Homogeneous and T800 Overlay Layers of UMCo50 Alloy Microstructure and Properties

Literature Overview

The study by Xue Haitao, Tang Qiang, Yan Chao, Jia Junya, Guo Weibing, and Lu Zhiming, published in 2020 in the journal "Hot Working Technology" and conducted by Hebei University of Technology and Beijing Aerospace Propulsion Institute, investigates the microstructure and mechanical properties of UMCo50 cobalt-based alloy overlay layers produced by homogeneous welding and T800 welding methods. This research is particularly relevant to aerospace applications where cobalt-based overlay alloys are used for high-temperature wear resistance, corrosion resistance, and thermal barrier protection in critical components such as turbine blades, exhaust nozzles, and combustion chamber liners.

UMCo50 is a cobalt-chromium-tungsten alloy that has been widely used in aerospace and power generation industries for its excellent combination of high-temperature strength, wear resistance, and oxidation resistance. The alloy typically contains approximately 17–21% Cr, 5–7% W, and 0.1–0.3% C, with the balance being cobalt. The research compares two different welding approaches—homogeneous welding (using the same alloy for both base and filler) and T800 welding (using a T800 nickel-based filler on a UMCo50 base or vice versa)—to evaluate their effects on microstructure, hardness, and high-temperature properties.

Material Characteristics and Process Comparison

Base Material Properties

Welding Process Parameters

Microstructural Analysis

Homogeneous Welding Microstructure

The homogeneous welding of UMCo50 produces a microstructure that is largely consistent with the base material, with minor variations due to the welding thermal cycle:

1. Matrix phase: The matrix consists of a solid solution of Cr and W in Co, which may exhibit some degree of carbide precipitation depending on the cooling rate.
2. Carbide phases: M₆C (Cr₇W₆C) and M₂₃C₆ (Cr₂₃C₆) carbides are present as primary and secondary phases. The cooling rate from the welding process affects the size and distribution of these carbides.
3. Solidification pattern: The weld metal exhibits a columnar dendritic structure in the first pass, transitioning to equiaxed grains in subsequent passes due to grain fragmentation and heterogeneous nucleation.
4. Heat-affected zone: The HAZ in homogeneous welding is minimal because the base and filler materials have similar melting points and thermal properties. The HAZ may exhibit slight grain growth and carbide coarsening but retains most of its original properties.

T800 Welding Microstructure

The T800 welding on UMCo50 base produces a more complex microstructure due to the compositional difference between the filler and base materials:

1. Dilution effects: The dilution from the UMCo50 base into the T800 filler introduces significant Cr and W into the weld metal, altering the phase equilibrium and carbide formation.
2. Carbide precipitation: The presence of Cr and W from the base metal promotes the formation of Cr-rich carbides (Cr₇C₃, Cr₂₃C₆) and mixed carbides (Cr₇W₆C), which are harder and more thermally stable than the Ni₃(Fe,Cr) type carbides in pure T800.
3. Phase distribution: The weld metal exhibits a gradient in composition from the T800-rich surface to the UMCo50-rich bond line, with corresponding variations in carbide type and distribution.
4. Bond line characteristics: The bond line is the critical region where the compositional mismatch is greatest. This region may exhibit brittle intermetallic phases or excessive carbide precipitation, which can compromise the bond strength.

Microstructural Comparison

Mechanical Properties and High-Temperature Performance

Room Temperature Properties

The T800 overlay exhibits higher hardness due to the increased carbide content resulting from dilution, but this comes at the expense of ductility and toughness. The homogeneous overlay maintains better ductility and impact resistance while providing a slight hardness improvement over the base material.

High-Temperature Properties