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Die design for stamping a notebook case with magnesium alloy sheetsContent SummaryIn the present study，the stamping processfor manufacturing anotebook top cover case with LZ91 magnesiumlithium alloy sheet at roomtemperature was examined using both the experimental approach and the finite element analysis. A four-operation stamping process was developed to eliminate both the fracture and wrinkle defects occurred in the stamping process of the top cover case. In order to validate the finite element analysis，an actua four-operation stamping process was conducted with the use of 0.6mm thick LZ91 sheetas the blank. A good agreement in the thickness distribution at various locations between the experimental data and the finite element results confirmed confirmed the accuracy and efficiency of the ementanalysis.The super or for mability of LZ91 sheet at room temperature was also demonstrated in the present study by successful manufacturing of the notebook topcover case. The proposed four operation process lend sit selftoan efficient approach to form the hinge in the notebook with less number of operational procedures than that required in the current practice. It also confirms that the notebook cover cases can be produced with LZ91 magnesium alloy sheet by the stamping process. It provides an alternative to the electronics industry in the application of magnesium alloys.Keywords: Notebook case；LZ91 magnesiumlithium alloy sheet；stamping； Multi-operation；Formability1. IntroductionDue to It slight weight and good performance in EMI resistance, magnesium alloy has been widely used for structural components in the electronics industry, such as cellular phones and notebook cases. Although the prevailing manufacturing process of magnesium alloy products has been die casting，the st- amping of magnesium all sheet has drawn interests from industry because of its competitive productivity and performance in the effective production of thin-walled structural components.As for stamping process，AZ31 magne siumalloy (aluminum 3%, zinc 1%) sheet has been commonly used for the forming process at the present time,even though it needs to be formed at elevated temperature due to its hexagonal closed packed (HCP) crystal structure Recently,the magnesiumlithium(LZ)alloy has also been successfully deve- loped to improve the formability of magnesium alloy at room temperature. The ductility of magnesium alloy can be improved with the addition of lit hium that develops the formation of body centered-cubic (BCC) crystal structure (Takuda et al., 1999a,b; Drozd et al,2004).In the present study, the stamping process of a notebook top cover case with the use of LZ sheet was examined. The forming of the two hinges in the top cover of a notebook, as shown in Fig.1(a and b),is the most difficult operation in the stamping process due to the small distance between the flanges and the small corner radii at the flanges, as displayed in Fig. 1(c). This geometri complexity was caused by a dramatic change in the corner radius when the flange of get stooclo set the notebook,which would easily cause fracture defect around the flange of hinge and requirea multi-operation stamping process to overcome this problem.In the present study, the formability of LZ magnesium alloy sheets was invest- igated and an optimum multi-operation stamping process was developed to reduce the number of operation all proced using both the experiment approach and the finite element analysis.Fig.1Flange of hinges at notebook top cover case.(a) Hinge, (b) top cover case and (c) flanges of hinge.2. Mechanical properties of magnesiumcontent of lithium increases. It is also observ from Fig. 2(a) that the curves of LZ91 sheet at room temperature and AZ31 sheet at 200,C are close to each other. LZ101 sheet at room temperature exhibit seven better ductility than LZ91 and AZ31 do at 200,C. Since the cost of lithium is very expensive, LZ91 sheet, instead of LZ101 sheet, can be considered as a suitable LZ magnesium alloy sheet to render favorable formability at room temperature. For this reason ,the present study adopted LZ91 sheet as the blank for the notebook top cover case and attempted to examine the formability of LZ91 at room temperature. In order to determine if the fracture would occur in the finite element analysis, the forming limit diagram for the 0.6mm thick LZ91 sheet was also established as shown in Fig. 2(b).alloy sheets The tensile test swereper formed for magnesiumlithiumalloy sheets of LZ61 (lithium 6%, zinc 1%), LZ91, and LZ101 at room temperature to compare their mechanical properties to those of AZ31 sheets at elevated temperatures. Fig. 2(a) shows the stressstrain relations of LZ sheets at room temperature and those of AZ31 sheets at both room temperature and 200?C. It is noted that the stressstrain curve tends to be lower. Fig. 2 Mechanical properties of magnesium alloy.(a) The stressstrain relations of magnesium alloy; (b) forming limit diagram (FLD) of LZ91 sheet.3. The finite element modelThe tooling geometries were constructed by a CAD software, PRO/E, and were converted into the finite element mesh ,as shown in Fig. 3(a), using the software DELTAMESH. The tooling was treated as rigid bodies, and the four-node shell element was adopted to construct the mesh for blank. The material lproper ties and forming limitd iagram sobtained from the experiments were used in the finite element simulations. The other simulation parameters used in the initial run were: punch velocity of 5mm/s, blank-holder force of 3kN, and Coulomb friction coefficient of 0.1. The finite element software PAM STAMP was employed to perform the analysis, and the simulations were performed on a desktop PC. A finite element model was first constructed to examine the oneoperation forming process of the hinge. Due to symmetry, only one half of the top cover case was simulated, as showninFig.3(a).The simulation result, as show ninFig.3(b),indicates that fracture occurs at the corners of flanges, and the minimum thickness is less than 0.35mm. It implies that the fracture problem is very serious and may not be solved just by enlarging the corner radii at the flanges. The finite element simulation swere performed to study the parameters .That affect the occurrence off racture. Several approaches were proposed to avoid the fracture as well.Fig. 3 The finite element simulations. (a) Finite element mesh and (b) fracture at the corners.4. Multi-operation stamping process designIn order to avoid the occurrence of fracture, a multi-operation stamping process is required. In the current industrial practice, itusually take satle ast tenoperational procedures to form the top cover case using the magnesium alloy sheet. In thepresent study, attempts were made to reduce the number of operational procedures. Several approaches were proposed to avoid the fracture, and the four-operation stamping process had demonstrated itself as a feasible solution to the fracture problem. To limit the length of this paper, only the two operation and the four-operation stamping processes were depicted in the following.4.1 Two-operation stamping processThe first operation in the two-operation stamp in process was side wall forming as shown in Fig.4(a),and the second one was the forming off lange of hing epresented in Fig.4(b),the height of the flange of hinge being 5mm .Fig.4(c)shows the thickness distribution obtained from the finite element simulation. The minimum thickness of the deformed sheet was 0.41mm and the strains were all above the forming limit diagram. It means the fractured effect could be avoided. Inaddition, the height of the flange conformed to the target goal to be achieved. How- ever, this process produced a critical defect of wrinkling, as shown in Fig. 4(d), on the flange of hinge, which induces a problem in the subsequent trimming operation. Hence, even though the two-operation stamping process solved the fracture problem at the corner of the bottom and the flange of hinge, a better forming process is still expected to solve the wrinkling of flange of hinge.Fig. 4 Two-operation stamping process. (a) Formation of sidewalls, (b) formation of hinges, (c) thickness distribution and (d) wrinkle.4.2. Four-operation stamping processThe four-operation forming process proposed in the present study starts with the forming of three side wall sand the flange of the hinge with a generous corner radius, as shown in Fig.5(a).Since the side wall close to the flange was open and the corner radius was larger than the desired ones, the flange was successfully formed without fracture. Such process success-fully avoided the difficulty of forming two geometric features simultaneously, but increased the material flow of the blank sheet. The next step was to trim the blank outside the side walls, and to calibrate the corner radius of 4mm to the desired value of 2.5mm. The hinge was thus formed, as shown in Fig. 5(b). The third step was to fold the open side, so that the sidewall could be completed around its periphery, as shown in Fig. 5(c). The effect of trimming the extra sheet outside the sidewalls in the second step on the third step was studied. When the extra sheet was not trimmed, the thickness at the corner was 0.381mm, as shown in Fig. 5(d). The thickness of Table Comparison of thickness measured ABCD Experiment 0.42mm 0.44mm 0.49mm 0.53mm Simulation 0.423mm 0.448mm 0.508mm 0.532mm Error 0.71% 1.79% 3.54% 0.38% the corner increased to 0.473mm, as shown in Fig. 5(e), if the trimming was implemented in the second step. The excessive material producedby the folding process in the third step was then trimmed off according to the parts design. The last step was the striking process that is applied to calibrate all the corner radii to the designed values. The minimum thickness at the corner of the final product was 0.42mm,and all the strains were above the forming limit diagram. It is to be noted that Fig. 5(ac) only shows the formation of one hinge. The same design concept was then extended to the stamping process of the complete top cover case. 5. Experimental validationIn order to validate the finiteel ement analysis,an actualfour operation stamping process was conducted with the use of 0.6mm thick LZ91 sheet as the blank. The blank dimension and the tooling geometries were designed according to the finite element simulation results. A sound product without fracture and wrinkle was then manufactured, as shown in Fig. 6(a). To further validate the finite element analysis quantitatively, the thickness at the corners around the hinge of the sound product, as shown in Fig. 6(b), were measured and compared with those obtained from the finite element simulations, as listed in Table 1. It is seen in Table 1 that the experimental data and the finite element results were consistent. The four-operation process design based on the finite element analysis was then confirmed by the experimental data.Fig. 6 The sound product. (a) Without fracture and wrinkle and (b) locations of thickness measured.Concluding remarksThe press forming of magnesium alloy sheets was studied in the present study using the experimental approach and the finite element analysis. The formability of both AZ31 and LZ sheets was examined first. The research results in dicated th a the LZ91 sheet has favorable formability at room temperature, which is similar to that of AZ31 sheet at the forming temper- ature of 200C.The superior formability of LZ91 sheet at room tempera Ture was also demonstrated in the present study by successful manufacturing of the notebook top cover case. The proposed four-operation process lends itself to an efficient approach to form the hinge in the notebook with fewer operational procedures than that required in the current practice. It also confirms that the notebook cover cases can be produced with LZ91 magne siumalloy LZ91sheet by the stamping process. It provides an alternative to the electronics industry in the application of magnesium alloys. Acknowledg ments The authors would like to thank the National Science Council of the Republic of China for financially supporting this research under the Project No. NSC-95-2622-E-002-019-CC3, which made this research possible. They would also like to thank ESI, France for the help in running the PAM STAMP program.References1 Chen. F.K.Huang.T.B.Chang. C.K.2003. Deep drawing of square cups with magnesium alloy AZ31 sheets. Int. J. Mach. Tools2 Manuf. 43.15531559.Drozd.Z.Trojanova .Z, Ku dela.S.2004. Deformation of behavior of MgLiAl alloy. J. Mater. Compd. 378. 192195.3Takuda.H.Yoshii.T. Hatta, N.1999a. Finite-element analysis of the formability of a based alloy AZ31 sheet. J.4 Mater. Process. Technol. 89/90. 135140.Takuda.H. Kikuchi.S. 5Tsukada.T.Kubota.K.Hatta.N.1999b.Effect of strain rate on deformation behavior of a Mg8.5 Li1Zn alloy sheet at room temperature. Mater. Sci. Eng. 271, 251256.笔记本上盖外壳的镁合金薄板冲压模具设计内容提要在本研究中，在室温下分别用实验方法和有限元分析对笔记本上盖的lz91镁合金薄板冲压工艺制造情况进行检查。四操作冲压工艺的开发消除了上盖冲压过程中的断裂和褶皱缺陷。为了验证有限元分析，以0.6毫米厚的LZ91薄板作为毛坯，执行了一个实际的四操作冲压工艺过程。在实验数据和有限元结果之间，恰当地符合不同单元中的厚度分布，证实了有限元分析的精确性和有效性。本研究还通过成功地制造笔记本上盖外壳论证了室温下LZ91薄板的最优可模锻性。本文提出的四操作过程有助于产生一种有效的方法，实现用比目前实际要求还要少的操作程序来设计笔记本铰链，也证实了笔记本外壳可以用LZ91镁合金薄板的冲压工艺来制造，提供了一个镁合金在电子工业应用中的选择方法。关键字：笔记本外壳；LZ91镁合金薄板；多操作冲压；可模锻性1. 绪论镁合金由于具有重量轻和在电磁干扰阻力下有良好性能的优点，已被广泛用于电子行业的结构部件，如手机和笔记本电脑外壳。虽然在主要的镁合金制造过程中产品是进行压铸的，但是由于镁合金薄板的冲压强竞争性的生产力和在有效生产薄壁结构单元时的性能，在工业领域里人们已对其产生兴趣。在冲压过程中，尽管由于它封闭的六角晶体结构以至它的形成需要高温，AZ31镁合金（铝3，锌1）薄板在当前形成过程中已被广泛应用。最近，镁锂（LZ）合金已研制成功，它可以改善室温下镁合金的可模锻性。镁合金的韧性可以通过增加锂成分得到改善，来发展以立方体为中心的晶体结构的坯体的形成。在本研究中，检验了LZ薄板在笔记本电脑上盖外壳的冲压过程中的应用。笔记本上盖外壳的两个铰链的形成显示在图1的a和b中，由于边缘和边缘的小角落半径之间微小的距离，铰链的形成成了冲压过程中最困难的运行部分，这些影响在图1的c中已表示出来。这种几何的复杂性是当铰链的边缘与笔记本的边缘太接近时，由角落半径的变化引起的，这将很容易造成铰链周围的破裂，此时需要一个多操作冲压过程来克服这个问题。 在本研究中， 研究了LZ镁合金薄板的可模锻性，并用实验方法和有限元分析两种方法开发了最优多操作冲压过程，来减少运行程序的数量。图1 笔记本上盖外壳铰链的边缘 （a）铰链 （b）上盖外壳 （c）铰链边缘2. 镁合金薄板的力学性质对室温下LZ61（锂6，锌1）、LZ91、LZ101镁合金薄板与高温下AZ31薄板在拉伸实验中的力学性质做比较。图2（a）表明了LZ薄板在室温下与AZ31薄板在室温和200摄氏度时的应力变化关系。据图可知，应力变化曲线随着锂的增加而降低。同时可从图2观察到，室温下LZ91薄板和200摄氏度下AZ31薄板的力学性质是很接近的，显示了室温下LZ101比室温下LZ91和200摄氏度下AZ31更好的延展性。由于锂的成本非常昂贵，可选LZ91作为合适的LZ镁合金薄板，而不选用LZ101，来反应室温下良好的可模锻性。基于此，本研究采用LZ91薄板作为笔记本上盖外壳的毛坯，并研究其在室温下的可模锻性。为了判定在有限元分析中是否会发生破裂，0.6毫米的LZ91薄板形成极限图在图2（b）中已给出。 图2 镁合金的力学性质 （a）镁合金的应力应变关系（b）LZ91薄板的形成极限图3. 有限元模型模具的几何结构是由CAD、PRO/E软件构造的，并用DELTAMESH软件修正为有限元网格，如图3（a）所示。模具可视为刚体，四节点外壳组成部分用来构建毛坯网格。从实验中获得的材料性能和成形极限图被用来做有限元模拟。其他用于初始运行的模拟参数有：冲床速度为5毫米/秒，压边力为3KN， 干摩擦系数为0.1 。有限元软件PAM-STAMP用来进行分析，模拟在台式电脑上完成。有限元模型的构造首先用来研究铰链的单操作成形过程。考虑大批上盖外壳的对称性，我们只对其一半进行模拟，如图3（a）所示。图3（b）所显示的模拟结果表明破裂发生在最小厚度小于0.35毫米的边缘的拐角处。这意味着破裂问题是非常严重的，可能无法通过扩大边缘的拐角半径得到解决。进行有限元模拟来研究影响发生破裂的参数，并提出了几种避免破裂的方法。图3 有限元模拟 （a）有限元网格 （b）拐角处的破裂4. 多操作冲压过程设计为了避免发生破裂，多操作冲压过程是必需的。在目前的工业实践中，使用镁合金薄板形成上盖外壳通常需要至少十个运行程序。在本研究中，我们试图减少运行程序数目，并提出了几种方法来避免破裂，证明了四操作冲压过程在破裂问题中是一个可行的解决办法。由于文章长度的限制，接下来只对两操作和四操作冲压过程进行描述。4.1 两操作冲压过程两操作冲压过程中的第一个运行程序是形成侧壁，如图4（a）所示，第二个运行程序是形成高度为5毫米的铰链边缘，如图4（b）所示。图4（c）显示了从有限元模拟中得到的厚度分布，变形薄板的最小厚度为0.41毫米，而且应力都高于成形极限，这意味着破裂是可以避免的。此外，边缘的高度符合要实现的目标。然而，如图4（d）所示，这一进程产生了一个关键的缺陷在铰链边缘处发生起皱，这将导致在后面去毛刺过程中产生问题。因此，尽管两操作冲压过程解决了底部和铰链边缘拐角处的破裂问题，仍期望有更好的形成过程来解决铰链边缘的起皱问题。图4 两操作冲压过程 （a）侧壁的形成（b）铰链的形成（c）厚度分布（d）起皱4.2 四操作冲压过程四操作形成过程在本研究中的提出，如图5（a）所示，是始于三个侧壁和具有大的拐角半径的铰链边缘的形成。由于边缘附近的侧壁是打开的，而且拐角半径比设计的要大，可成功形成边缘，而且无破裂现象。这样的过程成功地避免了同时形成两个几何特征的困难，但增加了毛坯薄板的材料流通量。下一步工作是对侧壁界外的毛坯进行修剪，并把4毫米的拐角半径修正到要求的2.5毫米。铰链就这样形成了，如图5（b）所示。第三步是把打开的一面折起来，这样侧壁就可以完成其周边区域了，如图5（c）所示。研究了第二步中修剪侧壁界外的毛坯对第三步的影响。当多余的薄板没有被修剪时，拐角的厚度是0.381毫米，如图5（d）所示，而当第二步中修剪工作实施后拐角厚度增加到0.473毫米，如图5（e ）所示。第三步中由折叠过程产生的多余的材料在接下来的零件设计中会被作修剪处理。最后一步是最重要的一步，要对所有拐角半径与设计值进行校准。最终产品的拐角最小厚度是0.42毫米，并且所有的应力都高于形成极限。这是应当指出的是，图5（a-c）只显示一个铰链的形成。同样的设计概念可延伸到完整的上盖外壳的冲压过程中去。图5冲压过程 （a）第一步操作（b）第二步操作（c）第三步操作（d）未修剪（e）已修剪5 实验确认为了证实有限元分析，以0.6毫米厚的LZ91薄板作为毛坯，进行了一个实际的四操作冲压过程。毛坯的尺寸和模具的几何形状是根据有限元模拟的结果设计的。一个无破裂无皱纹的完美的产品便制造出来了，如图6（a）所示。为了进一步定量验证有限元分析，如图6（b），对完美产品的铰链附近的拐角的厚度进行测量，并与有限元模拟中得到的数据进行比较，结果列在表1中。 从表1可以看出，实验数据和有限元结果是一致的。四操作过程是以有限元分析为基础设计的，并由实验数据进行验证。图6 完美产品（a）无破裂无起皱（b）厚度测量点总 结本研究使用实验方法和有限元分析两种方法对镁合金薄板的冲压进行了研究。首先对AZ31和LZ薄板的可模锻性进行检验。研究结果表明，LZ91薄板在室温下有良好的可模锻性，同样，AZ31薄板在200摄氏度的形成温度下也有该性质。本研究还通过成功地制造笔记本上盖外壳证实了LZ91薄板在室温下的良好的可模锻性。本文提出的四操作过程证实了它自身是一种有效的方法，可以运用比现在实际要求少的运行程序来制造笔记本的铰链。它也证实，笔记本外壳可通过对LZ91镁合金薄板进行冲压制造而成。这为镁合金在电子工业中的应用提供了选择方案。References1Chen. F.K.Huang.T.B.Chang. C.K.2003. Deep drawing of square cups with magnesium alloy AZ31 sheets. Int. J. Mach. Tools2Manuf. 43.15531559.Drozd.Z.Trojanova .Z, Ku dela.S.2004. Deformation of behaviorof MgLiAl alloy. J. Mater. Compd. 378. 192195.3Takuda.H.Yoshii.T. Hatta, N.1999a. Finite-element analysis of the formability of a based alloy AZ31 sheet. J.4Mater. Process. Technol. 89/90. 135140.Takuda.H. Kikuchi.S. 5Tsukada.T.Kubota.K.Hatta.N.1999b.Effect of strain rate on deformation behavior of a Mg8.5 Li1Zn alloy sheet at room temperature. Mater. Sci. Eng. 271, 251256.18
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