Copper Interlayer AZ31B TC4 Dissimilar Metal TIG Brazing Study

Literature Overview

This 2018 study by Liu Xiaoqing and colleagues from the School of Materials Science and Engineering, Xi'an University of Science and Technology, investigates the TIG brazing of AZ31B magnesium alloy to TC4 titanium alloy using a copper interlayer. Published in Hot Working Technology, this research addresses a highly specialized dissimilar metal joining challenge that combines two lightweight structural materials—magnesium and titanium—with fundamentally different metallurgical characteristics and thermal properties.

Core Technical Content

The AZ31B/TC4 dissimilar pair presents extraordinary joining challenges:

AZ31B: Magnesium alloy with melting point ~650 °C, high reactivity, low strength (~220 MPa tensile)
TC4 (Ti-6Al-4V): Titanium alloy with melting point ~1660 °C, high strength (~950 MPa), reactive above 400 °C
Copper interlayer: Melting point 1085 °C, acts as a diffusion barrier and brazing vehicle

The copper interlayer serves multiple critical functions: it provides a metallurgical buffer between the two incompatible materials, acts as the molten filler during brazing, and creates a diffusion barrier that prevents direct reaction between magnesium and titanium.

Thermal and Metallurgical Challenges

Challenge	Magnitude	Mitigation Strategy
Melting point difference	1010 °C (Ti vs Mg)	Brazing below Mg melting point
Thermal expansion mismatch	α_Mg ≈ 26×10⁻⁶/K vs α_Ti ≈ 9×10⁻⁶/K	Residual stress management
Diffusion reactions	Mg-Ti intermetallics extremely brittle	Copper barrier layer
Oxide formation	Both Mg and Ti form stable oxides	Inert atmosphere or vacuum
Galvanic coupling	Mg is highly active anode	Coating or isolation

Brazing Process Analysis

The brazing process for AZ31B/TC4 with copper interlayer requires careful control of several parameters:

Process Parameters

Parameter	Recommended Range	Rationale
Brazing temperature	400–500 °C	Below Mg melting point, above Cu solidus
Holding time	3–10 min	Complete wetting without excessive diffusion
Heating rate	2–5 °C/s	Uniform temperature distribution
Cooling rate	Controlled furnace cool	Minimize residual stress
Atmosphere	Argon or vacuum (<10⁻³ Pa)	Prevent oxidation of Mg and Ti
Copper interlayer thickness	0.2–0.5 mm	Adequate filler volume without excessive mass

Microstructural Analysis

The joint microstructure in AZ31B/TC4 copper-interlayer brazed joints consists of several distinct zones:

Joint Zone Architecture

Zone	Location	Composition	Characteristics
AZ31B base metal	Outer Mg side	Mg-3Al-1Zn	Unchanged from base
Mg reaction zone	Mg-Cu interface	Mg₂Cu, MgCu₂	Brittle intermetallics, thin layer
Copper braze zone	Center	Cu with dissolved Mg, Ti	Molten filler solidified
Ti reaction zone	Ti-Cu interface	Cu-Ti intermetallics	Cu₄Ti₃, CuTi
TC4 base metal	Outer Ti side	Ti-6Al-4V	Unchanged from base

The formation of intermetallic compounds at both interfaces is inevitable but must be controlled. Mg₂Cu and MgCu₂ at the magnesium interface are relatively thin and can be tolerated, while Cu₄Ti₃ and CuTi at the titanium interface form more readily and can compromise joint integrity if excessive.

Mechanical Performance

The mechanical properties of AZ31B/TC4 copper-interlayer brazed joints are governed by the weakest interface:

Property	Typical Value	Comparison to Base Metals
Shear strength	80–150 MPa	40–60% of Mg alloy strength
Peel strength	10–25 MPa	Significantly lower than base
Hardness (joint zone)	80–150 HV	Between Mg and Cu hardness
Fracture location	Mg-Cu interface	Weakest interface

Engineering Applications and Limitations

The AZ31B/TC4 joining technology has potential applications in aerospace structures where magnesium and titanium components must be joined, such as in satellite structures, rocket engine casings, and aircraft landing gear components. However, several limitations must be acknowledged:

Temperature limitation: The joint cannot withstand temperatures above 250 °C without significant degradation of intermetallic phases.
Corrosion concerns: Galvanic coupling between Mg and Ti in corrosive environments accelerates Mg dissolution.
Fatigue performance: The brittle intermetallic layers at interfaces are susceptible to fatigue crack initiation.
Scalability: The process is challenging to scale to large structural components due to atmosphere control requirements.

Key Questions and Reflections

This research raises important questions about the practical viability of dissimilar lightweight metal joining. While the copper interlayer approach demonstrates technical feasibility, the significant strength reduction (to 40–60% of the weaker base metal) and temperature limitations raise questions about real engineering applicability. The study contributes valuable fundamental knowledge about interfacial reactions, but translating this to production-ready technology requires additional work on process robustness, quality assurance, and long-term performance validation.

Study Insights and Implications

The most significant insight from this research is the demonstration that strategic use of interlayer materials can enable joining of fundamentally incompatible material pairs. This principle has direct relevance to cladding and bimetal product manufacturing, where interlayer materials (such as nickel-based transition layers between carbon steel and stainless steel) serve similar metallurgical buffering functions. The copper interlayer in this study performs an analogous role to the nickel transition layer in stainless steel cladding, preventing detrimental interfacial reactions while providing adequate metallurgical bonding.

The research also highlights the importance of understanding interfacial reaction kinetics in dissimilar metal joining. The thickness, composition, and continuity of intermetallic layers at each interface directly determine joint performance, and these layers can only be effectively controlled through precise management of temperature, time, and atmosphere conditions.