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Why do different pore structures affect the fatigue strength of titanium alloys?

Different pore shapes in titanium alloys can affect fatigue strength due to several factors:
• Stress Distribution: The shape of the pores influences the local stress distribution within the material. Different pore shapes can alter how stress is concentrated around the pores, affecting the likelihood of crack initiation and propagation.

• Melt Pool Conditions: Different pore structures can lead to variations in melt pool conditions during additive manufacturing. These variations affect the surface morphology and defect characteristics of the pore beams, which in turn influence the stress experienced by the porous structure and its fatigue life.

• Buckling and Bending: The cyclic deformation characteristics of porous materials are closely related to the coupling effect of buckling and bending components of the force applied to the unit pore beams. By adjusting the unit pore shape, the matching of these two components can be changed, improving the local stress distribution of the pore beam matrix. This reduces the accumulation of plastic deformation per cycle during cyclic deformation, retarding crack propagation within the pore beams and significantly improving fatigue strength.

• Relative Density and Geometry: The fatigue strength of porous titanium alloys with different pore shapes is related to the relative density and geometric shape of the structures.

• Surface Roughness and Defects: Variations in pore shapes can result in differences in surface roughness and the presence of defects, both of which can impact fatigue performance.

Fatigue sources tend to nucleate at defects such as unfused defects and pores. The larger the defect diameter and the closer it is to the surface, the more obvious the stress concentration, and the lower the fatigue life.

Optimizing the unit cell shape can enhance fatigue performance by altering the stress distribution and deformation behavior within the material.