Role of scanning strategy in defect formation for Ti–6Al–4V made via laser-based powder bed fusion: cross hatch versus island scanning

Island scanning is widely used in Laser-based Powder Bed Fusion; however, its influence on porosity formation compared with conventional cross-hatching has not been sufficiently investigated, particularly under varying hatch spacing. In this work, Ti6Al4V components were fabricated using crosshatch and island scanning strategies over a range of hatch spacing factors (A1 = 0.5–0.9), intentionally covering a regime associated with lack-of-fusion defects. Porosity distribution and microstructures were characterised using optical microscopy, scanning electron microscopy, and X-ray computed tomography, while Vickers hardness testing evaluated the mechanical response to porosity. Both strategies exhibited characteristic necklace-like porosity near boundaries; however, island scanned components showed substantially higher overall porosity, approximately five times that of crosshatch components, due to persistent island-boundary-related pores. Minimum porosity occurred at A1 = 0.6–0.7 for both strategies, whereas overheating-related irregular pores dominated at A1 = 0.5 and lack-of-fusion defects became prevalent at A1 = 0.8–0.9. Island scanning also showed a higher probability of extra-large pores (≥100 μm), which were absent in crosshatch components. Hardness decreased by approximately 10–20 HV at higher A1 values, consistent with increased defect content. Linear regression confirmed a negative correlation between porosity and hardness, with a stronger dependence for island scanning (R² = 0.69, p = 0.00012) than for crosshatch scanning (R² = 0.30, p = 0.034). Using the melt pool geometry criterion from literature, a target processing zone was identified, where a Tang-Pistorius-Beuth inequality range of 0.4–0.75 provides optimal conditions for defect reduction in crosshatch scanning, but does not apply to island scanning.