Zhuojin Pipe Fitting Co., LtdPartially Melted Zone Microstructure of 6005A/5083 Aluminum Alloy MIG Weld
Literature Overview
The paper by Xin Kai, Xing Yanshuang, Zhang Hao, and He Changshu, published in Light Alloy Fabrication Technology in 2021 (Volume 49, Issue 8, pages 51–62), investigates the microstructure of the partially melted zone (PMZ) in a two-layer, two-pass MIG weld joining 6005A aluminum alloy extrusion profiles to 5083 aluminum alloy plates. The research was supported by the Xingliao Talent Plan Project (XLYC1808038). The study focuses on grain boundary liquation behavior in the PMZ and its relationship to the original microstructure near the weld groove.
Core Technical Findings
The researchers observed grain boundary liquation in the PMZ that was clearly correlated with the original microstructure at the weld groove. The 6005A aluminum alloy extrusion profile exhibited a coarse grain layer of approximately 1000–3000 μm thickness at the groove surface. In the first pass PMZ (PMZ I), a continuous distribution of coarse grain structure was observed, while in the second pass PMZ (PMZ II), localized coarse grain structure was found. These coarse grain regions were identified as remnants of the original groove surface coarse grain layer.
PMZ Microstructural Analysis
| PMZ Region | Parent Material | Microstructural Feature | Liquation Mechanism |
|---|---|---|---|
| PMZ I (coarse grain) | 6005A | Continuous coarse grain | Pool metal infiltration along grain boundaries |
| PMZ II (coarse grain) | 6005A | Localized coarse grain | Pool metal infiltration along grain boundaries |
| PMZ I (fine grain) | 6005A | Fine grain | Pool infiltration + low-melting eutectic melting |
| PMZ II (fine grain) | 6005A | Fine grain | Primarily low-melting eutectic melting |
| PMZ (5083 side) | 5083 | Elongated grain boundary liquation | Pool metal infiltration along elongated grain boundaries |
Metallurgical Interpretation
The study reveals two distinct mechanisms of grain boundary liquation in the PMZ. In the coarse grain regions, the primary mechanism is the infiltration of pool metal along grain boundaries. In the fine grain regions, both pool metal infiltration and the melting of low-melting-point eutectic phases contribute to liquation, with the latter being the dominant mechanism for grain boundary liquation occurring farther from the weld pool.
The 5083 aluminum alloy plate exhibited a fine fibrous microstructure near the groove, and the PMZ on this side showed liquation along the boundaries of elongated grains. This is consistent with the anisotropic nature of the extrusion and rolling process that produces the elongated grain structure in 5083 plate.
Engineering Implications for Dissimilar Aluminum Alloy Welding
Dissimilar aluminum alloy welding, such as 6005A to 5083, is common in rail vehicle manufacturing, as evidenced by the involvement of CRRC Qingdao Sifang in this research. The PMZ is a critical region for crack susceptibility because grain boundary liquation can lead to hot cracking during solidification and to intergranular cracking under post-weld service conditions. The presence of a coarse grain layer at the groove surface of the 6005A extrusion profile exacerbates this risk because coarse grains provide longer paths for crack propagation and are more susceptible to liquation.
Key Reflections
This study provides valuable insights into the relationship between the original microstructure of aluminum alloy components and the PMZ liquation behavior in dissimilar welds. The finding that the groove surface coarse grain layer is preserved in the PMZ highlights the importance of pre-weld surface preparation, such as machining or grinding, to remove the coarse grain layer and reduce liquation susceptibility. Engineers working on aluminum alloy welding should pay close attention to the microstructural characteristics of the parent materials near the weld groove, as these characteristics directly influence the weldability and service performance of the joint.