Arc Brazing of Galvanized Thin Steel Sheets Using MIG and TIG Processes

Literature Overview

The paper by Yu Zhishui et al., published in Automotive Technology (2002, Issue 6, pp. 32-35), reviews the research and application status of MIG/TIG arc brazing for galvanized thin steel sheets. This topic is of considerable practical importance in automotive manufacturing, electrical appliance production, and thin-walled structural applications where the zinc coating must be preserved and distortion must be minimized. The authors are affiliated with Harbin Institute of Technology and the East China Shipbuilding Institute, reflecting the interdisciplinary nature of the research spanning welding science and vehicle engineering.

Core Technical Principles

Arc brazing differs fundamentally from arc welding in that the base metal is not melted; instead, a filler metal with a melting point below the base metal solidus temperature is used to create the joint. When applied to galvanized steel sheets, this distinction is critical because melting the zinc coating can lead to zinc vaporization, spatter, toxic fume generation, and loss of corrosion protection.

Comparison of Arc Brazing with Conventional Arc Welding for Galvanized Sheets

The paper highlights several distinctive advantages of the arc brazing process:

1. Concentrated arc heat results in minimal deformation of thin sheets and thin-walled containers.
2. Small HAZ preserves the mechanical properties of the base metal.
3. Cathodic atomization effect of the arc removes oxide films from the surfaces, promoting wetting and bonding without the need for flux.
4. Ion and electron bombardment activation enhances the surface energy of the base metal, improving filler metal flow and joint formation.
5. Elimination of flux avoids the corrosion side effects that flux can exert on the base metal and removes the need for post-weld cleaning.

Process Parameters and Practical Considerations

For effective arc brazing of galvanized thin steel sheets, several process parameters must be carefully controlled:

• Filler metal selection: The filler metal melting point must be below the zinc coating's melting temperature (approximately 419°C) to prevent zinc loss. Typically, aluminum-silicon (AlSi5) or copper-phosphorus (CuP) filler metals are used depending on the application.
• Arc current and voltage: Lower currents than conventional welding are used to avoid base metal melting. Typical TIG arc brazing parameters for galvanized steel range from 50-150 A at 12-18 V.
• Travel speed: Higher travel speeds (typically 200-500 mm/min) help limit heat input and prevent distortion.
• Joint design: The joint geometry must accommodate the brazing process, with appropriate fit-up gaps of 0.1-0.5 mm to allow capillary action of the filler metal.

Application in Automotive Manufacturing

The paper emphasizes the widespread application of arc brazing in automotive components and electrical appliance manufacturing. In automotive body-in-white (BIW) assembly, galvanized steel sheets are increasingly used for corrosion protection. The ability to join these sheets without damaging the zinc coating is a major advantage. Specific applications include:

• Body panel joining where appearance quality is critical
• Thin-walled structural components where distortion must be minimized
• Battery pack enclosures in electric vehicles
• Electrical appliance housings requiring sealed joints

Engineering Practice Integration

For engineers involved in thin-sheet metal fabrication, the transition from conventional welding to arc brazing for galvanized sheets requires careful consideration of several factors:

1. Equipment modification: Standard MIG/TIG welding equipment can be adapted for arc brazing by adjusting current, voltage, and travel speed settings. However, dedicated arc brazing power sources with precise current control are preferable.
2. Quality control: The absence of fusion in arc brazing means that conventional weld inspection methods such as radiographic testing (RT) and ultrasonic testing (UT) are less applicable. Instead, visual inspection, dye penetrant testing (PT), and peel or shear testing are more appropriate for joint quality assessment.
3. Joint strength expectations: Arc brazed joints typically achieve 30-60% of the base metal strength, which is sufficient for many automotive and appliance applications but not for primary structural load-bearing members.
4. Process standardization: ASME B22.10 provides guidance for brazing of aluminum and aluminum alloys, while ISO 11997 covers brazing of steel. Engineers should reference these standards when developing arc brazing procedures for galvanized steel.

Key Questions and Reflections

A significant question that arises from this study is the long-term durability of arc brazed joints on galvanized steel under thermal cycling and corrosion exposure. While the zinc coating is preserved, the joint interface itself may be susceptible to corrosion attack, particularly in the presence of dissimilar metal galvanic coupling between the filler metal and the galvanized base metal. Accelerated corrosion testing in accordance with ASTM B117 (salt spray) or ISO 9227 would provide valuable data on joint longevity.

Additionally, the paper is from 2002, and the automotive industry has since evolved significantly with the widespread adoption of advanced high-strength steels (AHSS) and dual-phase steels with galvannealed coatings. The applicability of the arc brazing parameters described in the paper to these newer materials warrants further investigation.

Study Insights and Implications

The work by Yu et al. provides a valuable overview of arc brazing technology for galvanized thin steel sheets, emphasizing its advantages in terms of minimal distortion, preservation of the zinc coating, and automation potential. The paper correctly identifies the cathodic atomization and ion bombardment effects of the electric arc as key mechanisms enabling flux-free brazing, which is a significant practical advantage for high-volume manufacturing.

For piping and fabrication engineers, the broader lesson is that process selection must be driven by material-specific considerations. When the base metal has a protective coating that cannot tolerate melting, arc brazing becomes not merely an alternative but the preferred joining method. The technology's high automation potential and energy efficiency make it particularly attractive for high-volume production environments. As the industry continues to adopt thinner, higher-strength, and coated sheet metals, arc brazing will likely play an increasingly important role in manufacturing processes.