Method for synergistically improving strength and plasticity of magnesium alloy sheet

Abstract

The application belongs to the technical field of material surface strengthening and heat treatment, and provides a method for synergistically improving the strength and plasticity of a magnesium alloy plate, which comprises the following steps: providing an initial Mg-2Zn-3Li-1Gd plate sample; performing laser shock peening treatment on the plate sample; after the laser shock peening treatment is completed, performing short-time annealing treatment on the treated plate and air cooling; through laser shock peening and short-time annealing composite treatment, the yield strength, tensile strength and elongation of the Mg-2Zn-3Li-1Gd magnesium alloy plate are simultaneously improved. The application introduces laser shock to strengthen the defect structure and realizes the optimization and regulation of the defect structure and the grain boundary state through short-time annealing, so as to realize the synergistic improvement of the strength and plasticity of the Mg-2Zn-3Li-1Gd alloy plate.

Description

Technical Field

[0001] This invention relates to the field of material surface strengthening and heat treatment technology, specifically to a method for synergistically improving the strength and plasticity of magnesium alloy sheets. Background Technology

[0002] Magnesium alloys, due to their low density and high specific strength, have significant application value in lightweight structures. However, in engineering applications, magnesium alloy sheets generally face a core contradiction: strength improvement is often accompanied by plasticity loss, while improved plasticity easily leads to a decrease in strength; that is, the material properties exhibit a typical inverse relationship between strength and plasticity. This contradiction is particularly prominent under room temperature conditions, directly restricting its application in room temperature forming and service reliability.

[0003] In engineering applications, critical load-bearing components typically require materials to have a certain elongation at room temperature to ensure formability and damage tolerance, while simultaneously needing high yield and tensile strength to meet load-bearing capacity and safety margins. For magnesium alloys, how to improve strength without sacrificing ductility, or how to further improve ductility while maintaining high strength levels, has become a key technical issue in the application of lightweight structural materials. To address this issue, researchers typically conduct studies from different directions, including alloy composition design, deformation processing strengthening, grain refinement strengthening, surface strengthening, and heat treatment. However, a single approach often struggles to overcome the inherent limitation of the trade-off between strength and ductility. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, this invention proposes a method for synergistically improving the strength and plasticity of Mg-2Zn-3Li-1Gd alloy plates. This method strengthens the defect structure by introducing laser shock and optimizes the control of the defect structure and grain boundary state through short-time annealing, thereby achieving a synergistic improvement in the strength and plasticity of Mg-2Zn-3Li-1Gd alloy plates.

[0005] A method for synergistically improving the strength and plasticity of magnesium alloy sheets includes the following steps:

[0006] S1. Provide initial samples of Mg-2Zn-3Li-1Gd plate;

[0007] S2. The plate sample is subjected to laser shock peening treatment;

[0008] S3. After completing the laser shock strengthening treatment, the treated plate is subjected to short-time annealing and air cooling.

[0009] The yield strength, tensile strength and elongation of Mg-2Zn-3Li-1Gd magnesium alloy sheet are simultaneously improved by the combined treatment of steps S2 and S3.

[0010] The Mg-2Zn-3Li-1Gd magnesium alloy plate sample, by mass percentage, contains: Zn 0.5%–2%, Li 1%–3%, Gd 0.1%–1%, with the balance being Mg and unavoidable impurities.

[0011] The process parameters for laser shock annealing and short-time annealing are as follows: Laser shock annealing parameters: double-sided shock, single pulse energy of 1J to 3J, spot diameter of 1mm to 6mm; Annealing parameters: annealing temperature of 250 to 350℃, holding time of 10 to 50min.

[0012] Preferably, the single-pulse energy of the laser shock enhancement is 0.5–2.5 J, more preferably 1 J;

[0013] The spot diameter is 3mm to 5mm, preferably 3mm;

[0014] The overlap rate is 30%–70%, preferably 50%, and a zigzag scanning method is used;

[0015] The absorbent layer is black tape, and the binding layer is a water layer with a thickness of 1-5 mm, preferably 2 mm.

[0016] The laser wavelength is 1064nm, and the pulse duration is 10-30ns, preferably 15ns.

[0017] Preferably, the short-time annealing temperature is 270–320°C, and the holding time is 5–60 min;

[0018] More preferably, the annealing temperature is 300℃, the holding time is 30min, and the annealing is followed by air cooling.

[0019] Furthermore, after the composite treatment, the resulting sheet material meets the requirements of yield strength ≥90MPa, tensile strength ≥270MPa, and elongation ≥37%.

[0020] The advantages of this application compared to the prior art are:

[0021] 1. By combining laser shock strengthening and short-time annealing, the yield strength, tensile strength and elongation of Mg-2Zn-3Li-1Gd alloy plates are simultaneously improved, alleviating the contradiction between strength and plasticity of magnesium alloys.

[0022] 2. Short-time annealing significantly improves elongation while maintaining high strength levels. The composite treatment state has a higher and more sustainable work hardening capacity, which helps to delay necking and improve uniform deformation capacity.

[0023] 3. Short-time annealing promotes the segregation of Zn and Gd at grain boundaries, improves grain boundary cohesion and crack resistance, and makes grain boundaries more tough, thus supporting the synergistic improvement of strength and plasticity at the interface level.

[0024] 4. This invention achieves a synergistic improvement in strength and plasticity by increasing and extending the effective work hardening stage and enhancing grain boundary cohesion and crack resistance, making it suitable for the engineering preparation of lightweight, high-strength, and high-plasticity magnesium alloy sheets. Attached Figure Description

[0025] Figure 1 The true strain versus true stress curves for different sheet materials used in the embodiments are shown.

[0026] Figure 2 The graphs showing the true strain versus work hardening rate for different sheet materials in the example are shown.

[0027] Figure 3 This is a schematic diagram of different types of dislocations inside the material after laser shock strengthening and short-time annealing combined treatment in the embodiment.

[0028] Figure 4 This is a schematic diagram of stacking faults and dislocations inside the material after laser shock strengthening and short-time annealing composite treatment in the embodiment.

[0029] Figure 5 This is a schematic diagram of element segregation at the grain boundaries of the initial plate material in the embodiment.

[0030] Figure 6 This is a schematic diagram showing the segregation of metal elements at the grain boundaries of the material after laser shock and short-time annealing combined treatment in the embodiment. Detailed Implementation

[0031] The embodiments of the technical solution of the present invention will now be described in detail with reference to the accompanying drawings. Unless otherwise stated, the technical or scientific terms used in this application have the ordinary meaning as understood by those skilled in the art.

[0032] Example 1: A cold-rolled sheet of Mg-2Zn-3Li-1Gd magnesium alloy was selected as the initial sample. The cold-rolled sheet was prepared according to the method disclosed in Chinese Invention Patent 202110752628.7.

[0033] Double-sided laser shock peening (LSP) treatment was performed on cold-rolled sheet metal. The single pulse energy was 1J, the spot diameter was 3mm, the overlap rate was 50%, and a zigzag scanning method was used. Black tape was used as the ablation absorption layer, and a 2mm water layer was used as the confinement medium. The laser wavelength was 1064nm, and the pulse duration was 15ns. The sheet metal after LSP treatment was recorded as LSP state.

[0034] The LSP state board was annealed at 300℃ for a short time and held at that temperature for 30 minutes. Then it was taken out and air-cooled to room temperature to obtain the composite treated board.

[0035] Test comparison: The original sheet material had a yield strength of 50.8 MPa, a tensile strength of 184.1 MPa, and an elongation of 35.7%; after laser shock strengthening, the sheet material had a yield strength of 139.3 MPa, a tensile strength of 285.8 MPa, and an elongation of 32.8%; after combined laser shock strengthening and short-time annealing treatment, the sheet material had a yield strength of 101.3 MPa, a tensile strength of 283.6 MPa, and an elongation of 39.1%. Figure 1 As shown, compared with the original sheet material, the sheet material after laser shock strengthening and short-time annealing composite treatment significantly improves both yield strength and tensile strength, while also increasing elongation, achieving a synergistic improvement in strength and plasticity.

[0036] Work hardening rate curve as shown Figure 2 As shown, the combined treatment of laser shock hardening and short-time annealing exhibits higher and more sustainable work hardening capacity, which helps to delay necking and improve uniform deformation capacity.

[0037] After laser shock hardening and short-time annealing combined treatment, defect structures such as dislocations and stacking faults are formed inside the material, such as... Figure 3 ( Figure 3 The diagram shows the dislocation pattern after laser shock annealing (LSA). It reveals that a significant number of dislocations remain after LSA, and different types of dislocations are activated during the LSA process. These dislocations interact with each other, forming a base-cone dislocation lock, which enhances work hardening ability and promotes a synergistic improvement in strength and ductility. (Arrows represent different types of dislocations.) Figure 4 As shown ( Figure 4 This indicates the presence of dislocations within stacking faults. The dislocations are blocked by stacking faults, thus promoting strength enhancement. (The arrows represent different types of dislocations). During subsequent tensile deformation, the interactions between dislocations, between dislocations and stacking faults, and between dislocations and grain boundaries are significantly enhanced. Dislocation movement is more prone to entanglement, pile-up, and pinning, leading to a decrease in the mean free path of dislocations and an increase in dislocation storage. Because dislocations are more difficult to slip over long distances and be eliminated by dynamic recovery, the stress required for continued deformation increases with strain more rapidly, resulting in a higher and more sustainable work hardening capacity. This increased work hardening capacity can delay necking and extend the uniform deformation stage, ultimately contributing to improved plasticity while maintaining a high strength level.

[0038] Meanwhile, short-time annealing promotes further segregation of Zn and Gd elements at grain boundaries, such as Figure 6 As shown, Figure 6 This indicates the segregation of elements at the grain boundaries after laser shock and short-time annealing combined treatment, and... Figure 5Compared to the initial elemental segregation at the grain boundaries of the sheet material, the increased elemental segregation at the grain boundaries after short-term annealing enhances fracture toughness. At this point, dislocations can accumulate in large quantities at the grain boundaries without causing cracks, thus promoting a synergistic improvement in strength and plasticity. In the figure, line AB represents the elemental content measured along this line. The elemental content changes abruptly at the grain boundaries. The lower right corner plot shows the content of each element, integrated into a single data graph. The horizontal axis represents the distance between points A and B, and the vertical axis represents the element's mass percentage. This segregation improves grain boundary cohesion and crack resistance, making the grain boundaries less prone to intergranular damage when subjected to dislocation pile-up and strain mismatch, thus providing interface protection for continuous plastic deformation and work hardening. On the other hand, the dragging and pinning effects generated by solute enrichment near the grain boundaries can suppress dislocation annihilation and recovery processes, promoting more complete storage of dislocations and maintaining a higher dislocation density during deformation. Therefore, the combined effect of grain boundary toughening and solute dragging further enhances the material's work hardening ability and supports a synergistic improvement in strength and plasticity.

[0039] The mechanism of this embodiment is as follows: a composite process of laser shock annealing and short-time annealing, which further stimulates and maintains the work hardening ability of the material while maintaining high strength, and at the same time improves the grain boundary damage resistance, thereby achieving a synergistic improvement in strength and plasticity. Magnesium alloy sheets obtained by this composite process exhibit more sufficient interaction and storage between dislocations, between dislocations and stacking faults, and between dislocations and grain boundaries during deformation, thus delaying necking and improving uniform deformation capability. Simultaneously, short-time annealing promotes further segregation of Zn and Gd at grain boundaries, improving grain boundary cohesion and crack resistance, making the grain boundaries more tough, and providing support for the synergistic improvement of strength and plasticity at the interface level.

[0040] The present invention has been disclosed above with reference to preferred embodiments, but it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed structure and technical content to create equivalent embodiments without departing from the scope of the present invention, and all such modifications or alterations shall still fall within the scope of the present invention.

Claims

1. A method for synergistically improving the strength and plasticity of magnesium alloy sheets, characterized in that: It includes the following steps: S1. Provide initial samples of Mg-2Zn-3Li-1Gd plate; S2. The plate sample is subjected to laser shock peening treatment; S3. After completing the laser shock strengthening treatment, the treated plate is subjected to short-time annealing and air cooling. The yield strength, tensile strength and elongation of Mg-2Zn-3Li-1Gd magnesium alloy sheet are simultaneously improved by the combined treatment of steps S2 and S3.

2. The method for synergistically improving the strength and plasticity of magnesium alloy sheets according to claim 1, characterized in that: The Mg-2Zn-3Li-1Gd magnesium alloy plate sample, by mass percentage, contains: Zn 0.5%–2%, Li 1%–3%, Gd 0.1%–1%, with the balance being Mg.

3. The method for synergistically improving the strength and plasticity of magnesium alloy sheets according to claim 1, characterized in that: The initial sample was a cold-rolled sheet.

4. The method for synergistically improving the strength and plasticity of magnesium alloy sheets according to claim 1, characterized in that: The laser shock peening process employs double-sided impact with a single pulse energy of 0.5–3 J.

5. The method for synergistically improving the strength and plasticity of magnesium alloy sheets according to claim 1, characterized in that: The diameter of the laser-strengthened spot is 1–6 mm.

6. The method for synergistically improving the strength and plasticity of magnesium alloy sheets according to claim 1, characterized in that, The overlap rate of the laser shock strengthening is 30% to 70%, and the scanning path is a Z-shaped scan.

7. The method for synergistically improving the strength and plasticity of magnesium alloy sheets according to claim 1, characterized in that, The laser shock strengthening process employs an absorption layer and a constraint layer, wherein the absorption layer is black adhesive tape.

8. The method for synergistically improving the strength and plasticity of magnesium alloy sheets according to claim 1, characterized in that, The laser wavelength is 1064nm and the pulse duration is 10-30ns.

9. The method for synergistically improving the strength and plasticity of magnesium alloy sheets according to claim 1, characterized in that, The short-time annealing temperature is 250–350 °C, and the holding time is 5–60 min.

10. A method for synergistically improving the strength and plasticity of magnesium alloy sheets according to any one of claims 1-9, characterized in that: After the composite treatment, the resulting sheet material meets the requirements of yield strength ≥ 90 MPa, tensile strength ≥ 270 MPa, and elongation ≥ 37%.

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