The Study Of Microstructure And Fatigue Propertie Of Titanium Alloy Joints After High-energy Beam Welding

Titanium alloy is an important aerospace structural material. Now more and more scholars have investigated Titanium alloy about its microstructure, common mechanical property and fatigue crack growth behavior after high energy beam welding. This dissertation focused on the characterization of the microstructures in welded joints and fatigue crack growth behavior of different regions of welded joints, and investigated the rule between microstructure and fatigue property, the change of microstructure and fatigue property of welded joints with different welding gap and fatigue crack growth behavior of fuse zone and heat affect zone.The important outcomes are as follows:①Perfected local gradient simulation technology, the method which used to describe the unhomogeneity of the microstructure of welded joins of TC4 alloy(Ti-6Al-4V). The unhomogeneity of the microstructure and microhardness and the relationship between the unhomogeneity of TC4 welding joints microstructure and the fatigue property had been studied. Gradient distributions about microstructure and microhardness of four shape TC4 welded joints show that, gradient distributions about length dimension of columnar grains and lateral microhardness have some regular pattern, while gradient distributions about width dimension of columnar grains and longitudinal microhardness have no regularity. Comprehensive analysis of gradient distributions about length dimension of columnar grains and lateral microhardness show that : The bell shaped weld joint has low unhomogeneity of microstructure , while the wedge shaped weld joint has high unhomogeneity of microstructure. And according to experiments , we have proved that the unhomogeneity of the microstructure and microhardness have some relationship with the fatigue property . ②Through fatigue fracture analysis, microstructure observation, studied the relationship between the weld gap and microstructure,fatigue property. With the increase of the welding gap, the width of heat affected zone decreased and the length of columnar crystal size gradually increased, while the size of intragranularα' martensite refined. Low-cycle fatigue test results showed that welding gap on the 3rd of 0.15mm sample welded joints have the highest low-cycle fatigue life, 0.20mm gap followed by. Through the analysis of fatigue fracture surface, indicated that higher sample life in the region near the source of fatigue the fatigue striation was more closed, and with little massive regions of different sizes, showing that the relatively good toughness.③Used the method of dynamic monitoring to observe fatigue cracks, the fatigue crack growth behavior of fuse zone and heat affect zone of TA15 is studied. Crack development in the weld zone is twists and turns, but smooth in the heat affected zone, and the crack tip plastic deformation is obvious in heat affected zone. The fatigue life of heat affected zone is significantly higher than that of the welded zone, that showed that the heat affected zone had the better anti-fatigue crack growth ability. Grain size and regional nature of the material of weld and heat affected zone affect the crack propagation path and fracture surface.