CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Aluminum Alloy and Galvanized Steel Pulsed MIG Arc Melt-Brazing Joints
Literature Overview
This paper, published in Acta Metallurgica Sinica in 2012 by researchers from the Key Laboratory of Liquid-Solid Structural Evolution and Processing for Materials, Ministry of Education, Shandong University, investigates the microstructure and mechanical properties of pulsed MIG arc melt-brazing joints between aluminum alloys and galvanized steel. The work was supported by the National Natural Science Foundation of China (Grant No. 50905099) and the Ministry of Education Doctoral Point Fund (Grant No. 20090131120027). The authors — Qin Guoliang, Su Yuhu, and Wang Shujun — addressed a critical joining challenge in the automotive and lightweight structural industries where aluminum and steel components are increasingly combined for weight reduction.
Core Technical Content
The fundamental challenge in joining aluminum alloys to galvanized steel is the formation of brittle iron-aluminum intermetallic compounds (IMCs) at the interface. The most common IMCs are FeAl, Fe₂Al₅, and FeAl₃, all of which are extremely brittle and have poor mechanical properties. The galvanized coating (typically Zn or Zn-Al alloy) on the steel surface adds another layer of complexity, as zinc also forms intermetallics with aluminum (such as Al₂Zn₃ and AlZn₃).
Intermetallic Compound Formation
| IMC Compound | Crystal Structure | Melting Point (°C) | Hardness (HV) | Brittleness |
|---|---|---|---|---|
| FeAl | BCC (CsCl type) | 1220 | 150–200 | Moderate |
| Fe₂Al₅ | Tetragonal | 1195 | 200–300 | High |
| FeAl₃ | Orthorhombic | 1345 | 250–350 | Very High |
| Al₂Zn₃ | Orthorhombic | 665 | 80–120 | Moderate |
| AlZn₃ | Tetragonal | 655 | 60–100 | Moderate |
The authors employed a pulsed MIG arc melt-brazing process that uses a two-step approach: first, a high-heat-input pulse melts the aluminum side while the steel side remains below the melting point; then, a lower-heat-input pulse completes the joint formation. This approach minimizes the thickness of the intermetallic layer while ensuring adequate metallurgical bonding.
Key Findings on Joint Microstructure
The microstructure of the joint consists of several distinct zones:
- Aluminum weld metal zone: Contains fine dendritic structures with Al-Si eutectic phases, exhibiting good ductility and strength.
- Intermetallic compound layer: A thin layer (typically 5–20 μm) of FeAl and Fe₂Al₅ at the aluminum-steel interface. The thickness is highly sensitive to heat input and holding time.
- Galvanized layer reaction zone: The original Zn coating reacts with the aluminum to form Al₂Zn₃ and AlZn₃ phases, which act as a buffer layer between the aluminum and steel.
- Steel heat-affected zone: Shows minimal microstructural change due to the low heat input on the steel side.
Interpretation of Technical Points
The pulsed MIG arc melt-brazing process described in this paper represents a significant advancement over conventional arc welding for aluminum-steel joints. The key innovation is the use of pulsed current modulation to independently control the heat input on the aluminum and steel sides. By adjusting the pulse frequency, peak current, and background current, the operator can:
- Maintain the aluminum side above its melting point (660°C) for complete wetting and bonding.
- Keep the steel side below its melting point (1538°C) but above the eutectic temperature with zinc (420°C) to promote interfacial reaction without excessive IMC growth.
- Minimize the thickness of the brittle IMC layer to below 15 μm, which is below the critical threshold for joint failure.
The authors demonstrated that the joint strength is primarily governed by the thickness and composition of the IMC layer. Joints with IMC layers thinner than 10 μm exhibit shear strengths of 150–180 MPa, comparable to the base aluminum alloy. Joints with IMC layers thicker than 30 μm exhibit shear strengths below 80 MPa, with brittle interfacial fracture.
Process Parameters for Optimal Joint Quality
| Parameter | Recommended Value | Effect on IMC Thickness |
|---|---|---|
| Pulse frequency | 100–150 Hz | Lower frequency: thicker IMC |
| Peak current (Al side) | 200–300 A | Higher current: thicker IMC |
| Background current | 30–50 A | Maintains arc stability |
| Travel speed | 400–600 mm/min | Higher speed: thinner IMC |
| Wire feed speed | 4.0–6.0 m/min | Matches arc length |
| Shielding gas | Ar + 5% CO₂ | Stable arc, reduced oxidation |
| Preheat (steel side) | 150–200°C | Reduces thermal gradient |
Connection with Engineering Practice
The aluminum-steel joining challenge is directly relevant to the fabrication of hybrid pressure vessels and heat exchangers where aluminum and steel components may be combined. For example, aluminum alloy tubes brazed to steel headers in heat exchangers, or aluminum alloy cladding on steel pressure vessels for specific corrosion environments, require careful control of intermetallic formation.
From a standards perspective, the acceptance criteria for aluminum-steel joints are not well established in most pressure vessel codes. ASME Section VIII and Section II Part D provide limited guidance on dissimilar metal joints involving aluminum and steel. The findings of this paper provide valuable data for developing qualification procedures and acceptance criteria for such joints.
The paper also highlights the importance of the galvanized coating in the joining process. The zinc coating acts as a diffusion barrier, slowing the growth of iron-aluminum IMCs. However, the zinc-aluminum IMCs formed during welding are themselves brittle and can serve as crack initiation sites. The optimal zinc coating thickness (typically 40–80 μm for hot-dip galvanizing) provides sufficient diffusion barrier without compromising the final joint strength.
Study Insights and Reflections
This paper makes a compelling case for pulsed MIG arc melt-brazing as a viable technique for joining aluminum alloys to galvanized steel. The key insight is that the pulsed current modulation allows independent control of heat input on each side of the joint, which is not possible with conventional continuous-current welding processes.
For engineers involved in the fabrication of hybrid aluminum-steel pressure vessels and heat exchangers, the paper provides practical guidance on process parameter selection. The critical finding is that the IMC layer thickness must be controlled below 15 μm for acceptable joint strength, which requires careful control of heat input and holding time. This is achievable with pulsed MIG welding but challenging with conventional processes.
The paper also raises important questions about the long-term durability of aluminum-steel joints. The IMC layer, even when thin, is susceptible to further growth during service at elevated temperatures. For pressure vessel applications operating above 150°C, the IMC layer may grow to critical thickness within the design life. This necessitates either conservative design margins or the use of alternative joining techniques such as mechanical fastening or adhesive bonding.