TIG Welding of Aluminum Plate and Copper Plate

Literature Overview

This technical article by Men Huihai from Hank (Tianjin) Electromechanical Co., Ltd., published in Welding Technology in 2006, addresses the practical challenges of joining aluminum plate to copper plate using TIG welding. Unlike the more extensively studied aluminum-steel and aluminum-titanium joints, aluminum-copper dissimilar metal welding remains a relatively uncommon application, primarily encountered in electrical busbar fabrication, heat exchanger tubesheets, and certain chemical processing equipment. The scarcity of published technical literature on this topic makes this contribution particularly valuable for practicing engineers.

Metallurgical Challenges

The aluminum-copper system presents unique metallurgical challenges that distinguish it from other dissimilar metal combinations. The aluminum-copper phase diagram shows extensive solid solubility of copper in aluminum (up to approximately 28 wt% at 548°C), but limited solubility of aluminum in copper (less than 0.1 wt% at room temperature). This asymmetry has profound implications for the joint microstructure and properties.

During TIG welding of aluminum to copper, the following metallurgical phenomena occur:

The aluminum side melts completely and forms the weld pool
The copper side may partially melt depending on the heat input, creating a mixed molten zone
Intermetallic compounds form at the interface, primarily Al₂Cu, AlCu, and Al₄Cu₉
The Al₂Cu phase (approximately 54 wt% Al) is relatively ductile, while AlCu and Al₄Cu₉ are brittle
A diffusion zone develops with a gradient of Al-Cu intermetallics extending into both parent metals

The formation of brittle intermetallics is the primary concern, as it can lead to catastrophic brittle fracture of the joint under mechanical loading. The thickness and composition of the intermetallic layer are strongly influenced by the welding parameters and the thermal cycle experienced by the joint.

Welding Process Considerations

TIG welding of aluminum-copper joints requires careful attention to several process aspects:

Process Aspect	Challenge	Solution
Heat input control	Excessive heat promotes intermetallic growth	Use higher travel speed with moderate current
Arc stability	Copper's high thermal conductivity cools the arc	Use AC TIG or DCEN with higher current
Oxide removal	Al₂O₃ on aluminum surface prevents wetting	AC TIG for cathodic cleaning, or mechanical cleaning
Filler selection	Pure Al may lead to excessive dilution	Al-Si or Al-Mg fillers improve wetting
Joint design	Thermal expansion mismatch causes distortion	Lap joint preferred over butt joint

The use of AC TIG is particularly advantageous for aluminum-copper joints because the AC cycle provides both cathodic cleaning (removing Al₂O₃) during the negative half-cycle and deeper penetration into the copper during the positive half-cycle. The balance between penetration and cleaning can be adjusted by varying the AC balance (the ratio of positive to negative time).

Typical Welding Parameters

For 3 mm aluminum plate joined to 3 mm copper plate using AC TIG:

Arc current: 180-220 A
AC balance: 40-50% positive time
Frequency: 50-100 Hz
Travel speed: 150-250 mm/min
Shielding gas: 100% Ar or 98% Ar / 2% He
Gas flow rate: 15-20 L/min
Filler wire: 1.6 mm Al-Si (or ER4043)
Preheat: 100-150°C on copper side to reduce thermal gradient

Microstructure and Mechanical Properties

The joint microstructure typically consists of the following zones from aluminum to copper:

Aluminum weld zone: equiaxed alpha-Al grains with Si particles (if Al-Si filler used)
Diffusion zone: gradient of Al-rich intermetallics (Al₂Cu, then AlCu, then Al₄Cu₉)
Copper HAZ: limited solid-state diffusion, minimal microstructural change

The intermetallic layer thickness is typically 20-50 μm for optimized TIG parameters. This is significantly thicker than in aluminum-steel welding-brazing joints (8-25 μm), reflecting the higher mutual solubility and reactivity between aluminum and copper.

Mechanical properties of the joint are dominated by the intermetallic layer:

Shear strength: 60-90 MPa (compared to 150-200 MPa for pure aluminum welds)
Hardness at interface: 150-200 HV (compared to 60-80 HV for aluminum and 100-120 HV for copper)
Fracture mode: typically intergranular within the intermetallic layer

The relatively low shear strength of aluminum-copper TIG welds limits their application to situations where mechanical loading is not critical, such as electrical connections where conductivity is the primary requirement. For structural applications, mechanical fastening or brazing with appropriate filler metals may be more appropriate.

Practical Recommendations

Based on this study and practical experience, I offer the following recommendations for aluminum-copper TIG welding:

Limit the application to non-structural or low-stress joints where electrical conductivity or thermal conductivity is the primary requirement
Use AC TIG with appropriate balance to achieve both oxide cleaning and adequate copper penetration
Preheat the copper side to reduce thermal distortion and improve aluminum wetting
Use Al-Si filler to promote wetting and reduce intermetallic thickness
Perform post-weld inspection using MT or PT to detect interfacial defects
For critical applications, consider alternative joining methods such as brazing with Cu-P or Ag-Cu fillers, or mechanical fastening with dissimilar metal gaskets

The study's practical orientation makes it particularly useful for fabrication shops encountering aluminum-copper joining requirements. The emphasis on process parameters rather than fundamental metallurgy reflects the applied nature of the work and its relevance to production environments.