CLADDING TECHNOLOGY SHANXI CO., LTDTIG Welding Process Research on Aluminized Steel
Literature Overview
This 2010 study by Guo Bixin and Du Xiaowei from the First Air Force Aviation Academy investigates the TIG (GTAW) welding of aluminized steel, a material increasingly used in automotive exhaust systems, structural panels, and certain pressure-retaining components where elevated-temperature oxidation resistance is required. Aluminized steel is produced by hot-dip diffusion of aluminum into a steel substrate, forming an Al-Fe intermetallic layer (typically containing FeAl, FeAl2, and Fe3Al phases) on the surface. The aluminum content in the diffusion layer ranges from approximately 25% to 65% by mass, depending on the heat treatment parameters. The study addresses the fundamental challenge of maintaining both the mechanical integrity of the weld and the corrosion/oxidation protection of the base material during GTAW joining.
Core Technical Challenges
The primary difficulty in welding aluminized steel lies in the extreme chemical reactivity of aluminum. At welding temperatures above 600°C, aluminum readily forms Al2O3, which has a melting point of 2050°C and is far denser than the molten weld pool. This oxide inclusion formation leads to porosity, lack of fusion, and reduced ductility in the heat-affected zone (HAZ). Additionally, the high thermal conductivity of the aluminum layer causes rapid heat dissipation, requiring higher energy input than conventional carbon steel welding.
| Parameter | Conventional Carbon Steel TIG | Aluminized Steel TIG (Recommended) |
|---|---|---|
| Current range | 60–150 A | 100–220 A |
| Voltage | 12–18 V | 14–22 V |
| Travel speed | 50–150 mm/min | 30–80 mm/min |
| Shielding gas | Ar or Ar/CO2 | Pure Ar (100%) |
| Filler wire | ER70S-6 | ER308L or ER347 |
| Preheat | Not required | 150–250°C |
| Interpass temp | <250°C | <200°C |
Process Analysis and Key Findings
The researchers identified that a two-step approach is most effective: first, mechanical or chemical removal of the aluminum coating from the weld preparation area (typically by grinding or pickling), followed by TIG welding with a stainless steel filler wire. When the coating is not removed, the weld metal becomes heavily contaminated with aluminum oxide inclusions, resulting in intergranular cracking along the HAZ due to the formation of brittle Al-Fe intermetallics at the weld boundary.
The optimal filler metal selection is critical. ER308L (304L equivalent) provides adequate ductility and corrosion resistance, while ER347 (347 equivalent, with Nb stabilization) offers superior resistance to sensitization and intergranular corrosion in the HAZ. The use of pure argon shielding is essential because any oxygen ingress accelerates aluminum oxidation in the weld pool.
Connection to Engineering Practice
In pressure vessel fabrication, aluminized steel is occasionally used for shell courses or heads in applications requiring moderate corrosion resistance at temperatures up to 600°C, such as certain heat exchanger shells or furnace components. The welding procedure specification (WPS) must account for the coating thickness variation across the plate surface. According to NB/T 47014, the qualification test coupon must represent the actual production conditions, including the presence or absence of the aluminum coating at the weld preparation area.
A critical engineering consideration is the post-weld inspection. Radiographic testing (RT) per JB/T 4730 may reveal oxide inclusions that appear as elongated indications, which can be misinterpreted as cracks. Ultrasonic testing (UT) is preferred for detecting lack of fusion at the weld root, as the oxide layer creates acoustic impedance mismatches that enhance defect sensitivity.
Key Questions and Reflections
The study raises an important question: under what conditions can the aluminum coating be preserved in the weld zone? In automotive applications, maintaining the coating is essential for corrosion performance. The researchers suggest that narrow-gap TIG with low heat input (below 0.5 kJ/mm) and high travel speed may partially preserve the coating while achieving adequate fusion. However, this approach requires precise control of arc stability and is not yet mature enough for pressure vessel applications where full fusion and mechanical reliability are paramount.
Another reflection concerns the long-term behavior of the weld in service. The Al-Fe intermetallic layer at the weld boundary may undergo further phase transformation during thermal cycling, potentially leading to microcracking. Long-term creep-rupture data for aluminized steel welds is scarce, and this represents a significant gap in the technical literature.
Study Insights and Implications
The practical takeaway is that aluminized steel welding remains a specialized operation requiring careful process control. For pressure vessel applications, the recommendation is to remove the coating from the weld preparation area, qualify the procedure per NB/T 47014 using stainless steel filler metal, and implement enhanced NDE protocols. The study provides a solid foundation for further investigation into low-dilution welding techniques that could preserve the coating while meeting pressure vessel code requirements.