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Acta Metall Sin  2017, Vol. 53 Issue (7): 842-850    DOI: 10.11900/0412.1961.2016.00421
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Effect of Hybrid Surface Nanocrystallization Before Welding on Microstructure and Mechanical Properties of Friction Stir Welded 2A14 Aluminum Alloy Joints
Jianhai YANG1,Yuxiang ZHANG1(),Liling GE2,Xiao CHENG3,Jiazhao CHEN1,Yang GAO1
1 Rocket Force University of Engineering, Xi'an 710025, China
2 School of Materials Science and Engineering, Xi'an University of Technology, Xi'an 710048, China
3 Chang Zheng Machinery Factory, China Aerospace Science and Technology Corporation, Chengdu 610100, China
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2A14 aluminum alloy is the important raw materials of aerospace, which belongs to the heat treatment aluminum alloy. Friction stir welding (FSW) can weld aluminum alloy with high quality, and can avoid the pores and cracks of fusion welding effectively. In order to obtain better mechanical properties of FSW joints, the surface nanocrystallization method is introduced into FSW technology. By means of the hybrid surface nanocrystallization (HSNC) method of both supersonic fine particles bombarding (SFPB) and surface mechanical rolling treatment (SMRT), a smooth gradient nanostructured (GNS) layer was formed on the surface of 2A14 aluminum alloy before FSW. The FSW joints microstructure and fracture morphology of the original and HSNC specimens were researched by OM, SEM and TEM. The results showed that nanostructure layer zone (NLZ) was formed when GNS with shape similar to the "S" line was distributed in the thermal-mechanical affected zone (TMAZ) and the nugget zone (NZ) of the HSNC specimen. The lowest micro-hardness and fracture position of the original specimen occurred on the TMAZ of advancing side (AS). The lowest micro-hardness and fracture position of the HSNC specimen occurred on the NZ. The tensile strength of HSNC specimen was 6.4% higher than the original sample. The elongation of HSNC specimen was 14.1% more than the original specimen. The fracture mode of both specimens was toughness fracture. The fracture morphology of the HSNC was isometric dimple when the fracture morphology of original specimen were non-isometric dimple and avulsion dimple. Analysis showed that the NLZ of the FSW joints was beneficial to improving the strength and the plastic deformation capability simultaneously.

Key words:  aluminum alloy      hybrid surface nanocrystallization      friction stir welding      microstructure      mechanical property     
Received:  21 September 2016     
Fund: Supported by National Natural Science Foundation of China (No.51275517), Science and Technology Project of Shaanxi Province (No.2009K06-22) and Special Project of Xi'an University of Technology (No.2014TS002)

Cite this article: 

Jianhai YANG,Yuxiang ZHANG,Liling GE,Xiao CHENG,Jiazhao CHEN,Yang GAO. Effect of Hybrid Surface Nanocrystallization Before Welding on Microstructure and Mechanical Properties of Friction Stir Welded 2A14 Aluminum Alloy Joints. Acta Metall Sin, 2017, 53(7): 842-850.

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Fig.1  Schematic of surface mechanical rolling treatment equipment[19]
Fig.2  Photo of friction stir welding (FSW) head
Fig.3  Cross-sectional OM images of the FSW joints of the original (a) and the hybrid surface nanocrystallization (HSNC) (b) specimens (TMAZ—thermal-mechanical affected zone, NZ—nugget zone, HAZ—heat affected zone, BM—base metal, AS—advancing side, RS—retreating side)
Fig.4  OM image of the 2A14 aluminum alloy BM
Fig.5  OM images of the original and HSNC specimens (NLZ—nanostructure layer zone)(a) TMAZ of the original specimen(b) TMAZ of AS of the HSNC specimen(c) TMAZ of RS of the HSNC specimen(d) NZ of the original specimen(e) NZ of the HSNC specimen
Fig.6  TEM images of the original and HSNC specimens (1—nanocrystalline, 2—submicron grain)(a) TMAZ of the original specimen(b) NZ of the original specimen(c) TMAZ of AS of the HSNC specimen(d) TMAZ of RS of the HSNC specimen(e) NZ of the HSNC specimen
Fig.7  Micro-hardness distributions of the FSW joints of 2A14 aluminum alloy
Fig.8  Fracture positions of FSW joints of the original (a) and HSNC (b) specimens
Sample Yield strength / MPa Ultimate strength / MPa Elongation / % Fracture position
Base metal 315 429 15.0
Original 210 362 7.8 TMAZ (AS)
HSNC 220 385 8.9 NZ
Table 1  Mechanical properties and fracture positions of FSW joints of the original and HSNC specimens
Fig.9  Low (a, c) and locally high (b, d) magnified fracture SEM images of FSW joints in the original (a, b) and HSNC (c, d) specimens
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