Research On The Mechanical Properties And Microstructure Of TC4/TA15 Joints Affected By Heat Treatment

The use of new materials and new structures of the aircraft is the key technologies and technical to improve the performance of aircraft structural, and is also the direction of design and development. The technology of electron beam welding titanium is widely used in the field of aerospace, especially in manufacturing the framework of aircraft and the load-bearing component of engines for its superiority.How to increase the stability of thick titanium alloy components welded by electron beam welding is one of the critical probeem. In this paper, two different titanium alloy materials, TC4/TA15, welded by the same parameters, but carried out by various of heat treatments. The trends were found out by investigating the changes of microstructure and mechanical properties that the heat treatment impacted on the microstructure and the mechanical properties (especially on the fatigue life).The results indicated that: the main microstructures of TC4 welded by EBW at the top and the roots of seam are martensite, the central microstructures are martensitee and clusterαstructures, the microstructure of HAZ areαphase and martensite. After the isothermal heat treatment, the seam microstructure is two-state structures which are high on the elongation and the fatigue performances; the structure of the base-metal is equiaxed structure. For TA15 EBW jionts, the higher annealing temperature, the fewer new-bornαand smallerα-axis, and the phaseαin the transitionβstructure is like the lens shape. With the annealing temperature increasing, the hardness and the intensity of TC4 reduced, but the toughness and the plastic improved. The strength of titanium joints welded by EBW was not lower than base metal, the tensile fracture was found on base metal. There was no split source on the surface of the tensile fracture plane, and the type of fracture was the micro- aggregation. The median fatigue life of TC4 / TA15 joints were significantly affected by heat treatment: On condition that the relative error was 5%, the degree of confidence was 90%, the median fatigue life of TC4 by the relief annealing exceed 24% compared to the welding joints, and the median fatigue life of TC4 by the isothermal annealing exceed 53% compared to the welding joints. The median fatigue life of TA15 joints by the relief annealing exceed 28% compared to the welding joints, but after the annealing heat treatment the median fatigue life of TA15 joints only improved 9% compared to welding joints. Welding defects (cold lap and pore) were the source of the fatigue fracture; most of the fatigue crack started from the surface if the size of defects too small.