Isothermal Gas Forming of Mg Alloy AZ31 Sheet

J Mater. Sci. Technol., Vol 18 No. 3, 2002227Isothermal Gas Forming of Mg Alloy Az31 SheetYung-llung Chen and Shyong LeT64 ADepartment of Mechanical Engineering, National Central University, Chung-li, Taiwan, ChinaJian-yih wangChung-Shan Institute of Science Technology, Lung-Lan, Taiwan, China[Manuscript received June 21, 2001, in revised form October 18, 2001There have been reports on sheet forming of Mg alloy in industry via the punch and die method; this paper isprobably the first formal one for studying the sheet formability of Az31 employing pressurized gas to press the sheetinto a female die cavity at various elevated temperatures. The results indicate it is feasible to form a rectangular boxvia pressurized gas from extruded sheets of 0.5 and 1.7 mm thick. the formed box has 1: 2 depth over width ratiowhich should be large enough when dealing with realistic industrial sheet forming parts. Presently, forming a sheetof 0.5 mm thick is considered a technical challenge by industry and it is conquered as demonstrated in this paperGas forming technique applied to mg alloy is unprecedented and shows potential for industrial utilizationKEY WORDS: AZ31 magnesium alloy, Sheet formability, Microstructure1. IntroductionMagnesium alloy is the lightest metal that can be employed for structural use. In the past, the demand for thisPressu IZed air inletalloy as a structural material was not high because of itsAir smling山less availability commercially as well as limited manufacturingethods. In recent years, die casting of Mg alloy has beerhe prevailing method for making parts in the automotiveAtmospher it at! outingindustry!l, 2 and as well as notebook computers and cellularPosting rinphones. However, this process is not ideal in making thinwalled Mg structure because excessive amount of waste ma-terial can be produced. a potential solution would be resort- Fig.1 Schematic of the rectangular die for isothermal gasig to sheet forming process. It is commonly recognized thatforming of sheetMg possesses poor formability at room temperature becauseof its hexagonal closed packed structure/ 3, 4. Fortunately, the peripheral rail was placed on to clamp the sheet, a chambersability of Mg alloy can be effectively improved by enhanc. was sealed for pressurized gas Lo mold it toward the contouring working temperature, for example, up to above 300 Cl2. of the die. The input gas pressure needed to be constantly adIn this paper, sheet formability of AZ31 at various elevated justed in accordance to the varying sheet configuration duringtemperatures was studied to assess the feasibility of forming the whole forming process. Some of the sheets were markedoroducts from extruded sheets. The gas forming tooling em- with icons so that local strain state could be determined byployed is described in Fig. 1, which uses pressurized gas to measuring the deformation of icons. Optical metallographypress the sheet into a female die cavity. This method has was performed to observe whether the material had proceededthe advantage of eliminating friction between work piece and microstructure transformation during the hot forming pro-unch tool so that the material stretch ability can be morecessenuinely exhibited. Strain distribution on various locationsof the formed product would be studied. Also, material flow 3. Results and Discussionpath would be traced and constructed3.1 Gas-forming of 1.7 mm thick sheet2.E3.1.1 Formability as a function of gas pressurization ratens formed by gas-pressing technique have beenompleted to study the formability of sheet at various com-hich the me alloys contains 3%Al as indicated in the binations of forming depth, temperature, as well as pressure.first numerical digit in the designation; and the last digit time (p-t)input. Two pieces had been successfully formed atThe material for sheet forming work was obtained by extrud. Fig. 2. For this shallow forming, only 90 s were needed utiliz.ing a bar, 8 inch diameter and 30 inch length, through a ing higher p-t as compared with other deeper cases.Furtherdie with 0.5 and 1.7 mm openings at 250C. The basic tooling to 12 mm depth was performed at the same temure with the p-t shown in Fig 3. This depth, created at anace offering desired isothermal condition. For the gas form- lower temperature of 310C, was also completed successfully,ing work, only one die was needed, which was in rectangularand longer time becausehape with a depth of 40 mm and a length of 120 mm. The中国煤化工s, For the 1e6 min case,depth of the die was 20 mm and could be adjusted to 8, 12CNMHGpicted in Fig 4. The fullor I6 mm by inserting dummy blocks. The pre-formed fatsheet was positioned on the die; when the cover plate with adepth, 20 mm, was tried with a p-t profile(Fig. 5)even higherthan the two 16 mm one, so it was doomed to fail. The twot To whom correspondence should be addressedunsuccessful specimens above were photographed and shownE-mail: shyong@cc ncu. edu. tw, Prin Fig. 6. It is seen that failure started at the middle of the28J Mater. Sci. Technol., Vol 18 No.3, 2002Fig2 Pressure-time profile leading to successful forming of Fig. 5 Pressure-time profile for the test to form a rectangu-rectangular shape box of 8 mm depth at 410C frorlar shape box of 20 mm depth at 410.C from 1.7 mm1. 7 mm thick sheetsa310℃020帕68100120140160Fig 3 Pressure-time profile leading to successful forming ofrectangular shape box of 12 mm depth at 410.C and310C form 1.7 nm thick sheetsFig 6 Failed gas forming on 1.7 mm thick sheet for the casesof 16(a)and 20 mm(b)depthsaicon was an array of identical circles of a 2.5 mm diameter,printed on the sheet surface prior to the forming work. Itbe seen from the deformed icons that the maximum stretchstrains are located at the middle of the long side on the uppercurved spot( Fig. 7). Failure would start at this position if thesheet suffered unfavorable pressurization rate and temperaThis measured strain is the firstindicating that the commonly assumed plane-strain-state inthe middle of the long side for deriving a p-t curve may notnoteworthy that the material undethe peripheral sealing rail was not firmly held, and actuallyFig4 Two different pt input for the 16 mm gas forming had the tendency to slide as shown in Fig 8. At high temat 410 C resulted in one success and one failure from peratures the peripheral rail on the cover plate indented the1.7 mm thick sheetssoftened sheet and created a groove under pressing load. Theouter and inner boundaries of the groove were originally paralel, however, some parts of the inner boundary was displacedng side on the die entrance. For all these forming jobs, inwardly indicating that the material under the peripheralgas pressure increased with time, which should be needed for sealing rail was still stretched even under the clamping loadmaintaining flow stress in the material at a constant levelThis mechanism may be important in achieving successful gascousidering the forming sheet as a part of spherical shell sur-formingace having instantaneous configuration and thickness as func-ion of time, then using the formula for calculating flow stress, 3.2Ga中国煤化工7=pr/2t(: gas pressure; r: curvature radius; t: thicknessCNMHGith the sheets beingwe can partially justify the above experimental pressurization 1.7 mmlobtain for arate. An advanced stress analysis with accurate modeling may ness of 0.5 mm. Success in this area would be considered asuggest a more ideal p-t curveechnical achievement from the industry point of view. Three1.2 Strain distribution and material flow locus Amongpecimen were experimented with the cavity depths beinghe seven gas-formed pieces, some were marked with icons 16 and 20 mm respectively. Working temperature was chosenO数瞎re the strain distribution. Originally, theto be 330C based on the previous feasibility for formingJ. Mater. Sci. Technol., vol 18 No. 3, 2002E=02722=001=0.02的0712L22=0.1s⊙1-1417620.0403981040050.100.15020Fig7 Inlarged view of the deformed icon at various locations in the formiug part of 12 mm depth, neasuringicons on the concave side (a) 16 mm depth, icons on the convex side,()ei designates the biggest localsurface strain and e? is that in the perpendicular direction, (e)shows the strain distributions in E1, ca axes回器Err of tn MetOriginal dentalisviginal indentationFig 8 Flow paths of the material under the peripheral seal.ing in the gas forming process--16mmFig 10 Microstructure of the extruded sheet for the shwe.quent gas forming, (a) 1.7 mm and (b)0.5 in thickpooed that stress gets much higher at the die entrance whenbending thicker sheet.3.3 Microstructure studyoxes of 12, 16 and 20 mm doptn at 330C from A Most specimens were examined by optical metallographyFig 9 Pressure-time profile for forming rectangular shapeigure 10 displays the microstructures of the pre-forming0.5 mm thick sheetssheet. The 1.7 mm sheet had more uniform grain size dis-ibution than that of the 0.5 mm one. Three of the formed1.7 nIm sheet, at 310 and 410.C. Initial pressurization rate wasboxes of 12 mm depth and one of 16set to 0.5 MPa considering that thickness was decreased by selected for optical metallography. Three locations. die en-more than two thirds relative to the previously working cascs. trance, bottom corner and bottom middle were examinedThe plots showing pressurization rate employed for the form- There was no sign of cavity formation(Figs. 1I and 12)whiching works are provided(Fig 9). For the 12 and 16 mm cases,the gas forming works are successful, only that much more中国煤化工 ing high strain. Grainas a result of sustainingtime was consumed compared to the previouscounterparts.CNMH Gm were formed at 410.CFor the 20 mm case, the forming was not successful, and fail-It seems that higherre started from the position under the peripheral rail, whichmperature is more effective in enhancing grain size(Figs. 11was more thinner at the beginning because of the indentation and 12). When prolonged time and high temperature con-n material due to clamping for sealing pressurized gas. This ditions were combined, grain growth phenomenon was evenfailure Inode is not the same as its 1. 7 mm thick counterpart,丹芳数撸 ure occurred at the die entrance. It can be pro-ain sites do not necessarily cause early fracture]. Mater. Sci. Technol. Vol 18 No. 3, 2002Fig11 Microstructure of the formed rectangular box (12 mm deep and 1.7 mm thick; 310C)at three locations(a) die entrance.(b) bottom corner, and (e) bottom middleFig 12 Microstructure of the formed rectangular box( 2 mu deep and 1.7 mm thick; 410"C) at three locations(a)die entrance, (b)bottom corner, and (e) bottom middle10mFig 13 Microstructure of the formed rectangular box(16 m deep and 1.7 mm thick: 410"C) at three locations.(a)die entrance, (b)bottom corner, and(e)bottom middle4. Summaryrelatively thicker sheet. Optical metallography shows that mi.crostructure could be changed during hot forming stage, butMagnesium alloy AZ31 in the sheet form can be formed no cavitation occurs even at the high strain sitesat elevated temperatures by pressurized gas mePerature is a maiu factor in determining whether the formingcan be successful. However there are still some secondaryduential factors such as lubrication, material pre-condition, [1N.A El-Mahallawy, M.A.Taha, E Pokora and F Klein: Jand gas-pressurization rate etc. Gas-forming applied to MgMater. Proc. Technol., 1998, 73, 125t his method, gas pressurization is an important techniquc. Its (3|H Tathan deeper ones, Failure mode by gas method is most likely4」H.Talto occur at the die entrance in the niddle of the long side with1998YHEN. Hatta:.. Mater. Proc. Technol.中国煤化工ater, Proc. Technol.CNMHG