Zhuojin Pipe Fitting Co., LtdComparative Study of FSW and MIG Joints in AZ31B Magnesium Alloy
Literature Overview
This paper by Xia Luosheng and Zhu Shuhong (2014, Ordnance Materials Science and Engineering, Vol. 37, No. 1, pp. 43-46) presents a systematic comparison between Friction Stir Welding (FSW) and Metal Inert Gas (MIG) welding for AZ31B wrought magnesium alloy. The work was funded by the Hunan Provincial Department of Education (11C1290) and the Zhangjiajie Aviation Vocational and Technical College research program (ZHKT2013-006). The study is particularly significant for aerospace and lightweight structural applications where magnesium alloys are increasingly substituted for aluminum alloys due to their superior specific strength.
Core Technical Findings
Microstructural Evolution
The fundamental difference between FSW and MIG joints lies in the thermal-mechanical cycle imposed on the base material. In FSW, the weld nugget zone experiences a combined action of frictional heat, mechanical stirring, and thermoplastic flow. This results in fine equiaxed recrystallized grains with clearly defined grain boundaries, averaging approximately 5 μm in diameter. The absence of melting in FSW prevents grain coarsening and avoids the formation of intermetallic phases that typically form during solidification.
In contrast, the MIG weld zone undergoes intense arc heating followed by rapid cooling. The resulting microstructure consists of equiaxed grains with an average size of approximately 20 μm — four times coarser than the FSW nugget zone. This coarsening is attributed to the high thermal gradient and rapid solidification rate inherent to arc welding processes.
| Parameter | FSW Joint | MIG Joint |
|---|---|---|
| Average grain size | ~5 μm | ~20 μm |
| Grain morphology | Equiaxed recrystallized | Equiaxed (solidification) |
| Melting involved | No | Yes |
| Thermal cycle | Moderate, no melting | High, rapid cooling |
| Grain boundary definition | Clear | Clear |
Mechanical Properties
The FSW joint demonstrated superior mechanical performance with an average tensile strength of 249.8 MPa, representing 96.1% of the base material strength. The average elongation reached 11.6%. This near-parent-material performance is a hathe writing systemark of FSW and is directly attributable to the fine recrystallized microstructure and the absence of solidification defects.
Fracture Analysis
Both joints exhibited fracture surfaces at approximately 45° to the tensile axis, consistent with shear-dominated failure mechanisms in ductile materials. However, the fracture locations and morphologies differed significantly:
- FSW joints fractured in the Heat-Affected Zone (HAZ), with fracture surfaces showing dimples of widely varying sizes, indicating a mixed ductile-brittle failure mode influenced by the tempering effect in the HAZ.
- MIG joints fractured in the weld zone, with fracture surfaces exhibiting both dimples and tear ridges, suggesting a more complex failure mechanism involving both microvoid coalescence and quasi-cleavage.
Engineering Practice Implications
For magnesium alloy structures in aerospace applications, the FSW process offers decisive advantages. The near-parent-material strength retention (96.1%) makes it suitable for load-bearing joints where MIG would introduce unacceptable soft spots. However, FSW is limited to lap joints and specific geometries, whereas MIG offers greater flexibility for butt joints and complex configurations.
The 45° fracture angle in both joints is consistent with the maximum shear stress criterion, which governs ductile fracture in materials with relatively low strain-rate sensitivity such as AZ31B. The presence of large dimples in the FSW HAZ fracture suggests that the tempering effect in the HAZ creates a localized zone of reduced strength but retained ductility, which can be exploited in fatigue-critical designs.
Key Reflections
This study underscores a fundamental principle in welding metallurgy: processes that avoid melting (such as FSW) inherently preserve the base material microstructure more effectively than fusion welding processes. For magnesium alloys, which are particularly susceptible to grain coarsening and intermetallic formation during solidification, this distinction is critical. In pipe fitting manufacturing, where FSW is currently impractical for butt-welded configurations, understanding these microstructural differences informs the selection of post-weld heat treatment to mitigate MIG weld softening.