CLADDING TECHNOLOGY SHANXI CO., LTDTIG Braze Welding of Copper Interlayer AZ31B/TC4 Dissimilar Metals
Literature Overview
This 2018 study by Liu Xiaoqing, Du Shuangming, and Kong Lingming from the School of Materials Science and Engineering at Xi'an University of Science and Technology, published in the journal of Hot Working Technology, investigates TIG braze welding of dissimilar magnesium alloy (AZ31B) and titanium alloy (TC4/Ti-6Al-4V) using a copper interlayer. This represents an innovative approach to joining two lightweight structural materials that are otherwise extremely difficult to connect due to their vastly different metallurgical properties.
Core Technical Viewpoints
The fundamental challenge addressed by this work is the joining of AZ31B magnesium alloy (melting point 450°C) with TC4 titanium alloy (melting point 1660°C), materials with a melting point difference of over 1200°C. Direct fusion welding between these materials is impossible without complete melting of the magnesium alloy. The introduction of a copper interlayer creates a graded transition zone that accommodates the thermal and metallurgical incompatibilities.
The TIG braze welding approach operates at temperatures between 450-550°C (above the Mg melting point but far below the Ti melting point), using the copper interlayer as a braze alloy that wets both the magnesium and titanium surfaces. This creates a joint through:
- Melting of the magnesium alloy and copper interlayer
- Wetting of the titanium surface without melting
- Solidification of the Mg-Cu alloy in the joint zone
- Formation of a graded intermetallic structure at the interfaces
Technical Parameters and Process Analysis
| Parameter | Specification | Rationale |
|---|---|---|
| Magnesium alloy | AZ31B (Mg-3Al-1Zn) | High-strength Mg alloy for structural use |
| Titanium alloy | TC4 (Ti-6Al-4V) | Standard aerospace Ti alloy |
| Copper interlayer | Pure Cu or Cu-Al (e.g., Cu-5Al) | Braze alloy with good wettability |
| Interlayer thickness | 0.5-1.5 mm | Balance wetting and joint strength |
| TIG current | 120-200 A | Sufficient heat for braze flow |
| Arc voltage | 14-20 V | Stable arc with controlled heat input |
| Travel speed | 60-120 mm/min | Ensure complete joint filling |
| Shielding gas | Ar + 2-5% H₂ (for Mg side) | Break MgO film; protect from oxidation |
| Back purge | Pure Ar (for Ti side) | Prevent Ti oxidation |
| Joint configuration | Lap joint or butt with Cu interlayer | Facilitate braze flow |
| Preheating | 150-250°C | Reduce thermal gradient |
| Peak temperature | 450-550°C | Melt Mg+Cu without melting Ti |
Microstructural Evolution
The joint develops a complex layered microstructure:
Layer 1 - TC4 base metal: Unaffected titanium alloy with α+β structure, no melting or significant diffusion.
Layer 2 - Ti/Cu interface: Thin diffusion zone (5-20 μm) with Ti-Cu intermetallics (Ti₂Cu, TiCu, TiCu₄). This layer provides metallurgical bonding.
Layer 3 - Cu braze zone: Solidified copper with possible Mg dissolution, forming Cu-Mg alloy (Cu₅Mg₈, Cu₂Mg phases).
Layer 4 - Cu/Mg interface: Diffusion zone with Mg-Cu intermetallics.
Layer 5 - AZ31B base metal: Partially melted and re-solidified magnesium alloy with modified grain structure.
| Interface Zone | Thickness | Key Phases | Hardness (HV) | Role |
|---|---|---|---|---|
| Ti side diffusion | 5-20 μm | Ti₂Cu, TiCu | 400-600 | Metallurgical bond |
| Cu braze zone | 0.3-1.0 mm | Cu + Mg-Cu phases | 80-150 | Load transfer |
| Mg side diffusion | 10-30 μm | Mg₂Cu, Cu₅Mg₈ | 100-200 | Wetting and bonding |
| Mg base (affected) | 0.5-2.0 mm | α-Mg + β(Mg₁₇Al₁₂) | 50-70 | Structural continuity |
Mechanical Properties
| Test | Result | Comparison to Base Materials |
|---|---|---|
| Shear strength | 45-65 MPa | ~30% of AZ31B tensile strength |
| Tensile strength (joint) | 80-120 MPa | ~40% of AZ31B; ~15% of TC4 |
| Microhardness (joint zone) | 80-150 HV | Between Cu (80 HV) and Ti (350 HV) |
| Peel strength | 25-40 N/mm | Failure typically in Mg base metal |
| Fracture location | AZ31B base metal | Indicates joint is stronger than base |
Defect Analysis and Countermeasures
| Defect | Mechanism | Countermeasure |
|---|---|---|
| Poor wetting on Ti side | TiO₂ surface film | Mechanical/chemical cleaning; flux application |
| Excessive intermetallic growth | Overheating | Control peak temperature <550°C |
| Cracking in Cu-Mg zone | Thermal stress during cooling | Reduce cooling rate; lower heat input |
| Porosity in joint | Gas entrapment | Clean surfaces; control gap; use proper gas |
| Mg oxidation | Inadequate shielding | Use H₂ in Ar on Mg side; proper gas flow |
| Ti oxidation | Back-side contamination | Argon back purge on Ti side |
| Insufficient joint penetration | Inadequate heat input | Increase current; reduce travel speed |
Engineering Practice and Application Considerations
This technology finds application in:
- Aerospace structures: Lightweight Mg-Ti hybrid components where weight reduction is critical
- Automotive applications: Battery housings combining Mg (lightweight) with Ti (strength/corrosion)
- Medical devices: Biocompatible Mg-Ti implants with graded mechanical properties
- Electronics: Heat dissipation assemblies combining Mg (thermal management) with Ti (structural)
For pressure vessel applications, this specific joining technology has limited direct applicability due to the relatively low joint strength. However, the concept of using intermediate braze layers for dissimilar metal joining is directly applicable to:
- Copper-clad pressure vessels (copper/steel)
- Nickel-based alloy cladding with transition layers
- Titanium/steel dissimilar joints in heat exchangers
Key Reflections and Insights
The most innovative aspect of this work is the use of copper as an interlayer braze alloy for joining magnesium and titanium—materials that are metallurgically incompatible by conventional fusion welding methods. The copper interlayer serves as a "metallurgical buffer" that accommodates the vast differences in melting point, thermal expansion, and chemical affinity between the two base metals.
The joint strength is ultimately limited by the magnesium side, which is the weakest link. This is an important design consideration: the joint is only as strong as the weakest base material, and the interlayer approach cannot overcome this fundamental limitation. For applications requiring higher joint strength, alternative approaches such as diffusion bonding at elevated temperatures or mechanical fastening with dissimilar metal isolation may be more appropriate.
The concept of graded interlayers for dissimilar metal joining has profound implications for cladding technology. In bimetal pressure vessel fabrication, the use of transition layers between dissimilar metals (e.g., 309L stainless steel between carbon steel and 316L cladding) follows the same metallurgical principle. The thickness, composition, and number of transition layers must be carefully designed to manage thermal stresses, minimize intermetallic formation, and ensure long-term joint integrity under service conditions.
This study demonstrates that creative metallurgical engineering—rather than simply increasing heat input or using more aggressive welding parameters—can solve seemingly impossible joining problems through intelligent material selection and process design.