Microstructure and Mechanical Properties of AZ31B Magnesium Alloy / Galvanized Steel TIG Brazing Joints

Literature Overview

The study by Chen Shijiang from Chongqing Vocational College of Electronic Engineering, published in 2017 in Hot Working Technology (Vol. 46, No. 17, pp. 198-201), investigates the microstructure and mechanical properties of TIG brazed joints between AZ31B magnesium alloy and galvanized steel. Funded by the Chongqing Basic and Frontier Research Program (Grant No. cstc2014jcyj A60002), this work addresses the challenge of joining dissimilar metals—specifically magnesium alloy to steel—using a TIG brazing process with AZ31B magnesium alloy filler wire.

Technical Background and Dissimilar Metal Joining Challenges

Joining magnesium alloy to steel presents fundamental metallurgical challenges:

1. Large melting point difference: Magnesium alloy melts at approximately 650°C while steel melts at approximately 1500°C, requiring a process that wets and bonds to the steel without melting it.
2. Chemical incompatibility: Magnesium and iron form brittle intermetallic compounds that are thermodynamically stable but mechanically detrimental.
3. Thermal expansion mismatch: The coefficient of thermal expansion of magnesium alloy (approximately 26 × 10⁻⁶/°C) is significantly higher than that of steel (approximately 12 × 10⁻⁶/°C), creating thermal stress during cooling.
4. Galvanized steel coating interaction: The zinc coating on galvanized steel introduces an additional interfacial layer that modifies the reaction chemistry at the joint.

TIG brazing (also referred to as TIG soldering or TIG brazing with low-melting-point filler) addresses these challenges by maintaining the base materials below their melting points while using a filler metal with a melting point between the two base materials. In this case, the AZ31B filler melts and flows onto the steel surface, creating a metallurgical bond through wetting and interfacial reaction.

Experimental Methodology

The study employs optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and universal tensile testing to characterize the joints. The key experimental variable is welding current, which controls the heat input and consequently the interfacial reaction extent.

Key Findings: Welding Current Effects

The study reveals a clear dependence of joint quality on welding current, with distinct regimes identified:

Low current regime (< 75 A):

• Extensive pore formation at the Mg/steel interface.
• Poor wetting and incomplete bonding.
• Low joint strength due to interface discontinuities.
• The galvanized steel surface may not be adequately cleaned or activated at low heat input.

Optimal current regime (> 75 A):

• Formation of intermetallic compounds: Mg₃₂(Al,Zn)₄₉, Al₂Mg, and AlFe₃ at the Mg/steel interface.
• Maximum tensile strength of 210 MPa achieved.
• Fish-scale (typical brazing) morphology at the interface.
• Adequate wetting and metallurgical bonding.

Excessive current regime (further increase beyond optimal):

• Progressive thickening of the brittle intermetallic reaction layer.
• Decreasing mechanical properties due to increased brittleness.
• Potential for Mg vaporization and porosity at excessive heat input.

Intermetallic Compound Analysis

The identification of specific intermetallic compounds provides critical insight into the joint metallurgy:

The presence of AlFe₃ is particularly significant because it forms at the steel side of the interface and may provide a metallurgical anchor that enhances adhesion. However, the accumulation of multiple brittle intermetallic phases creates a layered reaction zone that is inherently prone to brittle fracture.

The fish-scale morphology observed at the interface is characteristic of brazing joints and results from the oscillatory growth of intermetallic compounds during solidification. This morphology provides a larger effective bonding area compared to a planar interface but also creates stress concentration sites at the scale tips.

Mechanical Property Assessment

The maximum tensile strength of 210 MPa achieved in the optimal current regime represents approximately 60-70% of the AZ31B base material tensile strength (approximately 260-310 MPa depending on temper condition). This efficiency is reasonable for a dissimilar metal brazed joint and compares favorably with other magnesium-to-steel joining methods:

The fracture behavior of the joints, while not extensively detailed in the abstract, is expected to be interfacial or near-interfacial due to the presence of the brittle intermetallic layer. This is consistent with the general behavior of brazed dissimilar metal joints.

Engineering Application Considerations

For practical applications, several factors must be considered:

1. Galvanized steel coating thickness: The zinc coating thickness affects the interfacial reaction chemistry and the volume fraction of Mg-Zn intermetallics. Thicker coatings may produce more Mg₃₂(Al,Zn)₄₉, potentially enhancing bonding but increasing brittleness.
2. Surface preparation: The condition of the galvanized steel surface (cleanliness, oxide removal, coating integrity) significantly affects wetting and bonding. Mechanical or chemical cleaning may be required.
3. Joint design: The geometry of the joint affects heat input distribution and interfacial reaction extent. Lap joints, butt joints, and fillet joints will exhibit different behaviors.
4. Service environment: Magnesium alloy is highly susceptible to corrosion in most environments. The joint must be protected by coating or cathodic protection to ensure long-term durability.
5. Thermal cycling: The thermal expansion mismatch between Mg and steel creates cyclic stresses during temperature changes, which may lead to fatigue cracking at the intermetallic layer.

Critical Reflection

The study provides valuable fundamental data on the TIG brazing of AZ31B magnesium alloy to galvanized steel, establishing the existence of an optimal current window and identifying the key intermetallic phases involved. However, several limitations restrict the direct applicability of the findings:

• The study does not report the welding speed, torch angle, or gas shielding conditions, which are critical process parameters.
• No fatigue testing or corrosion testing is performed, which are essential for assessing long-term joint durability.
• The single filler metal (AZ31B) may not be optimal for all applications; alternative filler compositions could potentially reduce intermetallic thickness.
• The joint geometry is not specified, limiting the ability to extrapolate to specific structural configurations.

From a broader perspective, the use of galvanized steel as the counterpart material introduces an additional variable (the zinc coating) that complicates the interfacial metallurgy. The zinc coating may act as a diffusion barrier, reducing the formation of Fe-based intermetallics, or it may participate in the reaction to form Mg-Zn compounds. The net effect depends on coating thickness, welding heat input, and cooling rate.

Summary

This study demonstrates that TIG brazing can achieve reliable joining of AZ31B magnesium alloy to galvanized steel with a maximum tensile strength of 210 MPa at welding currents above 75 A. The joint interface exhibits a characteristic fish-scale morphology with the formation of Mg₃₂(Al,Zn)₄₉, Al₂Mg, and AlFe₃ intermetallic compounds. The welding current must be carefully controlled to balance adequate wetting and bonding against excessive intermetallic layer growth. While the process shows promise for lightweight structural applications combining magnesium and steel components, further investigation into fatigue behavior, corrosion resistance, and process parameter optimization is required before widespread industrial adoption.