Research on plastic forming of titanium alloy

In recent years, titanium alloys have been widely used due to their low density, high strength, good corrosion resistance, and good low-temperature performance. However, due to its poor cold plasticity, high resilience, and difficulty in processing, the current titanium alloy processing technology is mostly hot press forming. Since titanium alloys at high temperatures are prone to oxidation, wear and other problems, corresponding measures are also needed. High temperature resistant molds and heating equipment also require more costs in terms of operation. Therefore, studying the development status and future development trends of key technologies for plastic forming of titanium alloys is of great significance to improving the application level of titanium alloys.

⑴Isothermal forming technology

The isothermal forming process can effectively improve the plasticity and flow properties of the sheet, improve the uniformity of metal flow, and reduce deformation pressure. Some people have proposed using isothermal forming technology (a forming method in which the blank and the mold are heated to the deformation temperature, and the temperatures of the blank and the mold remain basically unchanged during the forming process) to manufacture sheet metal parts with titanium structures. The deformation temperature of titanium alloys It is very sensitive. For example, when the deformation temperature drops from 920°C to 820°C, the deformation resistance of titanium alloy almost doubles, and its superplastic deformation force is only about 1/30 to 1/10 of ordinary rolling. Among them, isothermal forming and superplastic forming are different, but the effect of isothermal forming on reducing the deformation resistance of the material and improving the plasticity of the material is not as significant as that of superplastic forming. The main advantage of superplastic pressure processing is that the material can achieve extreme deformation. However, many processes do not require 100% to 200% deformation. Usually the metal forging ratio is 5, which means that the deformation reaches 75%. In order to ensure high performance of parts, it is not always required to be optimal. Moreover, the coarse lamellar microstructure has better stability against fatigue crack expansion. Although isothermal forming technology can significantly improve the instability of materials, it is a one-step integrated forming technology, and it is difficult to guarantee that a good product will not have any local defects, such as forming local defects, etc. Once a product is formed in a local area If defects are found, the defect problem cannot be solved later, which will affect the quality of the entire titanium alloy product. This problem has also become one of the problems that need to be overcome in future technological development.

⑵ Creep forming technology

Creep deformation means that at a certain temperature, after the metal sheet is deformed under the action of the tool and mold to obtain the ideal shape, the temperature and load remain unchanged, so that stress relaxation occurs inside the workpiece, and the elastic strain changes to permanent plastic strain until the residual stress And springback is basically eliminated, and the ideal workpiece shape is obtained after final cooling. During the creep deformation process, the creep driving force is applied stress. As creep proceeds, the elastic strain decreases, resulting in a decrease in internal stress and a corresponding decrease in applied stress. Some researchers pointed out that the hot tensile creep process is a new type of thin-walled titanium alloy composite forming process. This process uses heating methods such as resistance heating to heat thin-walled metal sheets or profiles to the hot forming temperature and then stretch and bend them. As the final form is formed, the temperature remains constant and the material creeps in the tensile direction against the mold surface. This leads to a reduction in stress within the formed workpiece and online stress relaxation. The residual stress is reduced, thereby reducing the springback of the parts and improving the forming accuracy. The research status, process principles, key equipment, processing technology and advantages and disadvantages of new process technology are introduced. Finally, the application prospects of hot drawing-creep composite forming technology are prospected. Some researchers pointed out that titanium alloys are often used in aerospace applications, such as carrying airframes, due to their excellent mechanical and corrosion properties and relatively light weight. However, titanium alloys are notoriously difficult to form at room temperature. Therefore, the hot draw bending creep forming process is used in titanium alloy profile forming to improve formability and reduce springback. The principle of hot stretch bending and creep forming is to perform a stress relaxation phase by keeping the workpiece with the mold at a selected dwell time after the hot stretch bending phase. This allows for the benefits of low residual stress and minimal springback, including inexpensive tooling and good repeatability. The Arrhenius model was used to characterize the creep behavior, and a finite element model of the hot tensile bending creep deformation process was established in ABAQUS. Finite element simulation results show that the residual stress is greatly reduced during the stress relaxation stage, and low residual stress results in smaller springback. The predicted springback values are in good agreement with the experimental results. Some researchers pointed out that creep or stress relaxation is the main mechanism to reduce the hot forming springback of titanium alloy plates. So far, the differences and connections between these two phenomena have not been clearly explored. It conducted high-temperature short-term creep and stress relaxation tests on Ti6Al4V alloy. The microstructure of the alloy was observed using a transmission electron microscope. The effects of temperature, stress and time on creep and stress relaxation behavior were studied respectively. The correlations and differences between the two phenomena were compared based on the creep strain-time and strain rate-time relationships. The results show that atomic diffusion under low temperature and low stress controls the creep behavior, and dislocation slip and climb under high temperature and high stress control the creep behavior. The stress relaxation behavior is mainly controlled by dislocation creep. The stress relaxation behavior predicted from the creep data agrees well with the experimental results.

⑶ Springback control and pre-optimized precision plastic molding technology

① Controlling springback through various standards

Because titanium alloy materials have large deformation resistance, low elastic modulus, and strong anisotropy, controlling springback is of great significance in titanium plastic processing. It greatly affects the size and shape errors of the product. So far, we have made a lot of efforts to minimize the forming errors caused by springback. Finite element simulation combined with optimization techniques is the most commonly used method. An optimized method has been developed to reduce springback during cold stretch formation of TC1 aircraft coatings. In the optimization model, a mathematical formula of stress difference calculated by finite element is established as an indicator of rebound strength, instead of implicit rebound analysis, and a multi-island genetic algorithm is used (for genetic algorithms, the objective function is a multi-extreme value function, find the local optimal point by hypothesis,) to find the optimal loading parameters. The optimized design of process parameters effectively reduces the amount of springback and improves the forming accuracy. The research results provide guidance for springback control and technology in the sheet metal forming process. Someone proposed a finite element model based on the self-developed TA18 alloy (Ti-3AI-2.5V) numerical control (NC) rotating tube bending. Through multivariate step-by-step analysis, a quantitative relationship between the bending angle, material properties and springback angle was established. . Some scholars used Hill's anisotropy criterion to predict the elastic force of commercial pure titanium (CP-Ti) parts during the bending and forming process. Some scholars believe that the springback of TC4 bending rods depends on the size of the intermediate material zone that remains in the elastic state, which depends internally on processing and geometric parameters such as bending radius, bending angle, and diameter/thickness of the bending element. Some scholars use statistical methods to calculate the relationship between springback angle and forming parameters during the hydroforming process, which provides an effective method for tooling designers and technicians to shorten manufacturing lead time. Some scholars have studied the effect of temperature on springback compensation of CP-Ti sheets. The results show that as the temperature decreases, the resilience decreases significantly.

②Prefabricated parts and mold optimization methods

Prefabricated parts and mold optimization can greatly reduce body forming errors, which is of great significance to near-sheet forming. Reverse simulation technology is widely used in intervention molding design. Some scholars have proposed a stepwise reverse optimization method to optimize the initial billet of TA15 alloy. The study found that the selection of the correction surface used to determine the optimization object is the basis of optimization, and isometric offset determines the accuracy and reliability of optimization.

⑷ Defect control technology

Titanium alloys are prone to fracture during cold forming, such as TA18 titanium alloy rotational bending, TCI titanium alloy pure titanium stretch forming, single point incremental forming, etc. In the single-point incremental forming (SPIF) process of CP-Ti sheets, research results show that the thickness of the plastically formed workpiece of titanium alloy sheets decreases with increasing depth, avoiding fractures and excessive stretching during the forming process. The post-extension force will lead to the risk of fracture caused by cold stretching of TCI titanium alloy. Optimizing the force before and after stretching can avoid breakage. The velocity waveform of the strain rate cyclic superplasticity test of Ti-15-3 shows that fracture may occur where the deformation is uneven. Fracture can be avoided by using a reduction rate of 30% to 40% for the first time.

⑸Hot spinning technology

Spin forming refers to the rotational feed motion of one or more rotating wheels acting on the initial flat plate, gradually making the sheet material and the rotating mandrel fit together, and finally obtaining a hollow rotary body part with a relatively thin wall thickness. The forming process. Because the wheel is partially loaded during the spinning process, the load is significantly reduced compared to traditional sheet metal stamping. Spinning is a flexible sheet forming method that is suitable for producing final-formed parts of complex rotating parts, or near-finished parts, such as tapered parts, simple parts, etc. For light alloys that are difficult to form at room temperature, such as titanium alloys, magnesium alloys, etc., spinning needs to be performed under certain temperature conditions, which is called hot spinning. Because at the same temperature, different materials or the same material at different temperatures also have significant differences in their mechanical properties. Therefore, temperature control is very critical in hot spinning.

⑴ Mechanisms and rules in crystal structure evolution

The crystal structure of titanium alloys is formed due to continuous dislocation slip or the rotation of grain orientation of twins during deformation. The evolution of the deformation structure is highly sensitive to strain, temperature and deformation mode, which affects the subsequent microstructural evolution and corresponding mechanical properties of titanium alloys, such as strength, fatigue life, and corrosion resistance. The deformed structure is usually formed during cold forming and is affected by alloy composition, initial structure and processing parameters. Some scholars have studied the development of the sharp local structure of IMI834 alloy and found that this structure can greatly reduce the fatigue life. Some scholars conducted a series of compression tests to study the deformation structure evolution of CP-Ti at high temperatures. They found that both fine-grained and coarse-grained basal planes in the deformed samples tended to rotate from their initial orientation to an inclination angle of 45°. Some scholars have studied the effects of single aging, low-temperature, high-temperature double aging, aging heating rate and other processes on the texture evolution of Ti-10V-2Fe-3Al after thermal deformation. They observed that the initial phase structure evolves at small strains, while the a structure can be obtained at large deformations. Additionally, structures can be formed by recrystallization, which are known as recrystallized structures. Some scholars have found that changes in the texture evolution of CP-Ti during the recrystallization process are caused by secondary recrystallization. Some scholars have studied the evolution of the recrystallized structure of Ti-35Nb-7Zr-5Ta alloy during hot rolling. When the thickness decreased to more than 90%, they observed gradient structures. They believe that dynamic recrystallization caused by severe non-uniform deformation between the surface and center results in this type of structure.

⑵ Mechanisms and rules in morphological evolution

Microstructural morphology is sensitive to processing parameters such as temperature, strain, strain rate, strain path, and heat treatment route. Their combination is a typical morphology that affects the mechanical properties of titanium alloys. The volume fraction, particle size, and aspect ratio directly determine the microstructural morphology of titanium alloys. The transformed phase has good comprehensive properties and is widely used in aerospace, chemical processing, marine and maritime, transportation, and medicine. Furthermore, the microstructure has better duration and strength than the equiaxed microstructure, but its fatigue properties are inferior. Due to the large grain size of the phase, the integration of the interface, fracture toughness, duration and creep strength, etc., lead to extension in the bending direction and disperse the stress field around the fracture. However, due to the lack of confinement of the a phase, grain coarsening easily occurs, which may lead to disadvantages in tensile properties. Recently, some scholars have obtained a new three-mode microstructure, including about 15%, 50% to 60% lamellar and transformation matrix, showing high and low cycle fatigue performance, high creep fatigue interaction life, high fracture toughness and Approximate forging process for high service temperatures. Some scholars have studied the influence of strain distribution on microstructure morphology under the formation of near-local loads. They found that the microstructural morphology of TA15 titanium alloy changes with the degree of deformation and processing steps. Transformed particles with primary and lamellar phases are produced by small deformations during processing. And the aggregate transformation matrix with disordered lamellar phase is generated by the first large deformation. In the second step, the microstructural morphology of the transformed matrix and complete sphere were produced through moderate and large deformations, respectively.

⑶ Development of modeling methods

The internal state variable method uses a small number of internal state variables to describe its underlying phenomena, and has been widely used to simulate the microstructural evolution of titanium alloys during thermal processing. Some scholars have proposed a physics-based constitutive model to predict the flow stress and particle size changes of two-phase titanium alloys. In the model, it is assumed that the total stress consists of thermally activated stress and non-thermal stress, where the thermally activated stress is described by the Kock-Mecking model. Non-thermal stresses related to hardening effects are represented by two-parameter internal state variables, including dislocation density rate and grain size rate. The mixing rule and superposition theory are used to characterize the effect of phase a and tower phase flow stress. The predictions of this model are in good agreement with the experimental results of titanium alloys. Some scholars have also proposed a similar model for the two-phase TA6 alloy. Some scholars have proposed a model based on the evolution of two internal state variables, dislocation density and recrystallization fraction, to predict the changes in the grain size of the intermediate crystal phase during the thermal processing of TA15 alloy.

Crystal plasticity models can reflect physical mechanisms such as microscopic sliding and twinning, microscale inhomogeneous deformation, microstructure resistance deformation, and orientation evolution, and have thus been developed in depth and widely used. In this theory, rate-independent crystal plasticity (RICP) and rate-dependent crystal plasticity (RDCP) were proposed and developed respectively. The main problems in RICP numericalization are the non-uniqueness of the active slip system and the determination of the time-independent shear rate during the plastic deformation of single crystals. Some scholars have introduced a semi-implicit integration scheme to identify active slip systems before determining their shear rates and quantify the order in which a slip system becomes active. In the RDCP model, the problems caused by the RICP model are overcome by assuming that all slip systems are active. However, due to high-order nonlinear flow laws, serious numerical instability occurs in the integral of the RDCP model. The implicit algorithm for solving the RDCP model has good stability in solution. However, these schemes involve iteration at the local level to update stresses, and performing balancing globally requires significant computational effort. Therefore it can hardly be applied to simulate the three-dimensional formation process of thousands of elements. Therefore, an explicit algorithm is proposed to improve computational efficiency. Their work proved effective but requires further improvements for application to large deformations and complex loading conditions. When applying this theory to titanium alloys, there is another issue that needs to be addressed. Due to the hexagonal closest-packed structure of titanium alloys, sliding is the main deformation mode of α and β phases, while twinning is an optional mode of one phase. There are several methods to deal with the large number of new orientations generated by deformed twins, such as the main dual orientation (PTR) method, the volume fraction transfer (VFT) method and the full mesh method. Some scholars and others have reviewed modeling methods, problem processing methods and other applications.

The CA algorithm has been widely used in the modeling of microstructure evolution phenomena. Some scholars refer to cellular automata (C)ellular automata (CA) model and the DRX printing model were combined to simulate the microstructural evolution of TC4 alloy in positive field and field. They introduced changes in dislocation density calculated by the K-M model as integer states to relate mesoscopic structural features to actual processing conditions. In the CA model, important phenomena such as nucleation rate, growth kinetics, and the effects of processing parameters, as well as initial grain size, have been taken into account, enabling quantitative and topographic simulations of the growth kinetics and topology of each grain in the microstructure evolution. The predicted results of the flow stress curve shape, r-grain growth behavior and final microstructural morphology are very similar to the experimental results. Some scholars have simulated the static recrystallization of pure titanium during the cooling process through the CA method. They found that factors such as non-uniform deformation, non-uniform nucleation, etc. can lead to deviations in recrystallization kinetics from experimental observations. In order to introduce the non-uniform deformation gradient of each grain at the micro scale, some scholars coupled the CA model with the crystal plasticity finite element method (CPFEM) to simulate the evolution of the microstructure.

Titanium alloy has the advantages of low density, high strength, good corrosion resistance, high heat resistance, and good process performance. However, it has poor chemical reactivity with other materials at high temperatures and is very easy to absorb impurities such as hydrogen and oxygen. This property forces titanium alloys to be different from the traditional refining, melting and casting techniques, often even causing damage to the mold. If advanced titanium alloy plastic forming technology is applied, it can effectively reduce the forming force of the material, reduce the friction between the mold and the material, thereby improving the surface quality and dimensional accuracy of the parts, increasing the forming limit of the material, and improving the forming performance of the material. Etc. With further research on the plastic forming process of titanium alloys, by solving the problems of plastic forming of titanium alloys and improving the process performance of titanium alloys, the plastic forming technology of titanium alloys will become more mature, and titanium alloys will have a broader development and application space. . In recent years, titanium alloys have been widely used due to their low density, high strength, good corrosion resistance, and good low-temperature performance. However, due to its poor cold plasticity, high resilience, and difficulty in processing, the current titanium alloy processing technology is mostly hot press forming. Since titanium alloys at high temperatures are prone to oxidation, wear and other problems, corresponding measures are also needed. High temperature resistant molds and heating equipment also require more costs in terms of operation. Therefore, studying the development status and future development trends of key technologies for plastic forming of titanium alloys is of great significance to improving the application level of titanium alloys.