A melting device and a melting method for a corrosion-resistant magnesium alloy

Abstract

A kind of smelting device and smelting method of corrosion-resistant magnesium alloy, including furnace body, temperature control device, corundum tube and melting and casting crucible;Furnace body is wrapped and set in the outside of corundum tube, and melting and casting crucible is set in corundum tube;Temperature control device is set in the furnace body outside corundum tube;The bottom of melting and casting crucible is gas inlet, and the lower part is exhaust port, and gas inlet is provided with inlet valve, and exhaust port is provided with exhaust valve.The present application takes three-section temperature control vertical furnace as basic casting equipment, proposes a kind of method and device for improving the corrosion resistance of magnesium alloy, by genetic control inclusions and process control inclusions, the content of Fe, Si and other harmful elements in magnesium alloy can be reduced, and the purity of magnesium alloy is improved, and the corrosion resistance is greatly improved.

Description

Technical Field

[0001] This invention belongs to the field of magnesium alloy smelting technology, and specifically relates to a smelting apparatus and smelting method for corrosion-resistant magnesium alloys. Background Technology

[0002] Magnesium alloys, as the lightest metallic structural materials, are increasingly used in lightweight applications. However, magnesium alloys contain trace amounts of impurity elements such as Fe, Si, and Ni, and casting defects such as shrinkage cavities can occur during the casting process. These impurity elements and casting defects negatively impact the corrosion resistance of magnesium alloys. For example, a small amount (~50 ppm) of iron can significantly reduce corrosion resistance. Although the solid solubility of Si in magnesium is very low, it can form a brittle phase Mg2Si at the grain boundaries, which has a large potential difference with the matrix, severely reducing the alloy's corrosion resistance and plasticity, thus affecting its applications.

[0003] To address the aforementioned issues, current methods primarily employ flux methods, filtration methods, and pulsed current methods. Wu Guohua et al. conducted research on Fe removal from AZ91 magnesium alloy using fluxes containing TiO2, Na2B4O8, and MnCl2. These fluxes effectively reduced the Fe impurity content in magnesium alloys and improved the overall performance of the material. Their invented magnesium alloy zirconium compound silicon-removing flux effectively reduced the Si content in magnesium alloys, particularly showing good removal of Mg2Si. However, these methods all introduce new impurities and do not improve casting defects such as shrinkage porosity.

[0004] Existing methods for purifying magnesium alloys mainly fall into the following five categories.

[0005] Category 1: Flux purification for impurity removal. Category 2: Filtration for impurity removal. Category 3: Master alloy purification for impurity removal. Category 4: Pulsed current purification for impurity removal. Category 5: Specially designed purification equipment.

[0006] The existing technical solutions mainly have the following drawbacks and shortcomings:

[0007] 1. The flux method usually involves adding salts such as chlorides or fluorides or other new fluxes to magnesium alloys. This method introduces secondary impurities, which is not conducive to improving the purity of the alloy, and the amount used is relatively large.

[0008] 2. For filtration methods, ceramic filters are effective in removing fine inclusions and liquid flux inclusions, but it is still necessary to continuously modify the ceramic foam to improve its adsorption capacity for metal impurities.

[0009] 3. For the intermediate alloy method, the addition of intermediate alloy will also introduce other impurity elements. In addition, the metals used in intermediate alloy are generally rare earth metals, which are expensive and result in high smelting costs.

[0010] 4. For the pulsed current method, a large current or high-frequency pulsed current needs to be applied during the melting process, which will interfere with the control system and detection signals of the operating platform, posing certain safety hazards. The equipment is expensive and not conducive to energy saving.

[0011] 5. Specialized purification equipment, such as tantalum crucibles, is expensive and has high smelting costs, making it unsuitable for mass production. Summary of the Invention

[0012] The purpose of this invention is to provide a smelting apparatus and smelting method for corrosion-resistant magnesium alloys, so as to solve the problem that the prior art introduces new impurities and does not improve casting defects such as shrinkage porosity.

[0013] To achieve the above objectives, the present invention adopts the following technical solution:

[0014] A smelting apparatus for corrosion-resistant magnesium alloy includes a furnace body, a temperature control device, an alumina tube, and a casting crucible; the furnace body is wrapped around the outside of the alumina tube, and the casting crucible is placed inside the alumina tube; the temperature control device is placed inside the furnace body outside the alumina tube; the bottom of the casting crucible is an air inlet, and the bottom is an exhaust outlet; the air inlet is equipped with an air inlet valve, and the exhaust outlet is equipped with an exhaust valve.

[0015] Furthermore, the casting crucible has a cylindrical structure, comprising several sections of graphite tubing, which are connected by threads to form the casting crucible.

[0016] Furthermore, the interior of the casting crucible is arranged from top to bottom with a magnesium vapor trapping plate, a filter plate, and a single plate, dividing the interior of the casting crucible into four areas from top to bottom: a magnesium vapor trapping zone, a melting and slag removal zone, a clean and dense ingot casting zone, and a bottom support zone.

[0017] Furthermore, there are two magnesium vapor collection plates, and the collection area is located between the two magnesium vapor collection plates.

[0018] Furthermore, the magnesium vapor trapping plate is a porous plate.

[0019] Furthermore, the temperature control device is a three-stage temperature control device, which is evenly arranged in the furnace body from top to bottom.

[0020] Furthermore, both ends of the corundum tube are sealed by flanges, and the inlet valve and exhaust valve are both installed on the flanges at the corresponding positions.

[0021] Furthermore, the inlet valve is connected to an argon gas source, and a vacuum pump is installed at the exhaust valve. The vacuum pump is connected to the inside of the corundum tube through a pipeline, and a gas valve is installed on the pipeline of the vacuum pump.

[0022] Furthermore, a method for smelting a magnesium alloy includes the following steps:

[0023] Feeding: Select magnesium, aluminum, and zinc as raw materials, pickle and peel them, and then put them into the filter plate of the charging area of ​​the magnesium alloy smelting device;

[0024] Gas washing: Close the inlet valve and the exhaust valve, evacuate the gas to 10-15 Pa and then stop. Open the lower inlet valve and introduce argon gas. When the gas pressure in the sealed cavity reaches atmospheric pressure, close the lower gas valve. At this time, evacuate the gas to 10-15 Pa and then introduce argon gas again for gas washing. Repeat this cycle twice. Introduce argon gas for the third time until the gas pressure in the furnace reaches 1 atm to 1.002 atm, so that it is in a slightly positive pressure state. Open the exhaust valve at the top of the furnace body to allow the smelting to be carried out in an argon gas environment.

[0025] Melting: Start the furnace temperature control device and begin melting according to the following temperature settings:

[0026] The temperature is raised from room temperature to 820-860°C and held for 2-3 hours to ensure that the raw material on the filter plate melts and the slag is discharged through the filter plate. Then, the temperature is lowered to 730-750°C in 40-60 minutes and held for 1-1.5 hours to achieve static purification and further sedimentation of impurities such as Fe. Then, the temperature is lowered to 400-420°C in 120 minutes and held for 3-4 hours for homogenization. Finally, the temperature is cooled to room temperature in the furnace from 400-420°C.

[0027] Furthermore, the raw materials selected are 99.99% pure 4N grade high-purity magnesium, 99.99% pure 4N grade high-purity zinc, and 99.999% pure 5N grade high-purity aluminum. The melting and casting crucible device uses high-purity, high-density graphite, and the protective gas is 99.999% high-purity argon.

[0028] Compared with the prior art, the present invention has the following technical effects:

[0029] This invention uses a three-stage temperature-controlled vertical furnace as the basic casting equipment and proposes a method and device to improve the corrosion resistance of magnesium alloys. Through genetic impurity control and process impurity control, it can achieve (1) reduce the content of harmful elements such as Fe and Si and oxide inclusions in magnesium alloys, improve the purity of magnesium alloys, and greatly improve corrosion resistance; (2) improve shrinkage porosity and microsegregation, improve the density of magnesium alloy ingots, improve raw material utilization, and improve corrosion resistance and subsequent forming performance; (3) the method and device are simple to operate. After the raw materials are placed in the device, the entire melting process is carried out inside the device without manual operation, and the safety factor is high.

[0030] This invention improves the purity of magnesium alloys through genetic impurity control and process impurity control. Specifically, it improves the purity of raw materials to achieve genetic impurity control, effectively removing harmful impurity elements such as Fe, Si, and Ni. It uses high-purity graphite crucibles to achieve process impurity control, further avoiding the introduction of external Fe impurity elements.

[0031] In this device, the raw materials are melted and flow into the casting area below through a filter plate. The entire process is protected by argon gas, which removes oxide inclusions and improves density.

[0032] The AZ80 magnesium alloy obtained in the apparatus of this invention is prepared under slow cooling conditions, which not only improves corrosion resistance but also effectively improves casting defects such as shrinkage porosity and microsegregation. Attached Figure Description

[0033] Figure 1 Assembly diagram of a three-section temperature-controlled vertical furnace;

[0034] Figure 2 Assembly diagram of graphite crucible;

[0035] Figure 3 Macroscopic morphology of low-purity AZ80 (AZ80-LP) (left) and high-purity AZ80 (AZ80-HP) magnesium alloy ingots (right)

[0036] Figure 4 Low-purity AZ80 (AZ80-LP) (left) and high-purity AZ80 (AZ80-HP) (right) magnesium alloy filtration products

[0037] Figure 5 Comparison of impurity element content in AZ80 (Mg-8Al-0.5Zn) magnesium alloys of different purities

[0038] Figure 6 Comparison of corrosion rates of AZ80 (Mg-8Al-0.5Zn) magnesium alloys with different purities

[0039] Figure 7 Comparison of impurity element content in AZ80 (Mg-8Al-0.5Zn-0.2Mn) magnesium alloys with different purities

[0040] Figure 8 Comparison of corrosion rates of AZ80 (Mg-8Al-0.5Zn-0.2Mn) magnesium alloys with different purities Detailed Implementation

[0041] The present invention will be further described below with reference to the accompanying drawings:

[0042] Please see Figure 1A smelting apparatus for corrosion-resistant magnesium alloys is disclosed. The apparatus comprises a three-stage temperature-controlled vertical furnace (1), equipped with a three-stage temperature control device (2), corundum tubes (3), and flanges (4). Gas valves are installed on the upper and lower flanges; the lower inlet valve (5) connects to external argon gas, and the upper exhaust valve (7) connects to a vacuum pump (6). The corundum tubes and flanges form a sealed cavity. To control impurities during the process, a casting crucible (8) made of high-purity graphite is placed within this sealed cavity. The crucible is composed of multiple sections of graphite tubing connected by threads. The entire casting crucible is divided into four zones from top to bottom: a magnesium vapor trapping zone, a melting and slag removal zone, a clean and dense ingot casting zone, and a bottom support zone. Magnesium vapor trapping plates (9), filter plates (10), and single plates (11) are installed at the connection points. The magnesium vapor trapping plates trap magnesium vapor to prevent excessive escape; alloy raw materials are placed on the filter plates. As the temperature rises, the alloy raw materials melt and flow through the filter plates into the lower clean and dense ingot casting zone, ultimately yielding a magnesium alloy ingot on the single plate. This apparatus is described below. Figure 1 As shown, the detailed assembly diagram of the graphite crucible is as follows: Figure 2 As shown. To achieve genetic control of contamination, high-purity 4N-grade magnesium was selected as the raw material.

[0043] A method for smelting a magnesium alloy, specifically comprising:

[0044] (1) Feeding: The raw materials are selected as 4N grade high-purity magnesium, 4N grade high-purity zinc, and 5N grade high-purity aluminum. After acid washing and peeling, they are placed above the graphite filter plate in the feeding area of ​​the device.

[0045] (2) Gas washing: Close the upper and lower gas valves, evacuate the vacuum to 10-15 Pa and then stop. Open the lower gas valve and introduce argon gas. When the gas pressure in the sealed cavity is close to atmospheric pressure, close the lower gas valve. At this time, evacuate the vacuum to 10-15 Pa and then introduce argon gas again for gas washing. Repeat this cycle twice. Introduce argon gas for the third time until the gas pressure in the furnace reaches 1 atm to 1.002 atm, so that it is in a slightly positive pressure state. Open the exhaust valve on the top of the furnace body so that the melting can be carried out in an argon gas environment.

[0046] (3) Temperature setting: Set specific temperature control parameters. Heating: Raise the temperature from room temperature to 820-860°C and hold for 2-3 hours to ensure that the raw material on the filter plate melts and the slag is discharged through the filter plate; then cool down to 730-750°C in 40-60 minutes and hold for 1-1.5 hours to achieve static removal of impurities, allowing Fe and other impurity elements to settle further; then cool down to 400-420°C in 120 minutes and hold for 3-4 hours for homogenization treatment, and finally cool down to room temperature with the furnace from 400-420°C. The entire solidification process adopts slow cooling.

[0047] (4) Smelting: Start the three-stage temperature-controlled vertical furnace and complete the smelting according to the temperature program set above.

[0048] Example 1: High-purity AZ80 magnesium alloy, with an alloy ratio of 8% Al + 0.5% Zn.

[0049] 1. Feeding: The raw materials are selected as 4N grade high-purity magnesium, 4N grade high-purity zinc, and 5N grade high-purity aluminum. After acid washing and peeling, they are placed above the graphite filter plate in the feeding area of ​​the device.

[0050] 2. Gas washing: Close the upper and lower gas valves, evacuate the furnace to 10-15 Pa and then stop. Open the lower gas valve and introduce argon gas. When the gas pressure in the sealed cavity is close to atmospheric pressure, close the lower gas valve. At this time, evacuate the furnace to 10-15 Pa and then introduce argon gas again for gas washing. Repeat this cycle twice. Introduce argon gas for the third time until the gas pressure in the furnace reaches 1 atm to 1.002 atm, making it a slightly positive pressure. Open the exhaust valve at the top of the furnace to allow the smelting to proceed in an argon atmosphere.

[0051] 3. Temperature settings: Set specific temperature control parameters. Heating: Increase the temperature from room temperature to 850°C and hold for 2 hours to melt the raw material on the filter plate and discharge the slag through the filter plate. Cooling: Reduce the temperature from 850°C to 750°C in 60 minutes and hold for 1 hour to achieve static purification and further settle impurities such as Fe. Then, reduce the temperature to 400°C in 120 minutes and hold for 3 hours for homogenization. Finally, cool the furnace from 400°C to room temperature. The entire solidification process is achieved through slow cooling.

[0052] 4. Melting: Start the three-stage temperature-controlled vertical furnace and begin melting according to the temperature program settings.

[0053] Comparative Example 1: Low-purity AZ80 magnesium alloy, with an alloy ratio of 8% Al + 0.5% Zn.

[0054] The raw material was changed from 4N grade pure magnesium to 3N grade lower purity pure magnesium, while the equipment and smelting process remained unchanged, resulting in two AZ80 magnesium alloys with different purities: high purity AZ80 magnesium alloy AZ80-HP and low purity AZ80 magnesium alloy AZ80-LP.

[0055] The macroscopic morphology of AZ80-LP and AZ80-HP ingots is as follows: Figure 3 As shown in the figure, the surface of the ingot has no obvious shrinkage pores and is relatively dense overall. The filter material on the filter plate is as follows: Figure 4 As shown, the filter plate has a significant effect on removing oxide inclusions during the alloy smelting process.

[0056] The impurity content of the two magnesium alloys with different purities in this example was tested, and the results are as follows: Figure 5 As shown, the impurity content of AZ80-HP is significantly lower than that of AZ80-LP magnesium alloy, with the Fe content decreasing by about 87% and the Si content decreasing by about 54%. This indicates that the purification of magnesium alloy was effectively achieved through genetic and process-controlled impurity control.

[0057] Hydrogen evolution and weight loss corrosion properties of two magnesium alloys were tested in a 3.5% NaCl solution at a constant temperature of 25°C. The test results are as follows. Figure 6 As shown in the figure, the left figure compares the hydrogen evolution rates of two AZ80 magnesium alloys with different purities. It can be seen that the hydrogen evolution rate of low-purity AZ80 is much faster than that of high-purity AZ80, as shown in the right figure. Corrosion rates are expressed in mm / y. Calculated using the hydrogen evolution method, the corrosion rate of AZ80-HP is 18 mm / y, and that of AZ80-LP is 50 mm / y. Calculated using the weight loss method, the corrosion rate of AZ80-HP is 19 mm / y, and that of AZ80-LP is 51 mm / y. This means that the corrosion resistance of AZ80-HP is improved by approximately 63%. From the macroscopic corrosion morphology, compared with AZ80-HP, the AZ80-LP magnesium alloy has a larger corrosion area, deeper corrosion pits, and more severe corrosion. Therefore, the AZ80-HP magnesium alloy prepared in this embodiment shows a significant improvement in corrosion resistance compared to the comparative example.

[0058] Example 2: High-purity AZ80 magnesium alloy, with an alloy ratio of 8%Al + 0.2%Zn + 0.2%Mn.

[0059] The method was scaled up to industrial level using semi-continuous casting as the smelting process, the raw material was changed to 4N grade high-purity magnesium, and the crucible was changed from an iron crucible to a high-purity graphite crucible. That is, high-purity AZ80 magnesium alloy was prepared by genetic impurity control and process impurity control.

[0060] The smelting process is as follows:

[0061] Ingredient melting: Prepare the raw materials according to the alloy composition design. Heat 3N magnesium until it is completely melted, then add 3N aluminum, 3N zinc and anhydrous manganese chloride to the molten magnesium liquid. After melting, stir and test the composition. Once the composition meets the standard, start refining.

[0062] First refining: Add a refining agent to react chemically with impurities in the molten liquid, forming a solid that settles to the bottom of the crucible.

[0063] Slag removal: Remove impurities that have settled at the bottom of the crucible.

[0064] Secondary refining: Add refining agent and continue refining.

[0065] Cooling: Cool down to about 640℃ and let stand for a certain period of time to further remove Fe impurity elements.

[0066] Casting: The molten magnesium alloy is poured into the crystallizer and solidified by water cooling on the outer wall. After the initially cast magnesium ingot has basically taken shape, the ingot is pulled downward by the bottom plate of the crystallizer to achieve semi-continuous casting.

[0067] Comparative Example 2: Low-purity AZ80 magnesium alloy, with an alloy composition of 8% Al + 0.2% Zn + 0.2% Mn.

[0068] The raw material was changed from 4N grade pure magnesium to 3N grade lower purity pure magnesium, while the smelting process remained unchanged, resulting in two AZ80 magnesium alloys with different purities: high purity AZ80 magnesium alloy AZ80-HP and low purity AZ80 magnesium alloy AZ80-LP.

[0069] The impurity content of the two magnesium alloys in this example was tested, and the results are as follows: Figure 7 As shown, the impurity content of AZ80-HP is significantly lower than that of AZ80-LP magnesium alloy, with the Si content decreasing by approximately 69%.

[0070] Hydrogen evolution and weight loss corrosion properties of two magnesium alloys were tested in a 3.5% NaCl solution at a constant temperature of 25°C. The test results are as follows. Figure 8 As shown in the figure, the left figure compares the hydrogen evolution rates of two AZ80 magnesium alloys with different purities. It can be seen that the hydrogen evolution rate of AZ80-LP is faster than that of AZ80-HP, and the dispersion indicates that the corrosion resistance stability of AZ80-HP is much better than that of AZ80-LP. The right figure shows the corrosion rates calculated by the hydrogen evolution method and the weight loss method. The corrosion rates are in mm / y. The corrosion rate of AZ80-HP calculated by the hydrogen evolution method is 0.7 mm / y, and that of AZ80-LP is 2.8 mm / y. The corrosion rate of AZ80-HP calculated by the weight loss method is 0.85 mm / y, and that of AZ80-LP is 3.2 mm / y. That is, the corrosion resistance of AZ80-HP is improved by about 61%. From the macroscopic corrosion morphology, compared with AZ80-HP, at the same time, the AZ80-LP magnesium alloy shows obvious corrosion pits all over the surface, with deeper local corrosion pits, indicating more severe corrosion. Therefore, the AZ80-HP magnesium alloy prepared in this embodiment has significantly improved corrosion resistance compared with the comparative example, verifying the effectiveness of genetic and process-controlled impurities.

Claims

1. A method for smelting a magnesium alloy, characterized in that, The magnesium alloy smelting apparatus includes a furnace body (1), a temperature control device (2), an alumina tube (3), and a casting crucible (8); the furnace body (1) is wrapped around the outside of the alumina tube (3), and the casting crucible (8) is placed inside the alumina tube (3); the temperature control device is placed inside the furnace body (1) outside the alumina tube (3); the bottom of the casting crucible (8) is an air inlet, and the top is an exhaust outlet; the air inlet is equipped with an air inlet valve (5), and the exhaust outlet is equipped with an exhaust valve (7); The casting crucible (8) has a straight cylindrical structure and includes several sections of graphite tubes. The several sections of graphite tubes are connected by threads to form the casting crucible (8). The inside of the casting crucible (8) is arranged from top to bottom with a magnesium vapor trapping plate (9), a filter plate (10) and a single plate (11), dividing the inside of the casting crucible (8) into four areas from top to bottom: magnesium vapor trapping area, melting and slag removal area, clean and dense ingot casting area and bottom support area; There are two magnesium vapor trapping plates (9), and the magnesium vapor trapping zone is located between the two magnesium vapor trapping plates (9); the melting and slag removal zone is located between the lower magnesium vapor trapping plate (9) and the filter plate (10); the clean and dense ingot casting zone is located between the filter plate (10) and the single plate (11); the bottom support zone is located below the single plate (11); The magnesium vapor trapping plate (9) is a porous plate; The temperature control device (2) is a three-stage temperature control device, which is evenly arranged in the furnace body (1) from top to bottom; The method includes the following steps: Feeding: Select magnesium, aluminum, and zinc as raw materials, pickle and peel them, and then put them into the filter plate of the charging area of ​​the magnesium alloy smelting device; Gas washing: Close the inlet valve and the exhaust valve, evacuate the gas to 10-15 Pa and then stop. Open the lower inlet valve and introduce argon gas. When the gas pressure in the sealed cavity reaches atmospheric pressure, close the inlet valve and then evacuate the gas to 10-15 Pa and introduce argon gas again for gas washing. Repeat this cycle twice. For the third time, introduce argon gas until the gas pressure in the furnace reaches 1 atm to 1.002 atm, making it a slightly positive pressure state. Open the exhaust valve at the top of the furnace to allow the smelting to take place in an argon gas environment. Smelting: Start the furnace temperature control device and begin smelting according to the following temperature settings: raise the temperature from room temperature to 820-860°C and hold for 2-3 hours to ensure that the raw material on the filter plate melts and the slag is discharged through the filter plate; then lower the temperature to 730-750°C in 40-60 minutes and hold for 1-1.5 hours to achieve static impurity removal and further precipitation of Fe impurities; then lower the temperature to 400-420°C in 120 minutes and hold for 3-4 hours for homogenization treatment; finally, cool the furnace to room temperature from 400-420°C.

2. The method for smelting a magnesium alloy according to claim 1, characterized in that, The raw materials selected are 4N grade high-purity magnesium with a purity of 99.99%, 4N grade high-purity zinc with a purity of 99.99%, and 5N grade high-purity aluminum with a purity of 99.999%. The melting and casting crucible device uses high-purity, high-density graphite, and the protective gas is 99.999% high-purity argon.

3. The method for smelting a magnesium alloy according to claim 1, characterized in that, The two ends of the corundum tube (3) are sealed by flanges (4), and the inlet valve (5) and the exhaust valve (7) are both set on the flanges at the corresponding positions.

4. The method for smelting a magnesium alloy according to claim 1, characterized in that, The inlet valve (5) is connected to the argon gas source, and a vacuum pump is also installed at the exhaust valve (7). The vacuum pump is connected to the corundum tube (3) through a pipeline, and a gas valve (6) is installed on the pipeline of the vacuum pump.

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