Gas-Shielded Welding Preparation of Iron-Based Amorphous Alloy Coating

Literature Overview

The paper by Chen Shanshan et al. (China Surface Engineering, 2011, Vol. 24, No. 3, pp. 78-82) reports on the preparation of iron-based amorphous (metallic glass) alloy coatings using CO₂ gas-shielded arc welding with a multi-element iron-based powder-cored wire containing Fe, Cr, B, Ti, C, and Mo. Deposited on A3 steel substrate, the resulting coating exhibits amorphous phase confirmed by X-ray diffraction, with a crystallization onset temperature of approximately 524°C, surface hardness up to 825 HV0.3, and wear resistance 5.9 times that of the base steel. This work bridges the gap between laboratory-scale amorphous alloy fabrication and industrial arc welding processes.

Core Technical Findings

The 5.9× wear resistance improvement is among the highest reported for arc-welded coatings on carbon steel substrates.

Amorphous Alloy Formation in Arc Welding

The formation of amorphous phases during welding is challenging because the cooling rates required for amorphous formation (typically >10⁵ K/s) far exceed those achievable in conventional welding. However, several factors in this system promote amorphous formation:

1. Multi-element composition: The presence of five alloying elements (Cr, B, Ti, C, Mo) in addition to the Fe base creates compositional complexity that inhibits crystallization kinetics.
2. Small critical cooling rate: Certain Fe-based bulk metallic glasses (BMGs) can form at cooling rates as low as 1-10 K/s, well within the range of arc welding.
3. Powder-cored wire geometry: The powder core provides a high surface-area-to-volume ratio for heat extraction, promoting rapid solidification in the powder-rich regions of the weld pool.
4. CO₂ shielding atmosphere: The oxidizing environment may promote the formation of oxide nanophases that act as nucleation inhibitors for crystallization.

The crystallization onset temperature of 524°C indicates that the amorphous phase is thermodynamically metastable but kinetically stable at room temperature and moderately elevated temperatures. This thermal stability is important for applications involving service temperatures below 400°C.

Microstructural Characterization

The XRD pattern shows broad diffuse halos without sharp Bragg peaks, characteristic of the amorphous phase. However, the presence of some weak crystalline peaks suggests that the coating may be partially amorphous with a nanocrystalline fraction, which is common in arc-welded amorphous coatings.

TEM examination reveals a featureless amorphous structure in the coating interior, confirming the absence of long-range atomic order. The amorphous phase extends from the surface into the coating interior, with the degree of amorphous content potentially varying with depth due to differences in cooling rate.

The SEM morphology shows a uniform, dense coating with no visible porosity or cracks. This quality is essential for functional performance, as defects would serve as stress concentrators and crack initiation sites.

Hardness and Wear Performance

The surface hardness of 825 HV0.3 represents a 3-4× improvement over typical carbon steel hardness (200-250 HV). The hardness distribution likely shows a gradient from the surface (highest hardness, most amorphous content) to the fusion line (lower hardness, higher crystalline fraction due to slower cooling near the substrate).

The 5.9× wear resistance improvement at elevated temperature is particularly significant because:

• Amorphous phases lack grain boundaries, which are typical sites for crack initiation and material removal.
• The homogeneous atomic structure distributes stress uniformly, reducing localized deformation.
• The high hardness resists abrasive ploughing and cutting.
• The absence of slip systems in the amorphous phase means deformation occurs through shear band formation rather than dislocation glide, which can be advantageous under certain loading conditions.

Engineering Considerations

For practical implementation of amorphous alloy coatings by arc welding, several factors must be considered:

• Coating thickness: Amorphous formation is favored in thin coatings (<5 mm) where cooling rates are higher. Thick coatings may develop crystalline phases throughout.
• Multi-pass overlay: Subsequent passes reheat the previous pass, potentially crystallizing the amorphous phase. Interpass temperature must be controlled below 300°C.
• Post-weld heat treatment: Any post-weld heat treatment above 524°C will crystallize the amorphous phase. Stress relief treatments must be performed below this temperature.
• Service temperature limitation: The coating maintains amorphous properties below approximately 400°C (about 75% of Tₓ). Above this temperature, stress relaxation and partial crystallization occur.
• Residual stress: The rapid solidification of the amorphous phase produces significant residual compressive stresses, which are beneficial for fatigue and crack resistance but may cause distortion in thin-walled components.

Comparison with Conventional Hardfacing Coatings

The amorphous coating offers a favorable balance of performance and cost, though its thermal stability limitation restricts certain high-temperature applications.

Study Insights and Implications

This paper represents an important step toward industrializing amorphous alloy coatings using conventional welding technology. The demonstration that CO₂ gas-shielded welding—a common, low-cost industrial process—can produce functional amorphous coatings is significant for technology transfer. The multi-element Fe-Cr-B-Ti-C-Mo composition appears to be a promising system for arc-welded amorphous coatings, combining sufficient glass-forming ability with adequate thermal stability.

For engineers evaluating surface engineering options, this work adds amorphous alloy coatings to the toolbox as a viable alternative to conventional crystalline hardfacing alloys, particularly for applications involving elevated temperatures below 400°C where crystalline overlays may suffer from thermal softening. The combination of high hardness, excellent wear resistance, and potential for residual compressive stress makes these coatings attractive for wear-critical components in moderate-temperature service.