Fabrication and investigation of mechanical properties of FGM metal‐based materials with friction stir process and carbon nanotube particles

doi:10.21203/rs.3.rs-4266965/v1

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Fabrication and investigation of mechanical properties of FGM metal‐based materials with friction stir process and carbon nanotube particles

https://doi.org/10.21203/rs.3.rs-4266965/v1

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In this study, Al6061 aluminum sheets and carbon nanotubes with 0, 0.1, 0.5 and 1.5 volume percent were used to produce FGM materials using the friction-stir process and important parameters such as rotational speed, traverse speed and tool angle were investigated for the impact strength and microhardness of the produced composites. The process was carried out at rotational speeds of 950, 1225 and 1500 rpm, traverse speeds of 30, 37.5 and 45 mm/min and angles of 2, 3 and 4 degrees and the mechanical properties and microstructure of the composite material produced were investigated. In order to reduce costs, increase the accuracy of the experimental tests, determine the optimal conditions and also ensure the effectiveness of the parameters for the samples, the response surface design method was applied using Design Expert and Minitab software. In the impact test results, the fracture energy of the manufactured composite material is lower than that of the starting material, and as the rotational speed and tool angle increase, the fracture energy decreases, but as the traverse speed increases, the fracture energy increases. The results of the microhardness test show that the hardness of the produced composites decreases compared to the starting material. As the speed and angle of the tool increases, the hardness decreases, but as the traverse speed increases, the hardness of the composite material increases. The investigation of the effect of the percentage of carbon nanotubes on hardness showed that hardness also increases with increasing percentage of nanotubes.

Friction Stir Process

Carbon Nanotubes

Functionally Graded Materials

Composites

Composite materials are heterogeneous in their microstructure and their mechanical properties change continuously from one surface to another surface of the object, which is a continuous and variable combination of ceramic and metal obtained by mixing metal and ceramic powder (Kumar, Sharma and Singh, 2023). The change of metal and ceramic from one surface to another is completely continuous and gradual, so that one surface is pure ceramic material and one surface is pure metal, in other words, two surfaces are a continuous combination of both materials. The mechanical properties of these materials have continuous changes in the thickness of the material depending on the type of composition, and due to the continuity and gradualness of the composition of the constituent materials, they have effective mechanical properties compared to layered composite materials (Sai Charan et al., 2022). Due to the low heat transfer coefficient and high temperature resistance, ceramics can withstand very high temperatures and the metallic structure of the material causes the flexibility of the fabricated composite, in other words, the integration and interruption of ceramic and metal with continuous changes from one surface to another in an instrument that can be easily fabricated, and due to the continuous and gradual changes in mechanical properties, the problems of discontinuity that exist in composite structures do not occur in the case of FGM (Merdaci et al., 2022). There are different methods to produce FGMs, and the appropriate method, FSP, was chosen (Sai, 2021). The Friction stir process (FSP) was invented in 2003 to change the surface properties and create a surface bond (Mishra, Ma and Charit, 2003). The FSP process, like the FSW process, utilizes the motion of the rotating non-consumable tool to modify the structure (Bharti, Ghetiya and Patel, 2021). Severe deformation along with increasing temperature during the FSP process leads to microstructural changes at the desired location (Kumar and Goyat, 2023). For low melting point materials such as aluminum and magnesium, cooling can prevent the growth of recrystallized grains and the dissolution of solidification deposits in and around the agitated area. On the other hand, increasing the rotational speed of the tool leads to an increase in the applied strain rate and a reduction in the likelihood of defects in the workpiece (Jain et al., (Jain et al., 2023). The correct choice of the ratio between the rotational speed of the tool and the traverse speed of the tool is an important factor in creating a continuous flow and an area without defects. The time the materials are exposed to the temperature in the flow and the intensity of their plastic deformation depend on the traverse speed and the rotational speed of the tool (Jacquin and Guillemot, 2021). On the other hand, increasing the tool traverse speed of the tool increases the hardness and strength, which is due to a lower heat input and a higher degree of dynamic recrystallization (Saini et al., 2015). Increasing the number of passes leads to a reduction in the size of the particles and thus to the creation of a more homogeneous structure. Also, increasing the number of passes leads to an increase in the hardness of the composite and alloy sample due to the fineness and more uniform distribution of the particles (Golmohammadi, Atapour and Ashrafi, 2015).For this purpose, FSP is used to produce FGM, where the reinforcing particles are placed in the groove.On the other hand, by studying the mechanical properties and wear of AA6061-T6 aluminum alloy at three rotational speeds, 900, 1120 and 1400 rpm and a feed speed of 40 mm/min using FSP process and the effect of volume fraction of reinforcement particles and tool rotational speed on this alloy, a composite layer of silicon carbide and graphite was produced. As a result, increasing the volume fraction of SiC particles from 4% to 8% increased the microhardness of the composite layer (Devaraju, Kumar and Kotiveerachari, 2013). When investigating the microstructure and mechanical properties of aluminum 6061 with Al2O3 and SiC particles, the optimal conditions for maximum tensile strength were found at a rotational speed of 900 rpm, a linear speed of 15 mm/min and a combination of 60% SiC: and Al2O3: 40% (Rao and Saheb, undated) . On the other hand, carbon nanotubes (CNTs) were introduced into aluminum alloy Al5083 by FSP to produce metal matrix composites, where pure single-walled carbon nanotubes (CNTs) prepared by arc discharge method were used as dispersed phase to fill the groove. It was used on the workpiece and it was assumed that 70% of the groove was filled with CNT. Also, based on the preliminary tests, machining was carried out at a rotational speed of 1800 rpm and a linear speed of 25 mm/min and the composite material shows higher hardness compared to Al5083, which is due to grain refinement and integration of CNTs into Al5083 (Prabhakar et al., 2019). To investigate the potential of surface nanocomposites based on AA6061 with its reinforcement by carbon nanotubes (CNTs) to produce composite materials, grooves of different sizes were filled with CNTs using the friction stir process and microhardness and tensile tests were evaluated for the mechanical properties, resulting in a 47.3% increase in hardness and 32.4% in the ultimate tensile strength (UTS) of the base materials on which the process was carried out. Also, from a tool with a shoulder diameter of 25 mm, a cylindrical set screw (1.5 mm)10 degree taper with a base diameter of 4.5 mm and 4 mm length with rounded edges was used that increasing the purity (CNTs) increased the mechanical properties due to fewer defects in the purified CNTs. And increasing the rotational speed of the tool for FSP improved the mechanical properties as the CNTs in the material were better mixed at higher rotational speeds, as more rotational speeds per minute caused more movement of the material, resulting in better mixing of the reinforcements (Riaz, Mnazoor and Raza, 2019). By studying the effects of multiple FSP on the microstructure, microhardness and conductivity of CNTs reinforced copper matrix composites obtained by 3 passes of FSP with uniform dispersion, the hardness of CNTs reinforced copper is increased compared with the original product, on the other hand, the conductivity of carbon nanotube reinforced copper composite is slightly decreased compared with the original material, and the fabricated composite is still a good conductive material (Chen, Huang and Ke, 2012).By studying the ductility, mechanical and microstructural properties of Al5083 composites reinforced with MWCNT and B4C, 28% and 12% increase in tensile strength and hardness with B4C and without surface cracking, the bending ductility test was passed and the MWCNT composite showed a decrease in elongation at break of approx. 50% and an increase in hardness and tensile properties, and the hybrid composite showed an increase in tensile strength and hardness by 10 and 5, respectively, and a reduction in elongation at break by 16%, which can also be attributed to the poor surface bonding of MWCNTs and Al and the poor distribution of MWCNTs in the composite, in the bending test, the ductility failed, but the distribution of MWCNTs may increase in the composite with multiple FSP passes (Khan et al, 2017). By making 3.5 mm deep holes with different diameters in a 1016 aluminum sheet and filling them with different amounts of MWCNTs, using nanotubes with a diameter of about 10 to 40 nm and a length of 10 to 20 μm, a cylindrical tool A simple pin with a diameter and height of 12 and 8.7 mm, a rotational speed of 950 rpm and a traverse speed of 30 mm / min, the resulting structure was examined with optical, scanning electron and transmission electron microscopes, and the nanotubes were completely and uniformly distributed in the field with five FSP passes, and the surface composite was successfully fabricated (Liu et al, 2013). By investigating the effect of carbon nanotubes (CNTs) in Al matrix composites reinforced by the friction stir process (FSP) on the wear performance and hardness of aluminum matrix composites, the results have shown that CNTs are suitable particles for the production of aluminum matrix composites, reinforced by FSP, and that CNTs can effectively reinforce the matrix composites and significantly improve the hardness of the composites, and by increasing the CNT content, CNTs can also improve the wear performance of matrix composites using aluminum alloy (1060; 5 mm thickness) was used as the matrix material, nanotubes with a diameter of 10-20 nm and a length of 5-15 µm, tool pins with a constant diameter and a size of 8 mm and a length of 8 mm, a process with a constant linearity speed of 30 mm/min, a rotational speed of 950 rpm and an angle of 2° were used, and the volume fraction of carbon nanotubes was used as 0, 1.3, 3, 3.5 % and 5 %, respectively. When the volume fraction of CNTs is smaller, it is easier to produce a good composite material, and when the CNT content is increased, the phenomenon of separation of CNTs and even small holes outside the friction region of turbulence occurs (Weiping, Liming and Li, 2011).According to 2-12, the hardness of the composite material shows an increasing trend with the increasing content of carbon nanotubes, and the hardness is evenly distributed in the friction stirrer region (Weiping, Liming and Li, 2011). Fabrication of multi-walled carbon nanotube (CNT) reinforced aluminum alloy composite by encapsulating multi-walled CNT powder in a 0.3 × 2.3 mm groove in a lower plate covered by an upper plate before FSP, and a sheet of Al 6111-T4 alloy with a thickness of 1.1 mm as a cover plate to hold the CNT material in the groove during the process, and an Al 7075-T6 alloy bottom plate with a thickness of 6.35 mm was used. The CNTs had an outer diameter of 30-50 nm and a length of 10-20 μm, all samples were processed using a tool consisting of a comb with a diameter of 10 mm, a pin with a diameter of 4 mm and a length of 2.2 mm, and the rotational speed used was between 1500 and 2500 rpm and the linear speed was 2.5 mm/s. The comb penetrated 0.03-0.24 mm into the upper sheet of Al 6111, and it was observed that at a tool rotational speed of 1500 rpm, the depth shoulder penetration increased to 0.24 mm and the stirring zone was void-free and consisted of mixed layers of Al 6111 and Al 7075 materials. In addition, the rotational speed of the tool was increased to 2500 rpm and the penetration depth of the comb decreased to 0.24 mm thickness (Lim, Shibayanagi and Gerlich, 2009). However, evidence was observed that the nanotubes may have fractured during FSP, and it was found that increasing the rotational speed of the tool from 1500 and 2500 rpm and increasing the penetration depth of the tool shoulder improved the distribution of nanotubes in the alloy field, and when entangled nanotubes were used as the base material, a completely uniform distribution could not be achieved. it has been suggested that multiple passes may be necessary to further improve the dispersion of nanotubes in the matrix (Lim, Shibayanagi and Gerlich, 2009). For Al composites with AA7075 alloy and CNTs using FSP, microstructural improvements have shown that FSP causes grain refinement and uniform distribution of CNTs, which significantly increases the tensile properties but greatly reduces the elongation properties and increases the microhardness of the composite processed with CNTs by 20% compared to the base matrix and by 10% compared to the processed Al. Furthermore, the optimal parameters for improving the tensile properties and wear resistance using FSP were determined using the response surface method: a tool rotational speed of 1200 rpm, a linear speed of 40 mm/min and 0.6 g CNT to increase the tensile properties and wear resistance by up to 20%, the grain size of the composites can be reduced due to the dynamic recrystallization in the stirring zone during FSP (Ebrahimzad, Ghasempar and Balali, 2017).

All paragraphs should be indented and justified. The font of the entire document should be Cambria.

1.1. Introduction

In this study, Al6061-T6 sheets with dimensions of 120×100 mm and thickness of 10 mm were fabricated. Table 1 shows the chemical elements in this alloy.

The particles used in this study are multi-walled carbon nanotubes with an outer diameter of 20-30 nm and a length of 30-10 micrometres with a purity of over 95%, which were produced by CVD method. Table 2 shows some of the properties of these particles.

The SEM photo of these particles can be found in Figure 1.

A powerful milling machine was used for the friction stir process. One of the most important features of this machine is the rotational speed of the tool up to 1500 rpm, the possibility to move the machine table in three directions, the complete control by mechanical and adjustable devices, the possibility to assign a constant traverse speed in three directions, the possibility to tilt the spindle and the powerful motor. Figure 2 shows the clamping device used.

Considering that the aim of this research is to produce FGM material, this work was carried out layer by layer and each layer has its own percentage of carbon. To produce FGM material, the layers are stacked with increasing percentage of carbon nanotubes. This means that from the bottom to the top, the percentage of nanotubes in the layers is 0, 0.5, 1 and 1.5 percent by volume respectively.

To produce this composite material with the aforementioned properties, an FSP stitch is first made in the center of the sheet with a tool whose pin length is 9 mm and the first layer is produced without particles. Then, a groove with a depth of about 7.5 mm and a width of 1 mm is created in the center of the FSP area with a milling machine and a milling blade, as shown in the figure. 0.5 percent of the volume of the particles is placed in the groove according to the density of the sheet and particles obtained. Then the sheet is closed in the fixture and first an FSP pass is made on the groove with a pinless tool to close the groove. Then the process was carried out using a tool with a 7/5 mm long pin to form the next layer with half a percent carbon. After this step, a groove is created in the center of the sheet according to the previous step, this time with a depth of 5 mm and the same width as the previous one, that is, 1 mm. Place the particles in the groove according to the previous step. Considering that 0.5% particles were added in the previous step, the upper layers have 0.5% nanotubes. So to create a 1% layer, 0.5% of the particles are placed in the groove. The sheet is placed in the jig and according to the previous step, a pinless tool is used to close the groove. After the pinless tool, move on to the tool with a pin length of 5 mm and with this tool the FSP is performed to create the layer with 1% volume of particles. The 1.5% layer is produced in the same way as the previous layers, except that the groove is 2.5 mm deep and a tool with a 2.5 mm long pin is used.

In this study, the effects of three parameters were investigated: rotational speed, linear speed and inclination tilt of the tool. For the rotational speed, speeds of 950, 1180, 1500 rpm were considered, for the linear speed, speeds of 30, 37.5 and 47.5 mm per minute were considered and for the angle, 2, 3, 4 angles were considered. The penetration depth for all samples is assumed to be 0.2 mm. The experiments in this study were planned using the response level method.

Two software programmes, Minitab9 and Designexpert7, were used in this study to take advantage of the surface response and also to improve the graphical work. In this study, three main factors are considered as independent variables of the problem and one parameter is considered as the response. The three parameters traverse speed, rotational speed and spindle angle are considered as independent variables and the impact toughness is considered as the response to the problem. The response level statistical method was used to design the experiments and the BBD statistical scheme was used for this purpose. Three factors were examined at three different levels. Table 3 shows the experimental design matrix.

1.2. Mechanical tests

One of the mechanical tests carried out to evaluate the mechanical properties of materials is the impact test. The Charpy impact test is used in this investigation. The device used was the 200 Joule Charpy impact testing machine from Santam Iran. The impact test was performed according to the ASTM A370 standard. According to the ASTM A370 standard, the impact specimen is a piece with a length of 55 mm and a square cross-section of 10 x 10 mm, which has a V-shaped groove with an angle of 45 degrees and a depth of 2 mm and a root radius of 0.25. The specimens were tested with a wire cutter. The samples were cut with a wire cutter perpendicular to the direction of the FSP area. Hardness is one of the basic properties of a material. Hardness can be considered as the resistance of the material to a deformation of the material that involves a permanent or local change in shape. The hardness test was performed for each sample once in the direction of the thickness of the FSP region from bottom to top and in the direction of increasing carbon nanotube particles, and once perpendicular to the FSP region on the surface. To perform the hardness test, 16 points on the surface of the machined samples and 10 points in the direction of the thickness from bottom to top were selected and the hardness of these points was determined. Finally, the hardness of these points was plotted as a result of the hardness of the sample. The microhardness test of the samples was carried out using a microhardness tester from Bühler, America, in a time of 10 seconds under a load of 300 grams and at room temperature.

2.1. Introduction

The aim of this research is to produce a functional aluminum-based composite material with carbon nanotube particles using the FSP method and to investigate the properties of this composite material. The effect of process parameters, i.e. rotational speed, linear speed and tool angle, on fracture energy was investigated. Then, the effect of the percentage of particles on the microhardness and also the effect of the process parameters on the hardness of the composite is investigated and finally the microstructure of the produced composites is studied (Figure 3).

2.2. Impact Tests

In this study, the response level method and Design Expert and Minitab software were used to design and analyze the experiments. The BBD method was used to develop the experimental design matrix. A total of 14 samples were prepared and subjected to the process based on the experimental design used in the current study. Table 4 shows the experimental design matrix and the fracture energy resulting from each situation (problem solving). In studies based on the statistical design of experiments, the accuracy of the model is determined, among other things, by the value of the coefficient of determination (R-squared) and the adjusted coefficient of determination (adjusted R-squared). The closer the value of these components is to 1 or 100 percent, the higher the accuracy of the model fit and the higher the quality of the proposed model. The coefficients of the statistical model used in this study are shown in Table 5. A 95 confidence level was used in the statistical analysis and the ANOVA technique was used to test the involvement and effectiveness of the model factors. Table 6 shows the ANOVA for the fracture energy parameter.

The fracture energy for the starting material was measured to be 31.6, but the fracture energy for all manufactured samples was less than 60% of the base material. From the decrease in fracture energy, it can be seen that the reason for this is that our material was subjected to a thermal process and the change in particle size led to a decrease in fracture energy, as did the severe deformation of the plastic that the particles were subjected to during the process. Reduces the size of the aluminum grains. By reducing the grain size of the aluminum matrix, the grain boundary density has greatly increased, and since grain boundaries are one of the obstacles to the movement of dislocations, the increase in grain boundaries restricts mobility and reduces flexibility. Figure 4 shows the influence of the three parameters of rotational speed, linear speed and tool angle on the amount of fracture energy. Figure 4 shows that the higher the rotational speed and the more brittle the material, the lower the fracture energy. As the rotational speed decreases, the fracture energy increases and the optimum speed reaches 950 rpm. Since less heat is generated at low speeds, the disturbance is also more uniform and there is adequate plastic deformation by the pin and the shoulder of the tool. In other words, at higher rotational speeds, the heat and stress created by the comb and pin increase and cause structural porosity and create holes in the composite. On the other hand, the process carried out improves the casting defects caused by solidification, which helps increase the mechanical properties. Also, at rotational speeds higher than 1225 rpm, the rotation of the pin and shoulder of the tool with the sample causes a lot of unwanted heat, which results in a decrease in the cooling rate, which causes undesirable material flow. This undesirable material flow leads to holes and other defects in the FSP region. The presence of these defects is the main reason for the reduction of mechanical properties and fractured energy at a rotational speed of 1500 rpm. According to Figure 4, with the increase in the advance speed, the fracture energy decreased and as it can be seen, the fracture energy had a decreasing trend at a linear speed of 45 mm/min. Increasing the traverse speed causes the input heat to decrease and not reach the optimal value, and also the process is not performed well, as a result, it creates defects that cause the failure energy to decrease. According to Figure 4, it can be concluded that the higher the tilt angle, the lower the fracture energy. The tilting angle of the tool causes the materials that are trapped and plasticized under the shoulder of the tool during the penetration and movement of the tool to return to the part with high pressure using the forging force behind the tool. With the forging, the plastic flow collected in the tool trail area creates a suitable compression and interference, which will lead to the reduction of process defects such as defects and holes in the center of the FSP area. It should be noted that the tilt angle of the tool is also a factor with an optimal limit and should be established at its optimal limit. By increasing the tilt angle from the permissible limit, the amount of interaction and contact surface between the shoulder of the tool and the workpiece in the forehead region of the tool decreases. This issue will lead to a decrease in heat production and vertical plastic flow created by the tool shoulder, and will ultimately cause a decrease in the mechanical and metallurgical quality of the FSP region. Using an optimal tilt angle will increase the strength and create a homogeneous and uniform microstructure in the FSP region.

2.3. Validation of the model , Optimization and Microhardness test

To verify the validity of the model, three other tests that were not included in the test design table have been performed. Table 7 shows the parameters of three experiments with the size obtained from the model and the size obtained from the impact test. According to the numbers on Table 7 and the percentage of errors from the tests, it can be seen that the model is reliable.

To optimize the fracture energy, ten points with the highest fracture energy are considered. The obtained optimal points are shown in Table 8. According to Table 9, ten optimal points have been obtained, which show that the highest fracture energy occurs when the rotational speed is close to 950 rpm and the linear speed is 30 mm/min with an angle close to 2 degrees. The Microhardness test results of five samples 5, 8, 13, 11 and 12 and the base sample are shown on Table 9 . The backward region is shown with negative numbers and the forward region is shown with positive numbers.

As it is clear from Table 9 and, the highest and lowest hardness values were measured in the center of the agitation zone and near the base metal, respectively. Considering that the hardness value of the used aluminum alloy Al6061 T6 was measured as 106 Vickers, and since aluminum alloy 6061 is one of the alloys that can be heat treated, the frictional stir process of a soft area around the center of the processing area in a number of Aluminum alloys produce heat treatment, this condition is caused by the annealing conditions as a result of the dissolution of reinforcing deposits in the processed area, which have undergone severe plastic deformation and experienced high temperatures during the FSP process. So, by doing the process, the hardness of this alloy decreases. Also, due to the fact that plastic deformation is more in the regressive region and the grain size is finer, the hardness of this region is higher than the advancing region of the samples, and this issue can be understood according to the diagram and hardness values. According to the obtained results, it can be said that reducing the size of particles and increasing their number, the fineness of grains in the turbulent region caused by dynamic recrystallization, as well as creating a more homogeneous structure in stir regions, under the influence of heat and under the influence of mechanical work, are important reasons for increasing the hardness of the sample. The processed samples are in the disturbed area, which is clear in the graphs, and the samples have the highest hardness in the middle of the processed area. The hardness in the aluminum-based composite area depends on the grain size, density of dislocations, nano-reinforcement particles and heat input to this area. According to the Hall-Patch relation, hardness increases with decreasing grain size. The dislocations that are created as a result of the heterogeneous distribution of nano particles in the metal field causes an increase in hardness. Reinforcing particles has a double effect on hardness. Both the particles themselves have high hardness and the effect they leave due to the phenomenon of locking increases in the hardness of the samples. In the absence of nano particles, the only effective factor is the input heat, which, despite the granulation inside the FSP region, greatly reduces the hardness of this region. On the other hand, the input heat caused by the continuous friction stir process does not allow the hardness to increase to a great extent, the reason being the complete annealing effect. On the other hand, the heterogeneous distribution of nanotube particles in the FSP region, and as a result, the improper flow of materials during the process can be one of the main reasons for the strong clumping of reinforcing particles in these samples and sometimes local reduction of hardness.

2.4. Investigating effect of three important parameters

The two samples 5 and 6 were used to investigate the effects of the rotational speed of the tool. For these samples, the linear speed is the same and is 37.5 mm/min, and the tool angle is also 2 degrees for both samples. Sample 5 has a rotational speed of 950 rpm and sample 6 has a rotational speed of 1500 rpm. From Table 8, it can be seen that the hardness of sample 5 is higher than that of sample 6. It can be said that by increasing the rotational speed, the heat applied increases, and since the heat applied decreases the hardness, and also when the heat is higher than the optimum value, it causes the grains to increase in size and decreases the hardness, so the increase in rotational speed causes a decrease in hardness. Sample 6 is compared with sample 5. Two samples 11 and 12 were tested for the effects of traverse speed on hardness. Both samples have the same rotational speed and the same angle of 1225 rpm or 4 degrees. The traverse speed for sample 11 is 30 mm / min and for sample 12 ,45 mm / min. According to the results of the microhardness test of these two samples, the hardness of sample 12 is higher than that of sample 11. The reason for this is that by reducing the traverse speed, the heat input increases and the grains of the sample become larger, resulting in a decrease in hardness. Two samples 6 and 8 were used to investigate the influence of the tool angle on the hardness measurement. A rotational speed of 1500 rpm and the traverse speed of 37.5 mm / min were taken into account for the two samples 6 and 8. Sample 6 has an angle of 2 degrees and sample 8 has an angle of 4 degrees. From the results obtained, it can be concluded that the hardness of the tool decreased with the increase in the angle of inclination. This decrease is due to the fact that the contact area between the shoulder of the tool and the surface of the workpiece is smaller at an angle of 4 degrees, the force and heat required to carry out the process are not generated, thus causing a decrease in mechanical quality and hardness. As mentioned above, FGM material was produced in this study in which the proportion of particles increases from the lower layers to the top. Therefore, the effect of particle content on the hardness of the composite material was also investigated. For this purpose, the hardness of the points of three samples 8, 5 and 6 was measured from bottom to top. If you increase the percentage of particles and their quantity, the size of the particles decreases and they increase the hardness. The nanoparticles themselves also have a high hardness and the hardness increases with the increase in the volume percentage of the particles (Figure 5, Figure 6, Figure 7). SEM images of sample 5 with the highest fracture energy and sample 8 with the lowest fracture energy were taken to check the microstructure of the fracture region of the three base samples. Figure 14 shows the fracture surface of the samples mentioned. Figure 8 (a) shows the fracture cross-section of the base material. In this figure, depressions and pits can be seen, which shows that our base sample has a flexible fracture. In Figure 8 (b), where the fracture surface of sample 5 can be seen, the pits and pits have become larger and the crack line can also be seen, indicating a brittle and soft mixed fracture. Figure 8 (c) shows the fracture cross-section of sample 8, which indicates a brittle fracture based on the crack line. Examination of Figure 8 shows that the process has caused the soft fracture to transform into a brittle/soft fracture, indicating the presence of a pit. The larger cracks and the crack lines are evidence of this claim, and these results are consistent with the results of the notched impact test, which show that the basic sample is more flexible than the manufactured composites, and also sample 5 is tougher than sample 8. Figures 9 and 10 show the distribution of particles in the base material. In the electron micrographs, the back electron detector components of the structures that have a higher average atomic number can be seen brighter,

while the structural components that have a lower atomic number can be seen darker. Figures 9 and 10 show the distribution of the nanoparticles as seen in the SEM images. The background is aluminum alloy 6061 and has a higher average atomic number than the CNTs, it can be seen lighter and the particles dispersed in the alloy, which are darker, are the aggregation of the carbon nanotubes.

The examination of the composites materials shows an inhomogeneous distribution, in other words a clumping of the particles. This fact can be related to the low wettability of the aluminum alloy and the carbon nanotubes, as well as this dispersion, the non-uniform distribution and the different orientations of the CNTs caused by different types of deformation. FSP produces complex plastic deformation, and the flow direction is along the rotation direction of the pin, the pin is round, and the CNTs cannot be oriented in a certain direction. Another reason for this kind of uneven distribution is that when a reinforcing material is used in a metallic substrate, the bonding phase of the substrate and the reinforcing component is very important. Due to the fact that carbon nanotubes are highly agglomerated due to their nanometer size and high specific surface area (100 m2/gr). In sample 5 (Figure 9), the distribution of nanoparticles is more uniform than in sample 8, which is due to the process with more mechanical stress due to the lower angle of the tool and more optimal heat due to the lower rotational speed, which reduced the gaps between the common layers of the processing area and structural defects. The distribution of nanoparticles in sample 8 (Figure 10) is more in the form of an accumulation, mostly trapped at the sites of discontinuities and cracks in the bonding layer of some mixed layers, which is due to less disruption and weaker mechanical mixing of the nanotubes. Sample 8 is carbon with base metal material. Based on the larger clusters of carbon nanotubes in this sample, which can be seen in the images, it can be said that sample 8 is less flexible than sample 5. Looking at Figures 9 and 10, it can be seen that at low rotational speed, heat generation is lower and stirring is more uniform, resulting in adequate plastic deformation by the pin and shoulder of the tool. In other words, at a higher rotational speed, the heat generation and stress generated by the comb and pin increase, causing structural porosity and holes in the composite, as shown in Figure 10. In addition, the nanoparticles in sample 8 are larger than in sample 5 and agglomerated.

The use of metal-based functional materials is due to the fact that it is microscopically inhomogeneous and the structural characteristics, including the nature of the distribution, the size of the phases, gradually change from one surface to another, and this gradual change leads to a gradual change in the properties of FGM, is increasing . One of the methods for producing these materials is the friction stir method. This process is a surface improvement method developed for the production of surface composite materials. The friction stir process is a method of stirring the material based on the principles of friction stir welding, with the difference that only the change in microstructure is proposed and there is basically no welded joint. In this method, a non-consumable pin is used to generate heat and disturbance. Due to the fact that in aluminum and magnesium alloys, the friction process was carried out with a lot of movement, and also due to the very good properties of carbon nanotube particles such as high surface density, tensile strength, etc. It was decided to produce and investigate Al-CNT composites, where the parameters of rotational speed, linear speed and inclination angle of the tool were new parameters in the field. In this study, the effects of the process and process parameters, including the rotational speed of the tool, traverse speed and inclination angle of the tool, on the mechanical properties, including impact strength and microhardness, were comprehensively evaluated. Design Expert and Minitab software were used to test the impact strength. Microhardness was investigated both in the direction perpendicular to the weld and in the thickness direction from bottom to top due to the variable volume fraction of the particles. The fracture cross-section was examined using a scanning electron microscope to investigate the microstructure and the following results were obtained:

1. The 60% reduction in fracture energy of the composites produced compared to the base sample is due to the non-uniform distribution of the particles and the failure of the nanoparticles to achieve the barrier effect and the process-related complete annealing effect of the heat applied.

2. As the speed increases, the heat of the process increases and the cooling effect decreases, so that the material cannot flow well and the fracture energy decreases.

3. When the linear speed is increased, the process heat does not reach the optimum size and the process is not performed well, resulting in a decrease in fracture energy.

4. Increasing the angle of inclination of the tool reduces the fracture energy and microhardness, since increasing the angle of inclination reduces the amount of contact between the shoulder and the surface and reduces the heat of the process and causes defects in the structure.

5. The decrease in hardness of the produced composites compared to the base sample is due to the effect of full baking and the heat input caused by the process. In some places, the microhardness of the fabricated composite was measured to be less than 50% of the base sample.

6. By increasing the rotational speed and decreasing the linear speed, the microhardness decreases due to the higher heat input leading to an increase in the size of the grains.

7. The presence of carbon nanotube particles increases the hardness of the composites produced.

8. The hardness increases with the increase in the volume fraction of carbon nanotube particles.

9. During the microstructural investigation of the fracture surface, it was found that the process leads to the soft fracture of the base material becoming a soft-brittle and brittle fracture. In addition, the uneven distribution of carbon nanotubes in the produced composites and their agglomeration due to the low wettability of aluminum alloy and carbon nanotubes were observed.

10.From the microstructural studies of the two samples 5 and 8, it can be seen that sample 5 has a thermally uniform structure due to the lower and stronger wettability

optimal heat input compared to sample 8, and

the microstructural defects in sample 5 were also lower than in sample 8.

Funding Statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Conflict of Interest

The author declares that there is no conflict of interest regarding the publication of this article.

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Fabrication and investigation of mechanical properties of FGM metal‐based materials with friction stir process and carbon nanotube particles

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Abstract

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Introduction

Materials and methods

1.1. Introduction

1.2. Mechanical tests

Results and Discussion

2.1. Introduction

2.2. Impact Tests

2.3. Validation of the model , Optimization and Microhardness test

2.4. Investigating effect of three important parameters

Conclusions

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References

Tables

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Supplementary Files

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