The graphs of the feed forces measured during the milling of the Inconel 718 super alloy with CT and CU cutting tools are given in Fig. 3 and Fig. 4. When the graphs in Fig. 3 are examined, it is seen that feed forces increased in all machining conditions as feed rate increased. In the literature, feed rate that affect feed forces was discussed by many authors [9, 20, 24]. The feed forces and the feed rate increased in direct proportion. The reason for the increasing feed forces owing to the increasing feed rate is that the chip load per tooth increases as the feed rate increases [9, 20, 24]. When the graphs in Fig. 4 are examined to see the impact of cutting speed on the feed forces, it is seen that the feed forces increase as the cutting speed increases. In the literature, it was stated by the authors that the increase in the temperature in the cutting zone owing to the increase in the cutting speed causes the feed forces to increase [9, 14, 25]. Based on the increase in cutting speed, the change of cutting tool geometry, chip welding and BUE formation on the cutting tool cutting edge damage the machined surface, which at the same time contributes to the formation of heat in the cutting zone were investigated. In this case, it causes an increase in feed forces depending on the increase in cutting speed [9, 14, 25]. As a result of the experimental studies, BUE is seen on the rake face of the cutting tool illustrated in Fig. 10. At the same time, Inconel 718 increases the hardness of the machined surface by causing the heat occurred during machining to be trapped on the surface of the workpiece owing to their low thermal conductivity. This situation increases the temperatures in the cutting zone as the cutting speed increases. Therefore, as the cutting speed increases, the feed forces increase as well.
When the graphs in Fig. 3 - Fig. 4 showing the effect of the cryogenically treated tools on the feed forces are examined, it is determined that the feed forces obtained with the CT tools are lower than the CU tools. This situation is parallel to the studies in the literature [17]. When milling Inconel 718 workpiece with CT tools, the achievement of lower feed forces was attributed to the effect of the cryogenic treatment. The hardness provided by the cryogenic treatment for the cutting tools plays a crucial role in maintaining the sharpness of the cutting tool. At the same time, due to the good rigidity and high hardness that the CT brings to the cutting tool, it causes lower feed forces than the CU cutting tool [17].
3.1. Evaluation of Ff values
The graphs of the feed forces measured during the milling of the Inconel 718 super alloy with CT and CU cutting tools are given in Figure 3 and Figure 4. When the graphs in Figure 3 are examined, it is seen that feed forces increased in all machining conditions as feed rate increased. In the literature, feed rate that affect feed forces was discussed by many authors [9, 20, 24]. The feed forces and the feed rate increased in direct proportion. The reason for the increasing feed forces owing to the increasing feed rate is that the chip load per tooth increases as the feed rate increases [9, 20, 24]. When the graphs in Figure 4 are examined to see the impact of cutting speed on the feed forces, it is seen that the feed forces increase as the cutting speed increases. In the literature, it was stated by the authors that the increase in the temperature in the cutting zone owing to the increase in the cutting speed causes the feed forces to increase [9, 14, 25]. Based on the increase in cutting speed, the change of cutting tool geometry, chip welding and BUE formation on the cutting tool cutting edge damage the machined surface, which at the same time contributes to the formation of heat in the cutting zone were investigated. In this case, it causes an increase in feed forces depending on the increase in cutting speed [9, 14, 25]. As a result of the experimental studies, BUE is seen on the rake face of the cutting tool illustrated in Figure 10. At the same time, Inconel 718 increases the hardness of the machined surface by causing the heat occurred during machining to be trapped on the surface of the workpiece owing to their low thermal conductivity. This situation increases the temperatures in the cutting zone as the cutting speed increases. Therefore, as the cutting speed increases, the feed forces increase as well.
When the graphs in Figure 3 - Figure 4 showing the effect of the cryogenically treated tools on the feed forces are examined, it is determined that the feed forces obtained with the CT tools are lower than the CU tools. This situation is parallel to the studies in the literature [17]. When milling Inconel 718 workpiece with CT tools, the achievement of lower feed forces was attributed to the effect of the cryogenic treatment. The hardness provided by the cryogenic treatment for the cutting tools plays a crucial role in maintaining the sharpness of the cutting tool. At the same time, due to the good rigidity and high hardness that the CT brings to the cutting tool, it causes lower feed forces than the CU cutting tool [17].
3.2. Evaluation of Ra values
The graphs of the Ra values resulting from the milling of the Inconel 718 workpiece with the CT and CU cutting tools at three different cutting speeds and three feed values are given in Figure 5- Figure 6. When the Ra graphs based on the change of the feed values in Figure 5 are examined, it is determined that the Ra values increase in direct proportion as the feed rate increases. This result is an expected state. It is known in the literature that feed rate on Ra is the most effective parameter and it is affected by it directly proportionally [9, 24].
This situation was associated with the increase in material amount to be removed per unit time as the feed rate increased. It is thought that the increase in the vibration rate with the increase in the amount of material to be removed causes an increase in Ra values [9]. When the Ra graphs attained depending on the change of cutting speed in Figure 6 are examined, it is seen that the Ra values increases as the cutting speed increases. It is understood from the graphics in Figure 6 that the cutting speed has a crucial impact on the Ra. Cutting speed has a significant effect on the temperature in the cutting zone. The higher the cutting speed, the higher the temperature in the cutting zone [14]. The high temperature in the cutting zone causes chip welding to the cutting tool and built-up chip (BUE) formation, which is very common in Inconel 718 materials, causing the workpiece surface to deteriorate. At the same time, the low thermal conductivity of these materials causes heat trapped in the workpiece. Therefore, as the cutting speed increases, it is expected that the Ra values increase [14].
The Ra values measured from the machined surface of the workpiece as a result of milling Inconel 718 material with CT and CU cutting tools are illustrated in Figure 5 and Figure 6. When Ra graphs in Figure 5 and Figure 6 are examined, it is seen that the lowest Ra values were attained from CT cutting tools, while the highest Ra values were obtained from CU cutting tools. This situation is due to the properties that the CT brings to the cutting tool. CT cutting tools cause lower Ra values due to maintaining sharpness and rigidity of cutting tools compared to CU cutting tools, even at high cutting speed and feed rate.
3.3. Evaluation of vibrations
The graphs of vibration values measured during the machining of Inconel 718 workpiece with CT and CU cutting tools at constant depth of cut and three different feed rates and three cutting speeds are given in Figure 7 and Figure 8. When the graphs in Figure 7 showing the effect of the feed rate on vibration are examined, it is observed that the vibration values increase as the feed rate increases. Similar results are demonstrated by other authors [17]. The reason for the increase in vibration values owing to the increase in the feed rate is that the chip load for per tooth increases as the feed rate increases. The increase in the chip load for per tooth causes more strain on the cutting tool, which leads to an increase in vibration values.
When the graphs that give the effect of cutting speed on vibration in Figure 8 are examined, it is seen that vibration values increase as cutting speed increases. It is understood from the graphs in Figure 8 that the cutting speed has a crucial impact on the vibration values. Especially, when workpieces such as Inconel 718 are milled, chip welding to the sharp edge of the cutting tool and BUE formation increase due to the increase in cutting speed. In addition, as the chip removal process continues, work hardening occurs on the surface of Inconel 718 workpiece, which has a low thermal conductivity coefficient. At the same time, more vibrations occur owing to the increase of the cutting speed, for reasons such as more cutting tool wear and the inability to maintain the cutting-edge sharpness due to the BUE formation.
When the graphics in Figure 7 and Figure 8 are examined, it is observed that the cryogenic process has a significant effect on vibration. The lowest vibration values were determined when the workpieces were machined with CT cutting tools, while the highest vibration values were determined when the workpieces were machined with CU cutting tools. This situation is an expected one. The low tool wear, rigidity and strength, which the cryogenic process equips the cutting tool with, caused the lowest vibration values to be obtained when machining with CT cutting tools [17]. It was observed that the Ra values obtained from the surfaces machined with CT cutting tools were lower than the CU cutting tools (Figure 5- Figure 6). This situation reveals that there is an important relationship between vibration and surface roughness.
3.4. Evaluation of abrasive wear and cutting tool hardness
Abrasive wear and hardness measurements were conducted to see the properties that the CT brings to cutting tools. These measurements were made on new cutting tools never used in machining experiments before. Abrasive wear and hardness measurement values obtained from CT and CU cutting tools are given in Figure 9. When the graphs in Figure 6 are examined, it is determined that the hardness of the CT cutting tool is higher than the CU cutting tool. It has been emphasized in the literature that the CT gives hardness to the material [17]. Looking at the abrasive wear results in Figure 9, the material mass loss in the CU cutting tool was higher than the CT cutting tool. In addition, the hardness results of these cutting tools support abrasive wear tests. The lower material mass loss in the CT cutting tool with high hardness value can be referred to the wear resistance provided by the CT to the cutting tool.
3.5. Evaluation of cutting tool wear
The wears occurring in CT and CU cutting tools were compared under the depth of cut a=0.2 mm cutting, feed rate f=0.04 mm/tooth and speed V=55 m/min, which are the highest cutting parameters. The wears that occur in cutting tools when Inconel 718 workpiece is machined are given in Figure 10. When the graph showing the cutting tool wear is examined in Figure 10, it is established that the least wear occurred on the cutting edges of the CT cutting tool, while the highest wear occurred on the cutting edges of the CU cutting tool. Similar results were obtained by other researchers in the literature [17, 26]. Due to the low thermal conductivity of Inconel 718, untreated cutting tools cause more heat between cutting tool and chip than CT cutting tools, and this leads to layer-by-layer chip formation on the cutting tool. Also, chip welding on the sharp edge of the cutting tool causes BUE formation [17].
Therefore, it is expected that CT cutting tools will wear more than CU cutting tools. This situation is seen in Figure 10.b. EDS analysis was performed to determine the BUE formed on the CU cutting tool. The aim of the EDS analysis in this study is to analyze the material transferred from the workpiece to the cutting tool during chip removal. The element spectral analysis of the materials adhered to the cutting edge surface of the cutting tool is given in Figure 11. When looking at the element spectral analysis of the materials adhered to the cutting tool in Figure 11, it is established that the highest values of the elements belonging to this part are Ni, Cr and Fe, respectively. When the element composition of INCONEL 718 in Table 1 is checked, it is seen that it is Ni (53.45%), Cr (18.55) and Fe (17.64). This proves the occurance of build-up edge (BUE) on the cutting tool by element spectral analysis of the materials adhering to the cutting tool.
4. RESULTS
The results of feed forces, surface roughness, vibration, cutting tool wear, hardness and abrasive wear in milling INCONEL 718 with CT and CU cutting tools are briefly given below:
- It was determined that the Ff values obtained with CT cutting tools had lower values than CU cutting tools. It was found that the Ff values increased as the feed rate and cutting speed increased under all machining conditions.
- While the lowest Ra values were attained from the machined surfaces with CT cutting tools, the highest Ra values were attained from surfaces machined with CU cutting tools. As the feed rate and cutting speed increased, it was observed that Ra values increased in direct proportion.
- The lowest vibration values were measured when the workpieces were machined with CT cutting tools, while the highest vibration values were measured with CU cutting tools. As a result of the experiments, it was revealed that there was a significant relation between vibration and surface roughness.
- It was determined that the hardness of the CT cutting tool was higher than CU cutting tools. It was established that the material mass loss in CU cutting tools was higher than CT cutting tools. It was found that there was an inverse proportion between abrasive wear and hardness values.
- In cutting tool wear, it was established that the least wear occurred on CT tool and the highest wear on CU cutting tool.
- BUE formation was observed in untreated cutting tools, while BUE formation was not formed in cryogenically treated cutting tools.