according to the preparation method according to any one of claims 1 to 8.

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