Study On The Effects Of Electropulsing On Microstructures And Properties Of Titanium Alloys Sheet

Titanium alloys are widely used in aeronautics and astronautics applications taking advantage of their high specific strength, superior corrosion resistance, and stable performance at moderate temperatures. However, it is difficult that titanium alloy sheets are cold formed, so must be formed by heating. Titanium alloys have high chemical activity, the oxidation layer and the gas absorbing layer are formed on the metals surface when titanium alloy sheet is heated, and grain grows up easily during heating process, which lead to notably decrease in plasticity and increase in strength and hardness, and tend to brittle failure. There is a pressing need to seek a method to improve the formability of titanium alloy sheets. In the recent years, many researches indicate that elegtropulsing is an effective method for improving the microstructures of material and its properties. The objective of this study is to explore the effect of the electropulsing on microstructures and properties of titanium alloys with the ultimate aim of improving its formability.Firstly, annealed TC4 sheet is secleted as experimental material to investigate the effect of high density electropulsing on the damage for titanium alloys sheet. The specimens are cut from TC4 sheet along the rolling direction, are stretched on an MTS 810 test machine to produce 8% of pre-plastic deformation, and then unloaded for the electropulsing. Recovery degree of plasticity for titanium alloy sheet is studied using uniaxial tension tests. The effect of electropulsing on damage evolution of sheet metal is also investigated. Three specimens are mechanical polished before stretching so as to observe the morphology of microcracks and voids in the surface for deformed specimens using scanning electron microscopy (SEM). After electropulsing treatment, they are again observed by scanning electron microscopy (SEM). The optical microscope (OM) is used to examine the changes of the microstructure before and after the electropulsing. Experimental results indicate that the plasticity of the specimens with 8% of pre-plastic deformation is nearly recovered completely by electropulsing treatment under the certain condition. Microcracks resulted from plastic deformation are healed, the local recrystallization occurs in the sheet metal after the electropulsing treatment, and the forming limit of titanium sheet is increased.Secondly, a lot of dislocation is produced during cold rolling, which will increase the work hardening and reduce the workability of the materials. Therefore, the effect of the electropulsing on microstructures and mechanical properties of cold-rolled TA15 and TA1-A sheet are studied. The microstructure morphologies, fracture surfaces, dislocation and substructures of cold-rolled, annealed and the electropulsed specimens are observed using optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscope (TEM); mechanical properties are determined by uniaxial tensile test.Experimental results show that the electropulsing gave a significant increase in elongation of cold-rolled titanium alloy sheet, but tensile strength is not decreased. For TA15 sheet, dislocation tangles are decreased and fine recrystallization grain is formed after electropulsing, so mechanical properties are improved. The electropulsing can induce formation of lamellar microstructures, which lead to that strength of the electropulsed CP Ti is higher than that of the conventional annealed CP Ti. The electropulsing can provide a special and effective approach to enhance the strength of the pure titanium and titanium alloys sheet and to retain the required high ductility at the same time.Thirdly, the dislocation not only plays an important role in plastic deformation, but also influences strongly in recrystallization and microcrack initiation processes of real materials. However, there currently exist few reports about dislocation mobility in the as-electropulsed materials. A further purpose of this paper is to investigate the effect of the electropulsing on dislocation mobility. The internal friction and elastic modulus are measured by dynamic mechanical analyzer (DMA). Experimental results indicate that the damping peaks of the electropulsed CP Ti sheet shifts to lower temperatures compared with the conventional annealed CP Ti sheet. It is demonstrated that the electropulsing treatment can decrease dislocation tangle and enhance dislocation mobility.Finally, it being the fact that titanium aluminides are considered as the most promising lightmass high-temperature structural materials, using electropulsing to refine TiAl alloys grain is explored in this paper. The specimens cut from as- cast TiAl alloy are treated by high density electropulsing. Microstructures morphologies and grain size are observed using optical microscopy (OM) befoe and after electropulsing. The mechanism of refinement of TiAl alloy is discussed.