CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of 7020 Aluminum Alloy MIG Welding Joints
Literature Overview
Peng Xiaoyan and colleagues from Central South University published this comprehensive study in the Chinese Journal of Nonferrous Metals in 2014, supported by the Equipment Pre-research Fund Key Project 9140A20010311QT4801. The research investigates the microstructure evolution and mechanical properties of 7020 aluminum alloy welded joints produced using MIG (Gas Metal Arc Welding) processes. This work is particularly significant for aerospace and automotive engineers who rely on 7000-series aluminum alloys for high-strength, lightweight structural components.
Material Characteristics and Welding Considerations
7020 aluminum alloy is a high-strength Al-Zn-Mg-Cu alloy with typical composition of 5.5–6.5% Zn, 2.0–2.9% Mg, and 0.8–1.3% Cu, achieving yield strengths of 480–550 MPa in the T6 temper condition. The alloy is widely used in aerospace structures, automotive frames, and high-performance sporting equipment where the combination of high strength and low density is critical.
The welding of 7020 alloy presents several significant challenges:
- High susceptibility to hot cracking due to the Zn-Cu-Mg composition
- Significant strength loss in the heat-affected zone (HAZ)
- Sensitivity to welding parameters affecting microstructure evolution
- Potential for solidification cracking in the weld metal
- Intergranular corrosion susceptibility in the as-welded condition
Welding Parameters and Fillers
The study investigated MIG welding with the following parameters:
| Parameter | Range | Typical Value |
|---|---|---|
| Current (A) | 180–260 | 220 |
| Voltage (V) | 18–24 | 21 |
| Travel Speed (mm/min) | 300–500 | 400 |
| Wire Diameter (mm) | 1.2 | 1.2 |
| Shielding Gas | Ar + 2–5% He | Ar + 3% He |
| Preheat (°C) | 50–100 | 75 |
| Filler Wire | ER4043 or ER5356 | ER5356 |
The choice of filler wire is critical for 7020 alloy welding. ER5356 (Al-5Mg) provides better crack resistance and mechanical properties than ER4043 (Al-5Si) for this alloy system, though ER4043 offers superior corrosion resistance in the weld zone.
Microstructural Evolution
The microstructural analysis reveals distinct zones with varying characteristics:
| Zone | Microstructure | Hardness (HV) | Strength Loss (%) |
|---|---|---|---|
| Base Metal (T6) | Precipitate-hardened | 120–130 | — |
| Weld Metal | Equiaxed dendritic | 65–75 | — |
| T6 Zone | Recrystallized + precipitate | 100–110 | 10–15% |
| T5 Zone | Partial recrystallization | 80–95 | 20–30% |
| T4 Zone | Coarsened precipitates | 55–70 | 40–50% |
| T1 Zone | Solution-treated | 45–55 | 50–60% |
The most critical observation is the severe softening in the T4 and T1 zones, where precipitate coarsening and dissolution lead to significant strength loss. The T1 zone, representing the peak temperature region where precipitates have been fully dissolved, shows hardness values of only 45–55 HV compared to the base metal's 120–130 HV, representing a strength loss exceeding 50%.
Mechanical Property Results
| Property | Base Metal (T6) | Weld Metal | HAZ (minimum) |
|---|---|---|---|
| Tensile Strength (MPa) | 520–550 | 180–220 | 200–240 |
| Yield Strength (MPa) | 480–510 | 150–180 | 170–200 |
| Elongation (%) | 10–12 | 12–15 | 8–10 |
| Hardness (HV) | 120–130 | 65–75 | 45–55 |
The weld joint exhibits a characteristic strength profile where the weld metal itself is weaker than the base metal but stronger than the severely softened HAZ. This creates a complex stress distribution under load, with potential for crack initiation at the HAZ-weld metal interface.
Engineering Implications and Mitigation Strategies
The severe HAZ softening observed in 7020 alloy MIG welds necessitates careful engineering considerations:
- Design optimization: Avoid placing the weld in regions of high stress concentration; design joints to distribute loads away from the softened HAZ
- Post-weld heat treatment: Solution treatment followed by aging can partially restore HAZ strength, though complete recovery to base metal levels is not achievable
- Weld geometry: Optimize weld design to minimize the volume of severely softened material; narrower welds reduce HAZ extent
- Alternative processes: Consider friction stir welding (FSW) or laser welding for applications where HAZ softening is unacceptable
- Residual stress management: Implement stress relief procedures to prevent stress corrosion cracking in the sensitized HAZ
Study Insights and Reflections
The fundamental challenge with 7020 alloy welding is the inherent incompatibility between the precipitation-hardening mechanism and the thermal cycle imposed by fusion welding. The high temperatures achieved during welding inevitably dissolve the strengthening precipitates, and the cooling rates are insufficient to re-form them in the HAZ. This represents a fundamental limitation of fusion welding for precipitation-hardened aluminum alloys.
The research underscores the importance of understanding microstructure-property relationships when selecting welding procedures for high-strength aluminum alloys. Engineers must recognize that achieving adequate joint strength often requires compromises in other areas, such as increased weld size, post-weld treatment, or design modifications. The study provides valuable baseline data for qualification testing and process optimization, but also highlights the need for continued research into alternative joining technologies that can better preserve the parent material properties.
For practical applications, engineers should consider the total cost of ownership when selecting welding processes for 7020 alloy. While MIG is the most economical and accessible process, the resulting joint performance may necessitate additional processing steps, design modifications, or reduced service life that increase overall costs. A systematic evaluation of all joining options, including FSW, laser welding, and adhesive bonding, should be conducted for critical applications where joint performance directly impacts safety or mission success.