Mechanical and corrosion fatigue behaviors of gradient structured 7B50-T7751 aluminum alloy processed via ultrasonic surface rolling

doi:10.1016/j.jmst.2019.08.030

Abstract

Abstract:

In this work, ultrasonic surface rolling process (USRP) was utilized to produce a gradient structured layer on 7B50-T7751 aluminum alloy, and the mechanical properties and corrosion fatigue behavior of treated samples were studied. These results reveal that underwent USRP, a 425 μm thick gradient structure and a 700 μm deep compressive residual stress field are created, aluminum grain size become fine(~ 67 nm), and the corrosion rate of treated surface reduces by 60.08% owing to the combined effect of compressive residual stress and surface nanocrystallization. The corrosion fatigue strength is enhanced to 117% of that of 7B50 Al alloys by means of USRP due to the introduced compressive residual stress, which is considered as the major favorable factor in suppressing the initiation and early propagation of corrosion fatigue cracks. Besides, the gradient structure is an important factor in providing a significant synergistic contribution to the improvement of corrosion fatigue performance.

Key words: Aluminum alloy, Mechanical properties, Corrosion fatigue behavior, Gradient structure, Compressive residual stress, Ultrasonic surface rolling process

Xingchen Xu, Daoxin Liu, Xiaohua Zhang, Chengsong Liu, Dan Liu. Mechanical and corrosion fatigue behaviors of gradient structured 7B50-T7751 aluminum alloy processed via ultrasonic surface rolling[J]. J. Mater. Sci. Technol., 2020, 40: 88-98.

Figures/Tables 13

Fig. 1. Typical cross-sectional OM image (a) and surface secondary electron morphology (b) of the UR12 sample. Insets in (a) and (b) display the corresponding backscattered electron image of the rectangular area and the corresponding CLSM image of the surface, respectively.

Fig. 2. (a-d) BF TEM images of the microstructure occurring at a depth of 5 mm, 350 μm, 150 μm, and 40 μm, respectively, in the UR12 sample (insets show the corresponding DF TEM images); (e) SAED pattern corresponding to (d); (f) the statistics of grain size at a depth of 40 μm.

Fig. 3. Diffraction patterns of the BM [27] and UR12 samples.

Fig. 4. (a) Typical open circuit potential and (b) polarization curves for the different 7B50 aluminum alloy samples in a neutral 3.5% NaCl solution at 25 °C.

Table 1 Corresponding electrochemical corrosion parameters of the different samples obtained from Fig. 4(b).

Sample	E_corr (V_SCE)	i_corr (μA/cm²)	CR (μm/year)
BM	-0.735	1.92	20.94
UR12	-0.726	0.77	8.36
UR12-R	-0.729	1.34	14.55
UR12-P	-0.712	0.58	6.33
UR12-PR	-0.722	0.84	9.10

Fig. 5. Microhardness distribution of the UR12 sample.

Fig. 6. (a) σ-ε curves of the BM, UR12, and UR12-P samples, (b) σT-εT curves corresponding to the part of σ-ε curves before necking and (c) θ-εT curves corresponding to the uniform plastic deformation region. The meanings of points A, B, and C in (a) were also applied to the points in (b) and (c), and the BM sample was taken as an example for the analysis. (Note: the first yield point represents onset of plastic flow, and the nominal stress at this point is initial yield stress, which is lower than σ0.2).

Table 2 Mechanical properties of the different samples.

Sample	σ₀ (MPa)	σ_0.2 (MPa)	UTS (MPa)	ε_f	U_T (J/cm³)	n
BM	346.9	523.0	566.3	0.1072	61.38	0.19
UR12	375.6	512.4	577.7	0.1002	58.32	0.24
UR12-P	357.8	506.7	565.4	0.1056	59.91	0.21

Fig. 7. S?N curves for the 7B50-T7751 aluminum alloy samples.

Fig. 8. Depth distributions of axial residual stress for the 7B50-T7751 aluminum alloy samples.

Fig. 9. Comparison of CF life for the different samples when σa = 157.5 MPa.

Fig. 10. Typical SEM micrographs of corrosion fatigue fractography for the different samples: (a) BM [27], Nf =544,129; (b) UR12, Nf =600,653; (c) UR12-R, Nf =219,375; (d) UR12-P, Nf =740,822; (e) UR12-PR, Nf =384,046. σa for the untreated and treated samples are 83.25 MPa and 157.5 MPa respectively.

Fig. 11. Typical SEM micrographs of surface morphology near the corrosion fatigue fractography for the corresponding samples in Fig. 10: (a) BM; (b) UR12; (c) UR12-R; (d) UR12-P; (e) UR12-PR.

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