The application of grain refinement processing in titanium alloys

The refinement of grains by the large plastic deformation (SPD) method has attracted attention, and there have been many research reports in this regard. However, in practical applications, it still faces many difficulties in terms of cost and applicability to heat-treated alloys. Japanese researchers have developed a processing-heat treatment process (RMA-CREO, abbreviated as CREO) that can continuously carry out efficient production and refine grains. This method locally heats and cools the titanium rod in a high-frequency coil to form a local area with low deformation resistance. Torsional force is applied to the sample in this area, and the resulting shear deformation refines the grains. The refined structure is forcibly cooled by a water cooling device to prevent grain coarsening. Continuous fine-grained treatment can be carried out through continuous axial movement of heating torsional deformation. As long as a material can deform in the heating state, the CREO method can be used for grain refinement treatment. This method can be applied to metal materials such as aluminum, magnesium, steel, stainless steel, Ti-6Al-4V alloy, and copper alloy. As the CERO method is a non-contact process, it can apply strong strain at high temperatures followed by rapid cooling, making it an efficient heat treatment method. The previous SPD methods were often difficult to be applied to heat-treatable alloys, while the CREO method can carry out effective heat treatment to significantly improve the performance of the materials. The principles and effects of CREO processing can be roughly classified into the following three types.

The first is the grain refinement effect. That is, torsion causes shear deformation, which elongates the grains. This elongation deformation generates strain, thereby refining the grains. Due to the short-term heating, equiaxed fine recrystallized grains will be obtained after strain.

The second is to enhance the solid solution effect. That is, heating and the torsional strain of CREO cause the interatomic distance to increase, promoting solid solution and achieving solid solution strengthening.

The third is the control effect on the crystallization direction. That is, for HCP titanium alloys and others with strong anisotropy in the main slip plane (0001), the anisotropy of this material can be controlled by giving a certain texture at a given Angle through CREO treatment. Researchers confirmed through CREO treatment of pure titanium, Ti-6Al-4V, β titanium alloy, etc., that this method can refine the grains of titanium. If pure titanium undergoes CREO treatment, its grains can be refined from 20-100μm to 2-20μm. Researchers also forged 19mm diameter pure titanium rods treated with CREO into discs with a radius of approximately 90mm and a thickness of about 3mm. Studies show that forging forming can further refine the grains and improve their performance. To sum up, this RMA-CREO method is a practical processing-heat treatment process for grain refinement that can be expected for mass production. In the future, active practical discussions will be conducted on specific processed parts.