Zhuojin Pipe Fitting Co., LtdDissimilar 7075/5A06 Aluminum Alloy TIG Welded Joints — Microstructure and Mechanical Properties
Literature Overview
This paper by Liao Chuanqing, Su Guoyou, Gao Yanfang, Song Wei, Bao Hongwei, and Gao Jiashuang, published in the Chinese Journal of Nonferrous Metals (2015, Vol. 25, No. 1, pp. 43-48), investigates the microstructure and mechanical properties of TIG welded joints between 7075 and 5A06 aluminum alloys using two different filler wires (BJ380A and ER4043). Funded by a national international cooperation project (2013DFR504xx), this study addresses a practical challenge in aluminum structure fabrication where different alloy grades must be joined.
Core Technical Findings
The study reveals significant challenges inherent to dissimilar aluminum alloy welding:
| Property | BJ380A Filler | ER4043 Filler | Base Metal (5A06) | Base Metal (7075) |
|---|---|---|---|---|
| Weld cracks | None | None | — | — |
| Porosity | Scattered pores present | Scattered pores present | — | — |
| HAZ grain growth | More pronounced on 7075 side | More pronounced on 7075 side | — | — |
| Weld grain size | Coarser | Finer | — | — |
| Tensile strength | ~220 MPa | ~220 MPa | ~315 MPa | ~520 MPa |
| Strength ratio to 5A06 | ~70% | ~70% | 100% | — |
| Fracture location | Weld zone | Weld zone | — | — |
| HAZ hardness (7075 side) | Softening | Softening | — | — |
| HAZ hardness (5A06 side) | Hardening | Hardening | — | — |
| Weld hardness | Lower | Higher | — | — |
The tensile strength of approximately 220 MPa for both filler materials represents only 70% of the 5A06 base metal strength and less than 42% of the 7075 base metal strength. This strength reduction is a fundamental limitation of dissimilar aluminum alloy welding.
Microstructural Analysis
7075 Side HAZ Softening
The 7075-T6 aluminum alloy derives its exceptional strength from the precipitation of strengthening phases (MgZn2, η-phase) during the T6 temper. During TIG welding, the HAZ temperatures exceed the solution treatment temperature of 7075, causing complete dissolution of these precipitates. Since no artificial aging is performed after welding, the 7075 HAZ remains in a soft, overaged condition with significantly reduced hardness and strength. This phenomenon is well-documented in the literature and represents a fundamental limitation of welding 7075 aluminum alloy.
5A06 Side HAZ Hardening
The 5A06 (Al-Mg-Si) alloy exhibits hardening in the HAZ due to the precipitation of fine Mg2Si particles during the cooling cycle. The welding thermal cycle effectively provides a natural aging treatment to the 5A06 HAZ, increasing its hardness above the as-received condition. This creates an interesting asymmetry in the dissimilar joint where one side softens and the other hardens.
Weld Metal Microstructure
The use of ER4043 (Al-Si) filler wire produces finer weld grains and higher weld hardness compared to BJ380A filler. This is attributed to the higher silicon content in ER4043, which increases the nucleation rate during solidification and promotes the formation of fine eutectic Si particles. However, the higher silicon content also promotes the formation of brittle Al5FeSi intermetallic phases at grain boundaries, which may contribute to the scattered porosity observed.
Engineering Practice Considerations
For engineers designing aluminum structures that require joining 7075 and 5A06 components:
- Joint design optimization: The strength limitation at the weld zone necessitates careful joint design. Engineers should avoid placing the weld in primary load-bearing positions and consider using overlap joints or fillet welds where possible.
- Filler material selection: ER4043 provides better weld hardness and finer microstructure but does not significantly improve overall joint tensile strength. The selection should be based on specific service requirements rather than strength alone.
- Post-weld treatment considerations: Solution treatment and aging of the entire joint could potentially restore some strength to the 7075 HAZ, but this would also affect the 5A06 side and may cause distortion.
- Alternative joining methods: Friction stir welding (FSW) may offer superior results for this dissimilar combination, as it avoids complete melting and can preserve more of the base metal microstructure.
Key Questions and Study Insights
The fundamental challenge of this dissimilar joint is the inability to achieve strength values approaching either base metal. The 220 MPa tensile strength represents a significant design constraint that must be accounted for in structural analysis and fatigue assessment.
Key questions for further investigation include:
- Can post-weld heat treatment improve the joint strength without causing unacceptable distortion?
- How does the joint perform under fatigue loading, given the heterogeneous microstructure and hardness distribution?
- Are there alternative filler materials (e.g., Al-Zr or Al-Sc alloys) that could improve weld zone strength?
- How does the joint perform at elevated temperatures where the 7075 base metal would naturally soften?
This study provides valuable baseline data for engineers considering dissimilar 7075/5A06 aluminum alloy joints. The consistent tensile strength of approximately 220 MPa regardless of filler material suggests that the fundamental limitation lies in the base metal incompatibility rather than filler selection. Engineers must accept this limitation in design or seek alternative joining methods that can better accommodate the metallurgical differences between these two alloy systems.