CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Mechanical Properties of AZ31 Magnesium Alloy Galvanized Steel Dissimilar Metal TIG Braze Welding Joints
Literature Overview
Published in 2017 in the journal Hot Working Technology and funded by the Chongqing Basic and Frontier Research Program (Grant No. cstc2014jcyj A60002), this study investigates the microstructure and mechanical properties of AZ31 magnesium alloy to galvanized steel dissimilar metal joints produced by TIG braze welding. Conducted at Chongqing Electronic Engineering Vocational College, the research addresses a critical challenge in lightweight structural engineering: joining dissimilar metals with vastly different thermal properties, melting points, and metallurgical characteristics.
Technical Challenges of Dissimilar Metal Joining
The joining of AZ31 magnesium alloy to galvanized steel presents unique challenges that are directly relevant to bimetal product manufacturing:
| Property | AZ31 Mg Alloy | Galvanized Steel (Q235 base) | Challenge |
|---|---|---|---|
| Melting point | 650°C | 1510°C (steel), 419°C (Zn coating) | Extreme thermal mismatch |
| Thermal conductivity | 73 W/m·K | 50 W/m·K (steel), 116 W/m·K (Zn) | Uneven heat distribution |
| Coefficient of thermal expansion | 26 × 10⁻⁶/K | 12 × 10⁻⁶/K | Residual stress development |
| Oxidation tendency | Very high | Moderate | Oxide inclusion formation |
| Intermetallic formation | Mg-Zn, Mg-Fe phases | — | Brittle phase precipitation |
| Electrochemical potential | -1.6 V vs SHE | -0.76 V vs SHE | Galvanic corrosion risk |
The galvanized coating on the steel substrate introduces an additional layer of complexity. During welding, the zinc coating melts at 419°C, well below the melting point of the steel substrate, creating a liquid zinc pool that can flow into the weld zone and form brittle intermetallic compounds with both the magnesium alloy and the steel.
TIG Braze Welding Process Design
TIG braze welding is a hybrid process that combines the arc heating of TIG welding with the solidification characteristics of brazing. The key principle is that the filler metal melts and flows into the joint while the base metals remain solid, or only partially melted at the fusion boundary.
The process parameters investigated in this study likely included:
| Parameter | Typical Value | Rationale |
|---|---|---|
| Welding current | 60–150 A | Low heat input to minimize Mg melting |
| Travel speed | 400–800 mm/min | Controls heat input per unit length |
| Shielding gas | 100% Ar or He/Ar mix | Protects reactive Mg from oxidation |
| Filler material | Mg-based or Al-based braze alloy | Compatible with both base metals |
| Gap preparation | 0.5–2.0 mm | Allows filler metal flow |
| Preheat | 100–200°C (Mg side) | Reduces thermal gradient |
The braze welding approach is particularly suitable for this dissimilar metal combination because it avoids the formation of a fully melted weld pool that would contain both Mg and Fe, which would result in extensive brittle intermetallic compound formation throughout the joint.
Microstructural Characterization
The microstructure of the AZ31/galvanized steel TIG braze welding joint typically consists of the following zones:
- AZ31 base metal zone: Alpha-Mg matrix with beta-phase Mg₁₇Al₁₂ precipitates, potentially with grain coarsening near the joint interface due to thermal exposure.
- Mg-side reaction zone: A thin layer of Mg-Zn intermetallic compounds (MgZn₂, MgZn) formed at the interface between the AZ31 alloy and the molten zinc from the galvanized coating. This zone may be 5–50 μm thick depending on welding parameters.
- Filler metal zone: The braze alloy solidified in the joint gap, potentially containing dissolved Zn from the coating and possibly small amounts of Fe from the steel substrate. The microstructure depends on the filler alloy composition and cooling rate.
- Steel-side reaction zone: A thin layer of Fe-Zn intermetallic compounds (FeZn₁₃, Fe₃Zn₁₀) formed at the steel-zinc interface, potentially extending into the steel substrate as a diffusion zone.
- Galvanized coating remnants: Unmelted or partially melted zinc coating on the steel surface away from the joint, potentially with Fe-Zn compound layers.
The mechanical properties of the joint are primarily governed by:
- The integrity of the Mg-side reaction zone, which is typically the weakest link due to the brittleness of Mg-Zn intermetallics.
- The strength of the filler metal, which must be compatible with both base metals.
- The absence of voids, cracks, or lack of bonding at the interfaces.
Mechanical Property Evaluation
| Test Method | AZ31 Base Metal | Galvanized Steel | Joint Region |
|---|---|---|---|
| Tensile strength (MPa) | 220–260 | 350–400 | 150–200 |
| Elongation (%) | 10–15 | 25–30 | 5–10 |
| Hardness (HV) | 50–60 | 110–130 | 80–120 |
| Shear strength (MPa) | — | — | 80–150 |
The joint strength is typically limited by the weakest interface, which is often the Mg-Zn intermetallic reaction zone. Strategies to improve joint strength include:
- Minimizing the thickness of the intermetallic layer through reduced heat input and shorter welding times.
- Using filler alloys that are metallurgically compatible with both base metals.
- Applying surface treatments to the galvanized coating (e.g., zinc removal or conversion coating) to control interfacial reactions.
- Optimizing the welding sequence to control thermal cycling and residual stress development.
Galvanic Corrosion Considerations
The AZ31/galvanized steel joint creates a significant galvanic couple due to the large difference in electrochemical potential between magnesium and steel. In corrosive environments, the AZ31 alloy will act as the anode and corrode preferentially, potentially leading to rapid joint degradation.
Mitigation strategies include:
- Applying protective coatings to the joint area after welding.
- Using cathodic protection systems in aggressive environments.
- Selecting filler alloys that minimize the galvanic couple effect.
- Designing the joint geometry to prevent trapped moisture and electrolyte accumulation.
Engineering Practice and Quality Control
For engineers considering this type of dissimilar metal joint in structural or pressure-containing applications, the following quality control measures are essential:
- Visual inspection: Verify joint geometry, filler metal fill, and absence of visible defects.
- Cross-sectional metallography: Examine the interfacial reaction zones for excessive intermetallic layer thickness, cracking, or voids.
- Hardness mapping: Identify the distribution of brittle intermetallic phases across the joint cross-section.
- Tensile or shear testing: Validate joint strength against design requirements.
- Corrosion testing: Evaluate galvanic corrosion resistance in the intended service environment using salt spray testing, immersion testing, or electrochemical impedance spectroscopy.
The PDCA cycle should be applied to optimize the welding process: Plan the welding parameters based on preliminary trials, Do the welding and collect data, Check the results against acceptance criteria, and Act by adjusting parameters for improved performance.
Study Insights and Implications
This research highlights the complexity of dissimilar metal joining in lightweight structural applications and provides valuable insights into the metallurgical behavior of AZ31/galvanized steel joints. For engineers in the bimetal product and pressure vessel industry, the key lessons are:
- Interfacial metallurgy is critical: The strength and durability of dissimilar metal joints are primarily determined by the interfacial reaction zones, not the bulk properties of the base metals or filler material.
- Thermal control is paramount: Minimizing heat input and thermal gradients is essential to limit intermetallic compound formation and residual stress development.
- Corrosion considerations cannot be overlooked: The galvanic couple between dissimilar metals can be more detrimental to joint life than mechanical loading in many service environments.
- Process selection matters: TIG braze welding offers a viable approach for dissimilar metal joining when full fusion welding would result in unacceptable metallurgical degradation.
These principles extend to other dissimilar metal combinations encountered in bimetal pressure vessel fabrication, including titanium/steel, copper/steel, and nickel alloy/steel cladding joints, where interfacial metallurgy and corrosion compatibility are equally critical design considerations.