Zhuojin Pipe Fitting Co., LtdEffect of Natural Aging on Microstructure and Properties of A7N01 Aluminum Alloy Laser-Variable Polarity TIG Hybrid Welds
Literature Overview
This research by Qiao Junnan, Zou Jianglin, and Wu Shikai, published in the Chinese Journal of Welding (焊接学报) in 2018, investigates the effects of natural aging on the microstructure and mechanical properties of A7N01 aluminum alloy welds produced by fiber laser-variability polarity TIG (L-VP-TIG) hybrid welding. Funded by the Beijing Natural Science Foundation, this study represents an important contribution to the understanding of hybrid welding processes for high-strength aluminum alloys.
Core Technical Content
A7N01 (equivalent to 7075) is a high-strength aluminum alloy in the 7xxx series containing approximately 4.7% zinc, 1.5% magnesium, and 2.5% copper. Its excellent specific strength makes it indispensable in aerospace and high-performance structural applications. However, the combination of high strength and weldability presents a significant challenge, as the precipitation-hardened T6 temper is susceptible to softening in the HAZ during welding.
Hybrid Welding Process Characteristics
The L-VP-TIG hybrid process combines the deep penetration and high productivity of fiber laser welding with the wider weld pool and better mechanical properties of TIG welding. The variable polarity (alternating current) feature provides:
- Cathodic cleaning effect: The AC cycle provides periodic cathodic arc cleaning, removing oxide layers from the weld pool surface
- Reduced porosity: The cleaning action reduces gas entrapment and oxide inclusion formation
- Improved wetting: Better fusion and wetting of the base metal
For 4 mm thick A7N01 plates, the hybrid process achieves full penetration in a single pass, significantly improving productivity compared to conventional multi-pass TIG welding.
Natural Aging Results
The most significant finding is the dramatic improvement in mechanical properties after natural aging:
| Condition | Tensile Strength (MPa) | Elongation (%) | Strength/UTS Ratio to Base Metal |
|---|---|---|---|
| As-welded | 320 | 3.1 | ~73% |
| Naturally aged 30 days | 369 | 4.4 | 83% |
| After weld reinforcement removal | 394 | 7.6 | 94% |
The natural aging process promotes the precipitation of strengthening phases in the weld metal. The as-welded weld metal contains supersaturated solute atoms (Zn, Mg, Cu) that were dissolved during welding but did not have sufficient time to precipitate during solidification and cooling. Over time, these solute atoms nucleate and grow precipitates:
- GP zones: Supersaturated clusters that form first during aging
- η' (MgZn2) metastable phase: Provides significant strengthening
- η (MgZn2) stable phase: Final equilibrium phase, provides moderate strengthening
The TEM analysis confirmed that after 30 days of natural aging, the weld metal contained increased quantities and sizes of GP zones along with η' and η phases. These precipitates act as obstacles to dislocation motion, strengthening the weld metal through precipitation hardening.
Microstructural Evolution
The base metal of A7N01 in the T6 temper contains a well-dispersed population of fine precipitates (η', η, and S' phases) responsible for its high strength (approximately 500-520 MPa). During welding:
- Weld pool: Complete dissolution of precipitates; rapid solidification produces fine dendritic structure with supersaturated matrix
- HAZ: Peak temperatures above the solution treatment temperature cause precipitate dissolution; subsequent cooling produces coarse precipitates at grain boundaries
- Thermally affected zone (TAZ): Partial precipitate dissolution; overaging of remaining precipitates
After natural aging, the weld metal precipitates grow and strengthen, but the HAZ remains the weakest zone due to coarse precipitate distribution at grain boundaries. This explains why fracture always occurs in the weld region—the weld metal, despite aging, remains weaker than the properly aged base metal.
Engineering Practice Implications
- Aging schedule optimization: The finding that 30 days of natural aging achieves peak properties is practically significant. Engineers can plan production schedules to allow adequate aging time before final mechanical testing and component installation.
- Weld reinforcement removal: The additional 25 MPa improvement after grinding the weld reinforcement is notable. This suggests that the weld cap acts as a stress concentrator, and removing it not only eliminates the geometric discontinuity but also removes the zone with the most severe overaging.
- Process selection: For applications requiring high joint efficiency, the L-VP-TIG hybrid process with natural aging achieves 83-94% joint efficiency, which is excellent for aluminum alloys. This makes it suitable for aerospace and automotive structural applications.
- Fracture analysis: The ductile dimpled fracture morphology confirms that the weld metal has adequate ductility even after aging. The elongation improvement from 3.1% to 7.6% (after reinforcement removal) indicates significant improvement in strain tolerance.
Key Reflections
This study demonstrates that natural aging is a simple, cost-effective method to improve the mechanical properties of hybrid-welded A7N01 joints. Unlike artificial aging, which requires controlled furnace heating and carries risks of overaging or distortion, natural aging occurs at ambient temperature with no additional equipment costs. The 30-day stabilization period is long enough to be practical for most production schedules but short enough to not significantly delay project timelines.
However, engineers should be aware that natural aging is time-dependent and temperature-dependent. The aging rate increases with temperature, so storage conditions matter. A joint stored at 25°C will age more slowly than one stored at 40°C, and the final properties may differ. For critical applications, it is advisable to conduct aging studies at the actual service temperature to predict long-term property evolution.
The hybrid welding process itself represents a significant advancement over conventional TIG welding for aluminum alloys. The combination of laser deep penetration with TIG's wider weld pool and cathodic cleaning provides a synergistic effect that produces high-quality welds with reduced heat input and improved productivity. As fiber laser technology continues to advance with increasing power and beam quality, hybrid welding will become increasingly viable for thicker aluminum alloy sections.