面向航空高精度管材制備的冷變形-退火熱處理聯(lián)動(dòng)工藝調(diào)控TC4鈦合金微觀缺陷及綜合力學(xué)性能優(yōu)化研究

TC4鈦合金的名義成分為Ti-6Al-4V，是1954年由美國研制的一種α＋β型雙相鈦合金，兼具α型及β型鈦合金的優(yōu)點(diǎn)，具有高強(qiáng)度、耐腐蝕、耐高溫、抗氧化等特點(diǎn)，是全球應(yīng)用最為廣泛的鈦合金［1-3］ 。尤其在航空航天方面，如飛機(jī)機(jī)身所用的鈦合金，TC4占比可達(dá)70％～90％。

TC4鈦合金組織中的α?Ti為密排六方結(jié)構(gòu)，滑移系較少，冷變形相對困難，加上TC4合金的高比強(qiáng)度，導(dǎo)致該合金的冷變形能力較差，在冷軋制管過程中易發(fā)生表面開裂，極大增加了TC4鈦合金無縫管冷軋制備的難度。目前國內(nèi)TC4管材通常以熱擠壓或斜軋穿孔等方法制備管坯［4-6］ ，后續(xù)多采用溫軋方式生產(chǎn)TC4鈦合金管材制品［7］ ，但溫軋需在軋管機(jī)上加裝感應(yīng)加熱裝置，這種加工設(shè)備結(jié)構(gòu)復(fù)雜、工藝繁瑣、生產(chǎn)成本高。冷軋適合制備表面質(zhì)量和尺寸精度要求高的管材，能得到較好的成形效果和加工率，但對材料的塑性有較高的要求。李凱玥［8］研究了冷軋工藝對TC4鈦合金薄壁管材組織性能的影響，指出冷軋道次和道次間變形量對TC4軋制管材的表面質(zhì)量和力學(xué)性能有顯著影響。王國迪等［9］研究表明，TC4鈦合金冷變形量會(huì)直接影響合金的微觀組織和退火再結(jié)晶行為。目前國內(nèi)外針對TC4鈦合金無縫管的研究主要以熱變形為主，而冷變形工藝研究則鮮有報(bào)道。本文以熱擠壓φ86ｍｍ×5ｍｍ規(guī)格TC4管材為冷軋管坯，研究了道次不同冷變形量和退火溫度對TC4管坯微觀組織、織構(gòu)演變和力學(xué)性能影響規(guī)律，旨在為高精度鈦管冷軋工業(yè)化提供重要支撐。

1、試驗(yàn)材料與方法

試驗(yàn)選用3t的Φ750mmTC4鈦合金鑄錠,其化學(xué)成分見表1。采用自由鍛方式在45MN鍛機(jī)上將TC4鑄錠分別鍛成Φ230mm棒坯,棒坯經(jīng)過臥式擠壓機(jī)(45MN)在950℃熱擠壓成?90mmx9mm擠壓管,經(jīng)扒皮、鏜孔等處理得到?86mmx5mm冷軋用管坯,管坯在立式真空爐(VCQV-480)中于800℃退火2h后,再依次經(jīng)過兩輥(LG90、LG60)和三輥(LD60)軋機(jī)三道次冷軋成TC4成品管,冷軋變形量依次為29%、44%和22%,分別對應(yīng)Q值(相對減壁量與相對減徑量的比值)為1.8、1.0和2.5。每道次軋制完成后的鈦管先進(jìn)行退火(退火溫度:800℃,保溫時(shí)間:2h)后，再開展下一道次冷軋，直到軋至成No.3鈦管。對道次冷變形的No.1~No.3TC4管坯取樣并分別進(jìn)行750~850℃的退火熱處理試驗(yàn)，如表2所示，退火管隨爐冷卻。

表1 TC4鈦合金鑄錠的化學(xué)成分(質(zhì)量分?jǐn)?shù)，%)

Table 1 Chemical composition of the TC4 titanium alloy ingot(mass fraction,%)

表2 TC4鈦合金無縫管的工藝參數(shù)

Table 2 Processing parameters of the TC4 titanium alloy seamless tubes

冷軋及退火管沿軸向取10mm×10mm×10mm金相試樣,先用180~800目砂紙打磨,再進(jìn)行機(jī)械拋光10min制備成電子背散射衍射(EBSD)樣品,拋光液為SiO₂懸濁液(粒度40nm)。通過7900F型掃描電鏡觀察管材徑向(RD)-軸向(AD)面的微觀組織及織構(gòu)。采用Kroll試劑(4%HF和20%HNO3的水溶液)對試樣進(jìn)行腐蝕,并用光學(xué)顯微鏡(Axio Imager M2m)觀察試樣微觀組織。采用Instron AUTO-T型萬能試驗(yàn)機(jī)測試管材軸向室溫拉伸性能。

2、試驗(yàn)結(jié)果與討論

2.1擠壓管坯微觀組織

2.2冷軋對組織織構(gòu)的影響

表3 TC4冷軋管中孿晶界占比

Table 3 Twin boundary proportion in the TC4 cold rolled tubes

表4 TC4冷軋管不同滑移系的施密特因子

Table 4 Schmid factors of different slip systems in the TC4 cold rolled tubes

2.3退火過程再結(jié)晶與織構(gòu)演變

表5不同退火溫度下TC4冷軋管材的晶粒各取向及再結(jié)晶占比

Table 5 Grain orientation and recrystallization grain proportion of the TC4 cold rolled tubes after annealing at different temperatures

表6 不同退火溫度下TC4冷軋管材的晶粒尺寸分布

Table 6 Grain size distribution of the TC4 cold rolled tubes after annealing at different temperatures

2.4力學(xué)性能

2.5 斷口分析

3、結(jié)論

1)冷軋組織取決于道次變形量,小變形時(shí)組織中保留大量形態(tài)完整的等軸α晶粒;大變形量(44%)促使等軸α晶粒消失,形成纖維狀組織;

2)大變形會(huì)促使TC4鈦管中纖維狀α相形成并增強(qiáng)變形織構(gòu),而減壁為主的小變形可引發(fā)晶粒取向發(fā)生傾轉(zhuǎn),形成雙峰分裂織構(gòu);

3)退火可促進(jìn)生成基面織構(gòu)取向的再結(jié)晶晶粒,800℃退火可充分釋放TC4大變形試樣的變形儲(chǔ)能,實(shí)現(xiàn)完全再結(jié)晶,同時(shí)顯著降低變形織構(gòu)強(qiáng)度,提升塑性,850℃退火會(huì)略微增加合金管強(qiáng)度并降低伸長率;

4)建議工業(yè)生產(chǎn)中采用大變形結(jié)合800℃退火,可使TC4鈦合金管材獲得最佳力學(xué)性能;若需要更高的強(qiáng)度，則可選擇750℃退火。

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（注，原文標(biāo)題：冷變形及退火溫度對TC4無縫管組織演變的影響_李曉煜）

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