Tool Wear when Finish Turning Inconel 718 under Dry Conditions

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1 International Journal of Science Engineering and Technology Vol. 2, No. 3, 2009 ISSN: Available online at: ILRAM Publisher Tool Wear when Finish Turning Inconel 718 under Dry Conditions M.Z.A. Yazid 1, C.H. Che Haron 2, J.A. Ghani 2, G.A. Ibrahim 3, M.S.A. Yasir 4, 1 Institute of Product Design and Manufacturing, Universiti Kuala Lumpur, Kuala Lumpur, Malaysia 2 Department of Mechanics and Material, Universiti Kebangsaan Malaysia, Bangi, Malaysia 3 Department of Mechanical Engineering, Universitas Lampung, Indonesia 4 Department of Mechanical and Manufacturing, Universiti Kuala Lumpu-Malaysia France Institute (Received 16 October 2009; accepted 23 November 2009, published 01 December 2009) Abstract Cooling/lubricating fluid during machining heat resistant super alloy has always been a great concern to the metal removal industries. To improve overall performances and remain competitive, the solution is to cut heat resistant super alloy without cooling/lubricating fluid. In this experimental works, Inconel 718, a highly corrosive resistant, nickel-based super alloy, was finish-turning with CNC lathe under dry cutting conditions. The machining processes were carried out at three levels of cutting speed (V c =90 m/min, 120 m/min and 150 m/min), two levels of cutting depth (d=0.3 mm and 0.5 mm) and two levels of feed rate (f=0.10 and 0.15 mm/rev). The tool wear and wear progression were monitored and measured progressively at various time intervals. The objective was to evaluate the performance of PVD coated carbide tools when finish turning Inconel 718 at considerably higher cutting speed under dry conditions. The series of experiments indicated that thick PVD coated carbide tool may potentially cut Inconel 718 at high cutting speed. Most of the tool failures were due to flank wear and excessive chipping on the flank edge. Tool failure due to crater wear was not dominant. KEYWORDS: Inconel 718; coated carbide tools; Tool wear; Dry cutting 1. Introduction The feasibility of dry cutting in the material removal industries has received much attention due to the fact that the cost of cutting fluids is considerably high at about 17% of the total manufacturing cost [1]. Cutting fluid waste needs to be treated prior to disposal and prolonged exposure is hazardous to the machine operators due to risk of skin cancer and breathing difficulties [2]. Dry cutting is desirable because not only it reduces manufacturing cost but also eliminates all the adverse negative effects associated with the usage of cutting fluids for cooling/lubricating. The usage of cutting fluids in machining super alloy offers several important purposes especially to increase productivity and surface quality of the machined work-piece. This is possible because cutting fluids allow cutting processes to be carried out at much higher speeds, higher feed rates and greater cutting depth [3]. When use effectively, cutting fluids not only lengthen tool life, decrease surface roughness and increase dimensional accuracy, but also decrease the amount of power consumptions [4]. Furthermore, cutting fluids helps transport away the excessive heat and chip produced during the cutting processes, thus longer tool life may be achieved [5]. High speed machining to cut nickel based super alloy Inconel 718 has long been researched to increase cutting process productivity [6]. The challenge and difficulty to machine Inconel 718 is due to its profound characteristics such as high sheer strength, tendency to weld and form build-up edge [7], low thermal conductivity [8] and high chemical affinity. Inconel 718 also has the tendency to work harden and retain major part of it strength during machining [9]. Due to these characteristics, Inconel 718 is not easy to cut and thus has been regarded as difficult-to-cut materials. The unfavorable characteristics coupled with dry cutting condition causes severe and rapid tool wear when machining Inconel 718 [10]. For this reason, machining Inconel 718 is still performed at low cutting speed, using coated carbide tool with cutting fluid to cool the tool and work-piece [11]. At this lower speed, the productivity is sacrificed to gain acceptable surface quality which lead to expensive cutting processes [12]. The cutting speed usually employed under dry condition is in the range of 20~50 m/min for uncoated carbide tools where the feed rates are 0.1~0.2 mm/rev in turning [7]. Higher cutting speed, up to 100 m/min may be achieved with Corresponding Author: M.Z.A. Yazid, Institut of Product Designand Manufacturing, Universiti Kuala Lumpur. Kuala Lumpur, Malaysia.

2 Vb, mm Vb, mm M.Z.A. Yazid et al / Int. J. Eng and Sci.. Vol. 2, No. 3, 2009, coated carbide tools [10]. Much higher speeds are also possible using ceramics tools. This paper presents a series of experimental works of finish turning Inconel 718 using single layer PVD coated TiAIN carbide insert at high cutting speed. The effect of cutting speeds, cutting depth and feed rate on tool wear was investigated and analyzed. 2. Experimental Works Inconel 718 (103 mm (d) X 157 mm (l)) was finish-turning with COLCHESTER T CNC Lathe using single layer PVD coated TiAIN carbide insert. The work-piece was pre-machined prior to every trial run to remove any defect that would interfere with the experiment results using designated tool for roughing purposes. The machining parameters of the experimental works are shown in Table 1. During the process, the cutting was interrupted at pre-determined length and the insert was dismantled from the tool holders. Tool wear was measured using MITUTOYO Tool Maker microscope. The wear photographs were taken at magnification of 30x to monitor wear progression. The cutting process continued for another one pass turning at pre-determined length once the wear has been measured and recorded. The machining process was repeated until the tool failed. The tool is considered failed once the measured wear has reached one or a combination of the rejection criteria listed on Table 2. Table 1. Machining Parameters Work Piece Inconel 718 cylinder bar (103 mm (d) X 157 mm (l)) Cutting Tool CNMG QM 1105 Cutting Speed 90, 120, 150 m/min Cutting Depth 0.3, 0.5 mm Feed Rate 0.10, 0.15 mm/rev using Scanning Electron Microscopy (SEM) to observe wear mechanism. 3.1 Wear Progression From flank wear land versus cutting time in Fig. 1., it was obvious that the trend of the flank wear progression occurred rapidly at the initial phase, then gradually increased at the second phase, and extremely increased at the final phase before it failed. At the initial phase, the sharp new cutting tools quickly worn out due to small contact zone between the tool and work-piece, resulted in rapid wear progression. In the second phase, as the tool becomes worn out and sharp edge no longer exists, the wear progressed quite uniformly. At the final phase, the contact zone between the tool and work-piece has increased due to bigger worn out tool, thus friction and temperature increased at the cutting zone which caused serious damages to the tool which lead to rapid wear and failure of the tool. In Fig. 2., where the feed rate and cutting depth were 0.10 mm/rev and 0.30 mm, respectively, the wear progression displays similar pattern. This was also reported by Jindal et. al. [13] when turning Inconel 718. The same phenomenon was also observed by Jawaid et. al. [14] when turning Titanium alloy Flank Wear vs Cutting time f=0.15 mm/rev, d=0.5 mm Cutting time, s Fig. 1. Wear progress versus cutting time at feed= 0.15 mm/rev and cutting depth=0.5 mm Table 2. Rejection Criteria Average flank wear, V B 0.3 mm Maximum flank wear, VB max 0.7 mm Nose wear 0.5 mm Surface roughness 6 µm 3. Results and Discussions The measured average flank wear and maximum flank wear were used to decide on the end life of the tools. Wear progression was monitored and the photographs were taken. The effect of cutting speed, cutting depth and feed rate on the tool wear was evaluated. Finally, a micro analysis was conducted Flank Wear vs Cutting time f=0.1 mm/rev, d=0.3 mm Cutting time, s

3 Tool Life, min M.Z.A. Yazid et al / Int. J. Eng and Sci.. Vol. 2, No. 3, 2009, Fig. 2. Wear progress versus cutting time at feed= 0.10 mm/rev and cutting depth= 0.3 mm 3.2 Tool life Tool life is significantly influenced by temperature generated at the cutting zone especially for low thermal conductivity alloys such as Inconel 718. Increases the cutting speed effect the tool life and promote wear progression. The effect of cutting speed on tool life was quite apparent as displays in Fig. 1. and Fig. 2. Increases the cutting speed leads to shorter tool life. Kamata and Obikawa [15] conducted dry turning on Inconel 718 at 60 and 90 m/min also observed that tool life tends to decrease as cutting speed increases. Similar pattern was observed at lower feed rate and cutting depth. In both condition (from Fig. 1. and Fig. 2.) cutting speed of 150 mm/min shows the shortest life and cutting speed of 90 mm/min shows the longer life. Fig. 3. Compares the tool life at various cutting speed for two conditions: a) feed rate= 0.15 mm/rev and cutting depth=0.5 mm b) feed rate=0.10 mm/rev and cutting depth=0.3 mm. At lower feed rate and cutting depth, tool life improves for all cutting speeds however the increase in tool life is much more significant at 120 m/min and less significant at 150 m/min. shown in Fig. 4. At the initial stage of cutting, the flank wear developed uniformly and increased rapidly as the cutting approaches the end of tool life. This is also evident from the measured data from Fig. 1.and 2. As the cutting speed increased, it created higher temperature at the cutting zone resulted in severe wear at the flank face. It was observed that the flank wear progressed faster at 150 m/min compared to other lower cutting speeds. By examining closely the photo in Fig. 4., one would clearly notice the existence of a burnt mark near the edge of the flank wear. The burnt mark becomes noticeable at higher cutting speed. It is interesting to mention that at certain point of worn tool, the temperature was significantly high to the point where reddish-color chip was observed at cutting speed of 120 and 150 m/min. Burnt mark Tool Life vs Cutting Speed f=0.15, d=0.5 f=0.10, d= Burnt mark Cutting Speed, m/min Fig. 3. Tool life versus Cutting Speed 3.3 Tool Wear Inconel 718 has the tendency to work harden during cutting process and this generates high heat at the cutting zone thus giving rise to thermal stresses [16]. As a result, tools are exposed to high mechanical and thermal stress when cutting Inconel 718 resulting in tool wear and short tool life [17]. The typical wear patterns for each cutting speed when machined with feed rate of 0.15 mm/rev and cutting depth of 0.5 mm after reaching end of life are Fig. 4. Wear at rejection point at feed=0.15 mm/fev, depth=0.5 mm, A) 90 m/min B) 120 m/min C) 150 m/min (magnification 30x) In this experimental work, flank wear is the dominant wear observed. In all cases, the cause of end of tool life was the average flank wear and not maximum flank wear. Tool failure due to crater wear was not observed. 3.4 Wear Mechanism A SEM analysis was performed on the tool of run no.1 which was machined under the most extreme condition (speed 150 m/min, feed 0.15 mm/rev, depth 55

4 M.Z.A. Yazid et al / Int. J. Eng and Sci.. Vol. 2, No. 3, 2009, mm) to analyze the wear mechanism of cutting Inconel 718 at high speed under dry conditions. The SEM micrograph of the tool is shown in Fig. 5. The wear mechanism observed from the SEM analysis reveals that interactions of abrasive wear, chipping and micro-crack exist when cutting Inconel 718 at 150 m/min under dry condition. Abrasive wear, flaking and micro-crack occurs at the flank Fig. 5. SEM micrograph at speed 150 m/min, feed 0.15 mm/rev, depth 0.5 mm face. Chipping and flaking was observed at the nose radius. SEM shows that the dominant wear mechanism was abrasive wear and excessive chipping at the flank edge. 4. Conclusions This experimental works performed a series of trials of finish turning Inconel 718 using single layer PVD coated TiAIN carbide insert at high cutting speed. The effect of cutting speeds, cutting depth and feed rate on tool wear was investigated and analyzed. In machining Inconel 718 with PVD TiAlN coated carbide tool under dry condition at higher speed may be possible at lower feed rate and cutting depth. Pomising performance in term of tool life was observed at low feed rate and low cutting depth. The experimental works found that flank wear was the dominant wear that cause end of tool life when cutting Inconel 718 under dry condition with PVD TiAlN coated carbide tool. The micro analysis using References [1] M. Nalbant, A. Altin, and H. Gokkaya, "The effect of cutting speed and cutting tool geometry on machinability properties of nickel-base Inconel 718 super alloys," Materials & Design, vol. 28, pp , [2] N. R. Dhar, M. W. Islam, S. Islam, and M. A. H. Mithu, "The influence of minimum quantity of lubrication (MQL) on cutting temperature, chip and dimensional accuracy in turning AISI-1040 steel," Journal of Materials Processing Technology, vol. 171, pp , 56

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