Titanium alloy has become one of the most promising metal structural materials in the contemporary aerospace field due to its excellent comprehensive properties such as low density, high specific strength, high temperature resistance, corrosion resistance and non-magnetism. With the large number of applications of titanium alloys, their metallurgical quality issues have increasingly attracted widespread attention from the industry, so the metallurgical quality of titanium alloys has become increasingly important.
At present, more than 80% of industrial titanium alloys are used as deformed titanium alloys, such as forgings, forging rods and rolled profiles. Forging deformation is the most important means to ensure the ideal structure and properties of titanium alloy materials. However, incorrect forging processes often cause titanium alloy products to have some undesirable microstructure and metallurgical defects, thereby deteriorating their mechanical properties. Potential hazards caused by normal use, and at the same time cause a lot of waste for production and use manufacturers, so it is of great value to study and analyze the formation mechanism of various titanium alloy forging defects and take effective preventive measures.
Forging thermal effect
After a high-temperature titanium alloy ingot of a certain grade is forged on a fast forging machine, it is forged into a φ 165mm bar in multiple heats in the α + β two-phase region. After heat treatment, the low-magnification structure is a fuzzy crystal structure and the microstructure is The equiaxed structure is the ideal equiaxed structure of the α + β dual-phase titanium alloy. The micrograph is shown in Figure 1a. After cutting the above φ 165mm bar material and heating it at 50 ° C at the transformation point, it was forged on a 30kN hydraulic hammer into a φ 110mm × 110mm square billet. When the billet was subjected to anatomical analysis, the heart was found For clear crystals, the microstructure photo is shown in Figure 1b. The microstructure is α lath + β transition. It is a typical Weiss structure with clear grain boundaries. Α is a superheated structure in titanium alloy. The distance from the surface to 20 to 30 mm is Semi-clear crystal, the microstructure photo is shown in Figure 1c, the microstructure is α lath + α isometric + β rotation, the number of α isometric is scarce, the number of α laths is mostly, and there are intermittently distributed grain boundaries α; 0 from the surface In the range of ～ 20mm, it is fuzzy crystal.
The microstructure of a batch of φ80mm TC4 titanium alloy rods is a typical equiaxed alpha structure (see Figure 2a), and the primary alpha equiaxed content reaches more than 70%. After die forging on a heating hammer at 940 ° C (alloy transformation point 995 ° C), the microstructure of the core of the forged piece is shown in Figure 2b. The initial α equiaxed content is only about 15%, which is caused by overheating at the forging temperature.
Titanium alloy deforms above the transformation point (α + β / β transition temperature) to obtain the net basket structure or Weiss structure. The plasticity and fatigue performance are poor. Therefore, most titanium alloy product technical standards require near α type, α + β type. The microstructure of dual-phase titanium alloy products is generally an equiaxed or dual-state structure with good comprehensive properties. Therefore, forging of near-α and α + β dual-phase titanium alloy products is generally selected below the transformation point of 30 to 60 ° C. Heating forging. A large amount of research and engineering practice have proved that with the increase of the forging heating temperature, the content of primary α isoaxes in the microstructure of duplex titanium alloys decreases significantly, while the content of α slats increases significantly. That is to say, when a two-phase titanium alloy is heated below the transformation point, as the heating temperature increases, the primary α isoaxes in the structure gradually transition to the β phase, resulting in the primary α isoaxes in the microstructure of the titanium alloy after heating forging. The content decreases, the morphology becomes smaller, and the content of α-slab increases. When the heating forging temperature exceeds the transformation point of the titanium alloy, all the primary α isoaxes in the dual-phase titanium alloy structure disappear, and it is a slat-like net basket structure or Weiss structure. .
The thermal conductivity of titanium is 0.036c a l / c m · s · ℃ (1cal / cm · s · ℃ = 418.68W / cm · K). At room temperature, it is 1/15 of aluminum and 1/5 of iron. During the forging of titanium alloy on the hammer, due to the large instantaneous deformation rate (deformation on the hammer of 7 ~ 9m / s) and high striking frequency, the internal flow stress of the alloy is too large, which consumes a large amount of mechanical energy and is converted into internal heat in a short time. The core deformation is larger than the surrounding and the heat dissipation conditions are poor, which causes the internal temperature of the billet to rise, and the temperature in the center of the maximum deformation degree is close to, or even exceeds the alloy transformation point, resulting in a sharp reduction of the primary α isoaxes in the final microstructure of the billet, or even all Disappeared, the tissue was transformed into very poor Weiss' tissue when the overheating was severe. After forging the above two typical two-phase titanium alloys, the primary α equiaxed content in the microstructure decreased sharply, and the α lath content increased correspondingly. The microstructure changed from the ideal equiaxed structure to the poorer Wei. The main reason is that the titanium alloy is caused by overheating during the transient and severe deformation of the titanium alloy.
In the forging deformation of titanium alloy, the central part is generally a severely deformed area, so the center is the region with the highest temperature rise. The temperature rise of the central part is used as the main basis for formulating the forging process. When using a forging hammer with a faster forging speed to forge a titanium alloy, the central thermal effect in the forging process must be considered, and the billet cannot be continuously hit. Titanium alloy forging is recommended to use a press or a fast forging machine under conditions. This type of forging equipment has a low blow speed, a low instantaneous strain rate during the forging process, and the deformation heat generated is not very obvious. At the same time, there is enough time to deform. Thermal diffusion will not cause a significant increase in instantaneous heart temperature.
Uneven organization
During microstructure observation of a batch of TC17 titanium alloy die forgings, it was found that a certain large block α phase (commonly referred to as a coarse α block) exists in the basket structure, as shown in FIG. 3. The TC17 titanium alloy die forging is produced by a sub-beta forging process (heating die forging at 40 ° C at the transformation point, and air cooling after forging). It is expected that the microstructure is a uniform basket structure.
This coarse alpha block is also called a large white block. Compared with the small normal alpha strips in the basket structure, it appears morphologically coarse and uneven, growing from grain boundaries into the grains, and rarely staggered. Its crystal interface It is relatively rough, uneven, and the crystal interface of normal α bars is smooth. Studies have shown that the microhardness of such coarse α-blocks is about 10% lower than that of normal α-bars, which reduces the plasticity and thermal stability of the alloy and affects the quality of forgings. Therefore, it is necessary to prevent this uneven structure in titanium alloys. During the smelting and solidification process of titanium alloy, due to the equilibrium distribution coefficient of various alloying elements ≠ 1, the α-stabilizing element enrichment and segregation occur at the grain boundary after solidification, so the α phase first precipitates at its enrichment and follows the crystal. The boundary grows into the crystal, thereby forming a large alpha block, and the segregation of the micro-area composition is the root cause of this uneven structure.
The segregation of crystals in the micro area is caused by the equilibrium distribution coefficient k0> 1 or k0 <1. The segregation formed by the different solute concentration in the crystallization area of the alloy belongs to normal segregation. Such segregation is difficult to completely avoid, but can be controlled by appropriate measures. On the one hand, it is controlled by improving and optimizing the ingot melting process parameters; on the other hand, it is improved and eliminated through appropriate forging processes. For the forging process, firstly, during the ingot open-cast forging, appropriate high temperature homogenization treatment is adopted, and the micro-intracrystalline dendrite segregation in the columnar structure area of the ingot is improved and eliminated by homogenizing annealing or deformation recrystallization; secondly in alloy Appropriate post-forging cooling methods are used to control the blanks and finished products to prevent coarse α-blocks from appearing in their microstructures. After sub-β forging of the above TC17 titanium alloy forging, the use of air cooling is the reason for the appearance of coarse α blocks. After forging, the cooling rate is slow, the degree of subcooling is low, and the nucleation rate is low. Therefore, the α phase has sufficient time to grow to form coarse α. Piece.
After sub-β forging, rapid cooling (water cooling or oil cooling) can significantly reduce or suppress the appearance of large α blocks, accelerate the cooling rate and increase the degree of subcooling, and can increase the nucleation rate of α phase, although the localization of alloying elements in the local area has the growth The condition of the coarse alpha block, but the alpha phase has not yet grown up and merged. The phase transition process of the entire organization has ended. Controlling the cooling rate can significantly change the morphology and distribution of the precipitated alpha phase. After forging, the crystal defects (dislocations, sub-crystals) and deformed structures with increased dislocation density are fixed to room temperature in whole or in part by water cooling or oil cooling, and a large number of crystal cores are added for recrystallization during subsequent heat treatment. During the heat treatment, the precipitation mechanism of β phase changed from the induced nucleation mechanism under air-cooled condition to the independent nucleation method, and small, chaotic, intertwined strip-shaped primary α and secondary α were obtained. This structure can significantly improve the alloy's synthesis. performance.
Hollow defect
A batch of φ70mm gauge TA7 titanium alloy bars was found to have excessive defect waves during ultrasonic inspection at the factory. Lateral low magnification inspection was performed after dissecting the defect location. A large number of "pits" were found on the low magnification after corrosion, mainly concentrated on the rod. No "pits" were found in the central area of the steel and outside the 1/4 radius of the bar. Subsequently, high magnification observation was carried out on the pit, and it was found to be an intergranular void defect. The microstructure photograph of the defect is shown in Fig. 4. Some studies suggest that the phenomenon of "pits" is related to corrosion, and the more obvious the phenomenon of "pits" with the increase of the corrosion time; others have suggested that the "pits" may be related to the higher content of Fe as an impurity element. However, the above viewpoint is difficult to explain the phenomenon of the existence of excess defect waves in ultrasonic detection and the phenomenon of voids found in high-power analysis.
A large number of engineering practices have proved that TA7 forging process performance is worse than other TC4, TC11 and other titanium alloys, cracking is more likely to occur during forging than other titanium alloys, and the crack propagation rate is faster. Metal materials such as titanium and aluminum alloys are liable to induce loosening when they undergo large strains (such as superplastic forming). Cavities and even fractures occur in TA7 titanium alloys. At high strain rates, the rheological stress of TA7 titanium alloys increases significantly compared to static conditions, but the plasticity decreases significantly. As the strain rate increases, the flow stress and strain increase, but there is a critical strain rate. Above the critical value, the material will occur. Fracture; when the strain rate reaches a critical value, an adiabatic shear band is generated in the material, and microcavities are formed in the band. Under the action of applied stress, the cavities gradually grow up and even form microcracks. Micro-cavities are always formed along the maximum shear deformation zone. This is because in localized deformation, the deformation in the maximum shear zone is intense and the temperature is high, which softens the material in the zone and becomes an ideal place for defects such as cracks and voids. In the forging process of TA7 bar, the deformation of the bar in the central area is the largest, the deformation heat diffusion is the slowest, and the deformation temperature is the highest. Therefore, voids are most likely to occur during the large deformation process.
Studies have shown that during the plastic deformation of metal materials, accompanied by changes in microstructure, there are mainly grain growth, equiaxed crystal growth, grain rotation and sliding, dislocation proliferation, dynamic recovery and recrystallization, and void nucleation and growth. Wait. Grain boundary slip is the main mechanism of plastic deformation. Grain boundary slip will cause local stress concentration and hinder the further occurrence of grain boundary slip. When stress concentration cannot be eliminated by dislocation movement, voids will nucleate and then grow. Big. Cavities are preferentially nucleated at the triangular grain boundaries. As the amount of deformation increases, the cavities begin to grow, and the cavities do not grow in an equiaxed state, but grow in an elliptical manner. The cavity is easy to diffuse to the grain boundary shared by the parallel tensile stress, so that a directional vacancy flow is formed in the direction of the tensile stress, which continuously gathers to the center of the cavity, so that the cavity can grow in parallel to the tensile direction. A large number of literatures mentioned that "pitting" and voids are prone to occur during the forging of this alloy. By analyzing the mechanism of "pitting" and hole-like defects in TA7 titanium alloys, we have concluded a set of measures to prevent voiding defects in TA7 titanium alloy forgings The effective method is to strictly control the deformation amount per fire ≤ 50%, and strictly control the deformation rate. It is best to use hydraulic or hydraulic forging, and try to avoid using hammer forging. It has achieved good results in production.
4. Conclusion
At present, common forging defects in titanium alloys are mainly overheating and unevenness, voids, cracks, etc. These defects are generally easy to find in the microstructure inspection or ultrasonic inspection of titanium alloy products, mainly in the process of forging titanium alloy products. The parameters are formed by improper control, so during the forging process, the appropriate deformation rate (forging equipment), heating forging temperature, pass deformation and cooling rate after forging need to be selected according to titanium alloy materials with different characteristics.