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Article

The Role of Friction Stir Processing Travel Speed on the Microstructure Evolution and Mechanical Performance of As-Cast Hypoeutectic Al-5Si Alloy

by
Basma El-Eraki
1,
Mahmoud F. Y. Shalaby
2,
Ahmed El-Sissy
1,
Abeer Eisa
1,
Sabbah Ataya
3 and
Mohamed M. El-Sayed Seleman
4,*
1
Production Engineering and Mechanical Design Department, Faculty of Engineering, Menoufia University, Shebin Elkom 32511, Egypt
2
Department of Industrial Engineering, College of Engineering, Imam Mohammad Ibn Saud Islamic University, Riyadh 11432, Saudi Arabia
3
Department of Mechanical Engineering, College of Engineering, Imam Mohammad Ibn Saud Islamic University, Riyadh 11432, Saudi Arabia
4
Department of Metallurgical and Materials Engineering, Faculty of Petroleum and Mining Engineering, Suez University, Suez 43512, Egypt
*
Author to whom correspondence should be addressed.
Crystals 2025, 15(6), 546; https://doi.org/10.3390/cryst15060546
Submission received: 10 May 2025 / Revised: 2 June 2025 / Accepted: 3 June 2025 / Published: 6 June 2025
(This article belongs to the Special Issue Development of Light Alloys and Their Applications)
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Abstract

:
This study’s novelty lies in providing first-time insights into the isolated role of Friction Stir Processing (FSP) travel speed on microstructure evolution and mechanical performance (micro-hardness, tensile properties, impact energy, and wear behavior) specifically in hypoeutectic as-cast Al-5 wt.% Si alloys, addressing a critical unaddressed gap in previous works (typically on near-eutectic compositions of Si > 6.5 wt.%). FSP, a solid-state technique, is highly effective for enhancing the properties of cast materials. The FSP was conducted at a fixed rotational speed of 1330 rpm and various travel speeds (26, 33, 42, and 52 mm/min). The FSP improves the mechanical properties of as-cast Al-5Si alloy by refining its grain structure. This leads to higher hardness, ultimate tensile strength (UTS), yield strength (YS), and strain at fracture and toughness compared to the as-cast condition. The specimen processed at 26 mm/min achieved the highest values of YS, UTS, toughness, and wear resistance. The fracture surfaces of the tensile and impact test specimens were examined using scanning electron microscopy (SEM) and discussed. Results indicated that the fracture surfaces revealed a transition from predominantly brittle fracture in the as-cast alloy to ductile fracture at 26 mm/min, changing to a mixed fracture mode at 52 mm/min. These findings underscore the critical importance of optimizing FSP travel speed to significantly tailor and enhance the mechanical performance of as-cast hypoeutectic Al-5Si alloys for industrial applications.

1. Introduction

Aluminum–silicon (Al-Si) alloys have garnered significant attention in various industries owing to their exceptional properties, like being lightweight, having high thermal conductivity, and having excellent castability [1]. These attributes make Al-Si alloys particularly advantageous in marine, aerospace, and automotive applications, where minimizing weight without sacrificing strength is crucial [2,3]. However, the mechanical properties of as-cast hypoeutectic Al-Si alloys often require enhancement to meet the demanding service conditions of these sectors. The production of Al-Si alloys can be achieved through a range of techniques, including traditional casting [4,5,6,7], semi-solid metal processing [8,9,10,11,12], and modern additive manufacturing methods [13,14,15]. While various methods exist, gravity die casting is notable for producing high-quality components with excellent dimensional accuracy and reduced common casting defects such as porosity and shrinkage cavities [16,17]. The precision and efficiency of gravity die casting make it an ideal choice for producing complex and critical parts in various industrial applications [18].
One of the primary challenges in optimizing Al-Si alloys lies in controlling the microstructure, particularly the distribution and size of the silicon phase within the aluminum matrix. The as-cast structure typically exhibits coarse silicon particles, which lead to reduced ductility and impact strength. Traditional modification techniques such as the addition of alloying elements [19,20,21,22,23,24,25,26] and heat treatment [27,28,29,30,31,32] have long been employed to modify this cast structure. Additionally, techniques such as laser surface melting [33,34] and shot peening [35,36] have been explored to refine microstructures and improve surface characteristics, but these methods often result in trade-offs between strength and ductility. Among these approaches, friction stir processing (FSP) has gained recognition as a promising method for modifying the microstructure of Al-Si alloys [37,38,39,40], offering a unique approach to enhancing mechanical properties without the drawbacks of conventional methods [41,42,43,44,45,46,47]. FSP is a solid-state process that evolved from friction stir welding (FSW). This fundamental difference is crucial as it avoids melting and the associated issues of solidification shrinkage, elemental segregation, and the formation of brittle intermetallic compounds, which can often degrade the mechanical properties of the treated zone [48,49]. Thus, it has been widely adopted for modifying the internal structure and improving the mechanical performance of various metals [50,51,52,53,54,55,56,57,58,59,60]. FSP involves the localized plastic deformation of the material using different rotating and traveling tool speeds, which generates significant heat and mechanical work, leading to grain refinement and homogenization of the microstructure [61,62,63].
For aluminum alloys, FSP offers distinct advantages such as significant grain refinement, effective elimination of casting defects like porosity, and homogeneous distribution of existing phases, leading to superior mechanical properties without introducing foreign materials or altering the bulk composition significantly. Recent studies continue to underscore its versatility in enhancing the performance of various aluminum-based composites [59,64,65,66], and other research efforts have also focused on applying FSP for the surface modification of Al-Si alloys using FSP. Ma et al. [61] investigated the FSP of as-cast A356 alloy, varying tool design and rotation speed (700–1100 rpm) at a constant travel speed of 203 mm/min. They found that FSP enhanced ductility and strength by refining the microstructure, dispersing Si-phases, and reducing micro-voids. A higher tool rotation speed was identified as the primary factor for these improvements, whereas the effect of tool geometry was found to be non-systematic. Guru et al. [67] studied the impact of applying a tool speed of 800 rpm and a travel speed of 120 mm/min as FSP parameters on the machinability, hardness, and tensile properties of the hypoeutectic Al-6.5Si cast alloy. Results showed that the processed alloy had enhanced hardness, yield strength, ultimate tensile strength, and machinability over the as-cast alloy. Meenia et al. [41] investigated the influence of the number of passes of FSP at 800 rpm, 9 kN, and 120 mm/min process parameters on the microstructure, hardness, and tensile properties of both as-cast and solution-treated Al-6.5Si alloy. It was reported that multi-pass FSP significantly refined the microstructure, homogenized the material, and eliminated casting porosity in both initial conditions. Singh et al. [42] aimed to enhance the wear resistance and machinability of A356 through multi-pass FSP using the process parameters of 800 rpm tool rotating speed and 120 mm/min travel speed. Their results demonstrated that three FSP passes yielded the highest wear resistance under metallic conditions, while two passes were optimal for abrasive conditions. In dry drilling experiments, the drilling force and surface roughness decreased with up to two passes but increased with additional passes. Additionally, three FSP passes minimized burr formation at the entry and exit holes. Cui et al. [67] also aimed to study the role of number of passes during the FSP of a hypoeutectic Al-6.93Si alloy at scattered process parameters in terms of rotation and travel speeds. They found that FSP on this cast alloy effectively reduces porosity and breaks down/disperses large silicon particles, iron-rich phases, and Mg2Si particles, while also dissolving most of the Mg2Si. Multi-pass FSP, particularly two passes, significantly enhances the internal structural features and tensile characteristics compared to a single pass. A rotation speed of 1500 rpm and a travel speed of 30 mm/min for the two-pass FSP were identified as the optimal parameters for achieving the best mechanical properties. Alidokht et al. [68] examined the impact of applying various tool rotation speeds (500–1250 rpm), a tool tilt angle of 3°, and a constant travel speed of 50 mm/min during the FSP to modify the microstructure of as-cast A356 Al alloy material. They reported that higher tool rotation speed resulted in a more refined microstructure due to enhanced stirring. In a sample produced at 1250 rpm, the Si particles were significantly smaller (3.3 μm vs. 16.2 μm), less elongated (aspect ratio 1.1 vs. 8.5), and the grain size was much smaller (1 μm vs. 94 μm) compared to the untreated A356 alloy. In addition, significant improvements in hardness and wear resistance were detected at all applied rotation speeds.
Soleymanpour et al. [69] investigated the impact of FSP traverse speeds (15–240 mm/min) at 800 rpm on the microstructural and mechanical properties of pre-treated (heat-treated and cold-rolled) hypoeutectic Al-7Si alloy. The results indicated that the as-cast material exhibited higher average hardness compared to the FSP-treated samples, likely owing to the presence of larger second-phase particles. However, FSP improved tensile properties through microstructural refinement and particle modification. Furthermore, strength increased with traverse speed up to 120 mm/min due to smaller silicon and Fe-rich particles but decreased at higher speeds likely due to cavity formation. Baruch et al. [70] investigated the effects of multi-pass FSP (two and three passes) on a die-cast Al-7Si-3Cu alloy, using fixed parameters (600 rpm, 12 mm/min). The results indicated that FSP refines and redistributes Al-Si eutectic particles and Al dendrites, with more passes leading to finer particles. Specifically, three-pass FSP enhanced the tensile strength and ductility of the processed material, though the overall hardness after FSP was lower than that of the original as-cast alloy. In a single-pass FSP study, Kumar et al. [71] examined the influence of a 960-rpm tool rotation speed and a 50 mm/min travel speed on an as-cast Al-6.95Si-0.6Mg-0.31Cu-0.5Fe alloy. The processing led to a 40% enhancement in hardness and a 14% rise in tensile strength, while ductility decreased by 24% relative to the as-cast state.
Based on the above review, although numerous studies have been conducted on the effects of FSP variables on the properties of as-cast hypoeutectic Al-Si alloys, particularly near-eutectic compositions (Si content > 6.5 wt.%), limited attention has been given to exploring its impact on the hypoeutectic Al-5Si alloy.
Furthermore, previous works have predominantly employed fixed or scattered FSP parameters, with most focusing on the role of tool rotation speed and the number of FSP passes at a constant processing speed [70,71] without a systematic study of travel speed effects. To address these gaps, the present work aims to explore the impact of varying travel speeds (26, 33, 42, and 52 mm/min) at a constant high rotational speed (1330 rpm) on the microstructural evolution and overall mechanical properties, specifically hardness, tensile properties, impact strength, and wear behavior of the hypoeutectic Al-5Si alloy, to establish correlations between processing parameters and performance.

2. Materials and Methods

2.1. Casting Process

The experimental work began with the casting of hypoeutectic Al-5Si alloy plates using the gravity permanent die casting method. This technique was chosen for its capacity to yield high-quality parts characterized by precise dimensions and a surface finish. As shown in Figure 1, the process involved preparing a steel permanent mold designed to ensure uniform cooling and minimize casting defects. To ensure optimal thermal conditions and reduce the risk of thermal shock or stress-induced flaws, the mold was preheated to 220 °C using a heating flame. Mold preheating also enhanced the surface finish and dimensional accuracy of the final casting. For preparation of the alloy, 3 kg of pure Al (≥99.7% purity) was melted and 5% Si by weight was added to create the hypoeutectic Al-5Si alloy. The molten mixture was stirred for 5 min to homogenize the Si distribution within the Al matrix. This stirring process was critical for achieving uniform composition and reducing the segregation of silicon particles. The molten Al-Si alloy was heated to 750 °C and poured into the preheated mold using gravity. This controlled and steady filling process ensured uniform filling of the mold cavity, reducing the risk of porosity and other common casting defects. The pouring temperature was carefully selected to keep the alloy in a fully liquid phase throughout the process, ensuring enhanced flowability and filling properties. After the alloy solidified, the cast plates were carefully removed from the mold and immediately subjected to rapid cooling by immersion in water. This quenching process not only prevented the formation of coarse microstructures but also enhanced the mechanical properties and structural integrity of the cast plates by reducing residual stresses. Following solidification and cooling, the plates were precisely machined to dimensions of 240 mm × 140 mm × 6 mm using a CNC milling machine. This machining step was critical to achieving uniformity in sample preparation, ensuring that all plates had consistent dimensions and surface finishes. The high precision of the CNC milling process was vital for minimizing dimensional variability, which could otherwise impact the reliability of FSP experiments. The consistent surface quality and exact dimensions of the plates provided a uniform baseline, enabling accurate comparisons during subsequent mechanical testing and microstructural analysis. This meticulous preparation step ensured reproducibility and reliability in all downstream processes, including FSP and characterization.

2.2. Chemical Composition Analysis

The precise chemical composition of the as-cast hypoeutectic Al-5Si alloy was determined through analysis performed with a spectrometer (Belec Spektrometrie Opto-Elektronik GmbH model, Georgsmarienhütte, Germany). The analysis results, presented in Table 1, outline the elemental composition of the as-cast Al-5Si alloy and confirm its compliance with the intended specifications.

2.3. Friction Stir Processing (FSP)

The as-cast plates were then subjected to FSP to modify their microstructure using an FSP tool machined from H13 tool steel. This tool featured a cylindrical design with a 25 mm shoulder diameter, 5 mm pin diameter, and 3 mm pin length (Figure 2). The chosen shoulder-to-pin diameter ratio of was based on our lab experience and aligns with commonly accepted practices in published work for effective material flow and frictional heat generation [63].
The FSP was achieved using a milling machine at varying travel speeds of 26, 33, 42, and 52 mm/min and a constant tool rotation speed of 1330 rpm. The tilt angle was fixed at 3° for all of the processed specimens. Figure 3 reprents the FSP process and the processed plate.
The selection of the 1330 rpm rotation speed was optimized based on extensive experimental trials and supported by published work [41,69]. Our goal in this study was to systematically explore the impact of travel speed, and we specifically chose parameters within the lower processing range to investigate this less-explored window.

2.4. Sample Preparation for Microstructure, Mechanical, and Wear Testing

The plates were cut into specified samples using a Wire Cut Electrical Discharge Machining (EDM) machine, as shown in Figure 4. This precision cutting method was crucial for preparing samples for mechanical and wear tests while maintaining the integrity of the processed material. Following the FSP, samples were cut and prepared for microstructural analysis, tensile strength testing, and microhardness assessment. Microstructural observations were conducted on cross sections of the stir zone (SZ) perpendicular to the FSP direction. The samples were polished and etched with Keller’s Agent (45 mL HCl, 15 mL HF, 15 mL HNO3, 25 mL H2O) at room temperature for a few seconds until the desired contrast was achieved. The microstructural characterization was performed using an optical microscope (model Leco LX 31- St. Joseph, MI, USA). Tensile test specimens were extracted from the processed surface parallel to the FSP direction and prepared to the required dimensions according to the ASTM E8/E8M-11 standard. Figure 5 shows the typical specimen dimensions and the specimen under tensile testing. Tensile tests were conducted using a universal testing machine (WDW-300, Jinan, China) with a cross head of 1 mm/min. Hardness was measured in the SZ using a microhardness tester (model Leco LM 700, Michigan, USA) with a dwell time of 15 s and a 100 gf test load. Impact specimens were prepared to Charpy standard dimensions, as shown in Figure 6. The impact testing was performed using an impact tester machine (model JBW-500 N, Jinan Testing Equipment IE Corporation, Jinan, China). The tensile and impact specimens’ fracture surfaces were analyzed using a scanning electron microscope (SEM) Qunta FEG-250, Hillsboro, OR, USA.
The wear test was conducted using a pin-on-disk tribometer wear testing machine (model TM 260 GUNT, Gerätebau GmbH, Hamburg, Germany) in accordance with the ASTM G99-05 standard. The wear specimens were extracted from the SZ, with the pin axis oriented perpendicular to the FSP pass. Prismatic pins were prepared with dimensions of 4 mm diameter and 25 mm length. The counterface disk, made of hardened ground stainless steel, had a hardness of 62 HRC and a sliding track diameter of 40 mm. The tests were accomplished under dry conditions at a fixed load of 10 N and rotational speed of 300 rpm. Figure 7 shows the wear testing machine configuration.

3. Results and Discussion

3.1. Top Surface Features of Friction Stir Processed Specimens

Figure 8 shows photographs of the top surface of the friction stir processed tracks on the as-cast Al-5Si alloy plates, processed at a constant tool rotation speed of 1330 rpm and at different tool travel speeds using an H13-tool steel tool (25 mm shoulder, 5 mm pin diameter, 3 mm pin length). It can be seen that distinct surface characteristics correlate with the varying travel speeds of 26, 33, 42, and 52 mm/min. These variations likely reflect differences in material deformation, heat input, and the interaction of the tool shoulder and pin with the alloy surface [63]. As demonstrated in Figure 8a–d, at a constant tool rotation speed of 1330 rpm, roughness of the processed surface increased with processing travel speed from 26 to 52 mm/min. This increased the roughness and potential voids at higher travel speeds (42 and 52 mm/min, Figure 8b and Figure 8c, respectively) can be attributed to potentially insufficient time for adequate material flow and stirring. Notably, at the highest speed (52 mm/min), the resulting surface displayed spaced apart tool marks with raised characters and scattered protrusions (Figure 8d), suggesting insufficient heat input leading to incomplete mixing. Conversely, the lowest travel speed (26 mm/min) resulted in a surface displaying smooth ridges with pronounced shoulder-induced flow marks (Figure 8a), likely due to prolonged heat input and excessive material flow. Similar features appeared at a processing speed of 33 mm/min, indicating a more balanced heat input and material stirring.

3.2. Microstructure Characterization

Figure 9 presents the OM images of the microstructure of both the as-cast and the friction stir processed (FSPed) hypoeutectic Al-5Si alloy processed at a tool rotating speed of 1330 rpm and different travel speeds. As shown in Figure 9a, the as-cast Al-5Si alloy microstructure features coarse dendritic α-Al arms surrounded by eutectic Si. Porosity was randomly detected in some regions (Figure 9b), and no primary free-Si particles were observed. These microstructural features are similar to those reported by other researchers [41,44], with the difference being the absence of detected primary Si, which is typically found in near-eutectic Al-Si alloys with a Si content higher than 6.5 wt%. This coarse dendritic structure for the as-cast Al-Si alloys, while providing some strength, also results in limited ductility and reduced impact toughness [42]
The effect of FSP at different travel speeds (26–52 mm/min) on the microstructure of the Al-5Si alloy is evident through the significant modifications in its microstructure features, including grain size reduction, porosity elimination, and the breakdown of the primary and secondary arm spacing compared to the as-cast state, as given in Figure 9c–f. It can be noticed that the FSPed alloy exhibits distinct microstructural zones, each characterized by unique structural features. These zones include the stir zone (SZ), the thermomechanically affected zone (TMAZ), the heat-affected zone (HAZ), and the base metal (BM). The stir zone (SZ), situated at the center of the processed region, undergoes intense plastic deformation and dynamic recrystallization, resulting in significant grain refinement. This refined microstructure enhances mechanical properties, contributing to increased toughness, ductility, and uniformity in hardness distribution. The fine and equiaxed grains in this region indicate efficient recrystallization due to the combined effects of frictional heat and severe plastic deformation [42]. Adjacent to the stir zone, the thermomechanically affected zone (TMAZ) experiences both mechanical deformation and thermal exposure. However, unlike the SZ, the grains in this region are elongated and oriented along the direction of material flow, suggesting incomplete recrystallization. This zone serves as a transitional region between the SZ and HAZ, where the grain structure is partially refined but still retains some characteristics of the original dendritic microstructure. The heat-affected zone (HAZ), which lies beyond the TMAZ, undergoes thermal exposure without mechanical deformation. This leads to a partially modified grain structure where coarsening of grains may occur due to the heat input from FSP. The absence of significant plastic deformation in this zone differentiates it from the TMAZ, and the grain morphology largely depends on the extent of heat dissipation. Lastly, the base metal (BM) remains unprocessed and retains the coarse dendritic structure typical of as-cast Al-Si alloys. The contrast between the base metal and the processed zones highlights the substantial microstructural refinement achieved through FSP.
These microstructural modifications, as depicted in Figure 10, demonstrate the effectiveness of FSP in refining grain structure, improving homogeneity, and enhancing the overall mechanical properties of the hypoeutectic Al-5Si alloy. These findings are consistent with previous research, which has also reported significant microstructural and mechanical improvements in FSPed AA356 aluminum alloy [46].
Figure 10 and Figure 11 represent grain size histograms and the mean size of (α-Al), respectively, for the as-cast and processed specimens at 1330 rpm and different travel speeds (26–52 mm/min). The fragmentation of the dendritic structure in Al-Si alloys through FSP represents a significant advancement in microstructural modification techniques. In the as-cast condition, the alloy displays a coarse dendritic morphology, with a primary dendritic arm spacing (PDAS) of 170.47 ± 54.12 µm (Figure 10a) and a secondary dendritic arm spacing (SDAS) of 26.24 ± 4.79 µm (Figure 10b). Such features are characteristic of solidification in hypoeutectic Al-Si alloys and result in a non-uniform microstructure. Conversely, a significant grain size reduction is present in all of the processed Al-5Si alloys at different travel speeds. The α-Al grain sizes were 12.46 ± 1.59 µm (Figure 10c), 9.55 ± 1.51 µm (Figure 10d), 7.87 ± 1.38 µm (Figure 10e), and 7.75 ± 1.34 µm (Figure 10f) for the applied travel speeds of 26, 33, 42, and 52 mm/min, respectively. This notable refinement is primarily due to the intense plastic deformation and elevated thermal gradients induced during the stirring action of the FSP, which fragment the dendritic network and facilitate dynamic recrystallization [42,44]. This modification leads to a more refined and homogeneous microstructure and contributes to an increase in both tensile strength and impact energy, indicating an improvement in the overall mechanical properties. These findings are consistent with those reported in previous studies [42,44], demonstrating the potential of FSP in tailoring the as-cast solidified structure of Al-Si cast alloys.

3.3. Mechanical Properties of the As-Cast and the FSPed Al-5Si Alloy

3.3.1. Tensile Properties

To assess the tensile properties of the hypoeutectic Al-5Si alloy in both as-cast and FSPed conditions, tensile tests were performed. Figure 12 shows the stress–strain curves of the as-cast Al-5Si alloy samples before and after FSP at 1030 rpm and various travel speeds (26–52 mm/min). The resulting stress–strain curves, presented in Figure 12, provide crucial insights into the material’s overall deformation behavior under tensile loading. Monitoring the stress–strain curves reveals a significant improvement in tensile properties, specifically ultimate tensile strength (UTS) and strain at fracture, for all of the FSPed specimens across the investigated travel speed range (26–52 mm/min) compared to the as-cast Al-5Si alloy. Furthermore, the yield strength (YS) also generally increased in the processed samples relative to the as-cast state. This overall enhancement in mechanical properties can be attributed to beneficial microstructural modifications induced by FSP, such as porosity reduction or elimination, grain refinement within the stir zone, and the disruption of the coarse as-cast dendritic structure [41,42,61]. In addition, analysis of these curves will focus on key mechanical properties such as UTS, YS, and strain at fracture, as given in Figure 13, which can indicate differences in the material’s response to applied stress as a function of the FSP travel speed and the initial as-cast condition.
The relationship between UTS, YS, ductility, and grain size in the microstructure-modified Al-5Si alloy through FSP at various travel speeds reveals critical insights into the mechanical behavior of the material. In the as-cast condition, the alloy exhibits a relatively coarse grain size of 26.24 ± 4.79 µm, which correlates with the lowest observed UTS (108.7 MPa), YS (57 MPa), and strain at fracture (3.6 %) compared to the processed specimens. This reduced strength and ductility is primarily attributed to the dendritic microstructure and porosity, which lacks the ability to effectively resist applied loads during tensile testing. In the case of FSP, the alloy experiences significant grain refinement and porosity elimination which typically enhances its mechanical performance. For instance, at a travel speed of 26 mm/min, the grain size is refined to 12.26 ± 1.59 µm, accompanied by a notable increase in UTS (164.5 MPa), YS (73.5 MPa), and strain at fracture (10.2%). The sample FSPed at 33 mm/min shows slightly reduced, but still enhanced, mechanical properties compared to the 26 mm/min sample, with UTS at 157.4 MPa, YS at 65.3 MPa, and strain at fracture at 10.1%. This improvement in the lower travel speeds (26 and 33 mm/min) can be ascribed to multiple mechanisms associated with FSP, including the fragmentation and uniform redistribution of eutectic Si and the dendritic arms, which enhance the load-bearing capacity of the modified aluminum matrix. Grain refinement contributes not only to increased strength but also to improved ductility and toughness, as indicated by the strain at fracture (Figure 12 and Figure 13), as it promotes uniform plastic deformation and mitigates casting-related defects. At the higher speeds of 42 and 52 mm/min the improvement in UTS and strain at fracture compared to the as-cast Al-5Si is still observed (these values were 143 MPa, 10.9%, and 131.2 MPa, 10%, respectively), but the improvement is lower than that obtained at the lower speeds of 26 and 33 mm/min. For YS, a noticeable improvement is observed at the lower processing speeds of 26 and 33 mm/min. This enhancement can be attributed to improved material flow and dynamic recrystallization during FSP, which strengthens the matrix and hinders dislocation motion.
However, no improvement was detected at the higher speeds of 42 and 52 mm/min compared to the as-cast material. While increased travel speeds in this range continue to refine grain size, the reduced heat input per unit length and insufficient plasticization at these higher speeds become the dominant factors compromising mechanical properties. This can lead to incomplete material consolidation, less-effective material stirring, and the formation of residual defects such as micro-voids, tunnels, or incomplete grain boundary bonding, which are detrimental to mechanical integrity. Consequently, these processing-induced defects override the strengthening effect of further grain refinement, leading to the observed decrease in strength at higher travel speeds. These results underscore the critical role of travel speed in influencing YS during FSP, where higher speeds, despite finer grains, may diminish mechanical performance due to this complex interplay of heat input, material flow, microstructural stability, and defect formation. In fact, the mechanical properties are highly dependent on processing conditions, highlighting the importance of optimizing FSP parameters to maximize mechanical performance while preserving structural integrity.
Figure 14 presents SEM fractographic images of the tensile fracture surfaces of the Al-5Si alloy in both the as-cast state and after FSP at various travel speeds (26–52 mm/min) at a fixed rotation speed of 1330 rpm. The tensile fracture surfaces, as observed in the SEM micrographs, reveal distinct features that correspond to the variations in mechanical properties and microstructure induced by different FSP travel speeds. For the as-cast alloy, the fracture surface typically displays a mainly brittle failure mode, characterized by flat and faceted regions with limited plastic deformation, as shown in Figure 14a,b. The presence of coarse dendritic structures and casting micro-cavities contributes to crack initiation and propagation along weak interfaces, resulting in a relatively smooth fracture surface with cleavage planes and limited ductile behaviors [61,67]. In contrast, the FSPed sample at 26 mm/min, which exhibited the highest UTS and significant grain refinement, shows a predominantly ductile fracture mode. The SEM micrograph, as shown in Figure 14c,d, reveals numerous equiaxed deep and shallow dimples, indicative of substantial plastic deformation before failure. The deep and uniformly distributed dimples suggest enhanced toughness and ductility. The results are in agreement with previous works [68,71], which reported that FSP entirely disrupted the eutectic structure of the Al-Si alloy, thereby eliminating the network of preferential crack paths inherent to the as-cast material. Consequently, fracture initiation during tensile loading predominantly originated from the brittle eutectic Si particles. Finally, at the highest travel speed of 52 mm/min, the fracture surface reveals mixed modes of fractured features. The SEM images in Figure 14e,f show larger, irregular cleavage planes interspersed with micro-cracks and small and large dimples. This fracture morphology corresponds well with the lowest Ys and UTSs observed among the FSPed specimens, emphasizing the detrimental impact of insufficient heat input and plastic flow at the highest travel speed.

3.3.2. Microhardness

Hardness, which indicates a material’s ability to resist localized plastic deformation, is a key characteristic of the Al-5Si alloy. When subjected to FSP with a fixed rotation speed of 1330 rpm but different traverse speeds (26–52 mm/min), the as-cast alloy’s hardness is notably influenced by microstructure modification. Figure 15 represents the microhardness trend of the as-cast Al-5Si alloy and the FSP specimens at various travel speeds. The as-cast condition exhibits a microhardness of 45.5 ± 5.5 HV, which aligns with its coarse grain size, scattered eutectic phase, porosity formation, and relatively low tensile strength. Furthermore, it can be noted that the hardness values improved as a result of the FSP process at all applied travel speeds by an improvement rate of not less than 39% compared to the unprocessed material. Although there is also a slight improvement in the hardness values with increasing travel speed, the scattered measurements of hardness showed relatively large error bars for the as-cast material and the specimens processed at the higher travel speeds of 42 and 52 mm/min. The increase in microhardness may be attributed to the fact that the smaller the grain size, the higher the hardness, according to the Hall–Petch relationship. Conversely, the large error bars indicate a heterogeneous microstructure fracture which is due to insufficient heat input at high travel speeds [41,42,61].

3.3.3. Impact

The impact energy results provide additional insights into the mechanical behavior of the Al-5Si alloy after FSP, particularly in relation to the observed tensile properties, microhardness, and grain size. Figure 16 shows the impact energy before and after FSP of the as-cast Al-5Si alloy specimens processed at 1330 rpm and at different travel speeds. All the processed Al-Si alloys show higher toughness behavior than the as-cast materials at all the applied travel speeds. The as-cast alloy, with its coarse grain structure, exhibits a low impact energy of 4.2 ± 1 J, indicating poor toughness due to the brittle nature of the as-cast microstructure [7,62]. The microstructure features show the presence of cracks, micro-voids, dendrite arm fractures, cleavage facets, and, in some areas, large and shallow dimples, indicating a predominantly brittle fracture mode (Figure 17a). The large and shallow dimples suggest limited localized ductility of the as-cast alloy. The FSP significantly refines the microstructure (Figure 11), leading to improved toughness (Figure 16). However, fracture features vary with travel speed, as shown in Figure 17a,b for the specimens processed at 26 and 52 mm/min, respectively. The impact energy reached its highest value of 12.5 ± 0.57 J at a travel speed of 26 mm/min. This increase is consistent with the grain refinement and improved material ductility resulting from FSP, which enhances the alloy’s ability to absorb energy during impact. The fracture surface shows equiaxed deep dimples, indicating a mainly ductile fracture (Figure 17b). These observations are consistent with the grain refinement (Figure 11) and improved material ductility resulting from FSP (Figure 12), which enhances the alloy’s ability to absorb energy during impact. However, as the travel speed increases, the impact energy slightly decreases to be around the same value of 11.2 J for the two speeds of 33 and 42 mm/min. At the highest travel speed of 52 mm/min it attains the lowest value of 10 ± 0.64 J and the fracture surface shows dimples of different sizes and fewer cleavage facets compared to the as-cast alloy, indicating a mixed mode of the ductile brittle mode.

3.3.4. Wear Behavior

The wear behavior of the as-cast Al-5Si alloy is significantly influenced by its microstructure, bulk hardness, and toughness, which are in turn affected by processing conditions such as the travel speed when the rotation speed is constant. The wear test was performed at a 10 N wear load for the as-cast Al-5Si and the FSPed alloys at the different travel speeds of 26, 33, 42, and 52 mm/min. The wear behavior is plotted as a weight loss measured value against travel speed (Figure 18). It can be noticed that the applied load was enough to cause surface damage on all the tested samples, and the FSPed samples lost lower weight than the as-cast samples, showing a lower wear rate, which is shown in the wear rate as a function of FSP travel speed in Figure 19. For the as-cast state, the Al-5Si alloy exhibits a relatively high wear rate of 9.23 ± 0.84 × 10−6 g/m, indicating a lower wear resistance compared to the processed specimens under dry sliding conditions. This elevated wear rate is primarily attributed to the coarse microstructure of the alloy, which is characterized by a large SDAS of 26.24 ± 4.79 µm, which represents a lower hardness and toughness. The presence of a non-uniformly distributed network of a hard eutectic silicon phase within the softer Al matrix leads to microstructural inhomogeneity, which in turn contributes to uneven wear and excessive material loss during sliding.
At a travel speed of 26 mm/min, the wear rate is markedly reduced to 3.31 ± 0.8 × 10−6 g/m, representing a substantial improvement over the as-cast condition. This enhancement is mainly a result of grain refinement achieved through FSP. More refined and homogeneously distributed microstructural constituents lead to improved hardness, toughness, and resistance against wear by promoting a uniform distribution of stress during sliding. Thus, it can be concluded that among all the Al-5Si samples subjected to the wear test, the sample processed at a speed of 26 showed the highest wear resistance. Finally, findings presented in Figure 19 highlight the substantial improvement in wear resistance following FSP, with the extent of enhancement being strongly influenced by the processing parameters, particularly the tool travel speed. Generally, increased hardness correlates with improved wear resistance, whereas higher toughness can reduce crack propagation under abrasive conditions. Thus, the weight loss and wear rate are expected to vary depending on the balance between these competing mechanical properties at different speeds. These findings underscore the necessity of optimizing FSP parameters to achieve the best possible wear resistance for specific applications, particularly under different sliding conditions. These results are consistent with those described in the previous studies found in [61,69].
Figure 20 illustrates the coefficient of friction (COF) for the as-cast and FSPed Al-Si alloy samples under a dry sliding condition, revealing a complex relationship between microstructural changes and frictional behavior. The as-cast material exhibits a relatively high COF of 1.1 ± 0.13, which directly reflects its coarse, inhomogeneous microstructure characterized by SDAS. This non-uniform distribution of a hard eutectic silicon phase within the softer aluminum matrix leads to irregular interactions with the counter surface, resulting in increased friction (Figure 20) and higher wear rates (Figure 19). When subjected to FSP the Al-5Si alloy generally shows notable improvements in COF under a dry sliding condition, with the most significant reduction observed at a travel speed of 26 mm/min where the COF decreases to 0.66 ± 0.01. This substantial reduction is primarily attributed to the significant grain refinement and increased microstructural homogeneity achieved through FSP, leading to a smoother and more consistent contact surface that effectively reduces friction. While the COF values for samples processed at 33 mm/min and 42 mm/min are still lower than the as-cast condition, they show a slight increase compared to 26 mm/min, despite further grain refinement (Figure 11). This suggests that at these higher speeds factors like increased grain boundary density or residual stresses may begin to offset the benefits of refinement. At the highest travel speed of 52 mm/min, the COF is 0.9 ± 0.13, which is lower than that of the as-cast state but higher than the optimal FSP conditions. This indicates a less favorable balance of microstructural factors, resulting in higher friction and correlating with the increased wear rates observed at these less optimal FSP travel speeds.

4. Conclusions

The present study investigates the influence of FSP travel speed on the microstructure and mechanical performance of a hypoeutectic Al-5 wt.% Si alloy in its as-cast condition. FSP was conducted at a fixed rotational speed of 1330 rpm with varying travel speeds (26, 33, 42, and 52 mm/min). This work offers novel insights by systematically exploring the effect of varying travel speeds on this less-studied hypoeutectic Al-5Si alloy, filling a significant gap in the existing literature which is primarily focused on other Al-Si compositions or different processing parameters. The microstructure and mechanical properties (hardness, tensile strength, impact energy, and wear behavior) of the as-cast and processed materials were explored, and the following conclusions summarize the key outcomes of this study.
  • Friction stir processing (FSP) significantly modifies the microstructure and enhances the mechanical properties of as-cast Al-5Si alloy, with the most pronounced improvements generally observed at lower travel speeds. For instance, at a travel speed of 26 mm/min the hardness improved by up to 39%, ultimate tensile strength (UTS) by up to 51%, impact energy by up to 197%, and wear resistance by up to 64% compared to the as-cast state.
  • FSP results in a significant reduction in α-Al grain size. The as-cast Al-5Si alloy exhibited a coarse grain size of 26.24 ± 4.79 µm. Following FSP, the α-Al grain sizes were notably refined to 12.46 ± 1.59 µm, 9.55 ± 1.51 µm, 7.87 ± 1.38 µm, and 7.75 ± 1.34 µm for travel speeds of 26, 33, 42, and 52 mm/min, respectively.
  • Compared to the as-cast alloy, which exhibits a heterogeneous coarse structure and poor tensile properties (YS of 57 MPa, UTS of 108.7 MPa, and strain at fracture of 3.6%), FSP significantly enhances the UTS and strain at fracture of Al-5Si alloys, particularly at lower travel speeds (26 and 33 mm/min). The sample processed at 26 mm/min achieved the highest UTS (164.5 MPa) and strain at fracture (10.2%), primarily due to porosity elimination and the breakdown/dispersion of the dendrite arm structure and eutectic Si-phase. While higher travel speeds (42 and 52 mm/min) still showed improvement (e.g., UTS of 131.2 MPa and strain at fracture of 10% at 52 mm/min), the enhancement was less pronounced than at lower speeds. FPS improves the toughness of Al-5Si alloys at all travel speeds, and the impact energy reached its highest value of 12.5 ± 0.57 J at a travel speed of 26 compared to 4.2 ± 1 J for the as-cast state, indicating a predominantly ductile fracture mode.
  • Analysis of tensile and impact fracture surfaces confirmed changes in failure mechanisms. The as-cast Al-5Si alloy primarily exhibited brittle fracture, characterized by flat faced regions and limited plastic deformation. In contrast, the FSPed alloy, especially at a travel speed of 26 mm/min, showed a predominantly ductile fracture mode with numerous deep, equiaxed dimples, consistent with improved toughness. At the highest processing travel speed of 52 mm/min, a mixed mode fracture (combining ductile dimples and brittle cleavage facets) was observed.
  • The wear resistance of the Al-5Si alloy was significantly enhanced by FSP. The wear rate of the as-cast Al-5Si alloy was 9.23 ± 1.2 × 10−6 g/m, which is substantially higher than that of all FSPed materials. The lowest wear rate, indicating the highest wear resistance, was achieved at a travel speed of 26 mm/min (3.31 ± 0.8 × 10−6 g/m). While FSP generally improved wear resistance, the wear rate progressively increased with higher travel speeds, reaching 4.63 ± 0.9 × 10−6 g/m at 33 mm/min, 5.29 ± 0.7 × 10−6 g/m at 42 mm/min, and 8.6 ± 0.9 × 10−6 g/m at 52 mm/min. This trend highlights the optimal balance of microstructural refinement and heat input at lower travel speeds for superior wear performance.

Author Contributions

Conceptualization, A.E.-S., M.M.E.-S.S., A.E. and B.E.-E.; methodology, B.E.-E., M.M.E.-S.S. and S.A.; software, B.E.-E. and A.E.-S.; validation, B.E.-E., S.A., M.F.Y.S. and M.M.E.-S.S.; formal analysis, B.E.-E. and M.F.Y.S.; investigation, B.E.-E., M.M.E.-S.S. and S.A.; resources, A.E.-S., S.A. and A.E.; data curation, B.E.-E.; writing—original draft preparation, B.E.-E.; writing—review and editing, M.M.E.-S.S., S.A, M.F.Y.S. and A.E.-S.; visualization, B.E.-E., A.E. and M.F.Y.S.; supervision, A.E.-S., M.M.E.-S.S. and A.E.; project administration, M.F.Y.S. and S.A.; funding acquisition, S.A. and M.F.Y.S. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported and funded by the Deanship of Scientific Research at Imam Mohammad Ibn Saud Islamic University (IMSIU) (grant number IMSIU-DDRSP2503).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Steps of the gravity casting. (a) Design of the steel mold. (b) Gravity casting steps for the hypoeutectic Al-5Si alloy.
Figure 1. Steps of the gravity casting. (a) Design of the steel mold. (b) Gravity casting steps for the hypoeutectic Al-5Si alloy.
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Figure 2. Specification of the FSP tool.
Figure 2. Specification of the FSP tool.
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Figure 3. (a) Schematic diagram of the FSP and (b) a typical process.
Figure 3. (a) Schematic diagram of the FSP and (b) a typical process.
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Figure 4. Cutting samples with a wire cut EDM machine.
Figure 4. Cutting samples with a wire cut EDM machine.
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Figure 5. Tensile test sample, with all measurements provided in mm.
Figure 5. Tensile test sample, with all measurements provided in mm.
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Figure 6. Impact test specimen (all dimensions in mm).
Figure 6. Impact test specimen (all dimensions in mm).
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Figure 7. Wear testing machine (pin-on-disk).
Figure 7. Wear testing machine (pin-on-disk).
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Figure 8. Photos depict the top-surface of the Al-5Si samples after FSP at a fixed tool rotation speed of 1330 rpm and various travel speeds of (a) 26 mm/min, (b) 33 mm/min, (c) 42 mm/min and (d) 52 mm/min.
Figure 8. Photos depict the top-surface of the Al-5Si samples after FSP at a fixed tool rotation speed of 1330 rpm and various travel speeds of (a) 26 mm/min, (b) 33 mm/min, (c) 42 mm/min and (d) 52 mm/min.
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Figure 9. OM microstructure images of Al-5Si alloy: (a,b) as-cast condition and after FSP condition processed at a constant rotation speed of 1330 rpm and various travel speeds of (c) 26 mm/min, (d) 33 mm/min, (e) 42 mm/min, and (f) 52 mm/min.
Figure 9. OM microstructure images of Al-5Si alloy: (a,b) as-cast condition and after FSP condition processed at a constant rotation speed of 1330 rpm and various travel speeds of (c) 26 mm/min, (d) 33 mm/min, (e) 42 mm/min, and (f) 52 mm/min.
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Figure 10. (a) Primary dendritic arm spacing (PDAS) and (b) secondary dendritic arm spacing (SDAS) for the as-cast Al-5Si alloy, and (α-Al) grain size histograms after FSP at 1330 rpm and various travel speeds of (c) 26, (d) 33, (e) 42, and (f) 52 mm/min.
Figure 10. (a) Primary dendritic arm spacing (PDAS) and (b) secondary dendritic arm spacing (SDAS) for the as-cast Al-5Si alloy, and (α-Al) grain size histograms after FSP at 1330 rpm and various travel speeds of (c) 26, (d) 33, (e) 42, and (f) 52 mm/min.
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Figure 11. Represents the average α-Al grain size for all the Al-5Si alloy samples before and after FSP at a constant tool rotation speed of 1030 rpm and various travel speeds of 26, 33, 42, and 52 mm/min.
Figure 11. Represents the average α-Al grain size for all the Al-5Si alloy samples before and after FSP at a constant tool rotation speed of 1030 rpm and various travel speeds of 26, 33, 42, and 52 mm/min.
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Figure 12. Stress–strain curves of the as-cast Al-5Si alloy samples before and after FSP at 1030 rpm and at various travel speeds (26–52 mm/min).
Figure 12. Stress–strain curves of the as-cast Al-5Si alloy samples before and after FSP at 1030 rpm and at various travel speeds (26–52 mm/min).
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Figure 13. Ultimate tensile and yield strengths of the Al-5Si alloy before and after FSP at a constant rotation speed of 1030 rpm and various tool travel speeds (26–52 mm/min).
Figure 13. Ultimate tensile and yield strengths of the Al-5Si alloy before and after FSP at a constant rotation speed of 1030 rpm and various tool travel speeds (26–52 mm/min).
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Figure 14. Fracture-surface SEM images of the as-cast Al-5Si alloy sample (a) at 100× and (b) at 1000×, and that of the FSPed specimens at 1330 rpm and various travel speeds of 26 mm/min (c) at 500× and (d) at 1000×, and 52 mm/min (e) at 500× and (f) at 1000×.
Figure 14. Fracture-surface SEM images of the as-cast Al-5Si alloy sample (a) at 100× and (b) at 1000×, and that of the FSPed specimens at 1330 rpm and various travel speeds of 26 mm/min (c) at 500× and (d) at 1000×, and 52 mm/min (e) at 500× and (f) at 1000×.
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Figure 15. Microhardness of the as-cast Al-5Si alloy and the FSP specimens at a constant tool rotation speed of 1030 rpm and various travel speeds of 26, 33, 42, and 52 mm/min.
Figure 15. Microhardness of the as-cast Al-5Si alloy and the FSP specimens at a constant tool rotation speed of 1030 rpm and various travel speeds of 26, 33, 42, and 52 mm/min.
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Figure 16. Impact energy of the as-cast Al-5Si alloy and the FSP materials at a constant tool rotation speed of 1030 rpm and at the various travel speeds of 26, 33, 42, and 52 mm/min.
Figure 16. Impact energy of the as-cast Al-5Si alloy and the FSP materials at a constant tool rotation speed of 1030 rpm and at the various travel speeds of 26, 33, 42, and 52 mm/min.
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Figure 17. SEM fractography impact fracture surfaces of (a) the as-cast Al-5Si alloy at a magnification of 100×, and the FSPed specimens processed at a constant tool rotation speed of 1030 rpm and at the various travel speeds of (b) 26 mm/min at a magnification of 500× and of (c) 52 mm/min at a magnification of 500×.
Figure 17. SEM fractography impact fracture surfaces of (a) the as-cast Al-5Si alloy at a magnification of 100×, and the FSPed specimens processed at a constant tool rotation speed of 1030 rpm and at the various travel speeds of (b) 26 mm/min at a magnification of 500× and of (c) 52 mm/min at a magnification of 500×.
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Figure 18. Weight loss as a function of tool travel speed of the as-cast Al-5Si alloy and the FSPed materials at a constant tool rotation speed of 1030 rpm and at the various travel speeds of 26, 33, 42, and 52 mm/min.
Figure 18. Weight loss as a function of tool travel speed of the as-cast Al-5Si alloy and the FSPed materials at a constant tool rotation speed of 1030 rpm and at the various travel speeds of 26, 33, 42, and 52 mm/min.
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Figure 19. Wear rate against tool travel speed of the as-cast Al-5Si alloy and the FSPed specimens at a constant rotation speed of 1030 rpm and various travel speeds (26–52 mm/min).
Figure 19. Wear rate against tool travel speed of the as-cast Al-5Si alloy and the FSPed specimens at a constant rotation speed of 1030 rpm and various travel speeds (26–52 mm/min).
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Figure 20. Coefficient of friction (COF) against tool travel speed of the as-cast Al-5Si alloy and the FSPed specimens at a constant rotation speed of 1030 rpm and at various travel speeds (26–52 mm/min).
Figure 20. Coefficient of friction (COF) against tool travel speed of the as-cast Al-5Si alloy and the FSPed specimens at a constant rotation speed of 1030 rpm and at various travel speeds (26–52 mm/min).
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Table 1. Chemical composition of the produced hypoeutectic Al-5Si alloy.
Table 1. Chemical composition of the produced hypoeutectic Al-5Si alloy.
ElementSi%Fe%Cr%Mn%Zn%Cu%Ni%Mg%Al%
Percentage (%)50.720.030.020.060.010.0190.05Bal.
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MDPI and ACS Style

El-Eraki, B.; Shalaby, M.F.Y.; El-Sissy, A.; Eisa, A.; Ataya, S.; El-Sayed Seleman, M.M. The Role of Friction Stir Processing Travel Speed on the Microstructure Evolution and Mechanical Performance of As-Cast Hypoeutectic Al-5Si Alloy. Crystals 2025, 15, 546. https://doi.org/10.3390/cryst15060546

AMA Style

El-Eraki B, Shalaby MFY, El-Sissy A, Eisa A, Ataya S, El-Sayed Seleman MM. The Role of Friction Stir Processing Travel Speed on the Microstructure Evolution and Mechanical Performance of As-Cast Hypoeutectic Al-5Si Alloy. Crystals. 2025; 15(6):546. https://doi.org/10.3390/cryst15060546

Chicago/Turabian Style

El-Eraki, Basma, Mahmoud F. Y. Shalaby, Ahmed El-Sissy, Abeer Eisa, Sabbah Ataya, and Mohamed M. El-Sayed Seleman. 2025. "The Role of Friction Stir Processing Travel Speed on the Microstructure Evolution and Mechanical Performance of As-Cast Hypoeutectic Al-5Si Alloy" Crystals 15, no. 6: 546. https://doi.org/10.3390/cryst15060546

APA Style

El-Eraki, B., Shalaby, M. F. Y., El-Sissy, A., Eisa, A., Ataya, S., & El-Sayed Seleman, M. M. (2025). The Role of Friction Stir Processing Travel Speed on the Microstructure Evolution and Mechanical Performance of As-Cast Hypoeutectic Al-5Si Alloy. Crystals, 15(6), 546. https://doi.org/10.3390/cryst15060546

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