Influence of case and transition zone microstructures on tensile properties in laser powder bed fused and heat-treated Ti-5553 parts

In this work, laser powder bed fused Ti-5553 parts were characterized after solution treatment at 800 degrees C or 900 degrees C followed by aging at 500 to 700 degrees C for 2 h. Depending upon the temperature and duration of the aging treatment, a 20 to 50 mu m alpha case along with a 200 to 250 mu m thick transition zone formed, comprising coarse alpha rich grain boundaries and detrimental to the tensile performance. Four different microstructurally distinct regions were characterized within the peripheral alpha rich regions. Microhardness measurements revealed a hard and brittle outer case whose depth approximately matched the alpha case and the transition zone identified earlier. Electron backscatter diffraction exposed the significance of grain boundaries in controlling the alpha case region frontiers. During uniaxial tensile testing of the heat treated specimens with the peripheral alpha case regions, the specimens prematurely fractured at ultimate tensile strength (UTS) values ranging from 620 to 858 MPa. Fracture surfaces revealed a bi-modal fracture mode, exposing a brittle failure along an outer ring whose thickness was comparable with the combined thickness of the alpha case and transition zone. Away from the outer ring, the fracture was predominantly ductile in nature. Removing the alpha case and the transition zone by surface machining after heat treatment substantially improved the UTS by similar to 200% while maintaining an acceptable ductility. The samples aged at 600 degrees C for 2 h registered the best set of tensile properties with a UTS of 1325 +/- 19 MPa and a fracture strain of 12.7 +/- 0.8%.