A dense Ti-6Al-4V alloy

Abstract

This invention relates to the field of titanium alloy preparation technology, specifically to a dense Ti-6Al-4V alloy prepared by injection molding. The invention involves mixing a binder and Ti-6Al-4V pre-alloy powder to obtain a feedstock, then injection molding the feedstock to obtain a green compact. The green compact is then degreased and vacuum sintered to obtain a dense Ti-6Al-4V alloy. The feedstock contains 63-65 vol% Ti-6Al-4V pre-alloy powder. The binder is composed of polyoxymethylene, high-density polyethylene, ethylene-vinyl acetate copolymer, and ethylene bis-stearamide in a mass ratio of 85-90:1-2:5-10:1-10. The Ti-6Al-4V pre-alloy powder has a D... 10 It is 4.5~5.5μm and D 90 The thickness is 18~20μm. The process of this invention is simple and controllable, and the resulting product has excellent performance, making it easy for industrial applications.

Description

Technical Field

[0001] This invention relates to the field of titanium alloy preparation technology, specifically to a dense Ti-6Al-4V alloy prepared by injection molding. Background Technology

[0002] Metal injection molding (MIM) is a technology that involves mixing metal powder with a binder, molding the mixture using injection molding, and then debinding and sintering to obtain the final product. Due to its near-net-shape forming and high material utilization, it is increasingly used in the production of complex parts. Titanium alloys are lightweight, possess high mechanical properties, strong corrosion resistance, and good biocompatibility, making them important materials for applications in biomedicine, aerospace, and other fields. Traditional machining involves significant material waste, and the machining cost of complex parts is high, while machinability faces significant challenges. This has promoted the application of MIM technology in the production of titanium alloy parts.

[0003] In the field of MIM (Metal Injection Molding), adhesives are one of the core research areas. Currently, commonly used adhesives include three types: plastic-based, wax-based, and water-based. POM (polyoxymethylene) plastic-based adhesives are widely used in the MIM industry because they can decompose in an acidic atmosphere at lower temperatures, reducing carbon and oxygen impurities. This acid-catalyzed decomposition method is also beneficial for maintaining the shape of complex-shaped parts. For example, patent application CN120347211A involves using POM as an adhesive to prepare high-polish titanium alloy plug housings through injection molding, but it does not involve a scheme of multiple adhesive combinations.

[0004] In the MIM process, the feedstock also significantly impacts the quality of the final product. On one hand, insufficient feedstock flowability can prevent the mold cavity from filling completely during injection molding, leading to severe molding defects. On the other hand, poor compatibility between the binder and powder, as well as between binder components, can cause phase separation, resulting in sintering defects such as voids and cracks. This can severely reduce the mechanical properties, corrosion resistance, and fatigue performance of MIM titanium alloys. Furthermore, insufficient density can lead to severe surface porosity defects in MIM products during subsequent polishing and other aesthetic processes, resulting in a high scrap rate. This severely restricts the yield and industrial production of MIM titanium alloys.

[0005] This invention is proposed to prepare MIM Ti-6Al-4V alloys with higher density and superior mechanical properties. Summary of the Invention

[0006] To address the aforementioned problems, the present invention aims to provide a dense Ti-6Al-4V alloy prepared by injection molding. Under the synergistic effect of raw material size and appropriate amount and proportion of binder components, it can avoid bulging and large pore defects during the debinding and sintering process, and significantly improve the sintering density, thereby obtaining high-performance Ti-6Al-4V alloy products.

[0007] This invention, through in-depth research on the debinding and densification behavior in the MIM process, proposes the following binder optimization design criteria:

[0008] To avoid binder component separation and high-temperature decomposition recalcitrant component accumulation: During the catalytic degreasing stage, if the surface of the green body is enriched with binder components with high decomposition temperatures, the gases generated by the initial internal decomposition will have difficulty escaping, easily causing the surface to bulge and form defects. Therefore, appropriately reducing the amount of binder components and ensuring good compatibility among the binder components helps avoid significant phase separation between binders and between binder and powder during mixing and injection molding, thus minimizing the accumulation of certain components on the product surface and preventing gas escape. In particular, the content of high-decomposition-temperature components should be minimized. This not only prevents excessive impurities from remaining during high-temperature decomposition but also prevents the high pressure caused by hindering the escape of low-temperature decomposition components, which could enlarge porosity.

[0009] Improving green uniformity to enhance sintering density: Uneven flow and phase separation of the binder during injection molding lead to uneven powder distribution in the green body, with localized areas of low powder content and binder-rich zones. After debinding, these areas leave behind large pores. During the technical development process, this invention discovered that although atoms exhibit significant diffusion displacement along grain boundaries, when the "valley" region of the sintering neck is "filled," the pores surrounded by numerous particles tend to form a stable structure with regular curvature. Subsequently, diffusion dominated by pore surface smoothing and curvature adjustment limits effective densification. However, uniform powder distribution can better avoid this phenomenon because such a system has a connected grain boundary diffusion network, enabling rapid densification. Therefore, binder design should ensure uniform powder dispersion and good compatibility among components in the feedstock to obtain a uniform injection green body, which facilitates the gradual closure and filling of pores during sintering, thereby improving sintering density.

[0010] Based on the above design criteria, this invention selects a multi-component binder system with POM as the main component and optimizes the distribution ratio of each component to make the prepared Ti-6Al-4V feedstock have excellent flowability and the green body has a highly uniform and stable microstructure, thus providing the necessary conditions for obtaining high-density sintered products without bulges or large pores.

[0011] This invention discloses a dense Ti-6Al-4V alloy, which is obtained by mixing a binder and Ti-6Al-4V pre-alloy powder to obtain a feedstock, and then the feedstock is injection molded to obtain a green blank. After degreasing, the green blank is vacuum sintered to obtain a dense Ti-6Al-4V alloy.

[0012] The Ti-6Al-4V pre-alloyed powder loading in the feed is 63~65 vol%, preferably 64 vol%; the binder is composed of polyoxymethylene, high-density polyethylene, ethylene-vinyl acetate copolymer, and ethylene bis-stearamide in a mass ratio of polyoxymethylene:high-density polyethylene:ethylene-vinyl acetate copolymer:ethylene bis-stearamide = 85~90:1~2:5~10:1~10; the D of the Ti-6Al-4V pre-alloyed powder is... 10 It is 4.5~5.5μm and D 90 It is 18~20μm.

[0013] In the binder designed in this invention, an appropriate amount of polyoxymethylene (POM) can decompose in an acidic atmosphere below the oxidation temperature of titanium alloys, minimizing carbon and oxygen residues; a small amount and an appropriate amount of high-density polyethylene (HDPE) that decomposes at high temperatures provides support during degreasing; an appropriate amount of ethylene-vinyl acetate copolymer, as a plasticizer, not only improves feed flowability and formability but also enhances the compatibility between POM and HDPE; ethylene bis-stearamide improves the uniformity of powder distribution during mixing, and the high-density polyethylene and ethylene bis-stearamide that decompose at high temperatures account for 2% to 11% of the total mass of the binder, preferably 2% to 5.5%, and more preferably... The invention reduces the resistance to POM decomposition and low-temperature component decomposition during catalytic and thermal degreasing processes to an extremely low level. Specifically, the invention uses two appropriate amounts of high-temperature decomposition components in a specific ratio, so that the high-temperature decomposition components only perform the necessary skeletal support, lubrication, and powder dispersion functions, without forming dense residual areas and networks that hinder the escape of low-temperature component decomposition products. This reduces the resistance to POM decomposition during catalytic degreasing and low-temperature component decomposition during thermal degreasing to an extremely low level, significantly reducing exhaust obstruction, local pressure increase, and surface bubbling problems caused by high-temperature component migration, enrichment, or retention.

[0014] Preferably, the adhesive is composed of polyoxymethylene, high-density polyethylene, ethylene-vinyl acetate copolymer, and ethylene bis-stearamide in a mass ratio of 86~88:1~1.5:8~10:1~4.

[0015] As a further preferred option, the adhesive is composed of polyoxymethylene, high-density polyethylene, ethylene-vinyl acetate copolymer, and ethylene bis-stearamide in a mass ratio of 88:1:10:1.

[0016] The feed of the present invention is prepared by the following process:

[0017] The binder of the above formulation was mixed with Ti-6Al-4V alloy powder to obtain the feedstock. The titanium alloy powder used was a spherical pre-alloyed powder with a composition that met the Ti-6Al-4V standard, and a particle size of less than or equal to 20 μm (after optimization, the D of the spherical pre-alloyed powder meeting the Ti-6Al-4V standard was...). 10 4.5~5.5μm, D 50 10.5~12μm, D 90 The powder has a particle size of 19-20 μm and a powder loading of 63-65 vol%, preferably 64 vol% (corresponding to a powder mass fraction of approximately 86%). An inert gas (argon) protective environment is preferably used, and the powder is intensively mixed at 170-185°C at a rotor speed of 20-30 r / min, preferably 25 r / min, for 2-3 hours to ensure thorough mixing and uniform dispersion of the powder and binder components. The uniformly mixed material is then cooled to 130-140°C, preferably 135°C, and granulated by screw extrusion to obtain cylindrical feed pellets with a particle size of approximately 3 × 5 mm. The resulting feed exhibits good rheological properties in the range of 170-200°C, i.e., when the shear rate is greater than 10 s⁻¹. -1 At all temperatures, the viscosity is below 1000 Pa·s, exhibiting significant shear thinning characteristics, which is beneficial for injection molding filling. In this invention, the powder loading refers to the volume of Ti-6Al-4V standard spherical pre-alloyed powder in the feed accounting for 63-65% of the feed volume, preferably 64%.

[0018] As a preferred embodiment, the Ti-6Al-4V alloy comprises, by mass percentage, 5.9%~6.1% Al, 3.9%~4.1% V, ≤0.45% Fe, ≤0.2% O, ≤0.05% N, ≤0.01% H, ≤0.08% C, with the balance being Ti.

[0019] The injection molding process of this invention is as follows: the prepared feedstock is added to the barrel of an injection molding machine, and injection molding is performed using a threaded fastener (M6 screw) cavity mold. Before injection molding, the mold is heated to 85~110℃, preferably 88~95℃, and more preferably 90℃. The feedstock is heated to 175~185℃ and fully melted. Then, injection is performed using an injection pressure of 100~120MPa, preferably 110MPa. After holding the pressure for 10~30s, the mold is cooled, and the blank is demolded and removed.

[0020] When the threaded fastener is an M6 screw, the resulting Ti-6Al-4V alloy screw blank has a length of 25.9 mm, is intact, and has no visible defects. Due to the optimized binder formulation, the powder distribution inside the blank is uniform, and no obvious binder enrichment layer appears.

[0021] In this invention, catalytic degreasing involves placing the green blank in a sealed oxalic acid degreasing furnace for staged catalytic degreasing. In industrial applications, catalytic degreasing includes fully replacing the air in the furnace with nitrogen, then slowly heating to temperature A at a rate of 1.0~2.0℃ / min, preferably 1.5℃ / min, and holding the temperature for 90~150min, preferably 120min. Subsequently, the temperature is sequentially increased to temperature B, C, and D, with each temperature step held constant for 90~150min, preferably 120min. During this period, oxalic acid is introduced at a rate of 1~2g / min to fully catalytically decompose POM. After 8~12 hours, preferably about 10 hours (including heating time), more than 97.5% of the POM in the green blank is removed, resulting in a catalytically degreased blank without cracks, deformation, or surface defects, and with a complete shape. The value of A is 110~115, preferably 115; B minus A is 3~6, ​​C minus B is 3~6, ​​and D minus C is 3~6. In practical applications, B is 120℃, C is 125℃, and D is 130℃.

[0022] In this invention, thermal degreasing involves transferring the catalytically degreased blank into a thermal degreasing furnace to further remove remaining binder components and pre-sinter the blank. First, the vacuum level inside the furnace is evacuated to below 5 Pa at room temperature, then high-purity argon is introduced to prevent oxidation during thermal degreasing. Based on the thermogravimetric-differential scanning calorimetry (TGC) results of the catalytically degreased blank, a staged heating degreasing process is set: the temperature is increased to 280-320℃, preferably 300℃, at a rate of 1.0-3.0℃ / min, preferably 2℃ / min, and held for 25-35 min, preferably 30 min, to remove some of the low-temperature decomposed binder components. Then, the temperature is increased to E℃ and F℃ at the same rate, and held for 20-45 min, preferably 30 min, respectively, to remove the remaining binder. Finally, the temperature is rapidly increased to G℃ at a rate of 5℃ / min and held for 50-75 min, preferably 60 min, to form sintering necks between the powder particles, resulting in a pre-sintered blank with a certain strength. The value of E is 380~420, preferably 400; the value of F is 480~520, preferably 500; and the value of G is 650~720, preferably 700.

[0023] Vacuum sintering: The pre-sintered billet is placed on the molybdenum plate liner of the vacuum sintering furnace, and the vacuum is evacuated to ≤1×10⁻⁶. -4 After Pa, sintering begins. The temperature is increased to H℃ at a rate of 2~5℃ / min, preferably 3℃ / min, and held at that temperature for 5~8 hours, preferably 7 hours, for densification, followed by furnace cooling. The value of H is 1025~1075, preferably 1050.

[0024] This invention utilizes a high vacuum level (vacuum throughout the sintering process is better than 2×10⁻⁶). -3The increase in oxygen and other impurity element content during sintering is minimal, ensuring the purity of the product. The sintered Ti-6Al-4V alloy screws exhibit high dimensional accuracy and a smooth surface free of blistering defects. Microscopic analysis reveals that the internal structure of the product exhibits a fine equiaxed crystal structure (e.g., ...). Figure 3 As shown in the figure, only a very small number of extremely small pores are distributed, and these microstructures ensure that the product has high strength and elongation. According to the standard GB / T 3850-2015 / ISO 3369:2006, the relative density of the optimized sintered product is as high as 99.3% or more, close to fully dense, and the tensile strength is 1067.36±43.6MPa, the yield strength is 1000.61±15.38MPa, and the elongation is 15.16%±1.22%.

[0025] Beneficial effects:

[0026] The optimized binder design method described above significantly improves the sintering density of Ti-6Al-4V alloy MIM products and effectively avoids the formation of large pore defects during debinding and sintering. Specifically, the feedstock prepared by this invention has good flowability, the powder is evenly distributed in the green body, and the organic components decompose smoothly during debinding without binder enrichment or gas retention, thus preventing large pore defects. During sintering, uniform shrinkage occurs in all directions, and the pores gradually close, ultimately resulting in a dense product with near-full density.

[0027] The method of the present invention can also achieve high densification at a relatively low sintering temperature (about 1025~1075℃, preferably 1050℃), avoiding the problem of abnormal grain growth caused by excessively high temperature, which is beneficial to maintaining the good comprehensive mechanical properties of the material.

[0028] In summary, the binder design provided by this invention has significant innovation and practical value in improving sintering density and avoiding large pore defects, and can improve the yield and service performance of Ti-6Al-4V MIM products. Attached Figure Description

[0029] Figure 1 This provides a model for studying the complete dynamic process of Ti-6Al-4V alloy sintering using molecular dynamics methods, as well as snapshots of different sintering stages at a specific position parallel to the xz plane.

[0030] Figure 2 The image shows the cross-sectional morphology of the Ti-6Al-4V injection-molded screw blank prepared in Example 1.

[0031] Figure 3 The image shows the microstructure of the Ti-6Al-4V alloy screw product after polishing following sintering in Example 1.

[0032] Figure 4 This is a cross-sectional morphology diagram of the green blank prepared in Example 2.

[0033] Figure 5 This is a microstructure image of the sintered product after polishing in Example 2.

[0034] Figure 6 This is a diagram showing the distribution of powder and binder across the cross-section of the injection preform in Example 3.

[0035] Figure 7 This is a microstructure image of the sintered product after polishing in Example 3.

[0036] Figure 8 These are photographs of the catalytic degreasing blank and sintered blank of the Ti-6Al-4V screw prepared in Example 3.

[0037] Figure 9 To explore the microstructure morphology of the sintered product in Example 1 after polishing.

[0038] Figure 10 This is a microstructure image of the sintered product of Comparative Example 1 after polishing.

[0039] Figure 11 The image shows the microstructure of the sintered product after polishing, as shown in Comparative Example 2.

[0040] Figure 12 This is a microstructure image of the sintered product of Comparative Example 3 after polishing.

[0041] Figure 13 This is a microstructure image of the sintered product of Comparative Example 4 after polishing. Detailed Implementation

[0042] This invention discloses a method for designing and applying a binder for injection molding of high-density Ti-6Al-4V alloy. The implementation steps and effects are specifically illustrated through examples and comparative embodiments:

[0043] Example 1

[0044] Following the binder design guidelines provided in this invention, Ti-6Al-4V alloy MIM feedstock was prepared and sintered into screw products. The powder used was D... 10 =5.12μm, D 50 =11.33μm, D 90Ti-6Al-4V spherical pre-alloyed powder with a particle size of 19.67 μm (chemical composition: Al 5.97%, V 4.03%, Fe 0.023%, O 0.173%, N 0.003%, H 0.006%, C 0.08%, balance Ti). The binder formulation, by mass fraction, consists of 88% polyoxymethylene, 1% high-density polyethylene, 10% vinyl acetate copolymer, and 1% ethylene bis-stearamide, mixed with the powder at a volume fraction of 64%:36%. The specific steps are as follows: First, under an argon atmosphere with an oxygen content <1000 ppm, the powder and binder are fed into a mixer and mixed at 180°C and a rotor speed of 25 r / min for 3 hours. Then, the mixture is cooled to 135°C and granulated by screw extrusion to obtain cylindrical feed pellets with a diameter of approximately 3 mm and a length of approximately 5 mm. The melt flow index of the feed was measured to be 498.84 g / 10 min at 190℃ and a load of 21.6 kg. Rheological tests at 170℃, 180℃, 190℃, and 200℃ showed a shear rate >10 s. -1 With an viscosity of <1000 Pa·s (approximately 518 Pa·s), it exhibits good flowability and shear thinning characteristics.

[0045] The above feedstock was preheated at 90°C in the mold and 180°C in the barrel, and then injection molded at 110MPa to form an M6 screw preform (holding pressure for 10s, cooling for 10s, and demolding). The resulting preform cross-section is shown below. Figure 2 As shown, the powder is uniformly distributed and there is no obvious binder enrichment on the surface. Subsequently, the green body was placed in an oxalic acid catalytic degreasing furnace, and oxalic acid gas was introduced at stepped temperatures of 115℃, 120℃, 125℃, and 130℃ for 2 hours each (heating rate 1.5℃ / min, oxalic acid injection rate 2g / min) to remove the POM binder to the maximum extent, obtaining a catalytically degreased green body. After determining the decomposition peaks of the remaining binders by TG-DSC analysis, the body was pre-sintered in a vacuum thermal degreasing furnace at a rate of 2℃ / min to 300℃, 400℃, and 500℃ (holding for 30 min each), and then at a rate of 5℃ / min to 700℃ and held for 1 hour. Finally, it was sintered at 1×10⁻⁶... -4 The screws were sintered in a high-vacuum sintering furnace at a temperature increased to 1050℃ at a rate of 3℃ / min and held for 7 hours. After cooling to room temperature, the sintered samples were removed. The microstructure of the sintered screws is shown in the figure. Figure 3 As shown, the sintered sample exhibits uniform and fine equiaxed α-phase grains with almost no visible porosity defects. The relative density of the sintered sample was measured to be 99.37%, the tensile strength to be 1067.36 ± 43.6 MPa, the yield strength to be 1000.61 ± 15.38 MPa, and the elongation to be 15.16% ± 1.22%. Therefore, this embodiment demonstrates that by optimizing the binder formulation and process, a Ti-6Al-4V alloy MIM product with high density, no obvious defects, and superior comprehensive mechanical properties was successfully prepared at a relatively low sintering temperature.

[0046] Example 2

[0047] Ti-6Al-4V screw samples were prepared under the same process conditions as in Example 1 above, except that the binder formulation was changed to a binder with mass fractions of 84%, 1%, 10%, and 5% of polyoxymethylene, high-density polyethylene, vinyl acetate copolymer, and ethylene bis-stearamide. The melt flow index of this comparative feed was measured to be 663.96 g / 10 min. Under the same test conditions as in Example 1, the shear rate was measured to be >10 s. -1 At that time, its viscosity was <800 Pa·s (approximately 450 Pa·s). The cross-sectional morphology of the injection-molded green compact is as follows: Figure 4 As shown, the powder distribution uniformity is lower than in Example 1, with localized areas of binder enrichment. The microstructure morphology of the sintered sample (after polishing) is as follows. Figure 5 As shown, pores are clearly present on the surface (in Figure 5 Multiple pores appeared within the viewing frame. This indicates that while the increased ethylene bis-stearamide improved the feed flowability, it conversely worsened the uniformity of the green powder distribution during injection molding, making it difficult to achieve high densification during sintering. Furthermore, the resulting product surface exhibited noticeable micron-sized and irregular pores. The relative density of the sintered sample in this example was measured to be 99.1%. The tensile strength was 1057.71 ± 29.91 MPa, the yield strength was 998.7 ± 11.33 MPa, and the elongation was 12.31% ± 1.26%.

[0048] Example 3

[0049] Ti-6Al-4V screw samples were prepared under the same process conditions as in Example 1 above, except that the binder formulation was changed to a binder with mass fractions of 79%, 1%, 10%, and 10% of polyoxymethylene, high-density polyethylene, vinyl acetate copolymer, and ethylene bis-stearamide. The melt flow index of the feed in this example was measured to be 923.47 g / 10 min. Under the same test conditions as in Example 1, the shear rate was measured to be >10 s. -1 At that time, its viscosity was <800 Pa·s (approximately 376 Pa·s). The cross-sectional morphology of the injection-molded green compact is as follows: Figure 6 As shown, the powder distribution uniformity is lower than in Example 1, with localized areas of binder enrichment. The microstructure morphology of the sintered sample (after polishing) is as follows. Figure 7 As shown, the surface exhibits numerous pores. This indicates that while the increased ethylene bis-stearamide improves feed flowability, it conversely worsens the uniformity of green powder distribution during injection molding, making it difficult to achieve high densification during sintering. Furthermore, as... Figure 8The actual morphology of the screw is shown in the figure (the left side of the figure is the catalytically degreased blank, and the right side is the sintered blank). In this embodiment, bulging occurred after catalytic degreasing, and this bulging continued during the subsequent sintering process. This is related to phase separation between the powder and binder, and between binder components, as well as the accumulation of excessive high-temperature decomposed binder components on the surface, which hinders the escape of decomposition gases from the low-temperature decomposed components. The relative density of the sintered sample of this comparative example was measured to be only 98.04%. The tensile strength was 1071.35±29.91 MPa, the yield strength was 1014.6±8.91 MPa, and the elongation was 2.98%±0.24%. The study found that the binder formulation caused incomplete degreasing, resulting in a large amount of C and O residues. Although C and O impurities played a strengthening role, they also worsened the elongation. Therefore, while the strength of this embodiment increased, the elongation decreased sharply.

[0050] Exploration Example 1

[0051] Ti-6Al-4V screw samples were prepared under the same process conditions as in Example 1 above, except that the binder formulation was changed to 79%, 1%, 10%, and 10% by mass of polyoxymethylene, high-density polyethylene, vinyl acetate copolymer, and stearic acid, respectively. Specifically, the high-decomposition-temperature ethylene bis-stearamide that affected degreasing in Example 3 was replaced with stearic acid, which decomposes at a relatively low temperature. The melt index of the prepared feed was 906.72 g / 10min. The microstructure morphology of the sintered sample (after polishing) in this exploratory example is as follows. Figure 9 As shown, its porosity is reduced compared to Example 3. The sintering density of this comparative sample is 99.14%. However, the tensile strength of this sintered sample is 1060.05±17.96 MPa, the yield strength is 979.31±12.47 MPa, and the elongation is 14.81%±1.69%. These results further indicate that reducing high-temperature decomposition components or using components that decompose at lower temperatures is beneficial to avoiding the generation of bulging defects, but the uniformity of the green body remains the key factor determining the sintering density.

[0052] Comparative Example 1

[0053] Ti-6Al-4V screw samples were prepared under the same process conditions as in Example 1 above. The difference was that the binder formulation was changed to 80%, 5%, 10%, and 5% by mass of polyoxymethylene, high-density polyethylene, vinyl acetate copolymer, and stearic acid as binders. Specifically, ethylene bis-stearamide, which has a relatively high decomposition temperature, was replaced with stearic acid, which has a lower decomposition temperature, and the content of high-density polyethylene was increased. The effect of increasing the low-decomposition-temperature binder component on sintering densification was compared. The melt index of the prepared feedstock was 453.72 g / 10 min. The microstructure morphology (after polishing) of the comparative sintered sample is as follows. Figure 10As shown, the surface porosity is larger and more numerous compared to Example 1. This is related to the uniformity of the green powder distribution and the increased resistance of high-temperature decomposition components to the escape of low-temperature decomposition components. The density of the comparative sintered sample was measured to be 99.07%, the tensile strength to be 1042.05 ± 35.97 MPa, the yield strength to be 970.3 ± 20.97 MPa, and the elongation to be 14.2% ± 0.89%.

[0054] Comparative Example 2

[0055] Ti-6Al-4V screw samples were prepared under the same process conditions as in Example 1 above, except that the binder formulation was changed to 80%, 5%, 10%, and 5% by mass of polyoxymethylene, polypropylene, vinyl acetate copolymer, and stearic acid, respectively. In other words, the high-density polyethylene with a relatively high decomposition temperature in Comparative Example 1 was replaced in equal amounts with polypropylene, which also decomposes at a high temperature. The melt flow index of the prepared feed was 325.68 g / 10 min. The microstructure morphology of the sintered sample of this comparative example (after polishing) is as follows: Figure 11 As shown, the surface exhibits larger pores compared to Example 1, and the sintering density of this comparative sample is 99.01%. The results indicate that regardless of whether high-density polyethylene or polypropylene is used, the uniformity of the billet and the content of high-decomposition-temperature skeletal components both affect the sintering density to some extent. Furthermore, the tensile strength of this sintered sample is 1054.39 ± 25.05 MPa, the yield strength is 991.625 ± 7.14 MPa, and the elongation is 13.07% ± 1.83%.

[0056] Comparative Example 3

[0057] Ti-6Al-4V screw samples were prepared under the same process conditions as in Example 1 above, except that the binder formulation was changed to 80%, 5%, 10%, and 5% by mass of polyoxymethylene, polyethylene glycol, vinyl acetate copolymer, and stearic acid. That is, the high-density polyethylene with a high decomposition temperature in Comparative Example 1 was replaced with water-soluble decomposable polyethylene glycol. The melt index of the prepared feed was 650.64 g / 10 min. The microstructure morphology (after polishing) of the sintered sample of this comparative example is as follows. Figure 12 As shown, a small number of pores exist on the surface, and the sintering density of this comparative sample is 98.65%. This indicates that even if easily removable polyethylene glycol is used to replace some of the high-temperature decomposition components, if the green powder distribution is not uniform, a large number of large pores will still remain after sintering. Furthermore, the tensile strength of this sintered sample is 1061.09±22 MPa, the yield strength is 945.8±4.9 MPa, and the elongation is 10.44%±1.7%. Because the decomposition of the polyethylene glycol ether bond (COC) introduces additional oxygen, which is detrimental to plasticity, the elongation of this comparative example is lower than that of Comparative Example 1 and Comparative Example 2.

[0058] Comparative Example 4

[0059] Ti-6Al-4V screw samples were prepared under the same process conditions as in Example 1 above, except that the binder formulation was changed to 85%, 10%, and 5% by mass of polyoxymethylene, vinyl acetate copolymer, and stearic acid as binders, respectively. This meant that the high-density polyethylene that decomposes at high temperatures was removed compared to Comparative Example 1. The melt flow index of the prepared feed was 417.38 g / 10 min. The microstructure morphology of the sintered sample of this comparative example (after polishing) is as follows. Figure 13 As shown, the surface contains pores, and the sintering density of this comparative sample is 99.17%. The tensile strength of this sintered sample is 1055.28±30.59 MPa, the yield strength is 984.87±52.61 MPa, and the elongation is 13.77%±2.67%. These results further indicate that simply reducing the types of high-temperature components cannot improve sintering densification; the uniformity of the green body remains the key factor determining the sintering density.

[0060] Comparative Example 5

[0061] Ti-6Al-4V screw samples were prepared under the same process conditions as in Example 1 above, except that the binder formulation was changed to 68%, 20%, 10%, and 2% by mass of polyoxymethylene, polypropylene, high-density polyethylene, and stearic acid, respectively. This significantly increased the amount of polypropylene and high-density polyethylene that decompose at high temperatures, while reducing the amount of stearic acid that decomposes at low temperatures. The melt index of the prepared feedstock was 285.18 g / 10 min, and the sintering density of this comparative sample was 99%. However, the tensile strength of this sintered sample was 1068.45 ± 21.73 MPa, the yield strength was 1003.27 ± 3.22 MPa, and the elongation was 11.71% ± 1.2%. These results further indicate that the uniformity of the green body remains a key factor determining the sintering density, and that more binder components that decompose at high temperatures may increase the residual C and O after debinding, which is detrimental to plasticity.

[0062] Comparative Example 6

[0063] Ti-6Al-4V screw samples were prepared under the same process conditions as in Example 1 above, except that the powder particle size distribution was: D 10 =7~8μm, D 50 =13.5~14.5μm, D 90=23~24μm, and the binder formulation was changed to 68%, 20%, 10%, and 2% by mass of polyoxymethylene, polypropylene, high-density polyethylene, and stearic acid as binders (the binder formulation was the same as that of Comparative Example 5). The melt index of the prepared feed was 269.1 g / 10 min, and the sintering density of the comparative example sample was 99.03%. However, the tensile strength of the sintered sample was 1054.1±8.62 MPa, the yield strength was 992.55±7.21 MPa, and the elongation was 10.09%±1.11%. This result further shows that using Ti-6Al-4V powder with a wider particle size distribution cannot significantly improve the sintering density, which is still lower than that of Example 1. The uniformity of the green body is still the key factor determining the sintering density.

[0064] Comparative Example 7

[0065] Ti-6Al-4V screw samples were prepared under the same process conditions as Comparative Example 5 above, except that the sintering temperature was 1200℃. The sintering density of this comparative example sample was 99.5%. However, the tensile strength of this sintered sample was 1036.25±4.82 MPa, the yield strength was 966.84±20.63 MPa, and the elongation was 6.37%±1.04%. These results further indicate that significantly increasing the sintering temperature will increase the sintering density, but will result in a significant decrease in strength and plasticity.

[0066] Comparative Example 8

[0067] Ti-6Al-4V screw samples were prepared under the same process conditions as Comparative Example 6 above, except that the sintering temperature was 1200℃. The sintering density of this comparative example sample was 99.5%. However, the tensile strength of this sintered sample was 1013.97±3.78 MPa, the yield strength was 957.65±19.97 MPa, and the elongation was 5.82%±0.45%. This result further shows that regardless of the particle size distribution of Ti-6Al-4V powder used, significantly increasing the sintering temperature will increase the sintering density, but will result in a significant decrease in strength and plasticity.

[0068] The comparison of the above examples, exploratory examples, and comparative examples shows that the binder design method proposed in this invention has significant advantages, but the amount of ethylene bis-stearamide cannot be too high. The binder formulation in Example 1 takes into account the decomposition characteristics, compatibility, and dispersibility of each component, resulting in a uniformly distributed green body. No bulging occurred during debinding, and the highest density of 99.37% was achieved after sintering. The microstructure was a uniform equiaxed crystal, exhibiting the best overall mechanical properties. In contrast, Example 2, while avoiding bulging, had a sintering density of 99.1%. Example 3, due to the excessive addition of a high-temperature-difficult-to-decompose binder component and the aggregation of binder on the green body surface, resulted in poor debinding and bulging, reducing the sintering density to 98.04%, and significantly decreasing its elongation. Exploratory Example 1 replaced the high-temperature-decomposing ethylene bis-stearamide in Example 3 with low-temperature-decomposing stearic acid, avoiding bulging defects, but the sintering density was also lower than that of Example 1. The densities of Comparative Examples 1-6 were all slightly lower than that of Example 1, even with a relatively wide particle size distribution range. Comparative Examples 7 and 8 significantly increased the sintering temperature to 1200 °C. Although the sintering density increased slightly, the tensile strength, yield strength, and elongation all decreased significantly. Furthermore, even when the high-decomposition-temperature skeleton agent in Comparative Examples 3 and 4 was replaced with a more easily decomposed water-based binder component or the high-temperature decomposition component was removed, the sintering density was still lower than that of Example 1. This is mainly related to the uniformity of the powder distribution in the injection-molded preform, highlighting the importance and correctness of the proposed optimized binder for improving the uniformity of the injection-molded preform. Therefore, the binder formulation design of this invention has significant advantages in avoiding bulging defects, residual large-pore defects, and improving sintering density.

Claims

1. A dense Ti-6Al-4V alloy, characterized in that: The binder and Ti-6Al-4V pre-alloy powder are mixed to obtain a feedstock, which is then injection molded to obtain a green compact. After degreasing, the green compact is vacuum sintered to obtain a dense Ti-6Al-4V alloy. The Ti-6Al-4V pre-alloyed powder loading in the feed is 63~65 vol%; the binder is composed of polyoxymethylene, high-density polyethylene, ethylene-vinyl acetate copolymer, and ethylene bis-stearamide in a mass ratio of polyoxymethylene:high-density polyethylene:ethylene-vinyl acetate copolymer:ethylene bis-stearamide = 85~90:1~2:5~10:1~10; the D of the Ti-6Al-4V pre-alloyed powder is... 10 It is 4.5~5.5μm and D 90 It is 18~20μm.

2. The dense Ti-6Al-4V alloy according to claim 1, characterized in that: The adhesive is composed of polyoxymethylene, high-density polyethylene, ethylene-vinyl acetate copolymer, and ethylene bis-stearamide in a mass ratio of 86~88:1~1.5:8~10:1~4.

3. The dense Ti-6Al-4V alloy according to claim 1, characterized in that: The adhesive is composed of polyoxymethylene, high-density polyethylene, ethylene-vinyl acetate copolymer, and ethylene bis-stearamide in a mass ratio of 88:1:10:

1.

4. A dense Ti-6Al-4V alloy according to claim 1, characterized in that: The feed is prepared using the following process; The binder was mixed with Ti-6Al-4V alloy powder to obtain the feedstock; the titanium alloy powder used was a spherical pre-alloyed powder with a composition that met the Ti-6Al-4V standard, and the particle size was less than or equal to 20 μm. The powder loading was 63~65 vol%. An inert gas protective environment is used to knead the material at 170–185°C and a rotor speed of 20–30 r / min for 2–3 hours. The uniformly mixed material is then cooled to 130–140°C and granulated by screw extrusion to obtain cylindrical feed particles with a particle size of approximately 3 × 5 mm. The volume of Ti-6Al-4V standard spherical pre-alloyed powder in the feed accounts for 63–65% of the feed volume.

5. A dense Ti-6Al-4V alloy according to claim 1, characterized in that: D of standard spherical pre-alloyed powder of Ti-6Al-4V 10 4.5~5.5μm, D 50 10.5~12μm, D 90 It is 19~20μm. The Ti-6Al-4V alloy comprises, by mass percentage, 5.9%~6.1% Al, 3.9%~4.1% V, ≤0.45% Fe, ≤0.2% O, ≤0.05% N, ≤0.01% H, ≤0.08% C, with the balance being Ti.

6. A dense Ti-6Al-4V alloy according to claim 1, characterized in that: Add the prepared feed material to the injection molding machine barrel, and use a threaded fastener cavity mold for injection molding. Before injection molding, heat the mold to 85~110℃, heat the feed material to 175~185℃ and let it melt fully. Then, use an injection pressure of 100~120MPa for injection, hold the pressure for 10~30s, cool, demold and remove the blank.

7. A dense Ti-6Al-4V alloy according to claim 6, characterized in that: The green body was placed in a sealed oxalic acid degreasing furnace for staged catalytic degreasing. Catalytic degreasing included replacing the air in the furnace with nitrogen, raising the temperature to A℃ at a rate of 1.0~2.0℃ / min and holding it for 90~150min, then raising the temperature sequentially to B℃, C℃, and D℃, holding each temperature step constant for 90~150min. During this time, oxalic acid was introduced at a rate of 1~2g / min to fully catalytically decompose POM. After 6~10 hours of catalytic decomposition, more than 97.5% of the POM in the green body was removed, where A was 110~115; B minus A was 3~6, ​​C minus B was 3~6, ​​and D minus C was 3~6.

8. A dense Ti-6Al-4V alloy according to claim 7, characterized in that: The catalytically degreased green body is transferred to a hot degreasing furnace to further remove residual binder components and pre-sinter the green body. Specifically, the vacuum degree in the furnace is first evacuated to below 5 Pa at room temperature, then the temperature is increased to 280-320℃ at a rate of 1.0-3.0℃ / min and held for 25-35 min. Then, the temperature is increased to E℃ and F℃ at the same rate and held for 20-45 min respectively to remove the residual binder. Finally, the temperature is rapidly increased to G℃ at a rate of 5℃ / min and held for 50-75 min to form sintering necks between the powder particles, resulting in a pre-sintered green body with a certain strength. The values ​​of E, F, and G are 380-420, 480-520, and 650-720. Finally, the pre-sintered green body is placed in a vacuum sintering furnace for sintering.

9. A dense Ti-6Al-4V alloy according to claim 8, characterized in that: The pre-sintered billet is placed on the molybdenum plate liner of the vacuum sintering furnace, and the vacuum is evacuated to ≤1×10⁻⁶. -4 After Pa, the temperature is raised to sintering. During sintering, the temperature is raised to H℃ at a rate of 2~5℃ / min, and held for 5~8 hours for densification. Then, the furnace is cooled. The value of H is 1025~1075.

10. A dense Ti-6Al-4V alloy according to claim 9, characterized in that: The resulting sintered product has a relative density of over 99.3%, a tensile strength of 1067.36±43.6MPa, a yield strength of 1000.61±15.38MPa, and an elongation of 15.16%±1.22%.

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