Tool Wear and Surface Roughness in Turning of Metal Matrix Composite Built of Al2O3 Sinter Saturated by Aluminum Alloy in Vacuum Condition

Metal matrix composites (MMCs) are a special class of materials carrying combined properties that belongs to alloys and metals according to market demands. Therefore, they are used in different areas of industry, and the properties of this type of material are useful in engineering applications. Machining of such composites is of great importance to finalize the fabrication process with improved part quality. However, the process implies several challenges due to the complexity of the cutting processes and random material structure. The current study aims to examine machinability characteristics. Effects of turning a metal matrix composite built of Al2O3 sinter, saturated with an EN AC-44000 AC-AlSi11 alloy, are presented in this paper. In the present study, a turning process of new metal matrix composites was carried out to determine the state-of-the-art material for various engineering applications. During the turning process, the cutting forces, a tool’s wear, and surface roughness were investigated. Further, the SEM (scanning electron microscope) analysis of cutting inserts was performed. The influence of MMC structure on the machining process and surface roughness was studied. The Al2O3 reinforcements were used in different graininess. Effects of conventional turning of the metal matrix composite with Al2O3 sinter of FEPA (Federation of European Producers of Abrasives) 046 and FEPA 100 grade were compared. Results analysis of these tests showed the necessity of continuing research on turning metal matrix composites built of an AlSi alloy and Al2O3 ceramic reinforcement. The study showed the properties of MMCs that influenced machinability. In this paper, the influence of feed rate’s value on surface roughness was carried out. The significant tool wear during the turning of the MMC was proved.


Introduction
The composite is an example of an engineering material that is made of a minimum of two different components [1]. This type of material combines properties of its elements, or new and better properties could characterize it. Composites are built of a matrix and a reinforcing phase [2].
Metal matrix composites (MMCs) are engineering materials that are a mixture of a matrix made from metals such as aluminum, copper, and magnesium and reinforcement materials [3]. The reinforcing phase could be built of continuous fibers or discontinuous fibers [4]. Ceramic compounds, e.g., silicon carbide (SiC) [5], aluminum oxide (Al 2 O 3 ), boron carbide (B 4 C), titanium boride (TiB 2 ), and other compounds, could be mainly used as a reinforcement [6]. Due to their properties such as mechanical and thermal strength, stiffness, and wear resistance, metal matrix composites are successfully applied in various As a relatively new group of composites, metal matrix composites are examples of hardto-cut materials [22]. MMCs are characterized by heterogeneous structures. Furthermore, these materials have properties such as high hardness and abrasive resistance. Thus, machining MMCs with PCD (polycrystalline diamond), carbide, and ceramic tools should be performed. The most popular tool types used in the machining of MMCs are uncoated tungsten carbide, coated tungsten carbide, and polycrystalline diamond [5].
Tool wear takes place during the machining of metal matrix composites. It is because of heat load and the moving of particles or fibers [23]. Mainly, there is abrasive wear on the cutting edge occurring [24]. It is possible that in the case of this phenomenon taking place on the cutting edge, there are effects appearing such as micro-cutting, peeling, chipping, material discounting, microcracking, buildups, changing of material structure, and phase transitions [1].
The main problems with manufacturing pieces from MMCs are their machining because of intensive tool wear [25] and poor workpiece surface properties [26]. Researchers [25] carried out studies on the machining of MMCs reinforced with particles and whiskers. The information about the machining of metal matrix composites built of Al 2 O 3 sinter saturated by an aluminum alloy was not found in the available literature. Conventional tool materials such as HSS cannot be used to machine MMCs. The most effective results of machining MMCs can be achieved with ceramic and diamond tools [26]. It was observed that tool wear is caused by the abrasion of reinforcement particles which are characterized by excellent hardness [27]. Reinforcement particles in MMCs have an effect on the cutting edge similar to the grinding wheel. Poor surface finish is caused by that effect [28]. According to research [29], the most satisfactory effects on machining an Al/SiC composite are reached during turning with CBN (cubic boron nitride) inserts. Another study [30] showed that the lowest tool wear was observed using PCD and CVD (Chemical Vapor Deposition Diamond) inserts. Machining of an Al-SiC MMC with different types of diamond inserts proved a significant influence on the surface finish [27].
The results of the machining process of metal matrix composites depend on many factors [31]. Palanikumar et al. focused on the influence of machining technological parameters and cutting inserts' geometrical parameters on the geometrical structure of the MMC surface [32]. The quality of a machined surface is one of the most critical aspects of the properties of machines' parts [33]. Additives have an influence on tool wear [34]. Thus, the machined surface quality is a significant parameter that influences the application of metal matrix composites [17]. The surface roughness after turning MMCs depends on cutting speed and feed rate. Nataraj et al. [31] observed that the Ra parameter after turning an LM6/Al 2 O 3 composite reached the highest value when the cutting speed was 175 m/min. By decreasing the cutting speed to 125 m/min, the value of Ra increased. It was a result of greater cutting friction occurring during machining with lower cutting speed and required high shear energy. Concerning the correlation between feed rate and surface roughness, these research studies showed that the Ra parameter reached lower values while the feed rate was low [31]. The exact correlation between feed rate and surface roughness in turning an A359/B 4 C/Al 2 O 3 composite was presented by Srivastava et al. [19]. According to Kawalec et al. [4], another phenomenon could be noticed. These research studies prove that the surface roughness of an Al/SiC composite is reduced by increasing feed rate. It is assumed that this effect results from reducing tool wear with increasing the feed rate value. This report concludes that the surface roughness is improved at the highest values of cutting speed, feed rate, and depth of cut [35].
Pramanik et al. elaborated a mechanical model to predict the values of cutting forces during the machining of aluminum-based composites [8]. Other researchers developed a model to predict cutting forces while machining a SiC MMC [36]. Additionally, studies in the literature described the machining of composites manufactured by stir casting or powder metallurgy methods [37]. It could be observed that there are many studies on models for estimating cutting forces during machining processes [38]. Experimental studies on the machining of aluminum-based metal matrix composites show that the value of cutting force depends on plowing, chip formation, and fracture and displacement of particles. According to analytical models and experimental tests [8], it could be observed that increasing the feed rate and depth of the cut causes the growth of cutting forces. There are some discrepancies between various models and experimental tests, but the trend is constant. However, studies on the influence of cutting speed on cutting forces show that increasing the cutting speed value causes a decrease in cutting forces [8].
It should be noticed that different types of MMCs were machined in the past. However, there is no information in the available literature about studies on the machining of the material described in this paper because it is a new type of MMC, cast by the vacuum method. Nowadays, MMCs are fabricated by the stir casting method. In our investigation on the conventional turning of MMCs, the results of the turning process are shown. Described studies are basic research on the machinability of metal matrix composite casting by the vacuum method, which is not available in the literature. It is important to test the machinability of that group of materials because of the increasing industrial use of MMCs. It is reasonable to get to know the results of turning MMCs in conventional conditions before studies on machining this type of material in LAM conditions. These studies are mainly focused on investigating the machinability of a new type of MMC. The metallographic examination of a metal matrix composite built of Al 2 O 3 sinter saturated with an EN AC-44000 AC-AlSi 11 alloy was performed. The turning process of the MMC was completed. Cutting forces, tool wear, and surface roughness were measured.

Metal Matrix Composite
The present studies are mainly focused on a metal matrix composite built of Al 2 O 3 sinter saturated with an EN AC-44000 AC-AlSi 11 alloy. The metallographic examination of FEPA 100 was prepared. The microstructure analysis was performed on NIKON Eclipse MA200 ( Figure 1). The presented material is an example of the MMC group of composites. It was manufactured with a pressureless infiltration method. The reinforcement phase is built of electrocorundum, and the matrix phase is built of an AlSi 11 alloy. The grains of ceramic reinforcement have an irregular shape (Figure 1a,c). Some agglomerates of electrocorundum grains could be seen (Figure 1a). It should be noticed that porosity is visible in the microstructure of the material. The size of the fractions is measured in Figure 1b. Figure 1b,d show enlarged porosity.

Turning of Metal Matrix Composite
The turning process of the metal matrix composite built of Al 2 O 3 sinter saturated with an EN AC-44000 AC-AlSi 11 alloy was carried out on a laboratory station with a CNC lathe DMG CTX 310 ECOLINE produced by DMG Mori Seiki and piezoelectric dynamometer MW 2006-2. The lathe is equipped with the Siemens 840D Control System. In Figure 2, the lathe with measuring equipment and the sample after the turning process is presented. The shafts were located on the long pin between the spindle and tailstock, due to the geometry of the lathe. The mandrel was clamped in the jaws and supported by a tailstock. The spindle rotated clockwise. The variable diameter forced the spindle revolutions to change to keep the cutting speed constant. Two samples of diameter d = 35 mm and length l = 44 mm made of Al 2 O 3 FEPA 046 and Al 2 O 3 FEPA 100 with five separate measurement sections were prepared. The main difference between the two samples was the grain size of Al 2 O 3 ceramic: FEPA 046-355-425 µm and FEPA 100-125-150 µm. Turning process was stopped when each measurement section was machined. Cutting time t ranged between 10 and 12 s. for each measurement section.
Eclipse MA200 (Figure 1). The presented material is an example of the MMC group of composites. It was manufactured with a pressureless infiltration method. The reinforcement phase is built of electrocorundum, and the matrix phase is built of an AlSi11 alloy. The grains of ceramic reinforcement have an irregular shape (Figure 1a,c). Some agglomerates of electrocorundum grains could be seen (Figure 1a). It should be noticed that porosity is visible in the microstructure of the material. The size of the fractions is measured in Figure 1b. Figure

Turning of Metal Matrix Composite
The turning process of the metal matrix composite built of Al2O3 sinter saturated with an EN AC-44000 AC-AlSi11 alloy was carried out on a laboratory station with a CNC lathe DMG CTX 310 ECOLINE produced by DMG Mori Seiki and piezoelectric dynamometer MW 2006-2. The lathe is equipped with the Siemens 840D Control System. In Figure 2, the lathe with measuring equipment and the sample after the turning process is presented. The shafts were located on the long pin between the spindle and tailstock, due to the geometry of the lathe. The mandrel was clamped in the jaws and supported by a tailstock. The spindle rotated clockwise. The variable diameter forced the spindle revolutions to change to keep the cutting speed constant. Two samples of diameter d = 35 mm and length l = 44 mm made of Al2O3 FEPA 046 and Al2O3 FEPA 100 with five separate measurement sections were prepared. The main difference between the two samples was the grain size of Al2O3 ceramic: FEPA 046-355-425 µ m and FEPA 100-125-150 µ m. Turning process was stopped when each measurement section was machined. Cutting time t ranged between 10 and 12 s. for each measurement section.

Turning of Metal Matrix Composite
The turning process of the metal matrix composite built of Al2O3 sinter saturated with an EN AC-44000 AC-AlSi11 alloy was carried out on a laboratory station with a CNC lathe DMG CTX 310 ECOLINE produced by DMG Mori Seiki and piezoelectric dynamometer MW 2006-2. The lathe is equipped with the Siemens 840D Control System. In Figure 2, the lathe with measuring equipment and the sample after the turning process is presented. The shafts were located on the long pin between the spindle and tailstock, due to the geometry of the lathe. The mandrel was clamped in the jaws and supported by a tailstock. The spindle rotated clockwise. The variable diameter forced the spindle revolutions to change to keep the cutting speed constant. Two samples of diameter d = 35 mm and length l = 44 mm made of Al2O3 FEPA 046 and Al2O3 FEPA 100 with five separate measurement sections were prepared. The main difference between the two samples was the grain size of Al2O3 ceramic: FEPA 046-355-425 µ m and FEPA 100-125-150 µ m. Turning process was stopped when each measurement section was machined. Cutting time t ranged between 10 and 12 s. for each measurement section.  Depth of cut a p and cutting speed v c were constant technological parameters. The test was carried out with variable values of feed rate f = 0.05 mm/rev, f = 0.1 mm/rev, f = 0.125 mm/rev, and f = 0.15 mm/rev. Kennametal SNGN 120408 T01020 KYS25 ceramic cutting insert was used. Before each repetition, a new cutting tool was installed. Geometry and properties of the cutting insert are shown in Table 1.

Properties and application of insert material and coating
Mixed ceramic for hard machining, great hardness, thermal and wear resistance, excellent surface finish, lower cutting forces, and higher speeds; advanced TiCN CVD coating provides excellent chemical and depth-of-cut notch resistance.
Technological parameters of the turning process are shown in Table 2. The turning process was repeated five times. The results were analyzed with arithmetic average and dispersion. The measurements of the tool's wear were carried out on an optical microscope ZEISS SteREO Discovery.V20 ( Figure 3, Carl Zeiss AG, Oberkochen, Germany). The cutting inserts were located in a small holder. The microscope is able to digitally measure the tool's wear. Tests were carried out on the cutting edge.

Properties and application of insert material and coating
Mixed ceramic for hard machining, great hardness, thermal and wear resistance, excellent surface finish, lower cutting forces, and higher speeds; advanced TiCN CVD coating provides excellent chemical and depth-of-cut notch resistance.
Technological parameters of the turning process are shown in Table 2. The turning process was repeated five times. The results were analyzed with arithmetic average and dispersion. Table 2. Technological parameters of the cutting process used for tests.

Type of Inserts Sample
Symbol The measurements of the tool's wear were carried out on an optical microscope ZEISS SteREO Discovery.V20 ( Figure 3, Carl Zeiss AG, Oberkochen, Germany). The cutting inserts were located in a small holder. The microscope is able to digitally measure the tool's wear. Tests were carried out on the cutting edge.   The 3D surface roughness measurements of the workpiece were carried out with a HOMMEL-ETAMIC T-8000 profilometer (Jenoptik AG, Jena, Germany) equipped with a TKL 100/17 measuring tip ( Figure 4). The shafts were located in a prism on an automatic moving worktable. With HOMMEL-ETAMIC T-8000, it is possible to create 3D measurements of surface roughness. The tests were carried out according to ISO 11562:1996. The value of l p was 4.8 mm, λ c filter-0.8 mm. The value of λ s was not defined. The mapping section was set to 1.5 mm, and the traverse length was set to 80 µm. The value of traverse speed was set at 0.05 mm/s. TKL 100/17 measuring tip ( Figure 4). The shafts were located in a prism on an automatic moving worktable. With HOMMEL-ETAMIC T-8000, it is possible to create 3D measurements of surface roughness. The tests were carried out according to ISO 11562:1996. The value of lp was 4.8 mm, λc filter-0.8 mm. The value of λs was not defined. The mapping section was set to 1.5 mm, and the traverse length was set to 80 µ m. The value of traverse speed was set at 0.05 mm/s.

Tool's Wear after Machining Metal Matrix Composite
The studies were carried out with the methodology described in the previous paragraph. The research was planned according to the analyzed literature. Definition of future research courses was the main aim of these studies. Figure 5 shows the microscopic photo of the tool's wear measurement areas of the KENNAMETAL SNGN 120408 T01020 KYS25 cutting insert. The Aγ, Aα, and A'α are marked in these pictures.
In Figure 6a,b, the cutting insert after turning with f = 0.05 mm/rev of the FEPA 046 sample is shown. The abrasive wear is observed on the Aγ, Aα, and A'α surfaces. There is no built-up effect. The crater is observed on the cutting edge. The value of the tool's wear is equal to VB = 0.77 mm. Figure 6c,d show cutting inserts after turning with the same value of feed rate f but on the FEPA 100 sample. In that example, abrasive wear is also observed on the Aγ, Aα, and A'α surfaces. The tool's wear is equal to VB = 0.5 mm. No built-up effect occurring could be the reason for the low temperature of the turning process and the low plasticity of the machined material.

Tool's Wear after Machining Metal Matrix Composite
The studies were carried out with the methodology described in the previous paragraph. The research was planned according to the analyzed literature. Definition of future research courses was the main aim of these studies. Figure 5 shows the microscopic photo of the tool's wear measurement areas of the KENNAMETAL SNGN 120408 T01020 KYS25 cutting insert. The A γ , A α , and A' α are marked in these pictures. TKL 100/17 measuring tip ( Figure 4). The shafts were located in a prism on an automatic moving worktable. With HOMMEL-ETAMIC T-8000, it is possible to create 3D measurements of surface roughness. The tests were carried out according to ISO 11562:1996. The value of lp was 4.8 mm, λc filter-0.8 mm. The value of λs was not defined. The mapping section was set to 1.5 mm, and the traverse length was set to 80 µm. The value of traverse speed was set at 0.05 mm/s.

Tool's Wear after Machining Metal Matrix Composite
The studies were carried out with the methodology described in the previous paragraph. The research was planned according to the analyzed literature. Definition of future research courses was the main aim of these studies. Figure 5 shows the microscopic photo of the tool's wear measurement areas of the KENNAMETAL SNGN 120408 T01020 KYS25 cutting insert. The Aγ, Aα, and A'α are marked in these pictures. In Figure 6a,b, the cutting insert after turning with f = 0.05 mm/rev of the FEPA 046 sample is shown. The abrasive wear is observed on the Aγ, Aα, and A'α surfaces. There is no built-up effect. The crater is observed on the cutting edge. The value of the tool's wear is equal to VB = 0.77 mm. Figure 6c,d show cutting inserts after turning with the same value of feed rate f but on the FEPA 100 sample. In that example, abrasive wear is also observed on the Aγ, Aα, and A'α surfaces. The tool's wear is equal to VB = 0.5 mm. No built-up effect occurring could be the reason for the low temperature of the turning process and the low plasticity of the machined material. In Figure 6a,b, the cutting insert after turning with f = 0.05 mm/rev of the FEPA 046 sample is shown. The abrasive wear is observed on the A γ , A α , and A' α surfaces. There is no built-up effect. The crater is observed on the cutting edge. The value of the tool's wear is equal to VB = 0.77 mm. Figure 6c,d show cutting inserts after turning with the same value of feed rate f but on the FEPA 100 sample. In that example, abrasive wear is also observed on the A γ , A α , and A' α surfaces. The tool's wear is equal to VB = 0.5 mm. No built-up effect occurring could be the reason for the low temperature of the turning process and the low plasticity of the machined material. Similar signs of tool wear could be observed after turning with feed rate f = 0.125 mm/rev ( Figure 8). The cutting insert after turning the FEPA 046 shaft is shown in Figure  8a,b. There is abrasive wear on the Aα and A'α surfaces. The value of the tool's wear is equal to VB = 0.76 mm. Except for abrasive wear, mechanical wear could be observed on   (Figure 8c,d). The tool's wear is equal to VB = 0.48 mm, which is the smallest value of that parameter from FEPA 100 tests. the cutting edge of the cutting insert after turning the FEPA 100 sample (Figure 8c,d). The tool's wear is equal to VB = 0.48 mm, which is the smallest value of that parameter from FEPA 100 tests.   Figure 9c,d, where the cutting insert after turning the FEPA 100 sample is shown. A crater on the cutting edge could also be noticed. There was a furrowing phenomenon observed ( Figure  9d). The value of the tool's wear of cutting inserts used during the turning of the FEPA 046 shaft is the smallest value for these parameters, and it is equal to VB = 0.7 mm, while the tool's wear of the cutting insert used in the turning of the FEPA 100 shaft is equal to VB = 0.53 mm.   Figure 9c,d, where the cutting insert after turning the FEPA 100 sample is shown. A crater on the cutting edge could also be noticed. There was a furrowing phenomenon observed (Figure 9d). The value of the tool's wear of cutting inserts used during the turning of the FEPA 046 shaft is the smallest value for these parameters, and it is equal to VB = 0.7 mm, while the tool's wear of the cutting insert used in the turning of the FEPA 100 shaft is equal to VB = 0.53 mm. the cutting edge of the cutting insert after turning the FEPA 100 sample (Figure 8c,d). The tool's wear is equal to VB = 0.48 mm, which is the smallest value of that parameter from FEPA 100 tests.  It could be observed that turning an MMC in conventional conditions has a significant influence on tool wear. The values of that parameter and signs of tool wear could be reasons for difficult machining conditions. Results of measurements of the value of tool wear are unsatisfying in terms of machining economy. These phenomena confirm the necessity to continue studies on the technology of machining MMCs for achieving satisfying results.
The observations of optical microscope measurements were compared with scanning electron microscope (SEM) TESCAN VEGA 5135 (TESCAN, Brno, Czech Republic) measurements of cutting inserts used for turning the FEPA 046 sample (Figure 10a,b) and FEPA 100 sample (Figure 10c,d). The cutting inserts used did not conduct electricity. It was necessary to carburize them. For that reason, chemical analysis of cutting insert composition could be less precise. Generally, the sticking of Al was observed on the A γ , A α , and A' α surfaces. Particles of ceramic could also be noticed. It could be observed that the tool's wear after turning the FEPA 046 shaft is more significant than after turning the FEPA 100 shaft. The shaft made of the FEPA 046 composite is more hard-to-cut material than the FEPA 100 shaft. The influence of the grade of grains on the tool's wear could be proved. Shabani et al. showed that hard Al 2 O 3 reinforcing particles of the MMC material polished the surfaces of cutting inserts made of PCD [39]. Meanwhile, Durante et al. revealed the little influence of the size and hardness of abrasive particles on tool life [40]. Tool wear presented in this paper studies was manifested mostly by abrasive wear. The particles of ceramic reinforcement in FEPA 046 are larger than in FEPA 100. For that reason, the tool's wear signs are more noticeable after turning the FEPA 046 shaft. It was observed that larger particles of ceramic reinforcement caused a mechanical impact on cutting inserts. There is no simple correlation between the tool's wear VB and feed rate f that the machined material's heterogeneity could cause. It could be observed that turning an MMC in conventional conditions has a significant influence on tool wear. The values of that parameter and signs of tool wear could be reasons for difficult machining conditions. Results of measurements of the value of tool wear are unsatisfying in terms of machining economy. These phenomena confirm the necessity to continue studies on the technology of machining MMCs for achieving satisfying results.
The observations of optical microscope measurements were compared with scanning electron microscope (SEM) TESCAN VEGA 5135 (TESCAN, Brno, Czech Republic) measurements of cutting inserts used for turning the FEPA 046 sample (Figure 10 a,b) and FEPA 100 sample (Figure 10 c,d). The cutting inserts used did not conduct electricity. It was necessary to carburize them. For that reason, chemical analysis of cutting insert composition could be less precise. Generally, the sticking of Al was observed on the Aγ, Aα, and A'α surfaces. Particles of ceramic could also be noticed. It could be observed that the tool's wear after turning the FEPA 046 shaft is more significant than after turning the FEPA 100 shaft. The shaft made of the FEPA 046 composite is more hard-to-cut material than the FEPA 100 shaft. The influence of the grade of grains on the tool's wear could be proved. Shabani et al. showed that hard Al2O3 reinforcing particles of the MMC material polished the surfaces of cutting inserts made of PCD [39]. Meanwhile, Durante et al. revealed the little influence of the size and hardness of abrasive particles on tool life [40]. Tool wear presented in this paper studies was manifested mostly by abrasive wear. The particles of ceramic reinforcement in FEPA 046 are larger than in FEPA 100. For that reason, the tool's wear signs are more noticeable after turning the FEPA 046 shaft. It was observed that larger particles of ceramic reinforcement caused a mechanical impact on cutting inserts. There is no simple correlation between the tool's wear VB and feed rate f that the machined material's heterogeneity could cause.

Geometrical Structure of Surface after Machining of Metal Matrix Composite
Three-dimensional surface roughness measurements of the workpiece were carried out for each sample without machining and after machining with four different values of feed rate. Sa and Sz parameters were measured because they are the most useful in the industry. Figure 12 shows the results of measurements of samples FEPA 046 ( Figure 12a  The lowest value of Sa after turning shaft FEPA 046 (Figure 13a) was found after turning with f = 0.05 mm/rev, and it was equal to 1.23 µ m. Similar to Sa, the smallest value of Sz was found after turning with f = 0.05 mm/rev, and it was equal to 6.46 µ m. Figure 13b shows the results of turning the FEPA 100 sample with f = 0.05 mm/rev. The lowest value of Sa (Figure 13b) was equal to 0.92 µ m. Similar to Sa, the lowest value of Sz was found after turning with f = 0.05 mm/rev, and it was equal to 5.40 µ m. The periodic distribution of inequalities connected with the value of feed rate f is not observed. That phenomenon is not corresponding with the kinematic-geometric mapping of the machining process.

Geometrical Structure of Surface after Machining of Metal Matrix Composite
Three-dimensional surface roughness measurements of the workpiece were carried out for each sample without machining and after machining with four different values of feed rate. Sa and Sz parameters were measured because they are the most useful in the industry. Figure 12 shows the results of measurements of samples FEPA 046 ( Figure 12a) and FEPA 100 (Figure 12b) before turning. In this case, the scale of 3D measurements is limited to 100 µm. It is a result of the necessity to compare samples with different sizes of grades. After turning, the 3D measurements of the samples' surface roughness are shown on a scale limited to 50 µm for a more precise image. Before turning the FEPA 046 sample, the value of Sa reached 3.78 µm. The value of the Sz parameter before turning reached 17.04 µm. The value of Sa before turning the FEPA 100 sample reached 2.92 µm, and Sz was equal to 13.89 µm. The results of the Sa and Sz parameters are not satisfying because of the requirements which conform to machine parts used in the automotive and aeronautical industries.

Geometrical Structure of Surface after Machining of Metal Matrix Composite
Three-dimensional surface roughness measurements of the workpiece were carried out for each sample without machining and after machining with four different values of feed rate. Sa and Sz parameters were measured because they are the most useful in the industry. Figure 12 shows the results of measurements of samples FEPA 046 ( Figure 12a  The lowest value of Sa after turning shaft FEPA 046 (Figure 13a) was found after turning with f = 0.05 mm/rev, and it was equal to 1.23 µ m. Similar to Sa, the smallest value of Sz was found after turning with f = 0.05 mm/rev, and it was equal to 6.46 µ m. Figure 13b shows the results of turning the FEPA 100 sample with f = 0.05 mm/rev. The lowest value of Sa (Figure 13b) was equal to 0.92 µ m. Similar to Sa, the lowest value of Sz was found after turning with f = 0.05 mm/rev, and it was equal to 5.40 µ m. The periodic distribution of inequalities connected with the value of feed rate f is not observed. That phenomenon is not corresponding with the kinematic-geometric mapping of the machining process. The lowest value of Sa after turning shaft FEPA 046 (Figure 13a) was found after turning with f = 0.05 mm/rev, and it was equal to 1.23 µm. Similar to Sa, the smallest value of Sz was found after turning with f = 0.05 mm/rev, and it was equal to 6.46 µm. Figure 13b shows the results of turning the FEPA 100 sample with f = 0.05 mm/rev. The lowest value of Sa (Figure 13b) was equal to 0.92 µm. Similar to Sa, the lowest value of Sz was found after turning with f = 0.05 mm/rev, and it was equal to 5.40 µm. The periodic distribution of inequalities connected with the value of feed rate f is not observed. That phenomenon is not corresponding with the kinematic-geometric mapping of the machining process.    Dyzia [41] noticed that the machined surface of MMCs was treated by picking and deforming. For that reason, the scratches and the characteristic accumulations were observed. Some particles of ceramic reinforcement were crushed, and some were pressed into the metal matrix. Contrary to the research presented in this article, no tearing off of SiC particles was observed. Generally, it could be observed that the Sa and Sz parameters increased after increasing the feed rate f (Figure 17a,b). This correlation is disturbed by the roughness measurements of the FEPA 100 sample after turning with f = 0.15 mm/rev. Values of Sa and Sz on this test section were lower than after turning with f = 0.125 mm/rev.  Figure 18 shows the values of the cutting force Fc depending on the feed rate f. The values shown are the arithmetic mean of three tests carried out with the same parameters. In Figure 18a, it can be observed that the lowest value of cutting forces (Fc = 11.79 N) of the machining of the FEPA 046 sample was noticed during the process with feed rate f = 0.1 mm/rev and f = 0.15 mm/rev. The highest value of that parameter (Fc = 13.18 N) is observed during turning with f = 0.05 mm/rev. Figure 18b shows the results of the same test but for the turning of the FEPA 100 sample. The highest value of the cutting forces Fc was noticed during turning with f = 0.05 mm/rev, and it is equal to Fc = 17.95 N. The lowest value of the cutting forces of the machining of the FEPA 046 sample was noticed during the process with feed rate f = 0.1 (mm/rev), and it is equal to Fc = 10.93 N. The decrease in the cutting force Fc with the increase in the feed rate f is related to the increase in the susceptibility of the ceramic reinforcement grains to detachment from the material structure with the increase in feed rate f value.  Dyzia [41] noticed that the machined surface of MMCs was treated by picking and deforming. For that reason, the scratches and the characteristic accumulations were observed. Some particles of ceramic reinforcement were crushed, and some were pressed into the metal matrix. Contrary to the research presented in this article, no tearing off of SiC particles was observed. Generally, it could be observed that the Sa and Sz parameters increased after increasing the feed rate f (Figure 17a,b). This correlation is disturbed by the roughness measurements of the FEPA 100 sample after turning with f = 0.15 mm/rev. Values of Sa and Sz on this test section were lower than after turning with f = 0.125 mm/rev. Dyzia [41] noticed that the machined surface of MMCs was treated by picking and deforming. For that reason, the scratches and the characteristic accumulations were observed. Some particles of ceramic reinforcement were crushed, and some were pressed into the metal matrix. Contrary to the research presented in this article, no tearing off of SiC particles was observed. Generally, it could be observed that the Sa and Sz parameters increased after increasing the feed rate f (Figure 17a,b). This correlation is disturbed by the roughness measurements of the FEPA 100 sample after turning with f = 0.15 mm/rev. Values of Sa and Sz on this test section were lower than after turning with f = 0.125 mm/rev.  Figure 18 shows the values of the cutting force Fc depending on the feed rate f. The values shown are the arithmetic mean of three tests carried out with the same parameters. In Figure 18a, it can be observed that the lowest value of cutting forces (Fc = 11.79 N) of the machining of the FEPA 046 sample was noticed during the process with feed rate f = 0.1 mm/rev and f = 0.15 mm/rev. The highest value of that parameter (Fc = 13.18 N) is observed during turning with f = 0.05 mm/rev. Figure 18b shows the results of the same test but for the turning of the FEPA 100 sample. The highest value of the cutting forces Fc was noticed during turning with f = 0.05 mm/rev, and it is equal to Fc = 17.95 N. The lowest value of the cutting forces of the machining of the FEPA 046 sample was noticed during the process with feed rate f = 0.1 (mm/rev), and it is equal to Fc = 10.93 N. The decrease in the cutting force Fc with the increase in the feed rate f is related to the increase in the susceptibility of the ceramic reinforcement grains to detachment from the material structure with the increase in feed rate f value.   Figure 18 shows the values of the cutting force F c depending on the feed rate f. The values shown are the arithmetic mean of three tests carried out with the same parameters. In Figure 18a, it can be observed that the lowest value of cutting forces (F c = 11.79 N) of the machining of the FEPA 046 sample was noticed during the process with feed rate f = 0.1 mm/rev and f = 0.15 mm/rev. The highest value of that parameter (F c = 13.18 N) is observed during turning with f = 0.05 mm/rev. Figure 18b shows the results of the same test but for the turning of the FEPA 100 sample. The highest value of the cutting forces During the turning of aluminum matrix composites, an increase in cutting forces was noticed ( Figure 19). It is important to mention that in all cases, cutting force increased during the process. Intensive wedge wear was the cause of this effect. The highest value of cutting force Fc was noticed near 10 s of the cutting process. There is a peak of value.  After analysis of the graph (Figure 20a), it could be noticed that the value of the Sa parameter increases with the increase in VB and with the decrease in Fc. An inverse correlation is shown in Figure 20b. It could be stated that the Sz parameter increases with the decrease in VB and with the increase in Fc. Machined material had a heterogeneous structure. Larger grains of ceramic reinforcement were connected weaker with the metal matrix than smaller grains. For that reason, a lower value of cutting force Fc was needed to remove greater grains. However, due to the size of ceramic grains in this case, the Sa parameter of the surface roughness had a higher value. Tool wear was very high, given the short cutting time s. An increase in that parameter had a negative influence on surface roughness.  During the turning of aluminum matrix composites, an increase in cutting forces was noticed ( Figure 19). It is important to mention that in all cases, cutting force increased during the process. Intensive wedge wear was the cause of this effect. The highest value of cutting force F c was noticed near 10 s of the cutting process. There is a peak of value. During the turning of aluminum matrix composites, an increase in cutting forces was noticed ( Figure 19). It is important to mention that in all cases, cutting force increased during the process. Intensive wedge wear was the cause of this effect. The highest value of cutting force Fc was noticed near 10 s of the cutting process. There is a peak of value.  After analysis of the graph (Figure 20a), it could be noticed that the value of the Sa parameter increases with the increase in VB and with the decrease in Fc. An inverse correlation is shown in Figure 20b. It could be stated that the Sz parameter increases with the decrease in VB and with the increase in Fc. Machined material had a heterogeneous structure. Larger grains of ceramic reinforcement were connected weaker with the metal matrix than smaller grains. For that reason, a lower value of cutting force Fc was needed to remove greater grains. However, due to the size of ceramic grains in this case, the Sa parameter of the surface roughness had a higher value. Tool wear was very high, given the short cutting time s. An increase in that parameter had a negative influence on surface roughness.  After analysis of the graph (Figure 20a), it could be noticed that the value of the Sa parameter increases with the increase in VB and with the decrease in F c . An inverse correlation is shown in Figure 20b. It could be stated that the Sz parameter increases with the decrease in VB and with the increase in F c . Machined material had a heterogeneous structure. Larger grains of ceramic reinforcement were connected weaker with the metal matrix than smaller grains. For that reason, a lower value of cutting force F c was needed to remove greater grains. However, due to the size of ceramic grains in this case, the Sa parameter of the surface roughness had a higher value. Tool wear was very high, given the short cutting time s. An increase in that parameter had a negative influence on surface roughness.

Conclusions
The following conclusions can be drawn from the results of this research: • Structure of the metal matrix composite built of Al2O3 sinter saturated with an EN AC-44000 AC-AlSi11 alloy causes problems with the machining of this material.

•
Value of the feed rate f influences the geometrical structure of the MMC after conventional turning.

•
Tool's wear of cutting inserts used during tests is substantial. It is not possible to describe some correlations between the feed rate value and tool wear value, but abrasive and mechanical wear including craters could be observed in most of the samples.

•
The turning process contributed to the decrease in the Sa and Sz parameters of the machined surface compared to the not machined area. However, the results of this process are not satisfying. In general, some correlation between feed rate and surface roughness parameters could be observed. It can be stated that values of the Sa and Sz parameters increase with the growth of the feed rate's value, except turning the FEPA 100 shaft with f = 0.15 (mm/rev). During this test, Sa and Sz values decreased compared to turning with f = 0.125 (mm/rev).

•
Cutting force Fc has low values. In this case, any correlations with feed rate f could not be evidenced.

•
That correlation could not be noticed because of material heterogeneity. Reinforcement grains were pulled and crushed. The structure of the tested material caused the turning process to be uneven and results challenging to connect.

•
The results of the studies described in this article show that the conventional method of turning MMCs is not satisfactory in terms of technological results. The tool's wear was significant, and the surface roughness of the machined material was not satisfactory. The described tests were mainly focused on the results and possibilities of the conventional turning of MMCs.

•
The research showed results of machinability in the conventional condition of MMC casting by the vacuum method with shaped reinforcement. Machining of that material was not described in the available literature.

•
The carried out studies showed the necessity of continuing research on turning metal matrix composites built of Al2O3 sinter saturated with an EN AC-44000 AC-AlSi11 alloy. In future studies, LAM should be applied.

Conclusions
The following conclusions can be drawn from the results of this research: • Structure of the metal matrix composite built of Al 2 O 3 sinter saturated with an EN AC-44000 AC-AlSi 11 alloy causes problems with the machining of this material.

•
Value of the feed rate f influences the geometrical structure of the MMC after conventional turning. • Tool's wear of cutting inserts used during tests is substantial. It is not possible to describe some correlations between the feed rate value and tool wear value, but abrasive and mechanical wear including craters could be observed in most of the samples.

•
The turning process contributed to the decrease in the Sa and Sz parameters of the machined surface compared to the not machined area. However, the results of this process are not satisfying. In general, some correlation between feed rate and surface roughness parameters could be observed. It can be stated that values of the Sa and Sz parameters increase with the growth of the feed rate's value, except turning the FEPA 100 shaft with f = 0.15 (mm/rev). During this test, Sa and Sz values decreased compared to turning with f = 0.125 (mm/rev).

•
Cutting force F c has low values. In this case, any correlations with feed rate f could not be evidenced.

•
That correlation could not be noticed because of material heterogeneity. Reinforcement grains were pulled and crushed. The structure of the tested material caused the turning process to be uneven and results challenging to connect.

•
The results of the studies described in this article show that the conventional method of turning MMCs is not satisfactory in terms of technological results. The tool's wear was significant, and the surface roughness of the machined material was not satisfactory. The described tests were mainly focused on the results and possibilities of the conventional turning of MMCs.

•
The research showed results of machinability in the conventional condition of MMC casting by the vacuum method with shaped reinforcement. Machining of that material was not described in the available literature.

•
The carried out studies showed the necessity of continuing research on turning metal matrix composites built of Al 2 O 3 sinter saturated with an EN AC-44000 AC-AlSi 11 alloy. In future studies, LAM should be applied.

Conflicts of Interest:
The authors declare no conflict of interest.