Plasma Transfer Arc Welding of Spherical Tungsten Carbide Particle Reinforced Nickel-Based Alloy Cladding Layer Microstructure and Properties

Literature Overview

This paper by Deng Dewei and colleagues from Dalian University of Technology and Shenyang Blower Works Group, published in the journal Metal Heat Treatment (Volume 37, Issue 11, 2012, pp. 64-68), investigates the microstructure and mechanical properties of tungsten carbide (WC) particle reinforced nickel-based alloy cladding layers deposited on 304L stainless steel using plasma transfer arc welding (PTAW). The research was supported by the National 973 Program (2011CB013402) and the National Natural Science Foundation of China (11072045), indicating its significance within China's strategic materials research framework.

Core Technical Findings

The study systematically examines how varying WC particle content influences the microstructural evolution and microhardness of the cladding layer. The key findings can be summarized as follows:

• The cladding layer microstructure consists of dendritic crystals and interdendritic multicomponent eutectic structures.
• Initially added spherical WC particles settle at the bottom of the cladding layer due to density differences, leaving the top region devoid of intact WC particles.
• In the WC-free upper region, new fishbone-shaped and blocky structures appear, which are reaction products formed during the welding process.
• During PTAW, WC particles partially melt and interact with Ni-based alloy elements to form low-melting-point eutectic phases, which precipitate as blocky and elongated plate-like structures.
• Average hardness of the cladding layer increases with increasing WC content.
• The fishbone-shaped and blocky structures in the top region do not significantly affect overall cladding layer hardness.

Interpretation of Technical Points

Particle Settling Phenomenon

The settling of WC particles to the cladding layer bottom is a well-known phenomenon in powder metallurgy and cladding welding processes. The density of WC is approximately 15.63 g/cm³, while the liquid Ni-based alloy has a density of approximately 8.8-9.0 g/cm³. During the brief liquid phase lifetime in PTAW (typically 0.1-0.5 seconds), gravitational segregation causes the denser WC particles to migrate downward. This creates a compositional gradient through the cladding layer thickness, which has significant implications for wear resistance uniformity.

Reaction Mechanism of WC in Ni-Based Matrix

The interaction between WC and the Ni-based matrix during PTAW involves several concurrent reactions:

1. Partial melting of WC particles at the local high temperatures (arc temperature exceeding 10,000 K)
2. Dissolution of W and C into the Ni-based liquid
3. Formation of new carbide phases (Ni₃W, Ni₄W, Ni₇W₆, and complex eutectic carbides)
4. The fishbone-shaped structures likely represent dendritic Ni₃W or similar intermetallic compounds
5. The blocky structures correspond to eutectic carbide precipitates formed during solidification

Hardness Response to WC Content

The monotonic increase in average hardness with WC content is expected, as residual WC particles and reaction carbides serve as effective dispersion strengthening phases. However, the observation that the fishbone and blocky structures in the top region do not significantly affect hardness suggests that these phases are either too coarse or too sparse to provide meaningful strengthening, or their hardness is comparable to the surrounding matrix.

Process Parameters and Engineering Considerations

Integration with Engineering Practice

In industrial applications such as impeller repair in the blower industry (the context of Shenyang Blower Works Group), the non-uniform distribution of WC particles poses a challenge. The surface layer, which is the most critical for wear resistance, may lack sufficient WC reinforcement due to particle settling. This motivates alternative approaches such as:

• Using smaller WC particle sizes (below 25 μm) to reduce settling
• Employing in-situ synthesis methods to distribute carbides uniformly (as discussed in Topic 3)
• Applying multiple thin layers instead of a single thick layer
• Using pre-alloyed Ni-W-C powders instead of composite WC/Ni powders

Key Questions and Reflections

The paper raises an important question about the practical effectiveness of directly adding WC particles to PTAW cladding. If the top region (the wear-critical surface) lacks WC particles, the practical wear resistance improvement may be less than what bulk hardness measurements suggest. This is a common pitfall in research publications that report average hardness without depth-dependent wear testing.

Another critical consideration is the thermal cycling during subsequent service. The eutectic carbide phases formed during welding may undergo coarsening or decomposition during prolonged high-temperature exposure, potentially degrading the wear resistance over time.

Study Insights and Implications

This study provides valuable baseline data for understanding WC/Ni-based cladding behavior under PTAW. However, the particle settling phenomenon highlights a fundamental limitation of composite powder cladding approaches. For engineers designing repair and hardfacing procedures for critical components, this research underscores the importance of considering not just average properties but also the depth-dependent microstructural distribution. The findings directly support the motivation for in-situ synthesis approaches, where carbide phases are formed during solidification and can be distributed more uniformly throughout the cladding layer.