Multiscale microstructural heterogeneity and mechanical property scatter in Inconel 718 produced by directed energy deposition

Directed energy deposition (DED) is an additive manufacturing technique that enables rapid production and repair of metallic parts with flexible geometry. The complex nature of thermal and material transport during DED can yield unwanted microstructure heterogeneity, which causes scatter in parts performance. Here, we investigate microstructure variations at different length scales in Inconel 718 produced by powder-blown DED using different deposition rates. We quantify spatial trends in grain structure, texture, composition, and solidification structure within parts and correlate them with variations in hardness, yield strength, and Young's Modulus to highlight the effect of the thermal environment during solidification. We find that the high energy input employed when using high deposition rates is conducive to significant microstructure heterogeneity along both the build and transversal directions, which stems from the asymmetric cooling rates generated by the deposition strategy used. We also find that standard heat treatments employed on Inconel 718 are not suitable to homogenize the microstructure. These results have important implications for the development of industrially relevant build rate strategies for additively manufactured parts.Ministry of Education (MOE)Nanyang Technological UniversityNational Research Foundation (NRF)Published versionThis research was funded by the National Research Foundation (NRF) Singapore, under the NRF Fellowship program (NRFNRFF2018–05), and by STE Aerospace Pte. Ltd. YYC and EJ are supported by Nanyang Technological University Singapore (NTU) and the Ministry of Education of Singapore through an Industrial Postgraduate Programme (IPP) scholarship (M4062246) and the collaboration with the University of Strathclyde, UK, respectively

Repetitive operation of a dense plasma soft x-ray source for micromachining

This paper was presented at the 6th International Conference on Dense Z-Pinches, held in Oxford, United Kingdom from 25-28 Jul 2005The NX2 device, a low energy plasma focus, at the Nanyang Technological University in Singapore, was used as a soft X-ray (SXR) source for micromachining. The gas used was neon which produced SXRs in a narrow spectral range of 0.9 - 1.6 keV. The SXR yield from repetitive operation of the NX2 device was monitored and measured using a cost effective multi-channel SXR spectrometric system. The system consists of filtered BPX65 PIN diodes, with the associated electronics --- an integrator, sample and peak holder, analogue switch, an A/D converter and a microcontroller. The system enables easy shot-to-shot statistical analysis under repetitive operation at adjustable preset trigger frequencies. A total of 4000 shots were fired at 0.5 Hz, using the same gas filling. The SXR production was at an average yield of 60 J/shot and a maximum single-shot yield of more than 100 J. The SXRs emitted by the NX2 device was used for contact micromachining, producing structures with an excellent aspect ratio of up to 20:1 on 25 μm SU-8 resist.Published versio