Defect Induced Fatigue Behaviors of Selective Laser Melted Ti-6Al-4V via Synchrotron Radiation X-Ray Tomography

doi:10.11900/0412.1961.2018.00408

Acta Metall Sin

2019, Vol. 55

Issue (7): 811-820 DOI: 10.11900/0412.1961.2018.00408

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Defect Induced Fatigue Behaviors of Selective Laser Melted Ti-6Al-4V via Synchrotron Radiation X-Ray Tomography

Zhengkai WU¹,Shengchuan WU¹(

),Jie ZHANG²,Zhe SONG¹,Yanan HU¹,Guozheng KANG¹,Haiou ZHANG³

1. State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu 610031, China
2. AVIC Manufacturing Technology Institute, Beijing 100024, China
3. State Key Laboratory of Digital Manufacturing Equipment and Technology, Huazhong University of Science and Technology, Wuhan 430074, China

Cite this article:

Zhengkai WU, Shengchuan WU, Jie ZHANG, Zhe SONG, Yanan HU, Guozheng KANG, Haiou ZHANG. Defect Induced Fatigue Behaviors of Selective Laser Melted Ti-6Al-4V via Synchrotron Radiation X-Ray Tomography. Acta Metall Sin, 2019, 55(7): 811-820.

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Abstract

As a very promising additive manufacturing (AM) technique, selective laser melting (SLM) has gained considerable attentions due to the feasibility of producing light-weight metallic components directly from virtual design data. On the other hand, high strength, low density and high corrosion resistance Ti-6Al-4V alloy has been a preferred AM used material for the aviation and military industries. However, the fatigue damage behaviors of SLMed or AMed components usually suffer from interior defects such as incomplete fusion and gas pores due to unstable process or unsuitable processing parameters. Therefore, thorough investigations on process-induced and metallurgical defects and its influence on the fatigue behavior is required for robust designs and engineering applications of high performance SLM components. As an advanced characterization approach, synchrotron radiation micro computed X-ray tomography (SR-μCT) has been recently to investigate the fatigue damage behaviors of critical components with defects. Based on self-developed in situ fatigue testing rig fully compatible with the BL13W1 at Shanghai Synchrotron Radiation Facility (SSRF), several AMed specimens were prepared for in situ fatigue SR-μCT. The Feret diameter and extreme values statistics were then adopted to characterize the defect size, morphology, population, location and the influence on fatigue life. Fatigue fractography was also examined to further identify the defect to really initiate a fatigue crack. Results show that two types of defects including gas pores and the lack of fusion can be clearly distinguished inside SLM Ti-6Al-4V alloys. Fatigue crack with a typical semi-ellipse usually initiates from the defects at the surface and near the surface. Besides, the defects less than 50 μm and sphericity of 0.4~0.65 dominate for the SLM Ti-6Al-4V alloys. It is also found that the larger the characteristic size of the defect, the lower the fatigue life. Current results can provide a theoretical basis and support to predict the fatigue performance of SLM Ti-6Al-4V alloys. Further investigations should be performed on the relationship between the critical defect and fatigue strength by introducing the Kitagawa-Takahashi diagram.

Key words: additive manufacturing fatigue life assessment defect evolution characterization synchrotron radiation three-dimensional tomography high cycle fatigue

Received: 03 September 2018

ZTFLH:

TG146.2

Fund: National Natural Science Foundation of China(No.11572267)

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	Zhengkai WU
	Shengchuan WU
	Jie ZHANG
	Zhe SONG
	Yanan HU
	Guozheng KANG
	Haiou ZHANG

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https://www.ams.org.cn/EN/10.11900/0412.1961.2018.00408 OR https://www.ams.org.cn/EN/Y2019/V55/I7/811

Fig.1 Schematic of samples forming and location selection

Fig.2 High cycle fatigue specimen size (unit: mm)

Fig.3 Schematic of in situ fatigue experiment based on synchrotron radiation X-ray micro computed tomography (SR-μCT) showing the principle diagram of operation (a) and in situ fatigue specimen size (unit: mm) (b)

Fig.4 3D rendering of the defects within the gauge of in situ fatigue specimen

Fig.5 Distribution of effective diameter of defects and its cumulative frequency curve

Fig.6 Distribution of sphericity of defects and its normal curve fitting

Fig.7 Characterization of defects with different effective diameters

Fig.8 3D rendering of the crack induced by defects and corresponding fracture morphology of in situ fatigue specimen

Fig.9 High cycle fatigue specimen fracture morphologies of selective laser melted Ti-6Al-4V failed at σ _max =440 MPa, N _f=5.9×10⁴ cyc

Fig.10 Fracture morphologies of different defects in selective laser melted Ti-6Al-4V

Fig.11 Schematics of the defect position classification (h—the minimum distance between the boundary of the crack initiation defect and the free surface of the specimen)

Fig.12 Relationship between crack initiation defects and fatigue life of high cycle fatigue specimens of selective laser melted Ti-6Al-4V

Fig.13 Size parameter (α) and position parameter (λ) in Eq.(3) of extreme values statistical method obtained by linear fitting

Fig.14 Estimation curve of maximum defect feature size under certain volume

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