Research On High-energy Beam Rapid Prototyping Of TC4Titanium Alloy

TC4alloy (Ti-6A1-4V), as a α/β titanium alloy with medium strength and excellent comprehensive mechanical properties, has a great number of applications in the aerospace industry. But traditional processing methods in forming TC4alloy have disadvantages of high manufacturing cost, long manufacturing time and great technical difficulties. However, high-energy rapid prototyping, as a net shape forming technology, provides a convenient and efficient processing method for titanium alloys. In this thesis, the DLF and EBF TC4alloys experiments were investigated systematically to reveal the law of microstructure formation and development in high-energy rapid prototyping of TC4alloy. To improve the microstructure and comprehensive mechanical properties of the EBF TC4alloy, heat treatment and FSP were investigated. The main results are as follows:1. The macrostructures and microstructures of DLF TC4alloy and EBF TC4alloy have many similarities. In the aspect of macrostructures, they all exhibit a continuously columnar β cellular growth along the deposited direction and across several deposited layers, whose growth directions are highly influenced by the directions of heat flow. The base material has a limit influence on the grain growth of the DLF TC4alloy, only on the initial1-2layers near the base material. For the effect of the thermal cycles by high-energy beam, there are dark zones and bright zones along the deposited direction of the samples. The dark zones and bright zones are composed of "Basketwave" structure, and the dark zones transform to the bright zones by a phase coarsening in the contact zones between the dark zones and bright zones, because of the slow cooling rate induced by the thermal cycles, the microstructures have features of "Widmanstatten" structure.The mechanical properties of the DLF TC4alloy are as follows:hardness:335-370HV; tensile strength:935MPa; yield strength:860MPa; elongation:6.5%; which meet the standard of USA AMS499A.The mechanical properties of the EBF TC4alloy as are follows:hardness:310～352HV, tensile strength:845MPa, yield strength:770MPa, elongation:6.9%. The ductility of the EBF TC4alloy meets the USA AMS499A, but the strengths don’t meet this standard.2. After annealing treatment, the EBF TC4alloys’microstructures coarsen to some extent, and the mechanical properties are similar to that of as-deposited. After solution-aging, the EBF TC4alloys’ strengths increase significantly, but the ductility decrease. Under the solution-aging of960℃/1h WC+520℃/6h AC, the a lamellas split and globular, which result in a decrease in ductility, the mechanical properties are as follows:tensile strength:925MPa; yield strength:835MPa; elongation:5.0%.3. After friction stir processing, the EBF TC4alloys’microstructures are refined significantly, the strengths are highly increased and exceed the USA forging standard (ASTMB381-05), but the ductility are much lower than that standard. The heat input to the FSP processing zone can be changed by changing the processing parameters. Under100/25and150/50processing parameters, because of low heat input, the temperature of the FSP processing zones are under the β transforming temperature, which result in dynamic recovery and recrystallization, and the FSP processing zones are composed of fine equiaxed grains with low dislocation densities. Under200/75processing parameters, the heat input to the FSP processing zone increases, the temperature of the FSP processing zone is around the β transforming temperature, which results in fast growing of the β grains; after cooling, the FSP processing zones are composed of lamellar α+β and equiaxed a grains. Under300/100processing parameters, the heat input to the FSP processing zone is the highest, the temperature of the FSP processing zones is beyond the β transforming temperature, which results in fast growing of β grain, and β grain turn to lamellar α+β during the process of cooling, so the FSP processing zones are composed of lamellar α+β.