Additive manufacturing of Ni-based Superalloys- an analysis of parameter and strategy driven properties in Electron Beam Melting Process

Abstract

Metal AM processes produce significantly rough surfaces as compared to wrought or machined components. Currently, the as-EBM surfaces are extremely rough and consequently, the as-built components could not be directly used as commercial products. The state-of-the-art research has focused primarily on deeply machined (DM) samples. The mechanical properties of the EBM components as reported thus are not real representative of the process. This can largely dispute the main merits of the EBM process: print on demand and low buy-to-fly ratio. Hence, the research here was designed to bridge this research gap and systematically analyze the properties of net-shaped (NS) and near-net-shaped (NNS) components.

This thesis is divided into three levels, including the fundamental, validation and optimization level. The fundamental research work is performed on the commercialized processing parameters from Arcam, for Inconel 718 (IN718). The NNS samples made with and without the commercial multi-spot contour are tested for their mechanical properties. These samples achieved smoother surfaces and better tensile behavior. Thus, they were later compared with the NS and DM samples which were made with the same parameter settings but varied machining depth. The purpose of this work was to test the NS and NNS samples and analyze the cause of the failure. It was found that the premature failure of these samples was linked to the porous surface and subsurface region. Therefore, a machining protocol was set up according to the depth of the porous region which can theoretically improve the tensile strength of the machined samples to a level comparable to wrought material. The findings in case of IN718 were also validated for another superalloy, Inconel 625(IN625). The results again showed that the machining requirement of the as-EBM samples with contour is more than 2 mm. Therefore, a novel contouring strategy was later developed, optimized and tested in order to eliminate the machining requirement. To do so, the continuous contour with different energy, overlapping ratio and sequence to the hatching region was tested. The high-energy continuous contour applied after hatching was found to be the optimal strategy, which was able to maintain a comparable surface condition as the multi-spot contour whilst it could generate a denser subsurface region. Accordingly, the NNS samples with such contour achieved comparable tensile properties as DM samples.

It should be noted that all these research works were focused on the vertical samples (along the building direction) to avoid high thermal stresses in horizontal direction that seriously limit deformation free manufacturing of the samples. To deal with the challenge of thermal stresses and deformation, the last part of this research was designed to assess the thermal stresses and strains of EBM manufactured IN625 samples. The effects of different processing parameters and scanning strategies were tested to provide a set of guidelines about how to produce the horizontal and longer components. The results showed that one could effectively minimize the thermal stress/strain and deformation of the parts using a bidirectional scanning pattern with proper layer rotations angles to deliver shorter scans in each layer.

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