Preparation method of heat treatment free high strength and toughness AZ91 as-cast magnesium alloy

Abstract

The application discloses a preparation method of a heat-treatment-free high-strength and high-toughness AZ91 as-cast magnesium alloy, and belongs to the technical field of magnesium alloy material preparation, and solves the technical problem that a commercial AZ91 as-cast magnesium alloy needs to be subjected to a complicated heat treatment process to realize regulation and control of the structure and performance. The method comprises the following steps: S1, nano-grain refiner pre-attachment treatment; and S2, heat-treatment-free preparation of the high-strength and high-toughness AZ91 as-cast magnesium alloy. Through a special stirring casting process, liquid-state addition of the pre-attached nano-grain refiner, melt purification treatment, high-temperature air-cooling solidification technology, the AZ91 magnesium alloy ingot with a tensile strength greater than or equal to 200 MPa and an elongation greater than or equal to 7% is directly cast into a shape without subsequent heat treatment, and the problems of high energy consumption, coarse grains and mechanical property anisotropy of the traditional AZ91 alloy caused by T6 heat treatment are solved.

Description

Technical Field

[0001] This invention belongs to the field of magnesium alloy material preparation technology, specifically relating to a method for preparing a heat-free, high-strength and high-toughness AZ91 as-cast magnesium alloy. Background Technology

[0002] Magnesium and magnesium alloys possess characteristics such as low density, high specific strength and stiffness, high specific modulus, good corrosion resistance and dimensional stability, excellent electromagnetic shielding, and easy recyclability, making them promising for applications in aerospace, advanced weaponry, and medical devices. AZ91 magnesium alloy is an age-hardening magnesium alloy, a cast magnesium alloy with low density and high specific strength. Its main components include magnesium (Mg), aluminum (Al), and zinc (Zn), with small amounts of manganese (Mn) and other elements. Magnesium content is approximately 90%, aluminum content is between 8.3% and 9.7%, zinc content is between 0.45% and 0.9%, and manganese content is between 0.17% and 0.5%. Impurities such as silicon (Si), iron (Fe), copper (Cu), and nickel (Ni) are present in low amounts. It is widely used in transportation, aerospace, and 3C products.

[0003] Traditional AZ91 magnesium alloy ingots require T6 heat treatment (solution temperature 415℃, holding time 16h~24h; aging treatment temperature 175℃, holding time 16h) to improve performance, which suffers from high energy consumption, grain growth, and susceptibility to deformation and cracking. Existing technologies address these issues by adding rare earth elements or using rapid solidification techniques, which can partially eliminate the adverse effects of heat treatment, but these methods are costly, complex, and difficult to scale up. Therefore, a method for preparing high-strength and high-toughness AZ91 as-cast magnesium alloys without heat treatment is urgently needed. Summary of the Invention

[0004] The main objective of this invention is to overcome the shortcomings of the prior art and solve the technical problem that commercial AZ91 cast magnesium alloys require complex heat treatment processes to control their microstructure and properties. This invention provides a method for preparing high-strength and high-toughness AZ91 cast magnesium alloys without heat treatment.

[0005] This invention is achieved through the following technical solution: a method for preparing a heat-toughened high-strength and high-toughness AZ91 as-cast magnesium alloy, comprising the following steps: S1, Pre-attachment treatment of nanocrystal refiner: This step S1 is used to ensure that the nanocrystal refiner is uniformly dispersed in the melt. S1-1. Weigh the nano-crystal refiner and anhydrous ethanol according to the weight-volume ratio of 1g:(500mL~600mL). The nano-crystal refiner is spherical Fe3O4 particles. Mix the nano-crystal refiner and anhydrous ethanol evenly with electromagnetic stirring at a speed of 100r / min to obtain a suspension. S1-2. Add a solvent-based dispersant to the suspension prepared in step S1-1. The weight ratio of the solvent-based dispersant to the nanocrystal refiner is 1:(38~42). Stir continuously during the addition process to prevent the nanocrystal refiner from settling, and a mixture is obtained. S1-3. Add AZ91 magnesium alloy scrap to the mixture prepared in step S1-2. The weight ratio of AZ91 magnesium alloy scrap to nano-grain refiner is 100:1. The mixture is subjected to two ultrasonic treatments in sequence during continuous electromagnetic stirring. Each ultrasonic treatment lasts for 20 minutes and the time interval between the two ultrasonic treatments is 10 mm. The electromagnetic stirring speed is not less than 100 r / min and the ultrasonic power of the ultrasonic treatment is 250 W~300 W. The nano-grain refiner is then attached to the surface of the turning scrap. S1-4. The mixture prepared in step S1-3 is alternately filtered and washed with anhydrous ethanol at least twice. Finally, it is vacuum dried at a low temperature of 30℃~40℃ to complete the pre-attachment treatment of the nanocrystal refiner and obtain the pre-attached nanocrystal refiner for later use. S2. Preparation of high-strength and high-toughness AZ91 cast magnesium alloy without heat treatment: S2-1. Place the clean pure magnesium ingot with the surface oxide scale removed into a melting crucible and heat it to melt in a protective atmosphere. Then, according to the chemical composition and proportion of the high-strength and high-toughness AZ91 cast magnesium alloy, slowly add alloying elements and hold the temperature until all the alloy melts. Remove the slag from the surface of the melt. S2-2. After coating the surface of the mixer with fossil powder and zinc oxide slurry, dry it for later use. Preheat the casting mold to 450℃~500℃ for later use. Preheat the pre-attached nanocrystal refiner prepared in step S1 to 200℃ for later use. S2-3. Control the heating temperature of the alloy liquid to 720±2℃. Insert the stirrer after the pretreatment in step S2-2 into the alloy liquid 5-8cm below the liquid surface. Start the stirrer to start mechanical stirring in a single rotation direction. The speed of the stirrer should not be less than 600rpm. S2-4. During the mechanical stirring process of the stirrer in step S2-3, the pre-attached nanocrystalline grain refiner after preheating in step S2-2 is slowly added to the alloy liquid in small amounts and multiple times. The weight of the added pre-attached nanocrystalline grain refiner accounts for 0.05%-0.1% of the total weight of the alloy raw materials. After the pre-attached nanocrystalline grain refiner is added, the stirrer is kept at the stirring speed of step S2-3, rotating forward for 15 minutes and then in reverse for 10 minutes. The uniformity of dispersion of the pre-attached nanocrystalline grain refiner benefits from the pre-attachment treatment in step S1 and the application of the stirrer. S2-5. After mechanical stirring, the melt is subjected to ultrasonic treatment with an ultrasonic power of 1700W~2200W and an ultrasonic treatment time of 12min~15min. S2-6. Quickly pour the melt after ultrasonic treatment in step S2-5 into the casting mold after preheating in step S2-2, and apply a load of 350MPa~400MPa to obtain high-strength and tough AZ91 as-cast magnesium alloy.

[0006] Furthermore, in step S1-1, the average particle size of the spherical Fe3O4 particles is 100 nm.

[0007] Furthermore, in steps S1-2, the solvent-based dispersant is HH2017.

[0008] Furthermore, in steps S1-3, the dimensions of the AZ91 magnesium alloy shavings are 2mm in length × 2mm in width × 0.5mm in thickness.

[0009] Furthermore, in step S2-1, the alloying elements slowly added are Zn and Al.

[0010] Furthermore, in step S2-1, the protective atmosphere is a mixture of CO2 and SF6, and the volume ratio of CO2 to SF6 is 40:(5~6).

[0011] Further, in step S2-2, the stirrer includes a rotating shaft, impellers, and a shearing ring. At least three impellers are evenly arranged circumferentially at the lower end of the rotating shaft, and each impeller has the same inclination angle. The outer end of each impeller is fixedly connected to the inner wall of the shearing ring. Sawtooth teeth for shearing are provided on the upper and lower sides of the center line of the shearing ring, and these teeth are staggered. The impellers can drive the melt to flow rapidly upwards along the inner wall of the melting crucible, increasing the convection of the upper and lower layers of molten material and facilitating the vertical distribution of the pre-attached nanocrystalline grain refiner. Furthermore, the shearing ring provides lateral shearing force during rotation, breaking up agglomerated particles. In summary, under the high-speed rotation of the stirrer, the impellers and shearing ring work together to achieve a uniform distribution of the nanocrystalline grain refiner.

[0012] The beneficial effects of this invention are as follows: This invention uses a special stirring casting process, liquid addition of pre-attached nano-grain refiner, melt purification treatment, and high-temperature air-cooling solidification technology to directly cast the AZ91 magnesium alloy ingot without subsequent heat treatment, thereby obtaining an AZ91 magnesium alloy ingot with a tensile strength ≥220MPa and an elongation ≥8%. This solves the problems of high energy consumption, coarse grains, and anisotropy of mechanical properties caused by the traditional AZ91 alloy requiring T6 heat treatment. Attached Figure Description

[0013] Figure 1 OM image of the as-cast microstructure of AZ91 magnesium alloy ingot prepared in Example 1; Figure 2 SEM image of the as-cast microstructure of AZ91 magnesium alloy ingot prepared in Example 1; Figure 3 OM image of the as-cast microstructure of commercial AZ91 magnesium alloy ingot; Figure 4 This is a comparison of the mechanical property curves of AZ91 magnesium alloy ingots prepared in Examples 1 to 4 and commercially available as-cast AZ91 magnesium alloy ingots; Figure 5 This is a schematic diagram of the three-dimensional structure of the agitator; in the diagram, 1 is the rotating shaft, 2 is the impeller, 3 is the shearing ring, and 4 is the serration. Detailed Implementation

[0014] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments. Example 1

[0015] A method for preparing a heat-toughening-free high-strength and high-toughness AZ91 as-cast magnesium alloy includes the following steps: S1. Pre-adhesion treatment with nanocrystal refiner: S1-1. The nanocrystal refiner is spherical Fe3O4 particles with an average particle size of 100nm. Weigh 1g of the nanocrystal refiner and add it to 550mL of anhydrous ethanol. Mix it evenly with electromagnetic stirring at a speed of 100r / min to obtain a suspension. S1-2. Add a solvent-based dispersant, HH2017, to the suspension prepared in step S1-1 (HH2017 was purchased from Guangzhou Houhuan Chemical Additives Co., Ltd.). This is a solvent-based dispersion suitable for medium to high polarity systems. Agent The weight ratio of solvent-based dispersant to nanocrystal refiner is 1:40. During the addition process, the mixture is continuously stirred to prevent the nanocrystal refiner from settling, thus obtaining a mixed solution. S1-3. Add AZ91 magnesium alloy shavings to the mixture prepared in step S1-2. The dimensions of the AZ91 magnesium alloy shavings are 2mm in length × 2mm in width × 0.5mm in thickness. The weight ratio of AZ91 magnesium alloy shavings to nano-grain refiner is 100:1. The mixture is subjected to two ultrasonic treatments in sequence during continuous electromagnetic stirring. The duration of each ultrasonic treatment is 20min, and the time interval between the two ultrasonic treatments is 10mm. The speed of electromagnetic stirring is not less than 100r / min, and the ultrasonic power of ultrasonic treatment is 250W. The nano-grain refiner is then attached to the surface of the turning shavings. S1-4. The mixture prepared in step S1-3 is alternately filtered and washed with anhydrous ethanol at least twice. Finally, it is vacuum dried at a low temperature of 30°C to complete the pre-attachment treatment of the nanocrystal refiner and obtain the pre-attached nanocrystal refiner for later use. S2. Preparation of high-strength and high-toughness AZ91 cast magnesium alloy without heat treatment: S2-1. Place the clean pure magnesium ingot with the surface oxide scale removed into a melting crucible and heat it to melt in a protective atmosphere, which is a mixture of CO2 and SF6 with a volume ratio of CO2 to SF6 of 40:5. Then, according to the chemical composition and proportion of the high-strength and high-toughness AZ91 cast magnesium alloy, slowly add alloying elements (Zn and Al elements), hold the temperature until all the alloy melts, and remove the slag from the surface of the melt. S2-2. After coating the surface of the stirrer with fossil powder and zinc oxide slurry, dry it for later use. Preheat the casting mold to 500℃ for later use. Preheat the pre-attached nanocrystal refiner prepared in step S1 to 200℃ for later use. Figure 5 As shown, the stirrer includes a rotating shaft 1, blades 2, and a shearing ring 3. At least three blades 2 are evenly arranged at the lower end of the rotating shaft 1 along the circumferential direction, and each blade 2 has the same inclination angle. The outer end of each blade 2 is fixedly connected to the inner wall of the shearing ring 3. The upper and lower sides of the center line of the shearing ring 3 are respectively provided with serrations 4 for shearing, and the serrations 4 on the upper and lower sides are staggered. S2-3. Control the heating temperature of the alloy liquid to 720±2℃. Insert the stirrer after the pretreatment in step S2-2 into the alloy liquid 7cm below the liquid surface. Start the stirrer to start mechanical stirring in a single rotation direction. The speed of the stirrer should not be less than 600rpm. S2-4. During the mechanical stirring process of the stirrer in step S2-3, the pre-attached nano-grain refiner that was preheated in step S2-2 is slowly added to the alloy liquid in small amounts and multiple times. The weight of the added pre-attached nano-grain refiner accounts for 0.05% of the total weight of the alloy raw materials. After the pre-attached nano-grain refiner is added, the stirrer is kept at the speed of the stirrer in step S2-3, rotating forward for 15 minutes and then in reverse for 10 minutes. S2-5. After mechanical stirring, the melt is subjected to ultrasonic treatment with an ultrasonic power of 1800W and an ultrasonic treatment time of 15min. S2-6. The melt after ultrasonic treatment in step S2-5 is quickly poured into the casting mold after preheating in step S2-2, and a load of 400MPa is applied to obtain high-strength and high-toughness AZ91 as-cast magnesium alloy.

[0016] The high-strength and high-toughness AZ91 as-cast magnesium alloy prepared in Example 1 has a tensile strength ≥200MPa and an elongation ≥7%. Its as-cast microstructure OM image is shown below. Figure 1 As shown, the SEM image of the as-cast structure is as follows: Figure 2 As shown, combined with Figure 1 and Figure 2 It can be seen that the prepared AZ91 alloy does not have a network or continuous second phase, and a large number of uniformly distributed heterogeneous particles appear in the microstructure, indicating that the grain refiner is uniformly dispersed.

[0017] Commercially available as-cast AZ91 magnesium alloy was selected (OM photo as shown). Figure 3 As shown in the figure, compared with the high-strength and high-toughness AZ91 as-cast magnesium alloy prepared in Example 1, it can be seen that no second phase is generated in the as-cast microstructure of the AZ91 ingot prepared in Example 1, proving that the process of the present invention can eliminate the need for heat treatment process after solidification of AZ91 ingot. Example 2

[0018] A method for preparing a heat-free high-strength and high-toughness AZ91 cast magnesium alloy. The only difference between this Example 2 and Example 1 is that the ultrasonic power in steps S1-3 is 300W. The remaining steps and process parameters are the same as in Example 1, and will not be repeated here. Example 3

[0019] A method for preparing a heat-free high-strength and high-toughness AZ91 cast magnesium alloy. The only difference between this Example 3 and Examples 1 and 2 is that the preheating temperature of the casting mold in step S2-2 is 450℃. The other steps and process parameters are the same as in Example 1, and will not be repeated here. Example 4

[0020] A method for preparing a heat-free high-strength and high-toughness AZ91 cast magnesium alloy. The only difference between this Example 4 and Example 1 is that steps S1, S2-4 and S2-5 are deleted. The remaining steps and process parameters are the same as in Example 1, and will not be repeated here.

[0021] The comparison curves of the mechanical properties of the AZ91 magnesium alloy ingots prepared in Examples 1 to 4 above with those of commercially available as-cast AZ91 magnesium alloys are shown in the figure below. Figure 4 As shown, by Figure 4It can be seen that the present invention can significantly improve the strength and toughness of AZ91 as-cast magnesium alloy, and the added pre-attached nano-grain refiner can further improve the strength and toughness of AZ91 magnesium alloy in the as-cast state, providing technical support for the further application of commercial AZ91.

[0022] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing a heat-toughened, high-strength, high-toughness AZ91 as-cast magnesium alloy, characterized in that, Includes the following steps: S1. Pre-adhesion treatment with nanocrystal refiner: S1-1. Weigh the nano-crystal refiner and anhydrous ethanol according to the weight-volume ratio of 1g:(500mL~600mL). The nano-crystal refiner is spherical Fe3O4 particles. Mix the nano-crystal refiner and anhydrous ethanol evenly with electromagnetic stirring at a speed of 100r / min to obtain a suspension. S1-2. Add a solvent-based dispersant to the suspension prepared in step S1-1. The weight ratio of the solvent-based dispersant to the nanocrystal refiner is 1:(38~42). Stir continuously during the addition process to prevent the nanocrystal refiner from settling, and a mixture is obtained. S1-3. Add AZ91 magnesium alloy scrap to the mixture prepared in step S1-2. The weight ratio of AZ91 magnesium alloy scrap to nano-grain refiner is 100:

1. The mixture is subjected to two ultrasonic treatments in sequence during continuous electromagnetic stirring. Each ultrasonic treatment lasts for 20 minutes and the time interval between the two ultrasonic treatments is 10 mm. The electromagnetic stirring speed is not less than 100 r / min and the ultrasonic power of the ultrasonic treatment is 250 W~300 W. The nano-grain refiner is then attached to the surface of the turning scrap. S1-4. The mixture prepared in step S1-3 is alternately filtered and washed with anhydrous ethanol at least twice. Finally, it is vacuum dried at a low temperature of 30℃~40℃ to complete the pre-attachment treatment of the nanocrystal refiner and obtain the pre-attached nanocrystal refiner for later use. S2. Preparation of high-strength and high-toughness AZ91 cast magnesium alloy without heat treatment: S2-1. Place the clean pure magnesium ingot with the surface oxide scale removed into a melting crucible and heat it to melt in a protective atmosphere. Then, according to the chemical composition and proportion of the high-strength and high-toughness AZ91 cast magnesium alloy, slowly add alloying elements and hold the temperature until all the alloy melts. Remove the slag from the surface of the melt. S2-2. After coating the surface of the mixer with fossil powder and zinc oxide slurry, dry it for later use. Preheat the casting mold to 450℃~500℃ for later use. Preheat the pre-attached nanocrystal refiner prepared in step S1 to 200℃ for later use. S2-3. Control the heating temperature of the alloy liquid to 720±2℃. Insert the stirrer after the pretreatment in step S2-2 into the alloy liquid 5-8cm below the liquid surface. Start the stirrer to start mechanical stirring in a single rotation direction. The speed of the stirrer should not be less than 600rpm. S2-4. During the mechanical stirring process of the stirrer in step S2-3, the pre-attached nano-grain refiner preheated in step S2-2 is slowly added to the alloy liquid in small amounts and multiple times. The weight of the added pre-attached nano-grain refiner accounts for 0.05%-0.1% of the total weight of the alloy raw materials. After the pre-attached nano-grain refiner is added, the stirrer is kept rotating forward for 15 minutes and then reversed for 10 minutes. S2-5. After mechanical stirring, the melt is subjected to ultrasonic treatment with an ultrasonic power of 1700W~2200W and an ultrasonic treatment time of 12min~15min. S2-6. Quickly pour the melt after ultrasonic treatment in step S2-5 into the casting mold after preheating in step S2-2, and apply a load of 350MPa~400MPa to obtain high-strength and tough AZ91 as-cast magnesium alloy.

2. The method for preparing a heat-toughened high-strength and high-toughness AZ91 as-cast magnesium alloy according to claim 1, characterized in that, In step S1-1, the average particle size of the spherical Fe3O4 particles is 100 nm.

3. The method for preparing a heat-toughened high-strength and high-toughness AZ91 as-cast magnesium alloy according to claim 1, characterized in that, In steps S1-2, the solvent-based dispersant is HH2017.

4. The method for preparing a heat-toughened high-strength and high-toughness AZ91 as-cast magnesium alloy according to claim 1, characterized in that, In steps S1-3, the dimensions of the AZ91 magnesium alloy shavings are 2mm in length × 2mm in width × 0.5mm in thickness.

5. The method for preparing a heat-toughened high-strength and high-toughness AZ91 as-cast magnesium alloy according to claim 1, characterized in that, In step S2-1, the alloying elements added slowly are Zn and Al.

6. The method for preparing a heat-toughened high-strength and high-toughness AZ91 as-cast magnesium alloy according to claim 1, characterized in that, In step S2-1, the protective atmosphere is a mixture of CO2 and SF6, and the volume ratio of CO2 to SF6 is 40:(5~6).

7. The method for preparing a heat-toughened high-strength and high-toughness AZ91 as-cast magnesium alloy according to claim 1, characterized in that, In step S2-2, the stirrer includes a rotating shaft, blades, and a shearing ring. At least three blades are evenly arranged at the lower end of the rotating shaft along the circumferential direction, and each blade has the same inclination angle. The outer end of each blade is fixedly connected to the inner wall of the shearing ring. Saw teeth for shearing are provided on the upper and lower sides at the center line of the shearing ring, and the saw teeth on the upper and lower sides are staggered.

Images