Interface Microstructure and Mechanical Properties of TIG Brazed Aluminum Alloy/Galvanized Steel Joints

Literature Overview

The study by Lin Sanbao et al. (2009), published in Welding (Chinese journal), investigates the TIG brazing of aluminum alloy to galvanized steel using aluminum-based brazing filler metal. This is a critical research topic in the field of dissimilar metal joining, particularly relevant to automotive manufacturing where lightweight aluminum alloy components are increasingly used in conjunction with galvanized steel structures for corrosion protection. The research was conducted at the State Key Laboratory of Advanced Welding Production Technology, Harbin Institute of Technology, and was supported by the National Natural Science Foundation of China (grant 50874033).

Brazing Process and Wetting Behavior

The fundamental challenge in joining aluminum alloy to steel is the large difference in melting points and the tendency to form brittle intermetallic compounds at the interface. The TIG brazing approach addresses this challenge by using the aluminum-based filler metal as the wetting and bonding medium, while ensuring that the steel substrate remains in the solid state.

The wetting angle of less than 20 degrees on the galvanized steel surface indicates excellent wetting behavior of the aluminum-based filler metal. This is attributed to the zinc coating on the steel surface, which provides a more favorable surface energy for aluminum wetting compared to bare steel. The zinc layer acts as a diffusion barrier and reaction intermediate, moderating the formation of iron-aluminum intermetallic compounds.

Interface Microstructure Analysis

The interface microstructure reveals a complex sequence of intermetallic compound formation. From the weld seam side to the steel substrate side, the intermetallic compounds follow a specific transformation sequence:

This layered intermetallic structure is characteristic of diffusion-controlled reactions between aluminum and iron at elevated temperatures. The formation of multiple intermetallic phases indicates that the brazing temperature and holding time were sufficient to drive the reaction to multiple equilibrium phases. The total intermetallic layer thickness of less than 9.0 micrometers is relatively thin, which is beneficial for maintaining joint ductility and fracture resistance.

On the aluminum alloy side, the joint exhibits a fusion welding characteristic, with significantly enlarged grain size in the weld zone. This grain coarsening is a direct consequence of the thermal cycling during brazing, where the aluminum alloy side experiences temperatures above its solidus point. The enlarged grains reduce the grain boundary area, which can affect the mechanical properties and corrosion resistance of the joint.

Defect Analysis and Failure Mechanism

A critical finding of this research is the identification of local "non-brazed" defects at the joint interface, which serve as crack initiation sites. These defects occur where the brazing filler metal fails to achieve complete wetting and bonding with the substrate, creating weak interfaces that are prone to crack propagation under mechanical loading.

The fracture occurring at the weld root near the weld seam indicates that the weakest link in the joint is the interface region where the brazing connection transitions to the fusion-welded aluminum alloy side. The tensile strength of 90 MPa is relatively low, reflecting the combined effects of the brittle intermetallic layer, the non-brazed defects, and the grain coarsening in the aluminum alloy side.

Engineering Practice Considerations

For engineers designing aluminum alloy to galvanized steel joints using TIG brazing, several practical considerations emerge from this research:

1. Surface preparation – The zinc coating on galvanized steel is essential for achieving good wetting; any damage to the zinc layer during handling or fabrication can lead to non-brazed defects.
2. Thermal control – The brazing temperature must be carefully controlled to minimize intermetallic layer growth while ensuring complete wetting and bonding.
3. Joint design – The lap joint configuration used in this study is suitable for shear loading but may be less effective for tensile loading perpendicular to the joint plane.
4. Quality inspection – Non-brazed defects are difficult to detect by conventional non-destructive testing methods and may require specialized techniques such as ultrasonic testing or visual inspection of the joint edges.

The relatively low tensile strength of 90 MPa should be evaluated against the specific application requirements. For automotive body structures where shear loading is the dominant failure mode, the joint may be adequate if the shear strength is significantly higher than the tensile strength. However, for structural applications requiring high tensile strength, alternative joining methods such as resistance spot welding with dissimilar metal electrodes or mechanical fastening may be more appropriate.

Study Insights and Outlook

This research provides valuable fundamental data on the TIG brazing of aluminum alloy to galvanized steel, including the wetting behavior, intermetallic compound formation sequence, and failure mechanisms. The identification of non-brazed defects as the primary crack initiation sites is a critical quality control insight that should guide inspection procedures and process optimization.

Future research should focus on methods to eliminate or minimize non-brazed defects, such as improved surface preparation techniques, optimized filler metal composition, or controlled heating sequences. Additionally, the long-term durability of the joint under cyclic loading and corrosive environments should be investigated, as the intermetallic layers may be susceptible to stress corrosion cracking or intergranular corrosion. The development of reliable non-destructive testing methods for detecting non-brazed defects is also essential for industrial implementation.