Fraunhofer-Institut für Lasertechnik ILT
This contribution aims to give an insight into enhanced opportunities to tailor microstructures in metallic materials manufactured by Laser Powder Bed Fusion (LPBF) additive manufacturing process. Contrary to the majority of publications in LPBF research area, discrete energy deposition, established via pulsed wave (pw) laser emission, is used. Processing parameters (e.g. scanning speed, pulse duration, pulse frequency, pulse overlap) are adapted in a way, that, contrary to conventional LPBF manufacturing using continuous wave (cw) emission, discrete solidifaction mode is established for two exemplary alloys Inconel 718 and TiAl6V4, respectively. It can be demonstrated that by alternating the solidification mode from continuos to discrete solidification by pw emission, resulting solidification microstructures differ drastically from conventional LPBF solidification microstructures. Microstructural investigations via optical and scanning electron microscopy show, that epitaxial grain growth along the build direction which is commonly observed in cw-LPBF, is efficiently suppressed for both alloys when pw-emission is applied. Quasistatic tensile testing of specimen made of Inconel 718 in as built and heat treated (AMS 5662) condition reveals enhanced isotropy of static mechanic properties as well as increased values for yield strength and ultimate tensile strength compared to specimen manufactured by cw-LPBF which is assumed to be correlated with a more isotropic microstructure and suppressed epitaxial grain growth. For alloy TiAl6V4, similar results regarding epitaxial grain growth suppression are presented. Moreover, it can be demonstrated, that martensite lath orientation within finer and more globular grains is less texturized in as built condition compared to specimen manufactured by cw-LPBF. Specific challenges for manufacturing of specimen with a relative density > 99,5 %, that are correlated with increased keyhole instability due to discrete energy deposition in pw-LPBF are presented for alloy TiAl6V4. Conventional hot isostatic pressing (HIP) treatment is applied to alloy TiAl6V4 to demonstrate that microstructural differences between cw and pw manufactured microstructures are not affected or equalized by phase transformations during industrial applied post processing heat treatments.
Abstract
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