CLADDING TECHNOLOGY SHANXI CO., LTDMulti-Pass MIG Welding of Aluminum Alloys Interface Behavior Study Note
Literature Overview
This 2016 study by Jian Haigen, Tang Xianmin, Ou Ling, Wang Ying, and Yin Zhimin, published in Rare Metal Materials and Engineering, investigates the welding interface behavior in multi-pass MIG welding of aluminum alloys. Funded by the National Natural Science Foundation of China (Grant 51301065) and the Hunan Provincial Natural Science Foundation (Grant 14JJ7067), this research bridges the gap between welding process engineering and materials science, with collaborative input from Hunan University of Technology and Central South University.
Core Technical Content
Multi-pass MIG welding of aluminum alloys presents unique challenges compared to single-pass welding, particularly regarding the metallurgical behavior at the interface between successive weld passes. Each subsequent pass acts as a re-heat cycle on the previously solidified weld metal, potentially causing microstructural coarsening, precipitate dissolution and re-precipitation, and changes in mechanical properties.
Microstructural Evolution Across Passes
The interface between passes in aluminum alloy welds exhibits distinct zones that can be characterized as follows:
| Zone | Location | Microstructural Features | Mechanical Implication |
|---|---|---|---|
| Fusion zone of current pass | Center of bead | Fine equiaxed grains, dendritic | Highest hardness, lowest ductility |
| Remelted zone | Interface between passes | Partially remelted previous pass metal | Variable grain size, precipitate-free zones |
| Heat affected zone (HAZ) | Adjacent to fusion zone | Precipitate dissolution, grain growth | Softening, reduced strength |
| Previous pass solidified zone | Below interface | As-solidified from prior pass | Baseline properties |
Interpass Temperature Effects
The interpass temperature is a critical parameter governing interface behavior. In aluminum alloys, particularly 6061 and 7075 series, the interpass temperature should generally be maintained below 150°C to prevent excessive grain growth and loss of precipitate strengthening. However, maintaining such low interpass temperatures in practice requires either reducing the number of passes per layer or accepting longer cooling intervals, both of which reduce productivity.
Heat Input and Solidification Behavior
The total heat input in multi-pass welding accumulates with each successive pass. For aluminum alloys, the thermal conductivity is approximately 200–240 W/(m·K), which is roughly three times that of carbon steel. This high thermal conductivity results in:
- Rapid heat dissipation from the weld zone
- Narrower fusion zones compared to steel
- Higher cooling rates and finer microstructures
- Reduced risk of intergranular cracking but increased susceptibility to hot cracking due to the wider solidification range of many aluminum alloys
Process Analysis and Standards Compliance
For aluminum alloy pressure vessels fabricated under NB/T 47002 or ASME VIII Div.1, the multi-pass welding procedure must be qualified per NB/T 47014 or ASME IX. Key qualification requirements include:
- Procedure qualification: The WPS must specify maximum heat input per pass and total heat input, maximum interpass temperature, and the number of passes.
- Mechanical testing: Tensile, bend, and impact tests must demonstrate that the multi-pass weld meets or exceeds the base metal requirements.
- Non-destructive examination: Full radiographic or ultrasonic examination of the completed weld, with particular attention to the interpass interfaces where lack of fusion and porosity may concentrate.
- Microstructural verification: While not always required by code, many engineering specifications for critical aluminum alloy pressure vessels require metallographic examination of the multi-pass weld interface to verify sound bonding and acceptable microstructure.
Common Defects and Countermeasures
| Defect Type | Location | Root Cause | Countermeasure |
|---|---|---|---|
| Lack of fusion at pass interface | Between successive passes | Insufficient overlap, high travel speed | Increase overlap to 50% of bead width, reduce travel speed |
| Porosity at interface | Remelted zone boundary | Hydrogen pickup, gas entrapment | Increase shielding gas flow, clean base metal thoroughly |
| Hot cracking | Centerline of bead | Widening solidification range, restraint | Add Al-Si or Al-Mg-Si filler to narrow solidification range |
| Grain coarsening | Deep passes | Excessive cumulative heat input | Reduce interpass temperature, use lower current per pass |
| Undercut at pass boundary | Edge of bead | Excessive current, inadequate wire angle | Reduce current, adjust torch angle to 10–15° from vertical |
Engineering Practice Integration
In the fabrication of aluminum alloy heat exchangers and pressure vessels, multi-pass MIG welding is commonly employed for joints thicker than 6 mm. The interface behavior studied in this research directly impacts the long-term reliability of these components. For hydrogen service applications, where hydrogen embrittlement and stress corrosion cracking are concerns, the microstructural quality at pass interfaces is particularly critical.
The research findings suggest that a layered approach—completing each layer before beginning the next—produces more uniform microstructure than a single-layer multi-pass approach. However, this layered approach increases the total number of passes and thus the total heat input, creating a fundamental trade-off that must be managed through careful WPS development.
Study Insights and Implications
This research contributes valuable metallurgical understanding to what is often treated as a purely procedural exercise in aluminum alloy welding. The interface behavior between passes is not merely a quality concern but a fundamental determinant of the weld's long-term performance under cyclic loading, thermal cycling, and corrosive environments.
For engineers developing welding procedures for aluminum alloy bimetallic components, this work underscores the importance of treating each pass interface as a critical metallurgical boundary that requires deliberate control rather than passive acceptance. The accumulation of heat input across multiple passes demands a systematic approach to procedure development that accounts for the progressive microstructural evolution of the weld metal.