金屬半固態(tài)成形技術(shù),是一種介于鑄造和塑性成形之間的近凈成形技術(shù)[1,2] 目前,金屬半固態(tài)成形研究主要集中在低熔點(diǎn)的鋁、鎂合金[3~6]且較為成熟,已經(jīng)開始工業(yè)應(yīng)用 高熔點(diǎn)合金半固態(tài)成形研究,主要集中在鋼和鈦合金[7~9]

應(yīng)變誘導(dǎo)熔化激活法(SIMA法)是一種用固相法制備半固態(tài)坯料的方法,適用于高熔點(diǎn)合金 在用SIMA法制備半固態(tài)坯料的過程中,等溫時(shí)間是影響半固態(tài)坯料組織的重要工藝參數(shù)之一 合理的等溫時(shí)間能優(yōu)化合金的半固態(tài)組織,進(jìn)而提高其性能 Cao等[10]提出用旋鍛應(yīng)變誘導(dǎo)融化激活法(RSSIMA法)制備半固態(tài)銅合金并用LSW方程描述了球狀晶粒尺寸的演變,發(fā)現(xiàn)其力學(xué)性能比鑄態(tài)銅合金高；對(duì)用RSSIMA法制備的C3771半固態(tài)銅閥力學(xué)性能的研究結(jié)果表明,銅閥螺紋處的失效扭矩更高,具有更好的韌性[11] 王佳等[12]研究了用冷軋-重熔SIMA法制備的ZCuSn10合金半固態(tài)坯料,發(fā)現(xiàn)在預(yù)變形量為19.7%、在875℃等溫處理15 min時(shí),其組織最優(yōu) 制備半固態(tài)坯料是半固態(tài)成形的關(guān)鍵一步,坯料質(zhì)量的優(yōu)劣對(duì)零件的性能有極大的影響 Jiang等[13]研究了用新SIMA法制備的半固態(tài)坯料并觸變擠壓AZ61鎂合金零件的組織和性能 結(jié)果表明,隨著等溫溫度的提高和等溫時(shí)間的延長(zhǎng),成形零件的抗拉強(qiáng)度和伸長(zhǎng)率先提高后降低 Cao等[14]研究了用預(yù)變形對(duì)SIMA法制備C5191合金組織和力學(xué)性能的影響 結(jié)果表明,隨著變形量的增加合金的晶粒球化更快、尺寸更細(xì)小,硬度更高 Sankara Rao等[15]研究了用SIMA法制備的Al-10Cu-Fe合金的組織和摩擦學(xué)特性 結(jié)果表明,隨著等溫時(shí)間的延長(zhǎng)平均晶粒尺寸逐漸增大,預(yù)變形50%、在580℃等溫處理30 min后再進(jìn)行臨界退火的Al-10Cu-Fe合金,其磨損性能最優(yōu) Chen等[16]用再結(jié)晶重熔法制備2024鋁合金半固態(tài)坯料,研究了在間接觸變成形過程中顯微組織的演變 結(jié)果表明,對(duì)商用擠壓態(tài)2024鋁合金進(jìn)行二次重熔處理,可得到細(xì)小球晶組織的半固態(tài)坯料 Li等[17]用封閉式冷卻通道(ECSC)制備半固態(tài)漿料并擠壓鑄造CuSn10P1合金 結(jié)果表明,其顯微組織從鑄態(tài)的粗枝晶轉(zhuǎn)變?yōu)榘牍虘B(tài)的等軸晶 ECSC工藝的高冷卻速率和半固態(tài)漿料等溫處理抑制了錫擴(kuò)散,從而誘導(dǎo)了亞穩(wěn)相β'-Cu13.7Sn的形成 本文用CRITSIMA法制備半固態(tài)錫青銅坯料并將其擠壓得到錫青銅軸套,研究等溫時(shí)間對(duì)觸變擠壓錫青銅軸套微觀組織、物相及力學(xué)性能的影響

1 實(shí)驗(yàn)方法

實(shí)驗(yàn)用材料ZCuSn10P1錫青銅的化學(xué)成分(質(zhì)量分?jǐn)?shù),%)：Cu 88.45、Sn 9.82、P 0.94,其他元素的含量為0.79 使用STA449F3同步熱分析儀測(cè)出該合金的固、液相線溫度分別為876.1℃和1024℃

先用冷軋對(duì)ZCuSn10P1錫青銅進(jìn)行預(yù)變形處理,冷軋前長(zhǎng)方體坯料試樣的尺寸為25 mm×25 mm×100 mm,用兩輥軋機(jī)四道次冷軋后累積變形量為20% 將冷軋坯料放入溫度為910℃的井式電阻爐中進(jìn)行等溫處理,等溫時(shí)間分別為10、15、20、25 min 最后將半固態(tài)錫青銅坯料放入模具內(nèi)用四柱液壓機(jī)進(jìn)行擠壓,擠壓力為50 T,擠壓速率為15 mm/s,保壓時(shí)間為10 s 擠壓后取出水淬,得到觸變擠壓錫青銅軸套零件 模具的結(jié)構(gòu)示意圖如圖1所示

圖1



圖1觸變成形模具的示意圖

Fig.1Schematic diagram of thixoforming die. 1-upper pattern plate, 2-Punch fixing plate, 3-Punch, 4-Guide post, 5-Concave die, 6-Ejector rod, 7-Ejector rod fixing plate, 8-Push board, 9-Down pattern plate

用線切割機(jī)在軸套縱向中間位置截取金相式樣,將橫截面作為拍攝面 依次用400#、600#、800#、1000#目的水磨砂紙逐步打磨,然后用細(xì)度為2.5 μm的研磨膏在拋光機(jī)上將其拋光 待樣品表面沒有明顯的劃痕后用5%FeCl3溶液對(duì)樣品表面腐蝕3 s 然后迅速用酒精沖洗,最后用吹風(fēng)機(jī)將樣品的表面干燥

使用Nikon MA200光學(xué)顯微鏡觀察不同等溫時(shí)間觸變擠壓錫青銅軸套的金相組織并隨機(jī)拍攝100倍金相圖,使用Image Pro-Plus軟件計(jì)算平均晶粒尺寸、形狀因子(越接近1,晶粒圓整度越好),取多點(diǎn)測(cè)量結(jié)果的平均值[18] 使用CMT300萬能材料實(shí)驗(yàn)機(jī)進(jìn)行室溫拉伸,拉伸速率2 mm/min,每個(gè)試樣做3次,取其結(jié)果的平均值 拉伸試樣的取樣位置和尺寸,如圖2所示 使用HBE-3000A型電子布氏硬度計(jì)測(cè)試試樣的硬度,壓頭的直徑為10 mm,保壓時(shí)間為30 s,載荷為62.5 N,每個(gè)試樣做3次取其結(jié)果的平均值 用ZEISS EVO18掃描電鏡分析錫青銅形貌和拉伸斷口 用EMPYREAN型X射線衍射儀分析錫青銅的物相

圖2



圖2拉伸試樣的尺寸和取樣位置

Fig.2Sampling position and size of tensile sample

2 結(jié)果和討論2.1 錫青銅軸套的微觀組織

圖3給出了在910℃分別等溫處理10、15、20、25 min后觸變擠壓錫青銅軸套的金相組織 可以看出,整個(gè)組織由晶間液相和近球狀固相組成 液相為(α+δ+Cu3P)共析相,固相為固溶了Sn元素的α-Cu有限固溶體 由圖3a可見,固液兩相分布均勻,小晶粒的圓整度較好,大晶粒的圓整度不一 在圖中可見部分異常長(zhǎng)大的晶粒和“8”字形晶粒（圖中白圈所示）,其原因是在等溫處理初期距離較近且取向一致的晶粒發(fā)生合并長(zhǎng)大,且隨著等溫時(shí)間的延長(zhǎng)合并長(zhǎng)大的晶粒逐漸球化 由圖3b可以看出,晶粒尺寸明顯增大,形狀更加規(guī)則,圓整度更高 其原因是,隨著等溫時(shí)間的延長(zhǎng)在Ostwald熟化機(jī)制作用下大晶粒長(zhǎng)大而小晶粒熔化,熔化變小的晶粒沉積到大晶粒上促進(jìn)了晶粒的長(zhǎng)大和球化 同時(shí),更多晶界處的(α+δ+Cu3P)相熔化而使液膜增厚,分割晶粒并且增強(qiáng)潤(rùn)濕晶界的作用,晶界處枝晶熔斷促進(jìn)了晶粒的球化 由圖3c可見,液相率進(jìn)一步提高而固液均勻性下降,晶粒尺寸繼續(xù)增大而圓整度降低 晶粒大多呈現(xiàn)橢球狀并在晶界處出現(xiàn)枝晶組織,這是球化過程中熔斷的枝晶凝結(jié)生成的二次枝晶 由圖3d可見,組織粗化嚴(yán)重而圓整度進(jìn)一步降低,且出現(xiàn)了“U型”晶粒 其原因是,晶內(nèi)小液滴在界面能降低的驅(qū)動(dòng)下匯集并向晶界遷移,遷移到晶界處生成了“U型”晶粒 因?yàn)槿笨谔幍那瘦^大,在界面能的作用下晶間液相在缺口處不斷向內(nèi)侵蝕,最終將晶粒熔斷

圖3



圖3等溫時(shí)間不同的錫青銅的金相組織

Fig.3Microstructure of tin bronze of different isothermal time (a) 10 min; (b) 15 min; (c) 20 min; (d) 25 min

圖4給出了在910℃等溫處理不同時(shí)間的錫青銅軸套零件的晶粒尺寸和形狀因子 可以看出,隨著等溫時(shí)間的增加平均晶粒尺寸隨之增加而形狀因子先增加后降低,表明晶粒的圓整度先提高后降低 在910℃等溫處理15 min的組織最均勻,形狀因子最大為0.74、平均晶粒尺寸為63.56 μm 適當(dāng)?shù)牡葴靥幚?可提高組織的均勻性和半固態(tài)晶粒的球化效果 但是,等溫時(shí)間過長(zhǎng)反而降低組織的均勻性,使晶粒的粗化嚴(yán)重

圖4



圖4等溫時(shí)間不同的錫青銅的晶粒尺寸和形狀因子

Fig.4Grain size and shape factor of tin bronze of different isothermal time

圖5給出了平均晶粒尺寸D3隨等溫時(shí)間變化的散點(diǎn)圖和線性擬合 圖5是根據(jù)Lifshit、Slyozov、Wagner的Ostwald熟化理論[19]動(dòng)力學(xué)公式(Dt ) n -(D0) n =Kt(式中,Dt 為等溫處理t時(shí)的平均晶粒尺寸,D0為初始平均晶粒尺寸,K為晶粒粗化速率系數(shù),n為粗化經(jīng)驗(yàn)指數(shù))并使用Origin軟件進(jìn)行線性擬合得到的 由圖5可以看出,晶粒的長(zhǎng)大速度與等溫時(shí)間之間符合動(dòng)力學(xué)方程,擬合度R2為0.93676,粗化速率系數(shù)K為296 μm3/s

圖5



圖5錫青銅的平均晶粒尺寸D3與等溫時(shí)間關(guān)系的散點(diǎn)圖和線性擬合

Fig.5Scatter plot and linear fitting plot of average grain size D3 of tin bronze of isothermal time

圖6給出了在910℃等溫15 min后觸變擠壓錫青銅軸套零件不同位置的橫截面金相組織 由圖6可以看出,三個(gè)位置的晶粒尺寸和圓整度沒有顯著的差異,且固液兩相的均勻性較高 其原因是,在半固態(tài)合金觸變擠壓充型時(shí)晶間液相的潤(rùn)滑作用和固液兩相較大的協(xié)同流動(dòng)能力,使不同位置固液兩相均勻分布 從零件的頂部到底部液相逐漸增多,晶粒之間的間隙逐漸增大 其原因是,工藝的限制及固液兩相流動(dòng)特性不同使固液兩相分離 由模具的結(jié)構(gòu)可知,充型時(shí)合金流動(dòng)的前表面在擠壓力相反方向上不受到力的限制 這使得與頂面(接觸凸模一側(cè))之間出現(xiàn)較大的壓力梯度[20,21],使流動(dòng)性較好的液相優(yōu)先向型腔底部流動(dòng) 同時(shí),固相在頂部的聚集和粘結(jié)增大了與型腔內(nèi)壁的摩擦力,流動(dòng)阻力增大使零件頂部到低部的液相遞增

圖6



圖6在觸變擠壓錫青銅不同位置取樣的金相組織

Fig.6Microstructure of thixo-extruded tin bronze at different positions (a) top, (b) mid, (c) down

2.2 錫青銅軸套組織中元素的分布和物相

在910℃等溫處理15 min的觸變擠壓錫青銅軸套零件的XRD譜圖和SEM照片,分別在圖7和圖8中給出 由7圖可見,物相由α-Cu相、δ相(Cu41Sn11)及Cu3P相組成 由圖8可見,Cu元素的分布較為均勻,但是在固相中稍多 Sn元素在液相中的分布更為集中,出現(xiàn)了明顯的偏析 由XRD譜可見,液相中的Sn元素主要以δ相形式存在 P元素集中分布在晶間液相和固相邊界處,以Cu3P的形式存在

圖7



圖7觸變擠壓錫青銅的XRD譜

Fig.7XRD diffraction pattern of thixo-extruded tin bronze

圖8



圖8觸變擠壓錫青銅的面掃描圖

Fig.8Surface scan of thixo-extruded tin bronze

表1列出了由點(diǎn)掃描得到的固液兩相中三種元素的含量 由表1可見,點(diǎn)1處Cu元素的質(zhì)量分?jǐn)?shù)為93.84%,因?yàn)楣滔嘀饕獮棣?Cu有限固溶體 為了進(jìn)一步分析液相中的物相組成,需要定量分析液相內(nèi)的元素含量

Table 1

表1

表1觸變擠壓錫青銅中Cu、Sn和P 元素的含量

Table 1Cu, Sn, P element content in thixo-extruded tin bronze (mass fraction, %)

Element	Cu	Sn	P
Point1	93.84	4.23	0.14
Point2	70.90	25.45	0.20

根據(jù)公式

Ma=AmaAaAmaAa+AmbAb

(1)

Pa=MaMb

(2)

計(jì)算出Cu41Sn11中Sn元素的質(zhì)量分?jǐn)?shù)為33.40%,Sn、Cu的質(zhì)量比為1∶2(0.5) Cu3P中P元素的質(zhì)量分?jǐn)?shù)為13.97%,P、Cu質(zhì)量比為4∶25 式1中,Ma為組元a的質(zhì)量分?jǐn)?shù),Ama、Aa和Amb、Ab分別為組元a和組元b的相對(duì)原子質(zhì)量及原子比例 式2中,Pa為組元a與組元b的質(zhì)量分?jǐn)?shù)之比(以下簡(jiǎn)稱質(zhì)量比)

將液相中Cu的質(zhì)量分?jǐn)?shù)除以Cu3P中Cu的質(zhì)量分?jǐn)?shù)1.25%(由P、Cu質(zhì)量比及液相中P%得到),得到液相中Sn、Cu的質(zhì)量比為0.37 這個(gè)結(jié)果與Cu41Sn11中Sn、Cu質(zhì)量比0.5不符,可見液相中除了Cu41Sn11相及Cu3P相,還有一部分Cu元素以α-Cu相形式存在,表明液相是(α+δ+Cu3P)三元共析組織

2.3 等溫時(shí)間對(duì)錫青銅軸套布氏硬度的影響

圖9給出了在910℃等溫處理不同時(shí)間錫青銅軸套的布氏硬度 由圖9可見,隨著等溫時(shí)間由10 min延長(zhǎng)到15 min,布氏硬度由123 HBW提高到126 HBW,提高了2.4% 其原因是,等溫時(shí)間的延長(zhǎng)使晶粒更加圓整和細(xì)小,使固、液兩相的分布更加均勻(圖3b) 晶粒越圓整細(xì)小則晶界越多,對(duì)位錯(cuò)滑移的阻滯效應(yīng)越明顯,則晶粒的塑性變形可在更多的晶粒內(nèi)進(jìn)行,從而提高了零件的硬度 同時(shí),等溫時(shí)間的延長(zhǎng)有助于半固態(tài)組織中液相內(nèi)富集的Sn、P元素向初生α-Cu中擴(kuò)散,產(chǎn)生的固溶強(qiáng)化作用使更多的晶格發(fā)生畸變,增大了位錯(cuò)運(yùn)動(dòng)的阻力而使合金的硬度提高 等溫時(shí)間延長(zhǎng)到25 min,使布氏硬度由126 HBW降到118 HBW,降低了6.3% 其原因是,等溫時(shí)間過長(zhǎng)使晶粒在Ostwald熟化機(jī)制的作用下嚴(yán)重粗化(圖3d),且組織的均勻性顯著下降,使承擔(dān)塑性變形的晶粒減少,減弱了對(duì)位錯(cuò)滑移的阻滯效應(yīng),降低了固溶強(qiáng)化效果,使硬度降低

圖9



圖9等溫時(shí)間不同的錫青銅的布氏硬度

Fig.9Brinell hardness of tin bronze of different isothermal time

2.4 等溫時(shí)間對(duì)錫青銅軸套拉伸性能的影響

圖10給出了在910℃等溫不同時(shí)間錫青銅的抗拉強(qiáng)度和延伸率 可以看出,隨著等溫時(shí)間的延長(zhǎng)延伸率先提高后下降,抗拉強(qiáng)度在等溫15 min后顯著下降 等溫時(shí)間由10 min延長(zhǎng)到15 min,使抗拉強(qiáng)度由377 MPa下降到368 MPa,降低了2.4%,延伸率由2.5%提高到4.5%,提高了80% 此時(shí)固、液兩相分布均勻,晶粒有一定程度的長(zhǎng)大,球化效果最好(圖3b),更多的晶粒平均分配塑性變形使零件的塑性提高 同時(shí),Sn元素在Cu基體中的固溶強(qiáng)化使基體的強(qiáng)度提高,即在保證強(qiáng)度的同時(shí)塑性有較大幅度的提高 等溫時(shí)間由15 min延長(zhǎng)到20 min使抗拉強(qiáng)度由368 MPa下降到322 MPa,降低了12.5%,延伸率由4.5%提高到6.5%,提高了44.4% 隨著等溫時(shí)間的繼續(xù)延長(zhǎng)使固相晶粒進(jìn)一步長(zhǎng)大,晶界減少(圖3c),對(duì)位錯(cuò)滑移的阻礙大幅度降低和位錯(cuò)纏結(jié)降低,使零件的拉伸強(qiáng)度大幅降低和塑性提高 等溫時(shí)間延長(zhǎng)到25 min使抗拉強(qiáng)度進(jìn)一步下降到283 MPa,延伸率轉(zhuǎn)而下降到4.0% 其原因是,長(zhǎng)時(shí)間保溫使半固態(tài)組織發(fā)生粗化、晶粒尺寸不均勻,降低了軸套的強(qiáng)度和塑性

圖10



圖10等溫時(shí)間不同的錫青銅試樣的拉伸性能

Fig.10Tensile properties of tin bronze of different isothermal time

圖11給出了在910℃等溫不同時(shí)間錫青銅拉伸試樣斷口的形貌 由圖11a可以看出,斷口較為光滑平整,有少量的河流花樣但是沒有韌窩,斷裂方式應(yīng)為脆性斷裂 由圖11b可以看出,河流花樣的數(shù)量明顯增多并出現(xiàn)解理平臺(tái),斷裂方式應(yīng)為解理斷裂 在圖11c中可見韌窩及撕裂棱和解理平臺(tái),斷裂方式為韌性斷裂和解理斷裂的混合型斷裂,此時(shí)韌性斷裂的占比較高 在圖11d中可見光滑平整的斷裂面和解理平臺(tái),斷口韌窩變淺,斷裂方式仍為混合型斷裂,此時(shí)解理斷裂的占比較高

圖11



圖11等溫時(shí)間不同的錫青銅拉伸斷口的形貌

Fig.11Tensile fracture morphology of tin bronze at different isothermal times: (a) 10 min; (b) 15 min; (c) 20 min; (d) 25 min

3 結(jié)論

(1) 采用冷軋-等溫SIMA法和擠壓可制備出組織較好的錫青銅軸套 隨著等溫時(shí)間的延長(zhǎng)觸變擠壓錫青銅軸套的平均晶粒直徑增大,晶粒粗化速率為296 μm3/s；晶粒的形狀因子先增大后減小,圓整度先提高后降低

(2) 觸變擠壓錫青銅的物相由α相、δ相(Cu41Sn11)及Cu3P相組成 在910℃等溫15 min的觸變擠壓錫青銅軸套其組織最好,晶粒細(xì)小、圓整,平均晶粒尺寸為63.56 μm,形狀因子最大為0.74

(3) 隨著等溫時(shí)間的延長(zhǎng)觸變擠壓錫青銅軸套的抗拉強(qiáng)度降低,布氏硬度和延伸率先提高后降低 在910℃等溫15 min的錫青銅軸套綜合性能最優(yōu),抗拉強(qiáng)度為368 MPa,布氏硬度126 HBW,延伸率為4.5%

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Semi-solid billet of ZCuSn10 alloy is prepared by strain induced melt activation (SIMA) method which included the rolling and remelting process. Firstly, ZCuSn10 alloy is cast, and samples are cut from ingot casting. Secondly, the samples are rolled with 2~4 passes after holding at 450 ℃ for 15 min, then the new samples are cut from deformed alloy. Lastly, the new samples are reheated up to 850 ℃ or 875 ℃ for 15 min, then water quenching. Semi-solid microstructure is observed and compared with microstructure of ZCuSn10 alloy directly reheated after casting. The distribution of Sn element in microstructure under different conditions is measured by using EDS function of SEM, and the microstructure changes during the SIMA process are observed by means of OM and TEM. Based on the experiments, the microstructure evolution is synthetically analyzed and explained during the course of semi-solid billet of ZCuSn10 alloy prepared by SIMA method. The results indicate that semi-solid microstructure of ZCuSn10 alloy by rolling- remelting SIMA process is equal-fine grain, and spheroidization of solid particle is well. The optimum semi-solid microstructure is obtained when alloy deformed 19.7% is remelted at 875 ℃ for 15 min, the average grain diameter is 75.8 μm, shape factor is 1.62, and volume fraction of liquid phase is 17.28%. Deformation process plays a crucial role in grain refinement and spheroidization during SIMA process for preparing the semi-solid billet of ZCuSn10 alloy, as deformation and remelting temperature increases, the size and shape of solid phase in semi solid microstructure are smaller and more round, volume fraction of liquid phase increases. The main mechanism of SIMA process preparing semi-solid billet of ZCuSn10 alloy is that predeformation breaks dendrites and stores energy of deformation into dendrites, and promotes dendrites melting through remelting process. Meanwhile, liquid phase occupies sharp corners of solid particles by Sn element diffusing from liquid phase into α solid phase, so that fine, uniform and roundness α solid particles are gained.

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" />

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1

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