Design of metallic powders for additive manufacturing From scrap to resource

Daniel Cardenas del Rio

Research output: Book/ReportPh.D. thesis

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Abstract

The increased interest in powder-based metal additive manufacturing (MAM) has led to the need for novel feedstocks tailored to specific printing conditions in order to produce components with enhanced properties. The current feedstock used in additive manufacturing (AM) is powder which are not optimized for the distinctive characteristics of 3D printing. Consequently, understanding the relationship between the properties of the powder and the printed component, is essential to increase the overall quality of printed components and optimize the final product.

The present work focuses on the refurbishment of a 30 year old laboratory-scale vacuum induction gas atomizer to produce customized metal powders optimized for the MAM process. The primary objective of this study is to develop a system that allows users to explore diverse atomizing process conditions, thereby contributing to the development of metal powders and enhancing the performance of metal 3D printed components. The possibility of using scrap as raw material for the gas atomization process is also analyzed. The equipment was initially refurbished and made operational at Campus Risø, and later relocated to Lynby as part of the establishment of the new powder laboratory facilities.

Due to its high strength and corrosion resistance properties, the 316L stainless steel alloy is one of the preferred alloys used in MAM. It is used in this work as a benchmark material due to its wide range of applications in different industries, with the aim of improving its corrosion resistance properties. In order to achieve this goal, alloy development was carried out. Although the process conditions of MAM are different from those of conventional manufacturing, some of the problems observed during the welding processes may be comparable to those expected for printing. The segregation observed during solidification usually promotes the precipitation of phases that are detrimental to corrosion resistance properties. To address this, the effect of the addition of Nb and N2 as austenite stabilizing elements on phase stability and phase formation has been studied using thermodynamic simulations. CLPHAD (Thermo-Calc) and Phase Field (MICRESS) software are used in this work. The simulation results of phase fraction and phase stability of the new alloys are presented and compared using different compositions and process parameters. The proposed alloys have a chemical composition that meets the requirements to be considered as a modified 316L, with the purpose of having a quick material approval.

The size and shape of the metal powder particles are key parameters to pursue a stable printing process. This study evaluates various experimental methods for characterizing metal powders, aiming to propose a robust measurement trategy
to quantify particle size distribution and morphology relevant to metal MAM. Laser diffraction (LD), scanning electron microscopy (SEM), and high-resolution X-ray computed tomography (XCT) are employed to analyze commercial stainless steel 316L powders. The findings highlight the significant impact of both the powder sample and sample preparation on the obtained results. Moreover, the post-processing route applied in imaging methods, significantly affects the characterization. Proper sample preparation is essential in overcoming the post-processing challenges. It is shown that 2D and 3D methods generally lead to the same result when the 2D data are properly transformed to 3D space. Guidelines to overcome the shortcomings of the different techniques are suggested.

The refurbished equipment was used to atomize scrap-based material and powders with compositions similar to the simulated alloys. By atomizing under a 1.2 bar of nitrogen atmosphere, levels up to 0.3 wt% of N2 were achieved, and the addition of Nb proved to increases the nitrogen solubility. The importance of controlling the process conditions during gas atomization had shown to be highly relevant, as the powders are susceptible to contamination and oxygen pick up. It is possible to use scrap as raw material in the form of sheets or bulky samples, but since the melting process takes place in a closed chamber, clean scrap is required to avoid contamination and reduce the potential formation of inclusions.

Printed samples are fabricated from the atomized powders, followed by microstructural characterization and chemical composition analysis. Finally, potentiodynamic polarization corrosion tests are performed on the printed samples and compared to the performance of wrought 316L.
Original languageEnglish
PublisherTechnical University of Denmark
Number of pages145
ISBN (Electronic)978-87-7475-764-1
Publication statusPublished - 2023

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