Article
The Effect of Plug Rotation Speed on Micro-Structure of
Nugget Zone of Friction Plug Repair Welding Joint for 6082
Aluminum Alloy
Defu Li
1
and Xijing Wang
2,
*
Citation: Li, D.; Wang, X. The Effect
of Plug Rotation Speed on
Micro-Structure of Nugget Zone of
Friction Plug Repair Welding Joint for
6082 Aluminum Alloy. Materials 2021,
14, 5287. https://doi.org/10.3390/
ma14185287
Academic Editor: Arkadiusz Gola
Received: 3 August 2021
Accepted: 2 September 2021
Published: 14 September 2021
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1
State Key Laboratory of Advanced Processing and Recycling of Non-Ferrous Metals, Lanzhou University of
Technology, Lanzhou 730050, China; ldf0301@mail.lzjtu.cn
2
Faculty of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, China
* Correspondence: wangxj@lut.cn
Abstract:
This paper carried out the friction plug repair welding of 6082 aluminum alloy keyhole
defects by using the method of friction heating between shaft shoulder and base material. In addition,
a well-formed friction plug welding joint was obtained at different plug rotation speeds. In order to
study the influence mechanism of plug rotation speeds on the microstructure of the weld nugget
zone, EBSD technology was used to analyze the grain morphology, grain size and grain boundary
characteristics of the weld nugget zone under different rotation speeds of the plug rod. The results
show that in the nugget zone, the grain was fine and equated crystals refinement, and there was a
preferred orientation. The deformation texture components in the welded nugget zone increased
with the plug rotation speed from 1600 to 2000 rpm. However, the grain size first decreased and then
increased, while the components in the High-Angle Boundary first increased and then decreased.
Keywords:
auxiliary heating; friction plug repair welding; preferential orientation; dynamic
re-crystallization
1. Introduction
Friction stir welding (FSW) does not require filler material during the welding process.
If the welding temperature is lower than the melting point of the base metal, metallurgical
and crystallization defects can be effectively avoided [1]. Therefore, FSW is recognized as
an ideal welding method for aluminum alloy [
2
], which is widely used in aerospace, rail
transportation and other fields [
3
]. Based on the plastic flow characteristics of materials
in the FSW process, the joint can be divided into four zones: weld nugget zone (NZ),
thermo-mechanical affected zone (TMAZ), heat affected zone (HAZ) and base metal zone
(BM) [
4
]. Among these, the nugget zone is an “onion” ring structure [
5
] composed of
uniform and finely equated grains, finer than those in other regions [
6
]. Hofmann [
7
]
conducted underwater multi-channel friction stir machining of 6061 aluminum alloy, and
the results showed that the grain size in the weld nugget zone was related to the ambient
temperature. Zhao [
8
] found that the metal in the weld nugget zone was subjected to
plastic deformation and flow under the combined action of heat. However, the internal
dislocations in the grains increased; in addition, during the welding process these high-
density dislocation grains became the core of re-crystallization once again. When the
equated grains were refined, the location dislocation density in the weld nugget zone was
reduced. Zhang [
9
] analyzed the 6082 aluminum alloy FSW joint by adopting electron
back scattered diffraction (EBSD) technology; results showed that there were both long
strip grains and equated grains in the weld nugget zone, that there were small angle
grain boundaries in some equated grains, and that the composition of small angle grain
boundaries increased. In some regions, the size of the micro-structure of ultra-fine grains
was about 2 µm after complete dynamic re-crystallization.
Materials 2021, 14, 5287. https://doi.org/10.3390/ma14185287 https://www.mdpi.com/journal/materials
