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Optimized selection of vacuum annealing for titanium alloys

As we all know, titanium alloys are increasingly used in modern aircraft due to their outstanding advantages such as high specific strength and high thermal strength. On the other hand, due to the characteristics of titanium alloy itself, relatively frequent hydrogen penetration will occur during its ingot casting, pressure processing, heat treatment, welding and other processes. Generally speaking, an increase in the hydrogen content concentration in titanium alloys will lead to an increase in hydrogen embrittlement. The problem of hydrogen embrittlement (HIC-Hydrogen Induced Cracking) is not unique to titanium alloys. Other metal materials, such as steel, aluminum alloys, etc., also have hydrogen embrittlement problems.

As the application of titanium alloys in aircraft and engine structures continues to increase, its hydrogen embrittlement problem has received great attention from the aviation manufacturing industry. At present, vacuum annealing is the only process method used in the aviation manufacturing industry to remove hydrogen from titanium alloys. Vacuum annealing is required at the end of the manufacturing process of titanium alloy products and structures to reduce its hydrogen content concentration and bring it to a safe level. This ensures the safety and reliability of the titanium alloy structure during its service life. Currently, a trend in alloy vacuum annealing research is to establish a vacuum annealing specification selection based on dehydrogenation conditions (including structural, process, material conditions) and vacuum annealing specification parameters (including temperature, time and other parameters) multi-factor system, and optimize the system in order to obtain the best vacuum annealing specification in the optimization sense.

As mentioned above, the selection of vacuum annealing specifications for titanium alloys is a complex, multifactorial issue. It involves the principle of reversibility of the interaction between titanium alloy and hydrogen, such as the description of the relationship between the pressure of hydrogen in the gas medium and the hydrogen content in the titanium alloy, the quantitative relationship between the hydrogen content in the titanium alloy and its mechanical properties, titanium The relationship between the equilibrium concentration of hydrogen in the alloy and the annealing time, etc., also involves the structural form of the annealed structure or part, such as sphere, cylinder, thick plate and thin plate, as well as whether the structure has an internal cavity and whether the thickness is uniform (whether it is hypertrophic) end), etc., and even involves stress concentration issues in products and structures. Therefore, the selection of titanium alloy vacuum annealing specifications, which is affected by these many factors, itself has the so-called “optimal” problem. Optimization research on the above-mentioned multi-factor system will help to essentially clarify the impact of each factor on the selection of vacuum annealing specifications, which has obvious scientific significance in theory. On the other hand, vacuum annealing is an expensive process that requires expensive vacuum furnace equipment and consumes a lot of electricity. Especially for vacuum annealing of large titanium alloy structures, the increase in vacuum furnace size and electricity consumption will greatly increase production. cost, which requires strict analysis of various factors affecting vacuum annealing to ensure that the vacuum annealed titanium alloy structure has the desired mechanical properties and usability performance, and to reduce production costs. Therefore, optimizing the selection of vacuum annealing specifications also has important practical significance in actual production.

General principles for selecting vacuum annealing specifications for titanium alloys

In a given specific application, the vacuum annealing specifications for a certain titanium alloy are as follows:

(1) Determine the maximum safe allowable concentration Cf in this type of titanium alloy; the final concentration of hydrogen Ck can be taken to be equal to Cf;

(2) The equilibrium concentration of hydrogen Cp is determined by necessary calculations. In theory, it is the equilibrium concentration reached during vacuum annealing for an infinite time. It should be 1/2~1/3 of the final concentration;

(3) Taking into account the impact of dehydrogenation conditions and vacuum annealing specifications on the mechanical properties and service performance of titanium alloys, correct the vacuum annealing temperature;

(4) Under the selected temperature, calculate the duration of vacuum annealing based on the known diffusion coefficient of hydrogen in the titanium alloy to be annealed, as well as the calculated hydrogen concentration values Ck, Cp and initial hydrogen content Co;

(5) In the final stage of vacuum annealing, the injection of air during the cooling process of the annealed structure is specified to form a protective oxide film. For processed products, the heating speed should also be specified to prevent warping of the product structure.

The optimally selected vacuum annealing specification fully takes into account the influence of multiple factors, and its main parameter annealing temperature is more clear. For structures under different structural types and process conditions, the annealing duration is different. In order to ensure reasonable decomposition. Hydrogen and lower production costs offer the possibility.