Magnesium alloy grain boundary overburning structure and mechanical property repairing method
By combining controlled slow cooling, heating and remelting with surface pretreatment and hot deformation compaction, the grain boundary overheating caused by improper temperature control was repaired, solving the problem of decreased alloy mechanical properties, realizing the restoration of alloy properties and saving resources, and promoting the industrial application of high-strength and high-toughness magnesium alloys.
Patent Information
- Application Number
- CN202511080841.2
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-08-04
- Publication Date
- 2025-11-18
AI Technical Summary
Existing technologies are insufficient to effectively address the grain boundary overheating problem caused by improper temperature control during the heat treatment of Mg-RE-Zn alloys. This leads to a decline in the alloy's mechanical properties, which cannot be repaired and usually requires scrapping and remelting, resulting in waste of rare earth elements and increased production costs.
By combining controlled slow cooling, heating and remelting with surface pretreatment and hot deformation compaction, grain boundary overheating caused by improper temperature control is repaired, good bonding between the grain boundary phase and the matrix is rebuilt, grains are refined, and the mechanical properties of the alloy are restored.
This effectively avoids the remelting process of alloys that have been overheated at grain boundaries, saves raw materials and resources, improves production efficiency, and provides technical support for the industrial application of high-strength and high-toughness magnesium alloys.
Smart Images
Figure CN120967267A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of heat treatment technology for metallic materials, specifically providing a method for repairing the microstructure of Mg-RE-Zn alloys that have suffered grain boundary overburning due to overheating during homogenization heat treatment, thereby restoring the alloy's machinability and avoiding the scrapping and remelting of ingots. This method is applicable to the industrial production of lightweight alloy components for aerospace, rail transportation, and other applications. Background Technology
[0002] Magnesium alloys, as the lightest metallic structural materials, possess advantages such as low density, high specific strength / stiffness, and good damping properties. However, their low intrinsic strength and poor plasticity limit their application in load-bearing components. To meet the demand for high-performance lightweight materials in these fields, the research and application of high-strength and high-toughness rare-earth magnesium alloys (Mg-RE-Zn) have attracted widespread attention. Studies have shown that by introducing a long-period stacking ordered phase (LPSO phase), Mg-RE-Zn alloys exhibit significant potential for improving strength and plasticity. By rationally controlling the ratio of Zn to RE elements (such as Y, Gd, etc.) and the heat treatment process, LPSO phases can be formed at grain boundaries and within grains. This phase possesses high hardness, high elastic modulus, high thermal stability, and a coherent interface with the α-Mg matrix. It can not only effectively hinder dislocation movement and refine grains but also coordinate plastic deformation through lamellar twisting, achieving a synergistic improvement in strength and plasticity. This provides a key approach for developing high-strength deformable magnesium alloys with excellent comprehensive performance.
[0003] The formation and evolution of the LPSO phase are highly dependent on the heat treatment process. In as-cast Mg-RE-Zn alloys, eutectic phases (such as (Mg, Zn)3RE and Mg5RE) are usually present at grain boundaries. One of the core objectives of homogenization heat treatment is to promote the re-dissolution of these grain boundary eutectic phases and control the precipitation morphology (massive, lamellar) and distribution of the LPSO phase through subsequent cooling or aging. Although increasing the homogenization temperature can accelerate the dissolution of the eutectic phase and increase the solute supersaturation (which is beneficial for subsequent precipitation strengthening), furnace temperature fluctuations (±10~15℃) can easily trigger the melting of local low-melting-point second phases at grain boundaries, causing overheating (manifested as triangular grain boundaries, grain boundary widening, re-dissolution spheres, etc.), leading to irreversible deterioration of the material's mechanical properties.
[0004] This contradiction forces companies to generally adopt conservative processes (such as lowering the temperature or shortening the time). While this avoids the risk of overheating, it sacrifices the degree of eutectic phase re-dissolution and solute supersaturation, indirectly weakening the synergistic strengthening and toughening effect of LPSO phase and precipitate phase (such as β' phase) during subsequent aging. Studies have shown that the ideal microstructure of high-strength and high-toughness Mg-RE-Zn alloys must simultaneously contain: 1) fine lamellar LPSO phase within the grains (providing strengthening); 2) blocky / network LPSO phase at grain boundaries (coordinating plasticity); and 3) dispersed coherent precipitate phase in the matrix (age-hardening). This synergistic effect of multi-scale and multi-type strengthening phases is key to achieving both high strength and good plasticity (e.g., tensile strength > 400 MPa, elongation > 10%) in the alloy. Conservative processes struggle to fully activate this synergistic effect, severely limiting the upper limit of alloy performance.
[0005] Even more serious is the lack of effective repair methods in traditional technologies once grain boundary overheating occurs. Overheated alloys are prone to cracking during hot working and suffer a significant decline in mechanical properties due to grain boundary damage, usually requiring scrapping and remelting. This not only causes secondary losses of rare earth elements (REs are expensive and scarce) and energy waste, but also significantly increases production costs, becoming a major obstacle to industrial-scale application. Although existing technologies (such as multi-stage homogenization) attempt to prevent overheating, they cannot repair damaged structures, nor can they overcome the solid solution temperature limit to maximize solute supersaturation.
[0006] Therefore, a method for repairing overburnt grain boundary microstructure in Mg-RE-Zn alloys should be developed.
[0007] This is of great significance for unleashing the performance potential of Mg-RE-Zn alloys and promoting their industrial application in high-value-added fields. Summary of the Invention
[0008] The purpose of this invention is to provide a method for repairing the overheated microstructure and mechanical properties of magnesium alloy grain boundaries, thereby eliminating the cost of remelting overheated alloys, improving production efficiency and resource utilization, effectively salvaging overheated ingots or billets caused by improper temperature control, and providing technical support and safety margin for exploring heat treatment processes for high solid solubility magnesium alloys.
[0009] This invention is mainly aimed at Mg-Gd-Y-Zn alloys, but its applicability can be extended to other rare earth magnesium alloys containing LPSO phase. The alloy composition range is: Gd: 0-12wt.%, Y: 0-8wt.%, Zn: 1-3wt.%, Zr: 0.3-0.7wt.%, with the balance being Mg; Gd and Y are not simultaneously zero.
[0010] During homogenization heat treatment or other high-temperature heat treatment, the alloy may experience grain boundary overheating due to temperature control deviations causing the actual temperature to exceed the melting point of the low-melting-point second phase at the grain boundary.
[0011] The first aspect of this invention is to provide a heat treatment repair method for magnesium alloy ingots / bills in different initial overheated states: State A: The grain boundary phase overheats during high-temperature heat treatment and remains at high temperature. A method for repairing the overheated microstructure and mechanical properties of magnesium alloy grain boundaries, comprising the following steps: 1. Controllable slow cooling The alloy is cooled to 420°C to room temperature at a rate of 0.5°C / min to 5°C / min. This step aims to avoid overheating and melting of the grain boundary phase due to rapid cooling, which can cause shrinkage and cracking, forming pores or looseness. At the same time, it is beneficial to the diffusion and re-dissolution of grain boundary elements. 2. High-temperature repair The overheated alloy is reheated to 450~520℃ and held for 4~48 hours. The core purpose of this step is to allow the solidified non-equilibrium phase at the grain boundaries to undergo sufficient re-dissolution and transformation, and to rebuild the good bond between the grain boundary phase and the matrix. 3. Cooling After the heat preservation is completed, the material is cooled to room temperature using methods such as air cooling or furnace cooling.
[0012] State B: Alloy that has undergone grain boundary overheating and has been cooled to room temperature. A method for repairing the overheated microstructure and mechanical properties of magnesium alloy grain boundaries, comprising the following steps: 1. High-temperature repair The overheated alloy is reheated to 450~520℃ and held for 4~48 hours. The core purpose of this step is to allow the solidified non-equilibrium phase at the grain boundaries to undergo sufficient re-dissolution and transformation, and to rebuild the good bond between the grain boundary phase and the matrix. 2. Cooling After the heat preservation is completed, the material is cooled to room temperature using methods such as air cooling or furnace cooling.
[0013] The second aspect of the present invention is to provide a post-treatment method for overheated alloy ingots that have undergone heat treatment repair as described in the first aspect, in order to further restore their density and make them meet the requirements of subsequent processing or use, comprising the following steps: 1. Surface pretreatment: The alloy surface after repair heat treatment is machined, ground, or pickled to thoroughly remove the severe oxide layer and powdery parts caused by overheating; 2. Hot deformation compaction and microstructure refinement: The alloy is processed and deformed by hot forging / extrusion; the main purpose is to compact the microstructure, eliminate potential micro-porosity, break up coarse grain boundary phases, and refine the grains. Beneficial effects
[0014] This invention patent breaks through the traditional understanding that overheating is irreversible and provides a method for repairing the microstructure of overheated Mg-Gd-Y-Zn alloys, thereby avoiding the need for remelting of overheated alloys, effectively saving raw materials and resource consumption, and providing technical support and safety margin for exploring high solid solution heat treatment processes for magnesium alloys. It is applicable to the industrial production of lightweight alloy components in aerospace, rail transportation and other fields.
[0015] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but this does not imply any limitation on the scope of protection of the present invention. Attached Figure Description
[0016] Figure 1 The DSC curve of the as-cast alloy in Example 1 of this invention; Figure 2 These are typical metallographic and scanning microstructure images of the overburned magnesium alloy microstructure in Example 1 of this invention; Figure 3 These are typical photographs of the microstructure of the overburned magnesium alloy after heat treatment repair in Example 1 of this invention; Figure 4 The microstructure and mechanical properties of the overburned magnesium alloy after repair and subsequent hot extrusion in Example 1 of this invention. Detailed Implementation
[0017] Unless otherwise specified, the raw materials, additives or equipment used in the embodiments of the present invention are all commercially available products, and their models are conventional models in the art; the test methods used are all conventional methods in the art.
[0018] Example 1: The alloy composition in Example 1 is Mg-8Gd-4Y-1.6Zn-0.3Zr (wt.%). The grain boundary low melting point phase temperature of this cast alloy is 523℃. The normal industrial homogenization heat treatment temperature is 495~515℃. During this heat treatment, the actual temperature rose to 535℃ due to a temperature control failure. When the failure was discovered, the heat treatment had already been carried out at this temperature for 2.5 hours. For this alloy condition, a method for repairing the overheated microstructure and mechanical properties of magnesium alloy grain boundaries is provided, with the specific steps as follows: (1) Furnace cooling Turn off the heat treatment furnace and allow the alloy to cool to room temperature at a rate of 1°C / min; (2) High-temperature repair The alloy was heated to 520°C and held for 4 hours, then air-cooled to room temperature (~10°C). (3) Surface treatment Grind and clean the alloy surface to remove severely oxidized areas; (4) Hot extrusion compaction The alloy is deformed by hot extrusion, which compacts the structure and refines the grains to meet the requirements of subsequent deformation or use. The extrusion temperature is 450℃, the extrusion ratio is 25:1, and the extrusion rate is 1mm / min.
[0019] like Figure 1 The figure shows the DSC curve of the cast alloy in Example 1 of the present invention, which indicates that the melting point of the low melting point phase (analyzed as LPSO phase) is 523℃.
[0020] like Figure 2 The images shown are typical metallographic and scanning electron microscope (SEM) images of the overheated magnesium alloy structure in Example 1 of the present invention; wherein, (a) is a metallographic image of the overheated alloy structure, the microscope model is ZEISS AXIOVERT 2000MAT OM, and the shooting conditions are room temperature; (b) is a scanning electron microscope (SEM) image of the overheated alloy structure, the SEM model is JSM-7900 F SEM, and the shooting conditions are room temperature. like Figure 3 The image shown is a metallographic photograph of the microstructure of the overburned magnesium alloy after heat treatment repair in Example 1 of the present invention. It shows that the overburned microstructure has been completely restored, and the second phase microstructure is a typical blocky and layered LPSO phase. like Figure 4 As shown, this paper presents the microstructure and mechanical properties of the overburned magnesium alloy after repair and subsequent hot extrusion in Example 1 of this invention. Among them, (a) is a metallographic photograph of the hot extrusion microstructure of the alloy, taken with a ZEISS AXIOVERT 2000MAT OM microscope at room temperature; (b) shows the mechanical properties of the alloy extruded sheet in the transverse and longitudinal directions. The transverse yield strength, tensile strength and elongation of the alloy are 220MPa, 317MPa and 19.8% respectively, and the longitudinal yield strength, tensile strength and elongation are 253MPa, 343MPa and 20.7% respectively. Under this condition, after further aging treatment, the strength of the alloy can reach 450MPa and the elongation is over 10%, which is comparable to the current high-strength and high-toughness Mg-RE-Zn alloy. Example 2
[0021] The alloy composition in Example 2 is Mg-3Gd-8Y-1Zn-0.7Zr (wt.%). The normal industrial homogenization heat treatment temperature for this alloy is 495~520℃. During this heat treatment, the actual temperature rose to 540℃ due to a temperature control failure. When the failure was discovered, the alloy had already been heat-treated at that temperature for 2 hours. For this alloy condition, a method for repairing the overheated microstructure and mechanical properties of magnesium alloy grain boundaries is provided, with the specific steps as follows: (1) Furnace cooling Adjust the heating power of the heat treatment furnace to cool the alloy to room temperature at a rate of 5℃ / min; (2) High-temperature repair The alloy was heated to 450°C and held for 48 hours, then air-cooled to room temperature (~25°C). (3) Surface treatment Grind and clean the alloy surface to remove severely oxidized areas; (4) Hot forging and compaction The alloy is forged to compact the structure and refine the grains to meet the requirements of subsequent deformation or use. The specific forging process is as follows: two forging passes are performed at 450℃, with the forging direction of the second pass perpendicular to the first pass, and the reduction of the pass is 40%. Example 3
[0022] The alloy composition in Example 3 is Mg-6Y-1.5Zn-0.7Zr (wt.%). The normal industrial homogenization heat treatment temperature for this alloy is 495~520℃. During this heat treatment, the actual temperature rose to 540℃ due to a temperature control failure. When the failure was discovered, the alloy had already been heat-treated at this temperature for 4 hours. For this alloy condition, a method for repairing the overheated microstructure and mechanical properties of magnesium alloy grain boundaries is provided, with the specific steps as follows: (1) Furnace cooling Adjust the heating power of the heat treatment furnace to cool the alloy to 420°C at a rate of 0.5°C / min; (2) High-temperature repair The alloy was heated to 500°C and held for 24 hours, then cooled to room temperature in the furnace. (3) Surface treatment Grind and clean the alloy surface to remove severely oxidized areas; (4) Hot extrusion compaction The alloy is deformed by hot extrusion, which compacts the structure and refines the grains to meet the requirements of subsequent deformation or use. The extrusion temperature is 420℃, the extrusion ratio is 24:1, and the extrusion rate is 1.5mm / min. Example 4
[0023] The alloy composition in Example 4 is Mg-12Gd-2Y-2Zn-0.5Zr (wt.%). After the alloy homogenization heat treatment, it was found that the grain boundary phase was severely overburned, but the grains were intact, the connection between the grain boundary phase and the matrix phase was good, and there were no obvious pores or looseness. For this type of alloy, a method for repairing the overheated microstructure and mechanical properties of magnesium alloy grain boundaries is provided, and the specific steps are as follows: (1) High temperature repair The alloy was heated to 510℃, held for 24 hours, and then cooled to room temperature in the furnace at a cooling rate of ~2℃ / min. (2) Surface treatment The alloy surface is polished and acid-washed to remove severely oxidized areas, especially the powdery parts. (3) Hot forging and compaction The alloy is forged to compact the structure and refine the grains to meet the requirements of subsequent deformation or use. The specific forging process is as follows: four passes of multi-directional forging are carried out at 500℃, with a reduction of 50% per pass. After each pass, the alloy is flipped to ensure that different directions are compacted. The forging direction of the last pass is consistent with that of the first pass. Example 5
[0024] The alloy composition in Example 5 is Mg-5Gd-8Y-3Zn-0.6Zr (wt.%). During homogenization, due to equipment failure, it was overheated to 540°C and held at that temperature for 1.5 hours. Then it was cooled to room temperature with the furnace. Analysis of the microstructure revealed that the grain boundary phase was severely overheated, but the grains were intact. The connection between the grain boundary phase and the matrix phase was good, and there were no obvious pores or looseness. For this type of alloy, a method for repairing the overheated microstructure and mechanical properties of magnesium alloy grain boundaries is provided, and the specific steps are as follows: (1) High temperature repair Heat to 450℃, hold for 48 hours, and then cool to room temperature with the furnace at a cooling rate of ~1℃ / min; (2) Surface treatment The surface of the billet is machined to remove a 2mm surface oxide layer, and the severely oxidized parts are polished to remove the oxide layer. (3) Hot extrusion compaction The alloy is deformed by hot extrusion, which compacts the structure and refines the grains to meet the requirements of subsequent deformation or use. The specific extrusion temperature is 460℃, the extrusion ratio is 16:1, and the extrusion rate is 2mm / min.
[0025] This invention breaks through the traditional understanding that overheating of magnesium alloys is irreversible, and provides a method for repairing the microstructure of overheated Mg-Gd-Y-Zn alloys, thereby avoiding the need for remelting of overheated alloys, effectively saving raw materials and resource consumption, and providing technical support and safety margin for exploring high solid solution heat treatment processes for magnesium alloys.
[0026] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.
Claims
1. A method for repairing the overheated microstructure and mechanical properties of magnesium alloy grain boundaries, characterized in that, Includes the following steps: (1) Select the repair path based on the state of the overheated alloy: 1) If the alloy is above the overheating temperature, cool it to room temperature at a rate of 0.5~5℃ / min, then reheat it to 450~520℃ and hold it for 4~48 hours; 2) If the alloy has cooled to room temperature, heat it directly to 450~520℃ and hold for 4~48 hours; (2) After the heat preservation is completed, cool to room temperature; (3) Perform surface treatment on the repaired alloy surface to remove the oxide layer; (4) Perform hot deformation processing on the surface-treated alloy.
2. The method for repairing the overheated microstructure and mechanical properties of magnesium alloy grain boundaries according to claim 1, characterized in that: The hot deformation process described in step (4) is hot extrusion or hot forging.
3. The method for repairing the overheated microstructure and mechanical properties of magnesium alloy grain boundaries according to claim 1, characterized in that: In step (1) of 2), the cooling process to room temperature is carried out by furnace cooling or controlled power slow cooling.
4. The method for repairing the overheated microstructure and mechanical properties of magnesium alloy grain boundaries according to claim 1, characterized in that: In step (2), the cooling process to room temperature is carried out by furnace cooling or air cooling.
5. The method for repairing the overheated microstructure and mechanical properties of magnesium alloy grain boundaries according to claim 1, characterized in that: In step (1), the alloy is a Mg-Gd-Y-Zn alloy.
6. The method for repairing the overheated microstructure and mechanical properties of magnesium alloy grain boundaries according to claim 1, characterized in that: In step (1), the composition range of the alloy is: Gd: 0-12wt.%, Y: 0-8wt.%, Zn: 1-3wt.%, Zr: 0.3-0.7wt.%, with the balance being Mg. Gd and Y cannot be zero at the same time.
7. The method for repairing the overheated microstructure and mechanical properties of magnesium alloy grain boundaries according to claim 1, characterized in that: In step (2), the cooling is performed using either furnace cooling or air cooling.
8. The method for repairing the overheated microstructure and mechanical properties of magnesium alloy grain boundaries according to claim 1, characterized in that: In step (3), the surface treatment methods include turning, grinding or pickling.