In order to accurately obtain the fatigue performance parameters of the typical structural details of aircraft and verify the rationality of stress and durability life calculation method, two types of typical structural detail simulation specimens were designed with 7075-T7651 aluminum alloy and 05Cr13Ni8Mo2Al stainless steel as the research objects, and the fatigue test under the random spectrum was carried out. Based on the fracture surface observation results of scanning electron microscope, the fracture surface inversion model was established by combining Paris formula and trapezoidal method, and the inversion methods of crack initiation life, propagation life and stress amplitude were systematically studied, then the method effectiveness was verified by combining multiple engineering application cases. The results showed that the fracture surfaces of both types of material simulation specimens exhibited typical fatigue characteristics, and the crack sources were mostly located in the stress concentration area. Paris formula was suitable for life inversion in the stable propagation stage, and the trapezoidal method was more suitable for the calculation of full-cycle life under complex loads. The relative error absolute value between the fatigue life obtained by inversion and the measured value was less than 7%, and the stress amplitude calculation results matched well with the design load spectrum. This study provides reliable fracture surface inversion technical support for the fatigue performance evaluation of aircraft structural details.

In this paper, the mechanical properties, micro-Vickers hardness, electrical conductivity, exfoliation corrosion, and stress corrosion were tested at different positions of the cross-section of high-strength and high-toughness 7-series aluminum alloy sectional material, and the microstructures at different positions were compared and analyzed. Then, the laws governing the relationship between the mechanical properties, corrosion resistance and microstructure of high-strength and high-toughness 7-series aluminum alloy sectional material were explored.The results showed that the tensile properties and Vickers hardness values around the section of high-strength and high-toughness 7-series aluminum alloy sectional material were higher than those at the core, and the stronger the grain size was, the more obvious the strengthening effect was. The electrical conductivity of high-strength and high-toughness 7-series aluminum alloy sectional material at the non-reinforced areas was higher than that at the reinforced areas. The electrical conductivity at the grain change position was the lowest, and the electrical conductivity at the corner was the highest. The exfoliation corrosion resistance at the non-reinforced areas was slightly worse than that at the reinforced areas, which was related to the presence of the long and wide cake grains without recrystallization at the non-reinforced areas. While, the grains at the reinforced areas tended to be equiaaxial grains, and the equiaaxial grains had better exfoliation corrosion resistance. The stress corrosion resistance at the non-reinforced areas was better than that at the reinforced areas, the equiaxed grain was not good for stress corrosion resistance, and the elongated deformed grain was more favorable for stress corrosion resistance. In the design of high-strength and high-toughness 7-series aluminum alloy sectional materials for engineering applications, optimization based on the relationships among mechanical properties, corrosion resistance, and microstructure could be performed while satisfying stiffness and mass requirements, thereby achieving better comprehensive performance.

The indentation method is used to assess the mechanical properties of materials, while it could cause micro-damage indentations on the material surface. The impact of these indentations on the corrosion resistance of materials has become a focus in the industry. In this work, Inconel 718 alloy was taken as the research object, and the effect of surface micro-damage indentation defects on its corrosion resistance was investigated by means of surface indentation method and salt spray test. Unindented specimen and three types of indented specimens with indentation loads of 100, 300, and 500 N were subjected to salt spray test in 5 wt.% NaCl environment for 0-264 h. The results showed that the mass loss of indented specimens was 3-5 times that of unidented specimens, whereas the mass loss of specimens under different indentation loads exhibited little difference. Inside the corrosion pits, there were uniformly distributed oxides of elements such as Cr, Fe, Ni, and Nb, along with a small amount of enriched Fe₃O₄. There was a stress concentration at the indentation site, which reduced the corrosion resistance of Inconel 718 alloy.

The sample pretreatment method significantly affects the accuracy and repeatability of the intergranular corrosion depth measurement results. In this paper, the pretreatment methods for the corrosion depth measurement in standard of GB/T 7998-2023 and GB/T 7998-2005(The following were abbreviated as 2023 edition and 2005 edition, respectively), and the mechanical pretreatment method were adopted respectively for design of a comparative test to explore the effects of three pretreatment methods (nitric acid passivation, dilute nitric acid descaling, and mechanical removal of oxide film) on the test results of intergranular corrosion depth and corrosion grade of aluminum alloys. The results showed that after alkaline etching, if nitric acid (with mass fraction of approximately 68%, referring to concentrated nitric acid) was used for pretreatment, the sample would undergo passivation and no intergranular corrosion occured; after alkaline etching, if 25% (mass fraction) nitric acid was used for pretreatment, the depth of intergranular corrosion of the sample was significantly increased; if the oxide film on the sample surface was directly removed by mechanical method for the intergranular corrosion test, the depth of intergranular corrosion of the sample was also relatively obvious. The results of intergranular corrosion by 2023 edition standard pretreatment method showed significant differences compared to those by 2005 edition standard. It is recommended that the standard revisers clearly define the purpose of pretreatment based on actual production inspections, so that the standard could be better applied in production inspections.

Due to the dual constraints of insufficient strength of fixture materials in high-temperature environments and the incompatibility of sample configurations, the selection of fixture materials and the structural design for the high-temperature mechanical property testing of ceramic matrix composites (CMCs) have become a hot topic. Si₃N₄ ceramics with excellent high-temperature resistance, mechanical properties and phase structure stability under high-temperature conditions were selected as the fixture material. A high-temperature fixture suitable for the dovetail-shaped plate-like CMCs was designed and fabricated, and its feasibility in the mechanical property testing of CMCs at 1 000 ℃ was verified. The thermal stress distribution and stress state of Si₃N₄ ceramic fixture and silicon carbide composite were analyzed by Abaqus finite element analysis software. The results showed that the stress concentration in the fixture weak area was significantly reduced by combining the innovative dovetail-shaped sample clamping structure with the chamfered stress release design. Moreover, the fixture exhibited excellent load conduction and failure resistance property in the high-temperature service environment. The test results of high-temperature tensile and high-temperature fatigue properties of CMCs at 1 000 ℃ indicated that this fixture exhibited stable mechanical response and durability at high-temperatures.

The brazed rate of micro-honeycomb brazed composites is an important basis for optimizing the brazing process, quality inspection and product acceptance, and its non-destructive testing methods have always been a research hotspot in related fields. In this paper, a hexagonal honeycomb ring (with a single honeycomb diameter of 0.55 mm) that was brazed on the inner surface of high-temperature alloy ring with diameter of 300 mm and thickness of 20 mm was seclected as the research object. Through optimizing the parameters of ultrasonic C-scan probe, such as frequency, focal length, step size and focus size, the feasibility of quantitatively characterizing the brazed rate of micro-honeycomb brazed composites was explored by water-immersion ultrasonic C-scan technology. Based on the experience of frequency gradient design and parameter matching optimization, three types of probes with frequencies of 10, 20 and 30 MHz were selected respectively, and the corresponding focal lengths, step sizes and focus sizes were matched for experiments. The results showed that a relatively clear honeycomb image could be obtained with probe frequency of 30 MHz, the focal length of 31.75 mm, the focus size of 0.16 mm, and the step size of 0.05 mm, and the minimum resolution feature size reached 0.05 mm. The images were proportionally enlarged, and the diameters of these structures were measured to be 0.56 mm, which was consistent with the actual micro-honeycomb diameter of 0.55 mm. Combined with the definition of brazed rate (the ratio of the actual effective metallurgical bonding brazing seam area of the brazed joint to the total brazed area designed), water-immersion ultrasonic C-scan technology was adopted to calculate quantitatively the brazed rate by obtaining the ratio of the clear honeycomb image area to the total scanning area of the workpiece.

Direct current magnetic flux leakage flaw detector is widely used in steel pipe automatic testing, and its performance directly affects the accuracy of testing results. Currently, the evaluation is primarily conducted through comprehensive performance testing of equipment (flaw detectors, sensors, mechanical components) both domestically and internationally, with no established calibration standards for the instrument performance of direct current magnetic flux leakage flaw detector. By introducing the significance of implementing calibration for direct current magnetic flux leakage flaw detector, the paper analyzes the working principle of flaw detector and the composition of its hardware, and concludes the key components that affect the performance of direct current magnetic flux leakage flaw detector, including magnetization device, attenuator (or amplifier), overall noise, and demagnetizer. Meanwhile, scientific and reasonable tools should be used to test these key parts and provide corresponding metrological parameters: relative error of magnetization current, attenuation error of attenuator, electrical noise level, and residual magnetic induction intensity. Through research and analysis, certain quantitative indicators should be proposed for the metrological parameters and effective calibration methods should be established, aiming to provide a reference for the calibration of direct current magnetic flux leakage flaw detector for steel pipe testing.

The application of cast titanium alloys in steering knuckles represents a new direction, and the fracture analysis of these materials has also attracted much attention. The cast titanium alloy steering knuckle of a certain car fractured during vehicle testing. The fracture surface initiated at the chamfer surface and propagated toward the interior.This paper conducted a comprehensive analysis of the failed steering knuckle fracture surface by methods such as macroscopic analysis, chemical analysis, mechanical property analysis, metallographic analysis, and scanning electron microscope (SEM). The results showed that the fracture mode of the titanium alloy steering knuckle belonged to fatigue fracture. The chemical composition and mechanical properties of the steering knuckle both met the requirements for ZTC4 titanium alloy as stipulated in the relevant standards. The metallographic structure was normal. However, the processing quality of the outer surface at the fracture initiation position was poor, and there were rough machining tool marks. This led to stress concentration, and the crack initiated at the machining tool marks and rapidly propagated, causing the steering knuckle to fracture.

15-5PH stainless steel forging cracked after only 30 h of service, and the failure causes were analyzed by macroscopic fracture analysis, metallographic observation, scanning electron microscope (SEM) fracture analysis, chemical composition testing, and mechanical property testing, in order to ensure the safe and reliable operation of similar critical components. The results showed that no excessive inclusions that might cause cracks was detected by metallographic examination. The measured value of each chemical component in the sample met the reauirements in national standard GB/T 1220-2007 for 15-5PH steel. The cracking originated from the inner surface of ϕ1.5 mm through-hole, mainly due to the combined effect of the excessive surface roughness (R_a≥10.0 μm) of the inner surface of the hole and assembly residual stress, which resulted in significant stress concentration, inducing multiple fracture sources and continuous propagation. The measured lateral impact energy of the material ranged from 16.0 to 22.6 J, with some values below the standard requirement of 20.3 J specified in GB/T 1220-2007. The insufficient impact toughness further accelerated crack propagation, ultimately leading to failure of the forging.

In order to investigate the fracture causes and mechanisms of water-cooled wall tube in coal-fired boiler of a certain power plant, the failed water wall tubes were analyzed through macroscopic observation, mechanical property testing, material composition analysis, metallographic examination, scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). The results showed that the chemical composition of the sample and the yield strength on both the fire-facing and back sides complied with the requirements stipulated in the relevant standards for 20G steel. However, the hardness, the tensile strength and the elongation after fracture had not reached the standard requirements. There was a thick oxide film on the inner wall of water wall tubes. Around the burst fracture, there were numerous of intergranular microcracks (fissures) propagating into the matrix interior, and severe decarburization occurred around the cracks. All these were obvious characteristics of hydrogen corrosion. At the fracture surface, there were corrosive elements such as sulfur and phosphorus, indicating that acid corrosion also existed. In addition, the weld seam was an area prone to acid corrosion, and the fusion line area was the weakest zone of the entire welding joint. As the corrosion-induced microcracks gradually propagated, fracture preferentially initiated at the fusion line region. Based on the above, it could be concluded that the fracture of the water wall tubes was caused by the combined effects of hydrogen corrosion and acid corrosion.

The macroscopic observation and microstructure analysis of the cracked middle cylinder were conducted by metallographic microscope,scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). The cracking cause were comprehensively analyzed based on the results of chemical composition analysis and hardness testing. The results showed that the chemical composition of middle cylinder met the composition requirements of 30CrMnSiA steel specified in GB/T 3077-2015 standard. From macroscopic and microscopic morphological analysis, it was evident that the fracture originated from the base material at the fusion zone of the molten copper in the sample, indicating brittle cracking. According to the results of metallographic examination and energy dispersive spectroscopy, there were local cracks in the fracture source area, and the main component of the filler inside the cracks was copper, which was consistent with the molten copper layer and should have been generated during the molten copper welding process, namely copper penetration cracks. From the hardness test results, it was indicated that the martensite near the fusion line had a significantly higher hardness than that of the normal position. Its presence could significantly reduce the overall toughness of the material and increase the risk of fracture. Based on the above analysis, the failure of the middle cylinder was mainly caused by the infiltration cracks and hot cracks during the copper melting process. During the cooling process of the weld seam, the formation and propagation of cracks would be aggravated by the formation of structural stress due to phase transformation. It was recommended to preheat the parts of steel to be welded during copper melting, prevent excessive welding heat input, and shorten the time of molten copper to wet austenite grain boundaries, which would play a positive role in avoiding the production of cracks in the middle cylinder.

It is of great significance to carry out the cause analysis of black spots and warping defects of SAE1022 boron-containing steel surface for improving the quality and production stability of its hot-rolled plates. The microstructure morphology and composition analysis of the black spots and warping defects on the hot-rolled plate surface of SAE1022 boron-containing steel were analyzed by metallographic microscope, scanning electron microscope (SEM), and energy dispersive spectroscopy (EDS). The corresponding casting billet defects were also analyzed. At the same time, the on-site process investigations were conducted to examine the production-related factors. The characteristics, causes, and process control measures of the black spots and linear warping defects were discussed. The results showed that no abnormal inclusions or inclusion segregation were found at the locations of black spots and linear warping defects on the hot-rolled plate surface. Instead, obvious secondary oxidation, decarburization, and grain growth phenomena were observed, along with the segregation of copper, boron nitride, and aluminosilicate inclusions at the defect areas. The black spots and warping defects were caused by the uneven deformation of the star-shaped cracks and corner cracks in the continuous casting billet during the hot rolling process. These cracks were related to the detachment of copper coating in the continuous casting mold, the segregation of aluminosilicate inclusions, and the segregation of boron nitride at the grain boundaries. Based on the above analysis, corresponding control measures were taken for the steelmaking and hot rolling processes, resulting in a reduction of the defect occurrence rate from 2.48% to 0.85%.

Numerous delayed fracture failures have been observed in engine flywheel bolts of SCM435 produced by a manufacturing factory, which significantly degrades the reliability of the engine assembly. In this study, the failure causes leading to flywheel bolt fracture are systematically investigated through a series of analytical techniques, including fracture surface microscopic morphology observation, metallographic inspection, chemical composition analysis, tensile property testing and substrate hardness testing. Experimental results reveal that the fracture surface of the bolts displays typical characteristics associated with hydrogen embrittlement, accompanied by distinct precipitates distributed along grain boundaries. Surface corrosion induced during the pickling and phosphating process facilitates hydrogen penetration into the bolt matrix, and the resultant hydrogen embrittlement is identified as the primary contributor to fracture. Furthermore, chemical analysis results indicate that the cumulative content of low-melting-point elements of Pb and Sn in the flywheel bolt material exceeds the specified standard limits. The excessive low-melting-point elements tend to segregate at grain boundaries during the cooling stage following quenching and tempering, thereby weakening grain boundary cohesion and triggering intergranular brittle fracture. The synergistic interaction of these two factors ultimately gives rise to the delayed intergranular brittle fracture of the flywheel bolts. Nevertheless, the test results demonstrate that both the mechanical properties and matrix hardness of the bolts conform to the requirements of relevant national standards, and no abnormalities are detected in the quenched and tempered metallographic microstructure.

To address the issues of insufficient control accuracy and the limited manual optimization capabilities of deoxidation alloying during LF refining furnace refining process, an intelligent deoxidation alloying model is developed based on the metallurgical reaction mechanism and historical production data. This model integrates theoretical analysis and data-driven methods, comprehensively considering the alloy elements yield, market prices, and dynamic changes during the production process. It uses optimization algorithms to calculate the amount of alloy addition that yields the highest cost-effectiveness, thereby achieving precise control of deoxidation alloying and cost optimization. In the model design, the metallurgical mechanism part takes into account the dissolution and oxidation processes of alloy elements in the molten steel, providing a theoretical basis for the prediction of alloy elements yield. Meanwhile, the data-driven historical part uses linear regression analysis to extract the quantitative relationship between key operating parameters and alloy elements yield, compensating for the limitations of mechanism model in complex working conditions. Based on this, the model could dynamically adjust the amount of alloy addition to adapt to the fluctuations in the production process and ensure the stability of alloying effect. The results show that the prediction accuracy of this model for ferro-manganese and ferrochrome within±20 kg deviations reaches 90% and 96% respectively, and the accuracy of ferrosilicon deoxidation calculation within±20 kg deviations also exceeds 90%. The successful application of this model not only improves the efficiency and accuracy of deoxidation alloying in LF refining furnace, but also provides technical support for intelligent production in steel enterprises. Through precise control of the alloying process, enterprises could achieve efficient utilization of resources and significant cost optimization, providing important references for the green and intelligent transformation of steel industry.

The overall process flow and each process in automatic hot acid etching macrostructure inspection system of square billet were inveatigated. The design principles of the entire system, as well as the equipment composition and process flow of the sample receiving and processing system, acid etching and imaging system, environmental protection device and automatic control system, were expounded. The system had achieved the full-process intelligent detection of hot acid corrosion for square billet. After its commissioning, the work efficiency had increased by 40% compared with the manual mode, and the labor productivity had risen by 50%. Moreover, it had low operation and maintenance costs, and the material cost was only 1/3 of the high-speed circular saw solution. Finally, the key points of the laboratory design for the system were sorted out, and the development prospects of automatic macrostructure inspection system process were prospected.