A method for predicting the fatigue life of a welded joint of a superalloy for an aeroengine

Abstract

The present invention discloses a fatigue life prediction method for nickel-based high-temperature alloy welded joints for aircraft engines. During the service of electron beam welded structures of aircraft engines, the weld area is a hotspot where fatigue failure occurs in the structure. This method aims at the large dispersion of high-temperature fatigue life of electron beam welded joints. By considering the influence of service temperature and welding defects on fatigue life coupling, a full-condition high-temperature service fatigue life prediction model for nickel-based high-temperature alloy electron beam welded joints is established. This method uses the fatigue life data, Vickers hardness value and fracture morphology characteristics of welded joints at different temperatures as sample data; by fitting the mathematical relationship between temperature and Vickers hardness, and proposing the fatigue equivalent stress concept and calculation formula based on the defect size information of fatigue crack source of welded joints, the influence of service temperature and initial defect area on the fatigue performance of the joint is considered, and the life prediction of a certain type of high-temperature alloy electron beam welded joint under any fatigue service condition is realized. The present invention effectively improves the fatigue life prediction accuracy of high-temperature alloy electron beam welded structures.

Description

Technical Field

[0001] The present invention belongs to the field of aircraft engine structural strength safety, and in particular relates to a fatigue life prediction method for nickel-based high-temperature alloy welded joints used in aircraft engines. Background Art

[0002] GH4169, due to its excellent resistance to high temperatures, oxidation, corrosion, and fatigue, is widely used in high-temperature structures in aerospace, gas turbines, nuclear power, and other fields. Aircraft engines are hailed as the crown jewels of modern industry. With the increasing demands for performance, lightweighting, and environmental protection in recent years, welding continues to play a vital role as an efficient and practical joining method. GH4169, due to its slow precipitation of its primary strengthening phase, γ", and excellent weldability, is widely used in high-temperature, load-bearing components such as aircraft engine cases, turbine disks, and turbine shafts. During aircraft engine operation, most welded structures are subject to alternating loads, and their fatigue performance directly impacts engine reliability.

[0003] Welded joints are typically heterogeneous structures. Due to the unavoidable introduction of welding defects such as porosity, inclusions, and lack of fusion during the welding process, fatigue life of welded joints is highly scattering and cycle life is low. Currently, life prediction for butt-welded joints is primarily based on conducting extensive fatigue tests to obtain fatigue data and then fitting relevant life models. However, the uncertainty of internal weld defects often leads to scatter in fatigue data. This approach requires a large number of experiments, requires a long test cycle, and suffers from poor prediction accuracy. An alternative approach is to develop relevant life prediction models combined with fatigue tests on a small number of components to avoid excessive testing costs and long test cycles. Current research on the fatigue life of materials containing internal microcracks and defects is primarily based on the fatigue limit prediction model and fatigue crack growth life model for microcracked materials proposed by Murakami in his book Metal Fatigue: Effects of Small Defects and Nonmetallic Inclusions. However, there is no dedicated research on the impact of internal defects and weld microstructure on the fatigue life of electron beam welded nickel-based superalloy joints, limiting the comprehensive and reliable application of welding manufacturing technology in the aerospace engine field. Summary of the Invention

[0004] Aiming at the problem that the microstructure heterogeneity and internal welding defects of nickel-based high-temperature alloy welded joints of aircraft engines cause large dispersion of fatigue life, difficult life prediction and the need to consider the variable service temperature, the present invention aims to provide a fatigue life prediction method for high-temperature alloy welded joints based on high-temperature fatigue experimental data, high-temperature Vickers hardness of welded joints, and welding microstructure defects.

[0005] To achieve the above-mentioned object, the technical solution of the present invention is as follows: A method for predicting fatigue life of high-temperature alloy welded joints for aircraft engines, comprising the following steps:

[0006] Step 1: Using vacuum electron beam welding technology, a double-pass butt weld is performed on the solution-treated GH4169 sheet material. The weld quality is then assessed through nondestructive testing using fluorescence and X-rays. The weld joint undergoes a two-stage heat treatment to obtain an electron beam butt welded GH4169 sheet material that meets industry-standard process requirements.

[0007] Step 2: Conduct tensile test on GH4169 electron beam welded joint to obtain stress and strain curve of welded joint. Y According to the national standard of axial force control method for fatigue test of metal materials (GBT3075-2008), high temperature fatigue test was carried out on the welded joint, and the test data were fitted and analyzed.

[0008] Step 3: Use field emission scanning electron microscopy to observe the fracture morphology of all fatigue failure specimens, determine the location of the welding defect that causes fatigue crack initiation, and measure the crack source size using ImageJ image processing software to obtain the initial defect area (area) in μm. 2 .

[0009] Step 4: Using the measured crack source welding defect area, the fatigue limit of the welded joint is evaluated according to formula (1), which effectively considers the effect of the initial welding defect on the fatigue strength of the specimen:

[0010]

[0011] Where, σ w is the fatigue limit (MPa), C is a material-related constant and takes a value of 1.56 when the defect is located inside the specimen and 1.43 when it is located on the surface of the specimen, HV is the Vickers hardness (kgf / mm 2 )R is the stress ratio, and α is a constant related to the stress ratio.

[0012] Step 5: For materials with relatively uniform distribution and few defects, the following relationship between fatigue life and cyclic stress can be found according to the Basquin formula:

[0013]

[0014] Where σ is the cyclic stress, C1 and C2 are material constants obtained by fitting the measured data. According to existing research data, for defective materials, formula (2) can be modified to:

[0015]

[0016] Where C3 and C4 are material constants obtained by fitting the experimental data.

[0017] Step 6: Substituting equation (1) into equation (3) yields:

[0018]

[0019] Step 7: According to the fatigue limit prediction formula of defective materials, propose the modified stress σ m The definition expression is:

[0020]

[0021] Step 8: Before the fatigue test begins, the sample is scanned layer by layer using industrial CT technology. After the test, the fatigue fracture is observed to obtain the maximum defect size and location information of the material under the electron beam welding process, and σ m The value of .

[0022] Step 9: From this, at a specific temperature, when HV is a constant, the equivalent stress σ can be established m Functional relationship between cycle life:

[0023]

[0024] Where k1 and k2 are material constants obtained by fitting the experimental data.

[0025] According to the above formula (6), the fatigue life prediction model considering defects at a specific temperature can be obtained.

[0026] Step 10: Conduct a high-temperature Vickers hardness measurement test to obtain the average Vickers hardness of each area of the GH4169 welded joint at multiple temperatures, and construct a mathematical relationship between the average Vickers hardness (HV) of the welded joint and temperature (T):

[0027] HV=kT+b# (7)

[0028] Where T is the ambient temperature, k and b are the material constants obtained by fitting the measured data.

[0029] In summary, by using the optimization tool to perform global minimization optimization on the objective function and fitting k1 and k2, the following fatigue life prediction model considering both temperature and initial defects can be established:

[0030]

[0031] The advantages and beneficial effects of the present invention are as follows:

[0032] 1. This method addresses the phenomenon that fatigue failure often occurs at defects such as pores, inclusions, and lack of fusion in aircraft engine welded structures during service. When evaluating the life of GH4169 superalloy vacuum electron beam welded joints, the method comprehensively considers the effects of service temperature, cyclic loading, and the coupling effects of internal weld defects on the fatigue performance of welded joints. Based on Murakami's fatigue limit prediction formula for defective materials, a modified Basquin formula is proposed that accounts for defect space size and service temperature. This allows for more effective assessment and prediction of the fatigue strength and fatigue life of GH4169 vacuum electron beam welded joints.

[0033] 2. The method of the present invention aims at the uncertainty of welding defects, determines the maximum defect information through industrial CT scanning, and proposes a correction stress σ m The concept of a life prediction model was established by globally optimizing the parameters of the model through a unified fit of fracture analysis, high-temperature Vickers hardness testing, and fatigue experimental data. This method achieved life prediction for welded joints at any service temperature and load level, with a stress ratio R = 0.1. Furthermore, in practical engineering applications, the method can accurately predict the fatigue life of welded aircraft engine components under any given service condition based on only a small number of fatigue tests and a short duty cycle. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 GH4169 electron beam welded joint fatigue test piece processing dimensions;

[0035] Figure 2 GH4169 electron beam welded joint fatigue SN data diagram;

[0036] Figure 3 Different fatigue fracture morphologies of GH4169 electron beam welded (400℃);

[0037] Figure 4 Different fatigue fracture morphologies of GH4169 electron beam welded (500℃);

[0038] Figure 5 Different fatigue fracture morphologies of GH4169 electron beam welded (550℃);

[0039] Figure 6 Based on the modified stress σ m GH4169 electron beam welded joint fatigue S m -N data graph;

[0040] Figure 7 Relationship curve between Vickers hardness of welded joint and service temperature;

[0041] Figure 8 Based on the modified stress σm GH4169 electron beam welding life prediction results (400℃);

[0042] Figure 9 Based on the modified stress σ m GH4169 electron beam welding life prediction results (500℃);

[0043] Figure 10 Based on the modified stress σ m GH4169 electron beam welding life prediction results (550℃);

[0044] Figure 11 Based on the modified stress σ m Fatigue life prediction results of GH4169 electron beam welded joints and service temperature. DETAILED DESCRIPTION

[0045] The following is combined with Figure 1 —11 and specific embodiments further describe the present invention in detail.

[0046] This embodiment addresses the problem that fatigue life dispersion prediction of key aero-engine components made of GH4169 material due to welding defects in vacuum electron beam welding is difficult and the coupling effect of external service temperature structure and internal defects needs to be considered. Based on a small amount of material fatigue test sample data, according to the Murakami fatigue limit prediction formula for defective materials and the functional relationship between Vickers hardness and ambient temperature, combined with the Basquin fatigue life prediction model, fatigue life prediction of GH4169 vacuum electron beam joints under the combined action of internal defects and external stress in the welded joint is provided.

[0047] Specifically, this embodiment provides a fatigue life prediction method for nickel-based high-temperature welded joints used in aircraft engines. It is determined that fatigue failure of welded joints occurs when fatigue cracks initiate from weld defects under the influence of service temperature and expand to a critical size, leading to failure. The specific steps are as follows:

[0048] Step 1: After the 2.5mm thick GH4169 sheet material is solution treated at 970℃×1h, double-pass vacuum electron beam butt welding is carried out. After welding, the weld quality is tested by fluorescence detection and X-rad ray. The weld joints that pass the test are subjected to double aging heat treatment at 720℃×8h+620℃×8h to obtain GH4169 sheet butt electron beam welded joints that meet the quality requirements. Figure 1 Fatigue specimens were machined with the dimensions shown.

[0049] Step 2: Conduct tensile performance tests on GH4169 electron beam welded joints to obtain stress-strain curves and tensile strength UTS of the joints. According to the lifting method, fatigue performance tests of GH4169 welded fatigue specimens were conducted at three high temperatures of 400℃, 500℃, and 550℃ with a stress ratio of R=0.1. The stress-life data were fitted based on the test results to obtain the following: Figure 2 The SN curve is shown.

[0050] Step 3: If Figures 3 to 5 As shown in the figure, the fracture morphology of all fatigue failure specimens was observed under a field emission scanning electron microscope to determine the location of the welding defect that caused the fatigue crack initiation. The crack source size was measured using ImageJ image processing software to obtain the defect area (unit: μm). 2 The specific information of the samples at different temperatures is shown in Table 1.

[0051] Table 1 Summary of fatigue test sample data calculation

[0052]

[0053]

[0054] Step 4: Using the measured crack source initial weld defect area, the fatigue limit of the GH4169 welded specimen is evaluated according to the following formula:

[0055]

[0056] Where, σ w is the fatigue limit (MPa), C is a material-related constant and takes a value of 1.56 when the defect is located inside the specimen and 1.43 when it is located on the surface of the specimen, HV is the Vickers hardness (kgf / mm 2 )R is the stress ratio, and α is a constant related to the stress ratio.

[0057] Step 5: For materials with relatively uniform distribution and few defects, the following relationship between fatigue life and cyclic stress can be found according to the Basquin formula:

[0058]

[0059] Where σ is the cyclic stress, C1 and C2 are material constants obtained by fitting the measured data. According to existing research data, for defective materials, formula (2) can be modified to:

[0060]

[0061] Where C3 and C4 are material constants obtained by fitting the experimental data.

[0062] Step 6: Substituting equation (1) into equation (3), we can establish the relationship equation between fatigue life and cyclic stress including defect size and Vickers hardness:

[0063]

[0064] Step 7: According to the fatigue limit prediction formula of defective materials, propose the modified stress σ m The definition expression is:

[0065]

[0066] Where, σ max is the maximum cyclic stress;

[0067] Step 8: Before the fatigue test begins, the sample is scanned layer by layer using industrial CT technology. After the test, the crack source of the fatigue fracture is observed to obtain the maximum defect size and location information of the material sample under the electron beam welding process, and σ m The value of .

[0068] Step 9: Based on the experimental data in Table 1 above, establish the revised Curves, such as Figure 6 As shown. At a certain temperature, HV is a constant, and the high-temperature Vickers hardness measurement results of the welded joint are shown in Figure 7. In addition, during the low-cycle fatigue test, the fatigue crack sources of all specimens are located on the surface or subsurface. If the defect location is not considered, Equation (4) can be expressed as:

[0069]

[0070] Where, k1 and k2 are material constants obtained by fitting the test data;

[0071] In this embodiment, a life prediction model based on defect size is established for three temperatures of 400°C, 500°C, and 550°C according to formula (6), and the fatigue life prediction model containing defect information is optimized and solved. According to existing research data, the fatigue life prediction model for GH4169 electron beam welded joints is:

[0072]

[0073]

[0074]

[0075] Wherein: Formula (7) is the life prediction model at 400℃, Formula (8) is the life prediction model at 500℃, and Formula (9) is the life prediction model at 550℃. Figures 8-10The fatigue life model prediction results at three temperatures of 400℃, 500℃ and 550℃ respectively. The single temperature life prediction model based on corrected stress can achieve a prediction accuracy of ±2 times.

[0076] Step 10: In this embodiment, a high-temperature Vickers hardness measurement test of the welded joint is performed. The temperature gradient of the test is 25°C, 400°C, 500°C, 550°C, 650°C and 750°C, respectively. The average Vickers hardness of each area of the GH4169 welded joint at multiple temperatures is obtained, and a mathematical relationship between the average Vickers hardness (HV) of the welded joint and the temperature (T) is constructed:

[0077] HV=kT+b# (10)

[0078] Where T is the ambient temperature, k and b are the material constants obtained by fitting the measured data.

[0079] The overall Vickers hardness of the fitting joint can be obtained:

[0080] HV=-0.1317T+450# (11)

[0081] Therefore, the following fatigue life prediction model considering both temperature and initial defects can be established:

[0082]

[0083] Right now:

[0084]

[0085] The optimization tool is used to perform global minimization optimization on the objective function, and k1 and k2 are obtained by fitting. The following fatigue life prediction model considering both temperature and initial defects can be established:

[0086]

[0087] Table 2 Calculation summary of fatigue test sample data

[0088]

[0089]

[0090] The above is only an embodiment of the present invention, and common sense such as the specific structure and characteristics of the scheme are not described in detail here. For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-restrictive. The scope of the present invention is defined by the appended claims rather than the above description, and it is intended that all changes that fall within the meaning and scope of the equivalent elements of the claims are included in the present invention. Any figure mark in the claims should not be regarded as limiting the claim involved.

Claims

1. A method for predicting fatigue life of high-temperature alloy welded joints for aircraft engines, characterized by: Step 1: Use vacuum electron beam welding technology to perform double-layer butt welding on the solution-treated GH4169 sheet material, and then use fluorescence detection and X-rad ray non-destructive testing to evaluate the weld quality; Step 2: Conduct tensile test on GH4169 electron beam welded joint to obtain stress and strain curve of welded joint. Y , and the axial force control method of metal material fatigue test is used to conduct high temperature fatigue test on welded joints, and the test data are fitted and analyzed; Step 3: Use field emission scanning electron microscopy to observe the fracture morphology of all fatigue failure specimens, determine the location of the welding defect that causes fatigue crack initiation, and measure the crack source size using ImageJ image processing software to obtain the initial defect area (area) in μm. 2 ; Step 4: Using the measured crack source weld defect area, the fatigue limit of the welded joint is evaluated according to formula (1), which takes into account the effect of the initial weld defect on the fatigue strength of the specimen: Where σ w is the fatigue limit (MPa), C is a material-related constant and takes a value of 1.56 when the defect is located inside the specimen and 1.43 when it is located on the surface of the specimen, HV is the Vickers hardness (kgf / mm 2 )R is the stress ratio, α is a constant related to the stress ratio; Step 5: For materials with relatively uniform distribution and few defects, determine the relationship between fatigue life and cyclic stress according to the Basquin formula: Where σ is the cyclic stress, C1 and C2 are material constants obtained by fitting the measured data, and N is the fatigue life under a given cyclic stress. For materials with defects, formula (2) can be modified as follows: Where, C3 and C4 are material constants obtained by fitting the test data; Step 6: Substituting equation (1) into equation (3), we can establish the relationship equation between fatigue life and cyclic stress including defect size and Vickers hardness: Step 7: According to the fatigue limit prediction formula of defective materials, propose the modified stress σ m The definition expression is: Where, σ max is the maximum cyclic stress; Step 8: Before the fatigue test begins, the sample is scanned layer by layer using industrial CT technology. After the test, the fatigue fracture is observed to obtain the maximum defect size and location information of the material under the electron beam welding process, and σ m The value of Step 9: At a specific temperature, when HV is a constant, the equivalent stress σ can be established m Functional relationship between cycle life: Where, k1 and k2 are material constants obtained by fitting the test data; According to the above formula (6), the fatigue life prediction model considering defects at a specific temperature can be obtained; Step 10: Conduct a high-temperature Vickers hardness measurement test to obtain the average Vickers hardness of each area of the GH4169 welded joint at multiple temperatures, and construct a mathematical relationship between the average Vickers hardness (HV) of the welded joint and temperature (T): HV=kT+b#(7) Where T is the ambient temperature, k and b are the material constants obtained by fitting the measured data; The optimization tool is used to perform global minimization optimization on the objective function, and k1 and k2 are obtained by fitting. The following fatigue life prediction model considering both temperature and initial defects can be established:

2. The fatigue life prediction method for high-temperature alloy welded joints for aircraft engines according to claim 1, characterized in that: For GH4169 electron beam welded joints, the fatigue life prediction model is: Wherein: Formula (7) is the life prediction model at 400℃, Formula (8) is the life prediction model at 500℃, Formula (9) is the life prediction model at 550℃, and Formula (10) is the life prediction model at T℃.

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