TIG Welding Process Research on Aluminized Steel

Literature Overview

This study by Guo Bixin and Du Xiaowei (2010), published in Hot Working Technology, investigates the gas tungsten arc welding process for aluminized steel—a material system where a thin layer of aluminum or aluminum alloy is deposited on a carbon or low-alloy steel substrate to provide corrosion and oxidation resistance. The work originates from the First Air Force Aviation Academy and addresses the welding challenges associated with this dissimilar material combination, which is increasingly used in high-temperature structural applications.

Core Technical Points

The research addresses three primary challenges in welding aluminized steel:

Intermetallic compound formation: The welding process inevitably melts and redistributes the aluminum coating, leading to the formation of brittle iron-aluminum intermetallic phases (FeAl, Fe₂Al₅, FeAl₃) in the heat-affected zone and weld metal.
Cracking susceptibility: The mismatch in thermal expansion coefficients between the aluminum-rich layer and the steel substrate creates residual stresses that can initiate cracking.
Weld metal composition control: The dilution of the aluminum coating into the weld pool significantly alters the chemical composition of the deposited metal, affecting mechanical properties and corrosion resistance.

Interpretation of Technical Points

Intermetallic Compound Behavior

The formation of iron-aluminum intermetallics is the central metallurgical challenge in this welding system. The study examines how different welding parameters influence the morphology and distribution of these phases:

Intermetallic Phase	Crystal Structure	Hardness (HV)	Brittleness	Formation Temperature
FeAl	BCC (CsCl-type)	~200	Moderate	1221°C
Fe₂Al₅	Tetragonal	~300	High	1221°C
FeAl₃	Tetragonal	~250	Very high	1221°C
FeAl₆	Orthorhombic	~200	High	1221°C

The study demonstrates that:

Low welding current reduces the melting depth of the aluminum coating, limiting intermetallic formation but potentially resulting in incomplete fusion.
Higher travel speed reduces thermal input, decreasing the width of the intermetallic zone but requiring higher current to maintain penetration.
Electrode type and polarity influence the dilution ratio—DC electrode negative (DCEN) concentrates heat at the workpiece, promoting deeper penetration but also greater aluminum coating melting.

Welding Process Parameters

The study recommends the following process window for typical aluminized steel configurations (steel substrate 3-6 mm with 50-150 μm aluminum coating):

Parameter	Recommended Value	Rationale
Current	80-150 A (DCEN)	Balances penetration with coating dilution
Travel speed	150-300 mm/min	Controls thermal input
Shielding gas	Ar (100%) or Ar/CO₂ (80/20)	Adequate protection
Filler metal	ER70S-6 or similar	Compatible with base steel
Electrode	Pure tungsten, 2.4-3.2 mm	Stable arc
Gas flow	12-18 L/min	Complete shielding
Preheat	100-200°C	Reduces cracking risk

Engineering Practice Integration

Application in Pressure Vessel and Structural Components

Aluminized steel finds application in:

High-temperature structural components in aerospace and automotive industries
Corrosion-resistant cladding for chemical processing equipment
Oxidation-resistant panels for furnace linings and heat exchangers

For pressure vessel applications, the welding of aluminized steel presents additional concerns:

Bond strength verification: The interface between the aluminum layer and steel substrate must maintain adequate bond strength after welding.
Corrosion resistance retention: The weld zone must not become a preferential site for corrosion attack.
Mechanical property matching: The weld joint should achieve at least 80% of the base metal yield strength.

Defect Analysis and Countermeasures

Defect	Cause	Countermeasure
Cracking in HAZ	Residual stress from thermal mismatch	Preheat, post-weld stress relief
Excessive intermetallic	Overheating of aluminum coating	Reduce current, increase speed
Lack of fusion	Insufficient penetration	Increase current or reduce speed
Coating burn-off	Excessive arc energy	Use lower current, increase travel speed
Porosity	Gas entrapment in coating	Clean surface, adequate shielding

Key Questions and Reflections

The study raises important considerations for engineering practice:

What is the maximum allowable thickness of the aluminum coating that can be welded without unacceptable intermetallic formation?
How does the presence of alloying elements in the aluminum coating (such as silicon, magnesium, or zinc) affect weldability?
Can multi-pass welding strategies be designed to progressively manage the aluminum dilution?

From a practical perspective, I find that the study's emphasis on controlling the thermal input is well-founded. In my experience with bimetal fabrication, the key to successful welding of coated materials lies not in maximizing penetration but in finding the optimal balance between adequate fusion and minimal disturbance of the coating. This principle applies equally to aluminized steel, stainless steel cladding, and nickel alloy overlays.

Study Insights and Implications

This research provides valuable process guidance for welding aluminized steel, with particular emphasis on the metallurgical consequences of the welding process. The key insight is that welding aluminized steel requires a fundamentally different philosophy than welding homogeneous materials—the process must be designed to preserve the functional integrity of the coating while achieving structural continuity. For engineers working on bimetal products, this study reinforces the importance of understanding the metallurgical interactions at dissimilar material interfaces and designing welding procedures that account for these interactions from the outset rather than attempting to correct them after the fact.