CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Impact Mechanical Properties of Multi-Layer Multi-Pass TIG Welded Joints of Al-Zn-Mg Alloy Plates
Literature Overview
This paper by Gao Qing-wei, Shu Feng-yuan, He Peng, and Du Wen-bo, published in the Transactions of Nonferrous Metals Society of China in 2019, investigates the microstructural evolution and impact mechanical properties of multi-layer, multi-pass gas tungsten arc (GTAW/TIG) welded joints of Al-Zn-Mg alloy plates. The research was supported by the Shandong Province Natural Science Foundation (ZR2016EEQ03), the China Postdoctoral Science Foundation (2018M641822), and the Natural Scientific Research Innovation Foundation of Harbin Institute of Technology (HIT.NSRIF.201703). The collaboration between Shanghai University of Engineering Science, Harbin Institute of Technology at Weihai, and the State Key Laboratory of Advanced Welding and Joining represents a strong interdisciplinary approach to understanding aluminum alloy weld integrity.
Core Technical Content
The study focuses on the 5xxx series Al-Zn-Mg alloys, which are widely used in marine structures, automotive components, and cryogenic applications. The multi-layer, multi-pass welding configuration introduces complex thermal cycling that fundamentally alters the microstructure compared to single-pass joints. Key findings include:
- The weld zone, heat-affected zone (HAZ), and base metal exhibit distinctly different microstructural features after multi-pass welding.
- Grain refinement occurs in the weld metal due to the repeated thermal cycles, with subsequent passes acting as in-situ tempering treatments on previously deposited layers.
- Impact energy values vary significantly across the weld cross-section, with the HAZ typically showing the lowest toughness due to precipitate coarsening and grain growth.
Microstructural Analysis and Process Insights
The thermal history of multi-pass welding creates a non-uniform microstructural landscape. The first deposited pass experiences the most severe thermal input from subsequent passes, leading to over-aging of strengthening precipitates such as eta-phase (MgZn2) and eta-prime (MgZn) in Al-Zn-Mg alloys. The interpass temperature is a critical parameter, typically controlled between 150°C and 200°C for these alloys to prevent excessive grain growth while allowing adequate plasticity for crack resistance.
| Microstructural Zone | Grain Size | Precipitate State | Impact Energy (Typical) |
|---|---|---|---|
| Base Metal (BM) | 80-120 μm | Peak-aged (T6) | 60-80 J |
| Weld Metal (WM) | 30-60 μm | Over-aged (subsequent passes) | 35-55 J |
| HAZ (Peak Heat) | 150-250 μm | Dissolved and coarsened | 20-40 J |
| HAZ (Moderate Heat) | 100-150 μm | Partially dissolved | 40-55 J |
The nonlinear relationship between heat input and impact properties is particularly noteworthy. Excessive heat input above approximately 25 kJ/mm leads to dramatic grain coarsening in the HAZ, while insufficient heat input below 15 kJ/mm results in incomplete melting and poor fusion. The optimal window for 5083 or 5086 Al-Zn-Mg alloys typically lies between 18 and 22 kJ/mm for TIG welding at standard travel speeds of 8-15 mm/min.
Relevance to Cladding and Bimetal Engineering Practice
Although this study addresses aluminum alloy homogenous welding, the principles of multi-pass thermal cycling are directly applicable to overlay welding of aluminum-clad steel or aluminum-based cladding layers on dissimilar substrates. In bimetal pressure vessel fabrication involving aluminum-lined vessels for cryogenic service (such as liquid natural gas storage), understanding how interpass thermal input affects the overlay layer microstructure is essential for ensuring low-temperature toughness. The impact energy degradation observed in the HAZ mirrors concerns in clad plate pressure vessels where the cladding/substrate interface must maintain adequate fracture resistance under cryogenic conditions.
Key Questions and Reflections
The study raises several important questions for engineering practice. First, how does the interpass temperature control strategy differ for aluminum alloys compared to the more commonly encountered carbon steel and stainless steel systems in pressure vessel fabrication? Second, the role of welding sequence optimization in minimizing the HAZ embrittlement zone warrants further investigation, particularly for thick-section clad plates where multiple layers are deposited. Third, the relationship between grain refinement mechanisms in the weld metal and the resistance to hydrogen-induced cracking (HIC) in similar multi-pass configurations deserves attention, especially when aluminum overlays are applied to low-alloy steel substrates in hydrogen-containing environments.
The findings reinforce the importance of qualified welding procedures under standards such as ASME IX and NB/T 47014, where multi-pass configurations must be evaluated for impact properties at the qualification temperature. Engineers working on aluminum-clad pressure vessels should pay particular attention to the thermal management strategies that can preserve the beneficial microstructural features of the overlay layer while avoiding excessive softening of the substrate.