快速制造金屬導(dǎo)電物體的低熔點合金液相的三維印刷油墨外文文獻翻譯、中英文翻譯、外文翻譯,快速,制造,金屬,導(dǎo)電,物體,熔點,合金,三維,印刷,油墨,外文,文獻,翻譯,中英文 （附錄 1） 快速制造金屬導(dǎo)電物體的低熔點合金液相的三維印刷油墨 傳統(tǒng)的三維金屬印刷一般耗時以及缺乏高性能印刷油墨。另一種方式，這里我們提出了液相法的三維打印快速制作導(dǎo)電金屬物體。通過引入金屬合金的熔點略高于室溫的印刷油墨，幾個有代表性的結(jié)構(gòu)，從一個，兩個和三個維度的更為復(fù)雜的模式被證明是快速制造。在傳統(tǒng)的 3D 打印空冷相比，液相的制造提供了一個更高的冷卻速度，從而大大提高了速度在制造目標(biāo)的金屬物體。這一獨特的戰(zhàn)略也有效地防止液態(tài)金屬油墨從空氣氧化，這是難以避免的否則在一個普通的 3D 打印。關(guān)鍵的物理因素（如冷卻液，性質(zhì)中的空氣壓力注射器筒和針頭直徑，印刷油墨的種類和性能）和一些有趣的中間流體相互作用的現(xiàn) 象，金屬液與常規(guī)冷卻流體如水或乙醇之間，它明顯地影響印刷質(zhì)量，進行了披露。此外，為了使未來的液相的 3D 打印機結(jié)合注射泵和針陣列也提出了一個基本路線。液相三維打印，具有潛在的價值，在傳統(tǒng)的方法不可用，將很快在未來的時間制作導(dǎo)電金屬物體的一種有效方法。 1 引言： 作為一種添加劑制造（AM）技術(shù)，快速原型（RP）是在各種各樣的新興領(lǐng)域包括化學(xué)合成變得越來越重要，RP 技術(shù)的基本原理是通過制定可粉狀塑料材料連續(xù)層的創(chuàng)建一三維對象，金屬顆?；蚱渌澈蟿┎牧稀５侥壳盀橹?，已經(jīng)有一系列不同的是技術(shù)開發(fā)的金屬物體。在眾多的方法試過，三個典型的方法包括激光燒結(jié)（LS），激光熔化（LM）和激光金屬沉積（LMD）是最流行的，一般都能加工各種高熔點金屬，合金和金屬基復(fù)合材料（MMC）。在這些公司實現(xiàn)良好的金屬結(jié)構(gòu)，必須選擇合適的粉體材料（其屬性包括化學(xué)成分，顆粒大 小，和粉末的流動性）和激光加工（如激光型和功率，掃描速度，和粉層厚 度）。因此，印刷金屬油墨目前可用的類型相當(dāng)有限，如果你希望使用這種傳統(tǒng)的三維金屬印刷方法。 近年來，低熔點液態(tài)金屬，特別是室溫液態(tài)金屬不斷吸引更多的在計算機芯片冷卻領(lǐng)域更廣泛的關(guān)注。熱界面材料，微流體等這種材料也被提出作為在直接寫入電子和 3D 打印技術(shù)明顯價值油墨。 在這項研究中，進一步擴展用于制造金屬物體的 3D 打印技術(shù)，另一種方法稱為提出了液相三維打印，這不同于現(xiàn)有的風(fēng)冷式三維打印。為了說明的目 的，金屬，其熔點高于室溫低于 300°C，并作為印刷油墨。這種創(chuàng)新的傳統(tǒng)金屬印刷的方法在高熔點金屬油墨通常用于使一系列復(fù)雜的程序。 為了保證印刷質(zhì)量，概念的若干基本流體力學(xué)問題，如液-液系統(tǒng)中液滴的形成，滴噴射過渡等進行了系統(tǒng)的研究。在過去的幾年中，已經(jīng)出現(xiàn)了一系列的實驗在毛細管尖端。當(dāng)金屬液注入另一種不混溶的流體通過針，兩滴形成機制將觀察。如果金屬液的噴射速度低于某一臨界值時，液滴將直接在針尖形 成。但是如果注射速度大于臨界值時，液態(tài)金屬會形成噴射，然后分解成液滴由于瑞利不穩(wěn)定性。一般來說，在前一種情況下，液滴的大小是由浮力，粘度測定，表面張力和慣性的流體和下降，而在后者的液滴的大小是由射流穩(wěn)定性動力學(xué)確定。此外，從針的尖端也會影響印刷過程中的熔滴脫離的基本特征。更復(fù)雜的流體系統(tǒng)，本研究還提出了諸如滴液金屬基冷卻液以及精確地控制液相的最終金屬物沉積質(zhì)量的實踐策略之間的流體相互作用的有趣的科學(xué)問題。 2 實驗平臺和程序： 2.1 可印刷的金屬油墨的制備 目前，液相三維打印，所有的純金屬或合金的熔點在室溫至 300℃可能° 作為印刷油墨。這些包括鎵和銦，鉍基合金。納米材料如銅，銀顆粒在金屬液還提供了一種制備功能性油墨所需的。此外，金屬和非金屬材料組合在一起可以采取不同的印刷油墨。在這里，沿著這個方向的第一審判，這是專門選擇bi35in48.6sn15.9zn0.4 合金油墨表現(xiàn)出液相三維打印方法的基本工作原理。 為使這類功能性油墨的制備工藝為：四金屬鉍，銦，錫和鋅（高純度 99.99%） 重根據(jù) 35:48.6:15.9:0.4 比。把燒杯中的 5 h 245 C°電真空干燥箱，這些純金屬。然后，該混合物的燒杯放在水浴 85°–90 C 30 分鐘。最后攪拌，使燒杯在電真空干燥 2 小時的烤箱，可以進一步確?；旌虾玫暮辖鹑芤骸? 2.2 液相冷卻液的制備 液相冷卻流體可以從中選擇水，乙醇，煤油，膠水，硅油，硅膠等。這里為簡便起見，只有水和乙醇作為冷卻液的比較研究。 2.3 實驗裝置 本研究中所用的實驗裝置如圖 1 所示。由于 bi35in48.6sn15.9zn0.4 熔點略高于室溫，這種液態(tài)金屬容易受到由于凝固針頭阻塞。為了解決這個問題， 注射器通過康銅電阻絲安裝在鋁合金筒，加熱（62 歐姆每米）。溫度控制器是用來調(diào)節(jié)供給電力的康銅電阻絲的金屬筒保持恒溫。氮氣缸是用來在氣筒內(nèi)液態(tài)金屬油墨和壓力由電磁閥調(diào)節(jié)提供一個恒定的空氣壓力。注射器針頭浸入液相的冷卻液，這是在這個實驗中的乙醇水。滴或噴射過程用高速攝像機監(jiān)控 （尼康 nr-s3）可以捕捉每秒 30 幀設(shè)置為 1.999 秒曝光時間 圖 1 實驗裝置示意圖。 3 結(jié)果與討論： 圖 1 說明了目前液相三維打印制造金屬物體的基本實驗裝置。特別是，一四元合金的開發(fā)和應(yīng)用作為印刷油墨。它的一些性能進行了測試，在表 1 和圖2 提供?？梢钥闯觯琤i35in48.6sn15.9zn0.4 密度（7.898 g/cm3）與鐵（7.86 克/立方厘米）。這種合金的 DSC 曲線如圖 2（a）是通過使用一個空的坩堝通過差示掃描量熱計基準(zhǔn)測量（dsc200，耐馳，德國）在 10℃／min°下曲線的溫度變化率和上部曲線代表熔化和冷卻過程，分別。這些行為使其容易在印刷過程中的液固相變?？傊琤i35in48.6sn15.9zn0.4 作為一個理想的液態(tài)金屬油墨來實現(xiàn)液相三維打印本文提出。 bi35in48.6sn15.9zn0.4 表 1 的典型物理特 圖 2（色彩的基本特性 bi35in48.6sn15.9zn0.4 在線）。（一）差示掃描量熱法（DSC）曲線；（b）掃描電子顯微鏡（SEM）圖像；（c）能量色散譜 （EDS） 圖 3 給出了從簡單到復(fù)雜的尺寸的液相三維印刷法建立結(jié)構(gòu)的幾種典型的金屬物體。當(dāng)空氣壓力在注射器筒 34.5 的范圍內(nèi)變化–69 kPa（5–10 psi），和水平移動速度的注射針 0.26 毫米內(nèi)直徑為 5 毫米/秒，大量不同尺寸的金屬球可以迅速通過降低油墨在室溫下乙醇冷卻液形成（見圖 3（a））。這是在現(xiàn)有的焊球制造技術(shù)的概念創(chuàng)新。雖然類似的制備方法已經(jīng)提到過，它主要用于空氣冷卻的情況下。除了金屬球，線性結(jié)構(gòu)也可以容易地制造。當(dāng)空氣壓力使注射器筒內(nèi)設(shè)置為 103 kPa（15 psi），移動的注射針的內(nèi)徑是 0.26 毫米和乙醇冷卻液加熱到 32 C°，液態(tài)金屬棒（如圖 3（b））可以沿垂直方向打印。這些結(jié)構(gòu)是很難直接通過空氣冷卻或砂在傳統(tǒng)的三維印刷的冷卻方法。此外，許多其他結(jié)構(gòu)也可以在短時間內(nèi)以同樣的方式。例如，一個截頭圓錐結(jié) 構(gòu)，使金屬液缸的結(jié)構(gòu)在圖 3（c）和（d），分別。制作過程如下：首先，作為一個 CAD 生成的三維對象（計算機輔助設(shè)計）軟件在 SolidWorks 模型導(dǎo)入STL（固醇 Lithography）文件。然后 STL 文件導(dǎo)入到一個開源 slic3r（HTTP： ／／slic3r。org，進入十月十九日 2012）軟件生成的對象片為一組的水平層，每一層的刀具路徑生成。油墨是下降到室溫乙醇注射針的運動控制刀具路 徑和目標(biāo)最終是通過層印刷層冷卻液。在這里，針的內(nèi)徑為 0.11 毫米，使注射器筒內(nèi)空氣壓力為 69 kPa（10 psi）?？偟膩碚f，這些金屬物體的快速原型是歸因于液相的冷卻流體的高的熱導(dǎo)率和熱容量相比其他冷卻介質(zhì)。雖然目前可用的打印分辨率仍然較低，改進可以通過在不久的將來，調(diào)節(jié)各種印刷因素很可能將部分透露，在以后的章節(jié)?？偟膩碚f，液相印就可能使復(fù)雜的金屬結(jié)構(gòu)通過一個非常快速的方式。 圖 3（色在線）典型的三維金屬結(jié)構(gòu)的液相三維印刷法。（一）液體金屬球； （b）液體金屬棒；（c）一個錐形結(jié)構(gòu)體；（d）筒體結(jié)構(gòu)。 液滴的形成和沉積過程是液相三維印刷法的核心問題。圖 4 給出了這樣的液滴形成一個連續(xù)的過程。從圖 4 可以看出（一），當(dāng)液滴下落速度很小 （3.34 毫米/秒），墨水往往會 形成球形珠在針由于金屬液和冷卻液之間的表面張力大。隨著液滴下落速度的增加，相鄰的液滴越來越靠近，直到最后在一起。圖 4（b）顯示了這個有趣的長尾巴的蝌蚪狀的尾海鞘綱動物滴現(xiàn)象。 圖 4 動態(tài)液滴配置在液相。（一）在水冷卻液的液滴的形成過程（液滴下落速度是 3.34mm／s）；（b）長尾巴的蝌蚪滴由于乙醇的冷卻流體快速注射速度 （液滴下落的速度是 7.98 毫米/秒）。 所需的金屬物體是由熔滴沉積形成。由于本文中使用的印刷油墨的熔點略高于室溫，液滴易熔化和凝固。當(dāng)一滴掉到下面的實心柱，柱的頂部吸熱熔斷器與液滴。作為液滴冷卻的冷卻流體的溫度由于熱傳導(dǎo)，成為列然后將發(fā)育成特定的結(jié)構(gòu)所需的一部分。這是由液相印的典型過程可以用圖 5 所示的基本原理。 圖 5 液滴沉積過程（從（一）至（f））在乙醇中的冷卻流體（液滴下落的速度是 5.65 毫米/秒）。 圖 6 提供了乙醇冷卻印刷之間的比較，當(dāng)液滴以相同的速度落下，從相同的針。水滴在固態(tài)時接觸的底部為乙醇的冷卻方法是給予。這是由于乙醇的快速散熱和集群結(jié)構(gòu)可以容易地形成如圖 6 所示（一）。但對于空氣冷卻的印刷是 傳統(tǒng)的 3D 打印的情況下，液滴將保持在熔融狀態(tài)下在一個更長的時間當(dāng)他們到達底部和一個大的熔球?qū)⑿纬伞４送?，金屬氧化出現(xiàn)時相比，使用乙醇作為冷卻介質(zhì)更嚴(yán)重（見圖 6（b））。 圖 6 比較乙醇冷卻和空氣冷卻印刷之間。（一）由乙醇冷卻方法形成的柱； （b）熔球的形成空氣冷卻的方法。 表 2 比較表的導(dǎo)熱系數(shù)，密度，水的熱容量，乙醇和干燥的空氣，分別?？梢园l(fā)現(xiàn)，乙醇和水的相對密度為 828.22 和 655.02，分別。根據(jù)阿基米德原理， 等于流體的量，它無法取代的重量施加在一個浸沒在液體中的身體向上的浮 力。對金屬液滴浸沒在流體中的浮力，它可以表示為 Gdroplet,Pdroplet,Vdroplet 分別是:液滴的重力，密度和體積，g 是重力加 速度為 9.80665 米/ S2。因此，一個沉浸在水和乙醇液滴向上的浮力是 828.22 和 655.02 比空氣冷卻的情況下大時代。這是更大的浮力，有助于緩沖作用。水和乙醇的相對熱導(dǎo)率分別為 23.05 和 9.27，分別。與水和乙醇的相對熱容量是 4.16 和 2.41，分別。這些優(yōu)異的熱性能的液體（水，乙醇）使金屬液滴在液體 環(huán)境下快速冷卻。具有三維結(jié)構(gòu)的金屬物體，因此可以快速原型。 此外， 這種新方法可以防止金屬液滴的空氣氧化，從而保證印刷的金屬物體的質(zhì)量。值得一提的是，當(dāng)液相三維印刷法給藥，應(yīng)考慮幾個因素會顯著影響最終的印刷質(zhì)量。 第一個因素是冷卻流體的性質(zhì)。冷卻流體的溫度將直接影響印刷效果。如果這個值設(shè)置得太高，新的液滴脫落會融化連同以前。這樣做的結(jié)果是，形成的結(jié)構(gòu)是很難“成長”。但是，如果流體的溫度設(shè)置得太低，液滴下落將迅速冷卻和固化，由于傳熱過程是瞬間完成的。此外，冷卻液為提出的方法實際上是控制制造過程中某些聰明的方式。例如，三維金屬零件的一些獨特的結(jié)構(gòu)可以通過調(diào)節(jié)可能制造故意冷卻流體的速度和方向。此外，冷卻流體的粘度會影響墨水下降時液滴與冷卻流體的密度會影響液滴的浮力已經(jīng)解釋了上述。 水性能表 2，乙醇和 100 kPa 空氣干燥，20°C 第二，空氣壓力在注射筒和針頭直徑在目前的制造過程中扮演著重要的角色。結(jié)果發(fā)現(xiàn)，影響液滴的大小的兩相鄰的液滴之間的距離因素。這些問題已經(jīng)討論過關(guān)于空氣沉積情況。隨著注射速度的增加，液滴的直徑會變小，不同的液滴會更緊密。當(dāng)注射速度增大到一定值時，液滴的噴射將成為。滴噴射過渡的控制有重要影響的成型速度。 圖 7 粒度分布與 0.16 毫米針頭產(chǎn)生的液滴（一），0.34 毫米（B），0.51 毫米 （C）和 0.84 毫米（D）的內(nèi)徑，分別。 液滴直徑也由注射針頭大小的影響。圖 7 顯示了用不同大小的針產(chǎn)生的液滴大小分布的統(tǒng)計結(jié)果。實驗是在與乙醇冷卻使注射器筒內(nèi)空氣壓力在室溫下進行。可以看出，噴射液滴直徑變得與針的尺寸的增加，大。這些針直徑為 0.16 毫米，0.34 毫米，0.51 毫米和 0.84 毫米，所產(chǎn)生的液滴直徑范圍大 30 40 60––μm，80 m 80–μμ，100 M 和 180 M 100–μ，分別。 有了液滴直徑和相鄰的液滴，在圖 8 中顯示的時間間隔之間的近似線性關(guān)系（一）。當(dāng)間隔時間從 2.1 變化到零，整個過程顯示在圖 8（b）。隨著間隔時間越短，液滴越來越小。液滴形狀的變化從球的梭形，逐漸相鄰的液滴在一起。當(dāng)間隔時間最后變成零，滴噴射過渡完成。 圖 8 液滴直徑之間的關(guān)系和相鄰的液滴之間的間隔時間。（一）用乙醇冷卻方法內(nèi)徑 0.26 毫米針的統(tǒng)計結(jié)果；（b）過程（從 A 到 D）時的間隔時間從 2.1 變化的（滴）到零（噴射） 最后，印刷油墨的種類和性質(zhì)也主宰的制造工藝。原則上，所有的低熔點 （例如小于 300°C）金屬可選擇的條件下，適當(dāng)?shù)睦鋮s液可印刷油墨。油墨材料可以是鎵，鉍，銦基合金，甚至這些合金和納米顆粒的混合物。油墨的密 度，粘度，表面張力等，熔點，熱容量的一些性質(zhì)，導(dǎo)熱性能影響的三維金屬結(jié)構(gòu)的形狀和印刷速度。油墨的選擇應(yīng)考慮與冷卻液的類型在未來。 相比傳統(tǒng)的金屬成形方法，液相三維打印方法本文提出的提供了幾個明顯的優(yōu)勢。 （1） 快速制造速度。在 3D 印刷的金屬物體的方法在液相，采用流體控制機制和多元化的三維金屬結(jié)構(gòu)可以形成。此外，冷卻流體的溫度場和流場，可以靈活控制。通過調(diào)節(jié)冷卻流體流動的速度和方向，一些獨特的三維金屬結(jié)構(gòu)可以實現(xiàn)，例如三維旋轉(zhuǎn)體。 （2） 三維機電系統(tǒng)可以打印。導(dǎo)電液體金屬可以用在非金屬材料（如塑料）結(jié)合，形成三維功能裝置包括支撐結(jié)構(gòu)和導(dǎo)電裝置。液相三維印刷與傳統(tǒng)印刷相結(jié)合的方法可以更好地滿足各種打印需求。 （3） 金屬部件的制造業(yè)的能源消耗將減少。由于低熔點液態(tài)金屬油墨的介紹， 對熔融固體油墨所需的能量是很小的。因此，用這種方法制作的金屬部件的難度較小，使用高熔點金屬液通過常規(guī)方法。 然后將液相的 3D 打印機在未來的樣子？作為一種補充到現(xiàn)有的金屬印刷方法，液相的三維打印系統(tǒng)和方法有待進一步提高。液態(tài)金屬油墨可鎵基，基合金或其它低熔點合金鉍。印刷過程中進行溫度平衡空間的溫度應(yīng)高于液態(tài)金屬油墨。為了提高精度和 3D 打印速度，我們建議采用注射泵的注射器針頭陣列和陣列之間的組合。在這樣的系統(tǒng)中，注射器泵陣列是用來提取液的金屬溶液， 在注射器針陣列是在冷卻液注入液態(tài)金屬油墨。注射針可方便地更換不同大小的其他產(chǎn)品來滿足各種打印需求。設(shè)計與三維模型的離散化和每針注射速度的控制是通過計算機實現(xiàn)的過程中完成。注射針陣列部分的示意圖如圖 9 所示。在這種方式中，三維金屬物體上印刷的恒溫槽的底部，該冷卻流體可以是水， 乙醇或其他。毫無疑問，冷卻流體的溫度應(yīng)該設(shè)置一個合理的規(guī)模是低于液態(tài)金屬油墨。此外，冷卻流體流動的速度分布應(yīng)該是可控的、具有獨特結(jié)構(gòu)的一些金屬物體可以直接打印出來。 圖 9（色在線）未來的液相三維打印機注射針陣列。 4 結(jié)論： 總之，我們首次建立了液相三維打印快速制造金屬結(jié)構(gòu)的方法。用熔點高于室溫合金油墨作為印刷油墨。在典型情況下，液相冷卻方法表現(xiàn)出明顯的優(yōu)點比傳統(tǒng)的空氣冷卻的印刷。有了這個戰(zhàn)略，三維金屬結(jié)構(gòu)可以迅速由于液相的冷卻流體的熱導(dǎo)率高、熱容量的形成。此外，幾個關(guān)鍵的物理因素影響的液相 法，印刷質(zhì)量也澄清。液滴的大小和下降速度可通過改變針頭直徑的靈活控 制，使注射器筒內(nèi)的空氣壓力，和溫度，粘度和印刷油墨的表面張力和冷卻 液。的金屬物體的結(jié)構(gòu)可以通過改變冷卻流體的速度和方向調(diào)節(jié)。最后，提出 了一種新的液相的 3D 打印機。作為一個概念性的創(chuàng)新，現(xiàn)有的三維金屬印刷的方法，該液體冷卻印刷也提出了在未來的時間解決重要的基本理論以及實際問題。 （附錄 2） Liquid phase 3D printing for quickly manufacturing conductive metal objects with low melting point alloy ink Conventional 3D metal printings are generally time-consuming as well as lacking of high performance printable inks. From an alternative way, here we proposed the method of liquid phase 3D printing for quickly making conductive metal objects .Through introducing metal alloys whose melting point is slightly above room temperature as printing inks, several representative structures spanning from one, two and three dimension to more complex patterns were demonstrated to be quickly fabricated. Compared with the air cooling in a conventional 3D printing, the liquid- phase-manufacturing offers a much higher cooling rate and thus significantly improves the speed in fabricating the target metal objects. This unique strategy also efficiently prevents the liquid metal inks from air oxidation, which is hard to avoid otherwise in an ordinary 3D printing. The key physical factors (such as properties of the cooling fluid, air pressure within the syringe barrel and needle diameter, types and properties of the printing ink) and several interesting intermediate fluids interaction phenomena between liquid metal and conventional cooling fluids such as water or ethanol, which evidently affecting the printing quality, were disclosed. In addition, a basic route to make future liquid phase 3D printer incorporated with both syringe pump and needle arrays was also suggested. The liquid phase 3D printing, which owns potential values not available in a conventional method, opens an efficient way for quickly making conductive metal objects in the coming time. liquid phase 3D printing, liquid metal printer, rapid prototyping, low melting point metal, liquid cooling, oxidation 1 Introduction As a kind of additive manufacturing (AM) technology, the rapid prototyping (RP) is becoming increasingly important in a wide variety of newly emerging areas including chemi-cal synthesis [1], microfluidics [2], tissue engineering [3– 5],electronic circuit and device [6,7]. The basic principle of RP technology is to create a three dimensional object through laying down successive layers of materials which can be powdered plastic, metal particles or any other adhesive materials. So far, there are already a series of different AM techniques developed for making metal objects. Among the many methods ever tried, three typical ways including laser sintering (LS), laser melting (LM) and laser metal deposition (LMD) are the most prevailing ones which are generally capable of processing a variety of high melting point metals, alloys and metal matrix composites (MMCs) [8]. To achieve favorable metal structures during these fabrications,one has to select both appropriate powder materials (whose properties include chemical constituents, particle size, and powder flowability) and laser process (e.g. laser type and power, scan speed, and powder layer thickness) [8–11]. For such reason, the currently available types of printable metal inks are rather limited if one wishes to use this conventional 3D metal printing method. In recent years, the low melting point liquid metal, especially room temperature liquid metal kept attracting more and more extensive attentions in the areas of computer chip cooling, thermal interface material, microfluidics and so on [12–17]. Such material has also been proposed as printing ink with evident values in direct writing electronics and 3D printing technology. Zheng et al. [18] initiated a desktop printing of flexible circuits on paper via developing liquid metal ink and established a basic 3D printing scheme including computer controlled machine system for simultaneously manufacturing mechanical structure as well as conductive functional devices. Jin et al. [19] proposed an injectable 3-D fabrication method for directly depositing medical electronics at the target biological tissues. Yu et al.[20] discovered a channelless fabrication mechanism for large-scale preparation of room temperature liquid metal droplets. In these studies, the eutectic alloy made of gallium and indium (melting point ~15.7°C) was adopted as the writing ink or raw material. A limitation of such room temperature metal inks lies in that the printed objects are easily subject to melt, which therefore may restrict the application of the device to some extent. In this study, to further extend the 3D printing techniques for fabricating metal objects, an alternative approach termed as liquid phase 3D printing was proposed, which differs from the existing air-cooled 3D printing. For illustration purpose, the metals, whose melting points are above room temperature and less than 300°C, were identified and adopted as the printing ink. This innovates the traditional metal printing method where the high melting point metal inks are often used and thus a series of complicated procedures are involved. To ensure the printing quality, several conceptual basic fluid mechanics issues such as droplet formation in liquid-liquid systems, dripping-jetting transition and so on were systematically studied. Over the years, there have been a series of experimental and dynamic researches on the principle of the drop formation at a capillary tip [21– 23]. When the liquid metal is injected into another immiscible fluid via the needle, two drop-formation mechanisms will be observed. If the injection velocity of the liquid metal is lower than a certain critical value, the drops will be formed directly at the needle tip. However if the injection velocity is larger than the critical value, the liquid metal will form a jet which then breaks up into droplets because of Rayleigh instabilities [21]. Generally speaking, in the former case, the droplet size is determined by the buoyancy, viscosity, surface tension and inertial of the fluid and the drop, while in the latter the droplet size is determined by the jet stability dynamics [22]. Further, the basic features of the droplet detachment from a needle’s tip [23–25] would also affect the printing process. With more complex liquid systems together, the present study also raised interesting scientific issues such as the fluids interactions between dropping liquid metal and the base cooling fluid as well as the practical strategies to precisely control the deposition quality of the final metal objects in liquid phase. 2 Experiment platform and procedure 2.1 Preparation of printable metal inks For the present liquid phase 3D printing, all the pure metals or alloys whose melting points are from around room temperature to 300°C can potentially be adopted as the printing inks. These include gallium-, bismuth- and indium-based alloys. Addition of nanoparticles such as copper, silver particles into such metal fluids also offers a method to fabricate functional inks as desired. Besides, combination of metal and nonmetal material together can be adopted to make diverse printing inks. Here, as the first trial along this direction, the Bi35In48.6Sn15.9Zn0.4 alloy is specifically selected as printing ink to demonstrate the basic working principle of liquid phase 3D printing method. The preparation process for making this kind of functional ink was as follows: four metals of bismuth, indium, tin and zinc (with high purity of 99.99%) are weighed according to the ratio of 35:48.6:15.9:0.4. These pure metals are put in a beaker for 5 h at 245°C in an electric vacuum drying oven. Then, the mixture is stirred in the beaker which is put in water bath at 85–90°C for 30 min. Finally, keep the beaker in the electric vacuum drying oven for 2 h, one can further ensure a well-mixed alloy solution. 2.2 Preparation of liquid phase cooling fluid The liquid phase cooling fluid can be selected from among water, ethanol, kerosene, gluewater, silicone oil, silica gel and so on. Here for brevity, only water and ethanol are adopted as the cooling fluids in a comparative study. 2.3 Experimental devices The experimental device used in this study is illustrated in Figure 1. Because the melting point of Bi35In48.6Sn15.9Zn0.4 is slightly higher than the room temperature, such liquid metal would easily subject to blocking in the syringe needle due to solidification. To solve the problem, the syringe is installed in an aluminum alloy cylinder which is heated via constantan resistance wire (62 ohms per meter). A temperature controller is used to maintain constant temperature of the metal cylinder by adjusting the supply electrical power to the constantan resistance wire. The nitrogen cylinder is used to provide a constant air pressure upon the liquid metal ink inside the syringe and the pressure is regulated by a solenoid valve. The syringe needle is immersed into the liquid phase cooling fluid, which is water/ethanol in this experiment. The dripping or jetting process is monitored with a high speed camera (Nikon NR-S3) which can capture 30 frames per second with the exposure time set to 1.999 s. Figure 1 The schematic diagram of the experimental apparatus. 3 Results and discussion Figure 1 illustrates the basic experimental setup for the present liquid phase 3D printing in making metal objects. Particularly, a four-element alloy was developed and adopted here as the printing inks. Some of its properties are measured and provided in Table 1 and Figure 2. It can be seen that the density of Bi35In48.6Sn15.9Zn0.4 (7.898 g/cm3) is close to that of iron (7.86 g/cm3). The DSC curves of this alloy as shown in Figure 2(a) is measured by using an empty crucible as reference through a differential scanning calorimeter (DSC200, NETZSCH, Germany) at a temperature changing rate of 10°C/min. The lower curve and the upper curve represent the melting and the cooling processes, respectively. The melting point, which corresponds to the onset temperature of the exothermic peak in the lower curve, is 58.3°C. And the freezing point, which corresponds to the onset temperature of the endothermic peak in the upper curve, is 55.9°C. Therefore, the super-cooling degree of Bi35In48.6Sn15.9Zn0.4 defined as the difference between the melting point and the freezing point is 2.4°C. As the melting point of this alloy is slightly higher than room temperature and the supercooling degree is low,Bi35In48.6Sn15.9Zn0.4 in the liquid phase will be cooled quickly while the temperature is reduced in the range of 50–60°C. According to our measurements, the melting enthalpy which is calculated by the area of the exothermicpeak in the green curve and the specific heat capacity of Bi35In48.6Sn15.9Zn0.4 (28.94 J/g and 0.262 J/(g·°C), respectively) are much smaller than that of common metals such as iron (272.2 J/g and 0.46 J/(g·°C), respectively) and aluminum (393.0 J/g and 0.88 J/(g·°C), respectively). These behaviors enable its easy liquid-solid phase transition during the printing process. In a word, Bi35In48.6Sn15.9Zn0.4 serve as an ideal liquid metal printing ink to implement the liquid phase 3D printing as proposed in this paper. Table 1 Typical physical properties of Bi35In48.6Sn15.9Zn0.4 Figure 2 (Color online) Basic properties of Bi35In48.6Sn15.9Zn0.4 . (a) Differential scanning calorimetry (DSC) curves; (b) scanning electron microscopy (SEM) image; (c) energy dispersive spectrum (EDS) Figure 3 presents several typical metal objects with structures spanning from simple to complex dimensions built up by the present liquid phase 3D printing method.When the air pressure within the syringe barrel is varied within a range of 34.5–69 kPa (5–10 psi), and the horizontal moving velocity of the injection needle with 0.26 mm inner diameter is set as 5 mm/s, a large number of metal balls with different sizes can be rapidly formed through dropping the printing ink into the room temperature ethanol cooling fluid (see Figure 3(a)). This is a conceptual innovation over existing solder ball manufacturing technology. Although the similar preparation method has been mentioned before, it was mainly for air cooling case. Besides the metal balls,linear structure can also be easily manufactured. When the air pressure within the syringe barrel is set as around 103 kPa (15 psi), the inner diameter of the nonmoving injection needle is 0.26 mm and the ethanol cooling fluid is heated to about 32°C, liquid metal rods (as shown in Figure 3(b)) can be printed along the vertical direction. These structures are somewhat hard to directly make through air cooling or sand cooling method in a conventional 3D printing. Besides,many other structures can also be made in the same way in short time. For example, a frustum of a cone structure and a cylinder structure made of liquid metal are presented in Figure 3(c) and (d), respectively. The fabrication process is listed as follows: First, a 3D object generated as a CAD (computer-aided design) model in SolidWorks software is exported to STL (STereol Lithography) file. Then the STL file is imported into an open source Slic3r (http://slic3r.org,accessed 2012 October 19) software which generates slices of the object as a set of horizo