Zhuojin Pipe Fitting Co., LtdTIG Arc Additive Manufacturing of 5356 Aluminum Alloy Microstructure and Tensile Properties
Literature Overview
This study by Zhao Pengkang and colleagues from Xi'an University of Technology, published in the Journal of Welding (2020, Vol. 41, No. 5, pp. 65-70), investigates the microstructural evolution and mechanical properties of 5356 aluminum alloy deposited through TIG arc additive manufacturing. The research addresses a critical area of rapid prototyping and repair technology for aerospace-grade aluminum alloys, where the 5xxx series remains indispensable for structural applications requiring moderate strength, excellent corrosion resistance, and superior weldability. The work was supported by the China Postdoctoral Science Foundation (2017M613172) and the Shaanxi Provincial Department of Education Natural Science Foundation (17JK0562).
Core Technical Findings
The researchers deposited 5356 aluminum alloy wire onto a substrate using a TIG arc as the heat source, examining three distinct regions: the bottom region, the middle stable region, and the top region. A key structural observation is the alternating horizontal layering of fusion zones and deposited layers, which is characteristic of layer-by-layer additive processes.
| Parameter | Bottom Region | Middle Stable Region | Top Region |
|---|---|---|---|
| Deposited layer width | Maximum (~2.4 mm) | Minimum (~1.6 mm) | Intermediate |
| Grain size in deposited layers | Smallest | Moderate | Larger |
| Fusion zone defect density | Highest (porosity, shrinkage cavities) | Moderate | Lower |
| Hardness stability | Slightly higher than others | Stable | Stable |
| Fusion zone hardness variation | Significant fluctuation | Moderate | Moderate |
Microstructural Characteristics
The deposited layers predominantly exhibit equiaxed grain morphology, which is a direct consequence of the high cooling rates and repeated thermal cycling inherent to the additive process. The matrix contains dispersed black β-Al₃Mg₂ intermetallic phases distributed on a grey background, accompanied by minor quantities of Mg₂Si and (FeMn)Al₆ intermetallic compounds. This secondary phase distribution is consistent with the known precipitation behavior of the Al-Mg-Si alloy system, where the β-Al₃Mg₂ phase is the primary strengthening constituent in the 5xxx series.
The fusion zones, however, present a markedly different picture. These regions are riddled with porosity and shrinkage cavities, which represent the most significant quality concern in TIG arc additive manufacturing. The bottom region exhibits the highest concentration of these defects, likely because the first deposited layers experience the most severe thermal conditions as the entire thermal mass must be built from the substrate upward. The thermal accumulation in subsequent layers creates a more stable thermal environment, which explains the reduced defect density in upper regions.
Mechanical Properties
Tensile testing revealed that in the horizontal direction, strength and ductility show no significant variation across the three regions, with representative values of Rm = 274 MPa and A = 32.3%. In the vertical direction, tensile strength is approximately equal to the horizontal value, but the elongation drops to 26%. Fractographic analysis confirms that both horizontal and vertical fracture surfaces are dominated by equiaxed dimples, indicating ductile fracture mechanisms. The reduction in vertical elongation can be attributed to the increased volume fraction of fusion zone defects, particularly porosity, which acts as crack initiation sites during deformation.
Engineering Practice Implications
From a manufacturing perspective, this study highlights several critical considerations for TIG arc additive manufacturing of aluminum alloys:
- Thermal management is paramount. The progressive reduction in deposited layer width from bottom to top (2.4 mm to 1.6 mm) indicates thermal accumulation effects. In practice, interlayer temperature monitoring and controlled cooling strategies are essential to maintain dimensional accuracy and minimize defects.
- Fusion zone quality governs overall performance. While deposited layers exhibit acceptable mechanical properties comparable to wrought 5356 alloy, the fusion zones remain the weakest link. The porosity and shrinkage cavities identified in the study would need to be addressed through process optimization, such as pulse TIG parameters, wire feed rate adjustment, or preheating strategies.
- Anisotropy management. The directional difference in ductility (32.3% horizontal vs. 26% vertical) has practical implications for component design. Load-bearing paths should preferentially align with the deposition direction wherever possible.
- Process parameter sensitivity. The bottom region's combination of smallest grain size and highest defect density suggests that initial layer parameters require special attention. A lower heat input or modified wire feed strategy for the first few layers could improve fusion zone quality.
Key Questions and Reflections
The study raises important questions about the scalability of TIG arc additive manufacturing for aluminum alloy structural components. While the deposited layer properties are promising, the fusion zone defect population must be reduced to meet aerospace qualification standards. The interplay between thermal cycling, solidification microstructure, and defect formation in additive processes remains an active area of research. Furthermore, the relatively modest strength of 274 MPa for the as-deposited material suggests that post-deposition aging treatment could significantly enhance mechanical performance through controlled precipitation of the β-Al₃Mg₂ phase. The work provides a solid foundation for developing production-ready TIG arc additive processes for 5xxx series aluminum alloys, but substantial process refinement is still required before industrial deployment.