Method for enhancing fatigue performance of Ti175 porous titanium alloy

Abstract

The invention discloses a method for enhancing fatigue performance of a Ti175 porous titanium alloy, which comprises the following steps of: under the protection of inert atmosphere, quickly putting a sample into a central isothermal area of a tubular furnace preheated to a first target temperature, continuously heating to a second target temperature, preserving heat, converting an original structure into a double-state structure, and cooling to room temperature at a controlled cooling rate; under inert atmosphere protection, the sample is heated to a third target temperature and subjected to heat preservation, then cyclic loads within the elastic range are applied in a stress control mode, and after loading is finished, the sample is cooled to the room temperature at the controlled cooling rate; under the air or inert atmosphere, the sample is rapidly placed into a box-type furnace preheated to a fourth target temperature, the temperature continues to rise to a fifth target temperature, heat preservation is conducted for a short time, residual stress is released, a surface oxygen solid solution strengthening layer is formed, and then the sample is cooled to the room temperature at the controlled cooling rate.

Description

Technical Field

[0001] This invention relates to a method for enhancing the fatigue properties of Ti175 porous titanium alloy. Background Technology

[0002] Ti175 titanium alloy (Ti-6.5Al-2Sn-3.7Zr-4Mo-1.2W-0.2Si) is a near-α type high-temperature titanium alloy that exhibits excellent fatigue performance, creep resistance, and microstructural stability under high-temperature service conditions, with a long-term service temperature reaching 550 ℃. This alloy has been widely used in chemical, shipbuilding and marine engineering, and aerospace fields, and is particularly suitable for critical load-bearing components under high-temperature conditions. Research on porous titanium alloys is of great significance in further reducing structural mass and achieving even better comprehensive mechanical properties. While porosity aligns with the demand for lightweighting, achieving a balance between strength and plasticity while ensuring long-term service reliability directly impacts structural safety.

[0003] Existing research and engineering practice show that porous titanium alloy samples prepared by additive manufacturing often suffer from problems such as insufficient inter-channel fusion, unmelted defects, porosity, coarse columnar grains, segregation, and high residual stress, thus limiting the overall mechanical properties and consistency. Traditional optimization focuses on the geometric design of pore structure and porosity to improve strength, plasticity, and fatigue performance; however, relying solely on morphological parameters is insufficient to fundamentally suppress or repair microstructural defects. Therefore, it is urgent to achieve multi-objective improvements in strength, plasticity, and fatigue durability through controllable microstructure regulation (such as grain / phase ratio and distribution, dislocation / precipitate states, interface properties, and residual stress field). Summary of the Invention

[0004] The present invention provides a method for enhancing the fatigue properties of Ti175 porous titanium alloy in order to solve the problems existing in the prior art.

[0005] The technical solutions adopted in this invention are as follows:

[0006] A method for enhancing the fatigue properties of Ti175 porous titanium alloy includes the following steps:

[0007] (1) The Ti175 porous titanium alloy sample obtained by additive manufacturing was subjected to the first heat treatment:

[0008] Under an inert atmosphere, the sample is rapidly placed into the central isothermal zone of a tube furnace that has been preheated to the first target temperature, and then heated to the second target temperature and held to transform the original microstructure into a bimodal microstructure. The sample is then cooled to room temperature at a controlled cooling rate.

[0009] (2) The specimens treated in step (1) were subjected to fatigue cyclic loading within the long-term service temperature range of Ti175 alloy:

[0010] Under an inert atmosphere, the sample was heated to the third target temperature and held at that temperature. Then, a cyclic load within the elastic range was applied in a stress-controlled mode. After the loading was completed, the sample was cooled to room temperature at a controlled cooling rate.

[0011] (3) Perform a second heat treatment on the sample after step (2):

[0012] In air or an inert atmosphere, the sample is rapidly placed into a box furnace preheated to the fourth target temperature, and the temperature is further increased to the fifth target temperature and held for a short time to release residual stress and form a surface oxygen solid solution reinforcement layer. Then, it is cooled to room temperature at a controlled cooling rate.

[0013] Furthermore, the first target temperature is 920~940 ℃, the second target temperature is 950 ℃, and the holding time is 30 min.

[0014] Furthermore, the heating rate of the tube furnace is 1.5–2 °C / min after the sample is placed in.

[0015] Furthermore, in step (1), the controlled cooling rate is 80–100 °C / min.

[0016] Furthermore, the third target temperature is 520~580℃, and the holding time is 90 min.

[0017] Furthermore, the cyclic loading within the elastic range, with a stress amplitude of 5% to 8% of the yield strength of the Ti175 porous titanium alloy specimen, a stress ratio R = -0.1, a frequency of 10 Hz, and a cycle count of 2 × 10⁻⁶, is applied. 5 ~ 3×10 5 Second-rate.

[0018] Furthermore, the controlled cooling rate in step (2) is 100 ~ 120 ℃ / min.

[0019] Furthermore, the fourth target temperature is 670~690 ℃, the fifth target temperature is 700 ℃, and the holding time is 10~15 min.

[0020] Furthermore, in step (3), the controlled cooling rate is 100 ~ 120 ℃ / min.

[0021] Furthermore, the inert atmosphere is argon gas with a purity of ≥99.9%.

[0022] The present invention has the following beneficial effects:

[0023] (1) By fully decomposing martensite through high-temperature heat treatment and reconstructing the lamellar structure into a bimodal structure, the strength and plasticity are synergistically improved without changing the porosity and topology. At the same time, fatigue strength and low / high cycle fatigue life are improved. The bimodal structure has higher thermal stability, which will inhibit grain coarsening and reduce the performance decay rate during service, and improve anisotropy and strain uniformity, thereby expanding the safety margin and long-term service reliability under complex loads such as compression and tension.

[0024] (2) Under the action of high temperature fatigue cyclic loading, phase transformation strengthening is induced, thereby improving the strength of the material; the cyclic stress amplitude is controlled at a low level, so that the deformation is always limited to the elastic range, avoiding damage to the original structure and early crack initiation; the thermal activation dynamic recovery under high temperature environment promotes dislocation rearrangement and partial annihilation, reduces local high dislocation density and stress concentration, thereby inhibiting the accumulation of irreversible damage and achieving synergistic improvement of fatigue strength and fatigue life.

[0025] (3) Annealing the sample under high temperature conditions can effectively release / eliminate the residual stress introduced during forming and service, and promote the rearrangement and partial annihilation of fatigue-induced dislocation entanglements, dislocation walls / cells and other defects, thereby restoring the uniformity of the structure and plasticity reserves; then, under short-term high temperature in the air environment, oxygen elements are controlled to penetrate into the surface layer to form a thin oxygen-reinforced layer (thin α-shell / oxygen solid solution layer), which improves the surface hardness and yield strength and forms gradient hardening, enhances the surface layer's resistance to crack initiation, thereby improving the fatigue strength (life) of the material. Attached Figure Description

[0026] Figure 1 This is a picture of the actual sample 1.

[0027] Figure 2 The image shows the metallographic structure of sample 1 before treatment.

[0028] Figure 3 The image shows the metallographic image of sample 1 after processing.

[0029] Figure 4 This is a picture of the actual sample 2.

[0030] Figure 5 The image shows the metallographic structure of sample 2 before treatment.

[0031] Figure 6 The image shows the metallographic image of sample 2 after processing.

[0032] Figure 7 This is a picture of sample 3.

[0033] Figure 8 The image shows the metallographic structure of sample 3 before treatment.

[0034] Figure 9 The image shows the metallographic image of sample 3 after processing. Detailed Implementation

[0035] The invention will now be further described with reference to the accompanying drawings.

[0036] The process steps of this invention are applicable to various porous Ti175 samples, and the method for enhancing the strength, plasticity, and fatigue properties of porous titanium is as follows:

[0037] A method for enhancing the fatigue properties of Ti175 porous titanium alloy includes the following steps:

[0038] Step 1: Heat-treat the Ti175 porous titanium alloy sample obtained by additive manufacturing using a tube furnace.

[0039] By fully decomposing martensite through high-temperature heat treatment and reconstructing the lamellar structure into a bimodal structure, strength and plasticity are synergistically improved without altering porosity or topology. Simultaneously, fatigue strength and low / high cycle fatigue life are enhanced.

[0040] In addition, the dual-state structure has higher thermal stability, which can suppress grain coarsening and reduce the performance degradation rate during service, as well as improve anisotropy and strain uniformity, thereby expanding the safety margin and long-term service reliability under complex loads such as compression and tension.

[0041] To avoid the thermal shock effect causing the actual temperature of the tube furnace to exceed the target setting, the empty furnace was first heated to 920~940℃ (i.e., the first target temperature) at a heating rate of 10℃ / min, and then the sample was quickly placed into the furnace cavity to begin heat treatment. The sample was precisely placed in the isothermal zone at the center of the tube furnace to reduce the axial and radial temperature gradients and avoid measurement and microstructure deviations caused by uneven heating.

[0042] Before the heat treatment begins, 99.9% pure argon gas is continuously introduced into the tubular furnace as a protective gas to create an inert atmosphere and suppress oxidation reactions under high temperature conditions.

[0043] Continue heating to 950 ℃ (the second target temperature) at a heating rate of 1.5–2 ℃ / min, and hold for 30 min to ensure uniform temperature penetration and process repeatability.

[0044] To prevent sample oxidation and ensure controlled cooling, the sample was cooled in the furnace using argon air, with the cooling rate stably controlled at 80–100 °C / min until the sample cooled to room temperature.

[0045] Step 2: Use a fatigue testing machine to subject the heat-treated sample to high-temperature fatigue cyclic loading.

[0046] Under high-temperature fatigue cyclic loading, phase transformation strengthening is induced, thereby improving the material strength; the cyclic stress amplitude is controlled at a low level, so that the deformation is always limited to the elastic range, avoiding damage to the original structure and early crack initiation; the thermal activation dynamic recovery under high temperature environment promotes dislocation rearrangement and partial annihilation, reduces local high dislocation density and stress concentration, thereby inhibiting the accumulation of irreversible damage and achieving a synergistic improvement in fatigue strength and fatigue life.

[0047] The sample is placed in the center of the test stage to avoid eccentric loading of the fatigue cyclic load, thereby preventing uneven stress within the material. Before fatigue loading begins, 99.9% pure argon gas is continuously introduced into the heating furnace as a protective gas to inhibit oxidation and stabilize the atmosphere.

[0048] The heating furnace is rapidly heated to 520~550℃ (i.e., the third target temperature) and held at that temperature for 90 minutes to reduce the effects of thermal expansion and contraction and to achieve temperature uniformity.

[0049] The strain rate was 0.5 mm / min; the fatigue loading adopted the stress control mode, the stress ratio was R = 0.1, and the loading frequency was 10 Hz.

[0050] The applied cyclic stress is 5% to 8% of the yield strength of the raw material, and the number of fatigue cycles is controlled between 200,000 and 300,000.

[0051] To prevent sample oxidation and ensure controlled cooling, the samples were air-cooled with argon gas in the heating furnace of the fatigue machine, with the cooling rate stably controlled at 100~120 ℃ / min until the samples cooled to room temperature.

[0052] Step 3: Anneal the Ti175 porous titanium alloy sample obtained by additive manufacturing using a box-type resistance furnace.

[0053] Annealing samples under high-temperature conditions can effectively release / eliminate residual stress introduced during forming and service, and promote the rearrangement and partial annihilation of fatigue-induced dislocation tangles, dislocation walls / cells, and other defects, thereby restoring the uniformity of the microstructure and plastic reserves. Subsequently, under short-term high temperature in an air environment, oxygen is controlled to penetrate into the surface layer, forming a thin oxygen-reinforced layer (a rarefied α-shell / oxygen solid solution layer), which improves the surface hardness and yield strength and forms gradient hardening, enhancing the surface layer's resistance to crack initiation, thereby improving the fatigue strength (life) of the material.

[0054] To avoid the thermal surge effect causing the actual temperature of the box-type resistance furnace to exceed the set target, first heat the empty furnace to 670~690℃ (i.e., the fourth target temperature) at a heating rate of 10℃ / min. Then quickly place the sample into the furnace cavity to start the heat treatment. The sample loading action should be quick in and out and the furnace door should be closed to reduce thermal disturbance when the door is opened.

[0055] Continue heating at a heating rate of 1.5–2 °C / min to 700 °C (i.e., the fifth target temperature), and hold for 10–15 min.

[0056] The cooling rate was kept stable at 100–120 °C / min until the sample cooled to room temperature.

[0057] The porous structures prepared with the same printing parameters and Ti175 powder can all have their strength, plasticity and fatigue properties improved using the process of this invention. Figure 1 , Figure 4 and Figure 7 The additive manufacturing samples are from three batches (with different printing parameters and structures), which are respectively referred to as Sample 1, Sample 2 and Sample 3.

[0058] The yield strength, densification strain, and fatigue life at 40% yield strength of the three samples before and after treatment were tested, and the results are shown in Table 1.

[0059] Yield strength (MPa) Densification strain (%) Fatigue life (40% yield strength of raw material) (times) Sample 1 (before treatment) 49.24 46.21 91783 Sample 1 (after processing) 63.72 48.79 1886354 Sample 2 (before treatment) 39.51 48.16 42186 Sample 2 (after processing) 50.53 52.69 1052546 Sample 3 (before treatment) 21.86 42.14 84213 Sample 3 (after processing) 29.22 48.83 1745364

[0060] As can be seen from Table 1, compared with the original material, the treated material in the embodiments of the present invention has an increase in strength of about 25% to 35%, an increase in densification strain of about 5% to 10%, and a significant improvement in fatigue performance of about 20 to 25 times.

[0061] Figure 2 , Figure 5 and Figure 8 The images show the metallographic features of samples 1, 2, and 3 before processing, respectively. The overall structure exhibits very fine needle-like / lamellar characteristics, clearly indicating a fine needle-like transformation structure formed by rapid cooling, primarily composed of extremely fine blue-woven α-lamellae. Figure 3 , Figure 6 and Figure 9 The images show the metallographic images of samples 1, 2, and 3 after processing. The laths are noticeably thicker and longer, exhibiting the characteristics of an α+β lamellar structure. Comparison reveals that the method described in this patent can decompose and coarsen the fine needle-like structure at high cooling speeds, making the α+β lamellars clearer while retaining some martensitic phase, thereby significantly improving the fatigue performance of the material.

[0062] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several improvements without departing from the principle of the present invention, and these improvements should also be considered within the scope of protection of the present invention.

Claims

1. A method for enhancing the fatigue properties of Ti175 porous titanium alloy, characterized in that: Includes the following steps: (1) The Ti175 porous titanium alloy sample obtained by additive manufacturing was subjected to the first heat treatment: Under an inert atmosphere, the sample is rapidly placed into the central isothermal zone of a tube furnace that has been preheated to the first target temperature, and then heated to the second target temperature and held to transform the original microstructure into a bimodal microstructure. The sample is then cooled to room temperature at a controlled cooling rate. (2) The specimens treated in step (1) were subjected to fatigue cyclic loading within the long-term service temperature range of Ti175 alloy: Under an inert atmosphere, the sample was heated to the third target temperature and held at that temperature. Then, a cyclic load within the elastic range was applied in a stress-controlled mode. After the loading was completed, the sample was cooled to room temperature at a controlled cooling rate. (3) Perform a second heat treatment on the sample after step (2): In air or an inert atmosphere, the sample is rapidly placed into a box furnace preheated to the fourth target temperature, and the temperature is further increased to the fifth target temperature and held for a short time to release residual stress and form a surface oxygen solid solution reinforcement layer. Then, it is cooled to room temperature at a controlled cooling rate.

2. The method for enhancing the fatigue properties of Ti175 porous titanium alloy as described in claim 1, characterized in that: The first target temperature is 920 ~ 940 ℃, the second target temperature is 950 ℃, and the holding time is 30 min.

3. The method for enhancing the fatigue properties of Ti175 porous titanium alloy as described in claim 1 or 2, characterized in that: The heating rate of the tube furnace is 1.5–2 °C / min after the sample is placed in.

4. The method for enhancing the fatigue properties of Ti175 porous titanium alloy as described in claim 1, characterized in that: In step (1), the controlled cooling rate is 80–100 °C / min.

5. The method for enhancing the fatigue properties of Ti175 porous titanium alloy as described in claim 1, characterized in that: The third target temperature is 520 ~ 580℃, and the holding time is 90 min.

6. The method for enhancing the fatigue properties of Ti175 porous titanium alloy as described in claim 1, characterized in that: Cyclic loading within the elastic range, with a stress amplitude of 5% to 8% of the yield strength of the Ti175 porous titanium alloy specimen, a stress ratio R = -0.1, a frequency of 10 Hz, and a cycle count of 2 × 10⁻⁶. 5 ~ 3×10 5 Second-rate.

7. The method for enhancing the fatigue properties of Ti175 porous titanium alloy as described in any one of claims 1, 5, or 6, characterized in that: The controlled cooling rate in step (2) is 100 ~ 120 ℃ / min.

8. The method for enhancing the fatigue properties of Ti175 porous titanium alloy as described in claim 1, characterized in that: The fourth target temperature is 670~690 ℃, the fifth target temperature is 700 ℃, and the holding time is 10~15 min.

9. The method for enhancing the fatigue properties of Ti175 porous titanium alloy as described in claim 1 or 8, characterized in that: In step (3), the controlled cooling rate is 100 ~ 120 ℃ / min.

10. The method for enhancing the fatigue properties of Ti175 porous titanium alloy as described in claim 1, characterized in that: The inert atmosphere is argon gas with a purity of ≥99.9%.

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