The Influence of Metallurgical Pores on Fatigue Behaviors of Fusion Welded AA7020 Joints

doi:10.11900/0412.1961.2017.00448

Acta Metall Sin

2018, Vol. 54

Issue (8): 1131-1140 DOI: 10.11900/0412.1961.2017.00448

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The Influence of Metallurgical Pores on Fatigue Behaviors of Fusion Welded AA7020 Joints

Zhe SONG¹, Shengchuan WU¹(

), Yanan HU¹, Guozheng KANG¹, Yanan FU², Tiqiao XIAO²

1 State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu 610031, China
2 Shanghai Synchrotron Radiation Facility (SSRF), Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China

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Zhe SONG, Shengchuan WU, Yanan HU, Guozheng KANG, Yanan FU, Tiqiao XIAO. The Influence of Metallurgical Pores on Fatigue Behaviors of Fusion Welded AA7020 Joints. Acta Metall Sin, 2018, 54(8): 1131-1140.

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Abstract

With the rapid development of Chinese high-speed railway system, the urgent demand for lighter weight structures is increasing, and aluminum alloys are widely applied into manufacturing the railway train and critical safety components. As a medium strength aluminum alloy, the 7020 aluminum alloy shows a great potential. Hybrid laser welding has currently become one of the most important welding techniques for medium and high strength aluminum alloys. Nevertheless, intrinsic defects such as pores and shrinkages physically determine the fatigue resistance of the welded joint. Based on in situ synchrotron radiation X-ray computed microtomography (SR-μCT), the population, location and size of gas pores within AA7020 hybrid welded joints are firstly identified and counted. The critical size of gas pores, affecting the fatigue properties of welded joints, is acquired by combining the statistical results of the pores and the average grain size of the hybrid weld. Meanwhile, the qualitative relationship between pore size, effective stress and fatigue life is discussed through in situ fatigue life data via SR-μCT and fracture morphology. By using the finite element analysis, detailed works have been performed on the stress state near the pores of different positions inside the joint. Through the simulation analysis, the stress concentration coefficient around the pores firstly increases, then decreases, and finally tends to a stable trend as the location of the pore-like defect is transferred from the surface to the inside. Besides, the influence of porosity on fatigue crack initiation, fatigue crack growth and sudden breaking process is also analyzed using fatigue crack growth experiment. In conclusion, the results show that the critical pore size of hybrid laser welded joint can be qualitatively identified as 30 μm; the SR-μCT and fracture analysis show that larger surface and sub-surface pores are more likely to initiate fatigue cracks, and the fatigue crack propagation experiment further shows that the porosity has very little effect on the long crack growth but significant influence on the crack front.

Key words: synchrotron radiation X-ray microtomography welding defect fatigue life assessment laser hybrid welding high-speed railway train

Received: 26 October 2017

ZTFLH:

TG405

Fund: Supported by National Natural Science Foundation of China (No.11572267), Science and Technology Research and Development Project of Sichuan Province (No.2017JY0216), Open Research Project of State Key Laboratory for Strength and Vibration of Mechanical Structures of Xi'an Jiaotong University (No.SV2016-KF-21)

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	Zhe SONG
	Shengchuan WU
	Yanan HU
	Guozheng KANG
	Yanan FU
	Tiqiao XIAO

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https://www.ams.org.cn/EN/10.11900/0412.1961.2017.00448 OR https://www.ams.org.cn/EN/Y2018/V54/I8/1131

Fig.1 Schematics of in-situ fatigue loading device based on synchrotron radiation X-ray tomography
(a) principle diagram of operation (b) fatigue specimen size (unit: mm)

Fig.2 Statistical results of gas pores inside 10 in-situ imaging fatigue specimens at Shanghai synchrotron radiation facility (SSRF) and European synchrotron radiation facility (ESRF)

Fig.3 Position distribution of gas pores inside hybrid laser welded 7020 aluminum alloy from in situ imaging fatigue specimens at SSRF and ESRF

Fig.4 Synchrotron radiation X-ray based porosity imaging result of a typical 7020 aluminum alloy due to a hybrid laser welding process

Fig.5 Synchrotron radiation X-ray imaging result of process-induced pores inside a 7020 aluminum alloy due to a hybrid laser welding process

Fig.6 Schematics of the pore position classification in the welded joint (A—the square root of the projection area perpendicular to the load direction, h—the minimum distance between the boundary of the crack initiation pore and the free surface of the specimen)
(a) h=0, as surface defect
(b) h<A, as sub-surface defect
(c) h>A, as internal defect

Table 1 Fatigue life data related with the defect location and affected area under in situ fatigue microtomography conducted at ESRF and SSRF

Fig.7 Specimen fracture morphologies at N_f=77340 cyc and crack initiation from a surface pore
(a) macro morphology of fracture surface (b) fatigue source morphology
(c) fatigue striation of stable propagation zone (d) final fracture region

Fig.8 S-N plot of the maximum effective stress (σ_eff) and N_f in fatigue test and synchrotron radiation X-ray in situ fatigue test

Fig.9 The relationship between the position of the pore and the upper surface of the weld in the simulation model and the boundary conditions of the finite element simulation (s—the distance from the pore center to the free surface (X-Y plane);r—the pore radius;S1—X-Z plane; L1—the intersection line of the X-Z plane and the bottom plane; U1, U2 and U3—the translational degrees of freedom along coordinate axis X direction, along coordinate axis Y direction, along coordinate axis Z direction, respectively, σ—the applied stress)

Fig.10 Principal stress fields across the pore parallel to X-Z plane
(a) s/r =-0.2 (b) s/r =0 (c) s/r =2.7

Fig.11 Elastic and elastic-plastic simulations of stress concentration curves near the pores

Fig.12 Fatigue crack growth rates of three laser hybrid welded 7020 aluminum alloy joints under stress ratio R=0.1 and mean thickness t=1.84 mm

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