姚喆赫，沈奇艷，葛宏江，王振，董剛，葉鐘，李琳，姚建華

超聲對(duì)激光熔覆成形中熔池潤(rùn)濕行為的影響研究

姚喆赫1a,1b,2，沈奇艷1a,1b,2，葛宏江3，王振1a,1b,2，董剛1a,1b,2，葉鐘3，李琳4，姚建華1a,1b,2

（1.浙江工業(yè)大學(xué) a.激光先進(jìn)制造研究院 b.機(jī)械工程學(xué)院，杭州 310023；2.高端激光制造裝備省部共建協(xié)同創(chuàng)新中心，杭州 310023；3.杭州汽輪機(jī)股份有限公司，杭州 310020；4.英國(guó)曼徹斯特大學(xué) 激光加工研究中心，曼徹斯特 M13 9PL，英國(guó)）

對(duì)比有無(wú)超聲作用下熔體潤(rùn)濕行為，闡明超聲對(duì)于熔體潤(rùn)濕行為的影響及作用機(jī)制，為超聲輔助激光熔覆高質(zhì)量成形提供參考。基于高速相機(jī)拍攝研究了超聲對(duì)于液滴潤(rùn)濕性的作用效果，進(jìn)而研究了超聲輔助激光熔覆過(guò)程中熔池的潤(rùn)濕行為，使用Canny算法提取熔池輪廓，采用體視顯微鏡和共聚焦顯微鏡觀(guān)察試樣宏觀(guān)形貌，采用光學(xué)顯微鏡觀(guān)察分析微觀(guān)組織。將超聲振動(dòng)施加于金屬液滴，其在基板表面潤(rùn)濕性增強(qiáng)，金屬液滴與基板接觸面積增大39.3%。在熔覆過(guò)程中引入超聲振動(dòng)，熔池面積顯著增大。隨著超聲功率比的增強(qiáng)，熔覆層的熔高降低，熔深減小，熔寬逐漸增大，熔覆層逐漸由弧形輪廓變?yōu)楸馄健．?dāng)超聲功率比為80%時(shí)，熔覆層高度為無(wú)超聲下的75.2%，熔覆層和基體之間的潤(rùn)濕性顯著改善。在微觀(guān)形貌上，超聲能夠改變晶粒生長(zhǎng)方向，抑制枝晶外延生長(zhǎng)。超聲振動(dòng)作用于激光熔覆過(guò)程中，促進(jìn)了熔體的潤(rùn)濕作用且加大了熔池流動(dòng)性。熔體潤(rùn)濕行為的改變導(dǎo)致扁平熔覆層形狀的形成，熔覆過(guò)程中熔池面積增大，熔池流動(dòng)性增強(qiáng)，導(dǎo)致晶粒生長(zhǎng)方向與枝晶長(zhǎng)度的改變。

激光熔覆；超聲振動(dòng)；潤(rùn)濕行為；熔池；宏觀(guān)形貌；微觀(guān)組織

航空關(guān)鍵部件常服役于高溫高載荷等惡劣環(huán)境，易發(fā)生損傷和失效，損傷件的換新將產(chǎn)生巨大的成本與浪費(fèi)?；诩す馊鄹渤尚蔚募す庠霾脑僦圃旒夹g(shù)在損傷件的修復(fù)與再制造中展現(xiàn)出顯著優(yōu)勢(shì)和巨大潛力[1]，并已在航空發(fā)動(dòng)機(jī)葉片、旋轉(zhuǎn)部件、支撐結(jié)構(gòu)等關(guān)鍵航空部件的再制造中應(yīng)用[2-4]。激光熔覆與再制造技術(shù)具有稀釋率和熱影響區(qū)域小[5]、組織致密[6]、冶金結(jié)合強(qiáng)度高[7]、環(huán)保[8]、節(jié)省材料等優(yōu)點(diǎn)，同時(shí)激光熱源快熱快冷的特點(diǎn)導(dǎo)致成形過(guò)程常伴隨著微裂紋、氣孔、殘余拉應(yīng)力等缺陷的產(chǎn)生，嚴(yán)重影響了激光再制造的質(zhì)量。

為提高激光熔覆質(zhì)量，外加能場(chǎng)輔助成為當(dāng)前研究的熱點(diǎn)之一[9-10]。其中，超聲振動(dòng)作為外加能場(chǎng)引入激光成形在降低孔隙率、細(xì)化晶粒、提高機(jī)械性能等方面展示出了巨大的潛力。Cong等[11]在激光成形工藝中施加超聲波振動(dòng)，引起的聲流和空化的非線(xiàn)性效應(yīng)使孔隙率顯著降低，獲得了更細(xì)小的TiB晶粒，沿晶界分布且分布更均勻，同時(shí)晶粒細(xì)化進(jìn)一步提高了零件的顯微硬度。Todaro等[12]發(fā)現(xiàn)超聲波的使用促進(jìn)了具有隨機(jī)晶體結(jié)構(gòu)的細(xì)小等軸晶粒形成，施加超聲可以降低熔池主體的溫度梯度來(lái)增加凝固過(guò)程中的過(guò)冷度，進(jìn)而有利于晶粒成核和生長(zhǎng)。Wang等[13]研究了不同超聲頻率對(duì)熔覆層微觀(guān)組織和機(jī)械性能的影響，超聲頻率越高，晶粒度越小，顯微硬度越高，在超聲頻率25 kHz時(shí)氣孔抑制效果較好，獲得熔覆層耐磨性能、彈性模量更優(yōu)。王新洪等[14]在激光熔覆過(guò)程中施加超聲振動(dòng)降低了涂層的輪廓粗糙度，但涂層的稀釋度略有增加，超聲振動(dòng)改善了激光熔池中的傳熱傳質(zhì)，細(xì)化了微觀(guān)結(jié)構(gòu)，增加了陶瓷量，使陶瓷顆粒分布均勻。陳文靜等[15]發(fā)現(xiàn)超聲振動(dòng)作用下，熔覆層潤(rùn)濕鋪展程度提高，枝晶組織細(xì)化，元素分布均勻化，熔覆層顯微硬度更加均勻且提高了126.2HV0.2。姜風(fēng)春等[16]發(fā)現(xiàn)超聲振動(dòng)有效地削弱了織構(gòu)強(qiáng)度并使沉積層的晶粒結(jié)構(gòu)均勻化，晶粒結(jié)構(gòu)的改善提高了ER321不銹鋼的顯微硬度和屈服強(qiáng)度。

上述研究探討了超聲作用下激光熔覆成形微觀(guān)組織與性能的顯著變化，熔體流動(dòng)特性在超聲作用下的變化導(dǎo)致激光熔覆層宏觀(guān)形貌發(fā)生改變，將對(duì)多道搭接與路徑規(guī)劃策略產(chǎn)生不可忽視的影響。Cong等[17-18]利用超聲振動(dòng)輔助激光熔覆技術(shù)制備出了平整度更優(yōu)、熔池尺寸更大的熔覆層。莊棟棟等[19]發(fā)明超聲振動(dòng)能有效地改善熔覆層宏微觀(guān)成形質(zhì)量，降低了表面粗糙度和細(xì)化晶粒，提高了熔覆層的完整性和致密性。馬廣義等[20-21]將超聲振動(dòng)引入激光熔覆，隨著超聲輸出功率的增加，熔覆層深度增加，涂層與基材之間的潤(rùn)濕性增強(qiáng)，導(dǎo)致元素含量分布的變化。王維等[22]發(fā)現(xiàn)超聲的引入改善了搭接熔覆層的表面平整性，而疊高效率降低了36.7%。上述研究表明了超聲對(duì)熔覆層形貌的顯著影響，而當(dāng)前對(duì)于其影響規(guī)律及其機(jī)理的研究較為欠缺，仍待進(jìn)一步深入研究。

本文通過(guò)分析超聲作用下的液滴潤(rùn)濕過(guò)程，研究超聲在熔體中的作用效果及機(jī)理，采用高速相機(jī)監(jiān)測(cè)熔覆成形過(guò)程，并提取不同時(shí)刻熔池輪廓形狀，分析熔體潤(rùn)濕行為，同時(shí)，通過(guò)有無(wú)超聲作用下的對(duì)比，闡明超聲對(duì)于熔體潤(rùn)濕行為的影響規(guī)律。通過(guò)對(duì)比不同超聲功率比下的熔覆層宏微觀(guān)形貌探討形貌成形規(guī)律，討論超聲對(duì)于熔池潤(rùn)濕行為的影響機(jī)制，為超聲輔助高質(zhì)量激光熔覆成形提供參考。

1 試驗(yàn)

本文所采用的超聲輔助激光送絲熔覆試驗(yàn)裝置主要包括激光器、伺服送絲系統(tǒng)、冷卻系統(tǒng)、超聲振動(dòng)系統(tǒng)、控制系統(tǒng)等，試驗(yàn)過(guò)程由高速相機(jī)系統(tǒng)拍攝與記錄，試驗(yàn)系統(tǒng)示意圖如圖1所示。所用激光器為半導(dǎo)體光纖耦合激光器（武漢銳科），在本文試驗(yàn)中激光功率為1 kW，光斑直徑為2 mm。所用超聲振動(dòng)系統(tǒng)頻率為20 kHz，振幅大小通過(guò)功率比可調(diào)，100%功率比時(shí)振幅為50 μm，超聲從基體底部傳遞至熔池。

試驗(yàn)所用絲材與基板材料均為IN718鎳基高溫合金，基板尺寸為100 mm×60 mm×5 mm，絲材直徑為1.0 mm。在試驗(yàn)前，使用砂輪機(jī)打磨拋光基板表面去除表面雜質(zhì)，并用酒精清洗去除表面油污等。試驗(yàn)過(guò)程中采用高速相機(jī)拍攝熔池過(guò)渡行為。試驗(yàn)完成后，采用體視顯微鏡（尼康SMZ745T）和共聚焦顯微鏡（基恩士VK-X1000）觀(guān)測(cè)試樣宏觀(guān)形貌，采用光學(xué)顯微鏡（蔡司Axio Imager2）觀(guān)測(cè)分析熔覆區(qū)顯微組織。

圖1 超聲輔助激光送絲熔覆試驗(yàn)裝置示意圖

基于上述試驗(yàn)系統(tǒng)，在送絲方向?yàn)榍爸?，送絲角度為45°的情況下，開(kāi)展了激光熔覆工藝試驗(yàn)研究，以液橋過(guò)渡為目標(biāo)進(jìn)行工藝參數(shù)優(yōu)化，優(yōu)化后在本文中所用的掃描速度與送絲速度分別為8 mm/s和9.3 mm/s。

2 結(jié)果與分析

2.1 超聲對(duì)金屬液滴潤(rùn)濕行為的影響

為研究金屬熔體在超聲作用下的潤(rùn)濕行為，采用高速相機(jī)對(duì)金屬液滴在有無(wú)超聲作用下的潤(rùn)濕行為進(jìn)行觀(guān)測(cè)和對(duì)比研究。采用激光送絲在金屬板上預(yù)置IN718金屬半球，再對(duì)金屬半球進(jìn)行激光重熔，以1 kW的激光功率輻照1.8 s，使其完全熔化后凝固成形，采用高速相機(jī)在45°俯角下對(duì)有無(wú)超聲作用下的金屬半球重熔及凝固過(guò)程進(jìn)行觀(guān)測(cè)，結(jié)果如圖2所示。金屬半球在激光束加熱下逐漸熔化，直至完全熔化形成金屬液滴；在停止激光熱源輸入后，金屬液滴逐漸凝固成形。在無(wú)超聲作用下，金屬液滴形狀基本保持不變，未出現(xiàn)明顯的潤(rùn)濕現(xiàn)象；而在超聲作用下，金屬液滴表面呈現(xiàn)出明顯的波動(dòng)，在激光停止輻照后，液態(tài)金屬在超聲的作用下向四周潤(rùn)濕，最終形成扁平狀，潤(rùn)濕面積增大39.3%。上述結(jié)果顯示，超聲對(duì)于金屬液滴的潤(rùn)濕行為產(chǎn)生了顯著影響。

2.2 超聲對(duì)激光熔覆過(guò)程液橋過(guò)渡行為的影響

用高速相機(jī)拍攝有無(wú)超聲作用下的送絲熔覆液橋過(guò)渡過(guò)程，試驗(yàn)結(jié)果如圖3所示。當(dāng)未施加超聲振動(dòng)時(shí)，絲材前端的熔融金屬與基板接觸，潤(rùn)濕在基板表面，熔池呈現(xiàn)平穩(wěn)的液橋過(guò)渡，如圖3a所示。當(dāng)施加超聲振動(dòng)后，熔池在超聲作用下加劇了流動(dòng)，熔體與無(wú)超聲時(shí)相比在基板表面潤(rùn)濕性增強(qiáng)，隨后熔池沿激光掃描速度方向潤(rùn)濕，熔融區(qū)域進(jìn)一步擴(kuò)大。與此同時(shí)，超聲振動(dòng)加劇了液橋搭接處的流動(dòng)，使絲材熔融前端流動(dòng)加快，如圖3b所示。

圖2 金屬液滴重熔及凝固過(guò)程形態(tài)

圖3 激光熔覆過(guò)程液橋過(guò)渡行為監(jiān)測(cè)

針對(duì)有無(wú)超聲作用下的送絲熔覆液橋過(guò)渡過(guò)程的圖像開(kāi)展邊緣輪廓提取分析。如圖4所示，利用Canny算法[23-24]分析熔覆層，首先提取熔融區(qū)域高亮處的邊緣，獲得初步輪廓線(xiàn)，再對(duì)輪廓邊緣進(jìn)行優(yōu)化，獲得熔池高亮區(qū)域。在最后的結(jié)果圖中紅色輪廓線(xiàn)為熔池高亮區(qū)域輪廓，對(duì)應(yīng)熔池面積，藍(lán)色方框?yàn)楦吡羺^(qū)域的外接矩形，外接矩形的長(zhǎng)寬分別對(duì)應(yīng)熔池的橫向長(zhǎng)度和縱向長(zhǎng)度。觀(guān)測(cè)高速相機(jī)采集結(jié)果，可得：從0 s時(shí)刻激光輻照到絲材開(kāi)始，熔池面積逐漸增大，熔池的橫向縱向長(zhǎng)度均逐步增大；在無(wú)超聲作用下，0.43 s時(shí)刻液橋過(guò)渡趨于穩(wěn)定狀態(tài)，熔池面積較小，熔池拖尾較長(zhǎng)，橫縱比較大；在超聲作用下，熔池在超聲作用下加劇了流動(dòng)，熔池面積從0.5 s時(shí)刻液橋過(guò)渡趨于穩(wěn)定，熔融區(qū)域擴(kuò)大，橫縱比較小。

進(jìn)一步對(duì)熔池區(qū)域面積進(jìn)行統(tǒng)計(jì)，如圖5a所示。在0.15、0.45、0.60 s時(shí)刻，有無(wú)超聲情況下的熔池面積均顯著不同，超聲作用下熔池面積分別為無(wú)超聲作用下的1.52、2.07、2.80倍，結(jié)果顯示，同一時(shí)刻超聲作用下熔池區(qū)域面積增大。對(duì)圖中熔池面積和橫向縱向長(zhǎng)度進(jìn)行數(shù)值分析，如圖5b所示，超聲作用下熔池橫向與縱向長(zhǎng)度均顯著增大。

圖4 激光熔覆過(guò)程熔池圖像輪廓提取

圖5 激光熔覆過(guò)程熔池形貌分析

2.3 超聲對(duì)激光熔覆層宏觀(guān)形貌的影響

采用體視顯微鏡和共聚焦顯微鏡觀(guān)察試樣的宏觀(guān)形貌。圖6a為無(wú)超聲作用下的激光送絲熔覆層宏觀(guān)形貌，其表面較為平整光滑，無(wú)明顯缺陷，成形質(zhì)量好。圖6b—d表示超聲功率比（Ratio of Ultrasonic Power，RUP）分別為40%、60%、80%的試驗(yàn)結(jié)果，可見(jiàn)在超聲作用下，由于凝固過(guò)程中熔池表面的超聲波作用，熔覆層表面呈現(xiàn)波紋狀，且功率比增大后波紋形貌更為細(xì)小。

對(duì)有無(wú)超聲作用下的熔覆層表面輪廓進(jìn)行測(cè)量分析，如圖7所示。結(jié)果顯示，無(wú)超聲作用下的激光熔覆層的最大高度為627.7 μm，功率比為40%、60%、80%的超聲振動(dòng)下熔覆層最大高度分別為608.8、564.8、471.7 μm，80%超聲作用下熔覆層最大高度為無(wú)超聲下的75.2%。

2.4 超聲對(duì)激光熔覆層微觀(guān)形貌的影響

激光熔覆層的橫截面金相組織如圖8所示，無(wú)超聲作用下的熔覆層與基體冶金結(jié)合良好，晶粒沿著熱流反向生長(zhǎng)，形成長(zhǎng)條柱狀晶，晶粒生長(zhǎng)方向垂直于固液線(xiàn)。施加超聲振動(dòng)后，熔體向外潤(rùn)濕，熔寬明顯增大，熔高降低。施加40%、60%、80%功率比的超聲振動(dòng)后，熔覆層的金相組織發(fā)生顯著變化，熔池底部和中部的枝晶在高溫度梯度的作用下，仍為柱狀晶，但長(zhǎng)直柱狀晶明顯減少，且凝固組織生長(zhǎng)方向較為雜亂，枝晶寬度明顯變小。隨著超聲功率比的增大，枝晶細(xì)化的程度更為明顯。同時(shí)，在施加超聲振動(dòng)后，熔池頂部的轉(zhuǎn)向枝晶區(qū)域變窄。

對(duì)于激光熔覆層金相組織進(jìn)一步進(jìn)行了測(cè)繪，結(jié)果如圖9所示。無(wú)超聲振動(dòng)時(shí)，熔覆層高度為527 μm，熔覆層寬度為2 186 μm，熔覆層熔深面積為10.97×105μm2，此時(shí)熔覆層稀釋率為57.1%。當(dāng)施加的超聲功率比分別增加到40%、60%、80%時(shí)，熔深面積分別為9.29×105、10.97×105、8.33×105μm2，呈現(xiàn)下降趨勢(shì)。隨著超聲功率比的增大，熔高逐漸降低至483、419、435 μm，熔覆層寬度隨著超聲功率比的增大，逐漸增大至2 574、2 589、2 642 μm，稀釋率分別為50.9%、56.8%、48.5%，均小于無(wú)超聲作用下。隨著超聲功率比的增強(qiáng)，熔高降低，熔深減小，熔寬逐漸增大。無(wú)超聲振動(dòng)沉積的涂層輪廓呈現(xiàn)典型的弧形輪廓，而超聲波輸出功率比達(dá)到80%，涂層的輪廓底部較為平坦?？梢?jiàn)，在超聲振動(dòng)作用下，熔體和基體之間的潤(rùn)濕性顯著改善。

圖6 熔覆層宏觀(guān)形貌

圖7 熔覆層表面輪廓分析

圖8 激光熔覆層微觀(guān)組織形貌

圖9 超聲功率對(duì)熔覆層幾何形貌的影響

3 討論

通過(guò)對(duì)超聲作用下液滴潤(rùn)濕行為的觀(guān)測(cè)（如圖2所示），發(fā)現(xiàn)潤(rùn)濕角在超聲作用下顯著降低，可見(jiàn)超聲振動(dòng)破壞了液滴的初始界面力平衡。對(duì)超聲振動(dòng)作用下的液滴界面力變化進(jìn)行分析，如圖10所示，在無(wú)超聲時(shí)，液滴的氣液固三相邊界在邊界力的作用下處于平衡狀態(tài)；引入超聲振動(dòng)后，由于超聲振動(dòng)在液滴固液邊界處產(chǎn)生動(dòng)量傳遞層，引發(fā)了使液滴向三相邊界外側(cè)運(yùn)動(dòng)的額外作用力，進(jìn)而破壞了初始平衡狀態(tài)，提升了液滴在基板上的潤(rùn)濕效果[25]。在液滴潤(rùn)濕發(fā)生改變后，超聲振動(dòng)引起的額外作用力與新的三相邊界力達(dá)到平衡，該平衡可表示為[26-27]：

式中：σLV為液-氣界面表面張力；σSL為固-液界面表面張力；σSV為固-氣界面表面張力；F為由超聲振動(dòng)引起的附加聲張力；θ為動(dòng)態(tài)接觸角；U為基板振動(dòng)振幅；R為液滴接觸半徑；β?1為黏性動(dòng)量傳遞層，其值與液體黏度μ、密度u及振動(dòng)角頻率ω有關(guān)。由于超聲振動(dòng)引起的額外作用力平行于接觸線(xiàn)處的基板，根據(jù)分子動(dòng)力學(xué)理論[27]，接觸線(xiàn)的移動(dòng)與能量耗散有關(guān)，能量耗散通常為作用在移動(dòng)接觸線(xiàn)上的摩擦力做功。隨著三相線(xiàn)的外移，液滴接觸半徑R增大，動(dòng)態(tài)接觸角θ減小，三相邊界處徑向聲張力隨之增大，與液-氣界面表面張力σLV、固-液界面表面張力σSL、固-氣界面表面張力σSV實(shí)現(xiàn)表面力平衡，液滴三相線(xiàn)不再外移。圖4中熔池圖像輪廓的變化體現(xiàn)了熔體在超聲作用下潤(rùn)濕效果的提升，結(jié)合式（1），可見(jiàn)超聲振幅的增加使得超聲的額外作用力提升，因此在超聲功率增加時(shí)，熔覆層潤(rùn)濕效果提升，與圖8結(jié)果相符。

在激光熔覆過(guò)程中，熔池底部作為形核基底開(kāi)始凝固，枝晶生長(zhǎng)方向與熱流方向相反，在超聲作用下熔體流動(dòng)加速，熔池內(nèi)流場(chǎng)的變化改變了溫度場(chǎng)分布與熱流方向，使得枝晶生長(zhǎng)方向較為雜亂。前期研究[28]表明，隨著超聲功率比的增加，熔池表面溫度逐漸降低，熔池溫度梯度減小，在流場(chǎng)和溫度場(chǎng)的共同作用下，柱狀晶外延生長(zhǎng)受阻，枝晶長(zhǎng)度顯著變短，同時(shí)由于潤(rùn)濕性提升而產(chǎn)生的扁平熔覆層形狀也進(jìn)一步抑制了晶粒的外延生長(zhǎng)。無(wú)超聲作用下，當(dāng)凝固到頂部時(shí)，由于頂部與外界環(huán)境存在直接的熱交換，導(dǎo)致熱流方向發(fā)生變化，外延枝晶生長(zhǎng)方向和最大局部熱梯度方向之間的生長(zhǎng)偏角過(guò)大[29]，故在頂部能觀(guān)察到明顯的轉(zhuǎn)向枝晶。在超聲作用下，熔體表面高頻振動(dòng)，帶動(dòng)熔池頂部熱流方向的紊亂，抑制了轉(zhuǎn)向枝晶的形成。

超聲振動(dòng)引起的潤(rùn)濕作用也對(duì)激光熔覆成形工藝窗口和成形質(zhì)量產(chǎn)生影響。在激光送絲熔覆過(guò)程中，常出現(xiàn)工藝不匹配導(dǎo)致的頂絲而熔覆中止，高質(zhì)量的熔覆成形工藝窗口較窄[30-31]。而引入超聲振動(dòng)后，熔池波動(dòng)促進(jìn)了絲材與熔池間的熱量傳遞，加速了絲材在熔池內(nèi)部時(shí)的熔化，一定程度上避免了頂絲發(fā)生，增大了送絲熔覆工藝窗口。在超聲輔助激光送粉熔覆中，超聲引起的攪拌作用減少了熔覆中的球化現(xiàn)象，提高了熔覆層成形質(zhì)量[15,20]和粉末沉積過(guò)程中的粉末使用率[17]。低潤(rùn)濕角、小稀釋率的單道熔覆是實(shí)現(xiàn)表面平整、稀釋率小且無(wú)孔隙的多道搭接熔覆的必要條件[32-33]。已有學(xué)者模擬了不同潤(rùn)濕角和稀釋率的熔覆層在多道熔覆下的溫度場(chǎng)分布，模擬結(jié)果顯示，低潤(rùn)濕角、小稀釋率的熔覆層單位面積吸收能量升高，更利于多道搭接熔覆[34]。無(wú)超聲振動(dòng)沉積的涂層輪廓呈現(xiàn)典型的弧形輪廓，而隨著超聲功率比的增強(qiáng)，熔覆層形狀逐漸發(fā)生改變，呈現(xiàn)低潤(rùn)濕角和小稀釋率的單道形貌，為高質(zhì)量的多道搭接熔覆建立了基礎(chǔ)。

4 結(jié)論

1）超聲作用下，金屬液滴表面發(fā)生劇烈波動(dòng)，表面潤(rùn)濕性顯著提高，液滴趨于扁平，潤(rùn)濕面積增大39.3%。

2）熔池在超聲作用下加劇流動(dòng)，熔池往激光掃描速度方向潤(rùn)濕，熔融區(qū)域面積擴(kuò)大。超聲的引入加劇了液橋搭接處的流動(dòng)，使絲材熔融前端流動(dòng)加快，促進(jìn)了熔池的潤(rùn)濕行為，增大了熔體與空氣的接觸面積。

3）超聲的施加使熔覆層由典型的弧形輪廓變?yōu)楸馄?，隨著超聲功率比的增大，熔覆層最大高度逐漸減小，超聲功率比為80%作用下的熔覆層高度為無(wú)超聲情況下的75.2%。

4）施加超聲振動(dòng)使熔體潤(rùn)濕性增強(qiáng)且流速加快，加速了熔池與外界的熱交換，熔池頂部轉(zhuǎn)向枝晶減少，凝固組織生長(zhǎng)方向較為雜亂，長(zhǎng)直柱狀晶明顯減少。

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Influence of Ultrasound on the Wetting Behavior of Molten Pool in Laser Cladding

1a,1b,2,1a,1b,2,3,1a,1b,2,1a,1b,2,3,4,1a,1b,2

(1. a. Institute of Laser Advanced Manufacturing, b. College of Mechanical Engineering, Zhejiang University of Technology, Hangzhou 310023, China; 2. Collaborative Innovation Center of High-end Laser Manufacturing Equipment, Hangzhou 310023, China; 3. Hangzhou Steam Turbine Co., Ltd., Hangzhou 310020, China; 4. Laser Processing Research Centre, University of Manchester, Manchester M13 9PL, UK)

Laser additive remanufacturing technology based on laser cladding has shown significant advantages and great potential in the repair and remanufacturing of damaged parts. However, the rapid heating and cooling process during laser cladding leads to the formation of micro-cracks, pores, residual tensile stress, etc. in the cladding layer. In order to improve the quality of laser cladding, ultrasonic assisted laser cladding has become one of the hot spots in current research. In this study, the wetting behaviors of the molten pool with and without ultrasonic vibration were compared to investigate the influence and mechanism of ultrasonic vibration on the wetting behavior, providing reference for high-quality ultrasonic assisted laser cladding.

Inconel 718 substrates were polished and cleaned by alcohol to remove surface impurities. A laser beam with power of 1 kW and a spot diameter of 2 mm was used. Ultrasonic vibration with a frequency of 20 kHz and amplitude of 50 μm was transmitted from the bottom of the specimen to the molten pool. A forward wire feeding with a feeding angle of 45° was applied. The scanning speed and the wire feeding speed were 8 mm/s and 9.3 mm/s, respectively. During the experiments, the transition behaviors of the molten pool were captured by a high-speed camera, and the profiles of the molten pool were extracted by Canny algorithm. After the experiments, the macro morphology of the specimens was observed using a stereo microscope (Nikon, SMZ745T) and a confocal microscope (Keyence, VK-X1000). An optical microscope (Zeiss, Axio Imager2) was used to observe and analyze the microstructure of the cladding zone.

Significant fluctuation occurred on the surface of metal droplet with ultrasonic vibration. And the contact area between the metal droplet and the substrate increased by 39.3% with the effect of ultrasonic vibration, indicating the increase of wettability. The area of the molten pool in the laser cladding increased significantly caused by ultrasonic vibration. When a stable liquid-bridge transition was reached in the laser cladding, the area of the molten pool with ultrasonic vibration was 2.8 times of that without ultrasonic vibration. With the increase of ultrasonic power, the height and depth of the cladding layer decreased while the width increased. The dilution rate of the cladding layer was also reduced by ultrasonic vibration. And the cladding layer gradually varied from an arc profile to be relatively flat. When the ultrasonic power ratio was 80%, the height of the cladding layer was 75.2% of that without ultrasound, suggesting significant improvement of the wettability between the cladding and the substrate. In addition, the growth direction of the grains changed and the epitaxial growth of dendrites was inhibited. In addition, the turning dendritic structure at the top of the cladding layer became narrow with ultrasonic vibration. The mechanisms of ultrasonic vibration on the molten pool were discussed based on the experimental results.

In the laser cladding process, ultrasonic vibration is able to promote the wetting of melt and accelerate the melt flow of the molten pool, which leads to a relatively flat cladding layer. The area of molten pool increased, resulting in the variation of the grain growth direction and the length of dendrites.

laser cladding; ultrasonic vibration; wetting behavior; molten pool; macroscopic morphology; microstructure

V261.8

A

1001-3660(2022)10-0020-10

10.16490/j.cnki.issn.1001-3660.2022.10.003

2022–07–19；

2022–09–24

2022-07-19；

2022-09-24

國(guó)家自然科學(xué)基金（52175443、U1809220）；浙江省屬高?；究蒲袠I(yè)務(wù)費(fèi)專(zhuān)項(xiàng)資金（RF-B2020002）；浙江省公益技術(shù)應(yīng)用研究項(xiàng)目（LGG20E050019）

Supported by the National Natural Science Foundation of China (52175443, U1809220); the Fundamental Research Funds for the Provincial Universities of Zhejiang (RF-B2020002); Public Welfare Project of Zhejiang Province (LGG20E050019)

姚喆赫（1987—），男，博士，副研究員，主要研究方向?yàn)槟軋?chǎng)復(fù)合激光制造。

YAO Zhe-he (1987-), Male, Doctor, Associate professor, Research focus: energy field hybrid laser manufacturing.

姚建華（1965—），男，博士，教授，主要研究方向?yàn)榧す庵圃臁?/p>

YAO Jian-hua (1965-), Male, Doctor, Professor, Research focus: laser manufacturing.

姚喆赫, 沈奇艷, 葛宏江, 等.超聲對(duì)激光熔覆成形中熔池潤(rùn)濕行為的影響研究[J]. 表面技術(shù), 2022, 51(10): 20-29.

YAO Zhe-he, SHEN Qi-yan, GE Hong-jiang, et al. Influence of Ultrasound on the Wetting Behavior of Molten Pool in Laser Cladding[J]. Surface Technology, 2022, 51(10): 20-29.

責(zé)任編輯：萬(wàn)長(zhǎng)清