Effect of Spray Welding Process on Microstructure and Properties of Nickel-Based Coatings on Titanium Substrates

Literature Overview and Study Motivation

This literature investigates how spray welding parameters influence the microstructure, bonding quality, and mechanical properties of nickel-based coatings deposited on titanium substrates. Titanium and titanium alloys find extensive application in aerospace, marine, and chemical industries due to their outstanding specific strength and corrosion resistance. However, titanium surfaces are susceptible to wear and fretting corrosion under certain operating conditions, making surface engineering through cladding a critical technology. The study examines the intermetallic compound formation at the titanium-nickel interface, coating microstructure evolution under different spray welding regimes, and the resulting mechanical and corrosion performance.

Core Technical Findings

The fundamental challenge in spray welding nickel-based coatings onto titanium substrates lies in the formation of brittle intermetallic phases at the interface. Titanium and nickel exhibit limited mutual solubility, and during the high-temperature spray welding process, intermetallic compounds such as TiNi, Ti2Ni, Ti3Ni, and TiNi3 inevitably form. The study demonstrates that the thickness and morphology of this intermetallic layer are highly sensitive to spray welding parameters including current intensity, travel speed, powder feed rate, and torch oscillation amplitude.

Intermetallic Compound Formation and Interface Quality

The intermetallic layer thickness typically ranges from 50 to 300 micrometers depending on process parameters. At higher current levels and slower travel speeds, excessive heat input leads to deeper diffusion of titanium into the molten nickel pool, resulting in thicker intermetallic layers with reduced ductility. The study reveals that the intermetallic layer exhibits a columnar-to-equiaxed transition when the heat input is optimized, which improves bonding strength. The critical threshold identified is that intermetallic layers exceeding 150 micrometers begin to significantly degrade the coating's resistance to spalling under thermal cycling conditions.

Coating Microstructure Evolution

The nickel-based coating microstructure consists of a dendritic solidification structure with interdendritic phases. The study shows that rapid solidification during spray welding promotes a fine dendrite arm spacing of 3-8 micrometers, which enhances the coating's mechanical properties. The dilution rate from the titanium substrate into the coating typically ranges from 8% to 22%, and this dilution introduces titanium into the coating matrix, forming secondary phases that can either strengthen or embrittle the coating depending on their morphology and distribution.

Mechanical and Corrosion Properties

Bond strength testing reveals that optimized spray welding parameters yield interface shear strengths of 85-130 MPa, while poorly controlled processes produce values below 50 MPa. The corrosion resistance of the nickel-based coating in 3.5% NaCl solution is significantly improved compared to bare titanium, with polarization curves showing a shift in corrosion potential by 200-400 mV in the noble direction. However, the study notes that excessive intermetallic layer thickness creates galvanic couples that accelerate localized corrosion at the interface under aggressive environments.

Engineering Practice Implications

For engineering applications involving titanium components requiring wear-resistant or corrosion-resistant surfaces, the spray welding process must be carefully controlled. The key insight from this study is that the intermetallic layer, while inevitable, can be managed through parameter optimization to serve as a metallurgically sound bond rather than a defect-prone interface. In pressure vessel fabrication involving titanium-lined or titanium-clad components, understanding these interface characteristics is essential for predicting long-term service reliability. Preheating the titanium substrate to 200-300°C before spray welding has been shown to reduce residual stresses and improve coating adhesion. Post-weld annealing at 450-550°C can partially homogenize the intermetallic layer and relieve thermal stresses.

Study Insights and Conclusions

This literature provides valuable guidance for engineers working on titanium surface engineering applications. The central message is that the titanium-nickel interface is not merely a boundary to be minimized but a functional zone that can be engineered through precise process control. The intermetallic layer formation is thermodynamically driven and cannot be eliminated, but its thickness, morphology, and phase composition can be tuned to achieve acceptable mechanical and corrosion performance. For bimetal pressure vessel fabrication, where titanium-clad components may undergo cyclic thermal loading and internal pressure fluctuations, the spray welding parameter windows identified in this study should serve as baseline references for process qualification and production control. The study underscores the importance of integrating metallurgical understanding with process engineering to achieve reliable cladding results on dissimilar material combinations.