Titanium Alloy Forming Technology
Creep forming is one of the titanium alloy forming technologies. It means that at a certain temperature, after the metal sheet is deformed under the action of the tool and die to obtain the ideal shape, the temperature and load are kept constant, so that stress relaxation occurs inside the workpiece and elastic strain occurs. Transform into permanent plastic strain until the residual stress and springback are basically eliminated, and finally the ideal workpiece shape is obtained after cooling.
The driving force for creep during creep forming is the applied stress. As the creep progresses, the elastic strain decreases, so that the internal stress decreases accordingly, and the applied stress decreases accordingly. Some researchers pointed out that the thermal tensile creep process is a new forming process for thin-walled titanium alloy composites. The process uses heating methods such as electrical resistance heating to heat thin-walled metal sheets or profiles to thermoforming temperatures before stretching and bending. As the final shape is formed, the temperature remains constant and the material creeps in the tensile direction of the mold surface. This results in stress reduction and in-line stress relaxation within the formed workpiece. Residual stress is reduced, thereby reducing springback of parts and improving forming accuracy. The research status, process principle, key equipment, processing technology, advantages and disadvantages of the new process technology are introduced. Finally, the application prospects of hot drawing-creep composite forming technology are prospected.
Some researchers point out that titanium alloys are commonly used in aerospace applications, such as carrier 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, in the forming of titanium alloy profiles, hot draw bending creep forming technology is used to improve the forming performance and reduce springback. The principle of hot stretch bending and creep forming is that after the hot stretch bending stage, a stress relaxation stage is performed by keeping the workpiece with the mold for a selected dwell time. This has the advantages of low residual stresses and minimal springback, including cheap tooling and good repeatability. The Arrhenius model was used to characterize the creep behavior, and a finite element model of the thermal 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 rebound values are in good agreement with the experimental results. Some researchers pointed out that creep or stress relaxation is the main mechanism to reduce springback during hot forming of titanium alloy plates.
To date, the differences and connections between these two phenomena have not been explicitly explored. High-temperature short-term creep and stress relaxation tests were conducted 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 the creep behavior under low temperature and low stress is controlled by atomic diffusion, and the creep behavior under high temperature and high stress is controlled by dislocation slip and creep. The stress relaxation behavior is mainly controlled by dislocation climbing. The stress relaxation behavior predicted from the creep data agrees well with the experimental results.
