Key Technology Progress and Applications in Overlay Welding and Thermal Spraying

Literature Overview

This comprehensive review authored by Huang Zhiquan, Li Changjiu, He Dingyong, Chen Qian, Zhao Junjun, Gao Zhanqi, and Wang Chongyang—representing leading research institutions including the Zhengzhou Machinery Research Institute of China Machinery Engineering Corporation, Xi'an Jiaotong University, Beijing University of Technology, the Chinese Mechanical Engineering Society, and Zhejiang Barton Welding Technology Research Institute—was published in China Surface Engineering (2026). The paper provides a systematic overview of the latest advances in overlay welding and thermal spraying technologies, covering new materials, novel processes, advanced characterization techniques, and emerging applications.

The breadth of institutional representation reflects the interdisciplinary nature of the field, spanning materials science, welding engineering, surface engineering, and mechanical engineering. This diversity of expertise is essential for addressing the complex challenges of surface modification in modern manufacturing.

Advanced Overlay Welding Technologies

Hot-Wire TIG (HW-TIG) Cladding

Hot-wire TIG cladding represents one of the most rapidly developing overlay technologies in recent years. The process combines a tungsten inert gas arc with a continuously fed preheated filler wire, achieving deposition rates of 1.5–3.0 kg/h—significantly higher than conventional TIG cladding (0.3–0.8 kg/h). The preheated wire (typically heated to 400–600°C) enters the arc zone at a high velocity, creating a deep, narrow weld bead with a dilution ratio that can be as low as 15–25% depending on the process parameters.

The key advantage of HW-TIG is its ability to achieve low dilution with high deposition rates, making it suitable for applications requiring a high volume fraction of expensive alloying elements (e.g., nickel-based alloys on carbon steel). The authors highlight recent advances in the development of dedicated hot-wire feeders with precise temperature control and integrated wire straightening mechanisms.

Laser Cladding with High-Power Fiber Lasers

Laser cladding has evolved significantly with the availability of high-power fiber lasers (10–30 kW) and advanced powder feed systems. The process achieves dilution ratios as low as 5–10%, deposit thicknesses of 0.5–3.0 mm per pass, and surface roughness values below Ra 10 μm. Recent developments include:

• Multi-laser head systems: Two or three laser heads operating simultaneously to increase productivity while maintaining low dilution.
• Dual-powder feeding: Separate powder feeders for the base material and a reinforcing phase (e.g., WC particles or TiC particles) to achieve tailored microstructures.
• In-situ monitoring: Real-time optical or infrared sensors to monitor the melt pool geometry and detect defects during the cladding process.

Friction Stir Welding (FSW) Overlay

Although not a traditional welding process, FSW overlay has emerged as a promising technique for surface modification. The process involves rotating a non-consumable tool into the surface of the base material, where the plasticized material is displaced and consolidated by the tool geometry. FSW overlay produces defect-free deposits with no melting, no dilution, and no porosity. However, the process is limited to materials that are plastic at the processing temperature and requires high axial forces (10–50 kN).

Advanced Thermal Spraying Technologies

High-Velocity Oxy-Fuel (HVOF) Spraying

HVOF spraying continues to be the dominant thermal spraying process for high-performance coatings. The fuel-rich combustion of hydrogen and oxygen produces gas velocities exceeding 1000 m/s and particle velocities of 500–700 m/s. Recent improvements include:

• Torch geometry optimization: Computational fluid dynamics (CFD) simulations to design nozzles that maximize particle acceleration while minimizing particle dispersion.
• Powder feed rate control: Advanced powder feeders with mass flow controllers to maintain consistent deposition rates of 1.0–3.0 kg/h.
• Coating thickness monitoring: In-line laser displacement sensors to control coating thickness in real time.

High-Entropy Alloy (HEA) Coatings

The concept of high-entropy alloys, where five or more principal elements are mixed in equiatomic or near-equiatomic proportions, has been extended to thermal spraying. HEA coatings deposited by HVOF or cold spray exhibit exceptional combinations of hardness, wear resistance, and corrosion resistance. The authors report that CoCrFeMnNi coatings sprayed by HVOF achieve hardness values of 400–500 HV and exhibit excellent resistance to high-temperature oxidation at 800–1000°C.

Cold Spray with High-Strain-Rate Deformation

Cold spray technology, which deposits material through high-velocity particle impact without melting, has seen significant advances in recent years. The authors discuss the development of high-strain-rate cold spray processes that achieve particle velocities of 700–900 m/s, enabling the deposition of materials with high melting points (e.g., tungsten, tantalum) and reactive metals (e.g., titanium, aluminum) in an inert atmosphere.

Process Selection Framework

The authors propose a decision framework for selecting the appropriate surface modification technology based on application requirements:

Quality Assurance and Characterization Techniques

The paper emphasizes the importance of advanced characterization techniques in validating overlay and coating performance:

• Synchrotron X-ray diffraction: For detailed phase analysis of overlay deposits and coatings, particularly for identifying minor phases that may influence long-term performance.
• Focused ion beam (FIB) tomography: For three-dimensional characterization of pore structures, crack networks, and inclusion morphologies in coatings.
• Micro-electrochemical impedance spectroscopy (μ-EIS): For localized corrosion resistance assessment of individual phases within a coating microstructure.
• Nanoindentation mapping: For spatially resolved hardness and elastic modulus measurements across the coating cross-section.

Engineering Applications and Case Studies

The authors present several case studies illustrating the industrial application of advanced overlay and spraying technologies:

1. Hydropower turbine runners: HVOF-sprayed Stellite 6 coatings on runner surfaces extend service life from 5 years to 12–15 years in high-solids cavitation environments.
2. Petroleum refinery catalyst tubes: Laser-cladded nickel-based alloys on the inner surfaces of catalyst tubes improve resistance to coking and high-temperature corrosion.
3. Wind turbine main shafts: Cold-sprayed aluminum coatings on shaft surfaces provide corrosion protection in marine environments while maintaining fatigue strength.
4. Gas turbine hot sections: HVOF-sprayed MCrAlY (M = Ni, Co, Fe) coatings provide oxidation protection at 1100–1200°C.

Key Reflections and Implications

This review paper serves as an excellent reference for engineers seeking to understand the current state of the art in overlay welding and thermal spraying. The convergence of multiple disciplines—welding, surface engineering, materials science, and computational modeling—reflects the increasing complexity of surface modification challenges in modern manufacturing.

The emphasis on process monitoring and in-situ quality control is particularly noteworthy. As the industry moves toward Industry 4.0 and smart manufacturing, the integration of real-time sensing, data analytics, and adaptive process control will be essential for achieving consistent quality in overlay and coating production. The authors' discussion of digital twin concepts for overlay processes—where a virtual model of the process is continuously updated with sensor data to predict and optimize outcomes—points to a future where process optimization will be data-driven rather than experience-driven.

For practicing engineers, the key takeaway is that technology selection must be application-specific. No single process is universally superior; the optimal choice depends on the combination of performance requirements, cost constraints, production volume, and available equipment. The decision framework presented in the paper provides a practical starting point for this selection process.