Use of nitrogen gas in high-speed milling of Ti-6Al-4V

Available online at www.sciencedirect.comTransactions of款骂PScienceDirectNonferrous MetalsScienceSociety of ChinaEL SEVIER PressTrans. Noferrous Met. Soc. China 19(2009) 530-534w wu .nmsc.cnUse of nitrogen gas in high-speed milling of Ti-6Al-4VKE Ying-lin(柯映林)+2, DONG Hui- yue(董辉跃)', LIU Gang(刘刚), ZHANG Ming(章明)'1. Zhejiang Provinvicial Key Laboratory of Advanced Manufacturing Technology. Zhejiang University,Hangzhou 310027. China;2. State Key Laboratory of Fluid Powder Transmission and Control, Zhejiang University,Hangzhou 310027, ChinaReceived 30 June 2008; accepted 9 November 2008Abstract: To inhibit chips burning in the high-speed cutting of Ti-6AI-4V, nitrogen gas with 0.7 MPa pressure was ejected at themilling zone. The high speed flowing of nitrogen gas speeds up the chips lcaving, and prevents the chips from buming at the sametime. By this method the cutting force is reduced. Especally, the temperature increment of the finished surface is smaller than 5 C.This prevents the increase of hardness, improves the roughness of the finished surfacc, and reduces the tools wear. Comparing andanalyzing the morphology and color of chips, which are obtained from the high-speed machining of Ti-6AI-4V with and withoutnitrogen gas ejection, show the action mechanism of nitrogen gas during the high-speed machining of titanium alloy, and it isconcluded that nitrogen gas can be used to realizc the proper high-speed milling of Ti-6AI-4V titanium alloy.Key words: Ti-6Al-4V; high-speed machining; burming; nitrogen gasfinished surface of titanium alloys easily. Another1 Introductionimpairment caused by the high temperature is themelting of titanium alloy chips and adhesion to the toolTo improve the assembly accuracy and mobility ofand on the machined surface.fighter plane, stock is left after titanium alloy part isLots of studies have been done on the machining ofmachined in numerical control workshop, and this stocktitanium alloys, including tool wearing[1- -3]， chipis cut off during flexible assembly. To keep the positionforming (especially serrated chips)[4]， and theaccuracy of fighter plane, cutting force is demanded asoptimization of machining parameters[5-6]. ZHAO et almuch as small, and liquid lubrication oil is not allowed to[7] did some studies about tool wearing in high speeduse. The efficiency of original machining method is lowusing nitrogen-oil-mist as a cutting medium. The highestand cutting force is large, so it cannot meet the demandcutting speed was 400 m/min. BAKER[4], JIANG andof flexible assembly. The aim of the present work is toSHIVPURI[8] simulated the shaping process of titaniumsolve these problems.alloy. They found that the chips were discontinuousThe major application of titanium alloys is in thewhen cutting with slow speed, while they were serratedaerospace industry due to their high specific strength atwhen cutting with high speed. The highest machiningelevated temperatures and exceptional corrosionspeeds were 600 m/min in FEM mode and 240 m/min inresistance. However, titanium alloys were classified asexperiment. CHE-HARON and AWAID[6] researched“difficult-to-machine”material owing to their severalthe integrity, roughness, micro-hardness and micro-inherent properties, such as low thermal conductity,structure of the machined surface. The highest cuttinglow elastic modulus, high strength atelevatedspeed selected in their experiment was 100 m/min[6].temperature and high chemical reactivity with manyVARGAS PEREZ[3} studied the wear mechanism ofcutting tool materials during machining process. VeryWC inin face machining of titanium alloy. Hehigh temperature in the machining zone can damage thedemo中国煤化工.ed strain rate sensi-Foundation item: Projec(50705085) supported by the National Natural Science FoundTYHC N M H Gsppored by the NarionoHigh-tech Research and Development Program of ChinaCorresponding author: DONG Hui-yue; Tel: +86-571-87953929; E mail: donghuiyue@ zju. edu.cnDOI: 10.1016/S1003-6326(08)60307-6KE Ying-lin, et al/Trans Nonferrous Met. Soc. China 19(2009) 530-53431tivity and tendency to strain hardening when the face wasspeed operation with milling hole diameter enlargedmilled under certain cutting conditions[3]. The alloyfrom 20 mm to 26 mm. Fig.I shows the experimentalchips were prone to burning in high speed machining,setup. The cylinder workpiece was clamped with aand the chips would burn more severely if lubrication oilfixture mounted on the quartz three componentwas used. The burmt chips would adhere to cutting edges,dynamometer of KISTLER 9257B type. Experiments ofcausing tool to wear off rapidly, and the machinedcutting Ti-6Al-4V with and without nitrogen gas ejectionsurface integrity to be impaired at the same time.were done. The pressure of nitrogen gas was 0.7 MPa.Furthermore, liquid lubrication oil is forbidden to use inthe assembly process of airplane. So, the cutting speed of3 Abrasion on tool rake facetitanium alloy in most studies was very low, around 100m/min[9-10]. HONG et at[1I] studied cryogenicChips wrapped around the tool tip, and burningmachining of titanium alloy Ti-6AI-4V to improve toolhappened in dry machining of T-6AI-4V (withoutlife, and a special setup was designed. YILDIZ andnitrogen gas), as shown in Fig.2(a). Although thNALBANT[12] reviewed the cryogenic cooling inmajority of machining heat was taken away by chipsmachining processes of Ti-6AI-4V.during high-speed machining, if the chips could not beIn this study, Ti-6AI-4V was machined with theremoved immediately, the buming chips would adhere tocutting speed of 565 m/min, and nitrogen gas was ejectedthe cutting edge strongly, and the cutting edge wouldto the machining zone. This is a higher speed machiningwear off rapidly in the high speed revolution process. Iftechnology for cuting Ti-6AI-4V[13].the cutting operation was continued with the severe worntool, the workpiece material would melt.2 Experiment of high speed machiningThere was no flaming chips when nitrogen gas wasTi-6AI- 4Va)The workpiece material was Ti 6Al-4V. Table I liststhe machining conditions of cutting Ti-6AI-4V with highTable1 Machining conditions of cuting Ti-6AI-4V with highspeedToolNumberComerDiameter/MaterialCoatingof flutes radius/mmmmCemented0.TiAIN12carbideMachining parametersCuttingFeed rate/ Spindle speed/depth/mmwidth/mm(mmmin")(rmin )0.51.40015 000b可y.0中国煤化工。Fig.2YHCN M H GA14v: (2) Wihtoutnitrogen gas; (b) With nitrogen gas (1- -Nitrogen gas nozzle;Fig.1 Expermenlal setup of cuting Ti-6AI-4V2--Milling tol; 3- -Workpiece; 4- -Fixture)32KE Ying-lin, et al/Trans. Nonferrous Met. Soc. China 19(2009) 530-534ected to the machining zone (Fig.2()), because theflowing nitrogen gas with 0.7 MPa covered the whole80a)1machining zone and oxygen was prevented to approach.40-At the same time, the drainage of Ti-6AI-4V chips wasimproved. Without nitrogen gas, the cutting time wasless than 10 min before the tool was wom heavily; andthe melt chips were adhered to the tool rake face (parts inthe two elipses) (Fig.3(a)). Fig.3(b) shows the condition-40-of rake face when cutting with nitrogen gas ejection forabout 1 h. Because the cutting time was so long that the-80-tool was womn mildly, but there was not adherent chips inthe tool.-12号2728293031323334Time/s100pb)60-20--20--60-102728293031323334Fig.4 Measured cutting force in machining of Ti-6AI-4V withnitrogen: (a) InX direction (b) In Y directionaround the cutting edge, and the condition of chipdrainage was very poor. The amount of machining heatwas generated greatly and the chips were melted and0.1 mmadhered to the cutting edge. Then the cuting forceFig3 Microimages of tols rake face: () Without nitogen gas; increased consequently.(b) With nitrogen gasWhen machining was finished, the roughness of themachined surface was measured. The roughness of the4 Analysis of cutting force and surfacesurface was 0.22 um when using nitrogen while it wasroughness3.5 um without nitrogen gas. This is because that whennitrogen gas was not able to arrive at the machiningThe depth of the hole was 25 mm. Because thezone, lots of chips that absorbed a large amount of heatnitrogen gas nozzle was not designed specially, nitrogenand could not be removed immediately would burn. Thengas could not able to be ejected to the destination whenthe burning chips adhered to cutting edges and machinedthe hole was deeper than 20 mm. Cutting force, shape ofsurface. Fig.6 shows two morphologies of finishedchips, and the integnity of the machined surface whensurfaces. One was obtained when nitrogen gas was used,cutting with nitrogen gas were all different from thoseand the other was obtained when nitrogen gas could notwhen cutting without nitrogen gas. Fig.4 and Fig.5 showarrive at the machining zone (especially the partialthe cutting force curves when machining with andsurface in the ellipse).without nitrogen gas, respectively. It is obvious that the5 An中国煤化工ncutting force was greater when cutting without nitrogengas (the maximum of 162 N) than those when cutting.YHCNMHGwith nitrogen gas (the maximum of 109 N). The reasonThough the same machining parameters were used,of this is that without nitrogen gas, the chips congregatedthe configuration of chips obtained with and withoutKE Yinglin, et al/Trans. Nonferrous Met Soc. China 19(2009) 530-53433nitrogen gas ejection was different greatly. Fig.7 shows120()the difference. When there was no nitrogen gas, the80-shape of chips was continuous and undee. It was obviousthat the chip was composed of several serrated small40-chips. This characteristic was more obvious whenobserving the chip from its back side(Fig.8). The reasonwhy this configuration was formed during high speedmachining of Ti-6AI-4V can be explained as follows.-80-Chips that absorbed the majority of machining heat andcould not be removed away immediately from the-120 tmachining zone would be softened and bumt by the hightemperature when the edge cut into the material. The30 31 32334353637temperature softening effect in the primary shear zoneTime/swas especially stronger than the other part and resulted in1606)the plastic instability, leading to catastrophic shearfailure along the shear surface. So, after every constant8(length along the cutting edge trajectory, a serrated chipwould be formed[8,14]. A longer chip composed of-1690 引”立Fig.5 Measured cutting force in machining of Ti-6AI-4Vwithout nitrogen: (a) In X direction; (b) In Y direction0.11 mmFig.7 Configuration of chips under high speed machining ofTi-6Al-4V: (a) Without nitrogen gas; (b) With nitrogen gas中国煤化工J0.1 mmYHCNMH G57zmFig.6 Morphologies of finished surface of high-speedmachining of Ti-6Al-4V: (a) With nitrogen gas; (b) Withoutnitrogen gasFig.8 Back side of chip in Fig7(a)534KE Ying-lin, et aVTrans. Nonferrous Met. Soc China 19(2009) 530 -534several serrated parts was generated when an edge cut3) With nitrogen gas, the adhesion between chip andout the workpiece material. The fact that the chip lengthcutting edge was prevented, and the abrasion conditionwas almost the same as the length of cutting layerof tool was improved. The integrity of machined surfaceshowed that the chip did not break from an edgewas increased.cutting-into to cutting out workpiece material. The darkblue color of the chip showed it was bumt in theReferencesmachining process. The continuous and bumt chipsadhered to cutting edge and machined surface wouldmechanisms of coated carbide tools when face milling titanium alyresult in the poor integrity of machined surface, and小Jourmal of Materials Processing Technology. 2000, 99: 266-274.increased cutting force at the same time. This conclusion[2] DEARNLEY P A, SCHELLEWALD M, DAHM K L. Characteris-agreed well with the analysis of cutting force.tion and wear response of metal-boride cated WC-Co [小Wear,Fig.7(b) shows the shape of chips obtained when2005, 259: 861-869.nitrogen gas arrived at the machining zone. It was every[3] VARGAS PEREZ R G. Wear mechanisms of WC inserts in faceobvious that the color, length and shape of the chips weremillig of gamma titanium aluminides [] Wear, 2005, 259:1160-1 167.changed. The length was about 1/3-1/4 that with[4] BAKER M. 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Wear, 2006, 261: 760-766.was continuous and serrated. The chips were bumnt in the[14] BAKER M, ROSLER J, SIEMERS C. Finite clement simulation ofcutting process.segmented chip formation of T-6AI-4V []. Joumal of ManufacturingScience and Engineering, 2002. 124(5): 485 -488.2) When nitrogen gas arrived at machining zone[15] LI Liang. Study on the mechanism and proces of high speed miligwith high flowing speed, the chips did not bum and wereof titanium alloys [D]. Nanjing: Nanjing University of Aeronauticsbroke into 3-4 parts and were removed awayand Astronautics, 2004. (in Chinese)immediately.(Edited by YANG Bing)中国煤化工MYHCNMHG