外文翻译--AZ31和 AZ61镁合金的等温板料成形 英文版

Isothermal sheet formability of magnesiumalloy AZ31 and AZ61Shyong Leea,*, Yung-Hung Chena, Jian-Yih WangbaDepartment of Mechanical Engineering, National Central University, Chung-li, Taiwan, ROCbChung-Shan Institute of Science and Technology, Lung-tan, Taiwan, ROCReceived 18 February 2001AbstractThere have been reports on the forming of magnesium alloy sheet in industry, but this paper is probably the first formal paper for studyingthe sheet formability of AZ31 and AZ61 at various elevated temperatures. The results indicate that it is feasible to form products fromextruded sheets of 0.5, 1.3, 1.7 and 2 mm thickness. Presently, the forming of a sheet of 0.5 mm thickness is considered to be a technicalachievement by industry. There were two kinds of tooling employed, punch and punchless. The punchless die setting used pressurized gas topress the sheet into a female die cavity. This technique applied to Mg-alloy is unprecedented and shows potential for industrial utilization. Asthe stretch ability was demonstrated in gas forming, punchdie pressing should be achievable, numerous punching tests being performed toconfirm this. # 2002 Elsevier Science B.V. All rights reserved.Keywords: AZ31； Punchdie pressing； Isothermal sheet forming1. IntroductionMagnesium alloy is the lightest metal that can beemployed for structural use. In the past, the demand forthis alloy as a structural material was not high because of itsless availability commercially as well as limited manufac-turing methods. In recent years, the die casting of magne-sium alloy has been prevailing in the making of parts in theautomotive industry 1,2 and such items as the covers ofnotebook computers as well as cellular phones. However,this process is not ideal in making thin-walled magnesiumstructures because an excessive amount of waste materialcan result. A potential solution would be to resort to the sheetforming process. It is commonly recognized that magnesiumpossesses poor formability at room temperature because ofits hexagonal close-packed structure 3,4. Fortunately, theworkability of Mg-alloy can be effectively improved byincreasing the working temperature, e.g., increasing above300 8C 2. In this paper, the sheet formability of AZ31 andAZ61 at various elevated temperatures is studied to assessthe feasibility of forming products from extruded sheets.There are two kinds of tooling employed, punch and punch-less as described in Fig. 1. The punchless die setting usespressurized gas to press the sheet into a female die cavity.This method has the advantage of eliminating frictionbetween the workpiece and the punch tool, so that thematerials stretch ability can be more genuinely exhibited.The strain distribution on various locations of the formedproduct will be studied. Further, the material flow path willbe traced and constructed. On the other hand, the punchdiemethod involves much less stretching effect, but also anuneven load distribution, so that its failure mode may bequite different from that in the gas-forming process.2. Materials and experimental procedureThe alloys employed in the sheet forming work are AZ31and AZ61, in which the magnesium alloys contains 3 and 6%of aluminum, respectively, as indicated in the first numericaldigit in the designation: the last digit represents the zinccontent, which is 1% in the above cases. The material forsheet forming work was obtained by extruding a billet of 8 in.(203 mm) diameter C2 30 in. (762 mm) length through a diewith 0.5, 1.3, 1.7 and 2 mm openings at 250 8C (for AZ31)and 280 8C (for AZ61). The basic tool for the experiment is apress machine equipped with a furnace offering desiredisothermal conditions. For the gas forming (punchless) work,only one die is needed, which is in rectangular shape of40 mm width and 120 mm length. The depth of the die is20 mm, but could be adjusted to 8, 12 and 16 mm by insertingJournal of Materials Processing Technology 124 (2002) 1924*Corresponding author.E-mail address: .tw (S. Lee).0924-0136/02/$ see front matter # 2002 Elsevier Science B.V. All rights reserved.PII: S 0924-0136(02)00038-9dummy blocks. The pre-formed flat sheet was positionedon the die； the cover plate with a peripheral rail was placedon to clamp the sheet； and then the chamber was sealed toenable pressurized gas to mold the sheet towards the contourof the die. The input gas pressure needed to be adjustedconstantly in accordance with the varying sheet configurationduring the whole forming process. Some of the sheets weremarked with a grid so that local strain state could bedetermined by measuring the deformation of the grid. Forpunchdie pressing, the rectangular shape sheet used thesame die as that for gas forming but the die setting had a2 mm clearance between the punch and the die. A circular-shape pressing was also performed, where the die diameterwas 20 mm and there was a 2 mm clearance.3. Results and discussion3.1. Gas-forming of 1.7 mm thick AZ31(rectangular shape)3.1.1. Formability as a function of the gaspressurization rateSeveral specimens were formed by the gas-pressing tech-nique to study the formability of the sheet at various com-binations of forming depth and temperatures, as well aspressuretime (pt) input. Two pieces were formed success-fully at 410 8C with 8 mm depth following the pt profilesdepicted in Fig. 2. For this shallow forming, only 90 s wereneeded utilizing a higher pt profile as compared with otherdeeper cases. Further forming to 12 mm depth was performedat the same temperature with the pt profile shown in Fig. 3.This depth, formed at a lower temperature, 310 8C, was alsoFig. 1. Schematic diagram of the tools for isothermal sheet forming: (a) gas forming with a rectangular die； (b) rectangular punch and die； (c) circular punchand die.Fig. 2. Pressuretime profile leading to the successful forming of arectangular-shaped box of 8 mm depth at 410 8C.20 S. Lee et al. / Journal of Materials Processing Technology 124 (2002) 1924completed successfully, but it took a higher pressure and alonger time because the material had a larger flow stress. Forthe 16 mm case, two blow formings were done with onesuccess and one failure, due to the different pt inputsemployed as depicted in Fig. 4. The full depth, 20 mm,was tried with a pt profile (Fig. 5) that was even higherthan that for the two 16 mm cases, so it was doomed to fail.The above two unsuccessful specimens were photographedand are shown in Fig. 6. It is seen that failure started at themiddle of the long side on the die entrance. For all theseforming jobs, the gas pressure increased as the time increased,which should be needed for maintaining the flow stress in thematerial at a constant level. Considering the forming sheet asa part of spherical shell surface having an instantaneousconfiguration and thickness as a function of time； then usingthe equation for calculating the flow stress, i.e. s pr=2t,where p is the gas pressure, r the curvature radius and t thethickness, can partially justify the above experimental pres-surization rate. An advanced stress analysis with accuratemodeling may suggest a more ideal pt curve.3.1.2. Strain distribution and the materials flow locusAmong the seven gas-formed pieces, some were markedwith grids in order to measure the strain distribution. Origin-ally, the grid was an array of identical circles of 2.5 mmdiameter, printed on the sheet surface prior to the formingwork. It can be seen from the deformed grids that themaximum tensile strains are located at the middle of the longside on the upper curved spot (Fig. 7). Failure would start atthis position if the sheet suffered an unfavorable pressuriza-tion rate and temperature. This measured strains are the firstexperimental disclosure indicating that the commonlyFig. 3. Pressuretime profile leading to the successful forming of arectangular-shaped box of 12 mm depth at 410 and 310 8C.Fig. 4. Two different pt inputs for 16 mm depth gas forming at 410 8Cresulted in one success and one failure.Fig. 5. Pressuretime profile for the attempt to form a rectangular-shapedbox of 20 mm depth at 410 8C.Fig. 6. Failed gas forming of a 1.7 mm thick sheet for the cases of 16 and20 mm depths.S. Lee et al. / Journal of Materials Processing Technology 124 (2002) 1924 21Fig. 7. Enlarged view of the deformed grids at various locations in the forming of a part to: (upper) 12 mm depth, measuring the grids on the concave (upper) side；(lower) 16 mm depth, the grids being on the convex side (lower). e1denotes the greatest local surface strain whilst e2is that in the perpendicular direction.Fig. 8. Flow paths of the material under peripheral sealing in the gas-forming process.assumed plane-strain state in the middle of the long side forderiving a pt curve may not be absolutely correct. It wasnoteworthy that the material under the peripheral sealing railwas not firmly held, but actually had the tendency to slide asshown in Fig. 8. At high temperatures the peripheral rail onthe cover plate indented the softened sheet and created agroove under pressing load. The outer and inner boundariesof the groove were originally parallel, however, some parts ofthe inner boundary was displaced inwards, indicating thatthe material under the peripheral sealing rail was still beingstretched even under the clamping load. This mechanism maybe important in achieving successful gas forming.3.2. The gas-forming of 0.5 mm thick AZ31(rectangular shape)After completion of the gas forming with sheets of1.7 mm thickness, a more challenging task was that for0.5 mm thickness, where success would be considered atechnical achievement from the industrial point of view.Three pieces were tried with cavity depths of 12, 16 and20 mm. The working temperature was chosen to be 330 8Cbased on the previous feasibility with 310 and 410 8C.The initial pressurization rate was set to be 5 kgf/cm2(490 kN/m2) considering that the thickness is to bedecreased by more than two-thirds relative to that for thepreviously worked cases. Plots showing the pressurizationrate employed for the forming work are provided (Fig. 9).For the 12 and 16 mm cases, the gas forming work wassuccessful, only that much more time was consumed com-pared to the previous counterparts. For the 20 mm case,the forming was not successful, and failure started from theposition under the peripheral rail, which is thinner in thebeginning because of the indentation of the material due toclamping for sealing the pressurized gas. This failure modeis not the same as its 1.7 mm thick counterpart, in whichfailure occurred at the die entrance. It can be proposed thatthe stress is much higher at the die entrance when bendingthicker sheet.3.3. The rectangular punchdie forming of1.3 mm thick AZ31For the punchdie press work to C2416 mm depth on AZ31of 1.3 mm thickness in this category, nine pieces were testedat room temperature at 329 8C(five pieces) and at 435 8C(three pieces). The room temperature work was not asuccess, as was expected. At high temperature, 435 8C,all of the three specimens were formed successfully. Atintermediate temperature, only one out of the five workingpieces was formed successfully. This indicates that otherparameters such as lubrication, punch speed and clearancecan exhibit an influence when the temperature factor is notdominating. The failures in this category occurred mostly atthe corners (Fig. 10) where the punch exerted a concentrat-ing pulling force to drag the sheet down: this is just what thegas forming method can avoid.3.4. The rectangular punchdie forming of2 mm thick AZ61It is commonly stated that AZ61 is less formable thanAZ31 because of its greater aluminum content in the Mg-alloy. Two pieces of 2 mm thickness were punch-pressed toC2416 mm deep at 295 8C； one was successful and one failed.The working temperature was relatively lower than thatwhich was expected to be needed. The results strengthenthe observation above that there are some influential factorsother than temperature.Fig. 9. Pressuretime profiles for the forming of rectangular-shaped boxesof depth: (a) 12 mm, (b) 16 mm and (c) 20 mm at 330 8C from 0.5 mmthick sheets.S. Lee et al. / Journal of Materials Processing Technology 124 (2002) 1924 233.5. The circular punchdie forming of 2 mm thick AZ61The circular punching of 16 mm depth of up to 18 pieceswas performed. The yield as a function of temperature islisted in the following:That the success and failure results are dictated by thetemperature seems quite clear. However, there is a transitionzone in which the temperature is not the only decisive factor.Lubrication, the materials pre-condition, the punchingspeed, and the punchdie clearance, etc., may be additionalaffecting factors.4. ConclusionsMagnesium alloys AZ31 and AZ61 in sheet form can beformed at elevated temperatures by pressurized gas as wellas by punchdie methods. Temperature is the main factor indetermining whether the forming can be successful. How-ever, there are still some secondary influential factors suchas lubrication, th