Laser Remelting and Aging Treatment of Co-Based Alloy Overlay Layer: Microstructure and Performance Study

Literature Overview

The 2011 study by Hong Yongchang and Wang Minghui from Anhui University of Technology and the Anhui Provincial Key Laboratory of Metal Materials and Processing investigates the microstructural evolution and mechanical performance of cobalt-based alloy overlay layers subjected to laser remelting and subsequent aging treatment. This research was funded by the Anhui Provincial Department of Education Natural Science Research Project (2005KJ036) and published in the Journal of Materials Heat Treatment. The work addresses a critical challenge in cobalt-based overlay engineering: achieving the optimal balance between hardness, wear resistance, and thermal stability through post-deposition thermal processing.

Background: Co-Based Alloy Overlay Systems

Cobalt-based alloys, particularly the Stellite family (Stellite 6, Stellite 21, Stellite 25), are widely used for overlay applications requiring exceptional wear resistance, corrosion resistance, and high-temperature strength. These alloys typically contain 55–65% Co, 20–30% Cr, and 5–10% W or Mo, with the hardening phase being Cr7C3 carbides. The microstructure of as-deposited overlay layers—whether produced by thermal spraying, plasma arc welding, or laser cladding—is often characterized by a dendritic or columnar structure with primary and secondary carbides, and may contain residual stresses and microcracks.

Laser remelting is a post-deposition process that uses a focused laser beam to rapidly melt and resolidify the surface layer of the overlay. This process eliminates residual stresses, refines the grain structure, and can homogenize the carbide distribution. However, the rapid cooling rates associated with laser remelting can lead to very fine but potentially brittle microstructures. Aging treatment is then applied to temper the microstructure, relieve any remaining stresses, and optimize the hardness and toughness balance.

Technical Content: Laser Remelting Process Parameters

The laser remelting process parameters used in this study are summarized in the following table:

The authors investigated the effect of laser power and scanning speed on the remelted layer microstructure and hardness. A key finding was that there exists an optimal energy density window—typically 15–30 J/mm² for Co-based alloys—above which excessive melting depth leads to dilution with the base metal, and below which incomplete remelting leaves residual columnar structures and porosity.

Aging Treatment: Microstructural Evolution

The aging treatment was conducted at temperatures ranging from 700 °C to 950 °C for holding times of 1 to 8 hours, followed by air cooling or furnace cooling. The following table summarizes the microstructural changes observed at different aging conditions:

The optimal aging condition identified by the authors was 800 °C for 4 hours, which produced a microstructure with well-distributed Cr7C3 carbides in a tempered matrix, yielding a hardness of approximately 520 HV and improved fracture toughness compared to the as-remelted condition. The aging process promoted the precipitation of fine secondary carbides and the partial dissolution of primary carbides, resulting in a more homogeneous microstructure.

Performance Evaluation

The following table compares the mechanical and tribological properties of the Co-based overlay layer under different processing conditions:

The combined laser remelting and aging treatment produced the best overall performance, with the lowest wear rate and the highest fracture toughness among the tested conditions. The reduction in residual stress after aging is particularly beneficial for applications where thermal cycling or cyclic loading is expected, as it reduces the risk of fatigue cracking at the overlay-substrate interface.

Defect Analysis and Process Considerations

A critical observation from this study is that the aging response of Co-based overlays is highly sensitive to the initial microstructure produced by the deposition process. Overlays produced by laser cladding typically have finer microstructures than those produced by thermal spraying or arc welding, which means they require lower aging temperatures and shorter holding times to achieve the same level of microstructural evolution. This has direct implications for process planning in production environments where multiple overlay processes may be used on the same component.

Integration with Engineering Practice

In industrial applications, Co-based overlay layers are commonly used on components subjected to severe wear and corrosion conditions, such as valve seats, pump impellers, extruder barrels, and mining equipment. The laser remelting and aging treatment sequence described in this study offers a post-deposition route to significantly improve the performance of as-deposited overlays without requiring re-deposition.

For pressure vessel and heat exchanger applications, where overlay layers may be subjected to thermal cycling during normal operation, the reduction in residual stress and the improvement in fracture toughness achieved through aging are particularly valuable. The standard NB/T 47014 qualification procedure for overlay welding procedures includes requirements for post-weld heat treatment, and the findings of this study provide a basis for optimizing such PWHT parameters for Co-based alloy overlays.

Key Questions and Reflections

The study raises several important questions for further research. First, the long-term stability of the aged microstructure under cyclic thermal loading is not fully addressed. In service conditions where temperatures fluctuate between ambient and several hundred degrees Celsius, repeated thermal cycling could induce additional microstructural changes, including carbide coarsening and phase transformations. Second, the effect of the underlying base metal on the aging response of the overlay layer warrants investigation. Dilution during the initial deposition process introduces base metal alloying elements into the overlay, which can alter the aging kinetics and the final microstructure.

The work by Hong and Wang demonstrates that post-deposition thermal processing is a powerful tool for optimizing the performance of Co-based alloy overlays. The systematic approach to varying aging parameters and correlating them with microstructural and mechanical property changes provides a valuable methodology that can be applied to other overlay alloy systems.

Study Insights and Implications

This study contributes significantly to the understanding of post-deposition thermal processing for Co-based overlay layers. The key insight is that the combination of laser remelting and aging treatment can produce overlay layers with superior wear resistance and fracture toughness compared to as-deposited conditions, while also reducing residual stresses that could compromise long-term performance. For engineers involved in the design and fabrication of components with Co-based overlays, this research provides a clear process roadmap: deposit the overlay, remelt the surface to refine the microstructure and relieve stresses, and then age to optimize the carbide distribution and mechanical properties.

The practical implications extend to maintenance and repair operations, where existing overlay layers that have degraded in service can potentially be restored through laser remelting and aging, extending component life and reducing replacement costs. This represents a significant economic benefit in industries where Co-based overlay components are expensive and difficult to replace.