Zhuojin Pipe Fitting Co., LtdDC Magnetic Field Assisted TIG Welding of Copper-Steel Dissimilar Joints Microstructure and Mechanical Properties
Literature Overview
This paper published in Materials Reports (2021, Vol. 35, Issue 22, pp. 22137-22140) investigates the effect of direct current magnetic field (DCMF) assistance on the microstructure and mechanical properties of copper-steel butt joints welded by gas tungsten arc welding (GTAW/TIG). The research team from Nanchang Hangkong University and Nanchang University employed a systematic approach to optimize the magnetic field strength for improving joint performance. The work is funded by the National Natural Science Foundation of China (Grant No. 51965045), reflecting its significance in the field of dissimilar metal welding technology.
Core Technical Findings
The study reveals that when the magnetic field strength reaches B = 10 mT, the joint exhibits excellent surface morphology with no porosity or other welding defects. The tensile strength follows a trend of first increasing and then decreasing as the magnetic field strength increases, reaching an optimal value of 194.6 MPa at B = 10 mT, which represents a 25.79% improvement compared to the non-magnetic condition. Additionally, the micro-Vickers hardness in the fusion zone increased by 8.8% at the optimal magnetic field strength.
Microstructural Analysis of the Fusion Zone
The joint structure comprises six distinct zones: steel base metal, steel-side heat-affected zone (HAZ), steel-side fusion zone, weld metal zone, copper-side HAZ, and copper base metal. The fusion zone microstructure is composed of copper-steel solid solutions with two distinct layers:
| Layer | Microstructural Feature | Description |
|---|---|---|
| Layer I | Spherical large particles | Contains iron-rich phase with copper particles |
| Layer II | Strip-shaped small particles | Contains iron-rich phase with (α+ε) duplex structure |
At B = 10 mT, the spherical iron-rich phase in Layer I exhibits obvious agglomeration with increased copper particle content. The vortex-shaped (α+ε) duplex organization becomes more prevalent with increased vorticity, and the iron matrix transforms from a network structure to small strip-shaped morphology. In Layer II, the iron-rich phase disappears and the thickness of the (α+ε) duplex organization becomes thinner.
Mechanism of Strength Enhancement
The increase in vorticity of the (α+ε) duplex organization and the agglomeration of iron-rich phase in the fusion zone contribute to mechanical interlocking and enhanced second-phase strengthening effects. This dual mechanism explains the simultaneous improvement in joint strength and hardness observed at the optimal magnetic field strength of 10 mT.
Engineering Practice Implications
For engineers working on dissimilar metal welding applications, particularly in aerospace and electrical engineering where copper-steel joints are common, this research provides valuable guidance. The optimal magnetic field strength of 10 mT is practically achievable with existing electromagnetic equipment. The mechanical interlocking mechanism identified in this study suggests that similar magnetic field assistance strategies could be applied to other dissimilar metal combinations, such as aluminum-steel or titanium-steel joints, where interface bonding strength is a critical concern.
Process Parameter Recommendations
Based on the findings, the following process recommendations emerge for practical implementation:
- Magnetic field strength should be controlled within the range of 8-12 mT to achieve optimal results without introducing defects.
- The TIG welding parameters (current, voltage, travel speed) should be maintained at values suitable for the specific copper and steel grades being joined.
- Post-weld inspection should focus on the fusion zone region where the most significant microstructural changes occur.
- Quality control procedures should include hardness profiling across the joint to verify the expected 8.8% improvement in the fusion zone.
Study Insights and Reflections
This research demonstrates the potential of electromagnetic field assistance as a non-contact, non-invasive method to improve welding quality. The mechanism of action through Lorentz force effects on the weld pool fluid dynamics, which promotes better mixing and reduces segregation, is particularly noteworthy. The identification of mechanical interlocking and second-phase strengthening as the primary mechanisms for strength improvement provides a clear physical understanding that can guide further optimization efforts. Engineers should note that while the 25.79% improvement in tensile strength is significant, it should be evaluated in the context of the specific application requirements and the relative costs of implementing magnetic field assistance equipment.