The influence of cutting parameters on the durability of carbide tools in internal finishing turning of Inconel 718 parts

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1 The influence of cutting parameters on the durability of carbide tools in internal finishing turning of Inconel 718 parts ION CIOCAN Romanian Research & Development Institute for Gas Turbines COMOTI 220D Iuliu Maniu Bd., CP , sector 6, Bucharest ROMANIA TABITA-DANA POPOVICI University Politehnica of Bucharest, Faculty of Engineering and Management of Technological Systems 313 Splaiul Independentei, CP , sector 6, Bucharest Abstract: - This paper plan to offer recommendations for the appropriate choice of cutting parameters for internal finishing turning for parts made of Inconel 718, on the unique and short series production. The central idea traced was the maximum productivity of cutting tools with minimum costs. Cutting tools manufacturers suggest, in general, the maximum possible cutting parameters without indicating resulting durability. In this case, the right choice of the cutting parameters knowing the durability of cutting tools used, have a great influences on the processing cost. Key-Words: -Inconel 718, durability, internal turning, costs 1 Introduction Inconel 718 is part of super-alloys, materials developed for service at high temperatures under high mechanical stress. These materials are usually alloys of Ni, Cr, Co or Fe with various amounts of W, V, Ta, Nb, Ti, Al, B, Zr, Hf. Doing just a quick review of alloying elements, we can understand why they have superalloy features as: high resistance to prolonged stress at high temperatures, to corrosion and hot erosion. Inconel 718 is not an exception and is much used in the aviation industry, the hot areas of turbo engines, rocket engines, nuclear reactors, pumps etc. Special properties from high temperature to cryogenic temperatures or corrosive environments make it a valuable material, although the price is not low! It is an nickel base alloy (Ni + Co, 50-55%) with the main alloying element chromium (17-21%). It Is hardened by precipitation with exceptional mechanical strength up to temperatures of C [1] [2] [3] [4]. Inconel 718 is in class of difficult to cut materials. After heat treatment, the hardness obtained is close to 40 HRC, which creates big machinability issues. In the process of cutting are developed important cutting forces that contribute to early tool wear. Ddifficulties are caused in the same time by very high temperatures in the cutting zone, because of low thermal conductivity of this material. Even though this nickel-based alloy contains carbide particle (Crc, TiC, and WC) as to improve the creep resistance, these particles are abrasive material that contribute to reduction in tool life. In the process of cutting this material tendency is to create quickly major deposits on the cutting edge and modify tool geometry. Inappropriate choice of cutting regime parameters may cause rapid wear or damage to the tool with effects on surface quality, the level of tension in parts and costs. If we talk about cutting process of Inconel 718, general recommendations are to use low cutting speeds and a lower number of crossings.this is necessary to avoid the hardening on surfaces and the deformation of the workpiece, especially for thin-walled parts which are characteristic to aviation. 2 Problem Formulation Generally available information does not provide much qualitative data about the durability of cutting tools for mechanical processing of this material, with some exceptions[3] [4]. This paper aims to provide guidance about the durability of carbide cutting tools for internal finishing turning, using a certain type of cutting insert. ISBN:

2 3 Problem Solution 3.1 Experimental scheme Experiment has proposed to determine the optimum durability for interior finish turning under cost optimization tool purchase. For this purpose it will determine the link between factors that characterize the phenomenon studied, namely the study of the relations: y=f(x1, x2,, x n ) (1) Where y is dependent variable and x n the independent variables. In this experiment we shall use the following variables: T= tool durability; [min.] Q= the quantity of chips produced in unit time; [cm 3 /min] V= the quantity of chips cut with an edge of insert [cm 3 /min] f= cutting feed; [mm/rev.] a p = depth of cut; [mm] s= speed; [m/min], And we shall determine some dependent functions like: T= f(s, f, a p ) (2) Q= f(s,f, a p ) (3) To determine the quantity of chips results V, on each experiment, the following assumptions were considered: - if we note by D, the total distance made by the cutting edge of tool for durability T, with velocity s, the value of D is given by the next mathematical expression: ;[m] (4) Chips produced can be considered having a dimensional cross-section area, defined by the variable S: S= a p ; [mm 2 ] (5) Where "d" is considered a variable having the same value with the feed used. Therefore, if we consider the resulting chip as a continuous parallelepiped with height equal to the distance D, and with the base area defined by the parameter S, the formula for total volume of chips produced in time of durability T, can be considered V, with the expression: [cm 3 ] (6) In that case, expression of Q is: (7) 3.2 Experimental Procedure For experiment we use the following equipment: -Gildemeister CTX 620 machining centre with CNC Siemens 840D; -Screw lock boring bar for 35 rhombic inserts 5 clearance,s 40T SVQBL 16, 110 mm active length; Fig. 1 Boring bar S40 T SVQBL Fig. 2 Geometry of VBMT inserts - VBMT inserts, TT9020 quality, PVD coated, with 2 cutting edges on insert and central screw locking [5]. Coolant was used.the cutting was continuous on each test and the speed constant. Fig. 3 The cutting area with the test part Experimental basis for this study is the response surface method [6] and the experimental program is presented in Table 1: Table 1 Experimental program Nr Inputs Results i X 1 X 2 X 3 Y 1 Y ISBN:

3 Experiment data To determine the range of values for the speed, feed and depth of cut was done preliminary tests, starting from the provider catalog recommendations. Table 2 presents the results for these preliminary tests Table 2 Preliminary tests a p f s T remarks mm mm/rev m/min. sec broken insert broken insert broken insert We can see that the time for cutting was very short and the final was the broken of the inserts. The decision was to use the next testing intervals: a p mm mm/rev m/min The results of the experiments are shown in table 3. Table 3 Experimental results Nr Inputs Results i a p f s T i Q i V i mm mm/ m/ min cm 3 / cm 3 rev min min Influence of feed on durability If we choose the experiments where the speed is constant, the result is shown in the diagrams on Fig.4 6. Fig. 4 Fig. 5 Fig. 6 In all situations the durability decreases with the increasing on depth of cut and the cutting speed. A small exception is on fig. 4 where we can see that a small feed is worse than a big feed at small depth of cut! Influence of speed on durability On fig. 7 9, can be observed how decrease the T variable when speed is variable. In all situations the great speed is worse for durability than the small one. 3.4 Short analysis of influence of each parameter ISBN:

4 Fig. 7 Small feed determine an almost equal durability no matter the speed used. Fig. 10 we use small cutting speed, small depth of cut and big feeds, when we have a local growing of durability. Fig. 8 Fig. 11 Fig. 9 The biggest difference is at maximum feed at small depth of cut. Small depth of cut is not a good option at high speed of cut and big feeds. In the same time, the maximum depth of cut determines at high feed the same durability like the small Influence of depth of cut on durability In the diagrams on fig.10 12, can be seen the same decrease durability, in that case when the depth of cut is growing. An unexpected situation is when Fig Durability in absolute value If we make an analyse of the absolute durability depending of each parameter, we obtain the diagrams of fig Depending on depth of cut, the best durability is on small values, see fig. 13. A good durability can be obtained on mean and high values on feed, see fig. 14. When we analyse the influence of speed, recommendation is to use small or mean values like in fig. 15. ISBN:

5 Fig. 13 Fig. 14 Fig Cutting productivity If we note with: - V T = the total of chips in all experiments, - V i = the total quantity of chips on each experiment, - n pi = the total consumption of cutting edges of the inserts to perform all work using the parameters in the experiment i, - t ai = total cutting time of the experiment using cutting parameters from experiment i, Then: V T =, (8) n pi =V T /V i (9) t ai n pi (10) In that conditions we can find the total consumption of tools or time for total experiment, if we use the cutting parameters on each experiment i. Using these relationships, the results are in table 4. Nr. of cutting edges used, necessary time i Q i V i n pi t ai cm 3 /min. cm 3 pcs. min In that moment, we can analyse the function of maxim productivity (time function minimum) g =min(t ai ) (11) or for minimum costs(number of cutting edges min.) f =min(n pi ) (12) Giving an equal weighting of each parameter, we can use the next function: h= min(n pi x t ai ) (13) Using this criteria, the results are in table 5. i n pi t ai n pi x t ai pcs. min. pcs.x min Table 5 Table 4 ISBN:

6 If we sort the table 5, using criteria (13), the results are shown in table 6. Table 6 Top cutting parameters Nr. exp. a p f s n pi x t ai Conclusion This work allows the following conclusions: - to optimize cutting processing on an Inconel 718 parts is necessary to use all the cutting parameters. - in all the experiments, the result was an unfragmented chips. - for interior finishing turning of parts made of Inconel 718 is recommended to use the maximum feed recommended by the tool producer. For finishing is recommended to avoid of feeds under 0.1mm/rev. - the maximum cutting depth recommended by the tool manufacturer is need to be used. A reduced cutting depth is not profitable in terms of costs. - Low cutting speed determine very good durability, but using average or high values is recommended because lead to the benefit of reduced processing time and a good productivity. References: [1] B. Fang, S. Guoyue, K. Fanya, Y. Ke. Room Temperature Deformation Behavior of Inconel718, Materials Science Forum Vols (2005) pp , Trans Tech Publications, Switzerland [2] X.S.Xie, S.H.Fu,, S.Q.Zhao, and J.X.Dong, The Precipitation Strengthening Effect of Nb, Ti and Al in Cast/Wrought Ni-Base Superalloys, Materials Science Forum Vols (2010) pp , Trans Tech Publications, Switzerland [3] I.Mrkvica, R. Konderla, M. Faktor, Turning of Inconel 718 by Cemented Carbides, Key Engineering Materials Vol. 496 (2012) pp , Trans Tech Publications, Switzerland [4] S. Karabulut, A. Gullu, Dynamic Chip Breaker Design for Nickel-Base, Inconel 718, Alloy with Coated Carbide Tools Using Negative Angle Tool Holder, Solid State Phenomena Vols (2009) pp , Trans Tech Publications, Switzerland [5] TaeguTec metalworking cutting tools catalog [6] M. Gheorghe, Research on cutting machinability on malleable cast iron, IPB-TCM, Bucharest, 1984 ISBN: