CLADDING TECHNOLOGY SHANXI CO., LTDStudy Note on Microstructure and Properties of Aluminum Alloy-Galvanized Steel Pulse MIG Arc Brazing Joints
Literature Overview and Research Background
This 2012 publication from Shandong University, published in the Acta Metallurgic Sinica, investigates the microstructural evolution and mechanical properties of aluminum alloy to galvanized steel joints produced by pulse MIG arc brazing. The authors Qin Guoliang, Su Yuhu, and Wang Shujun conducted a comprehensive study of the interfacial reaction zone, weld metal microstructure, and overall joint performance. The research was supported by the National Natural Science Foundation of China (Grant No. 50905099) and the Ministry of Education Doctoral Point Fund (Grant No. 20090131120027).
This research is particularly significant for the automotive industry, where aluminum-to-steel joints are increasingly used to achieve lightweighting targets while maintaining structural integrity. The principles investigated also have direct relevance to bimetal pressure vessel engineering, where controlled interfacial reactions are essential for long-term serviceability.
Process Parameters and Experimental Configuration
The study employed a systematic approach to investigate the effects of key process parameters on joint quality. The experimental matrix included variations in arc current, travel speed, and filler wire composition, with metallographic examination, microhardness mapping, and mechanical testing used to characterize the resulting joints.
Experimental Parameters
| Parameter | Range Investigated | Optimal Value |
|---|---|---|
| Arc current | 120-200 A | 160 A |
| Travel speed | 300-800 mm/min | 500 mm/min |
| Filler wire | AlSi5, AlMg5, AlSi12 | AlSi12 |
| Shielding gas | Ar, Ar/CO₂ (80/20) | 100% Ar |
| Wire diameter | 0.8, 1.0, 1.2 mm | 1.0 mm |
| Preheat temperature | 100-250 °C | 180 °C |
| Stickout length | 8-18 mm | 12 mm |
| Pulse frequency | 100-300 Hz | 200 Hz |
The selection of AlSi12 filler wire was found to be optimal for this application because the high silicon content promotes the formation of a thin, uniform intermetallic layer and provides good wetting characteristics on both aluminum and steel substrates. The silicon also acts as a crack arrester in the weld metal, reducing susceptibility to hot cracking during solidification.
Interfacial Microstructure and IMC Formation
The interfacial reaction zone is the most critical region in aluminum-to-steel joints, as it determines the mechanical strength and long-term durability of the connection. The IMCs formed at this interface are inherently brittle, with limited ductility and fracture toughness, and their thickness must be carefully controlled to achieve acceptable joint performance.
IMC Layer Characteristics
| IMC Phase | Crystal Structure | Morphology | Hardness (HV) | Ductility |
|---|---|---|---|---|
| Fe₂Al₅ | Orthorhombic | Plate-like; columnar | 400-500 | Very low |
| FeAl₃ | Tetragonal | Needle-like; lamellar | 350-450 | Very low |
| FeAl | Orthorhombic | Thin layer; discontinuous | 300-400 | Low |
| FeZn₁₃ | Hexagonal | Granular; dispersed | 150-200 | Low |
| Al-Zn solid solution | FCC | Matrix phase | 80-120 | Moderate |
The formation sequence follows a predictable pattern: Fe₂Al₅ forms first at the steel side due to the high thermodynamic driving force for iron-aluminum compound formation, followed by FeAl₃ on the aluminum side. In the presence of zinc from the galvanized coating, additional Fe-Zn intermetallics form at the steel surface, creating a complex multi-phase interfacial region.
Effect of Heat Input on IMC Thickness
| Heat Input (J/mm) | IMC Thickness (μm) | Joint Strength (N/mm) | Failure Mode |
|---|---|---|---|
| 1500 | 3-5 | 45-55 | Interface (IMC) |
| 2000 | 5-10 | 50-65 | Interface (IMC) |
| 2500 | 8-15 | 55-70 | Interface (IMC) |
| 3000 | 12-25 | 45-55 | Interface (IMC) |
| 3500 | 20-40 | 30-40 | Interface (IMC) |
| 4000 | 30-60 | 20-30 | Interface (IMC) |
The relationship between heat input and IMC thickness is nonlinear: initially, increasing heat input promotes diffusion and IMC growth, but beyond an optimal threshold, excessive heat input causes rapid IMC thickening that degrades joint strength. The optimal heat input range of 2000-2500 J/mm produces IMC layers of 5-15 μm, which provide adequate mechanical bonding while maintaining acceptable joint ductility.
Mechanical Properties and Failure Analysis
The mechanical performance of the joints was evaluated through shear testing, tensile-shear testing, and lap joint testing. The results reveal that joint strength is primarily governed by the interfacial IMC layer, with failure consistently occurring at or near the aluminum-steel interface.
Joint Performance Summary
| Test Method | Maximum Strength | Failure Location | Failure Mode |
|---|---|---|---|
| Single shear | 55-70 MPa | Interface | Brittle interfacial fracture |
| Tensile-shear | 60-75 MPa | Interface | Mixed interfacial/IMC fracture |
| Lap joint | 50-65 MPa | Interface | Brittle interfacial fracture |
| Peel test | 30-45 N/mm | Interface | Interfacial separation |
| Fatigue (10⁷ cycles) | 25-35 MPa | Interface | Fatigue crack initiation at IMC |
The joint strength is significantly lower than the base metal strength of either material, which is expected for a brazed joint with a brittle interfacial layer. However, the achieved strengths of 55-75 MPa are adequate for many automotive structural applications where the joint is designed to yield before the base metal fails, providing a controlled energy absorption mechanism during crash events.
Zinc Coating Effects and Corrosion Considerations
The galvanized coating introduces unique challenges and opportunities in aluminum-to-steel joining. During brazing, the zinc coating melts and diffuses into the joint, affecting both the microstructure and the corrosion behavior of the interface.