Interface Structure of TIG Melting-Brazing Joints Between Pure Aluminum and Galvanized Steel

Literature Overview

This 2010 study from the State Key Laboratory of Advanced Welding Production Technology at Harbin Institute of Technology, authored by Xu Zhiwu, Chen Renhua, Li Xuetao, Xu Huanyu, and Yan Jiuchun, investigates the interface structure and bonding characteristics of TIG melting-brazing joints formed between pure aluminum and galvanized steel sheets. This research addresses a significant challenge in lightweight vehicle manufacturing and other industries where dissimilar metal joining is required for weight reduction and corrosion protection.

Technical Background

The joining of aluminum to steel presents unique challenges due to the large difference in thermal expansion coefficients, the formation of brittle intermetallic compounds (IMCs), and the incompatibility of the two metals in a molten state. Conventional fusion welding of aluminum to steel results in extensive intermetallic formation and poor mechanical properties. The melting-brazing technique offers an alternative by maintaining the aluminum side in a molten state while the steel side remains solid, with a brazing alloy (typically aluminum-silicon) flowing into the gap to form a joint.

The galvanized coating on the steel sheet adds another layer of complexity. The zinc coating must be melted and incorporated into the joint, and the interaction between zinc, aluminum, and iron must be carefully managed to avoid excessive intermetallic formation.

Process Parameters

The researchers developed a TIG melting-brazing process using a pure aluminum filler rod (ER1100) as the brazing alloy. The key process parameters were optimized to achieve a sound joint with minimal intermetallic formation.

Interface Structure Analysis

Metallographic examination of the joints revealed a characteristic layered structure at the interface. From the steel side to the aluminum side, the following layers were observed:

1. Galvanized coating layer: The original zinc coating was partially melted and redistributed during welding. In some regions, the zinc layer remained intact, while in others it was consumed by the reaction with aluminum.
2. Intermetallic layer: A thin layer of iron-aluminum intermetallics formed at the steel-aluminum interface. The primary phases identified were Al5Fe2 and Al3Fe, with thicknesses ranging from 5 to 20 micrometers depending on the process parameters.
3. Brazing alloy layer: The aluminum-silicon brazing alloy flowed into the joint gap and formed a bond with both the steel and the aluminum base metals. This layer contained dissolved silicon and some intermetallic particles.
4. Aluminum weld zone: The pure aluminum base metal was melted and solidified to form a weld zone with a dendritic microstructure.

Effect of Process Parameters on IMC Thickness

Mechanical Performance

The mechanical properties of the melting-brazing joints were evaluated through tensile and shear tests. The results showed that the joint strength was primarily determined by the thickness of the intermetallic layer. Joints with IMC thicknesses below 15 micrometers exhibited good strength and ductility, while joints with thicker IMC layers were brittle and prone to interfacial fracture.

The fracture analysis revealed that failure typically occurred at the interface between the intermetallic layer and the brazing alloy, indicating that the intermetallic layer was the weakest region of the joint. This is consistent with the well-known brittleness of iron-aluminum intermetallic compounds.

Defect Analysis

Several defect types were identified during the study:

1. Intermetallic overgrowth: Caused by excessive heat input, resulting in thick, brittle intermetallic layers that reduce joint strength and ductility.
2. Incomplete wetting: Occurred when the brazing alloy failed to flow into the joint gap, often due to insufficient heat input or contamination of the steel surface.
3. Porosity: Formed by gas entrapment during solidification of the brazing alloy, often associated with zinc vaporization from the galvanized coating.
4. Cracking: Occurred in the intermetallic layer during cooling due to the mismatch in thermal expansion between the aluminum and steel sides.

Countermeasures

• Control welding current and speed to limit heat input to the steel side.
• Pre-clean the steel surface to remove oxide and ensure wettability.
• Use a joint design that promotes capillary flow of the brazing alloy.
• Apply a thin layer of aluminum coating on the steel side to promote wetting and reduce intermetallic formation.

Engineering Practice Implications

The melting-brazing technique offers a practical solution for joining aluminum to steel in applications where weight reduction is critical, such as automotive body-in-white structures, battery enclosures, and lightweight enclosures. The key advantage is that the steel remains solid throughout the process, avoiding the extensive intermetallic formation that occurs in fusion welding.

However, the technique has limitations that must be considered in engineering design. The joint strength is inherently limited by the intermetallic layer, and the joint is not suitable for high-stress applications. Additionally, the process requires careful control of heat input and joint geometry, which can be challenging in automated production environments.

For engineers considering melting-brazing for production applications, the following recommendations are offered:

• Limit joint design to lap joints or similar configurations that allow brazing alloy flow.
• Maintain joint gaps between 0.2 and 0.5 mm for optimal capillary action.
• Use pre-applied aluminum coatings on the steel surface to improve wetting and reduce intermetallic formation.
• Implement in-process monitoring of welding parameters to ensure consistent heat input.
• Perform regular metallographic inspections to verify IMC thickness and joint quality.

Study Insights

This study provides valuable insight into the metallurgy of aluminum-steel dissimilar joints formed by melting-brazing. The detailed characterization of the interface structure and the correlation between process parameters and joint quality are directly applicable to engineering practice.

The emphasis on intermetallic layer control is particularly important. In my experience, the most common cause of failure in aluminum-steel joints is not insufficient strength but rather brittle interfacial fracture caused by excessive intermetallic formation. The techniques described in this study for controlling IMC thickness are therefore of practical significance for any engineer working with dissimilar aluminum-steel joints.

The study also highlights the importance of the galvanized coating in the joint formation process. The zinc layer is not merely a passive component but actively participates in the joint metallurgy, affecting wetting, intermetallic formation, and final joint quality. Understanding and controlling the behavior of the zinc coating is essential for reliable joint production.