Enhancing the mechanical performance of Al-Zn-Mg alloy builds fabricated via underwater friction stir additive manufacturing and post-processing aging

doi:10.1016/j.jmst.2021.08.050

Abstract

Abstract:

Our previous studies have demonstrated that underwater friction stir additive manufacturing (FSAM) could effectively suppress the macroscale softening of the fabricated Al-Zn-Mg-Cu alloy build from top to bottom. However, the accompanying local softening problem, i.e., a low-hardness region at the bottom of each stir zone, becomes prominent. In this study, an Al-Zn-Mg alloy with low quench sensitivity was used to fabricate a multilayered build via underwater FSAM. In-process water cooling could effectively solve the macroscale and local softening problems in the FSAM of the Al-Zn-Mg alloy and improve the mechanical performance of the build. The microhardness and ultimate tensile strength (UTS) of the water-cooled build in as-fabricated and aged states were more uniform along the building direction and higher than those of their counterparts. After 90 days of natural aging, the UTS of the water-cooled build in building and traveling directions reached 398 and 400 MPa, respectively, slightly higher than that of the base metal (392 MPa). The enhancement in the mechanical performance of the water-cooled build was attributed to a high degree of supersaturation and age-strengthening ability because of a high cooling rate of the underwater FSAM process and low quench sensitivity of the base metal.

Key words: Additive manufacturing, Friction stir additive manufacturing, Water cooling, Al-Zn-Mg alloy, Microstructure, Mechanical property

Changshu He, Ying Li, Jingxun Wei, Zhiqiang Zhang, Ni Tian, Gaowu Qin, Xiang Zhao. Enhancing the mechanical performance of Al-Zn-Mg alloy builds fabricated via underwater friction stir additive manufacturing and post-processing aging[J]. J. Mater. Sci. Technol., 2022, 108: 26-36.

Figures/Tables 15

Fig. 1. Schematic of (a) the cross-section of the multilayered build, (b) macroscale softening phenomenon, and (c) local softening phenomenon.

Table 1. Chemical composition of the 7N01 Al alloy (wt.%).

Zn	Mg	Cu	Mn	Cr	Ti	Fe	Si	Al
4.66	1.09	0.12	0.32	0.20	0.06	0.10	0.03	Bal.

Table 2. Mechanical properties of the 7N01 Al alloy.

Ultimate tensile strength/MPa	Yield strength/MPa	Elongation/%	Microhardness/HV
392	322	30.6	112

Fig. 2. (a) Experimental setup of the underwater FSAM process, and (b) dimensions of the tool used in this study.

Fig. 3. Schematic of the temperature measurement.

Fig. 4. Schematic of the extraction position and dimension of the tensile samples. Unit: mm.

Fig. 5. Microhardness distributions on the cross section of the air-cooled (AC) and water-cooled (WC) builds along the building direction in the as-fabricated and NA-90d states.

Fig. 6. Macro and microstructures of the air-cooled (AC) and water-cooled (WC) builds: (a) macrostructure of the air-cooled build, EBSD mappings performed at the top region (b) and the bottom region (c) of the air-cooled build, (d) macrostructure of the water-cooled build, EBSD mappings performed at the top region (e) and the bottom region (f) of the water-cooled build.

Table 3. Average grain sizes of the top and the bottom regions in the air- and water-cooled builds.

Samples	Average grain size (Standard deviation)/μm
Samples	Top	Bottom
Air-cooled build	3.3 (1.69)	4.2 (2.39)
Water-cooled build	2.4 (1.11)	2.8 (1.47)

Fig. 7. STEM images in the (a) top and (c) bottom regions of the air-cooled build, (b) top and (d) bottom regions of the water-cooled build, (e) corresponding mapping images of Al, Zn, Mg, Cr, and Mn, and (f) STEM-EDS results of the particles.

Fig. 8. Nanoscale precipitate characteristics at the top and bottom regions of the air-cooled and water-cooled builds after NA-90d: HRTEM results at the (a) top and (b) bottom regions of the air-cooled build, (c) top and (d) bottom regions of the water-cooled build; (e) inverse FFT image after masking and (f) FFT pattern of the GP-I zone, as outlined in (a); (g) inverse FFT image after masking and (h) FFT pattern of the GP-II zone, as outlined in (b). All the micrographs were captured along the 〈110〉Al orientation.

Fig. 9. (a) Schematic of the temperature test location, temperature test results of (b) the air-cooled build and (c) the water-cooled build.

Fig. 10. Schematic of the relationship of microhardness-microstructure-thermal exposure in the (a) air-cooled build, and (b) water-cooled build.

Fig. 11. Tensile properties of the samples extracted from the air-cooled (AC) and water-cooled (WC) builds along traveling (H) and building (Z) directions: (a, b) ultimate tensile strength and elongation of the samples in the as-fabricated and NA-90d states, respectively; (c, d) engineering stress-strain curves of the samples in the as-fabricated and NA-90d states, respectively.

Fig. 12. Fracture features of tensile samples along the building direction: (a) air-cooled build, and (b) water-cooled build (As-fabricated samples).

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