A post-processing method for improving fatigue performance of additive manufacturing TA15 alloy

Abstract

The application relates to the field of metal material fatigue performance optimization, and particularly relates to a post-treatment method for improving the fatigue performance of additive manufacturing TA15 alloy. The method comprises the following steps: annealing printing state material of the additive manufacturing TA15 alloy to obtain stress-free state material of the additive manufacturing TA15 alloy; then, performing hot isostatic pressing treatment; taking the material after the hot isostatic pressing treatment to perform solid solution treatment; and then, performing aging treatment. The application proposes that, on the basis of the HIP, the solid solution and aging treatment are combined, on the basis of eliminating pores, the coarsening of alpha lath is inhibited, the distribution density of grain boundary alpha enrichment in the additive manufacturing TA15 alloy is reduced, and the microstructure of the alloy is further optimized. The method can effectively improve the fatigue strength of the additive manufacturing TA15 alloy, and the process is simple and reliable.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of metal material fatigue performance optimization, and particularly relates to a post-processing method for improving fatigue performance of additive manufacturing TA15 alloy. BACKGROUND

[0002] As a high-aluminum-equivalent near-alpha titanium alloy, TA15 alloy (Ti-6.5Al-2Zr-1Mo-1V) 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, welding performance, and thermal stability. However, traditional manufacturing techniques generally face the dual dilemma of high manufacturing cost and long production cycle when forming complex TA15 components, which severely restricts the engineering application of the alloy. In recent years, additive manufacturing (AM) technology provides a revolutionary solution for the rapid manufacturing of complex titanium alloy components in the aerospace field due to its unique layer-by-layer accumulation forming characteristics. Compared with traditional methods, it has the advantages of fewer production steps, shorter production cycle, and higher material utilization. Therefore, additive manufacturing technology has significantly promoted the application of TA15 alloy in the aerospace field.

[0003] During actual use, additive manufacturing aircraft parts often bear cyclic alternating loads. To ensure the long-term safe use of aircraft, additive manufacturing titanium alloy materials need to have excellent fatigue performance. However, due to the high-temperature rapid heating and cooling process, the complex interaction between the laser and the powder material, defects are easily generated in the alloy during the forming process. These defects act as fatigue sources and easily expand rapidly under high cyclic loads, leading to fatigue failure.

[0004] To solve this problem, hot isostatic pressing (HIP) as a kind of pressurized heat treatment technology has been proved to be effective in closing the pores inside the material. However, the high-temperature long-time holding and low cooling rate of HIP inevitably lead to microstructure coarsening, and fatigue cracks are initiated from coarse alpha lath or clusters, thereby introducing new fatigue "short boards". In addition, high-temperature heat treatment easily leads to grain boundary alpha enrichment, thereby initiating grain boundary alpha phase fatigue cracks. SUMMARY

[0005] The present application provides a post-processing method for improving the fatigue performance of additive manufacturing TA15 alloy. Through this method, the alpha lath is refined, and the distribution density of grain boundary alpha enrichment in the additive manufacturing TA15 alloy is reduced, thereby achieving a substantial improvement in the fatigue strength of the additive manufacturing TA15 alloy.

[0006] To achieve the above-mentioned purposes, the technical solutions adopted by the present application are as follows:

[0007] A post-processing method for improving the fatigue performance of additive manufacturing TA15 alloy, comprising the following steps:

[0008] (1) annealing the as-printed material of additive manufacturing TA15 alloy to obtain a stress-relieved material of additive manufacturing TA15 alloy;

[0009] (2) hot isostatic pressing the stress-relieved material of additive manufacturing TA15 alloy;

[0010] (3) solid solution treatment of the material after hot isostatic pressing;

[0011] (4) aging treatment of the material after solid solution treatment.

[0012] The additive manufacturing TA15 alloy is the as-printed material of additive manufacturing TA15 alloy in step (1) prepared by laser powder bed fusion 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%, Ti is the balance.

[0013] Further, the annealing treatment in step (1) is at a temperature of 600-650 ℃, the holding time is 3-5 h, and the cooling method is air cooling to room temperature.

[0014] Further, the hot isostatic pressing in step (2) is at a temperature of 900-950 ℃, a pressure of 145-160 MPa, a holding time of 2-5 h, and air cooling to room temperature after furnace cooling to 400-500 ℃.

[0015] Further, the solid solution treatment in step (3) is at a temperature of 50-80 ℃ higher than the phase transition point of the stress-relieved material of additive manufacturing TA15 alloy, the time is 0.5-3 h, and the cooling method is water cooling to room temperature.

[0016] Further, the phase transition point of the stress-relieved material of additive manufacturing TA15 alloy is determined by differential scanning calorimetry analysis: finding the endothermic peak 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 additive manufacturing TA15 alloy, the temperature is in the range of 1010-1030 ℃.

[0017] Further, the aging treatment in step (4) is at a temperature of 400-600 ℃, the holding time is 4-6 h, and the cooling method is air cooling to room temperature.

[0018] The present application proposes to combine solid solution and aging treatment on the basis of HIP, to inhibit the coarsening of alpha lath on the basis of eliminating pores, to reduce the distribution density of grain boundary alpha enrichment in additive manufacturing TA15 alloy, to further optimize the microstructure of the alloy, and to develop a method capable of reducing fatigue "short board" to effectively improve the fatigue performance of additive manufacturing TA15 alloy.

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

[0020] 1. The present application can effectively improve the fatigue performance of additive manufacturing TA15 alloy, and the fatigue strength is increased by 35.3% compared with the stress relief state.

[0021] 2. The present application points out the direction for the current realization of additive manufacturing TA15 alloy hot isostatic pressing combined post-processing optimization, that is, to realize the improvement of fatigue performance by further refining the structure and reducing the distribution density of grain boundary alpha enrichment. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 EBSD image and grain size statistics of the microstructure of the stress relief state material, wherein a is the EBSD image of the alpha lath in the grain of the stress relief state material, b is the EBSD image of the grain of the stress relief state material, c is the size statistics of the alpha lath in the grain of the stress relief state material, and d is the size statistics of the grain of the stress relief state material.

[0023] Figure 2 XRT hole distribution image and size statistics of the stress relief state material, wherein a is the XRT hole distribution image of the stress relief state material, and b is the hole size statistics result of the stress relief state material.

[0024] Figure 3 EBSD image and grain size statistics of the microstructure of the hot isostatic pressing state material, wherein a is the EBSD image of the alpha lath in the grain of the hot isostatic pressing state material, b is the EBSD image of the grain of the hot isostatic pressing state material, c is the size statistics of the alpha lath in the grain of the hot isostatic pressing state material, and d is the size statistics of the grain of the hot isostatic pressing state material.

[0025] Figure 4 XRT hole detection result of the hot isostatic pressing state material.

[0026] Figure 5 Differential scanning calorimetry (DSC) heating curve of the stress relief state material.

[0027] Figure 6EBSD images and size statistics of α laths of the additive manufacturing TA15 alloy material treated by the present application, wherein a is the EBSD image of α laths in a grain of the additive manufacturing TA15 alloy material treated by the present application, b is the EBSD image of a grain of the additive manufacturing TA15 alloy material treated by the present application, and c is the size statistics of α laths in a grain of the additive manufacturing TA15 alloy material treated by the present application.

[0028] Figure 7 Fatigue performance test results of the stress relieved material and the additive manufacturing TA15 alloy material obtained by the present application.

[0029] Figure 8 Fatigue cracking positions of the stress relieved material, the hot isostatic pressed material and the additive manufacturing TA15 alloy material obtained by the present application.

[0030] Figure 9 Microstructure, constant stress amplitude fatigue performance test results and fatigue cracking positions of the material prepared in Comparative Example 1 and the additive manufacturing TA15 alloy material obtained by the present application, 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 positions of the material prepared in Comparative Example 1 and the additive manufacturing TA15 alloy material obtained by the present application, and the number 3 in the figure represents that 3 samples are made.

[0031] Figure 10 Fatigue performance test results of the hot isostatic pressed material (Comparative Example 2) and the additive manufacturing TA15 alloy material obtained by the present application.

[0032] Figure 11 Constant stress amplitude fatigue performance test results of the material prepared in Comparative Example 3 and the additive manufacturing TA15 alloy material obtained by the present application, and the number 3 in the figure represents that 3 samples are made.

[0033] Figure 12 Constant stress amplitude fatigue performance test results of the material prepared in Comparative Example 4 and the additive manufacturing TA15 alloy material obtained by the present application, and the number 3 in the figure represents that 3 samples are made. DETAILED DESCRIPTION

[0034] The present application will be further described below through specific embodiments and the accompanying drawings.

[0035] The raw material used in the following examples and comparative examples is a printed material of additive manufacturing TA15 alloy prepared by a laser powder bed fusion process purchased from Platinum Group, and the material composition is (mass percent): 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 (each less than 0.01%); the size of the printed material is a rod with a height of 90 mm and a diameter of 15 mm.

[0036] Example 1

[0037] Step 1: 50 rods of printed material with a size of 90 mm in height and 15 mm in diameter were stress relieved at 630 ℃ in a high-temperature box-type electric furnace (BLMT-XA-9-74), and the holding time was controlled at 4 h before being taken out and cooled to room temperature in air to remove the residual stress in the printing process, and the obtained material was named as stress relieved material. According to the standard GB / T 9455-2008, the stress relieved material was subjected to a tensile test at room temperature, and the tensile strength was 1, 100 MPa, the elongation was 5.5%, and the fracture toughness was 48 MPa·m1 / 2. Figure 1 As can be seen, fine α laths are distributed in the grains, and the average width of the laths is 2.5 μm (the width distribution range is 1-8 μm). Non-destructive hole detection (the accuracy is 2.26 μm) was performed by XRT (X-ray three-dimensional imaging system), and as shown in FIG. 2, the existence of printing defects in the stress relieved material was obviously detected, and the average diameter of the defects was 9.2 μm (the defect diameter distribution range is 5-30 μm). Figure 2

[0038] Step 2: The stress relieved material was subjected to hot isostatic pressing treatment at 920 ℃ under the action of isotropic high-pressure argon gas with a pressure of 150 MPa (QIH48 URC equipment produced by Quintus, Germany, equipped with a uniform rapid cooling system), and after holding for 3 h, the furnace was cooled to 500 ℃ and then air-cooled to room temperature, and the obtained material was named as hot isostatic pressed material, as shown in FIG. 3. Figure 3 As can be seen, the grain size is obviously increased, and the α laths in the grains are coarsened. A sample was randomly taken after the hot isostatic pressing treatment for XRT non-destructive hole detection, and as shown in FIG. 4, no defects were detected under the accuracy of XRT (2.26 μm), indicating that the holes in the stress relieved material were successfully eliminated after the hot isostatic pressing, and a hole-free material was preliminarily obtained. Figure 4

[0039] Step 3: The phase transition point of the stress relieved material was determined by differential scanning calorimetry (DSC) detection, and as shown in FIG. 5, the phase transition point was 1, 012 °C. Figure 5 ​​As shown, a clear endothermic peak appears during the heating process, the extrapolated starting temperature of the endothermic peak is 889 ℃ (the starting point of the endothermic peak), and the extrapolated ending temperature is 1012 ℃ (the ending point of the endothermic peak), indicating that when the heating temperature reaches 889 ℃, part of the alpha phase begins to transform, and when the temperature reaches 1012 ℃, all alpha phases are completely transformed into beta phases. The hot isostatic pressing state material obtained in step 2 is subjected to solid solution treatment at a temperature higher than the phase transition point 1012 ℃ of the stress relief state material (high-temperature box-type electric furnace of BLMT-XA-12-74 type), that is, after being kept at 1080 ℃ for 1 h, it is water-cooled.

[0040] Step 4: The sample after solid solution treatment and water cooling is subjected to low-temperature aging treatment, kept at 500 ℃ for 6 h, taken out and air-cooled to room temperature, so as to achieve the double 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 additive manufacturing TA15 alloy material obtained by the present application has a large phase transition driving force, the alpha lath in the grain is obviously refined, the average width of the lath is 1.9 μm (the width distribution range is 0.5-7 μm), and the grain is significantly grown, and the grain size is obviously higher than 150 μm.

[0042] According to Figure 7 It can be seen that the additive manufacturing TA15 alloy material obtained by the present application exhibits a significant improvement in fatigue strength in the tensile-tensile fatigue performance test with a stress ratio R=0.1 (test standard: GB / T 3075-2008 “Metallic Materials Fatigue Test Axial Force Control Method”). Figure 7 As shown, the fatigue strength is tested and determined using the standard lifting method, and compared with the stress relief state material, the fatigue strength of the additive manufacturing TA15 alloy treated by the present application reaches 862.5 MPa, which is improved by 35.3% compared with the stress relief state (test standard: GB / T 24176-2009 “Metallic Materials Fatigue Test Data Statistical Scheme and Analysis Method”). From Figure 8It can be seen that the fatigue cracks of the stress relieved material all initiate from the pores. For the material after the step 2 hot isostatic pressing treatment, although the grain size does not significantly increase, the alpha lath is obviously coarsened, which leads to the change of the fatigue cracking mechanism, and the fatigue cracks initiate from the coarsened alpha lath or alpha cluster. Further combined with the solid solution and aging treatment, the grain size is significantly increased while the alpha lath is refined, and part of the pores is reproduced, so that the fatigue cracks of the additive manufacturing TA15 alloy treated by the method of the application again initiate from the defects with extremely small size, and in addition, a small amount of fatigue cracks initiate from the grain boundary alpha enrichment. It is worth noting that the grain boundary growth during the solid solution and aging process leads to the decrease of the distribution density of the enriched alpha cluster, and the refinement of the alpha lath in the grain effectively hinders the crack propagation path. The coupling mechanism of "grain boundary enrichment dilution-grain refinement" finally realizes the significant improvement of the fatigue strength, which not only provides an efficient and operable way for the performance improvement of the additive manufacturing TA15 alloy, but also provides solid technical support for its practical application in high stress applications, and has important industrialization potential.

[0043] Comparative Example 1

[0044] In the comparative example, the additive manufacturing TA15 alloy is prepared by the method same as that in example 1, and the difference is that the heat treatment mode of step 3 is changed to the conventional solid solution and aging heat treatment, that is, water cooling after 1 min of heat preservation at 1080 DEG C.

[0045] The microstructure characteristics of the material prepared in the comparative example are shown in Figure 9 a. The fatigue performance test is carried out at room temperature, and the test adopts the stress ratio R=0.1, and the constant stress amplitude is 800 MPa. The test standard is GB / T 3075-2008 "Metallic materials fatigue test axial force control method". The fatigue life distribution is shown in Figure 9 b. It can be seen that the fatigue life of the material prepared in comparative example 1 is dispersed and lower than that in example 1. This is mainly due to the fact that the grain boundary is relatively stable under high temperature and short time heat treatment, and there is grain boundary relaxation, and there is grain boundary alpha enrichment, so that the fatigue cracks are easy to initiate from the coarse grain boundary alpha phase.

[0046] Comparative Example 2

[0047] In the comparative example, the additive manufacturing TA15 alloy is prepared by the method same as that in example 1, and the difference is that steps 3 and 4 are not carried out.

[0048] The fatigue performance test was carried out at room temperature, and the stress ratio was R = 0.1. The test standard was GB / T 3075-2008 "Metallic materials - Fatigue tests - Axial force control method". The fatigue strength was determined by using the standard step-up method, and the fatigue strength was determined according to GB / T 24176-2009 "Metallic materials - Fatigue tests - Data statistical scheme and analysis method". The fatigue strength is shown in Table 1, and it can be seen that the fatigue strength of the material prepared in Comparative Example 2 reaches 737.5 MPa and is significantly lower than that of Example 1. Figure 10

[0049] Comparative Example 3

[0050] In this comparative example, the preparation method of the additive manufactured TA15 alloy is the same as that of Example 1, except that the hot isostatic pressing temperature of step 2 is changed, and the stress-free material is subjected to hot isostatic pressing treatment under the action of isotropic high-pressure argon gas with a pressure of 150 MPa at 800 and 850 ℃, respectively, and then cooled to 500 ℃ in the furnace and then air-cooled to room temperature after holding for 3 h.

[0051] The fatigue performance test was carried out at room temperature, and the stress ratio was R = 0.1. The test standard was GB / T 3075-2008 "Metallic materials - Fatigue tests - Axial force control method". The fatigue strength was determined by using the standard step-up method, and the fatigue strength was determined according to GB / T 24176-2009 "Metallic materials - Fatigue tests - Data statistical scheme and analysis method". The fatigue strength is shown in Table 1, and it can be seen that the fatigue strength of the material prepared in Comparative Example 2 reaches 737.5 MPa and is significantly lower than that of Example 1. Figure 11

[0052] Comparative Example 4

[0053] In this comparative example, the preparation method of the additive manufactured TA15 alloy is the same as that of Example 1, except that the heat treatment temperature of step 3 is changed, i.e. water cooling after holding for 1 h at 950, 970, 990, 1020 and 1050 ℃, respectively. The fatigue performance test was carried out at room temperature, and the stress ratio was R = 0.1. The test standard was GB / T 3075-2008 "Metallic materials - Fatigue tests - Axial force control method". The fatigue strength was determined by using the standard step-up method, and the fatigue strength was determined according to GB / T 24176-2009 "Metallic materials - Fatigue tests - Data statistical scheme and analysis method". The fatigue strength is shown in Table 1, and it can be seen that the fatigue strength of the material prepared in Comparative Example 2 reaches 737.5 MPa and is significantly lower than that of Example 1. Figure 12

[0054] ​​​In summary, the core innovation of the present application is that the traditional single optimization strategy is broken through, and the synergistic regulation and synchronous optimization of material organization and defects are proposed and realized, the inherent process barriers such as coarse lath and grain boundary alpha phase enrichment in the hot isostatic pressing and heat treatment process are effectively overcome, the size of alpha lath is refined, and the distribution density of grain boundary alpha phase enrichment area is significantly reduced, so as to reduce the preferential initiation source of fatigue crack (i.e. fatigue ''short plate''). The method can effectively improve the fatigue strength of the additive manufacturing TA15 alloy, and the process is simple and reliable.

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 a stress-relieved material of the additively manufactured TA15 alloy; the annealing treatment is performed 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; (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 68~80℃, which is higher than the phase transition point of the stress-relieved 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 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℃.

4. 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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