EFFECTS ON FORCES AND SURFACE ROUGHNESS DURING MACHINING INCONEL 718 ALLOY USING MINIMUM QUANTITY LUBRICATION

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1 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India EFFECTS ON FORCES AND SURFACE ROUGHNESS DURING MACHINING INCONEL 718 ALLOY USING MINIMUM QUANTITY Bikash Chandra Behera 1*, Chetan 2, Sudarsan Ghosh 3, P Venkateswara Rao 4 1,2,3,4 Department of Mechanical Engineering, IIT Delhi, * E Mail:bikash.iitd@gmail.com 2 chetan.harry@gmail.com 3 sudarsan.ghosh@gmail.com 4 pvrao@mech.iitd.ernet.in Abstract Turning operations are one of the most versatile secondary manufacturing processes. In industry, manufacturing processes are contrived and improved in order to obtain maximum quality and minimum cost. Requirements of higher machining quality and manufacturing efficiency have led to a great deal of researches aimed at controlling and planning the metal cutting processes. In manufacturing industries new techniques have been developed to improve manufacturing performances through better processes, and use of advanced cutting tools. However, the environmental aspects are increasingly becoming a significant issue. Thus, in parallel with manufacturing process optimization, efforts must be made to reduce the impact of industrial activity on environment and health. The increasing requirements for environment-friendly cutting processes enabled use of new techniques like the minimal quantity or even the complete omission of cutting fluids. The most promising solution to these requirements is MQL machining, which has been performed in the metal removal process such as milling, drilling and turning. In the present study we compared the effect of forces and surface roughness data during turning of Inconel 718 under dry, conventional fluid and conventional fluids under Minimum Quantity Lubrication process. Key words: MQL Turning, cutting force, surface finish, Inconel Introduction Turning is a one of the most commonly used machining technique. In turning process, the heat is produced in the cutting zone because of the deformation brought about by flow stress, compressive mean stress due to shearing of chips and also by friction between tool-chip and tool-work interfaces. The generated heat strongly affects the tool life. The abnormal generation of heat during the machining becomes more of a concern especially, if the work material has poor thermal property and high strength. All engineering materials are not always easily machinable due to their mechanical, chemical and thermal properties. These materials are called difficult to machine materials. Inconel 718 (UNS N07718) is one of them due to its high strength at elevated temperature, low thermal diffusivity and high chemical affinity. Inconel 718 has been widely used in manufacturing of the components of aeronautical engines, gas turbines, nuclear devices etc (Reed, 2006). To evaluate the machinability of a material, SAE 1020 grade steel is used as a reference material which has been assigned the machinability rating of 100%. According to Pratt and Whitney aircraft division and research Centre of United Techno Corporation (Schirra and Viens, 1994), Ni based superalloys have been assigned the machinability rating of less than 25% whereas wrought Inconel 718 has been rated as 14% with average material removal rate of 3.28 cm 3 /sec (in processes such as milling, drilling, turning etc.). 35% of the total manufacturing cost is used to machine the alloy 718. The poor machinability of Inconel 718 is due to its rapid work hardening ability during machining and also due to the presence of hard carbides in its microstructure. It possesses unusually high dynamic shear strength and poor thermal diffusivity which results in increase in temperature at machining zone (Donachie and Donachie, 2002; Dudzinski et al., 2004; Ezugwu et al., 1998). The temperature rise during machining is not totally avoidable. The rate is minimizing by using proper environment such that the temperature in machining zone should be minimum.in order to achieve proper cooling and lubrication in the machining zone, several methods of application have been adopted. The coolants and lubricants reduce the friction force between toolchip interfaces and also carry away the heat from the 116-1

2 EFFECTS ON FORCES AND SURFACE ROUGHNESS DURING MACHINING INCONEL 718 ALLOY USING MINIMUM QUANTITY machining zone. Today s machining performs in industry in flood cooling environment. The high amount of cutting fluids create environmental hazards and expensive. The cutting fluids often have detrimental impact on the operator s health and sometimes may also have a negative impact on production rate due cleaning of machine tool and shop floor. From the literature survey (Shokrani et al., 2012; Yildiz and Nalbant, 2008) it has been found that minimum quantity lubrication (MQL) is an efficient cooling technique, which is more environmental friendly. MQL is also known as near to dry lubrication or micro- lubrication (Klocke and Eisenblätter, 1997). It refers to the use of cutting fluids for only about a minute during the machining operation. This (10-100ml/h) flow rate is about one fifth or one tenth of magnitude lower than the flow rate commonly used for conventional flood cooling. The air mixed with the lubricant forms an aerosol which is then delivered to the clearance face or rake face of the tool. (A.S.Varadharajan et al., 1999; Kamata and Obikawa, 2007). In MQL technique, because of evaporation by tiny droplets and forced convection by compressed air, the heat transfer from the machining zone takes place in an efficient manner. This technique has been suggested about a decade ago as a means of addressing the issues of environmental officiousness and occupational endangerments associated with the airborne cutting fluid particles within the factory shop floors. The minimization of cutting fluid also minimize the costs and work piece/tool/machine cleaning cycle time Kamata and Obikawa (2007) experimentally studied the effect of MQL during the machining of Inconel 718 alloy on tool life and the surface finish using three different coated carbide inserts. The researchers compared the results with dry and wet cutting conditions and concluded that the surface finish with MQL is at par with that done under wet machining condition. Obikawa et al. (2008) investigated the performance of the focused spraying of oil mist in micro litter lubrication machining (<1ml/h) of Inconel 718 based on the tool life and surface finish and compared the results with that of wet turning. It was examined that the focused spraying of oil mist with an especially designed nozzle was effectively increasing the tool life in the micro-litter lubrication range. It was also reported that the control of the flow of oil mist and the distance from the nozzle to the tool tip were the important parameters which could enhance the cutting performance during MQL machining. Yazid et al. (2011) experimentally investigated the effect of cutting parameters and machining conditions on surface quality of Inconel 718. The authors investigated the surface quality by applying various flow rate (MQL 50ml/h and MQL 100ml/h) and reported that a flow rate of 50ml/h produced better surface finish than dry and a flow rate of 100ml/h was beneficial at cutting speeds of m/min. Settineri et al. (2008) experimentally studied the performance of different coated carbide inserts during the machining of the nickel based super alloy under both dry and MQL environments. It was found that all coated carbide tools performed better than uncoated tools under both environmental conditions. In the present work a newly MQL set up is developed for turning operation. The present turning experiments have been conducted under wet, MQL and dry environments. The effects of process parameters and environments have been studied during turning of Inconel 718 alloy with TiAlN coated carbide tool. 2 MQL Setup Aerosols used in MQL are generated using a process called atomization, which is the conversion of bulk liquid into a spray or mist, by passing the soluble oil carried by pressurized air through a nozzle (Figure 1). The design of the atomizer is critical in MQL as it determines the concentration of the aerosol and the size of droplets. The aerosol is sprayed onto the tool from outside via a nozzle fitted close to the machining zone as shown in figure 2. Other equipment needed for the MQL set up are listed below: A compressor for sending pressurized air. A pressure gauge fitted close to the nozzle for measuring pressure of incoming air to the nozzle. A pump and oil reservoir for sending oil continuously to the nozzle in minute quantity. A flow valve for checking and measuring oil flow rate. A frame stand for supporting all the above in a convenient manner. Figure 1Sectional view and 3 D solid view of Nozzle 116-2

3 5 th International & 26 th All India Manufacturing Guwahati, Assam, India Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT dynamometer and the surface roughness evaluated through a surface profile meter. The range of the experimental process parameters used in the experiments have been chosen from literature. The cutting inserts used in this experiment was TiAlN coated carbide cutting insert. The details of experimental set up and experimental conditions are giving in Table 2. Table 2 Description of components and parameters Figure2 Experimental set up with MQL arrangement and application of aerosol at tool-chip interface 3 Experimental conditions A 100mm diameter and 300mm length Inconel 718 round bar is turned in the present experiments. The composition of the material is given in table 1. Eleme nts Table 1 Inconel 718 composition (Wt %) Ni C Cr Fe Nb Experiments are conducted under dry, wet and MQL environments in a CNC turning centre. In each experiment a fresh insert has been used to turn 20 mm length of the work piece. The forces have been measured with the help of tool force piezoelectric M o Ti Al Components / parameters Cutting insert Tool holder Tool signature after fixing on tool holder CNC Turning centre Dynamometer Surface profilo meter Nozzle Cutting Fluid MQL flow rate Cutting velocity Feed Depth of cut Descriptions 4 Results and Discussions 4.1 Surface roughness TiAlN coated carbide, 13 rake, manufacturer Kennametal PCLN2020, -6 rake Effective rake angle = 7, Back clearance angle = 0, Approach angle = 95, End cutting edge angle = 5, Nose radius = 8mm Leadwell-Fanuc Series Oi Mate-TD KISTLER Dyno Ware (model no AA). Talysurf Taylor Hobson, England Internally mixed micro-nozzle with twin fluid atomization Water Soluble Oil (1: 80 of oil by volume) 250 ml/hr m/min mm/rev 0.5mm Surface finish of the work material mainly depends upon cutting condition and environment. Surface roughness plays vital role on machined component function, its service life especially when it is in conjugation with other mating part. Surface roughness values have been measured perpendicular to the feed marks. In each cut three R a (arithmetic mean deviation) values were recorded at different locations and its average value has been taken for analysis. Figure 3 shows the variation of surface roughness for various cutting speed (V C ) and feed (S) at a constant depth of cut of 0.5mm under different environments. It is clear that as cutting speed increases the value of surface roughness decreases. In MQL environment variation of R a value is more homogeneous and it clearly follows the trend as mentioned above. The surface roughness reduces by 15-40% in MQL cutting compared to dry 116-3

4 EFFECTS ON FORCES AND SURFACE ROUGHNESS DURING MACHINING INCONEL 718 ALLOY USING MINIMUM QUANTITY cutting. This is because in MQL the tiny air suspended cutting fluid particles entered and spread easily in the tool-chip interface and formed a thin layer. That thin layer prevents the abrasion wear and formation of built up edge which normally predominates during machining of Inconel 718 alloy. At lower cutting speed of 40m/min it is observed that the surface finish improved by about 40% compared to dry condition whereas at higher cutting speed the improvement of surface finish is only about 15%. At lower cutting speed the elastic contact between tool and the chip is higher than the plastic or bulk contact. At lower cutting speed, droplets of cutting fluid reach the chip-tool interface by capillary action whereas at higher speed the plastic contact zone increases and prevents such capillary motion of the droplet inside the tool chip interface. So the effectiveness of MQL has been found to be more at lower cutting speed. From figure 4 it is clearly seen that with increase in feed, R a values increase, in accordance with the theoretical equation 1(Bhattacharya, 2008) (where h m is the theoretical roughness parameter and Ø and Ø 1 are principal and auxiliary cutting edge angle) and it is also seen that under MQL mode the surface roughness values are the lowest. Such decrease in surface roughness under MQL may again be attributed to the better cooling and lubrication. Surface roughness (µm) h m =S/{Cot(Ø)+Cot(Ø 1 )} (1) Speed (m/min) Figure 3Variation of surface roughness with increase in cutting speed at feed 0.04mm/rev Surface roughness (µm) Figure 4 Variation of surface roughness with increase in feed at cutting speed 80m/min. 4.2 Cutting forces Feed (mm/rev) The magnitude of tangential cutting force (P Z ) is an important reference of machinability which has the vital roles on power and specific energy consumption. So it is imperative to study the effect of MQL on the cutting forces. As shown in figure 5 the force increases with increase in speed beyond 100m/min under dry condition. The increase in force with speed especially at higher speeds may be because at higher speeds temperature generated is high which resulted in accelerated tool wear (especially the diffusion and the adhesion wear mechanisms increase and dominate). Consequently because of the higher tool wear, the cutting ability hampers leading to an increase in force. Also possibly at the higher speed range built up edge (BUE) formation increases. That BUE on the tool edge reduces the effective rake angle and also further reduces the sharpness of the tool. It has also been observed from the figure 5 that the main cutting force in the cutting speed range 40-80m/min under MQL environment is lower as compared to wet and dry machining. This is possibly due to the cooling and lubricating effects of MQL. Beyond 100m/min, the force under wet condition is found to be lower than the MQL and dry machining. This is because the BUE formation in wet machining is effectively prevented by the flood cooling which utilizes its pressure head to dislodge the BUE. In figure 6 it is observed that with increase in feed the cutting force increases under all the environments. Also there is not significant effect of MQL environment on cutting force with variation in feed. Overall, from the Figure 5 it can be concluded that the effect of MQL machining on main cutting force is superior to that under wet and dry machining of Inconel 718 alloy especially for cutting speeds below 80 m/min

5 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India Force, P Z (N) Figure 5Variation of tangential cutting force with increase in cutting speed at feed 0.04mm/rev Force (N) Speed, V c (m/min) Feed (mm/rev) Figure 6 Variation of tangential cutting force with increase in feed at cutting speed 80m/min. 5 Comparison of chip forms The chip form during machining is one of the parameters to determine the machinability. Kaldor et al. (1979) suggested that there are two types of chips (a) Favorable or acceptable (b) Unfavorable or unacceptable based on easy to handling. Favorable chips do not interfere in work or tool. This is easy to handle whereas unfavorable chips are entangling with work and tool. So unfavorable chips cause poor surface finish and sometimes unexpected tool failure. Here in the chips form are compared at higher speeds and higher feeds under dry, wet and MQL. As shown in figure 7 it is observed that at higher speeds coiled chips are formed under dry machining and closely coiled chips are formed under MQL machining. In wet machining long snarled chips are formed whereas at higher feeds coiled chips are observed for all the cases. However more closed coil chips are obtained in MQL machining due to effectiveness cooling of MQL at tool chip interface. 6 Conclusions Inconel 718 turning was conducted under MQL environment using a newly fabricated set up. The cutting performance of the alloy improved under MQL condition.mql has been found to be more effective at lower cutting speed possibly due to better penetration inside the tool-chip interface. However as the cutting speed increased the effect of MQL got reduced due to higher plasticization of the chip tool contact area which prevented effective penetration of the mist. It was also found that with change in feed both the force and the surface roughness decreased appreciably under MQL environment compared to those under dry and wet environments. Closely coiled chips were formed during MQL turning at higher feed (0.2mm/rev) and speed (120m/min). Such coiled ships indirectly proved the effectiveness of the MQL as the coiling of the chips have happened due to effective cooling at the chip tool interface. The present experiments established that the overall machining performance of the Inconel alloy improved under MQL condition. Figure 7Comparison of chip sample during dry, wet and MQL turning

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