Compressive fatigue properties of additive-manufactured Ti-6Al-4V cellular material with different porosities

Selective laser melting (SLM) is a novel additive manufacturing (AM) technique for producing cellular metallic materials with designed pore structures. SLM Ti-6Al-4V cellular alloys have been extensively studied for biomedical and other applications. The objective of this study was to investigate the effects of porosity (33 vol%, 50 vol%, and 84 vol%) on the compressive fatigue performance and fracture mechanism of an SLM Ti-6Al-4V cellular material with a new cuboctahedron COH-Z unit cell. Furthermore, the influences of hot isostatic pressing (HIP) at 1000 degrees C/150 MPa on the fatigue properties and fracture were also examined in this study. The results showed that the fatigue strain slowly accumulated when the stress cycle was below the 70% fatigue life. After the 70% fatigue life, the ratcheting strain rate (d epsilon/dN) gradually increased. A shear band along a direction inclined at similar to 45 degrees to the stress axis could be definitely identified at about 95% fatigue life, as demonstrated by the digital image correlation technique. With further increases in the stress cycle, the shear band was intensified and finally led to rapid fatigue failure. The HIP process did not alter the fracture behaviors of the SLM cellular alloy, although the fatigue life was much improved by several times. On the other hand, the fatigue endurance ratio of the SLM cellular alloy with 33 vol% was 0.5. Raising the porosity from 33 vol% to 84 vol% reduced the endurance ratio from 0.5 to 0.15 due mainly to the high notch sensitivity of alpha'-martensite in the SLM cellular alloy. The HIP treatment changed the microstructure from alpha'-martensite to lamellar alpha+beta phases and thus significantly improved the fatigue endurance performance. With the combination of the COH-Z unit cell and HIP treatment, the endurance ratios at 10(6) cycles of the Ti-6Al-4V cellular alloy with porosities of 33 vol% to 84 vol% were as high as 0.5. This superior endurance ratio is advantageous to the long-term application of SLM Ti-6Al-4V cellular alloy.