Interrupted fatigue testing with periodic tomography to monitor porosity defects in wire + arc additive manufactured Ti-6Al-4V

Porosity defects remain a challenge to the structural integrity of additive manufactured materials, particularly for parts under fatigue loading applications. Although the wire + arc additive manufactured Ti-6Al-4 V builds are typically fully dense, occurrences of isolated pores may not be completely avoided due to feedstock contamination. This study used contaminated wires to build the gauge section of fatigue specimens to purposely introduce spherical gas pores in the size range of 120–250 micrometres. Changes in the defect morphology were monitored via interrupted fatigue testing with periodic X-ray computed tomography (CT) scanning. Prior to specimen failure, the near surface pores grew by approximately a factor of 2 and tortuous fatigue cracks were initiated and propagated towards the nearest free surface. Elastic-plastic finite element analysis showed cyclic plastic deformation at the pore root as a result of stress concentration; consequently for an applied tension-tension cyclic stress (stress ratio 0.1), the local stress at the pore root became a tension-compression nature (local stress ratio −1.0). Fatigue life was predicted using the notch fatigue approach and validated with experimental test results.