Zhuojin Pipe Fitting Co., LtdTIG Welding Process Development for Aluminized Steel
Literature Overview
This paper by Guo Bixin and Du Xiaowei (2010), published in Hot Working Technology (Vol. 39, No. 19, pp. 173-174), investigates the weldability of aluminized steel and develops a TIG welding process that effectively prevents weld cracking while preserving the corrosion resistance of the welded joint. Aluminized steel, produced through the pack cementation process (typically at 900-1100 °C for 2-4 hours), develops a diffusion layer of iron-aluminum intermetallic compounds (FeAl, Fe₂Al₅, FeAl₂) beneath a pure aluminum surface layer. This structure provides excellent oxidation and corrosion resistance but introduces significant welding challenges.
Weldability Analysis of Aluminized Steel
The primary welding difficulties of aluminized steel include:
- High susceptibility to solidification cracking: The aluminum diffusion layer contains high concentrations of Fe₂Al₅, which is a brittle intermetallic phase that can form low-melting-point eutectics during welding.
- Rapid aluminum burn-off: The pure aluminum surface layer has a significantly lower melting point (660 °C) than the base steel (approximately 1500 °C), leading to preferential vaporization and loss of aluminum content in the weld zone.
- Oxide inclusion formation: Aluminum oxide (Al₂O₃) has a melting point of 2050 °C and forms rapidly in the weld pool, potentially causing porosity and inclusions.
- Reduced toughness in the heat-affected zone: The intermetallic diffusion layer near the weld can embrittle the HAZ due to localized softening and grain coarsening.
Developed TIG Welding Process
The authors propose the following welding parameters and procedures:
| Parameter | Recommended Value | Rationale |
|---|---|---|
| Shielding gas | Pure argon (99.99%) | Minimizes oxidation of aluminum-rich pool |
| Current type | DCEN | Provides stable arc and controlled penetration |
| Current range | 80-160 A | Depends on base metal thickness |
| Travel speed | 200-400 mm/min | Controls heat input to limit Al burn-off |
| Electrode | Pure tungsten (2.0-3.2 mm diameter) | Resists contamination from aluminum |
| Preheating | Not required for thin sections | Avoids further intermetallic growth |
| Interpass temperature | Below 150 °C | Prevents softening of diffusion layer |
| Filler metal | ER308L or matching aluminized alloy | Low carbon to reduce cracking risk |
Key Process Features
The welding process emphasizes several critical control points:
- Low heat input strategy: Minimizing the heat input (typically below 1.5 kJ/mm) reduces the extent of aluminum vaporization and limits the softening of the intermetallic diffusion layer in the HAZ.
- Precise arc control: The TIG process provides excellent control over the arc location, which is essential for keeping the arc centered on the joint and avoiding excessive heating of the aluminized surface.
- Back-purging: For thicker sections where root access is available, helium back-purging helps prevent root oxidation and maintains the aluminum content in the root weld bead.
- Post-weld treatment: A controlled cooling rate (air cooling or light wind cooling) is recommended to avoid excessive grain growth in the weld metal, which could further reduce toughness.
Metallurgical Considerations
The weld metal microstructure in aluminized steel joints typically shows:
- A dendritic ferrite-pearlite structure in the weld metal
- A transition zone where the intermetallic phases partially dissolve
- A tempered martensite or bainite structure in the HAZ of the steel substrate
- Potential formation of brittle Fe₂Al₅ phases at the weld fusion line if heat input is excessive
The authors note that the developed process successfully prevents transverse and longitudinal weld cracking, which are the most common defects in aluminized steel welding. The corrosion resistance of the weld joint is maintained through careful control of the aluminum content in the weld metal and minimization of oxide inclusions.
Engineering Practice Implications
For engineers working with aluminized steel components—common in automotive exhaust systems, architectural applications, and industrial equipment—the following practical recommendations emerge:
- WPS development: A dedicated welding procedure specification should be developed for aluminized steel, incorporating the low heat input parameters and specific shielding gas requirements identified in this study.
- Welder qualification: Welders must be trained specifically in aluminized steel welding, as the process is sensitive to arc wander and parameter deviations that would be less critical in standard carbon steel welding.
- Non-destructive testing: Visual inspection should be supplemented with magnetic particle testing (MT) or dye penetrant testing (PT) to detect fine cracks that may not be visible to the naked eye.
- Corrosion testing: Post-weld corrosion resistance should be verified through salt spray testing (ASTM B117) or electrochemical testing, particularly for applications where the corrosion performance of the joint is critical.
Summary
This paper demonstrates that aluminized steel can be successfully welded using TIG welding with appropriate parameter control, despite the inherent challenges posed by the iron-aluminum intermetallic diffusion layer. The key to success lies in minimizing heat input to limit aluminum loss and intermetallic embrittlement, while maintaining excellent gas shielding to prevent oxide inclusion formation. For production welding of aluminized steel components, the principles established in this study should be incorporated into formal WPS development and welder qualification programs.