Abstract
In this study, friction stir processing (FSP) was used to create new surface properties of the Al 6101-SiC composite, and different types of friction stir process tools are used like cylindrical with thread and conical with thread. The SEM and EDAX analyses made it evident that texture appeared in FSPed Al6101 both with and without SiC. Due to the refined grain structure and the inclusion of SiC in the composite, both FSPed samples’ hardness and tensile strength were found to be higher than those of the base alloy. In contrast to FSPed and base material, the composite had a lower percentage of elongation. Drilling experiments led to machining research that showed FSPed Al6101 to have larger cutting forces than the base and composite materials. These findings revealed that SiC served as reinforcement in the material, increasing hardness and cutting forces during drilling. Electrochemical tests on the corrosion behavior showed that the composite had less corrosion resistance than the FSPed alloy but had slightly better corrosion resistance than the base material. Al6101-SiC composites were found to have superior mechanical characteristics and greater machinability. However, when creating an Al6101-SiC composite via FSP, it is crucial to consider corrosion resistance degradation.
1. Introduction
Friction stir processing is a popular solid-state method for creating surface metal matrix composites (MMCs), as recently documented in the literature [1]. Friction stir processing is the extension solid-state procedure of friction stir welding (FSW) to change the microstructure of the surface. In-depth explanations of grain refining and material flow mechanisms during FSW and FSP can be found elsewhere [2, 3]. Certain material properties of the composites are influenced by microstructure alteration and the incorporation of other metals in the form of surface reinforcement by friction stir process [4–6]. For the development of surface properties of composites by FSP, a variety of material systems, including aluminum, magnesium, copper, steels, and titanium alloys, have been used as matrix materials. TiO2, TiC, SiC, B4C, SiO2 Al2O3, carbon nanotubes (CNTs), graphene, metallic powders, hydroxyapatite, and fly ash have been used as dispersing phases.
To create components and structures, the composites must also go through several manufacturing procedures, such as machining, welding, and shaping operations, to be used in engineering applications. To qualify these composites as raw materials for structural application in difficult settings, such as highly corroding environments, it is also required to study their behavior in such environments. One of the crucial manufacturing processes used in creating various applications for manufacturing machine components in the engineering sector. Compared to monolithic parts, multiphase and composite materials are more difficult to machine. Composites are multiphase materials, and because the matrix and reinforcement have different physical characteristics, they behave differently during machining processes such as cutting, chip removal, and surface finishing; the tool wear is more and is affected by the reinforcement particles to metal matrix composites [7–9]. Al2219/SiC and Al2219/SiC-graphene composites were created by the conventional casting technique by Basavarajappa et al. [10], and enhanced machining properties were noted for the components containing reinforcement of graphene. In drilling AZ91 Mg alloy processed by FSP, Surya Kiran et al. [11] showed the major impact of the reinforcement phase on the surface machining and separation into constituent layers. When compressed [12, 13], the SiC-reinforced A359 alloy underwent increased strain hardening, and after that, an impact was observed on size, shape, and quantity of refinement on the particles on the strain hardening of the composite when it was compressed. On the other hand, the microstructure of metals and alloys also significantly impacts their strength to be machined.
Better machining was shown for pure titanium of commercial grade compared with the original base by Lapovok et al. [14]. In a similar vein, Venkataiah et al. [15] observed that during drilling, fine-grained ZE41 Mg alloy had lower cutting forces than coarse-grained base alloy. Additional investigations [16] confirmed the importance of grain refining in changing machinability. A common commercial alloy used in marine, automotive, and aerospace applications is Al6101. On the other hand, nanosize SiC has recently attracted a lot of interest in the field of materials research as a promising material with distinctive features [17, 18].
As a result, AAl6101 aluminum was taken for the matrix material in the current study. Nanosize SiC as dispersing phase was then used to produce surface MMCs by FSP, to examine the impact of adding nanosize SiC on the composites’ mechanical properties, machining, and corrosion behavior. To improve the mechanical properties of AA6101 composite at the required area, FSP is the most flexible process. With FSP, only top surface proprieties can be modified by incorporating the reinforcement particles in the surface area. This study was performed with different types of tools to enhance the quality of the surface.
2. Materials and Procedures
Chemical composition AA6101 of 20 mm × 6 mm aluminum alloy (commercial grade) strips purchased from Agarwal Aluminum in Visakhapatnam, 98.9% pure aluminum, 0.6% magnesium, and 0.50% silicon. Al6101 strip measuring 20 mm × 6 mm and 4 mm long was cut into pieces 320 mm long, and the vertical milling machine’s stroke length was 350 mm. An FSP tool manufactured by EN 8 tool steel was used for friction stir processing. The FSP tools and 4 various types of tools are used: cylindrical, cylindrical with thread, conical, and conical with thread, and the probe did 6 mm probe height 5 mm. A vertical milling machine with an automatic feeding system (BFW Ltd., India) was used to perform FSP at various speeds and feed rates, including 495, 675, and 850 RPMs, and 42, 55, and 74 mm/min, respectively. For making surface metal matrix composites, the workpiece surface grooves are machined, 1 mm in width by 4 mm in depth. The groove was filled with nanoSiC, and the open side was lapped with a lapping tool to ensure an even distribution of reinforced material during the stirring process. Before applying genuine FSP to create the composites, the groove was closed using a tool with no pin that was specially built for FSP. The characteristics of the workpiece were not significantly altered by the initial groove closing procedure [19]. Its goal is to seal the groove so that nanosize SiC cannot escape it. The friction stir process lapped tool without a probe rotated at 1660 RPM while moving at 42 mm/min with negligible penetration, the lapping tool was in less contact with the workpiece, and adequate vertical movement of the work table was applied. Up until the groove was entirely closed, contact was kept with the workpiece. The composites were then produced at various speeds and feeds using genuine FSP tools, which included pins with the aforementioned dimensions. The processing settings for developing surface MMC were chosen based on past research [20, 21]. Al6101-SiC was the name of the composite that FSP created after a single pass. Without SiC, the base alloy and the FSPed Al6101 alloy were given the designation Al6101. Figure 1(a) displays the image captured during the FSP process, and Figure 1(b) displays the surface MMC of Al6101-nano-SiC created during the current investigation. FSPed composite machining tests are depicted through photographs. The base alloy, the treated areas of base metal, and Al6101-SiC workpieces were used as the sources for samples for microstructural and microhardness tests. The samples were polished using various emery sheet grades, followed by diamond paste polishing, according to standard SEM and EDAX methods. This polishing was performed suitable for the scanning process. Following the procedure, the samples were cleaned in ethanol and allowed to dry, and the samples were etched with Keller’s reagent. Using a microscope (Leica, Germany), photographs taken with it were produced. Image analysis software was used to do the microstructure study. SEM and EDAX were used in this investigation to observe the Al 6101-SiC microstructure. The friction stir method using a vertical milling machine is shown in Figure 1.
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The stir zone of the specimens was sliced across the FSPed sections so that it would be in the center of the tensile sample. The specimen test samples are shown in Figure 2. The OPTIMU—UF100 MASCHINEN—GERMAN vertical milling machine with automatic spindle speed and a tool feed system capable of tool feed length up to 350 mm stroke are used. The figure displays the images and the tensile samples’ measurements (as per ASME standards). By tensile testing machine ZwickRoell, Germany-based uniaxial tensile testing was carried out under ambient conditions with a strain rate of 0.01 s−1. Mechanical characteristics were determined from the tensile test data by drawing stress-strain curves. Mean values and standard deviation were computed from the data and compared. All of the samples were subjected to drilling tests to evaluate the machining behavior. A 6-mm-diameter twist drill bit was mounted in a vertical milling machine. The prepared samples for SEM and EDAX analysis are shown in Figure 3. The corrosion test samples are shown in Figure 4.
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High carbon steel is used for manufacturing, and friction stir process tools with different tool shapes such as cylindrical, cylindrical with thread, conical, and conical with thread are tested in this experimentation work, as shown in Figure 5. The Kistler dynamometer and the samples tested are shown in Figure 6. The cutting force versus time plots of the dynamometer test are shown in Figure 7.
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For testing, samples are mounted on the dynamometer with required fixing clamps that were set up on the dynamometer’s platform. Also, the dynamometer was fixed on the drilling machine (Kistler of Switzerland). Two different speeds of 120 and 300 RPM with two different feeds of 15 and 42 mm/min were considered for drilling trials without the use of coolant. Before the drill bit made contact with the work piece’s surface, the cutting forces were measured. This measurement lasted until the cutting forces stabilized. Mean cutting forces were found by calculating the cutting forces vs. time curve. The corrosion test was conducted to understand the corrosion properties in the newly processed material and to investigate the influence of the additive on corrosion resistance. Using potentiodynamic polarization (PDP) tests, the samples’ corrosion behavior was evaluated. The stir zone of FSPed workpieces was used as the source of the specimens (n = 2) for the corrosion studies. These specimens undergo ethanol cleaning and polishing before being dried. Using a solution of 3.5% NaCl as the electrolyte, experiments were carried out at ambient temperature. The counter electrode was made of graphite rod, and the reference electrode was made of saturated calomel electrode (SCE). The working electrode was assumed to be a workpiece (one square cm exposed to the electrolyte). Open circuit potential (OCP), which was produced before the studies and maintained for 30 min, served as the basis for the investigations, which were carried out at a scan rate of 1 mV/s. The Tafel exploration was used to extract the electrochemical parameters from the PDP curves, including the corrosion potential and current (Ecorr and Icorr, respectively) [21]. A scanning electron microscope (SEM, Carl ZEISS, Germany) operating at 30 kV was used to study the samples’ developing Al6101-corroded surfaces to determine the type of corrosion attack that occurred on the samples’ surfaces during PDP experiments.
3. Results and Discussions
Al6101 is a well-known aluminum alloy that is widely used in marine and automotive applications due to its excellent corrosion resistance [22]. SiC is a novel material that has recently demonstrated great promising material for several industrial applications, including food processing, electronics, water treatment, chemical engineering, sensors, and materials engineering. SiC has a larger surface energy than other metals and is more likely to aggregate when introduced as a dispersing material, making it harder to introduce SiC with varied Wt% (2%, 4%, and 6%) into metals to create composites by liquid state pathways. This problem is addressed by solid-state processes like friction stir processing (FSP), which allows reinforcements to be added to the solid state without melting the matrix material. Furthermore, according to Huang et al. [23], by combining stir casting and subsequent FSP, the agglomeration problems related to nanodispersion phases can be resolved. This is because FSP causes extreme plastic deformation. Furthermore, FSP causes grain refinement in the majority of alloys. Kumar et al. [24] studied the mechanical characteristic efficiency of nanosized reinforcements by promoting more implicit particle hardening mechanisms compared to micron-sized reinforcements. In this study, the SEM image of SiC is employed. The comparable selective area electron diffraction (SAED) pattern of the SiC is displayed in the figures. SiC’s nanosize and crystalline have been confirmed by SEM and EDAX patterns. The shape of SiC resembles a flake.
The optical microscope pictures of the Al6101, FSP Al6101, and Al6101-SiC samples are shown. After FSP, the grain is refined from a starting size of 115 4.6 lm to 8 2.6 lm. Bands of grain-refined regions and SiC-rich regions may be seen in the composites. For the composite, the average grain size was 6.9 1.5 lm. FSP’s development of surface MMCs has two significant benefits. First, it gets rid of the problems with liquid process routes, and second, it delivers the benefit of grain refining, as shown in the current work. We can find more information about the mechanics involved in grain refinement during FSP elsewhere [4]. Al6101-nano-SiC exhibits material flow in addition to SiC showing black and grey patterns in the stir zone, which is a typical observation in the development of surface composite through FSP. It has been discovered that SiC is not distributed uniformly throughout the composite. Bands of distorted material are seen in the stir zone as a result of the way that material flows during FSP. These bands may have varying degrees of grain refinement because of the differences in material flow in the thickness direction around the rotating pin circumference [25]. The additional secondary phases may also be spread within the bands during the creation of the composites [26].
Due to the way that composites are formed, it is challenging to determine the precise composition of the composites that FSP produces. The area of the created composite over a specified length and the volume fraction of the dispersing phase, on the other hand, can be used to compute the approximate volume fraction. In the current investigation, the total volume of grooves is filled with SiC, and the composite area is measured using SEM and EDAX.The parameter set during SEM analysis for sample 1 is as follows: RHT 15.00 KV, WD 8.0 mm and signal A set 1, Mag 1.00kx; for sample 2, it is Mag 500X, for sample 3, it is 500X, and for sample 4, it is 1.00kx. This shows the equal distribution of SiC surface structure in the EDAX for samples 1, 2, 3, and 4. The scanning electron microscope images of the AA6101-SiC composite are shown in Figure 8. The EDAX images of the AA6101-SiC composite are shown in Figure 9.
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The AA6101-SiC microhardness measurements are shown in Figure 10. Both the FSP Al6101 and Al6101-SiC samples had higher hardness distributions than base alloy, according to measurements. The average hardness values of FSP Al6101 and Al6101-SiC are 66.1 and 67.23, respectively. The sample A 495 RPM spindle speed and 42 mm/min feed with 4% SiC have significantly higher hardness values than Al6101 (31.4). In comparison to the other samples, the composite has demonstrated greater toughness. The presence of grains in the fine form in the friction stir pressed and base samples can be attributable to the enhanced properties of hardness. Because of the combination process of SiC reinforcement and grain refining, SiC addition has promoted increased hardness in the composite. In comparison to the other two samples, the composite’s hardness distribution is seen to be less uniform and to vary more from measured values. This is explained by the addition of SiC to the tiny grains in the composite. When the indents are positioned in the region of the composite with a greater SiC concentration, and enhancement of hardness and grain structure inclusions its role is crucial, the measured hardness values are higher. The potentiodynamic polarization curves of the samples are shown in Figure 11.
There are more fluctuations in the surface structure properties of composite because grain refining is the main factor increasing hardness in other sections of the composite metal matrix. Also, this process is comparable to the previous results from the FSP production of Mg-CNT composites that Sai Krishna et al. [21] described. The mechanical characteristics were discovered through tensile tests. In comparison to Al6101, FSP Al6101 has demonstrated better yield strength and ultimate tensile strength (UTS). Comparing SiC to FSP Al6101 and the base material, SiC has little impact on raising yield strength but a considerable impact on boosting ultimate tensile strength.
It has been noted that the SiC distribution is not uniform throughout the composite, which may have had an impact on the mechanical response, especially yield strength. However, it was shown that as compared to Al6101, the percentage of elongation was much lower for FSP Al6101 and Al6101-SiC. Grain refining increases the mechanical properties of yield strength and ultimate tensile strength because of the mechanism for grain refinement and modification, which has been thoroughly proven [27]. As a result, FSP Al6101 displays greater yield strength and UTS than Al6101. SiC by FSP incorporation offers the benefit of adding refinement material for strengthening, and the presence of different materials causes plastic deformation of the material and shows the tensile strength behavior of Al6101-SiC composite material. Although its effect is greater in the composite, increased strength causes a decrease in elongation in both FSPed samples. However, these types of composites are good for manufacture, and properties of material higher strength are required for the expense of decreasing some ductility.
The machining forces collected when the drilling is summarized and displayed average cutting forces (Fz) derived by its shape in the stabilized area. Reduced machining forces are seen in all feed rates (15 and 42 mm/min) as the cutting speed increases from 120 to 300 RPM and distribution with greater deviations from the mean. At both cutting speeds, it is discovered that these differences are less pronounced at maximum feed rates which are 42 mm/min than at minimum feed rates which are 15 mm/min. In comparison to Al6101 and the composite, maximum cutting forces are found in the friction stir process Al6101 sample of all compositions and parameters. Also, it observed that the machining forces are slightly higher for Al6101-SiC than for FSP Al6101 but slightly lower for the composite than for Al6101.
The effect of grain boundary strengthening in boosting resistance to material shear during material removal is crucial when machining grain-refined materials. Higher forces are thus required for metals that have been machine-ground and refined. This is consistent with observations made in past papers while processing 304L stainless steel [28], ZE41 Mg alloy [13], and AZ91 Mg alloy [21]. SiC acts as a dry lubricant during the drilling of the Al6101-SiC composite, which helps to reduce the cutting forces. The resistance to cutting material removal by mechanical methods is observed in activities such as the machining process, material wear, material cutting, and surface grinding when the secondary phases in the composite have lubricating in material properties [29]. PDP curves for the samples and electrochemical parameters derived from the PDP curves are included, and in comparison to Al6101 and the composite, FSP Al6101 has demonstrated higher corrosion resistance, as evidenced by a lower average Icorr value.
When compared to both Al6101 and FSP Al6101, the FSP-SiC composite across all the samples displayed a modest Icorr value. When compared to base alloys, FSP Al6101 and Al6101-SiC exhibit less corrosive surface deterioration. However, considerably deteriorated patches can be seen in all of the samples (as indicated by arrows). This is explained by the fact that secondary phase particles are present near the grain boundaries, where they cause localized corrosion to increase. In addition, SiC promotes galvanic corrosion, causing significant portions of the composite to deteriorate. The results strongly indicate that the Al6101-SiC composite’s corrosion performance is worse than that of FSP Al6101. This corrosion susceptibility is also observed from the SEM and EDAX analysis. The higher sensivity to corrosion for Al6101-SiC is due to a significant amount of alloy presnt at lower density planes [30]. Furthermore, greater grain boundaries that result from grain refining improve Al alloys’ resistance to corrosion. The improvement in corrosion resistance brought on by refinement is more significant than the degradation in corrosion caused by the textural effect. As a result, the FSP Al6101 has better corrosion resistance as a result of the cumulative effect. Surprisingly, compared to FSP Al6101, SiC addition reduces corrosion resistance. The stress-strain curve of the AA6101-SiC sample is shown in Figure 12.
The results significantly support the idea that the galvanic effect had a part in the composite’s lower corrosion resistance when compared to FSP Al6101 and Al6101. A drawback of the created Al6101-SiC composite is that because SiC serves as the cathode, the Al6101 matrix serves as the anode, and the composite experiences increased galvanic corrosion. Thus, it is clear that the Al6101-SiC composite made by FSP has superior mechanical and machining behavior. However, when constructing the structures, it is important to take into account the composite’s poor corrosion performance caused by the presence of SiC.
4. Conclusion
In the present investigation, FSP created Al6101-nano-SiC surface metal matrix composites (MMCs) to examine the impact of silicon addition on mechanical, machining, and corrosion properties. Both the FSP Al6101 and the composite appeared to have a robust (200) texture based on the SEM and EDAX investigations. The grain refinement is the source of the perfect surface reorganization technique. Due to the combined effects of grain refinement and the addition of silicon carbide, the composite’s hardness was measured to be higher than that of FSP Al6101 and Al6101. With a lower percentage of elongation than the basic alloy, the FSPed Al6101 had relatively greater yield strength and UTS. Comparing the composite to FSP Al6101 (base metal), a slight increase in strength was seen. Lower cutting forces were observed during drilling even though the composite had higher hardness and tensile strength, indicating that the silicon-incorporated material was more machinable. On the other hand, galvanic corrosion brought on by the inclusion of silicon in the composite decreased corrosion resistance. Therefore, it can be said that Al6101-silicon carbide composites created may reach the increased mechanical and machinability due to friction stir processing. A special mention must be made, nevertheless, regarding the poor corrosion performance of these composites, which precludes their use in corrosive environments.
Data Availability
The data are available in the manuscript.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
The authors would like to express their gratitude to the Department of Mechanical Engineering, GITAM (Deemed to be University) School of Technology, Visakhapatnam Campus, for providing laboratory facilities for the research. The authors also sincerely thank the Karpagam Academy of Higher Education (KAHE), India, for providing facilities to carry out the research work.