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熱沖壓鋼的熱機(jī)械性能的調(diào)查
M. Merklein 1, J. Lechler ?
埃爾蘭根 - 紐倫堡大學(xué)， Egerlandstrasse11，91058埃爾蘭根，
德國制造技術(shù)系
摘要
熱成沖壓是一種創(chuàng)新的熱加工成形過程，它也許可以在一個(gè)步驟中實(shí)現(xiàn)熱處理和成形的有效結(jié)合。這給目前在汽車行業(yè)中應(yīng)用的幾何形狀復(fù)雜，高強(qiáng)度和最小的回彈組件的制造提供了一個(gè)機(jī)會。作為熱沖壓的標(biāo)準(zhǔn)物質(zhì)，quenchenable高強(qiáng)度鋼22MnB5被廣泛地使用。為了建立工藝的數(shù)字模型，掌握材料熱機(jī)械性能的知識是必須的。為了確定22MnB5鋼的熱機(jī)械材料特性，本文通過導(dǎo)電處理研究了22MnB5鋼在奧氏體狀態(tài)下的流動特性以及采用根據(jù)時(shí)間 - 溫度特性原理研究熱沖壓過程的Gleeble1500系統(tǒng)進(jìn)行熱拉伸試驗(yàn)。
@2006愛思唯爾B.V.保留所有權(quán)利。
關(guān)鍵詞：熱沖壓；高強(qiáng)度鋼;22mnB5;熱機(jī)械性能，流動行為
1、介紹
今后幾年汽車行業(yè)最重要的挑戰(zhàn)之一是在實(shí)現(xiàn)油耗在降低的同時(shí)滿足安全性能的增加。這可以初步實(shí)現(xiàn)了由白色組件通過使用更薄的材料滿足更高的強(qiáng)度及重量的減少。因此越來越高和超高強(qiáng)度鋼越來越多地使用在汽車行業(yè)，由于其成形性??能改善[1]。例如與應(yīng)用quenchenable超高強(qiáng)度鋼22MnB5，復(fù)雜的碰撞加固等有關(guān)組件部分，前保險(xiǎn)杠等與最終強(qiáng)度約1500MPa [2]可以同時(shí)降低材料的厚度，制造。但是，使用高強(qiáng)度鋼通常也導(dǎo)致一些缺點(diǎn)像工具的影響，降低成形性和回彈趨勢。為了提高材料成形性，如熱沖壓過程中的quenchenables鋼已開發(fā)的技術(shù)。熱沖壓是一個(gè)非等溫金屬板材，在那里形成和淬火形成的過程，需要在一個(gè)組合的過程中一步到位。圖1為熱沖壓過程圖。作為交付22MnB5具有的抗拉強(qiáng)度約600MPa級鐵素體 - 珠光體組織的基礎(chǔ)材料。通過熱成型過程后，終于組件展品，強(qiáng)度約1500MPa馬氏體顯微組織。所需的最終高強(qiáng)度馬氏體組??織的一個(gè)先決條件，是空白必須第一奧氏體約5-10分鐘一爐約900-950?C。后達(dá)到均勻的奧氏體微觀空白自動轉(zhuǎn)移到水冷卻三年內(nèi)死亡秒，形成和淬火同時(shí)發(fā)生。采取的減少流動應(yīng)力由于溫度升高，從而優(yōu)勢。通過發(fā)生接觸的熱與冷?？瞻?，可實(shí)現(xiàn)高冷卻速率和非擴(kuò)散馬氏體相變的發(fā)生。
有了可靠的流程建模，除了摩擦條件和機(jī)械特性如楊氏模量，泊松比等熱機(jī)械材料特性的知識，在熱成型時(shí)的溫度特性的依賴方面，過程中，是必需的。在下面的章節(jié)22MnB5的熱機(jī)械的流動性的調(diào)查結(jié)果，根據(jù)熱沖壓工藝要求
和參數(shù)的影響，將提交。由于熱拉伸試驗(yàn)已進(jìn)行了修改，伺服液壓機(jī)械采用Gleeble 1500測試系統(tǒng)。
圖1 直接熱沖壓工藝的示意圖
2材料和實(shí)驗(yàn)程序
2.1材料的特性
在汽車行業(yè)的直接和間接的熱沖壓的quenchenable超高強(qiáng)度鋼22MnB5常用。本文的范圍內(nèi)與材料厚度的1.75毫米由阿塞洛產(chǎn)生的冷軋帶鋼。硼/錳微合金鋼，所以稱為USIBOR1500P，展品與鐵素體 - 珠光體組織的171 HV10的硬度，屈服強(qiáng)度400MPa級和拉伸強(qiáng)度約為600MPa級[3]。軋制方向和應(yīng)變率的依賴基體材料中的流動性方面，它應(yīng)該被稱為到Merklein等。 [4]。 [4]顯示，USIBOR1500P表現(xiàn)方面軋制方向和變形速度交付顯著沒有對flowbehavior的靈敏度。為了防止氧化和脫碳的空白，在熱處理爐轉(zhuǎn)移到模具USIBOR1500P是預(yù)涂鋁層。涂層厚度一般是23和32之間有根據(jù)供應(yīng)商提供的。為了實(shí)現(xiàn)所需的同質(zhì)
淬火前的奧氏體微觀結(jié)構(gòu)，根據(jù)[5]，一爐 停留時(shí)間至少3.5分鐘為1.75毫米厚空白是必不可少的。以下連續(xù)時(shí)間溫度轉(zhuǎn)變（TTT）圖 2，冷卻速率，至少27Ks-1是為避免貝氏體相變，并實(shí)現(xiàn)了全熱沖壓件的馬氏體組織所必需的。
圖2 根據(jù)Arcelor [3]的USIBOR 1500P TTT 圖表.
2.2實(shí)驗(yàn)設(shè)置和程序
根據(jù)熱沖壓過程中的微觀結(jié)構(gòu)轉(zhuǎn)變過程中，實(shí)際形成過程的溫度窗口限于
22MnB5奧氏體相。由于馬氏體開始溫度（MS）的約400?C和轉(zhuǎn)移依賴空氣冷卻，形成的空白，通常在850和400之間發(fā)生?C。為熱成型過程的數(shù)值模擬，確定材料的熱機(jī)械特性影響參數(shù)如溫度，加熱和冷卻速度，真應(yīng)變和應(yīng)變率的依賴的先決條件，是必不可少的。與傳統(tǒng)的機(jī)械測試系統(tǒng)面臨的挑戰(zhàn)是難以滿足。因此，采用Gleeble1500測試系統(tǒng)已被修改，是能夠重現(xiàn)導(dǎo)電熱拉伸試驗(yàn)USIBOR1500P的流動性為特征的熱沖壓過程中的有關(guān)工藝參數(shù)的依賴，。調(diào)整后的伺服液壓采用Gleeble1500系統(tǒng)原理圖顯示。 3。更精確的武力或脅迫的數(shù)據(jù)，分別由于在高溫下，外部的，更敏感的50千牛稱重傳感器的應(yīng)力值降低，根據(jù)自建夾緊裝置，用于接收，實(shí)現(xiàn)在本機(jī)的試驗(yàn)室。為了實(shí)現(xiàn)更高的冷卻速率比27Ks-1，兩個(gè)壓縮空氣噴嘴已集成。作為這些修改熱拉伸試驗(yàn)的結(jié)果與 時(shí)間 - 溫度特性，適合熱沖壓過程中，冷卻速度高達(dá)8090Ks-1就可以實(shí)現(xiàn)。測量試樣的伸長率，實(shí)現(xiàn)了使用光學(xué)變形系統(tǒng)，阿拉米斯（墨西哥灣，德國）。
在這項(xiàng)工作單軸，導(dǎo)電熱拉伸試驗(yàn)已執(zhí)行 USIBOR1500P的流動性，以確定在滾動的依賴 風(fēng)向，溫度（500，650，700和800?C）和應(yīng)變率（0.01，0.1和1個(gè)S-1），根據(jù)DIN EN10002第5部分指引。因此，標(biāo)本已被判處以下的熱機(jī)械測試程序方面的鋼鐵供應(yīng)商的建議已測試樣品加熱到奧氏體化溫度950?CNOT速度比約6Ks-1。離開試樣在950?C的180秒，以保證一個(gè)完整的，均勻的奧氏體[5]，快速冷卻，并在800和500之間的溫度穩(wěn)定?C的五秒鐘后發(fā)生的。事后等溫條件下進(jìn)行拉伸試驗(yàn)。使用鎳/鉻鎳熱電偶樣品到現(xiàn)場焊接長度的一半（比較圖3），溫度測量的實(shí)現(xiàn)。變形的標(biāo)本進(jìn)行檢測，使用光學(xué)測量系統(tǒng)阿拉米斯。試樣的幾何形狀遵循EN482-2[6]，在變形過程中的照片已采取頻率為10赫茲的建議。對于每個(gè)調(diào)查參數(shù)已經(jīng)進(jìn)行了至少5個(gè)測試運(yùn)行。流量計(jì)算的曲線基本的應(yīng)力應(yīng)變數(shù)據(jù)，已收到來自50 kN載荷細(xì)胞和阿拉米斯系統(tǒng)的，分別為。最后計(jì)算的真實(shí)應(yīng)力應(yīng)變值之后[7,8]。
圖3 修改的測試室的伺服液壓1500Gleeble機(jī)械系統(tǒng)示意草圖
3 實(shí)驗(yàn)結(jié)果
3.1軋制方向在奧氏體狀態(tài)的流動性的影響
????軋制方向22MnB5流動行為的影響被調(diào)查的在不同溫度下的奧氏體狀態(tài)，繼前一節(jié)中提到的測試路徑。對于上下測試極限溫度，500和800?，軋制方向上的流動性USIBOR 1500P的影響，代表性的圖所示。 4。測試溫度為模范流量曲線說明三個(gè)軋制方向0°，45°和90°，后冷卻速度與約80Ks-1和模范應(yīng)變率0.1 S-1的快速冷卻。此外，試驗(yàn)溫度為650?C流量曲線在軋制方向的依賴。根據(jù)所有曲線顯示，該材料表現(xiàn)出的軋制方向上在奧氏體相沒有依賴。各種應(yīng)變和冷卻速率進(jìn)一步的實(shí)驗(yàn)證實(shí)了這一結(jié)果。在此基礎(chǔ)上，進(jìn)一步拉伸試驗(yàn)已進(jìn)行了帳戶沒有采取滾動方向。因此，同一個(gè)方向平行于軋制方向的唯一標(biāo)本已被使用。
3.2在奧氏體狀態(tài)的流動性溫度的影響
????溫度對試驗(yàn)材料的流動性能的影響USIBOR 1500P已研究不同溫度和應(yīng)變率在奧氏體狀態(tài)后快速冷卻。圖5顯示溫度敏感的材料。對于500和800之間的不同溫度?C后迅速冷卻，代表真實(shí)的應(yīng)力 - 應(yīng)變曲線顯示模范應(yīng)變率1個(gè)S-1。流量曲線特性表明，溫度有重大影響力上鋼quenchenable的成形行為。溫度增加導(dǎo)致的流動應(yīng)力顯著減少和降低加工硬化指數(shù)，在一個(gè)真實(shí)的應(yīng)力 - 應(yīng)變曲線的斜率明顯減少。低應(yīng)變速率一樣，例如，0.1 S-1的材料顯示，在溫度相同的依賴，但額外的，同時(shí)發(fā)生的動態(tài)毀滅和恢復(fù)過程，在溫度高于650?C在變形（比較圖6）。這將導(dǎo)致鈑金增加的趨勢，表現(xiàn)出幾乎是平面的流量曲線隨著溫度的升高后的初始應(yīng)變硬化的特點(diǎn)。由于時(shí)間和溫度的依賴，這種效果更加明顯較高的溫度和變形速度較低。據(jù)在圖中顯示的結(jié)果。 5和6，應(yīng)變率似乎影響以及22MnB5的流動性，除了溫度升高溫度，因而被認(rèn)為是表征材料的成形行為。
圖5、6 1500 P、應(yīng)變速率為1 s?1條件下溫度對流曲線USIBOR屬性的影響。
3.3 應(yīng)變率中的流動性的影響奧氏體狀態(tài)
一直依賴應(yīng)變率材料的流動行為在三個(gè)不同的應(yīng)變率0.01，0.1和1個(gè)S-1在奧氏體階段快速冷卻后，調(diào)查。模范的圖，這個(gè)影響??參數(shù)的敏感性。 7應(yīng)變硬化函數(shù)顯示在各種變形速度的依賴于溫度650?C。對于每個(gè)應(yīng)變率有代表性的流量曲線顯示。根據(jù)曲線的特征，這是明顯的應(yīng)變率有USIBOR1500P形成行為產(chǎn)生重大影響。應(yīng)變率增加導(dǎo)致的應(yīng)力水平，并強(qiáng)迫材料加工硬化的后果曲線的斜率明顯增加。進(jìn)一步可以看出，測試速度下降，從而增加變形時(shí)，流量曲線表現(xiàn)出一種傾向，初始應(yīng)變硬化后達(dá)到一個(gè)穩(wěn)定的狀態(tài)。出現(xiàn)這種情況的應(yīng)變硬化功能幾乎漸近趨勢與方法 逐步延伸。這種效應(yīng)可以發(fā)生擴(kuò)散平衡的應(yīng)變硬化[9,10]的依賴微觀復(fù)蘇的進(jìn)程。對于更高的溫度高達(dá)800?C時(shí)，變形速度相媲美的材料靈敏度可檢測[4]。
圖7 USIBOR 1500 P,650?C條件下應(yīng)變速率對流動特性的影響。
4 結(jié)論
熱機(jī)械覆膜22MnB5的流動性，本文進(jìn)行了調(diào)查，在熱沖壓過程的時(shí)間 - 溫度特性的依賴。從這些測試中接收的數(shù)據(jù)是必要的有關(guān)材料的成形行為在熱沖壓過程的數(shù)值模擬。在第三章顯示的結(jié)果，對于材料的流動行為的數(shù)學(xué)描述在高溫奧氏體狀態(tài)，軋制方向具有不被考慮。在對面的溫度和應(yīng)變率以及22MnB5超高強(qiáng)度鋼的成形行為的影響。溫度增加導(dǎo)致的流動應(yīng)力值顯著下降和初始應(yīng)變硬化的斜坡。對于敏感性 關(guān)于應(yīng)變率材料的成形性能，變形速度增加導(dǎo)致的應(yīng)力水平顯著增加和加工硬化。
5 總結(jié)和展望
在此工作的quenchenable超高強(qiáng)度鋼USIBOR由阿塞洛制造的1500P的熱機(jī)械的流動性進(jìn)行了研究。因此，伺服液壓采用Gleeble1500系統(tǒng)進(jìn)行了修改，是能夠表征22MnB5形成的行為，在奧氏體狀態(tài)的熱沖壓過程中的時(shí)間 - 溫度特性。因此導(dǎo)電熱拉伸試驗(yàn)已進(jìn)行出，像rollingdirection，溫度和應(yīng)變速率從950?C至溫測試溫度后迅速冷卻下來的影響參數(shù)的依賴。結(jié)果表明，該材料對溫度和應(yīng)變速率具有高靈敏度?？纱_定軋制方向無顯著影響。對于一個(gè)數(shù)值模型來描述材料的流動行為的產(chǎn)生，溫度和應(yīng)變率必須考慮到。在今后的工作中，將影響加熱和冷卻速度的流動特性一個(gè)數(shù)值的工藝設(shè)計(jì)可靠的材料模型方面的研究。此外，實(shí)驗(yàn)解決方案，將開發(fā)用于測定根據(jù)顯著的特點(diǎn)斷裂準(zhǔn)則熱沖壓過程。
鳴謝
作者感謝德國研究基金會東風(fēng)集團(tuán)項(xiàng)目的財(cái)政支持，這是東風(fēng)集團(tuán)成立科研單位“quenchenable鋼板材熱成形原則”的一部分。此外，作者感謝他們就免費(fèi)供應(yīng)充足USIBOR1500P阿塞洛的支持。
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Cold stamping formability of AZ31B magnesium alloy sheet undergoing repeated unidirectional bending process
Lei Zhanga,b, Guangsheng Huanga,b, * , Hua Zhanga,b , Bo Song a,b
a National Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing 400030, China
b College of Material Science and Engineering, Chongqing University, Chongqing 400030, China
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a r t i c l e i n f o
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Article history:
Received 11 April 2010
Received in revised form 7 November 2010
Accepted 28 November 2010
Available online 7 December 2010
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a b s t r a c t
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The repeated unidirectional bending (RUB) process was carried out on an AZ31B magnesium alloy in order to investigate its effects on the cold stamping formability. The limiting drawing ratio (LDR) of the RUB processed magnesium alloy sheet with an inclination of basal pole in the rolling direction can reach 1.5 at room temperature. It was also con?rmed that cell phone housings can be stamped successfully in crank press using the RUB processed AZ31B magnesium alloy sheet. The improvement of the stamping formability at room temperature can be attributed to the texture modi?cations, which led to a lower yield strength, a larger fracture elongation, a smaller Lankford value (r-value) and a larger strain hardening exponent (n-value).
? 2010 Elsevier B.V. All rights reserved.
Keywords: Magnesium alloy sheet;RUB;Texture;Stamping formability；
Cell phone housing
1. Introduction
Nowadays, the products of magnesium alloys, mainly formed by casting and die-casting, are used in the aerospace, automobile, civilian household appliances. Compared with casting and die-casting, plastic forming technology seems to be more attractive because of its competitive productivity and performance. Among the fabrication
processes of plastic forming, stamping of magnesium alloy sheets is especially important for the production of thin-walled structural components (Chen and Huang, 2003). However, magnesium alloy sheets have low ductility at room temperature due to its
strong (0002) basal texture, as shown in the literature (Doege and Droder, 2001). Mori and Tsuji (2007) investigated cold deep drawing of commercial magnesium alloy sheets, they demonstrated that the limiting drawing ratio of rolled AZ31 magnesium alloy sheets
annealed at 773 K can reach 1.7. Mori et al. (2009) have shown that a two-stage cold stamping process are also helpful for forming magnesium alloy cups. Watanabe et al. (2004) suggested the ductility of magnesium alloy sheets can be improved by reducing (0002) basal texture at room temperature. The limiting drawing ratio for the cold deep drawing of commercial magnesium alloy sheets can be improved from 1.2 to 1.4 by reducing (0002) basal plane texture (Iwanaga et al., 2004). It is well-known that equal channel angular pressing (ECAP) is an effective method to obtain a tilted basal texture, which improved signi?cantly the tensile elongation (Kim et al., 2003). But it is hard for ECAP to fabricate thin sheet. Recently, it is reported that a rolled magnesium alloy sheet, with a tiled texture obtained by cross-roll rolling (Chino et al., 2006) and different speed rolling (DSR) process (X.S. Huang et al., 2009), exhibit higher stamping formability compared with a rolled magnesium alloy sheet by normal-roll rolling. It is therefore important to improve the formability at room temperature for a wide use of magnesium alloy sheets by changing or weakening the basal texture.
Older versions of the ASM Metals Handbook (1969) on forming refer to a “special bending sheet,” which was produced by Dow Magnesium. The special bending sheet with a modi?ed crystallographic texture, had better forming characteristics than conventional AZ31 sheet.
Previous study (G.S. Huang et al., 2009) revealed that the RUB process also improved the stretch forming of magnesium alloy sheets by weakening basal texture of sheets. The Erichsen values of the RUB processed sheet signi?cantly increased from 3.53 to 5.90 in comparison with the cold-rolled magnesium alloy sheet. However, up to now, few researchers made efforts to study the cold stamping formability of the magnesium alloy sheets. Cold stamping products, such as housings of laptop computers and cell phones, have not been reported in other investigations. Hence, it is important to investigate the cold deformation behaviors so as to establish fundamental knowledge of the cold forming technology of magnesium alloy.
In this paper, an investigation of the drawability of RUB processed AZ31 magnesium alloy sheet was performed at room temperature using uniaxial tensile tests, deep drawing and cold stamping of a cell phone housing. The performance of RUB sheet was compared with that of the as-received sheet.
2. Experimental material and procedure
2.1. The preparation of experimental material
Commercial AZ31B magnesium alloy sheets with a thickness of 0.8 mm, cut into 1000 mm × 100 mm (length × width) pieces, were used in the experiments. Fig. 1 shows the schematic diagram of the RUB process. The radius of the cylindrical support was 1 mm andthe bending angle was 90 ? . The magnesium alloy sheet was bent on a cylindrical support under a constant force T with a constant speed v. There was six-pass bending, which indicated that there were six bending operations in all at two orientations in the experiment. This meant that after each bending pass, the sheet was turned over and the bending orientation was also changed in the next pass. The RUB processed sheets were annealed at 533 K for 60 min, and then were subjected to tensile tests, deep drawing, and cold stamping of cell phone housing investigation.
Fig. 1. Schematic diagram of the RUB process.
Fig. 2. {0002} Pole ?gures of as-received sheet and RUB sheet. (a) as-RUB sample, max density = 8.66; (b) RUB sample, max density = 7.31.
Previous studies (Song et al., 2010; Huang et al., 2010) have investigated microstructure and texture evolution of AZ31 magnesium alloy sheets underwent RUB. For the as-received sheet, the grains were ?ne. After the magnesium alloy sheet underwent
RUB was annealed at 260 ? C, the grains near the surface of sheet grew obviously, while those in the central region had little growth. The average grain size of two state sheets was almost the same. Fig. 2 shows {0002} pole ?gures of two state sheets. The asreceived
sheets exhibit a strong basal texture, where the majority of grains are oriented with their
{0002} basal planes parallel to the rolling plane of the sheet. In contrast, the RUB processed sheets exhibit a large inclination of c-axis around the normal direction
(ND) towards the RD, which weakens basal texture of the sheet.
2.2. Uniaxial tensile tests
The specimens for tensile tests had a parallel length of 57 mm, a width of 12.5 mm and a thickness of 0.8 mm. The specimens were cut along planes coinciding with at the angles of 0 ? (RD) and 45 ? and 90 ? (TD) to the rolling direction. Prior to testing, all specimens were polished by the abrasive paper to remove major scratches to avoid fracture occurring at an undesired location of the specimen. The uniaxial tensile tests were carried out on a CMT6305-300 KN testing machine with an initial strain rate of 3 × 10 -2 s -1 to examine the mechanic properties such as the yield strength, the ultimate tensile strength and the fracture elongation. The strain hardening exponent values (n-value) were obtained by power law regression (_ = _e n ) of the tensile test data within a uniform strain of e =15%. The Lankford values (r-value), r= e w /e t , where the variables e w and e t denote the strains in the tensile specimens’ transverse and thickness directions, respectively, were measured on the specimens at a uniform plastic deformation of e =15%.
2.3. Limiting drawing ratio (LDR) tests
To evaluate the deep drawability of the RUB processed AZ31 magnesium alloy sheet, limiting drawing ratio (LDR) tests were carried out on a 600 kN hydraulic press to examine the stamping formability at room temperature. The schematic diagram and geometry dimension of mold are shown in Fig. 3 and Table 1, respectively. Magnesium alloy sheets were processed into circular specimens with various diameter dimensions using wire-cutting. Before deep drawing, all circular specimens should be polished by the abrasive paper in order to avoid crack in them. Special positioning ring was adopted to ?x the specimens. A rigid blank holder was used on the molds, which can offer suf?cient blank holder force to press the blank tightly by adjusting the spring. Consequently, the
blank holder and die were uniformly lubricated with oil. The punch was not lubricated.
Fig. 3. Schematic diagram of mold.
Table 1 Parameters of punch and die used in the experiment.
Punch diameter, d p
(mm)
Punch shoulder
radius, r p (mm)
Die clearance,z (mm)
Die shoulder
radius, r d (mm)
50
5
128
9.1
2.4. Cold stamping of cell phone housing
The as-received sheets and the RUB processed AZ31 magnesium alloy sheets with a thickness of 0.6 mm were used in these experiments; three sets of stamping dies for cell phone housing manufacture were used, the blanking die, deep drawing die and piercing die. Compared with the blanking and piercing die, the structure of deep drawing die was more complex. The main parameters of deep drawing die were as follows: punch radius r p = 1 mm; die radius r d = 2 mm; die clearance in the straight wall C = 0.6 mm; die clearance in the corner C = 0.66 mm. The three sets of dies driven by the crank press completed the blanking, the deep drawing and the piercing process in turn.
Fig. 4. The true stress–strain curves of the as-received specimens and the RUB processed
specimens in the tensile directions of RD, 45 ? and TD (RD, rolling direction;
TD, transverse directions).
3. Results
3.1. Mechanical properties
Fig. 4 shows that the true stress–strain curves of the as-received specimens and the RUB processed specimens in the tensile directions of RD, 45 ? and TD. Compared with the as-received specimens, the RUB processed specimens exhibit larger in-plane anisotropy,
and the signi?cant differences can be observed from the true stress–strain curves at the beginning stage of the tensile deformation. The work-hardening effects are stronger for the tensile specimens in the tensile directions of RD, 45 ? and TD after the yield deformation. The yield strength, tensile strength and the fracture elongation are shown in Fig. 5. The tensile strengths of the RUB processed specimens are nearly the same as that of the as-received specimens regardless of the tensile directions. While yield strength of the RUB processed specimens is signi?cantly lower than that of the as-received specimens especially in the RD. These results indicate that the RUB process has a strong effect on the yield strength but not the tensile strength. Additionally, the fracture elongations of the RUB processed specimens are improved in the tensile directions of RD, 45 ? and TD in comparison with those of the as-received specimens, especially in the RD with the largest increase from 19.2% to 26.7%. These are mainly due to the RUB processed spec-imens with stronger work-hardening effects which contribute to the increase in the fracture elongation. Above all, the inclination of the c-axis toward the RD lowers the yield strength but elevates work-hardening effects which contribute to improve the uniform elongation.
The r-value and the n-value of the as-received specimens and the RUB processed specimens are shown in Fig. 6. Compared with the as received specimens, the RUB processed specimens show a much smaller r-value and a larger n-value especially in the RD, which decreases from 2.15 to 0.92 and increases from 0.20 to 0.29, respectively.
The difference between r-values as well as that between n-values of the as received specimens and the RUB processed specimens decreases with increasing the tensile angle. The average r-value (ˉ r = (r RD + 2r 45 ? + r TD )/4) falls from 2.45 to 1.36, and the average n-value ( ˉ n = (n RD + 2n 45 ? + n TD )/4) rises from 0.175 to 0.225 in comparison with those of the as-received specimens. The decrease in ˉ r indicates that it is easier to reduce or increase the thickness of sheet during the plastic deformation. Furthermore, the improvement in the fracture elongation was mainly due to the high
ˉ n which resulted in a low sensitivity to strain localization in the
form of necking.
Fig. 5. (a) Tensile strength and yield strength, (b) fracture elongation of the as-received specimens and the RUB processed specimens in the tensile directions of RD, 45 ? and
Fig. 6. r-Value and n-value of the as-received specimens and the RUB processed
specimens in the tensile directions of RD, 45 ? and TD.
3.2. LDR
Drawing ratio (DR) is commonly expressed by RD = d 0 /d p , where d 0 and d p are the blank diameter and punch diameter, respectively. The LDR is the one when the specimen is on the verge of fracture.
Fig. 7 shows cold deep drawn cups of the as-received specimens and the RUB processed specimens for DR = 1.5. The as-received specimens fractured at the punch shoulder, and the drawing depth was only 7.2 mm. However, the drawn cup of the RUB processed specimens showed a good appearance at a drawing depth of 11.8 mm. Compared with the as-received specimens, the RUB processed specimens show better stamping formability. These are mainly due to the RUB processed specimens with a tiled texture, which contribute to the increase in the drawing depth. If the drawing depth went up to 14.8 mm, the fracture occurred at the edge of the ?ange for the RUB processed specimens during deep drawing. Yang et al. (2008) investigated die as shown in Fig. 8(a), the force was not applied onto the edge using the ?at blank holder. To apply the force onto the edge even in passing though the die corner, the blank holder was exchanged for
that having a ring-shaped projection in an intermediate stage of the deep drawing as shown in Fig. 8(b) (Mori and Tsuji, 2007). Additionally, for magnesium alloy sheets, the fracture happened in the top of the cup during bending–unbending as the material passes over the die radius. Those previous observations point out that compared with aluminum-alloy sheets (including AA2024, 6061,7075), magnesium alloys exhibit poor bending ductility due to their strong in-plane anisotropy and mechanical twinninginduced
tension–compression strength asymmetry in two sides of the bending blank (Agnew et al., 2006). The blank holder with a ring-shaped projection is employed instead of the ?at bank holder after the edge of the ?ange breaking out of the ?at bank holder, which is helpful to improve unbending ductility of the sheet in the die corner. Fig. 9 shows cold deep drawn cup using the blank holder with a ring-shaped projection in an intermediate stage of the
deep drawing as shown in Fig. 8(b). The LDR of the RUB processed specimens is 1.5 under present experimental conditions. However, compared to a circular cup deep drawing, the depth of cell phone housing is only 6 mm, thus the subsequent cold stamping process
of cell phone housing is carried out using one-step and ?at blank holder.
Fig. 7. Cold deep drawn cups with different drawing depth of as-received specimen and the specimen undergoing RUB process for DR = 1.5.
Fig. 8. The edge of the blank passes though the corner of the die at different pressure situations: (a) No blank holder force; (b) action of blank holder force.
Fig. 9. Cold deep drawn cup using the blank holder with a ring-shaped projection.
Fig. 10 shows the thickness strain at the angles of 0 ? (RD), 45 ? and 90 ? (TD) to the rolling direction of cold deep drawn cup for the RUB processed specimens. The valleys of the curves represent the sections of the cup corners. Despite of the different r-values in the
three directions, the values at the cup corners are approximately the same. It is well known that the stresses in the hoop directions around the ?ange of the cup resulted in the increase in thickness during deep drawing. For the RUB processed sheets with a tilted basal texture, the thickness strain can be generated by basal
slip.
Fig. 10. Distributions of wall thickness strain of drawn cups for ? = 1.5.
3.3. Cold stamping of cell phone housings
Preliminary experimental results demonstrate that the RUB process has an important in?uence on the stamping formability of AZ31 magnesium alloy sheets. Fig. 11 shows the results of cold stamping of cell phone housings. The as-received specimen was drawn unsuccessfully, as shown in Fig. 11(a). It can be found that the critical section at the punch shoulder was broken before the ?ange of the specimen was fully dragged into the die cavity. While the RUB processed specimen was drawn successfully, the critical section at the punch shoulder and the ?ange was excellent, as shown in Fig. 11(b). The experimental results show that the RUB process improved the shallow drawing formability of magnesium alloy sheets. Besides, certainly, cell phone housings can be obtained successfully in crank press using the RUB processed AZ31 specimens by the cold stamping process.
Fig. 11. The results of cold stamping of cell phone housings: (a) as-received sample;
(b) the RUB processed specimen.
4. Discussion
G.S. Huang et al. (2009) revealed that mechanical properties and stretch formability of magnesium alloy sheets with a tilted basal texture obtained by the RUB process were improved at room temperature. Agnew and Duygulu (2005) and Koike et al. (2003) have
noted that for magnesium alloy sheets with a very strong basal texture, the width strain e w can be generated by prismatic slip, while the thickness strain is generated by pyramidal slip and twinning. Therefore, this led to the high r-value and the poor deformation capability of sheet thinning for the as-received sheets in the work of X.S. Huang et al. (2009). In contrast, the thickness strain of magnesium alloy sheets, with a tilted basal texture obtained by the RUB process, can be generated by basal slip, which resulted in a lower r-value. It is generally expected that high r values favor sheet
formability and will lead to higher limiting drawing ratios (Lee, 1984). However, the RUB processed sheet exhibits a lower r value and better drawability at room temperature. The results indicate that the relationship between the r values and sheet formability of
magnesium alloys should be interpreted in a different way than is usually done for cubic metals. The lower r value means the tendency of increase in the thickness strain, which favors the formability of the drawn cup corners. Previous studies (Cheng et al., 2007; Yi et al., 2010) have reached the same conclusion, but the relation between drawability and a lower r value is unclear and further research is needed. It is reported that the sheets with a favored texture for the basal slip exhibited a superior formability in both stretch forming (G.S. Huang et al., 2009) and deep drawing (Cheng et al., 2007). Therefore, formability of magnesium alloy sheets can be improved by the RUB process weakening basal texture of the sheet. Compared with the as-received sheets, the Erichsen values of the RUB processed sheets increased to 5.90 from 3.53, which increased by 67% at most. The LDR of the RUB processed sheets can reach 1.5 from 1.2 which was proved in other study of Chino et al. (2006) at room temperature. The larger Erichsen values for the RUB processed sheets were attributed to the larger n-value and the smaller r-value, which enhanced the capability of sheet thinni