Additive manufacturing of nickel-base superalloy IN738LC by laser powder bed fusion

IN738LC belongs to the class of γ’-precipitation strengthened nickel-base superalloys which are used for load bearing parts in the hot section of gas turbines and have excellent mechanical properties at elevated temperatures. Main manufacturing process for these alloys is investment casting. Nowadays, laser powder bed fusion (LPBF) is one of the most important additive manufacturing (AM) technologies for metals. In comparison to casting, this layer-based manufacturing technology allows for a larger geometrical complexity in the part design while simultaneously reducing lead times due to the toolless manufacturing. Originally developed for the solidification conditions in casting processes, the high thermal gradients and rapid solidification during LPBF of superalloys can cause micro cracking during processing as well as macro cracking during post-heat treatment. Additionally, the resulting microstructure exhibits small grains with a preferred orientation in build direction, leading to anisotropic mechanical properties and especially a reduced creep strength in comparison to casting. The objectives in this work are (1) to prevent crack formation by reducing the process induced stresses through preheating up to temperatures of 1100°C, (2) to achieve an isotropic grain structure for isotropic mechanical properties by generation of a discontinuous solidification front through pulse-modulated laser radiation and (3) to consistently analyze the microstructure evolution along the processing route in order to evaluate its influence on the mechanical properties. This study is done on the example of IN738LC. For each processing condition (cw laser radiation with and without preheating, pulse-modulated laser radiation without preheating) process parameters are determined which allow to manufacture samples with a combined area fraction of pores and lack of fusions of 600°C) causes in-situ γ’ precipitation resulting in strain age cracking in the LPBF process. The extent of in-situ γ’ precipitation is sufficiently reduced at a temperature of 1050°C and, thus, allows to manufacture crack free samples at high preheating temperatures. A super solvus post-heat treatment at 1245°C is developed, which leads to recrystallization in samples with a sufficiently high dislocation density (residual stresses) via bulging of high angle grain boundaries. This recrystallization is homogenous in samples which with a low crystallographic texture (pulse-modulated) leading to an isotropic grain structure with isotropic mechanical properties. Inhomogeneous recrystallization is observed in the presence of a strong crystallographic texture (cw) leading to large elongated grains in build direction. Ultimately, these large grains allow for a similar creep strength in build direction as in cast parts