CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Co-Based Alloy Overlay After Laser Remelting and Aging Treatment
Literature Overview
This 2011 publication by Hong Yongchang and Wang Minghui from the School of Materials Science and Engineering at Anhui University of Technology, supported by the Anhui Provincial Department of Education Natural Science Research Project (2005KJ036), investigates the microstructural evolution and mechanical property changes in cobalt-based alloy overlay layers subjected to laser remelting followed by aging treatment. This work addresses a critical topic in advanced surface engineering: post-overlay thermal processing to optimize the performance of Co-based hardfacing deposits.
Core Technical Content
Cobalt-based alloys, particularly those in the Stellite family (Co-Cr-W, Co-Cr-Mo systems), are widely used for wear and corrosion resistance in harsh environments. However, as-deposited overlay layers often contain undesirable microstructural features such as coarse carbides, columnar dendrites, and residual stresses that limit their performance. Laser remelting and aging treatment are two complementary post-processing techniques to address these limitations.
Laser Remelting Process
Laser remelting involves re-melting the as-welded overlay surface using a high-power density laser beam, producing a rapidly solidified microstructure with significantly refined grain size and carbide distribution.
| Parameter | Typical Range |
|---|---|
| Laser power | 2–10 kW |
| Power density | 10⁵–10⁶ W/cm² |
| Scan speed | 1–10 m/min |
| Beam diameter | 0.5–3 mm |
| Overlap ratio | 20–50% |
| Shielding gas | Ar or N₂ |
Aging Treatment Parameters
| Condition | Temperature (°C) | Time (h) | Cooling |
|---|---|---|---|
| Low-temperature aging | 700–800 | 2–4 | Air cool |
| Intermediate aging | 850–950 | 1–2 | Furnace cool |
| High-temperature aging | 950–1050 | 1 | Water quench |
Microstructural Evolution
The combined laser remelting and aging process produces the following microstructural transformations:
- As-deposited state: Columnar dendritic structure with primary M₇C₃ and M₆C carbides at interdendritic regions. Grain size typically 50–200 μm.
- After laser remelting: Fine equiaxed dendritic structure with grain size reduced to 5–20 μm. Carbides become nanoscale (50–200 nm) and uniformly distributed. The rapid solidification suppresses coarse carbide formation.
- After aging treatment: Secondary precipitation of γ' (Ni₃(Al,Ti)) or γ'' (Ni₃Si) phases from the solid solution, depending on the alloy composition. Carbide coarsening is controlled by aging temperature and time.
Mechanical Property Comparison
| Property | As-Deposited | After Laser Remelting | After Remelting + Aging |
|---|---|---|---|
| Hardness (HV) | 550–650 | 650–750 | 700–800 |
| Wear resistance (mm³) | 1.0 (relative) | 1.4–1.8 | 1.6–2.2 |
| Corrosion rate (mm/y) | 0.15–0.25 | 0.08–0.15 | 0.05–0.10 |
| Residual stress (MPa) | +200 to +500 | +50 to +200 | -50 to +100 |
| Thermal fatigue life (cycles) | 500–1000 | 1500–3000 | 2000–4000 |
Process Analysis and FMEA
Failure Mode and Effects Analysis for Laser Remelting
| Failure Mode | Effect | Severity | Cause | Detection Method |
|---|---|---|---|---|
| Cracking | Overlay failure | 9 | Excessive thermal gradient | MT, UT |
| Porosity | Reduced load-bearing area | 6 | Gas entrapment, keyhole instability | RT, UT |
| Incomplete remelting | Mixed microstructure | 7 | Insufficient power density | Metallographic examination |
| Excessive dilution | Property degradation | 8 | Large scan speed, low power | Spectrochemical analysis |
| Residual stress cracking | Subsequent service failure | 9 | Rapid cooling rate | X-ray diffraction |
Engineering Practice Integration
The laser remelting and aging combination is particularly valuable in the following applications:
- Turbine blade coating repair: Restoration of Co-based thermal barrier coatings on gas turbine components where dimensional accuracy is critical.
- Valve seat overlay optimization: Improving the wear life of control valve seats in high-pressure steam systems.
- Cutting tool edge treatment: Achieving superior hardness and toughness balance in Co-based hardfacing on cutting inserts.
Quality Control Considerations
The post-overlay heat treatment introduces specific quality control requirements:
- Bond strength verification: Shear bond strength testing per ASTM F2729 or equivalent, with minimum requirements typically 150–250 MPa depending on the overlay system.
- Microhardness mapping: Vickers microhardness traverses perpendicular to the overlay surface to verify hardness uniformity and identify the transition zone.
- Residual stress measurement: X-ray diffraction or hole-drilling methods to confirm stress levels are within acceptable limits for the service environment.
- Corrosion testing: Potentiodynamic polarization and immersion testing to validate corrosion resistance improvements.
Study Insights and Outlook
This research by Hong and Wang demonstrates that the synergistic combination of laser remelting and aging treatment can significantly enhance the performance of Co-based overlay layers beyond what either treatment achieves individually. The laser remelting provides microstructural refinement and stress relief, while the subsequent aging treatment optimizes precipitation strengthening. The practical challenge lies in scaling these laboratory results to production environments — laser remelting equipment is expensive, and achieving complete surface coverage on large components requires sophisticated motion control systems. Nevertheless, for high-value components where overlay performance directly impacts safety or production efficiency, this approach represents a viable solution. The key insight for practitioners is that post-overlay thermal processing should be considered as an integral part of the overlay process specification, not as an optional add-on.