TC4 titanium alloy is the most widely used a+β type titanium alloy. Due to its characteristics of high specific strength, good corrosion resistance and superior comprehensive performance, it is widely used in aerospace, chemical machinery, medical engineering and other industries. Common defects in TC4 titanium alloys include segregation and inclusion of alloying elements, residue of cast structure, embrittlement layer, hydrogen embrittlement, and brittleness and forging cracks.
In recent years, more and more countries have begun to develop medium-strong and high-damage-tolerant titanium alloys. Compared with other titanium alloy rods, it has higher fracture toughness and crack propagation resistance under the conditions of similar strength and plasticity, and can be widely used as key bearing members for important models.
Two-phase zone annealing, two-phase zone solid solution + aging, and single-phase zone solid solution + aging heat treatment process are the commonly used heat treatment process. By studying the influence of these concentrated heat treatment processes on the microstructure and mechanical properties of TC4 titanium alloy rods, the best matching heat treatment process of the TC4 alloy can be obtained.
(1) TC4 titanium alloy undergoes ordinary annealing and recrystallization in the two-phase region, recrystallization occurs after annealing treatment, and the size of phase a increases; and the higher the annealing temperature, the slower the cooling rate, the more complete recrystallization; The a phase aggregates and grows, resulting in a significant decrease in alloy strength and an increase in plasticity.
(2) Applying solid solution + aging treatment in the two-phase region to obtain a two-state structure; as the aging temperature increases, the secondary a-phase content decreases, the a-sheet size increases, the strength and fracture toughness of the alloy decrease, and the plasticity decreases. When the solid solution dissolves, the cooling rate increases, and the secondary sheet-like phase formed during aging is finer, resulting in an increase in the strength of the alloy and a decrease in plasticity and fracture toughness.
(3) Martensite structure is obtained by solid solution water cooling in the single-phase zone, followed by aging treatment, the plasticity of the alloy decreases sharply; and the aging temperature increases, the sheet is thickened, but precipitated. The secondary a-phase content is reduced, and the strength and fracture toughness of the titanium alloy titanium rod are decreased. However, in the single-phase region, solid solution air cooling obtains the Widmanstätten structure, followed by high-temperature aging treatment, which further increases the thickness of the a-sheet layer, and The degree of chaos is reduced, resulting in a decrease in strength and an increase in both plasticity and fracture toughness.
(4) TC4 titanium alloy rods are heat-treated by 950C/1h/WQ+550C/6h/AC, which can achieve the optimal matching of strength, plasticity and toughness, thus obtaining excellent comprehensive mechanical properties.
The TC4 titanium rod is forged is heated at 950 degrees to be forged into a disk. However, the disk will be expanded into a ring. The defects may be caused by cracking due to the improper forging process, or there may be metallurgical defects such as inclusions, segregation, pores, etc. The forging temperature determines the microstructure of the titanium alloy and it is inevitable to produce β phase transformation during the forging process of titanium alloy. The following conclusions can be drawn from the research in this article:
(1) There is no difference in the main chemical components between the black spot area of the TC4 titanium rod and the normal area, and there is no component segregation phenomenon. Therefore, the black spot is not a phase change structure caused by chemical segregation.
(2) The p-phase morphology of the TC4 titanium rod black spot area is quite different from that of the normal area, which is an abnormal phase change structure formed by the single-phase area.
(3)TC4 titanium rod black spot tissue is an uneven structure in which the titanium rod is deformed unevenly during the forging process to form different phase transitions.
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