Iron-Based Amorphous Alloy Coatings Prepared by GMAW Cladding

Literature Overview

This study, published in 2011 by researchers from the Surface Science and Technology Research Institute of the Corrosion and Protection Center at Beijing University of Science and Technology, investigates the preparation of iron-based amorphous (metallic glass) alloy coatings using gas metal arc welding (GMAW) cladding. The work represents a significant advancement in the field of rapidly solidified surface engineering, addressing the long-standing challenge of producing bulk amorphous coatings through conventional welding methods rather than the more energy-intensive and equipment-intensive techniques typically required for bulk metallic glass fabrication.

Core Technical Content

The formation of amorphous structures in weld deposits requires cooling rates exceeding critical thresholds (typically >10³ K/s for iron-based alloys). Conventional welding processes generally produce cooling rates in the range of 10–10³ K/s, making the formation of fully amorphous structures challenging. The authors demonstrate that by carefully selecting the alloy composition and optimizing the welding parameters, it is possible to achieve amorphous or near-amorphous microstructures in GMAW cladding deposits.

Alloy Design Principles

The alloy design follows the Inokutai criterion for glass-forming ability (GFA), which requires that the difference between the liquidus and glass transition temperatures (ΔTₓ = Tₗ - Tₓ) exceeds a critical value. For iron-based alloys, compositions in the Fe-Cr-Mo-C-B-Ni system have been identified as having favorable GFA. The specific composition examined in this study contains approximately Fe-8Cr-4Mo-2C-1B-1Ni (wt%), which is designed to maximize the supercooled liquid region while maintaining weldability.

Composition Element	Content (wt%)	Role in Amorphization
Fe	Balance	Matrix element; provides structural continuity
Cr	6–10	Enhances GFA; suppresses crystallization
Mo	3–5	Widens ΔTₓ; increases viscosity of melt
C	1.5–2.5	Lowers melting point; promotes liquidus depression
B	0.5–1.5	Suppresses crystalline phase formation
Ni	0.5–2.0	Modifies phase diagram; improves ductility

Welding Process Parameters

The GMAW cladding was performed using a self-shielded flux-cored wire (FCAW-S) or solid wire with CO₂ or Ar/CO₂ shielding gas. The key process parameters are summarized below:

Parameter	Value	Rationale
Shielding gas	80% Ar / 20% CO₂	Balances arc stability and carbon pickup
Wire diameter	1.2 mm	Fine wire promotes rapid solidification
Current	180–220 A	Moderate heat input for fine grain structure
Voltage	22–26 V	Controls arc length and penetration
Travel speed	200–300 mm/min	Higher speed increases cooling rate
Wire stick-out	12–15 mm	Reduces heat transfer to deposited metal
Interpass temperature	<150 °C	Prevents crystallization in prior pass

Microstructural Characterization

X-ray diffraction (XRD) analysis of the as-deposited overlay reveals a broad halo pattern characteristic of an amorphous structure, with no discernible crystalline peaks. Transmission electron microscopy (TEM) confirms the absence of long-range atomic order, with the amorphous matrix exhibiting a featureless contrast and diffuse diffraction rings. The amorphous structure is stable up to temperatures of approximately 450–500 °C, corresponding to the crystallization onset temperature (Tc₁).

The amorphous nature of the coating confers several advantageous properties:

Corrosion resistance: The absence of grain boundaries, inclusions, and segregation zones eliminates preferential corrosion pathways, resulting in corrosion rates in 3.5% NaCl solution that are 3–5 orders of magnitude lower than those of conventional austenitic stainless steel cladding.
Uniform hardness: Hardness values of 700–800 HV₀.₃ are uniform across the cross-section, without the soft/brittle phase variations common in crystalline overlays.
High strength: Tensile strength values exceeding 1800 MPa have been reported for similar compositions, significantly surpassing conventional weld metals.

Engineering Considerations and Defect Analysis

While the amorphous coating concept is technologically compelling, several practical challenges must be addressed for industrial implementation:

Common Defects and Countermeasures

Defect	Cause	Countermeasure
Partial crystallization	Excessive cooling rate reduction (thick deposit, high interpass temp)	Reduce wire diameter; increase travel speed; lower interpass temp
Hydrogen-induced cracking	High carbon content; slow cooling	Use low-hydrogen flux; preheat to 100–150 °C
Porosity	Gas entrapment from flux decomposition	Optimize flux composition; ensure proper arc stability
Cracking in amorphous zone	Intrinsic brittleness of amorphous phase	Limit overlay thickness to <2 mm; apply multiple thin passes
Delamination from substrate	Thermal expansion mismatch; poor wetting	Apply transition layer of compatible composition

Process Optimization Strategy

A systematic approach to process optimization is recommended:

Step 1 – Substrate preparation: Grind the base material to a smooth, oxide-free surface with a minimum Ra of 3.2 μm. Preheat to 100–150 °C to reduce thermal shock.
Step 2 – First pass deposition: Use a lower current (160–180 A) and higher travel speed (250–300 mm/min) to minimize heat input and promote rapid solidification.
Step 3 – Subsequent passes: Maintain interpass temperatures below 150 °C. Each subsequent pass should be thinner (0.5–1.0 mm) to limit the total heat accumulation.
Step 4 – Post-deposition treatment: Avoid any heat treatment that exceeds 350 °C. If stress relief is required, apply a low-temperature treatment (200–250 °C) for a short duration (≤1 hour).
Step 5 – Inspection: Perform XRD on each pass to verify amorphous content. Use hardness mapping to identify any crystallized zones that may indicate process deviation.

Integration with Corrosion Protection Engineering

The primary application target for iron-based amorphous coatings is in the corrosion protection of process equipment in the chemical, petrochemical, and marine industries. The amorphous structure eliminates the microstructural inhomogeneities that drive localized corrosion (pitting, crevice corrosion, intergranular corrosion). In aggressive chloride environments, the coating exhibits no measurable pitting up to 24 hours of immersion in 3.5% NaCl at room temperature, compared to extensive pitting observed in 316L stainless steel cladding under the same conditions.

However, the inherent brittleness of the amorphous phase limits the coating thickness to practical ranges of 0.5–2.0 mm. For applications requiring thicker protective layers, a hybrid approach combining an amorphous top layer with a crystalline intermediate layer is recommended. The intermediate layer provides toughness and bonding strength, while the thin amorphous top layer provides the corrosion-resistant barrier.

Study Insights and Implications

This research demonstrates that conventional GMAW/FCAW equipment can be used to produce amorphous coatings, which is a significant step toward the industrialization of metallic glass surface engineering. The key enabler is the careful selection of alloy composition with sufficient glass-forming ability to compensate for the relatively low cooling rates achievable in welding processes. The work bridges the gap between laboratory-scale amorphous material research and practical surface engineering applications.

For engineers, the main takeaway is that amorphous coatings offer a fundamentally different corrosion protection mechanism compared to conventional crystalline overlays. The elimination of microstructural defects provides a level of corrosion resistance that is difficult to achieve with traditional cladding alloys, even those with high alloy content. The challenge lies in maintaining the amorphous structure during processing and in managing the brittleness inherent in glassy phases. Future work should focus on developing multi-layer strategies and on extending the stable amorphous temperature range through further alloy optimization.