A method for synergistically enhancing the strength and plasticity of an additively manufactured titanium alloy by Si and Fe

Abstract

The present invention relates to a method for synergistically enhancing the strength and plasticity of an additively manufactured titanium alloy by Si and Fe. The method comprises the following steps: (1) mixing a silicon source powder, a titanium source powder and an iron source powder to obtain a mixed powder; (2) carrying out ball milling and mixing on the prepared mixed powder under argon protection; (3) adopting a forming process for the ball-milled mixed powder to obtain a titanium alloy block. The forming process is additive manufacturing, specifically direct laser energy deposition or laser powder bed fusion. The present invention realizes the improvement of strength and plasticity, wherein Si exists in a solid solution form and does not form brittle silicides.

Description

Technical Field

[0001] The present invention relates to a method for synergistically enhancing the strength and plasticity of an additively manufactured titanium alloy by Si and Fe, and belongs to the technical field of metal preparation. Background Art

[0002] Due to its high specific strength, excellent fatigue resistance, outstanding corrosion resistance and oxidation resistance, as well as excellent damage tolerance characteristics, titanium alloys have been widely used in many fields such as aerospace, chemical engineering, ocean engineering and medical devices. Although titanium alloys have these advantages, their characteristics such as low thermal conductivity, high deformation resistance and narrow forging temperature range pose significant challenges to the manufacturing of complex parts. These characteristics may lead to difficulties in forming complex-shaped parts and increase the manufacturing cost. Additive manufacturing technology, with its high material utilization rate and forming ability not limited by the shape and size of parts, not only effectively overcomes these technical bottlenecks, but also realizes the lightweight, high-performance and low-cost of parts.

[0003] The additive manufacturing technology of titanium alloys is gradually becoming mature, but there are still some common problems. Coarse columnar grains will be formed during the traditional titanium alloy printing process, resulting in anisotropic mechanical properties, which is not conducive to application. At the same time, the coarse structure is also not conducive to obtaining high mechanical properties (fine grain strengthening). The structure of as-printed titanium alloy is mostly coarse martensite, which has high strength but poor plasticity (low elongation). It is necessary to improve the plasticity by subsequent heat treatment to decompose or completely eliminate part of the martensite, but this is at the cost of strength loss, and at the same time, the production process and cost are also increased.

[0004] Taking the most widely used Ti6Al4V alloy as an example, the main solution is to introduce alloying elements to inhibit the formation of columnar crystals by strengthening constitutional supercooling, thereby achieving a fine equiaxed grain structure. These elements include Fe, Co, Cu, Mo, etc. The effectiveness of this method has been verified by a large number of experiments. Si is a eutectoid-type β-stabilizing element, which theoretically also has the effect of eliminating columnar crystals and refining grains. Research on traditional cast / forged titanium alloys has confirmed that Si easily reacts with titanium to form brittle titanium silicide compounds, which will lead to a decrease in the plasticity of titanium alloys. Therefore, Si is usually avoided in titanium alloys, and even if introduced, it is below the solubility (0.4 wt%), so the Si content in titanium alloys is lower than this theoretical value. Additive manufacturing titanium alloys themselves have problems with poor plasticity, so Si is not considered an effective element for improving the properties of additive manufacturing titanium alloys. The key to Si improving the properties of additive manufacturing titanium alloys is to ensure that Si exists in a solid solution form and does not form titanium silicide compounds. In addition, there is a limit to supersaturated Si, and the higher the supersaturation degree of Si, the higher the risk of precipitation of brittle silicides during high-temperature use. However, when the supersaturation degree of Si is low, it is not conducive to obtaining higher strength and plasticity. For the problem of the martensite structure in printed titanium alloys, at present, in addition to heat treatment and adjusting the printing process, there is no effective method to solve it through composition design. Summary of the Invention

[0005] The object of the present invention is to provide a method for synergistically enhancing the strength and plasticity of additive manufacturing titanium alloys by Si and Fe in view of the current problems of coarse microstructure and poor strength and plasticity of additive manufacturing titanium alloys. This method introduces Si and Fe into additive manufacturing titanium alloys. By using the extremely high growth restriction factor of Si element to refine the primary β grains and martensite lamellae, and at the same time using the characteristics of high cooling rate in additive manufacturing to dissolve supersaturated silicon in the titanium alloy, thus achieving extremely high grain refinement strengthening effect and solid solution strengthening effect; in addition, using the β-stabilizing element Fe to stabilize the β phase and coordinating material deformation by increasing the β phase content, thereby realizing excellent strength-ductility matching of the material. The present invention realizes the improvement of strength and plasticity, in which Si exists in a solid solution form and does not form brittle silicides.

[0006] The technical solution of the present invention is as follows:

[0007] A method for synergistically enhancing the strength and plasticity of additive manufacturing titanium alloys by Si and Fe, comprising the following steps:

[0008] (1) Prepare a mixed powder

[0009] Mix the silicon source powder, titanium source powder and iron source powder to obtain a mixed powder.

[0010] Among them, taking the sum of the masses of Si, Fe, and Ti in the mixed powder as the mass of the target component, the mass of silicon is 0.4% - 3% of the mass of the target component, and the mass of iron is 0.01% - 4% of the mass of the target component;

[0011] The silicon source is pure silicon powder, SiO 2 , SiC or silicon-titanium master alloy (TixSiy);

[0012] The iron source is pure iron, iron oxide or titanium-iron master alloy (TixFey);

[0013] The titanium source is pure titanium or titanium alloy; the titanium alloy is specifically α-titanium alloy, β-titanium alloy or α + β-titanium alloy;

[0014] Preferably, it is Ti6Al4V, TA7 or Ti1023.

[0015] (2) Ball milling treatment of the mixed powder

[0016] Under argon protection, the prepared mixed powder is ball milled and mixed. The ball milling parameters are: the ball-to-material ratio is 2:1 - 15:1, the rotation speed is 100 - 500 revolutions per minute, and the ball milling time is 0.5 - 6 hours;

[0017] (3) Forming process

[0018] The ball milled mixed powder is processed by a forming process to obtain a titanium alloy block.

[0019] The forming process is additive manufacturing, specifically direct laser energy deposition or laser powder bed fusion.

[0020] When using the direct laser deposition technology, it includes the following steps: the laser power is 600 - 1500 W, the single pass width is 1.5 - 3 mm, the pass spacing is 0.9 - 1.8, the scanning speed is 5 - 15 mm / s, and the angle between each layer alternates between 0 - 90°.

[0021] When using the laser powder bed fusion technology, it includes the following steps: the laser power is 100 - 500 W, the scanning speed is 1000 - 2000 mm / s, the rotation angle between each layer is 20 - 90°, and the layer thickness is 0.3 - 0.8 mm.

[0022] The substantial features of the present invention are:

[0023] Since metals, especially in the additive manufacturing process of titanium alloys, the inherent high cooling rate and high thermal gradient usually lead to high-density micron-sized martensite and almost completely columnar grains, resulting in a serious mismatch between the strength and plasticity of as-deposited titanium alloys. Some early additive manufacturing works have reported the addition of different elements (such as B, O, Mo, and Cu, etc.) to achieve grain refinement. Si is not considered an effective additive for improving the strength and plasticity of additively manufactured titanium alloys due to its low solubility and easy formation of brittle silicides. However, the higher the supersaturation degree of Si, the higher the risk of brittle silicide precipitation during high-temperature applications.

[0024] In the present invention, by introducing Si with a low supersaturation degree (for example, reducing from 3.5 times the solid solubility of the comparative sample to 2 times the solid solubility of Example 1), and simultaneously introducing Fe, the strength and plasticity are improved by the synergistic effect of the two. This not only reduces the risk of supersaturated silicon precipitation to form brittle silicides, but also can achieve a better improvement effect of strength and plasticity than that of high supersaturation degree Si, and the present invention does not require heat treatment. Therefore, the design concept of the present invention is to utilize the synergistic effect of supersaturated Si and solid-solution Fe to achieve the improvement of the strength and plasticity of additively manufactured titanium alloys through solid-solution strengthening, fine-grain strengthening, and phase ratio regulation.

[0025] The beneficial effects of the present invention are as follows:

[0026] Ideal metallic structural materials maintain good plasticity and toughness while having high strength, but there is a significant inverse relationship between the strength and plasticity of the materials, that is, the strength increases while the plasticity and toughness decrease. Taking the most typical Ti6Al4V alloy as an example, by introducing 1.25 wt% of supersaturated Si (3.5 times the solid solubility), the strength is increased from 1110 MPa to 1407 MPa, and the elongation is increased to 9.61%. By introducing 0.75 wt% of Si (2 times the solid solubility) and 0.75 wt% of Fe according to the present invention, the tensile strength is further increased to 1454 MPa, and the fracture elongation is further increased to 10.66% (Example 1), that is, the strength and plasticity are further improved. In addition, due to the decrease in the Si content, the risk of brittle silicide precipitation during high-temperature applications is reduced (by 40%). Description of the Drawings

[0027] Figure 1 Optical microscope grain diagrams of Example 1, Example 2, and Ti6Al4V prepared by the same process.

[0028] Figure 2 Transmission electron microscope morphology diagrams of Example 1, Example 2, and Ti6Al4V prepared by the same process.

[0029] Figure 3 XRD diagrams of Example 1, Example 2, and Ti6Al4V prepared by the same process.

[0030] Figure 4Energy spectrum element analysis diagram of Example 1.

[0031] Figure 5 Stress-strain curve diagrams of Example 1, Example 2, Ti6Al4V prepared by the same process, and Ti6Al4V + 1.25 wt% Si.

[0032] Figure 6 Physical diagrams of the titanium alloys of Example 1 and Example 2. Detailed implementation manners

[0033] The present invention will be further described below in conjunction with examples.

[0034] Aiming at the problems of coarse microstructure and poor strength and plasticity of additively manufactured titanium alloys, the present invention provides a method for improving the strength and plasticity of additively manufactured titanium alloys by utilizing the synergistic effect of Si and Fe. Its action mechanism: ① Utilize Si and Fe to increase the constitutional supercooling degree of the molten pool to achieve the transformation from columnar crystals to equiaxed crystals and refine the microstructure; ② Supersaturated Si and dissolved Fe can play a significant solid solution strengthening role; ③ The introduction of Fe will increase the proportion of β phase in the alloy, which is more conducive to coordinating deformation. First, the technical route of the present invention will be described.

[0035] (1) Prepare mixed powder

[0036] Mix the silicon source powder, titanium source powder and iron source powder to obtain mixed powder.

[0037] Among them, based on the total mass of the three elements Si, Fe and Ti in the mixed powder as the target component mass, the mass of silicon is 0.4% - 3% of the target component mass, and the mass of iron is 0.01% - 4% of the target component mass.

[0038] The silicon source is pure silicon powder, SiO 2 , SiC or silicon-titanium master alloy (TixSiy);

[0039] The iron source is pure iron, iron oxide or titanium-iron master alloy (TixFey);

[0040] The titanium source is pure titanium or a titanium alloy; the titanium alloy is specifically an α-titanium alloy, a β-titanium alloy or an α + β-titanium alloy;

[0041] Preferably, it is Ti6Al4V, TA7 or Ti1023.

[0042] (2) Perform ball milling treatment on the mixed powder

[0043] Under argon protection, perform ball milling and mixing on the prepared mixed powder. Ball milling parameters: ball-to-powder ratio 2:1 - 15:1, rotation speed 100 - 500 revolutions per minute, ball milling time 0.5 - 6 hours;

[0044] (3) Forming process

[0045] The ball-milled mixed powder is processed by a forming process to obtain a titanium alloy block;

[0046] The forming process is additive manufacturing, such as direct laser energy deposition, laser powder bed melting, etc.;

[0047] Here, the direct laser deposition technology in the additive manufacturing process is selected for illustration. Direct laser deposition technology: the laser power is 600 - 1500 W, the single-pass width is 1.5 - 3 mm, the pass spacing is 0.9 - 1.8, the scanning speed is 5 - 15 mm / s, and the angle between each layer alternates between 0 and 90°.

[0048] When using the laser powder bed melting technology, the following steps are included: the laser power is 100 - 500 W, the scanning speed is 1000 - 2000 mm / s, the rotation angle between each layer is 20 - 90°, and the layer thickness is 0.3 - 0.8 mm.

[0049] The purity of the silicon powder and iron powder is greater than 99%, and the Ti6Al4V titanium alloy powder is a conventional commercial powder for additive manufacturing.

[0050] The present invention will be further described below in conjunction with embodiments,

[0051] Example 1

[0052] Weigh 1.5 g of silicon powder, 1.5 g of iron powder, and 197 g of Ti6Al4V titanium alloy powder (containing 177.3 g of titanium), place them in a 500 - milliliter stainless - steel ball - milling jar, and fill it with argon. Use a planetary ball mill to mill for 2 hours at 350 revolutions per minute, with a ball - to - material ratio of 2.5:1.

[0053] Take out the ball - milled mixed powder and use the direct laser energy deposition process for sample preparation. The laser power is 900 W, the scanning speed is 10 mm / s, the angle between each layer alternates 90°, the powder feeding speed is 3 g / min, the single - pass width is 2 mm, and the pass spacing is 1.2 mm. Obtain a Si and Fe co - enhanced Ti6Al4V sample with a length of 80 mm, a width of 30 mm, and a height of 20 mm. (Among them, based on the sum of the masses of the Si, Fe, and Ti three elements in the alloy as the target component mass, the mass of silicon is 0.83% of the target component mass, and the mass of iron is 0.83% of the target component mass)

[0054] Example 2

[0055] Weigh 2.2 g of silicon powder, 2.2 g of iron powder, and 195.6 g of Ti6Al4V titanium alloy powder (containing 176.04 g of titanium), place them in a 500-milliliter stainless-steel ball milling jar, and fill it with argon. Use a planetary ball mill to ball mill for 2 hours at 350 revolutions per minute, with a ball-to-material ratio of 2.5:1. Take out the ball-milled mixed powder and use the direct laser energy deposition process to prepare the sample. The laser power is 900 W, the scanning speed is 10 mm / s, the angle between each layer alternates by 90°, the powder feeding speed is 3 g / min, the single-pass width is 2 mm, and the track spacing is 1.2 mm. Obtain a Si and Fe co-reinforced Ti6Al4V sample with a length of 80 mm, a width of 30 mm, and a height of 20 mm.

[0056] Example 3

[0057] Weigh 1.5 g of silicon powder, 1.5 g of iron powder, and 197 g of Ti6Al4V titanium alloy powder (containing 177.75 g of titanium), place them in a 500-milliliter stainless-steel ball milling jar, and fill it with argon. Use a planetary ball mill to ball mill for 2 hours at 350 revolutions per minute, with a ball-to-material ratio of 2.5:1. Take out the ball-milled mixed powder and use the laser powder bed fusion technology to prepare the sample. The laser power is 350 W, the scanning speed is 1200 mm / s, the rotation angle between each layer is 60°, and the layer thickness is 0.4 mm. Obtain a supersaturated Si and Fe co-reinforced Ti6Al4V sample with a length of 80 mm, a width of 10 mm, and a height of 10 mm. The effect is similar to that of Example 1.

[0058] The optical microscope test results of the Ti6Al4V samples and Si and Fe co-reinforced Ti6Al4V samples prepared by the additive manufacturing process in Example 1 are as follows Figure 1 shown. The Ti6Al4V alloy has columnar grains, while the alloy in Example 1 shows fine equiaxed grains. Further increasing the content of Si and Fe (Example 2), the grains will be further refined, indicating that the addition of Si and Fe can inhibit the formation of columnar crystals and achieve the effect of grain refinement, which is one of the reasons for promoting the simultaneous improvement of the strength and plasticity of the titanium alloy;

[0059] Figure 2 This is the morphology of nano-martensite in the titanium alloy of Example 1 observed by transmission electron microscopy. The width of the martensite is less than about 200 nm, which is another reason for promoting the simultaneous improvement of the strength and plasticity of the titanium alloy.

[0060] Figure 3 This is the XRD pattern of the titanium alloy in Example 1. Using XRD for phase calculation, it is found that the addition of Fe can significantly increase the volume fraction of the β phase. The β phase, as the ductile phase in the titanium alloy, can play a role in coordinating plastic deformation, thereby improving the plasticity of the material.

[0061] Figure 4 It is the energy spectrum diagram of the micro-region of the titanium alloy in Example 1. The results show that Si is uniformly distributed in the matrix, and no silicide appears, which plays the role of solid solution strengthening, while Fe segregates in the β phase.

[0062] The tensile test results of Example 1 are as Figure 5 shown (the test is carried out based on GB / T228.1-2021). It can be seen from the figure that compared with the Ti6Al4V alloy prepared by the same process, the strength and plasticity of the total titanium alloy in Example 1 are significantly improved. Compared with the sample of Example 1, the Ti6Al4V alloy prepared by the same process has lower strength due to its coarse columnar grains and micron-sized martensite. The sample of Example 1 has excellent strength-plasticity matching due to the synergistic strengthening effect of fine equiaxed grains and nano-sized martensite, the solid solution strengthening effect of Si, and the back stress strengthening effect caused by Fe.

[0063] Figure 6 It is the physical picture of the printed sample, indicating that the sample has high shape stability and good surface quality, which is beneficial to the preparation of complex structural parts.

[0064] Titanium alloys have high specific strength, excellent fatigue resistance, outstanding corrosion resistance and oxidation resistance, but there are problems in processing. Additive manufacturing technology can effectively reduce the production cost and manufacturing cycle of complex titanium alloy parts, but the titanium alloys manufactured by additive manufacturing have problems such as coarse grains and poor strength-plasticity. Taking the most common Ti6Al4V alloy as an example, through the present invention, the tensile strength of this alloy can be increased from 1110 MPa to 1454 MPa, and the fracture elongation can be increased from 7.26% to 10.66%. A method for synergistically enhancing the strength and plasticity of additive manufacturing titanium alloys by Si and Fe of the present invention can effectively solve the bottleneck and problems of coarse grains and low strength-plasticity of additive manufacturing titanium alloys, and is expected to be applied in the fields of aerospace and military equipment.

[0065] Matters not covered by the present invention are well-known technologies.

Claims

1. A method for synergistically enhancing the strength and plasticity of titanium alloys manufactured by additive manufacturing using Si and Fe, characterized in that the method comprises the following steps: (1) Preparation of mixed powder The silicon source powder, titanium source powder and iron source powder are mixed to obtain a mixed powder. The target component mass is calculated based on the sum of the mass of Si, Fe and Ti in the mixed powder. The mass of silicon is 0.4% to 3% of the target component mass, and the mass of iron is 0.01% to 4% of the target component mass. The silicon source is pure silicon powder, SiO2, SiC or silicon-titanium master alloy; The iron source is pure iron, iron oxide or titanium-iron master alloy; The titanium source is pure titanium or titanium alloy; the titanium alloy is TA7, Ti1023 or Ti6Al4V; (2) Ball milling of mixed powder The prepared mixed powder is ball-milled under argon protection, and the ball-milling parameters are as follows: ball-to-material ratio 2:1-15:1, rotation speed 100-500 rpm, and ball-milling time 0.5-6 hours; (3) Molding process The mixed powder after ball milling is subjected to a molding process to obtain a titanium alloy block; The molding process described is additive manufacturing.

2. The method for synergistically enhancing the strength and plasticity of titanium alloys in additive manufacturing using Si and Fe as described in claim 1 is characterized in that the molding process is direct laser energy deposition or laser powder bed melting.

3. The method for synergistically enhancing the strength and plasticity of titanium alloys manufactured by additive manufacturing of Si and Fe as claimed in claim 1, characterized in that when it is a direct laser deposition technology, it comprises the following steps: the laser power is 600-1500 W, the single track width is 1.5-3 mm, the track spacing is 0.9-1.8, the scanning speed is 5-15 mm / s, and the layers are alternated by 0-90°; When the laser powder bed melting technology is used, the following steps are included: the laser power is 100-500 W, the scanning speed is 1000-2000 mm / s, the rotation angle between each layer is 20-90°, and the layer thickness is 0.3-0.8 mm.

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