Microstructure and Properties of UMCo50 Homogeneous and T800 Surfacing Layers

Literature Overview

The research by Xue Haitao et al. (2020), published in Hot Working Technology, presents a comparative study of two cobalt-based surfacing alloys—UMCo50 and T800—deposited on a UMCo50 substrate using tungsten inert gas (GTAW) welding and plasma arc powder surfacing (PAW), respectively. This work is particularly relevant to engineers working in high-temperature and wear-resistant applications where cobalt-based alloys are the material of choice.

Core Technical Approach

The study employed two different surfacing processes to deposit two different cobalt-based alloys:

• UMCo50 wire was deposited on UMCo50 substrate using GTAW, creating a homogeneous surfacing layer
• T800 cobalt-based alloy powder was deposited on UMCo50 substrate using PAW, creating a heterogeneous surfacing layer

The characterization methods included X-ray diffraction (XRD) for phase identification, scanning electron microscopy (SEM) for microstructural analysis, microhardness testing for hardness distribution, and high-temperature friction wear testing for wear performance evaluation. Heat resistance and corrosion resistance were also assessed.

Key Technical Parameters and Results

Metallurgical Analysis

The phase composition is the key differentiator between the two surfacing layers. The UMCo50 homogeneous layer consists of α-Co (face-centered cubic cobalt) and ε-Co (hexagonal close-packed cobalt) phases, which are typical for cobalt-based alloys with moderate alloying. These phases provide good toughness and moderate wear resistance.

The T800 layer introduces the Laves phase (a topologically close-packed intermetallic compound, typically Co₂W or Co₂Cr in this context), which dramatically increases hardness and wear resistance. The Laves phase acts as a hard reinforcement within the cobalt matrix, similar to the role of carbides in steel or ceramics in ceramic-metal composites. This phase is formed due to the higher concentration of alloying elements such as W, Cr, and Mo in the T800 composition.

The trade-off identified in this study is significant: the T800 layer offers superior high-temperature wear resistance but inferior heat resistance and corrosion resistance compared to the UMCo50 layer. This trade-off is primarily attributed to the lower chromium content in T800. Chromium is essential for forming protective oxide scales (Cr₂O₃) that provide oxidation and corrosion resistance at elevated temperatures.

Process Considerations

The choice of surfacing process is critical for cobalt-based alloys:

• GTAW provides excellent control over heat input and dilution, making it suitable for homogeneous surfacing where composition matching is desired. However, it is a relatively slow process with limited deposition rates.
• PAW offers higher deposition rates and can produce denser, more homogeneous deposits due to the high-energy plasma arc and the controlled feeding of powder. The plasma arc provides a more concentrated heat source, reducing dilution and allowing for the formation of harder phases.

The PAW process is particularly advantageous for T800 deposits because the high-energy arc promotes the formation of the Laves phase, which requires sufficient thermal energy and appropriate cooling rates.

Engineering Practice Implications

For engineers selecting surfacing alloys for high-temperature wear applications, this study provides a clear decision framework:

1. If high-temperature wear resistance is the primary requirement and the operating environment is not severely corrosive, T800 surfacing is the preferred choice.
2. If oxidation resistance and corrosion resistance are critical, UMCo50 surfacing provides better protection, albeit with lower wear resistance.
3. For applications requiring both wear and corrosion resistance, a multi-layer approach combining UMCo50 as a barrier layer with T800 as a wear layer could be considered, though this would require additional qualification testing.
4. The choice of process (GTAW vs. PAW) should be guided by the required deposit thickness, production rate requirements, and the specific alloy system being deposited.

Quality Control Considerations

For cobalt-based surfacing applications, the following quality control measures are recommended:

• XRD analysis to verify phase composition and detect unwanted phases such as brittle intermetallics
• Microhardness mapping to verify hardness distribution and identify soft spots or incomplete melting
• High-temperature wear testing on coupon samples to validate performance claims
• Oxidation testing at the maximum service temperature to verify heat resistance
• Corrosion testing in the expected service environment to verify corrosion resistance
• Visual and dye penetrant inspection for surface cracks, porosity, and lack of fusion

Study Reflections

This paper provides a valuable comparative analysis of two cobalt-based surfacing systems that are commonly used in aerospace and industrial applications. The clear identification of the Laves phase as the key factor in enhanced wear resistance, and chromium content as the key factor in heat and corrosion resistance, provides a straightforward basis for material selection.

The use of PAW for T800 deposition is particularly noteworthy. Plasma arc powder surfacing is a relatively advanced technique that offers significant advantages over conventional arc surfacing for cobalt-based alloys. The ability to produce dense, homogeneous deposits with controlled phase formation makes PAW the preferred process for high-performance surfacing applications.

One area for further investigation would be the effect of cooling rate on phase formation in both systems. The cooling rate in PAW is typically higher than in GTAW, which can influence the morphology and distribution of the Laves phase. Understanding this relationship would allow for more precise control of the final microstructure and properties.