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1防止活塞销冷挤压工艺中出现流动缺陷的新方法防止活塞销冷挤压工艺中出现流动缺陷的新方法D.J.Lee ,D.J.Kim, B.M.Kim精密机械工程系，研究生院，釜山国家大学，釜山，韩国机械设计工程部门，研究生院，釜山国家大学，釜山，韩国机械工程系，工程研究中心，釜山国家大学，釜山，韩国编号 3Janjeon-董，Kumjeong-顾，釜山 609-735，韩国摘要：这份报告主要研究的是作为汽车零部件之一的活塞销的流动缺陷。在联合冷挤压制活塞销的工艺中，起皱就是一种流动缺陷，它是由死金属区引起的。具有这种缺陷的部件带有很明显的外部特征，特征是被一微小而且厚的块状物嵌入材料中，这种缺陷对保证尺寸精度和降低材料损失是不利的，活塞销的这种缺陷对于其强度和疲劳寿命也有不利的影响。因此，在工艺设计的早期预测并防止这种缺陷是非常重要的。防止其产生的最好方法就是通过控制材料流动来限制或减少死金属区。有限元模拟分析方法被应用于流动缺陷研究分析当中，这份研究报告提出了通过去除死金属区防止产生流动缺陷的新工艺方法有限元分析法。将有限元分析的结果与实验结果做比较，结果表明有限元分析的结果与实验结果相符合。关键词：流动缺陷；活塞销钉；材料流动控制；前后双向冷挤压；死金属区；有限元分析1、序言冷加工是一种及其重要而且经济的加工方法，尤其对于大批量制件的加工，其优点更为突出。由于冷加工具有高的成品率、精确的尺寸精度、良好的表面光洁度，优良的机械加工性和冶金工艺性等优点，因此冷加工是工业生产当中应用最为广泛的零件加工工艺。冷锻制件广泛应用于飞机制造、摩托车、螺母和螺栓等生产制造。但是，冷锻制件也有可能产生缺陷，这主要取决于金属材料的变形过程、成形加工的外部条件2和材料的流动方式等。可延伸的裂纹缺陷是由材料的引应力状态和变形过程引起的；流动缺陷是由不稳定的材料流动引起的；低的尺寸精度是由低的模具尺寸精度和摩擦情况引起的，总之，锻压制件的缺陷主要包括两类，分别是内部缺陷和外部缺陷。这些缺陷危害到产品的质量和制造成本，因此，在工艺设计中的早期预防是非常重要的。利用有限元分析法中的不同可用标准来研究大型锻件的可延伸裂纹缺陷。KIM 和 KIM 对两道加强筋进行冷挤压件的内部和外部缺陷研究，并还在进行一种防止产生这些缺陷的加工工艺设计。这份报告是一份关于汽车活塞销产生的缺陷的测试报告，而这种活塞销是采用前后双向联合挤压的方式支撑的。这份报告中也提出了新的工艺方法可在工艺设计的早期防止产生流动缺陷，而这些新工艺方案是通过有限元分析研究得出的，实验证明，这些新工艺方案是可行的。2、成形工艺与缺陷形成分析2.1、成形工艺活塞销是汽车零部件当中用来连接活塞与曲轴的并传递动力的部件，当采用冷冲压制活塞销时，设计要求必须保证前后双向冲压时具有相同的高度并且不能出现锻压缺陷，因为活塞销在周期性大载荷作用下工作。制作活塞销的材料是 AISI-4135H 合金钢，它具有如下材料流动性 768.06*0.139 ，润滑措施是采用润滑油类的磷镀在活塞销表面进行润滑，经试验测试摩擦系数 M 为 0.1。加工活塞销钉以前用的是多步骤加工法（如图 3 所示） ，前两步通过导圆角和冲出非圆形的基准孔等预处理工序来减少缺陷的产生，从而可以提高尺寸精度和模具寿命，第三步和第四步相同，分别是从前后双向冲出圆形的腹板，最后一步是修整工序，从而得到活塞销的形状，然而，用普通加工方法加工的结果显示：第三步的早期会在腹板部位形成缺陷，更严重的是在缺陷产生的部位出现了一种不一致的流动形式，这种形式是一种非常坏的流动形式的延伸3 图 1 活塞销钉的形状和尺寸 图 2 活塞销钉的流动缺陷 图 3 活塞销钉传统的形成过程2.2 用有限元分析预测缺陷的产生塑性变形组织分布和有效应力对比图的应用，暗示着有限元精密塑造程序在成形与缺陷分析领域中的商业价值。最初的坯料直径为 30mm，深度为 61mm，最终成品的体积为 43.118，这种成形工艺看上去类似于普通加工结果。最大的裂缝值可以结算出断裂缺陷产生的可能性，在这个冲压过程中，其大小只有 0.08mm，而且分布在坯料和冲床活塞冲头接触的端部。因此，可以避免流动缺陷的产生，因此这种缺陷并不能产生可延展的裂纹。金属流动的流线图是由 Altan和 Knoerr 提出的，他们正在从事这种缺陷的分析研究，随着冲头冲压深度的增加，剧烈变动的流线出现了不同的流动速度，从而导致实验中缺陷的产生（如图 5 所示） 。所以金属流动只出现在第四步的反向冲压而不出现在正向冲压，并且在靠近腹板处的金属被拔起形成一条筋，很像是重叠缺陷，因此，活塞销的流动缺陷产生并发展的原因是：正反冲压时由于死金属区域产生而造成的金属流动速度的不同，这4种现象在像活塞销这种薄壁件冲出尺寸精度高，材料损耗少的孔的制件中是非常明显的。对于活塞销这类工作温度高，载荷大而且为交变载荷的零件来说，这种流动缺陷的产生会对其强度和疲劳寿命产生有害的影响。因此，有必要研究一种新工艺来防止产生流动缺陷。 图 4 有效的负荷和裂缝价值的关系图 5 金属流动和速度的关系3.防止缺陷的工艺分析与设计流动缺陷产生的原因是金属限制死金属区域的流动。为了在传统工艺中早期的冲压部位(第三步)消除死金属区，正冲压或反冲压工艺被改为联合正反冲压工艺，这种工艺在两个完全相反的方向上同时进行同样地动作。由于正反两向不同的冲压率和冲压长度，要使两个方向上同时完成材料流动是很困难的，因此在提前完成材料流动就会出现传统工艺一样出现的死金属区。因此，在活塞销成形这种情况下，两个方向的冲压率和冲压长度都是 1.89 和551mm。目前，一项关于活塞销的冲压长度的调查研究正在进行开模正反冲压工艺的分析，两个方向上的冲压长度是不同的，正向冲压长度长为 24.9mm，反向冲压长度如图 6 所示要比正向的短。反向金属流动必须强制性的被限制才能满足设计要求，而这就意为着死金属区会产生。因此，要想在两个方向上得到相同的冲压长度，提出了三种控制金属流动的方法，这三种方法都不同程度的强制限制金属流动。图 6 反向冲压长度3.1 改变初加工的形状在正反双向冲压之前，为了保证从腹板中心处起正反两个方向的冲压长度相等，就得要求初加工要将反向冲压筋的长度设计与双向冲压长度 24.9mm 有所不同。图 7展示了这种改进的工艺的结果，图 8 展示了在这种情况下采用正反双向冲压工艺时最后一步中金属的流动。从模拟实验的结果可以得出，两个方向的冲压筋的长度都是 51mm，这恰好满足设计要求和活塞销的尺寸要求。另外，死金属区的金属流动形式相同，而不像采用普通加工时会产生流动缺陷，而且在两个方向上的流动速度也是连续变化的，这就意为着金属流动在整个过程中是一致的，不会出现限制其流动的死金属区。6 图七 多级样板的修改过程 图八金属网的流动 3.2 驱动冲压模膛驱动模膛工艺被用来控制金属流动从而满足设计要求，这种设备采用向相反方向运动的模膛先与已经冲压成形的一侧接触（如图 9 所示） ，这样就有助于加快后冲压方向上的金属流动而减慢先冲压方向上的金属流动速度，采用这种工艺制作的活塞销，由于反方向冲压提前完成，而此时活塞正沿着这个方向移动从而增加了金属沿着这个方向的流动，这个工艺的首要变化因素是冲头与活塞的相对速率和金属材料与活塞之间的摩擦条件。在这个研究中，由于摩擦系数 m0.1（在毛胚材料和模膛之间） ，模拟实验只与相对速率这一变量有关。如果相对速率小于满足同时成型最合适的速率，则在反向方向上的冲压过程就会比正向冲压提前完成，这样的话就会像采用普通加工一样在相同部位产生流动缺陷，相反，如果相对速率大于最适宜的速率，则正向冲压过程就会比反向冲压过程提前完成，这样就会在相反地部位产生缺陷。因此，为了满足设计要求，采用半分法可以找出最佳的相对速率，从结果来看，最佳的相对速率是 0.48，图 10 和 11 显示了相对速率分别为 0.1 、0.48、1.0 时采用一次冲压变形过程和金属流动情况。图 11（c）显示了当采用最佳相对速率 0.48时的金属流动形式，它记录了一个可以防止缺陷产生的流动形式。7图 9 轴向移动的箱体示意图图 10 根据相对速度比率变化的活塞销钉形态图 11 根据相对速度比率比较的金属3.3 修改模具结构这种被提出的修改模具结构的工艺可以限制金属在反方向上的流动，而在这个方向上容易提前完成变形，从而可以实现在两个方向上同时完成变形，采用这种工艺时，为了能在两个方向上同时完成变形过程而得到相同的变形长度，卸料器又被设计者重新采用，它是一种使冲头从制件中抽出的装置。如果采用普通加工工艺中的固定式卸料器，则由于材料流动受到限制，会出现死金属区，而此时产生的部位与采用双向冲压时产生在中间位置不同。8因此，一种利用弹簧弹力的结构可以推迟金属材料沿反方向的流动。图 12显示了这种模具结构，采用这种方法，选用合适的弹簧弹力对于满足变形同时完成的要求来讲是很重要的，因而有限元模拟可以计算出这种必要地弹力。从模拟结果来看，需要给卸料器施加 5 吨的弹力。图 13 展示了这种工艺下金属流动形式，与其它改进的工艺方法相比，这种工艺在死金属区没有出现不连续的流动速度，此处的金属流动形式是相同的。 图 12 使用冲压模板的凹模模子结构示意图 图 13 使用冲压模板的金属流动4.结果和实验通过有限元分析法分析出的三种方法中是适合防止金属的流动缺陷。每个方法的情况如下。第一种方法是初步加工的产品需要三级过程(预制， 正反压挤，穿孔)并且有一个简单的模具结构；第二方法是使用沿轴方向移动的冲孔模板；第三种方法是轴向移动的箱体需要二级过程(前后压挤，穿孔)并且有一个复杂的模具结构。关于在里面形成的负荷，这三个方法都非常相似。特别是在沿轴方向移动的大约10吨的箱体情况下形成最大的负荷比其他方法小，因为在穿孔过程中沿轴方向移动的箱体会增加材料的流动。通过表1分析出的方法为形成做出了比较。在这项研究过程中，一个用在初步加工产品的实验被进行，并且为了证实模拟结果所以使用一个250吨能力的多级样板。在穿孔之前，为了金属的观察蚀刻流动能够正常被进行，所以必须为活塞销做一个流动缺陷检查。图14就是表示这个实验结果，这种方法改变了初步加工的产品。实验结果证明了在缺陷区域内金属流动的缺陷是相同的，并且满足形成同时完成和在两个挤压方向长度相同。这9种过程和模拟的结果相符。传统方法初步加工的产品的使用冲压模板的使用移动箱体的用途 最大负荷（吨）97.296.396.184.0挤压的过程2个阶段2个阶段1个阶段1个阶段缺陷存在不存在不存在不存在表1 各个方法的比较 图14 对流动缺陷的消除5.结论在这项研究过程中，流动缺陷过程和预防缺陷的过程都已经被有限元分析重新设计。，缺陷的原因已经被分析，并且通过分析已经模拟出了结果。从模拟结果中可以看出，有限元分析方法是可以防止流动缺陷并且满足生产过程中控制材料的流动状态。通过有限元分析的结果和实验的结果做比较，可以得出以下几个结论：（1）活塞销里存在流动缺陷的原因是材料限制死金属区域的流动。消除这个区域最重要的是控制材料的流动。（2）初步加工的产品设计和改变模具结构是使用轴向运动的挤压箱来消除挤压过程中出现的流动缺陷。（3）被提出的方法满足了工艺的要求，向前挤压的长度部分和落后的部分都是相同的，这些已经由实验所证实。10参考文献：1 T.Altan，S.I.Oh，L.Gegel，Metal forming，ASM(1983).2 T. Okamoto，T. Fukuda，H. Hagita，Source Book on Cold Forming，ASTM，1997，pp. 216226.3 S.W.Oh，T.H.Kim，B.M.Kim，J.C.Choi，KSME 19 (12) (1995) 31213129.4 R.C.Batra，N.V.Nechitailo，Int.J.Plast. 13 (4) (1997) 291306.5 A.S. Wifi，A.Abdel-Hamid，N. El-Abbasi， J. Mater. Process. Technol.77 (1998) 285293.6 D.J. Kim，B.M. Kim，J. KSTP 8 (6) (1999) 612619.7 D.C. Ko，Pusan National University Dissertation，1998.8 T. Altan，M. Knoerr，J. Mater. Process. Technol. 35 (1992) 275302.9 K. Osakata，X. Wang，S. Hanami，J. Mater. Process. Technol. 71 (1997) 105112. 摘要冲压制品已在工业，农业，国防和日常生活中的方面得到广泛应用,特别是在机械业中则为突出。机械产品的外壳大部分是冲压制品，产品性能的提高要求高素质的冲压模具和冲压性能，成型工艺和制品的设计。 冲压制品的成型方法很多。其主要用于是冲孔，落料，弯曲，拉伸等。而冲压模，约占成型总数的 60%以上。当然如利用电气控制，可实现半自动化或自动化作业。冷冲冲裁模主要用于金属制品的成型，它是冲压制品生产中十分重要的工艺装置。冲压模的基本组成是：上下模座、下模垫板、下模固定板、凹模镶块、抬料钉、导料板、卸料板,导柱导套、卸料板弹钉、卸料板等。冲裁模成型的广泛适用，正是我这个设计的根本出发点。关键词：冲孔、落料 AbstractAbstract Stamping products has been extensively applied in the industry, agriculture, national defense and in the daily lives of area, especially in the machinery industry. Mechanical products is the most pressing housing products, and the improvement of product performance requires of high-quality performance stamping molds ,stamping,process and product design. There are many ways of molding products of stamping. Piercing is mainly used for blanking, bending, stretching, etc. And Stamping molds almost form more than 60 percent of the total number. For example ,Electrical Control can be realized as semi-automatic or automatic operation. Cold-metal stamping die mainly used for the molding products, and it is very important in the production of stamping technology devices. The basic component of stamping molds is block model from top to bottom, mould plate, fixed-plate of mould plate, die inserts, raising nails, I. plate, plate unloading I. Introduction sets column, unloading bombs nail plate, plate Discharge and so on. The widely application of blanking moulding is exactly the basic perpose of my design.Key words: Piercing、Blanking 目 录1 绪论.11.1 冲压的概念、特点及应用 .11.2 冲压的基本工序及模具 .21.3 冲压技术的现状及发展方向 .31.3.1 冲压成形理论及冲压工艺方面.31.3.2 冲模是实现冲压生产的基本条件.41.3.3 冲压设备和冲压生产自动化方面.51.3.4 冲压标准化及专业化生产方面.61.4 设计要求 .72 冲裁工艺设计.82.1 冲裁件的工艺分析 .82.1.1 材料特性分析.82.1.2 冲裁件的结构工艺性分析.82.2 冲压工艺方案的确定 .93 排样设计及材料利用率计算.113.1 排样方案的确定 .113.2 搭边的选取 .113.3 送料步距、条料宽度及导料销与条料间距计算 .123.4 材料利用率的计算 .134 冲裁工艺计算.144.1 冲裁力和压力中心的计算 .144.1.1 冲裁力的计算.144.1.2 卸料力、推料力和顶件力的计算.154.1.3 压力中心的计算.16 4.1.4 压力机的选择 .174.1.5 曲柄压力机的主要技术参数.184.1.6 曲柄压力机的选用 .194.2 凸凹模刃口尺寸的计算 .194.2.1 落料刃口尺寸的计算.214.2.2 冲孔刃口尺寸计算.225 模具主要零部件结构和设计.245.1 卸料装置 .245.1.1 橡胶的选用.255.2 出件装置 .285.3 定位零件 .285.4 凹模的设计 .295.5 凸凹模的设计 .315.6 凸模的设计 .315.7 模架 .325.7.1 模板.325.7.2 导向零件.335.8 联接与固定零件 .345.8.1 模柄.345.8.2 凸模固定板与垫板.345.8.3 螺纹紧固件.345.8.4 圆柱销.356 确定装配基准.377 总结.38致谢.39参考文献.40 1 绪论1.1 冲压的概念、特点及应用 冲压是利用安装在冲压设备（主要是压力机）上的模具对材料施加压力，使其产生分离或塑性变形，从而获得所需零件（俗称冲压或冲压件）的一种压力加工方法。冲压通常是在常温下对材料进行冷变形加工，且主要采用板料来加工成所需零件，所以也叫冷冲压或板料冲压。冲压是材料压力加工或塑性加工的主要方法之一，隶属于材料成型工程术。 冲压所使用的模具称为冲压模具，简称冲模。冲模是将材料（金属或非金属）批量加工成所需冲件的专用工具。冲模在冲压中至关重要，没有符合要求的冲模，批量冲压生产就难以进行；没有先进的冲模，先进的冲压工艺就无法实现。冲压工艺与模具、冲压设备和冲压材料构成冲压加工的三要素，只有它们相互结合才能得出冲压件。 与机械加工及塑性加工的其它方法相比，冲压加工无论在技术方面还是经济方面都具有许多独特的优点。主要表现如下:(1) 冲压加工的生产效率高，且操作方便，易于实现机械化与自动化。这是因为冲压是依靠冲模和冲压设备来完成加工，普通压力机的行程次数为每分钟可达几十次，高速压力要每分钟可达数百次甚至千次以上，而且每次冲压行程就可能得到一个冲件。（2）冲压时由于模具保证了冲压件的尺寸与形状精度，且一般不破坏冲压件的表面质量,而模具的寿命一般较长,所以冲压的质量稳定,互换性好,具有“一模一样”的特征。（3）冲压可加工出尺寸范围较大、形状较复杂的零件，如小到钟表的秒表，大到汽车纵梁、覆盖件等，加上冲压时材料的冷变形硬化效应，冲压的强度和刚度均较高。（4）冲压一般没有切屑碎料生成，材料的消耗较少，且不需其它加热设备，因而是一种省料，节能的加工方法，冲压件的成本较低。但是，冲压加工所使用的模具一般具有专用性，有时一个复杂零件需要数套模 具才能加工成形，且模具 制造的精度高，技术要求高，是技术密集形产品。所以，只有在冲压件生产批量较大的情况下，冲压加工的优点才能充分体现，从而获得较好的经济效益。 冲压地、在现代工业生产中，尤其是大批量生产中应用十分广泛。相当多的工业部门越来越多地采用冲压法加工产品零部件，如汽车、农机、仪器、仪表、电子、航空、航天、家电及轻工等行业。在这些工业部门中，冲压件所占的比重都相当的大，少则 60%以上，多则 90%以上。不少过去用锻造=铸造和切削加工方法制造的零件，现在大多数也被质量轻、刚度好的冲压件所代替。因此可以说，如果生产中不谅采用冲压工艺，许多工业部门要提高生产效率和产品质量、降低生产成本、快速进行产品更新换代等都是难以实现的。1.2 冲压的基本工序及模具 由于冲压加工的零件种类繁多，各类零件的形状、尺寸和精度要求又各不相同，因而生产中采用的冲压工艺方法也是多种多样的。概括起来，可分为分离工序和成形工序两大类；分离工序是指使坯料沿一定的轮廓线分离而获得一定形状、尺寸和断面质量的冲压（俗称冲裁件）的工序；成形工序是指使坯料在不破裂的条件下产生塑性变形而获得一定形状和尺寸的冲压件的工序。 上述两类工序，按基本变形方式不同又可分为冲裁、弯曲、拉深和成形四种基本工序，每种基本工序还包含有多种单一工序。 在实际生产中，当冲压件的生产批量较大、尺寸较少而公差要求较小时，若用分散的单一工序来冲压是不经济甚至难于达到要求。这时在工艺上多采用集中的方案，即把两种或两种以上的单一工序集中在一副模具内完成，称为组合的方法不同，又可将其分为复合-级进和复合-级进三种组合方式。 复合冲压在压力机的一次工作行程中，在模具的同一工位上同时完成两种或两种以上不同单一工序的一种组合方法式。 级进冲压在压力机上的一次工作行程中，按照一定的顺序在同一模具的不同工位上完面两种或两种以上不同单一工序的一种组合方式。 复合-级进在一副冲模上包含复合和级进两种方式的组合工序。 冲模的结构类型也很多。通常按工序性质可分为冲裁模、弯曲模、拉深模和成形 模等；按工序的组合方式可分为单工序模、复合模和级进模等。但不论何种类型的冲模，都可Journal of Materials Processing Technology 139 (2003) 422427New processes to prevent a flow defect in the combinedforwardbackward cold extrusion of a piston-pinD.J. Leea, D.J. Kimb, B.M. Kimc,aDepartment of Precision Mechanical Engineering, Graduate School, Pusan National University, Pusan, South KoreabDepartment of Mechanical Design Engineering, Graduate School, Pusan National University, Pusan, South KoreacDepartment of Mechanical Engineering, Engineering Research Center for Net Shape and Die Manufacturing, Pusan National University, No. 3,Janjeon-Dong, Kumjeong-Ku, Pusan 609-735, South KoreaAbstractA flow defect of a piston-pin for automobile parts are investigated in this study. In the combined cold extrusion of a piston-pin, a lappingdefect,whichisakindofflowdefect,appearsbythedeadmetalzone.Thisdefectisevidentinproductswithasmallthicknesstobepiercedand is detrimental to dimensional accuracy and decrease of material loss. The flow defect that occurs in the piston-pin has bad effects onthe strength and the fatigue life of the piston-pin. Therefore, it is important to predict and prevent the defect in the early stage of processdesign. The best method that can prevent the flow defect is removing or reducing dead metal zone through the control of material flow.Finite element simulations are applied to analyze the flow defect. This study proposes new processes which can prevent the flow defect byremoving the dead metal zone. Then the results are compared with the results of experiments for verification. These FE simulation resultsare in good agreement with the experimental results. 2003 Elsevier Science B.V. All rights reserved.Keywords: Flow defect; Piston-pin; Material flow control; Forwardbackward extrusion; Dead metal zone; FE simulation1. IntroductionCold forming is extremely important and economical pro-cesses, especially for producing parts in large quantities.Because of advantages of cold forming such as high pro-duction rates, excellent dimensional tolerances and surfacefinish, mechanical and metallurgical properties, cold form-ing is by far the largest application of industry for producingparts.However, cold forged parts are also used in manufactur-ing aircraft, motorcycles, nuts and bolts 1, but it is possiblefor defects to occur in forged parts, depending on the de-formation history, forming conditions and material flow pat-tern, etc. The kind of defects are ductile fracture caused bythe state of stress and the deformation history, flow defectscaused by unstable material flow, and poor dimensional tol-erances caused by inferiority of the die and friction condi-tion. Further, defects in forged parts are classified as internaldefects and external defects 24.These defects have harmful effects on the quality of theproduct and an increase in the cost of production. Therefore,Corresponding author. Tel.: +82-51-510-3074; fax: +82-51-514-7640.E-mail address: bmkimpusan.ac.kr (B.M. Kim).it is important to predict and prevent defects in the earlystage of process design.Wifiet al. 5 studied ductile fracture in bulk formedparts, using different workability criteria by the finite ele-ment method. Kim and Kim 6 studied internal and exter-nal defects of cold extruded products with double ribs andperformed process design to prevent these defects.In this study is examined a defect which occurs in produc-ing a forwardbackward extrusion product, a piston-pin foran automobile part, and new processes are designed to pre-vent the defect by finite element method in the early stageof process design. Then the results are compared with theresults of experiments for verification.2. Forming and defect-occurrence analysis2.1. Forming processThe piston-pin is an automobile components used in thetransmission of power between the connecting rod and thecrankshaft. In the cold extrusion of a piston-pin, the designrequirements are to keep the same height of the forwardextruded part and the backward part (Fig. 1) without anydefect in the forged product, for use under high and repeated0924-0136/03/$ see front matter 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0924-0136(03)00515-6D.J. Lee et al./Journal of Materials Processing Technology 139 (2003) 422427423Fig. 1. Shape and dimension of the piston-pin.Fig. 2. Photograph of a flow defect of a piston-pin.load. The material used for the piston-pin is AISI-4135H(Fig. 2) alloy steel, with the following flow stress behavior: = 768.06 0.139(MPa)Fig. 4. Distribution of effective strain and fracture value.Fig. 3. Conventional forming process for a piston-pin.The lubricant used is phosphate coating and bond lube. Thefriction factor, m, is assumed to be 0.1, which is confirmedby the ring compression test.The sequence of the conventional process for thepiston-pin is performed using a multi-stage former (Fig. 3).The first and second stages are pre-upsetting to eliminatedefects by the cropping process such as ovality and ec-centricity of the billet for improvement of dimensionaltolerances and die life whilst the third and forth stages areforward or backward extrusion for the forming of one di-rection from the web, and final stage is the piercing processfor the pin shape.However, the results of experiment for the conventionalprocessdisplayedadefectinthewebpartformedearlyinthethird process (Fig. 3). Especially, a nonuniform flow patternis observed in part of the defect occurrence, which lookslike a flow defect similar to lapping with an undesirable flowpattern.2.2. Prediction of defects by FE analysisDEFORM is used, which is commercial code of arigid-plastic FE program for forming and defect analy-sis. The diameter of the initial billet is 30mm and theheight is 61mm, the whole volume of final product being424D.J. Lee et al./Journal of Materials Processing Technology 139 (2003) 422427Fig. 5. Metal flow and velocity distribution, where a defect occurs according to stroke.43,118mm3. The forming is simulated with a conventionalprocess sequence.The maximum fracture value that can estimate the occur-rence of a crack 7 is small at 0.08 and is distributed ina position within the head part of the punch, so that a de-fect does not occur. Thus this defect is not one due to duc-tile fracture (Fig. 4). Then flow line-tracking scheme thatwas proposed by Altan and Knoerr 8 is performed for de-fect analysis. According to the progress of the punch stroke,severe variation of flow lines appears and discontinuity ofvelocity occurs in the part that a defect occurred in the ex-periment (Fig. 5).Consequently, the metal flows only in the backward di-rection without flow to the forward direction in the fourthprocess and metal near the web part is pulled up in the ribpart like a lapping defect. Therefore, the cause of the ini-tiation and development of the flow defect that occurred inpiston-pin is the velocity discontinuity between backwardand forward direction by the formation of a dead metalzone. This appearance evidently occurs in products like apiston-pin with a low thickness to be pierced for the dimen-sional accuracy and the decrease of material loss. A flowdefect occurring in a piston-pin has harmful effects on thestrength and the fatigue life of a piston-pin that has high andrepeated load at high temperature. Therefore, it is necessaryfor a new process to prevent the flow defect.3. Process redesign and analysis for the preventionof defectThe cause of the initiation and development of the flowdefect is the restriction of metal flow by the dead metalzone. For the elimination of the dead metal zone in theearly extruded part (3rd process) in the conventional process,the forward or backward extrusion process is modified tocombined forwardbackward extrusion, which is performedsimultaneously in the two directions. Because of the varietyof extrusion ratios and lengths in the forward and backwarddirections, the simultaneous completion of the material flowin the both directions is very difficult. Consequently, one ofthe directions is completed early, then material flow stoppedand dead metal zone appears in this part just like that in theconventional process.Therefore, in the case of piston-pin forming, the extrusionratio and the length of both directions are the same at 1.89and51mm.First,analysisofopendieforwardbackwardex-trusion is performed for an investigation of extrusion lengthsof the piston-pin. The difference of two extruded ribs is24.9mm and the backward extruded rib is shorter than theforward extruded rib as shown in Fig. 6.The metal flow of backward direction must be restrictedcompulsorily for the satisfaction of the design conditionsand this means the occurrence of a dead metal zone. There-fore, for the same extrusion length in both directions, threeFig. 6. Extrusion length in forwardbackward extrusion.D.J. Lee et al./Journal of Materials Processing Technology 139 (2003) 422427425Fig. 7. Modified process sequence for a multi-stage former.methods are proposed to control the metal flow without thecompulsory restriction of metal flow3.1. Change of preform shapeTo secure the same length of both directions from thecenter of web, it is required that the backward extrudedrib is performed by preform design as the difference ofboth-direction lengths at 24.9mm from the above results,before forwardbackward extrusion. Fig. 7 shows the mod-ified process sequence, and Fig. 8 shows the metal flow ofthe final stage of forwardbackward extrusion in this case.From the results of simulation, the lengths of two extrudedribs are 51mm, which is the dimension of the piston-pin andsatisfied the design condition. In addition, the metal flow isuniform in the defect zone where the flow defect occurredin the conventional process, and there is not a discontinu-ity of velocity in both extrusion directions. This means thatmetal flows uniformly in the whole process without a deadmetal zone by restriction of metal flow.Fig. 8. Metal flow of web in case of using preform.Fig. 9. Schematic diagram of the axially moving container die structure.3.2. Driving of extrusion containerThe driving extrusion container method 9 is used formetal flow control for the satisfying of the design condi-tion. This structure is that the extrusion container is movedin the counter direction to the early extruded one (Fig. 9).This has the effect of increasing the metal flow in the lateextruded direction and restricting metal flow in the early ex-truded direction. In the case of the piston-pin, because ofthe early completion of backward extrusion, the extrusioncontainer is moved in the forward direction for the increaseof metal flow to this direction. In this process, the princi-pal process variables are the relative velocity ratio of thepunch and the moving extrusion container, and the frictioncondition between the material and the moving extrusioncontainer.In this study, because the friction factor, m, is 0.1 be-tween the material and container, simulation is performedonly according to the variation of the relative velocity ratio(VC/VP= 0.1, 0.25, 0.5, 0.75, 1.0). If the relative velocityratio is smaller than the optimum which can complete form-ing simultaneously, extrusion in the backward direction iscompleted earlier than in the forward direction and a flowdefect occur in the same part as in the conventional process.Otherwise, if the relative velocity ratio is larger than the op-timum one, extrusion in the forward direction is completeearlier than backward direction and a flow defect occurs inthe opposite part to where a defect occurs in the conven-tional process.Therefore, for satisfaction of the design conditions, theoptimum relative velocity ratio is searched for by an opti-mization technique, the bisection method. From the result,the optimum relative velocity ratio is 0.48. Figs. 10 and 11show the deformation modality and metal flow according tothe relative velocity ratio (0.1, 0.48, 1.0) for a punch strokeof 42.7mm, respectively. Fig. 11(c) shows the metal flow426D.J. Lee et al./Journal of Materials Processing Technology 139 (2003) 422427Fig. 10. Deformation modality of the piston-pin according to the relative velocity ratio.Fig. 11. Comparisons of metal flow according to the relative velocity ratio.at the optimum relative velocity (0.48) where an improvedflow pattern without a flow defect can be noted.3.3. Modification of die structureA modification of the die structure is proposed whichcan restrict the metal flow of backward direction, which isdeformed early, for simultaneous completion of extrusion inboth directions. In this case, for simultaneous completionand the same length in both directions, the stripper, which isFig. 12. Schematic diagram of die structure using stripper.equipmentforpunchextractionfromproducts,isredesigned.If a fixed stripper of conventional type is used, a dead metalzone appears from the middle stage of backwardforwardextrusion by the restriction of material flow.Therefore, a structure is used that can delay the metal flowin the backward direction by spring force. Fig. 12 showsthe die structure. For this method, it is very important todecide the proper spring force for simultaneous completionof forming. Therefore, the necessary spring force for this iscalculated by FE simulation. From the simulation result, itwas 5t to be applied load to stripper. Fig. 13 shows metalflow in this case. The metal flow is similarly uniform at thedefect zone without discontinuity of velocity in comparisonwith other modification methods.Fig. 13. Metal flow of web in case of using stripper.D.J. Lee et al./Journal of Materials Processing Technology 139 (2003) 422427427Table 1Comparison process for each of the proposed methodConventional methodUse of preformUse of stripperUse of moving containerMaximum load (t)97.296.396.184.0Process of extrusion2 stage2 stage1 stage1 stageDefectExistNoneNoneNone4. Results and experimentFrom the FE simulation, the three proposed methodsare proper to prevent a flow defect by metal flow control.The characteristics of each process are as follows. The firstmethod that uses a preform needs three stage processes (pre-forming, forwardbackward extrusion, piercing) and has asimple die structure; however, the second method that usesa stripper and the third method that uses an axially mov-ing container need two-stage processes (forwardbackwardextrusion, piercing) and have a complex die structure. Inrespect of the forming load, the processes are similar toeach other.Especially,themaximumformingloadissmallerthanthatof other processes by about 10t in the case of the axiallymoving container, because the axially moving container in-creases material flow in the direction punch movement. It iscompared with the proposed method for forming by a pressin Table 1. In this study, an experiment using a preform isperformed and uses a mult