Titanium alloy is favored by people for its excellent room temperature, high temperature mechanical properties, outstanding corrosion resistance and high strength. It has become an important structural material in the aviation and aerospace industries. Titanium alloys are divided into α titanium alloys, β titanium alloys and α + β titanium alloys according to their annealed structure. At present, many parts of the new models use these three materials.
Titanium alloy undergoes strengthening heat treatment. This process can significantly increase the strength of the alloy and obtain the comprehensive performance of high strength and good plasticity. So what exactly does the titanium alloy behave when it encounters heat treatment, and what principles need to be followed?
Titanium alloy heat treatment mainly plays a role in adjusting the structure
The structure of titanium alloy is determined by thermal deformation, and heat treatment mainly only plays a role of adjustment. For example, heat treatment can only adjust the ratio of α phase and β phase obtained by thermal deformation, and the sheet structure produced by overheating cannot be changed into a dual-state structure by heat treatment.
The strengthening heat treatment of titanium alloy is limited by the alloy phase composition
Most near-α and stable β-type titanium alloys (except for a very small number such as Ti-2Cu) cannot be strengthened by heat treatment, and only α+β-type titanium alloys can be strengthened by heat treatment.
Strictly control heating temperature and time
When the titanium alloy is heated above the β transformation temperature, the crystal grains grow rapidly; in the subsequent cooling, the α phase first nucleates and grows on the grain boundary and grows into the inside of the grain. The β crystal grain size obtained after heat treatment in the β zone is relatively large, which can generally reach the degree that is visible to the naked eye. Moreover, the method of heat treatment cannot eliminate the coarse-grained structure of the titanium alloy, and the forging deformation must be used to change the structure. Therefore, when heating before forging or heat treatment in the β zone, the heating temperature and time should be strictly controlled to prevent excessive grain growth.
Titanium alloys easily combine with oxygen, nitrogen, etc. at high temperatures, forming an oxygen-rich embrittlement layer on the surface. Therefore, forgings should generally be heat treated in a micro-oxidizing atmosphere; for some forgings whose surfaces are no longer processed, such as engine precision forged blades, vacuum heat treatment should generally be used to avoid surface oxidation.
Control hydrogen absorption
Titanium alloys tend to absorb hydrogen at high temperatures. Therefore, electric furnaces should be used as much as possible when heating or heat treatment before titanium alloy forging; if oil furnaces or gas furnaces must be used, the furnace gas should be slightly oxidizing. For some important parts, especially thin-walled forgings, the temperature and time should be strictly controlled during chemical milling to prevent excessive hydrogen absorption.
Pay attention to controlling the heating and cooling rate
Titanium alloy has low thermal conductivity. When cooling after heat treatment, the thin section of the forging will cool faster than the thick section, causing uneven microstructure; in some cases, the temperature difference of the section of the titanium alloy during the heating-cooling process is too large. , May produce excessively high residual stress, resulting in warping and deformation of the workpiece. In the past, when the titanium alloy ingot with poor plasticity was heated before forging, the internal thermal stress was too large, which caused the ingot to fracture. Therefore, it is recommended to adopt the method of segmented heating to minimize the thermal stress inside the ingot or billet.
In summary, the correct and reasonable use of titanium alloy heat treatment process is of great significance to prevent the failure of titanium alloy parts.
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