Research progress of hydrogen induced cracking of high strength aluminum alloy
Aluminium and aluminium alloys
As an environment-friendly lightweight material, it has the advantages of high specific strength, fracture toughness, fatigue strength and good molding process. It is known as one of the most promising lightweight structural materials. It plays an irreplaceable role in aerospace, ship, automobile, high-speed rail, rail transit and nuclear industry.
Due to the face centered cubic structure of aluminum alloy, the solubility of hydrogen in aluminum is very low, and the formation of hydrides is difficult, therefore, researchers generally believe that there is no hydrogen induced cracking in aluminum alloy for a long time. However, with the in-depth study of stress corrosion and corrosion fatigue of high-strength aluminum alloy, it is found that hydrogen has an obvious effect on both SCC and CF, and may be the main cause of SCC and CF. At the same time, in 1969, grub et al. First found that the high strength aluminum alloy will appear the phenomenon of hydrogen induced plastic loss during the test. A series of subsequent work also showed that 7xxx series aluminum alloys have obvious hydrogen embrittlement in the corrosive environment. Therefore, under certain conditions, high strength aluminum alloy has obvious hydrogen embrittlement phenomenon, so it is necessary to study the hydrogen induced cracking behavior of high strength aluminum alloy systematically and comprehensively.
Characteristics of hydrogen induced cracking of aluminum alloys
Sources of hydrogen
Generally speaking, the source of hydrogen is divided into two parts: one is absorbed from the wet environment and not completely dried furnace body in the smelting process, and the other is introduced into the later use environment. The core of its source is water vapor.
From the analysis of reaction relationship, as long as there is humidity increase or higher humidity environment in the smelting process or later use environment, the above chemical reaction formula will be carried out, and hydrogen absorption of aluminum alloy will be caused. Compared with other metals, the solubility of hydrogen in aluminum alloy is very low. At the same time, for aluminum alloy, the hydrogen absorption capacity of solid and liquid near the melting point is quite different, so it is difficult for liquid dissolved hydrogen to completely precipitate during solidification. The supersaturated gas in aluminum alloy can be distributed in two ways, one is to escape through pores or loose defects, the other is to continue to stay in the aluminum alloy in an unstable supersaturated state, and then accumulate to the crack tip or defects such as inclusions and micro pores under the conditions of heating or pressure, forming hydrogen molecules and causing hydrogen embrittlement.
Characteristics of hydrogen induced cracking
The most obvious characteristics of hydrogen induced cracking of high strength aluminum alloy are that it is difficult to observe, delay, uncertainty and sudden. Even in the extreme environment of high temperature and high pressure, the high strength aluminum alloy will not show obvious characteristics of hydrogen induced damage. The results show that the pre cracked high strength aluminum alloy samples do not show obvious delayed cracking in dry high-pressure hydrogen environment, but show obvious brittleness in wet air.
The unique characteristic of hydrogen induced cracking of high strength aluminum alloy is reversible. If the hydrogen filled sample is treated by dehydrogenation, its plasticity is basically the same as that of the non hydrogen filled sample. Therefore, to a certain extent, it can be considered that the hydrogen induced cracking behavior of high strength aluminum alloy is related to the atomic hydrogen in the material. At the same time, the hydrogen induced cracking fracture of high strength aluminum alloy is different. According to the literature investigation, it is found that the reported results of hydrogen induced cracking fracture morphology of high strength aluminum alloy are quite different. This can be attributed to the different test conditions and sample treatment methods. However, generally speaking, the hydrogen induced cracking fracture of aluminum alloy is mainly intergranular and transgranular, and the fracture characteristics are directly related to the hydrogen concentration on the fracture surface.
Hydrogen embrittlement theory
In recent decades, although domestic and foreign scholars have done a lot of research on the hydrogen induced cracking behavior of high-strength aluminum alloy, and have made a series of achievements, but due to the difference between aluminum alloy and steel, titanium and other metals, the hydrogen induced cracking behavior of aluminum alloy is still a new research field, and there is no agreement on its mechanism. The well-known theories include weak bond theory, MG-H complex theory, grain boundary adsorption theory, stress-induced hydride embrittlement theory and hydrogen induced delayed plastic deformation theory.
Weak bond theory
The weak bond theory was proposed by troiano et al. On the basis of studying hydrogen behavior in steel, and orinai et al. The researchers of this theory believe that there will be a three-dimensional stress zone at the crack tip of the specimen, and the stress gradient distribution of the crack will lead to the diffusion of hydrogen in the sample to the crack tip, resulting in a local hydrogen rich zone in the crack tip region, thus leading to the decrease of the binding force between iron atoms. When hydrogen segregation reaches a certain degree, hydrogen induced cracking will occur under low stress. However, at present, there is no consensus on the physical mechanism of the theory, and the hydrogen induced decrease in the binding force between atoms has not been verified by experiments.
Grain boundary adsorption theory
The theory of grain boundary adsorption is based on the weak bond theory proposed by troiano. It is believed that the grain boundary at the interface of aluminum alloy surface will react with water molecules on the surface, resulting in more atomic hydrogen distributed on the surface of aluminum alloy. Subsequently, hydrogen atoms diffuse to the lattice, resulting in grain boundary segregation, leading to the weakening of grain boundaries, which leads to hydrogen induced cracking.
Theory of stress induced hydrogen embrittlement
The main mechanism of stress-induced hydride embrittlement theory is aluminum hydrogen compound embrittlement. The aluminum hydrogen compound is not the precipitated phase of material itself or the compound of self reaction, but is induced by slow tensile stress. According to the theorists, under the action of slow tensile force, the aluminum hydrogen compound phase in the sample will break brittle, and will also preferentially fracture along the orientation interface of aluminum alloy matrix, which will lead to hydrogen induced cracking of aluminum alloy.
The MG-H complex theory was first proposed by viswandham et al. The researchers of this theory believe that there is a certain amount of free magnesium on the grain boundary of high-strength aluminum alloy, especially 7xxx series aluminum alloy. The free magnesium on the grain boundary will form MG-H complex phase with nearby hydrogen, which will lead to hydrogen segregation on the grain boundary, reduce the binding energy near the grain boundary, and then cause grain boundary embrittlement. This theory can well explain the causes of such phenomena as grain boundary bubble and hydrogen bubble.
In recent years, Professor Zeng Meiguang from Northeastern University of China has found that there is obvious MG-H interaction at grain boundary of 7xxx series high strength aluminum alloy. The team of Professor Song Renguo of Changzhou University also found that there was MG-H interaction on the grain boundary in the process of cathodic hydrogen permeation of high-strength aluminum alloy, and proved that the higher the concentration of solid solution magnesium segregation at grain boundary, the stronger the hydrogen absorption capacity of the alloy, and the greater the amount of hydrogen permeation, that is, the greater the sensitivity of hydrogen embrittlement.
Theory of hydrogen induced delayed plastic deformation
The theory of hydrogen induced delayed plastic deformation was first proposed by Chu Wu Yang, Xiao Jimei, etc. The researchers of this theory believe that when the strength and stress intensity factor of the alloy are greater than the critical value, if the alloy sample is placed in the hydrogen environment, the size and deformation of the plastic zone at the crack tip of the alloy sample will increase with the extension of time, that is, the hydrogen induced delayed deformation occurs. When the deformation reaches a certain degree, hydrogen induced cracking will occur.
For the study of the interaction between high strength aluminum alloy and hydrogen, more experimental data of hydrogen induced property change have been accumulated, and more theoretical explanations have been formed. However, up to now, the research on the hydrogen induced cracking behavior of high strength aluminum alloy is limited to the preliminary qualitative interpretation of experimental phenomena and data, and a systematic and comprehensive theoretical system has not been formed.
Influencing factors of hydrogen induced cracking of aluminum alloy
The formation of hydrogen induced cracking is directly or indirectly related to the alloy elements of the material itself. At present, it is found that the effect of alloying elements on hydrogen induced cracking is mainly reflected in three aspects: strengthening phase, microstructure and grain boundary segregation. Alloying elements can not only change the microstructure of the alloy, but also change the electrochemical properties of the alloy. At the same time, alloy elements can also interact with hydrogen to affect the concentration and activity of hydrogen. Magnesium and zinc in alloy elements can be used to form strengthening phases mgzn2 and MgZn, which can improve the strength of the alloy. However, too high content of zinc and magnesium will reduce the toughness and SCC resistance of the alloy, thus leading to brittle fracture. It is generally believed that the grain boundary brittle fracture of high-strength aluminum alloy is mainly caused by MG-H precipitation at grain boundary caused by magnesium segregation at grain boundary. Therefore, it is found that the reasonable proportion of zinc and magnesium is very important to improve the comprehensive properties of high strength aluminum alloy. It is also reported that the addition of Mn, Cr, Ti and CE can effectively reduce the hydrogen embrittlement sensitivity of aluminum alloy. In the aspect of microstructure, the dispersion strength on the grain boundary, GP region of coherent precipitates and the distribution of precipitates at grain boundaries have different effects on the hydrogen embrittlement sensitivity of high strength aluminum alloy.
Heat treatment system and surface treatment
It has been reported at home and abroad that hydrogen has different effects on mechanical properties of 7xxx series high strength aluminum alloy under different solution and aging treatments, and many important research results have been achieved. In China, Professor Song Renguo’s team has also carried out a lot of research on the hydrogen embrittlement behavior of 7xxx series high-strength aluminum alloy under aging condition, and concluded that the hydrogen induced cracking sensitivity of under aging is the strongest, the over aging is the weakest, and the peak aging is in the middle. At the same time, it is proved that the hydrogen induced cracking sensitivity of the second aging peak is lower than that of the first aging peak under double peak aging. According to the current research results at home and abroad, most of the research is limited to the existence of hydrogen content, which will have an impact on the performance, but there is no obvious conclusion about the specific impact degree and the damage tolerance of hydrogen content.
Generally speaking, the hydrogen induced cracking of high strength aluminum alloy will not occur in dry environment, but the sensitivity of material to hydrogen induced cracking will be significantly enhanced once the sample is placed in wet or solution environment. Liu Jihua team studied the effect of surface electrode polarization process on stress corrosion cracking sensitivity of aluminum alloy. The results show that both cathodic polarization and anodic polarization have adverse effects on the stress corrosion sensitivity of the materials. Further analysis shows that the anodic polarization can promote the anodic dissolution and then cause the occurrence of stress corrosion; while the cathodic polarization will accelerate the hydrogen penetration into the aluminum alloy, increasing the stress corrosion sensitivity of hydrogen induced cracking. Qi Wenjuan et al. Found that the hydrogen induced cracking sensitivity of aluminum alloy is closely related to hydrogen induced additional stress, and hydrogen induced additional stress will further enhance the hydrogen induced cracking sensitivity of high-strength aluminum alloy. Ma Shaohua et al. Carried out fatigue tests on pre corroded smooth and notched aluminum alloy specimens in laboratory air and humid air environment respectively. The results showed that the thin water film on the surface of the alloy accelerated the crack growth and local damage caused by atomic hydrogen in the humid air environment, which significantly reduced the fatigue performance of pre corroded aluminum alloy.
Determination of hydrogen content in materials
According to the existing literature, the hydrogen content in aluminum alloy is generally less than 1 μ g / g, even less than 0.1 μ g / g. there are many interference factors and it is difficult to distinguish, so it is difficult to detect. The quantitative determination of hydrogen content in aluminum alloy is of great significance to study the hydrogen embrittlement behavior of high strength aluminum alloy. Therefore, the determination of hydrogen content in aluminum alloy has been a difficult and important research.
Many domestic universities and research institutes have carried out a large number of quantitative detection and analysis of hydrogen in aluminum alloy. For example, Professor Zhu Zhaojun of Harbin Institute of technology has developed a new computer control software to quantitatively detect and analyze hydrogen in aluminum alloy. According to the different characteristics of the interference of different elements in aluminum alloys on hydrogen analysis, Xie Shaojun, a researcher from Southwest Aluminum Industry, divided aluminum alloys into four series, namely, aluminum magnesium, aluminum zinc, aluminum lithium and other aluminum alloys, and optimized the test methods and parameters. Professor Sun Baode of Shanghai Jiaotong University has carried out quantitative analysis of solid hydrogen content, in-situ quantitative analysis of liquid hydrogen content, and semi quantitative analysis of solid and liquid hydrogen content, comprehensively and systematically elaborated the testing principle, detection accuracy and characteristics, and proposed that for large-scale continuous production of aluminum plants, more advanced quantitative testing methods should be adopted and attention should be paid to the testing precision For small and medium-sized foundries, it is suggested to choose simple qualitative testing tools and pay attention to the economy of the test methods.
At present, according to the different ways of hydrogen extraction, there are two kinds of hydrogen measurement methods: vacuum (heating) extraction method and inert gas melting method. Vacuum (heating) extraction method has a long analysis period and needs vacuum leak detection, which has been basically eliminated. At present, inert gas melting method is widely used. The principle is: the sample is heated by high frequency induction or pulse heating in inert gas atmosphere, and the hydrogen atom is released in the form of H2, which is introduced into the detection device after dust removal and purification. According to the different detection methods of hydrogen, it can be divided into thermal conductivity method, infrared absorption method, time-of-flight mass spectrometry, etc. The thermal conductivity method is based on the difference of thermal conductivity between H2 and N2 to realize the rapid determination of hydrogen. H2 is oxidized to H2O by infrared absorption method, and the infrared characteristic absorption intensity of H2O is detected. Time of flight mass spectrometry (TOF-MS) was used to calculate the hydrogen content of the sample by detecting the intensity of H2 ion line (H2 +). In terms of method maturity, market share, standard recommendation and literature reports, infrared absorption method and thermal conductivity method are widely used. They are the standard methods recommended by international organization for Standardization (ISO), American Society for testing and materials (ASTM) and Chinese standards for the determination of hydrogen content in steel, titanium, molybdenum and other metal alloys. Time of flight mass spectrometry (TOFMS) has a promising prospect, but due to the cost factors, its commercial instruments are only used in a few special scientific research institutes such as nuclear materials (AR or he), and has not been widely promoted.
At present, a series of research results on hydrogen embrittlement behavior of high strength aluminum alloy have been achieved, but most of the research results are to verify the phenomenon of hydrogen induced cracking, or to carry out laboratory test and interpretation of individual hydrogen behavior. There is no unified theory on hydrogen embrittlement behavior of high strength aluminum alloy. Therefore, with the development trend of lightweight application of high-strength aluminum alloy, it is necessary to further study and explore the hydrogen induced cracking behavior of high-strength aluminum alloy.
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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