CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Laser Remelting Scanning Speed on Microstructure and Wear Resistance of Co-Based Alloy Overlay Layer
Literature Overview and Research Context
This study examines the influence of laser remelting scanning speed on the microstructure evolution and tribological performance of cobalt-based alloy overlay layers. Laser remelting is a post-processing technique applied to existing overlay deposits (typically produced by plasma transferred arc welding, submerged arc welding, or hot-wire TIG) to refine the microstructure, reduce porosity, and enhance surface integrity. The Co-based alloy system, exemplified by Stellite 6 (Co-Cr-W-Mo) and its variants, is widely used in high-temperature, high-wear applications including gas turbine components, hot-section valves, and mining equipment. The scanning speed is a critical laser parameter that directly controls the heat input per unit length, which in turn governs the solidification rate, grain morphology, and carbide precipitation behavior.
Core Technical Findings
The research systematically varies the laser remelting scanning speed across a range of 0.2 to 2.0 m/min and evaluates the resulting microstructural changes and wear resistance through metallographic examination, X-ray diffraction analysis, and pin-on-disk wear testing.
| Scanning Speed (m/min) | Linear Energy Input (J/mm) | Cooling Rate (°C/s) | Primary Microstructure | Hardness (HV30) | Wear Rate (mg/1000 cycles) |
|---|---|---|---|---|---|
| 0.2 | 450 | 800 | Coarse columnar grains, large carbides | 420 | 18.5 |
| 0.4 | 225 | 1600 | Refined columnar grains, moderate carbides | 485 | 12.3 |
| 0.6 | 150 | 2400 | Fine columnar grains, fine carbides | 530 | 8.2 |
| 0.8 | 112 | 3200 | Very fine grains, nano-carbides | 555 | 6.1 |
| 1.0 | 90 | 4000 | Fine equiaxed grains, nano-carbides | 540 | 7.8 |
| 1.5 | 60 | 6000 | Ultra-fine grains, incomplete remelting | 460 | 15.2 |
| 2.0 | 45 | 8000 | Partial remelting, unmelted regions | 380 | 22.1 |
The data reveals a clear trend: wear resistance increases with scanning speed up to an optimal value of approximately 0.8 m/min, beyond which it deteriorates due to incomplete remelting and the presence of unmelted regions.
Microstructure Evolution with Scanning Speed
The laser remelting process produces a range of microstructural features depending on the scanning speed:
- Low scanning speed (0.2-0.4 m/min): High heat input produces coarse columnar grains growing perpendicular to the substrate. Carbide precipitation occurs primarily at grain boundaries and within grains, forming relatively large Cr7C3 and Cr23C6 carbides (5-15 μm). The coarse microstructure provides moderate hardness but poor wear resistance due to the large carbide size and coarse grain boundaries that serve as crack initiation sites.
- Optimal scanning speed (0.6-0.8 m/min): The heat input is sufficient to achieve complete remelting while providing rapid cooling that produces fine columnar or equiaxed grains (1-5 μm). Carbides precipitate as fine particles (0.5-2 μm) distributed uniformly within the matrix and at grain boundaries. The fine microstructure maximizes hardness (530-555 HV30) and wear resistance through the combined effect of fine grain strengthening and hard carbide dispersion.
- High scanning speed (1.0-2.0 m/min): Insufficient heat input leads to incomplete remelting, where unmelted regions of the original overlay deposit coexist with remelted zones. The unmelted regions retain the original coarse microstructure, creating a heterogeneous deposit with variable properties. The remelted regions exhibit ultra-fine grains but the overall wear resistance is compromised by the unmelted areas and the increased porosity associated with rapid solidification.
Carbide Precipitation Behavior
The Co-based alloy system is characterized by the precipitation of chromium carbides (primarily Cr7C3 and Cr23C6) that provide the primary wear resistance. The scanning speed significantly influences carbide precipitation:
| Scanning Speed | Carbide Type | Carbide Size (μm) | Distribution | Volume Fraction (%) |
|---|---|---|---|---|
| 0.2 | Cr7C3, Cr23C6 | 5-15 | Grain boundary and intragranular | 18-22 |
| 0.6 | Cr7C3, Cr23C6 | 1-3 | Uniformly distributed | 20-25 |
| 0.8 | Cr7C3, Cr23C6 | 0.5-2 | Nanodispersed | 22-28 |
| 1.5 | Cr7C3 (predominantly) | 0.3-1 | Intragranular | 15-18 |
| 2.0 | Cr7C3 (predominantly) | 0.2-0.8 | Sparse | 10-14 |
At optimal scanning speeds, the rapid cooling suppresses the growth of large carbides and promotes the formation of fine, uniformly distributed carbides. The increased volume fraction of fine carbides at 0.6-0.8 m/min is attributed to the high supersaturation of carbon and chromium in the rapidly solidifying melt, which drives extensive carbide precipitation upon cooling.
Wear Mechanism Analysis
The wear behavior of the laser-remelted Co-based overlay layers was analyzed through scanning electron microscopy of the wear tracks:
- Low scanning speed (0.2-0.4 m/min): The wear mechanism is predominantly abrasive, with deep plowing grooves and material removal by micro-cutting. The large carbides are easily dislodged, creating voids that accelerate further wear. Oxidative wear is also significant at high temperatures.
- Optimal scanning speed (0.6-0.8 m/min): The wear mechanism shifts to a combination of mild abrasive wear and adhesive wear. The fine carbides resist micro-cutting effectively, and the fine grain structure provides high strength that resists plastic deformation. A protective oxide layer forms on the surface, reducing further material loss.
- High scanning speed (1.0-2.0 m/min): The wear mechanism is dominated by abrasive wear in the unmelted regions and a combination of abrasive and adhesive wear in the remelted regions. The unmelted areas act as weak links, failing preferentially and exposing the substrate.
Engineering Practice Implications
Process Parameter Optimization
Based on the study findings, the following guidelines can be established for laser remelting of Co-based alloy overlays:
| Parameter | Recommended Range | Rationale |
|---|---|---|
| Scanning speed | 0.6-0.8 m/min | Optimal balance of complete remelting and fine microstructure |
| Laser power | 2-4 kW | Sufficient for complete remelting of typical overlay thicknesses (2-4 mm) |
| Spot diameter | 2-4 mm | Adequate overlap for uniform treatment |
| Overlap ratio | 50-70% | Ensures complete coverage without excessive heat input |
| Preheating temperature | 100-200°C | Reduces thermal gradient and cracking risk |
| Gas shielding | Ar or He (99.99%) | Prevents oxidation of the remelted surface |
Application-Specific Recommendations
| Application | Recommended Scanning Speed | Key Consideration |
|---|---|---|
| Gas turbine hot-section components | 0.6-0.8 m/min | Fine microstructure for high-temperature wear resistance |
| Mining equipment (bucket teeth, chutes) | 0.6-0.8 m/min | Maximum wear resistance with fine carbide dispersion |
| Hydraulic valve components | 0.8-1.0 m/min | Surface integrity and low roughness for sealing |
| Pump impellers and vanes | 0.6-0.8 m/min | Erosion-corrosion resistance with fine microstructure |
| Large-area equipment (hopper liners) | 0.4-0.6 m/min | Balance between productivity and quality |
Quality Control and Inspection
The laser remelting process requires careful quality control to ensure consistent results:
- Pre-remelting inspection: Verify the integrity of the underlying overlay layer, checking for cracks, porosity, and insufficient thickness. Any defects should be repaired before remelting.
- In-process monitoring: Monitor laser power, scanning speed, and spot diameter in real time. Deviations from set parameters can result in incomplete remelting or excessive heat input.
- Post-remelting inspection: Perform visual inspection, magnetic particle testing (if applicable), and hardness testing to verify the quality of the remelted surface. Cross-sectional metallography should be performed on test coupons to confirm complete remelting and microstructure quality.
- Surface roughness verification: The remelted surface should exhibit a smooth, uniform appearance with roughness (Ra) typically below 1.6 μm. Excessive roughness indicates incomplete remelting or excessive spatter.
Key Questions and Reflections
Several questions arise from this study that merit further consideration:
- How does the scanning speed interact with the underlying overlay microstructure? If the base overlay was produced by a different process (e.g., PTA vs. SAW), the remelting behavior may differ. A coarse-grained PTA deposit may require different scanning speeds than a fine-grained GTAW deposit to achieve the same final microstructure.
- What is the effect of multiple remelting passes? Some applications may require multiple passes to achieve uniform treatment over large areas. The interaction between successive passes and the resulting microstructure evolution needs further investigation.
- How does the laser remelting affect the residual stress state of the overlay? While remelting can relieve some residual stresses through thermal cycling, it can also introduce new stresses due to thermal contraction. The net effect on fatigue life and cracking resistance is not fully characterized.
- What is the long-term stability of the fine microstructure at elevated service temperatures? The fine carbides and grains produced by optimal scanning speeds may coarsen during prolonged high-temperature service, reducing the wear resistance advantage. Understanding the thermal stability of the microstructure is critical for high-temperature applications.
Study Insights and Implications
This study demonstrates that laser remelting is a powerful tool for enhancing the wear resistance of Co-based alloy overlays, but the scanning speed must be carefully optimized. The optimal range of 0.6-0.8 m/min produces a fine, uniform microstructure with nano-dispersed carbides that maximizes hardness and wear resistance. Deviations from this range, in either direction, result in suboptimal performance.
The finding that incomplete remelting at high scanning speeds significantly degrades wear resistance is particularly important for engineering practice. It underscores the need for careful process qualification and in-process monitoring. Engineers should not assume that higher scanning speeds are always beneficial; the balance between productivity and quality must be carefully managed.
For bimetal pressure vessel and equipment fabrication, laser remelting can be used as a finishing operation to enhance the surface integrity of critical components such as valve seats, pump impellers, and heat exchanger tubes. The process is particularly valuable for repairing damaged overlays or enhancing the performance of existing deposits.
In conclusion, the laser remelting scanning speed is a critical parameter that governs the microstructure and wear resistance of Co-based alloy overlay layers. The optimal range of 0.6-0.8 m/min produces fine, uniformly distributed carbides in a fine-grained matrix, maximizing wear resistance. Engineers must carefully control this parameter and verify the results through appropriate quality control measures to ensure consistent performance in service.