High power laser powder bed fusion of Inconel 718 alloy: Effect of laser focus shift on formability, microstructure and mechanical properties

Abstract

In this study, a 1 kW high-power laser powder bed fusion (HP-LPBF) equipment was employed for the additive manufacturing of Inconel 718 (IN718) alloy, and the effects of laser focus shift (F s 0, +5, +10, +15, +20 mm) on the formability, microstructure and mechanical properties of the HP-LPBF samples were investigated. Results suggest that HP-LPBF is able to produce near full-dense samples with high build rates by applying larger scanning speeds and hatch spacings. The process limit can be further expanded when a suitable F s is adopted through its impact on the energy density, laser spot size and laser intensity distribution. As the increase of F s , the surface roughness of HP-LPBF sample decreases initially but increases subsequently. The corresponding melting mode transforms from keyhole to transition and then to conduction. Besides, the F s is found to be influential in the mechanical properties by changing the primary dendritic arm spacing, the fraction of Laves phase and the grain size. The optimal performance at the F s of +10 mm is comparable to the low-power laser powder bed fusion (LP-LPBF), whereas its maximal build rate (16 mm 3 /s) rises to around 3-12 times that of the latter. Moreover, the scaling law of energy density for LP-LPBF remains applicable for HP-LPBF when the focal plane is located at the build surface, but not if the focal plane is shifted away. Based on those findings, the potential efficiency and limitations of HP-LPBF with higher laser powers (≥2 kW) were briefly prospected.