A forming process for high-strength and corrosion-resistant magnesium alloy pipe

Abstract

The invention provides a forming process of a high-strength and corrosion-resistant magnesium alloy pipe, and belongs to the field of alloy materials. The magnesium alloy pipe in the invention comprises the following raw materials in terms of mass percentage: Gd 3.0% to 5.0%, Y 1.5 to 3.0%, Al 1.0 to 3.0%, Zr 0.3 to 0.8%, Mn 0.05 to 0.1%, Cu≤0.2%, Fe≤0.15%, Zn0.5 to 0.8% and the balance Mg, wherein the total amount of Gd, Y and Al is >9%, the mass ratio of Gd, Y and Zr is (6.5 to 10): (3.5 to 6.8): 1, and (Fe+Zn) / Cu is 4.5 to 5.3; the raw materials are mixed in the above proportions, and then smelting, refining, casting, solution heat treatment, pipe extrusion, room temperature rotary forging, aging heat treatment and surface processing are performed in sequence to obtain the magnesium alloy pipe. The present invention significantly improves the corrosion resistance and mechanical properties of the magnesium alloy pipe by optimizing the alloy composition and combining extrusion and room temperature rotary forging.

Description

Technical Field

[0001] The invention belongs to the field of alloy materials, and in particular relates to a forming process for a high-strength and corrosion-resistant magnesium alloy pipe. Background Art

[0002] As an important lightweight metal material, magnesium alloy plays a pivotal role in modern industry, especially in weight-sensitive applications such as aerospace, new energy vehicles and consumer electronics. These application fields have extremely high requirements for materials, requiring not only excellent mechanical properties, but also the ability to meet the use requirements in specific environments. Magnesium alloy is one of the ideal choices due to its unique physical and chemical properties. It has the characteristics of light weight, high specific strength, good electrical conductivity and thermal conductivity, which makes it particularly critical in the design and manufacture of high-performance structural parts.

[0003] However, despite the many advantages of magnesium alloys, they still face some challenges in practical applications. Especially in the preparation process of seamless magnesium alloy tubes, although traditional manufacturing methods such as extrusion and drawing technology can achieve the basic forming of tubes, these processes also bring a series of problems. First, due to the characteristics of magnesium alloys themselves and the limitations of existing processing technologies, seamless tubes produced by extrusion or drawing often exhibit lower mechanical properties and cannot meet the requirements of certain high-strength applications. Secondly, the quality stability of such products is also relatively poor, and there may be significant differences between different batches, which is an obvious shortcoming for industrial applications that pursue consistency and reliability.

[0004] In addition, magnesium alloys also have the problem of insufficient corrosion resistance, especially commonly used commercial magnesium alloy grades such as AZ31 and AZ61, which perform poorly in highly corrosive environments. This deficiency limits their application range under harsh working conditions and may lead to a shortened service life of the finished product, increasing maintenance costs and replacement frequency. In order to solve these problems, researchers have been exploring new alloy formulations and improved processing technologies to improve the overall performance of magnesium alloy materials, enhance their corrosion resistance, and thus expand their applicability in various harsh environments. Summary of the invention

[0005] The purpose of the present invention is to provide a forming process for a high-strength and corrosion-resistant magnesium alloy pipe to solve the problems of poor quality stability, poor mechanical properties and poor corrosion resistance of the above-mentioned magnesium alloy pipe.

[0006] In order to achieve the above object, the present invention provides the following technical solutions:

[0007] A forming process for a high-strength, corrosion-resistant magnesium alloy pipe comprises the following steps:

[0008] (1) Alloy composition design

[0009] The magnesium alloy pipe includes the following raw materials in terms of mass percentage: Gd 3.0%~5.0%, Y 1.5~3.0%, Al 1.0~3.0%, Zr 0.3~0.8%, Mn 0.05~0.1%, Cu≤0.2%, Fe≤0.15%, Zn0.5~0.8% and the balance Mg, wherein the total amount of Gd, Y and Al>9%, the mass ratio of Gd, Y and Zr is (6.5~10):(3.5~6.8):1, (Fe+Zn) / Cu is 4.5~5.3;

[0010] (2) Melting

[0011] The raw materials in step (1) are added into an induction melting furnace according to a certain proportion for melting, and a protective atmosphere is used for protection during the melting process;

[0012] (3) Refining

[0013] Refining agents are added during the smelting process to remove impurities and obtain a molten alloy;

[0014] (4) Casting

[0015] The molten alloy is formed by two-stage extrusion casting to obtain an ingot with a diameter of 85-95 mm;

[0016] (5) Solution heat treatment

[0017] subjecting the ingot to solution heat treatment;

[0018] (6) Extruded pipe

[0019] The ingot after solution heat treatment and the die used for extrusion are coated with a lubricant, and then subjected to a two-stage extrusion process to obtain a pipe;

[0020] (7) Rotary forging at room temperature

[0021] After the extruded pipe is demoulded, the lubricant is evenly applied to its inner and outer surfaces again, the die steel is installed inside the pipe, and the sheath material is added to the outside of the extruded pipe;

[0022] During rotary forging, the spindle speed is 600~1200rpm, the bar feed speed is 200~500mm / min, and after the bar reaches the bottom, the holding mechanism carries the bar back along the original path for secondary rotary forging; the pipe needs to be rotary forged to an outer diameter of 14.3~14.6mm to obtain a rotary forging deformation of more than 30%;

[0023] (8) Aging heat treatment

[0024] After rotary forging, the outer sheath and the inner die steel support are removed, and the pipe is subjected to aging treatment;

[0025] (9) External surface processing

[0026] The tube after aging heat treatment is subjected to surface finishing to obtain a magnesium alloy tube.

[0027] The present invention adds Gd, Y, Al, Zr, Mn, Cu, Fe and Zn to the magnesium alloy. The various metal raw materials work together to form a balanced state at a specific addition amount, thereby improving the mechanical properties of the magnesium alloy pipe. Analysis shows that the addition of the two rare earth elements Gd and Y mainly plays the role of fine grain strengthening and precipitation strengthening. Gadolinium and yttrium can form a stable intermediate phase (such as Mg 12 Gd、Mg 24 Y5, etc.), these intermediate phases can effectively hinder dislocation movement, thereby improving the strength of the alloy. In addition, rare earth elements can also promote grain refinement, and fine grains help improve the toughness of the material. Aluminum can form strengthening phases such as Mg2Al with magnesium, and the presence of these phases can significantly improve the strength of magnesium alloys. Zirconium can form MgZr intermediate phases with magnesium, and these intermediate phases can serve as the core of heterogeneous nucleation during the solidification process to promote grain refinement. The fine grain structure can improve the strength and toughness of the material. Trace added elements such as manganese can purify the alloy, reduce impurities, and improve the purity of the material, thereby improving the overall performance. The present invention also found that Gd, Y, Al, Zr, Cu, Fe and Zn can further improve the corrosion resistance of magnesium alloy pipes in specific proportions. Analysis shows that Gd, Y and Al can form a variety of strengthening phases, such as Mg 12 Gd、Mg 24 Y5 and Mg2Al, etc. The presence of these strengthening phases can hinder the penetration of corrosive media and improve the corrosion resistance of the material. The present invention can ensure that the synergistic effect of these elements in the alloy is maximized by controlling the total amount of Gd, Y and Al to be greater than 9%, and the interaction between rare earth elements and aluminum can form a denser protective film to improve the corrosion resistance of magnesium alloys. The present invention can make these strengthening phases evenly distributed in the matrix by controlling the ratio of Gd, Y and Zr to form a balanced state, so that the effects of rare earth elements and zirconium in the alloy reach the best effect, thereby improving the corrosion resistance of the entire material. When the ratio of Fe and Zn to Cu is controlled within a certain range in the present invention, they can form a relatively stable phase distribution in the magnesium matrix, reducing the possibility of local corrosion, because the presence of Fe and Zn can inhibit the precipitation of Cu, thereby reducing the corrosion active area formed due to Cu enrichment. The ratio of (Fe+Zn) / Cu in the present invention helps to form a more uniform alloy phase distribution, which can reduce the increase in corrosion sensitivity caused by element segregation. Especially when the ratio of (Fe+Zn) / Cu is between 4.5 and 5.3, this combination helps to form a denser corrosion product layer, which can effectively prevent the corrosive medium from further eroding the magnesium matrix.

[0028] Preferably, in step (4), the specific process of the two-stage extrusion casting is:

[0029] The first stage of extrusion casting is to maintain the temperature and pressure at 710~720℃ and 60~80MPa for 20~30s;

[0030] The second stage of extrusion casting is to maintain the temperature and pressure at 660~680℃ and 90~100MPa for 40~60s.

[0031] Preferably, in step (6), the specific process of the two-stage extrusion treatment is:

[0032] The first extrusion process is to extrude a 90 mm ingot into a 20 mm solid bar at 320-350 °C and an extrusion speed of 400-500 mm / min, and then keep it at 280-300 °C for 1-2 hours;

[0033] The second extrusion process is to extrude the 20mm solid bar into a tube with an outer diameter of 17mm and an inner diameter of 12mm at 280~300℃ and an extrusion speed of 300~400mm / min;

[0034] After extrusion, keep the temperature at 260~280℃ for 2~4h to prepare for tissue control at room temperature forging.

[0035] In order to further optimize the performance of magnesium alloy pipes, the present invention also specifically arranges two-stage extrusion casting treatment and two-stage extrusion-insulation treatment to improve the mechanical properties and corrosion resistance of magnesium alloy pipes. In the two-stage extrusion casting process of the present invention, the temperature gradually decreases and the pressure gradually increases. Such temperature and pressure settings are conducive to densification, and fine grains can improve the strength and toughness of the material. Through multi-stage treatment, the stress inside the material can be gradually released, defects such as thermal cracking and pores can be reduced, the density of the material can be improved, and the corrosion resistance of magnesium alloy pipes can be improved.

[0036] In the two-stage extrusion-insulation treatment process of the present invention, the temperature gradually decreases and the extrusion speed also decreases accordingly. This gradual decrease in temperature and speed helps to control the grain refinement process, making the internal structure of the material more uniform, and the fine grains and uniform organization help to form a denser protective film, reduce the erosion of corrosive media, and thus improve the corrosion resistance of the material. The insulation treatment after each stage of extrusion helps to release the internal stress of the material, reduce cracks and pores caused by rapid deformation, and improve the density of the material. Through multi-stage treatment, the pores and cracks inside the material can be reduced, forming a denser organization, which helps to improve the corrosion resistance of the material.

[0037] Preferably, in step (2), the protective atmosphere is one of nitrogen or argon, and the melting temperature is 700-750°C.

[0038] Preferably, in step (5), the solution temperature is 350-375° C., and the solution time is 10-12 h.

[0039] Preferably, in step (6), the lubricant is a mixture of graphite powder and lubricating oil, and the mass ratio of lubricating oil to graphite powder is 8-12:88-92.

[0040] In step (7), a die steel with a diameter of 12.0 mm is installed inside the tube, and an aluminum alloy tube with an inner diameter of 17 mm and an outer diameter of 18 mm is installed outside the extruded tube as a sheath material.

[0041] The die steel in the present invention is mainly used as a structural support and limits the deformation of the inner diameter, and the sheath material is mainly used to provide a three-dimensional stress state for the pipe rotary forging process and reduce the excessive impact load of large rotary forging equipment on the pipe.

[0042] Preferably, in step (8), the temperature of the aging heat treatment is 125-175° C., and the time is 18-24 h.

[0043] Compared with the prior art, the advantages and beneficial effects of the present invention are:

[0044] 1. The present invention adds Gd, Y, Al, Zr, Mn, Cu, Fe and Zn to the magnesium alloy through alloy composition design, which helps to form a strengthening phase, can significantly improve the strength, toughness and corrosion resistance of the magnesium alloy, and enhance the comprehensive mechanical properties of the material.

[0045] 2. The present invention performs solution heat treatment at a specific temperature and time, which can make the alloy elements evenly distributed in the matrix and further improve the strength of the material.

[0046] 3. The addition of rare earth elements such as Gd and Y in the present invention can significantly improve the corrosion resistance of magnesium alloys. The compounds formed by these elements can form a dense protective film on the surface of magnesium alloys to prevent the invasion of corrosive media, thereby improving the corrosion resistance of the material.

[0047] 4. In the present invention, the multi-stage extrusion casting method can gradually refine the grain structure and improve the uniformity and density of the material. This refined grain structure is beneficial to improving the tensile strength and fatigue performance of the material.

[0048] 5. The present invention can further improve the microstructure of the material, enhance its plasticity and toughness through rotary forging, and also correct the dimensional deviation that may occur during the extrusion process.

[0049] 6. The present invention performs multi-stage extrusion-insulation treatment at different temperatures and speeds, which can effectively control the evolution of the microstructure of the material, so that the material obtains a more uniform structure during the molding process, thereby improving its mechanical properties and corrosion resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 This is a diagram of the tensile mechanical properties of the magnesium alloy tubes prepared in Examples 1-3. DETAILED DESCRIPTION

[0051] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0052] Example 1

[0053] This embodiment provides a forming process for a high-strength and corrosion-resistant magnesium alloy tube, comprising the following steps:

[0054] (1) Alloy composition design

[0055] The magnesium alloy tube includes the following raw materials in terms of mass percentage: Gd 4.8%, Y 2.5%, Al 2.5%, Zr 0.5%, Mn 0.06%, Cu 0.15%, Fe 0.1%, Zn 0.65% and the balance Mg, wherein the total amount of Gd, Y and Al is >9%, the mass ratio of Gd, Y and Zr is 9.6:5:1, and (Fe+Zn) / Cu is 5;

[0056] (2) Melting

[0057] The raw materials in step (1) are added into an induction melting furnace according to a certain proportion for melting, and a protective atmosphere is used for protection during the melting process;

[0058] The protective atmosphere is argon and the melting temperature is 735°C;

[0059] (3) Refining

[0060] A refining agent is added during the smelting process to remove impurities, the refining agent is NaF, and the amount of the refining agent added is 10wt%, to obtain a molten alloy;

[0061] (4) Casting

[0062] The molten alloy is formed by two-stage extrusion casting to obtain an ingot with a diameter of 90 mm;

[0063] The first stage of squeeze casting is to maintain the temperature and pressure at 715℃ and 70MPa for 25s;

[0064] The second stage of squeeze casting is to maintain the temperature and pressure at 670℃ and 95MPa for 60s;

[0065] (5) Solution heat treatment

[0066] subjecting the ingot to solution heat treatment;

[0067] The solution temperature is 350°C and the solution time is 12h;

[0068] (6) Extruded pipe

[0069] Applying a lubricant to the ingot after solution heat treatment and the die used for extrusion, the lubricant is a mixture of graphite powder and lubricating oil, the mass ratio of lubricating oil to graphite powder is 10:90, and then performing two-stage extrusion treatment to obtain a pipe;

[0070] The lubricating oil can be purchased from Suzhou Benair Chemical Technology Co., Ltd., or lubricating oil commonly used in the art can be used;

[0071] The first extrusion process is to extrude a 90 mm ingot into a 20 mm solid bar at 330 °C and an extrusion speed of 450 mm / min, followed by keeping at 290 °C for 1.5 h;

[0072] The second extrusion process is to extrude the 20 mm solid bar into a tube with an outer diameter of 17 mm and an inner diameter of 12 mm at 290°C and an extrusion speed of 350 mm / min.

[0073] After extrusion, keep the temperature at 270℃ for 3h to prepare for the structure control of room temperature rotary forging;

[0074] (7) Rotary forging at room temperature

[0075] After the extruded tube is demoulded, the inner and outer surfaces are evenly coated with lubricant again. The lubricant is a mixture of graphite powder and lubricating oil. The mass ratio of lubricating oil to graphite powder is 10:90. A die steel with a diameter of 12.0 mm is installed inside the tube as a structural support and to limit the deformation of the inner diameter. At the same time, an aluminum alloy tube with an inner diameter of 17 mm and an outer diameter of 18 mm is installed outside the extruded tube as a sheath material to provide a three-dimensional stress state for the rotary forging process of the tube and reduce the excessive impact load of the large rotary forging equipment on the tube.

[0076] During rotary forging, the spindle speed is 900rpm, the bar feed speed is 300mm / min, and after the bar reaches the bottom, the holding mechanism carries the bar back along the original path for secondary rotary forging; the pipe needs to be rotary forged to an outer diameter of 14.4mm to obtain a rotary forging deformation of more than 30%;

[0077] (8) Aging heat treatment

[0078] After rotary forging, the outer sheath and the inner die steel support are removed, and the pipe is subjected to aging treatment;

[0079] The temperature of aging heat treatment is 125℃ and the time is 18h;

[0080] (9) External surface processing

[0081] The tube after aging heat treatment is subjected to surface finishing to obtain a magnesium alloy tube with an outer diameter of 14 mm.

[0082] Example 2

[0083] This embodiment provides a forming process for a high-strength and corrosion-resistant magnesium alloy tube, comprising the following steps:

[0084] (1) Alloy composition design

[0085] The magnesium alloy tube includes the following raw materials in terms of mass percentage: Gd 3.8%, Y 3.0%, Al 2.8%, Zr 0.45%, Mn 0.1%, Cu 0.18%, Fe 0.15%, Zn 0.714% and the balance Mg, wherein the total amount of Gd, Y and Al is >9%, the mass ratio of Gd, Y and Zr is 8.44:6.66:1, and (Fe+Zn) / Cu is 4.8;

[0086] (2) Melting

[0087] The raw materials in step (1) are added into an induction melting furnace according to a certain proportion for melting, and a protective atmosphere is used for protection during the melting process;

[0088] The protective atmosphere is argon and the melting temperature is 720°C;

[0089] (3) Refining

[0090] A refining agent is added during the smelting process to remove impurities, the refining agent is NaF, and the amount of the refining agent added is 10wt%, to obtain a molten alloy;

[0091] (4) Casting

[0092] The molten alloy is formed by two-stage extrusion casting to obtain an ingot with a diameter of 90 mm;

[0093] The first stage of squeeze casting is to maintain the temperature and pressure at 710℃ and 80MPa for 20s;

[0094] The second stage of squeeze casting is to maintain the temperature and pressure at 660°C and 100MPa for 50s;

[0095] (5) Solution heat treatment

[0096] subjecting the ingot to solution heat treatment;

[0097] The solution temperature is 355°C and the solution time is 12h;

[0098] (6) Extruded pipe

[0099] Applying a lubricant to the ingot after solution heat treatment and the die used for extrusion, the lubricant is a mixture of graphite powder and lubricating oil, the mass ratio of lubricating oil to graphite powder is 10:90, and then performing two-stage extrusion treatment to obtain a pipe;

[0100] The first extrusion process was to extrude a 90 mm ingot into a 20 mm solid bar at 325 °C and an extrusion speed of 400 mm / min, followed by keeping at 280 °C for 1 h;

[0101] The second extrusion process is to extrude the 20 mm solid bar into a tube with an outer diameter of 17 mm and an inner diameter of 12 mm at 280°C and an extrusion speed of 320 mm / min.

[0102] After extrusion, keep the temperature at 260℃ for 4h to prepare for the structure control of room temperature rotary forging;

[0103] (7) Rotary forging at room temperature

[0104] After the extruded tube is demoulded, the inner and outer surfaces are evenly coated with lubricant again. The lubricant is a mixture of graphite powder and lubricating oil. The mass ratio of lubricating oil to graphite powder is 10:90. A die steel with a diameter of 12.0 mm is installed inside the tube as a structural support and to limit the deformation of the inner diameter. At the same time, an aluminum alloy tube with an inner diameter of 17 mm and an outer diameter of 18 mm is installed outside the extruded tube as a sheath material to provide a three-dimensional stress state for the rotary forging process of the tube and reduce the excessive impact load of the large rotary forging equipment on the tube.

[0105] During rotary forging, the spindle speed is 900rpm, the bar feed speed is 300mm / min, and after the bar reaches the bottom, the holding mechanism carries the bar back along the original path for secondary rotary forging; the pipe needs to be rotary forged to an outer diameter of 14.35mm to obtain a rotary forging deformation of more than 30%;

[0106] (8) Aging heat treatment

[0107] After rotary forging, the outer sheath and the inner die steel support are removed, and the pipe is subjected to aging treatment;

[0108] The temperature of aging heat treatment is 135℃ and the time is 20h;

[0109] (9) External surface processing

[0110] The tube after aging heat treatment is subjected to surface finishing to obtain a magnesium alloy tube with an outer diameter of 14 mm.

[0111] Example 3

[0112] This embodiment provides a forming process for a high-strength and corrosion-resistant magnesium alloy tube, comprising the following steps:

[0113] (1) Alloy composition design

[0114] The magnesium alloy tube includes the following raw materials in terms of mass percentage: Gd 5.5%, Y 3.0%, Al 3.0%, Zr 0.8%, Mn 0.05%, Cu 0.13%, Fe 0.126%, Zn 0.55% and the balance Mg, wherein the total amount of Gd, Y and Al is >9%, the mass ratio of Gd, Y and Zr is 6.875:3.75:1, and (Fe+Zn) / Cu is 5.2;

[0115] (2) Melting

[0116] The raw materials in step (1) are added into an induction melting furnace according to a certain proportion for melting, and a protective atmosphere is used for protection during the melting process;

[0117] The protective atmosphere is argon and the melting temperature is 745°C;

[0118] (3) Refining

[0119] A refining agent is added during the smelting process to remove impurities, the refining agent is NaF, and the amount of the refining agent added is 10wt%, to obtain a molten alloy;

[0120] (4) Casting

[0121] The molten alloy is formed by two-stage extrusion casting to obtain an ingot with a diameter of 90 mm;

[0122] The first stage of squeeze casting is to maintain the temperature and pressure at 720℃ and 60MPa for 30s;

[0123] The second stage of squeeze casting is to maintain the temperature and pressure at 680℃ and 90MPa for 60s;

[0124] (5) Solution heat treatment

[0125] subjecting the ingot to solution heat treatment;

[0126] The solution temperature is 360°C and the solution time is 11h;

[0127] (6) Extruded pipe

[0128] Applying a lubricant to the ingot after solution heat treatment and the die used for extrusion, the lubricant is a mixture of graphite powder and lubricating oil, the mass ratio of lubricating oil to graphite powder is 10:90, and then performing two-stage extrusion treatment to obtain a pipe;

[0129] The first extrusion process is to extrude a 90 mm ingot into a 20 mm solid bar at 345 °C and an extrusion speed of 560 mm / min, followed by keeping at 295 °C for 2 h;

[0130] The second extrusion process is to extrude the 20 mm solid bar into a tube with an outer diameter of 17 mm and an inner diameter of 12 mm at 295°C and an extrusion speed of 380 mm / min.

[0131] After extrusion, keep the temperature at 280℃ for 2h to prepare for the structure control of room temperature rotary forging;

[0132] (7) Rotary forging at room temperature

[0133] After the extruded tube is demoulded, the lubricant is evenly applied to its inner and outer surfaces again. The lubricant is a mixture of graphite powder and lubricating oil. The mass ratio of lubricating powder to graphite powder is 10:90. A die steel with a diameter of 12.0 mm is installed inside the tube as a structural support and to limit the deformation of the inner diameter. At the same time, an aluminum alloy tube with an inner diameter of 17 mm and an outer diameter of 18 mm is installed outside the extruded tube as a sheath material to provide a three-dimensional stress state for the tube rotary forging process and reduce the excessive impact load of large rotary forging equipment on the tube.

[0134] During rotary forging, the spindle speed is 1000rpm, the bar feed speed is 350mm / min, and after the bar reaches the bottom, the holding mechanism carries the bar back along the original path for secondary rotary forging; the pipe needs to be rotary forged to an outer diameter of 14.45mm to obtain a rotary forging deformation of more than 30%;

[0135] (8) Aging heat treatment

[0136] After rotary forging, the outer sheath and the inner die steel support are removed, and the pipe is subjected to aging treatment;

[0137] The temperature of aging heat treatment is 150℃ and the time is 18h;

[0138] (9) External surface processing

[0139] The tube after aging heat treatment is subjected to surface finishing to obtain a magnesium alloy tube with an outer diameter of 14 mm.

[0140] Comparative Example 1

[0141] The difference between this comparative example and Example 1 is that the composition of the magnesium alloy tube is different, and the addition amount of Y and Al is reduced. In this comparative example, the magnesium alloy tube includes the following raw materials in terms of mass percentage: Gd 4.8%, Y 1.5%, Al 1.0%, Zr 0.5%, Mn 0.06%, Cu 0.15%, Fe 0.1%, Zn 0.65% and the balance Mg, wherein the total amount of Gd, Y and Al is <9%, the mass ratio of Gd, Y and Zr is 9.6:3:1, and (Fe+Zn) / Cu is 5.

[0142] Comparative Example 2

[0143] The difference between this comparative example and Example 1 is that the composition of the magnesium alloy tube is different, and the amount of Zr added is increased. In this comparative example, the magnesium alloy tube includes the following raw materials in terms of mass percentage: Gd 4.8%, Y 1.5%, Al 1.0%, Zr 1.5%, Mn 0.06%, Cu 0.15%, Fe 0.1%, Zn 0.65% and the balance Mg, wherein the total amount of Gd, Y and Al is >9%, the mass ratio of Gd, Y and Zr is 3.2:1:1, and (Fe+Zn) / Cu is 5.

[0144] Comparative Example 3

[0145] The difference between this comparative example and Example 1 is that the composition of the magnesium alloy tube is different, and the addition amount of Cu and Zn is reduced. In this comparative example, the magnesium alloy tube includes the following raw materials in terms of mass percentage: Gd 4.8%, Y 2.5%, Al 2.5%, Zr 0.5%, Mn 0.06%, Cu 0.05%, Fe 0.1%, Zn 0.4% and the balance Mg, wherein the total amount of Gd, Y and Al is greater than 9%, the mass ratio of Gd, Y and Zr is 9.6:5:1, and (Fe+Zn) / Cu is 10.

[0146] Comparative Example 4

[0147] The difference between this comparative example and Example 1 is that the extrusion casting process is different. The extrusion casting process of this comparative example is:

[0148] The molten alloy was kept at 715°C and 55 MPa for 85 seconds to obtain an ingot with a diameter of 90 mm.

[0149] Comparative Example 5

[0150] The difference between this comparative example and Example 1 is that the process of extruding the pipe is different. The specific process of extruding the pipe in this comparative example is:

[0151] The lubricant for the die coating used for the ingot after solution heat treatment and extrusion is a mixture of graphite powder and lubricating oil, and the mass ratio of lubricating oil to graphite powder is 10:90. Then, at 330°C and an extrusion speed of 450mm / min, a 90mm ingot is extruded into a 20mm solid bar, and then the 20mm solid bar is extruded into a tube with an outer diameter of 17mm and an inner diameter of 12mm.

[0152] Performance Testing

[0153] The yield strength, tensile strength and elongation of the magnesium alloy tubes prepared in the examples and comparative examples were tested in accordance with ASTM E8 / E8M-13a “Metallic Materials Tensile Test Methods”.

[0154] The magnesium alloy tubes prepared in the embodiment and the comparative example were immersed in a 3.5wt% NaCl solution for 7 days, and then the corrosion rate was calculated. The test results are shown in Tables 1 and Figure 1 .

[0155] Corrosion rate = (mass of magnesium alloy tube before immersion - mass of magnesium alloy tube after immersion) / (specific surface area of ​​magnesium alloy tube × number of immersion days).

[0156] Table 1 Performance test results

[0157]

[0158] It can be seen from the above performance test results that the magnesium alloy tubes of Examples 1-3 have excellent mechanical properties and good corrosion resistance, especially the comprehensive performance of Example 3 is the most outstanding. This is mainly because the corrosion resistance and mechanical properties of the magnesium alloy tube are significantly improved by optimizing the alloy composition and combining extrusion and room temperature rotary forging.

[0159] The comparative example, because it did not adopt the necessary technical solution, was significantly inferior to the embodiment in the corresponding performance test. In comparative example 1, the addition amount of Y and Al was changed. Although the addition amount was still within the scope of the present invention, the change in the addition amount resulted in the total amount of Gd, Y and Al <9%, and the mass ratio of Gd, Y and Zr also changed, destroying the equilibrium state of Gd, Y, Al and Zr in the magnesium alloy. From the results, it can be seen that the mechanical properties and corrosion resistance of the magnesium alloy pipe were significantly reduced, proving that the total amount of Gd, Y and Al and the mass ratio of Gd, Y and Zr have an important influence on the comprehensive performance of the magnesium alloy pipe. In comparative example 2, the addition amount of Zr was increased, which exceeded the scope of the present invention. From the results, it can be seen that the mechanical properties and corrosion resistance of the magnesium alloy pipe prepared therefrom were reduced to varying degrees, indicating that the limitation of the addition amount of various raw materials added in the present invention has an important influence on the comprehensive performance improvement of the magnesium alloy pipe. In comparative example 3, the addition amount of copper and zinc was changed, and the ratio of (Fe+Zn) / Cu exceeded the range defined in the present invention. From the results, it can be seen that the mechanical properties and corrosion resistance of the magnesium alloy tube were significantly reduced, proving that the ratio of (Fe+Zn) / Cu has an important influence on the improvement of the comprehensive performance of the magnesium alloy tube. In comparative example 4, the extrusion casting process was changed to one stage. From the results, it can be seen that the comprehensive performance of the magnesium alloy tube prepared by it was reduced, indicating that the casting process defined in the present invention has a very important influence on improving the mechanical properties and corrosion resistance of the alloy. In comparative example 5, the two-stage extrusion-insulation treatment process was changed to one stage. From the results, it can be seen that the comprehensive performance of the magnesium alloy tube prepared by it was significantly reduced, indicating that the two-stage extrusion-insulation treatment process in the present invention has a very important influence on the preparation of the magnesium alloy tube, and also proves the synergistic effect of the two-stage extrusion casting and the two-stage extrusion-insulation treatment process in the present invention. The above experimental results further prove the importance of the technical scheme defined in the present invention for its technical effect.

[0160] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A forming process for a high-strength and corrosion-resistant magnesium alloy tube, characterized in that: The following steps are involved: (1) Alloy composition design The magnesium alloy pipe includes the following raw materials in terms of mass percentage: Gd 3.0%~5.0%, Y 1.5~3.0%, Al 1.0~3.0%, Zr 0.3~0.8%, Mn 0.05~0.1%, Cu≤0.2%, Fe≤0.15%, Zn0.5~0.8% and the balance Mg, wherein the total amount of Gd, Y and Al>9%, the mass ratio of Gd, Y and Zr is (6.5~10):(3.5~6.8):1, (Fe+Zn) / Cu is 4.5~5.3; (2) Melting The raw materials in step (1) are added into an induction melting furnace according to a certain proportion for melting, and a protective atmosphere is used for protection during the melting process; (3) Refining Refining agents are added during the smelting process to remove impurities and obtain a molten alloy; (4) Casting The molten alloy is formed by two-stage extrusion casting to obtain an ingot with a diameter of 85-95 mm; (5) Solution heat treatment subjecting the ingot to solution heat treatment; (6) Extruded pipe The ingot after solution heat treatment and the die used for extrusion are coated with a lubricant, and then subjected to a two-stage extrusion process to obtain a pipe; (7) Rotary forging at room temperature After the extruded pipe is demoulded, the lubricant is evenly applied to its inner and outer surfaces again, the die steel is installed inside the pipe, and the sheath material is added to the outside of the extruded pipe; During rotary forging, the spindle speed is 600~1200rpm, the bar feed speed is 200~500mm / min, and after the bar reaches the bottom, the clamping mechanism carries the bar back along the original path for secondary rotary forging; the pipe needs to be rotary forged to an outer diameter of 14.3~14.6mm to obtain a rotary forging deformation of more than 30%; (8) Aging heat treatment After rotary forging, the outer sheath and the inner die steel support are removed, and the pipe is subjected to aging treatment; (9) External surface processing The tube after aging heat treatment is subjected to surface finishing to obtain a magnesium alloy tube.

2. The forming process of a high-strength and corrosion-resistant magnesium alloy tube according to claim 1 is characterized in that: In step (2), the protective atmosphere is one of nitrogen or argon, and the melting temperature is 700-750°C.

3. The forming process of a high-strength and corrosion-resistant magnesium alloy tube according to claim 1 is characterized in that: The specific process of two-stage squeeze casting is: The first stage of extrusion casting is to maintain the temperature and pressure at 710~720℃ and 60~80MPa for 20~30s; The second stage of extrusion casting is to maintain the temperature and pressure at 660~680℃ and 90~100MPa for 40~60s.

4. The forming process of a high-strength and corrosion-resistant magnesium alloy tube according to claim 1, characterized in that: In step (5), the solution temperature is 350-375°C and the solution time is 10-12 hours.

5. The forming process of a high-strength and corrosion-resistant magnesium alloy tube according to claim 1, characterized in that: In step (6), the lubricant is a mixture of graphite powder and lubricating oil, and the mass ratio of lubricating oil to graphite powder is 8-12:88-92.

6. The forming process of a high-strength and corrosion-resistant magnesium alloy tube according to claim 1, characterized in that: In step (6), The specific process of the two-stage extrusion process is: The first extrusion process is to extrude a 90 mm ingot into a 20 mm solid bar at 320-350 °C and an extrusion speed of 400-500 mm / min, and then keep it at 280-300 °C for 1-2 hours; The second extrusion process is to extrude the 20mm solid bar into a tube with an outer diameter of 17mm and an inner diameter of 12mm at 280~300℃ and an extrusion speed of 300~400mm / min; After extrusion, keep the temperature at 260~280℃ for 2~4h to prepare for tissue control at room temperature.

7. The forming process of a high-strength and corrosion-resistant magnesium alloy tube according to claim 1, characterized in that: In step (7), a die steel with a diameter of 12.0 mm is installed inside the tube, and an aluminum alloy tube with an inner diameter of 17 mm and an outer diameter of 18 mm is installed outside the extruded tube as a sheath material.

8. The forming process of a high-strength and corrosion-resistant magnesium alloy tube according to claim 1, characterized in that: In step (8), the temperature of the aging heat treatment is 125-175°C and the time is 18-24 hours.

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