CLADDING TECHNOLOGY SHANXI CO., LTDBypass Shunting MIG Arc Brazing of Magnesium and Steel Dissimilar Metal Joints
Literature Overview
This paper by Miao Yugang, Wu Bintao, Han Duanfeng, Xu Xiangfang, and Li Xiaoxu from the School of Naval Architecture and Ocean Engineering at Harbin Engineering University was published in the Transactions of the China Welding Institute in 2014. Funded by the National Natural Science Foundation of China, the research investigates a novel bypass shunting MIG arc brazing technique for joining magnesium alloys to steel substrates. This is a significant contribution to dissimilar metal joining technology, which has applications in marine engineering, lightweight structural design, and bimetal component manufacturing.
Core Technical Principles
The bypass shunting MIG arc brazing technique involves redirecting a portion of the welding current through an auxiliary shunting path, thereby reducing the direct thermal input to the joint while maintaining sufficient arc energy for filler metal melting and wetting. This approach addresses the fundamental challenge of joining magnesium alloys to steel: the large difference in melting points, thermal expansion coefficients, and chemical reactivity between the two metals.
Process Mechanism
In conventional MIG welding of dissimilar metals, the high thermal input causes excessive intermetallic compound formation at the interface, leading to brittle weld joints susceptible to cracking. The bypass shunting technique mitigates this by:
- Reducing the peak temperature at the joint interface below the melting point of the magnesium alloy
- Controlling the cooling rate to limit the thickness of intermetallic layers
- Maintaining a liquid filler pool that wets both substrates through capillary action
- Allowing the formation of a brazed joint rather than a fully fused weld
| Parameter | Conventional MIG | Bypass Shunting MIG Brazing |
|---|---|---|
| Peak temperature | Above Mg melting point | Below Mg melting point |
| Intermetallic thickness | Thick, brittle | Thin, controlled |
| Joint strength | Low, prone to cracking | Improved |
| Dilution | High | Minimal |
| Residual stress | High | Reduced |
Metallurgical Analysis
The study reveals that the bypass shunting technique produces a joint microstructure consisting of a thin intermetallic layer at the magnesium interface, a bronze-like reaction zone at the steel interface, and a filler metal core. The intermetallic compounds formed include Mg₂Al₃, Mg₅Al₈, and Mg₁₇Al₁₂, with their thickness and morphology directly influenced by the shunting current ratio.
The optimal shunting current ratio was found to be in the range of 30-50% of the total welding current, depending on the joint configuration and filler metal composition. At lower shunting ratios, insufficient heat input leads to poor wetting and incomplete joint formation. At higher shunting ratios, excessive heat causes excessive intermetallic growth and potential melting of the magnesium substrate.
Engineering Practice and Quality Control
For engineers working on bimetal component fabrication, this technique offers a viable solution for joining lightweight magnesium alloys to structural steel without resorting to mechanical fastening or adhesive bonding. The joint strength achieved through bypass shunting brazing is typically 60-80% of the base metal strength, which is often adequate for structural applications.
From a non-destructive testing perspective, the brazed joint presents unique challenges. Since the joint does not involve full fusion, conventional ultrasonic testing methods may not provide reliable indications. Magnetic particle testing and dye penetrant testing are more suitable for detecting surface and near-surface defects such as lack of fusion, incomplete wetting, and porosity.
Study Insights and Implications
The bypass shunting concept represents a fundamental shift in thinking about dissimilar metal joining. Rather than trying to create a fully fused weld joint between incompatible metals, the technique embraces the brazing paradigm where the filler metal acts as a bridge between the two substrates. This philosophy is consistent with the approach used in explosion cladding and roll-bonded cladding, where the bond is achieved through mechanical interlocking rather than metallurgical fusion.
For pressure vessel applications, the bypass shunting technique could be extended to create clad joints between dissimilar materials, such as titanium-to-steel or zirconium-to-steel interfaces. The key challenge would be developing appropriate filler metals and process parameters for each material combination, along with establishing qualified welding procedures and acceptance criteria under relevant standards.