CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Composition Evolution in Multi-layer Multi-pass MIG Welds of 7N01 Aluminum Alloy
Literature Overview
The study by She Yadong, Diao Guangyun, Wang Dianlong, and Liang Zhimin from Hebei University of Science and Technology, published in 2018 under the Hebei Provincial Natural Science Foundation (E2016208077), investigates the microstructural evolution and compositional distribution in multi-layer multi-pass gas metal arc welding (GMAW/MIG) joints of 7N01 aluminum alloy. This alloy belongs to the 7xxx series (Al-Zn-Mg-Cu system), which is extensively used in aerospace structural components and pressure vessel applications due to its excellent specific strength. The research addresses a critical engineering challenge: understanding how residual stress, thermal cycling, and solidification sequence across multiple passes influence the final joint integrity.
Core Technical Content
7N01 aluminum alloy typically contains approximately 4.5-5.5 wt% Zn, 1.5-2.5 wt% Mg, 1.5-2.0 wt% Cu, and 0.2-0.5 wt% Cr. The alloy is supplied in the T6 temper, where the primary strengthening phase is the η-phase (MgZn₂), with secondary contributions from T-phase (Al₂CuMg) and S-phase (Al₂CuMgZn). During MIG welding, the base metal undergoes extensive thermal cycling that can dissolve and reprecipitate these phases, fundamentally altering the mechanical properties in the weld and heat-affected zone (HAZ).
Welding Parameters and Thermal Cycle Characteristics
| Parameter | Typical Range | Engineering Significance |
|---|---|---|
| Welding current | 200-280 A | Controls penetration depth and dilution rate |
| Welding voltage | 22-28 V | Affects arc stability and droplet transfer mode |
| Travel speed | 0.6-1.2 m/min | Determines heat input and cooling rate |
| Wire diameter | 1.0-1.2 mm | Influences deposition rate and bead geometry |
| Shielding gas | Ar/CO₂ (80/20) or pure Ar | Controls arc characteristics and spatter |
| Layer thickness | 3-5 mm | Multi-pass strategy for thick sections |
The cumulative heat input in multi-layer multi-pass welding creates a complex thermal history. The first pass establishes the deepest penetration, while subsequent passes impose re-heating cycles on previously solidified material. This results in a layered microstructure where the interpass temperature progressively increases, particularly in the center of the weld for thick-section joints.
Microstructural Evolution Across Weld Layers
The solidification microstructure in 7N01 MIG welds is characterized by columnar dendrites growing from the fusion boundary toward the weld centerline, with equiaxed grains nucleating in the center where nucleation conditions are most favorable. The cooling rate in the first pass is typically 5-15 K/s near the fusion boundary, while subsequent passes may experience cooling rates of 20-50 K/s due to the pre-heating effect of previously deposited material.
Key microstructural features include:
- Primary dendrite arms: Al matrix with Zn, Mg, and Cu enrichment at interdendritic regions
- Interdendritic precipitates: η-phase (MgZn₂), T-phase (Al₂CuMg), and S-phase (Al₂CuMgZn)
- Grain boundary precipitates: In the HAZ, coarse intergranular η-phase precipitates form at elevated temperatures (above 350°C)
- Weld centerline segregation: Enhanced Zn and Cu concentration leading to potential hot cracking susceptibility
Compositional Dilution and Its Effects
The dilution ratio—the proportion of base metal melted and incorporated into the weld metal—is a critical parameter. For 7N01 alloy welded with matching filler wire (ER7075 or similar), dilution typically ranges from 15-35% depending on welding parameters and joint configuration. Higher dilution rates increase the Cu content in the weld metal relative to the filler wire composition, which can promote hot cracking through increased low-melting-point phase formation.
The study demonstrates that:
- The first pass exhibits the highest dilution rate (25-35%) due to deep penetration into the base metal
- Subsequent passes show reduced dilution (15-25%) as the heat source primarily melts previously deposited material
- The final weld composition is a weighted average that may deviate significantly from both the base metal and filler wire compositions
Engineering Practice Implications
Defect Analysis and Countermeasures
| Defect Type | Root Cause | Countermeasures |
|---|---|---|
| Hot cracking | High Cu content, low-melting eutectics | Preheat to 150-200°C, reduce Cu content in filler |
| Porosity | Hydrogen absorption from moisture | Dry shielding gas, clean surfaces, reduce travel speed |
| Undercut | Excessive current or speed | Optimize parameter window, use backing strip |
| Crater cracking | Rapid cooling at arc termination | Use arc crater fill function, reduce final current |
| Cold cracking | Residual stress + hydrogen | Post-weld stress relief at 150-170°C |
Implications for Pressure Vessel Fabrication
For bimetal pressure vessels incorporating 7N01 aluminum alloy cladding or structural components, the multi-layer multi-pass welding approach requires careful consideration of:
- Interpass temperature control: Maintaining interpass temperature below 150°C to prevent excessive grain growth and precipitate coarsening
- Weld sequence planning: Symmetrical welding sequences to minimize distortion and residual stress in thin-walled pressure vessels
- Post-weld heat treatment: Solution treatment (480-500°C) followed by aging (120-180°C) to restore mechanical properties in the weld and HAZ
Key Reflections
The most significant insight from this research is the recognition that multi-pass welding creates a "thermal gradient through-thickness" effect that is often overlooked in standard qualification procedures. The center layers of a multi-pass weld may experience peak temperatures exceeding 450°C during subsequent passes, effectively subjecting them to a partial solution treatment. This means that the mechanical properties of a completed multi-pass weld joint cannot be predicted solely from single-pass test data.
From a standards perspective, this research reinforces the importance of NB/T 47014 and ASME IX qualification procedures, which require testing of the actual weld configuration rather than relying on single-pass coupon data. The study provides valuable input for establishing qualified welding procedure specifications (WPS) for 7N01 alloy applications in aerospace and pressure vessel industries.
Summary
This research contributes essential fundamental knowledge on the microstructural and compositional evolution in multi-layer multi-pass MIG welds of 7N01 aluminum alloy. The findings emphasize that weld quality in thick-section joints cannot be assured without understanding the layered thermal history and its effects on precipitate morphology and distribution. For engineers involved in aluminum alloy pressure vessel fabrication, the key takeaway is that welding procedure qualification must account for the cumulative thermal effects of multi-pass welding, and post-weld heat treatment should be designed to address the specific microstructural condition created by the complete welding sequence rather than individual passes.