CLADDING TECHNOLOGY SHANXI CO., LTDInfluence of La2O3 on Microstructure and Properties of Nickel-Based Spray Welding Coating on Titanium Alloy
Literature Overview
This research examines the effect of lanthanum oxide (La2O3) addition on the microstructure, bonding quality, and functional properties of nickel-based spray welding (oxy-fuel or plasma arc) coatings applied to titanium alloy substrates. Titanium alloys, particularly Ti-6Al-4V, are widely used in aerospace and chemical industries for their high specific strength and excellent corrosion resistance, yet they face challenges in tribological applications due to low hardness and poor wear resistance. Nickel-based overlay coatings offer a viable solution, and the incorporation of rare earth oxides such as La2O3 is investigated as a means to further optimize coating performance.
Core Technical Findings
The La2O3 addition modifies the solidification behavior of the nickel-based overlay by acting as a grain refiner and influencing the precipitation sequence during cooling. Metallographic analysis reveals that La2O3 particles promote equiaxed grain formation and reduce the columnar grain ratio in the coating cross-section. The coating microstructure transitions from a predominantly columnar dendritic morphology in the unmodified coating to a more equiaxed structure with finer grain size upon La2O3 addition, which generally correlates with improved toughness and reduced cracking susceptibility.
XRD analysis confirms the presence of La2O3 in the coating matrix, along with nickel-rich solid solution and intermetallic phases such as Ni3Ti and NiTi at the coating-substrate interface. The interface region is critical, as titanium-nickel intermetallics form inevitably during the high-temperature spray welding process. La2O3 appears to moderate the thickness of the intermetallic reaction zone, potentially reducing the brittle layer that can compromise interfacial bond strength. Bond strength tests demonstrate improved peel strength values for La2O3-modified coatings compared to the baseline, indicating enhanced metallurgical bonding at the interface.
| Property | Without La2O3 | With La2O3 (Optimal) |
|---|---|---|
| Coating hardness (HV) | 380–420 | 450–500 |
| Peel bond strength (MPa) | 45–55 | 60–75 |
| Intermetallic zone thickness (μm) | 30–50 | 15–25 |
| Grain morphology | Columnar dominant | Equiaxed dominant |
| Wear resistance (relative) | 1.0 (baseline) | 1.4–1.8 |
Standards and Quality Control Considerations
In the context of pressure vessel and heat exchanger fabrication, the application of nickel-based spray welding coatings to titanium substrates must comply with relevant standards including NB/T 47014 for welding procedure qualification and JB/T 4730 for non-destructive examination. The interfacial bond strength must be verified through peel testing or adhesion testing, and the coating thickness uniformity should be confirmed by ultrasonic thickness measurement or eddy current testing. The presence of intermetallic phases at the interface requires careful evaluation, as excessive brittle Ni3Ti or NiTi phases can lead to interfacial cracking under mechanical or thermal loading.
Study Insights and Reflections
This work underscores the importance of rare earth modification in surface engineering of dissimilar metal systems, particularly where the substrate and overlay have significant differences in thermal expansion coefficients and melting points. The La2O3 addition not only improves the coating microstructure but also mitigates the interfacial reaction that is often the weakest link in spray welding applications on reactive substrates like titanium. For engineers involved in titanium-lined pressure vessel fabrication, understanding the La2O3 modification mechanism provides a valuable tool for optimizing overlay process parameters and ensuring long-term structural integrity.