Zhuojin Pipe Fitting Co., LtdMicrostructure and Properties of TIG Thermal Conduction Welded Joints Between 6061 Aluminum Alloy and H60 Copper Alloy
Literature Overview
This paper by Zuo Yong, Tian Yu, and Liu Tao from China Petroleum Pipeline Engineering Corporation, published in "Welding" (2017, Vol. 8, pp. 43-47), investigates a novel approach to joining dissimilar metals—specifically 6061 aluminum alloy and H60 copper alloy—using TIG thermal conduction welding. The technique relies on heating the copper side with the TIG arc while achieving aluminum melting through thermal conduction alone, without direct arc contact on the aluminum. This approach addresses a fundamental challenge in dissimilar metal joining: the vast difference in melting points between aluminum and copper.
Core Technical Concept
The TIG thermal conduction welding method exploits the high thermal conductivity of copper to transfer heat from the arc-heated copper surface to the aluminum workpiece. The aluminum, having a lower melting point (660°C) compared to copper (1085°C), melts through conductive heat transfer while the copper remains in a solid state at the arc contact point. This creates a diffusion-bonded joint with intermetallic compound layers at the interface.
Interfacial Microstructure Analysis
| Layer (from Cu side to Al side) | Phase Composition | Formation Mechanism |
|---|---|---|
| H60 copper base metal | α-Cu + CuZn (brass) | Unaffected base metal |
| Al₄Cu₉ reaction layer | Al₄Cu₉ intermetallic | Solid-state reaction, high-temperature diffusion |
| Al₂Cu reaction layer | Al₂Cu intermetallic | Diffusion-controlled growth |
| α-Al + Cu₅Zn₈ + Al₂Cu | Mixed phase zone | Aluminum-side reaction products |
| 6061 aluminum base metal | α-Al + Mg₂Si precipitates | Unaffected base metal |
Effect of Welding Current on Joint Performance
The study systematically varied welding current to evaluate its influence on interface microstructure and mechanical performance. The following trends were observed:
| Welding Current (A) | Al₄Cu₉ + Al₂Cu Layer Thickness | Crack Presence | Tensile Load (kN) |
|---|---|---|---|
| Low (e.g., 70-80) | Thin | None | Moderate |
| Medium (e.g., 90-100) | Moderate | None | Increasing |
| 110 | Thick | Visible cracking | Maximum: 1.67 |
| Above 110 | Very thick | Severe cracking | Decreasing |
The maximum tensile load of 1.67 kN was achieved at 110 A, which also marks the threshold where cracking becomes observable at the interface. This indicates that the optimal process window is narrow—centered around 100-110 A—where sufficient interfacial bonding is achieved without excessive intermetallic growth that promotes brittle failure.
Metallurgical Interpretation
The formation of intermetallic compounds at the Al-Cu interface follows classic diffusion bonding kinetics. The Al₄Cu₉ phase forms at higher temperatures and is thermodynamically less stable than Al₂Cu, which explains why Al₂Cu appears closer to the aluminum side where temperatures are lower. The Cu₅Zn₈ phase observed on the aluminum side results from zinc diffusion from the H60 brass (which contains approximately 40% Zn) into the aluminum matrix, forming a zinc-rich intermetallic.
The cracking observed at 110 A and above is attributed to the brittleness of the thick intermetallic layers. Intermetallic compounds such as Al₄Cu₉ and Al₂Cu are inherently brittle phases with limited plastic deformation capacity. When their combined thickness exceeds a critical value (approximately 10-15 μm based on literature correlations), the joint becomes susceptible to intergranular cracking along the intermetallic grain boundaries.
Engineering Practice Implications
For pipeline engineering applications, this technology could find use in:
- Joining aluminum heat exchanger tubes to copper headers
- Fabricating electrical connections between aluminum and copper busbars in pipeline instrumentation
- Manufacturing lightweight components where aluminum-copper transitions are required for thermal management
The key process control parameters for production implementation include: welding current (target 95-105 A to avoid cracking), torch travel speed (must be precisely controlled to maintain consistent heat input), gas shielding (high-purity argon with flow rate 15-20 L/min to prevent aluminum oxide formation), and joint fit-up (gap tolerance within ±0.1 mm to ensure uniform thermal contact).
Study Insights and Reflections
This paper demonstrates that dissimilar metal joining between aluminum and copper—historically considered extremely challenging due to the large melting point differential and intermetallic brittleness—can be achieved through careful process parameter selection. The thermal conduction approach is elegant in its simplicity: by avoiding direct arc contact on the aluminum, it prevents excessive melting and mixing that would lead to thick, brittle intermetallic layers. However, the narrow process window and the inherent brittleness of the joint limit its applicability to low-stress, non-cyclic applications. For pipeline service involving pressure cycling or thermal fatigue, this joint type would require extensive qualification testing. The maximum load of 1.67 kN is relatively low, indicating that the joint strength is governed by the intermetallic layer rather than the base metals.