1. Forging thermal effect
After a certain grade of high-temperature titanium alloy ingot was forged on a fast forging machine, it was heated and forged into a φ 165mm bar in the α+β two-phase region for multiple times. After heat treatment, its macrostructure was observed to be a fuzzy crystal structure, and its microstructure was an equiaxed structure, which is an ideal equiaxed structure of α+β two-phase titanium alloy. The microstructure photo is shown in Figure 1a. After the φ 165 mm bar was sawed and cut, it was heated at 50°C below the phase transformation point and forged into a φ 110 mm × 110 mm square billet on a 30 kN hydraulic hammer. When the square billet was subsequently dissected and analyzed, it was found that the core was clear crystal. The microstructure photo is shown in Figure 1b. The microstructure is α lath + β rotation, which is a typical Widmanstatten structure with clear grain boundaries. α belongs to the overheated structure in titanium alloy. The semi-clear crystal is 20 to 30 mm away from the surface. The microstructure photo is shown in Figure 1c. The microstructure is α lath + α equiaxed + β rotation. The number of α equiaxed is rare, the number of α laths is large, and there are discontinuously distributed grain boundaries α; the range of 0 to 20 mm from the surface is fuzzy crystal.
A batch of φ80mm TC4 titanium alloy bars has a typical equiaxed α microstructure (see Figure 2a), and the primary α equiaxed content is more than 70%. After die forging on a heating hammer at 940℃ (alloy transformation point 995℃), the microstructure of the core of the die forging is shown in Figure 2b, and the primary α equiaxed content is only about 15%, which is caused by overheating of the forging temperature.
Titanium alloys deformed above the phase transition point (α+β/β transformation temperature) to obtain basketweave structures or Widmanstatten structures with poor plasticity and fatigue properties. Therefore, most titanium alloy product technical standards require that the microstructures of nearly α-type and α+β-type dual-phase titanium alloy products are generally equiaxed structures or dual-state structures with good comprehensive performance. Therefore, nearly α-type and α+β-type dual-phase titanium alloy products are generally heated and forged at 30-60°C below the phase transition point. A large number of studies and engineering practices have shown that with the increase of forging heating temperature, the content of primary α equiaxed in the microstructure of dual-phase titanium alloys is significantly reduced, while the content of α laths is significantly increased. That is to say, when the duplex titanium alloy is heated below the phase transformation point, as the heating temperature increases, the primary α equiaxed phase in the structure gradually transforms to the β phase, resulting in a decrease in the primary α equiaxed content, a smaller morphology, and an increase in the α lath content in the microstructure of the titanium alloy after heating and forging. When the heating and forging temperature exceeds the phase transformation point of the titanium alloy, the primary α equiaxed phase in the duplex titanium alloy structure disappears completely, and becomes a lath-like basketweave structure or Widmanstatten structure.
The thermal conductivity of titanium is 0.036cal/cm·s·℃ (1cal/cm·s·℃=418.68W/cm·K), which is 1/15 of aluminum and 1/5 of iron at room temperature. During the forging process of titanium alloy on the hammer, due to the high instantaneous deformation rate (7~9m/s on the hammer) and high frequency of striking, the internal flow stress of the alloy is too large, and a large amount of mechanical energy is consumed and converted into internal heat in a short time. Since the deformation of the core of the billet is larger than that of the surrounding area and the heat dissipation conditions are poor, the temperature inside the billet rises and the temperature of the center area with the maximum deformation degree approaches or even exceeds the alloy phase transition point, resulting in a sharp decrease or even complete disappearance of the primary α equiaxed microstructure in the center of the final billet. When overheating is serious, the organization is transformed into a very poor performance Widmanstatten organization. After the above two typical dual-phase titanium alloys are forged by hammer, the primary α equiaxed content in their microstructures decreases sharply, and the α lath content increases accordingly. The microstructure is transformed from an ideal equiaxed structure to a poor Widmanstatten structure. The main reason is that the titanium alloy is overheated during the instantaneous and severe deformation process.
During forging deformation of titanium alloy, the center is generally the violent deformation zone, so the center is the area with the highest temperature rise. The temperature rise in the center is used as the main basis for formulating the forging process. When forging titanium alloy with a forging hammer with a faster forging speed, the central heat effect during the forging process must be considered, and the blank cannot be hit continuously. It is recommended to use a press or a fast forging machine for titanium alloy forging if conditions permit. This type of forging equipment has a low striking speed, and the instantaneous strain rate of the blank during forging is low. The deformation heat generated is not very obvious. At the same time, there is enough time for the deformation heat to diffuse, which will not cause a significant increase in the instantaneous core temperature.
2. Uneven organization
When observing the microstructure of a batch of TC17 titanium alloy die forgings, it was found that there were certain large blocky α phases (commonly known as coarse α blocks) in the basket structure, see Figure 3. The TC17 titanium alloy die forgings were produced using a sub-β forging process (heating die forging at 40°C above the phase transition point, air cooling after forging), and the microstructure was expected to be a uniform basket structure.
This kind of coarse α block is also called big white block. Compared with the fine normal α strip in the basket structure, it is coarse and uneven in shape. It grows from the grain boundary to the grain, rarely interlacing, and its crystal interface is relatively rough and uneven, while the crystal interface of the normal α strip is relatively smooth. Studies have shown that the microhardness of this coarse α block is about 10% lower than that of the normal α strip, which causes the alloy plasticity and thermal stability to decrease, affecting the quality of forgings, so it is necessary to prevent this kind of uneven structure in titanium alloy. During the melting and solidification process of titanium alloy, due to the equilibrium distribution coefficient of various alloy elements ≠ 1, there is enrichment and segregation of α stabilizing elements at the post-solidification grain boundaries, so the α phase precipitates first at its enrichment and grows along the grain boundary to the grain, thus forming a coarse α block. Micro-region component segregation is the fundamental reason for the generation of this uneven structure.
Micro-region component crystallization segregation is caused by the equilibrium distribution coefficient k0>1 or k0<1. The segregation formed by different solute concentrations in the successive crystallization regions of the alloy is normal segregation. This segregation is difficult to avoid completely, but it can be controlled by appropriate measures. On the one hand, it can be controlled by improving and optimizing the ingot smelting process parameters, and on the other hand, it can be improved and eliminated by appropriate forging processes. In terms of forging process, firstly, during the forging of the ingot, appropriate high-temperature homogenization treatment is used. The microscopic intracrystalline dendrite segregation in the columnar structure area of the ingot is improved and eliminated by homogenization annealing or deformation recrystallization; secondly, during the die forging process of the alloy billet and the finished product, appropriate post-forging cooling methods are used to control it and inhibit the appearance of coarse α blocks in its microstructure. The use of air cooling after the sub-β die forging of the above-mentioned TC17 titanium alloy forgings is the cause of the appearance of coarse α blocks. The post-forging cooling rate is slow, the undercooling degree is small, and the nucleation rate is low, so the α phase has enough time to grow and form coarse α blocks.
Rapid cooling (water cooling or oil cooling) after sub-β forging can significantly reduce or inhibit the appearance of coarse α blocks. Accelerating the cooling rate and increasing the degree of supercooling can increase the nucleation rate of α phase. Although there is alloy element segregation in the local area, which has the conditions for the growth of coarse α blocks, the α phase has not had time to grow and merge, and the phase transformation process of the entire organization has ended. Controlling the cooling rate can significantly change the morphology and distribution of the precipitated α phase. Water cooling or oil cooling after forging fixes all or part of the crystal defects (dislocations, subgrains) and deformed organizations with increased dislocation density generated by forging to room temperature, adding a large number of crystal nuclei for recrystallization in the subsequent heat treatment process. In the subsequent heat treatment, the precipitation mechanism of the β phase changes from the induced nucleation mechanism under air cooling conditions to an independent nucleation mode, and small, chaotic, interwoven strips of primary α and secondary α are obtained. This organization can significantly improve the comprehensive properties of the alloy.
3. Cavity-type defects
During the factory ultrasonic inspection of a batch of φ 70mm TA7 titanium alloy bars, excessive defect waves were found. After dissecting the defect location, a low-power horizontal inspection was conducted. After corrosion, a large number of "pits" were found on the low-power inspection, mainly concentrated in the center area of the bar, and no "pits" were found outside the 1/4 radius of the bar. Subsequently, the pits were observed at high power and found to be intergranular void defects. The microstructure photos of the defects are shown in Figure 4. Some studies believe that the "pit" phenomenon is related to corrosion, and the "pit" phenomenon becomes more obvious as the corrosion time increases; some studies believe that the "pit" may be related to the high content of impurity element Fe. However, the above viewpoints are difficult to explain the phenomenon of excessive defect waves in ultrasonic testing and the void phenomenon found in high-power analysis.
A large number of engineering practices have proved that the forging process performance of TA7 is worse than that of other titanium alloys such as TC4 and TC11. It is more prone to cracking during the forging process than other titanium alloys, and the crack propagation rate is faster. Metal materials such as titanium and aluminum alloys are prone to induce looseness, voids, and even fractures when undergoing large strains (such as superplastic forming). The voids in TA7 titanium alloy are induced by large strains. Under high strain rates, the flow stress of TA7 titanium alloy increases significantly compared to static conditions, but the plasticity decreases significantly; as the strain rate increases, the flow stress strain increases, but there is a critical strain rate. When the critical value is exceeded, the material will fracture; when the strain rate reaches the critical value, an adiabatic shear band is generated in the material, and micro-voids are formed in the band. Under the action of external stress, the voids gradually gather and grow and even form micro-cracks. Microvoids are always formed along the maximum shear deformation band. This is because in localized deformation, the maximum shear band deforms violently, resulting in a high temperature, which softens the material in the band and becomes an ideal place for defects such as cracks and voids. During the forging process, the deformation of the central area of the TA7 bar is the largest, the deformation heat diffusion is the slowest, and the deformation temperature is the highest. Therefore, voids are most likely to appear during large deformation.
Studies have shown that the plastic deformation of metal materials is accompanied by changes in organizational morphology, mainly grain growth, equiaxed grain elongation, grain rotation and sliding, dislocation proliferation, dynamic recovery and recrystallization, and void nucleation and growth. Grain boundary sliding is the main mechanism of plastic deformation. Grain boundary sliding can cause local stress concentration and hinder the further occurrence of grain boundary sliding. When the stress concentration cannot be eliminated by dislocation movement, the void will nucleate and then grow. The void preferentially nucleates at the triangular grain boundary. As the deformation increases, the void begins to grow, and the void does not grow in an equiaxed state, but grows in an elliptical manner. The voids are easy to diffuse to the grain boundaries that share parallel tensile stress, thus forming a directional vacancy flow in the direction of tensile stress, which continuously gathers toward the center of the void, allowing the void to grow parallel to the tensile direction. A large number of literatures mention that "pitting" and voids are prone to appear during the forging process of this alloy. Through the analysis of the formation mechanism of "pitting" and void defects of TA7 titanium alloy, we have summarized a set of effective methods to prevent void defects in TA7 titanium alloy forgings, which is to strictly control the deformation amount of each fire to ≤50%, strictly control the deformation rate, and preferably use hydraulic or hydraulic press forging, and try to avoid hammer forging, which has achieved good results in production.
4. Conclusion
At present, the common forging defects in titanium alloys mainly include overheating and uneven structure, voids, cracks, etc. These defects are generally easy to find in the microstructure inspection or ultrasonic testing of titanium alloy products. They are mainly caused by improper control of process parameters during the forging process of titanium alloy products. Therefore, in the forging process, it is necessary to select appropriate deformation rate (forging equipment), heating forging temperature, deformation amount per pass and cooling speed after forging according to the different characteristics of titanium alloy materials.
