Method for preparing titanium alloy by additive manufacturing

Abstract

A method for preparing a titanium alloy by additive manufacturing, relating to the technical field of material preparation. The technical solution used is: first, eliminating holes in a printing material by means of additive manufacturing and hot isostatic pressing processes; and then on the basis of the critical temperature and time for grain growth and phase transformation, carrying out high-temperature short-time solution treatment on the printing material, and carrying out aging treatment on a sample obtained by means of the solution treatment and water cooling, to optimize the material structure. The method for preparing a titanium alloy by additive manufacturing overcomes the technical barriers of printing holes, lath coarsening and grain boundary α phase that are inevitable in traditional printing processes, hot isostatic pressing and traditional heat treatment, does not require fine optimization of the printing and hot isostatic pressing processes, and can conveniently and efficiently obtain an additively manufactured titanium alloy having an ultrahigh fatigue strength, greatly reducing costs and improving efficiency. Also provided is an additively manufactured titanium alloy.

Description

A preparation method for additively manufacturing titanium alloy Technical Field

[0001] The present invention relates to the technical field of fatigue-resistant material preparation, and in particular to an additively manufactured titanium alloy material and a preparation method thereof. Background Art

[0002] Additive manufacturing (AM), a digital forming technology, holds broad application prospects in rail transportation, aerospace, biomedical applications, and other fields due to its advantages, including high design freedom, minimal material waste, short delivery times, and environmental friendliness. It can directly produce engineering components with complex geometries. However, titanium alloys, which possess high specific strength and stiffness and excellent corrosion resistance, are expensive and have poor formability in traditional manufacturing. Therefore, AM technology is in high demand to achieve fast delivery times and low buy-to-fly ratios (BTFs), which are the ratio of the amount of raw material required to produce a part to the amount of material contained in the final part. However, the large-scale application of AM titanium alloys remains a challenge, as they generally exhibit inferior fatigue resistance compared to traditionally manufactured parts, especially to high-cycle fatigue cycles (greater than 105 cycles). Therefore, the widespread adoption of AM titanium alloy components in the aerospace, defense, and military sectors will depend on their fatigue reliability.

[0003] Given that fatigue cracks in AM alloys usually initiate from holes generated during the printing process, especially near-surface holes, a large number of studies have been conducted to minimize the size or density of holes in the printed materials through alloying, adjusting the printing process, and developing advanced printing equipment. However, holes in the printed materials are still unavoidable, which seriously restricts the improvement of the fatigue resistance of additively manufactured titanium alloys.

[0004] Meanwhile, with the rapid development of additive manufacturing (AM), hot isostatic pressing (HIP) has become increasingly mature as an effective method for closing internal pores in materials. Currently, HIP processing can completely close internal pores smaller than 200μm within printed materials. However, the long-term high-temperature holding time and slow cooling rate lead to severe microstructural coarsening, losing the uniform and fine microstructure characteristic of additive manufacturing. After HIP treatment, fatigue cracks initiate in coarse laths or clusters. Consequently, research has shifted to optimizing the HIP microstructure, specifically improving the HIP process and equipment to minimize microstructural coarsening. Reducing the HIP temperature and combining it with advanced HIP systems with rapid cooling rates (1-2°C / s) have reduced microstructural coarsening to some extent. However, compared with cooling rates of traditional water quenching (120-410°C / s), the degree of microstructural refinement remains very limited. To eliminate the coarsening caused by HIP, researchers are also focusing on post-HIP processing control. However, due to the unclear principles for optimizing the fatigue properties of titanium alloys, the regulation of HIP post-processing is mainly aimed at obtaining a structure similar to that of traditional manufacturing, resulting in limited improvements in microstructure refinement and fatigue performance. Furthermore, traditional heat treatment tends to cause coarsening of primary β grains and enrichment of α phase at grain boundaries, which can trigger fatigue cracks in the α phase at grain boundaries. Therefore, while HIP and post-HIP treatment can effectively close pores within the material, they introduce a number of fatigue shortcomings that are not inherent to the additively manufactured structure itself.

[0005] In summary, current research efforts to optimize fatigue performance in AM titanium alloys focus on optimizing the printing process and HIP post-processing to eliminate fatigue shortcomings such as pores and lath coarsening, grain boundary α phase, and heat-treated pore regeneration. However, a suitable post-processing process for simultaneous optimization remains lacking. Therefore, the core of this invention is to develop a simple and convenient method to effectively eliminate pores and microstructural fatigue shortcomings in AM titanium alloys, thereby producing AM titanium alloys with ultra-high high-cycle fatigue resistance.

[0006] Summary of the Invention

[0007] In view of the current lack of suitable post-processing technology to achieve the simultaneous optimization of AM titanium alloy pores and microstructure and the vague AM fatigue performance optimization principle, based on a large number of attempts and the revelation of fatigue damage mechanism, this study proposes a step-by-step regulation of the microstructure and pores of AM materials to achieve the simultaneous optimization of the two. Based on this, a simple and feasible post-processing process named NAMP (Net-AM preparation) technology is developed, which aims to eliminate the pores in AM titanium alloy tissue while restoring the AM microstructural characteristics, avoiding the coarsening of laths and clusters, grain boundary α phase and heat treatment recurrence pores, which are not fatigue shortcomings of the AM tissue itself, thereby obtaining titanium alloy materials with fatigue resistance far higher than that of traditional manufacturing.

[0008] To achieve the above object, the technical solutions adopted by the present invention are as follows:

[0009] A method for preparing a titanium alloy by additive manufacturing, the method comprising the following steps:

[0010] (1) By fixing the values ​​of the printing layer thickness t and the scanning spacing h of the selected titanium alloy additive manufacturing, the printing process is optimized by adjusting the values ​​of the printing power P and the scanning speed v (the selection of P and v depends on the parameters of the additive manufacturing equipment itself), thereby controlling the initial hole size in the printed material; the samples printed at different printing powers P and different scanning speeds v are polished and observed under an optical microscope, and the sample printing parameters with the smallest number of holes (hole density) (less than 3 holes / mm3) and hole size (equivalent diameter less than 120μm) are selected as the optimal printing parameters, which are named as the printing state;

[0011] (2) At a certain temperature, the printed material is subjected to isotropic high-pressure argon gas hot isostatic pressing treatment. After keeping the temperature for a certain period of time, the material is cooled to a certain temperature in the hot isostatic pressing furnace and then taken out of the furnace and air-cooled to room temperature, thereby eliminating the printed holes in the material;

[0012] (3) Through high-temperature laser confocal microscopy, it was found that the grain boundary relaxation phenomenon of the additively manufactured titanium alloy existed above the phase transition point, that is, above the phase transition point, the grain boundary did not migrate and grow immediately, and the grain boundary began to migrate gradually after a certain period of holding temperature, while the phase transition process occurred immediately after the phase transition point was exceeded. However, the titanium alloy prepared by the forging process did not have the phenomenon of grain boundary relaxation. Based on the above asynchronous nature of grain boundary migration and phase transition of additively manufactured materials, the materials after hot isostatic pressing were solution treated at a certain temperature above the phase transition point of the selected titanium alloy and held for different periods of time, and then water-cooled. The samples were then polished, corroded, and observed under an optical microscope. By comparing the grain size and lath size of the samples treated at different temperatures and holding times, it was determined that the grain size did not change, and the lath width within the grain became thinner, that is, the critical temperature and critical time for grain non-growth and lath refinement.

[0013] (4) Take the sample that has been solution treated at critical temperature and critical time and then water-cooled for aging treatment.

[0014] In step (1), additive manufacturing adopts a laser powder bed melting process, the upper limit of the selected value of the printing power P is 400-500W, the lower limit is 100-200W, and 40-60W is an increasing gradient. From the lower limit, it increases in sequence according to the gradient to the upper limit, and the printing operation is performed as the printing process parameter of the printing power P. The upper limit of the selected value of the scanning speed v is 2000-2200mm / s, the lower limit is 1000-1200mm / s, and 180-220mm / s is an increasing gradient. From the lower limit, it increases in sequence according to the gradient to the upper limit, and the printing operation is performed as the printing process parameter of the scanning speed v. The selected printing power P corresponds to different scanning speeds v, and the printing layer thickness t and the scanning spacing h are fixed at 20-100μm and 70-120μm, respectively. The process optimization ensures that the hole size in the printed material is controlled below 120μm, which can be determined by the non-destructive hole detection method of XRT.

[0015] In step (2), the hot isostatic pressing temperature is controlled between 915°C and 925°C, the pressure is controlled between 150MPa and 200MPa, and the holding time is controlled between 3h and 5h; the material is cooled to 400-500°C in the furnace and then air-cooled to room temperature; after the hot isostatic pressing treatment, the material needs to be subjected to XRT non-destructive hole detection to ensure that the holes in the material are completely eliminated.

[0016] In step (3), the phase transition point and phase transformation process of the titanium alloy can be determined and studied by differential scanning calorimetry (DSC); a temperature range of 20°C to 120°C higher than the phase transition point of the titanium alloy is selected to determine the critical temperature, with a lower limit temperature of 20-30°C higher than the phase transition point of the titanium alloy and an upper limit temperature of 100-120°C higher than the phase transition point of the titanium alloy. The solution treatment temperature is selected as an increasing gradient from 20-30°C, and the solution treatment temperature is increased from the lower limit to the upper limit according to the gradient, and the solution treatment temperature is measured respectively. The solution treatment time is selected with 0.5-2min as the lower limit time and 8-20min as the upper limit time, and 0.5-2min is an increasing gradient. It increases from the lower limit to the upper limit according to the gradient, and is measured as the solution treatment time. The selected solution treatment temperatures correspond to different solution treatment times. The solution treatment adopts a hanging heat treatment method to ensure that the material is heated evenly in the heat treatment furnace; the quenching adopts ice water cooling (liquid nitrogen cooling is used to control the water temperature below 0℃) to ensure that the material obtains higher hardenability.

[0017] In step (4), the critical temperature and critical time solution treatment is a high temperature short time solution treatment, and the sample after the solution treatment is water-cooled is subjected to aging treatment. The aging treatment adopts low temperature aging treatment, the temperature is 500-550°C, and the holding time is 4-6h to completely remove the quenching residual stress and ensure that the material is fully strengthened.

[0018] The titanium alloy is an α+β alloy.

[0019] The critical temperature is 40 to 65° C. higher than the phase transition point of the titanium alloy, and the critical time is 3 to 5 minutes.

[0020] The printing layer thickness t and scanning spacing h are fixed at 50-70 μm and 90-110 μm, respectively.

[0021] The titanium alloy prepared by the preparation method has ultra-high resistance to high cycle fatigue.

[0022] The design mechanism and beneficial effects of the present invention are as follows

[0023] 1. The present invention proposes a method of step-by-step regulation to synchronously optimize tissue pores, overcoming the inevitable printing pores, lath coarsening, and grain boundary α-phase barriers in traditional printing processes, hot isostatic pressing, and heat treatment. The present invention can conveniently and simply prepare additively manufactured titanium alloy materials with ultra-high high-cycle fatigue resistance. There is no need to strictly control the pore density in the printed material and the tissue coarsening during hot isostatic pressing, avoiding the traditional fine optimization of the printing process and hot isostatic pressing process. A new anti-fatigue design concept for additively manufactured titanium alloys is proposed, namely, relaxed printing and hot isostatic pressing plus precise high-temperature short-time heat treatment, which greatly saves costs and improves efficiency.

[0024] 2. The present invention can effectively eliminate voids in AM titanium alloy materials while restoring the microstructural characteristics of AM, avoiding fatigue shortcomings that are not inherent in the additive manufacturing structure itself, such as coarsening of laths and clusters, α phase at grain boundaries, and recurrence of voids during heat treatment. This clarifies the ultra-high fatigue resistance of the additive manufacturing structure itself and the huge potential of additive manufacturing technology in manufacturing fatigue-resistant structural parts.

[0025] 3. The ideas and clean (hole-free) additive manufacturing tissue material design concepts proposed in this invention will not be limited to titanium alloys, and may be extended to other additive manufacturing metal materials, such as aluminum alloys, steels, high-entropy alloys, etc., to obtain excellent fatigue performance.

[0026] 4. This invention points out a direction for current research on printing process optimization. That is, if the holes in the printing process are eliminated through continuous technological innovation, net additive manufacturing materials with ultra-high fatigue resistance can be directly produced, which will greatly promote their application as load-bearing structural parts in future aerospace and other fields.

[0027] 5. The present invention points out the direction for the current research on hot isostatic pressing and hot isostatic pressing post-processing optimization, that is, by further refining the structure to obtain the structure type of additive manufacturing, titanium alloys with high fatigue resistance can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 shows the metallographic hole distribution under different printing powers and scanning speeds.

[0029] Figure 2 shows the XRT hole distribution and statistical results of the printed material.

[0030] Figure 3 is the microstructure EBSD image of the printed material.

[0031] Figure 4 shows the XRT hole detection results of the printed material after hot isostatic pressing.

[0032] FIG5 is a differential scanning calorimetry (DSC) temperature rise curve.

[0033] Figure 6 shows the grain boundary migration behavior of additively manufactured and forged titanium alloys at high temperatures, captured by a high-temperature laser tool.

[0034] Figure 7 shows the initial grain EBSD images of additively manufactured and forged titanium alloys and the grain EBSD images after being kept at the same temperature for the same time.

[0035] Figure 8 shows the grain morphology and lath size after quenching at different temperatures and holding times.

[0036] FIG9 shows fatigue performance test results and performance curves of the α+β titanium alloy (grade Ti6Al4V) in the printed state and prepared according to the present invention.

[0037] FIG10 is a comparison of fatigue properties of the α+β titanium alloy (brand Ti6Al4V) prepared according to the present invention, titanium alloys in the literature, and other materials.

[0038] FIG11 shows the structure and metallographic pore distribution of the printed materials at the layer thickness of Comparative Example 1 and Comparative Example 2.

[0039] FIG12 shows the structure, fatigue properties and fatigue cracking location of the material prepared in Comparative Example 4.

[0040] FIG13 shows the structure, fatigue properties and fatigue cracking location of the material prepared in Comparative Example 5. DETAILED DESCRIPTION

[0041] The present invention is described in more detail below with reference to an example. This example uses the most widely used α+β titanium alloy (brand Ti6Al4V) as an example to prepare the material. The specific steps are as follows:

[0042] Example 1

[0043] Step 1: By fixing the powder layer thickness (printing layer thickness) t and scanning spacing h of the laser powder bed melt at 60μm and 100μm, the printing power P and scanning speed v were adjusted to optimize material preparation and the printing process. Selecting a powder layer thickness t = 60μm ensures complete interlayer fusion and reduces the temperature gradient between printed layers, thereby achieving equiaxed crystals (smaller grains relative to columnar crystals) and reducing printing defects. A specific demonstration is shown in Comparative Example 1. Given that the maximum power of the additive manufacturing equipment used is 500W, with a common power range of 200-400W, a gradient of 50W was selected. Specifically, a sample gradient of printing process parameters with a printing power P of 200W, 250W, 300W, 350W, and 400W was established. To improve additive efficiency, the scanning speed can be increased as much as possible. However, for this example, the scanning speed of the equipment exceeds 2200mm / s, and the laser beam is unstable and distorted. Furthermore, the selected titanium alloy powder cannot be fully fused due to the excessively fast scanning speed. Therefore, in this example, a scanning speed v range of 1200 to 2000 mm / s was selected, with a gradient of 200 mm / s. Specifically, a printing process parameter gradient of scanning speed v was established at 1200 mm / s, 1400 mm / s, 1600 mm / s, 1800 mm / s, and 2000 mm / s. Each printing power P corresponded to five different scanning speeds v, ultimately resulting in 25 print samples. As shown in Figure 1, samples printed at different printing powers P and scanning speeds v were polished and observed under an optical microscope. It can be seen that lower energy density (corresponding to smaller P and larger v) is more likely to produce unfused pores, while higher energy density (corresponding to larger P and smaller v) is more likely to produce pores. Based on the printing results in Figure 1, the sample with the lowest pore density (an average of 2 pores / mm3) and the smallest pore size was selected. The optimal printing process was determined to be power P = 300W and scanning speed v = 1200 mm / s, which is named the printing state. Through XRT non-destructive hole detection, as shown in Figure 2, the equivalent diameter of the holes in the printed material is all below 120μm. According to Figure 3, the structure of the printed material is uniform equiaxed crystals with fine α laths distributed inside the grains, indicating that the printing process parameters are good.

[0044] Step 2: The as-printed material was hot isostatically pressed (HIP) at 920°C using isotropic high-pressure argon at 150 MPa. After holding the HIP for 3 hours, the material was furnace-cooled to 500°C and then air-cooled to room temperature. This is designated the HIP state. Two samples were randomly selected after HIP for nondestructive XRT porosity testing. As shown in Figure 4, no porosity was detected in either sample, indicating that the HIP process completely eliminated any pores in the as-printed material, resulting in a preliminarily void-free material.

[0045] Step 3: Differential scanning calorimetry (DSC) testing of the material confirmed the phase transition point of the Ti6Al4V titanium alloy to be 994°C, as shown in Figure 5. At 852°C, only a portion of the α phase begins to transform. Above 994°C, all α phase in the titanium alloy completely transforms to the β phase. High-temperature laser confocal microscopy was used to investigate the grain boundary migration behavior of the additively manufactured and wrought titanium alloys at 1400°C. Rapidly moving grain boundaries were not captured as grain boundary traces; only stable grain boundaries were captured as traces, indicated by red arrows, as shown in Figure 6. When heated to the same temperature, a large number of grain boundaries in the wrought titanium alloy initially migrated rapidly, while only a few stable grain boundaries could be captured as grain boundary traces. However, in the additively manufactured titanium alloy, a large number of grain boundaries did not migrate immediately; instead, they remained stable at high temperatures, exhibiting grain boundary relaxation. Grain boundary migration only began gradually after a certain holding period. Consequently, after a final holding period of 200 seconds, the grain size of the additively manufactured titanium alloy was smaller than that of the wrought titanium alloy, as shown in Figure 7. The phase transformation of forged titanium alloys and additively manufactured titanium alloys occurs immediately after the phase transition point is exceeded and driven by superheat. Based on the asynchrony of grain growth and phase transformation in additive manufacturing, the material after hot isostatic pressing in step 2 was solution treated at different temperatures higher than the phase transformation point of Ti6Al4V titanium alloy at 994°C for different times and then water-cooled. The temperature 26-106°C higher than the phase transformation point of Ti6Al4V titanium alloy was selected, and 1020°C was used as the initial temperature. The solution treatment temperature was selected as a gradient of 20-30°C, specifically 1020°C, 1050°C, 1080°C, and 1100°C. Seven different solution treatment times were measured for each solution temperature. The initial time was 30s, and the solution treatment time was selected as a gradient of 0.5-2min, specifically 30s, 1min, 2min, 4min, 6min, 8min, and 10min. Finally, 28 samples after solution treatment were obtained. After grinding, polishing, and metallographic etching, the grain morphology and size as well as the changes in the lath size within the grains were observed under an optical microscope. As shown in Figure 8, at 1020°C, the temperature is too low and the driving force for phase transformation is small. It takes at least 8 minutes to completely refine the laths within the grains. If it exceeds 8 minutes, the grains will tend to grow.At the same time, tiny defects such as vacancies and holes have enough time to diffuse and aggregate during this period, causing holes to reappear in the material [S. Tammas-Williams, PJ Withers, Todd, PBPrangnell, Porosity regrowth during heat treatment of hot isostatically pressed additively manufactured titanium components; Script Materialia 122 (2016) 72-76]; at high temperatures, the grain growth rate is too fast. As shown in Figure 8, at 1080°C and 1100°C, the grains began to grow abnormally within only 1 minute. Excessively rapid grain growth prevents the phases within the grains from fully transforming in time, thus preventing complete lath refinement [O.M. Ivasishin, R.V. Teliovich, Potential of rapid heat treatment of titanium alloys and steels; Materials Science and Engineering A263 (1999) 142-154]. Figure 8 shows that when holding at 1050°C for more than 4 minutes, the grains begin to grow abnormally, and 4 minutes is sufficient for complete lath refinement within the grains. Therefore, the appropriate critical temperature and time are determined to be 1050°C and 4 minutes. Solution treatment, i.e., high-temperature, short-time (1050°C, 4 minutes) solution treatment, is performed at these critical temperatures and times. This high-temperature, short-time solution treatment utilizes a hanging heat treatment method to ensure uniform heating of the material in the heat treatment furnace. The quenching is performed using ice water cooling (liquid nitrogen cooling is used to keep the water temperature below 0°C) to ensure high hardenability.

[0046] Step 4: The sample after solution treatment and water cooling at critical temperature and critical time is subjected to aging treatment. The aging treatment adopts low-temperature aging treatment, the temperature is 500℃, and the holding time is 6h to completely remove the residual stress of quenching and ensure that the material is fully strengthened.

[0047] The additively manufactured Ti6Al4V material prepared by the present invention was further subjected to tensile fatigue performance testing with a stress ratio of R = 0.1. As shown in Figure 9a, the fatigue life of the additively manufactured Ti6Al4V material prepared by the present invention was significantly improved compared to the as-printed state. The fatigue strength was tested and determined using the standard lifting method. As shown in Figure 9b, the fatigue strength of the additively manufactured Ti6Al4V material prepared by the present invention reached an astonishing 978MPa, an increase of 106% compared to the original state. The additively manufactured Ti6Al4V material prepared by the present invention was further compared with literature data, as shown in Figure 10. It can be seen that the additively manufactured Ti6Al4V material prepared by the present invention is far superior to the additively manufactured Ti6Al4V alloys reported in the literature, and even higher than traditional forged titanium alloys (Figure 10a). A comparison of the net additively manufactured Ti6Al4V structure prepared by the present invention with other materials revealed (Figure 10b) that the net additively manufactured Ti6Al4V alloy prepared by the present invention exhibited the highest resistance to high-cycle fatigue, far exceeding all metal materials. This demonstrates that the present invention can quickly and easily produce ultra-high-cycle fatigue-resistant additively manufactured titanium alloys, eliminating the need for strict control of pore density within the printed material and microstructure coarsening during hot isostatic pressing (HIP), significantly reducing costs and improving efficiency. The present invention also clarifies the ultra-high fatigue resistance of additively manufactured structures and demonstrates the enormous potential of additive manufacturing technology for the manufacture of fatigue-resistant structural components.

[0048] Comparative Example 1

[0049] The preparation method of additively manufactured titanium alloy is the same as that in Example 1, except that the printing layer thickness t in step 1 is 30 μm. The experimental results are shown in Figure 11a. When the thickness is 30 μm, the powder is too thin, resulting in a large temperature gradient between layers, which leads to the directional growth of grains to form columnar crystals.

[0050] Comparative Example 2

[0051] The preparation method of additively manufactured titanium alloy is the same as that in Example 1, except that the printing layer thickness t in step 1 is 90 μm. The experimental results are shown in Figure 11c. When the thickness is 90 μm, the powder is too thick, resulting in poor bonding between the powder layers and easy formation of cracks and holes.

[0052] Comparative Example 3

[0053] The additive manufacturing method for preparing titanium alloys is the same as that in Example 1, except that the hot isostatic pressing (HIP) treatment in step 2 is omitted. Because the printed holes are not completely eliminated before the subsequent heat treatment for microstructure optimization, stress concentration is likely to occur around the holes during fatigue, leading to premature initiation of fatigue cracks and fatigue cracking. This prevents the fatigue resistance of the microstructure from being fully developed. This means that the effects of the subsequent heat treatment microstructure optimization in steps 2 and 3 will be completely masked by the harmful effects of the holes, significantly reducing their effectiveness.

[0054] Comparative Example 4

[0055] The additive manufacturing method for preparing the titanium alloy was the same as that of Example 1, except that step 3 was omitted. The microstructure characteristics are shown in Figure 12a, and the fatigue performance is shown in Figure 12b. It can be seen that the fatigue performance is significantly lower than that of Example 1. This is primarily due to the severe microstructure coarsening during the hot isostatic pressing process, which leads to fatigue crack initiation from the coarse microstructure and poor fatigue resistance, as shown in Figure 12c.

[0056] Comparative Example 5

[0057] The preparation method of the additively manufactured titanium alloy is the same as that of Example 1, except that the heat treatment method in step 3 is changed, and the critical temperature and time determination in step 2 are not performed. Instead, a conventional solution-aging heat treatment is used. The solution temperature and aging method are the same as those in Example 1, and the solution time is 1 hour, which is not the critical time for the corresponding temperature. Its microstructure characteristics are shown in Figure 13a, and its fatigue performance is shown in Figure 13b. It can be seen that its fatigue performance is far lower than that of Example 1. This is mainly due to the abnormal growth of grains during traditional heat treatment, which leads to grain boundary coarsening and enrichment of dendritic α phase at the grain boundaries. As a result, fatigue cracks are easily initiated from the coarse grain boundary α phase and poor fatigue resistance, as shown in Figure 13c.

[0058] To sum up, the innovation of the present invention lies in that it takes into account the coupled influence of material organization and defects and adopts distributed control to achieve synchronous optimization. It does not simply reduce defects and ignore the influence of organization, nor does it simply refine the organization and ignore the influence of defects. Instead, it comprehensively considers the shortcomings of printing process, hot isostatic pressing process and traditional heat treatment process in organization and defects. The coupled control method is used to overcome the inevitable printing holes, lath coarsening and grain boundary α phase barriers in traditional printing process, hot isostatic pressing and heat treatment. It can easily and quickly prepare ultra-high high-cycle fatigue resistant additive manufacturing titanium alloy materials. There is no need to strictly control the hole density in the printed material and the organization coarsening of hot isostatic pressing, which avoids the traditional fine optimization of printing process and hot isostatic pressing process, greatly saving costs and improving efficiency.

[0059] The above description is merely a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Any simple modifications, equivalent changes and modifications made to the above embodiment based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing titanium alloy by additive manufacturing, characterized in that: The method comprises the following steps: (1) The printing process is optimized by fixing the values ​​of the printing layer thickness t and the scanning spacing h of the selected titanium alloy additive manufacturing, and adjusting the values ​​of the printing power P and the scanning speed v. The selection of P and v is based on the parameters of the additive manufacturing equipment itself, thereby controlling the initial hole size in the printed material; the samples printed under different printing powers P and different scanning speeds v are polished and observed under an optical microscope, and the sample printing parameters with the least number of holes and the smallest hole size are selected as the optimal printing parameters, which are named printing states; (2) At a certain temperature, the printed material is subjected to isotropic high-pressure argon gas hot isostatic pressing treatment, and after keeping the temperature for a certain period of time, the material is cooled to a certain temperature in the hot isostatic pressing furnace, and then taken out of the furnace and air-cooled to room temperature, thereby eliminating the printed holes in the material; (3) Through high-temperature laser confocal microscopy, it was found that the grain boundary relaxation phenomenon of the additively manufactured titanium alloy existed above the phase transition point, that is, above the phase transition point, the grain boundary did not migrate and grow immediately, and the grain boundary began to gradually migrate after a certain period of heat preservation, while the phase transformation process occurred immediately after the phase transition point was exceeded, but the titanium alloy prepared by the forging process did not have the phenomenon of grain boundary relaxation; Based on the above asynchronism of grain boundary migration and phase transformation of additively manufactured materials, the materials after hot isostatic pressing were solution treated at a certain temperature above the phase transition point of the selected titanium alloy and kept for different periods of time, and then water-cooled, and then the samples were polished, corroded, and observed under an optical microscope. By comparing the grain size and lath size of the samples treated at different temperatures and holding times, it was determined that the grain size did not change, and the lath width in the grain became thinner, that is, the critical temperature and critical time for grain non-growth and lath refinement; (4) Take the samples that have been solution treated at critical temperature and critical time and then water cooled for aging treatment.

2. The preparation method according to claim 1, characterized in that: In step (1), the additive manufacturing adopts a laser powder bed melting process, the upper limit of the selected value of the printing power P is 400-500w, the lower limit is 100-200w, and the incremental gradient is 40-60w. From the lower limit, the value increases in sequence according to the gradient to the upper limit, and the printing operation is performed as the printing process parameters of the printing power P. The upper limit of the selected value of the scanning speed v is 2000-2200mm / s, the lower limit is 1000-1200mm / s, and the incremental gradient is 180-220mm / s. From the lower limit, the value increases in sequence according to the gradient to the upper limit, and the printing operation is performed as the printing process parameters of the scanning speed v. The selected printing power P corresponds to different scanning speeds v, and the printing layer thickness t and the scanning spacing h are fixed at 20-100μm and 70-120μm, respectively. The process optimization ensures that the hole size in the printed material is controlled below 120μm, which can be determined by the non-destructive hole detection method of XRT.

3. The preparation method according to claim 1, characterized in that: In step (2), the hot isostatic pressing temperature is controlled between 915°C and 925°C, the pressure is controlled between 150MPa and 200MPa, and the holding time is controlled between 3h and 5h; the material is cooled to 400-500°C in the furnace and then air-cooled to room temperature; after the hot isostatic pressing treatment, the material needs to be subjected to XRT non-destructive hole detection to ensure that the holes in the material are completely eliminated.

4. The preparation method according to claim 1, characterized in that: In step (3), the phase transition point and phase transformation process of the titanium alloy can be determined and studied by differential scanning calorimetry; a temperature range of 20°C to 120°C higher than the phase transition point of the titanium alloy is selected to determine the critical temperature, with a lower limit temperature of 20-30°C higher than the phase transition point of the titanium alloy, an upper limit temperature of 100-120°C higher than the phase transition point of the titanium alloy, and a solution treatment temperature of 20-30°C as an increasing gradient, which is increased from the lower limit to the upper limit according to the gradient, and is measured as the solution treatment temperature and the solution treatment time. The solution treatment time is selected with 0.5-2min as the lower limit time, 8-20min as the upper limit time, and 0.5-2min as an increasing gradient. The time is increased from the lower limit to the upper limit according to the gradient, and the solution treatment time is measured respectively. The selected solution treatment temperatures correspond to different solution treatment times. The solution treatment adopts a hanging heat treatment method to ensure that the material is heated evenly in the heat treatment furnace. The quenching adopts ice water cooling, and liquid nitrogen cooling is used to control the water temperature below 0°C to ensure that the material obtains a higher hardenability.

5. The preparation method according to claim 1, characterized in that: In step (4), the critical temperature and critical time solution treatment is a high temperature short time solution treatment, and the sample after the solution treatment is water cooled is subjected to aging treatment. The aging treatment adopts low temperature aging treatment, the temperature is 500-550°C, and the holding time is 4-6h, so as to completely remove the quenching residual stress and ensure that the material is fully strengthened.

6. The preparation method according to claim 1, characterized in that: The titanium alloy is an α+β alloy.

7. The preparation method according to claim 1 or 4, characterized in that: The critical temperature is 40 to 65°C higher than the phase transition point of the titanium alloy, and the critical time is 3 to 5 minutes.

8. The preparation method according to claim 1 or 2, characterized in that: The printing layer thickness t and scanning spacing h were fixed at 50-70 μm and 90-110 μm, respectively.

9. A titanium alloy prepared according to the preparation method according to any one of claims 1 to 8, characterized in that: The titanium alloy has ultra-high resistance to high cycle fatigue.

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