Effect of stress relief and inherent strain-based pre-deformation on the geometric accuracy of stator vanes additively manufactured from inconel 718 using laser powder bed fusion

Abstract

Laser powder bed fusion (LPBF) enables additive manufacturing (AM) of complex three-dimensional objects. Inconel 718 is a widely used aerospace material and a material that has been well adopted for the LPBF process. However, when manufacturing geometrically complex Inconel 718 parts in the aerospace industry, some aspects of the LPBF processes still require optimisation. This is because high residual stresses arise during the LPBF process, which remain within the part, in turn affecting its geometric accuracy. Therefore, the paper presents the results of a multicriteria evaluation of the geometric accuracy of LPBF-made stator vanes from Inconel 718. The part represents a group of thin-walled, axisymmetric parts. The results are based on measurements made with the use of X-ray computed tomography (XCT). XCT was used as a coordinate measurements system. Measurement data was collected both before and after build plate cut-off. The parts were prepared in variants, and considered: the build plate cut-off (before and after cutting), two types of stress relief annealing (standard – 1065 °C/1 h and high-temperature – 1150 °C/6 h), and a pre-deformation simulated by the inherent strain method. Stress relief annealing, particularly high-temperature variants, ensures the relaxation of macro- and micro-stresses, thus minimising part distortion during the build plate cut-off. This results in a very high symmetry of the stator vanes (angular pitch deviation of ±0.01° and the chord angle deviation of ±0.1°). Stress relief before the cut-off allows a twofold reduction of the cut-off caused distortion. The LPBF process residual stresses simulation allows for developing a pre-deformed model and a threefold reduction of the global distortion of the parts and a total cumulative deviation of 98% of the part's surface below 0.169 mm. A critical aspect to the geometric accuracy of LPBF-made stator vanes is the trailing edge of the vanes. The presented study shows that a careful LPBF process design combined with pre- and post-process operations allows precise manufacturing of thin-walled, axisymmetric parts within the aerospace specified tolerances.