Hongjian Wu

Cold spraying is a solid-state deposition technology with great potential and application value for repairing metallic components. As a key parameter, the process gas temperature governs gas and particle velocity as well as the heat input to both the powder particles and the component surface. However, to avoid nozzle clogging and maintain process stability, the process gas temperature is typically kept below the solidus temperature of the feedstock material. This constraint limits the usable parameter regime, particularly with respect to further heating and thus softening of powder particles as well as component surfaces, which both would promote better bonding. In this study, a novel powder pre-cooling approach is proposed to shift the thermal limits, enabling stable cold spray conditions at elevated process gas temperatures by using Nitrogen as process gas. As a case study, Al6061 powder was pre-cooled using a custom dry-ice cooling system prototype prior to spraying. The results show that stable deposition could be maintained even at a process gas temperature of 720 °C, thereby widening so far applicable ranges from conventional cold spraying of Al6061 by 220 °C. In addition, the increased process gas temperature increased the component surface temperature from 144 °C to 209 °C, enhancing the surface softening and thereby improving overall deposit performance.

Cold spraying is a solid-state deposition technology with great potential and application value for repairing metallic components. As a key parameter, the process gas temperature governs gas and particle velocity as well as the heat input to both the powder particles and the component surface. However, to avoid nozzle clogging and maintain process stability, the process gas temperature is typically kept below the solidus temperature of the feedstock material. This constraint limits the usable parameter regime, particularly with respect to further heating and thus softening of powder particles as well as component surfaces, which both would promote better bonding. In this study, a novel powder pre-cooling approach is proposed to shift the thermal limits, enabling stable cold spray conditions at elevated process gas temperatures by using Nitrogen as process gas. As a case study, Al6061 powder was pre-cooled using a custom dry-ice cooling system prototype prior to spraying. The results show that stable deposition could be maintained even at a process gas temperature of 720 °C, thereby widening so far applicable ranges from conventional cold spraying of Al6061 by 220 °C. In addition, the increased process gas temperature increased the component surface temperature from 144 °C to 209 °C, enhancing the surface softening and thereby improving overall deposit performance.

The fabrication of aluminum/steel heterogeneous metals is challenging due to interfacial cracking from brittle intermetallic compounds produced in high-heat-input processes and poor heterogeneous interfacial bonding in cold spray (CS). In this work, laser-assisted cold spray (LACS) was used to deposit 316 stainless steel (316SS) coatings on 7050Al substrates, and the microstructure and properties were systematically investigated. An L9(3³) Taguchi design was first employed to optimize the CS parameters, followed by an evaluation of the effect of laser power on coating performance. The results showed that 600 W was the optimal laser power, reducing the coating porosity from ~1.0% to 0.2%. Laser irradiation also markedly improved heterogeneous interfacial bonding, increasing the bond strength from 35 MPa for CS coatings to over 65 MPa. X-ray diffraction confirmed that LACS retained the characteristics of solid-state deposition, with only γ-Fe detected and no δ-Fe or Al-Fe intermetallic compounds formed. EBSD analysis showed that LACS promoted grain refinement, substructure evolution, and grain boundary transformation, leading to a more homogeneous microstructure. In addition, compared with CS coatings, LACS coatings exhibited superior wear and corrosion resistance, with the friction coefficient decreasing from 0.75 to 0.65, the wear rate dropping from 7.5 × 10⁻⁴ to 7.2 × 10⁻⁵ mm³·N⁻¹·m⁻¹, and the corrosion current density decreasing from 2.71 × 10⁻⁵ to 8.77 × 10⁻⁶ A/cm². These results demonstrate that LACS provides an effective solid-state route for fabricating high-performance 316SS/7050Al heterogeneous coatings.

In today’s increasingly competitive global marketplace, thermal spray processes are needed to support higher-value user manufacturing techniques and to satisfy continuously changing customer demands. These require thermal spray systems that are flexible enough to be produced at different scales of manufacturing batches, while still achieving high-quality coatings at a low cost. Therefore, the established and standard thermal spray processes are in the need to be upgraded to catch up with the stringent requirements of Industry 4.0. The purpose of this study is to provide a comprehensive review on the technologies of Industry 4.0 already integrated with thermal spray processes to uplift their applications through advanced digitization and automation, leading to enhanced efficiency and sustainability. To this purpose, the main current challenges facing the thermal spray family are described. Specific technologies including process control for improving the stability of the spray system, characterization for tailoring coatings to the specific requirements of various applications, modeling and simulation for enhanced replicability, analysis and optimization of the process, in situ observation for monitoring and data collection, artificial intelligence for allowing efficient and effective control and decision making and finally robotics for realizing hybrid thermal spray processes are detailed. These progressive technological aspects can play an irreplaceable role in enhancing the quality and applicability of thermal spray processes. Ultimately, the potential for incorporating these advanced technologies into thermal spray process chains is comprehensively evaluated, and the prospective trajectory of thermal spray technology’s evolution is discussed.

Low-pressure cold spray (LPCS) has broadened the application field of cold spray owing to its portability and low cost. For additive manufacturing (AM) based on LPCS (LPCSAM), it is important to investigate the effects of parameters such as temperature and pressure of the gas, stand-off distance, gun traverse speed, and the number of scanning passes of the gun on the deposition and properties. This study aims to determine the optimal kinematic spray parameters for spraying Cu+Al ₂ O ₃ powder onto an aluminum substrate through LPCS, so as to prepare for the next AM work. The deposition mass, deposition rate, microhardness, and roughness under different spraying conditions were studied. The best spraying effect, with a high deposition rate and without nozzle clogging, was obtained at a nitrogen pressure of 0.9 MPa and a temperature of 400°C.