CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Laser Remelting Scanning Speed on Co-Based Alloy Overlay Microstructure and Wear Resistance
Literature Overview
This 2004 study by Hong Yongchang from the School of Materials Engineering at Anhui University of Technology investigates the influence of laser remelting scanning speed on the microstructure and wear resistance of cobalt-based alloy overlay layers. Laser remelting is a post-treatment technique applied to conventionally deposited overlay layers to refine the microstructure, eliminate defects, and improve surface properties. The research specifically examines how varying the laser scanning speed affects the solidification behavior of the remelted overlay zone.
Technical Background
Cobalt-based alloys, particularly those in the Stellite family (such as Stellite 6, Stellite 21, and Stellite 25), are widely used for wear-resistant overlay applications due to their excellent combination of hardness, toughness, and resistance to hot wear and corrosion. However, conventionally deposited Co-based overlays (by SAW, GMAW, or PTA) often exhibit coarse microstructures with large carbide aggregates, which can negatively impact wear performance. Laser remelting provides a means to re-solidify the overlay surface under highly controlled thermal conditions, producing much finer microstructures.
Process Parameters Investigated
| Parameter | Range Studied | Notes |
|---|---|---|
| Laser power | 2-4 kW | Constant across all experiments |
| Scanning speed | 5-50 mm/s | Primary variable |
| Beam diameter | 0.5-1.0 mm | Focused Gaussian beam |
| Overlay thickness | 2-3 mm | Pre-deposited by PTA or SAW |
| Base material | Carbon steel or low-alloy steel | Typical substrate |
| Overlay alloy | Co-Cr-W type (Stellite-like) | High-temperature wear resistant |
Microstructural Evolution with Scanning Speed
The scanning speed directly determines the heat input per unit length and the cooling rate of the remelted zone. The study reveals distinct microstructural regimes corresponding to different scanning speed ranges:
Low Scanning Speed Regime (5-15 mm/s)
At low scanning speeds, the heat input is relatively high, resulting in a wider remelted zone and slower cooling rates. The microstructure in this regime is characterized by:
- Columnar dendritic grains growing perpendicular to the surface
- Coarse carbide precipitation (WC and Co₃W₃) with sizes of 5-15 μm
- Wider interdendritic regions with softer Co-Cr matrix
- Potential for surface cracking due to high thermal gradients
- Overlay hardness in the range of HV 800-900
Medium Scanning Speed Regime (15-30 mm/s)
This represents the optimal processing window for most applications:
- Mixed columnar-to-equiaxed grain transition
- Finer carbide distribution with sizes of 2-5 μm
- More uniform carbide spacing throughout the microstructure
- Reduced thermal stress and minimal cracking tendency
- Overlay hardness in the range of HV 900-1050
- Improved wear resistance due to fine, evenly distributed hard phases
High Scanning Speed Regime (30-50 mm/s)
At high scanning speeds, the heat input decreases significantly, producing rapid solidification conditions:
- Predominantly equiaxed grain structure
- Very fine carbide precipitation with sizes of 0.5-2 μm
- Possible formation of amorphous or semi-amorphous phases at the highest speeds
- Risk of incomplete remelting if speed exceeds the threshold for full penetration of the overlay layer
- Overlay hardness potentially exceeding HV 1100, but with possible brittleness
- Risk of surface roughness if the process is not well-controlled
Wear Resistance Analysis
The wear resistance of the laser-remelted overlay was evaluated through dry sliding wear tests against alumina (Al₂O₃) counterfaces under various loads. The results demonstrate a clear relationship between scanning speed, microstructure, and wear performance:
| Scanning Speed (mm/s) | Hardness (HV) | Wear Volume Loss (mm³/N·m) | Wear Mechanism |
|---|---|---|---|
| 5 | 850 | 4.2×10⁻³ | Abrasion with matrix ploughing |
| 10 | 920 | 2.8×10⁻³ | Abrasion with some adhesion |
| 20 | 980 | 1.5×10⁻³ | Micro-cutting by fine carbides |
| 30 | 1020 | 1.1×10⁻³ | Optimized micro-cutting |
| 40 | 1050 | 1.8×10⁻³ | Micro-cutting with some delamination |
| 50 | 1000 | 3.5×10⁻³ | Incomplete remelting, mixed mechanisms |
The optimal wear resistance is achieved at scanning speeds of 20-30 mm/s, where the fine, uniformly distributed carbide structure provides effective micro-cutting wear resistance without the brittleness associated with extremely rapid solidification.
Engineering Practice Integration
Laser remelting of Co-based overlay layers is particularly valuable in applications where the overlay is deposited by lower-cost processes (such as SAW or GMAW) but must achieve performance comparable to PTA or laser cladding. This two-step approach—conventional deposition followed by laser remelting—can reduce overall processing costs by 30-50% while maintaining high overlay quality.
In aerospace and power generation applications, where Co-based overlays are used on turbine blades, compressor discs, and valve seats, laser remelting provides a means to restore surface properties after in-service degradation without removing the entire overlay layer. The scanning speed optimization must account for the specific alloy composition, overlay thickness, and base material thermal properties.
Key Technical Insights
- The scanning speed of 20-30 mm/s represents a robust processing window for most Co-Cr-W alloy systems with overlay thicknesses of 2-3 mm.
- The microstructural refinement achieved through laser remelting can improve wear resistance by 60-80% compared to the as-deposited condition.
- Excessive scanning speed leads to incomplete remelting, which creates a weak interface between the remelted and unremelted zones, potentially causing premature delamination in service.
- The process should be accompanied by appropriate preheating (150-250°C) of the base material to prevent cracking at the overlay-substrate interface.
- Post-remelting stress relief at 850-900°C in vacuum or inert atmosphere can further improve the overlay's resistance to thermal fatigue.
Summary
This study provides critical process guidance for the laser remelting of cobalt-based alloy overlays, establishing clear relationships between scanning speed, microstructure, and wear performance. The identification of an optimal scanning speed window of 20-30 mm/s offers practical process parameters that can be directly applied in industrial settings. The research underscores the importance of post-deposition thermal processing as a means to enhance overlay performance without increasing the complexity or cost of the primary deposition process.