Cracking behaviour of high-strength AA2024 aluminium alloy produced by Laser Powder Bed Fusion

Most wrought aluminium alloys of the 2000 series are difficult to manufacture by laser powder bed fusion (L-PBF) due to the formation of cracks during building. To date, the effects of processing regimes on crack formation are still not well understood. In this study we performed a detailed microstructural characterisation of crack development in the AlCuMg alloy AA2024 to quantify the extent of cracking and porosity arising from a range of different process parameters. Two samples, produced with different build parameters, were selected for in-depth study by scanning and transmission electron microscopy; these had similar low levels of porosity but high (H - 2.6 ± 0.4 mm/mm2) and low (L - 1.5 ± 0.3 mm/mm2) crack densities respectively. Based on distinct morphological features and characteristic length, we differentiate hot cracks from solid state cracks. Hot tears form at high angle grain boundaries and are associated with micron-sized gas pores as well as intermetallic phases in both samples. The solidification and cracking behaviour are modelled with the aid of a Scheil-Gulliver model that includes solute trapping. This approach predicts differences in hot-crack-susceptibilities, due to different solidification velocities, in line with experimental observations. The sample H, of high crack density, also experiences the higher cooling rate, and hence strain rate, which contributes to the greater propagation of cold cracks in the low fracture toughness AA2024. The observation of cracking associated with microporosity and the use of a Scheil-based solidification model, including solute trapping, provide new insights into the complex problem of hot tearing during L-PBF.