材料成型及控制工程外文文獻(xiàn)翻譯--AZ31鎂合金在高溫下的吹塑成型

1、.畢業(yè)設(shè)計(jì)（論文）的外文文獻(xiàn)翻譯原始資料的題目/來(lái)源： Blow forming of AZ31 magnesium alloys at elevated temperatures/ORIGINAL RESEARCH 翻譯后的中文題目： AZ31鎂合金在高溫下的吹塑成型 院 （系） 材料科學(xué)與工程學(xué)院 專 業(yè) 材料成型及控制工程 中文翻譯AZ31鎂合金在高溫下的吹塑成型1.摘要：本文研究和報(bào)道了關(guān)于AZ31B鎂合金商業(yè)片在高溫下的成形行為。實(shí)驗(yàn)分兩個(gè)階段進(jìn)行。第一階段是分析自由脹形實(shí)驗(yàn)，第二階段是分析板材填充封閉模具的能力。施加不同的壓力和溫度，用標(biāo)本圓拱高度表征參數(shù)，在相同時(shí)實(shí)驗(yàn)中，使用分析

2、的方法來(lái)計(jì)算應(yīng)變速率敏感指數(shù)。因此適當(dāng)?shù)某尚螀?shù)如溫度和壓力，對(duì)于隨后的成形實(shí)驗(yàn)是有用處的。第二階段中，在帶有棱形空腔的密閉模具中進(jìn)行成形實(shí)驗(yàn)。同時(shí)分析了相關(guān)的過(guò)程參數(shù)對(duì)成形結(jié)果中壁厚填充、最終樣品上圓角半徑及分布的影響。閉模成形實(shí)驗(yàn)證明：如果工藝參數(shù)選擇適當(dāng)，所研究的商業(yè)鎂板可成形復(fù)雜的幾何形狀。2.關(guān)鍵詞：吹塑成形、材料特性、AZ31鎂合金3.簡(jiǎn)介在眾多結(jié)構(gòu)材料中，鎂合金以其低比重得到了行業(yè)廠家越來(lái)越多個(gè)興趣，鎂合金在具有最低的密度，在輕量化上也有很高的潛力，尤其是在移動(dòng)正在使用運(yùn)動(dòng)部件的領(lǐng)域。在這些應(yīng)用中，越來(lái)越多地為輕質(zhì)合金材料，尤其是傳統(tǒng)的成形方法無(wú)法快速有效的成形鎂合金等合金，使得

3、超塑性成形（SPF）成為一種有吸引力的成形方法。事實(shí)上，具有極其復(fù)雜形狀的輕量化部件可以由具有超塑性的單層板通過(guò)超塑性成形制造。輕金屬合金如鋁、鈦和鎂，有些難以在傳統(tǒng)的成形條件下成形，吹塑成形（BF）的應(yīng)用隨之越來(lái)越多。吹塑過(guò)程主要是將坯料放入模具型腔中，并在其上施加成形氣體（如空氣、氬氣）。相比基于成形操作的流體，在高溫成形的區(qū)域，氣體的耐熱性為實(shí)現(xiàn)更高的溫度提供了可能1。該氣體可完全替代傳統(tǒng)沖壓工序中的驅(qū)動(dòng)沖頭，并且允許具有高細(xì)節(jié)層次的不同種類材料的變形。在過(guò)去，吹塑成型的理念主要應(yīng)用在傳統(tǒng)的玻璃吹制上，它的主要原理是在材料溫度高于其軟化點(diǎn)后成形。目前，BF的原理已被廣泛應(yīng)用到塑料的制造上

4、。這個(gè)過(guò)程進(jìn)行的金屬板料成形相比傳統(tǒng)的成形方法潛力是顯著的。它有如下優(yōu)點(diǎn)：（i）在具有高細(xì)節(jié)層次的單一操作中成形大而形狀復(fù)雜的組件；（ii）他們用近似網(wǎng)狀的制造方法大大減少了后續(xù)的成本和在裝配操作上花費(fèi)的時(shí)間；（iii）無(wú)需人力損耗；（iv）成品有更高的尺寸精度；（v）回彈對(duì)成形部分影響小。然而，金屬的超塑性吹塑成形并沒(méi)有在工業(yè)上得到廣泛的應(yīng)用，因?yàn)槠湓牧虾蜕a(chǎn)過(guò)程成本較高，這使得它的競(jìng)爭(zhēng)力要比其他的傳統(tǒng)技術(shù)弱。為了克服這些缺點(diǎn)，高應(yīng)變速率超塑性和其他技術(shù)比如快速塑料成形應(yīng)用而生，并且為實(shí)現(xiàn)大批量生產(chǎn)一直在改進(jìn)。成功實(shí)施QPF技術(shù)需要一個(gè)遠(yuǎn)離小批量假設(shè)的轉(zhuǎn)換，這種假設(shè)與之前應(yīng)用在航空航天和利

5、基汽車產(chǎn)品的BF技術(shù)是相關(guān)聯(lián)的。另一方面，原材料的準(zhǔn)備更加苛刻：控制極小平均晶粒尺寸的顯微結(jié)構(gòu)也是有要求的2,3。在這項(xiàng)工作中，分析了在高溫下通過(guò)BF技術(shù)的裝置一塊商業(yè)鎂板的成形行為。所使用的剛發(fā)包括兩個(gè)方面：（i）通過(guò)脹形測(cè)試得出的第一相表征；（ii）第二階段在封閉模具中成形過(guò)程的分析。該工作主要目標(biāo)是結(jié)合BF技術(shù)分析鎂合金在工業(yè)生產(chǎn)過(guò)程中的應(yīng)用潛力。這些合金已經(jīng)顯示出在高溫下具有超塑性4,7。最終目標(biāo)是看這種成形技術(shù)對(duì)商業(yè)開發(fā)的影響，一旦工藝參數(shù)的優(yōu)化有了顯著的成果，首先可是實(shí)現(xiàn)循環(huán)時(shí)間的減少。4.實(shí)驗(yàn)裝置：在實(shí)驗(yàn)室規(guī)模的設(shè)備上已經(jīng)進(jìn)行了材料特性和封閉模具成形測(cè)試，這個(gè)設(shè)備嵌在INSTRO

6、N萬(wàn)能材料試驗(yàn)機(jī)的圓筒形裂解爐中。設(shè)備包括：（i）一個(gè)壓邊器，（ii）具有多個(gè)不同型腔的凹模，來(lái)滿足不同的成形條件，（iii）連通氬氣缸的供應(yīng)氣體的氣動(dòng)回路，在靠近成形腔室應(yīng)有成比例電磁閥和鋼管，在低溫地方有靈活的聚氨酯管，（iv）具有電子控制器的電爐，為補(bǔ)償熱擴(kuò)散，需在上部、中部、下部設(shè)置三種不同的溫度，（v）監(jiān)測(cè)板材和工具熱條件的熱電偶，（vi）一個(gè)傳感器用于測(cè)量在脹形試驗(yàn)期試樣的圓頂高度（vii）采集I/O設(shè)備上溫度，壓力，壓邊力數(shù)據(jù)的PC機(jī)，可以用來(lái)監(jiān)控和管理。對(duì)于材料特性，脹形試驗(yàn)在一個(gè)直徑為45mm的圓筒形模腔中進(jìn)行，板材在其中可以自由擴(kuò)散。在整個(gè)實(shí)驗(yàn)過(guò)程中通過(guò)數(shù)字采集的位置傳感器

7、信號(hào)檢測(cè)試樣的圓拱高度。設(shè)備上的更多詳情可見(jiàn)參考文獻(xiàn)8。在閉模成形試驗(yàn)中，使用的是具有14mm深棱柱空腔的模具。該空腔截面為方形，邊長(zhǎng)為40mm，兩側(cè)之間的圓角半徑為5mm。設(shè)備示意圖如圖1。圖1 封閉模具成形試驗(yàn)的試驗(yàn)裝置5.材料特性商業(yè)AZ31B鎂合金板材在收獲狀態(tài)中已經(jīng)得到測(cè)試。該材料未經(jīng)機(jī)械和熱處理；板材在退火條件下已被壓緊，平均晶粒尺寸為153m，厚度0.75mm。 關(guān)于材料的超塑性特性，拉伸試驗(yàn)通常是在不同的溫度和應(yīng)變速率條件下進(jìn)行，目的是為了得到材料具有最高斷裂伸長(zhǎng)率的最佳條件。這個(gè)可以通過(guò)拉伸試驗(yàn)測(cè)量斷裂伸長(zhǎng)率和跳應(yīng)變速率實(shí)驗(yàn)測(cè)量應(yīng)變速率敏感性指數(shù)得到9。一些學(xué)者證明：當(dāng)晶界滑

8、移（GBS）為主要變形機(jī)制時(shí)，應(yīng)力和應(yīng)變條件對(duì)材料特性影響不大10。其他學(xué)者也證明了單軸拉伸應(yīng)力和應(yīng)變條件不能有效地獲得材料的參數(shù)，因?yàn)槌尚瓦^(guò)程中板材存在這樣一個(gè)現(xiàn)象，與模具相互作用，然后再經(jīng)過(guò)一定的應(yīng)力應(yīng)變條件，結(jié)果完全不同。此外，鎂合金具有晶粒粗大的趨勢(shì)，并且一些例子證明GBS不能作為主要的變形機(jī)制11。同時(shí)，在超塑性條件下進(jìn)行單軸拉伸試驗(yàn)必須合理設(shè)計(jì)實(shí)驗(yàn)裝置和試樣幾何形狀。一些標(biāo)準(zhǔn)的存在，如ISO20032和ASTM E2448給試驗(yàn)步驟和設(shè)備以很好的指示。在超塑性條件下，拉伸試驗(yàn)的顯著優(yōu)勢(shì)是在試驗(yàn)過(guò)程中，有可能可以是更加精確的控制應(yīng)變速率，但是另一方面，可以說(shuō)：試樣的切割精度必須非常高

9、，因?yàn)榍懈罴夹g(shù)也會(huì)影響試驗(yàn)結(jié)果；機(jī)械切割流程必須優(yōu)先熱切割，因?yàn)楹笳邥?huì)改變靠近切割邊緣材料的顯微結(jié)構(gòu)；試樣尺寸和形狀（標(biāo)準(zhǔn)長(zhǎng)度和寬度，平行和夾緊部分的圓角半徑）會(huì)影響實(shí)驗(yàn)結(jié)果；爐子必須足夠大來(lái)容納變形過(guò)程中的大應(yīng)變 為了克服這些困難和在應(yīng)力條件下更接近真實(shí)過(guò)程的測(cè)試板材，取代單軸拉伸的一些試驗(yàn)已經(jīng)有了提議和報(bào)道；這些中有一個(gè)是基于BF技術(shù)來(lái)進(jìn)行脹形試驗(yàn)12-14。在這項(xiàng)工作中，材料通過(guò)吹塑成型試驗(yàn)表征：使用上述實(shí)驗(yàn)設(shè)備，在不同的溫度和壓力條件下進(jìn)行恒壓脹形試驗(yàn)。根據(jù)材料特性和設(shè)備能力，試驗(yàn)中，壓力從0.2MPa到0.8MPa（分七步），溫度從360到520（分四步）。在整個(gè)過(guò)程中，壓力保持恒定

10、直到破裂。試驗(yàn)中，如果與預(yù)期時(shí)間有3000s的出入則被排除在計(jì)劃之外。整個(gè)試驗(yàn)期間，每個(gè)階段的圓拱高度使用之前的位置傳感器來(lái)測(cè)定。試樣的最終高度也在試驗(yàn)結(jié)束后進(jìn)行了測(cè)量。圖2顯示了試驗(yàn)試樣，破裂前的高度也繪制在內(nèi)。 圖2 四種溫度條件下 破裂時(shí)圓拱高度和成形壓力的函數(shù) 具有研究意義的最高溫度和壓力為520和0.2MPa。盡管使用了惰性氣體來(lái)成形，但成形試樣在經(jīng)過(guò)2825s后還是出現(xiàn)了明顯氧化。較好的實(shí)驗(yàn)結(jié)果，就破裂圓拱高度而言，在460也有出現(xiàn)，尤其是在低壓條件下，相比之前的條件無(wú)明顯氧化。另一個(gè)試驗(yàn)明顯說(shuō)明：降低溫度，成形壓力對(duì)試樣破裂的圓拱高度影響很小。 根據(jù)15，試驗(yàn)中的等效應(yīng)變速率值可

11、通過(guò)下面的公式進(jìn)行計(jì)算： （1）其中，h為圓拱高度，為h相對(duì)時(shí)間的導(dǎo)數(shù)（潮高比），R是模腔的半徑。例如，在圖3中，顯示了460和0.3MPa下的H-T曲線和應(yīng)變速率的變化。圖3 0.3MPa的恒壓下，460脹形試驗(yàn)中頂點(diǎn)處圓拱高度和應(yīng)變率與成形時(shí)間的關(guān)系圖 該圖的初始部分的特征是圓拱高度快速增加相對(duì)于應(yīng)變速率的增加（高于3x103S1）;該圖的第二部分顯示當(dāng)保持一個(gè)恒定的應(yīng)變速率（約2x104S3）時(shí)，高度時(shí)間關(guān)系曲線斜率基本不變；結(jié)束部分，高度和應(yīng)變速率再次增長(zhǎng)直到破裂。這個(gè)現(xiàn)象也存在與其他不同溫度，壓力，應(yīng)變速率的試驗(yàn)中。在試驗(yàn)的第二階段，從0.3MPa到0.8MPa改變壓力，保持恒定的溫

12、度，發(fā)現(xiàn)應(yīng)變速率保持在一個(gè)幾乎恒定的值上，從2x104S3升到4x104S3。 圖4顯示了460，從0.3MPa到0.8MPa6個(gè)不同壓力水平下圓拱高度的變化情況?？梢钥闯?，當(dāng)需要相同的圓拱高度時(shí)，成形時(shí)間會(huì)隨壓力水平的提高呈線性減少。類似的現(xiàn)象在其他溫度也出現(xiàn)過(guò)?？紤]恒壓水平和分析溫度對(duì)成形行為的影響，應(yīng)考慮的因素是，在自由膨脹試驗(yàn)中的高度速率相比溫度更加線性的增加，如圖4b。通過(guò)分析H-T曲線接近線性的那部分，可以根據(jù)斜率計(jì)算出圓拱高度速率。由圖4b可知，溫度由360升到410會(huì)使高度速率由0.006mm/s增加到0.03mm/s，當(dāng)溫度由460升到520時(shí)會(huì)使高度速率由0.11mm/s升

13、到0.57mm/s。試驗(yàn)中，每個(gè)計(jì)算出的高度速率被認(rèn)為與應(yīng)變速率是成比例的，得出如下結(jié)論，應(yīng)變速率的增加隨溫度呈線性關(guān)系。圖4 a 圓拱高度在460和六個(gè)不同壓力下與成形時(shí)間的關(guān)系圖 b 圓拱高度在0.8MPa和四個(gè)不同溫度下與成形時(shí)間的關(guān)系圖 在熱成形過(guò)程中一個(gè)最重要的參數(shù)就是應(yīng)變速率敏感性指數(shù)，m，它可以通過(guò)不同應(yīng)變速率下的拉伸試驗(yàn)很容易計(jì)算出來(lái)。在氣脹成形領(lǐng)域，Jovane和其他學(xué)者如Enikeev和Kruglov16,17提出了評(píng)估脹形試驗(yàn)本構(gòu)參數(shù)的分析方法。例如，測(cè)量?jī)蓚€(gè)不同壓力條件下脹形試驗(yàn)后的高度，應(yīng)變敏感性指數(shù)可通過(guò)如下公式得到：m=lnp2p1lnt1t2 （2）其中，下標(biāo)1

14、、2分別表示第一組、第二組壓力水平，p為壓力值，t為需要得到與凹模圓角半徑相等的圓拱高度所需的成形時(shí)間。如之前提到的，鎂合金在高溫下的吹塑成型受到微觀結(jié)構(gòu)的變化也會(huì)影響m值。因此，通過(guò)（2）式計(jì)算出的m值是一個(gè)平均值，但它可以被認(rèn)為是分析壓力如何影響成形性能的良好的開端參數(shù)。 m的最高值出現(xiàn)在最高溫度（460和520），最低壓力（0.2MPa-0.3MPa），最高壓力（0.7MPa-0.8MPa）。確認(rèn)指數(shù)的重要性，破裂的最高圓拱高度對(duì)應(yīng)最大的m值，通過(guò)計(jì)算不同試驗(yàn)溫度和相同壓力條件下的m的平均值，可以看到在460，該合金表現(xiàn)出更大的m的平均值，如圖5所示。采用高的成形溫度會(huì)導(dǎo)致晶粒變粗變大；

15、此外板材的氧化，成形過(guò)程中溫度的降低，會(huì)帶來(lái)更好的最終制件18。因此，根據(jù)試驗(yàn)結(jié)果和這些因素，在那些被檢測(cè)的結(jié)果中，這種鎂合金的最佳成形溫度為460，在這個(gè)溫度可以實(shí)現(xiàn)等效斷裂伸長(zhǎng)率和后成形材料的良好折中。圖5 應(yīng)變速率敏感性指數(shù)與成形溫度的函數(shù)曲線6.封閉模具成形為了分析封閉模腔的填充，2k多因子實(shí)驗(yàn)方案應(yīng)用而生。這兩個(gè)因素分兩個(gè)層面，壓力為0.4MPa和0.8MPa，成形時(shí)間為500S和1000S。另外，為了得出在模腔填充（正比于填充在封閉模腔中板材的體積）和檢測(cè)的因素中是否存在非線性，補(bǔ)充了一個(gè)中心點(diǎn)。實(shí)驗(yàn)方案如圖6a，各自具有成形壓力和時(shí)間。增加了兩點(diǎn)是在單一的壓力條件下（0.8MPa

16、），成形時(shí)間為50S和2000S的兩點(diǎn)也被加入。在吹塑成型之后，用數(shù)字圖像相關(guān)系統(tǒng)采集變形的輪廓來(lái)得到板材的形狀，并且很容易測(cè)量該組件的主要幾何參數(shù)，來(lái)量化填充模具的板材。圖6 閉模成形試驗(yàn) a有一個(gè)中心點(diǎn)，兩組成形壓力水平和兩組成形時(shí)間 b 在已檢驗(yàn)的壓力水平下，成形后沿方形板材方向的圓角半徑和成形時(shí)間的函數(shù)曲線 為了定量填充，進(jìn)行了3個(gè)幾何參數(shù)的測(cè)量：（i）板材和閉模底部的接觸面積，（ii）沿著中間軸線所成形的的板材的圓角半徑，（iii）沿對(duì)角線的平方部分所成形的板材的圓角半徑。 比較二者可以看出，后者的值要大于前者。這兩個(gè)參數(shù)之間的差異隨著成形時(shí)間的減少而減小：0.8MPa50S后差異為

17、11%，2000S后掉到4%。 盡管該材料達(dá)不到超塑性成形的目的，閉模試驗(yàn)已經(jīng)證實(shí)了其在高溫條件下具有很高的延展性：在0.8MPa的恒壓條件下，經(jīng)過(guò)2000S，板材在不破裂的情況下達(dá)到了最小的圓角半徑約為1.2mm（沿中軸方向）。盡管成形時(shí)間對(duì)于傳統(tǒng)工業(yè)應(yīng)用來(lái)說(shuō)并無(wú)太高成本效益，但對(duì)獲得形狀復(fù)雜的材料和成形過(guò)程來(lái)說(shuō)是很有啟發(fā)的。所有已分析的幾何參數(shù)證明，充模和溫度、壓力這兩個(gè)因素之間為非線性關(guān)系：觀測(cè)試驗(yàn)可以看出，0.8MPa條件下，50S到500S過(guò)程中形成的圓角半徑要比500S到2000S過(guò)程的大很多（如圖6b所示）。中心點(diǎn)的結(jié)果表示充模和成形壓力之間為非線性關(guān)系：相比0.6MPa條件下獲

18、得的結(jié)果，比0.4MPa條件下的，它更接近于0.8MPa條件下所獲得的結(jié)果。類似的結(jié)果也在自由脹形試驗(yàn)中獲得，證明板材的應(yīng)變速率比成形壓力會(huì)更加線性的增加。 眾所周知，在恒壓條件下，當(dāng)板材接觸模腔底部時(shí)，坯料的平均應(yīng)變率會(huì)突然下降。常見(jiàn)的SPF應(yīng)用中，當(dāng)在最佳壓力周期時(shí)，板材模腔底部接觸后壓力會(huì)越來(lái)越大，來(lái)保持接近目標(biāo)值的應(yīng)變率19。在恒壓測(cè)試中，板材迅速接觸模腔底部，而0.8MPa條件下經(jīng)過(guò)50S后，板材已經(jīng)接觸了模腔底部，但需要更多的時(shí)間來(lái)校準(zhǔn)和接觸模壁（如圖6b）。在這些情況下，使用高壓條件，板材的應(yīng)變率在接觸后還是會(huì)變得很低。7.結(jié)論 高溫條件下商用AZ31鎂合金的成形行為在脹形和閉模

19、試驗(yàn)中得到了分析。這些試驗(yàn)的結(jié)果得出： 為了具有超塑性行為，即使對(duì)材料不進(jìn)行前處理，它也會(huì)和原來(lái)一樣在斷裂錢表現(xiàn)出大的等效伸長(zhǎng)率； 最大伸長(zhǎng)率出現(xiàn)在最高的溫度和最低的壓力下：在已進(jìn)行試驗(yàn)的眾多溫度水平下，460可以實(shí)現(xiàn)斷裂伸長(zhǎng)率、應(yīng)變速率敏感性指數(shù)和材料后成形條件的良好折衷； 降低成形溫度，壓力對(duì)斷裂圓拱高度的影響也會(huì)減??；在恒溫條件下，把應(yīng)變率作為壓力的函數(shù)或者在恒壓條件下，把應(yīng)變率作為溫度的函數(shù)都可以得出很明顯的線性關(guān)系； 在閉模成形中，材料在高溫下可以達(dá)到非常小的圓角半徑，表現(xiàn)出很高的延展性； 在檢驗(yàn)溫度和壓力時(shí)，充模和成形壓力有很好的線性關(guān)系，與成形時(shí)間幾乎不存在線性關(guān)系。為了更好的了

20、解鎂合金在高溫下吹塑成型的影響因素，還需進(jìn)一步的試驗(yàn)研究。由于微觀結(jié)構(gòu)的變化和氣蝕現(xiàn)象，后成形的特點(diǎn)也許更深入的分析。在加快成形周期和優(yōu)化板材厚度分布的過(guò)程中，壓力是可以調(diào)節(jié)的，吹塑成形被認(rèn)為在生產(chǎn)形狀復(fù)雜的薄壁鎂合金組件中有很高的競(jìng)爭(zhēng)力。8.鳴謝 作者感謝意大利機(jī)構(gòu)”Ministero dellIstruzione, dellUniversit e della Ricerca”和 “Fondazione Cassa di Risparmio di Puglia”對(duì)該研究活動(dòng)的投資和支持。 外文文獻(xiàn)的原稿Blow forming of AZ31 magnesium alloys at elev

21、ated temperaturesDonato Sorgente & Leonardo Daniele Scintilla &Gianfranco Palumbo & Luigi TricaricoReceived: 29 April 2009 / Accepted: 15 June 2009 / Published online: 25 June 2009# Springer/ESAFORM 2009Abstract In this work, the forming behaviour of a commercial sheet of AZ31B magnesium alloy at el

22、evated temperatures is investigated and reported. The experimental activity is performed in two phases. The first phase consists in free bulging test and the second one in analysing the ability of the sheet in filling a closed die. Different pressure and temperature levels are applied. In free bulgi

23、ng tests, the specimen dome height is used as characterizing parameter; in the same test, the strain rate sensitivity index is calculated using an analytical approach. Thus, appropriate forming parameters, such as temperature and pressure, are individuated and used for subsequent forming tests. In t

24、hesecond phase, forming tests in closed die with a prismatic shape cavity are performed. The influence of relevant process parameters concerning forming results in terms of cavity filling, fillet radii on the final specimen profile are analysed. Closed die forming tests put in evidence how theexamin

25、ed commercial magnesium sheet can successfully be formed in complicated geometries if process parameters are adequately chosen. Keywords Blow forming . Material characterization .Magnesium alloys . AZ31IntroductionMagnesium (Mg) alloys are receiving increasing interest from industry manufacturers pr

26、incipally because of their low specific weight: among structural materials, Mg alloys have the lowest density and offer the highest potential for saving weight, especially in areas where moving components are in use. In these applications, the increasing demand for lightweight alloys, in particular

27、for Mg alloys and the inability of conventional forming techniques to effectively form these alloys make Superplastic Forming (SPF) an attractive forming technique. In fact, lightweight components with extremely complex shapes can be manufactured by SPF from a single sheet of superplastic material.

28、The application of the Blow Forming (BF) process is particularly interesting and innovative considering light metallic alloys such as aluminium, titanium and magnesium ones, some of which are hard to form using conventional conditions. The BF process consists in the application of aforming gas (e.g.

29、 air, argon) pressure on the blank that is forced in a die cavity. In comparison with fluid based forming operations, in the area of forming at elevated temperatures, gas offers the chance to provide higher temperatures due to its high temperature resistance in contrast to most fluids 1. The gas rep

30、laces completely the driven punch of conventional stamping processes, andallows deforming different kinds of materials with the highest level of detail. In the past, the concept of forming by blowing was applied in the traditional glassblowing, whose fundamental principle is based on forming the mat

31、erial at a temperature greater than the softening point.Currently, the basic principle of BF is widely used in the manufacture of plastics, for example in Blow Moulding processes. In sheet metal forming, potentialities of this process compared with conventional forming techniques are significant. It

32、 gives several advantages: (i) forming components with large and complex shapes, in a single operation with a high level of details; (ii) their manufacturing in nearly netshape, drastically reducing subsequent costly and time consuming assembly operations; (iii) the absence of male tools costs; (iv)

33、 better dimensional accuracy of finished products; (v) low springback effects on the formed part. However, SPF with BF for metals has not a large scale application in the industry, owing to the high cost of the process and raw materials, which made this type of process globally less competitive comp

34、ared with other conventional technologies. In order to overcome these limits, high strain rate superplasticity (HSRSP) and some techniques as Quick Plastic Forming (QPF) have been developed and are continuously improved for achieving high volumes production requests. The successful implementation of

35、 QPF technology requires a shift away from the low-volumeassumptions typically connected with prior applications of BF processes to aerospace or niche automotive products. On the other hand, material preparation is much more restrictive: controlled microstructure with very small mean grain size are

36、generally required 2, 3. In this work, the forming behaviour of a commercial Mg sheet has been analyzed at elevated temperatures by means of the BF technique. The used approach consists in a first phase of characterization by bulging tests and in a second part of process analysis by closed die formi

37、ng tests. The main objective is to analyze the potentialities of the Mg alloy combined with the BF technique in industrial manufacturing processes. These alloys have already demonstrated to have a superplastic behaviour at elevated temperatures in different conditions 47. The ultimate goal is to loo

38、k at this forming technique towards its commercially exploiting, once significant results in the optimization of process parameters and, above all, in the cycle time reduction could be achieved.Experimental setupBoth the material characterization and the closed die forming tests have been performed

39、on a laboratory scale equipment embedded in the cylindrical split furnace of an INSTRON universal testing machine. The equipment consists in: (i) a blank-holder, (ii) a female die with different cavity shapes for generating on the blank different forming conditions, (iii) a pneumatic circuit for gas

40、 supply with an Argon cylinder, proportional electronic valves, steel tubes in proximity of the forming chamber and flexible polyurethane tubes in colder zones, (iv) an electric furnace with its electronic controller for upper, central and lower zones which can be set with three different temperatur

41、es for compensating thermal dispersion, (v) thermocouples to monitor thermal condition on the sheet and on the tools, (vi) a transducer for measuring, during bulging test, the dome height on the specimen and (vii) a PC with a data acquisition I/O device by which pressure, temperature,blank holder fo

42、rce can be monitored and managed. For material characterization, bulge tests are performed with a cylindrical die cavity (diameter 45 mm) in which the sheet can freely expand; the dome height of the specimen is monitored during the whole test by the digital acquisition of a position transducer signa

43、l. Further details on the equipment can be found in 8.In closed die forming tests, a die with a 14.5 mm deep prismatic cavity has been used. The cavity has a squared section with a side length of 40 mm and a fillet radius between sides of 5 mm. A schematic representation of the equipment is shown in

44、 Fig. 1 Fig. 1 Experimental setup for closed die forming testMaterial characterizationCommercial AZ31B Mg Sheets have been tested in the asreceived conditions. No mechanical or thermal treatment has been carried out on the material; sheet has been purchased in the annealed conditions with an average

45、 grain size of 153 m and a thickness of 0.75 mm. In superplastic material characterization, usually tensile tests at different temperatures and strain rates are performed in order to get optimal conditions in which material has the best performances with the highest elongation to failure. This can b

46、e done by measuring the elongation to failure instandard tensile tests and the strain rate sensitivity index in jump strain rate tests 9. Some authors have demonstrated that, when grain boundary sliding (GBS) is the predominant deformation mechanism, the stress and strain condition has a marginal ro

47、le in the material characterization 10. Some other authors have demonstrated also that uni-axial tensile stress and strain conditions are not effective for obtaining material parameters due to the fact that during a forming process the sheet, interacting with the die, undergoes to a stress and strai

48、n condition that is completely different.Moreover, Mg alloys have a great tendency to grain coarsening and in several cases GBS cannot be considered as the predominant deformation mechanism 11. Furthermore, testing setup and specimen geometry for uni-axial tensile tests in superplastic conditions ha

49、ve to be properlydesigned. Some standards exists, such as ISO 20032 and ASTM E2448, giving indications on the best test procedures and equipments. In superplastic conditions, the great advantage of tensile tests is the possibility of controlling in a sufficiently accurate way the strain rate during

50、the test, but on the other hand it can be said that: cutting accuracy in the specimen preparation must be very high, since also the cutting technique can influence test results; mechanical cutting processes have to be preferred to thermal cutting processes that can modify the material microstructure

51、 near the cutting edge; specimen dimensions and shape (gauge length and width, fillet radius between the parallel portion and the clamp section) can affect test results furnace must be sufficiently large to accommodate the large strain that the specimen undergoes during the testIn order to overcome

52、these problems and to test the sheet in a strain condition more similar to the real process one, several tests alternative to uni-axial tensile tests have been proposed and reported; one of most common is based on bulge tests by means of the BF technique 1214.In this work, the material has been char

53、acterized with blow forming tests: constant pressure bulge tests at different temperatures and different pressure levels have been performed using the aforementioned laboratory equipment. Tests have been performed ranging pressure from 0.2 MPato 0.8 MPa (with 7 different levels) and temperature from

54、 360C and 520C (with 4 different levels), according to material physical properties and to the equipment capabilities. Pressure has been kept constant during the whole test until rupture occurred. Tests with an expected forming time to failure greater than 3000 s have been excluded from the experime

55、ntal plan. For each test, the dome height has been acquired during the whole test using the position transducer that described before. Final height of the specimen has also been measured after the test. In Fig. 2, tested specimens are shown and their height to failure is plotted. Fig. 2 Dome height

56、at failure as a function of the forming pressure applied on the sheet, plottedfor four different temperatures The highest value of the height is reported for 520C and 0.2 MPa. In spite of the use of an inert forming gas, the formed specimen, after a forming time of 2825 s, appears markedly oxidised.

57、 Good results, in terms of dome height at failure, can be found also at 460C, especially for low pressure levels, with a less evident grade of oxidation on the formed sheet. Another result that can be highlighted is that, reducing the temperature, the height of the specimen at which the material fails, is less influenced by the forming pressure. Value of the equivalent strain rate during the test, according to 15, can be calculated by the following expression: (1)where h is the dome height; his the derivative of h with respect to time (height rate) and R is the die cavity radius.