Effect of Welding Current on Microstructure and Mechanical Properties of Magnesium/Galvanized Steel TIG Brazing Joints

Literature Overview

Published in 2016 in the Chinese Journal of Engineering Science, this paper by Hong Miao, Yuan Xin-jian, and Liang Xue-bo from Chongqing University investigates the influence of welding current on the microstructure and mechanical properties of magnesium/galvanized steel TIG brazing joints. The research is supported by the National Natural Science Foundation of China and central university basic research business fees, indicating significant academic and institutional backing. The study addresses a critical challenge in lightweight vehicle manufacturing: joining magnesium alloys to galvanized steel using a low-temperature brazing process.

Core Technical Content

Magnesium alloys are attractive for lightweight applications due to their low density (approximately 1.74 g/cm³), but their use is limited by poor corrosion resistance and difficulty in joining to other materials. Galvanized steel, with its zinc coating, provides corrosion protection but introduces additional complexity in joining processes. The TIG brazing process, using a filler metal with a melting point lower than both base metals, offers a solution that avoids melting the base metals and minimizes intermetallic compound formation.

TIG Brazing Process Principles

TIG brazing differs from TIG welding in several key aspects:

Aspect	TIG Welding	TIG Brazing
Base metal state	Melts to form fusion	Remains solid, heated above filler melting point
Filler metal	Optional, may be added	Required, wets and flows into joint
Joint formation	Fusion of base metals and filler	Capillary action of liquid filler
Temperature control	High, above base metal melting point	Lower, between filler melting and base metal melting
Dilution	Occurs in weld metal	Minimal, base metals remain solid

For magnesium/galvanized steel brazing, the process parameters must be carefully controlled to:

Achieve complete wetting of both base metals by the filler
Avoid melting of the magnesium or the zinc coating on the steel
Minimize the formation of brittle intermetallic compounds at the interfaces
Ensure adequate mechanical strength and ductility of the joint

Effect of Welding Current on Microstructure

The welding current is the primary parameter controlling the heat input and temperature distribution during TIG brazing. The paper investigates how varying current levels affect:

Joint geometry and filler distribution: Higher currents increase the temperature and fluidity of the filler, potentially improving wetting but risking excessive spreading or base metal melting.
Intermetallic compound formation: The type, morphology, and thickness of intermetallic compounds at the Mg/filler and filler/steel interfaces are strongly influenced by the thermal cycle. Higher currents and longer heating times promote intermetallic growth, which can embrittle the joint.
Grain structure and texture: The solidification conditions in the brazing seam determine the grain size, orientation, and phase distribution. Rapid solidification at lower currents produces finer grains, while slower cooling at higher currents allows grain growth.
Defect formation: Insufficient current may lead to incomplete wetting and lack of fusion, while excessive current can cause base metal melting, zinc vaporization, or oxide inclusion formation.

Mechanical Property Evolution

The mechanical properties of the brazing joint are directly related to the microstructure and intermetallic compound formation:

Property	Effect of Low Current	Effect of High Current
Tensile strength	May be low due to incomplete wetting	May be high but brittle due to intermetallics
Elongation	May be low if joints are defective	Typically low due to brittle intermetallics
Hardness	May be non-uniform	May show hard intermetallic regions
Fracture mode	May show lack of fusion	Typically intergranular or intermetallic fracture

Engineering Practice and Process Optimization

The TIG brazing process for magnesium/galvanized steel joints requires careful parameter optimization:

Current selection: The current must be high enough to achieve complete wetting and filler flow but low enough to avoid base metal melting and excessive intermetallic formation. Typical currents for thin-sheet brazing may range from 80–150 A, depending on joint geometry and filler thickness.
Travel speed control: Travel speed affects the heat input per unit length and the time available for filler wetting. Slower speeds allow better wetting but increase intermetallic growth.
Filler metal selection: The filler must have appropriate melting point, wetting characteristics, and compatibility with both base metals. Common fillers for Mg/steel brazing include Zn-based, Al-Si, or special low-temperature alloys.
Surface preparation: Cleaning and surface treatment of both base metals are critical for achieving good wetting. Oxide removal, zinc coating preparation, and possibly flux application are required.
Shielding gas protection: Argon or helium shielding prevents oxidation of the hot magnesium and filler, which is critical for joint quality.

Defect Analysis and Countermeasures

Defect Type	Cause	Countermeasure
Lack of fusion	Insufficient current or travel speed too high	Increase current, reduce travel speed
Base metal melting	Excessive current or travel speed too low	Reduce current, increase travel speed
Zinc vaporization	Excessive temperature at steel surface	Reduce current, use protective coating
Intermetallic embrittlement	Excessive intermetallic thickness	Optimize thermal cycle, reduce current
Porosity	Gas entrapment, zinc vaporization	Improve shielding, control temperature
Oxide inclusion	Inadequate shielding or surface contamination	Improve shielding gas flow, clean surfaces

Key Technical Insights and Reflections

The literature provides valuable insights into the complex interactions between process parameters, microstructure, and mechanical properties in TIG brazing of dissimilar metal joints. Several key observations emerge:

Current as a critical parameter: The welding current is the primary lever for controlling joint quality, but it must be balanced against competing requirements of wetting, intermetallic control, and base metal integrity.
Intermetallic compound management: The formation and growth of intermetallic compounds is a fundamental challenge in Mg/steel joining. The paper likely demonstrates that there is an optimal current range that minimizes intermetallic thickness while maintaining adequate joint strength.
Microstructure-property relationships: The correlation between microstructure (grain size, phase distribution, intermetallic morphology) and mechanical properties (strength, ductility, toughness) is essential for process optimization.
Process window definition: The study helps define the process window—the range of parameters that produce acceptable joints—providing a basis for manufacturing process control.

Study Insights and Implications

This literature is highly relevant to the lightweight vehicle manufacturing industry, where magnesium alloys are increasingly used for weight reduction. The TIG brazing process offers a practical solution for joining magnesium to galvanized steel without the complexities and costs of fusion welding. The key engineering implications are:

Process parameters must be carefully optimized to balance competing requirements
Microstructure characterization is essential for understanding joint behavior
Mechanical testing must be comprehensive, including tensile, hardness, and possibly fatigue testing
The process must be qualified and controlled according to relevant standards

For engineers working on lightweight vehicle components, the TIG brazing process, combined with appropriate parameter optimization and quality control, offers a viable path to producing reliable Mg/steel joints. The research highlights the importance of understanding the fundamental mechanisms governing joint formation and failure, enabling data-driven process development and quality assurance.