Key Technology Progress and Applications in Overlay Welding and Thermal Spraying

Literature Overview

This comprehensive review article by Huang Zhiquan and colleagues, published in China Surface Engineering (Vol. 39, No. 1, 2026, pp. 275-297), provides an authoritative assessment of the state of the art in overlay welding and thermal spraying over the past decade. The authors represent leading Chinese research institutions including the Zhengzhou Machinery Research Institute, Xi'an Jiaotong University, Beijing University of Technology, and the Zhejiang Baton Welding Technology Research Institute. As a surface engineering practitioner, I regard this as one of the most thorough reviews available in the Chinese-language literature, and it serves as an essential reference for engineers seeking to understand the current technological landscape and future development directions in surface protection technology.

Arc Overlay Welding Technology Development

The paper provides a detailed overview of arc welding-based overlay processes, categorizing them into conventional arc overlay, high-energy beam overlay, and specialized overlay techniques.

Conventional Arc Overlay Welding

Conventional arc overlay welding remains the workhorse of industrial surface protection. The key developments in recent years include:

The trend toward automated and robotic overlay welding systems has significantly improved process consistency and productivity. Multi-axis robotic systems can now handle complex geometries including large-diameter pipes, turbine blades, and pressure vessel internals with high precision.

High-Energy Beam Overlay

High-energy beam processes including laser cladding and electron beam cladding have seen rapid development. Laser cladding is particularly notable for its ability to produce overlay layers with dilution rates as low as 3-5%, near-equilibrium microstructures, and excellent metallurgical bonding. The key technological advances include:

• High-power fiber lasers (10-30 kW) enabling practical industrial deposition rates exceeding 1 kg/h.
• Multi-layer and multi-track cladding strategies for building up thick functional layers with controlled microstructure gradients.
• In-situ alloying through the addition of ceramic powders (TiC, WC, Al2O3, SiC) to create composite overlay layers with tailored wear and corrosion properties.

Specialized Overlay Technologies

The review highlights several specialized overlay techniques gaining industrial traction:

1. Cold Metal Transfer (CMT) Welding: Utilizes short-circuiting arcs with low heat input, enabling low-dilution overlay on thin substrates and dissimilar material joints. It has found applications in repair welding of thin-walled components and overlay welding on aluminum and copper substrates.
2. Friction Stir Welding (FSW)-based surface modification: Although primarily a solid-state joining process, FSW has been adapted for surface modification to create work-hardened and refined microstructures on component surfaces, enhancing wear and fatigue resistance.
3. Electroslag Overlay Welding: Offers high deposition rates and deep penetration, making it suitable for building up thick overlay layers on large components such as ship propellers and marine engineering structures.

Thermal Spraying Technology Progress

The paper devotes substantial attention to thermal spraying developments, covering both established and emerging technologies.

Cold Spray Technology

Cold spray (CS) and high-velocity oxy-fuel (HVOF) cold spray have emerged as transformative technologies for surface engineering. Unlike conventional thermal spray processes, cold spray deposits material through high-velocity kinetic impact at temperatures below the melting point, preserving the original microstructure of the coating material. Key developments include:

• High-velocity cold spray (HVCS) achieving particle velocities exceeding 800 m/s, enabling deposition of difficult-to-spray materials such as copper, titanium, and high-strength steels.
• Cold spray for additive manufacturing (CS-AM) of near-net-shape components with full density and excellent mechanical properties.
• Application of cold spray for repair of aerospace components including turbine blades and landing gear.

Plasma Spray and Related Technologies

Plasma spray remains the dominant thermal spray process for oxide and ceramic coatings. Recent advances include:

• Suspension Plasma Spray (SPS): Enables coating of sub-micron powders and nanocomposite materials, producing coatings with superior density and mechanical properties compared to conventional atmospheric plasma spray (APS).
• Plasma Spray Physical Vapor Deposition (PSP-PVD): Combines the advantages of plasma spray with PVD techniques to produce ultra-fine grain coatings with unique microstructures and properties.
• High-Power Plasma Spray (HPPS): Achieves particle velocities of 500-700 m/s and coating densities exceeding 98%, suitable for demanding applications including thermal barrier coatings (TBCs) on gas turbine blades.

Explosive Spray Technology

Explosive spray (ES) continues to evolve as a technology for producing high-density, high-bond-strength coatings. Recent developments include:

• Application to nickel-based superalloy coatings for hot section components in aero-engines.
• Development of ES for copper and aluminum coatings in the electronics industry for electrical contact applications.
• Integration with laser and plasma processes in hybrid surface engineering strategies.

Application Overview

The paper provides a comprehensive survey of industrial applications across multiple sectors:

Future Development Directions

The authors identify several key directions for future development:

1. High-end equipment overlay materials: Development of new overlay materials capable of withstanding extreme conditions including ultra-high temperatures (>1200°C), severe corrosion environments, and combined wear-corrosion-fatigue loading.
2. Composite overlay technology: Multi-layer and functionally graded overlay designs combining different materials to achieve synergistic property combinations not attainable with single materials.
3. Automation, intelligence, and flexibility: Integration of sensor-based process monitoring, adaptive control systems, and robotic platforms to achieve fully automated overlay welding and spraying operations with real-time quality assurance.
4. New materials and processes for thermal spray: Development of novel coating materials including high-entropy alloys, nanocomposites, and bio-inspired materials, combined with advanced spray processes to achieve near-theoretical coating densities.
5. Improved coating density: Reducing porosity and improving the compactness of thermal spray coatings through optimized process parameters, advanced powder metallurgy, and hybrid processing approaches.

Study Insights and Implications

This review is exceptionally valuable as a roadmap for surface engineering research and development in China. From my perspective as a practicing engineer, the most impactful trend is the convergence of overlay welding and thermal spraying into a unified surface engineering discipline. The distinction between "welding" and "spraying" is becoming increasingly blurred as hybrid processes combine the metallurgical bonding of welding with the material versatility of spraying. Engineers must develop a broader understanding of both process families to make optimal technology selections for specific applications.

The emphasis on automation and intelligence is particularly relevant given China's manufacturing industry's push toward digital transformation and Industry 4.0. The integration of machine vision, sensor networks, and data analytics into overlay welding and spraying processes will be essential for achieving the consistency and reliability required in high-value applications such as aerospace and power generation. However, I caution that the path to full automation is not merely a matter of adding sensors and software—it requires a fundamental understanding of the metallurgical processes involved and the ability to translate that understanding into robust control algorithms that can handle the inherent variability of arc and spray processes.

This review should be required reading for any engineer involved in surface engineering technology selection, process development, or equipment specification.