Magnesium alloy heat treatment method based on thermal expansion self-pressurization

Abstract

The invention provides a magnesium alloy heat treatment method based on thermal expansion self-pressurization, and relates to the technical field of metal material heat treatment. The invention aims to solve the problems of insufficient density, shrinkage porosity, shrinkage cavity and other defects in magnesium alloy castings or blanks, slow heat treatment dynamics, insufficient heat treatment strengthening effect and the like. The method comprises the steps that an ejection assembly is installed at one end of a mold, a magnesium alloy blank is placed in a mold cavity, and the mold is made of low-expansion alloy; a side pulling sliding block assembly is inserted into an inlet of the mold and used for closing a mold cavity; heating the closed mold and the magnesium alloy blank to a heat treatment temperature, and preserving heat; taking out the mold for cooling after the heat preservation time is up, and pulling out the side-pulling slide block for opening the mold; and the densified magnesium alloy blank is taken out through an ejection assembly. The method can be used for densifying the as-cast magnesium alloy, eliminating the defects of shrinkage porosity, shrinkage cavity and the like of the as-cast magnesium alloy blank and improving the heat treatment strengthening capacity of the magnesium alloy.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metal material heat treatment, in particular to a magnesium alloy heat treatment method based on thermal expansion self-pressurization. BACKGROUND

[0002] Magnesium alloy has a wide application prospect in the fields of aerospace, automobile, 3C products, etc. due to its low density, high specific strength and other advantages. However, magnesium alloy is prone to internal defects such as shrinkage porosity and shrinkage cavity during casting or preforming, which can significantly reduce the mechanical properties, fatigue life and air tightness of the material. In addition, the magnesium alloy has low heat treatment precipitation kinetics, and the heat treatment time is often long, and the heat treatment strengthening effect is insufficient, which limits its application in high-performance structural parts.

[0003] At present, the mainstream method to eliminate internal porosity defects of metal is hot isostatic pressing (HIP) technology. The technology places the blank in a sealed high-pressure container and applies isotropic high temperature and high pressure through inert gas (such as argon) to make the material creep and diffuse, thereby pressing the internal defects. Although the HIP technology has significant effect, its equipment is extremely expensive, the running cost is high, and the process cycle is long. In addition, there are also some processes that cold or warm extrude the blank through rigid molds, aiming to consolidate the internal porosity through large plastic deformation. Such methods usually require large-tonnage presses, which have high energy consumption, large stress on the mold, easy wear, and difficulty in ensuring the uniformity of the internal stress state of the blank.

[0004] Therefore, the industry urgently needs to develop a device and method that is novel in principle, simple in structure, low in cost, and can effectively eliminate internal defects of magnesium alloy or improve the heat treatment strengthening effect of magnesium alloy. SUMMARY

[0005] The present application is to solve the problem of insufficient density of magnesium alloy castings or blanks, internal defects such as shrinkage porosity and shrinkage cavity, and slow heat treatment kinetics, insufficient heat treatment strengthening effect, and further proposes a magnesium alloy heat treatment method based on thermal expansion self-pressurization.

[0006] The technical solution adopted by the present application to solve the above problems is: The present application proposes a magnesium alloy heat treatment method based on thermal expansion self-pressurization, comprising the following steps: Step one, install an ejection assembly at one end of the mold, put the magnesium alloy blank into the mold cavity, and the blank and the mold are in transition fit; the mold is made of low expansion alloy; Step two, insert a side extraction slide block assembly at the entrance of the mold for closing the mold cavity; Step 3: Heat the mold and magnesium alloy billet after mold closing to the heat treatment temperature and hold them at that temperature. During this process, the magnesium alloy billet expands in volume because its coefficient of thermal expansion is much greater than that of the mold material. This expansion is constrained by the rigid mold cavity, thereby generating uniform isostatic pressure inside the billet, driving the healing of internal defects and promoting the formation of precipitates. Step 4: After the heat preservation time is reached, remove the mold and let it cool. Pull out the side slide block to open the mold and remove the densified magnesium alloy billet through the ejector assembly.

[0007] Furthermore, the expansion alloy mentioned in step one is an Invar alloy.

[0008] Furthermore, the mold includes a mold body with a cavity, one end of the mold body is provided with an ejector assembly, and the other end is provided with a side-pull slider assembly; before the magnesium alloy billet is placed into the mold cavity, lubricant is uniformly sprayed onto the outer surface of the magnesium alloy billet, the inner surface of the mold cavity, the side-pull slider assembly, and the ejector assembly.

[0009] Furthermore, the lubricant is a water-based graphite lubricant.

[0010] Furthermore, the side-pull slider assembly includes two wedge-shaped sliders, the inclined surfaces of the two wedge-shaped sliders are in contact with each other, and the flat surfaces are in contact with the mold mating surfaces.

[0011] Furthermore, the wedge angle α of each wedge slider is 5°-8°.

[0012] Furthermore, the ejection assembly includes a cylindrical push plate and an ejection rod. The cylindrical push plate is located at the end of the cavity and transitionally fits with the cavity. One end of the ejection rod passes through the mold sidewall and is fixed to the cylindrical push plate.

[0013] Furthermore, the heating temperature in step three is 200℃-450℃, and the holding time is 2h-5h.

[0014] Furthermore, the cooling method described in step four is furnace cooling or air cooling to room temperature.

[0015] Furthermore, the ratio of the mold wall thickness to the cavity radius is not less than 2:1, to ensure that a pressure of not less than 300MPa can be generated inside the blank during the heat treatment process.

[0016] The beneficial effects of this invention are: I. This invention enables low-cost and high-efficiency isostatic pressing densification of magnesium alloys. Traditional hot isostatic pressing (HIP) relies on extremely expensive external ultra-high pressure gas sources and complex containers. This invention utilizes the inherent thermal expansion characteristics of magnesium alloys, generating uniform pressure within the billet using a conventional heating furnace and mold. This causes defects such as shrinkage porosity and shrinkage cavities to undergo plastic flow and diffusion healing at heat treatment temperatures, resulting in densification of the magnesium alloy. Furthermore, compared to traditional hot isostatic pressing, the required equipment costs and operating energy consumption are significantly reduced.

[0017] Second, this invention can improve the heat treatment strengthening ability of magnesium alloys. Because the magnesium alloy is always under pressure during the heat treatment process, the precipitation kinetics of the magnesium alloy can be improved, which not only shortens the heat treatment time but also enhances the precipitation strengthening ability of the magnesium alloy.

[0018] Third, the heat treatment process of this invention possesses high safety and operational reliability. The mold inlet consists of a pair of wedge-shaped sliders at a specific angle. When the internal pressure rises due to the expansion of the magnesium alloy, this pressure is converted into a radial force that makes the inclined surfaces of the sliders mesh more tightly, achieving a self-locking effect under friction. This self-locking device requires no external hydraulic or bolt locking devices, is simple to operate, and offers extremely high safety. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the mold structure in the magnesium alloy densification treatment method based on thermal expansion self-compacting of the present invention. Figure (a) is a three-dimensional schematic diagram; Figure (b) is a cross-sectional view.

[0020] In the diagram: 1-Ejector pin; 2-Push plate; 3-Mold body; 3-1-Cavity; 4-Wedge slider.

[0021] Figure 2 The difference in microstructure between unconstrained and mold-constrained magnesium alloys after holding at 425℃ for 5 hours in the embodiments of the present invention is as follows: (a) represents no mold constraint, and (b) represents a mold constraint.

[0022] Figure 3 The difference in performance between magnesium alloys without mold constraints and those with mold constraints after heat treatment at 425℃ for 5 hours is shown in the embodiments of the present invention. Detailed Implementation

[0023] This embodiment proposes a magnesium alloy heat treatment method based on thermal expansion self-pressurization. Due to the high coefficient of thermal expansion of magnesium alloy, the magnesium alloy billet is placed in a rigid mold cavity made of a low-expansion alloy, initially in a micro-transition fit. When the entire billet is heated to the plastic deformation temperature range of magnesium alloy (200℃-450℃), the magnesium alloy undergoes a dramatic volume expansion due to its much higher coefficient of expansion than the mold material. However, this expansion is constrained in all directions by the rigid inner wall of the mold, thereby generating uniform pressure inside the billet. This pressure is the fundamental driving force for densification, forcing the material at defects to undergo high-temperature plastic flow, thus achieving defect compression and healing. Furthermore, since pressure can promote atomic diffusion, it can improve the heat treatment strengthening ability of magnesium alloy. Based on the difference in the coefficient of thermal expansion between magnesium alloy and Invar alloy, and considering the elastic deformation of the mold, the theoretical elastic pressure generated inside the magnesium alloy at different temperatures is calculated. P as follows:

[0024] The coefficient of thermal expansion of magnesium alloy = 26×10 -6 / °C, coefficient of thermal expansion of Invar alloy = 1.5×10 -6 / °C, bulk modulus of magnesium alloy K = 45GPa, C / V is the normalized compliance of the mold, a parameter related to the mold geometry, size, and material properties. It represents the elastic increase in the volume of the mold cavity under unit internal pressure. According to the elastic theory of thick-walled cylinders, the normalized compliance C / V of the mold is given by the formula C / V = (2 / E)[ (1 + ν)(r o ² / (r o ² - r i ²)) +(1-ν) ] calculate, where r i and r o Here, ν represents the inner and outer radii of the mold cavity, respectively, and E and ν are the elastic modulus and Poisson's ratio of the mold material. To generate a pressure of not less than 250 MPa inside the workpiece, it is recommended that the ratio of mold wall thickness to cavity radius be not less than 2:1, i.e., r o / r i ≥ 3, thus making C / V less than approximately 3.5 × 10 -11 Pa -1 Here, we take C / V = 3.00 × 10⁻⁶. -11 Pa -1 .

[0025] After calculation, at 200°C, the pressure P generated by the mold on the inside of the blank is approximately 250 MPa; at 300°C, the pressure P generated by the mold on the inside of the blank is approximately 395 MPa; at 450°C, the pressure P generated by the mold on the inside of the blank is approximately 605 MPa. This calculation theoretically verifies that within the designed mold and process temperature range, a pressure of 250–605 MPa can be generated inside the magnesium alloy solely by utilizing the difference in thermal expansion coefficients. This pressure is much higher than the yield strength of the magnesium alloy at the corresponding high temperature (usually several tens of megapascals), which is sufficient to drive the plastic flow healing of defects such as internal shrinkage porosity and promote the diffusion of alloy atoms, while achieving the densification of the material and enhancing the precipitation kinetics of the material.

[0026] Meanwhile, the pressure generated by the expansion of the magnesium alloy acts on a pair of wedge-shaped sliders at the mold inlet. When the design of the sliders meets the condition of friction self-locking, the pressure on the sliders will be converted into the force that compresses the meshing surfaces of the two. Based on the static friction coefficient (μ is usually 0.15 - 0.25) of the low-expansion invar alloy in the temperature range of 200°C - 450°C, to ensure a reliable self-locking effect under the internal high pressure, the wedge angle α must satisfy the self-locking condition: α < arctan(μ). Therefore, the designed range of the wedge angle α is 5° - 8°. This angle range can ensure self-locking under conservative friction coefficients and at the same time make it easy to remove the sliders after the heat treatment.

[0027] The specific operation steps of this method are as follows: Step 1: Mold preparation As Figure 1 shown, provide a set of special molds. The mold includes a mold body 3, a side-drawing slider assembly, and an ejection assembly. The mold body 3 is rectangular and has a cylindrical cavity 3-1 inside; the ejection assembly is installed at one end of the mold body 3 and includes a cylindrical ejector plate 2 and an ejection rod 1. The end of the cylindrical ejector plate 2 located inside the cavity is in transitional fit with the cavity 3-1, and one end of the ejection rod 1 passes through the mold side wall and is fixed to the cylindrical ejector plate 2; the side-drawing slider assembly includes two wedge-shaped sliders 4, and the inclined surfaces of the two wedge-shaped sliders 4 are mutually adhered, and the flat surfaces are in contact with the mating surfaces of the mold body 3; the wedge angle of each wedge-shaped slider 4 is 8°; Preferably, a water-based graphite lubricant is evenly sprayed on the inner wall of the cavity 3-1, the outer surface of the wedge-shaped slider 4, and the outer surface of the cylindrical ejector plate 2; Preferably, there is a transitional fit between the side of the ejector plate 2 in the ejection assembly and the mold body, and between the side of the ejection rod 1 and the mold body.

[0028] Preferably, the materials of the mold body, the ejection assembly, and the side-drawing slider assembly are all invar alloy.

[0029] Step 2: Blank filling Water-based graphite lubricant is uniformly sprayed onto the outer surface of the magnesium alloy billet, and the magnesium alloy billet is placed into the mold cavity; The magnesium alloy is a solid solution Mg-6Gd-4Y-0.5Zn-0.3Zr alloy; When the magnesium alloy billet is placed into the mold cavity, the magnesium alloy billet and the mold are in a transition fit.

[0030] Step 3: Insert a pair of lubricated wedge sliders crosswise from both sides of the mold. The mating surfaces between the sliders are inclined, and the mating surfaces between the sliders and the mold are flat, until the cavity entrance is completely closed. At this point, the sliders and the mold, as well as the two wedge sliders, are in a tight transition fit.

[0031] Step 4: Heat treatment for densification The assembled device after mold closing is placed in a muffle furnace, and the mold and magnesium alloy billet are heated to 425°C; the holding time is 5 hours; during this process, the magnesium alloy billet (coefficient of thermal expansion) Approximately 26 × 10 -6 Significant expansion occurs at 0°C, due to the Invar alloy mold (coefficient of thermal expansion is approximately 1.5 × 10⁻⁶). -6 At / °C, expansion is minimal. The expansion of the billet is constrained by the rigid mold cavity and wedge-shaped slider, thus generating uniform isostatic pressure within it. This high-pressure environment promotes the healing of pre-existing micropores and shrinkage porosity through plastic rheology and diffusion at high temperatures. Simultaneously, the pressure accelerates the diffusion of solute atoms, promoting the precipitation kinetics of the strengthening phase.

[0032] Step 5: Remove the part after cooling After the heat preservation is completed, remove the mold as a whole from the furnace and allow it to cool naturally in the air to below 100°C. Use a special tool to gently tap outwards and pull out the wedge-shaped sliders on both sides. Push the ejector rod to push the magnesium alloy billet out of the cavity in a balanced manner and allow the billet to continue cooling to room temperature.

[0033] Magnesium alloy samples treated by the method of this invention were tested and compared with samples of the same composition, at the same temperature and time, but without mold constraints (i.e., conventional heat treatment): like Figure 2 As shown, the magnesium alloy treated by the method of the present invention precipitates a higher density long periodic phase (LPSO phase), and the grain boundaries become less obvious after corrosion due to the reduced density of micro-defects at the grain boundaries.

[0034] like Figure 3 As shown, mechanical property tests indicate that the magnesium alloy of the present invention has significantly improved strength and elongation, with the yield strength increasing from 175MPa to 200MPa and the elongation increasing from 9.1% to 9.8%.

[0035] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, 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 technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent substitutions, and improvements made to the above embodiments without departing from the scope of the present invention, based on the technical essence of the present invention and within the spirit and principles of the present invention, shall still fall within the protection scope of the present invention.

Claims

1. A heat treatment method for magnesium alloys based on thermal expansion and self-pressurization, characterized in that, Includes the following steps: Step 1: Install an ejector assembly at one end of the mold and place the magnesium alloy billet into the mold cavity; the mold is made of a low-expansion alloy. Step 2: Insert the side sliding block assembly at the other end of the mold to close the mold cavity; Step 3: Heat the mold and magnesium alloy billet after mold closing to the heat treatment temperature and hold them at that temperature. During this process, the magnesium alloy billet expands in volume because its coefficient of thermal expansion is much greater than that of the mold material. This expansion is constrained by the rigid mold cavity, thereby generating uniform isostatic pressure inside the billet, driving the healing of internal defects and promoting the formation of precipitates. Step 4: After the heat preservation time is reached, remove the mold to cool it, pull out the side slide block to open the mold, and take out the densified magnesium alloy billet through the ejector assembly.

2. The method for heat treatment of magnesium alloys based on thermal expansion and self-pressurization according to claim 1, characterized in that, The low-expansion alloy mentioned in step one is an Invar alloy.

3. The method for heat treatment of magnesium alloys based on thermal expansion and self-pressurization according to claim 1, characterized in that, The mold includes a mold body with a cylindrical cavity, an ejector assembly at one end of the mold body, and a side-pull slider assembly at the other end; before the magnesium alloy billet is placed into the mold cavity, lubricant is uniformly sprayed onto the outer surface of the magnesium alloy billet, the inner surface of the mold cavity, the side-pull slider assembly, and the ejector assembly.

4. The method for heat treatment of magnesium alloys based on thermal expansion and self-pressurization according to claim 3, characterized in that, The lubricant is a water-based graphite lubricant.

5. The method for heat treatment of magnesium alloys based on thermal expansion and self-pressurization according to claim 3, characterized in that, The side-pull slider assembly includes two wedge-shaped sliders, with the inclined surfaces of the two wedge-shaped sliders fitting together and the flat surfaces contacting the mold mating surfaces.

6. The method for heat treatment of magnesium alloys based on thermal expansion and self-pressurization according to claim 5, characterized in that, The wedge angle of each wedge slider is 5°-8°.

7. The method for heat treatment of magnesium alloys based on thermal expansion and self-pressurization according to claim 3, characterized in that, The ejection assembly includes a cylindrical push plate and an ejection rod. The cylindrical push plate is located at the end of the cavity and transitions into the cavity. One end of the ejection rod passes through the mold sidewall and is fixed to the cylindrical push plate.

8. The method for heat treatment of magnesium alloys based on thermal expansion and self-pressurization according to claim 1, characterized in that, The heating temperature in step three is 200℃-450℃, and the holding time is 2h-5h.

9. The method for heat treatment of magnesium alloys based on thermal expansion and self-pressurization according to claim 1, characterized in that, The cooling method described in step four is furnace cooling or air cooling to room temperature.

10. The method for heat treatment of magnesium alloys based on thermal expansion and self-pressurization according to claim 1, characterized in that, The ratio of the mold wall thickness to the cavity radius is not less than 2:1 to ensure that a pressure of not less than 250MPa can be generated inside the blank during the heat treatment process.

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