Method for determining sensitivity of titanium alloy to fatigue with load retention

Abstract

The application discloses a method for judging the sensitivity of a titanium alloy to sustained fatigue, which comprises the following steps: step (1): a mathematical model formula y=kx+b is simulated by using titanium alloy forging samples with different feather organization contents and a sustained fatigue sensitivity coefficient; in the formula, x represents the feather organization content, and y represents the sustained fatigue sensitivity coefficient; and step (2): the feather organization content of a titanium alloy forging sample to be detected is substituted into the mathematical model formula to obtain the result; the feather organization is a flaky cluster alpha phase in a net basket organization observed by a metallographic microscope, and the content of the feather organization is a ratio of an area of the feather organization to a total area of the net basket organization in a metallographic micrograph taken. The judging method can quantitatively judge the sustained fatigue sensitivity coefficient of the titanium alloy forging sample according to the microstructure characteristics of the titanium alloy forging sample in engineering, and the sustained fatigue value of the material does not need to be measured when mass-produced forgings or a process window is adjusted.

Description

Technical Field

[0001] This invention specifically relates to a method for determining the fatigue sensitivity of titanium alloys under load. Background Technology

[0002] Compared to narrow-body aircraft engines, wide-body passenger aircraft engines experience increased stress and temperature in their high-pressure compressor components, thus demanding higher performance from these components. In particular, the high-temperature, high-stress operating conditions place stringent requirements on the thermal creep performance of the high-pressure compressor disc's drum region. Ti-6Al-2Sn-4Zr-2Mo alloy (domestic equivalent TA19, hereinafter referred to as Ti6242) is a high-temperature titanium alloy with excellent comprehensive properties. Its long-term operating temperature can reach up to 510℃, and its high-temperature strength and creep performance are superior to Ti-6Al-4V alloy (domestic equivalent TC4). It has been widely used in the manufacture of aero-engine casings, compressor blades, discs, and other components. With relatively low production costs, it has become the second most in-demand high-temperature titanium alloy for aero-engines after Ti-6Al-4V.

[0003] Titanium alloy forging processes can be broadly classified into two-phase forging and β forging based on the pre-forging heating temperature.

[0004] Ti6242 is best suited for α+β forging / high-temperature solution aging treatment to achieve comprehensive properties such as tensile strength, creep, and fatigue resistance. Due to Ti6242's sensitivity to load-bearing fatigue in the low-temperature, high-pressure range, it is more suitable for discs and drums operating at temperatures above 200℃. Considering the high content of the β-stabilizing element Mo in Ti6242 alloy and its poor high-temperature creep performance, β forging is chosen to obtain a basket-like microstructure to improve its high-temperature creep performance. The lamellar structure of β-forged basket-like titanium alloys determines their high fracture toughness, giving the material high damage tolerance. β forging has significant advantages in reducing the deformation resistance of titanium alloys, improving formability, enhancing damage tolerance, and improving high-temperature creep resistance. Especially with the increasing thickness, projected area, and forming difficulty of aerospace titanium alloy forgings, β forging is receiving increasing attention. There is no domestic experience in the trial production of β forging process for Ti6242. β forging requires a higher billet heating temperature and a narrower forging window. The drum part has the characteristics of thin wall and large diameter. The temperature drops faster during forging and the microstructure is more difficult to control. Therefore, the β forging technology of Ti-6Al-2Sn-4Zr-2Mo alloy is more difficult to realize.

[0005] Near-alpha type Ti6242 titanium alloy exhibits general load-holding fatigue sensitivity. As the test temperature increases, the rate of reduction in fatigue life under load-holding conditions compared to conventional cyclic conditions gradually decreases. A significant load-holding effect is typically observed below 200°C, hence this phenomenon is termed the cold load-holding effect. Near-alpha type titanium alloys and some two-phase titanium alloys commonly exhibit cold creep; even at room temperature, strain accumulation occurs under sustained loads below the material's yield strength.

[0006] The microstructure of titanium alloys has a significant impact on the load-holding fatigue effect:

[0007] ① For bimodal or isometric structures, the loading effect increases with the increase of the volume fraction of primary α content;

[0008] ② The elongated primary α microstructure at the grain boundaries has a higher load-bearing fatigue sensitivity than the equiaxed microstructure because it has obvious local grain concentration orientation (microtexture), which makes it easier to form a stress redistribution mechanism and cause earlier crack initiation.

[0009] ③ Compared with the bimodal structure, the Widmanstätten and basketweave structures of Ti6242 alloy obtained by β forging have significantly lower load-holding fatigue sensitivity;

[0010] ④ Compared with pure lamellar tissue, the Widmanstätten + basketweave mixed tissue with α-bundles has lower load retention sensitivity.

[0011] Excessive, undamaged residual α grains (α bundles) increase the holding fatigue effect. Based on the above literature review, it can be concluded that the basket-like structure of Ti6242 alloy produced by β forging should have lower holding fatigue sensitivity than that produced by α+β forging. However, the β forging process of general titanium alloys is prone to producing parallel needle-like α bundles, i.e., feather-like structures, which increase holding fatigue sensitivity. Summary of the Invention

[0012] This invention aims to overcome the shortcomings of existing technologies, such as long holding-load fatigue testing and high costs, by providing a method for determining the holding-load fatigue sensitivity of titanium alloys. This method allows for quantitative engineering judgment of the holding-load fatigue sensitivity coefficient of titanium alloy forging samples based on their microstructure characteristics during mass production of forgings or adjustment of process windows, without the need to measure the material's holding-load fatigue values.

[0013] The present invention mainly solves the above technical problems through the following technical solutions.

[0014] This invention provides a method for determining the load-bearing fatigue sensitivity of titanium alloys, which includes the following steps:

[0015] Step (1) uses titanium alloy forged samples with different feather tissue content and fatigue sensitivity coefficients to simulate and obtain the mathematical model formula:

[0016] y = kx + b;

[0017] In the formula, x is the feather tissue content, and y is the fatigue sensitivity coefficient.

[0018] Step (2): Substitute the feather structure content of the titanium alloy forging sample to be tested into the mathematical model formula to obtain the y value, which is the fatigue sensitivity coefficient under load.

[0019] The feather tissue is a sheet-like clustered α phase in the basket tissue observed under a metallographic microscope, and the feather tissue content is the ratio of the area of ​​the feather tissue to the total area of ​​the basket tissue in the metallographic micrograph.

[0020] In this invention, those skilled in the art will know from the aforementioned determination method that the determination method generally includes the following steps: obtaining titanium alloy forged samples with different feather tissue contents using different forging processes; detecting the feather tissue content in the cross-section of each titanium alloy forged sample using a metallographic microscope; simultaneously calculating the load-holding fatigue sensitivity coefficient of each titanium alloy forged sample; obtaining the feather tissue content and load-holding fatigue sensitivity coefficient of each titanium alloy forged sample; plotting a curve with feather tissue as the abscissa and load-holding fatigue sensitivity coefficient as the ordinate; and using the plotted curve to simulate a linear relationship, thereby obtaining the aforementioned mathematical model formula.

[0021] In this invention, those skilled in the art will know that, in order to obtain accurate test results, when measuring the feather tissue content of titanium alloy forged samples prepared by different forging processes, the metallographic micrographs taken are dissected at the same location of different titanium alloy forged samples, and the cross-sections at the same location are photographed with the same magnification to obtain the same area.

[0022] In this invention, in the formula y = kx + b, the calculated value of k is preferably 0.4 to 0.614, for example 0.49; and the value of b is preferably 3.047 to 4.46, for example 3.42.

[0023] In this invention, the mathematical model formula is preferably y = 0.49x + 3.42.

[0024] In this invention, R is calculated based on different feather tissue contents and fatigue sensitivity coefficients. 2 =0.992451, indicating that there is a linear positive correlation between the feather structure content and the fatigue sensitivity coefficient in the titanium alloy forging sample.

[0025] In this invention, the titanium alloy can be a conventional titanium alloy in the art, generally referring to Ti-6Al-2Sn-4Zr-2Mo alloy.

[0026] In this invention, the method for calculating the fatigue sensitivity coefficient in step (1) is conventional in the art, and its calculation formula is generally as follows: N represents lifespan, and Life Debit represents the fatigue sensitivity coefficient under load.

[0027] In this invention, those skilled in the art will know from the determination method described above that the titanium alloy forging samples with different feather tissue contents can generally be obtained through different β forging processes.

[0028] The different β forging processes preferably include the forging temperature and / or the solution treatment temperature.

[0029] The forging temperature can be 20-30°C above the β-phase transformation point of the titanium alloy, that is, the β-phase transformation point of the titanium alloy plus 20-30°C.

[0030] The forging process can take, for example, 100 minutes.

[0031] The solution treatment temperature can be 20 to 50°C below the β-phase transformation point of the titanium alloy, or 20 to 50°C below the β-phase transformation point of the titanium alloy.

[0032] The solution treatment temperature is, for example, 1 hour.

[0033] Those skilled in the art will know that, in order to establish more accurate judgments, it is preferable to use three or more different β forging processes to prepare titanium alloy forged samples.

[0034] The β forging process is conventional in the field and generally includes the following steps: subjecting the titanium alloy to the forging process, preheating process, solution treatment and aging treatment in sequence.

[0035] In the β forging process, the forging deformation amount can be, for example, 50%.

[0036] The preheating temperature is, for example, 900°C. The preheating time is, for example, 90 minutes.

[0037] The aging treatment temperature can be, for example, 595°C. The aging treatment time can be, for example, 1 hour.

[0038] Those skilled in the art will know that air cooling is generally performed after the forging process, the solution treatment, or the aging treatment.

[0039] Based on common knowledge in the field, the above-mentioned preferred conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention.

[0040] The reagents and raw materials used in this invention are all commercially available.

[0041] The significant advantages of this invention are: fatigue testing under load is time-consuming and costly. The method of this invention, when mass-producing forgings or adjusting process windows, allows for quantitative engineering judgment of fatigue sensitivity based on the characteristics of the microstructure of the dissected part or test ring, without the need to measure the fatigue values ​​of the material under load. This saves research and development and manufacturing time, and reduces costs. Attached Figure Description

[0042] Figure 1 This is an example image for identifying feather structure in a titanium alloy forged sample. Figure 1 (a) Feather structure originating from the original, straight, continuous β grain boundaries; Figure 1 (b) refers to the small-angle grain boundaries originating from the original β grains. Figure 1 (c) Feather structure around the original, straight, and continuous β grain boundaries. Figure 1 (d) shows the feather structure within a larger, poorly woven β-grain. The arrows in the figure indicate the location of the feather structure.

[0043] Figure 2 The image shows the feather structure in a metallographic micrograph of the titanium alloy forged sample obtained by the β forging process in Example 1.

[0044] Figure 3 The image shows the feather structure in a metallographic micrograph of the titanium alloy forged sample obtained by the second β forging process in Example 1.

[0045] Figure 4 The image shows the feather structure in a metallographic micrograph of the titanium alloy forged sample obtained by the β forging process in Example 1.

[0046] Figure 5 The waveforms used in Example 1 are the triangular waveform for low-cycle fatigue testing and the trapezoidal waveform for load-holding fatigue testing.

[0047] Figure 6 This is a graph showing the relationship between feather structure content and properties of the three titanium alloy forged samples in Example 1. Figure 6 (a) is a graph with feather tissue content as the abscissa and yield strength, tensile strength and elongation as the ordinates; Figure 6 (b) is a graph with feather tissue content as the horizontal axis and fatigue sensitivity coefficient, low-cycle fatigue and fatigue as the vertical axes. Detailed Implementation

[0048] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or as selected according to the product instructions.

[0049] The Ti-6Al-2Sn-4Zr-2Mo alloy used in Example 1 below was sourced from... The chemical composition of Ti-6Al-2Sn-4Zr-2Mo rods is shown in Table 1 below:

[0050] Table 1

[0051] element Content (wt.%) element Content (wt.%) Al 5.93 N <0.02 Sn 2 C 0.003 Zr 4.02 Mo 1.97 O 0.1 / /

[0052] The specific preparation processes of β forging process one, β forging process two, and β forging process three in Example 1 are shown below:

[0053] (1) The preparation process of β forging process one is as follows: the above Ti-6Al-2Sn-4Zr-2Mo bar is subjected to forging treatment, solution treatment and aging treatment in sequence.

[0054] The forging process involves a temperature of Tβ+20℃ (Tβ refers to the temperature of the β phase transformation point of the titanium alloy), a holding time of 100 minutes, a forging deformation of 50%, and a forced-air cooling process after forging.

[0055] Solution treatment: Place in the furnace at 900℃, preheat for 90 minutes, then raise the temperature to Tβ-20℃, hold for 1 hour, and air cool.

[0056] Aging treatment: heating temperature 595℃, holding time 8h, air cooling.

[0057] (2) The preparation process of β forging process 2 is as follows: the above Ti-6Al-2Sn-4Zr-2Mo bar is subjected to forging treatment, solution treatment and aging treatment in sequence.

[0058] The forging process involves a temperature of Tβ+30℃, a holding time of 100 minutes, a forging deformation of 50%, and subsequent forced air cooling.

[0059] Solution treatment: Place in the furnace at 900℃, preheat for 90 minutes, then raise the temperature to Tβ-20℃, hold for 1 hour, and air cool.

[0060] Aging treatment: heating temperature 595℃, holding time 8h, air cooling.

[0061] (3) The preparation process of β forging process three is as follows: the above Ti-6Al-2Sn-4Zr-2Mo bar is subjected to forging treatment, solution treatment and aging treatment in sequence.

[0062] The forging process involves a temperature of Tβ+30℃, a holding time of 100 minutes, a forging deformation of 50%, and subsequent forced air cooling.

[0063] Solution treatment: Place in a furnace at 900℃, preheat for 90 minutes, then raise the temperature to Tβ-50℃, hold for 1 hour, and air cool.

[0064] Aging treatment: heating temperature 595℃, holding time 8h, air cooling.

[0065] Example 1

[0066] (1) Identification and location determination of feather tissue

[0067] By adjusting the β-forging process of the Ti-6Al-2Sn-4Zr-2Mo alloy, feather-like structures with different contents and sizes were obtained. The feather-like structure is essentially a cluster of lamellar α-phase within the basketweave structure of titanium alloys.

[0068] Features are Figure 1 The microscopic images show that the feather structure is characterized by large, clustered α-phase lamellar structures, with the growth of the clusters originating from the original, straight, and continuous β-grain boundaries. Figure 1 (a)) or small-angle grain boundaries in the original β grains (adjacent grains with a phase difference of less than 10°) Figure 1 (b)).

[0069] Unlike the woven structure of the basket mesh in the forged titanium alloy sample, the feather structure is a relatively large, non-uniform microstructure, which is easily identifiable under a metallographic microscope. In fatigue fracture images, the feather structure is characterized by brittle fracture cleavage, showing clear morphology along lamellar α-phase fracture, and growing along concave or convex ridges. Figure 1 (c) and 1(d)) exhibit a half-feather morphology. Feather tissue often appears around the original straight and continuous β grain boundaries, growing into the grain interior, or in larger original β grains with poor weaving.

[0070] (2) Forged samples prepared using the three β-forging processes described above were dissected, and the metallographic microstructure of the cross-sections was observed using a DMI500 metallographic microscope. The feather-like morphology was identified, and the feather tissue content was analyzed and calculated. Next, scanning electron microscopy was used to analyze the fracture surface after fatigue testing, further identifying whether the crack path passed through the feather tissue. The feather tissue is a crack propagation path and accelerates crack propagation. The feather-like tissue identification results for different β-forging processes are as follows: Figure 2 , Figure 3 , Figure 4 As shown in Table 1, the content calculation results are as follows.

[0071] (3) Selecting low-cycle fatigue test parameters and load-holding fatigue test parameters

[0072] Since both low-cycle fatigue and load-holding fatigue strength are positively correlated with yield strength, different yield strengths result in different fatigue strengths. To avoid the influence of yield strength caused by different processes on fatigue performance, room temperature tensile tests were performed on each titanium alloy forging sample.

[0073] Meanwhile, the fatigue specimens, tensile specimens, and metallographic microscopy were all tested using the same method (the same specimens refer to the same sample taken from the same location). In accordance with GB / T 15248 "Metallic Materials - Axial Constant Amplitude Low Cyclic Fatigue Test Method", room temperature low-cycle fatigue and load-holding fatigue performance tests were carried out on an Instron 8801 testing machine. The stress control mode was adopted, and the stress close to the yield strength of titanium alloy was selected as the peak stress in the fatigue test, i.e., 0.95Rp0.2.

[0074] The fatigue test loading waveform is as follows Figure 5 As shown, one test used a triangular waveform for low-cycle fatigue testing, with a loading frequency of 1 Hz and a stress ratio of 0.1; the other test used a trapezoidal waveform for sustained-load fatigue testing, with a sustained-load time of 2 minutes and load increase / decrease times of 1 second each. Other test conditions were the same as for low-cycle fatigue. The test results for low-cycle fatigue and sustained-load fatigue are shown in Table 2.

[0075] Table 2 Mechanical properties and feather tissue content under different β forging processes

[0076]

[0077] (4) Plotting the feather structure content of the three titanium alloy forged samples as the abscissa, and the yield strength, tensile strength, elongation, fatigue sensitivity coefficient under load, low-cycle fatigue, and fatigue under load as the ordinate, as shown in the figure. Figure 6 The diagram shown is a relational diagram.

[0078] Among them, sustained-load fatigue is a fatigue phenomenon that occurs under sustained loading at low temperatures. Fatigue cracks initiate at the subsurface of the specimen, with the initiation point being a flat cleavage plane. The life of sustained-load fatigue is shorter than that of ordinary fatigue. The decrease in sustained-load fatigue life (i.e., the strength of the sustained-load effect, also known as sustained-load fatigue sensitivity) is usually evaluated using the following formula:

[0079] N represents lifespan, and Life Debit represents the fatigue sensitivity coefficient under load.

[0080] Because feather tissue is a coarse and anomalous structure, it reduces the static strength of the material. Therefore, the higher the feather tissue content, the lower the yield strength and tensile strength of the forged titanium alloy. Figure 6 (a). The effect of different material yield strengths on fatigue strength has been reduced by selecting a fatigue test stress of 0.95 times the yield strength.

[0081] Simultaneously, by dividing the fatigue life under load by the low-cycle fatigue life, the influence of low-cycle fatigue on the fatigue life under load is eliminated. A coefficient is then used to reflect the influence of different material microstructures on the sensitivity to fatigue under load, yielding... Figure 6 (b) shows the relationship diagram. Feather tissue content is positively correlated with the fatigue sensitivity coefficient, and in this data analysis, the relationship is linear. The mathematical model established based on this linear relationship is as follows:

[0082] y = 0.49x + 3.42; where x is the feather tissue content and y is the fatigue sensitivity coefficient.

[0083] Based on the linear relationship between feather tissue content and the fatigue sensitivity coefficient, given this mathematical model, the fatigue sensitivity coefficient of the sample can be calculated simply by detecting the feather tissue content using a metallographic microscope and substituting it into the above formula.

Claims

1. A method for determining the fatigue sensitivity of titanium alloys under load, characterized in that, It includes the following steps: Step (1): Using titanium alloy forged samples with different feather tissue content and fatigue sensitivity coefficients under load, the mathematical model formula was obtained through simulation: y = kx + b; In the formula, x is the feather tissue content, and y is the fatigue sensitivity coefficient. The titanium alloy forged samples with different feather tissue contents were obtained through different β forging processes, including forging temperature and / or solution treatment temperature. The feather tissue content in the titanium alloy forged samples was statistically analyzed using metallographic microscopy, and the fracture surface of the titanium alloy forged samples after fatigue test was analyzed using scanning electron microscopy to identify whether the crack path passes through the feather tissue. Step (2): Substitute the feather structure content of the titanium alloy forging sample to be tested into the mathematical model formula to obtain the y value, which is the fatigue sensitivity coefficient under load. The feather tissue is a sheet-like clustered α phase in the basket tissue observed under a metallographic microscope, and the feather tissue content is the ratio of the area of ​​the feather tissue to the total area of ​​the basket tissue in the metallographic micrograph.

2. The method for determining the fatigue sensitivity of titanium alloys under load as described in claim 1, characterized in that, In the formula y = kx + b, k has a value of 0.4 to 0.614, and b has a value of 3.047 to 4.

46.

3. The method for determining the fatigue sensitivity of titanium alloys under load as described in claim 1, characterized in that, In the formula y = kx + b, k is 0.49 and b is 3.

42.

4. The method for determining the fatigue sensitivity of titanium alloys under load as described in any one of claims 1 to 3, characterized in that, The titanium alloy is a Ti-6Al-2Sn-4Zr-2Mo alloy.

5. The method for determining the fatigue sensitivity of titanium alloys under load as described in claim 1, characterized in that, In step (1), the fatigue sensitivity coefficient is calculated according to the following formula: The N represents the lifespan.

6. The method for determining the fatigue sensitivity of titanium alloys under load as described in claim 1, characterized in that, The forging temperature is 20-30°C above the β phase transformation point of the titanium alloy; And / or, the forging process takes 100 minutes.

7. The method for determining the fatigue sensitivity of titanium alloys under load as described in claim 1, characterized in that, The solution treatment temperature is 20 to 50°C below the β phase transformation point of the titanium alloy; And / or, the solution treatment time is 1 hour.

8. The method for determining the fatigue sensitivity of titanium alloys under load as described in claim 1, characterized in that, The method for preparing titanium alloy forged samples by the β forging process includes the following steps: sequentially subjecting the titanium alloy to the forging treatment, preheating treatment, solution treatment, and aging treatment.

9. The method for determining the fatigue sensitivity of titanium alloys under load as described in claim 8, characterized in that, The preheating temperature is 900℃; And / or, the preheating treatment time is 90 min; And / or, the aging treatment temperature is 595°C; And / or, the time for the time-sensitive processing is 1 hour; And / or, in the β forging process, the forging deformation is 50%.

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