| dc.description.abstract | This study focuses on the structure after vacuum brazing using Ti-6Al-4V as the base material and TiCuNi as the filler material (metal foils with a thickness of 30 µm). The first part aims to establish the optimal combination of process parameters. Quality parameters, including tensile strength, fracture toughness, and microhardness, are analyzed using both single-objective and multi-objective optimization methods. The second part aims to establish fatigue crack growth rate data, investigating the influence of average stress and overload conditions. Through fatigue experiments under constant load amplitudes, the study obtains data on material fatigue constants and the effect of stress ratio. Utilizing the results from variable load amplitude fatigue experiments, various fatigue crack growth models are validated, including stress ratio effects, load interaction effects, and crack closure effects, to establish the optimal fatigue crack growth prediction model.
The research results indicate that the optimal parameter combination, targeting tensile strength, consists of a preheating temperature of 890°C, a preheating time of 60 minutes, a brazing temperature of 975°C, and a holding time of 45 minutes. Experimental validation yields a tensile strength of 1265 MPa, with a deviation of only 0.24% from the predicted value of 1262 MPa. When utilizing tensile strength, fracture toughness, and microhardness as quality objectives, the optimized parameter combination includes a preheating temperature of 890°C, a preheating time of 60 minutes; a brazing temperature of 1005°C, and a holding time of 15 minutes. Notably, when the overload load ratio exceeds 2.0, a significant fatigue crack growth delay effect is observed. With respect to the fracture surface observation of welded joints, those with lower tensile strength exhibit a fracture surface predominantly characterized by small cleavage facets. On the contrary, specimens with higher tensile strength display a fracture surface composed of ductile dimples and small cleavage facets, with increased density and compactness of the ductile dimples corresponding to higher strength levels. The application of overload stress induces a significant decrease in the crack propagation rate, revealing the applied stress location and the crack front at the time of fracture on the fracture surface. As the applied overload stress increases, the arcuate curve of the crack front becomes more pronounced on the fracture surface. In terms of fatigue crack growth models: (1) The rainflow cycle counting method is superior to the simple range method. (2) In the crack closure effect correction model: The predicted results of the Elber model are similar to the uncorrected Paris model and close to experimental values, while the Schijve model′s predicted life values are excessively small. (3) The fatigue crack growth prediction model closest to experimental values is the static failure mode. (4) The Willenborg model corrected with the plastic deformation zone yields worse results than the uncorrected model. In the verification of sequence effects, the High-to-Low (HtoL) transition, occurred in the early stages, induces significant tensile stresses, leading to an overload effect on the material. The residual stress field generated as a result of overloading diminishes the effective stress for subsequent loading, consequently reducing the crack propagation rate and enhancing the fatigue life. | en_US |