Optimisation of silicon content in Fe-Si alloys processed via Laser Powder Bed Fusion for an additively manufactured soft magnetic core

Additive Manufacturing (AM) of electric motors, specifically, Laser Powder Bed Fusion (LPBF), for rotating soft magnetic cores is of research interest because of its potential benefits in industry sectors such as energy, aerospace and automotive. AM, also commonly known as 3D printing (3DP), offers unrivalled design freedom and the capability to produce components with complex geometries from metal alloys that cannot be processed with conventional manufacturing methods (casting, injection moulding etc.). However, before AM becomes the norm in the production of novel electric drives and power generators, it is necessary to understand how the selection of materials in the motor affects the performance of the 3D printed active components (rotor, stator, windings). This thesis aims at enabling the AM of more compact, lightweight, reliable and efficient electric machines through the development of a comprehensive understanding of the metallurgy and material properties of the additively manufactured components of an electric drive. It focuses on two materials: a soft ferromagnetic alloy, namely silicon steel, for the soft magnetic core and high purity copper for the windings of the electric motor. The study investigated the mechanical, thermal and magnetic properties of high silicon steel (from Fe-3.5%wt Si up to Fe-6.9%wt Si) by adjusting – for the first time – the alloys’ chemistry in order to improve ductility and avoid the risk of in-process cracking; this is achieved by mixing pre-alloyed Fe-6.9%wt Si powder with high-purity Fe powder. Another material investigated, which has lately received increased interest both for electrical applications and heat exchangers, was pure copper. Although high purity copper is challenging to process with LPBF due to its high reflectivity, oxidation and high thermal conductivity, it was included in the study due the potential to further increase a motor’s performance by optimising the design of the windings in a 3DP electric motor. the materials under investigation were subjected to heat treatments. Annealing of the soft magnetic parts produced by LPBF changed their microstructure by increasing the grain size and increased their permeability. Experiments were also performed to investigate how the performance of a Switched Reluctance Motor (SRM) could be improved by manufacturing the soft magnetic rotor core using LPBF. A prototype SRM soft magnetic core was additively manufactured from 5%w.t. silicon steel and tested. We compared the efficiency of the motor with the 3D-printed rotor core to a motor with an identical but traditionally laminated rotor. This investigation has therefore, developed an understanding of the various aspects of the LPBF process for the successful manufacturing of a prototype functional electric motor. The results from this work can be used to advance the implementation of AM in the production of lightweight high-performance electrical machines and revolutionise the way electrical motors are designed and manufactured

Optimisation of silicon content in Fe-Si alloys processed via Laser Powder Bed Fusion for an additively manufactured soft magnetic core

Additive Manufacturing (AM) of electric motors, specifically, Laser Powder Bed Fusion (LPBF), for rotating soft magnetic cores is of research interest because of its potential benefits in industry sectors such as energy, aerospace and automotive. AM, also commonly known as 3D printing (3DP), offers unrivalled design freedom and the capability to produce components with complex geometries from metal alloys that cannot be processed with conventional manufacturing methods (casting, injection moulding etc.). However, before AM becomes the norm in the production of novel electric drives and power generators, it is necessary to understand how the selection of materials in the motor affects the performance of the 3D printed active components (rotor, stator, windings). This thesis aims at enabling the AM of more compact, lightweight, reliable and efficient electric machines through the development of a comprehensive understanding of the metallurgy and material properties of the additively manufactured components of an electric drive. It focuses on two materials: a soft ferromagnetic alloy, namely silicon steel, for the soft magnetic core and high purity copper for the windings of the electric motor. The study investigated the mechanical, thermal and magnetic properties of high silicon steel (from Fe-3.5%wt Si up to Fe-6.9%wt Si) by adjusting – for the first time – the alloys’ chemistry in order to improve ductility and avoid the risk of in-process cracking; this is achieved by mixing pre-alloyed Fe-6.9%wt Si powder with high-purity Fe powder. Another material investigated, which has lately received increased interest both for electrical applications and heat exchangers, was pure copper. Although high purity copper is challenging to process with LPBF due to its high reflectivity, oxidation and high thermal conductivity, it was included in the study due the potential to further increase a motor’s performance by optimising the design of the windings in a 3DP electric motor. the materials under investigation were subjected to heat treatments. Annealing of the soft magnetic parts produced by LPBF changed their microstructure by increasing the grain size and increased their permeability. Experiments were also performed to investigate how the performance of a Switched Reluctance Motor (SRM) could be improved by manufacturing the soft magnetic rotor core using LPBF. A prototype SRM soft magnetic core was additively manufactured from 5%w.t. silicon steel and tested. We compared the efficiency of the motor with the 3D-printed rotor core to a motor with an identical but traditionally laminated rotor. This investigation has therefore, developed an understanding of the various aspects of the LPBF process for the successful manufacturing of a prototype functional electric motor. The results from this work can be used to advance the implementation of AM in the production of lightweight high-performance electrical machines and revolutionise the way electrical motors are designed and manufactured

Electrical resistivity of pure copper processed by medium-powered laser powder bed fusion additive manufacturing for use in electromagnetic applications

Pure copper is an excellent thermal and electrical conductor, however, attempts to process it with additive manufacturing (AM) technologies have seen various levels of success. While electron beam melting (EBM) has successfully processed pure copper to high densities, laser powder bed fusion (LPBF) has had difficulties achieving the same results without the use of very high power lasers. This requirement has hampered the exploration of using LPBF with pure copper as most machines are equipped with lasers that have low to medium laser power densities. In this work, experiments were conducted to process pure copper with a 200 W LPBF machine with a small laser spot diameter resulting in an above average laser power density in order to maximise density and achieve low electrical resistivity. The effects of initial build orientation and post heat treatment were also investigated to explore their influence on electrical resistivity. It was found that despite issues with high porosity, heat treated specimens had a lower electrical resistivity than other common AM materials such as the aluminium alloy AlSi10Mg. By conducting these tests, it was found that despite having approximately double the resistivity of commercially pure copper, the resistivity was sufficiently low enough to demonstrate the potential to use AM to process copper suitable for electrical applications

Additive manufacturing and testing of a soft magnetic rotor for a switched reluctance motor

Additive manufacturing is acknowledged as a key enabling technology, although its adoption is still constrained to niche applications. A promising area for this technology is the production of electrical machines (EMs) and/or their main components (e.g. magnetic cores, windings, heat exchangers, etc.) due to the potential of creating lightweight, highly efficient rotating motors, suitable for applications requiring a low moment of inertia. This work investigates the readiness of metal additive manufacturing, specifically Laser Powder Bed Fusion (LPBF), applied to the field of EMs to bridge the gaps of how to use this technological approach in this field. A soft magnetic material featuring high silicon content (Fe-5.0%w.t.Si) has been developed for LPBF and a rotor has been 3D-printed for a switched reluctance machine. The printed rotor was assembled into a conventionally laminated stator and the performance of the whole machine was evaluated. Its performance was compared against an identical machine equipped with a laminated rotor of the same dimensions made of conventional non-oriented silicon steel. A comparative study was carried out through both finite element simulations and experimental tests. The efficiency of the two machines was assessed together with the principal electrical and mechanical quantities under several operating conditions

Novel Powder Feedstock towards Microstructure Engineering in Laser Powder Bed Fusion: A Case Study on Duplex/Super Duplex and Austenitic Stainless-Steel Alloys

Additive manufacturing of Duplex Stainless Steels (DSS) and Super Duplex Stainless Steels (SDSS) has been successfully demonstrated using LPBF in recent years, however, both alloys feature an almost fully ferritic microstructure in the as-built condition due to the fast cooling rates associated with the Laser Powder Bed Fusion (LPBF) process. Blends of DSS and SDSS powders were formulated with austenitic stainless-steel 316L powder, aiming to achieve increased austenite formation during in the LPBF as-built condition to potentially minimize the post heat treatments (solution annealing and quenching). Powder characteristics were investigated and process parameters were optimized to produce near fully dense parts. Nanoindentation (NI) tests were conducted to measure, not only the local mechanical properties and correlate them with the as-built microstructure, but also to gain a deeper understanding in the deformation behavior of individual phases that cannot be studied directly by macroscopic tensile tests. Scanning Electron Microscopy (SEM) and Electron Backscatter Diffraction (EBSD) were employed for microstructural analysis and phase quantification. The microstructural analysis and EBSD phase maps revealed an increase in austenite in the as-built microstructures. Blend 1 resulted in a duplex microstructure consisting of 10% austenite at the XY plane and 20% austenite at the XZ plane. The austenite content increased with increasing proportion of 316L stainless steel in the powder blends. The DSS blend required a much higher volumetric energy density for the fabrication of near fully dense parts. This imposed a slower solidification and a higher melt pool homogeneity, allowing for adequate diffusion of the austenite stabilizing elements. The presented workflow and findings from this study provide valuable insights into powder mixing for the development of custom alloys for rapid material screening in LPBF

A Comparative Investigation of Duplex and Super Duplex Stainless Steels Processed through Laser Powder Bed Fusion

The aim of this paper was to compare duplex (DSS) and super duplex stainless steel processed by laser powder bed fusion (LPBF) based on the process parameters and microstructure–nanomechanical property relationships. Each alloy was investigated with respect to its feedstock powder characteristics. Optimum process parameters including scanning speed, laser power, beam diameter, laser energy density, and layer thickness were defined for each alloy, and near-fully dense parts (>99.9%) were produced. Microstructural analysis was performed via optical (OM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The samples were subjected to stress relief and high-temperature annealing. EBSD revealed the crystallographic orientation and quantified the phases in the as-built and annealed sample conditions. The as-built samples revealed a fully ferritic microstructure with a small amount of grain boundary austenite in the SDSS microstructure. High-temperature solution annealing resulted in the desired duplex microstructure for both alloys. There were no secondary phases present in the microstructure after both heat treatments. Nanoindentation generated nanomechanical (modulus) mapping grids and quantified the nanomechanical (both hardness and modulus) response; plasticity and stress relief were also assessed in all three conditions (as-built, stress-relieved, and annealed) in both DSS and SDSS. Austenite formation in the annealed condition contributed to lower hardness levels (~4.3–4.8 Gpa) and higher plastic deformation compared to the as-built (~5.7–6.3 Gpa) and stress-relieved conditions (~4.8–5.8 Gpa) for both alloys. SDSS featured a ~60% austenite volume fraction in its annealed and quenched microstructure, attributed to its higher nickel and nitrogen contents compared to DSS, which exhibited a ~30% austenite volume fraction

Φορολογική πρόβλεψη φυσικών προσώπων με νευροασαφή λογική, νευρωνικά δίκτυα και γενετικούς αλγόριθμους

Μη διαθέσιμη περίληψηNot available summarizatio

A Tool for Rapid Analysis Using Image Processing and Artificial Intelligence: Automated Interoperable Characterization Data of Metal Powder for Additive Manufacturing with SEM Case

A methodology for the automated analysis of metal powder scanning electron microscope (SEM) images towards material characterization is developed and presented. This software-based tool takes advantage of a combination of recent artificial intelligence advances (mask R-CNN), conventional image processing techniques, and SEM characterization domain knowledge to assess metal powder quality for additive manufacturing applications. SEM is being used for characterizing metal powder alloys, specifically by quantifying the diameter and number of spherical particles, which are key characteristics for assessing the quality of the analyzed powder. Usually, SEM images are manually analyzed using third-party analysis software, which can be time-consuming and often introduces user bias into the measurements. In addition, only a few non-statistically significant samples are taken into consideration for the material characterization. Thus, a method that can overcome the above challenges utilizing state-of-the-art instance segmentation models is introduced. The final proposed model achieved a total mask average precision (mAP50) 67.2 at an intersection over union of 0.5 and with prediction confidence threshold of 0.4. Finally, the predicted instance masks are further used to provide a statistical analysis that includes important metrics such as the particle size distinction

Chemical recovery of spent copper powder in laser powder bed fusion

In laser powder bed fusion (LPBF), recovered unfused powder from the powder bed often degrades upon sequential processing through mechanisms like thermal oxidation and particle satelliting from ejected weld spatters and particle-laser interactions. Given the sensitivity of LPBF performance and build quality to powder properties, spent powder is generally discarded after a few build cycles, especially for materials that are sensitive towards surface oxidation. This increases feedstock material costs, as well as costs associated with machine downtime during powder replacement. Here, a new method to chemically reprocess spent LPBF metal powder is demonstrated under ambient conditions, using a heavily oxidised Cu powder feedstock recovered from prior LPBF processing as a model material. This is compared to an equivalent virgin Cu powder. The near-surface powder chemistry has been analysed, and it is shown that surface oxide layers present on spent Cu powder can be effectively reset after rapid reprocessing (from 5 to 20 min). Diffuse reflectance changes on etching, reducing for gas-atomised virgin Cu powder due to the formation of anisotropic etch facets, and increasing for heavily oxidised spent Cu as the highly absorptive oxide layers are removed. The mechanism of powder degradation for moisture sensitive materials like Cu has been correlated to the degradation of LPBF deposits, which manifests as widespread and extensive porosity. This extensive porosity is largely eliminated after reprocessing the spent Cu powder. Chemically etched spent powder is therefore demonstrated as a practical feedstock in LPBF in which track density produced is comparable to virgin powder.ISSN:2214-860

The interaction of volatile metal coatings during the laser powder bed fusion of copper

The high optical reflectance of Cu at near-infrared wavelengths narrows the process window to fabricate Cu parts by laser powder bed fusion (LPBF). Coating powders with optically absorptive materials has been investigated to improve processability and enhance part properties. However, given the intense heat localization and thin coating layers relative to the powder, the mechanisms of thin film coating interaction in LPBF remain unclear, despite recent work showing the importance of the near-track environment in deposition behavior. In this study, optically absorptive Zn-coated Cu powders were prepared by physical vapor deposition and characterized. Single LPBF tracks were fabricated to elucidate material incorporation phenomena influenced by the volatile Zn coating. It is shown that Zn-coated powder enhances accretion at fastest effective scan speed tested (100 mm/s), where mean track volumes are increased from 0.72 ± 0.05 mm3 (as-received) to 0.91 ± 0.01 mm3 (Zn-coated). This has been correlated to the stronger vapor jet from the volatile Zn-coating, which denudes the surrounding powder bed. This exhausts the powder bed at slower effective scan speeds, causing instability and balling when compared to the as-received powder. It is shown that Zn is localized at the track surface and is undetectable in the track bulk, indicating Zn vaporization on interaction with the incident beam. Zn present mainly occurs through secondary deposition mechanisms like spatter and condensation, rather than in-process alloying. Coating powder feedstocks for use in LPBF therefore affects composition, laser beam absorptivity, and the near-track vapor environment that is known to influence material incorporation behavior