曹繼偉, 王沛, 劉志遠(yuǎn), 劉長(zhǎng)勇, 吳甲民, 陳張偉

基于粉末成形的激光增材制造陶瓷技術(shù)研究進(jìn)展

曹繼偉1,2, 王沛1,2, 劉志遠(yuǎn)1,2, 劉長(zhǎng)勇1,2, 吳甲民3,4, 陳張偉1,2

(1. 深圳大學(xué) 增材制造研究所, 深圳 518060; 2. 廣東省電磁控制與智能機(jī)器人重點(diǎn)實(shí)驗(yàn)室, 深圳 518060; 3. 華中科技大學(xué) 材料科學(xué)與工程學(xué)院, 材料成形與模具技術(shù)國(guó)家重點(diǎn)實(shí)驗(yàn)室, 武漢 430074; 4. 華中科技大學(xué) 增材制造陶瓷材料教育部工程研究中心, 武漢 430074)

陶瓷以其優(yōu)異的熱物理化學(xué)性能在航空航天、能源、環(huán)保以及生物醫(yī)療等領(lǐng)域具有極大的應(yīng)用潛力。隨著這些領(lǐng)域相關(guān)技術(shù)的快速發(fā)展, 其核心零件部件外形結(jié)構(gòu)設(shè)計(jì)日益復(fù)雜, 內(nèi)部組織逐步走向定制化、梯度化。陶瓷具有硬度高、脆性大等特點(diǎn), 較難通過傳統(tǒng)的加工成形方法實(shí)現(xiàn)異形結(jié)構(gòu)零件的制造, 最終限制了陶瓷材料的工程應(yīng)用范圍。激光增材制造技術(shù)作為一種快速發(fā)展的增材制造技術(shù), 在復(fù)雜精密陶瓷零部件的制造中具有顯著優(yōu)勢(shì): 無模、精度高、響應(yīng)快以及周期短, 同時(shí)能夠?qū)崿F(xiàn)陶瓷零件組織結(jié)構(gòu)靈活調(diào)配, 有望解決上述異形結(jié)構(gòu)陶瓷零件成形問題。本文綜述了多種基于粉末成形的激光增材制造陶瓷技術(shù): 基于粉末床熔融的激光選區(qū)燒結(jié)和激光選區(qū)熔化; 基于定向能量沉積的激光近凈成形技術(shù)。主要討論了各類激光增材陶瓷技術(shù)的成形原理與特點(diǎn), 綜述了激光選區(qū)燒結(jié)技術(shù)中陶瓷坯體后處理致密化工藝以及激光選區(qū)熔化和激光近凈成形技術(shù)這兩種技術(shù)中所打印陶瓷坯體基體裂紋開裂行為分析及其控制方法的研究進(jìn)展, 對(duì)比分析了激光選區(qū)燒結(jié)、激光選區(qū)熔化以及激光近凈成形技術(shù)成形陶瓷零件的技術(shù)特征, 最后展望了激光增材制造陶瓷技術(shù)的未來發(fā)展趨勢(shì)。

激光增材制造; 激光選區(qū)燒結(jié); 激光選區(qū)熔化; 激光近凈成形技術(shù); 陶瓷; 綜述

陶瓷材料具有優(yōu)異的機(jī)械性能、熱穩(wěn)定性能以及其他物理性能, 在航空航天、能源環(huán)保、生物醫(yī)療等領(lǐng)域具有廣泛的應(yīng)用前景。隨著各應(yīng)用領(lǐng)域的發(fā)展, 對(duì)陶瓷構(gòu)件的制造技術(shù)水平提出了更高的要求: 日益復(fù)雜的結(jié)構(gòu)設(shè)計(jì)、性能的定制化以及組織功能梯度化等。陶瓷材料硬度高、脆性大, 采用減材加工技術(shù)對(duì)刀具性能要求較高、成本昂貴, 且通常很難實(shí)現(xiàn)零件定制化以及組織與性能的靈活調(diào)配。增材制造(Additive manufacturing, AM)也稱3D打印, 通過累加成形原理, 能夠?qū)崿F(xiàn)復(fù)雜結(jié)構(gòu)零件成形及組織性能調(diào)控[1-2]。目前該技術(shù)已廣泛應(yīng)用于聚合物、金屬、陶瓷等材質(zhì)零部件的制造, 并發(fā)展演變出各具特色的增材制造技術(shù)。其中, 激光增材制造技術(shù)具有能量密度高、打印速度快、后處理工藝少等特點(diǎn), 國(guó)內(nèi)外科研機(jī)構(gòu)與學(xué)者圍繞該技術(shù)展開了大量深入的科研探索。

激光增材制造陶瓷技術(shù)通?；诜勰Y(jié)原理, 利用大功率激光束提供熱能, 對(duì)含有松散堆積陶瓷/固態(tài)黏結(jié)劑顆粒的粉床表面進(jìn)行選區(qū)燒結(jié)/熔化, 或利用激光在沉積區(qū)產(chǎn)生熔池, 通過同軸噴頭送粉方式將陶瓷顆粒熔化/沉積, 最后基于層層堆疊原理實(shí)現(xiàn)陶瓷零件的成形。依據(jù)陶瓷送粉方式, 激光增材制造陶瓷技術(shù)分為鋪粉式的粉末床熔融(Powder bed fusion, PBF)和送粉式的定向能量沉積(Directed energy deposition, DED)技術(shù)。按照激光對(duì)陶瓷粉末的加熱程度, 粉末床熔融技術(shù)又包括激光選區(qū)燒結(jié)(Selective laser sintering, SLS)和激光選區(qū)熔化(Selective laser melting, SLM); 定向能量沉積技術(shù)也稱為激光近凈成形技術(shù)、激光熔化沉積技術(shù)、激光立體成形技術(shù), 本文中統(tǒng)稱為激光近凈成形技術(shù)(Laser engineered net shaping, LENS)[3-4]。

本文主要介紹SLS、SLM以及LENS這三種基于粉末成形的激光增材制造陶瓷技術(shù)成形原理與國(guó)內(nèi)外研究現(xiàn)狀, 重點(diǎn)分析討論了SLS技術(shù)中陶瓷致密化工藝以及SLM和LENS 技術(shù)中陶瓷裂紋控制方法研究進(jìn)展, 對(duì)比了SLS、SLM以及LENS技術(shù)成形陶瓷的技術(shù)特征與優(yōu)劣勢(shì), 最后探討了各激光增材陶瓷技術(shù)目前存在的問題及未來發(fā)展趨勢(shì)。

1 激光增材制造陶瓷技術(shù)現(xiàn)狀

1.1 陶瓷激光選區(qū)燒結(jié)技術(shù)(Selective laser sintering, SLS)

陶瓷粉末一般具有很高的熔點(diǎn), 直接燒結(jié)陶瓷顆粒所需激光能量較大, 燒結(jié)溫度較高。一種可行的方法是在基質(zhì)陶瓷粉末上涂上或混合其他熔點(diǎn)/軟化點(diǎn)較低的材料作為陶瓷粉末的黏結(jié)劑。SLS技術(shù)[5]就是通過激光束加熱粉床表面, 使得黏結(jié)劑熔化并在陶瓷顆粒周圍形成玻璃相, 對(duì)陶瓷顆粒實(shí)現(xiàn)低溫黏結(jié)。隨后在前一打印層表面涂覆新的粉末層以備打印下一層, 如此往復(fù)循環(huán)直到完整打印設(shè)計(jì)的三維零件。SLS過程中已打印成形的結(jié)構(gòu)始終被粉床中的粉末支撐, 所以無需額外設(shè)計(jì)和制造支撐結(jié)構(gòu)。圖1所示為SLS工藝原理。

1990年, 德克薩斯大學(xué)奧斯汀分校的Laksh-minarayan等[6-7]基于氧化鋁的混合粉末體系, 首次論證了采用SLS制造復(fù)雜結(jié)構(gòu)3D陶瓷零件的可行性。該研究將磷酸銨(NH4H2PO4)和氧化硼(B2O3)作為低溫黏結(jié)劑(熔點(diǎn)分別為190和460 ℃), 最終成功制作了齒輪、鑄造模具等三維陶瓷零件。SLS打印所需的黏結(jié)劑可以是有機(jī)聚合物材料[8-10]也可以是無機(jī)材料, 如金屬基低熔點(diǎn)材料和玻璃[7,11-16]。當(dāng)黏結(jié)劑為有機(jī)聚合物時(shí), 可將SLS陶瓷打印件放入高溫爐進(jìn)行脫脂工藝來分解/去除有機(jī)黏結(jié)劑, 繼續(xù)升溫?zé)Y(jié)成陶瓷零件; 當(dāng)黏結(jié)劑為無機(jī)材料時(shí), 依靠熱處理工藝無法完全去除黏結(jié)劑, 黏結(jié)劑只能殘留在基體中, 但可通過其與基質(zhì)粉末反應(yīng)轉(zhuǎn)化形成新的所需復(fù)合陶瓷材料。

圖1 陶瓷激光選區(qū)燒結(jié)技術(shù)(SLS)示意圖[1]

SLS成形的陶瓷零件坯體性能主要與材料自身特性以及激光–材料的相互作用有關(guān)。一方面, 陶瓷基質(zhì)和黏結(jié)劑粉末要具有良好的流動(dòng)性, 球形度較高的微米級(jí)顆粒, 其流動(dòng)性能較好[17]。研究發(fā)現(xiàn), 涂有黏結(jié)劑的復(fù)合粉末比混合黏結(jié)劑的復(fù)合粉末的零件強(qiáng)度更高。主要原因是黏結(jié)劑涂在陶瓷表面時(shí)復(fù)合粉末的分散性更好, 最終陶瓷制件中的缺陷更少, 強(qiáng)度也更高[18-20]。另一方面, SLS打印中激光束與材料之間的反應(yīng)是一個(gè)非常復(fù)雜的過程。在激光快速燒結(jié)熔合過程中局部微觀相互作用的各種瞬態(tài)情況會(huì)影響所制造零件的微觀結(jié)構(gòu)、機(jī)械性能和幾何尺寸, 必須予以重視。其中一個(gè)關(guān)鍵因素是作用于粉床的激光能量, SLS打印時(shí), 所需的激光能量取決于混合粉末成分、粉末的熱力學(xué)性能, 如材料的熔點(diǎn)和導(dǎo)熱系數(shù)以及粉床的填充密度等條件。激光能量過低時(shí), 黏結(jié)劑熔化不足, 會(huì)引起相鄰層粘合不牢, 進(jìn)而導(dǎo)致生坯強(qiáng)度低; 而激光能量過高則容易引起黏結(jié)劑過度熔化甚至蒸發(fā), 將會(huì)產(chǎn)生較大的幾何尺寸誤差, 最終導(dǎo)致零件打印失敗[21-22]。

SLS打印陶瓷零件時(shí), 最終零件的燒結(jié)收縮率和孔隙率均較高。眾所周知, 結(jié)構(gòu)陶瓷必須接近完全致密才能達(dá)到最佳的機(jī)械性能。為提升陶瓷件致密度, 可在SLS打印后使用浸漬、浸滲或冷/熱等靜壓等工藝對(duì)打印工件進(jìn)行致密化處理[23]。圖2為提高陶瓷零件密度和機(jī)械強(qiáng)度的后處理工藝流程圖。

為提高零件致密度, Shahzad等[24-28]通過高溫準(zhǔn)等靜壓工藝獲得了致密度高達(dá)94%的氧化鋁陶瓷[25]。而在制備ZrO2陶瓷零件時(shí), 該團(tuán)隊(duì)將 SLS與溫等靜壓(Warm isostatic pressing, WIP)結(jié)合, 最終零件致密度達(dá)到92%。圖3所示為致密化前后的零件[28]及其微觀組織, 可以看到經(jīng)過一系列致密化工藝處理后, 零件尺寸收縮十分顯著, 內(nèi)部組織經(jīng)過等靜壓處理之后, 孔隙數(shù)量顯著減少。Wang等[29]將SLS成形的Si3N4陶瓷經(jīng)過冷等靜壓(Cold isostatic pressing,CIP)處理, 也實(shí)現(xiàn)了打印陶瓷致密度和強(qiáng)度的提高。

圖2 陶瓷零件SLS工藝流程及其它后處理工藝[1]

The process marked with asterisk * is optional. SLS: Selective laser sintering

華中科技大學(xué)史玉升團(tuán)隊(duì)[16, 30-32]研究了冷等靜壓工藝對(duì)SLS打印的Al2O3陶瓷坯體致密度的影響。研究表明: 壓力越大, 陶瓷顆粒排布越密實(shí), 陶瓷坯體的孔隙在很大程度被消除, 最終燒結(jié)后Al2O3陶瓷致密度可達(dá)92%。除通過復(fù)合等靜壓工藝提高SLS陶瓷致密度, 熔體浸滲方法也可以提高陶瓷坯體的致密度。該研究團(tuán)隊(duì)在制備SiC陶瓷時(shí), 首先采用SLS技術(shù)實(shí)現(xiàn)碳纖維預(yù)制體(Carbon fiber (Cf) preform)的成形, 如圖4(a～d)所示, 隨后進(jìn)行液硅反應(yīng)熔滲(Liquid silicon infiltration, LSI), 纖維預(yù)制體通過硅碳反應(yīng)燒結(jié)成SiC陶瓷基復(fù)合材料, 其致密度可達(dá)99%以上[33-34]。采用反應(yīng)熔滲工藝不僅能夠提高陶瓷坯體致密度, 而且陶瓷基體在熔滲前后幾乎無收縮(<1%)。然而, 反應(yīng)熔滲工藝僅適合特定陶瓷復(fù)合材料, 同時(shí)在滲硅過程中基體內(nèi)部和表面會(huì)引入脆性相的游離Si, 進(jìn)而降低成形陶瓷零件的表面質(zhì)量與力學(xué)性能。哈爾濱理工大學(xué)成夙 和汕頭大學(xué)曾濤等[35-37]通過SLS成形SiC等陶瓷基坯體后, 再經(jīng)過多次先驅(qū)體浸漬裂解(Precursor infiltration pyrolysis, PIP)工藝完成了對(duì)SiC陶瓷零件的致密化(圖4(e～h))。與前述反應(yīng)燒結(jié)SiC陶瓷工藝相比, PIP工藝制備的陶瓷零件表面質(zhì)量較好, 但內(nèi)部閉氣孔隙率較高, 制備周期較長(zhǎng)。為降低氣孔率, 該團(tuán)隊(duì)在SLS成形的SiC陶瓷坯體進(jìn)行PIP工藝前, 引入冷等靜壓工藝。最終所制備的SiC陶瓷氣孔率從28.95%降低至22.03%。

圖3 SLS結(jié)合等靜壓制備ZrO2陶瓷零件及其微觀形貌[28]

(a, d) ZrO2ceramic green bodies and their morphologies printed by SLS; (b) Warm isostatic pressure equipment; (c, e) ZrO2ceramics and their microstructures after warm isostatic pressing sintering. SLS: Selective laser sintering; WIP: Warm isostatic pressing

為保證結(jié)構(gòu)承載陶瓷零件強(qiáng)度, 需要保證其較高的致密度, 因此SLS成形坯體后需要大量的后處理致密化工藝。然而, 對(duì)于多孔功能、功能–結(jié)構(gòu)陶瓷, 功能性往往依賴于其多孔結(jié)構(gòu)特點(diǎn), 而對(duì)其強(qiáng)度要求較低。因此, 在燒結(jié)多孔陶瓷時(shí)往往并不需要后續(xù)繁多的致密化工藝。華中科技大學(xué)報(bào)道了多孔莫來石(Mullite)、Al2O3、Si3N4陶瓷SLS成形方法的研究[38-40], 如圖5所示。該研究中, 通過表面改性等方法在陶瓷顆粒表面制備各種有機(jī)、無機(jī)涂層, 最終獲得具有核殼結(jié)構(gòu)的可打印陶瓷粉末。其中, 有機(jī)涂層不光能夠?qū)崿F(xiàn)SLS過程中粘結(jié)陶瓷顆粒的作用, 脫脂后有機(jī)涂層脫除后會(huì)形成新的孔道結(jié)構(gòu), 提高了多孔陶瓷的孔隙率與比表面積。而無機(jī)涂層在SLS過程中會(huì)形成晶須納米線, 有望提高多孔陶瓷的機(jī)械性能。

與其他增材制造多孔陶瓷一樣, SLS打印的多孔陶瓷在生物醫(yī)學(xué)應(yīng)用中也越來越受歡迎, 特別是在組織工程中打印具有一定生物相容性的復(fù)雜結(jié)構(gòu)支架, 如圖6所示, 其中黏結(jié)劑含量體積分?jǐn)?shù)可達(dá)60%。例如由陶瓷–聚合物混合粉末制成的骨植入物, 如羥基磷灰石–磷酸三鈣(HA-TCP)[10], 羥基磷灰石–聚碳酸酯(HA-PC)[42], 碳酸鈣–聚乳酸(CC-PLLA)[43], 羥基磷灰石–聚醚醚酮(PA-PEEK)[9]和二氧化硅–聚酰胺(SiO2-PA)[44]; 以及陶瓷–玻璃復(fù)合材料如羥基磷灰石–磷酸鹽玻璃[45], 磷灰石–莫來石[46-47]和磷灰石–硅灰石[14]。在SLS打印過程中, 這些材料的黏結(jié)劑一般選擇低熔點(diǎn)聚合物和玻璃。

在結(jié)構(gòu)陶瓷領(lǐng)域, 盡管SLS打印的陶瓷坯體孔隙率較高, 但通過優(yōu)化打印工藝參數(shù), 并結(jié)合浸漬和等靜壓以及反應(yīng)熔滲等后處理工藝, 仍能夠制造出具有較高強(qiáng)度和高致密度的陶瓷零件。而在多孔陶瓷領(lǐng)域, 尤其是多孔生物陶瓷, SLS打印件所需后處理工藝較少, 且可打印材料種類較多。因此, 在功能和結(jié)構(gòu)多孔陶瓷制造中均有廣泛的應(yīng)用。

圖4 SiC陶瓷及其復(fù)合材料零件SLS制備過程[33,36-37]

(a-d) Reaction sintering of Cf/SiC ceramic matrix composites by SLS technology; (e-h) SLS preparation process of SiC/SiC ceramics

PF: Phenolic resin; Cf: Carbon fiber; SLS: Selective laser sintering; LSI: Liquid silicon infiltration; PIP: Precursor infiltration pyrolysis

圖5 多孔陶瓷SLS制備方法[38-41]

(a) Pre-treatment of ceramic particles and SLS; (b) Sintering of porous ceramic; (c) Porous mullite ceramic; (d) Porous Al2O3ceramic; (e) Porous Si3N4ceramic SLS: Selective laser sintering

圖6 SLS打印的多孔陶瓷在生物醫(yī)學(xué)上的應(yīng)用

(a, b) CC-PLLA porous skull scaffolds and their mechanical properties[43]; (c, d) Porous biological ceramic scaffolds and their micromorphologies[48]

SLS: Selective laser sintering

1.2 陶瓷激光選區(qū)熔化技術(shù)(Selective laser melting, SLM)

1996年, 德國(guó)弗勞恩霍夫激光技術(shù)研究所(ILT)研發(fā)成功SLM技術(shù)[49], 該技術(shù)通常被認(rèn)為是基于SLS技術(shù)演變而來。與SLS成形陶瓷的工作原理相似, SLM也是通過高能束激光加熱粉床粉末實(shí)現(xiàn)零件打印。不同的是, SLM使用的激光源能量密度更高, 激光逐層掃描將粉床中陶瓷粉末完全熔化再凝固成形, 整個(gè)成形過程不需要借助低熔點(diǎn)黏結(jié)劑的融合作用。由于陶瓷粉末被完全熔化, 因此也不需要經(jīng)過后處理加熱燒結(jié)工藝。圖7所示為SLM工藝示意圖。

影響SLM陶瓷零件整體質(zhì)量的因素也很多, 如陶瓷粉末特性、打印工藝參數(shù)、打印策略和取向、后處理工藝以及打印過程中激光–粉末和粉末–粉末之間相互的物化作用等。在打印工藝參數(shù)中, 分層厚度是一個(gè)重要的因素, 它對(duì)零件的打印總時(shí)長(zhǎng)和表面質(zhì)量有很大影響。較小的分層厚度會(huì)降低零件表面粗糙度, 但會(huì)使零件打印總時(shí)間更長(zhǎng); 而較大的分層厚度盡管能提高打印效率, 但會(huì)導(dǎo)致顯著的臺(tái)階效應(yīng)。而分層厚度設(shè)置依賴于熔化深度, 這與光固化增材制造過程中分層厚度與透射深度的關(guān)系類似。因此, 為兼顧打印質(zhì)量與效率, 仍需要對(duì)SLM各種打印工藝參數(shù)進(jìn)行優(yōu)化組合。

圖7 陶瓷激光選區(qū)熔化技術(shù)(SLM)示意圖[1]

SLM技術(shù)存在一個(gè)重要問題: 激光掃描時(shí)每次極短的局部劇烈升溫和急速冷卻會(huì)導(dǎo)致打印件基體內(nèi)產(chǎn)生較大的熱應(yīng)力[50]。陶瓷材料的抗熱震性能有限, 因此, 其燒結(jié)件極易在熱應(yīng)力的作用下產(chǎn)生裂紋和變形。Shishkovsky等[51]報(bào)道了使用SLM制造ZrO2零件的研究, 結(jié)果在陶瓷基體上出現(xiàn)了明顯的裂紋和變形(圖8(a))。Deckers等[52]使用SLM制造Al2O3零件, 其相對(duì)密度僅為85%。盡管對(duì)粉末涂覆處理, 并優(yōu)化了激光掃描參數(shù), 最終陶瓷組織產(chǎn)生較大的殘余氣孔, 基體內(nèi)出現(xiàn)較大裂紋, 如圖8(b, c)所示, 且打印過程中粉末完全熔化(圖8(d))。Bertrand等[53]報(bào)道了采用SLM制造ZrO2-Y2O3陶瓷零件, 最終零件相對(duì)密度也極低, 僅為56%, 即使經(jīng)過進(jìn)一步熱處理也無法改善。Mercelis等[50]研究了SLM制造零件中殘余應(yīng)力的來源, 并建立了一個(gè)簡(jiǎn)單的理論模型來預(yù)測(cè)殘余應(yīng)力的分布。結(jié)果表明, 掃描方式對(duì)殘余應(yīng)力有較大影響, 垂直于掃描方向的應(yīng)力大于平行于掃描方向的應(yīng)力。此外, 激光掃描時(shí)間過短也會(huì)導(dǎo)致粉末熔化不足, 最終陶瓷基體中產(chǎn)生較大的殘余氣孔, 零件表面質(zhì)量較差。

圖8 SLM打印的陶瓷及其微觀缺陷[51-52]

(a) ZrO2sample; (b, c) Al2O3samples and cracks; (d) Un-melted alumina balls

西北工業(yè)大學(xué)蘇海軍團(tuán)隊(duì)[54]采用SLM技術(shù)制備了Al2O3/GdAlO3/ZrO2三元共晶陶瓷, 并探索了SLM過程中陶瓷基體閉氣孔以及表面凹點(diǎn)的形成原因, 如圖9所示。陶瓷粉末本身含有大量的氣體, 在激光作用下, 陶瓷顆粒熔化成熔池的瞬間, 陶瓷顆粒周圍的氣體即被陶瓷熔池所包裹。而在陶瓷凝固時(shí), 氣泡沿著固液界面向熔池中心與表面運(yùn)動(dòng)。當(dāng)氣泡運(yùn)動(dòng)速率低于凝固速率時(shí), 即在陶瓷基體內(nèi)形成閉氣孔; 而當(dāng)氣泡運(yùn)動(dòng)速率與凝固速率相當(dāng)時(shí), 氣泡隨著凝固界面一起發(fā)展到試樣表面, 最終在試樣表面形成凹坑。該研究最終通過優(yōu)化打印工藝發(fā)現(xiàn), 當(dāng)掃描速度小于12 mm/min時(shí), 可有效抑制這些閉氣孔、凹坑等缺陷。

至今, 研究人員基于SLM打印工藝, 衍生發(fā)展了多種改進(jìn)的增材制造方法, 并將其用于制造陶瓷零件。為了提高SLM的粉床堆積密度, 避免出現(xiàn)低燒結(jié)密度和開裂現(xiàn)象, 研究人員還開發(fā)了基于泥漿的SLM[55-56]技術(shù), 如漿體形態(tài), 以代替干粉涂覆。相比于粉末形態(tài), 漿體形態(tài)具有更高填充率和均勻性的優(yōu)點(diǎn), 應(yīng)用前景較好。Gahler等[56]已經(jīng)制備了固相含量體積分?jǐn)?shù)高達(dá)63%的高流動(dòng)性Al2O3-SiO2混合陶瓷水基漿料, 打印時(shí), 使用刮刀刮平漿料表層。由于SiO2熔點(diǎn)低, 打印時(shí)產(chǎn)生液相, 因此最終制造的零件表面光滑, 相對(duì)密度高達(dá)92%。在隨后的發(fā)展中他們又通過相同工藝打印了各種結(jié)構(gòu)陶瓷制品, 但均無法實(shí)現(xiàn)完全致密化[57-58]。

盡管研究人員已經(jīng)在陶瓷SLM方面做了大量工作, 但目前來看該技術(shù)成形的陶瓷零件應(yīng)用領(lǐng)域仍然十分有限。主要原因是所成形陶瓷零件仍然有較多缺陷: 較大孔隙率、較粗糙表面及較大精度誤差等。同時(shí)SLM很難實(shí)現(xiàn)致密、各向同性陶瓷零件的制造。因此, 還需要進(jìn)一步在初始粉末材料設(shè)計(jì)、打印制備工藝等方面做更多的研究, 為實(shí)現(xiàn)真正無缺陷、高精度、全致密陶瓷零件的制造提供理論與技術(shù)指導(dǎo)。

圖9 SLM陶瓷基體內(nèi)部閉氣孔和表面凹點(diǎn)形成的原因[54]

(a) SLM printing process and Al2O3/GdAlO3/ZrO2ternary eutectic ceramics; (b) Formation process of the closed pores and pits

1.3 陶瓷激光近凈成形技術(shù)(Laser engineered net shaping, LENS)

美國(guó)Sandia 國(guó)立實(shí)驗(yàn)室于1998年將激光增材制造和激光溶覆工藝相結(jié)合提出激光近凈成形技術(shù)(LENS)[59-60]。在LENS打印過程中, 激光束移動(dòng)的同時(shí), 陶瓷粉末被同軸/單側(cè)沉積到指定的激光光斑區(qū)域, 隨即形成陶瓷熔池[61]并進(jìn)行打印, 如圖10所示。

Balla等[60]利用LENS制備了圓柱形、立方體和齒輪狀的Al2O3零件, 其相對(duì)密度達(dá)到94%, 但獲得的陶瓷力學(xué)性能具有各向異性。盡管后續(xù)采用了熱處理工藝, 但未能改變其強(qiáng)度和各向異性特質(zhì), 晶粒尺寸反而從6 μm增大到200 μm。當(dāng)施加拉伸載荷時(shí)發(fā)現(xiàn)陶瓷沿柱狀晶界出現(xiàn)裂紋。大連理工大學(xué)吳東江團(tuán)隊(duì)[62, 64-68]利用LENS打印了具有微晶結(jié)構(gòu)的全致密簡(jiǎn)單形狀A(yù)l2O3-YSZ/YAG零件。在快速升溫熔化與冷卻凝固過程中, 片狀集落的共晶間距達(dá)到100 nm, 最終所制備陶瓷的力學(xué)性能與傳統(tǒng)燒結(jié)方法制備的陶瓷相當(dāng), 如圖11所示。

圖10 陶瓷激光近凈成形技術(shù)(LENS)示意圖[62-63]

圖11 LENS打印的陶瓷試樣[62,67]

(a) Al2O3spherical particles; (b, c) Large-sized cylindrical Al2O3ceramic, stress-strain curve and fracture morphology of Al2O3ceramic; (d) Single-bead wall part fabricated with different laser power; (e) Typical geometry of the cross-section of a single-bead wall part

西北工業(yè)大學(xué)蘇海軍團(tuán)隊(duì)[69-71]研究了Al2O3- YAG、Al2O3/GdAlO3/ZrO2等Al2O3基多元共晶陶瓷的LENS成形工藝以及所制備陶瓷材料微觀組織特征形成機(jī)制及力學(xué)性能。目前, 該研究團(tuán)隊(duì)采用LENS技術(shù)已制備了直徑為45 mm、高度大于250 mm的大型Al2O3/GdAlO3/ZrO2三元共晶陶瓷棒(圖12), 并通過熱處理的方法消除了沉積態(tài)共晶陶瓷的組織不均一性。研究發(fā)現(xiàn)共晶陶瓷的粗化行為符合Graham-Kraft模型, 共晶間距隨熱處理時(shí)間呈線性增大。不過, 從已報(bào)道的文獻(xiàn)中可以看出, LENS技術(shù)尚處于研發(fā)初期階段, 目前均是完成了對(duì)棒狀以及單道陶瓷薄壁制件的成形, 其對(duì)異形結(jié)構(gòu)零件的成形能力仍較低。

2 激光增材制造陶瓷過程中的熱致裂紋控制

采用立體光固化、材料噴射/擠出等增材制造技術(shù)打印陶瓷零件時(shí), 陶瓷漿料/粉末處于常溫/低溫且溫差變化較小的條件下, 因此成形的陶瓷坯體沒有熱致裂紋的問題。然而在基于粉末成形的激光增材制造技術(shù)中, 陶瓷粉末經(jīng)歷高能激光束的瞬態(tài)加熱和冷卻, 在固–液–固轉(zhuǎn)化過程中存在收縮變形效應(yīng), 進(jìn)而在成形構(gòu)件內(nèi)部產(chǎn)生復(fù)雜應(yīng)力。而陶瓷材料自身的硬脆特性最終在應(yīng)力的作用下極易產(chǎn)生裂紋。由于SLS技術(shù)使用的陶瓷–黏結(jié)劑復(fù)合粉末對(duì)溫度梯度的耐受性更為良好, 因此在打印陶瓷零件時(shí)較少產(chǎn)生裂紋。然而采用SLM和LENS技術(shù)打印陶瓷零件時(shí), 陶瓷層之間瞬間溫差急劇變化會(huì)產(chǎn)生巨大的梯度熱應(yīng)力進(jìn)而在陶瓷基體中誘發(fā)裂紋。因此, SLM和LENS成形陶瓷過程中的熱應(yīng)力控制與裂紋抑制是目前研究的重點(diǎn)與難點(diǎn)。

圖12 Al2O3/GdAlO3/ZrO2共晶陶瓷[70]

(a) Ceramic shaping process; (b) Eutectic ceramic sample; (c) Annealed eutectic ceramic sample

研究發(fā)現(xiàn), 預(yù)熱陶瓷粉床可以有效減少由熱應(yīng)力引起的裂紋和變形等[72-73]。德國(guó)弗勞恩霍夫激光技術(shù)研究所的Wilkes等[72, 74]采用SLM打印陶瓷時(shí), 將純ZrO2和Al2O3粉末的共晶混合物作為初始粉末, 通過CO2激光高溫預(yù)熱系統(tǒng)對(duì)粉床進(jìn)行預(yù)熱(預(yù)熱溫度可達(dá)1700 ℃), 使用Nd:YAG激光進(jìn)行陶瓷粉床選區(qū)熔化(圖13(a)), 該方法可有效防止溫度梯度引起的裂紋, 并且無需后處理即可獲得抗彎強(qiáng)度大于500 MPa的全致密均勻微觀組織陶瓷零件。盡管如此, 高熔點(diǎn)陶瓷材料要求預(yù)熱溫度高于1000 ℃,而接近熔點(diǎn)的溫度將產(chǎn)生較大的熔池尺寸, 使得低粘度熔融陶瓷材料滲透到周圍未熔化的粉末間隙中, 最終導(dǎo)致打印件的尺寸誤差增大, 零件表面質(zhì)量較差; 同時(shí)陶瓷零件在高度方向上的溫度梯度仍然存在, 這種自上而下的整體激光粉床預(yù)熱只能制造壁高<3 mm的小型零件。在LENS技術(shù)中, 為克服CO2激光預(yù)熱在高度方向上的溫度梯度, 第四軍醫(yī)大學(xué)和西北工業(yè)大學(xué)等[75]打印ZrO2/Al2O3陶瓷時(shí), 提出了自下而上的感應(yīng)預(yù)熱法(圖13(b)), 這種預(yù)熱方法在制造較大零件時(shí)可達(dá)到最小化溫度梯度的目的, 最終抑制了ZrO2/Al2O3陶瓷在凝固過程中產(chǎn)生的凝固缺陷和縮孔。此外, 在冷卻凝固過程中對(duì)陶瓷進(jìn)行同步結(jié)晶可能會(huì)產(chǎn)生微晶結(jié)構(gòu), 引發(fā)晶界強(qiáng)化, 使打印陶瓷件獲得優(yōu)異的力學(xué)性能[76]。

除了采用預(yù)熱方法外, 國(guó)內(nèi)外研究團(tuán)隊(duì)還通過打印掃描策略、引入超聲振動(dòng)等方式來抑制打印陶瓷熱裂紋, 如圖14所示。Mishra等[78]采用激光直接沉積工藝打印Al2O3陶瓷塊體時(shí)發(fā)現(xiàn), 當(dāng)打印掃描角度=45°或者67°時(shí), 陶瓷基體內(nèi)雖然仍然有閉氣孔, 但內(nèi)部的裂紋明顯減少。而Wu和Cong等[63, 66]在LENS技術(shù)上, 引入超聲輔助振動(dòng)來減少打印過程中Al2O3-ZrO2陶瓷基體形成的裂紋, 并從晶體學(xué)角度討論了超聲細(xì)化晶粒的作用。研究發(fā)現(xiàn), 在LENS工藝中引入超聲振動(dòng), 產(chǎn)生了非線性聲流和瞬態(tài)空化作用, 有助于均勻化材料彌散, 平滑熱梯度, 細(xì)化晶粒, 抑制了裂紋的萌生和沿沉積方向的擴(kuò)展, 最終可提高陶瓷力學(xué)性能。未來的研究應(yīng)集中在成形陶瓷零件的表面質(zhì)量, 進(jìn)一步提高機(jī)械強(qiáng)度等研究上, 并促進(jìn)該技術(shù)從實(shí)驗(yàn)室向工業(yè)應(yīng)用轉(zhuǎn)化。

圖13 SLM-CO2激光預(yù)熱方式和LENS-感應(yīng)預(yù)熱方式及其制備的陶瓷

(a) CO2laser preheating method and ZrO2/Al2O3ceramic prepared by SLM[77]; (b) Induction preheating method and ZrO2/Al2O3ceramic prepared by LENS[75]

圖14 掃描策略和超聲振動(dòng)對(duì)裂紋缺陷的影響[63,78]

(a) Scanning strategy; (b) Ultrasonic vibration

3 激光增材制造陶瓷技術(shù)對(duì)比

表1從可打印材料、后處理工藝以及打印質(zhì)量等對(duì)比了鋪粉式的SLS/SLM技術(shù)和送粉式的LENS陶瓷增材制造技術(shù)。相比于SLM技術(shù), SLS由于有較低的激光功率(打印溫度/溫度梯度)以及低熔點(diǎn)黏結(jié)劑復(fù)合作用, 因此打印陶瓷材料時(shí)產(chǎn)生的熱應(yīng)力較低, 適合打印的陶瓷材料種類較多, 且成形后的陶瓷零件精度相對(duì)較高, 表面質(zhì)量也較好。但SLS成形后的陶瓷尚處于生坯或多孔狀態(tài), 仍需后處理工藝來實(shí)現(xiàn)陶瓷零件的致密化以保證結(jié)構(gòu)陶瓷零件的機(jī)械性能。目前采用的致密化工藝制備周期長(zhǎng)且成本較高。對(duì)SLS技術(shù)而言, 未來研究應(yīng)沿著縮短SLS致密化工藝周期、提高零件致密度等方向進(jìn)行。雖然SLM與LENS技術(shù)中陶瓷的送粉方式不同, 但陶瓷在打印過程中, 成形和致密化工藝均是同步完成, 因此無需引入過多的后處理致密化工藝, 打印效率更高、周期較短。這樣可加快陶瓷零件研制迭代速率。這兩種技術(shù)中, 陶瓷零件內(nèi)部在激光快速加熱和冷卻下引起的殘余應(yīng)力仍然是導(dǎo)致裂紋和變形等缺陷的主要因素, 同時(shí)陶瓷從疏松的粉體熔融成致密陶瓷時(shí)收縮率較大, 最終打印件尺寸精度較差、表面質(zhì)量較低。后續(xù)應(yīng)深入研究激光與陶瓷顆粒之間的動(dòng)態(tài)作用和熔化過程, 這將有助于實(shí)現(xiàn)控制和制造陶瓷零件結(jié)構(gòu)。目前, 這方面的工作還稍顯欠缺。

4 結(jié)束語

目前, 國(guó)內(nèi)外學(xué)者已針對(duì)基于粉末成形的激光增材制造陶瓷技術(shù)展開了一系列研究, 取得了一定的研究進(jìn)展和成果。然而, 該技術(shù)尚處于研發(fā)階段, 所制造的大部分零件性能、質(zhì)量以及精度遠(yuǎn)未達(dá)到應(yīng)用要求, 仍需開展大量的研究工作。SLS、SLM以及LENS技術(shù)發(fā)展進(jìn)程因各自陶瓷零件的成形特征差異而有所區(qū)別, 其后續(xù)具體的研究方向也有很大不同。但在應(yīng)用領(lǐng)域中, 隨著航天航空、能源環(huán)保、核能軍工以及生物醫(yī)療等領(lǐng)域的需求不斷擴(kuò)大, 發(fā)展快速、高性能和低成本陶瓷零件制造技術(shù)變得尤為迫切, 其對(duì)各基于粉末成形的激光增材制造陶瓷技術(shù)提出了共同的發(fā)展方向。具體應(yīng)著重關(guān)注以下三方面的研究:

1) 大型復(fù)雜結(jié)構(gòu)陶瓷零件制造。

大型復(fù)雜結(jié)構(gòu)陶瓷零件廣泛用于國(guó)家重大戰(zhàn)略裝備、核心支柱產(chǎn)業(yè), 但其整體制造仍然為世界性技術(shù)難題。相比于其他增材制造技術(shù), 激光增材制造技術(shù)以陶瓷粉末堆積成形、無需支撐設(shè)計(jì)制造, 在大尺寸零部件的成形中具有天然優(yōu)勢(shì)。SLS技術(shù)在打印陶瓷時(shí), 基體熱應(yīng)力與收縮均較低, 非常適合大尺寸復(fù)雜結(jié)構(gòu)零件的成形, 可成形零件尺寸由粉末床大小決定。但SLS成形后的大尺寸陶瓷零件坯體致密度較低, 其后處理致密化工藝不僅面臨周期較長(zhǎng)、致密度低的挑戰(zhàn), 且大型復(fù)雜結(jié)構(gòu)陶瓷零件坯體在脫脂過程中的發(fā)氣量更大, 燒結(jié)過程中的收縮變形效應(yīng)更為顯著, 表面質(zhì)量與裂紋控制難度也更高。未來應(yīng)以大型復(fù)雜結(jié)構(gòu)陶瓷零件坯體為研究對(duì)象, 探索抑制脫脂開裂與燒結(jié)收縮變形的控制方法; 對(duì)于LENS技術(shù), 其同步送粉的成形模式原則上對(duì)零件沒有尺寸限制。然而, LENS技術(shù)與SLM技術(shù)在陶瓷零件成形過程中產(chǎn)生的熱應(yīng)力與裂紋, 制約了其在大型復(fù)雜結(jié)構(gòu)陶瓷零件的應(yīng)用。目前, 仍需進(jìn)一步研究LENS與SLM成形陶瓷過程中裂紋形成與擴(kuò)展機(jī)理, 發(fā)展有效的應(yīng)力控制與裂紋抑制方法, 最終將該技術(shù)應(yīng)用到高性能大型復(fù)雜結(jié)構(gòu)陶瓷零件的制造。

2) 陶瓷復(fù)合材料與多材料激光增材制造。

目前, 國(guó)內(nèi)外大多數(shù)增材制造研究往往圍繞著單一陶瓷材料的成形與制備。均質(zhì)、單相的陶瓷性能單一, 且陶瓷脆性大的問題一直困擾著其在結(jié)構(gòu)承載部件上的應(yīng)用, 應(yīng)發(fā)揮激光增材制造的技術(shù)優(yōu)勢(shì), 實(shí)現(xiàn)陶瓷零件材料的復(fù)合化以提高其損傷容限; 實(shí)現(xiàn)陶瓷零件的多尺度結(jié)構(gòu)與多材料打印以拓寬其功能應(yīng)用范圍。如將不同陶瓷顆?；旌稀⑻沾深w粒與復(fù)合增韌相材料混合或陶瓷顆粒與金屬顆?；旌? 再經(jīng)激光增材制造技術(shù)實(shí)現(xiàn)陶瓷復(fù)合材料與多材料打印; 將不同材料通過不同送粉方式進(jìn)行打印, 如陶瓷LENS技術(shù), 其特有的送粉方式能夠?qū)崿F(xiàn)多種陶瓷材料在陶瓷層內(nèi)與層間的打印工藝。

表1 基于粉末成形的激光增材制造陶瓷技術(shù)對(duì)比

3) 新型陶瓷激光增材制造裝備研發(fā)。

相比金屬材料, 陶瓷材料所需激光功率要求更高。陶瓷激光增材制造設(shè)備造價(jià)非常昂貴, 提高了激光增材制造陶瓷技術(shù)的研制門檻, 限制了該技術(shù)的推廣與應(yīng)用。應(yīng)在保證成形效率的同時(shí), 研發(fā)新型激光增材制造裝備, 降低設(shè)備的成本。相比于傳統(tǒng)機(jī)加工工藝, 激光增材制造成形的陶瓷零件精度與表面質(zhì)量仍較低, 后續(xù)仍需必要的精加工工藝, 為降低后續(xù)精加工難度, 縮短加工周期, 提高加工效率, 可將激光增材制造與傳統(tǒng)減材技術(shù)(磨銑削等)結(jié)合, 搭建新型陶瓷增減材設(shè)備, 統(tǒng)籌增減材加工優(yōu)勢(shì), 在實(shí)現(xiàn)異形陶瓷零件成形的同時(shí)克服零件表面質(zhì)量較差的問題。

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Research Progress on Powder-based Laser Additive Manufacturing Technology of Ceramics

CAO Jiwei1,2, WANG Pei1,2, LIU Zhiyuan1,2, LIU Changyong1,2, WU Jiamin3,4, CHEN Zhangwei1,2

(1. Additive Manufacturing Institute, Shenzhen University, Shenzhen 518060, China; 2. Guangdong Key Laboratory of Electromagnetic Control and Intelligent Robot, Shenzhen 518060, China; 3. State Key Laboratory of Materials Processing and Die & Mould Technology, College of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China; 4. Engineering Research Center for Additive Manufacturing Ceramic Materials, Ministry of Education, Huazhong University of Science and Technology, Wuhan 430074, China)

Ceramics, with its excellent thermal, physical and chemical properties, have great potential applications in various fields, such as aerospace, energy, environmental protection and bio-medicine. With the development of relevant technology in these fields, the structural design of core components is increasingly complex, and the internal microstructures gradually become customized and gradient. However, the hard and brittle features of ceramics make it difficult to realize the forming of special-shaped parts by traditional manufacturing methods, which in turn limits further application. As a rapidly developing additive manufacturing technology, laser additive manufacturing technology presents a momentous advantage in the manufacturing process of extremely precision ceramic components: free molding without mold and support, quick response feature and short developing cycle,. At the same time, the technology can realize the flexible deployment of ceramic parts, which is expected to solve the problems mentioned above. Three kinds of powder-based laser additive manufacturing techniques of ceramic were reviewed in this paper: selective laser sintering and selective laser melting based on powder bed fusion technology; laser engineered net shaping based on direct energy deposition technology. The forming principle and characteristics were mainly discussed; the research progress of ceramic green body densification process in selective laser sintering technology and the forming principle, propagation mechanism and control methods of ceramic green body cracks in selective laser melting, and laser engineered net shaping technology were reviewed; the technical characteristics of selective laser sintering, selective laser melting and laser engineered net shaping technologies in shaping of ceramic parts were compared and analyzed; and the future development trends of laser additive manufacturing technology of ceramic parts were prospected.

laser additive manufacturing; selective laser sintering; selective laser melting; laser engineered net shaping; ceramic; review

TQ174

A

1000-324X(2022)03-0241-14

10.15541/jim20210590

2021-09-26;

2021-10-18;

2021-11-01

國(guó)家自然科學(xué)基金(51975384, 51975230); 廣東省自然科學(xué)基金(2020A1515011547); 深圳市基礎(chǔ)研究基金(JCYJ20190808144009478); 深圳市高校穩(wěn)定支持項(xiàng)目(20200731211324001); 深大-臺(tái)北科大合作項(xiàng)目(2021007)

National Natural Science Foundation of China (51975384, 51975230); Natural Science Foundation of Guangdong Province (2020A1515011547); Basic Research Foundation of Shenzhen (JCYJ20190808144009478); University Support Fund of Shenzhen City (20200731211324001); NTUT-SZU Joint Research Program (2021007)

曹繼偉(1989–), 男, 博士. E-mail: caojiwei@szu.edu.cn

CAO Jiwei (1989–), male, PhD. E-mail: caojiwei@szu.edu.cn

吳甲民, 副教授. E-mail: jiaminwu@hust.edu.cn; 陳張偉, 教授. E-mail: chen@szu.edu.cn

WU Jiamin, associate professor. E-mail: jiaminwu@hust.edu.cn; CHEN Zhangwei, professor. E-mail: chen@szu.edu.cn