High power laser powder bed fusion of Ti6Al4V alloy: The control of defects, microstructure, and mechanical properties

Laser powder bed fusion (LPBF) is the most widely used metal additive manufacturing technology, but it still faces challenges in manufacturing efficiency. To address the issue, high-power LPBF (HP-LPBF) which employs kilowatt-level lasers emerges in recent years. In this study, a 4 kW Flat-top laser was used for the HP-LPBF of Ti6Al4V alloy. The samples in as-built state and annealed states (750 degrees C/2h, 850 degrees C/2h, 950 degrees C/2h, 1050 degrees C/2h) were investigated in terms of defects, microstructure, and mechanical properties. Results show that keyhole mode melting is avoided by adopting the Flat-top laser, so the relative density of as-built Ti6Al4V is generally positively correlated with the employed laser energy density. The minimum laser energy density to obtain the high-density (>= 99.9%) sample is 50.0 J/mm3, and the highest build rate is 288 cm3/h. The microstructure in asbuilt state exhibits a unique alternating pattern of "bright bands/dark bands". The dark bands consist of needleshaped alpha ' martensite, while the bright bands consist of needle-shaped alpha+(3, which results from the decomposition of alpha ' under the in-situ annealing effect of the 4 kW Flat-top laser. After annealing at 750 degrees C, most of the residual alpha ' decomposes into needle-shaped alpha+(3. As the annealing temperature increases from 750 degrees C to 1050 degrees C, the microstructure undergoes an evolution from needle-shaped alpha+(3 to lamellar alpha+(3 then to lamellar alpha+(3 with some globular alpha and finally to duplex alpha+(3. An optimum strength-ductility balance is achieved after annealing at 850 degrees C, both the tensile strength and the elongation exceed the standard values of Ti6Al4V forgings.