热等静压SLM Ti-6Al-4V合金的缺口敏感性
Effect of Hot Isostatic Pressing on Notch Sensitivity of Ti-6Al-4V Ti-alloy Prepared by Selective Laser Melting
通讯作者: 杨朋飞,yangpf@zjnu.edu.cn,研究方向为金属增材制造赵元,副教授,zhaoyuan@zjnu.edu.cn,研究方向为金属疲劳
收稿日期: 2024-11-25 修回日期: 2025-04-08
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Corresponding authors: YANG Pengfei, Tel:
Received: 2024-11-25 Revised: 2025-04-08
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作者简介 About authors
臧 涛,男,1999年生,硕士
研究了用激光选区熔化(SLM)制备的Ti-6Al-4V合金经热等静压(HIP)处理后在含缺口(应力集中系数Kt = 3)条件下的高周疲劳性能,并揭示了其对缺口的高敏感性机制。结果表明,虽然HIP处理的SLM Ti-6Al-4V合金静态拉伸强度和硬度都优于锻造态合金,但是在含缺口条件下其疲劳性能显著降低,疲劳极限从锻造态的250 MPa降低到150 MPa。SLM+HIP Ti-6Al-4V合金对缺口高度敏感的原因有:一是SLM+HIP合金的裂纹萌生区呈现准解理断裂的脆性形貌(显著不同于锻造态合金的典型韧性断裂形貌),使其发生脆性断裂的裂纹更易萌生;二是晶粒尺寸的影响,SLM+HIP合金的晶粒是特有的板条状,其晶粒尺寸(比锻造态合金的)较大。SLM+HIP合金在 X 方向和 Z 方向的平均晶粒尺寸分别为2.34和2.58 μm (显著高于锻造态合金的1.63 μm),较大的晶粒加剧了对缺口的敏感性,使其在疲劳过程中裂纹更易萌生和扩展。
关键词:
The block Ti-6Al-4V alloy was fabricated by selective laser melting (SLM) and then subjected to hot isostatic pressing (HIP) treatment. Next, the influence of HIP on the high cycle fatigue performance of the alloy with notch (stress concentration factor Kt = 3) was assessed, while elucidating the mechanisms related with the high notch sensitivity of the SLM+HIP Ti-6Al-4V alloy. The results indicate that although the SLM+HIP Ti-6Al-4V alloy exhibits superior static tensile strength and hardness compared to wrought counterparts, its fatigue performance under notched conditions is significantly compromised, with the fatigue limit decreasing from 250 MPa (wrought) to 150 MPa. Further analysis reveals that the high notch sensitivity of the SLM+HIP Ti-6Al-4V alloy may mainly be attributed to two factors: (1) differences in crack initiation mechanisms, i.e., as the crack initiation region of SLM+HIP alloy exhibits cleavage-like brittle fracture morphology, distinctly different from the ductile fracture morphology of the forged alloy, leading to a higher propensity for brittle crack initiation; and (2) the influence of grain size characteristics, namely the SLM+HIP alloy exhibits a unique lath-like grain structure, with an overall larger grain size compared to the forged alloy. Specifically, the average grain size in the X and Z directions is 2.34 and 2.58 μm, respectively, which is significantly larger than 1.63 μm in the forged alloy. This increased grain size exacerbates its notch sensitivity, making cracks more prone to initiation and propagation during the fatigue process.
Keywords:
本文引用格式
臧涛, 杨朋飞, 赵元, 高英, 鄂世举, 刘洋, 齐山贺, 张烨, 张嘉振.
ZANG Tao, YANG Pengfei, ZHAO Yuan, GAO Ying, E Shiju, LIU Yang, QI Shanhe, ZHANG Ye, ZHANG Jiazhen.
钛合金的比强度高、密度低、耐高温和耐腐蚀性能优异,在航空航天、汽车制造和生物医疗等领域得到了广泛的应用[1~3]。但是,传统钛合金的制造工艺成本高昂且流程复杂,使其应用受到了限制。增材制造(Additive manufacturing, AM)技术的自由度高和材料的浪费较少,可替代传统工艺制造钛合金[4~6]。AM技术中的激光选区熔化(Selective laser melting, SLM)技术基于粉末床的熔化和固化特性,应用较广[7]。但是,金属AM工艺仍有一定的局限性。用AM工艺制造的部件内部往往出现大量的缺陷,常见的有未熔合和气孔[8,9]。航空航天和工程机械等领域设备的部件承受复杂的动态载荷,这些缺陷成为应力集中源和疲劳裂纹的萌生点,从而使其疲劳性能降低[10]。除了内部的固有缺陷,工程机械零部件中还常有几何形状突变的区域,如凹槽、轴肩和切口等,这些部位统称为缺口。在缺口的根部,局部应力集中产生极大的应力梯度,是疲劳损伤高发区域[11]。这种应力集中显著降低材料缺口区域的抗疲劳性能,尤其是较小的缺口曲率半径使其尖锐度提高,从而使缺口的应力集中系数增大和材料疲劳失效的风险提高[12~14]。
1 实验方法
表1 Ti-6Al-4V合金的成分
Table 1
| Material | Al | V | O | C | N | Fe | Ti |
|---|---|---|---|---|---|---|---|
| Wrought | 6.00 | 4.07 | 0.152 | 0.016 | 0.004 | 0.050 | Bal. |
| SLM+HIP | 6.11 | 4.05 | 0.08 | 0.01 | 0.01 | 0.11 | Bal. |
用EOS-M280增材制造试件,采用双向平行扫描,在预热至约200 ℃的基板上沿Z方向逐层构建Ti-6Al-4V毛坯零件。为了提高质量,SLM工艺的优化参数为:激光功率为280 W,扫描速度为1400 mm/s,单层厚度为30 μm,舱口间距为90 μm。在打印过程用氩气保护。
打印完成后,将试件退火:(1) 在800 ℃真空炉中保温2 h,以释放残余应力;(2) 依据ASTM F2924标准在氩气环境中进行HIP处理:在920 ℃施加150 MPa的压力,保温保压3 h,加热速率为9 ℃/min,冷却过程是自然对流。依据ASTM标准E466和E647将热处理后的试件加工成标准几何形状的拉伸试样和含缺口(Kt = 3)的高周疲劳试样。图1a、b分别给出了拉伸试样和高周疲劳试样的尺寸。
图1
图1
试样的尺寸形状和实验方法
Fig.1
Dimensions of tensile (a) and fatigue (b) specimens, and experimental method (c)
用MTS-370伺服液压疲劳实验机进行拉伸测试和高周疲劳寿命测试。使用位移加载模式进行拉伸测试,设定加载速率为0.5 mm/min,将预校准的引伸计安装在试样的工作段,实时记录力载荷变化,以分析试样的拉伸变形行为和力学性能。用力控制模式进行高周疲劳测试,用夹具固定试样并施加正弦波载荷。疲劳测试的频率为30 Hz,应力比为0.06,试样循环加载直至失效,失效时的循环次数由试验机传感器自动记录,测试场景在图1c中给出。
表征和分析失效试样断口的微观组织。用扫描电子显微镜(SEM,COXEM EM-30AX)观察断口的形貌,以识别疲劳裂纹的萌生和扩展路径,揭示材料失效机制。将硬度测试样品用400#、800#、1500#砂纸逐级打磨至表面光洁,用无水乙醇清洗后干燥。用维氏硬度计(TMVS-S1-ALC)进行压痕实验测试硬度,载荷为1 kg,保载时间为15 s;沿样品均匀区域选取12个测试点,剔除结果的最大和最小值后取剩余10个点硬度的均值。用电子背散射衍射(EBSD,eFlash FS-Bruker)技术表征试样的微观组织。从钛合金棒材上截取试样并用砂纸(180至1200目)逐级打磨后进行电解抛光。抛光电压为14 V,阳极为试样,阴极为铁极,抛光液为甲醇(90 mL)与高氯酸(10 mL)的混合液,抛光温度为-30 ℃,抛光时间100 s。将抛光后的试样用清水和乙醇清洗后吹干,进行EBSD扫描,步长为0.15 µm,放大倍数为2000,扫描区域的分辨率高于90%。将试样电解抛光后,用Kroll试剂HF∶HNO3∶H2O = 1∶3∶6 (体积比)腐蚀10 s,用酒精冲洗并气吹干燥后用光学显微镜(Leica,DMI 3000M)观察其金相组织。
2 实验结果
2.1 金相组织和显微硬度
图2
图2
Ti-6Al-4V合金的金相组织
Fig.2
Metallographic microstructure of Ti-6Al-4V alloy (a) wrought Ti-6Al-4V alloy, (b) HIP-X Ti-6Al-4V alloy, (c) HIP-Z Ti-6Al-4V alloy
图3给出了Ti-6Al-4V合金的显微硬度。经SLM+HIP处理的Ti-6Al-4V合金在X向和Z向的显微硬度均约为339HV1.0。传统锻造Ti-6Al-4V合金的硬度约为322HV1.0,比SLM+HIP处理合金的硬度略低。
图3
图3
Ti-6Al-4V合金在不同条件下的显微硬度
Fig.3
Hardness of Ti-6Al-4V under different conditions
2.2 力学性能
表2列出了静力拉伸力学测试的结果。可以看出,锻造态Ti-6Al-4V合金与SLM+HIP Ti-6Al-4V合金的弹性模量没有显著的不同,SLM+HIP处理的合金拉伸强度显著高于锻造态。两种合金的断后伸长率都高于15%,表明其塑性和延展性良好。
表2 不同状态Ti-6Al-4V合金的静力拉伸力学性能
Table 2
| Material | Modulus / GPa | Strength / MPa | Elongation / % |
|---|---|---|---|
| Wrought | 115 ± 3.6 | 886 ± 17 | 19 ± 2.4 |
| HIP-X | 121 ± 6.3 | 1015 ± 8.7 | 18 ± 1.7 |
| HIP-Z | 117 ± 8.5 | 1022 ± 5.7 | 16 ± 2.5 |
图4
2.3 断口形貌
图5
图5
Ti-6Al-4V合金的拉伸断口形貌
Fig.5
Tensile fracture morphology of Ti-6Al-4V alloy (a, b) wrought Ti-6Al-4V alloy, (c, d) HIP-X Ti-6Al-4V alloy, (e, f) HIP-Z Ti-6Al-4V alloy
图6给出了在不同条件下处理的Ti-6Al-4V合金疲劳断口和裂纹源区域的宏观形貌。对比图6a、d和g可见,含缺口的SLM+HIP工艺Ti-6Al-4V合金试件的断裂面比锻造态试件更为粗糙。这表明,SLM+HIP工艺对Ti-6Al-4V合金的疲劳裂纹扩展路径的影响更为复杂,使其断口更曲折。值得注意的是,与光滑试件的高周疲劳中常见的单一裂纹源萌生不同,含缺口的Ti-6Al-4V合金的高周疲劳具有多裂纹源萌生的特点。对裂纹源区域的进一步观察表明,锻造态Ti-6Al-4V合金的裂纹起源于表面,具有典型的韧性断裂形貌,没有出现明显的缺陷(图6b和c)。SLM+HIP处理的Ti-6Al-4V合金虽然也表现出多裂纹源萌生特征,但是在裂纹萌生处有光滑刻面,呈现出准解理断裂特征(图6e、f、h、i)。
图6
图6
疲劳试件断口的宏观形貌和裂纹萌生位置的微观形貌
Fig.6
Macro fracture morphology and microstructure at crack initiation site in fatigue specimens (a-c) wrought Ti-6Al-4V alloy, (d-f) HIP-X Ti-6Al-4V alloy, (g-i) HIP-Z Ti-6Al-4V alloy
图7
图7
不同位置的疲劳裂纹扩展的微观形貌
Fig.7
Microstructure of fatigue crack propagation zone (a-c) wrought Ti-6Al-4V alloy, (d-f) HIP-X Ti-6Al-4V alloy, (g-i) HIP-Z Ti-6Al-4V alloy
图8
图8
不同状态下Ti-6Al-4V合金疲劳瞬断区的微观形貌
Fig.8
Microstructure of fatigue instantaneous fracture zone (a) wrought Ti-6Al-4V alloy, (b) HIP-X Ti-6Al-4V alloy, (c) HIP-Z Ti-6Al-4V alloy
3 讨论
3.1 微观组织的不同
HIP处理消除了SLM Ti-6Al-4V合金内部的缺陷,使试件的致密程度与锻造件相同,图6中断口的照片可以佐证。裂纹源处未见划痕或硬质颗粒,表明裂纹的萌生与表面加工质量无关。同时,对所有试件的热处理消除了残余应力,因此在缺口应力集中条件下SLM+HIP Ti-6Al-4V合金的疲劳性能低于锻造件与微观组织的不同有关。
图9a、b、c中的EBSD图给出了不同条件下Ti-6Al-4V合金的典型微观结构。锻造态Ti-6Al-4V合金具有等轴晶和随机取向晶粒的形貌,而SLM+HIP Ti-6Al-4V合金具有复杂的板条状形貌。使用Azteccrystal软件统计了晶粒尺寸:用等效圆直径表征等轴晶,用等效椭圆短轴长度表征板条状组织。图9d、e、f表明,SLM+HIP态Ti-6Al-4V合金的大尺寸晶粒比例显著高于锻造件。统计结果表明,X向(2.34 μm)与Z向(2.58 μm)的平均晶粒尺寸都显著比锻造件的(1.63 μm)高。根据Hall-Petch关系[27, 28],粗大晶粒的晶界处塞积的位错较多和应力集中严重,易于启动相邻晶粒的位错源使塑性变形容易;晶粒细小则晶界增多,晶界对位错运动的阻力使变形困难。
图9
图9
不同状态下Ti-6Al-4V合金样品的晶粒尺寸和微观结构
Fig.9
Grain structure (a-c) and size distribution (d-f) of Ti-6Al-4V under different processing conditions (a, d) wrought Ti-6Al-4V alloy, (b, e) HIP-X Ti-6Al-4V alloy, (c, f) HIP-Z Ti-6Al-4V alloy
3.2 缺口应力集中条件下Ti-6Al-4V的断裂模式
在本文的实验中观察到3种类型的疲劳裂纹萌生机制,分别命名为I型、II型和III型。I型为锻造态Ti-6Al-4V合金的疲劳裂纹萌生机制,裂纹萌生区域具有凹凸不平的“小峰”形貌(图10a和b),因为在塑性变形过程中微裂纹扩展并连接。SLM+HIP Ti-6Al-4V合金的疲劳裂纹萌生机制为II型和III型,两者都与微观结构中的刻面相关,与HIP导致的α板条粗化和变形有关[31](图10c~f)。其中II型刻面的分布较为复杂,由多个“T”形构成,而III型则由多条平行刻面构成,且附近出现了细小的次级裂纹(图10d和f)。刻面的形成机制遵循Bache[32]和Neal[33]提出的钛合金软/硬晶粒裂纹形核模型,涉及穿晶断裂和沿晶断裂两种模式。
图10
图10
缺口条件下Ti-6Al-4V合金的裂纹萌生机制
Fig.10
Different crack initiation mechanisms of Ti-6Al-4V alloy (a, b) ductile fracture, (c, d) transgranular fracture, (e, f) intergranular fracture
图11
图11
缺口应力集中条件下Ti-6Al-4V合金的断裂模式
Fig.11
Fracture modes of Ti-6Al-4V subjected to notch stress concentration (a-d) wrought, (e-i) SLM+HIP
结果还表明,缺口应力集中的锻造Ti-6Al-4V合金与SLM+HIP Ti-6Al-4V合金疲劳性能的不同,其原因是微观组织和裂纹萌生机制的不同。
4 结论
(1) 与传统的锻造件的拉伸强度相比,SLM+HIP技术制备的Ti-6Al-4V合金在X向和Z向的拉伸强度更高。两者的延伸率都比较高(超过15%),表明其延展特性优异。
(2) 与传统锻造件相比,SLM+HIP Ti-6Al-4V合金的缺口敏感性更高。SLM+HIP Ti-6Al-4V合金的疲劳缺口敏感性,主要源于其裂纹萌生机制及晶粒尺寸效应。SLM+HIP合金表现出准解理断裂特征,其危害性更高。其较大的晶粒尺寸,进一步使其疲劳性能降低。
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