Efficient Abrasive Water Jet Milling For Near-net-shape .

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The International Journal of Advanced Manufacturing Technology (2020) 074-3ORIGINAL ARTICLEEfficient abrasive water jet milling for near-net-shape fabricationof difficult-to-cut materialsEckart Uhlmann1,2 · Constantin Männel1 · Thomas Braun1Received: 2 June 2020 / Accepted: 9 September 2020 / Published online: 2 October 2020 The Author(s) 2020AbstractThe utilization of materials with high strength to density ratio enables efficiency improvements and is therefore demandedfor many applications, particularly in the aerospace and other mobility sectors. However, the machining of these typicallydifficult-to-cut materials poses a challenge for conventional manufacturing technologies due to the high tool wear. Abrasivewater jet (AWJ) machining is a promising alternative manufacturing technology for machining difficult-to-cut materials,since the tool wear is low and material independent. However, AWJ machining is limited regarding the producible geometrieswhen conducting cuts through a material. This limitation can be resolved with AWJ milling operations which on the otherhand are time-consuming. To approach this challenge, an enhanced AWJ milling operation is presented and investigated inthis paper with the aim to expand the producible geometries. This operation consists of two kerfs, inserted from differentsides of the workpiece, which intersect at their kerf ground. Consequently, a piece of material is separated without the cutmaterial being entirely chipped. Thus, the operation possesses a high aggregated material removal rate. The investigationspresented in this paper show and evaluate the effects that occur during the milling of kerfs with variable depths on titaniumaluminide TNM-B1. Furthermore, a method to compensate these effects is introduced and thus the producible geometriesfor effective AWJ milling could be enhanced.Keywords Abrasive water jet · Near-net-shape fabrication · Titanium aluminide · AWJ millingNomenclatureAbrasive flow rateṁAAdjusted angle of cutα cConstant angles of cutαc(y)Actual cutting angleαc,realαcAngle of cutOpening angle of the jet forerun for concaveαjf,ashapesαjf,xOpening angle of the jet forerun for convex shapesOpening angle of the jetlag for concave shapesαjl,aαjl,xOpening angle of the jetlag for convex shapesAWJAbrasive water jetKerf development coefficient 1c1c2Kerf development coefficient 2 Constantin Männelmaennel@iwf.tu-berlin.de1Technische Universität Berlin, Institute for Machine Tools andFactory Management (IWF), Berlin, Germany2Institute for Production Systems and Design Technology(IPK), Fraunhofer-Gesellschaft, Berlin, K,pdK,sdK1dKdoeEDMfafxfpsKerf development coefficient 3Power coefficientCarbon fibre-reinforced polymersCeramic matrix compositesFocus tube diameterJet diameterAverage cumulated kerf depthDifference between lowest and the deepest kerfdepthMaximum kerf depthMinimum kerf depthMeasured kerf depthKerf depths on the primary target partKerf depths on the secondary target partKerf depth for one passKerf depthOrifice diameterDeviation of the kerf depth to the target kerfElectrical discharge machiningConcave geometry factorConvex geometry factorFrames per second

686ldDepth of specimenlfFocus tube lengthStand of distancelsLength of specimenlWMMC Metal matrix compositespWater pressurepWWorkpiece positionPTPPrimary target partrRadius of shapervfRelative velocitysStandard deviationSTPSecondary target partTNM-B1 Gamma-TiAl Ti-43,5Al-4Nb-1Mo 0,1BvfFeed speedzNumber of passes1 IntroductionThe AWJ technology inheres some desirable advantagesover the conventional cutting processes milling, drillingand turning. These are, for example, the independence ofthe tool wear from the workpiece material, the absence ofrepercussions of the material surface on the cutting abilityof the AWJ and the possibility to cut almost all kindsof materials brittle [1] and ductile [2]. Thus, AWJ is apromising technology, particularly to manufacture difficultto-cut materials such as metal matrix composites (MMC),nickel base alloys [3], titanium aluminides [4], ceramics,ceramic matrix composites (CMC) [5] or carbon fibrereinforced polymers (CFRP) [6]. Since the applicationof such materials is continuously increasing due to thedemands of light weight design and efficiency requirements,the AWJ technology has attracted further attention andthe market is continuously growing over the last years.However, the attainable surface quality of AWJ machiningis limited. If a very high surface roughness is requiredfor example for aerodynamic parts, AWJ machining mightnot fulfil these requirements. Consequently, a finishingoperation, e.g. grinding, is required [7]. Therefore, theinvestigated AWJ technology is considered to be a near-netshape fabrication technology.Conventionally, the AWJ technology is a cutting processfor sheet metal and other flat materials (Fig. 1a) [1].However, since the producible geometries of this processare limited, further AWJ operations have been proposed andstudied. Turning, milling and drilling [8] are some of theprocesses that can be adopted with the AWJ and are appliedtoday. Milling without masks, shown in Fig. 1b, is a processof particular interest since this operation enables furthergeometrical design leeways [9]. AWJ milling operationshave been tested and qualified for a number of materialsincluding titanium aluminides [10]. To apply AWJ milling,the process parameters are usually changed. The waterInt J Adv Manuf Technol (2020) 111:685–693pressure p is reduced, the feed speed vf and the number ofpasses z are increased. These parameter settings generatea better surface quality but reduce the material removalrate. Because of the high feed speed acceleration anddeceleration procedures of the manufacturing machine haveto be considered for this operation [11]. To quickly designAWJ milling operations, van Bui et al. [12] have suggestedthe use of the Gaussian curve and its superposition todescribe the material removal of the operation. Although alot of fundamental and application knowledge about AWJmilling operations has been achieved [13], the applicationof the technology for industrial purposes is limited. The lowuse of the technology might be due to the decreased materialremoval rate and thus the long manufacturing time [9]. Inorder to increase the efficiency of the AWJ milling process,the superposition of two kerfs, cut from different sides ofthe workpiece (Fig. 1c), was suggested [14] and studied indetail by Faltin [15] and in previous investigations regardingthe modelling possibilities [4], the implementation [16] andthe cost-effectiveness [17] of the approach.Faltin [15] demonstrated the feasibility of the approachand provided fundamental knowledge for its application.Furthermore, a model has been introduced to effectivelydesign these AWJ milling operations in a previous work [4].Following this approach, the AWJ milling operations can bedesigned and predicted by the use of a power coefficient cpand three coefficients c1 to c3 for the development of a kerfover the number of passes z. The formulae can be convertedto formula 1, which calculates the feed speed vf (dK )necessary to attain a certain kerf depth for a given waterpressure p and the number of passes z.vf (dK ) p · cp · dK (z)p · cp · (c1 c2 · z c3 · z2 ) dK · dK (z1 )dK · (c1 c2 c3 )(1)Both the modelling study [4] and the analysis by Faltin[15] consider only the cutting of constant kerfs depths(Fig. 1c). In order to further increase the produciblegeometries and to enhance the effectiveness of the presentedefficient AWJ milling operation, it is necessary to adjustthe kerf depth depending on the part design. Consequently,kerfs with variable kerf depths are investigated in thispaper. Kerfs with variable kerf depths are necessary, e.g.for the manufacturing of a turbine blade. The operationsof interest are cuts A and B (Fig. 1d). Once all cutscan be designed, the entire turbine blade including theblade and the fir tree connection can be manufacturedby the efficient AWJ milling operation. In this study,titanium aluminide is considered workpiece material. Thismaterial is one of the above-described high-performance butdifficult-to-cut materials. This material resists high stressesas well as high temperatures while offering a better strength

Int J Adv Manuf Technol (2020) 111:685–693687vfvfvfVrb)a)xAyc)vf (x)zCvfd)Bvf (x)Fig. 1 AWJ process variations: a cutting; b controlled depth milling; csegment removal by controlled depth cutting; d machining of a turbineblade using segment removal by controlled depth cuttingto density ratio than nickel base alloys and thus promisesfurther improvements in light weight design. Furthermore,titanium aluminides are partly used in gas turbines already.However, the manufacturing of titanium aluminides usingconventional cutting remains a challenge [15]. Hence, theapplication of titanium aluminides might be promoted by amore efficient manufacturing technology.To achieve variable kerf depths dK (x), the number ofpasses z, the water pressure p or the feed speed vf can beadjusted. Since the feed speed vf (dK ) can be adjusted veryprecisely, this parameter is tested to ensure a homogeneousjet at all times. In the previous investigations [4], the useof an angle of cut αc for a milling operation has showna distinct influence on the kerf parameters. Comparableeffects must be expected for kerfs with variable kerfdepths dK (x) as well.2 Experimental setupIn order to investigate the possibility of cutting kerfs withvariable kerf depths dK (x), a series of experiments werecarried out. The investigation comprised a detailed analysisof the water jet’s behaviour and its deflection when cuttingthe desired concave and convex shapes. First, the generalbehaviour of the water jet was examined and visualized witha high-speed camera to better understand the fundamentaleffects occurring during the cutting. Secondly, an analogytest was performed in order to evaluate the strength of theseeffects. Third, a test plan was carried out cutting the desiredkerfs with variable depths.All experiments were performed by a robot-guidedwater jet machine type HRX 160 L by STM STEINMOSER GMBH, Schweinfurt, Germany (Fig. 2c). Thecutting head was equipped with an orifice with a diameterof do 0.25 mm, and a focus tube with a length oflf 76.2 mm and a diameter of df 0.76 mm. Astand of distance of ls 2 mm was applied for alltests (Fig. 2a). Garnet mesh size 120 of GMA GARNET(EUROPE) GMBH, Hamburg, Germany, was used to cutthe test material titanium aluminide, type Gamma-TiAlTi-43,5Al-4Nb-1Mo 0,1B (TNM-B1). All kerf depths dKwere measured using an optical measurement deviceMICROPROF MPR 100 by FRT GMBH, Bergisch Gladbach,Germany. Three measurements were conducted per run.For the high-speed recording experiments, the camerawas placed in front of the specimens, which were preparedin convex and concave shapes with a radius of r 30 mmand a depth of ld 1 mm. The length of the specimenwas lW 60 mm (Fig. 2a). The specimens were fixed inbetween two acrylic glass panes which had a squared shape.Thus, a convex and concave kerf was constructed whichenables the recording of the AWJ effects in the kerf withthe high-speed camera. Each video showed one specimenbeing machined once, number of passes z 1, by the AWJfrom one edge to the other for all parameter combinationgiven in Table 1, except the given angle of cut αc . Everyvideo was analysed in regard to the opening angle of thejetlag as well as the opening angle of the jet forerun atseveral workpiece positions pW . Additionally, the ratio ofthe intensity of the jetlag and the jet forerun was evaluated.The analysed positions were set in 5-mm steps betweenthe specimen’s edges. Thus, 176 samples of the AWJ’sdistributions were collected. The high-speed camera used toperform recordings of these first cutting experiments wasa FASTCAM SA1.1 by PHOTRON DEUTSCHLAND GMBH,Reutlingen. The FASTCAM SA1.1 records video datawhich allowed frame-by-frame analysis. This providedthe possibility to select valid image data by manuallychoosing an appropriate frame. The video was recordedwith 10,000 frames per second (fps) and a resolution of512 512 pixels. For the lighting of the experimentalsetup, spotlights from the front and from the backwere used to obtain sufficient brightness in the imagery(Fig. 2b).The analogy tests were conducted applying the sametest plan from the high-speed recording test regarding theparameters water pressure p, feed speed vf and abrasive flowrate ṁA . The setup was modified in a way that the angleof cut αc remains constant for one cut (Fig. 2d). Hence, thefactor “Shape” (Table 1) was replaced by the constant anglesof cut αc(y) . The setup comprised a primary target part (PTP)

688Int J Adv Manuf Technol (2020) 111:685–693Fig. 2 Experimental setup: ainput and target values of thehigh-speed recording andapplication tests; b setup of thehigh-speed recording tests; cwater jet machine HRX 160 Lby STM STEIN-MOSERGMBH, Schweinfurt, Germany;d input and target values of theanalogy testvfyacrylicglass panelsdK, minαjfxzpWαjldK, maxconvexspecimen15 lled beneath the jet under the angles of cut αc(y) . Thejet moved along the x-axis causing a kerf and a jet deflectiontowards a secondary target part (STP). To investigate theintensity of the jet and the strength of its deflection, theresulting kerf depths on the primary dK,p and the secondarydK,s target parts were measured for 32 parameter settings.The application tests comprise the milling of kerfs withvariable kerf depth dK (x). The tests were carried out bythe adaption of the feed speed vf (dK ). The applicationtests were set up to mill the shapes described in the highspeed recording experiments with a radius of r 30 mm, amaximum kerf depth of dK,max 30 mm and a minimumkerf depth of dK,min 15 mm (Fig. 2a). The values fordK, pdK, sthe feed speed vf (dK ) were derived using formula 1 withthe coefficients and parameters given in Table 2. Theseparameters predict the kerf depth dK of constant kerfs. Themilling of variable kerf depths dK (x) is likely to influencethe kerf depth beyond the effects described by formula 1.This influence can be expected since the cutting conditionsare changed compared with the cutting of constant kerfdepth. Therefore, a test plan (Table 2) was performed tofind suitable parameters. The tests were preformed twiceto ensure repeatability. In order to measure the kerf depths,the specimens were separated along the kerf using EDM.Afterwards, the kerf depth was measured on the remainingkerf profiles every 2 mm.Table 1 Experimental design for high-speed recording and analogy testsParameterLevels Water pressure pFeed speed vfAbrasive flow rate ṁAShapeAngle of cut αc(y)Number of passes zMPamm/ming/min -22.51 1003000150Convex45 2005000250Concave67.590

Int J Adv Manuf Technol (2020) 111:685–693689Table 2 Experimental design for the application testsParameterLevelsWater pressure pMax. feed speed vf , minMin. feed speed vf , maxShapeAbrasive flow rate ṁANumber of passes zPower coefficient cp [4]Coefficient c1 [4]Coefficient c2 [4]Coefficient c3 [4]MPamm/minmm/ming/min-3 ResultsThe main effects of the opening angles observed duringthe high-speed recording investigations are shown in Fig. 3.The diagram shows that in average the opening angles areapproximately two times higher for the concave geometries.In addition, the opening angle of the jet forerun for concaveProcess:AWJ millingTools:Garnet, Mesh 120, GMAdO 0.25 mmdF 0.76 mmlF 76.2 mmWorkpiece:TNM-B1 γ-TiAlvfProcess parameters:mmls 2z 1Opening angle of jetlag concave αjl, aOpening angle of jetlag convex αjl, xOpening angle of jet forerun concave αjf, aOpening angle of jet forerun convex αjf, xopening angle αj80 402000146341530mmposition on the workpiece pW6011890 34angle of cut αc for a concave shape6214690 angle of cut αc for a convex shapeFig. 3 Main effects of the jetlag and the jet forerun opening angles 10024004800Convex1503007.490.1360.101 0.22e 412530006000Concaveshapes αjf,a is higher for high angles of cut (αc 90 ) atthe beginning of the workpiece. The jetlag of the concavegeometry αjl,a shows a reversed behaviour and has higheropening angles for lower angles of cut (αc 90 ) atthe end of the workpiece. The convex geometry shows anopposite behaviour compared with the concave geometry,considering the position on the workpiece pW . If the angleof cut αc is considered, the opening angle of the jet forerunαjf,x is as well higher for high angles of cut αc 90 . Inaddition, the opening angle of the jetlag αjl,x is higher forlower angles of cut αc 90 .Besides the opening angles, the intensity of the jetlag andthe jet forerun have been analysed. This observation resultedin a linear increase of the jetlag’s intensity. The increase wasfound for the concave geometry between the position of theworkpiece pW 15 to 45 mm. Correspondingly, the effectsare reversed for the convex geometry and the jet forerun.The main effects of the kerf depth of the analogy testare depicted in Fig. 4. The diagram shows that the primarykerf depth dK,p increases with increasing angle of cut αc ,starting at αc 22.5 until the kerf depth reaches a peak atαc 67.5 . For the angle of cut αc 90 , the kerf depthis reduced. The secondary kerf depth dK,s continuouslydecreases with increasing angle of cut αc until dK,s 0 mmat an angle of cut of αc 90 . Considering the setup ofthe tests, the results can be mirrored by αc 90 to higherangles. Thus, the value of the angle of cut of αc 67.5 also applies for the angle of cut of αc 112.5 and theangle of cut of αc 45 for the angle of cut of αc 135 ,and the angle of cut of αc 157.5 equals the angle ofcut of αc 22.5 . In Fig. 4, only the values of the angleof cut of αc 67.5 are mirrored towards the angle of cutof αc 112.5 .Figure 5 shows the results of the convex kerfs of theapplication experiments. The black line marks the targetkerf. The arrow bars indicate the standard deviation s of

690Int J Adv Manuf Technol (2020) 111:685–693Process:AWJ millingWorkpiece:TNM-B1 γ-TiAlProcess parameters:ls 2mmz 1Tools:Garnet, Mesh 120, GMAdO 0.25 mmdF 0.76 mmlF 76.2 mmxPrimary kerf depth dK, pSecondary kerf depth dK, s zCombined kerf depth dK, p svfySTPαc(y)PTPkerf depth dK0.16mm0.080.04022.54567.5 112.5angle of cut αcFig. 4 Main effects of the kerf depth caused by the primary and thesecondary jetthe kerf depth. The diagram shows that the results are welldistributed around the target kerf. All kerfs seem to fit thetarget kerf in a sufficient manner. The difference caused bythe parameter settings does not change the shape of the kerfand the average difference between lowest and the deepestkerf depth is dK,diff 10.7 mm. The best kerf regardingthe convex shape seems to be the parameter with high feedspeed and high water pressure p.The kerf depth results of the concave shape are morediversified (Fig. 6). In comparison with the convex shape,the kerf depths of the different parameter combinationsare much further apart, with an average difference of thekerf depth of dK,diff 20.4 mm. Furthermore, none of theparameter settings was able to fit the concave shape flawlessregardless of the depth. Notably, most of the curves seem tohave a flattened beginning and end.4 DiscussionThe results of the high-speed recording investigationsdemonstrated that there is a general difference regarding theopening angles between convex and concave geometries.Fig. 5 Results of the application test: kerf depth dK of the convex kerfswith variable kerf depthFurthermore, the test reveals that all opening angles arelow at very high αc 146 and very low αc 34 angles of cut. The cutting intensity of the jetlag or the jetforerun is likely to depend on the opening angles and

However, the machining of these typically difficult-to-cut materials poses a challenge for conventional manufacturing technologies due to the high tool wear. Abrasive water jet (AWJ) machining is a promising alternative manufacturing technology for machining difficult-to-cut materials,

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