CLADDING TECHNOLOGY SHANXI CO., LTDMIG Melt-Braze Welding of Pure Aluminum to Galvanized Steel Sheet
Literature Overview
This 2013 research published in the Journal of Aeronautical Materials by Guo Xuming, Wang Jianmei, and Xu Rongzheng from Shenyang Aerospace University investigates the MIG melt-braze welding process for joining pure aluminum to galvanized steel sheet. This is a challenging dissimilar metal joining problem because aluminum and steel have vastly different physical and metallurgical properties. Conventional fusion welding of aluminum to steel typically produces brittle intermetallic compounds (FeAl, FeAl₃, Fe₂Al₅, FeAl₆) that severely compromise joint strength and ductility. The melt-braze welding approach offers a solution by creating a hybrid joint where the aluminum side undergoes fusion while the steel side is brazed (wetted) without melting.
Technical Principles and Process Parameters
The fundamental principle of MIG melt-braze welding is that the welding arc simultaneously melts the aluminum workpiece and filler wire while heating the galvanized steel sheet to a temperature sufficient to wet its surface with the molten aluminum without achieving full fusion. The zinc coating on the steel sheet plays a crucial role in the wetting process, acting as an intermediate layer that facilitates adhesion between the aluminum and the steel substrate.
Critical Process Parameters
| Parameter | Recommended Value | Rationale |
|---|---|---|
| Wire diameter | 1.0–1.2 mm | Balances deposition rate and heat input control |
| Current | 120–180 A | Sufficient to melt aluminum, insufficient to melt steel |
| Voltage | 16–20 V | Controls arc length and wetting characteristics |
| Travel speed | 300–600 mm/min | Determines heat input and wetting zone width |
| Shielding gas | 100% Ar or Ar/CO₂ (95/5) | Protects molten aluminum from oxidation |
| Wire stick-out | 8–12 mm | Affects arc stability and heat distribution |
| Preheating | 50–100°C on steel side | Improves wetting and reduces thermal gradient |
The key challenge is maintaining the steel surface temperature in the range of 400–550°C—high enough for the molten aluminum to wet the zinc coating but low enough to avoid steel melting and excessive intermetallic formation. The zinc layer acts as both a wetting agent and a diffusion barrier, limiting the extent of intermetallic compound growth at the interface.
Interface Metallurgy and Joint Characterization
The joint structure in a melt-braze weld typically consists of:
- Fused aluminum zone: Fully solidified aluminum weld metal with good ductility
- Wetting/bonding layer: A thin layer of aluminum-zinc alloy at the steel surface
- Limited intermetallic layer: A controlled thickness of Fe-Zn intermetallics (η-phase, δ-phase) at the zinc-steel interface
- Steel substrate: Heated but not melted, retaining its original microstructure
The thickness of the intermetallic layer is critical to joint performance. Layers thinner than 5 μm generally provide good ductility and strength, while layers exceeding 15 μm become brittle and prone to fracture. The MIG melt-braze process, when properly controlled, typically produces intermetallic layers in the 3–10 μm range, which represents an acceptable compromise between bond strength and ductility.
Joint Strength Characteristics
| Test Method | Typical Result | Acceptance Criteria |
|---|---|---|
| Shear strength | 30–60 MPa | Depends on application requirements |
| Peel strength | 100–300 N/mm | Indicates bonding quality |
| Ductility (extension) | 5–15% | Depends on joint design |
| Corrosion resistance | 200–500 h in salt spray | Depends on coating integrity |
Engineering Practice and Quality Control
From my experience with bimetal products and clad plate fabrication, the melt-braze welding concept shares important principles with explosive cladding and roll-bonded cladding, where the goal is to create a strong metallurgical bond between dissimilar materials while controlling intermetallic formation. The following quality control measures are essential:
- Pre-weld cleaning: Both the aluminum surface and the galvanized steel surface must be thoroughly cleaned to remove oxide films and contamination. Aluminum oxide is particularly problematic because it has a melting point of 2050°C, far above the aluminum melting point, and must be removed by mechanical or chemical means before welding.
- Parameter monitoring: Real-time monitoring of current, voltage, and travel speed is essential to maintain the delicate balance between aluminum fusion and steel non-fusion. Process deviations can result in either incomplete wetting (insufficient bond) or steel melting (excessive intermetallic formation).
- Post-weld inspection: Visual inspection should verify complete wetting of the steel surface without burn-through. Cross-sectional metallographic examination should confirm acceptable intermetallic layer thickness. Mechanical testing (shear, peel, or tensile) should verify joint strength meets requirements.
Common Defects and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Incomplete wetting | Insufficient heat input or contaminated surface | Increase current, improve cleaning |
| Burn-through of steel | Excessive heat input or high travel speed | Reduce current, increase travel speed |
| Excessive intermetallic | Overheating of zinc coating | Optimize heat input, reduce dwell time |
| Porosity in aluminum weld | Oxide inclusion or gas entrapment | Improve shielding gas coverage, clean surfaces |
| Cracking at interface | Thermal stress or brittle intermetallic | Reduce heat input, add preheat |
Study Insights and Implications
The melt-braze welding approach represents an elegant solution to the dissimilar metal joining challenge that is relevant to multiple engineering applications. In the context of bimetal pressure vessels, the same principles apply when joining dissimilar materials in heat exchanger tubesheets or when applying corrosion-resistant overlays to structural steel. The key insight is that controlled partial melting can create stronger and more ductile joints than full fusion welding, which inevitably produces brittle intermetallic phases.
For engineers working on aluminum-steel composite structures in aerospace and automotive applications, this study provides a practical welding process that avoids the need for expensive fasteners, adhesives, or intermediate joining layers. The process is compatible with high-speed automated production and can be integrated into robotic welding systems. The limitations include sensitivity to surface preparation quality and the requirement for precise parameter control, which demands skilled operators or sophisticated process monitoring systems.