Al-Zn-Mg-Cu(7000系)高強(qiáng)鋁合金具有高比強(qiáng)度、比剛度和良好的耐腐蝕性能，在航空航天、船舶制造、軌道交通等領(lǐng)域得到了廣泛的應(yīng)用[1,2] 添加稀土元素Sc對(duì)鋁合金有顯著的改性作用，使其具有高強(qiáng)度、高耐熱性和良好的抗蝕性[3,4] 在傳統(tǒng)7055、7075等高強(qiáng)鋁合金中加入微量Sc元素的Al-Zn-Mg-Cu-Sc鋁合金可使其具有更加優(yōu)異的綜合力學(xué)性能[5,6]，但是用熔焊焊接高強(qiáng)7000系(包括含Sc)鋁合金時(shí)產(chǎn)生的氣孔和熱裂紋等缺陷使接頭的力學(xué)性能嚴(yán)重降低[7] 攪拌摩擦焊(Friction Stir Welding，F(xiàn)SW)是一種固相連接技術(shù)，特別適于焊接Al-Zn-Mg-Cu-Sc鋁合金 [8]

雖然FSW的熱輸入明顯小于熔焊，但是其接頭仍出現(xiàn)軟化現(xiàn)象 先前的研究表明，提高時(shí)效強(qiáng)化鋁合金接頭力學(xué)性能的三種方式，分別是提高焊接速度[9,10]、采用冷卻介質(zhì)增加FSW過程的冷卻速率[11]和焊后熱處理[12~15] 焊后熱處理是最有效的方式 對(duì)5.4 mm厚2219-T6鋁合金FSW接頭的研究表明[12]，焊后人工時(shí)效熱處理能顯著提高焊速接頭的力學(xué)性能 對(duì)6 mm厚2014-T6鋁合金FSW接頭的研究表明[12]，焊后固溶＋人工時(shí)效(T6)熱處理可將接頭硬度恢復(fù)到母材的水平，但是接頭焊核區(qū)(Nugget zone, NZ)的晶粒發(fā)生了異常長(zhǎng)大[13] Robert等[14]對(duì)2519-0.36Sc鋁合金進(jìn)行FSW，發(fā)現(xiàn)焊后T6熱處理使接頭的整體硬度明顯提高，但是NZ區(qū)的晶粒異常長(zhǎng)大，且在NZ與熱機(jī)影響區(qū)(Thermal-mechanically affected zone, TMAZ)的界面出現(xiàn)了孔洞，使接頭的疲勞性能大幅降低 以上研究表明，焊后熱處理能提高時(shí)效強(qiáng)化鋁合金FSW接頭的硬度，但是T6熱處理使某些合金接頭NZ出現(xiàn)晶粒異常長(zhǎng)大和孔洞 在焊后T6熱處理過程中FSW接頭的晶粒是否異常長(zhǎng)大，與接頭的原始晶粒尺寸有密切的關(guān)系[15]，而工藝參數(shù)是影響FSW接頭晶粒尺寸的關(guān)鍵因素 Sc能抑制晶粒長(zhǎng)大 基于此，本文對(duì)11 mm厚7055-0.1Sc-T4鋁合金進(jìn)行FSW，研究焊后人工時(shí)效和T6熱處理對(duì)接頭的組織和性能的影響

1 實(shí)驗(yàn)方法

實(shí)驗(yàn)用母材(Base material, BM)為11 mm厚的7055-0.1Sc-T4軋制板材，其化學(xué)成分和力學(xué)性能分別列于表1和表2

Table 1

表1

表17055-0.1Sc-T4軋制板材的化學(xué)成分

Table 1Chemical composition of 7055-0.1Sc-T4 plate (mass fraction, %)

Zn	Mg	Cu	Sc	Zr	Si	Fe	Mn	Al
7.6~8.4	1.8~2.3	2.0~2.6	0.1	0.08~0.25	≤0.1	≤0.15	≤0.05	Bal.

Table 2

表2

表27055-0.1Sc-T4軋制板材的拉伸性能

Table 2Tensile properties of 7055-0.1Sc-T4

Yield Strength / MPa	Tensile Strength / MPa	Elongation / %
595±28	635±36	9±5

進(jìn)行FSW前，用砂紙將試板表面打磨以除去表面的氧化膜，再用酒精清除表面的油污 用FSW-SDLM 50型焊機(jī)對(duì)100 mm×300 mm板材進(jìn)行對(duì)焊，攪拌頭的主軸傾角為2.75°，轉(zhuǎn)速為500 r/min，焊速分別為100 mm/min和250 mm/min，焊后的接頭在空氣中自然冷卻至室溫

將接頭在室溫下放置20 d后，切取試樣分別進(jìn)行焊后人工時(shí)效(120℃×24 h)和T6(470℃×1.5 h+水淬+120℃×24 h)熱處理 分別將焊態(tài)(As welded, AW)接頭、焊后人工時(shí)效態(tài)(Artificial aging, AA)接頭和固溶＋人工時(shí)效態(tài)(T6)接頭簡(jiǎn)稱AW接頭、AA接頭和T6接頭 例如，500-100-AW表示轉(zhuǎn)速為500 r/min，焊速為100 mm/min的焊接態(tài)接頭

在垂直于焊接方向切取金相樣品，將其磨拋后用Keller試劑(2 mL HF+3 mL HCl+5 mL HNO3+190 mL H2O)腐蝕，然后分別用體視顯微鏡和Leica DMI光學(xué)顯微鏡觀察其宏觀和微觀組織

沿焊接樣品板厚的中線，使用Leco LM-247AT型硬度儀測(cè)試樣品的顯微硬度，載荷為300 g，保壓時(shí)間15 s，各點(diǎn)間距為1.5 mm 將TEM試樣預(yù)減薄至約60 μm，然后將其沖成直徑為3 mm的圓片，再將其在30% HNO3＋70% CH3OH溶液中雙噴減薄，用液氮冷卻后使用JEOL JEM-2100F型透射電鏡(TEM)觀察和分析接頭各區(qū)域的第二相特征，加速電壓為200 kV

在垂直于焊接方向切取室溫拉伸樣品，其尺寸在圖1中給出 用INSTRON 5982型拉伸機(jī)測(cè)試?yán)煨阅?，?yīng)變速率為1×10-3 s-1，測(cè)試3個(gè)標(biāo)準(zhǔn)試樣，取其結(jié)果的平均值 用Quanta 450型掃描電鏡(SEM) 觀察拉伸后斷口的形貌



圖1拉伸試樣的尺寸

Fig.1Configuration and sizes of tensile specimen

2 結(jié)果和討論2.1 接頭的形貌

圖2給出了各種FSW 7055-0.1Sc-T4接頭橫截面的宏觀形貌 可以看出，焊接態(tài)接頭均沒有出現(xiàn)任何焊接缺陷(圖2a和b)，可分為NZ、TMAZ和熱影響區(qū)(Heat affected zone, HAZ)三個(gè)區(qū)域 焊后T6熱處理沒有改變低焊速接頭500-100-T6的宏觀形貌(圖2c)，但是高焊速接頭500-250-T6底部的顏色發(fā)生明顯的改變(圖2d)且出現(xiàn)了晶粒異常長(zhǎng)大 接頭500-100-T6中心和接頭500-250-T6底部(圖2c和d中黑色箭頭所示)的拋光態(tài)局部放大圖顯示，在兩個(gè)區(qū)域均出現(xiàn)了微小孔洞(圖3a和b)，其位置均位于接頭“S”線上 與AW接頭相比，人工時(shí)效熱處理的FSW接頭宏觀和微觀形貌沒有變化



圖2FSW 7055-0.1Sc-T4焊接態(tài)和T6態(tài)接頭橫截面的宏觀形貌

Fig.2Cross-sectional macrostructure of FSW 7055-0.1Sc-T4 joints (a) 500-100-AW, (b) 500-250-AW, (c) 500-100-T6, (d) 500-250-T6



圖3圖2c, d中黑色箭頭所示位置的放大圖

Fig.3Magnified of zones marked by the black arrows in Fig.2c, d

溫度較低(120℃)的人工時(shí)效熱處理后，F(xiàn)SW接頭的宏觀和微觀形貌與焊接態(tài)接頭的組織基本相同，而FSW鋁合金接頭在焊后T6熱處理過程中晶粒異常長(zhǎng)大[13~15] 鋁合金的晶粒穩(wěn)定性與晶粒尺寸和第二相分布有關(guān)[15~17] 在FSW過程中較高的熱輸入和劇烈塑性變形使NZ發(fā)生動(dòng)態(tài)再結(jié)晶，NZ晶粒為等軸細(xì)小的再結(jié)晶晶粒，此時(shí)晶界處于高自由能狀態(tài)，焊后晶界仍保持較高的能量狀態(tài)，NZ底部晶粒越細(xì)其晶界能越高[18] 本文的研究中，低焊速接頭500-100-AW的晶粒較大 NZ中Al3(Zr, Sc)等難溶顆粒對(duì)晶粒長(zhǎng)大的抑制，使焊后T6熱處理沒有改變低焊速接頭500-100-T6的晶粒形貌 但是，高焊速500-250-AW接頭的晶粒更加細(xì)小，高角晶界能給二次再結(jié)晶極大的驅(qū)動(dòng)力，細(xì)小晶粒與周圍晶粒合并以降低晶界能，達(dá)到更穩(wěn)定的組織狀態(tài)[19]；Al3(Zr, Sc)等相對(duì)晶界的釘扎已不能抑制其晶粒長(zhǎng)大，因此在接頭500-250-T6 NZ底部晶界能最高的位置發(fā)生晶粒異常長(zhǎng)大(圖2d和圖4e)



圖4FSW 7055-0.1Sc-T4焊接態(tài)和T6態(tài)接頭不同位置的微觀組織

Fig.4OM of different regions in the FSW joints (a) BM, (b)~(e) magnified NZ of position B~E in Fig.2

圖5給出了FSW 7055-0.1Sc-T4接頭中的“S”線分布 “S”線源自于板材原始面，由高密度的Al2O3顆粒組成[20] 從圖5可以看出，轉(zhuǎn)速為500 r/min、焊接速度為100 mm/min時(shí)“S”線從NZ的底部開始偏向后退側(cè)，最后呈弧狀向上曲折延伸到接頭上部(圖5a) 焊速由100 mm/min提高到250 mm/min，NZ材料的流動(dòng)弱化[21]，使后退側(cè)更多的金屬材料參與NZ成型，使接頭“S”線的彎曲程度提高且集中在NZ中心區(qū)域(圖5b) 與AW態(tài)相比，T6熱處理的接頭其“S”線形狀不變，但更加直觀、易于觀察(圖5c和d)，“S”線上分布著許多黑點(diǎn) 本研究中的黑點(diǎn)形貌分布與Ren等[22]對(duì)FSW 7075-T651鋁合金接頭的報(bào)道類似 該報(bào)道稱，T6熱處理后接頭沿“S”線開裂，周圍有許多小孔洞 Hu等[23]觀察了T6熱處理后FSW接頭“S”線周圍出現(xiàn)的小孔洞，認(rèn)為FSW接頭的殘余應(yīng)力和氧化物與基體之間線膨脹系數(shù)的差異產(chǎn)生的熱應(yīng)力，是T6熱處理后“S”線周圍產(chǎn)生小孔洞的原因 這可用于解釋本文實(shí)驗(yàn)中T6熱處理后接頭出現(xiàn)孔洞缺陷(圖3a和b)



圖5FSW 7055-0.1Sc-T4焊接態(tài)和T6態(tài)接頭的“S”線分布

Fig.5Pattern of “S” line in FSW 7055-0.1Sc-T4 joints (a) 500-100-AW, (b) 500-250-AW, (c) 500-100-T6, (d) 500-250-T6

2.2 硬度分布

圖6給出了各個(gè)狀態(tài)FSW 7055-0.1Sc-T4接頭橫截面中心線的硬度分布 可以看出，對(duì)于各焊接參數(shù)，AW態(tài)和AA態(tài)接頭的硬度呈“W”型分布，最低硬度區(qū)(Low hardness zone，LHZ)位于HAZ，隨著焊速由100 mm/min提高到250 mm/min，LHZ的硬度隨之提高，位置內(nèi)移(圖6a和b) 焊后人工時(shí)效熱處理明顯提高了500~100和500~250兩種接頭NZ和BM的硬度，但是沒有改變LHZ的硬度 而焊后T6處理使兩個(gè)接頭各區(qū)硬度的提高相同(約195 Hv)，明顯高于BM(T4態(tài))的硬度



圖6FSW 7055-0.1Sc-T4接頭橫截面中心線的顯微硬度分布

Fig.6Microhardness profile of FSW 7055-0.1Sc-T4 joints

沉淀強(qiáng)化鋁合金FSW接頭的硬度分布，主要受各區(qū)域沉淀強(qiáng)化相演變的影響 7000系(Al-Zn-Mg-Cu)鋁合金是可熱處理強(qiáng)化鋁合金，在T6熱處理過程中這種合金的析出相序列為SSSS→GP Zone(I or II)→η'(MgZn2)→η(MgZn2)[9] 在7055鋁合金中添加微量Sc元素，生成的Al3(Zr, Sc)與基體保持良好的共格關(guān)系[4]，彌散分布在基體中有穩(wěn)定晶界和彌散強(qiáng)化的作用 Al-Zn-Mg-Cu-Sc鋁合金在較低的溫度(25~100℃)時(shí)效GP區(qū)和Al3(Zr, Sc)是主要強(qiáng)化相[9]，而在高于120℃[9]的溫度時(shí)效則η'相、η相和Al3(Zr, Sc)相是主要強(qiáng)化相

圖7給出了FSW 7055-0.1Sc-T4接頭BM、NZ、LHZ的典型TEM明場(chǎng)像 BM沉淀相為球狀A(yù)l3(Zr, Sc)和GP區(qū)[24]，對(duì)基體有彌散強(qiáng)化作用(圖7a和b) 在FSW過程中NZ經(jīng)歷了峰值溫度>475℃的熱循環(huán)[25]，使GP區(qū)溶解，板條狀的η相基于Al3(Zr, Sc)顆粒相形核并析出(圖7c)，在后續(xù)自然時(shí)效過程中NZ中的GP區(qū)重新析出[24]，使NZ的硬度略比BM低但是遠(yuǎn)高于LHZ



圖7FSW 7055-0.1Sc-T4焊接態(tài)和T6接頭沉淀相的分布

Fig.7TEM micrographs (a) BF and (b) associated diffraction patterns of BM; (c) NZ and (d) LHZ of 500-100-AW; (e) NZ and (f) LHZ of 500-100-T6

在FSW過程中HAZ經(jīng)歷峰值溫度為360~370℃的熱循環(huán)[26]，使GP區(qū)溶解并析出粗化的η'相和η相(圖7d)，粗化沉淀相破壞了與基體之間的共格關(guān)系[27]，使硬度降低，因此HAZ的硬度最低 470℃×1.5 h固溶處理使500~100接頭各區(qū)的沉淀相重新溶進(jìn)Al基體中，提高了基體中溶質(zhì)原子的濃度、相變驅(qū)動(dòng)力和形核率，在隨后的人工時(shí)效過程中接頭各區(qū)的沉淀相均勻析出細(xì)小且高密度的η'和η相(圖7e和f)，因此T6處理后接頭的硬度分布呈“一”字分布，硬度整體提高

2.3 拉伸性能和斷裂特征

表3列出了各FSW 7055-0.1Sc-T4接頭的拉伸性能 可以看出，F(xiàn)SW各接頭的抗拉強(qiáng)度明顯低于BM；焊后人工時(shí)效熱處理使500~100和500~250接頭的抗拉強(qiáng)度分別小幅度提高了15.9 MPa和9.1 MPa，塑性小幅度降低；而焊后T6處理使500-100-T6和500-250-T6接頭的抗拉強(qiáng)度分別大幅度提高了156.6 MPa和86.5 MPa，其最高強(qiáng)度系數(shù)可達(dá)87.6%，但是接頭幾乎沒有塑性而直接脆斷

Table 3

表3

表3FSW 7055-0.1Sc-T4的接頭熱處理前后的拉伸性能

Table 3Transverse tensile properties of FSW 7055-0.1Sc-T4 joints under AW, AA and T6 states

Number	Sample	Tensile Strength / MPa	Elongation / %	Strength coefficient / %
1	500-100-AW	399.9±1.4	8.9±0.5	63.0%
2	500-250-AW	468.5±7.8	6.3±0.5	73.8%
3	500-100-AA	415.8±2.1	4.3±0.4	65.5%
4	500-250-AA	477.6±3.5	3.8±0.4	75.2%
5	500-100-T6	556.5±1.5	Brittle fracture	87.6%
6	500-250-T6	555.0±2.0	Brittle fracture	87.4%

圖8給出了各FSW 7055-0.1Sc-T4接頭拉伸斷裂的位置，黑色箭頭所指為接頭斷裂時(shí)產(chǎn)生的裂紋 AW態(tài)和AA態(tài)FSW接頭的斷裂位置均位于LHZ，斷裂路徑呈“之”字形，中部路徑與接頭的拉伸方向呈45°夾角，上部和下部路徑與中部垂直(圖8a) 而T6態(tài)FSW接頭的斷裂位置均位于NZ(圖8b和c) 進(jìn)一步觀察可以發(fā)現(xiàn)，500-100-T6接頭斷裂路徑的中下部與“S”線基本重合(圖8b)，500-250-T6接頭的斷裂路徑只在根部與少量“S”線重合(圖8c)



圖8FSW 7055-0.1Sc-T4焊接態(tài)和T6接頭的斷裂位置

Fig.8Fracture locations of FSW 7055-0.1Sc-T4 joints (a) 500-100-AW, (b) 500-100-T6, (c) 500-250-T6

焊后T6熱處理后，接頭都在NZ斷裂 低焊速的T6態(tài)接頭“S”線上的微孔洞(圖3a)在拉伸過程中成為裂紋源且沿著與基體結(jié)合強(qiáng)度較弱的“S”線擴(kuò)展[23]，因此500-100-T6接頭的斷裂路徑與“S”線部分重合 高焊速的500-250-T6接頭根部的斷裂路徑(圖8c中紅框位置)與圖3b中的根部“S”線對(duì)應(yīng)，表明該位置為接頭拉伸時(shí)先斷裂的部位 與低焊速接頭相比，高焊速接頭的“S”線分布更加彎曲，使500-250-T6接頭的斷裂路徑只有少部分與“S” 線重合[22]

圖9給出了FSW 7055-0.1Sc-T4 接頭的典型斷口形貌 從圖9可見，500-100-AW接頭斷口的宏觀形貌較為平坦(圖9a)，而500-100-T6接頭的宏觀斷口上、下明顯不同 圖9b中左側(cè)黑色箭頭一側(cè)為接頭下部，呈臺(tái)階狀，而上部凹凸不平 500-100-AW斷口中心位置C放大后的微觀形貌表明，其斷裂形式主要是穿晶斷裂，可觀察到明顯的韌窩組織，韌窩底部有沉淀相顆粒，是典型的鋁合金斷口形貌(圖9c) 在500-100-T6接頭的拉伸過程中，其NZ中部“S”線上的孔洞成為裂紋源，裂紋沿“S”線向接頭上下擴(kuò)展 因此，位置D和E放大后的微觀形貌為解理斷裂，沒有明顯的韌窩組織特征，為典型的脆性斷裂特征(圖9d和e) 需要強(qiáng)調(diào)的是，接頭500-250-AW、500-100-AA和500-250-AA與接頭500-100-AW斷口的形貌類似，接頭500-250-T6與接頭500-100-T6的斷口形貌類似



圖9FSW 7055-0.1Sc-T4焊接態(tài)和T6接頭斷口的形貌

Fig.9Fracture morphologies of FSW 7055-0.1Sc-T4 joints, macrographic of (a) 500-100-AW, (b) 500-250-T6; magnified micrographs of positions C-E: (c) position C, (d) position D, (e) position E

3 結(jié)論

(1) 在轉(zhuǎn)速為500 r/min、焊接速度為100~250 mm/min條件下，7055-0.1Sc-T4的FSW接頭沒有焊接缺陷，其最低硬度區(qū)位于熱影響區(qū)

(2) 焊后人工時(shí)效熱處理使焊核區(qū)和母材區(qū)域的硬度提高，但是低硬度區(qū)的硬度沒有明顯變化，接頭的抗拉強(qiáng)度小幅提高而塑性降低

(3) T6熱處理后低焊速接頭焊核區(qū)的晶粒沒有異常長(zhǎng)大，但是高焊速接頭焊核區(qū)底部的晶粒異常長(zhǎng)大；T6熱處理將FSW 7055-0.1Sc-T4接頭各區(qū)的硬度提高到均一水平，使其強(qiáng)度大幅提高、強(qiáng)度系數(shù)提高到87.6%，但是其塑性極低

(4) 焊接態(tài)“S”線不影響FSW 7055-0.1Sc-T4接頭的拉伸性能，但是T6熱處理使接頭出現(xiàn)沿“S”線分布的孔洞，拉伸時(shí)接頭沿“S”線脆斷

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1

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