Predictive modelling of laser powder bed fusion of Fe-based nanocrystalline alloys based on experimental data using multiple linear regression analysis Merve G. Özden, Xianyuan Liu, Tom J. Wilkinson, Meryem S. Üstün-Yavuz, Nicola A. Morley Heliyon, 2024 This study harnessed bivariate correlational analysis, multiple linear regression analysis and tree-based regression analysis to examine the relationship between laser process parameters and the final material properties (bulk density, saturation magnetization (Ms), and coercivity (Hc)) of Fe-based nano-crystalline alloys fabricated via laser powder bed fusion (LPBF). A dataset comprising of 162 experimental data points served as the foundation for the investigation. Each data point encompassed five independent variables: laser power (P), laser scan speed (v), hatch spacing (h), layer thickness (t), and energy density (E), along with three dependent variables: bulk density, Ms, and Hc. The bivariate correlational analysis unveiled that bulk density exhibited a significant correlation with P, v, h, and E, whereas Ms and Hc displayed significant correlations exclusively with v and P, respectively. This divergence may stem from the strong influence of microstructure on magnetic properties, which can be impacted not only by the laser process parameters explored in this study but also by other factors such as oxygen levels within the build chamber. Furthermore, our statistical analysis revealed that bulk density increased with rising P, h, and E, while decreased with higher v. Regarding the magnetic properties, a high Ms was achievable through low v, while low Hc resulted from high P. It was concluded that P and v were considered as the primary laser process parameters, influencing h and t due to their control over the melt-pool size. The application of multiple linear regression analysis allowed the prediction of the bulk density by using both laser process parameters and energy density. This approach offered a valuable alternative to time-consuming and costly trial-and-error experiments, yielding a low error of less than 1 % between the mean predicted and experimental values. Although a slightly higher error of approximately 6 % was observed for Ms, a clear association was established between Ms and v, with lower v values corresponding to higher Ms values. Additionally, a further comparison was conducted between multiple linear regression and three tree-based regression models to explore the effectiveness of these approaches.
Enhancing Soft-Magnetic Properties of Fe-Based Nanocrystalline Materials with a Novel Double-Scanning Technique Merve G. Özden, Nicola A. Morley Advanced Engineering Materials, 2023 This article presents a novel scanning technique for the laser powder bed fusion (LPBF) of Fe‐based soft‐magnetic alloys, which have low glass forming ability, and microstructural change happens during LPBF process. This technique involves double scanning where 1) the first scan applied uses high energy density (E = P/vht, where P is the laser power, v is the laser scan speed, h is the hatch spacing, and t is the layer thickness) with different process parameters (P: 30, 40, and 50 W, v: 500, 600, and 700 mm s−1, h: 20 and 30 μm, and t: 50 μm) to achieve high density and 2) the second scan employed before the spreading subsequent powder layer uses low E (=20 J mm−3, P = 20 W, v = 1000 mm s−1, h = 20 μm, and t = 50 μm) to refine the microstructure and thus reduce coercivity. This increases the saturation magnetization to a maximum value of 226.81 Am2 kg−1 and reduces the coercivity to a lowest value recorded (130 A m−1). Likewise, the bulk density (94.59–99.25%) is enhanced significantly with double scanning, especially the samples produced using high P (50 W) resulting from the relieving of the mechanical and thermal stress evolving during the process.
Optimizing laser additive manufacturing process for Fe-based nano-crystalline magnetic materials Merve G. Özden, Nicola A. Morley Journal of Alloys and Compounds, 2023 Fe-based amorphous magnetic alloys offer new opportunities for magnetic sensors, actuators and magnetostrictive transducers due to their high saturation magnetostriction (λs = 20–40 ppm) compared with that of amorphous Co-based alloys (λs = −3 to −5 ppm). Due to the conventional production limitations of Fe-based glassy alloys, including dimensional limitations and poor mechanical properties, this has led to a search for novel fabrication techniques. Recently, the laser powder bed fusion (LPBF) technique has attracted attention for the production of Fe-based magnetic bulk metallic glasses (BMGs) as it provides high densification, which brings about excellent mechanical properties, and high cooling rate during the process. Optimization of process parameters in the LPBF technique have been studied using the volumetric energy input (E), which includes the major build parameters; laser power (P), scan speed (v), layer thickness (t) and hatch spacing (h). This study investigates how the major process parameters influence the physical and magnetic properties of LPBF-processed Fe-based amorphous/nanocrystalline composites ((Fe87.38Si6.85B2.54Cr2.46C0.77 (mass %)). Various process parameter combinations with P (90, 100, 120 and 150 W) and v (700, 1000 and 1300 mm/s) were applied with t of 30, 50 and 70 µm and h of 20, 30, 40, 50 and 60 µm. It was found that bulk density improves as P and t increases, v and h decreases, i.e., high E is necessary, however, 99.45% of bulk density was achieved with E of 61.22 J/mm3 (P = 150 W, v=700 mm/s, h=50 µm and t = 70 µm), which indicates the importance of understanding how parameters affect the specific materials. In addition, the magnetic properties differ significantly due to the nanocrystalline phases present in the microstructure, with their size depending on the process parameters considerably. Owing to the laser scanning nature, the microstructure evolves as molten pools (MP) and heat affected zones (HAZ) due to the high thermal gradient that occurred between laser tracks. MP form around the scans, containing α-Fe(Si) nanograins mainly, whereas HAZ generally contains Fe2B and Fe3Si nanocrystalline clusters. The size and quantities of those nanocrystallites determine the magnetic properties. With the same E (60 J/mm3), v (1000 mm/s) and t (50 µm), only changing P and h caused samples to have different saturation magnetization; 206 emu/gr (P: 90 W and h: 30 µm) and 150 emu/gr (P: 150 W and h: 50 µm). In general, the saturation magnetisation, Ms of LPBF-processed samples changes between 130 and 206 emu/gr, which is much higher than that of feedstock powder (102 emu/gr) due to their nanocrystalline structures. The coercivity (Hc) is in the range of 14.55 and 34.68 Oe, which is considered high for soft-magnetic behaviour (Hc ≤ 12.5 Oe), resulting from the larger crystallite size and the presence of defects (pores and cracks) in the microstructure.
Soft-Magnetic Behavior of Fe-Based Nanocrystalline Alloys Produced Using Laser Powder Bed Fusion Merve G. Özden, Felicity SHB Freeman, Nicola A. Morley Advanced Engineering Materials, 2023 Herein, an extensive experimental study is presented on the influence of the major process parameters of the laser powder bed fusion (LPBF) technique on the bulk density and soft‐magnetic properties of Fe‐based bulk metallic glasses (BMGs). For this purpose, 81 samples are manufactured using the combinations of different process parameters, that is, layer thickness (t: 50–70 μm), laser power (P: 70–130 W), laser scan speed (v: 900–1100 mm s−1), and hatch spacing (h: 20–40 μm). High bulk density (≥99%) is achieved utilizing low P and v combined with low h and t in order to decrease energy input to the powder, preventing cracks associated with the brittle nature of BMGs. Furthermore, it is indicated that h = 30 μm and v = 1000 mm s−1 play a determining role in acquiring high saturation magnetization (≥200 Am2 kg−1). Due to the laser scanning nature of the process, two distinct microstructures evolve, melt‐pool (MP) and heat‐affected zone (HAZ). According to thermal modeling performed in this study, laser power has the major effect on the thermal development in the microstructure (thermal gradient evolved between the two hatches and the cooling rate from MP through HAZ).
Laser additive manufacturing of Fe-based magnetic amorphous alloys Merve G. Ozden, Nicola A. Morley Magnetochemistry, 2021 Fe-based amorphous materials offer new opportunities for magnetic sensors, actuators, and magnetostrictive transducers due to their high saturation magnetostriction (λs = 20–40 ppm) and low coercive field compared with polycrystalline Fe-based alloys, which have high magnetostriction but large coercive fields and Co-based amorphous alloys with small magnetostriction (λs = −3 to −5 ppm). Additive layer manufacturing (ALM) offers a new fabrication technique for more complex net-shaping designs. This paper reviews the two different ALM techniques that have been used to fabricate Fe-based amorphous magnetic materials, including the structural and magnetic properties. Selective laser melting (SLM)—a powder-bed fusion technique—and laser-engineered net shaping (LENS)—a directed energy deposition method—have both been utilised to fabricate amorphous alloys, owing to their high availability and low cost within the literature. Two different scanning strategies have been introduced by using the SLM technique. The first strategy is a double-scanning strategy, which gives rise to maximum relative density of 96% and corresponding magnetic saturation of 1.22 T. It also improved the glassy phase content by an order of magnitude of 47%, as well as improving magnetic properties (decreasing coercivity to 1591.5 A/m and increasing magnetic permeability to around 100 at 100 Hz). The second is a novel scanning strategy, which involves two-step melting: preliminary laser melting and short pulse amorphisation. This increased the amorphous phase fraction to a value of up to 89.6%, and relative density up to 94.1%, and lowered coercivity to 238 A/m. On the other hand, the LENS technique has not been utilised as much as SLM in the production of amorphous alloys owing to its lower geometric accuracy (0.25 mm) and lower surface quality, despite its benefits such as providing superior mechanical properties, controlled composition and microstructure. As a result, it has been commonly used for large parts with low complexity and for repairing them, limiting the production of amorphous alloys because of the size limitation. This paper provides a comprehensive review of these techniques for Fe-based amorphous magnetic materials.
