Microstructure and Properties of TIG Clad Fe-based Amorphous Alloy Layer

Literature Overview and Research Significance

This research investigates the microstructure evolution and mechanical properties of Fe-based amorphous alloy overlay layers produced by gas tungsten arc welding (GTAW/TIG) cladding. Amorphous alloys, also known as metallic glasses, possess unique properties including high hardness, excellent corrosion resistance, and superior magnetic characteristics. However, their application in overlay welding is challenging because the amorphous structure is thermally unstable and tends to crystallize during the welding thermal cycle. This study addresses the fundamental challenge of preserving or controlling the amorphous-to-crystalline transformation in weld overlay deposits, providing insights into the cooling rate requirements, phase evolution, and resulting mechanical behavior.

Core Technical Points: Amorphous Structure Stability

The central finding of this research is that the TIG welding thermal cycle inevitably causes partial or complete crystallization of Fe-based amorphous alloy deposits, with the degree of crystallization strongly dependent on cooling rate, alloy composition, and process parameters. The study identifies critical cooling rates above which amorphous structures can be partially preserved in the upper regions of the deposit, while the fusion line and lower portions inevitably undergo crystallization due to the slower cooling rates near the substrate.

The microstructural analysis reveals that crystallized regions exhibit a mixture of BCC ferrite, intermetallic phases (such as Fe2B, Fe3B, and Fe2Si), and retained austenite, depending on the specific alloy composition. The amorphous regions, when present, display characteristic featureless contrast under transmission electron microscopy and exhibit significantly higher hardness values compared to crystallized regions.

Mechanical Property Characterization

Hardness measurements across the clad layer reveal a pronounced gradient, with values ranging from 600–800 HV in regions with significant amorphous content to 350–500 HV in fully crystallized regions. The fusion line area typically shows the lowest hardness (300–400 HV) due to complete crystallization and dilution with the substrate material.

Tensile and bond strength tests demonstrate that the clad layer maintains adequate adhesion to the substrate, with bond strengths exceeding 250 MPa. However, the brittle nature of amorphous regions can lead to intergranular fracture modes during tensile testing, particularly when the amorphous-to-crystalline ratio is high. Fracture surface analysis reveals a mixed mode of fracture, with river patterns characteristic of ductile failure in crystallized regions and flat, featureless surfaces indicative of brittle fracture in amorphous regions.

Corrosion resistance testing in simulated industrial environments shows that amorphous regions exhibit superior pitting resistance compared to crystallized regions, attributed to the absence of grain boundaries and the homogeneous chemical distribution. This finding has significant implications for applications where localized corrosion is a primary concern.

Process Optimization and Engineering Considerations

The study identifies several process strategies to maximize amorphous content in TIG-clad deposits:

1. Minimizing deposit thickness per pass to ensure rapid solidification
2. Using high travel speeds to increase cooling rates
3. Avoiding substrate preheating or using minimal preheat temperatures
4. Employing active cooling of the substrate during welding
5. Selecting alloy compositions with high glass-forming ability (GFA)

The concept of glass-forming ability is critical here, as alloys with higher GFA (indicated by higher Tx/Tg ratios and larger ΔTx values) are more resistant to crystallization during the welding thermal cycle. The study evaluates several Fe-based alloy compositions, finding that those containing higher concentrations of B, Si, and C exhibit better amorphous retention.

Defect Analysis and Quality Control

Common defects observed in TIG-clad amorphous alloy deposits include:

• Crystallization-induced microcracking: Occurs when amorphous regions transform to brittle crystalline phases during cooling or post-weld heat treatment
• Porosity: Gas porosity from hydrogen pickup, particularly in high-silicon alloys
• Lack of fusion: Resulting from insufficient heat input or poor wetting
• Spatter: More prevalent at higher current settings

Quality control recommendations include metallographic examination of cross-sections to assess amorphous content distribution, microhardness profiling to map property gradients, and differential scanning calorimetry (DSC) to quantify crystallization temperatures and enthalpies.

Key Reflections and Study Insights

This research contributes significantly to the understanding of how welding thermal cycles interact with the metastable nature of amorphous alloys. The finding that complete preservation of amorphous structure is practically unachievable in conventional TIG cladding is both a limitation and an opportunity, as the resulting semi-amorphous microstructure may offer a favorable combination of properties.

The study's emphasis on the cooling rate as the dominant factor controlling amorphous retention aligns with fundamental solidification theory but provides specific quantitative data for Fe-based alloys that are valuable for process design. The identified critical cooling rates (above approximately 100 °C/s for partial amorphous retention) offer practical benchmarks for evaluating the feasibility of amorphous overlay in different component geometries.

A notable observation is the potential for using the crystallization process deliberately, as post-weld heat treatment can be used to control the degree and type of crystallization, thereby tailoring the final properties of the clad layer. This opens avenues for developing amorphous-based overlay processes that exploit controlled crystallization to achieve specific microstructures and properties.

Summary

This study provides essential insights into the microstructure-property relationships of TIG-clad Fe-based amorphous alloy overlays, establishing that while complete amorphous retention is challenging, semi-amorphous deposits with enhanced hardness and corrosion resistance are achievable through careful process control. The research highlights the cooling rate as the critical parameter governing amorphous content and offers practical process strategies for maximizing the beneficial properties of amorphous overlays in engineering applications. Future work should focus on developing hybrid processes that combine TIG welding with rapid solidification techniques to further enhance amorphous retention in thicker deposits.