Zhuojin Pipe Fitting Co., LtdEffect of Aging Heat Treatment on Microstructure and Properties of 7075 Aluminum Alloy Dual-Pulse MIG Welding Joints
Literature Overview
This 2026 study by Yang Hao and colleagues from the Beijing Institute of Electromechanical Technology, China National Machinery Industry Corporation, published in Materials Reports, investigates the effect of aging heat treatment on the microstructure and mechanical properties of 7075 aluminum alloy welded joints produced by dual-pulse MIG welding. The paper spans pages 582 to 586 of Volume 40, Issue S1.
Core Technical Content
7075 aluminum alloy is widely used in aerospace applications due to its excellent mechanical properties and corrosion resistance. However, its weldability is poor, with significant property degradation in conventional welded joints. This study addresses the critical challenge of improving welded joint properties through post-weld heat treatment.
Microstructural Evolution: As-Welded vs. Heat Treated
| Region | As-Welded Microstructure | Heat Treated Microstructure |
|---|---|---|
| HAZ | Coarsened CuAl2 precipitates, ~2 μm | Fine dispersed MgZn2 platelet precipitates |
| Fusion zone | Sharp decrease in intragranular precipitates | Equiaxed grains with dispersed nanoscale MgZn2 |
| Weld zone | Widened grain boundaries | Equiaxed grains, nanoscale MgZn2, micron MgZn2 at boundaries |
Mechanical Property Improvement
| Property | As-Welded | After Heat Treatment | Improvement |
|---|---|---|---|
| Tensile strength | Baseline | 486.81 MPa | 68% increase |
| Strength ratio to base metal | Below 50% | Exceeds 80% | Significant |
| Fracture location | Weld and fusion zones | Entirely within base metal | Major improvement |
| Fracture mode | Tear cup and quasi-cleavage | Transgranular with fine dimples | More ductile |
Heat Treatment Process
The study employs a solution treatment followed by dual-stage artificial aging process. The solution treatment dissolves coarse precipitates formed during welding, while the dual-stage aging allows controlled reprecipitation of strengthening phases at optimal sizes and distributions.
Fracture Behavior Analysis
In the as-welded condition, cracks propagate along the weld zone and fusion zone, with microcracks forming along coarsened fusion zone grain boundaries. The fracture surface shows a mixture of tear-cup ductile dimples and quasi-cleavage facets.
After heat treatment, cracks propagate entirely within the base metal without entering the weld region. The upper crack is located near the base metal HAZ, and fracture is predominantly transgranular with fine dimples on the fracture surface. This indicates that the heat treatment has effectively restored the strength of the weld and fusion zones to levels exceeding those of the base metal HAZ.
Engineering Practice Implications
For aerospace and high-performance structural applications using 7075 aluminum alloy:
- Dual-pulse MIG welding provides improved arc stability and heat input control compared to conventional MIG welding.
- Post-weld solution treatment and dual-stage aging is essential for achieving acceptable joint strength.
- The 80% strength ratio to base metal achieved after heat treatment meets many aerospace application requirements.
- The fracture mode shift from intergranular to transgranular indicates improved ductility and damage tolerance.
Study Insights
This research demonstrates that the combination of advanced welding technology (dual-pulse MIG) and optimized post-weld heat treatment can substantially mitigate the weldability challenges of 7075 aluminum alloy. The microstructural analysis reveals that the key to property restoration lies in the dissolution of coarsened CuAl2 precipitates during solution treatment and the controlled reprecipitation of fine MgZn2 phases during aging. The dual-stage aging process is particularly effective because it allows for both fine intragranular precipitation and controlled boundary precipitation, providing a balanced strength-ductility combination.