Experimental investigation and FEM analysis of chip morphology in the turning of ASTM F-75 CoCrMo al
* Author to whom correspondence should be addressed.
Sigma Journal of Engineering and Natural Sciences 2024, Vol. 42, Issue 3, pp. 679-691; doi.org/10.14744/sigma.2023.00130
Abstract
Keywords: ASTM-F75 CoCrMo Alloy; Chip Morphology Modelling; FEM; Turning
Introduction
CoCrMo alloys are widely used in fracture bone joints, knee and hip prosthesis applications due to their high biocompatibility and mechanical properties. However, in
mechanical properties of the elements in the alloy can cause problems in the machining of the final product. One of these problems is tool wear and deterioration of surface
*Corresponding author. *E-mail address: ender.emir@istiklal.edu.tr This paper was recommended for publication in revised form by Editor in-Chief Ahmet Selim Dalkilic Published by Yıldız Technical University Press, İstanbul, Turkey Copyright 2021, Yıldız Technical University. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
Sigma J Eng Nat Sci, Vol. 42, No. 3, pp. 679−691, June, 2024
integrity due to the high temperature during chip removal. These problems that occur during the manufacturing process increase both the production time and cost. Therefore, it is very important to problems estimate and control any that may arise during fully production. One of the most important issues examined in the literature on the machining of hard materials is tool wear [1-5]. Tool wear in machining has an economically important role in chip removal. Given that machining is a complex process, predicting tool wear is very difficult and complex. The turning of hard alloys such as CoCrMo rises to approximately 850-1200°C depending on the temperature processing parameters that occur between the tool and the workpiece. Therefore, knowledge of tool wear mechanisms and the ability to predict tool life are important in metal cutting. It should be noted that contact conditions and areas in the cutting zone also affect tool wear [6]. Determining these accurances by experimental studies affects important parameters such as processing time and cost. However, changes in parameters such as feed rate and cutting speed during the experiments especially affect the force distributions, heat generation and tool wear. The workpiece material and its physical properties affect the cutting force for the applied cutting conditions. The optimum performance of a cutting tool requires an accurate combination of machining parameters and cutting conditions [7]. The use of FEM software to determine the ideal conditions for chip removal is necessary for the optimization of experimental studies. Therefore, FEM software is important in the optimization of machining processes. Thanks to these softwares, it is possible to obtain various results such as cutting force, cutting temperature, stress, tool temperature, chip formation, and heat transfer when cutting. When the studies on this subject are examined in the literature, it is seen that numerical analyses performed in the FEM environment are similar to the experimentally obtained results [8,9]. On the other hand, the chips produced during the manufacturing process adversely affect the surface quality, tool wear, and the resulting forces of the final product. Tool wear and surface quality deterioration can be prevented by microscopic examination of the chips acquired as a result of the experiments. It is known in the literature that chip morphology resulting from the machining of hard alloys is commonly caused by adhesion and cracks. In addition, it is necessary to determine the surface and subsurface stresses for the detection of damaging effects such as wear, fatigue, fracture and collapse in the contact that occurs during machining. [10,11]. For this reason, studies have been carried out in the literature to investigate the effects of machining parameters on tool wear and chip morphology. Zhang et al. (2014) investigated the relationship between chip morphology and tool wear in ultra-precision raster milling (UPRM). A cutting experiment was performed to explore chip morphologies under different flank wear area widths. Theoretical and experimental results revealed that the occurrence of tool flank wear can cause cutting chips to be cut on both the cutting edge and
reduce the length of the cutting chips in the feed direction [12]. Japtag et al. (2018) have worked on the chip formation mechanism in CNC turning of CoCrMo alloy [13]. They conducted experiments at different feed rate, cutting speed, and cutting depths. In the results they obtained, they stated that the chip thickness ratio increased due to the increase in the feed rate. Parida and Maity (2018) Monel-400 material was seen as a result of turning the microscope images saw the formation of saw tooth form [14]. They also stated that the rate of progress is an important parameter in the formation of continuous and discontinuous chips. Zhao et al. (2018) studied the effects of Ti6Al4V alloy on the chip by cryogenic turning [15]. During the cryogenic cutting process, with the increase of cutting speed, the chip height ratio and serrated pitch of titanium alloy chip increase. Tang et al. (2019) studied the tool wear performance in dry turning of AISI D2 steel at various hardness levels [5]. In the experimental results, they observed that lateral abrasions and crater abrasions occured significantly due to the increase in the hardness of the workpiece. Bolat et al. (2021) the processing properties of pumice reinforced AA7075 syntactic foams produced by sandwich infiltration technique were investigated by performing face turning at different cutting speeds (25, 50, and 100 m/min) and feed rates (0.05, 0.10, and 0.15 mm/rev). In the results they obtained, they said that the shape of the metal chips changed from long/continuous character to saw tooth morphology depending on the increasing cutting speed levels, while the pumice particles showed a tendency to break as their feed rate increased [16]. Pop and Titu (2021) investigated the effect of clearance angle change on chip formation in the turning process using FE analysis. As a result of their study, they said that with the increase of the radius of the cutting edge, the stress distribution is radial over a larger area of the part, and therefore the deformation of the chip is smaller [17]. Wakjira and Ramulu (2022) investigated the analysis of chip morphology using the FE method in the turning of CSN 12050 carbon steel with various tool geometries (tool rake and flank angles). In the experiments, rake angles of 0, 5, and 10, edge angles of 0 and 6, and depths of cut of 0.2 mm and 0.5 mm were determined using adaptive meshing for the tool. In the FE analysis results, they estimated increasing von Mises stresses and decreasing cutting forces with 10 rake angles and 6 edge angles [18]. Okokpujie et al. (2022) carried out an experimental study and FEM analysis to examine tool wear during turning of Al-Si-Mg alloy. In particular, they investigated the effect of process parameters and machining conditions on tool wear. They said that the depth of cut is the most effective process parameter in terms of tool wear [19]. The machining of ASTM-F75 CoCrMo alloy affects the dimensional accuracy deterioration and forces due to irregularities in chip removal and tool wear, creating extra costs to reach the desired standards. One of the factors affecting the yield of the product to be obtained in the chip removal process is sawdust. Therefore, it is important to examine the effect of machining parameters on chip formation. The aim of this study is to investigate the effects of different feed
Sigma J Eng Nat Sci, Vol. 42, No. 3, pp. 679−691, June, 2024
rate increases on chip types, residual stresses and tool wear in the machining of CoCrMo material and to compare the results obtained with the FEM model. Unlike the literature, the effects of feed rate on chip morphology, residual stress, and the occurrence of tool wear are discussed together. In this context, the results obtained by turning operations at three different feed rates at a constant cutting speed and FEM results are presented together. Significant differences were observed in chip types, especially with the increase in feed rate. In addition, significant wear was observed in the cutting tool with the force and temperature distribution during the turning operation.
implemented for the flow stress of the workpiece and can be represented as Eq. (1) (1) where 𝜎 is the flow stress, 𝜀 is the plastic strain, ̇𝜀 is the plastic strain rate and 𝜀⋅0 is the reference plastic strain rate. T, Tr and Tm are the work temperature, reference temperature and material melting temperature, respectively. A, B, n, C and m are the material constants. The flow chart for FEM analysis is given in Figure 2.
Experimental Procedure
Experimentel Setup and FE Modelling The results of the study were obtained by using two different methods. In the first method, machinig process was performed on turning machine at different feed rates (Figure 1). SEM images were taken of the chip and the cutting tool used as a result of the experiment. In the second method, chip removal simulation was performed by using ThirdWave software with FEM (Figure 1). As a result of this method, temperature changes and force distributions that occured in the cutting tool were obtained. The chemical composition of the CoCrMo-F75 alloy used in the study is given in Table 1. The mechanical properties of the alloy are given in Table 2. The cutting process was made of tungsten carbide (WC) material with a 55° tool tip angle and a radius of 0.08 mm. In the study, the processing parameters are given in Table 3. In particular, the cutting speed was set low for tool and workpiece integrity. In addition, a mesh with a tetrahedral element type, an element size of 0.1 mm, 3517 nodes, and 10265 elements were used for FEM simulations. In order to analyze the analyses, linear motion was given to the cutting tool, while the workpiece was fixed. The Johnson-Cook material model was
0.25. (max)
Table 2. Mechanical properties of CoCrMo [21] Young’s Modulus (GPa)
Sigma J Eng Nat Sci, Vol. 42, No. 3, pp. 679−691, June, 2024
Table 3. Machining parameters of CoCrMo Cutting speed (m/min)
Result And Discussion
Simulated Force and Temperature Distribution Figure 3 shows the force changes resulting from a series of FEM simulations performed for three different feeds (0.1
mm/rev, 0.2 mm/rev, 0.3 mm/rev) at a constant cutting speed of 80 m/min. In the results obtained, it was seen that the force value in the X direction was highest at the feed rate of 0.3 mm/rev. Considering the force variation range, it was higher at a feed rate of 0.3 mm/rev than at a feed
Figure 3. Force distribution during chip removal; a) 0.1 mm/rev, b) 0.2 mm/rev, c) 0.3 mm/rev.
Sigma J Eng Nat Sci, Vol. 42, No. 3, pp. 679−691, June, 2024
Figure 4. Temperature distribution during chip removal; a) 0.1 mm/rev, b) 0.2 mm/rev, c) 0.3 mm/rev.
rate of 0.2 and 0.1 mm/rev. Due to thermal softening, the heating effect reduces workpiece stability [14]. As a result of FEM analysis, the maximum temperatures were 877.25°C at 0.1 mm/rev, 983.024°C at 0.2 mm/rev, and 1132.38° C at
0.3. mm/rev (Figure 4). As a result of the FEM analysis, it
was seen that the temperature increased with the increase of the feed rate. In this case, it was seen in the results of the analysis that the forces that occurred during cutting were effective. Effect of Machining Parameters on Chip Morphology Characteristics Figure 5 shows typical chip morphologies at different feed rates. The size of the chips was directly related to the feed rate. If the feed rate was 0.1 mm/rev, continuous chips were observed. On the other hand, discontinuous chips were observed at feed rates of 0.2 mm/rev and 0.3 mm/rev respectively. From the acquired results, it can be concluded that an increase in the feed rate facilitates discontinuous chip formation. This means that more chip thickness will be
removed with increased feed during chip removal, resulting in the chips being broken down into smaller pieces instead of curling or striping. Figure 6 shows SEM images of chip morphologies that occur at a constant 80 m/min of cutting speed at different feeds. In the obtained images, the surface of the chip was seen to be smooth at 0.1 mm/rev. At 0.2 mm/rev, it was observed that the chip surfaces caused cracks due to the increased heat generation during chip removal. A further feed rate of 0.3 mm/rev caused an increase in the amount of chips removed per unit feed, resulting in increased temperatures between the tool and the chip. As a result, it was observed that there was significant adhesion on the surface of the chip. It is seen that chip-shaped saw-tooth formations occur due to the natural brittleness of the material when removing chips from the surfaces of hard materials [8]. Similar results were also seen from the performed FEM analysis (Figure 7). For detailed analysis of the effect of machining parameters on chip morphology, optical images from chip
Sigma J Eng Nat Sci, Vol. 42, No. 3, pp. 679−691, June, 2024
Figure 5. Chip images at a constant cutting speed of 80 m/min; a) 0.1 mm/rev, b) 0.2 mm/rev, c) 0.3 mm/rev.
Figure 6. Chip SEM images: a) smooth, b) crack, c) adhesion. surfaces are as shown in Figure 8. In the study, it was seen that saw-tooth structures were formed in chip morphology due to the increase in progress. This is explained by the localization of the stresses in the cutting zone during the turning of the alloy and the instability of the plastic flow of the workpiece material [12, 22]. In fact, in the FEM analysis formed according to the orthogonal cutting model, it was observed that a localized stress zone was formed in the primary deformation region during metal removal (Figure 9).
Chip morphologies are not uniform due to various feed rates. SEM images showed that lamellar structures were formed on the free surface of the chips (Figure 10). This type of structure extends along the width of the chips, which is normal to the chip flow direction. While the outer edges of the chips have saw-tooth structures, when the inner parts are examined, a transition to the lamellar structure is observed (Figure 10a). Saw-tooth structures are not clear in this region. The increase in the feed rate reveals the
Sigma J Eng Nat Sci, Vol. 42, No. 3, pp. 679−691, June, 2024
Figure 7. Saw tooth formation as a result of FEM analysis at 0.3 mm/rev. Figure 8. Saw tooth formation at 0.3 mm/rev.
formation of regular lamellar structure (Figure 10b, Figure 10c). This is due to the cutting band formed due to the high feed rate when the material in front of the cutting tool is moved in the cutting direction [23-25]. Figure 11 shows that the chip morphologies change significantly at different feed rates. Continuous chip formation is observed compared to the chips obtained at a low feed
rate (f = 0.1 mm/rev) and other feed rates (f = 0.2 mm/rev and f = 0.3 mm/rev) (Figure 11a). With an increase in the feed rate, the chip increasingly scalloped on a takes shape (Figure 11b and Figure 11c). This is due to cyclic cracking, creating very intensive shear bands. In fact, similar results have emerged in the on the machining of hard metals literature [26]. The schematic diagrams of the chip height ratio and serrated pitch measurement obtained as a result of both experimental and numerical work are shown in Figure 12. The chip deformation degree can be characterized by the chip height ratio (Gs) and the chip height ratio is calculated as given in Eq.2: [15]: (2) where Gs is chip height ratio, h1 is the height from the bottom of chip to the peak of serrated chip, and h2 is the height from the bottom of chip to the lowest valley of serrated chip. The serrated pitch (Pc) can be obtained by measuring the distance between the addenda of two adjacent
Figure 10. Lamella structure formation at; a) transition of saw teeth and lamella structure, b) 0.3 mm/rev, c) 0.2 mm/rev.
Sigma J Eng Nat Sci, Vol. 42, No. 3, pp. 679−691, June, 2024
Figure 11. Chip morphology at varying feed rate; a) 0.1 mm/rev, b) 0.2 mm/rev, c) 0.3 mm/rev.
Figure 12. Comparison of modeled and experimental chip; a) modelled chip, b) experimental chip. teeth in the chip. Each of the geometric characteristic parameters of the chip was measured 5 times and the average values of Gs and Pc were used. In Figure 13, the chip height ratio and tooth pitch variation of the chips obtained from both experimental and numerical analyses at different feed rates are given. When
the change in the tooth height ratios is examined, this ratio decreases with an increase in the feed rate. With the increase of the feed rate, the cutting area between the tool and the cutting workpiece increases, which increases the chip pit height (h2) while decreasing the chip top height (h1). It was observed that it decreased by 31% at a feed rate of 0.2 mm/
Sigma J Eng Nat Sci, Vol. 42, No. 3, pp. 679−691, June, 2024
Figure 13. Comparison of saw-tooth degree of modeled and experimental chip; a) chip height ratio, b) tooth pitch.
Figure 14. Formation mechanism of residual stress; a) f=0.1 mm/rev, b) f=0.2 mm/rev, c) f=0.3 mm/rev
Sigma J Eng Nat Sci, Vol. 42, No. 3, pp. 679−691, June, 2024
rev compared to a feed rate of 0.1 mm/rev, and by 20% at a feed rate of 0.3 mm/rev compared to a feed rate of 0.2 mm/ rev. Conversely, the tooth pitch values increase with the increase in the feed rate. It was observed that it increased 20% at 0.2 mm/rev feed rate compared to 0.1 mm/rev feed rate, and 15% increased at 0.3 mm/rev feed rate compared to 0.2 mm/rev feed rate. In fact, similar results have been observed in studies focused on the investigation of chip morphology by researchers in the literature [27,28].
zone 1. The stress occurring in this region was in the opposite direction to the shear plane. Zone 3 occurred on the surface and lower layers of the workpiece, and especially in this region, there was a mechanical load accumulation. It was also greatly affected by the thermal load in this region, as heat generation occurred due to plastic deformation during chip removal and friction between the tool and the workpiece. However, the heat gradient formed on the surface was limited by the subsurface layers, thus compressive plastic stress occurred in the surface layer.
Residual Stress The variation of the simulated residual stress along the depth of the workpiece at various feed rates was analyzed, as shown in Figure 14. The maximum compressive residual stress was observed on the machined surface. Three stress zones occurred as a result of the contact between the tool and the workpiece. Zone 1 was formed in front of the cutting edge, and less tensile stress occurred in zone 2 compared to
Tool Wear During the machining of Co alloys, crater wear, lower tungsten particles stick between the workpiece and the cutting tool, also high levels of diffusion speeds occur [29]. In this study, after the machining of CoCrMo alloy, microscopic images were taken from carbide cutting tool at 3 different feed rates (0.1, 0.2, 0.3 mm/rev) at the of
Figure 15. Change of crater wear dimensions due to increased feed rate.
Figure 16. Distribution of temperature influence area due to increase in progress; a) 0.1 mm/rev, b) 0.2 mm/rev, c) 0.3 mm/rev.
Sigma J Eng Nat Sci, Vol. 42, No. 3, pp. 679−691, June, 2024
increase in the temperature between the workpiece and the cutting tool [30-32]. Thus, the size of the crater wear gradually increases. Table 4 gives the measurement results of the wear areas that occur in the cutting tool. The values support the microscopic images. Furthermore, from the FEM analysis, with the increase of the progression on the cutting tool (Figure 16). It was seen that the effect area of the temperature increased significantly. As a result of the analysis, it was seen that the temperature formed at 0.1 mm/rev advance was approximately significant to 1.212 mm distance from the tool tip, 1.328 mm distance at 0.2 mm/rev and 1.409
manufacturing. Microscop images showed a significant increase in the progression of tool wear. Images showed a significant increase in crater sizes due to the increase in progression at a constant 80 m/min (Figure 15). Increasing the feed rate during production increases the amount of chips removed per unit time, which in turn leads to an
Figure 17. Peak temperature change in the tool at different feed rates; a) 0.1 mm/rev, b) 0.2 mm/rev, c) 0.3 mm/rev.
Sigma J Eng Nat Sci, Vol. 42, No. 3, pp. 679−691, June, 2024
mm distance at 0.3 mm/rev feed rate. Additionally, the peak temperature changes that occurred during the manufacturing process of the cutting tool were examined (Figure 17). The highest feedrate used in the analyses was increased to 1200°C with a maximum peak temperature of 0.3 mm/ rev. While the other feed rates increased to a maximum of 1000°C. Thus, it was observed that the high temperature values were directly associated with each other during crater wear and chip removal. On the other hand, it has been observed that the interaction between the tool nose radius and the feed rate affects the tool wear area. The increase in the cutting zone temperature with the increase in feed rate did not significantly affect the tool nose radius. It has been found that the maximum temperature occurs in the middle of the tool-chip contact length and in the interaction of the corner radius of the tool with the side face. As a matter of fact, similar results were obtained in studies on the tool nose radius of machining parameters [33].
Data Availability Statement
The authors confirm that the data that supports the findings of this study are available within the article. Raw data that support the finding of this study are available from the corresponding author, upon reasonable request.
Conflict Of Interest
The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Ethics
There are no ethical issues with the publication of this manuscript.
Share and Cite
EMİR, E.; ÖZDEMİR, B.; BAHÇE, E. Experimental investigation and FEM analysis of chip morphology in the turning of ASTM F-75 CoCrMo al. Sigma Journal of Engineering and Natural Sciences 2024, Vol. 42, pp. 679-691. https://doi.org/10.14744/sigma.2023.00130