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

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,