Research on the Formability of High-Strength Titanium Alloy Sheets at Room Temperature

Titanium sheets are widely recognized for their high strength, light weight, and good structural rigidity. The high-strength titanium alloy Ti-6Al-4V can not only be used in the aerospace field, but is also an important candidate material for structural components in other industrial fields such as automobiles and chemical industries. The Ti-6Al-4V alloy sheets have very limited formability at room temperature, and the post-forming deformation has a large amount of rebound, which brings many problems to traditional stamping and pressure forming. Although the formability limit of Ti-6Al-4V alloy sheets increases at high temperatures and the rebound decreases, room temperature forming still has great advantages in cost savings. Rolling forming is a forming method that uses rotating rolls to gradually deform the metal billet to form the workpiece. It is suitable for forming structural components with high strength and limited formability and is increasingly applied in the automotive industry, mainly for forming ultra-high-strength steel, high-strength steel, etc. Due to the small rebound angle during the rolling forming process and the possibility of simple and feasible rebound compensation, rolling forming is an effective method for the room temperature forming of Ti-6Al-4V alloy sheets. Therefore, Ossama et al. conducted laboratory research on the forming and rebound behavior of 2 mm thick high-strength Ti-6Al-4V alloy sheets after annealing at 820 ℃ at room temperature.

The original microstructure of the Ti-6Al-4V alloy sheets selected in the experiment was composed of 93.86% equiaxed α phase and 6.14% β phase, with an average grain size of 1.3 μm ± 0.7 μm. The results of the room temperature tensile test showed that it had a large anisotropy, and when the test specimen was in the direction at a 45° angle to the rolling direction, the yield strength was the lowest, the elongation was higher, and when reaching the ultimate strength, the specimen would quickly break. The forming limit test was completed on a device equipped with a hemispherical punch, with the diameter of the hemispherical punch being 60 mm. An optical strain measurement system "Autogrid Vario" equipped with 4 advanced CCD cameras was used to record the complete deformation history of each specimen. By designing different specimen shapes, the deformation behavior of different strain paths was tested. The experiment found that all specimens suddenly fractured at the top of the hemispherical punch, without obvious necking phenomenon before fracture, indicating that the room temperature formability of this alloy is very limited. Comparative analysis was conducted on the deformation behavior of Ti-6Al-4V alloy sheets during room temperature bending and rolling forming. The results showed that the minimum bending radius of the pendulum folding bending test and the V-shaped mold bending test was 9 mm, while the minimum bending radius of the rolled forming was 7.51 mm, an increase of more than 15%. Rolling forming can form smaller radius sizes and has less rebound than simple bending forming. This is mainly because rolling forming is a cumulative deformation process with multiple steps, which can inhibit the growth of cracks and make the deformation of the material more sufficient than a single deformation. In addition, shape defects that often occur during the rolling of high-strength steel are relatively rare in the rolling forming of Ti-6Al-4V alloy. It can be seen that rolling forming is a very promising process scheme for room temperature forming of high-strength titanium alloy sheets for aerospace and automotive structures.