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Article

The Influence of Grain Structure on Mechanical Properties of LPBF AlSi10Mg Alloy Obtained via Conventional and KOBO Extrusion Process

by
Przemysław Snopiński
1,*,
Paweł Ostachowski
2,
Krzysztof Matus
3 and
Krzysztof Żaba
4
1
Department of Engineering Materials and Biomaterials, Silesian University of Technology, Konarskiego Street 18A, 44-100 Gliwice, Poland
2
Faculty of Non-Ferrous Metals, AGH University of Krakow, A. Mickiewicza 30 Av., 30-059 Krakow, Poland
3
Materials Research Laboratory, Faculty of Mechanical Engineering, Silesian University of Technology, Konarskiego Street 18A, 44-100 Gliwice, Poland
4
Department of Metal Working and Physical Metallurgy of Non-Ferrous Metals, AGH University of Science and Technology, Al. Adama Mickiewcza 30, 30-059 Cracow, Poland
*
Author to whom correspondence should be addressed.
Symmetry 2025, 17(5), 709; https://doi.org/10.3390/sym17050709
Submission received: 9 April 2025 / Revised: 29 April 2025 / Accepted: 2 May 2025 / Published: 6 May 2025
(This article belongs to the Special Issue Chemistry: Symmetry/Asymmetry—Feature Papers and Reviews)
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Versions Notes

Abstract

:
This study compares the microstructures and mechanical properties of the AlSi10Mg alloy processed by laser powder bed fusion (LPBF) after undergoing different post-processing techniques. These techniques include conventional extrusion at 350 °C (CE-350) and KOBO extrusion at both room temperature (KOBO-RT) and 350 °C (KOBO-350). The extrusion processes, regardless of the method used, effectively densified the alloy, fragmented the primary silicon network, and refined the grain structure. Notably, a microstructure analysis indicated that the CE-350 method produced the finest grains, whereas the KOBO-350 method resulted in the largest grains. From a mechanical perspective, extrusion significantly increased ductility—rising from 2.4% to more than 14% elongation—while decreasing strength compared to the as-built state. Among the extruded samples, CE-350 provided the best balance of strength and ductility, exhibiting a yield strength of 186 MPa and a ductility of 18.1% elongation. Overall, the results demonstrate that while extrusion enhances ductility, it does so at the expense of strength, with conventional extrusion yielding a more favorable property balance for this alloy under the tested conditions.

1. Introduction

Symmetry in materials science refers to the regular and repeating arrangement of atoms in a material’s structure, as well as the design of manufacturing tools. In crystallography, symmetry describes the repeating patterns of atoms in crystal structures, governed by symmetry elements like rotation axes and mirror planes [1]. These symmetrical arrangements at the atomic level can determine a material’s physical properties [2]. In the context of manufacturing, symmetry in design, i.e., biaxial symmetry, allows for a uniform metal flow [3], which is particularly important in processes involving high pressures and temperatures, such as hot extrusion.
The hot extrusion process is a metal forming technique where a heated billet is forced through a die to create a product with a specific cross-sectional shape. This process is typically performed above the material’s recrystallization temperature [4]. Key parameters in hot extrusion process include the billet temperature, die temperature, extrusion ratio, extrusion speed, and the applied pressure [5]. Hot extrusion has a wide range of applications, primarily used to manufacture profiles with various types of sections, both solid and hollow flow [3]. Consequently, hot-extruded profiles made from materials such as aluminum [6,7], titanium [8], and magnesium alloys are commonly utilized in the automotive and aerospace industries.
One of the primary effects of hot extrusion is refinement of the grain microstructure. The severe plastic deformation introduces a high density of dislocations, which subsequently rearrange and annihilate through dynamic recovery. Beyond a critical strain, dynamic recrystallization (DRX) occurs, leading to the formation of new, strain-free grains [9]. In addition DRX and other softening mechanisms like dynamic recovery and geometric dynamic recrystallization (gDRX) can contribute to microstructural evolution during hot extrusion. The size of the recrystallized grains is influenced by processing parameters. Generally, higher strain rates tend to promote finer grain sizes, while lower strain rate and higher temperature will generate larger recrystallized grains [10]. Furthermore, hot extrusion significantly affects the crystallographic texture. The directional flow of material through the die aligns the grains along the extrusion direction, often resulting in a strong fiber textures (typically, fiber axes are aligned parallel to the extrusion direction (ED)) [11]. Furthermore, the morphology and distribution of secondary phases are also modified during hot extrusion. Existing precipitates may be fractured, dispersed, or spheroidized depending on the imposed strain and deformation temperature [12].
The hot extrusion of conventionally manufactured Al-Si [13,14,15,16,17] alloys has been extensively studied. In this context, Ding et al. [13] produced an Al-20Si alloy from heat-treated powder via hot extrusion and achieved a UTS of ~170 MPa and an elongation of ~10.8%. In another study, Jin et al. [15] used melt-spun foils which were post-processed via hot extrusion. They reported that the yield stress of the melt-spun sample was about 102% higher than that of the DC casting sample after hot extrusion. In another study, Ke et al. [16] studied the effect of hot extrusion on the mechanical properties and microstructure of a near-eutectic Al–12.0%Si–0.2%Mg alloy, and reported a good combination of tensile strength (256.3 MPa) and elongation (15.0%) when the hot-extruded sample was aged for 12 h. Additionally, Noga et al. [17], who studied the mechanical properties of rapidly solidified Al-Si alloys after extrusion, reported a ~130 MPa increase in tensile strength for the Al-20Si alloy. However, there is a significant research gap regarding the application of bulk deformation processes, particularly hot extrusion, as a post-processing method specifically for materials fabricated by laser powder bed fusion (LPBF). While some research exists on the post-processing of LPBF materials, it has primarily focused on techniques like hot isostatic pressing (HIP) [18,19,20], which offer benefits such as the reduction in crack density [17], modification of grain boundary character distribution [18], and the improvement of fatigue life [19]. Bulk deformation methods like conventional extrusion have received considerably less attention as a means to refine the unique microstructure of LPBF materials and enhance their bulk mechanical properties. To our knowledge, the effect of severe plastic deformation (SPD) methods, such as the KOBO extrusion technique, as a post-processing route for LPBF materials is almost entirely unexplored, with only one preceding study by the current author documenting its application to LPBF AlSi10Mg alloy [21].
The KOBO method, was developed at the AGH University of Science and Technology [22]. The fundamental principle of KOBO is to achieve complete control over the material’s substructure evolution and its associated mechanical properties by externally manipulating the way it undergoes plastic flow. The core mechanism of the KOBO method involves periodic changes in the deformation path [23]. This is typically accomplished using a mechanism that applies an additional torque cyclically, operating within a predetermined plane and at a specific frequency [24]. This cyclic action leads to a permanent destabilization of the metal’s structure, characterized by the dominance of localized plastic flow occurring within shear bands that intersect cyclically. This unique approach offers several significant advantages. It results in a several-fold reduction in the material’s resistance to plastic deformation, which translates directly into lower processing forces and a significant decrease in energy consumption. Critically, it enables the formation of a desirable, fine-grained, stable, and homogeneous microstructure, leading to high functional properties of the final product [25].
Building upon the identified research gap regarding bulk deformation post-processing of LPBF materials, and specifically addressing the limited understanding of SPD application to LPBF alloys, this article presents a novel comparative study. We examine the microstructure and mechanical properties of LPBF AlSi10Mg alloy processed using conventional and KOBO extrusion method. The primary originality of this work lies in this direct comparison of conventional hot extrusion with the KOBO extrusion. The purpose is to clarify how the unique thermomechanical paths imposed by these three methods influence the microstructural characteristics and consequently modify the bulk mechanical performance of the LPBF AlSi10Mg alloy.

2. Methodology

2.1. Material Fabrication via Laser Powder Bed Fusion (LPBF)

The specimens for this experimental work were manufactured from commercially available spherical AlSi10Mg alloy powder using a selective laser melting (SLM) process on a TruPrint 1000 system from Trumpf (Ditzingen, Germany). The following process parameters were utilized (Table 1).
The chemical composition of the AlSi10Mg alloy powder and the as-built LPBF material are documented in Refs. [26,27].

2.2. Post-Processing via Conventional and KOBO Extrusion

Three groups of LPBF samples underwent post-processing. The details are given in Table 2.

2.3. Metallographic Preparation and Microstructural Characterization

A consistent and high-quality surface finish for microstructural analysis was achieved by subjecting all specimens to a standardized metallographic preparation procedure. This procedure was performed using a LaboPol-20 automatic polishing device (Struers, Copenhagen, Denmark) and commenced with grinding using 800 and 1200 grit silicon carbide (SiC) papers under running tap water. Subsequently, the samples were polished using diamond suspensions with particle sizes of 6 µm, 3 µm, and 1 µm. The final polishing step involved mirror-polishing with a 0.04 μm colloidal silica suspension (OP-U suspension, Struers, Copenhagen, Denmark) for a duration of one hour.
Initial microstructural characterization was performed using a Zeiss Axio Observer Z1 inverted optical microscope (Carl Zeiss NTS GmbH, Oberkochen Germany). Observations were conducted in brightfield (BF) illumination mode. Prior to observation, samples were etched using Keller’s reagent (2.5 mL HNO3, 1.5 mL HCl, 1 mL HF, and 95 mL distilled water) to reveal the microstructure.
Quantitative analysis of Si particle size and morphology was performed using the ImageJ software version 1.53. Representative light microscopy micrographs were utilized as input. Prior to the analysis, the image scale was calibrated based on the micrograph scale bars. Then, the images were converted to 8-bit grayscale format. To separate touching particles, the thresholded images were converted to binary format and processed using the Watershed algorithm. The resulting data were exported for further statistical analysis and comparison between processed samples.
The detailed characterization of the grain structure was performed using electron backscatter diffraction (EBSD) in a Zeiss Supra 35 scanning electron microscope (Carl Zeiss NTS GmbH, Oberkochen, Germany), which was equipped with an EDAX NT EBSD detector. EBSD scans were conducted on longitudinal sections parallel to the extrusion direction to assess the microstructural evolution resulting from the different processing strategies. Data acquisition was performed in the central region of each sample with a step size of 100 nm. Following EBSD map acquisition and initial automated indexing, the raw datasets were processed and cleaned to remove artifacts and improve data quality prior to quantitative analysis. This processing was performed using the EDAX OIM Analysis 9 software package. The initial cleanup involved partitioning points with a Confidence Index (CI) below 0.1. Standard neighbor correlation routines, including grain dilation requiring 5 neighbors, were applied to refine orientations and fill non-indexed pixels. Subsequently, grains comprising fewer than 9 pixels were removed. Final grain reconstruction defined grains based on a 5° critical misorientation angle threshold. The resulting cleaned data were used for microstructural analysis.

2.4. Mechanical Properties Characterization

Uniaxial tensile tests were performed on a Zwick/Roell Z050 universal testing machine at a strain rate (έ) of 0.008 s−1 on cylindrical specimens with a gauge length of 50 mm. During the test, an extensometer was used for the precise determination of the yield strength. Tensile test were performed at room temperature in accordance with the ASTM E8/ASTM E8M standard [28]. For each sample, repeated tensile teste were performed.
Vickers microhardness measurements were carried out on the longitudinal section plane of the specimens using a Future-Tech microhardness tester. A test load of 300 g-force (gf) was applied for a dwell time of 15 s for each indentation. Multiple (10) indentations were performed across the cross-section of each sample to obtain an average hardness value.

3. Results

3.1. External Surface Observations

Typical surface morphologies of the extrudates are given in (Figure 1). Significant differences in surface quality were observed between the samples produced using KOBO extrusion at 350 °C (KOBO-350), KOBO extrusion at room temperature (KOBO-RT), and conventional extrusion at 350 °C (CE-350). The KOBO-350 extrudate exhibited relatively good surface quality, showing only minimal defects manifesting as a few small, localized cracks. Conversely, the KOBO-RT sample displayed notable surface damage characterized by more extensive cracking. The conventionally extruded CE-350 sample presented the smoothest surface finish of the three conditions, appearing largely defect-free at this magnification level.

3.2. Cellular Network Fragmentation

Figure 2 show typical light microscopy images of LPBF-produced AlSi10Mg alloy, sectioned longitudinally—along the build direction. The micrographs show typical microstructural features resulting from the LPBF process. Specifically, Figure 2a evidences the semi-circular (“fish-scale”) boundaries. These boundaries become visible after etching due to underlying microstructural differences, such as variations in the Si morphology.
Upon examining the interior of individual melt pools (Figure 2b), a fine and well-defined cellular microstructure can be observed. As revealed, the microstructure of LPBF-produced AlSi10Mg comprises primary α-Al cells, which appear relatively bright in the optical micrograph, surrounded by a continuous network. This network, identified as the Al-Si eutectic phase, appears darker due to preferential etching which creates visible contrast when sample is observed under reflected light.
The cellular microstructure morphology is a consequence of the rapid solidification kinetics inherent to the LPBF process. The prevailing high cooling rates, estimated to be in the range of 105–107 K/s [29], result from the combination of steep thermal gradients (G) and high solidification front velocities (R) [30]. These conditions drastically limit the time available for solute diffusion ahead of the advancing solid–liquid interface. Consequently, the solidification behavior deviates significantly from equilibrium [31]. The limited solute redistribution suppresses the formation of coarse dendrites, promoting instead the observed cellular/cellular-dendritic growth mode [32]. This leads to a substantial reduction in microstructural length scales, with cellular spacing (λ) of 0.5–1 µm frequently reported for LPBF Al-Si alloys [33,34,35].
A microstructural analysis following extrusion revealed significant fragmentation of the Al-Si eutectic network. The AlSi10Mg alloy sample subjected to conventional extrusion at 350 °C (CE-350) exhibited the smallest average Si particle size, approximately 0.18 μm2 (Figure 3a,b). In contrast, the room-temperature KOBO-processed sample exhibits a microstructure with Si particles of an intermediate average size of approximately 0.34 µm2 (refer to Figure 3c,d). The KOBO extrusion performed at 350 °C creates a microstructure with the largest Si particles among the three conditions, measuring approximately 0.47 µm2 (refer to Figure 3e,f). To chemically verify the identity of the measured particles, an EDS microanalysis was conducted. The results of this analysis, presented in Figure 4, confirm that these microstructural features are silicon precipitates.
The fragmentation of the silicon (Si) cellular network during the extrusion process is primarily driven by the significant difference in plastic deformation behavior of the ductile aluminum (α-Al) matrix and the brittle Si phase [36]. The mismatch in yield strength between aluminum and silicon results in pronounced strain partitioning, where the majority of the imposed macroscopic deformation is absorbed by the Al matrix, which has a lower flow stress. As a result, substantial stress concentrations build up within the constrained Si network. These stresses can lead to decohesion and the formation of microvoids, which facilitate crack propagation through the cellular Si network [37]. Furthermore, cyclic die rotation generates turbulent plastic flow, expected to randomize Si particle distribution. Nevertheless, the KOBO-processed sample contained larger Si particles. This indicates that frictional heating at the tool–workpiece interface, driven by the die oscillation, probably caused localized temperature spikes well above the nominal extrusion temperature of 350 °C. This could facilitate diffusion-controlled particle coarsening (e.g., via Ostwald ripening) [38]. Moreover, as highlighted in the study by Yeh et al. [39], the reciprocating nature of the extrusion process promotes strain-assisted diffusion, which could further facilitate the growth of Si particles.
Beyond the refinement of the cellular Al-Si eutectic network, light microscopy analysis reveals deformation-induced macroscopic changes in the microstructure of the LPBF samples, most notably a substantial reduction in porosity (densification). As shown in Figure 2a, the as-built sample exhibits a certain volume fraction of intrinsic porosity, with some defects, including small gas pores and lack-of-fusion voids (common artifacts of layer-wise additive manufacturing) [39,40,41,42], reaching lengths of 30–40 μm. Following extrusion, the light microscopy images reveal substantial densification. For all extruded samples, only small defects, typically in the size range of 2–5 μm, can be observed. Only in the KOBO-350 sample, several defects with a length of approximately 20 μm are still present. The observed substantial densification is attributed to the high hydrostatic compressive stresses inherent to the extrusion process. These conditions facilitate the collapse of the internal voids and promote intimate contact and potential diffusion bonding between the opposing pore surfaces, leading to a marked improvement in the material’s density compared to the initial LPBF state.

3.3. Analysis of the Grain Microstructure

In the next phase of investigation, electron backscatter diffraction was employed to characterize the grain microstructures of the extruded samples. The EBSD inverse pole figure (IPF) maps are presented in Figure 5. It is important to note that these maps exclusively identify the Al phase, as indexing Si using EBSD is technically challenging due to the close similarities between the diffraction patterns of Al and Si phases. It should also be noted that the microstructure of the as-built LPBF samples was characterized in a previous study [21].
A microstructural analysis of the extrudates revealed a banded (elongated) grain microstructure aligned parallel to the extrusion direction (ED) (Figure 6). This grain morphology develops due to the combined effects of tensile stress along the ED and compressive stress along the normal direction (ND) [43]. As revealed, the width of the elongated grains was significantly influenced by both the extrusion technique and the deformation temperature. Consequently, the CE-350 sample exhibits the smallest grain width (~1.9 µm), followed by the KOBO-20 sample with an intermediate width of (~3.4 µm), while the KOBO-350 sample has the largest grain width (~6.3 µm).
The EBSD IPF map of the CE-350 sample (Figure 5a) displays grains elongated along the ED. The dominant color in this map is green, reflecting [101] directions parallel to the TD. The histogram of grain boundary misorientation distribution (Figure 6a) reveals that the fractions of high-angle grain boundaries (HAGBs) and low-angle grain boundaries (LAGBs) are comparable in this condition. HAGBs (excluding twins) account for approximately 46.4% of the total boundary length fraction, while LAGBs comprise about 47.0%. Twin boundaries, which range between 55° and 65°, comprise roughly 6.6% of the total boundary length fraction (Figure 7a). The relatively high proportion of twin boundaries is likely a result of dynamic recrystallization (DRX) events [44] or static annealing processes that may occur concurrently or subsequently at the HAGBs.
The KOBO-RT sample microstructure is dominated by HAGBs, accounting for approximately 63.6% of the total boundary length fraction (excluding twins) (Figure 6b). In addition, this sample possesses the highest fraction of twin boundaries of ~9.4% (Figure 7b). It appears that the unique strain path in the KOBO process promotes development of grains with higher misorientations (specific orientation relationships with the parent matrix grains), likely due to enhanced grain rotation and boundary migration during deformation. These grains can be observed predominantly at the boundaries of the elongated grains. The presence of more “green” grains in the KOBO-RT sample indicates the development of a stronger deformation texture (Figure 6b).
The microstructure of the KOBO-350 sample is less refined than that of the KOBO-RT sample (Figure 5c). However, similarly to the KOBO-RT sample, HAGBs dominate the microstructure, constituting approximately 68.2% of the total boundary length fraction (excluding twins) (Figure 6c). At 7.8%, the twin boundary fraction is lower than in the KOBO-RT sample but higher than in the CE-350 sample (Figure 7c). The lower fraction of LAGBs at approximately 24.0% (lower than in KOBO-RT) indicates that DRV is not the dominant restoration mechanism. In addition, the significantly lower LAGB fraction compared to the CE-350 condition (despite the same extrusion temperature) suggests the potent effect of the cyclic change in the deformation path in stimulating HAGB generation/migration over subgrain formation. The largest grain width of approximately 6.3 µm correlates with the enhanced grain boundary mobility and dynamic recrystallization (DRX) kinetics under these conditions.

3.4. Mechanical Properties

Vickers hardness (HV) measurements were performed on the LPBF AlSi10Mg alloy to evaluate the influence of different extrusion routes (CE-350, KOBO-RT, KOBO-350) on mechanical properties. The results, summarized in Table 3, indicate that all extrusion processes led to a decrease in hardness compared to the as-built condition. Among the extruded samples, the highest hardness of 76 HV was measured for the CE-350 condition. The KOBO-RT and KOBO-350 samples exhibited lower hardness values of approximately 72 HV and 61 HV, respectively. To further characterize the mechanical response, subsequent uniaxial tensile testing was conducted.
The as-built sample demonstrates the highest strength, with a yield strength (YS) of 255 MPa and a tensile strength (TS) of 422 MPa; however, it exhibits limited ductility, with an elongation at break of only 2.4% (see Table 1). In contrast, all extruded samples present significantly lower strength values but markedly improved elongation. The room temperature KOBO produces extrudate with a yield strength of 133 MPa, a tensile strength of 238 MPa, and an elongation of 14.8%. Increasing the KOBO extrusion temperature to 350 °C leads to a further reduction in strength (YS: 114 MPa; UTS: 215 MPa) and a slightly lower elongation at break of 14.1%. The conventional extrusion produces extrudate with a yield strength of 186 MPa, a tensile strength of 285 MPa, and the highest elongation of 18.1%, as illustrated in Figure 8.
The substantial reduction in strength following extrusion process, observed across all processed conditions, can be understood by considering the microstructural evolution occurring during the thermo-mechanical post-processing. The high strength of the AlSi10Mg alloy in as-built condition is well-established to originate from unique, fine cellular microstructure, significant Si supersaturation within the α-Al matrix, and the existence of high density dislocations [45,46,47,48].
During both KOBO and conventional extrusion, the α-Al matrix grains are refined and the Si cellular network (which is beneficial to the mechanical properties of AlSi10Mg alloy) become fragmented (see Figure 3, Figure 4 and Figure 5). Furthermore, the grain microstructure is refined. While grain refinement typically contributes to strengthening via the Hall–Petch mechanism [49], this effect appears to be overshadowed by concurrent softening phenomena. The input of thermal energy—arising from friction and sample preheating (particularly for KOBO at 350 °C and conventionally extruded at 350 °C samples)—promotes dynamic recovery, recrystallization, and dislocation annihilation, thereby reducing the high dislocation density inherited from the rapid solidification condition in LPBF process. Furthermore, elevated deformation temperature facilitates the precipitation of excess Si from the supersaturated solid solution, which is followed by the coarsening of these Si precipitates (due to Ostwald ripening). It is recognized that coarser precipitates are less effective obstacles moving dislocations, than the Si cellular network (the absence of eutectic Si network reduces work hardening) [50,51,52]. The conventionally extruded sample showed the highest tensile strength (TS) and yield strength (YS). This elevated strength was likely due to Orowan strengthening and resulted from the highly dispersed silicon (Si) particles. In addition to this effect, grain boundaries with small-angle disorientation also contributed to grain boundary strengthening, as the CE-350 sample exhibited the highest fraction of low-angle grain boundaries (LAGBs), accounting for 46%.
Conversely, the dramatic improvement in ductility observed post-extrusion aligns with extensive literature on the post-processing of Al-Si alloys [53,54,55,56]. Several key factors contribute to the observed enhanced ductility of the LPBF AlSi10Mg alloy: (i) Extrusion promotes microstructural homogenization and assists in closing internal defects, such as micro-porosity and lack-of-fusion defects which are common in LPBF parts, reducing sites for premature crack initiation [57]. (ii) The process effectively breaks down the brittle, interconnected eutectic Al-Si network characteristic of the as-built state into more equiaxed, discrete particles dispersed within the significantly more ductile α-Al matrix (the stress localization can be alleviated by the refinement of the eutectic Si network). This morphological change allows the composite-like structure to accommodate greater plastic strain before fracture [58]. (iii) Significant relief of the high residual stresses inherent to the LPBF process occurs during extrusion, further contributing to improved ductility [59]. (iv) While the overall dislocation density is reduced (lowering strength), the resultant microstructure, potentially containing dynamically recovered or recrystallized grains, facilitates more uniform plastic deformation. (v) The deformed samples showed a notable presence of grains with misorientation angles ranging from 55 to 63.5 degrees, which is a characteristic of annealing twins. This finding, along with the previously observed high percentage of low Σ-CSL boundaries in KOBO-processed samples [21], indicates that twinning-related features have a significant impact on the enhanced elongation properties. The formation of these annealing twins is typically associated with recrystallization during hot extrusion. Research has demonstrated that a higher proportion of pre-existing annealing and deformation twins can improve the synergy between strength and ductility. This enhancement occurs because dislocations can glide and accumulate more easily along the twin boundaries [60].
While the above described factors collectively enhanced ductility, the specific strength–ductility balance achieved varied significantly, correlating with microstructural features reported in this study. All extrusion processes induced a characteristic (101)//TD texture, known to influence deformation anisotropy [61]; notably, this texture was more pronounced in the KOBO-RT sample compared to CE-350. However, the dominant factor explaining the variations in measured strength across the conditions was the final grain size. The CE-350 sample, combining the finest grain width (~1.9 µm) with the strong (101)//TD texture, exhibited the highest yield strength and overall best property balance. Conversely, despite also developing a fiber texture, the substantial grain coarsening (~6.3 µm) in KOBO-350 led to the lowest strength. The intermediate grain size (~3.4 µm) and stronger texture of KOBO-RT yielded intermediate properties. Thus, while the development of texture is integral to the extrusion process and affects deformation pathways, grain size refinement appeared paramount in controlling the strength levels observed in this comparative study.

4. Conclusions

This study presented a comparative analysis of the microstructure and resulting mechanical properties of an LPBF AlSi10Mg alloy after post-processing extrusion. The investigation focused specifically on characterizing and evaluating materials extruded via the KOBO method (at 20 °C and 350 °C) and through conventional extrusion. The main findings from this study are as follows:
  • Post-processing via extrusion substantially densified the LPBF AlSi10Mg alloy, eliminating the majority of the initial porosity inherent to the additive manufacturing process.
  • The degree of eutectic Si particle refinement was highly sensitive to the specific extrusion parameters. Conventional extrusion at 350 °C (CE-350) yielded the highest degree of Si particle refinement, resulting in a mean particle area of ≈0.18 µm2. In contrast, KOBO extrusion at the same nominal temperature (KOBO-350) led to significant particle coarsening, yielding a mean Si particle area of ≈0.47 µm2.
  • The resulting grain structure varied markedly with the applied extrusion conditions. CE-350 sample possessed the finest microstructure (average grain width ≈ 1.9 µm). In contrast, KOBO extrusion yielded microstructures, with average widths of ≈3.4 µm after room-temperature processing (KOBO-RT) and ≈6.3 µm after processing at 350 °C (KOBO-350).
  • Extrusion significantly enhanced the ductility of the LPBF AlSi10Mg alloy (elongations exceeding 14% compared to 2.4% as-built), albeit accompanied by a reduction in yield strength. Conventional extruded (CE-350) sample demonstrated the most advantageous property combination within this study, delivering the maximum elongation (18.1%) and a yield strength (186 MPa) superior to that of the materials processed by the KOBO method under the investigated parameters.

Author Contributions

P.S.: conceptualization, methodology, validation, formal analysis, resources, data curation, writing—original draft, visualization, supervision, project administration, founding acquisition; K.M.: data curation, investigation. P.O.: data curation, conceptualization, investigation, writing—original draft. K.Ż.: data curation, conceptualization, investigation, writing—original draft. All authors have read and agreed to the published version of the manuscript.

Funding

The research was funded by the National Science Centre, Poland, based on the decision number 2021/43/D/ST8/01946. Research project supported by program “Excellence initiative—research university” for the AGH University.

Data Availability Statement

Data available on request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Stereoscopic images of the surface morphology of AlSi10Mg alloy samples after extrusion: (a) KOBO-350, (b) KOBO-RT, and (c) CE-350.
Figure 1. Stereoscopic images of the surface morphology of AlSi10Mg alloy samples after extrusion: (a) KOBO-350, (b) KOBO-RT, and (c) CE-350.
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Figure 2. Light microscopy images of the longitudinal section of microstructure of LPBF-fabricated AlSi10Mg alloy: (a) Overview at 200× magnification showing characteristic overlapping melt pool boundaries and the general cellular structure within. (b) Detailed view at 1000× magnification resolving the fine cellular morphology, consisting of primary α-Al cells surrounded by the intercellular eutectic Si network.
Figure 2. Light microscopy images of the longitudinal section of microstructure of LPBF-fabricated AlSi10Mg alloy: (a) Overview at 200× magnification showing characteristic overlapping melt pool boundaries and the general cellular structure within. (b) Detailed view at 1000× magnification resolving the fine cellular morphology, consisting of primary α-Al cells surrounded by the intercellular eutectic Si network.
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Figure 3. Microstructures of the longitudinal section of (a,b) CE-350 sample, (c,d) KOBO-RT sample, and (e,f) KOBO-350 sample.
Figure 3. Microstructures of the longitudinal section of (a,b) CE-350 sample, (c,d) KOBO-RT sample, and (e,f) KOBO-350 sample.
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Figure 4. Results of EDS chemical composition microanalysis performed for KOBO-RT sample: (a) SEM image, (b) results of pointwise chemical composition microanalysis, and (c–e) the EDS spectra corresponding to measurement points p1–p3.
Figure 4. Results of EDS chemical composition microanalysis performed for KOBO-RT sample: (a) SEM image, (b) results of pointwise chemical composition microanalysis, and (c–e) the EDS spectra corresponding to measurement points p1–p3.
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Figure 5. TD EBSD IPF maps of the LPBF AlSi10Mg alloy: (a) conventional extrusion at 350 °C (CE-350); (b) after KOBO extrusion at 20 °C (KOBO-RT); (c) KOBO extrusion at 350 °C (KOBO-350); (d) the corresponding color-coded stereographic triangle (black—HAGBs; red—LAGBs).
Figure 5. TD EBSD IPF maps of the LPBF AlSi10Mg alloy: (a) conventional extrusion at 350 °C (CE-350); (b) after KOBO extrusion at 20 °C (KOBO-RT); (c) KOBO extrusion at 350 °C (KOBO-350); (d) the corresponding color-coded stereographic triangle (black—HAGBs; red—LAGBs).
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Figure 6. The grain boundary misorientation distributions for the three extruded samples: (a) AlSi10Mg alloy after conventional extrusion at 350 °C; (b) AlSi10Mg alloy after KOBO extrusion at 20 °C; (c) AlSi10Mg alloy after KOBO extrusion at 350 °C. Obvious deviation of the misorientation angle distribution from the theoretical MacKenzie distribution can be seen in histograms, which confirm a strong texture resulting from deformation.
Figure 6. The grain boundary misorientation distributions for the three extruded samples: (a) AlSi10Mg alloy after conventional extrusion at 350 °C; (b) AlSi10Mg alloy after KOBO extrusion at 20 °C; (c) AlSi10Mg alloy after KOBO extrusion at 350 °C. Obvious deviation of the misorientation angle distribution from the theoretical MacKenzie distribution can be seen in histograms, which confirm a strong texture resulting from deformation.
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Figure 7. TD EBSD IPF map revealing the 55°–65° boundary distribution (black lines) and reconstructed Al/Si phase interfaces (red lines) for (a) AlSi10Mg alloy after conventional extrusion at 350 °C, (b) AlSi10Mg alloy after KOBO extrusion at 20 °C, and (c) AlSi10Mg alloy after KOBO extrusion at 350 °C. The EBSD analysis reveals that the detected twin boundaries are not attributable to twinned Si precipitates.
Figure 7. TD EBSD IPF map revealing the 55°–65° boundary distribution (black lines) and reconstructed Al/Si phase interfaces (red lines) for (a) AlSi10Mg alloy after conventional extrusion at 350 °C, (b) AlSi10Mg alloy after KOBO extrusion at 20 °C, and (c) AlSi10Mg alloy after KOBO extrusion at 350 °C. The EBSD analysis reveals that the detected twin boundaries are not attributable to twinned Si precipitates.
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Figure 8. Experimental (representative) stress strain curves obtained from the uniaxial tensile tests.
Figure 8. Experimental (representative) stress strain curves obtained from the uniaxial tensile tests.
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Table 1. LPBF process parameters used to fabricate the AlSi10Mg alloy samples.
Table 1. LPBF process parameters used to fabricate the AlSi10Mg alloy samples.
ParameterValue
Laser Power175 W
Layer Thickness20 μm
Scanning Speed1400 mm/s
Scan Rotation67°
Table 2. Extrusion parameters used in current study.
Table 2. Extrusion parameters used in current study.
SampleKOBO-350KOBO-RTCE-350
Post-processing MethodKOBO ExtrusionKOBO ExtrusionConventional Extrusion
Extrusion Temperature350 °CRoom Temperature (~20 °C)350 °C
Die Oscillation Frequency5 Hz5 HzN/A
Die Twist Angle±8°±8°N/A
Billet Heating Duration/Method15 min in press containerN/A15 min in press container
Table 3. Summary of mechanical properties of analyzed samples.
Table 3. Summary of mechanical properties of analyzed samples.
As-BuiltCE-350KOBO-RTKOBO-350
Hardness, HV0.3117 ± 276 ± 1.772 ± 1.961 ± 1.2
Yield strength (YS), [MPa]255 ± 3186 ± 3133 ± 3114 ± 3
Tensile strength (TS), [MPa]422 ± 4285 ± 3238 ± 4215 ± 4
Elongation, %2.4 ± 0.118.1 ± 0.214.8 ± 0.214.1 ± 0.4
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Snopiński, P.; Ostachowski, P.; Matus, K.; Żaba, K. The Influence of Grain Structure on Mechanical Properties of LPBF AlSi10Mg Alloy Obtained via Conventional and KOBO Extrusion Process. Symmetry 2025, 17, 709. https://doi.org/10.3390/sym17050709

AMA Style

Snopiński P, Ostachowski P, Matus K, Żaba K. The Influence of Grain Structure on Mechanical Properties of LPBF AlSi10Mg Alloy Obtained via Conventional and KOBO Extrusion Process. Symmetry. 2025; 17(5):709. https://doi.org/10.3390/sym17050709

Chicago/Turabian Style

Snopiński, Przemysław, Paweł Ostachowski, Krzysztof Matus, and Krzysztof Żaba. 2025. "The Influence of Grain Structure on Mechanical Properties of LPBF AlSi10Mg Alloy Obtained via Conventional and KOBO Extrusion Process" Symmetry 17, no. 5: 709. https://doi.org/10.3390/sym17050709

APA Style

Snopiński, P., Ostachowski, P., Matus, K., & Żaba, K. (2025). The Influence of Grain Structure on Mechanical Properties of LPBF AlSi10Mg Alloy Obtained via Conventional and KOBO Extrusion Process. Symmetry, 17(5), 709. https://doi.org/10.3390/sym17050709

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