This work investigates the fatigue strength of AISI 4140 steel with different porosity levels manufactured by laser - powder bed fusion and looks into the influence of hot isostatic pressing on defects and fatigue strength. A detailed investigation of microstructural phases, defect distributions, fatigue behaviour, fractography, and residual stress was conducted to study the dominant influencing factors on the fatigue strength of AISI 4140. Results show that the defect size could vary in a certain range without deteriorating the fatigue strength. Although the hot isostatic pressing heat treatment can improve fatigue strength, some remaining defects may reduce the densification efficiency.
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