A magnesium alloy die-casting and its preparation method and application

Abstract

The present invention provides a magnesium alloy die-cast part, a preparation method thereof and an application thereof, belonging to the technical field of alloys. The magnesium alloy die-cast part includes a magnesium alloy matrix and a composite layer prepared on the surface of the magnesium alloy matrix by using a plasma spraying technique; the composite layer includes nickel powder and modified titanium dioxide, and the mass ratio of the nickel powder to the modified titanium dioxide is (1.7 - 1.9):1. The present invention can effectively improve the long-term corrosion resistance of the obtained magnesium alloy die-cast part.

Description

Technical Field

[0001] The present invention belongs to the technical field of alloys, and particularly relates to a magnesium alloy die-casting and its preparation method and application. Background Art

[0002] Magnesium alloys have characteristics such as light weight and recyclability, and are widely used in fields such as electronic devices and automobiles, and have gradually become an ideal material for modern industrial products. Magnesium alloys have excellent shock absorption performance, can efficiently absorb road vibration energy, and reduce the impact transmitted to the vehicle body. Due to the strong damping and noise reduction ability of magnesium alloys, magnesium alloy die-castings can be used as the bottom cylinder of shock absorbers in the shock absorbers of electric tricycles or electric two-wheelers.

[0003] Magnesium alloys themselves have poor corrosion resistance and are prone to corrosion in humid / salt spray environments; while nickel layers can physically isolate corrosion media and use the passivation characteristics of nickel to slow down the corrosion rate, which can improve the corrosion resistance of magnesium alloys to a certain extent in the short term (such as the corrosion resistance time can reach more than 72 hours in the standard salt spray test).

[0004] However, as the time in the humid / salt spray environment prolongs, due to the large potential difference between nickel and magnesium, a strong galvanic couple is formed, and conventional nickel layers are prone to micropores / breakages, which will form local corrosion cells and instead accelerate pitting corrosion / corrosion of the substrate. Summary of the Invention

[0005] To solve the problems in the background art, the present invention provides a magnesium alloy die-casting and its preparation method and application, which can effectively improve the long-term corrosion resistance of the obtained magnesium alloy die-casting.

[0006] To achieve the above object, in the first aspect, the present invention provides a magnesium alloy die-casting, including a magnesium alloy substrate and a composite layer prepared on the surface of the magnesium alloy substrate by using plasma spraying technology; the composite layer includes nickel powder and modified titanium dioxide, and the mass ratio of the nickel powder to the modified titanium dioxide is (1.7 - 1.9):1.

[0007] Further, the preparation method of the modified titanium dioxide is as follows:

[0008] A1. Add 750 - 800 mL of sodium hydroxide solution with a concentration of 9 - 10 mol / L to a three-neck flask, and then add 18 - 20 g of nano-titanium dioxide, and stir to obtain a mixture;

[0009] A2. Heat the mixture obtained in A1 under reflux to obtain a reactant;

[0010] A3. After centrifugally separating the reactant obtained in A2, wash it with distilled water until it is neutral, and dry it to obtain modified titanium dioxide.

[0011] Further, in A1, the stirring speed is 300 - 360 r / min, and the stirring time is 15 - 20 min.

[0012] Further, in A2, the mixture is heated to 100 °C and refluxed at 100 °C for 10 - 12 h.

[0013] Further, in A3, the drying temperature is 96 - 100 °C, and the drying time is 50 - 70 min.

[0014] Further, the magnesium alloy matrix, by mass percentage, comprises the following components: 0.4% - 0.42% Si, 0.05% - 0.065% Al, 0.02 - 0.04% RE, 0.01% - 0.012% Mn, 0.01% - 0.015% Zn, and 0.009% - 0.011% Ca, with the balance being Mg and unavoidable impurities; the RE includes La and / or Ce.

[0015] In the second aspect, the present invention provides a method for preparing the above - mentioned magnesium alloy die - casting, comprising the following steps:

[0016] S1. Weigh raw materials according to the component ratio in the magnesium alloy matrix, melt them, cast to obtain a magnesium alloy ingot, and then melt - die - cast the magnesium alloy ingot to form a magnesium alloy matrix.

[0017] S2. Polish and grind the surface of the magnesium alloy matrix, perform ultrasonic cleaning, and dry it to obtain a pretreated matrix.

[0018] S3. Mix nickel powder and modified titanium dioxide, ball - mill for 3.5 - 4.5 h to achieve uniform dispersion, and then dry in a vacuum oven at 80 ± 2 °C for 11 - 12 h to obtain a composite powder.

[0019] S4. Using plasma spraying, spray the composite powder obtained in S3 onto the pretreated matrix obtained in S2, and then put it into a sintering furnace for vacuum sintering to obtain the magnesium alloy die - casting.

[0020] Further, in S4, the plasma spraying power is controlled at 35 - 45 kW, the working gas is a mixed gas of argon and hydrogen, the spraying distance is 90 - 100 mm, the flow rate of argon is 42 - 47 L / min, the flow rate of hydrogen is 5 - 6 L / min, the powder feeding rate is 30 - 40 g / min, and the preheating temperature is 120 - 130 °C.

[0021] Further, in S4, the sintering furnace is heated to 410 - 460 °C at a rate of 10 °C / min, held for 0.6 - 1 h, and the vacuum degree in the sintering furnace < 1×10 -2 Pa.

[0022] In a third aspect, the present invention provides an application of the above-mentioned magnesium alloy die-casting, which is used in a shock absorber of an electric tricycle or an electric two-wheeled vehicle.

[0023] This application has the following beneficial effects:

[0024] The invention comprises a magnesium alloy substrate and a composite layer prepared on the surface of the magnesium alloy substrate by utilizing plasma spraying technology; the composite layer comprises nickel powder and modified titanium dioxide.

[0025] Nano-titanium dioxide fills the gaps in nickel powder and forms a "brick-mud" structure after plasma spraying, reducing the porosity. In addition, the semiconductor properties of titanium dioxide are between those of nickel and magnesium, which can construct a step-type potential gradient, reducing the local current density and promoting the improvement of long-term corrosion resistance.

[0026] Nano-titanium dioxide is soaked in sodium hydroxide solution and then refluxed to create modified titanium dioxide. This increases the surface hydroxylation density, strengthening chemical bonding with nickel powder. The reflux treatment also induces a phase transition, creating a layered structure that further blocks the diffusion paths of corrosive media. Furthermore, the abundant -OH groups on the modified titanium dioxide surface form hydrogen bonding networks with corrosion products, promoting the formation of a dense complex salt layer in the corroded area, achieving a self-healing effect and synergistically improving long-term corrosion resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 , a comparative trend chart of average corrosion rate test data of the test pieces of Examples 1 to 3 and Comparative Examples 1 to 5 in the test examples of the present invention immersed in 3.5% sodium chloride solution at 25°C for 45 days. DETAILED DESCRIPTION

[0028] The present application is further described in detail below with reference to the embodiments.

[0029] Unless otherwise specified, the raw materials used in the examples and comparative examples of the present application are all commercially available.

[0030] Example 1: (1) Preparation of modified titanium dioxide, the preparation method is as follows:

[0031] A1. Add 780 mL of 9.5 mol / L sodium hydroxide aqueous solution to a three-necked flask, then add 19 g of nano-titanium dioxide. Stir at 320 rpm for 18 minutes to obtain a mixture. The nano-titanium dioxide (AEROXIDE P25-2) has a purity of 99.5% and an average particle size of 21 nm and was purchased from Borida (Dongguan) New Materials Co., Ltd.

[0032] A2. Heat the mixture obtained in A1 to 100°C and reflux at 100°C for 11 h to obtain the reactant.

[0033] A3. After centrifuging the reactants obtained in A2 (8000 r / min), wash them with distilled water until neutral, and then place them in a drying oven for drying. The drying temperature is about 98 °C and the drying time is 100 min to obtain modified titanium dioxide.

[0034] (2) Prepare a magnesium alloy die-casting, and its preparation method includes the following steps:

[0035] S1. Prepare a magnesium alloy matrix, which, by mass percentage, includes the following components: 99.45% Mg, 0.415% Si, 0.059% Al, 0.023% Ce, 0.012% La, 0.012% Mn, 0.015% Zn, and 0.011% Ca, and the balance is inevitable impurities. The specific preparation method is the prior art. First, weigh the raw materials according to the component ratio in the magnesium alloy matrix, then melt them, then cast to obtain a magnesium alloy ingot, and then melt and die-cast the magnesium alloy ingot to obtain a magnesium alloy matrix.

[0036] S2. Polish and grind the surface of the magnesium alloy matrix. Specifically, first perform rough polishing: a 400-mesh sand belt, a linear speed of 15 m / s, and a feed rate of 0.1 mm / pass; then perform fine polishing: 2000-mesh diamond grinding paste, a pressure of 0.2 MPa, and Ra ≤ 0.8 μm. Use a cleaning agent (a mixture of acetone and ethanol with a volume ratio of 1:1) for ultrasonic cleaning, with an ultrasonic frequency of 40 kHz and a cleaning time of 15 min. Dry in a hot air circulation drying oven at 80 °C for 30 min to obtain a pretreated matrix.

[0037] S3. Mix nickel powder and modified titanium dioxide in a mass ratio of 1.8:1, and use a planetary ball mill (zirconia pot and grinding balls) for ball milling. The ball-to-material ratio is 10:1, the rotation speed is 240 rpm, and pause for 5 min every 30 min to prevent overheating. Ball mill for a total of 8 h to achieve uniform dispersion. Then dry in a vacuum oven at about 80 °C for 11.5 h to obtain a composite powder. Among them, the particle size of the nickel powder is 5 - 10 μm, and it is purchased from Zhuyu New Materials Technology Co., Ltd.

[0038] S4. Adopt plasma spraying to spray the composite powder obtained in S3 on the pretreated matrix obtained in S2. The plasma spraying power is controlled at 40 kW, the working gas is a mixed gas of argon and hydrogen, the spraying distance is 100 mm, the flow rate of argon is 45 L / min, the flow rate of hydrogen is 5 L / min, the powder feeding rate is 35 g / min, and the preheating temperature of the pretreated matrix is 125 °C. Then place it in a sintering furnace for vacuum sintering. Specifically, heat up the sintering furnace to 420 °C at a rate of 10 °C / min, hold for 0.8 h, and the vacuum degree in the sintering furnace < 1×10 -2Pa, and finally cooled to 150 ° C in the furnace and filled with high-purity argon gas to eventually form a composite layer (about 52 μm), thus obtaining a magnesium alloy die-casting.

[0039] Example 2: This example differs from Example 1 in that a magnesium alloy die casting is prepared, and the preparation method includes the following steps:

[0040] S1. Prepare a magnesium alloy matrix comprising the following components, calculated by mass percentage: 99.45% Mg, 0.415% Si, 0.059% Al, 0.023% Ce, 0.012% La, 0.012% Mn, 0.015% Zn, and 0.011% Ca, with the remainder being unavoidable impurities. The specific preparation method is based on existing techniques, comprising weighing raw materials according to the composition ratio of the magnesium alloy matrix components, smelting, and then casting to obtain a magnesium alloy ingot. The magnesium alloy ingot is then melted and die-cast to obtain a magnesium alloy matrix.

[0041] S2. Polishing and grinding the surface of the magnesium alloy substrate, ultrasonically cleaning, and drying to obtain a pretreated substrate.

[0042] S3. Mix nickel powder and modified titanium dioxide in a mass ratio of 1.7:1, ball mill for 3.5 hours to achieve uniform dispersion, and then dry in a vacuum oven at about 80°C for 11 hours to obtain composite powder.

[0043] S4: Plasma spraying was used to spray the composite powder obtained in S3 onto the pre-treated substrate obtained in S2. The plasma spraying power was controlled at 40 kW, the working gas was a mixture of argon and hydrogen, the spraying distance was 100 mm, the argon flow rate was 45 L / min, the hydrogen flow rate was 5 L / min, the powder feeding rate was 35 g / min, and the preheating temperature of the pre-treated substrate was 125°C. The composite powder was then placed in a sintering furnace for vacuum sintering. Specifically, the temperature in the sintering furnace was raised to 420°C at a rate of 10°C / min and kept at this temperature for 0.8 h. The vacuum degree in the sintering furnace was <1×10 -2 Pa, and finally cooled to 150 ° C in the furnace and filled with high-purity argon gas to form a composite layer, thus obtaining a magnesium alloy die casting.

[0044] Example 3: This example differs from Example 1 in that a magnesium alloy die casting is prepared, and the preparation method includes the following steps:

[0045] S1. Prepare a magnesium alloy matrix, which, by mass percentage, includes the following components: 99.45% Mg, 0.415% Si, 0.059% Al, 0.023% Ce, 0.012% La, 0.012% Mn, 0.015% Zn, and 0.011% Ca, with the balance being inevitable impurities. The specific preparation method is a prior art. First, weigh the raw materials according to the component ratio in the magnesium alloy matrix, then melt them, and then cast to obtain a magnesium alloy ingot. Then, melt and die-cast the magnesium alloy ingot to form a magnesium alloy matrix.

[0046] S2. Polish and grind the surface of the magnesium alloy matrix, ultrasonically clean it, and dry it to obtain a pretreated matrix.

[0047] S3. Mix nickel powder and modified titanium dioxide in a mass ratio of 1.9:1, ball-mill for 4.5 h to achieve uniform dispersion, and then dry in a vacuum oven at about 80 °C for 12 h to obtain a composite powder.

[0048] S4. Using plasma spraying, spray the composite powder obtained in S3 onto the pretreated matrix obtained in S2. The plasma spraying power is controlled at 40 kW, the working gas is a mixed gas of argon and hydrogen, the spraying distance is 100 mm, the flow rate of argon is 45 L / min, the flow rate of hydrogen is 5 L / min, the powder feeding rate is 35 g / min, and the preheating temperature of the pretreated matrix is 125 °C. Then, put it into a sintering furnace for vacuum sintering. Specifically, the temperature in the sintering furnace is raised to 420 °C at a rate of 10 °C / min, held for 0.8 h, and the vacuum degree in the sintering furnace < 1×10 -2 Pa. Finally, cool it in the furnace to 150 °C and then fill it with high-purity argon to finally form a composite layer, thus obtaining a magnesium alloy die-casting.

[0049] Comparative Example 1: The difference between this comparative example and Example 1 is that the composite layer is deleted (delete the composite powder, and the magnesium alloy matrix is the magnesium alloy die-casting).

[0050] Comparative Example 2: The difference between this comparative example and Example 1 is that the modified titanium dioxide is deleted (the composite powder is replaced with nickel powder).

[0051] Comparative Example 3: The difference between this comparative example and Example 1 is that the composite powder includes nickel powder and nano-titanium dioxide, and the mass ratio of nickel powder to nano-titanium dioxide is 2.2:1.

[0052] Comparative Example 4: The difference between this comparative example and Example 1 is that the composite powder includes nickel powder and nano-titanium dioxide, and the mass ratio of nickel powder to nano-titanium dioxide is 1.8:1.

[0053] Comparative Example 5: The difference between this comparative example and Example 1 is that in the composite powder, the mass ratio of nickel powder to modified titanium dioxide is 2.2:1.

[0054] Test Example: Test subjects: Specimens prepared from Examples 1-3 and Comparative Examples 1-5. Test method: Dry weight m1, soaked in 3.5% sodium chloride solution at 25°C for 45 days, then dried to obtain weight m2. Average corrosion rate = (m1 - m2) / 30 / specimen surface area. Test results: See Table 1.

[0055] Table 1. Test data of experimental example

[0056]

[0057] Result analysis: Analyze Example 1-Example 3 and combine the data in Table 1 and Figure 1 It can be seen that the average corrosion rate test data of the magnesium alloy die castings (test pieces) prepared in the present invention (Examples 1 to 3) immersed in 3.5% sodium chloride solution at 25°C for 45 days is as low as 0.40 mg·cm -2 ·d -1 The following fully meets the application requirements of shock absorbers on electric three-wheeled vehicles or electric two-wheeled vehicles.

[0058] Analyze Example 1 and Comparative Examples 1-5 and combine the data in Table 1 and Figure 1 By comparing Comparative Example 1 with Comparative Example 2, it can be seen that compared with Comparative Example 1, Comparative Example 2 adds a nickel layer, and the average corrosion rate is 0.59 mg·cm -2 ·d -1 (Comparative Example 1) increased to 0.63 mg·cm -2 ·d -1 (Comparative Example 2) shows that the preparation of the nickel layer by (nickel powder) plasma technology actually leads to an increase in the average corrosion rate of the magnesium alloy die-casting (test piece) immersed in a 3.5% sodium chloride solution at 25°C for 45 days; that is, simply adding a nickel layer will cause the long-term corrosion resistance of the magnesium alloy test piece to decrease rather than increase.

[0059] This is because magnesium alloys themselves have poor corrosion resistance and are easily corroded in humid / salt spray environments. The nickel layer can improve the corrosion resistance of the magnesium alloy to a certain extent in the short term by physically isolating the corrosive medium. However, as the time in the humid / salt spray environment increases (increased to 45 days in this test example), the large potential difference between nickel and magnesium forms a strong galvanic pair, and conventional nickel layers are prone to micropores / damage, which will form localized corrosion cells and accelerate pitting / corrosion of the substrate. Ultimately, the long-term corrosion resistance of the magnesium alloy specimens decreases instead of increases.

[0060] Comparing Comparative Example 3 and Comparative Example 4, it can be seen that after adding nano-titanium dioxide to nickel powder to make a composite powder and then using ion technology to prepare a composite layer, the average corrosion rate of the obtained magnesium alloy die-casting (specimen) immersed in 3.5% sodium chloride solution at 25°C for 45 days can be reduced; that is, adding nano-titanium dioxide to nickel powder can improve the long-term corrosion resistance of magnesium alloy specimens. Moreover, the improvement effect of the mass ratio of nickel powder to nano-titanium dioxide being 2.2:1 (Comparative Example 3) is significantly greater than that of the mass ratio of 1.8:1 (Comparative Example 4).

[0061] This is because nano-titanium dioxide fills the gaps in nickel powder, forming a "brick-mud" structure after plasma spraying, reducing the porosity; and the semiconductor properties of titanium dioxide are between nickel and magnesium, which can build a stepped potential gradient and reduce the local current density. Only when the proportion of nickel powder is high enough can the conductive network of the composite layer be complete enough to better avoid local charge accumulation; if the proportion of nickel powder is low, it is not conducive to better improving the corrosion resistance.

[0062] Comparing Comparative Example 5 and Example 1, it can be seen that making nano-titanium dioxide into the modified titanium dioxide of the present invention can produce a synergistic effect with nickel powder, synergistically improving the long-term corrosion resistance of magnesium alloy specimens. Moreover, the synergistic improvement effect of the mass ratio of nickel powder to modified titanium dioxide being 1.8:1 (Example 1) is significantly greater than that of the mass ratio of 2.2:1 (Comparative Example 5).

[0063] This is because making nano-titanium dioxide into the modified titanium dioxide of the present invention, on the one hand, the surface hydroxylation density increases, enhancing the chemical bonding with nickel powder; and the reflux treatment induces a phase transformation, and the layered structure is more conducive to blocking the diffusion path of corrosive media. On the other hand, the surface of the modified titanium dioxide has surplus -OH groups that can form a hydrogen bond network with corrosion products, promoting the formation of a dense double salt layer in the corrosion area to achieve a self-repair effect. Only when the proportion of modified titanium dioxide is high enough can it better play a synergistic role with nickel powder and better play the self-repair ability; if the proportion of modified titanium dioxide is low, it is not conducive to better improving the long-term corrosion resistance.

[0064] In addition, it should be noted that among the various specific technical features described in the above specific embodiments, they can be combined in any suitable way without contradiction. To avoid unnecessary repetition, the present invention will not separately describe various possible combination methods.

[0065] In addition, any combination can be made between various different embodiments of the present invention, as long as it does not violate the idea of the present invention, it should also be regarded as the content disclosed by the present invention.

Claims

1. A magnesium alloy die casting, characterized in that: The invention comprises a magnesium alloy substrate and a composite layer prepared on the surface of the magnesium alloy substrate by plasma spraying technology; the composite layer comprises nickel powder and modified titanium dioxide, and the mass ratio of the nickel powder to the modified titanium dioxide is (1.7-1.9):1; The preparation method of the modified titanium dioxide is as follows: A1. Add 750-800 mL of 9-10 mol / L sodium hydroxide solution to a three-necked flask, then add 18-20 g of nano-titanium dioxide and stir to obtain a mixture; A2, heating the mixture obtained in A1 to reflux to obtain a reactant; A3. Centrifuge the reaction product obtained in A2, wash it with distilled water until it becomes neutral, and dry it to obtain modified titanium dioxide.

2. The magnesium alloy die casting according to claim 1, characterized in that: In A1, the stirring speed is 300-360 r / min and the stirring time is 15-20 min.

3. The magnesium alloy die casting according to claim 1, characterized in that: In A2, the mixture was heated to 100°C and refluxed at 100°C for 10-12 h.

4. The magnesium alloy die casting according to claim 1, characterized in that In A3, the drying temperature is 96-100°C and the drying time is 50-70 minutes.

5. The magnesium alloy die casting according to claim 1, characterized in that: The magnesium alloy matrix includes the following components in percentage by mass: 0.4%-0.42% Si, 0.05%-0.065% Al, 0.02-0.04% RE, 0.01%-0.012% Mn, 0.01%-0.015% Zn and 0.009%-0.011% Ca, with the remainder being Mg and unavoidable impurities; the RE includes La and / or Ce.

6. A method for preparing a magnesium alloy die casting according to any one of claims 1 to 5, characterized in that: The following steps are involved: S1. Weighing raw materials, melting, casting, and melt die-casting to obtain a magnesium alloy matrix; S2. polishing and grinding the surface of the magnesium alloy substrate, ultrasonically cleaning, and drying to obtain a pretreated substrate; S3, mixing nickel powder and modified titanium dioxide, ball milling for 3.5-4.5 hours to achieve uniform dispersion, and then drying in a vacuum oven at 80±2°C for 11-12 hours to obtain a composite powder; S4. Using plasma spraying, the composite powder obtained in S3 is sprayed on the pretreated substrate obtained in S2, and then placed in a sintering furnace for vacuum sintering to obtain a magnesium alloy die-casting.

7. The method for preparing a magnesium alloy die casting according to claim 6, wherein: In S4, the plasma spraying power is controlled at 35-45kW, the working gas is a mixture of argon and hydrogen, the spraying distance is 90-100mm, the argon flow rate is 42-47L / min, the hydrogen flow rate is 5-6L / min, the powder feeding amount is 30-40g / min, and the preheating temperature is 120-140℃.

8. The method for preparing a magnesium alloy die casting according to claim 6, wherein: In S4, the temperature in the sintering furnace is raised to 410-460°C at a rate of 10°C / min and kept at this temperature for 0.6-1h. The vacuum degree in the sintering furnace is less than 1×10 -2 Pa.

9. An application of the magnesium alloy die casting according to any one of claims 1 to 5, characterized in that: Used in shock absorbers of electric tricycles or electric two-wheelers.

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