袁 潔,王快社,趙 凱,王 文
(西安建筑科技大學(xué)冶金工程學(xué)院,陜西西安710055)
鈦合金攪拌摩擦焊接最新研究進(jìn)展
袁 潔,王快社,趙 凱,王 文
(西安建筑科技大學(xué)冶金工程學(xué)院,陜西西安710055)
鈦合金具有密度低、比強(qiáng)度高,耐蝕性好,加工性能優(yōu)異等優(yōu)點(diǎn),主要應(yīng)用于航空航天、交通運(yùn)輸和石油化工等領(lǐng)域。當(dāng)鈦合金作為結(jié)構(gòu)材料應(yīng)用在不同領(lǐng)域時(shí),傳統(tǒng)的熔融焊接方法會(huì)產(chǎn)生較大殘余應(yīng)力,組織粗化,變形大,裂紋和孔隙等缺陷;而采用攪拌摩擦焊接技術(shù)可以避免傳統(tǒng)熔融焊接方法產(chǎn)生的缺陷,從而大幅度提高鈦合金焊接接頭質(zhì)量。目前,鈦合金的攪拌摩擦焊接技術(shù)已成為國(guó)內(nèi)外研究熱點(diǎn)。主要介紹攪拌摩擦焊接的原理、工藝特點(diǎn),國(guó)內(nèi)外關(guān)于鈦合金FSW焊接接頭的宏觀形貌、微觀組織(晶粒大小、織構(gòu))和力學(xué)性能等方面的最新研究進(jìn)展,最后展望了鈦合金FSW未來(lái)的研究方向。
鈦合金;攪拌摩擦焊;微觀組織;力學(xué)性能
鈦合金具有密度小、比強(qiáng)度高、耐熱、耐蝕等優(yōu)異性能,在航空、航天、石油化工等領(lǐng)域得到了廣泛應(yīng)用[1-4]。鈦合金傳統(tǒng)焊接方法有鎢極氬弧焊、等離子弧焊、電子束焊、激光焊、電阻焊、釬焊等[5-8],但這些方法的焊接條件苛刻且工藝復(fù)雜,焊后存在較大殘余應(yīng)力,變形大,組織粗化,易產(chǎn)生裂紋、氣孔及脆性化合物等缺陷,嚴(yán)重影響接頭質(zhì)量,大大降低焊接接頭的力學(xué)性能[9]。攪拌摩擦焊接(Friction stirwelding,F(xiàn)SW)技術(shù)作為一種固相焊接技術(shù),已經(jīng)在鎂合金、鋁合金等低熔點(diǎn)合金上得到了廣泛的應(yīng)用[10-15]。隨著攪拌頭工具的發(fā)展,近幾年開(kāi)始對(duì)高熔點(diǎn)合金如銅、鋼、鎳及鈦合金的FSW展開(kāi)研究[16-19]。本研究主要對(duì)國(guó)內(nèi)外鈦合金FSW接頭宏觀組織、微觀組織及力學(xué)性能等進(jìn)行綜述,并展望了鈦合金FSW未來(lái)的研究方向。
鈦合金FSW接頭表面形貌和橫截面的宏觀組織如圖1所示。表面光滑發(fā)亮,橫截面宏觀組織分為焊核區(qū)(Stir Zone,SZ),熱影響區(qū)(Heat Affected Zone,HAZ)和母材(Base Material,BM),而熱機(jī)械影響區(qū)(Thermo-Mechanically Affected Zone,TMAZ)的存在則存在較大的爭(zhēng)議[18-24]。鈦合金FSW宏觀組織的特點(diǎn):一方面,鈦合金強(qiáng)度較高,難以發(fā)生塑性變形,塑性變形過(guò)程較復(fù)雜;另一方面,鈦合金在FSW過(guò)程中發(fā)生了相變,相變掩蓋了TMAZ或HAZ[25]。交界區(qū)較窄,通常只有幾十到幾百微米,這是因?yàn)殁伜辖馃釋?dǎo)率較低,僅為17 W/(m·K),而鋁合金的熱導(dǎo)率高達(dá)210 W/(m·K)。SZ呈“碗狀”形狀,且整個(gè)接頭形貌基本上沿焊縫中心對(duì)稱,焊縫兩側(cè)形貌差別較小。
圖1 Ti6Al4V FSW接頭宏觀形貌[29]Fig.1 Macro-appearance of Ti6Al4V FSW welds
鈦合金FSW接頭微觀組織的研究主要集中在相變和晶粒取向方面。李繼忠等人[26]研究了轉(zhuǎn)速對(duì)FSW TC4接頭組織的影響。當(dāng)焊速為30 mm/min、轉(zhuǎn)速為150 r/min時(shí),SZ主要為α相等軸超細(xì)晶;轉(zhuǎn)速增至200 r/min時(shí),SZ為α相等軸超細(xì)晶和片層狀α+β的雙態(tài)組織;轉(zhuǎn)速繼續(xù)增至250 r/min,SZ全部由片層狀α+β組織組成,α等軸晶消失。王快社等人[27]研究表明,SZ組織為細(xì)密的等軸晶組織,焊縫最大溫度未超過(guò)相變溫度。王文[18]和Liu等人[24]研究結(jié)果表明,SZ組織為片層狀α+β結(jié)構(gòu),組織轉(zhuǎn)變主要為β相向片層α+β相轉(zhuǎn)變,TMAZ為等軸晶α和α+β片層的雙態(tài)組織,分析認(rèn)為組織轉(zhuǎn)變受動(dòng)態(tài)再結(jié)晶和相變共同作用。TC4 FSW接頭未超過(guò)相變溫度的兩種組織形態(tài)如圖2所示。
圖2 TC4焊核區(qū)TEM微觀組織[29]Fig.2 Microstructures of SZ in TC4 by FSW
由于鈦合金低的導(dǎo)熱系數(shù),在厚度方向上也呈現(xiàn)出不同的組織特征。Yoon[28]和姬書(shū)得等人[29]研究了Ti6Al4V板材FSW接頭厚度方向上的組織,結(jié)果表明:在焊速50 mm/min、轉(zhuǎn)速300 r/min下,近焊縫表面組織由完全的片層狀β相組成,表明在該區(qū)域峰值溫度已經(jīng)超過(guò)了β相變溫度;中間部分的組織由等軸的α相及片層狀的α+β晶粒組成,這個(gè)區(qū)域的峰值溫度未超過(guò)β相變溫度[18,22,24,30]。最底層的組織主要由完全等軸α相組成。當(dāng)轉(zhuǎn)速逐漸下降時(shí),近焊縫表面的完全片層狀組織范圍減?。划?dāng)轉(zhuǎn)速降為225 r/min時(shí),SZ組織全部為α等軸晶。
焊接參數(shù)直接影響焊接溫度。當(dāng)焊接峰值溫度低于β相變溫度時(shí),鈦合金FSW后的組織分為兩種,一種為等軸細(xì)晶α相,另一種為等軸細(xì)晶α相和片層狀α+β的雙態(tài)組織;當(dāng)峰值溫度超過(guò)β相變溫度時(shí),組織為完全的層狀β相。
鈦是密排六方(hcp)晶體結(jié)構(gòu),晶格對(duì)稱性較低,滑移系相對(duì)較少,在FSW過(guò)程中易形成變形織構(gòu)。Zhou等人[31]研究了FSW Ti6Al4V織構(gòu)演變過(guò)程,結(jié)果表明:Ti6Al4V鈦合金母材為明顯軋制織構(gòu),F(xiàn)SW后的SZ發(fā)生明顯的動(dòng)態(tài)再結(jié)晶過(guò)程,形成{φ1=30°,φ=62°,φ2=30°}織構(gòu),且高角度晶界比例明顯提升。
Fonda等人[32]研究了近α態(tài)鈦合金FSW接頭的織構(gòu)演變規(guī)律。結(jié)果表明,在低焊速下焊接接頭主要為hcp p1型剪切織構(gòu),在高焊速下主要為bcc D1型剪切織構(gòu),hcp p1型織構(gòu)強(qiáng)度高于bcc D1型。究其原因是在高焊速下,熱量在焊縫處停留時(shí)間短,攪拌頭到焊縫剪切變形傳輸過(guò)程比較慢,所以產(chǎn)生了織構(gòu)強(qiáng)度稍低的bcc D1型織構(gòu)。
Liu等人[33]對(duì)于工業(yè)純鈦FSW接頭的微觀織構(gòu),如圖3所示。
圖3 EBSD測(cè)試結(jié)果[38]Fig.3 The results of EBSD[38]
結(jié)果表明,BM為明顯的軋制織構(gòu),SZ區(qū)為P1{10}<10>型剪切織構(gòu)。從前進(jìn)側(cè)(Advancing Side,AS)到后退側(cè)(Retreating Side,RS),P1型織構(gòu)發(fā)生了轉(zhuǎn)動(dòng),AS側(cè)的[0001]α織構(gòu)與焊接方向垂直,而RS側(cè)的α織構(gòu)與焊接方向平行,這與Zhou[30]和Yoon等人[34]的研究結(jié)果一致。而在鎂合金中,從AS到RS的[0001]α織構(gòu)并沒(méi)有發(fā)生轉(zhuǎn)動(dòng)[35-36],這表明織構(gòu)不僅與晶體結(jié)構(gòu)有關(guān),還與FSW過(guò)程中材料的塑性變形有重要關(guān)聯(lián)。再者,F(xiàn)SW接頭高角度晶界比例明顯提升,在焊接過(guò)程中伴隨著亞晶的轉(zhuǎn)動(dòng),晶粒發(fā)生了動(dòng)態(tài)再結(jié)晶,晶粒顯著細(xì)化。組織演變一方面與應(yīng)變和不連續(xù)動(dòng)態(tài)再結(jié)晶有關(guān),另一方面與織構(gòu)誘導(dǎo)的晶粒匯聚有關(guān)[37-38]。
力學(xué)性能是衡量FSW接頭質(zhì)量的關(guān)鍵指標(biāo),目前針對(duì)FSW鈦合金力學(xué)性能的研究主要包括顯微硬度、拉伸性能,針對(duì)疲勞性能的研究相對(duì)較少。
Fujii等人[39]研究了工業(yè)純鈦FSW后SZ區(qū)晶粒尺寸、顯微硬度和拉伸性能的關(guān)系,如表1所示。隨著焊速的提升,SZ區(qū)晶粒尺寸呈減小趨勢(shì),顯微硬度值呈增大趨勢(shì)。同時(shí),隨著焊速的增大,抗拉強(qiáng)度(Ultimate Tensile Strength,UTS)先增大后減?。ㄒ?jiàn)圖4),當(dāng)焊速為300 mm/min時(shí),雖然晶粒得到細(xì)化,但熱輸入量的降低使塑性變形不充分,導(dǎo)致UTS減小。
Zhang等人[22]對(duì)Ti6Al4V FSW接頭的力學(xué)性能進(jìn)行了評(píng)價(jià),如圖5所示。拉伸試驗(yàn)分為兩部分,一種拉伸試樣平行段包含整個(gè)焊縫,另一種拉伸試樣平行段只包含SZ區(qū)。結(jié)果表明:第一種拉伸試樣的UTS、YS(屈服強(qiáng)度)都達(dá)到BM的95%以上,伸長(zhǎng)率可達(dá)55%。隨著轉(zhuǎn)速的增大,α相和β相尺寸增大,完全片層狀組織使得強(qiáng)度和伸長(zhǎng)率略有降低。第二種拉伸試樣由于SZ為細(xì)小的α相,相比BM, UTS與屈服強(qiáng)度增約15%,伸長(zhǎng)率增約42%。Liu等人[24,40-41]對(duì)TC4 FSW接頭力學(xué)性能分析表明,F(xiàn)SW接頭UTS均能達(dá)到BM的92%以上,斷裂位置位于SZ區(qū),這是因?yàn)樵贔SW過(guò)程中經(jīng)歷了回復(fù)與再結(jié)晶,材料發(fā)生軟化。
圖4 不同參數(shù)條件下焊接接頭的抗拉強(qiáng)度[39]Fig.4 Tensile strength of the samples at different welding speeds
表1 不同焊接條件Ti6Al4V焊接接頭的晶粒尺寸與硬度關(guān)系表[44]Table 1 Summary of the grain size and hardness of the samples welded under different conditions
圖5 不同轉(zhuǎn)速下的拉伸性能[27]Fig.5 Effect of rotational speed on tensile properties[27]
Edwards等人[23,42-43]對(duì)Ti6Al4V FSW的研究結(jié)果表明,F(xiàn)SW后接頭疲勞性能相比BM降低了50%,對(duì)焊縫進(jìn)行熱處理后相比BM提升了約20%,同時(shí)UTS、YS均得到提升,故對(duì)于鈦合金FSW接頭,選擇合理的焊后熱處理能夠提升接頭的疲勞壽命。
分析總結(jié)目前關(guān)于鈦合金FSW力學(xué)性能的相關(guān)文獻(xiàn),鈦合金FSW力學(xué)性能呈現(xiàn)以下特點(diǎn):(1)鈦合金FSW后力學(xué)性能指標(biāo)良好,UTS最高能達(dá)到BM的99%以上;(2)在焊縫無(wú)缺陷狀態(tài)下,通過(guò)合理的熱處理制度,UTS、YS可進(jìn)一步提高;(3)FSW后晶粒得到細(xì)化,高角度晶界比例增加,有利于材料強(qiáng)塑性的結(jié)合??傊?,鈦合金FSW后的力學(xué)性能與多種因素有關(guān),包括晶粒大小,α與β相含量、尺寸及分布,晶粒取向的變化。
鈦合金FSW的開(kāi)發(fā)和應(yīng)用剛剛起步,具有廣闊的應(yīng)用前景。對(duì)鈦合金FSW的研究應(yīng)從以下方面深入研究:(1)鈦合金FSW組織演變機(jī)制及控制;(2)鈦合金FSW目前主要集中于薄板的研究,應(yīng)實(shí)現(xiàn)不同板厚的連接,滿足實(shí)際工業(yè)化應(yīng)用;(3)對(duì)于鈦合金FSW的力學(xué)性能,除考慮到晶粒尺寸與相變外,還要深入研究織構(gòu)對(duì)接頭力學(xué)性能的影響,同時(shí)開(kāi)展超塑性變形機(jī)制的研究,以擴(kuò)大鈦合金作為結(jié)構(gòu)件的應(yīng)用范圍;(4)開(kāi)展FSW接頭腐蝕性能研究,滿足在腐蝕工況條件下的應(yīng)用。
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Latest research progress of friction stir welding of titanium alloys
YUAN Jie,WANG Kuaishe,ZHAO Kai,WANG Wen
(College of Metallurgy Engineering,Xi'an University of Architecture and Technology,Xi'an 710055,China)
Titanium alloys with some advantages of low density,high specific strength,good corrosion resistance and excellent processing performance are mainly used in the aerospace,transportation,petrochemical and other fields.When using titanium alloys as structural materials in different fields,the traditional fusion welding method will produce a larger residual stress,microstructure coarsening,large deformation,cracks,pores and other defects,while the friction stir welding(FSW)technology can avoid these defects,thus sharply improves the quality of the welded joints of titanium alloys.In addition,the FSW technology of the titanium alloy has become a research focus.In this paper,the theory and technology characteristics of FSW are introduced,and the latest research progress of the welded joints in FSW for titanium alloys at home and abroad is also introduced from the aspects of the macrostructure,microstructure(grain size,texture)and mechanical properties.And the further trends of FSW of titanium alloys are discussed.
titanium alloys;friction stir welding;microstructure;mechanical properties
TG457.19
C
1001-2303(2017)05-0067-06
10.7512/j.issn.1001-2303.2017.05.14
2017-02-10;
2017-02-23
國(guó)家自然科學(xué)基金(U1360105);國(guó)家自然科學(xué)基金(51574192)
袁 潔(1991—),女,在讀碩士,主要從事攪拌摩擦焊接的研究工作,E-mail:yuanjxauat@126.com。
本文參考文獻(xiàn)引用格式:袁潔,王快社,趙凱,等.鈦合金攪拌摩擦焊接最新研究進(jìn)展[J].電焊機(jī),2017,47(05):67-72.