劉小超,武傳松
(山東大學(xué)材料連接技術(shù)研究所,濟(jì)南 250001)
攪拌摩擦焊(Friction stir welding,F(xiàn)SW)已經(jīng)在航空航天、軌道交通、船舶、汽車、軍工等制造領(lǐng)域取得了應(yīng)用[1—4]。作為一種新型的固相連接技術(shù),其在焊接低熔點(diǎn)高性能輕金屬材料(鋁合金和鎂合金等)時(shí),能夠避免傳統(tǒng)熔焊方法容易引起的氣孔、裂紋等焊接缺陷,具有接頭質(zhì)量高、焊接變形小和焊接過程綠色環(huán)保等優(yōu)點(diǎn)[5—7]。近年來,隨著攪拌摩擦焊技術(shù)的快速發(fā)展,高熔點(diǎn)材料(鈦合金、鐵基合金、鎳基合金等)的攪拌摩擦焊接也越來越受到重視[8—12]。然而,常規(guī)攪拌摩擦焊主要依靠攪拌頭與工件之間的摩擦產(chǎn)熱和材料塑性變形功產(chǎn)熱來軟化待焊區(qū)材料并形成連接,焊接時(shí)需要施加很大的軸向壓力和旋轉(zhuǎn)扭矩才能產(chǎn)生足夠的熱量和軟化,從而造成焊接載荷(包括軸向壓力、旋轉(zhuǎn)扭矩及前進(jìn)阻力)大、工藝柔性差、裝夾要求嚴(yán)、設(shè)備龐大、攪拌頭磨損(甚至折斷)等問題,同時(shí)也限制了焊接速度的進(jìn)一步提高,影響了焊接效率[13—16]。如果軸向壓力不夠大或者焊接速度超過一定值,會(huì)導(dǎo)致產(chǎn)熱不足,造成材料的塑性流動(dòng)性變差,難以獲得優(yōu)異的焊縫成形和接頭性能。為了解決這些問題,研究者試圖通過優(yōu)化焊接工藝參數(shù)和改進(jìn)攪拌頭的設(shè)計(jì)[13,16—21]來改善材料流動(dòng)和產(chǎn)熱,減小攪拌頭磨損,但是效果有限。另一種思路是通過施加輔助能量來協(xié)助軟化待焊材料。隨著時(shí)間的推移,先后出現(xiàn)了激光、電弧、感應(yīng)、電流、超聲等能量輔助攪拌摩擦焊方法[53—75,80—98]。根據(jù)輔助能量的類型,大致可以將其劃分為熱能輔助攪拌摩擦焊(如激光、電弧、感應(yīng)、電流等)和機(jī)械能輔助攪拌摩擦焊(如超聲振動(dòng))。文中首先簡要回顧攪拌摩擦焊及其衍生的焊接技術(shù)的發(fā)展現(xiàn)狀,然后詳細(xì)綜述外加能量輔助攪拌摩擦焊的研究進(jìn)展,并著重介紹3種超聲振動(dòng)輔助攪拌摩擦焊的工藝機(jī)理和工藝效果,最后對外加能量輔助攪拌摩擦焊的發(fā)展趨勢進(jìn)行展望。
攪拌摩擦焊技術(shù)誕生于1991年。經(jīng)過20余年的發(fā)展,攪拌摩擦焊的相關(guān)技術(shù)和工藝?yán)碚撊照橥晟啤H藗円呀?jīng)認(rèn)識到,攪拌摩擦焊是一個(gè)熱-力耦合的塑性變形加工工藝[5]。如圖1所示,一個(gè)帶有攪拌針和軸肩的高速旋轉(zhuǎn)的攪拌頭被向下的軸向力壓入兩塊對接的工件接縫中,并沿著對接線行走。在攪拌針和軸肩的摩擦和擠壓作用下,工件材料局部被加熱至塑性流動(dòng)狀態(tài),并隨著攪拌頭的旋轉(zhuǎn)發(fā)生遷移,從而在攪拌頭經(jīng)過之后形成可靠的連接。由于攪拌頭的旋轉(zhuǎn)和移動(dòng),在焊縫中心線兩側(cè)材料流動(dòng)呈現(xiàn)出明顯的差異,為了區(qū)分這種差異,將旋轉(zhuǎn)方向的切向與焊接移動(dòng)方向一致的一側(cè)稱為前進(jìn)側(cè)(Advancing side,AS),而相反的一側(cè)則被稱為后退側(cè)(Retreating side,RS)。
圖1 攪拌摩擦焊接原理[5]Fig.1 Schematic of FSW
在攪拌摩擦焊過程中,接頭中的不同區(qū)域經(jīng)歷了不同程度的塑性變形和熱循環(huán),形成了具有不同微觀組織結(jié)構(gòu)和性能的區(qū)域,在接頭橫截面上,其分布如圖2所示。從中心向兩側(cè)依次是焊核區(qū)(weld nugget zone,WNZ)、熱力影響區(qū)(thermo-mechanically affected zone,TMAZ)、熱影響區(qū)(heat affected zone,HAZ)和母材(base metal,BM)。焊核區(qū)的材料經(jīng)歷了攪拌頭的劇烈攪拌,晶粒發(fā)生嚴(yán)重破碎,在此過程中又發(fā)生了完全的動(dòng)態(tài)再結(jié)晶,從而形成細(xì)小的等軸晶晶粒組織,一般具有良好的力學(xué)性能;熱力影響區(qū)的材料經(jīng)歷了加熱和塑性變形的共同影響,而塑性變形的程度遠(yuǎn)小于焊核區(qū),因而該區(qū)域的晶粒組織發(fā)生了部分的動(dòng)態(tài)回復(fù)和再結(jié)晶,呈現(xiàn)出具有明顯變形痕跡的晶粒取向,力學(xué)性能一般稍差于焊核區(qū);熱影響區(qū)的材料僅僅是經(jīng)歷了焊接加熱和冷卻,晶粒在此過程中發(fā)生了粗化,晶粒內(nèi)部的強(qiáng)化相也可能因此而溶解或長大,因而熱影響區(qū)的力學(xué)性能一般都較差;母材既未發(fā)生塑性變形,也不受焊接熱影響,力學(xué)性能一般不發(fā)生變化。
圖2 攪拌摩擦焊接頭微觀組織結(jié)構(gòu)區(qū)域劃分[6]Fig.2 Microstructure of FSW joints
隨著人們對攪拌摩擦焊認(rèn)識的不斷加深,一些基于攪拌摩擦焊基本原理的新技術(shù)也被不斷研發(fā)出來。攪拌摩擦點(diǎn)焊(Friction Stir Spot Welding,F(xiàn)SSW)便是其中的一種[22—24]。其原理與攪拌摩擦焊類似,即將高速旋轉(zhuǎn)的攪拌頭插入被焊的上下兩層工件,利用摩擦產(chǎn)熱和攪拌頭高速旋轉(zhuǎn)所引起的材料塑性流動(dòng)形成接頭。隨著攪拌摩擦點(diǎn)焊技術(shù)的進(jìn)一步發(fā)展,又演變出了回填式 FSSW[25—28]、擺動(dòng)式 FSSW[29]、掃描式FSSW[30]、無針式 FSSW[31—34](也被稱為流動(dòng)摩擦點(diǎn)焊)等多種點(diǎn)焊新技術(shù)?;靥钍紽SSW通過起始階段材料的吸入和結(jié)束階段材料的壓填而得到完整無匙孔的焊點(diǎn)。擺動(dòng)式FSSW中攪拌頭以一定距離為半徑,繞攪拌頭夾持端擺動(dòng)一定角度來實(shí)現(xiàn)點(diǎn)焊,從而消除焊接中常見的界面畸變現(xiàn)象,并增加有效連接寬度。掃描式FSSW是指攪拌頭在旋轉(zhuǎn)的同時(shí)并以一定的運(yùn)動(dòng)軌跡進(jìn)行移動(dòng),這樣可以增大焊點(diǎn)截面積,提高接頭力學(xué)性能。無針式FSSW(流動(dòng)摩擦點(diǎn)焊)則類似于攪拌摩擦加工(Friction Stir Processing,F(xiàn)SP),通過特殊設(shè)計(jì)的軸肩壓入工件并旋轉(zhuǎn)來帶動(dòng)上下兩層工件材料的塑性流動(dòng),形成焊點(diǎn)。值得一提的是,上述無針式FSSW的攪拌頭設(shè)計(jì)也可用于正常的攪拌摩擦焊連接[35]。
雙軸肩攪拌摩擦焊(Bobbin Tool Friction Stir Welding)也是一種新型的 FSW 焊接技術(shù)[36—38]。它擁有上下兩個(gè)軸肩并通過攪拌針相連,焊接時(shí)上下兩個(gè)軸肩分別與工件的上下表面相接觸,并形成相互支撐。其優(yōu)點(diǎn)是不需要額外的背部墊板即可進(jìn)行攪拌摩擦焊接,可解決一些中空結(jié)構(gòu)和不便施加背部支撐的結(jié)構(gòu)的攪拌摩擦焊接問題,并從根本上消除未焊透等根部缺陷,是對常規(guī)攪拌摩擦焊的有力補(bǔ)充。
差速攪拌摩擦焊[39—52]也是對常規(guī)攪拌摩擦焊的進(jìn)一步發(fā)展,通過特殊的傳動(dòng)機(jī)構(gòu)設(shè)計(jì),實(shí)現(xiàn)軸肩與攪拌針的旋轉(zhuǎn)速度和旋轉(zhuǎn)方向的獨(dú)立控制,即可根據(jù)需要控制其焊接熱輸入。在差速攪拌摩擦焊中,當(dāng)軸肩和攪拌針旋轉(zhuǎn)方向相同時(shí),即為同向差速攪拌摩擦焊[41—42],當(dāng)軸肩轉(zhuǎn)速為零時(shí),即為靜止軸肩攪拌摩擦焊[39,43—49],當(dāng)軸肩和攪拌針旋轉(zhuǎn)方向相反時(shí),即為逆向差速攪拌摩擦焊[50—52]。其中靜止軸肩攪拌摩擦焊又可以用于特殊場合,如角焊縫的攪拌摩擦焊接[47]和旨在降低焊接載荷的高轉(zhuǎn)速攪拌摩擦焊(轉(zhuǎn)速超過10000 r/min)[48—49]。在角焊縫的攪拌摩擦焊接中,通過設(shè)計(jì)特殊的軸肩形狀,使軸肩與工件的兩個(gè)直角面緊密切合,并在焊接過程中不旋轉(zhuǎn),即可防止軸肩破壞工件和材料被擠出,形成無缺陷的角焊縫接頭。在旨在降低焊接載荷的高轉(zhuǎn)速攪拌摩擦焊中,靜止軸肩的引入能夠有效地減小孔洞、飛邊等缺陷,使焊接過程保持穩(wěn)定。逆向差速攪拌摩擦焊工件受到的來自攪拌針和軸肩旋轉(zhuǎn)的扭矩還可以相互抵消一部分,使工件裝夾變得更加容易。
復(fù)合攪拌摩擦焊技術(shù)借鑒了其他復(fù)合焊接如激光-MIG復(fù)合焊、激光-TIG復(fù)合焊等的設(shè)計(jì)思路,利用其他的小功率輔助能量促進(jìn)攪拌摩擦焊接過程中的材料軟化和塑性流動(dòng),從而降低焊接載荷、減小攪拌頭磨損、提高焊接速度,甚至實(shí)現(xiàn)常規(guī)攪拌摩擦焊難以焊接的高熔點(diǎn)材料的同種或異種材料攪拌摩擦焊接,達(dá)到“1+1>2”的目的。常見的攪拌摩擦焊輔助能源有激光、電弧、感應(yīng)、電流、超聲等。
在復(fù)合攪拌摩擦焊技術(shù)中,絕大多數(shù)是通過使用額外的加熱裝置在焊接時(shí)對工件待焊區(qū)域進(jìn)行局部預(yù)熱,然后再進(jìn)行常規(guī)的攪拌摩擦焊接。輔助熱源的引入可以提高焊接溫升速率,降低材料塑性變形抗力,減小攪拌頭磨損等。常見的熱源形式主要有激光、電弧、感應(yīng)加熱和電阻熱等。
Kohn等人[53]于2002年率先提出了激光輔助攪拌摩擦焊(Laser-assisted friction stir welding,LAFSW)。其原理如圖3所示,激光束被精確地作用在攪拌頭前方的待焊工件局部進(jìn)行預(yù)熱,然后再由攪拌頭進(jìn)行常規(guī)的攪拌摩擦焊接,形成固相接頭。激光的精確預(yù)熱能夠軟化待焊材料,使工件在焊接時(shí)受到的扭矩有所降低,這就降低了工件對焊接工裝的要求;同時(shí)攪拌頭所需的進(jìn)給力也有減小,因此能夠減輕攪拌頭的磨損,提高焊接速度。
圖3 激光輔助攪拌摩擦焊原理[53]Fig.3 Schematic of laser assisted FSW
Kohn 等人[53]、Zaeh 等人[54]和 Casalino 等人[55]研究了鋁、鎂等輕合金的LAFSW焊接,發(fā)現(xiàn)激光輔助能夠明顯消除焊縫內(nèi)部缺陷,提高焊接速度,獲得良好的接頭力學(xué)性能。Sun等[56]研究了S45C鋼板的LAFSW接頭的微觀組織和力學(xué)性能。研究發(fā)現(xiàn)當(dāng)激光束聚焦在攪拌頭前方10 mm的焊接線上時(shí),焊接速度能夠由常規(guī)的400 mm/min提高到800 mm/min。當(dāng)焊接速度低于600 mm/min時(shí),能夠避免焊縫中形成脆性的馬氏體相。此外,當(dāng)預(yù)熱位置處于前進(jìn)側(cè)時(shí),會(huì)顯著降低攪拌頭與工件之間的摩擦產(chǎn)熱,而預(yù)熱位置處于后退側(cè)時(shí),則會(huì)獲得較高的熱輸入。此外,異種材料攪拌摩擦連接時(shí)可以利用激光集中加熱熔點(diǎn)較高的一側(cè)金屬。Merklein和Giera[57]的研究表明,使用激光預(yù)熱能夠提高鋼/鋁異種金屬攪拌摩擦焊連接的力學(xué)性能,原因是激光預(yù)熱增強(qiáng)了材料流動(dòng),降低了金屬間化合物層的脆性。Chang等[58]研究了在對接面上添加鎳箔的鋁/鎂異種材料LAFSW焊接,發(fā)現(xiàn)激光輔助能夠顯著提高接頭抗拉強(qiáng)度。原因是韌性較好的鎳基金屬間化合物相取代了原來的脆性相Al12Mg17,而激光加熱了鎳箔,改善了焊縫成形,促進(jìn)了鎳基金屬間化合物相的形成。
激光輔助攪拌摩擦焊也存在一些問題,如激光在金屬表面會(huì)發(fā)生反射,造成了能源浪費(fèi)。
電弧輔助攪拌摩擦焊主要是指TIG電弧輔助攪拌摩擦焊和等離子弧輔助攪拌摩擦焊,其原理與激光輔助攪拌摩擦焊類似,如圖4所示。先由焊槍在攪拌頭前方不遠(yuǎn)處對工件局部進(jìn)行預(yù)熱,然后再由攪拌頭進(jìn)行正常焊接。焊槍既可以是TIG焊槍,也可以是等離子弧焊槍。
圖4 電弧輔助攪拌摩擦焊原理[60]Fig.4 Schematic of arc assisted FSW
劉會(huì)杰等人[59—60]率先采用了等離子弧作為輔助熱源焊接了2219-T6鋁合金,發(fā)現(xiàn)在保證接頭力學(xué)性能的前提下,焊接速度能夠成倍增加。然而,由于雙重?zé)嵫h(huán)的影響,焊核區(qū)沉淀相發(fā)生明顯粗化,成為接頭最薄弱區(qū)。
Yaduwanshi等人[61]研究了1100鋁合金等離子弧輔助攪拌摩擦焊過程中的焊接軸向力、熱循環(huán)、峰值溫度、冷卻速率、接頭力學(xué)性能和顯微組織等,發(fā)現(xiàn)焊核區(qū)的晶粒組織得到細(xì)化,焊接軸向力顯著降低,接頭力學(xué)性能有所提高。Bang等人[62—63]嘗試采用TIG電弧輔助焊接不銹鋼(STS304)/鋁合金(Al6061)和鋁合金(Al6061)/鈦合金(Ti-6%Al-4%V)異種金屬,以期提高接頭力學(xué)性能。TIG電弧分別被用來在不銹鋼和鈦合金一側(cè)加熱工件,所獲接頭的橫向抗拉強(qiáng)度都超過了鋁母材強(qiáng)度的91%。分析認(rèn)為這是由于電弧預(yù)熱增強(qiáng)了材料的塑性流動(dòng),在不銹鋼與鋁合金的連接中電弧預(yù)熱還形成了局部退火,使焊縫伸長率顯著增大。
電弧輔助攪拌摩擦焊的缺點(diǎn)是強(qiáng)烈的弧光會(huì)干擾正常的攪拌摩擦焊接,使工作環(huán)境變差,弱化了常規(guī)攪拌摩擦焊綠色環(huán)保的優(yōu)點(diǎn)。
Midling等人[64]在早期申請的專利中提出了采用感應(yīng)加熱作為輔助熱源的攪拌摩擦焊技術(shù)。其原理如圖5所示,感應(yīng)線圈安裝在攪拌頭前方并隨著攪拌頭向前移動(dòng)。感應(yīng)加熱可以塑化待焊金屬,降低材料流變抗力,改善焊接工藝效果。
圖5 感應(yīng)熱輔助攪拌摩擦焊原理[64]Fig.5 Schematic of induction heat assisted FSW
申志康等人[65]研究了Q235鋼的感應(yīng)加熱輔助攪拌摩擦焊接,并對預(yù)熱與否的焊縫顯微組織、力學(xué)性能和顯微硬度等進(jìn)行了測試對比。發(fā)現(xiàn)感應(yīng)加熱預(yù)熱可以提高鋼的塑性,降低攪拌頭磨損,提高攪拌頭壽命;但是預(yù)熱同時(shí)也粗化了攪拌區(qū)的晶粒尺寸,降低了接頭的力學(xué)性能。álvarez等人[66]研究了感應(yīng)加熱輔助攪拌摩擦焊接超級雙相不銹鋼(GX2CrNiMoN26-7-4),對比了常規(guī)焊接與感應(yīng)加熱輔助焊接條件下焊縫的微觀組織結(jié)構(gòu)和機(jī)械性能的變化。研究發(fā)現(xiàn),感應(yīng)預(yù)熱能夠使焊接所需的軸向壓力降低31%,或者在相同軸向壓力下成倍地提高焊接速度。焊縫中沒有發(fā)現(xiàn)σ相,攪拌區(qū)的晶粒尺寸顯著減小,引起了接頭平均硬度值和抗拉強(qiáng)度的提高,鐵素體比例介于50%~70%之間。
圖6 感應(yīng)加熱輔助攪拌摩擦焊點(diǎn)焊工藝原理[67]Fig.6 Schematic of induction heat assisted FSSW
Sun等人[67]將高頻感應(yīng)加熱應(yīng)用于S12C鋼板的攪拌摩擦焊點(diǎn)焊中,工藝原理如圖6所示。其研究發(fā)現(xiàn),盡管高頻感應(yīng)加熱預(yù)熱增大了攪拌區(qū)的晶粒尺寸,但同時(shí)也增大了焊點(diǎn)的體積,即連接界面增大,從而增大了點(diǎn)焊接頭的粘接強(qiáng)度;而且,攪拌頭和工件之間的摩擦產(chǎn)熱也因高頻感應(yīng)預(yù)熱而減小。
感應(yīng)加熱的弊端在于不能對工件待焊局部進(jìn)行精確加熱,凡是能夠產(chǎn)生感應(yīng)電流的區(qū)域都會(huì)被加熱,如夾具、攪拌頭等。此外,感應(yīng)加熱對非導(dǎo)磁性材料的加熱效率非常低,且不能加熱非導(dǎo)電性材料。
電流輔助攪拌摩擦焊是指利用電流的焦耳熱效應(yīng)和所謂的“電塑性”來輔助軟化待焊材料,提高攪拌摩擦焊過程中材料的塑性流動(dòng)性。其中電流的運(yùn)用非常靈活,衍生出了多種變體。
Luo等人[68—71]采用如圖7所示的電流輔助攪拌摩擦焊方法,分別以攪拌頭和工件端部作為兩個(gè)電極,使電流流過工件內(nèi)部進(jìn)而產(chǎn)生電阻熱。焊接了AZ31B鎂合金、7075鋁合金的對接接頭,以及不銹鋼2Cr13Mn9Ni4和普通碳鋼Q235B的搭接接頭,發(fā)現(xiàn)電阻熱細(xì)化了鎂合金焊核區(qū)晶粒組織,提高了其顯微硬度,增強(qiáng)了塑性變形。而對于鋁合金,焊核區(qū)和熱影響區(qū)的晶粒尺寸隨著電流密度的增大而輕微增加。另外還指出電流輔助攪拌摩擦焊對于鋼等高強(qiáng)合金的焊接有很大的應(yīng)用潛力。Santos等人[73—74]改進(jìn)了電流輔助攪拌摩擦焊,如圖8所示。其在攪拌頭的軸心增加一個(gè)銅芯,并在背部墊板上相應(yīng)地增加一個(gè)銅鍵,使電流在工件中的分布更加集中,很好地解決了常規(guī)攪拌摩擦焊由于底部產(chǎn)熱不足引起的未焊透等根部缺陷。
圖7 電流輔助攪拌摩擦焊原理(電阻熱)[68—71]Fig.7 Schematic of electric current assisted FSW(Joule heat)
圖8 改進(jìn)的電流輔助攪拌摩擦焊原理(電阻熱)[73]Fig.8 Schematic of improved electric current assisted FSW(Joule heat)
Liu等人[75]研發(fā)了如圖9所示的電流輔助攪拌摩擦焊,兩個(gè)直接施加在工件上的電極可以形成一個(gè)隨攪拌頭移動(dòng)的電流場,而不需要攪拌頭作為其中一個(gè)電極。其研究了電塑性(高密度電流引起的金屬軟化)對6061鋁合金和TRIP780鋼異種金屬連接時(shí)的影響。發(fā)現(xiàn)輔助電流能夠顯著減小焊接軸向力,并且當(dāng)攪拌頭轉(zhuǎn)速較低或者攪拌頭向鋁側(cè)偏移量較小時(shí),這一現(xiàn)象更加明顯。微觀結(jié)構(gòu)觀察顯示,在鋁/鐵界面上金屬間化合物薄層的形成和微觀上的互鎖特點(diǎn)被增強(qiáng),因而提高了接頭質(zhì)量。
圖9 電流輔助攪拌摩擦焊(電阻熱和電塑性)[75]Fig.9 Schematic of electric current assisted FSW(Joule heat and electro-plastic effect)
電流輔助攪拌摩擦焊存在的問題是:金屬材料的導(dǎo)電性一般都較好,導(dǎo)致電流的熱效率非常低,往往需要很大的電壓[71]或電流[73]才能產(chǎn)生作用,即使是利用電塑性,所需電流強(qiáng)度仍然非常大(超過500 A[75])。
此外,盡管輔助熱源能夠促進(jìn)材料軟化,但是也會(huì)導(dǎo)致工件材料經(jīng)歷雙重的熱循環(huán),如果熱源參數(shù)不當(dāng),會(huì)導(dǎo)致焊縫熱影響區(qū)擴(kuò)大,焊縫金屬的強(qiáng)化相長大或溶解,從而降低了接頭的力學(xué)性能,縮小了可用工藝參數(shù)的范圍。
超聲振動(dòng)作為一種機(jī)械能,具有頻率高、方向性強(qiáng)和能量集中等特點(diǎn)。早在20世紀(jì)50年代,人們就發(fā)現(xiàn)超聲振動(dòng)能夠降低金屬材料塑性變形時(shí)的屈服應(yīng)力和流變應(yīng)力,但是熱作用并不明顯。Langenecker[79]在研究高純度鋁單晶拉伸時(shí)發(fā)現(xiàn),在室溫下對鋁單晶施加一定能量密度的超聲振動(dòng),其變形時(shí)所需的拉伸應(yīng)力會(huì)顯著降低,與加熱至一定溫度條件下的拉伸應(yīng)力相當(dāng),如圖10所示。當(dāng)超聲振動(dòng)的能量密度達(dá)到50 W/cm2時(shí),其所需的拉伸應(yīng)力幾乎下降為0,與加熱至600℃的條件相當(dāng)。而在對不銹鋼和鈹?shù)臏y試中發(fā)現(xiàn),如要將拉伸應(yīng)力降至“零”,所需的超聲能量密度約為80~100 W/cm2。類似的現(xiàn)象在鋅、鎘、鐵、鈦、鎢等金屬材料中也存在[76—79]。
圖10 單晶鋁拉伸應(yīng)力應(yīng)變曲線(超聲頻率20 kHz)[79]Fig.10 Stress vs.elongation for aluminum single crystals(ultrasonic frequency 20 kHz)
將超聲振動(dòng)引入攪拌摩擦焊,利用超聲振動(dòng)降低焊接區(qū)材料的屈服應(yīng)力,提高其塑性流動(dòng)性,從而降低焊接軸向壓力和主軸轉(zhuǎn)矩,減小攪拌頭磨損,提高焊接速度,成了最近興起的復(fù)合攪拌摩擦焊技術(shù)。目前主要有3種超聲振動(dòng)的施加方式,即從橫向施加于攪拌頭上、從軸向施加于攪拌頭上以及直接施加在工件上。
Park等人[80—82]將超聲振動(dòng)從橫向施加于攪拌頭上,研發(fā)了超聲輔助攪拌摩擦焊工藝(Ultrasonic assisted FSW,UaFSW),其原理如圖11所示。首先超聲波發(fā)生器將市電轉(zhuǎn)換為超聲頻的電信號,然后換能器將其轉(zhuǎn)化為超聲頻的機(jī)械振動(dòng),再經(jīng)超聲傳遞裝置及振動(dòng)耦合裝置將縱向的超聲振動(dòng)傳遞至攪拌頭。超聲傳遞裝置即為通常所說的變幅桿,起到聚焦超聲能量、放大振幅的作用。振動(dòng)耦合裝置由兩個(gè)對稱的滾動(dòng)軸承組成,并通過兩根軸緊固在超聲傳遞裝置上,隨其做同頻率同方向的超聲振動(dòng)。滾動(dòng)軸承的外圈再與攪拌頭柱面緊密配合,其在攪拌頭旋轉(zhuǎn)的帶動(dòng)下做高速轉(zhuǎn)動(dòng)。這樣,來自換能器的縱向的超聲振動(dòng)就變成攪拌頭的橫向振動(dòng),因此,攪拌頭的運(yùn)動(dòng)狀態(tài)即為橫向的超聲振動(dòng)疊加在周向的旋轉(zhuǎn)運(yùn)動(dòng)上,焊接時(shí)再由攪拌頭將超聲振動(dòng)傳遞至待焊工件中。其進(jìn)行的6061鋁合金平板對接和1018鋼平板對接工藝試驗(yàn)表明,UaFSW工藝不僅能夠降低鋁合金焊接阻力,提高焊接接頭的力學(xué)性能,減小甚至消除焊縫成形缺陷,同時(shí)也能夠減小鋼材攪拌摩擦焊的軸向壓力,提高焊接溫度。
圖11 超聲振動(dòng)從橫向施加于攪拌頭[82]Fig.11 Schematic of superposing ultrasonic vibration on FSW tool in horizontal direction
Rostamiyan等人[83]研究了6061鋁合金超聲振動(dòng)輔助攪拌摩擦焊點(diǎn)焊(UaFSSW),其原理與UaFSW類似。研究結(jié)果表明超聲振動(dòng)能夠顯著提高搭接接頭的抗剪切強(qiáng)度和顯微硬度。Ahmadnia等人[84]研究了UaFSW工藝參數(shù)對6061鋁合金焊接接頭的力學(xué)性能和摩擦特性的影響。其優(yōu)化了超聲功率、攪拌頭轉(zhuǎn)速、焊接速度和軸向壓力以獲得最優(yōu)的接頭抗拉強(qiáng)度和焊縫成形性,以及最小的表面粗糙度和滑動(dòng)磨損率。結(jié)果表明超聲功率對焊接接頭的力學(xué)性能和摩擦特性都有非常重大且積極的影響。
應(yīng)當(dāng)指出,這種從橫向施加超聲振動(dòng)的方式得到的振動(dòng)效果一般,超聲振動(dòng)的能量在傳遞至待焊工件的過程中損失(包括超聲傳遞裝置自耗、耦合損失和攪拌頭自耗等)較大,能量利用率較低。超聲傳遞裝置和振動(dòng)耦合裝置的設(shè)計(jì)也相對復(fù)雜,加工難度較大。
賀地求等人[85—89]提出了超聲攪拌復(fù)合摩擦焊方法(Ultrasound Stir Compound Welding)。其原理如圖12所示,將攪拌針與超聲的換能器變幅桿集成為一體,這樣攪拌針上會(huì)疊加軸向的超聲振動(dòng),再以正常的攪拌摩擦焊程序進(jìn)行焊接,從而將超聲振動(dòng)的能量導(dǎo)入到焊縫深層。超聲振動(dòng)能量的導(dǎo)入能夠降低焊接流變的抵抗力,減小焊后殘余應(yīng)力,改善了焊縫組織,提高了焊縫強(qiáng)度,同時(shí)超聲的加入還可以起到細(xì)化晶粒,改善金屬宏觀和微觀偏析的效果[85]。Amini等人[90]采用了類似的原理設(shè)計(jì)了一體化的超聲變幅桿和攪拌頭,并進(jìn)行了6061-T6鋁合金的焊接工藝試驗(yàn)研究,結(jié)果表明超聲振動(dòng)降低了焊接載荷,提高了焊接溫度。
圖12 超聲振動(dòng)從軸向施加于攪拌頭[85]Fig.12 Schematic of superposing ultrasonic vibration on FSW tool in axial direction
這種設(shè)計(jì)方法的優(yōu)點(diǎn)是能夠保證超聲作用的部位位于攪拌區(qū),超聲能量能夠作用于焊縫深層。但是在這種設(shè)計(jì)中,焊接時(shí)很大的軸向壓力會(huì)對攪拌針的超聲振動(dòng)產(chǎn)生不利影響,這將影響超聲的實(shí)際作用效果,目前相關(guān)的報(bào)道中僅限于薄板焊接[85—90]。在將攪拌針和超聲的換能器變幅桿連為一體時(shí),需要采用相對復(fù)雜的安裝機(jī)構(gòu),對攪拌摩擦焊機(jī)改動(dòng)較大;因?yàn)槠鋽嚢桀^的設(shè)計(jì)與常規(guī)不同,因而該系統(tǒng)所用攪拌頭與通用的攪拌頭不具有互換性。為了使超聲振動(dòng)系統(tǒng)工作在諧振頻率上,需要針對不同厚度的工件制作不同的攪拌頭以及與攪拌頭相匹配的超聲振動(dòng)系統(tǒng),增加了工藝的復(fù)雜性。
武傳松等人[91—92]提出了直接將超聲振動(dòng)通過工具頭施加在攪拌頭前方的待焊工件上的工藝方法,并稱之為超聲振動(dòng)強(qiáng)化攪拌摩擦焊(Ultrosonic vibration enhanced FSW,UVeFSW),其原理如圖13所示。超聲波發(fā)生器傳輸?shù)碾娦盘栆来谓?jīng)過換能器、變幅桿和工具頭,轉(zhuǎn)變成超聲頻的機(jī)械振動(dòng),直接作用在攪拌頭前方的待焊區(qū)域。
圖13 超聲振動(dòng)強(qiáng)化攪拌摩擦焊原理[92]Fig.13 Schematic of UVeFSW
Liu等人[93—96]開展了 6061 和 2024 鋁合金的超聲振動(dòng)強(qiáng)化攪拌摩擦焊工藝試驗(yàn),發(fā)現(xiàn)施加超聲能夠改善焊縫表面成形效果,增大焊縫截面積,減小甚至消除焊縫內(nèi)部缺陷,優(yōu)化接頭微觀組織結(jié)構(gòu),提高接頭力學(xué)性能和焊接速度。
在此基礎(chǔ)上,Liu等人[97—98]進(jìn)一步開展了超聲振動(dòng)強(qiáng)化攪拌摩擦焊的材料流動(dòng)試驗(yàn)。其將1060純鋁箔以不同的配置方式夾在基體材料之間,通過攪拌頭“急?!奔夹g(shù)和焊縫“切片”技術(shù)以及特殊設(shè)計(jì)的焊接行程來觀察攪拌頭周圍材料的瞬態(tài)和準(zhǔn)穩(wěn)態(tài)流動(dòng)。研究結(jié)果表明,超聲振動(dòng)的施加能夠顯著增加攪拌針周圍變形材料的體積,提高攪拌區(qū)材料的流動(dòng)速度以及應(yīng)變和應(yīng)變速率。
目前該技術(shù)正處在積極的研究中,存在的主要問題是如何進(jìn)一步優(yōu)化超聲振動(dòng)系統(tǒng)參數(shù),爭取更加顯著的工藝效果;以及超聲工具頭作為一個(gè)消耗品,如何有效地保證其使用壽命。
攪拌摩擦焊技術(shù)因其相較于傳統(tǒng)熔焊方法的獨(dú)特優(yōu)勢,越來越多地受到人們的青睞。相信在倡導(dǎo)低碳經(jīng)濟(jì)的今天,攪拌摩擦焊技術(shù)一定會(huì)得到大力的發(fā)展。通過綜述攪拌摩擦焊接技術(shù)的發(fā)展現(xiàn)狀和能量輔助攪拌摩擦焊的研究進(jìn)展,可以得到以下結(jié)論。
1)攪拌摩擦焊逐漸由最初的單一技術(shù)發(fā)展為全方位、多元化、綜合性的焊接加工技術(shù),為實(shí)際應(yīng)用提供了更多選擇,成為材料成形領(lǐng)域不可或缺的一分子。
2)能量輔助攪拌摩擦焊技術(shù)作為對常規(guī)攪拌摩擦焊技術(shù)的延伸和補(bǔ)充,有效地拓寬了攪拌摩擦焊技術(shù)的應(yīng)用領(lǐng)域。
3)熱能輔助攪拌摩擦焊受制于輔助熱源的特性,其進(jìn)一步發(fā)展將依賴于新的熱源形式或者熱源施加方式。
4)與熱能輔助攪拌摩擦焊相比,機(jī)械能(超聲)輔助使攪拌摩擦焊能夠避免雙重?zé)嵫h(huán)對焊接接頭帶來的不利影響,具有廣闊的發(fā)展前景。
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