A method for refining grains of a magnesium-aluminum alloy

Abstract

This invention relates to the field of non-ferrous metal processing technology, and in particular to a composite treatment method for refining the grain size of magnesium-aluminum alloys, comprising the following specific steps: step (1) raw material preparation; step (2) protective gas melting; step (3) two-stage refining treatment; step (4) controlled-rate cooling solidification; and step (5) homogenization annealing. The composite treatment method for refining the grain size of magnesium-aluminum alloys described in this invention combines a two-stage refining treatment with ultrasonic-assisted dispersion. Through the synergistic effect of Al-Zr inhibiting grain growth and Al-Ti-B providing heterogeneous nucleation, combined with controlled-rate cooling and homogenization annealing, high-precision refining with a grain size ≤50μm and a standard deviation ≤10μm is achieved. This fills the technical gap in the performance stability of magnesium-aluminum alloys under complex stress scenarios and solves the problems of low grain refining efficiency, poor microstructure uniformity, and insufficient mechanical properties in existing technologies.

Description

Technical Field

[0001] This invention relates to the field of non-ferrous metal processing technology, and in particular to a composite treatment method for refining the grain size of magnesium-aluminum alloys. Background Technology

[0002] Magnesium-aluminum alloys, as one of the lightest structural metal materials, are increasingly widely used in aerospace (such as drone frames), new energy vehicles (such as battery trays), and 3C products (such as laptop casings). However, the problem of grain coarsening has always constrained performance improvement: when the grain size exceeds 100μm, the tensile strength of the alloy is generally lower than 250MPa, and the elongation is less than 8%, making it difficult to meet the requirements of high-end equipment for both lightweight and high strength.

[0003] In the prior art, CN108949987A discloses a method for refining magnesium alloys using Al-Ti-B master alloys. However, single refining agents are prone to agglomeration in magnesium melts, resulting in a nucleation efficiency of only 30%-40%. Traditional processes lack precise control over the order of refining agent addition and synergistic effects. For example, CN112176348A only discloses a single-stage refining process, with a grain size standard deviation of over 20 μm and poor microstructure uniformity.

[0004] To address the above problems, this invention proposes a composite treatment method for refining the grain size of magnesium-aluminum alloys. Summary of the Invention

[0005] The main objective of this invention is to provide a composite treatment method for refining the grain size of magnesium-aluminum alloys, which can effectively solve the problems in the background art.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for refining the grain size of magnesium-aluminum alloys, characterized by the following steps: raw material preparation, protective gas melting, two-stage refining treatment, controlled-rate cooling solidification, and homogenization annealing, as follows: Step (1) Raw material preparation: Weigh magnesium ingots with a purity ≥99.9% and aluminum ingots with a purity ≥99.7% according to the target composition ratio, wherein the Al content is 4%-10% (mass fraction); Step (2) Protective gas melting: Heat the raw materials to 700-750℃ in an argon atmosphere to melt them into a homogeneous melt, and stir with a graphite stirring paddle at 100-300rpm for 5-10min to ensure that the compositional uniformity deviation is ≤1.5%; Step (3) Two-stage refining treatment: ① First refining: Add 0.3%-0.8% (mass fraction) of Al-5Zr or Al-10Zr master alloy to the melt at 720-740℃ and stir for 3-5min. ① Complete dissolution to form Zr-Mg compound particles to inhibit grain growth; ② Secondary refinement: Add 0.2%-0.6% (mass fraction) of Al-3Ti-0.2B or Al-5Ti-0.5B master alloy at 700-730℃, and simultaneously apply ultrasonic treatment at 20-40kHz and 1-3kW for 3-8min to break the primary phase through cavitation effect and promote the uniform distribution of TiB2 nucleation particles; Step (4) Controlled cooling solidification: Pour the treated melt into a metal mold preheated to 150-250℃ or a sand mold dried at 200-300℃ for 2-4h, and use circulating water to control the cooling rate at 5-20℃ / s, wherein the heat transfer coefficient of the metal mold is 500-1500W / (m²・K); Step (5) Homogenization annealing: Heat the ingot to 400-450℃, and heat at 5-15℃ / min Heat up and hold for 4-8 hours (furnace temperature fluctuation ≤ ±5℃), then cool to room temperature before removing from the furnace.

[0008] Preferably, the time interval between the two-stage refining processes is ≤15 min to ensure that the melt maintains the effective operating temperature range of the refining agent in the liquid state.

[0009] Preferably, the ultrasonic treatment breaks down the primary α-Mg phase through cavitation during the secondary refinement process, and refines the TiB2 particle size to 1-5 μm, increasing the distribution density to 10. 6 -10 7 pcs / mm³.

[0010] Preferably, during the controlled-speed cooling and solidification process, the cooling rate of the metal mold is controlled in conjunction with the circulating water flow rate (5-15 m³ / h) and the mold preheating temperature, and the sand mold achieves directional solidification by setting cooling channels.

[0011] Preferably, after homogenization annealing, the size of the second phase in the alloy is ≤2μm, and the number of divorced eutectic structures distributed along the grain boundaries is reduced by ≥80%.

[0012] Preferably, the Al-Zr master alloy is prepared by aluminothermic reduction, with a ZrO2 reduction rate ≥95% and the resulting ZrB2 particle size ≤3μm.

[0013] Preferably, the Ti / B atomic ratio in the Al-Ti-B master alloy is 3:1 to 5:1, and the lattice mismatch between the TiB2 particles and the α-Mg matrix is ​​≤6%.

[0014] Preferably, the cooling rate control is optimized by mold temperature field simulation software to ensure that the temperature gradient at the melt solidification front is ≥10℃ / mm.

[0015] Preferably, after homogenization annealing, the alloy has a tensile strength ≥280MPa, a yield strength ≥150MPa, and an elongation ≥10%.

[0016] Preferably, after the composite treatment, the average grain size of the magnesium-aluminum alloy is ≤50μm, the grain size grade is ≥5 (ASTM E112 standard), and the standard deviation of the grain size is ≤10μm.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] 1. Two-stage refinement of synergistic effect:

[0019] The Zr-Mg compounds (such as Mg3Zr) generated from the Al-Zr master alloy (Al-5Zr / Al-10Zr) during the primary refinement process act as grain growth inhibitors, reducing the grain growth rate by 40%-60% (compared to single Al-Ti-B treatment).

[0020] TiB2 particles (size 1-5 μm, distribution density 10) formed from Al-Ti-B master alloys (Al-3Ti-0.2B / Al-5Ti-0.5B) during secondary refinement. 6 -10 7 Using (number / mm³) as a highly efficient nucleation core, the nucleation rate is increased to over 75%. Combined with the ultrasonic cavitation effect to break up the primary α-Mg phase, the grain nucleation density is increased by 3 times compared to traditional processes.

[0021] 2. Enhanced precision temperature control technology:

[0022] Controlled cooling (5-20℃ / s) combined with mold pretreatment (metal mold preheating 150-250℃, sand mold drying 200-300℃) ensures that the temperature gradient at the solidification front is ≥10℃ / mm, inhibiting columnar crystal growth and achieving an equiaxed crystal ratio of over 90%.

[0023] Homogenization annealing (400-450℃×4-8h) reduces the size of the second phase to ≤2μm, decreases the eutectic structure by 80%, eliminates compositional segregation, and further refines the grains by 10%-15% through grain boundary migration.

[0024] 3. Significant performance improvement:

[0025] Average grain size ≤50μm (ASTM E112 ≥5 grade), grain size standard deviation ≤10μm, and microstructure uniformity improved by 50% compared with existing technologies;

[0026] Tensile strength ≥280MPa (20% higher than traditional process), yield strength ≥150MPa, elongation ≥10%, meeting the highest performance grade requirements of the ASTM B93 standard for aerospace magnesium alloys. Attached Figure Description

[0027] Figure 1 This is a flowchart of a composite treatment method for refining the grain size of magnesium-aluminum alloys according to the present invention;

[0028] Figure 2 This is a comparison diagram of the grain structure of a magnesium-aluminum alloy grain refinement composite treatment method according to the present invention. Detailed Implementation

[0029] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments.

[0030] A method for refining the grain size of magnesium-aluminum alloys, characterized by the following steps: raw material preparation, protective gas melting, two-stage refining treatment, controlled-rate cooling solidification, and homogenization annealing, as follows: Step (1) Raw material preparation: Weigh magnesium ingots with a purity ≥99.9% and aluminum ingots with a purity ≥99.7% according to the target composition ratio, wherein the Al content is 4%-10% (mass fraction); Step (2) Protective gas melting: Heat the raw materials to 700-750℃ in an argon atmosphere to melt them into a homogeneous melt, and stir with a graphite stirring paddle at 100-300rpm for 5-10min to ensure that the compositional uniformity deviation is ≤1.5%; Step (3) Two-stage refining treatment: ① First refining: Add 0.3%-0.8% (mass fraction) of Al-5Zr or Al-10Zr master alloy to the melt at 720-740℃ and stir for 3-5min. ① Complete dissolution to form Zr-Mg compound particles to inhibit grain growth; ② Secondary refinement: Add 0.2%-0.6% (mass fraction) of Al-3Ti-0.2B or Al-5Ti-0.5B master alloy at 700-730℃, and simultaneously apply ultrasonic treatment at 20-40kHz and 1-3kW for 3-8min to break the primary phase through cavitation effect and promote the uniform distribution of TiB2 nucleation particles; Step (4) Controlled cooling solidification: Pour the treated melt into a metal mold preheated to 150-250℃ or a sand mold dried at 200-300℃ for 2-4h, and use circulating water to control the cooling rate at 5-20℃ / s, wherein the heat transfer coefficient of the metal mold is 500-1500W / (m²・K); Step (5) Homogenization annealing: Heat the ingot to 400-450℃, and heat at 5-15℃ / min Heat up and hold for 4-8 hours (furnace temperature fluctuation ≤ ±5℃), then cool to room temperature before removing from the furnace.

[0031] Example 1 (Mg-6Al alloy, metal mold)

[0032] Step (1) Raw material preparation:

[0033] Magnesium ingots with a purity of 99.95%, aluminum ingots with a purity of 99.8%, and a total batch weight of 100 kg (Al content 6%).

[0034] Step (2) Protective gas melting:

[0035] Argon protection in the resistance furnace, melting temperature 720℃, stirring with graphite paddle at 200 rpm for 8 minutes, compositional uniformity deviation 1.2%;

[0036] Step (3) Two-stage refinement process:

[0037] First refinement: Add 0.5 kg of Al-5Zr master alloy (0.5% by mass) at 730℃ and stir for 4 min until completely dissolved;

[0038] Secondary refinement: 0.4 kg of Al-3Ti-0.2B master alloy (0.4% mass fraction) was added at 710℃, followed by ultrasonic treatment at 30 kHz / 2 kW for 5 min, resulting in a TiB2 particle distribution density of 8 × 10⁻⁶. 6 pcs / mm³;

[0039] Step (4) Controlled cooling and solidification:

[0040] Preheat the metal mold to 200℃, with a circulating water flow rate of 10m³ / h, a cooling rate of 12℃ / s, and a solidification time of 15min;

[0041] Step (5) Homogenization annealing:

[0042] The temperature is increased at a rate of 10℃ / min to 420℃, held for 6 hours (furnace temperature fluctuation ±3℃), and then cooled to room temperature with the furnace.

[0043] Test results:

[0044] Grain size: average 35 μm, standard deviation 8 μm.

[0045] Mechanical properties: tensile strength 295 MPa, yield strength 165 MPa, elongation 13%.

[0046] Tissue observation: equiaxed crystal ratio 92%, second phase size 1-2μm, and dimorphic eutectic reduced by 85%.

[0047] Example 2 (Mg-8Al alloy, sand mold)

[0048] Step (1) Raw material preparation:

[0049] Magnesium ingots with a purity of 99.92%, aluminum ingots with a purity of 99.75%, and a total batch weight of 200 kg (Al content 8%).

[0050] Step (2) Protective gas melting:

[0051] Argon protection in induction furnace, melting temperature 740℃, stirring at 300 rpm for 10 minutes with graphite paddle, composition uniformity deviation 1.0%;

[0052] Step (3) Two-stage refinement process:

[0053] First refinement: Add 1.0 kg of Al-10Zr master alloy (0.5% by mass) at 740℃ and stir for 5 min;

[0054] Secondary refinement: 0.8 kg of Al-5Ti-0.5B master alloy (0.4% mass fraction) was added at 720℃, and ultrasonic treatment was carried out at 40 kHz / 3 kW for 8 min to refine the TiB2 particle size to 3 μm;

[0055] Step (4) Controlled cooling and solidification:

[0056] Dry the sand mold (with built-in graphite cooling channel) at 250℃ for 4 hours, with a cooling rate of 20℃ / s and a solidification time of 25 minutes.

[0057] Step (5) Homogenization annealing:

[0058] Heating rate 15℃ / min to 450℃, hold for 8 hours, furnace temperature fluctuation ±5℃.

[0059] Test results:

[0060] Grain size: average 40 μm, standard deviation 9 μm.

[0061] Mechanical properties: tensile strength 310 MPa, yield strength 170 MPa, elongation 11%.

[0062] Tissue observation: TiB2 particles are uniformly distributed with a lattice mismatch of 5.5% (with the α-Mg matrix).

[0063] Comparison table of process data

[0064] process parameters Scope of the invention Comparison of existing technologies Technological advantages Method of adding finer agent Dual-stage + ultrasound-assisted Single-stage ultrasound-free The dispersibility of the finer agent is improved by 60%. Grain size (μm) ≤50 (standard deviation ≤10) 60-120 (standard deviation ≥20) Tissue uniformity improved by 50% Tensile strength (MPa) ≥280 200-250 Strength increased by 25%-40% Cooling speed control 5-20℃ / s (mold collaboration) Single water cooling (10-30℃ / s) The proportion of equiaxed crystals increased by 30%. homogenization annealing effect Different eutectic reduced by ≥80% Reduce by 50%-60% Grain boundary strength increased by 40%.

[0065] The above embodiments and data demonstrate that the present invention achieves a technological breakthrough in refining the grain size and improving the performance of magnesium-aluminum alloys through the combined effects of a two-stage grain refiner, ultrasonic dispersion strengthening, and precise temperature control. In particular, it exhibits significant stability advantages under complex stress conditions and has broad prospects for industrial applications.

[0066] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for composite treatment of magnesium-aluminum alloy grain refinement, characterized in that: The process includes the following steps: raw material preparation, protective gas melting, two-stage refining, controlled-rate cooling solidification, and homogenization annealing. Specifically: Step (1) Raw material preparation: Weigh magnesium ingots with a purity ≥99.9% and aluminum ingots with a purity ≥99.7% according to the target composition ratio, wherein the Al content is 4%-10% (mass fraction); Step (2) Protective gas melting: Heat the raw materials to 700-750℃ in an argon atmosphere to melt them into a homogeneous melt. Stir with a graphite stirring paddle at 100-300rpm for 5-10min to ensure that the compositional uniformity deviation is ≤1.5%; Step (3) Two-stage refining: ① First refining: Add 0.3%-0.8% (mass fraction) of Al-5Zr or Al-10Zr master alloy to the melt at 720-740℃ and stir for 3-5min until completely dissolved to form Zr-Mg compound particles to inhibit grain growth; ② Secondary refinement: Add 0.2%-0.6% (mass fraction) of Al-3Ti-0.2B or Al-5Ti-0.5B intermediate alloy at 700-730℃, and simultaneously apply ultrasonic treatment at 20-40kHz and 1-3kW for 3-8min to break the primary phase through cavitation effect and promote the uniform distribution of TiB2 nucleation particles; Step (4) Controlled cooling solidification: Pour the treated melt into a metal mold preheated to 150-250℃ or a sand mold dried at 200-300℃ for 2-4h, and use circulating water to control the cooling rate at 5-20℃ / s, wherein the heat transfer coefficient of the metal mold is 500-1500W / (m²・K); Step (5) Homogenization annealing: Heat the ingot to 400-450℃, increase the temperature at 5-15℃ / min, hold for 4-8h (furnace temperature fluctuation ≤±5℃), and cool to room temperature before taking it out of the furnace.

2. The method for composite grain refinement treatment of magnesium-aluminum alloy according to claim 1, characterized in that: The time interval between the two-stage refining processes is ≤15 min to ensure that the melt maintains the effective operating temperature range of the refining agent in the liquid state.

3. The method for composite grain refinement treatment of magnesium-aluminum alloy according to claim 1, characterized in that: The ultrasonic treatment, during the secondary refinement process, breaks down the primary α-Mg phase through cavitation, and refines the TiB2 particle size to 1-5 μm, increasing the distribution density to 10. 6 -10 7 pcs / mm³.

4. The method for composite grain refinement treatment of magnesium-aluminum alloy according to claim 1, characterized in that: During the controlled-speed cooling and solidification process, the cooling rate of the metal mold is controlled by the circulating water flow rate (5-15 m³ / h) and the mold preheating temperature, while the sand mold achieves directional solidification by setting cooling channels.

5. The method for composite grain refinement treatment of magnesium-aluminum alloy according to claim 1, characterized in that: After homogenization annealing, the size of the second phase in the alloy is ≤2μm, and the number of divorced eutectic structures distributed along the grain boundaries is reduced by ≥80%.

6. The method for composite grain refinement treatment of magnesium-aluminum alloy according to claim 1, characterized in that: The Al-Zr master alloy was prepared by aluminothermic reduction, with a ZrO2 reduction rate of ≥95% and the resulting ZrB2 particle size ≤3μm.

7. The method for composite treatment of magnesium-aluminum alloy grain refinement according to claim 1, characterized in that: The Ti / B atomic ratio in the Al-Ti-B master alloy is 3:1 to 5:1, and the lattice mismatch between TiB2 particles and the α-Mg matrix is ​​≤6%.

8. The method for composite treatment of magnesium-aluminum alloy grain refinement according to claim 1, characterized in that: The cooling rate control is optimized through mold temperature field simulation software to ensure that the temperature gradient at the melt solidification front is ≥10℃ / mm.

9. The method for composite grain refinement treatment of magnesium-aluminum alloy according to claim 1, characterized in that: After homogenization annealing, the alloy has a tensile strength ≥280MPa, a yield strength ≥150MPa, and an elongation ≥10%.

10. The method for composite treatment of magnesium-aluminum alloy grain refinement according to claim 1, characterized in that: After the composite treatment, the average grain size of the magnesium-aluminum alloy is ≤50μm, the grain size grade is ≥5 (ASTM E112 standard), and the standard deviation of the grain size is ≤10μm.

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