Hot cracking evolution and formation mechanism in 2195 Al-Li alloy printed by laser powder bed fusion

Al-Li alloys printed by laser powder bed fusion (LPBF) have huge potential for industrial application due to their remarkable advantages. However, the high hot cracking susceptibility (HCS) due to the addition of Li is still a key restraining factor for their rapid development towards such industrial application. Here in this report, hot cracking behaviour of 2195 Al-Li alloys fabricated by LPBF process was investigated by a three-dimensional (3D) X-ray micro-tomography technique. The relationship between the microstructural evolution and the high HCS was established to reveal the hot cracking mechanism for the first time. Observations by X-ray tomography showed large, interconnected cracks with a 3D reticular structure in the printed samples, extending layer by layer from the lamellar cracks in the previous single tracks along the building direction. Contributions to hot cracking were found to origin from the stable liquid film and the stress concentration. The segregation between the Al6CuLi3 and alpha-Al matrix contributed to the Al-Cu eutectics along the high-angle grain boundaries in form of intergranular liquid films. Furthermore, it was found that the interfacial layer (intradendritic liquid film) between the Al2Cu and the adjacent LiAlSi or AlCuMgAg exhibited the reduced micro-crack resistance in the interior of the grains. It was pointed out on the basis of the calculation that the higher stability of the intergranular liquid film led to a higher HCS at the grain boundaries as compared to that in the interior of the grains. Furthermore, high internal residual tensile stress provided the driving force for the crack initiation and propagation. In summary, this work contains a practical guide to optimize the powder composition and processing steps of high-quality Al-Li alloys produced by LPBF.