CLADDING TECHNOLOGY SHANXI CO., LTDStudy Note on Laser and TIG Cladding Cobalt-Based Alloy Properties
Literature Overview
Published in 2013 in the Chinese Journal of Welding, this study by researchers from Shenyang University of Technology and Shenyang SIASUN Robot & System Co., Ltd. compares the performance of laser cladding and gas tungsten arc (GTAW/TIG) cladding for cobalt-based alloy overlays. Funded by the Liaoning Provincial Natural Science Foundation (Grant 20092047), this work addresses a practical manufacturing question: which process delivers superior overlay properties for cobalt-based alloys in terms of microstructure, hardness, corrosion resistance, and bonding quality?
Core Technical Content
Cobalt-based alloys—particularly Stellite 6, Stellite 21, and CoCr-based alloys—are valued for their exceptional combination of hot hardness, wear resistance, and corrosion resistance in aggressive environments. They are widely used in valve trim, pump impellers, cutting tools, and heat exchanger tube sheets. The choice between laser cladding and TIG cladding fundamentally affects the dilution level, cooling rate, and resulting microstructure.
Process Comparison
| Parameter | Laser Cladding | TIG Cladding |
|---|---|---|
| Energy density | 10⁴–10⁶ W/cm² | 10²–10⁴ W/cm² |
| Dilution ratio | 2–10% | 15–35% |
| Cooling rate | 10³–10⁶ °C/s | 10–100 °C/s |
| Layer thickness | 0.1–1.0 mm | 1.0–3.0 mm |
| Heat-affected zone | Minimal (<0.5 mm) | Significant (1–3 mm) |
| Residual stress | High (tensile) | Moderate |
| Equipment cost | High | Moderate |
| Production rate | Lower | Higher |
Microstructural Differences
The laser cladding process produces a fine, columnar dendritic microstructure with significantly reduced grain size compared to TIG cladding. The rapid solidification inherent to laser processing promotes the formation of fine carbides (e.g., Co₃W, Co₇W₆ in CoCrW alloys) that contribute to enhanced hardness. Typical hardness values for laser-cladded cobalt-based alloys range from 800 to 1100 HV, compared to 500–700 HV for TIG-cladded counterparts.
In TIG cladding, the higher heat input and slower cooling rate result in coarser microstructures with larger grain sizes and more extensive carbide networks. While this may reduce hardness, it can improve ductility and reduce the propensity for cracking in thick overlay applications.
Corrosion Resistance Evaluation
Both processes were evaluated for corrosion resistance, typically through potentiodynamic polarization tests in simulated service environments (e.g., 3.5% NaCl solution or acidic media). Laser-cladded overlays generally exhibit superior corrosion resistance due to:
- Lower dilution with the carbon steel substrate, preserving the noble character of the cobalt alloy.
- Finer microstructure with fewer microsegregation zones susceptible to galvanic attack.
- More uniform elemental distribution, reducing local galvanic couples.
However, TIG cladding can achieve acceptable corrosion performance when the cobalt alloy composition is designed with sufficient Cr and Mo content to compensate for dilution effects.
Engineering Practice Considerations
The selection between laser and TIG cladding for cobalt-based overlays depends on the specific application requirements:
- Laser cladding is preferred when low dilution, thin layers, and minimal substrate distortion are critical—such as in repair of precision valve seats or aerospace components.
- TIG cladding is more economical for thick overlays on large components where high deposition rates are needed, such as in mining equipment or heavy-duty industrial parts.
A practical hybrid approach involves using TIG cladding for the first few passes to build up bulk material, followed by laser cladding for the final surface layer to achieve a fine microstructure and low dilution. This sequential strategy balances cost, productivity, and performance.
Key Reflections
The comparative study underscores that process selection is not merely a matter of technological preference but must be driven by the functional requirements of the end application. Engineers must carefully weigh the trade-offs between dilution control, production rate, equipment investment, and final performance. For cobalt-based alloys specifically, the superior dilution control of laser cladding translates directly into better retention of the alloy's designed properties, making it the preferred choice for high-performance applications where even small amounts of substrate dilution can significantly degrade wear or corrosion resistance.