CLADDING TECHNOLOGY SHANXI CO., LTDExperimental Study on Improving Cladding and Thermal Spray Quality by Arc Ultrasonic
Literature Overview and Scope
This study investigates the application of arc ultrasonic technology to enhance the quality of both weld overlay cladding and thermal spray coatings. Arc ultrasonic refers to the introduction of ultrasonic vibrations (typically in the range of 20–40 kHz) into the arc welding or thermal spray process, which modifies the solidification behavior, microstructure, and mechanical properties of the deposited material. The research addresses a persistent challenge in the cladding industry: achieving uniform, defect-free overlay layers with fine-grained microstructures and excellent metallurgical bonding to the substrate.
Core Technical Findings
The study demonstrates that the application of arc ultrasonic vibrations during the overlay welding process produces several beneficial effects on the deposited layer. These effects include grain refinement, reduction of porosity, elimination of micro-cracking, and improvement of the metallurgical bond between the overlay and substrate. The ultrasonic energy is introduced either through the electrode or through a dedicated horn coupled to the workpiece, and the amplitude is typically controlled in the range of 50–200 μm.
| Parameter | Without Ultrasonic | With Ultrasonic (20–40 kHz) | Improvement |
|---|---|---|---|
| Average grain size | 80–150 μm | 30–60 μm | 50–60% reduction |
| Porosity rate | 2–5% | < 0.5% | Significant reduction |
| Micro-crack density | Moderate to high | Negligible | Near elimination |
| Hardness uniformity (HV) | ±15–25 HV | ±8–12 HV | Improved uniformity |
| Dilution rate | 15–25% | 10–18% | Reduced dilution |
Mechanism of Grain Refinement
The grain refinement achieved through arc ultrasonic is attributed to two primary mechanisms. First, the ultrasonic vibrations induce acoustic streaming within the molten pool, which enhances the transport of nucleation sites and promotes heterogeneous nucleation. Second, the ultrasonic energy disrupts the thermal gradient at the solidification front, reducing the constitutional supercooling and promoting equiaxed grain formation. The result is a fine, equiaxed microstructure that exhibits superior mechanical properties compared to the columnar microstructure typically observed in conventional overlay welding.
Effects on Thermal Spray Coatings
For thermal spray coatings, the application of ultrasonic vibration to the substrate during deposition improves the bonding strength between the coating and substrate by 20–40%. The ultrasonic energy cleans the substrate surface, removes oxide layers, and promotes mechanical interlocking between the splats and the substrate. Additionally, the ultrasonic vibrations during the deposition of the coating layer promote densification of the coating by promoting the plastic deformation and spreading of individual splats.
Defect Analysis and Countermeasures
| Defect Type | Conventional Process | Ultrasonic-Assisted Process | Root Cause and Countermeasure |
|---|---|---|---|
| Porosity | 2–5% | < 0.5% | Ultrasonic streaming promotes bubble escape; maintain proper shielding gas flow |
| Micro-cracking | Common in high-alloy overlays | Rare | Reduced thermal stress due to uniform heat distribution |
| Poor bonding | Occasional lack of fusion | Consistent metallurgical bond | Ultrasonic cleaning of substrate surface |
| Segregation | Moderate elemental segregation | Reduced segregation | Enhanced mixing in molten pool |
Process Parameters and Optimization
The optimization of arc ultrasonic parameters is critical to achieving the desired quality improvements. The study employs a systematic approach, varying the ultrasonic frequency, amplitude, and coupling method to identify the optimal process window. The results indicate that a frequency of 25 kHz with an amplitude of 100–150 μm provides the best balance between grain refinement and process stability. Higher amplitudes can lead to arc instability and excessive dilution, while lower amplitudes do not provide sufficient energy to significantly alter the solidification behavior.
The interaction between ultrasonic parameters and conventional welding parameters (current, voltage, travel speed, and torch angle) is also investigated. The study finds that the optimal welding parameters for ultrasonic-assisted overlay differ from those of conventional overlay. Specifically, a slightly lower current and a faster travel speed are recommended to compensate for the enhanced heat transport induced by the ultrasonic vibrations.
Engineering Practice Implications
From an engineering perspective, the integration of arc ultrasonic technology into existing cladding and thermal spray operations offers a promising pathway to improve quality without requiring significant changes to equipment or process flow. The technology is particularly advantageous for applications where the overlay layer is subjected to cyclic loading, thermal fatigue, or corrosive environments, as the improved microstructure and reduced defect density translate directly into enhanced service life.
However, the implementation of arc ultrasonic in production environments faces several challenges. The ultrasonic generator and horn assembly add complexity to the welding setup, and the coupling of ultrasonic energy to the workpiece requires careful attention to contact resistance and alignment. Additionally, the technology may not be suitable for all substrate geometries, particularly those with complex shapes or restricted access.
Key Questions and Reflections
One of the most interesting aspects of this study is the observation that arc ultrasonic can simultaneously improve multiple quality metrics — grain size, porosity, crack resistance, and bonding strength — in a single process step. This multi-benefit effect makes the technology particularly attractive for high-value applications such as nuclear components, aerospace structures, and high-pressure reactors. However, the question remains whether the benefits justify the added cost and complexity, particularly for large-scale production runs.
The study also raises important questions about the scalability of the technology. While the experimental results are promising, the transition from laboratory-scale to production-scale application requires careful consideration of factors such as ultrasonic energy delivery, process automation, and operator training. The literature acknowledges these challenges but does not provide a comprehensive roadmap for industrial implementation.
Summary
The experimental study on arc ultrasonic-enhanced cladding and thermal spray quality provides compelling evidence that ultrasonic vibration can significantly improve the microstructure, mechanical properties, and defect resistance of overlay layers. The technology offers a practical solution to longstanding quality challenges in the cladding industry, particularly for high-alloy and high-performance overlay applications. Engineers considering the adoption of this technology should carefully evaluate the specific requirements of their application, the available process parameters, and the potential cost-benefit analysis before committing to full-scale implementation.