Microstructure and Properties of Co-Based Alloy Surfacing Layer After Laser Remelting and Aging Treatment

Literature Overview

This study, published in 2011 in the journal Transactions of Materials and Heat Treatment, investigates the microstructural evolution and mechanical performance of Co-based alloy surfacing layers subjected to laser remelting followed by different aging treatments. The research is conducted by researchers from Anhui University of Technology and the Key Laboratory of Metallic Materials and Processing in Anhui Province. The study employs a comprehensive suite of characterization techniques, including optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), microhardness testing, and wear resistance evaluation, to elucidate the relationship between processing parameters, microstructure, and tribological performance.

Core Technical Content

The experimental procedure involves first depositing a Co-based alloy surfacing layer on a substrate through conventional arc surfacing, then subjecting the surfacing layer to laser remelting to refine the microstructure, and finally applying various aging treatments to further optimize the hardness and wear resistance. The key findings reveal that laser remelting significantly refines the grain structure of the Co-based alloy surfacing layer, resulting in improved hardness and wear resistance compared to the as-deposited state. The microstructure of the remelted layer is predominantly composed of a gamma-Co (γ-Co) matrix with dispersed strengthening phases such as Cr7C3 and Cr23C6 carbides. Among the various aging treatments investigated, the specimen aged at 900 °C for 6 hours exhibits the best wear resistance, indicating that this thermal treatment optimally balances the precipitation hardening effect with the risk of over-aging or grain coarsening.

Interpretation of Technical Points

The microstructural refinement achieved through laser remelting can be attributed to the extremely rapid heating and cooling rates associated with laser processing. The high cooling rate suppresses grain growth and promotes the formation of fine, uniformly distributed carbide precipitates within the γ-Co matrix. These fine carbides act as effective strengthening agents by impeding dislocation motion, thereby enhancing the hardness and wear resistance of the surfacing layer. The subsequent aging treatment further optimizes the microstructure by promoting the precipitation of additional fine carbide particles and relieving residual stresses introduced during the laser remelting process. However, excessive aging temperatures or durations can lead to carbide coarsening and grain growth, which degrade the mechanical properties.

From a welding metallurgy perspective, the laser remelting process creates a narrow heat-affected zone (HAZ) with minimal dilution of the base material, preserving the integrity of the substrate while fully transforming the surfacing layer microstructure. The thermal cycle imposed by the laser is characterized by a peak temperature well above the melting point of the Co-based alloy, followed by rapid solidification. This thermal history is fundamentally different from that of conventional arc surfacing, resulting in a markedly different microstructural outcome.

Connection with Engineering Practice

In engineering practice, Co-based alloy surfacing layers are widely used in components subjected to severe wear and corrosion conditions, such as turbine blades, pump impellers, valve seats, and mining equipment. The conventional arc surfacing process often produces a coarse microstructure with inadequate hardness and wear resistance. The application of laser remelting followed by optimized aging treatment offers a practical solution to enhance the tribological performance of these components without requiring complete re-manufacturing. The recommended processing sequence for engineering applications is as follows: first, deposit the Co-based alloy surfacing layer using a suitable arc welding process (such as SMAW or SAW); second, apply laser remelting with appropriate parameters (typically laser power of 1–3 kW, scanning speed of 1–5 m/min, and spot diameter of 0.5–1.5 mm); and third, perform aging treatment at 900 °C for 6 hours in a protective atmosphere to prevent oxidation.

Key Questions and Reflections

Several questions merit further consideration. First, the long-term stability of the precipitate-hardened microstructure under service conditions involving thermal cycling or sustained high-temperature loading requires investigation. Second, the effect of the laser remelting process on the bond strength between the surfacing layer and the substrate should be evaluated, as excessive thermal input could potentially cause interfacial cracking. Third, the scalability of the laser remelting process for large-area surfacing applications, such as those encountered in the repair of large industrial components, needs to be assessed in terms of productivity and cost-effectiveness.

Study Insights and Implications

This study demonstrates that the combination of laser remelting and optimized aging treatment is a highly effective approach for enhancing the microstructure and wear resistance of Co-based alloy surfacing layers. The findings provide a solid scientific basis for the development of surface engineering processes aimed at extending the service life of components subjected to severe wear and corrosion. Engineers working in the field of surface hardening and overlay welding should consider incorporating laser remelting into their process toolbox, particularly for high-value components where performance improvements can justify the additional processing cost. The research also highlights the importance of systematic microstructural characterization in understanding and optimizing the performance of surfacing materials.