Due to a series of advantages such as high specific strength, good corrosion resistance and high temperature resistance, titanium alloys have been widely used in aerospace, naval and chemical industries.
Industrial pure titanium is a kind of metal structure material with high specific strength and good corrosion resistance, has high process plasticity and is capable of large deformation even under cold deformation conditions. In recent years, pure titanium foils have been used in an increasingly wide range of applications, such as plate heat exchangers, corrugated plates for applications in the electrolysis industry and titanium curtain wall panels.
Cold press forming is an important forming method for foils. For example, TA1 titanium plate heat exchanger sheets are generally formed by cold stamping, and the strip must have good expansion properties. Pure titanium foil has the following disadvantages in the process of stamping and forming: (1) large rebound, titanium's flexural strength is relatively high, the modulus of elasticity is small, so the elastic strain is high, and the rebound after forming is large; (2) plastic anisotropy is large, due to the fact that the titanium alloy strips and foils are rolled by the strip method, and it is not possible to change the direction of the strip in the process of strip production, so the strips and foils generally have significant mechanical property anisotropy phenomenon, especially the plasticity. The anisotropy will significantly affect the press forming, and usually the transverse plasticity is lower than the longitudinal direction, leading to press cracking.
In this paper, TA1 titanium alloy is taken as the test material, sliced in the process of strip production, and then cross-rolled to study the effect of cross-rolling on the organisation and properties of TA1 titanium alloy foil.
1 Test materials and methods
In this paper, TA1 titanium alloy as a test material, its composition is shown in Table 1. titanium sponge is used to prepare TA1 titanium alloy ingot by vacuum self-consumption melting, and then forged into a slab with a thickness of 155 mm, and then made into a thin slab by hot rolling, and finally cold rolled the thin slab into a foil with a thickness of 0.1 mm. In the process of cold rolling foil from thin slabs, two rolling methods are used: one is ordinary rolling, i.e., rolling using the strip method, which is not possible to change the direction of cross-rolling; the other is cross-rolling, i.e., cutting a portion of the flake specimen, changing the direction of rolling (rotating by 90 °) during the subsequent rolling process, and then rolling it again in the original direction to the final product. After rolling, the foils using normal rolling and cross rolling were annealed in a vacuum annealing furnace at 680, 700 and 720 °C for 1 h, respectively.
After completing the annealing, an optical microscope was used to observe the microstructure of the test foils, and a tensile machine was used to test the room temperature mechanical properties of the test foils. The tensile test was carried out according to GB/T 228.1-2010 standard.
Conclusion
(1) The microstructure of TAl titanium alloy specimens annealed by cross rolling and normal rolling both consisted of equiaxed α grains; comparing the two rolling methods, it can be found that the size of the grains is smaller and the organisation is more homogeneous after cross rolling.
(2) As the annealing temperature increases, the strength of the material gradually decreases and the plasticity gradually increases.
(3) After cross rolling, the strength of the specimen decreases and the plasticity increases, and the degree of strength and plasticity anisotropy decreases significantly; at the same time, the transverse flexural strength ratio decreases significantly.
(4) The grain size and yield strength follow the Hall-Petch relationship. After cross-rolling, the σ0 values are significantly lower and the K values are all positive; after ordinary rolling, the σ0 values are higher and the K values are all negative.
