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                          Structure and hardness of industrial pure titanium processed by ECAP at room temperature

                          Industrial pure titanium belongs to the alpha alloy with a hexagonal close-packed (hcp) lattice structure. It has good corrosion resistance, mechanical properties and welding properties. It is widely used in aerospace, chemical industry, marine engineering and other fields. Through ECAP deformation, the crystal grains can be significantly refined, the structure is improved, and the mechanical properties of the material can be improved. Therefore, ECAP technology has received extensive attention from domestic and foreign scholars in recent decades. At present, there have been many reports in the literature that ECAP has successfully prepared ultra-fine-grained (UFG) materials, including face-centered cubic structure metals, body-centered cubic structure metals, and close-packed hexagonal structure metals.

                          Hardness is defined as the ability of the surface of a metal material to resist the intrusion of other hard objects, which can reflect the material's physical and mechanical properties such as elasticity, plastic strength, toughness, and wear resistance. Among them, the commonly used hardness index Vickers hardness is widely used in precision industry and material science research, especially to determine the hardness of metal film layers or surface layers after chemical treatment, as well as the hardness of smaller and thinner workpieces. There are many reports on the change of material hardness after ECAP technology processing, but because industrial pure titanium is a close-packed hexagonal structure metal, its slip system is less than that of cubic structure metal, and its plasticity is poor, so ECAP deformation is difficult. Most of the foreign studies are in It is carried out at high temperature, and it is rare for successful extrusion at room temperature. Researchers mainly study the changes in the structure and hardness of industrial pure titanium after one ECAP extrusion with different molds at room temperature, and analysis of its influencing factors will help improve the processing technology.

                          The experimental material is industrial pure titanium (TA1) in hot rolled state. Its chemical composition (mass fraction, %) is: 0.10O, 0.001H, 0.01N, 0.007C, 0.03Fe. The average grain size is 23μm, and it has an equiaxed single-phase structure. The tensile strength of the experimental material is 407MPa, the Vickers hardness is 1588MPa, and the elongation is 35%. Cut the hot-rolled sheet into ECAP specimens of 15mm×15mm×70mm, and use two channels with angles of Φ=90° and Φ=120°, and the outer fillet angles are all Ψ=20° molds, and perform one ECAP at room temperature. Deformed. When the mold Φ=90°, the equivalent strain of one cycle is about 1.08; when the mold Φ=120°, the equivalent strain of one cycle is about 0.635; before the experiment, the self-made compound lubricant is coated on the mold channel and the sample. The extrusion speed is 120mm/min. Research result:

                          (1) Successfully achieved one ECAP deformation of 90° and 120° mold industrial pure titanium at room temperature, and obtained lath-like structures elongated in one direction.

                          (2) The room temperature ECAP deformation of industrial pure titanium can significantly refine the grains and increase the hardness, and the hardness values after one extrusion with different molds (90° and 120°) have little difference.

                          (3) The hardness value of the upper and lower surfaces of the sample after 90° die extrusion is slightly lower than the hardness value in the middle of the sample, while the surface hardness distribution of the sample after 120° die extrusion is more uniform.

                          The hardness of the original CP-Ti sample and different molds after one ECAP deformation (MPa)

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