AlSi5 Galvanized Steel TIG Brazing Wetting Spreading and Interface Behavior

Literature Overview

The research conducted by Shi Zhongxing, Li Ruifeng, Yu Shuizhi, Ge Qinglei, and Wu Mingfang, published in Welding Technology in 2014, investigates the wetting, spreading, and interfacial behavior of AlSi5 brazing alloy applied to galvanized steel substrates using the TIG (GTAW) brazing process. This study addresses a critical practical challenge in the automotive and appliance industries, where the brazing of galvanized steel sheets is essential for producing corrosion-resistant joints with minimal distortion. The presence of the zinc coating on the steel substrate introduces complex metallurgical interactions during brazing, including zinc dissolution into the molten brazing alloy, zinc vaporization, and the formation of brittle intermetallic compounds at the joint interface.

Core Technical Content

The study systematically examines the following aspects of AlSi5-on-galvanized-steel TIG brazing:

• The wetting angle and spreading diameter of the AlSi5 brazing alloy on the galvanized steel surface as a function of brazing temperature and holding time.
• The microstructural evolution at the AlSi5/zinc coating/steel substrate interface, including the identification and characterization of intermetallic phases.
• The effect of zinc content in the molten brazing pool on the joint strength and ductility.
• The influence of TIG brazing parameters (current, travel speed, arc voltage) on the joint quality and defect formation.

Wetting and Spreading Behavior

The wetting behavior of AlSi5 on galvanized steel was evaluated using the sessile drop method and in-situ observation during TIG brazing. The key findings are summarized below:

The study reveals that the optimal brazing temperature window for AlSi5 on galvanized steel is approximately 610–630°C, which is above the melting point of zinc (419°C) and the zinc-iron intermetallic phases but below the melting point of the steel substrate. At this temperature range, the zinc coating melts and dissolves into the AlSi5 brazing alloy, promoting good wetting and spreading. However, temperatures above 640°C lead to vigorous zinc vaporization, which can cause porosity in the joint and zinc deposition on the torch nozzle, potentially leading to arc instability.

Interfacial Microstructure and Phase Analysis

Metallographic examination and X-ray diffraction (XRD) analysis of the brazed joints revealed the following interfacial microstructure:

• A thin layer of Fe-Al intermetallic compounds (FeAl, Fe2Al5, and FeAl2) formed at the interface between the molten AlSi5 brazing alloy and the steel substrate.
• The zinc coating acted as a diffusion barrier during brazing, but at temperatures above 600°C, zinc dissolved into the brazing alloy and diffused through the molten pool, ultimately reacting with iron at the interface to form Fe-Zn intermetallics (such as Gamma and Delta phases).
• The total intermetallic layer thickness increased with brazing temperature and holding time, following a parabolic growth kinetics with a diffusion coefficient in the range of 10^-13 to 10^-12 m²/s.
• At brazing temperatures above 630°C, the intermetallic layer exceeded 15 μm in thickness, leading to significant joint embrittlement and a reduction in shear strength by more than 50% relative to the base metal.

TIG Brazing Process Parameters

The TIG brazing parameters optimized in the study are presented below:

The study emphasizes that the use of a flux-free brazing process is feasible with AlSi5 on galvanized steel because the silicon in the alloy acts as a natural flux, dissolving the zinc oxide layer on the coating surface. However, the flux-free approach requires careful control of the brazing temperature to prevent excessive zinc vaporization and oxide reformation.

Defect Analysis and Quality Control

The following defects were identified and analyzed in the study:

The study recommends a multi-step quality control procedure for AlSi5-on-galvanized-steel TIG brazed joints:

1. Visual inspection for wetting coverage and surface appearance.
2. Shear strength testing in accordance with ASTM B333 to verify joint strength.
3. Cross-sectional metallographic examination to assess intermetallic layer thickness (acceptance criterion: total intermetallic layer thickness < 10 μm).
4. Corrosion testing (salt spray test per ASTM B117) to evaluate the corrosion resistance of the brazed joint in the presence of zinc coating remnants.

Engineering Practice and Implications

The findings of this study are directly applicable to the brazing of galvanized steel components in automotive body-in-white assembly, appliance manufacturing, and HVAC systems. The following practical implications emerge:

• The optimal brazing temperature window of 610–630°C is narrow and requires precise thermal control. In production environments, this can be achieved through preheating the substrate to 200–300°C and using a TIG arc with controlled current and travel speed.
• The zinc coating thickness (80–120 g/m²) significantly influences the joint quality. Thicker zinc coatings require higher brazing temperatures to achieve complete dissolution, which increases the risk of excessive intermetallic formation and zinc vaporization. Engineers should consider reducing the zinc coating thickness to 60–80 g/m² for brazable applications.
• The flux-free brazing approach simplifies the production process and reduces post-brazing cleaning requirements, but it demands strict control of the brazing alloy purity and substrate surface condition.
• The intermetallic layer thickness must be monitored as a key quality indicator. A thickness exceeding 10 μm is associated with unacceptable joint brittleness, and a thickness below 2 μm may indicate insufficient metallurgical bonding.

Study Insights and Reflections

This study provides valuable insights into the complex metallurgical interactions that occur during the TIG brazing of galvanized steel with AlSi5 alloy. The most significant finding is that the zinc coating, while providing corrosion protection, introduces substantial metallurgical complexity during brazing. The dissolution of zinc into the brazing alloy and the subsequent formation of Fe-Zn and Fe-Al intermetallics at the interface represent a fundamental challenge that must be managed through careful control of the brazing temperature and holding time.

The study also highlights the importance of process parameter optimization in achieving a balance between adequate wetting and spreading, and the suppression of excessive intermetallic growth. The recommended temperature window of 610–630°C represents a compromise that satisfies both requirements, but it leaves little margin for process variation in production environments. Engineers should consider implementing in-process monitoring techniques, such as arc voltage sensing or thermal imaging, to ensure consistent brazing temperatures throughout the production run.

The research also opens avenues for further investigation, including the development of alternative brazing alloys with reduced zinc solubility, the application of surface treatments to the galvanized steel to improve wettability without zinc dissolution, and the exploration of alternative brazing processes such as induction heating or resistance brazing that may offer better thermal control for galvanized steel applications.