Effects of process parameters and die geometry on longitudinal welds quality in aluminum porthole di

J. Cent. South Univ. Technol. (2010) 17: 688- 696包SpringerDOI: 10. 1007/11771-010-0541-1Effects of process parameters and die geometry on longitudinal welds quality inaluminum porthole die extrusion processLIU Jian(刘健), LIN Gao-yong(林高用), FENG Di(冯迪), Z0U Yan-ming(邹艳明), SUN Li-ping(孙利平)School of Materials Science and Engineering, Central South University, Changsha 410083, ChinaC Central South University Press and Springer-Verlag Berlin Heidelberg 2010Abstract: By using the rigid-visco-plasticity finite element method, the welding process of aluminum porthole die extrusion to forma tube was simulated based on Deform-3D software. The welding chamber height (H), back dimension of die leg (D), processvelocity and initial billet temperature were used in FE simulations so as to determine the conditions in which better longitudinalwelding quality can be obtained. According to K criterion, the local welding parameters such as welding pressure, effective stress andwelding path length on the welding plane are linked to longitudinal welds quality. Simulation turns out that pressure-to-effectivestress ratio (p!o) and welding path length (L) are the key factors affecting the welding quality. Higher welding chamber best andsharper die leg give better welding quality. When H=10 mm and D=0.4 mm, the longitudinal welds have the best quality. Higherprocess velocity decreases welds quality. The proper velocity is 10 mm/s for this simulation. In a certain range, higher temperature isbeneficial to the longitudinal welds. It is found that both 450 and 465 °C can satisfy the requirements of the longitudinal welds.Key words: aluminum alloy; longitudinal welds; porthole die; die geometry; extrusion process; K criterionafter introducing the time factor. This criterion greatly1 Introductionemphasizes the dead zone in the welding chamber. Basedon this observation, DONATI and TOMESANI [6]In recent years, aluminum alloy hollow profiles areintroduced speed as a correction factor and proposed theincreasingly used in manufacturing processes, mainly forK criterion (pressure -time -flow criterion). By applyinglightweight components in the vehicles for ground, seathese criteria to experiments performed by VALBERGand air transportation. The hollow profile producedet al [7-8] and FE simulation [6], it was found that Kthrough porthole die contains a number of seam weldscriterion can discriminate the welding quality efficiently.along its length, which are called the longitudinal welds.Meanwhile, FANG et al [9] demonstrated that 3D FEMIt is a common view that cracking happens preferentiallysimulation is a viable predictive tool in both dienear the welding lines of the extruded profiles [1]. Thedesigning and process optimization for any extrudedfailure of hollow extruded products mostly occurs alongshapes. LIU et al [10] simulated the longitudinal weldsthe weld lines when the products are subject to severeformation process during porthole die extrusion andinternal pressure or expansion in the practical use [2]. So,evaluated the factors determining the quality of the weldthe welds often represent the weakest points of theseams. LI et al [11] suggested that the ratio of theprofile's section. Therefore, it is of particular importancemaximum normal pressure in the welding chamber to theto increase the welding quality of longitudinal welds.flow stress of the billet material on the welding planeSo far, few studies have been done on longitudinaldetermined the weld quality. DONATI and TOMESANIwelds. Earlier researches about longitudinal welds12] defined the workability area under differentformation process were performed by AKERET [3- 4],processing conditions and used tensile strength andwho considered once the maximum pressure inside theequivalent fracture strain to assess the effectiveness ofwelding chamber exceeds a critical limit, which dependslongitudinal welds.on the physical state of the material at that point, thewelding could be assessed as available. In FE analysis,parameters and die geometries on longitudinal weldsthe criterion can be normalized by rating pressure toquality were analyzed by means of thermo-mechanicaleffective stress at a specific point. PLATA and PIWNIKFE simulation and according to K criterion. The process[5] proposed the Q criterion (pressure- time criterion)parameters an中国煤化工the weldingFoundation item: Project(2007BAE38BO4) supported by the National Science and Technology PillMYHCNMHGReceived date: 2009- 11-25; Accepted date: 2010-03- 12Corresponding author: LIN Gao-yong, PhD, Professor; Tel: +86- -731- 88879341; Fax: +86- 731-88876692; E-mail: mater218@ 163.comJ. Cent. South Univ. Technol. (2010) 17: 688- -696_689chamber height (H), back dimension of die leg (D),Table 1 Conditions in porthole die extrusionprocess velocity and initial billet temperature. TheCondition_Valueprocess conditions and parameters for achieving goodBillet temperature/C420, 450, 465, 480 .longitudinal welds quality were found.Extrusion velocity/(mms ")1,5, 10, 152 FE simulationWelding chamber height/mm4, 7, 10Extrusion ratio222.1 Conditions of FE simulationHeat transfer cofficient/(W-m-2.K)3 000According to the rigid-visco-plasticity FE method,Shear factor0.8the welding process of porthole die extrusion isCoulomb factorsimulated by Deform-3D software. The schematic tooassembly used in the porthole die extrusion is shown inhe dimensions of the porthole die used in thisFig.1. During the extrusion process, materials dividedwork are shown in Fig.3. An AA6061 billet of d32 mmXthrough portholes are gathered within the welding25 mm is preheated and then extruded through thchamber and then welded together, and the weldabilityporthole die to produce tube profile, which is 1.5 mm incan be affected by process parameters and die geometries.thickness. The billet is meshed with tetrahedral elements, .The process variables used in FE analysis are presentedand its heat exchanges with the tools are allowed. Tcin Table 1, and back dimensions of die leg are shown inmaintain the efficiency and accuracy of FE simulation,Fig.2.areas in the welding chamber and near the bearing aremeshed with higher element density and finer elements.The total number of elements is 38 144. Adhesion isRamconsidered throughout the tool system, except on theContainerbearing, where sliding friction is adopted [13].Billet2.2 Metal flow in porthole die extrusionThe extrusion process can be divided into threestages: dividing stage, welding stage and forming stage.The dividing stage is shown in Fig.4(a), in which thebillet is divided into six portholes. The welding stage isshown in Fig.4(b), in which divided materials flowUpper diethrough the welding chamber and are welded together togenerate the welding plane. The forming stage is shownin Fig.4(c), in which the welded materials flow throughBottom diethe die exit to form the required tube profile. Consideringthe symmetry of the die assembly, only one-sixth of thebillet is selected to evaluate the factors that determine theFig.1 Tool assembly used in porthole die extrusionquality of the longitudinal welds. Fig 4(d) shows one-.09王中国煤化工a)(b)(c)MYH.CNMHGFig.2 Structures of die legs (Unit: mm): (a) D=4.0 mm; (b) D-2.4 mm; (C) D=1.2 mm; (d) D=0.4 mm690J. Cent, South Univ. Technol. (2010) 17: 688- -696.36, d25d27\Ad36(一t7 d13y8JRII(a)b)Fig.3 Dimensions of porthole die used in this work (Unit: mm): (a) Plane view; (b) Vertical viewa)*y%'C)d)Weldingplane*%"Fig.4 Procedure of porthole die extrusion: (a) Dividing stage; (b) Welding stage; (c) Forming stage; (d) One-sixth of billetsixth of the billet, and the welding plane is pointed outK= I Pdt.v= jPdL， where p is the contact pressure; σby an arrow.中国煤化工is the effectivepath from the3 Results and discussionentrance on theMYHC N M H Git. K criterionAccording to Ref.[6], K criterion can be written as:is composed of two functions: the pressure-to-effectiveJ. Cent. South Univ. Technol. (2010) 17: 688- -696691 _stress ratio (/o) and the welding path length (L). So,better welding quality could be obtained by either(a)increasing the pressure-to-effective stress ratio (p/o) orMean stress/MPathe welding path length (L). The welding plane wasdivided into three regions: dead region, main weldingregion and bearing region. Either H profile or tubeprofile was chosen, this classification method is available.-100In this work, dead region is characterized by a very small-130velocity, the pressure reaches its maximum value from a-17lower one and the effective stress increases from a-200minimum to a higher one. Main welding regioni-230characterized by a decreasing value of contact pressureand an increase in material velocity, meanwhile, the440 Mneffective stress increases slightly. In the bearing region,100 Maxhighest, the effective stress increases to the highest value,and deformation reaches its maximum.I 6)3.1 Effect of welding chamber height on seam weldsqualityDifferent lengths of welding chamber mainly affectthe welding path length and the maximum pressure in thewelding chamber. Simulation was conducted under threedifferent welding chamber heights of 4, 7 and 10 mm,-170initial billet temperature of 450 °C, extrusion velocitychose 10 mm/s and the back dimension of die leg D=-231.2 mm. During the extrusion processing, the welding-270pressure in the chamber is proximately equal to means-300stress.110 MaxWelding pressure distribution on the welding planeis shown in Fig.5. It is easy to find that contact pressureon the welding plane increases as welding chamberC)height rises, resulting in materials being more severelyMeanstressiMPpressured before reaching the die exit. In addition, tharea where the divided materials are pressurizedincreasedly, indicates that the divided materials havemore chance to contact, so the welding path length Lincreases. In the main welding region, the maximumpressure is located at the upper height of the welding4-170plane where two ports encounter. When H=4 mm, themaximum pressure is 180 MPa; when H=7 mm, themaximum pressure is 230 MPa; while H=10 mm, themaximum pressure increases to 250 MPa.-510 MnEffective stress distribution on the welding plane is140 Maxpresented in Fig.6. It can be seen from Fig.6 thateffective stress in the main welding region decreases asthe welding chamber height increases. DeformationFig.5 Welding pressure distribution on welding plane withdecreases from the encounter point toward the die exit,different welding chamber heights: (a) H=4 mm; (b) H=7 mm;and then increases when approaching the die exit where(C) H=10 mmmaterial are pressured to form the required profile shape.Higher welding chamber results in decreasing of theAs mentioned above, in the main welding region,deformation and the effective stress. When H=4 mm, thevith increasing中国煤化Intact pressureminimum effective stress is 52 MPa; when H=7 mm, theincreases whileMYHCNM H Gadecreasingminimum effective stress is 45 MPa; while H=10 mm,tendency, so the pros-tinit ouss ratio (p/o)the minimum effective stress is 43 MPa.increases greatly, and the maximum p/σ values are692J. Cent. South Univ. Technol. (2010) 17: 688- 696path length L increase at the same time, so integral valuea)K increases greatly as the welding chamber heightincreases, and the bigger the welding chamber height, theEffective stress/MPabetter quality the longitudinal welds. Meanwhile, anextra-high welding chamber height (H) greatly increasesextrusion force and prevents the material from flowing58toward the die exit, so, proper welding chamber height5(H) should be chosen during production.3.2 Effect of die leg on seam welds qualityOne major factor regarding die design is legstructure in hollow profile production. The leg representsan obstacle to the material flow towards the die exit,0.03which modifies the distribution of contact pressure and120 Maxmaterial flow [2]. If the shape of the bridge is improperlydesigned, a gas pocket may be formed in this area,b)leading to poor welds quality [11].Effctive stress/MPaFour different leg structures are used in thissimulation, as shown in Fig.2. FromFigs.2(a)- (d), the66back dimensions of die leg (D) gradually decrease fromD=4.0 mm to D=0.4 mm. For this group of simulation,the initial biller temperature is fixed at 450 C, weldingchamber height is 7 mm and extrusion velocity i10 mm/s.47The velocity fields on the welding plane arepresented in Fig.7. The simulation results indicate thatdead zone under the leg decreases as D minishes, whichmeans that the encounter point of two ports on thewelding plane is closer to the back leg, the main weldingregion expands and the welding path length L increases.With decreasing D, the back of the die leg getsC)sharper, and the encounter materials are more severelyEffectivestress/MPapressed to each other, leading to the increase of thewelding pressure in the main welding region. Thewelding pressure distribution on the welding plane canbe seen clearly in Fig.8. In Fig.9, the effective stressdistribution on the welding plane is presented. Just like .the phenomena described in section 3.1, as the weldingath length L increases, there is more chance for thedeformation to decrease before approaching the die exit,so effective stress in the main welding region decreasesslightly with decreasing D, just as shown in Fig.9.According to the analysis above, p/σ and weldingK increases with decreasing D, and the sharper the backFig.6 Effective stress distribution on welding plane withleg, the better the weldability of longitudinal welds. Atdifferent welding chamber heights: (a) H=4 mm; (b) H=7 mm;the same time, the strength and stiffness of die leg will(c) H=10 mmbe maintained.respectively 3.46, 5.11 and 5.81 when welding chamber3.3 Efect of p”中国煤化Ids qualityheights are 4, 7 and 10 mm. In addition, the welding pathProcess vher importantlength (L) increases from the entrance to the die exitparameter afceTYHCNMHGhisthemain.synchronously. According to K criterion, p/o and weldingfactor that influences the time for the material flowingJ. Cent. South Univ. Technol. (2010) 17: 688- -696693b)velocity/(mm:sI)Velocity/(mm-sI)Velocity/(mm-s-)Velocity/(mm-s-")6Fig.7 Velocity fields on welding plane with different back dimensions of die leg: (a) D=4.0 mm; (b) D=2.4 mm; (C) D=1.2 mm;(d) D=0.4 mmMean sressMPa. Mee Sesss-170-200-230300110M|120 Mak00中国煤化工THC NMHGFig.8 Welding pressure distribution on welding plane with different back dimensions or ane 1eg: (a) D=4.U mm; (b) D=2.4 mm;(c) D=1.2 mm; (d) D=0.4 mm694J. Cent, South Univ. Technol. (2010) 17: 688- -696bEffective StressMPaEfcctive SesSMPa505015160 MaxCdJ， Efective stesiMPaEffective stresSMPa0售0|30Fig.9 Effective stress distribution on welding plane with different back dimensions of die leg: (a) D=4.0 mm; (b) D=2.4 mm;(c) D=1.2 mm; (d) D=0.4 mm .through the welding chamber. But LIU et al [10] thoughtcontainer and the porthole die; when v=5 mm/s, heatthe contact time between two metal streams just has ageneration oversets the heat loss, leading to the rise ofminor effect on the welding quality, instead, the yieldingtemperature, and the maximum temperature near the diestrength of the material, which is a function ofexit is 490 C; while v=10 mm/s and v=15 mm/s, thetemperature and welding pressure is of more importancemaximum temperatures near the die exit are respectivelyto the welds quality. Therefore, the effect of process510 and 530 C. According to Ref.[14], the temperaturevelocity on the welds quality is actually exerted throughnear the die exit should be lower than the solutionits effect on temperature, welding pressure and effectivetemperature, which is 530 C for AA6061. Meanwhile,stress rather than on contact time.niger temperature is beneficial to bonding, so, 510 °CFour different process velocities are chosen in thiswill be proper, and 10 mm/s is the proper velocity forsimulation, which are 1, 5, 10 and 15 mm/s; the initialthis simulation.billet temperature is 450 °C, welding chamber height isOther numerical simulation results are shown in7 mm and die leg is the type in Fig.2(c).Table 2. The results reveal that, in the main weldingThe temperature distribution on welding plane isregion, welding pressure and effective stress increaseshown in Fig.10. It can be seen clearly that withsynchronously with process velocity increasing, whenextrusion proceeding, the temperature on the weldingv=1 mm/s, the maximum pressure is 210 MPa, theplane increases continuously, anhe maximumminimum effective stress is 41 MPa, when v values are 5,temperature appears in front of the die bearing, which is10 and 15 mm/s, the maximum pressures are respectivelystrongly influenced by process velocity. When =1 mm/s,220, 230 and 2中国煤化工ctive stresses .the maximum temperature near the die exit is 440 C,pressure-to-effei YHCN M H Ghe maximumtemperature decreases because the heat generated duringrauus Viuj ale 5.25, 5.12,deformation is not enough to offset the heat loss to the5.11 and 5.00 when v values are respectively 1,5, 10 andJ. Cent. South Univ. Technol. (2010) 17: 688- -696695(a)b)Temperature/"CTemperature/C52》5305205105149049484704746444342140 Mn500 MaxC)d)480440430 I420 I40 MinFig.10 Temperature distribution on welding plane with different process velocities: (a) 1 mm/s; (b) 5 mm/s; (C) 10 mm/s; (d) 15 mm/sTable 2 Effect of process velocity on extrusion processwelds seam formation. The effect of temperature on theProcesMaximumMinimumlongitudinal welds quality is actually exerted through itsvelocity/weldingeffective stress/effect on the welding pressure and yielding behaviorp/(mm's l) pressure/MPaMPawhich is represented by the effective stress. In thissimulation four different initial billet temperatures are2105.25420, 450, 465 and 480 C, process velocity is 10 mm/s,22035.12the welding chamber height is 7 mm and die leg is the10230455.11type in Fig.2(c).30l6The simulation results are shown in Table 3. Theyreveal that with increasing billet temperature,15 mm/s. The maximum p/σ decreases slightly, while dietemperature near the die exit increases, the maximumstructure and welding path length L keep the same.welding pressure and the minimum effective stress inAccording to K criterion, integral value K decreases withmain welding zone decrease simultaneously, but p/σprocess velocity proceeding, so higher velocity decreasesincreases slightly, indicating that higher temperature isthe quality of longitudinal welds, which is in accordancefavorable to longitudinal welds quality. Despite this,with Valberg's experiment results [7- 8]. Integral valuestemperature must be controlled in a certain rangeare almost the same when v=5 mm/s and 10 mm/s, forbecause too high a temperature may bring surface defectthe purpose of productivity, v=10 mm/s will be suitable.the prof中国煤化Ile，massive .recrystallizationYHCNMH Gasily formed3.4 Effect of billet temperature on seam welds qualityin longitudinal welds at mgner temperature and decreaseTemperature is another important parameter inthe strength and deformability of profile [14]. According696J. Cent. South Univ. Technol. (2010) 17: 688- 696Table 3 Effect of billet temperature on extrusion processTemperature near dieMaximum weldingMinimum effectiveBillet temperature/'CMaximum p/σexit/°Cpressure/MPastress/MPa420490- -500240485.0045051023045.11465510-520220435.12480520- 530210415.13to the analysis above, both 450 and 465 C can satisfyTechnology Seminar. Chicago, ET, 1992: 319- -336.the demand.PLATA M, PIWNIK J. Theoretical and experimental analysis ofseam weld formation in hot extrusion of aluminum alys [C]//Proceedings of the Seventh International Aluminum Extrusion4 ConclusionsTechnology Seminar. Chicago, ET: 2000: 205- 211.[6] DONATI L, TOMESANI L. The prediction of seam welds quality in(1) Pressure-to-effective stress ratio (p/o) andaluminum extrusion []. Journal of Material Processing Technology,welding path length (L) are key factors that determine the2004, 153/154: 366-373.VALBERG H. Extrusion welding in aluminium extrusion [welding quality. Better welding quality could bMaterials and Product Technology, 2002, 17(7): 497-56.obtained by increasing either of them.[8] VALBERG H, LOEKEN T, HVAL M. The extrusion of hollow(2) In the main welding region, as welding chamberprofiles with a gas pocket behind the bridge []. Materials andheight (H) increases, p/σ and L increase at the same time.Product Technology, 1995, 10(3/6): 222- -267.When H=10 mm, welds have the best quality. Sharper9] FANG G, ZHOU J, DUSZCZYK J. Extrusion of 7075 aluminiumdie leg also increases p/σ and L at the same time. Whenalloy through double-pocket dies to manufacture a complex profile[田. Journal of Materials Processing Technology, 2009, 209:D=0.4 mm， welds have the best quality: after3050- 3059.maintaining the strength and stiffness of die leg, it should10] LIU G, ZHOU J, DUSZCZYK J. FE analysis of metal flow and weldbe made as sharp as possible.seam formation in a porthole die during the extrusion of a(3) With process proceeding, welding pressure andmagnesium aloy into a square tube and the effet of ram speed oneffective stress increase at the same time, but p/weld strength []. Journal of Materials Processing Technology, 2008,200: 185-198decreases. The increase in extrusion velocity gives worse川] LI L, ZHANG H, ZHOUI, LI G Y, ZHONG Z H. Numerical andwelding quality. In this work, V= 10 mm/s can satisfy theexperimental study on the extrusion through a porthole die todemand. With temperature increases, welding pressureproduce a hollow magnesium profile with longitudinal weld seamsand effective stress decrease, but p/σ increases, while[]. Materials and Design, 2008, 29: 1190- 1198.temperature should be controlled in a certain range,12] DONATI L, TOMESANI L. The efet of die design on thewhich facilitates bonding. Both 450 and 465 C areproduction and seam weld quality of extruded aluminum profiles [JJournal of Materials Processing Technology, 2005， 164/165:proper for AA6061 extrusion.1025-1031.Referencesbonding in extrusion and FSW: Process mechanics and analogies [].Journal of Materials Processing Technology, 2006, 177: 344- -347.[1] DONATI L, TOMESANI L, MINAK G. Characterization of seam14] CHEN Jing-chun. Production and control of the welding mark atweld quality in AA6082 extruded profiles []. Journal of Materialsaluminium extrusion [D]. Shenyang: Northeastern University, 2006.Proessing Technology, 2007, 191: 127-131.(in Chinese)[2] KIM KJ, LEE C H, YANG D Y. Investigation into the improvement[15] JO H H, JEONGC s, LEE s K, KIM B M. Determination of weldingof welding strength in three-dimensional extrusion of tubes usingpressure in the non-steady-state porthole die extrusion of improvedporthole dies [J]. Journal of Materials Processing Technology, 2002,A17003 hollow section tubes []. Journal of Materials Processing130/131:426- -431.Technology, 2003, 139: 428 -433.[3AKERET R. Properties of pressure welds in extruded aluminum16] JOH H, LEES K, JUNGC S, KIM B M. A non-steady state FEalloy sections [U]. Journal of the Institute of Metals, 1972, 10: 202-analysis of Al tubes hot extrusion by a porthole die []. Journal of210.Materials Pocssing Technology, 2006, 173: 2232 231.[4] AKERET R. Extrusion welds-quality aspects are now center stage(Edited by YANG You-ping)[C]/ Proceedings of the Fith International Aluninum Extrusion中国煤化工MHCNMH G