Post-treatment method for improving fatigue performance of additive manufacturing TA15 alloy

Abstract

The invention relates to the field of metal material fatigue performance optimization, in particular to a post-treatment method for improving the fatigue performance of an additive manufacturing TA15 alloy. The method comprises the following steps that a printing state material of the additive manufacturing TA15 alloy is taken to be subjected to annealing treatment, and a destressing state material of the additive manufacturing TA15 alloy is obtained; then carrying out hot isostatic pressing treatment; carrying out solution treatment on the material subjected to hot isostatic pressing treatment; and then aging treatment is carried out. According to the method, solution aging treatment is combined on the basis of HIP, on the basis of eliminating pores, coarsening of alpha laths is inhibited, the grain boundary alpha enrichment distribution density in the additive manufacturing TA15 alloy is reduced, the microstructure of the alloy is further optimized, the fatigue strength of the additive manufacturing TA15 alloy can be effectively improved, and the process is simple, convenient and reliable.

Description

Technical Field

[0001] The present invention relates to the field of fatigue performance optimization of metal materials, and specifically to a post-processing method for improving the fatigue performance of additively manufactured TA15 alloy. Background Art

[0002] TA15 alloy (Ti-6.5Al-2Zr-1Mo-1V), a high-aluminum equivalent, near-alpha titanium alloy, is an ideal choice for complex components such as aircraft load-bearing frames, missile engine casings, and launch vehicle fuel tanks due to its excellent mechanical properties, weldability, and thermal stability. However, conventional manufacturing techniques for forming complex TA15 components generally face the dual dilemma of high manufacturing costs and lengthy production cycles, severely restricting the alloy's engineering applications. In recent years, additive manufacturing (AM) technology, with its unique layer-by-layer forming properties, has provided a disruptive solution for the rapid fabrication of complex titanium alloy components in the aerospace industry. Compared with traditional methods, it offers advantages such as fewer production steps, shorter production cycles, and higher material utilization. Therefore, AM has significantly advanced the application of TA15 alloy in the aerospace industry.

[0003] Additively manufactured aerospace components are often subjected to cyclical loads during actual use. To ensure the long-term safety of aircraft, additively manufactured titanium alloys must exhibit excellent fatigue properties. However, due to the high-temperature, rapid heating and cooling processes, and the complex interaction between the laser and the powdered material, defects can easily form during the forming process. These defects, acting as fatigue sources, can rapidly expand under high cyclic loads, leading to fatigue failure.

[0004] To address this issue, hot isostatic pressing (HIP), a pressurized heat treatment technique, has been shown to effectively close pores within the material. However, the long, high-temperature hold and low cooling rate of HIP inevitably lead to microstructural coarsening, causing fatigue cracks to initiate from coarse α-laths or clusters, thus introducing a new fatigue "weakness." Furthermore, high-temperature heat treatment can easily lead to α-enrichment at grain boundaries, which can trigger α-phase fatigue cracks within grain boundaries. Summary of the Invention

[0005] The present invention provides a post-processing method for improving the fatigue performance of additively manufactured TA15 alloy. By this method, the α laths are refined while the distribution density of grain boundary α enrichment in the additively manufactured TA15 alloy is reduced, thereby achieving a significant improvement in the fatigue strength of the additively manufactured TA15 alloy.

[0006] To achieve the above object, the technical solutions adopted by the present invention are as follows:

[0007] A post-processing method for improving fatigue performance of additively manufactured TA15 alloy comprises the following steps:

[0008] (1) Annealing the printed material of the additively manufactured TA15 alloy to obtain the stress-relieved material of the additively manufactured TA15 alloy;

[0009] (2) subjecting the stress-relieved material of the additively manufactured TA15 alloy to hot isostatic pressing;

[0010] (3) taking the material after hot isostatic pressing and performing solution treatment;

[0011] (4) Aging treatment is performed on the material after solution treatment.

[0012] The additively manufactured TA15 alloy is a printed material of the additively manufactured TA15 alloy in step (1) prepared by a laser powder bed melting process; the alloy composition is Al: 5.8~6.8%, Zr: 1.8~2.3%, Mo: 0.8~1.5%, V: 0.8~1.8%, O: ≤0.15%, Fe≤0.30%, N≤0.05%, H≤0.015%, C≤0.05%, Si≤0.05%, and Ti is the balance.

[0013] Furthermore, the annealing treatment in step (1) is carried out at a temperature of 600-650°C, a holding time of 3-5 hours, and a cooling method of cooling to room temperature in air.

[0014] Furthermore, the hot isostatic pressing treatment in step (2) is carried out at a temperature of 900-950°C, a pressure of 145-160 MPa, a holding time of 2-5 h, and is then cooled to 400-500°C in the furnace and then cooled to room temperature in air.

[0015] Furthermore, the solution treatment in step (3) is performed at a temperature of 50 to 80° C., which is higher than the phase transition point of the stress-relieved material of the additively manufactured TA15 alloy, for a time of 0.5 to 3 h, and the cooling method is water cooling to room temperature.

[0016] Furthermore, the phase transition point of the stress-relieved material of the additively manufactured TA15 alloy is determined by differential scanning calorimetry: an endothermic peak is found in the differential scanning calorimetry heating curve, and the end point of the endothermic peak is the phase transition point of the stress-relieved material of the additively manufactured TA15 alloy, and the temperature is in the range of 1010~1030℃.

[0017] Furthermore, the aging treatment in step (4) is carried out at a temperature of 400-600°C, a holding time of 4-6 hours, and a cooling method of air cooling to room temperature.

[0018] The present invention proposes to combine HIP with solid solution aging treatment, suppress the coarsening of α laths on the basis of eliminating porosity, reduce the grain boundary α enrichment distribution density in the additively manufactured TA15 alloy, further optimize the microstructure of the alloy, and develop a method that can reduce fatigue "shortboard" and thus effectively improve the fatigue performance of the additively manufactured TA15 alloy.

[0019] The beneficial effects of the present invention are as follows

[0020] 1. The present invention can effectively improve the fatigue performance of additively manufactured TA15 alloy, and the fatigue strength is increased by 35.3% compared with the stress-relieved state.

[0021] 2. This invention points out the direction for the current research on hot isostatic pressing combined with post-processing optimization in additive manufacturing of TA15 alloy, namely, to achieve the improvement of its fatigue performance by further refining the structure and reducing the distribution density of α enrichment at the grain boundaries. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Figure 3 is the organizational EBSD image of the stress-relieved material and the statistical results of the α lath and grain size within the grains, where a is the EBSD image of the α lath within the grains of the stress-relieved material, b is the EBSD image of the grains of the stress-relieved material, c is the α lath size statistics within the grains of the stress-relieved material, and d is the grain size statistics of the stress-relieved material.

[0023] Figure 2 Figure 2 is the XRT hole distribution image and size statistics of the stress-relieved material, where a is the XRT hole distribution image of the stress-relieved material, and b is the hole size statistics of the stress-relieved material.

[0024] Figure 3 Figure 3 is the EBSD image of the hot isostatically pressed material and the statistical results of the α lath and grain size within the grains. a is the EBSD image of the α lath within the grains of the hot isostatically pressed material, b is the EBSD image of the grains of the hot isostatically pressed material, c is the statistical result of the α lath size within the grains of the hot isostatically pressed material, and d is the statistical result of the grain size of the hot isostatically pressed material.

[0025] Figure 4 This is the XRT hole detection result of hot isostatically pressed material.

[0026] Figure 5 This is the differential scanning calorimetry (DSC) heating curve of the stress-relieved material.

[0027] Figure 6These are the organizational EBSD images and α-lath size statistics of the additively manufactured TA15 alloy material processed by the present invention, wherein a is the intra-grain α-lath EBSD image of the additively manufactured TA15 alloy material processed by the present invention, b is the grain EBSD image of the additively manufactured TA15 alloy material processed by the present invention, and c is the intra-grain α-lath size statistics of the additively manufactured TA15 alloy material processed by the present invention.

[0028] Figure 7 These are the fatigue performance test results of the stress-relieved material and the additively manufactured TA15 alloy material obtained by the present invention.

[0029] Figure 8 These are the fatigue cracking locations of the stress-relieved material, the hot isostatically pressed material, and the additively manufactured TA15 alloy material obtained by the present invention.

[0030] Figure 9 The microstructure, constant stress amplitude fatigue performance test results and fatigue cracking location of the material prepared in Comparative Example 1 and the additively manufactured TA15 alloy material obtained in the present invention, wherein a is the metallographic structure of the material prepared in Comparative Example 1, b is the constant stress amplitude fatigue performance test results and fatigue cracking location of the material prepared in Comparative Example 1 and the additively manufactured TA15 alloy material obtained in the present invention. The number 3 in the figure represents that 3 samples were made.

[0031] Figure 10 These are the fatigue performance test results of the hot isostatically pressed material (Comparative Example 2) and the additively manufactured TA15 alloy material obtained in the present invention.

[0032] Figure 11 These are the constant stress amplitude fatigue performance test results of the material prepared in Comparative Example 3 and the additively manufactured TA15 alloy material obtained in the present invention. The number 3 in the figure represents that three samples were made.

[0033] Figure 12 These are the constant stress amplitude fatigue performance test results of the material prepared in Comparative Example 4 and the additive TA15 alloy material obtained in the present invention. The number 3 in the figure represents that three samples were made. DETAILED DESCRIPTION

[0034] The present invention will be further described below with reference to specific embodiments and accompanying drawings.

[0035] The raw materials used in the following examples and comparative examples are printed materials of additively manufactured TA15 alloy prepared by laser powder bed melting process purchased from Polylite Company. The material composition is (mass percentage): Ti (balance), Al (6.39%), Zr (2.01%), Mo (1.26%), V (1.69%), O (0.11%), Fe (0.036%), N (0.018%), H (0.004%), C and Si (content of each is less than 0.01%); the printed material size is a rod with a height of 90 mm and a diameter of 15 mm.

[0036] Example 1

[0037] Step 1: 50 rod-shaped printed materials with a size of 90 mm in height and 15 mm in diameter were subjected to stress relief annealing at 630°C (high-temperature box-type electric furnace model BLMT-XA-9-74). The holding time was controlled at 4 hours and then taken out and cooled to room temperature with air to remove the residual stress during the printing process. The obtained material is named stress-relieved material. Figure 1 It can be seen that there are fine α laths distributed inside the grains, with an average width of 2.5 μm (width distribution range is 1~8 μm). Non-destructive hole detection (accuracy is 2.26 μm) is performed using XRT (X-ray three-dimensional imaging system). Figure 2 As shown in the figure, printing defects are clearly detected inside the stress-relieved material, with an average diameter of 9.2 μm (the defect diameter distribution range is 5~30 μm).

[0038] Step 2: At 920 ° C, the stress-relieved material is subjected to isotropic high-pressure argon gas with a pressure of 150 MPa for hot isostatic pressing (QIH48 URC equipment equipped with a uniform rapid cooling system produced by Quintus, Germany). After keeping the temperature for 3 hours, it is cooled to 500 ° C in the furnace and then air-cooled to room temperature. The obtained material is named hot isostatically pressed material, such as Figure 3 It can be clearly seen that the grain size has increased significantly and the α lath inside the grain has coarsened. After hot isostatic pressing, a sample was randomly selected for XRT non-destructive hole detection. Figure 4 As shown in the figure, no defects were detected under the accuracy of XRT (2.26 μm), indicating that the pores in the stress-relieved material were successfully eliminated after hot isostatic pressing, and a pore-free material was initially obtained.

[0039] Step 3: The phase transition point of the stress-relieved material is determined to be 1012 °C by differential scanning calorimetry (DSC). Figure 5As shown in the figure, a distinct endothermic peak appears during the heating process, with an extrapolated onset temperature of 889°C (the starting point of the endothermic peak) and an extrapolated end temperature of 1012°C (the end point of the endothermic peak). This indicates that when the heating temperature reaches 889°C, part of the α phase begins to transform, and when the temperature reaches 1012°C, all of the α phase completely transforms into the β phase. The hot isostatically pressed material obtained in step 2 was solution treated (in a BLMT-XA-12-74 high-temperature box-type electric furnace) at a temperature 1012°C above the phase transformation point of the stress-relieved material. This treatment was followed by water cooling at 1080°C for 1 hour.

[0040] Step 4: The sample after solution treatment and water cooling is subjected to low-temperature aging treatment, kept at 500 °C for 6 h, taken out and cooled to room temperature with air to achieve the dual goals of completely removing the residual stress of quenching and ensuring that the material is fully strengthened.

[0041] according to Figure 6 It can be seen that the additively manufactured TA15 alloy material obtained by the present invention has a large phase transformation driving force, and the α laths in the grains are significantly refined. The average width of the laths is 1.9 μm (the width distribution range is 0.5~7 μm), and the grains are significantly grown, with the grain size being significantly higher than 150 μm.

[0042] according to Figure 7 It can be seen that the additively manufactured TA15 alloy material obtained by the present invention exhibits a significant improvement in fatigue strength in a tension-tension fatigue performance test with a stress ratio of R=0.1 (test standard: GB / T 3075-2008 "Axial Force Control Method for Fatigue Test of Metallic Materials"). Figure 7 As shown in the figure, the fatigue strength of the TA15 alloy processed by the additive manufacturing method was tested and determined using the standard lifting method. Compared with the stress-relieved material, the fatigue strength of the additive manufacturing TA15 alloy processed by the present invention reached 862.5 MPa, which is 35.3% higher than the stress-relieved state (the test standard is: GB / T 24176-2009 "Statistical Scheme and Analysis Method for Fatigue Test Data of Metallic Materials"). Figure 8It can be seen that fatigue cracks in the de-stressed material all initiate from pores. For the material treated with hot isostatic pressing in step 2, although the grain size did not grow significantly, the α laths significantly coarsened, resulting in a shift in the fatigue cracking mechanism, with fatigue cracks initiating from coarsened α laths or α clusters. Further solution aging treatment significantly increases the grain size while refining the α laths and partially reappearing pores, causing fatigue cracks in the additively manufactured TA15 alloy treated with this method to initiate from extremely small defects. In addition, a small number of fatigue cracks initiate from α-enriched areas at grain boundaries. It is worth noting that the growth of grain boundaries during the solution-aging process leads to a decrease in the distribution density of enriched α clusters, while the refinement of α laths within the grains effectively hinders the crack propagation path. This "grain boundary enrichment dilution-grain refinement" coupling mechanism ultimately achieves a significant improvement in fatigue strength. This not only provides an efficient and operational approach to improving the performance of TA15 alloys in additive manufacturing, but also provides solid technical support for its practical application in high-stress applications, and has important industrial potential.

[0043] Comparative Example 1

[0044] The preparation method of the additively manufactured TA15 alloy in this comparative example is the same as that in Example 1, except that the heat treatment method in step 3 is changed to a conventional solution aging heat treatment, i.e., keeping the temperature at 1080°C for 1 minute and then cooling with water.

[0045] Its organizational characteristics such as Figure 9 As shown in a. The fatigue performance test was carried out at room temperature, using a tension-tension fatigue performance test with a stress ratio of R=0.1 and a constant stress amplitude of 800 MPa (test standard: GB / T 3075-2008 "Method of Axial Force Control in Fatigue Tests of Metallic Materials"). The fatigue life distribution is shown in Figure 9 As shown in Figure 2b, it can be seen that the fatigue life of the material prepared in Comparative Example 1 is scattered and lower than that of Example 1. This is mainly due to the fact that the grain boundaries are relatively stable under high-temperature and short-time heat treatment, the phenomenon of grain boundary relaxation exists, and there is also α enrichment at the grain boundaries, which makes fatigue cracks easily initiate from the coarse α phase at the grain boundaries.

[0046] Comparative Example 2

[0047] The preparation method of additively manufactured TA15 alloy in this comparative example is the same as that in Example 1, except that step 3 and step 4 are not performed.

[0048] Fatigue performance testing was conducted at room temperature using a tension-tension fatigue performance test with a stress ratio of R=0.1 (test standard: GB / T 3075-2008 "Method for Axial Force Control in Fatigue Testing of Metallic Materials"). Measurement and analysis were performed in accordance with GB / T 24176-2009 "Statistical Scheme and Analysis Method for Fatigue Test Data of Metallic Materials" and the standard lift method was used to determine the fatigue strength. Figure 10 As shown, it can be seen that the fatigue strength of the material prepared in Comparative Example 2 reaches 737.5 MPa and the fatigue strength is significantly lower than that of Example 1.

[0049] Comparative Example 3

[0050] The preparation method of additively manufactured TA15 alloy in this comparative example is the same as that in Example 1, except that the hot isostatic pressing temperature in step 2 is changed, and the stress-relieved material is subjected to isotropic high-pressure argon gas with a pressure of 150 MPa for hot isostatic pressing at 800 and 850°C, respectively. After holding the temperature for 3 hours, the material is furnace-cooled to 500°C and then air-cooled to room temperature.

[0051] The fatigue performance test was carried out at room temperature, using a tension-tension fatigue performance test with a stress ratio of R=0.1 and a constant stress amplitude of 800 MPa (test standard: GB / T 3075-2008 "Method of Axial Force Control in Fatigue Tests of Metallic Materials"). The fatigue life distribution is as follows: Figure 11 As shown, it can be seen that the fatigue life of the material prepared in Comparative Example 3 is significantly lower than that in Example 1.

[0052] Comparative Example 4

[0053] The preparation method of additively manufactured TA15 alloy in this comparative example is the same as that in Example 1, except that the heat treatment temperature in step 3 is changed to 950, 970, 990, 1020, and 1050°C for 1 hour and then water-cooled. Fatigue performance testing was carried out at room temperature using a tension-tension fatigue performance test with a stress ratio of R = 0.1 and a constant stress amplitude of 800 MPa (test standard: GB / T 3075-2008 "Method of Axial Force Control in Fatigue Testing of Metallic Materials"). The fatigue life distribution is shown in Figure 2. Figure 12 As shown, it can be seen that the fatigue life of the material prepared in Comparative Example 4 is more dispersed and significantly lower than that of Example 1.

[0054] In summary, the core innovation of this invention lies in its breakthrough of the limitations of traditional single optimization strategies by proposing and implementing the coordinated control and simultaneous optimization of material structure and defects. This effectively overcomes inherent process barriers such as lath coarsening and α-phase enrichment at grain boundaries during hot isostatic pressing and heat treatment. By refining the α-lath size and significantly reducing the distribution density of α-phase-rich regions at grain boundaries, this method reduces the preferential initiation source of fatigue cracks (i.e., the fatigue "short board"). This method can effectively improve the fatigue strength of additively manufactured TA15 alloys with simplicity and reliability.

Claims

1. A post-processing method for improving fatigue performance of additively manufactured TA15 alloy, characterized in that: The method comprises the following steps: (1) Annealing the printed material of the additively manufactured TA15 alloy to obtain the stress-relieved material of the additively manufactured TA15 alloy; (2) The stress-relieved material of the additively manufactured TA15 alloy is subjected to hot isostatic pressing at a temperature of 900-950°C, a pressure of 145-160 MPa, and a holding time of 2-5 h. The material is then cooled to 400-500°C in the furnace and then cooled to room temperature in air. (3) The material after hot isostatic pressing is subjected to solution treatment. The solution temperature is 50~80℃ higher than the phase transition point of the stress relief material of the additively manufactured TA15 alloy. The time is 0.5~3h, and the cooling method is water cooling to room temperature. (4) Aging treatment is performed on the material after solution treatment.

2. The post-processing method for improving fatigue performance of additively manufactured TA15 alloy according to claim 1, characterized in that: The additively manufactured TA15 alloy is a printed material of the additively manufactured TA15 alloy described in step (1) prepared by a laser powder bed melting process; the alloy composition is Al: 5.8~6.8%, Zr: 1.8~2.3%, Mo: 0.8~1.5%, V: 0.8~1.8%, O: ≤0.15%, Fe≤0.30%, N≤0.05%, H≤0.015%, C≤0.05%, Si≤0.05%, and Ti is the balance.

3. The post-processing method for improving fatigue performance of additively manufactured TA15 alloy according to claim 1, characterized in that: The annealing treatment in step (1) is carried out at a temperature of 600-650°C, with a holding time of 3-5 hours, and is cooled to room temperature in air.

4. The post-processing method for improving fatigue performance of additively manufactured TA15 alloy according to claim 1, characterized in that: The phase transition point of the stress-relieved material of the additively manufactured TA15 alloy in step (3) is determined by differential scanning calorimetry: an endothermic peak is found in the differential scanning calorimetry heating curve, and the end point of the endothermic peak is the phase transition point of the stress-relieved material of the additively manufactured TA15 alloy, and the temperature is in the range of 1010~1030℃.

5. The post-processing method for improving fatigue performance of additively manufactured TA15 alloy according to claim 1, characterized in that: The aging treatment described in step (4) is carried out at a temperature of 400-600°C, with a holding time of 4-6 hours, and is cooled to room temperature by air.

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