Method for forming high-toughness rare-earth magnesium alloy cylindrical castings

Abstract

This invention provides a forming method for a high-strength and high-toughness rare-earth magnesium alloy cylindrical casting, comprising the following steps: molding to obtain a mold; melting the magnesium alloy to obtain a molten alloy; pouring the molten alloy into the mold obtained after molding through a gating system; cooling to obtain a formed workpiece; and heat-treating the formed workpiece to obtain a magnesium alloy cylindrical casting. The gating system is a vertical cylindrical inlet gate type, and sequential solidification is used during pouring. The heat treatment employs an embedded clamping mechanism for the formed workpiece. The casting obtained by the forming method of this invention has a dense internal structure, reducing defects such as segregation, shrinkage porosity, and cracks, resulting in high internal and surface quality. Furthermore, the casting deformation is precisely controlled, ensuring the dimensional accuracy of the casting.

Description

Technical Field

[0001] This invention relates to the field of magnesium alloy casting processing technology, and more specifically to a forming method for high-strength and high-toughness rare earth magnesium alloy cylindrical castings. Background Technology

[0002] In recent years, research on high-strength and high-toughness rare-earth wrought magnesium alloys has attracted widespread attention both at home and abroad, especially Mg-Gd-Y series rare-earth magnesium alloys. Due to the excellent solid solution strengthening and age hardening effects of Gd and Y, ultra-high strength and plasticity close to those of high-strength aluminum alloys can be obtained.

[0003] However, Mg-Gd-Y rare earth magnesium alloy cylindrical castings still have many defects during the forming process, such as cracks and shrinkage cavities inside and outside the casting, and linear defects on the surface of the casting (e.g. Figure 1 and Figure 2 (As shown in the image); factors such as large deformation during heat treatment of castings and difficulty in controlling casting dimensions result in a low casting qualification rate, which does not meet the requirements of high efficiency and low cost in casting production.

[0004] In aerospace modules, castings are required to have few internal and surface defects, high deformation and dimensional accuracy. At the same time, there are also some large, complex cylindrical castings with significant differences in wall thickness in aerospace modules, which places higher demands on the casting process.

[0005] Therefore, there is an urgent need for a forming method to improve the internal and surface quality of high-strength and high-toughness rare-earth magnesium alloy cylindrical castings, as well as deformation control. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by providing a forming method for high-strength and high-toughness rare-earth magnesium alloy cylindrical castings. The castings obtained by this forming method have a dense internal structure, reducing defects such as segregation, shrinkage porosity, and cracks, and have high internal and surface quality. Furthermore, the deformation of the castings is precisely controlled, ensuring the dimensional accuracy of the castings.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A method for forming a high-strength and high-toughness rare-earth magnesium alloy cylindrical casting includes the following steps:

[0009] The shape is cast into a mold, and the magnesium alloy is melted to obtain the alloy melt;

[0010] The molten alloy is poured into a mold obtained after molding using a gating system, and the molded workpiece is obtained after cooling.

[0011] After heat treatment, the formed workpiece is obtained as a magnesium alloy cylindrical casting.

[0012] The gating system is an in-slot gating system for vertical cylinders, and the casting process uses a sequential solidification method; the heat treatment uses an embedded clamping system to form the workpiece.

[0013] Preferably, the gating system includes a gating gate, an annular runner, a vertical tube, a slotted gating system, and a riser.

[0014] Preferably, the solidification sequence is as follows: casting, slit ingate, vertical cylinder, horizontal runner, and riser pipe.

[0015] Preferably, the principles for determining the dimensions of the gap and the vertical cylinder are as follows: gap width a: when the wall thickness δ>10mm, a=(0.8~1.0)*δ; gap length b=20~40mm; vertical cylinder diameter D=(3~5)*a.

[0016] Preferably, the gating system is an open gating system.

[0017] Preferably, the casting is cooled by chilling with a chiller.

[0018] Preferably, the chill is placed inside the casting cavity.

[0019] Preferably, the thickness of the chill is 1.2-1.5 times the thickness of the casting boss.

[0020] Preferably, the magnesium alloy comprises, by mass fraction, the following components: 0.4-0.8% Zr, 0.1-0.2% Zn, 10.0-11.0% Gd, 0-3.5% Y, ≤0.01% Si, ≤0.01% Al, ≤0.01% Fe, with the balance being Mg.

[0021] The beneficial effects of this invention are as follows:

[0022] 1. The forming method of the high-strength and tough rare-earth magnesium alloy cylindrical casting of the present invention establishes a reasonable solidification sequence during pouring, adopts a sequential solidification method, and ensures that the parts of the casting structure solidify in the following order: the parts furthest from the riser solidify last, then the parts closer to the riser, and finally the riser itself. This is combined with chilling to cool the thicker parts of the casting. Simultaneously, during pouring, the temperature of the top riser is higher than the temperature of the casting, the temperature of the casting is lower than the temperature of the gating system, and the temperature of the thicker parts of the casting is lower than the temperature of the rest of the casting due to the presence of chills, thus establishing a temperature gradient. This ensures that there is no temperature difference or the temperature difference between different parts of the casting structure is minimized. Through the temperature gradient and sequential solidification method, the stress caused by uneven thickness during solidification is reduced, shrinkage porosity and crack defects are eliminated, and segregation defects are also eliminated.

[0023] 2. This invention involves heat treatment by embedding the cast part into a tooling fixture. The diameter of the heat-treated casting increases by only about 0.1% compared to the diameter before heat treatment, and the deformation is controlled within ±1mm, resulting in higher casting dimensional accuracy. Attached Figure Description

[0024] Figure 1 It is a linear defect diagram of castings in the existing technology.

[0025] Figure 2 This is a diagram of crack defects in castings in the existing technology.

[0026] Figure 3 It is a defect image of a casting on a flaw detection film in the existing technology.

[0027] Figure 4 This is a schematic diagram of the process flow for forming the high-strength and tough rare-earth magnesium alloy cylindrical casting of the present invention. Detailed Implementation

[0028] To better understand the technical content of the present invention, specific embodiments are described below in conjunction with the accompanying drawings.

[0029] Various aspects of the invention are described in this disclosure with reference to the accompanying drawings, in which numerous illustrative embodiments are shown. The embodiments of this disclosure are not necessarily intended to encompass all aspects of the invention. It should be understood that the various concepts and embodiments described above, as well as those described below in more detail, can be implemented in any of a number of ways.

[0030] High-strength and high-toughness rare-earth magnesium alloy cylindrical castings, especially in the thicker parts, exhibit various internal defects such as shrinkage porosity, point segregation, strip segregation, and cracks. Additionally, thin-walled skin areas may contain minute linear defects that are undetectable by X-rays but appear during fluorescence inspection. The morphology of these defects on the flaw detection film is as follows. Figure 3 .

[0031] For example, when using high-strength and high-toughness rare-earth magnesium alloy to cast the third-stage tail section, the main problems in the early stages are as follows: Figure 3 The segregation defects shown have macroscopic morphologies including banded, cloud-like, and linear forms. Severe segregation defects can lead to cracks. Banded segregation appears as white fishtail patterns on scratched radiographs and generally occurs near the two thick, large spherical sections at the tail end. Cloud-like and linear segregation appear as cloud-like and line-like patterns on flaw detection radiographs, respectively, and occur in the thick areas near the rear end frame of the third-stage tail end.

[0032] High-strength and high-toughness rare-earth magnesium alloys have a wide crystallization range and solidify in a paste-like manner. When the temperature drops to the crystallization temperature, a framework of interconnected coarse equiaxed crystals gradually forms. As the temperature decreases, the solid framework gradually grows, and the liquid is divided into small molten pools. The areas of the casting that solidify first have low solute content, while the liquid phase that solidifies later has high solute content. Towards the end of solidification, the small molten pools enriched with solute Gd replenish the shrinkage porosity and crack defects caused by solid shrinkage, healing them. At the same time, a eutectic reaction occurs, producing segregation. Therefore, when observed on the flaw detection film, the cloud-like segregation is very similar to the shrinkage porosity morphology, and the linear segregation is very similar to the crack morphology, only differing in color on the film (shrinkage porosity and cracks appear black on the film, while segregation appears white).

[0033] Therefore, this invention provides a forming method for high-strength and tough rare-earth magnesium alloy cylindrical castings. By establishing a reasonable solidification sequence during pouring, the method ensures that the parts of the casting structure solidify last, followed by the parts closer to the gating and riser, and finally the gating and riser itself. At the same time, a good temperature gradient is established, and the method ensures that there is no temperature difference or the temperature difference between the parts of the casting structure is minimized during solidification, thus guaranteeing the internal and surface quality of the casting. Furthermore, the method uses embedded tooling to clamp the formed part for heat treatment, ensuring the dimensional accuracy of the casting.

[0034] like Figure 4 As shown, in an exemplary embodiment of the present invention, a method for forming a high-strength and high-toughness rare-earth magnesium alloy cylindrical casting is provided, comprising the following steps:

[0035] The shape is cast into a mold, and the magnesium alloy is melted to obtain the alloy melt;

[0036] The molten alloy is poured into a mold obtained after molding using a gating system, and the molded workpiece is obtained after cooling.

[0037] After heat treatment, the formed workpiece is obtained as a magnesium alloy cylindrical casting.

[0038] The gating system is an in-slot gating system for vertical cylinders, and the casting process uses a sequential solidification method; the heat treatment uses an embedded clamping system to form the workpiece.

[0039] In a preferred embodiment, the gating system includes a gating gate, an annular runner, a vertical column, a slotted gating system, and a riser.

[0040] In a preferred embodiment, the solidification sequence is as follows: casting, slit ingate, vertical cylinder, horizontal runner, and riser pipe.

[0041] In a preferred embodiment, the dimensions of the gap and the vertical cylinder are determined according to the following principles: gap width a: when the wall thickness δ>10mm, a=(0.8~1.0)*δ; gap length b=20~40mm; vertical cylinder diameter D=(3~5)*a.

[0042] In a preferred embodiment, the gating system is an open gating system.

[0043] In a preferred embodiment, the casting is cooled by chilling with a chiller.

[0044] In a preferred embodiment, a chill is disposed within the inner cavity of the casting.

[0045] In a preferred embodiment, the thickness of the chill is 1.2-1.5 times the thickness of the casting boss.

[0046] In a preferred embodiment, the magnesium alloy comprises, by mass fraction, the following components: 0.4-0.8% Zr, 0.1-0.2% Zn, 10.0-11.0% Gd, 0-3.5% Y, ≤0.01% Si, ≤0.01% Al, ≤0.01% Fe, with the balance being Mg.

[0047] To facilitate better understanding, the present invention will be further described below with reference to specific examples, but the forming method is not limited thereto, and the content of the present invention is not limited thereto.

[0048]

Example 1

[0049] The dimensions of the cast workpiece are Diameter Φ1000mm, height 550mm, wall thickness The thickest part of the casting wall is about 35mm, and the remaining parts are three-stage tail section castings with walls between 15 and 30mm.

[0050] Gating system

[0051] Based on the section diameter and internal structural characteristics, this casting has eight sprues, evenly distributed around the circumference. Following the general principles for determining gap and column dimensions, the basic gap width is determined to be 20mm, with wider gaps at the front and rear ends and in thicker sections; the length is 40mm; the column diameter is... The bottom horizontal sprue is trapezoidal with a cross-sectional dimension of 75×70×65, and there are 8 vertical tubes.

[0052] Furthermore, in the low-pressure casting process design, on the one hand, a corresponding radial horizontal runner is designed at the bottom of the vertical cylinder to enhance the metal flow of the vertical cylinder and the sprue gate, thereby improving the filling and feeding capacity of the vertical cylinder and the sprue gate. On the other hand, an additional section of the vertical cylinder is added at the top of the casting to enhance the filling and feeding capacity of the top of the casting, and also to collect gas and slag.

[0053] Through the design of the shape and position of the above gating system, molten aluminum can be used to smoothly fill the side wall of the casting shell, and during the solidification process, the vertical cylinder of this gating system can effectively compensate for the shrinkage of the side wall of the casting shell.

[0054] Cooling system

[0055] Because of the large size of the castings, it is not easy to control the solidification direction during the solidification process. Therefore, in order to achieve sequential solidification of the castings during the low-pressure casting process, in addition to designing a reasonable gating system, according to the structural characteristics of the castings, forming chills are set in the thick parts of the castings. The thickness of the chills is 1.2 to 1.5 times the thickness of the boss.

[0056] The gating system consists of a gating gate, annular runner, vertical runner, sprue, and risers. Forming chills are installed in the thicker sections of the casting's inner wall. The chills are made of aluminum, and the risers are made of the same material as the casting.

[0057] Heat treatment embedded tooling

[0058] To control the gap between the heat treatment fixture and the inner cavity of the casting end frame, four round bars are used to fix the upper and lower end frame plates. Two 250mm diameter vent holes are opened in the middle of the upper and lower end frame plates to facilitate air flow and temperature uniformity within the casting during heat treatment. Considering that the inner cavity of the casting has two cylindrical surfaces with a diameter of about 300mm, to prevent the heat treatment from affecting the casting dimensions due to the significant difference in wall thickness in this area, two plates are used to support the two planes when making the fixture.

[0059] Forming process

[0060] The molding process yields a casting mold, and the magnesium alloy (composition: 0.4% Zr, 0.2 Zn, 10.0 Gd, 2.0% Y, 0.005% Si, 0.01% Al, 0.01% Fe, balance Mg) is melted to obtain an alloy melt.

[0061] The alloy molten liquid is poured into the mold obtained after molding through the above-mentioned gating system, and the molded workpiece is obtained after cooling; wherein, the casting pouring temperature is 730℃, the initial temperature of the mold and chill is 25℃, the filling speed is 0.6KPa / s, the filling time is 90 seconds, the holding pressure is 0.08MPa, and the holding time is 2000 seconds.

[0062] The formed workpiece is clamped in an embedded tooling and, after heat treatment, a magnesium alloy cylindrical casting is obtained.

[0063] Internal quality and surface quality of castings

[0064] Verification methods: 100% of the castings undergo fluorescent testing, and the surface quality is checked according to the HB / Z 61 penetrant testing method. Cold shuts, undercasting, cracks, penetrating porosity, and penetrating voids throughout the entire wall thickness are not permitted on the castings. 100% of the castings undergo X-ray inspection, and the internal quality is checked according to the HB / Z 60 X-ray inspection method. The internal quality of the castings meets the requirements for Class I parts.

[0065] The test results show that its internal quality and surface quality meet the requirements of Class I parts in the "Technical Conditions for Manufacturing and Acceptance of Level III Tail Sections".

[0066] Casting performance testing

[0067] The casting was dissected. The cross-section properties at room temperature were σb≥300MPa; σ0.2≥200MPa; δ5≥4%. At 250℃, the properties were σb≥240MPa; σ0.2≥180MPa; δ5≥6%.

[0068] The results are shown in Tables 1 and 2.

[0069] Table 1. Anatomical Properties of Castings

[0070]

[0071]

[0072] Table 2 High-temperature performance of dissected specimens (250℃):

[0073]

[0074]

[0075] The test results show that the castings meet the requirements for both room temperature and high temperature performance.

[0076] Deformation of castings

[0077] The deformation of the third-stage tail section casting is basically controlled within ±1mm, and the casting wall thickness meets the requirements of subsequent processing.

[0078] While the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the invention. Those skilled in the art can make various modifications and refinements without departing from the spirit and scope of the invention. Therefore, the scope of protection of the present invention shall be determined by the claims.

Claims

1. A forming method of a high-toughness rare earth magnesium alloy cylindrical casting, characterized by, Includes the following steps: The shape is obtained by casting, and the magnesium alloy is melted to obtain the alloy melt; The molten alloy is poured into the mold obtained after molding through a gating system, and the molded workpiece is obtained after cooling. After heat treatment, the formed workpiece is obtained as a magnesium alloy cylindrical casting. The gating system is a vertical cylinder slit gating system, which includes a gating gate, an annular runner, a vertical cylinder, a slit gating system, and a riser. During pouring, a sequential solidification method is adopted, and the solidification sequence is as follows: casting, slit ingate, vertical cylinder, horizontal runner and riser pipe; The heat treatment uses an embedded clamping fixture to form the workpiece. The gap between the heat treatment fixture and the inner cavity of the casting end frame is controlled by four round bars to fix the upper and lower end frame plates. Two 250mm diameter vent holes are opened in the middle of the upper and lower end frame plates to facilitate air flow and temperature uniformity within the casting during heat treatment. Considering that the inner cavity of the casting has two 300mm diameter cylindrical surfaces, to prevent the heat treatment from affecting the casting dimensions due to the significant difference in wall thickness in this area, two plates are used to support the two planes when making the fixture. During pouring, the temperature of the top riser is higher than that of the casting, the temperature of the casting is lower than that of the sprue, and the temperature of the thick parts of the casting is lower than that of the rest of the casting by laying chills, thus establishing a temperature gradient. Through the temperature gradient and sequential solidification, the stress caused by uneven thickness during solidification is reduced, shrinkage porosity and crack defects are eliminated, and segregation defects are also eliminated.

2. The high-toughness rare earth magnesium alloy cylindrical casting forming method according to claim 1, characterized by, The principles for determining the dimensions of the gap and the vertical cylinder are as follows: Gap width a: when the wall thickness δ>10mm, a=(0.8~1.0)*δ; Gap length b=20~40mm; Vertical cylinder diameter D=(3~5)*a.

3. The high-toughness rare earth magnesium alloy cylindrical casting forming method according to claim 1, characterized by, In the casting system, the lower part of the vertical cylinder is provided with radial horizontal runners, and the upper part of the vertical cylinder is equal to the height of the casting.

4. The high-toughness rare earth magnesium alloy cylindrical casting forming method according to claim 1, characterized by, The gating system is an open gating system.

5. The high-toughness rare earth magnesium alloy cylindrical casting forming method according to claim 1, characterized by, The casting is cooled by using chilled iron.

6. The high-toughness rare earth magnesium alloy cylindrical casting forming method according to claim 5, characterized by, Chips are placed inside the casting.

7. The high-toughness rare earth magnesium alloy cylindrical casting forming method according to claim 5, characterized by, The thickness of the chill is 1.2-1.5 times the thickness of the casting boss.

8. The high-toughness rare earth magnesium alloy cylindrical casting forming method according to any one of claims 1 to 7, characterized by, The magnesium alloy comprises, by mass fraction, the following components: 0.4-0.8% Zr, 0.1-0.2% Zn, 10.0-11.0% Gd, 0-3.5% Y, ≤0.01% Si, ≤0.01% Al, ≤0.01% Fe, with the balance being Mg.

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