A method for improving the strength and plasticity of additively manufactured titanium alloys by Si

Abstract

The present invention relates to a method for improving the strength and plasticity of additive manufacturing titanium alloys by Si. The method comprises the following steps: (1) mixing a silicon source powder and a titanium source powder to obtain a mixed powder; (2) taking ball milling mixing as an example: ball milling and mixing 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, including direct laser deposition, laser powder bed melting technology, etc. The present invention realizes a significant improvement in the strength and plasticity of titanium alloys without heat treatment.

Description

Technical Field

[0001] The present invention relates to a method for improving the strength and plasticity of additively manufactured titanium alloys by Si, belonging to the technical field of metal preparation. Background Art

[0002] Titanium alloys have been widely used in many fields such as aerospace, chemical industry, marine engineering, and medical devices due to their high specific strength, excellent fatigue resistance, outstanding corrosion resistance and oxidation resistance, as well as excellent damage tolerance characteristics. Despite these advantages of titanium alloys, 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 parts with complex shapes and increase the manufacturing cost. Additive manufacturing technology, with its high material utilization rate and forming ability not limited by part shape and size, 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. During the traditional titanium alloy printing process, coarse columnar grains will be formed, resulting in mechanical property anisotropy, 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 as-printed titanium alloy structure 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 partially decompose or completely eliminate the martensite, but this comes at the cost of strength loss, and also increases the production process and cost.

[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 grains by strengthening constitutional supercooling, so as to achieve 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, and theoretically it also has the effect of eliminating columnar grains 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 reduce 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. The additively manufactured titanium alloys themselves have the problem of poor plasticity, so Si is not considered an effective element to improve the properties of additively manufactured titanium alloys. The key to improving the additively manufactured titanium alloys by Si element is to ensure that Si exists in a solid solution form and does not form titanium silicide compounds. For the problem of the as-printed titanium alloy structure being martensite, 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 improving the strength and plasticity of additively manufactured titanium alloys by Si, aiming at the problems of coarse microstructure and poor strength and plasticity of current additively manufactured titanium alloys. This method introduces supersaturated Si into the titanium alloy without forming brittle titanium silicide compounds, and utilizes Si to increase the constitutional supercooling degree of the alloy to achieve the transformation from columnar crystals to fine equiaxed crystals. At the same time, the lamellar structure is refined to maximize the fine grain strengthening. By taking advantage of the characteristics of high cooling rate in additive manufacturing, Si is forced to dissolve in the titanium alloy without forming brittle titanium silicide compounds. The present invention realizes a significant improvement in the strength and plasticity of titanium alloys without heat treatment.

[0006] To achieve the above object, the present invention is implemented through the following technical solutions:

[0007] A method for improving the strength and plasticity of additively manufactured titanium alloys by Si, comprising the following steps:

[0008] (1) Prepare mixed powder

[0009] Mix according to silicon source powder: titanium source powder to obtain mixed powder;

[0010] Among them, based on the sum of the masses of Si and Ti in the mixed powder as the target component mass, the mass of Si is 0.4% - 4% of the target component mass; the balance is Ti;

[0011] The silicon source is pure silicon powder, SiO2, SiC or silicon-titanium master alloy (TixSiy);

[0012] The titanium source is pure titanium or titanium alloy; the specific titanium alloy is α-titanium alloy, β-titanium alloy or α + β-titanium alloy;

[0013] Preferably, it is Ti6Al4V, TA7 or Ti1023.

[0014] (2) Carry out mechanical mixing treatment on the mixed powder

[0015] Taking ball milling mixing as an example: Under argon protection, ball milling mixing is carried out on the prepared mixed powder. The ball milling parameters are: ball-to-material ratio 2:1 - 15:1, rotation speed 100 - 500 revolutions per minute, and ball milling time 0.5 - 6 hours;

[0016] (3) Forming process

[0017] The ball-milled mixed powder is processed by a forming process to obtain a titanium alloy block.

[0018] The forming process is additive manufacturing, including direct laser deposition, laser powder bed fusion technology, etc.

[0019] When using the direct laser deposition technique, the following steps are included: 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 alternates between 0 - 90° between each layer.

[0020] When using the laser powder bed fusion technique, 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.

[0021] The substantial features of the present invention are as follows:

[0022] In traditional cast / forged titanium alloys, adding silicon easily forms brittle silicides, thereby reducing plasticity, so silicon is usually avoided. However, silicon is beneficial for high-temperature properties, so a small amount of silicon is added to some high-temperature alloys, that is, below the solid solubility, to ensure that silicon does not form brittle silicides. The microstructure of additively manufactured titanium alloys is mostly composed of coarse columnar grains and martensite, which results in significant anisotropy in properties and low strength and plasticity. Currently, mainly by introducing alloying elements such as Fe, B, O, Mo, Cu, etc., the undercooling degree of the molten pool composition is increased, and then the microstructure refinement and the improvement of strength and plasticity are realized. The solubility of Si in titanium is relatively low (0.4 wt%) and it is extremely easy to form brittle silicides, resulting in plastic loss. Therefore, Si is not introduced into titanium alloys as much as possible, and even if introduced, it is controlled below the solid solubility.

[0023] The inventors have found through research that by introducing Si into titanium alloys through additive manufacturing technology, at the high cooling rate of additive manufacturing, Si far exceeding the solubility will also dissolve in the titanium matrix (in Example 1, it is more than 3.5 times the solid solubility). This not only avoids the adverse effect of the formation of brittle silicides on plasticity, but also refines the microstructure, realizing a significant improvement in the strength and plasticity of titanium alloys. Therefore, the substantial feature of the present invention is to successfully introduce supersaturated Si into titanium alloys through additive manufacturing technology without forming brittle silicides, realizing microstructure refinement and a significant improvement in strength and plasticity.

[0024] The beneficial effects of the present invention are as follows:

[0025] In the current technology, it is difficult for additively manufactured Ti6Al4V alloys using other elements as modifiers to simultaneously meet the requirements that the tensile strength is greater than 1300 MPa and the elongation is greater than 10%, while an ideal metallic structural material should maintain good plasticity and toughness at high strength.

[0026] The present invention can significantly improve the strength of materials without sacrificing the elongation of the materials. Taking the most widely used Ti6Al4V alloy as an example, the tensile strength of this titanium alloy printed by the same process as in Example 1 is 1110 MPa, and the fracture elongation is 7.26%. This result is comparable to the currently reported results. The tensile strength of the supersaturated Si-reinforced Ti6Al4V alloy prepared by direct laser deposition in Example 1 is increased to 1407 MPa, and the fracture elongation is increased to 9.61% ( Figure 4 ), and the present invention does not require heat treatment. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is the optical microscope grain diagram of Example 1 and Ti6Al4V prepared by the same process.

[0028] Figure 2 It is the transmission electron microscope morphology diagram of Example 1 and Ti6Al4V prepared by the same process.

[0029] Figure 3 It is the energy spectrum element analysis diagram of Example 1.

[0030] Figure 4 It is the stress-strain curve diagram of Example 1, Example 2 and Ti6Al4V prepared by the same process.

[0031] Figure 5 It is the physical diagram of the titanium alloy of Example 1 and Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0032] The present invention will be further described below in conjunction with the embodiments.

[0033] The preparation process of the Si-reinforced titanium alloy by additive manufacturing of the present invention is as Figure 1 shown. We designed the ball milling of titanium powder and silicon powder, realized the transformation from columnar crystal to equiaxed crystal through subsequent preparation processes, and realized the nanocrystallization of martensite and the solid solution strengthening of supersaturated silicon. Thereby improving the mechanical properties of the additive manufacturing titanium alloy, the present invention provides a method for preparing a titanium alloy with ultra-high strength and plasticity matching by additive manufacturing process without post-treatment. First, the technical route of the present invention will be described.

[0034] (1) Prepare the mixed powder

[0035] Mix the silicon source powder and the titanium source powder to obtain the mixed powder;

[0036] Among them, taking the sum of the masses of Si and Ti in the mixed powder as the target component mass, the mass of silicon is 0.4% - 4% of the target component mass; the balance is Ti;

[0037] The silicon source is pure silicon powder, SiO2, SiC or silicon-titanium master alloy (TixSiy);

[0038] The titanium source is pure titanium or titanium alloy; the specific titanium alloy is α-titanium alloy, β-titanium alloy or α+β-titanium alloy;

[0039] Preferably, it is Ti6Al4V, TA7 or Ti1023.

[0040] (2) Ball milling treatment is carried out on the mixed powder

[0041] Under argon protection, the prepared mixed powder is ball milled and mixed. The ball milling parameters are: ball-to-material ratio of 2:1 to 15:1, rotation speed of 100 to 500 revolutions per minute, and ball milling time of 0.5 to 6 hours;

[0042] (3) Forming process

[0043] The ball-milled mixed powder is processed by a forming process to obtain a bulk titanium alloy.

[0044] The forming process is additive manufacturing, including direct laser deposition, laser powder bed melting technology, etc.

[0045] Using the direct laser deposition technology: the laser power is 600 to 1500 W, the single-pass width is 1.5 to 3 mm, the track spacing is 0.9 to 1.8, the scanning speed is 5 to 15 mm / s, and the angle between each layer alternates between 0 and 90°.

[0046] When using the laser powder bed melting technology, it includes the following steps: the laser power is 100 to 500 W, the scanning speed is 1000 to 2000 mm / s, the rotation angle between each layer is 20 to 90°, and the layer thickness is 0.3 to 0.8 mm.

[0047] The purity of the silicon powder is 99%, and the Ti6Al4V titanium alloy powder is a conventional commercial alloy powder.

[0048] The present invention will be further described below in conjunction with embodiments.

[0049] Example 1

[0050] Weigh 2.5 g of silicon powder and 197.5 g of Ti6Al4V titanium alloy powder (containing 177.75 g of titanium), place them in a 500-ml 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.

[0051] Take out the mixed powder after ball milling and prepare samples by laser deposition process. The laser power is 900 W, the scanning speed is 10 mm / s, the angle alternates by 90° between layers, 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-reinforced 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 two elements Si and Ti in the alloy as the mass of the target component, the mass of silicon is 1.4% of the mass of the target component;)

[0052] Example 2

[0053] Weigh 1 g of silicon powder and 199 g of Ti6Al4V titanium alloy powder, 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, and the ball-to-material ratio is 2.5:1.

[0054] Take out the mixed powder after ball milling and prepare samples by laser deposition process. The laser power is 900 W, the scanning speed is 10 mm / s, the angle alternates by 90° between layers, 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-reinforced Ti6Al4V sample with a length of 80 mm, a width of 30 mm, and a height of 20 mm. The effect is similar to that of Example 1. (Among them, based on the sum of the masses of the two elements Si and Ti in the alloy as the mass of the target component, the mass of silicon is 0.56% of the mass of the target component;)

[0055] Example 3

[0056] Weigh 2.5 g of silicon powder and 197.5 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, and the ball-to-material ratio is 2.5:1. Take out the mixed powder after ball milling and prepare samples by laser powder bed fusion technology. The laser power is 350 W, the scanning speed is 1200 mm / s, the rotation angle between layers is 60°, and the layer thickness is 0.4 mm. Obtain a Si-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.

[0057] The optical microscope test results of Ti6Al4V and Si-reinforced Ti6Al4V prepared by additive manufacturing process in Example 1 are as Figure 1 shown. The Ti6Al4V alloy shows columnar grains, while the alloy in Example 1 shows fine equiaxed grains, indicating that the addition of Si 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 titanium alloy;

[0058] Figure 2 In the titanium alloy of Example 1, the morphology of nano-martensite by transmission electron microscopy shows that the average width of martensite is refined to about 200 nm, which is another reason for the simultaneous improvement of the strength and plasticity of the titanium alloy.

[0059] Figure 3 The energy spectrum of the micro-region of the titanium alloy in Example 1 shows that Si is uniformly distributed in the matrix and no silicide appears, which has the effect of solution strengthening.

[0060] The tensile test results of Example 1 and Example 2 are as Figure 4 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 of Example 1 and Example 2 is significantly improved, and the elongation is also significantly increased. 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 higher strength and non-reduced elongation due to the synergistic strengthening effect of fine equiaxed grains and nano-martensite.

[0061] Figure 5 This is a physical picture of the printed sample, indicating that the sample has high shape stability and good surface quality, which is beneficial for the preparation of complex structural parts.

[0062] The inherently high cooling rate and high thermal gradient of the titanium alloy additive manufacturing process usually lead to high-density micron-sized martensite and almost completely columnar grains, resulting in serious anisotropy of its properties and serious mismatch between strength and plasticity. Preparing an isotropic titanium alloy with both strength and toughness has always been the persistent pursuit of scientific researchers. The present invention can significantly improve the strength and plasticity of existing additive manufacturing titanium alloys. Taking the most typical Ti6Al4V alloy as an example, Example 1 can increase the tensile strength from 1110 MPa to 1407 MPa and the elongation from 7.29% to 9.96%. A method for improving the strength and plasticity of additive manufacturing titanium alloys by supersaturated Si of the present invention can effectively solve the bottleneck and problems of columnar crystal growth and strength-plasticity mismatch of additive manufacturing titanium alloys, and is expected to be applied in the fields of aerospace and military equipment.

[0063] Matters not covered by the present invention are well-known technologies.

Claims

1. A method for improving the strength and plasticity of additively manufactured titanium alloys by Si, characterized in that the method comprises the following steps: (1) Prepare a mixed powder Mix the silicon source powder and the titanium source powder to obtain a mixed powder. Among them, based on the sum of the masses of Si and Ti in the mixed powder as the mass of the target component, the mass of silicon is 0.56% - 4% of the mass of the target component; the balance is Ti. The silicon source is pure silicon powder, SiO2, SiC or silicon-titanium master alloy. 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. (2) Perform mechanical mixing treatment on the mixed powder Taking ball milling and mixing as an example: Under argon protection, ball mill and mix 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. (3) Forming process Use the forming process on the ball-milled mixed powder to obtain a titanium alloy block. The forming process is additive manufacturing. The titanium alloy is Ti6Al4V, TA7 or Ti1023. The forming process is direct laser deposition or laser powder bed fusion technology.

2. The method for improving the strength and plasticity of additively manufactured titanium alloys by Si according to claim 1, characterized in that when using the direct laser deposition technology, it comprises the following steps: laser power is 600 - 1500 W, single-pass width is 1.5 - 3 mm, track spacing is 0.9 - 1.8, scanning speed is 5 - 15 mm / s, and the angle between each layer alternates between 20 - 90°. When using the laser powder bed fusion technology, it comprises the following steps: laser power is 100 - 500 W, 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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