面向航空高精度管材制备的冷变形-退火热处理联动工艺调控TC4钛合金微观缺陷及综合力学性能优化研究

TC4钛合金的名义成分为Ti-6Al-4V，是1954年由美国研制的一种α＋β型双相钛合金，兼具α型及β型钛合金的优点，具有高强度、耐腐蚀、耐高温、抗氧化等特点，是全球应用最为广泛的钛合金［1-3］ 。尤其在航空航天方面，如飞机机身所用的钛合金，TC4占比可达70％～90％。

TC4钛合金组织中的α⁃Ti为密排六方结构，滑移系较少，冷变形相对困难，加上TC4合金的高比强度，导致该合金的冷变形能力较差，在冷轧制管过程中易发生表面开裂，极大增加了TC4钛合金无缝管冷轧制备的难度。目前国内TC4管材通常以热挤压或斜轧穿孔等方法制备管坯［4-6］ ，后续多采用温轧方式生产TC4钛合金管材制品［7］ ，但温轧需在轧管机上加装感应加热装置，这种加工设备结构复杂、工艺繁琐、生产成本高。冷轧适合制备表面质量和尺寸精度要求高的管材，能得到较好的成形效果和加工率，但对材料的塑性有较高的要求。李凯玥［8］研究了冷轧工艺对TC4钛合金薄壁管材组织性能的影响，指出冷轧道次和道次间变形量对TC4轧制管材的表面质量和力学性能有显著影响。王国迪等［9］研究表明，TC4钛合金冷变形量会直接影响合金的微观组织和退火再结晶行为。目前国内外针对TC4钛合金无缝管的研究主要以热变形为主，而冷变形工艺研究则鲜有报道。本文以热挤压φ86ｍｍ×5ｍｍ规格TC4管材为冷轧管坯，研究了道次不同冷变形量和退火温度对TC4管坯微观组织、织构演变和力学性能影响规律，旨在为高精度钛管冷轧工业化提供重要支撑。

1、试验材料与方法

试验选用3t的Φ750mmTC4钛合金铸锭,其化学成分见表1。采用自由锻方式在45MN锻机上将TC4铸锭分别锻成Φ230mm棒坯,棒坯经过卧式挤压机(45MN)在950℃热挤压成∅90mmx9mm挤压管,经扒皮、镗孔等处理得到∅86mmx5mm冷轧用管坯,管坯在立式真空炉(VCQV-480)中于800℃退火2h后,再依次经过两辊(LG90、LG60)和三辊(LD60)轧机三道次冷轧成TC4成品管,冷轧变形量依次为29%、44%和22%,分别对应Q值(相对减壁量与相对减径量的比值)为1.8、1.0和2.5。每道次轧制完成后的钛管先进行退火(退火温度:800℃,保温时间:2h)后，再开展下一道次冷轧，直到轧至成No.3钛管。对道次冷变形的No.1~No.3TC4管坯取样并分别进行750~850℃的退火热处理试验，如表2所示，退火管随炉冷却。

表1 TC4钛合金铸锭的化学成分(质量分数，%)

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

表2 TC4钛合金无缝管的工艺参数

Table 2 Processing parameters of the TC4 titanium alloy seamless tubes

冷轧及退火管沿轴向取10mm×10mm×10mm金相试样,先用180~800目砂纸打磨,再进行机械抛光10min制备成电子背散射衍射(EBSD)样品,抛光液为SiO₂悬浊液(粒度40nm)。通过7900F型扫描电镜观察管材径向(RD)-轴向(AD)面的微观组织及织构。采用Kroll试剂(4%HF和20%HNO3的水溶液)对试样进行腐蚀,并用光学显微镜(Axio Imager M2m)观察试样微观组织。采用Instron AUTO-T型万能试验机测试管材轴向室温拉伸性能。

2、试验结果与讨论

2.1挤压管坯微观组织

2.2冷轧对组织织构的影响

表3 TC4冷轧管中孪晶界占比

Table 3 Twin boundary proportion in the TC4 cold rolled tubes

表4 TC4冷轧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退火过程再结晶与织构演变

表5不同退火温度下TC4冷轧管材的晶粒各取向及再结晶占比

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

表6 不同退火温度下TC4冷轧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力学性能

2.5 断口分析

3、结论

1)冷轧组织取决于道次变形量,小变形时组织中保留大量形态完整的等轴α晶粒;大变形量(44%)促使等轴α晶粒消失,形成纤维状组织;

2)大变形会促使TC4钛管中纤维状α相形成并增强变形织构,而减壁为主的小变形可引发晶粒取向发生倾转,形成双峰分裂织构;

3)退火可促进生成基面织构取向的再结晶晶粒,800℃退火可充分释放TC4大变形试样的变形储能,实现完全再结晶,同时显著降低变形织构强度,提升塑性,850℃退火会略微增加合金管强度并降低伸长率;

4)建议工业生产中采用大变形结合800℃退火,可使TC4钛合金管材获得最佳力学性能;若需要更高的强度，则可选择750℃退火。

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（注，原文标题：冷变形及退火温度对TC4无缝管组织演变的影响_李晓煜）

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