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Effect of titanium forging on properties and microstructure of TC4 titanium rod and titanium alloy rod

TC4 titanium alloy is the most widely used a+ beta titanium alloy at home and abroad. It is widely used in aerospace, chemical machinery, medical engineering and other industries due to its characteristics of high strength, good corrosion resistance and superior comprehensive performance.The common defects of TC4 titanium alloy include segregation of alloy elements and inclusions, residual of casting structure, embrittlement layer, hydrogen embrittlement, brittleness and forging cracks.Developed in China during the "tenth five-year plan started with a strong high damage tolerance of titanium alloy, compared with other strong in titanium alloy titanium rods, which under the condition of strength, plasticity level, have higher fracture toughness and crack propagation resistance, can be widely used in our country the fourth generation machine, large aircraft and other important key bearing component models.The effects of different heat treatment processes on the microstructure and mechanical properties of TC4 titanium alloy rod were studied by adopting the heat treatment processes of two-phase annealing, two-phase solid solution + aging, and single-phase solid solution + aging.

  1. (1) the recrystallization of TC4 titanium alloy occurred after general annealing and recrystallization annealing in the two-phase zone. The size of phase a increased, and the higher the annealing temperature, the slower the cooling speed, the more sufficient recrystallization occurred, and the aggregation of phase a grew up, leading to a significant decrease in the strength of the alloy and an increase in the plasticity.

  2. (2) the two-state structure was obtained by solid solution + aging treatment in the two-phase zone. With the aging temperature increasing, the content of secondary a phase decreased, the size of a slice increased, the strength and fracture toughness of the alloy decreased, and the plasticity increased.However, when the solid solution is dissolved, the cooling speed increases, and the secondary flake a phase formed during aging becomes smaller, which results in the increase of strength, plasticity and fracture toughness of the alloy.

  3. (3) the martensite structure was obtained by solid-solution-water cooling in the single-phase zone, followed by aging treatment. The plasticity of the alloy deteriorated sharply and brittle fracture occurred.With the aging temperature increasing, the lamellar thickness increased, but the precipitated secondary a phase content decreased, and the strength and fracture toughness of titanium alloy titanium rod decreased.However, the weinbach structure was obtained by solid solution cooling in the single-phase zone, followed by high-temperature aging treatment, which further increased the thickness of the a-slice layer and reduced the degree of chaos interweaving, resulting in the decrease of strength, and the improvement of plasticity and fracture toughness.

  4. (4)TC4 titanium alloy rod can be heat treated with 950C/1h/WQ+550C/6h/AC to achieve optimal matching of strength, plasticity and toughness, thus obtaining excellent comprehensive mechanical properties.

TC4 titanium rods to ring forging process are two fire in 950 - degree heat, forging into bread, cake material flaring loop material, maybe it's because of the defect formation change caused by improper forging technology craze, it is possible that the bar itself exists the metallurgical defects such as inclusions, segregation, porosity, etc., and in the subsequent change caused by metallurgical defect in the process of forging cracking.Through theoretical calculation, it is concluded that the design of this set of special jig for computer flat crane bridge is completely in line with the theoretical design requirements.So the author manufactured the real object of this set of jig and installed it to the work table of vcentei-70 high-precision CNC machining center for actual processing verification.Then the finished parts of the horizontal crane overpass processed into the three coordinate measuring machine for accurate precision measurement, the Angle error of the two included angles in 0.2° ~ 0.6° range, in line with the technical requirements.Because this set of jig can be very convenient and fast to realize the processing and positioning of bridge parts, and do not need to find the correct through other means, so it is suitable for the mass production of bridge parts.

The forging temperature determines the microstructure of titanium alloy, and it is inevitable to produce beta phase transition in the forging process of titanium alloy. The following conclusions can be drawn through the study in this paper:

  1. (1) there was no difference in the main chemical composition between TC4 titanium bar black spot area and normal area, and no segregation phenomenon was observed. Therefore, the black spot was not the phase transition tissue caused by chemical composition segregation.

  2. (2) the p-phase morphology in the dark spot area of TC4 titanium rod was significantly different from that in the normal area, and it was an abnormal phase transformation structure formed in the single phase area.

  3. (3) the dark spot structure of TC4 titanium rod is an uneven structure formed by different phase transitions due to uneven deformation of titanium rod in the forging process.Excluding the influence of experimental errors, according to the test results, there was no significant difference in the content of Al, V, Fe and titanium matrix elements between the low-power dark spot area and the normal area of the titanium bar, indicating that there was no component segregation in the dark spot area.Therefore, there is no direct relationship between the dark spots of titanium rod and the segregation of chemical components.

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