Defect-organized microstructure-stress synergy in laser in-situ forging additive manufacturing (LIF-AM): A pathway to fatigue-resistant Ti-6Al-4V

Laser powder bed fusion (LPBF) technology has achieved industrial application in the lightweight manufacturing of aerospace complex components due to its high dimensional accuracy and excellent static mechanical properties. However, the non-steady-state thermal effects during processing induce heterogeneous tensile stresses, coarse grains, and fusion defects, severely degrading the fatigue performance of as-built LPBF components and limiting their structural applications. In this paper, a novel technique, laser in-situ forging additive manufacturing (LIF-AM), is proposed to improve the fatigue endurance of metal by applying in-situ layer-by-layer femtosecond laser shock during the LPBF process. The results show that LIF-AM technology can reduce un-melted defects, refine the grain, and introduce a compressive residual stress field into Ti-6Al-4V alloy. The maximum defect size decreases from 65 μm in LPBF to 27 μm in LIF-AM due to the femtosecond laser surface cleaning. Under the action of the shock wave, β columnar grains transform into equiaxed grains, and a gradient compressive residual stress field with a depth of ∼800 μm forms, contributing to the dynamic recrystallization. Combined with reducing defects, the crack initiation and propagation are suppressed, causing the high fatigue limit in LIF-AM Ti-6Al-4V alloy which is 19.3 % higher than that of conventional LPBF Ti-6Al-4V alloy. The LIF-AM technology will provide a novel and transformative approach for the high-performance manufacturing of aerospace load-bearing components.