Effects of vector length on the melt pool morphology and grain structure characteristics of Alloy 718 in laser powder bed fusion

This study investigated the effects of vector length, including Short-vector (80 µm), Long-vector (4000 µm), Long-Short vector (4000 µm and 80 µm), and Incremental vector (ranging from 80 µm to 400 µm), on the melt pool morphology and grain structure characteristics of Alloy 718 during laser powder bed fusion (LPBF). The melt pool morphologies and microstructure characteristics at different vector lengths were examined via optical microscopy (OM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). It was shown that the melt pool depth for Short-vector printing (about 138 µm) exceeded that of Long-vector printing (about 85 µm). This was due to that the energy densities were not the same at different vector lengths when considering the actual laser motion. The slow scanning speed caused excessive energy input in short vectors, resulting in narrow and deep melt pools. As the vector length increased, the preferred < 001 > growth direction along the scanning direction was not significantly affected by vector length, whilst grains were likely to rotate around the scanning direction to < 011 > ⊥BD. In addition, with the increase in vector length, grain size correspondingly increased from less than 20 µm in Short-vector printing to over 50 µm in Long-vector printing. However, this increase in grain size was accompanied by a reduction in hardness, declining from approximately 347.6 ± 7.5 to 298.2 ± 4.4 HV. This is the first time in the literature to report the vector length effect on the grain structure characteristics in LPBF.