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Powder spreading is a crucial step in the powder bed fusion process, which controls the quality of powder bed and consequently affects the quality of printed parts. To date, however, powder spreadability has received very little attention and substantial fundamental work is still needed, largely because of the lack of experimental studies. Therefor...
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... particle size distributions of the powders under investigation are shown in Fig. 2. As expected, powder A has the wider particle size distribution. It is comprised of very large particles (≈100 µm) and a considerable number of fines. Powder B presented a gaussian type of distribution which is generally considered optimal for the SLM process. A similar distribution is seen in powder C. However, the distribution is ...
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... were supplied having the same particle size distribution. Still, prior to carrying out the experimental work, these powders underwent sieving to remove the variable of different particle size distribution to enable an accurate study of the investigated factors and correlation between their physical and flow characteristics. However, as seen in Fig. 2 there was still exist a small difference between the powders particle size distributions and their volume densities. As can be seen, it is difficult to remove these two variables completely when comparing powders. Nevertheless, their influence on the responses can be minimised via sieving as addressed in this ...
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Fused Filament Fabrication (FFF) technology is used to create metal parts in this paper. A binder formula is developed for 316L stainless steel powder, composed of polypropylene (PP), styrene ethylene butylene styrene (SEBS) and paraffin wax (PW). The binder is mixed with the 316L stainless steel powder to produce mixture which is then extruded int...
Citations
... This is caused by the lower energy input and convergence feature of the laser beam when using the negative defocus strategy. Unlike the loose powder LPBF, where a roller or blade can control or balance the layer thickness of raw material to be solidified [34], the solidified layer thickness of MAPS printed samples changes with the process parameters that are Fig. 7. OM cross-section of MAPS printed samples using irregular powder sheet material, processed with different scanning speeds and defocus settings. employed (Fig. 8b). ...
... The concentration of smaller size powder (< 25 µm) is also increased after adding WC particles. The mechanical interlocking between low spherical powders agglomeration reduces the flowability during manufacturing processing, resulting in higher pore concentration and larger pore size in the LPBF MMCs [23][24][25]. Additionally, the addition of WC compromises the ductility of the MMCs. For example, Kang et al. [26] found that adding 5 wt% WC to LPBF maraging steel increases the ultimate tensile strength from 1200 MPa to 1300 MPa, whereas it results in a 5 % reduction in fracture strain. ...
The main challenge of particle reinforced metal matrix composites (MMCs) is balancing strength and ductility. This research uses type 420 stainless steel and spherical cast tungsten carbide (WC/W2C) with a similar powder size and range as raw powders to manufacture laser powder bed fusion (LPBF) 420 + 5 wt% WC/W2C MMCs. LPBF 420 + 5 wt% WC/W2C MMCs contain austenite, martensite, and W-rich carbides (WC/W2C, FeW3C, M6C, and M7C3) from nanometre to micrometre scale. The well-balanced composition creates a crack-free reaction layer between the reinforced particles and matrix. This reaction layer consists of two distinct layers, depending on the element concentration. The LPBF 420 + 5 wt% WC/W2C MMCs achieved an excellent compressive strength of ∼5.5 GPa and a considerable fracture strain exceeding 50 %. The underlying mechanisms for the improved mechanical properties are discussed, providing further insight to advance the application of MMCs via additive manufacturing.
... Achieving the desired surface finish is essential for optimizing performance and ensuring consistent product quality. Surface roughness, similar to density, is also influenced by the interplay between process parameters [16,25,32,33] and material properties such as cohesion, particle size distribution [14] and particle shape [12,34]. In particular, cohesion strongly influences the surface roughness during spreading. ...
... Using this variation as a metric of uniformity, surface roughness is calculated as the mean deviation of surface height from the powder bed average height [14,18]. Experimentally, the surface height can be determined using optical 3D digital microscopy [32] or high-speed laser profilometry [33]. Surface roughness can be quantified either using planar profile measurements in two dimensions [12,25] or areal measurements in three dimensions [18,35]. ...
Producing dense and homogeneous powder layers with smooth free surface is challenging in additive manufacturing, as interparticle cohesion can strongly affect the powder packing structure and therefore influence the quality of the end product. We use the Discrete Element Method to simulate the spreading process of spherical powders and examine how cohesion influences the characteristics of the packing structure with a focus on the fluctuation of the local morphology. As cohesion increases, the overall packing density decreases, and the free surface roughness increases, which is calculated from digitized surface height distributions. Local structural fluctuations for both quantities are examined through the local packing anisotropy on the particle scale, obtained from Voronoï tessellation. The distributions of these particle-level metrics quantify the increasingly heterogeneous packing structure with clustering and changing surface morphology.
... Yao et al. [33] further determined the separation mechanism of the powders. Mussatto et al. [34] studied the comprehensive effect of spreading velocity and powder shape on the powder bed uniformity. The results revealed that the uniformity of the powder bed is better when the spreading velocity is less than 0.08 m/s. ...
To reveal the relationship between tool geometries and process parameters in Powder Bed Fusion (PBF), discrete element method is utilized for numerical simulation of PBF. The powder bed flatness and coefficient of variation are proposed to evaluate the quality of the powder bed. Results indicate that the powder bed voidage increases with the increase in the gap height, the decrease in the velocity and the increase in the blade fillet radius. When the blade inclination angle decreased from 45°to 30°and the average powder diameter is decreased from 50 to 30 µm, the porosity of the powder bed is decreased by 3.47% and 8.19%, respectively. The voidages in PBF with blade, roller and blade-roller are 55.78%, 49.08% and 47.57%. When the gap height is higher than 0.15 mm, the voidage in case of roller is obviously smaller than that in case of blade. The optimized combination of the parameters and tool can improve the powder bed flatness by 4.29% and reduce the voidage and coefficient of variation by 0.77% and 2.26%.
... The flowability and spreadability of the powder are directly influenced by factors including particle size, morphology, and surface properties of the particles [13,14]. The particle size and morphology of the powder, in turn, are determined by the preparation process and post-processing methods employed. ...
The powder bed packing density of metal powders plays a crucial role in additive manufacturing as it directly affects the defect and mechanical properties of the fabricated parts. Powder bed packing density is related to powder flowability and spreadability. In this study, we introduced a new method to improve powder flowability and spreadability, where Haynes 230 powder with exceptional flowability was successfully produced using an in situ micro-oxidation gas atomization process. Compared to conventional gas atomization, the powder exhibited improved flowability and spreadability, measuring at 11.8 s/50 g. Additionally, the angle of repose was reduced by 25%, resulting in a powder bed packing density of 5.67 g/cm3, corresponding to 63.7% of the theoretical density. Notably, the oxygen content in the powder was only 180 ppm, as confirmed by XRD testing, and no oxide peaks were detected. Furthermore, the depth of the oxide layer on the particle surface increased by less than 20 nm. As a result, the in situ micro-oxidation process reduces the number of pores and cracks in the Haynes 230 alloy formed specimens and improves the relative density of the built specimens. This study highlights the potential of in situ micro-oxidation gas atomization as a promising method for producing powders with high flowability and spreadability for laser powder bed fusion (LPBF) processes.
... From the parametric studies performed on the effect of powder size distribution, it was observed that increasing the number of fine particles increases the powder packing density. Mussatto et al. [43] also found similar results in his work, where powder spreading process was extensively studied for powder bed fusion process and the influence of surface morphology of powder, spreader velocity and layer thickness was studied. It was reported that having increased number of fine particles improved the packing fraction of the powder bed. ...
Laser powder bed fusion (LPBF) is a subset of the additive manufacturing process in which a laser beam selectively joins the metal powder into a desired part in a sequential layer process. Owing to its complex nature of rapid heating and cooling of the melt pool, there is a need to understand the melt pool behavior and its effects on the final manufactured part. A densely packed powder bed is highly desirable for fabricating a superior part using the LPBF process. In this work, discrete element model was used to generate powder beds with realistic powder properties and various factors affecting the packing density were studied. The powder beds generated were then irradiated with a high-power laser source to selectively melt the powder particles to study the melt pool dynamics using volume of fluid method. The effect of laser parameters like laser power and scan speed on the melt pool was studied using single-track and multi-track simulations. Multi-layer simulations were performed to replicate the actual LPBF process. The simulations were in close agreement with actual experimental efforts.
... As the process condition limits the number of available samples (fourteen), a more detailed analysis from a statistical point of view would be recommended, including employing more specific models [26] to extrapolate fatigue life limit values. However, due to the large amount of information already contained in this work, this analysis will be left for later work, including comparing materials from PA and HDH powders [5,[27][28][29]. ...
... The arrow indicates the presence of a small peak of residual hcp (Ti-alpha phase) in the NT-HDH sample, adapted from [28]. could be formed, with a higher probability regarding a non-cohesive powder, during the deposition of a thin layer of this type of powder [27,31,32]. After a series of powder deposition-laser fusion cycles, this non-uniformity could eventually form defects in the final parts produced by L-PBF, especially defects caused by lack-of-fusion. ...
... Microfractography analysis (section 3.8) suggests that porosities are harmful if they μm diameter), Fig. 12-a and 12-b (sample #8 NT-PA), which occurred in all analysed samples, and irregularly shaped lack-of-fusion pores, also seen in all samples, Fig. 12-c and 12-d (sample #7 NT-HDH), which were identified in large numbers particularly in the sample #7 NT-HDH. This result is consistent with the previous discussion in 3.1 regarding the higher probability of the formation of lack-of-fusion defects in NT-HDH samples due to the higher cohesiveness of the HDH powder [27,32]. Fig. 13-a and 13-c show the typical microstructure pattern induced by the L-PBF process in both NT-PA and NT-HDH sets. ...
... The laser powder bed fusion (LPBF) process is a widely recognized additive manufacturing method that provides several benefits over wrought production techniques. These benefits include swift manufacturing speed, design flexibility, and reduced material wastage [1][2][3]. The LPBF technique generates a distinctive microstructure due to its rapid heating and cooling cycles [4], resulting in prominent characteristics such as porosity, grain morphology, cellular structure, and nano-sized inclusions [5,6]. ...
The microstructure and corrosion performance of wrought, laser powder bed fusion (LPBF), and MoNi overalloyed
LPBF type 316L stainless steel is compared. Adding Mo and Ni to LPBF 316L stainless steel resulted in
ẟ-ferrite. Potentio-dynamic polarisation was used to rank all microstructures, with LPBF + MoNi has more
resistant than LPBF and wrought material. Bipolar electrochemistry was applied, with LPBF samples having the
lowest critical pitting potential and highest pit growth kinetics. The best corrosion performance was for LPBF +
MoNi stainless steel. Different pitting corrosion resistance as function of process directions were found. The tiny
pores negatively impacted the pitting performance.
... These voids can subsequently introduce bubbles, which compromise the structural integrity [39]. Their formation is influenced by factors such as powder size distribution, spreader speed, and texture of the preceding layers [40,41]. ...
... The prominent diffraction peaks of the α-Mg matrix exhibit a rightward shift as Zn content increases. This observation aligns with Bragg's law of diffraction [41]: ...
... The prominent diffraction peaks of the α-Mg matrix exhibit a rightward shift as Zn content increases. This observation aligns with Bragg's law of diffraction[41]: 2d sin(θ) = nλ ...
This study investigated the effects of Zinc (Zn) content, specifically in the range of 1 wt.% to 7 wt.%, on the powder characteristics, porosity, microstructure, and corrosion behavior of Mg-xZn-0.2Mn alloys produced using selective laser melting (SLM). To evaluate the porosity of the printed parts and various powder attributes, such as size, circularity, void spaces between powders, and inherent imperfections, scanning electron microscopy (SEM) and optical microscopy (OM) were employed. The alloy microstructure, composition, and phase were examined using energy dispersive X-ray (SEM-EDX) and X-ray Diffraction (XRD). The corrosion resistance and degradation behavior were assessed through electrochemical corrosion tests and immersion tests in Hanks’ solution at 37.5 °C, respectively. Finally, OM and SEM-EDX were used to characterize the corrosion products. The findings of this study indicated that the powder size increased with Zn content, maintaining a 0.8 circularity. Powder defects were minimal, with occasional satellite particles. For the SLM-printed samples, it was evident that porosity characteristics could be influenced by Zn content. As Zn content increased, the pore fraction rose from 1.0% to 5.3%, and the pore size grew from 2.2 μm to 3.0 μm. All printed samples consisted of an α-Mg matrix. Additionally, a higher Zn content resulted in more distinct grain boundaries. Corrosion resistance decreased with Zn, leading to more pronounced localized corrosion after immersion in Hanks’ solution. Ca-P was found as white corrosion products on all samples.
... Nan et al. reported that the packing density of the powder bed increased with decreasing flowability owing to the suppressed particle motion [17]. Mussatto et al. pointed out that the quality of the powder bed was dominated by the particle morphology rather than the flowability [18]. Yim et al. reported that the superior spreadability and packing quality of the powder were mainly attributed to cohesive forces during the spreading process [19]. ...
Providing a dense powder bed is necessary to ensure the rationality of the final product prepared by powder bed fusion-based additive manufacturing under broad building parameters. In this study, the powder spreading mechanism was elucidated using particle image velocimetry and discrete element simulations. The particle flow regimes in the spreading process were identified based on the particle displacement: alignment, rotation, and deposition. The alignment regime was dominant in gas-atomized stainless steel 304 powder piles, whereas the rotation regime was dominant in plasma rotating electrode processed stainless steel 304 powder piles. A high-fidelity spreading simulation model was developed to clarify the critical factors resulting in the different flow regimes of different stainless steel 304 powders. The dominant rotational regime of the plasma rotating electrode processed powder was substituted for alignment by increasing the cohesive force. The particle supply in the spreading process was further suppressed by increasing the cohesive force, owing to the formation of a strong force arch and agglomeration. The high cohesive force in the gas-atomized powder was mainly attributed to the electrostatic force caused by the thick oxide film. Therefore, it was proven that the oxide film thickness is a key factor in determining the powder spreading mechanism and powder bed quality in the powder bed fusion additive manufacturing process.
