Mg-Sn-RE-Al-Ag magnesium alloy suitable for low-temperature forging and preparation method thereof

Abstract

The invention discloses an Mg-Sn-RE-Al-Ag magnesium alloy suitable for low-temperature forging and a preparation method of the Mg-Sn-RE-Al-Ag magnesium alloy. The magnesium alloy comprises the following components in percentage by mass: 6-9% of Sn, 1-5% of Ce, 0.2-1% of La, 0.2-1% of Nd, 0.2-1% of Pr, 0.5-3% of Al, 0.1-1% of Ag and the balance of Mg and inevitable impurity elements. Sn is added in a pure metal form, Ce, La, Nd and Pr are added in a mixed rare earth form, Al and Ag alloy elements are added in a high-temperature stable intermediate alloy form, and the alloy cost is reduced while the alloy is highly solubilized. After a high-quality cast ingot is prepared, the problems that in the forging process of the alloy, the temperature loss is large, the temperature difference of different parts is large and the structure non-uniformity is large are solved through the homogenizing heat treatment and the low-temperature forging mode with the temperature lower than the aging temperature, the machining cost is saved, the machining steps are simplified, and meanwhile the strength and plasticity of the alloy are further improved; and thus, the Mg-Sn-RE-Al-Ag magnesium alloy suitable for low-temperature forging is obtained.

Description

Technical Field

[0001] The invention relates to a Mg-Sn-RE-Al-Ag magnesium alloy suitable for low-temperature forging and a preparation method thereof. The invention can be used in the fields of automobiles and aerospace, and belongs to the technical field of industrial magnesium alloy applications. Background Art

[0002] my country has abundant magnesium resources, accounting for more than 70% of the world's magnesium ore resources. The application prospects of my country's magnesium resources are broad. Magnesium alloy has the advantage of low density and is one of the important materials for lightweighting. The development of high-performance magnesium alloy materials is of great significance to further meet the urgent needs of aerospace, transportation, national defense and military industries. Magnesium alloy materials are listed as key strategic materials for the development of rail transit equipment. The application of magnesium alloy materials can greatly reduce the energy consumption of rail transit equipment throughout its life cycle. Actively carry out research and engineering applications of magnesium alloy materials technology, improve the technical level of rail transit equipment, promote the development of China's light alloy materials industry, and strongly support the improvement of China's high-end equipment technology level. In addition, the large-scale application of magnesium alloy forgings in weapons and equipment, aerospace and other fields has great strategic significance for improving the mobility and combat effectiveness of weapons and equipment, reducing the launch cost of spacecraft and improving the carrying capacity, and for national defense and security. The low elongation and difficulty of plastic processing of magnesium alloys have always been problems that need to be solved in the field of material research and development. The development of high-strength and plastic magnesium alloy materials that are easy to process and form is currently a hot research topic. Summary of the invention

[0003] The purpose of the present invention is to provide a Mg-Sn-RE-Al-Ag magnesium alloy suitable for low-temperature forging. The alloy material has the characteristics of wide processing range, low processing cost, high strength and high plasticity, and can meet the demand for lightweight in the fields of automobiles, aerospace, etc.

[0004] Another object of the present invention is to provide a method for preparing the Mg-Sn-RE-Al-Ag magnesium alloy suitable for low temperature forging.

[0005] Based on the above purpose, the inventors have found that by adding tin, aluminum, mixed rare earth and a small amount of silver and other high ductility metals to magnesium, the plasticity of the magnesium alloy can be greatly improved, and the strength of the alloy can be greatly improved, thereby obtaining excellent mechanical properties. The price of the main alloying elements added is relatively low, which can greatly reduce the production and manufacturing cost of the magnesium alloy. At the same time, after adding the above elements, the magnesium alloy can be forged and deformed at low temperatures. On the one hand, it greatly improves the uniformity of the alloy temperature and structure, and on the other hand, it greatly simplifies the deformation processing degree of the magnesium alloy, reduces the difficulty of plastic deformation of the alloy, and further reduces the production and manufacturing cost of the alloy. It can further expand the application volume of rare earth-containing magnesium alloys in the development process of high-end equipment and enhance the application value of magnesium alloys in the field of non-ferrous metals.

[0006] Therefore, in order to achieve the above object, the present invention adopts the following technical solutions: A Mg-Sn-RE-Al-Ag magnesium alloy suitable for low temperature forging, wherein the components and mass percentages are: Sn 6-9%, Ce 1-5%, La 0.2-1%, Nd 0.2-1%, Pr 0.2-1%, Al 0.5-2%, Ag 0.1-1%, and the rest are Mg and unavoidable impurity elements.

[0007] In the Mg-Sn-RE-Al-Ag magnesium alloy suitable for low temperature forging of the present invention, the content of Sn is preferably in the range of 6-8%.

[0008] The present invention also provides a method for preparing the Mg-Sn-RE-Al-Ag magnesium alloy suitable for low temperature forging, comprising the following steps: (1) Ingredients: All alloying elements are added in mass percentage. Sn is added in the form of pure metal, Ce, La, Nd, and Pr are added in the form of mixed rare earths, and Al and Ag alloying elements are added in the form of high-temperature stable master alloys. (2) Melting: Grind the surface of all raw materials clean and dry them in a drying furnace at 180℃-200℃ for 12~24h; melt part of the pure magnesium ingot, and heat the molten magnesium to 825-865℃ and keep it warm for 10-20min, perform electromagnetic stirring and mechanical stirring, add the mixed rare earth, intermediate alloy and the remaining pure magnesium ingot and tin ingot in proportion, quickly reverse mechanical stirring after adding, filter and cool with water mist for 30min after reverse mechanical stirring; after the metal is completely melted, add magnesium alloy flux, keep it warm for 20~45min and then remove the slag; (3) Casting: After standing for 20 minutes, the crucible containing the magnesium alloy melt is mist-cooled, and then water-cooled to obtain a magnesium alloy ingot; (4) Homogenization heat treatment: The ingot is covered with aluminum silicate insulation cotton and subjected to homogenization heat treatment at 460°C-530°C for 48-96 hours to obtain a homogenized ingot sample; (5) Forging: The ingot prepared in step (4) is heated at 150°C-200°C for 4-8 hours, and then forged at low temperature.

[0009] In the step (1), a pure magnesium ingot with a purity of 99.90% or more, a pure tin ingot with a purity of 99.95% or more, a mixed rare earth containing 40-50wt.% of cerium, 5-10wt.% of lanthanum, 5-10wt.% of neodymium, and 5-10wt.% of praseodymium, and an aluminum-silver master alloy containing 30-40wt.% of silver and 60-70% of aluminum are selected.

[0010] In the step (2), a portion of the pure magnesium ingot is heated to 200-250° C., kept warm for 25-35 minutes, and melted under the protection of a gas atmosphere of tetrafluoroethane and argon.

[0011] In step (2), the mixed rare earth and the intermediate alloy are added in proportion in the stirring process, so that the alloy elements can be evenly distributed during the stirring process; at the same time, the melting point difference of the alloy elements and the formation characteristics of the metal compounds between them are used to stir so that the melt maintains a uniform distribution of solutes, and then the low-temperature magnesium ingot and the tin ingot are added along the center of the melt; after all the metal elements are added, mechanical stirring and reverse mechanical stirring are quickly performed, and after reverse mechanical stirring, filtering and water mist cooling for 30 minutes are performed, and the temperature gradient is used to accelerate the freeing of crystal nuclei, induce grain formation and uniform distribution of elements. After slag removal, the top, middle and bottom of the melt are sampled and tested for composition using inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis technology to ensure that the difference in alloy elements added to different parts of the ingot is within 10%. In step (2), the speeds of mechanical stirring and reverse mechanical stirring are preferably 2500-3500 r / min, respectively.

[0012] In the step (6), the forging temperature is 150°C-200°C, and the forging process is six forgings (after roughening the ingot, three forgings and three drawing, and finally returning the center point to its original state) followed by reverse extrusion, and finally forming a cylinder or a conical barrel; during the six forgings, the single-pass pressing amount is not less than 60%, the reverse extrusion ratio is 4:1-10:1, and the forging / forging speed is 0.5-2.0 mm / s.

[0013] Compared with the prior art, the present invention has the following beneficial effects: (1) In the present invention, the main alloying elements are tin and mixed rare earth, and the added elements all have a large solid solubility in magnesium, which achieves high solid solubility of the alloy while reducing the cost of the alloy.

[0014] (2) The addition method of pure metal, mixed rare earth and intermediate alloy can not only refine the original grain of the alloy, but also ensure the uniformity of the added metal elements in the matrix. The added alloy elements do not react with each other in the matrix, which provides a favorable organizational basis for subsequent deformation and improvement of strength and plasticity.

[0015] (3) The addition of a variety of high-ductility metals changes the structure evolution law of the alloy during the deformation process, solves the problem of low elongation and difficulty in deformation processing of magnesium alloys, and ensures that the alloy can be deformed in a wider temperature range.

[0016] (4) The low-temperature forging method, which is lower than the aging temperature, solves the problems of excessive temperature loss, large temperature differences in different parts and large structural inhomogeneity during the forging process. It saves processing costs, simplifies processing steps and further improves the strength and plasticity of the alloy.

[0017] In summary, the present invention adds alloy elements such as tin, cerium, lanthanum, neodymium, praseodymium, silver, and aluminum to pure magnesium in the form of pure metal, mixed rare earth, and intermediate alloy, and combines electromagnetic stirring, mechanical stirring, and reverse mechanical stirring, low temperature compensation, element compensation, low temperature forging below the aging temperature, reverse extrusion, etc., so that the prepared alloy finally has the characteristics of high strength, high plasticity, and easy processing. After forging, the magnesium alloy product has a tensile strength of greater than 500MPa, a yield strength of greater than 400Mpa, and an elongation of greater than 10% in the main direction of the alloy; the tensile strength of the alloy in other directions is greater than 420MPa, the yield strength is greater than 280Mpa, and the elongation is greater than 8%. This type of magnesium alloy has superior strength and plasticity than traditional rare earth series magnesium alloys, is easy to process and shape, and has a lower preparation and processing cost. It can meet the application needs of high-strength and high-plastic magnesium alloys in different industrial fields. DETAILED DESCRIPTION

[0018] The Mg-Sn-RE-Al-Ag magnesium alloy suitable for low temperature forging of the present invention mainly contains tin as the alloying element, and the limiting solid solubility of tin in the magnesium matrix is ​​14.48 wt.%, which can well improve the strength and plasticity of the alloy. By adding mixed rare earth and metal elements such as aluminum and silver with high ductility, the plastic deformation ability of the alloy can be further improved, so that the magnesium alloy of this type can be forged and deformed at low temperature, and uniform organization and performance can be obtained at the same time. The use requirements of high-strength and high-plasticity magnesium alloys in different fields can be met.

[0019] The present invention is described in detail below with reference to specific embodiments.

[0020] Example 1 The composition ratio of the alloy is Mg-6Sn-1.5Ce-0.2La-0.2Nd-0.2Pr-0.5Al-0.1Ag (wt.%), and the Mg-Sn-RE-Al-Ag magnesium alloy forging suitable for low temperature forging is prepared. The preparation process is as follows: (1) All raw materials were polished and dried in a preheated furnace at 180°C for 12 hours. Part of the pure magnesium ingot was melted under the protection of tetrafluoroethane and argon gas atmosphere, and then the molten magnesium was kept at 825°C for 10 minutes, and then electromagnetic stirring (10 minutes) and mechanical stirring (20 minutes, 2500r / min) were performed. During the stirring process, mixed rare earths containing 40wt.% cerium, 5wt.% lanthanum, 5wt.% neodymium, and 5wt.% praseodymium, and aluminum-silver master alloy containing 30wt.% silver were added in proportion, so that the metal elements could be evenly diffused during the stirring process, thereby achieving a uniform distribution effect. At the same time, by taking advantage of the difference in melting points of alloy elements and the formation characteristics of metal compounds between each other, the melt is kept evenly distributed with solutes during the stirring process, and then low-temperature tin ingots and remaining magnesium ingots are added along the center of the melt; after all metal elements are added, reverse mechanical stirring is quickly carried out (10 minutes, 2500r / min), and then filtering and water mist cooling are carried out for 30 minutes. The temperature gradient is used to accelerate the liberation of crystal nuclei, induce grain formation and uniform distribution of elements. After the metal is completely melted, RJ-2 flux is added in a weight ratio of 20:1 to the melt, and the slag is removed after 20 minutes of heat preservation; after removing the slag, the top, middle and bottom of the melt are sampled for composition testing using inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis technology, and the difference in alloy elements added in different parts is within 6%.

[0021] (2) Casting: After standing for 20 minutes, the crucible containing the magnesium alloy melt is mist-cooled and then water-cooled to obtain a magnesium alloy ingot.

[0022] (4) Homogenization heat treatment: The ingot was covered with aluminum silicate insulation cotton and subjected to homogenization heat treatment at 460 °C for 48 h to obtain a homogenized ingot sample.

[0023] (5) Forging: The ingot prepared in step (4) was heated at 150° C. for 4 h, and then forged at 150° C. The forging process was six forgings followed by reverse extrusion to finally form a forged cylinder. During the six forgings, the single-pass pressing amount was 60%, the reverse extrusion ratio was 4:1, and the forging / extrusion speed was 0.5 mm / s.

[0024] After the tensile test, it was found that the tensile strength of the alloy in the main direction (longitudinal direction) was 500MPa, the yield strength was 400Mpa, and the elongation was 12%; the tensile strength in other directions (transverse or high direction) of the alloy was 435MPa, the yield strength was 280Mpa, and the elongation was 10%.

[0025] Example 2 The composition ratio of the alloy is Mg-7Sn-2Ce-0.2La-0.2Nd-0.2Pr-0.5Al-0.2Ag (wt.%), and the Mg-Sn-RE-Al-Ag magnesium alloy forging suitable for low temperature forging is prepared. The preparation process is: (1) All raw materials were polished and dried in a preheated furnace at 190°C for 16 hours. Pure magnesium ingots were melted in a gas atmosphere of tetrafluoroethane and argon, and then the molten magnesium was kept at 830°C for 10 minutes, and then electromagnetically stirred (15 minutes) and mechanically stirred (20 minutes, 2800 r / min). During the stirring process, mixed rare earths containing 45wt.% cerium, 4.5wt.% lanthanum, 4.5wt.% neodymium, and 4.5wt.% praseodymium, and aluminum-silver intermediate alloy containing 35wt.% silver were added in proportion, so that the metal elements could be evenly diffused during the stirring process, thereby achieving a uniform distribution effect. At the same time, by taking advantage of the difference in melting points of alloy elements and the formation characteristics of metal compounds between each other, the melt is kept evenly distributed with solutes during the stirring process, and then low-temperature tin ingots and magnesium ingots are added along the center of the melt; after all metal elements are added, reverse mechanical stirring is quickly performed (10 minutes, 2800r / min), and then filtering and water mist cooling are performed for 30 minutes after reverse mechanical stirring. The temperature gradient is used to accelerate the liberation of crystal nuclei, induce grain formation and even distribution of elements. After the metal is completely melted, RJ-2 flux is added in a weight ratio of 20:1 to the melt, and the slag is removed after 30 minutes of heat preservation. After the slag is removed, the top, middle and bottom of the melt are sampled for composition testing using inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis technology. The difference in alloy elements added in different parts is within 9%.

[0026] (2) Casting: After standing for 20 minutes, the crucible containing the magnesium alloy melt is mist-cooled and then water-cooled to obtain a magnesium alloy ingot.

[0027] (4) Homogenization heat treatment: The ingot was covered with aluminum silicate insulation wool and subjected to homogenization heat treatment at 485 °C for 60 h to obtain a homogenized ingot sample.

[0028] (5) Forging: The ingot prepared in step (4) was heated at 175° C. for 4 h, and then forged at 175° C. The forging process was six forgings followed by reverse extrusion to finally form a forged cylinder. During the six forgings, the single-pass pressing amount was 70%, the reverse extrusion ratio was 4.5:1, and the forging / extrusion speed was 0.8 mm / s.

[0029] After the tensile test, it was found that the tensile strength of the alloy in the main direction (longitudinal direction) was 515MPa, the yield strength was 430Mpa, and the elongation was 11.5%; the tensile strength in other directions (transverse or high direction) of the alloy was 440MPa, the yield strength was 288Mpa, and the elongation was 9.5%.

[0030] Example 3 The composition ratio of the alloy is Mg-8Sn-4Ce-0.5La-0.5Nd-0.5Pr-2Al-1Ag (wt.%), and the Mg-Sn-RE-Al-Ag magnesium alloy forging suitable for low temperature forging is prepared. The preparation process is as follows: (1) All raw materials were polished and dried in a preheated furnace at 200°C for 24 hours. Pure magnesium ingots were melted in a gas atmosphere of tetrafluoroethane and argon, and then the molten magnesium was kept at 865°C for 20 minutes, and then electromagnetically stirred (20 minutes) and mechanically stirred (30 minutes, 3500 r / min). During the stirring process, mixed rare earths containing 48wt.% cerium, 6wt.% lanthanum, 6wt.% neodymium, and 6wt.% praseodymium, and aluminum-silver intermediate alloy containing 38wt.% silver were added in proportion, so that the metal elements could be evenly diffused during the stirring process, thereby achieving a uniform distribution effect. At the same time, by taking advantage of the difference in melting points of alloy elements and the formation characteristics of metal compounds between each other, the melt is kept evenly distributed with solutes during the stirring process, and then low-temperature tin ingots and magnesium ingots are added along the center of the melt; after all metal elements are added, reverse mechanical stirring is quickly carried out (10 minutes, 3500r / min), and then filtering and water mist cooling for 30 minutes are carried out after reverse mechanical stirring. The temperature gradient is used to accelerate the liberation of crystal nuclei, induce grain formation and uniform distribution of elements. After the metal is completely melted, RJ-2 flux is added in a weight ratio of 20:1 to the melt, and the slag is removed after keeping warm for 30 minutes. After removing the slag, the top, middle and bottom of the melt are sampled for composition testing using inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis technology. The difference in alloy elements added in different parts is within 8%.

[0031] (2) Casting: After standing for 20 minutes, the crucible containing the magnesium alloy melt is mist-cooled and then water-cooled to obtain a magnesium alloy ingot.

[0032] (4) Homogenization heat treatment: The ingot was covered with aluminum silicate insulation wool and subjected to homogenization heat treatment at 530 °C for 96 h to obtain a homogenized ingot sample.

[0033] (5) Forging: The ingot prepared in step (4) was heated at 200°C for 8h, and then forged at 200°C. The forging process was six forgings followed by reverse extrusion to finally form a forged cylinder. During the six forgings, the single-pass pressing amount was 65%, the reverse extrusion ratio was 8:1, and the forging / extrusion speed was 1 mm / s.

[0034] After the tensile test, it was found that the tensile strength of the alloy in the main direction (longitudinal direction) was 535MPa, the yield strength was 455Mpa, and the elongation was 10%; the tensile strength in other directions (transverse or high direction) of the alloy was 450MPa, the yield strength was 295Mpa, and the elongation was 8%.

[0035] Example 4 The composition ratio of the alloy is Mg-6.5Sn-2.5Ce-0.5La-0.5Nd-0.5Pr-2Al-0.3Ag (wt.%), and the Mg-Sn-RE-Al-Ag magnesium alloy forging suitable for low temperature forging is prepared. The preparation process is: (1) All raw materials were polished and dried in a preheated furnace at 200°C for 16 hours. Pure magnesium ingots were melted in a gas atmosphere of tetrafluoroethane and argon, and then the molten magnesium was kept at 845°C for 20 minutes, and then electromagnetic stirring (20 minutes) and mechanical stirring (20 minutes, 3500r / min) were performed. During the stirring process, mixed rare earths containing 50wt.% cerium, 10wt.% lanthanum, 10wt.% neodymium, and 10wt.% praseodymium, and aluminum-silver intermediate alloy containing 30wt.% silver were added in proportion, so that the metal elements could be evenly diffused during the stirring process, thereby achieving a uniform distribution effect. At the same time, by taking advantage of the difference in melting points of alloy elements and the formation characteristics of metal compounds between each other, the melt is kept evenly distributed with solutes during the stirring process, and then low-temperature tin ingots and magnesium ingots are added along the center of the melt; after all metal elements are added, reverse mechanical stirring is quickly performed (10 minutes, 3500r / min), and then filtering and water mist cooling are performed for 30 minutes after reverse mechanical stirring. The temperature gradient is used to accelerate the liberation of crystal nuclei, induce grain formation and even distribution of elements. After the metal is completely melted, RJ-2 flux is added in a weight ratio of 20:1 to the melt, and the slag is removed after 45 minutes of heat preservation. After the slag is removed, the top, middle and bottom of the melt are sampled for composition testing using inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis technology. The difference in alloy elements added in different parts is within 8%.

[0036] (2) Casting: After standing for 20 minutes, the crucible containing the magnesium alloy melt is mist-cooled and then water-cooled to obtain a magnesium alloy ingot.

[0037] (4) Homogenization heat treatment: The ingot was covered with aluminum silicate insulation wool and subjected to homogenization heat treatment at 525 °C for 72 h to obtain a homogenized ingot sample.

[0038] (5) Forging: The ingot prepared in step (4) was heated at 185° C. for 6 h, and then forged at 185° C. The forging process was six forgings followed by reverse extrusion to finally form a forged cylinder. During the six forgings, the single-pass pressing amount was 60%, the reverse extrusion ratio was 9:1, and the forging / extrusion speed was 1.5 mm / s.

[0039] After the tensile test, it was measured that the tensile strength of the alloy in the main direction (longitudinal) was 525MPa, the yield strength was 410Mpa, and the elongation was 13.5%; the tensile strength in other directions (transverse or high direction) of the alloy was 425MPa, the yield strength was 285Mpa, and the elongation was 9.5%.

[0040] Example 5 The composition ratio of the alloy is Mg-7.5Sn-2.5Ce-0.5La-0.5Nd-0.5Pr-1Al-0.2Ag (wt.%), and the Mg-Sn-RE-Al-Ag magnesium alloy forging suitable for low temperature forging is prepared. The preparation process is: (1) All raw materials were polished and dried in a preheated furnace at 180°C for 24 hours. Pure magnesium ingots were melted in a gas atmosphere of tetrafluoroethane and argon, and then the molten magnesium was kept at 850°C for 20 minutes, and then electromagnetic stirring (10 minutes) and mechanical stirring (30 minutes, 3000r / min) were performed. During the stirring process, mixed rare earths containing 40wt.% cerium, 8wt.% lanthanum, 8wt.% neodymium, and 8wt.% praseodymium, and aluminum-silver master alloy containing 40wt.% silver were added in proportion, so that the metal elements could be evenly diffused during the stirring process, thereby achieving a uniform distribution effect. At the same time, by taking advantage of the difference in melting points of alloy elements and the formation characteristics of metal compounds between each other, the melt is kept evenly distributed with solutes during the stirring process, and then low-temperature magnesium ingots and tin ingots are added along the center of the melt; after all metal elements are added, reverse mechanical stirring is quickly carried out (10 minutes, 3000r / min), and then filtering and water mist cooling for 30 minutes are carried out after reverse mechanical stirring. The temperature gradient is used to accelerate the liberation of crystal nuclei, induce grain formation and uniform distribution of elements. After the metal is completely melted, RJ-2 flux is added in a weight ratio of 20:1 to the melt, and the slag is removed after 30 minutes of heat preservation. After the slag is removed, the top, middle and bottom of the melt are sampled for composition testing using inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis technology. The difference in alloy elements added in different parts is within 5%.

[0041] (2) Casting: After standing for 20 minutes, the crucible containing the magnesium alloy melt is mist-cooled and then water-cooled to obtain a magnesium alloy ingot.

[0042] (4) Homogenization heat treatment: The ingot was covered with aluminum silicate insulation wool and subjected to homogenization heat treatment at 515 °C for 72 h to obtain a homogenized ingot sample.

[0043] (5) Forging: The ingot prepared in step (4) was heated at 160°C for 6 h, and then forged at 160°C. The forging process was six forgings followed by reverse extrusion to finally form a forged cylinder. During the six forgings, the single-pass pressing amount was 70%, the reverse extrusion ratio was 10:1, and the forging / extrusion speed was 1 mm / s.

[0044] After the tensile test, it was measured that the tensile strength of the alloy in the main direction (longitudinal) was 532MPa, the yield strength was 412Mpa, and the elongation was 14%; the tensile strength in other directions (transverse or high direction) of the alloy was 426MPa, the yield strength was 282Mpa, and the elongation was 8.5%.

[0045] Example 6 The composition ratio of the alloy is Mg-6Sn-3.5Ce-0.7La-0.7Nd-0.7Pr-2Al-0.1Ag (wt.%), and the Mg-Sn-RE-Al-Ag magnesium alloy forging suitable for low temperature forging is prepared. The preparation process is: (1) All raw materials were polished and dried in a preheated furnace at 180°C for 24 hours. Pure magnesium ingots were melted in a gas atmosphere of tetrafluoroethane and argon, and then the molten magnesium was kept at 865°C for 20 minutes, and then electromagnetically stirred (20 minutes) and mechanically stirred (30 minutes, 3500r / min). During the stirring process, mixed rare earths containing 40wt.% cerium, 8wt.% lanthanum, 8wt.% neodymium, and 8wt.% praseodymium, and aluminum-silver master alloy containing 35wt.% silver were added in proportion, so that the metal elements could be evenly diffused during the stirring process, thereby achieving a uniform distribution effect. At the same time, by taking advantage of the difference in melting points of alloy elements and the formation characteristics of metal compounds between each other, the melt is kept evenly distributed with solutes during the stirring process, and then low-temperature magnesium ingots and tin ingots are added along the center of the melt; after all metal elements are added, reverse mechanical stirring is quickly carried out (10 minutes, 3500r / min), and then filtering and water mist cooling are carried out for 30 minutes. The temperature gradient is used to accelerate the liberation of crystal nuclei, induce grain formation and uniform distribution of elements. After the metal is completely melted, RJ-2 flux is added in a weight ratio of 20:1 to the melt, and the slag is removed after keeping warm for 45 minutes. After removing the slag, the top, middle and bottom of the melt are sampled for composition testing using inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis technology. The difference in alloy elements added in different parts is within 6%.

[0046] (2) Casting: After standing for 20 minutes, the crucible containing the magnesium alloy melt is mist-cooled and then water-cooled to obtain a magnesium alloy ingot.

[0047] (4) Homogenization heat treatment: The ingot was covered with aluminum silicate insulation wool and subjected to homogenization heat treatment at 525 °C for 72 h to obtain a homogenized ingot sample.

[0048] (5) Forging: The ingot prepared in step (4) was heated at 150°C for 8h, and then forged at 150°C. The forging process was six forgings followed by reverse extrusion to finally form a forged cylinder. During the six forgings, the single-pass pressing amount was 60%, the reverse extrusion ratio was 6:1, and the forging / extrusion speed was 2 mm / s.

[0049] After the tensile test, it was found that the tensile strength of the alloy in the main direction (longitudinal direction) was 528MPa, the yield strength was 406Mpa, and the elongation was 12%; the tensile strength in other directions (transverse or high direction) of the alloy was 426MPa, the yield strength was 287Mpa, and the elongation was 11%.

Claims

1. A Mg-Sn-RE-Al-Ag magnesium alloy suitable for low temperature forging, characterized in that: Its components and mass percentages are Sn 6-9%, Ce 1-5%, La 0.2-1%, Nd 0.2-1%, Pr 0.2-1%, Al 0.5-2%, Ag 0.1~1%, and the rest are Mg and unavoidable impurity elements.

2. The Mg-Sn-RE-Al-Ag magnesium alloy suitable for low temperature forging according to claim 1, characterized in that: The Sn content ranges from 6-8%.

3. The Mg-Sn-RE-Al-Ag magnesium alloy suitable for low temperature forging according to claim 1 or 2, characterized in that: The lowest forging temperature can reach 100~300℃.

4. The Mg-Sn-RE-Al-Ag magnesium alloy suitable for low temperature forging according to claim 3, characterized in that: The lowest forging temperature can reach 150~200℃.

5. The Mg-Sn-RE-Al-Ag magnesium alloy suitable for low temperature forging according to claim 1 or 2, characterized in that: After forging, the tensile strength of the alloy in the main direction is greater than 500MPa, the yield strength is greater than 400Mpa, and the elongation is greater than 10%; the tensile strength of the alloy in other directions is greater than 420MPa, the yield strength is greater than 280Mpa, and the elongation is greater than 8%.

6. The method for preparing a Mg-Sn-RE-Al-Ag magnesium alloy suitable for low temperature forging according to any one of claims 1 to 5, characterized in that: The steps include: (1) Ingredients: All alloying elements are added in mass percentage. Sn is added in the form of pure metal, Ce, La, Nd, and Pr are added in the form of mixed rare earths, and Al and Ag alloying elements are added in the form of high-temperature stable master alloys. (2) Melting: Grind the surface of all raw materials clean and dry them in a drying furnace at 180℃-200℃ for 12~24h; melt part of the pure magnesium ingot, and heat the molten magnesium to 825-865℃ and keep it warm for 10-20min, perform electromagnetic stirring and mechanical stirring, add mixed rare earth, intermediate alloy and remaining pure magnesium ingot and tin ingot in proportion, and quickly perform reverse mechanical stirring after adding. After reverse mechanical stirring, filter and cool with water mist for 30min. After the metal is completely melted, add magnesium alloy flux in proportion, keep it warm for 20~45min and remove the slag; (3) Casting: After standing for 20 minutes, the crucible containing the magnesium alloy melt is mist-cooled, and then water-cooled to obtain a magnesium alloy ingot; (4) Homogenization heat treatment: The ingot is covered with aluminum silicate insulation cotton and subjected to homogenization heat treatment at 460°C-530°C for 48-96 hours to obtain a homogenized ingot sample; (5) Forging: The ingot prepared in step (4) is heated at 150°C-200°C for 4-8 hours, and then low-temperature forged.

7. The method for preparing the Mg-Sn-RE-Al-Ag magnesium alloy suitable for low temperature forging according to claim 6, characterized in that: In the step (1), a pure magnesium ingot with a purity of 99.90% or more, a pure tin ingot with a purity of 99.95% or more, a mixed rare earth containing 40-50wt.% of cerium, 5-10wt.% of lanthanum, 5-10wt.% of neodymium, and 5-10wt.% of praseodymium, and an aluminum-silver master alloy containing 30-40wt.% of silver and 60-70% of aluminum are selected.

8. The method for preparing the Mg-Sn-RE-Al-Ag magnesium alloy suitable for low temperature forging according to claim 6, characterized in that: In the step (2), a portion of the pure magnesium ingot is heated to 200-250° C., kept warm for 25-35 minutes, and melted under the protection of a gas atmosphere of tetrafluoroethane and argon.

9. The method for preparing the Mg-Sn-RE-Al-Ag magnesium alloy suitable for low temperature forging according to claim 6, characterized in that: In step (2), the speeds of mechanical stirring and reverse mechanical stirring are 2500-3500 r / min respectively.

10. The method for preparing the Mg-Sn-RE-Al-Ag magnesium alloy suitable for low temperature forging according to claim 6, characterized in that: In the step (5), the forging temperature is 150°C-200°C, and the forging process is six forgings followed by reverse extrusion to finally form a cylinder or a conical barrel; during six forgings, the single-pass pressing amount is not less than 60%, the reverse extrusion ratio is 4:1-10:1, and the forging / forging speed is 0.5-2.0 mm / s.