Effects of process parameters on numerical control bending process for large diameter thin-walled al

Available online at www.sciencedirect.comTransactions ofScienceDirectNoferrous Metals总ScienceSociety of ChinaELSEVIER PressTrans. Nonferous Met. Soc. China 19(2009) 668-673wwu .tnmsc.cnEffects of process parameters on numerical control bending process forlarge diameter thin-walled aluminum alloy tubesLI Cheng(李成)', YANG He(杨合)', ZHAN Mei(詹梅)', XU Xu-dong(许旭东), LI Guang-jun(李光俊)'1. State Key Laboratory of Solidification Processing. School of Materials Science and Engineering,Northwestem Polytechnical University, Xi'an 710072, China;2. Chengdu Aircraft Industry (Group) Corporation Ltd, Chengdu 610092, ChinaReceived 23 May 2008; accepted 16 October 2008Abstract: Numerical control(NC) bending experiments with different process parameters were caried out for 50520 aluminumalloy tubes with outer diameter of 70 mm, wall thickness of 1.5 mm, and centerline bending radius of 105 mm. And the effects ofprocess parameters on tube wall thinning and cross section dstortion were investigated. Meanwhile, acceptable bending of the50520 aluminum tubes was accomplished based on the above experiments. The results show that the effects of process parameterson bending process for large diameter thin-walled aluminum alloy tubes are similar to those for small diameter thin-waled tubes, butthe forming quality of the large diameter thin-walled aluminum aloy tubes is much more sensitive to the process parameters and thusit is more dificult to form.Key words: large diameter thin-walled aluminum alloy tubes; small bending radius; numerical control bending processwrinkling. And thus, it is necessary to study forming1 Introductionlaws in NC bending processes for large diameterthin-walled aluminum alloy tubes with small bendingThe research and development of numerical controlradi, particularly, effects of process parameters. Much(NC) bending processes for large diameter thin-walledresearch work, using FEM and experiments, has beenaluminum alloy tubes with small bending radi is notcarried out on the thin-walled tube bending processes,only one of the demands urgently to be solved in theand the stress and strain distribution, tube wall thinning,manufacturing advanced airplanes, but also one of thecross section ovalization and other defects in the NCfrontiers in advanced plastic forming fields. However,bending bave been studied[3- -16]. However, there haveNC bending processes of thin-walled tubes are verynot been sufficient studies in the literature on NCcomplex tri-nonlinear physical processes withbending processes for large diameter thin-walledmulti-factor coupling interactive effectsunderaluminum alloy tubes with small bending radii.multi-die[1- -2]. Thus, if there is any deviation of processIn this work, the experiments were carried out toparameters, it is possible for the tubes to produceinvestigate the effects of process parameters on tube wallover-thinning, section distortion or even wrinkling, andthinning and CrOss section distortion during NC bendingthese phenomena occur more easily for large diameterprocesses for large diameter thin-walled aluminum alloytubes with small bending radii.radi. The key to realize stable and precise bendingforming is to select sound process parameters in order to2 Experimentalcontrol the stress and strain states, and thus the degreesof ovalization and thinning of the bending tubes can be2.1 Exneriment equninmentcontrolled to some acceptable extent under free中国煤化工aried out using theFoundation item: Project(50225518) supported by the National Science FoundationYHC N M H Golars Pise965650175092) supported by the National Natural Science Foundation of China; Project(04H53057) suppored by the Aviation ScienceFoundation ofChinaCorresponding author: YANG Hc, Tel: +86 29-88495632; Fax: +86 29-88495632; E-mail: yanghe@nwpu.edu.cnDOI: 10.1016/51003- 632608)6033-3LI Cheng, et al/Trans. Nonferrous Met. Soc. China 19(2009) 668 -673669NC hydraulic tube bender W27YPC-159, which wasthickness ranged from 0.15 mm to 25.40 mm; and theequipped with push assistant and booster, as shown insound wave ranged from 10 MHz to 22 MHz. Due to theFig.1. Its bending capacity was up to d159 mm (outerhigh precision degree, the error was only about 1%.diameter)X 12 mm (wall thickness). The largest and theTherefore, the ultrasonic thickness meter PX-7DLsmallest centerline bending radii were 450 mm and 105satisfies the requirement to the experiment precision.mm, respectively. The rotational velocity of the bendingCross section distortion can be calculated by thedie (bending velocity) ranged from 0 r/min to 0.8 r/min.equation:T= Dmax二Dmin x 100%(1)Dmaxwhere Dmin is the minimum diameter of the formed tube,and Dmax is the maximum diameter of the formed tube.The larger the T, the more serious the cross sectiondistortion.3 Results and discussion3.1 Forming quality of tubesIn this work, acceptable bending of the 50520Fig.1 NC hydraulic tube bender W27YPC-159aluminum tubes with size d70 mm (outer diameter)X 1.5mm (wall thickness)XR105 mm (centerline bending2.2 Materials and experimental parametersThe 50520 aluminum tubes (d70 mmX1.5 mm)radius) has been accomplished on the basis of correctlystudied in this work are widely used in aerospace, sincesetting tooling (Fig.2). The bent tubes have goodtheir strength can be improved by cold-work hardening.out-surface quality without wrinkling, cracking or hump.The basic mechanical properties (Table 1) of the tubesThe measurement results show that the maximum wallwere obtained using uniaxial tensile test, in which thethinning of the tubes with different bending angles is lessspecimans were intercepted along the tube axes.than 25%, and the maximum cross section distortion isless than 5%.Table 1 Mechanical properties of 50520 aluminum tubeE/GPa80:,2/MPaσyMPa0.3378203.9E is clastic modulus; is Possion rati; 8 is elongation percentage; 02 isyield stess; and 6% is strength sress.The experimental parameters were as follows: thebending velocity ranged from 1.0 ()/s to 3.2 ()/s; thepush assistant velocity of the pressure die ranged from3.50 mm/s to 6.10 mm/s; the mandrel extension lengthranged from 10 mm to 20 mm; and the mandrel had 2 orFig.2 Formed thin- walled aluminum alloy tubes with different3 balls.bending anglesIn addition, these experiments were carried out atroom temperature. Draw oil S980B diluted with coal oil3.2 Effects of push assistant on tube wall thinningwas used as antifriction material between the mandrelPush assistant applies an axial pushing force on theand the inwall of tube. The wiper die was lubricated withtube outside to reduce the tension stress in the bendinga ltte thinned oil. Dies were set properly to reduce theprocesses, so that the tube wall thinning can be reducedrisk of wrinkling and the cross-section distortion degree.and acceptable bending is obtained. Thus, the pushassistant parameters have influence on the forming2.3 Measurement methodsquality, In this wnrk. two imnortant parameters wereIn this work, an ultrasonic thickness meter PX-7DLexperin中国煤化工he push assistantwas used to measure the thickness of the bent tube wall.velocityMHCNMHG_essure die and tbeThe performance parameters were as follows: thout-surtresolution factor was up to 0.01 mm; the sonic speedThe results show that the push assistant velocity hasranged from 1.25 km/s to 10 km/s; the measurablea neglectable infuence on the tube wall thinning. Fig.3670LI Cheng, et al/Trans. Nonferrous Met. Soc. China 19(2009) 668-673indicates that the maximum wall thinning is less thanThese experimental results are similar to those of24% in every experiment and the differences betweenthe aluminum alloy tubes with size d50 mmX1 mmXtwo experiments are all less than 3%, which satisfies theR100 mm[8]. However, the effects of the friction on therequirement of the aerial standard. Consequently, it iswall thinning are more remarkable in this work.comparatively easy to set the push assistant velocity toobtain acceptable bending in either experiments or actual3.3 Effects of bending velocity on bending qualityproduction.The bending velocity has a significant influence onthe forming quality of the bent tubes. In order to study26 rthe influence, experiments were carried out at differentbending velocities. The other experimental conditions22were as follows: the push assistant velocity of thepressure die was 3.86 mm/s; the mandrel extension8-length was 14.5 mm; the centerline bending radius wasR105 mm (1.5D); and the friction between the pressure4die and the out- surface of tube was dry friction.It is found that the 50520 aluminum tubes are0textremely sensitive to the bending velocity, and they mayeasily crack when the velocity is larger than 3.9 (°)/s一Push assistant velocity of 5.86 mm/s一Push assistant velocity of 4.5 I mm/s(Fig.5), since the hardening value of aluminum alloy- Push assistant velocity of 3.80 mm/sis small. Furthermore, the hydraulic tube bender mays0operate astatically after long-time working, which causesAngle between certain section andfast heightening of the oil temperature and instability ofbending plane/(' )the hydraulic and electric systems. Experimental resultsshow that the bending velocity of 1.9 ()/s selected in thisFig.3 Wall thinning rate with different push assistant velocitieswork is reasonable, at which the risk of cracking can bereduced and the forming quality is fine.However, it is found that the friction between thepressure die and the out-surface of tube has a significantinfluence on the tube wall thinning (Fig.4). It can be seenthat 1) larger friction can increase the pushing force ofthe push assistant and improve the wall thinning, while itcan also increase the inside wall thickening， andaccordingly wrinkling may occur; 2) smaller friction canreduce the pushing force and the inside wall thickeningto lessen the risk of wrinkling， while it can alsoaggravate the outside wall thinning. So, the key toobtaining acceptable bending is to select suitable frictionaccording to the requirement of actual manufacturing.Fig.5 Tube crack shape at high bending velocity0.3-3.4 Effects of mandrel extension length on tube wall0.2thinningThe mandrel extension length is an important0.1parameter in the tube bending processes. Wrinkling mayInner thickness strain, small fricktionoccur when the length is too small, and otherwise the口一Outer thickness strain, small fricktionoutside of tube may fracture with over large length. InInner thickness strain, large fricktionthis work, bending processes were investigated with the-0.1Outer thickness strain, large fricktionmandrel extension lengths of 10 mm, 15 mm and 19 mm,respectively when ) mandrel halls are used. It is foundthat n中国煤化工in these cases.20405080100YHCNMHGthinning rate withdiffercil lsallulcl CALCIISIUII lcl1guis.s found that thelarger the mandrel extension length, the more serious theFig.4 Thickness strain with different pressure frictionwall thinning. When the length is 10 mm, the maximumLI Cheng, et al/Trans. Nonfrrous Met. Soc. China 19(2009) 668 67367wall thinning is 22.7%; when the length is 15 mm, theo the requirements of experiments or actualmaximum wall thinning is 23.5%; and when the length ismanufacturing. To investigate the effects of ball numbers19 mm, the maximum wall thinning is 25.7%. Theon the bending quality, experiments were carried oureason for this is that larger mandrel extension lengthwith 2 and 3 balls, respectively, when the mandrelincreases the friction between the front-end and the tubeextension length was 14.5 mm. The results are shown ininwall to bafftle the flowing of the material, then largerFig.7 and Fig.8.tangent strain and tube wall thinning are caused. Overlarge length may cause hump or even cracking.28 r0r- Mandrel extension length of 19 mm●一Mandrel extension length of I5 mm4|8-10-:二骨2/of 10mmxension lengh204080~100Angle between certain section and5030bending plane/(" )Fig.7 Infuence of ball numbers (n) on wall thinning rateFig.6 Wall thinning rate with different mandrel extensionlengthsThese results are similar to those of the stainlesssteel tubes with size d50 mmX 1 mmX R75 mm[6] andthe aluminum alloy tubes with size d50 mmXl mmXR100 mm[8], while the effects of the mandrel extensionlength on the tube wall thinning are more obvious in thiswork.To avoid over-thinning, proper mandrel extension:二length should be selected in bending processes. Anequation to calculate the maximum mandrel extension0length is put forward[7]:bending plane/(°)emax=v(R+D/2-1)2 -(R+d/2)2 +r(2)Fig.8 Infuence of ball numbers () on cross section distortionwhere D is the outer diameter of the tube; d is theFig.7 shows the tube wall thinning rate with variousmandrel diameter; r is the round radius of the mandrel; Iball numbers. It can be seen that the maximum wallis the tube wall thickness; and R is the centerline bendingthinning with 2 balls is 21 3%, while the maximum wallradius.thinning with 3 balls is 26.9%. Consequently, the tubeIn particular, for the tubes with extremely lowall thinning is more serious with increasing ballelongation percentage, test bending after calculatingnumbers. It can be proved that more balls can increasemandrel extension length by the equation is needed,the friction between the balls and the tube inwall, whichotherwise cracking may easily occur. In this work, thebaffles the flowing of material, and accordingly causesmaximum mandrel extension length calculated is 21.8larger tangent strain and more serious tube wall thinning.mm, which is in agreement with the experimental results.Fig.8 s中国煤化工ection distortion is3.86%um cross section3.5 Effects of ball numbers on bending qualitydistortiYHCNMHGbesenthatmoreMandrel balls used in this work are detachable, thusmandrel balls improve the cross section distortionit is convenient to change the number of balls accordingefficiently, because more mandrel balls can support a672LI Cheng, et al/Trans. Nonferrous Met. Soc. China 19(2009) 668-673wider range of curved surface.because larger mandrel ball diameter increases thThese results are similar to those of the stainlessfriction between the balls and the tube inwall, andsteel tubes with size d38 mmX1 mmX R57 mm[6],accordingly causes larger tangent strain and more seriouswhile the effects of ball numbers on bending quality aretube wall thinning.more remarkable in this work.From Fig.10, it is found that the larger the mandrelball diameter, the smaller the cross section distortion.3.6 Effects of ball diameters on bending qualityThe maximum cross section distortion with mandrel ballThe mandrel ball diameter is a significantdiameter of 65.2 mm is more than 3.8%; however, thedimension parameter since it greatly influences themaximum cross section distortion with diameter of 66.5bending quality. In this work, bending processes weremm is only 3%. It can be concluded that larger mandrelinvestigated with the mandrel ball diameters of 65.2 mmball diameter can support the inwall of the tube moreand 66.5 mm, respectively, when 2 mandrel balls wereeffectively to improve cross section distortion.used. The results are shown in Fig.9 and Fig.10.4 Conclusions281) It is found that the effects of process parameterson bending processes for large diameter thin-walledaluminum alloy tubes are similar to those for smalldiameter thin-walled tubes, but the forming quality of thelarge diameter thin-walled aluminum alloy tubes is muchmore sensitive to the process parameters and thus it ismore diffcult to form.2) In NC bending processes of large diameterF 10■一Ball diameter of 65.2 mmthin-walled aluminum alloy tubes, the friction between●一Ball diameter of 66.5 mmhe pressure die and the out-surface of tube has asignificant influence on tube wall thinning, while the68000push assistant velocity affects tube wall thinning slightly.Angle between certain section andLarger friction can improve the outside tube wallbending plane/( )thinning, while it can also increase the inside tube wallFig9 Influence of ball diameters on wall tinning ratethickening, which is possible to produce wrinkling.3) The mandrel parameters have more obviousinfluence on the bending quality for the large diameterthin-walled aluminum alloy tubes than for the smalldiameter thin-walled tubes. Larger mandrel extensionength aggravates tube wall thinning, even causescracking. More mandrel balls or larger mandrelball2-diameter can improve cross section distortion, andaggravate tube wall thinning.4) Acceptable bending of the 50520 aluminumtubes (d70 mmX 1.5 mmX R105 mm) is accomplished: - Ball diameter of 65.2 mmsuccessfully, showing that though the NC bending ofBall diameter of 66.5 mmlarge diameter thin-walled tubes with smaller bending4060100radi is difficult to realize, and qualified bent tube can beobtained when proper process parameters are selectedbending plane/(°)and adopted.Fig.10 Influence of ball diameters on cross section distortionReferencesFig.9 shows that the maximum wall thinning withmandrel ball diameter of 65.2 mm is 21.3%; and the[1]中国煤化iviewv orndvmcd plusiesmaximum wall thinning with diameter of 66.5 mm isfacing 21st cenury [CV/ .27.1%. 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Journal of Materialsof oval twbes using rotary draw bender for hydroforming applicationsProcessing Technology, 1996, 58(2/3): 274 -285.[小Journal of Materials Processing Technology, 2005, 168(2):(Edited by YANG Hua)中国煤化工MYHCNMHG