Fine martensite and beta-grain variational effects on mechanical properties of Ti–6Al–4V while laser parameters change in laser powder bed fusion

In this study, several variations of Ti–6Al–4V β-grains and α′-martensites were observed while changing the combinations of laser parameters and keeping the energy density (ED) constant in the laser powder bed fusion (LPBF) process. Several combinations of laser power, scanning speed, and hatch spacing were considered, resulting in high product density between 99.3 % and 100.0 %. Accordingly, tensile specimens were fabricated to observe the above strategic fabrication and microstructural effects on tensile properties. At the same time, microhardness was also measured to observe the similarities. However, the size and shape of the β-grains differed significantly, while the scanning speed gradually decreased along with the laser power, with the shape changing from irregular to classically hexagonal and the size increasing sharply. Using the results of differential thermal analysis (DTA), it can be said that most of the tertiary and quaternary α′-martensites formed after the following few thermal cycles below 370 °C. Similarly, the α′-particles decomposed and formed β-particles due to thermal treatment at 370 °C. Therefore, a denser and higher number of tertiary and quaternary α′-martensites and a lower number of primary and secondary α′-martensites occurred, while the cooling rate decreased and the number of thermal cycles increased due to a lower scanning speed. These phenomena increased the hardness 370–395 HV and deteriorated the yield strength 1250-840 MPa. Changes in track overlap (hatch spacing) of 30–50 % affect mechanical and microstructural characteristics more than track overlap of 10–25 %. The columnar β-grains became longer and wider as the lane overlap increased from 30-50 %. At the same time, some β-grains also changed to contain finer and fewer α′-martensites. These factors reduced both hardness and tensile properties.