活塞環(huán)、活塞桿的密封試驗(yàn)臺(tái)設(shè)計(jì)【說明書+CAD+UG】

活塞環(huán)、活塞桿的密封試驗(yàn)臺(tái)設(shè)計(jì)【說明書+CAD+UG】,說明書+CAD+UG,活塞環(huán)、活塞桿的密封試驗(yàn)臺(tái)設(shè)計(jì)【說明書+CAD+UG】,活塞環(huán),活塞桿,密封,試驗(yàn)臺(tái),設(shè)計(jì),說明書,仿單,cad,ug 防止活塞銷冷擠壓工藝中出現(xiàn)流動(dòng)缺陷的新方法 D.J.Lee ,D.J.Kim, B.M.Kim 精密機(jī)械工程系，研究生院，釜山國(guó)家大學(xué)，釜山，韓國(guó) 機(jī)械設(shè)計(jì)工程部門，研究生院，釜山國(guó)家大學(xué)，釜山，韓國(guó) 機(jī)械工程系，工程研究中心，釜山國(guó)家大學(xué)，釜山，韓國(guó)編號(hào)3 Janjeon-董，Kumjeong-顧，釜山609-735，韓國(guó) 摘要： 這份報(bào)告主要研究的是作為汽車零部件之一的活塞銷的流動(dòng)缺陷。在聯(lián)合冷擠壓制活塞銷的工藝中，起皺就是一種流動(dòng)缺陷，它是由死金屬區(qū)引起的。具有這種缺陷的部件帶有很明顯的外部特征，特征是被一微小而且厚的塊狀物嵌入材料中，這種缺陷對(duì)保證尺寸精度和降低材料損失是不利的，活塞銷的這種缺陷對(duì)于其強(qiáng)度和疲勞壽命也有不利的影響。因此，在工藝設(shè)計(jì)的早期預(yù)測(cè)并防止這種缺陷是非常重要的。防止其產(chǎn)生的最好方法就是通過控制材料流動(dòng)來限制或減少死金屬區(qū)。有限元模擬分析方法被應(yīng)用于流動(dòng)缺陷研究分析當(dāng)中，這份研究報(bào)告提出了通過去除死金屬區(qū)防止產(chǎn)生流動(dòng)缺陷的新工藝方法——有限元分析法。將有限元分析的結(jié)果與實(shí)驗(yàn)結(jié)果做比較，結(jié)果表明有限元分析的結(jié)果與實(shí)驗(yàn)結(jié)果相符合。 關(guān)鍵詞： 流動(dòng)缺陷；活塞銷釘；材料流動(dòng)控制；前后雙向冷擠壓；死金屬區(qū)；有限元分析 1、序言 冷加工是一種及其重要而且經(jīng)濟(jì)的加工方法，尤其對(duì)于大批量制件的加工，其優(yōu)點(diǎn)更為突出。由于冷加工具有高的成品率、精確的尺寸精度、良好的表面光潔度，優(yōu)良的機(jī)械加工性和冶金工藝性等優(yōu)點(diǎn)，因此冷加工是工業(yè)生產(chǎn)當(dāng)中應(yīng)用最為廣泛的零件加工工藝。 冷鍛制件廣泛應(yīng)用于飛機(jī)制造、摩托車、螺母和螺栓等生產(chǎn)制造。但是，冷鍛制件也有可能產(chǎn)生缺陷，這主要取決于金屬材料的變形過程、成形加工的外部條件和材料的流動(dòng)方式等。可延伸的裂紋缺陷是由材料的引應(yīng)力狀態(tài)和變形過程引起的；流動(dòng)缺陷是由不穩(wěn)定的材料流動(dòng)引起的；低的尺寸精度是由低的模具尺寸精度和摩擦情況引起的，總之，鍛壓制件的缺陷主要包括兩類，分別是內(nèi)部缺陷和外部缺陷。 這些缺陷危害到產(chǎn)品的質(zhì)量和制造成本，因此，在工藝設(shè)計(jì)中的早期預(yù)防是非常重要的。利用有限元分析法中的不同可用標(biāo)準(zhǔn)來研究大型鍛件的可延伸裂紋缺陷。KIM和KIM對(duì)兩道加強(qiáng)筋進(jìn)行冷擠壓件的內(nèi)部和外部缺陷研究，并還在進(jìn)行一種防止產(chǎn)生這些缺陷的加工工藝設(shè)計(jì)。 這份報(bào)告是一份關(guān)于汽車活塞銷產(chǎn)生的缺陷的測(cè)試報(bào)告，而這種活塞銷是采用前后雙向聯(lián)合擠壓的方式支撐的。這份報(bào)告中也提出了新的工藝方法可在工藝設(shè)計(jì)的早期防止產(chǎn)生流動(dòng)缺陷，而這些新工藝方案是通過有限元分析研究得出的，實(shí)驗(yàn)證明，這些新工藝方案是可行的。 2、成形工藝與缺陷形成分析 2.1、成形工藝 活塞銷是汽車零部件當(dāng)中用來連接活塞與曲軸的并傳遞動(dòng)力的部件，當(dāng)采用冷沖壓制活塞銷時(shí)，設(shè)計(jì)要求必須保證前后雙向沖壓時(shí)具有相同的高度并且不能出現(xiàn)鍛壓缺陷，因?yàn)榛钊N在周期性大載荷作用下工作。制作活塞銷的材料是AISI-4135H合金鋼，它具有如下材料流動(dòng)性 σ＝768.06*ε0.139 ，潤(rùn)滑措施是采用潤(rùn)滑油類的磷鍍?cè)诨钊N表面進(jìn)行潤(rùn)滑，經(jīng)試驗(yàn)測(cè)試摩擦系數(shù)M為0.1。 加工活塞銷釘以前用的是多步驟加工法（如圖3所示），前兩步通過導(dǎo)圓角和沖出非圓形的基準(zhǔn)孔等預(yù)處理工序來減少缺陷的產(chǎn)生，從而可以提高尺寸精度和模具壽命，第三步和第四步相同，分別是從前后雙向沖出圓形的腹板，最后一步是修整工序，從而得到活塞銷的形狀，然而，用普通加工方法加工的結(jié)果顯示：第三步的早期會(huì)在腹板部位形成缺陷，更嚴(yán)重的是在缺陷產(chǎn)生的部位出現(xiàn)了一種不一致的流動(dòng)形式，這種形式是一種非常壞的流動(dòng)形式的延伸 圖1 活塞銷釘?shù)男螤詈统叽? 圖2 活塞銷釘?shù)牧鲃?dòng)缺陷 圖3活塞銷釘傳統(tǒng)的形成過程 2.2用有限元分析預(yù)測(cè)缺陷的產(chǎn)生 塑性變形組織分布和有效應(yīng)力對(duì)比圖的應(yīng)用，暗示著有限元精密塑造程序在成形與缺陷分析領(lǐng)域中的商業(yè)價(jià)值。最初的坯料直徑為30mm，深度為61mm，最終成品的體積為43.118，這種成形工藝看上去類似于普通加工結(jié)果。 最大的裂縫值可以結(jié)算出斷裂缺陷產(chǎn)生的可能性，在這個(gè)沖壓過程中，其大小只有0.08mm，而且分布在坯料和沖床活塞沖頭接觸的端部。因此，可以避免流動(dòng)缺陷的產(chǎn)生，因此這種缺陷并不能產(chǎn)生可延展的裂紋。金屬流動(dòng)的流線圖是由Altan和Knoerr提出的，他們正在從事這種缺陷的分析研究，隨著沖頭沖壓深度的增加，劇烈變動(dòng)的流線出現(xiàn)了不同的流動(dòng)速度，從而導(dǎo)致實(shí)驗(yàn)中缺陷的產(chǎn)生（如圖5所示）。 所以金屬流動(dòng)只出現(xiàn)在第四步的反向沖壓而不出現(xiàn)在正向沖壓，并且在靠近腹板處的金屬被拔起形成一條筋，很像是重疊缺陷，因此，活塞銷的流動(dòng)缺陷產(chǎn)生并發(fā)展的原因是：正反沖壓時(shí)由于死金屬區(qū)域產(chǎn)生而造成的金屬流動(dòng)速度的不同，這種現(xiàn)象在像活塞銷這種薄壁件沖出尺寸精度高，材料損耗少的孔的制件中是非常明顯的。對(duì)于活塞銷這類工作溫度高，載荷大而且為交變載荷的零件來說，這種流動(dòng)缺陷的產(chǎn)生會(huì)對(duì)其強(qiáng)度和疲勞壽命產(chǎn)生有害的影響。因此，有必要研究一種新工藝來防止產(chǎn)生流動(dòng)缺陷。 圖4有效的負(fù)荷和裂縫價(jià)值的關(guān)系 圖5金屬流動(dòng)和速度的關(guān)系 3.防止缺陷的工藝分析與設(shè)計(jì) 流動(dòng)缺陷產(chǎn)生的原因是金屬限制死金屬區(qū)域的流動(dòng)。為了在傳統(tǒng)工藝中早期的沖壓部位(第三步)消除死金屬區(qū)，正沖壓或反沖壓工藝被改為聯(lián)合正反沖壓工藝，這種工藝在兩個(gè)完全相反的方向上同時(shí)進(jìn)行同樣地動(dòng)作。由于正反兩向不同的沖壓率和沖壓長(zhǎng)度，要使兩個(gè)方向上同時(shí)完成材料流動(dòng)是很困難的，因此在提前完成材料流動(dòng)就會(huì)出現(xiàn)傳統(tǒng)工藝一樣出現(xiàn)的死金屬區(qū)。 因此，在活塞銷成形這種情況下，兩個(gè)方向的沖壓率和沖壓長(zhǎng)度都是1.89和51mm。目前，一項(xiàng)關(guān)于活塞銷的沖壓長(zhǎng)度的調(diào)查研究正在進(jìn)行開模正反沖壓工藝的分析，兩個(gè)方向上的沖壓長(zhǎng)度是不同的，正向沖壓長(zhǎng)度長(zhǎng)為24.9mm，反向沖壓長(zhǎng)度如圖6所示要比正向的短。 反向金屬流動(dòng)必須強(qiáng)制性的被限制才能滿足設(shè)計(jì)要求，而這就意為著死金屬區(qū)會(huì)產(chǎn)生。因此，要想在兩個(gè)方向上得到相同的沖壓長(zhǎng)度，提出了三種控制金屬流動(dòng)的方法，這三種方法都不同程度的強(qiáng)制限制金屬流動(dòng)。 圖6反向沖壓長(zhǎng)度 3.1 改變初加工的形狀 在正反雙向沖壓之前，為了保證從腹板中心處起正反兩個(gè)方向的沖壓長(zhǎng)度相等，就得要求初加工要將反向沖壓筋的長(zhǎng)度設(shè)計(jì)與雙向沖壓長(zhǎng)度24.9mm有所不同。圖7展示了這種改進(jìn)的工藝的結(jié)果，圖8展示了在這種情況下采用正反雙向沖壓工藝時(shí)最后一步中金屬的流動(dòng)。從模擬實(shí)驗(yàn)的結(jié)果可以得出，兩個(gè)方向的沖壓筋的長(zhǎng)度都是51mm，這恰好滿足設(shè)計(jì)要求和活塞銷的尺寸要求。另外，死金屬區(qū)的金屬流動(dòng)形式相同，而不像采用普通加工時(shí)會(huì)產(chǎn)生流動(dòng)缺陷，而且在兩個(gè)方向上的流動(dòng)速度也是連續(xù)變化的，這就意為著金屬流動(dòng)在整個(gè)過程中是一致的，不會(huì)出現(xiàn)限制其流動(dòng)的死金屬區(qū)。 圖七 多級(jí)樣板的修改過程 圖八金屬網(wǎng)的流動(dòng) 3.2 驅(qū)動(dòng)沖壓模膛 驅(qū)動(dòng)模膛工藝被用來控制金屬流動(dòng)從而滿足設(shè)計(jì)要求，這種設(shè)備采用向相反方向運(yùn)動(dòng)的模膛先與已經(jīng)沖壓成形的一側(cè)接觸（如圖9所示），這樣就有助于加快后沖壓方向上的金屬流動(dòng)而減慢先沖壓方向上的金屬流動(dòng)速度，采用這種工藝制作的活塞銷，由于反方向沖壓提前完成，而此時(shí)活塞正沿著這個(gè)方向移動(dòng)從而增加了金屬沿著這個(gè)方向的流動(dòng)，這個(gè)工藝的首要變化因素是沖頭與活塞的相對(duì)速率和金屬材料與活塞之間的摩擦條件。 在這個(gè)研究中，由于摩擦系數(shù)m＝0.1（在毛胚材料和模膛之間），模擬實(shí)驗(yàn)只與相對(duì)速率這一變量有關(guān)。如果相對(duì)速率小于滿足同時(shí)成型最合適的速率，則在反向方向上的沖壓過程就會(huì)比正向沖壓提前完成，這樣的話就會(huì)像采用普通加工一樣在相同部位產(chǎn)生流動(dòng)缺陷，相反，如果相對(duì)速率大于最適宜的速率，則正向沖壓過程就會(huì)比反向沖壓過程提前完成，這樣就會(huì)在相反地部位產(chǎn)生缺陷。 因此，為了滿足設(shè)計(jì)要求，采用半分法可以找出最佳的相對(duì)速率，從結(jié)果來看，最佳的相對(duì)速率是0.48，圖10和11顯示了相對(duì)速率分別為0.1 、0.48、1.0時(shí)采用一次沖壓變形過程和金屬流動(dòng)情況。圖11（c）顯示了當(dāng)采用最佳相對(duì)速率0.48時(shí)的金屬流動(dòng)形式，它記錄了一個(gè)可以防止缺陷產(chǎn)生的流動(dòng)形式。 圖9軸向移動(dòng)的箱體示意圖 圖10根據(jù)相對(duì)速度比率變化的活塞銷釘形態(tài) 圖11根據(jù)相對(duì)速度比率比較的金屬 3.3 修改模具結(jié)構(gòu) 這種被提出的修改模具結(jié)構(gòu)的工藝可以限制金屬在反方向上的流動(dòng)，而在這個(gè)方向上容易提前完成變形，從而可以實(shí)現(xiàn)在兩個(gè)方向上同時(shí)完成變形，采用這種工藝時(shí)，為了能在兩個(gè)方向上同時(shí)完成變形過程而得到相同的變形長(zhǎng)度，卸料器又被設(shè)計(jì)者重新采用，它是一種使沖頭從制件中抽出的裝置。如果采用普通加工工藝中的固定式卸料器，則由于材料流動(dòng)受到限制，會(huì)出現(xiàn)死金屬區(qū)，而此時(shí)產(chǎn)生的部位與采用雙向沖壓時(shí)產(chǎn)生在中間位置不同。 因此，一種利用彈簧彈力的結(jié)構(gòu)可以推遲金屬材料沿反方向的流動(dòng)。圖12顯示了這種模具結(jié)構(gòu)，采用這種方法，選用合適的彈簧彈力對(duì)于滿足變形同時(shí)完成的要求來講是很重要的，因而有限元模擬可以計(jì)算出這種必要地彈力。從模擬結(jié)果來看，需要給卸料器施加5噸的彈力。圖13展示了這種工藝下金屬流動(dòng)形式，與其它改進(jìn)的工藝方法相比，這種工藝在死金屬區(qū)沒有出現(xiàn)不連續(xù)的流動(dòng)速度，此處的金屬流動(dòng)形式是相同的。 圖12使用沖壓模板的凹模模子結(jié)構(gòu)示意圖 圖13使用沖壓模板的金屬流動(dòng) 4.結(jié)果和實(shí)驗(yàn) 通過有限元分析法分析出的三種方法中是適合防止金屬的流動(dòng)缺陷。每個(gè)方法的情況如下。第一種方法是初步加工的產(chǎn)品需要三級(jí)過程(預(yù)制， 正反壓擠，穿孔)并且有一個(gè)簡(jiǎn)單的模具結(jié)構(gòu)；第二方法是使用沿軸方向移動(dòng)的沖孔模板；第三種方法是軸向移動(dòng)的箱體需要二級(jí)過程(前后壓擠，穿孔)并且有一個(gè)復(fù)雜的模具結(jié)構(gòu)。關(guān)于在里面形成的負(fù)荷，這三個(gè)方法都非常相似。 特別是在沿軸方向移動(dòng)的大約10噸的箱體情況下形成最大的負(fù)荷比其他方法小，因?yàn)樵诖┛走^程中沿軸方向移動(dòng)的箱體會(huì)增加材料的流動(dòng)。通過表1分析出的方法為形成做出了比較。在這項(xiàng)研究過程中，一個(gè)用在初步加工產(chǎn)品的實(shí)驗(yàn)被進(jìn)行，并且為了證實(shí)模擬結(jié)果所以使用一個(gè)250噸能力的多級(jí)樣板。在穿孔之前，為了金屬的觀察蝕刻流動(dòng)能夠正常被進(jìn)行，所以必須為活塞銷做一個(gè)流動(dòng)缺陷檢查。圖14就是表示這個(gè)實(shí)驗(yàn)結(jié)果，這種方法改變了初步加工的產(chǎn)品。實(shí)驗(yàn)結(jié)果證明了在缺陷區(qū)域內(nèi)金屬流動(dòng)的缺陷是相同的，并且滿足形成同時(shí)完成和在兩個(gè)擠壓方向長(zhǎng)度相同。這種過程和模擬的結(jié)果相符。 傳統(tǒng)方法 初步加工的產(chǎn)品的使用 沖壓模板的使用 移動(dòng)箱體的用途 最大負(fù)荷（噸） 97.2 96.3 96.1 84.0 擠壓的過程 2個(gè)階段 2個(gè)階段 1個(gè)階段 1個(gè)階段 缺陷 存在 不存在 不存在 不存在 表1 各個(gè)方法的比較 圖14 對(duì)流動(dòng)缺陷的消除 5.結(jié)論 在這項(xiàng)研究過程中，流動(dòng)缺陷過程和預(yù)防缺陷的過程都已經(jīng)被有限元分析重新設(shè)計(jì)。，缺陷的原因已經(jīng)被分析，并且通過分析已經(jīng)模擬出了結(jié)果。從模擬結(jié)果中可以看出，有限元分析方法是可以防止流動(dòng)缺陷并且滿足生產(chǎn)過程中控制材料的流動(dòng)狀態(tài)。通過有限元分析的結(jié)果和實(shí)驗(yàn)的結(jié)果做比較，可以得出以下幾個(gè)結(jié)論： （1）活塞銷里存在流動(dòng)缺陷的原因是材料限制死金屬區(qū)域的流動(dòng)。消除這個(gè)區(qū)域最重要的是控制材料的流動(dòng)。 （2）初步加工的產(chǎn)品設(shè)計(jì)和改變模具結(jié)構(gòu)是使用軸向運(yùn)動(dòng)的擠壓箱來消除擠壓過程中出現(xiàn)的流動(dòng)缺陷。 （3）被提出的方法滿足了工藝的要求，向前擠壓的長(zhǎng)度部分和落后的部分都是相同的，這些已經(jīng)由實(shí)驗(yàn)所證實(shí)。 參考文獻(xiàn)： [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. 216–226. [3] S.W.Oh，T.H.Kim，B.M.Kim，J.C.Choi，KSME 19 (12) (1995) 3121–3129. [4] R.C.Batra，N.V.Nechitailo，Int.J.Plast. 13 (4) (1997) 291–306. [5] A.S. Wifi，A.Abdel-Hamid，N. El-Abbasi， J. Mater. Process. Technol. 77 (1998) 285–293. [6] D.J. Kim，B.M. Kim，J. KSTP 8 (6) (1999) 612–619. [7] D.C. Ko，Pusan National University Dissertation，1998. [8] T. Altan，M. Knoerr，J. Mater. Process. Technol. 35 (1992) 275–302. [9] K. Osakata，X. Wang，S. Hanami，J. Mater. Process. Technol. 71 (1997) 105–112. 10 Journal of Materials Processing Technology 139 (2003) 422427 New processes to prevent a flow defect in the combined forwardbackward cold extrusion of a piston-pin D.J. Lee a , D.J. Kim b , B.M. Kim c, a Department of Precision Mechanical Engineering, Graduate School, Pusan National University, Pusan, South Korea b Department of Mechanical Design Engineering, Graduate School, Pusan National University, Pusan, South Korea c Department 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 Korea Abstract A flow defect of a piston-pin for automobile parts are investigated in this study. In the combined cold extrusion of a piston-pin, a lapping defect, which is a kind of flow defect, appears by the dead metal zone. This defect is evident in products with a small thickness to be pierced and is detrimental to dimensional accuracy and decrease of material loss. The flow defect that occurs in the piston-pin has bad effects on the strength and the fatigue life of the piston-pin. Therefore, it is important to predict and prevent the defect in the early stage of process design. 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 by removing the dead metal zone. Then the results are compared with the results of experiments for verification. These FE simulation results are 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 simulation 1. Introduction Cold 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 surface finish, mechanical and metallurgical properties, cold form- ing is by far the largest application of industry for producing parts. However, cold forged parts are also used in manufactur- ing aircraft, motorcycles, nuts and bolts 1, but it is possible for 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 by the state of stress and the deformation history, flow defects caused 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 internal defects and external defects 24. These defects have harmful effects on the quality of the product 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 early stage of process design. Wifietal.5 studied ductile fracture in bulk formed parts, 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 and performed 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 for an automobile part, and new processes are designed to pre- vent the defect by finite element method in the early stage of process design. Then the results are compared with the results of experiments for verification. 2. Forming and defect-occurrence analysis 2.1. Forming process The piston-pin is an automobile components used in the transmission of power between the connecting rod and the crankshaft. In the cold extrusion of a piston-pin, the design requirements are to keep the same height of the forward extruded part and the backward part (Fig. 1) without any defect in the forged product, for use under high and repeated 0924-0136/03/$ see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0924-0136(03)00515-6 D.J. Lee et al. / Journal of Materials Processing Technology 139 (2003) 422427 423 Fig. 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. The friction factor, m, is assumed to be 0.1, which is confirmed by the ring compression test. The sequence of the conventional process for the piston-pin is performed using a multi-stage former (Fig. 3). The first and second stages are pre-upsetting to eliminate defects by the cropping process such as ovality and ec- centricity of the billet for improvement of dimensional tolerances and die life whilst the third and forth stages are forward or backward extrusion for the forming of one di- rection from the web, and final stage is the piercing process for the pin shape. However, the results of experiment for the conventional process displayed a defect in the web part formed early in the third process (Fig. 3). Especially, a nonuniform flow pattern is observed in part of the defect occurrence, which looks like a flow defect similar to lapping with an undesirable flow pattern. 2.2. Prediction of defects by FE analysis DEFORM is used, which is commercial code of a rigid-plastic FE program for forming and defect analy- sis. The diameter of the initial billet is 30 mm and the height is 61 mm, the whole volume of final product being 424 D.J. Lee et al. / Journal of Materials Processing Technology 139 (2003) 422427 Fig. 5. Metal flow and velocity distribution, where a defect occurs according to stroke. 43,118 mm 3 . The forming is simulated with a conventional process sequence. The maximum fracture value that can estimate the occur- rence of a crack 7 is small at 0.08 and is distributed in a 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 that was 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 of velocity 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 fourth process and metal near the web part is pulled up in the rib part like a lapping defect. Therefore, the cause of the ini- tiation and development of the flow defect that occurred in piston-pin is the velocity discontinuity between backward and forward direction by the formation of a dead metal zone. This appearance evidently occurs in products like a piston-pin with a low thickness to be pierced for the dimen- sional accuracy and the decrease of material loss. A flow defect occurring in a piston-pin has harmful effects on the strength and the fatigue life of a piston-pin that has high and repeated load at high temperature. Therefore, it is necessary for a new process to prevent the flow defect. 3. Process redesign and analysis for the prevention of defect The cause of the initiation and development of the flow defect is the restriction of metal flow by the dead metal zone. For the elimination of the dead metal zone in the early extruded part (3rd process) in the conventional process, the forward or backward extrusion process is modified to combined forwardbackward extrusion, which is performed simultaneously in the two directions. Because of the variety of extrusion ratios and lengths in the forward and backward directions, the simultaneous completion of the material flow in the both directions is very difficult. Consequently, one of the directions is completed early, then material flow stopped and dead metal zone appears in this part just like that in the conventional process. Therefore, in the case of piston-pin forming, the extrusion ratio and the length of both directions are the same at 1.89 and 51 mm. First, analysis of open die forwardbackward ex- trusion is performed for an investigation of extrusion lengths of the piston-pin. The difference of two extruded ribs is 24.9 mm and the backward extruded rib is shorter than the forward extruded rib as shown in Fig. 6. The metal flow of backward direction must be restricted compulsorily for the satisfaction of the design conditions and this means the occurrence of a dead metal zone. There- fore, for the same extrusion length in both directions, three Fig. 6. Extrusion length in forwardbackward extrusion. D.J. Lee et al. / Journal of Materials Processing Technology 139 (2003) 422427 425 Fig. 7. Modified process sequence for a multi-stage former. methods are proposed to control the metal flow without the compulsory restriction of metal flow 3.1. Change of preform shape To secure the same length of both directions from the center of web, it is required that the backward extruded rib is performed by preform design as the difference of both-direction lengths at 24.9 mm from the above results, before forwardbackward extrusion. Fig. 7 shows the mod- ified process sequence, and Fig. 8 shows the metal flow of the final stage of forwardbackward extrusion in this case. From the results of simulation, the lengths of two extruded ribs are 51 mm, which is the dimension of the piston-pin and satisfied the design condition. In addition, the metal flow is uniform in the defect zone where the flow defect occurred in the conventional process, and there is not a discontinu- ity of velocity in both extrusion directions. This means that metal flows uniformly in the whole process without a dead metal 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 container The driving extrusion container method 9 is used for metal flow control for the satisfying of the design condi- tion. This structure is that the extrusion container is moved in the counter direction to the early extruded one (Fig. 9). This has the effect of increasing the metal flow in the late extruded direction and restricting metal flow in the early ex- truded direction. In the case of the piston-pin, because of the early completion of backward extrusion, the extrusion container is moved in the forward direction for the increase of metal flow to this direction. In this process, the princi- pal process variables are the relative velocity ratio of the punch and the moving extrusion container, and the friction condition between the material and the moving extrusion container. In this study, because the friction factor, m, is 0.1 be- tween the material and container, simulation is performed only according to the variation of the relative velocity ratio (V C /V P = 0.1, 0.25, 0.5, 0.75, 1.0). If the relative velocity ratio is smaller than the optimum which can complete form- ing simultaneously, extrusion in the backward direction is completed earlier than in the forward direction and a flow defect 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 complete earlier than backward direction and a flow defect occurs in the opposite part to where a defect occurs in the conven- tional process. Therefore, for satisfaction of the design conditions, the optimum 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 11 show the deformation modality and metal flow according to the relative velocity ratio (0.1, 0.48, 1.0) for a punch stroke of 42.7 mm, respectively. Fig. 11(c) shows the metal flow 426 D.J. Lee et al. / Journal of Materials Processing Technology 139 (2003) 422427 Fig. 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 improved flow pattern without a flow defect can be noted. 3.3. Modification of die structure A modification of the die structure is proposed which can restrict the metal flow of backward direction, which is deformed early, for simultaneous completion of extrusion in both directions. In this case, for simultaneous completion and the same length in both directions, the stripper, which is Fig. 12. Schematic diagram of die structure using stripper. equipment for punch extraction from products, is redesigned. If a fixed stripper of conventional type is used, a dead metal zone appears from the middle stage of backwardforward extrusion by the restriction of material flow. Therefore, a structure is used that can delay the metal flow in the backward direction by spring force. Fig. 12 shows the die structure. For this method, it is very important to decide the proper spring force for simultaneous completion of forming. Therefore, the necessary spring force for this is calculated by FE simulation. From the simulation result, it was 5 t to be applied load to stripper. Fig. 13 shows metal flow in this case. The metal flow is similarly uniform at the defect zone without discontinuity of velocity in comparison with 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) 422427 427 Table 1 Comparison process for each of the proposed method Conventional method Use of preform Use of stripper Use of moving container Maximum load (t) 97.2 96.3 96.1 84.0 Process of extrusion 2 stage 2 stage 1 stage 1 stage Defect Exist None None None 4. Results and experiment From the FE simulation, the three proposed methods are proper to prevent a flow defect by metal flow control. The characteristics of each process are as follows. The first method that uses a preform needs three stage processes (pre- forming, forwardbackward extrusion, piercing) and has a simple die structure; however, the second method that uses a stripper and the third method that uses an axially mov- ing container need two-stage processes (forwardbackward extrusion, piercing) and have a complex die structure. In respect of the forming load, the processes are similar to each other. Especially, the maximum forming load is smaller than that of other processes by about 10 t in the case of the axially moving container, because the axially moving container in- creases material flow in the direction punch movement. It is compared with the proposed method for forming by a press in Table 1. In this study, an experiment using a preform is performed and uses a multi-stage former having 250 t ca- pacity for the verification of simulation. Etching for obser- vation of metal flow is performed to examine for a flow defect for the piston-pin before piercing. Fig. 14 shows the experiment result, based on the first proposed method, changing the preform. The experiment result shows that metal flow is uniform in the defect zone where the flow de- fect had occurred, and satisfied the simultaneous completion of forming and the same length in both extrusion directions. This tendency is in good agreement with the simulation result. Fig. 14. The elimination of the flow defect by the first proposed method. 5. Conclusions In this study, the flow defect that occurs in the manu- facturing process of the piston-pin is examined and a new process to prevent the defect is redesigned by FE analysis. First, the cause of the defect is investigated, and the analyti- cal approach is verified by comparison of experimental and simulation results. From these results, it is possible to de- sign processes that can prevent the flow defect and satisfy the design condition to control the material flow. Comparing the experiment and FE analysis for the pro- posed new processes, several conclusions can be drawn: (1) The cause of the flow defect that occurs in the piston-pin forming is a dead metal zone by restriction of material flow, and it is very important to control the material flow for eliminating this zone. (2) Design of the preform and change of the die structure and the use of an axially moving extrusion container are proposed to secure simultaneous filling for elimination of the flow defect in the combined forwardbackward extrusion process. (3) The proposed methods satisfy the requirements of pro- cess design, i.e. the same length of the forward extru- sion part and the backward one, and these are verified by experiment. Acknowledgements The authors wish to thank the Engineering Research Cen- ter for Net Shape and Die Manufacturing, located in Pusan National University, Pusan, South Korea, for the support of this research. References 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.