Effect of Nickel Interlayer on Cavitation Erosion Resistance of NiTi Cladding Deposited by TIG Surfacing

Literature Overview

This research paper, authored by Zhen-Ping Shi and colleagues from the Institute of Metal Research of the Chinese Academy of Sciences, the University of Science and Technology of China, and Jiangsu University of Science and Technology, was published in Acta Metallurgica Sinica (English Letters) in 2020. The study investigates the effect of a nickel interlayer on the cavitation erosion resistance of NiTi-based cladding deposits produced by Tungsten Inert Gas (TIG) surfacing. The work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA13040500) and the Opening Project of the Material Corrosion and Protection Key Laboratory of Sichuan Province (No. 2017CL18).

Motivation and Application Context

Cavitation erosion is a significant degradation mechanism in hydraulic machinery, marine propellers, hydroelectric turbines, and nuclear reactor coolant components. It occurs when vapor bubbles form and collapse in a liquid, generating localized pressure spikes that can reach hundreds of megapascals, causing material fatigue and surface damage. Nickel-titanium (NiTi) alloys, particularly NiTi shape memory alloys, have been investigated for cavitation erosion resistance due to their unique combination of high strength, good ductility, and shape memory effect that may contribute to self-healing of micro-damage. However, the direct deposition of NiTi alloys by arc welding often results in brittle intermetallic phases, cracking, and poor bonding with ferrous substrates, limiting their practical application.

The introduction of a nickel interlayer between the substrate and the NiTi cladding layer is a well-established strategy in cladding technology to mitigate these issues. Nickel, being more ductile and having better metallurgical compatibility with both steel substrates and NiTi alloys, can act as a diffusion barrier and crack-arresting layer. This study specifically examines how the presence of a Ni interlayer influences the cavitation erosion performance of the final NiTi cladding deposit.

Experimental Methodology

TIG Surfacing Parameters

The TIG surfacing process was conducted using DCEN polarity with the following typical parameter ranges:

The multi-pass cladding build-up typically consists of a nickel interlayer (2–3 passes) followed by the NiTi cladding layer (3–5 passes). The interpass temperature is carefully controlled to avoid excessive thermal cycling that could promote grain coarsening or cracking in the interlayer.

Cavitation Erosion Testing

The cavitation erosion tests were conducted using a standard ultrasonic cavitation erosion test apparatus, typically operating at a frequency of 20 kHz with a peak-to-peak amplitude of 100 μm. The test duration is generally 1–10 hours, with mass loss measured at intervals. The cavitation erosion rate is expressed as mass loss per unit area per hour (mg/cm²·h) or as a normalized erosion rate relative to a reference material (usually SUS304 stainless steel).

Results and Analysis

Metallurgical Characteristics

The presence of the Ni interlayer significantly alters the microstructure of the NiTi cladding deposit. Without the interlayer, the NiTi deposit directly on steel typically exhibits a columnar grain structure with significant mixing of iron into the cladding layer, forming brittle Fe-Ni and Fe-Ti intermetallic phases at the interface. These phases are prone to cracking and provide preferential sites for cavitation damage initiation.

With the Ni interlayer, the diffusion of iron into the NiTi layer is substantially reduced. The Ni interlayer acts as a diffusion barrier, limiting the penetration of iron atoms into the NiTi cladding. Metallographic examination typically reveals a more equiaxed grain structure in the NiTi deposit when deposited on a Ni interlayer, as the thermal gradient is moderated by the interlayer's thermal properties. The Ni interlayer itself shows a gradient microstructure, with Ni-rich regions adjacent to the substrate transitioning to regions with increasing NiTi content at the interface with the cladding layer.

Cavitation Erosion Performance

The cavitation erosion resistance of the NiTi cladding with Ni interlayer is significantly improved compared to the case without an interlayer. The key mechanisms responsible for this improvement include:

1. Crack suppression: The Ni interlayer prevents the formation of interfacial cracks that would otherwise serve as initiation sites for cavitation damage.
2. Reduced brittle phase formation: By limiting iron diffusion, the Ni interlayer reduces the formation of brittle intermetallic phases within the NiTi cladding, resulting in a more ductile and damage-tolerant microstructure.
3. Enhanced bonding strength: The Ni interlayer provides a metallurgically sound bond between the steel substrate and the NiTi cladding, preventing delamination under cavitation loading.
4. Shape memory effect preservation: The NiTi cladding layer deposited on a Ni interlayer retains more of its intrinsic shape memory characteristics, which may contribute to self-repair of micro-damage during cavitation exposure.

The cavitation erosion rate of the NiTi/Ni/steel system is typically 50–70% lower than that of the NiTi/steel system without an interlayer, and the NiTi/Ni/steel system demonstrates cavitation erosion resistance comparable to or better than conventional cavitation-resistant materials such as 13Cr stainless steel and nickel-based alloys.

Microstructural Evolution During Cavitation

After cavitation erosion testing, the surface of the NiTi cladding with Ni interlayer shows a characteristic work-hardened layer with a thickness of approximately 50–150 μm. The surface exhibits a smooth, compact morphology with minimal material loss, indicating that the cavitation-induced damage is primarily accommodated by plastic deformation rather than material removal. In contrast, the NiTi cladding without a Ni interlayer shows deep cavitation pits, subsurface microcracks, and spalling of brittle intermetallic phases, indicating a fundamentally different damage mechanism.

Engineering Significance

This study has direct relevance to the design and fabrication of cavitation-resistant components in nuclear power plants, hydroelectric equipment, and marine applications. The NiTi-based cladding with Ni interlayer offers a promising alternative to conventional nickel-based alloy cladding for cavitation erosion protection, with the added benefit of potential shape memory self-healing properties. The TIG surfacing process used in this study is well-suited for on-site repair and cladding of large components, as it requires relatively simple equipment and can be adapted to various joint geometries.

For pressure vessel and piping applications, the Ni interlayer approach provides a practical solution to the metallurgical incompatibility between NiTi alloys and carbon steel substrates. The interlayer thickness should be optimized based on the specific application requirements—thicker interlayers provide better diffusion barrier performance but increase the total cladding thickness and cost. A typical interlayer thickness of 1.5–3.0 mm is recommended for most cavitation erosion protection applications.

Study Insights and Implications

The key insight from this study is that the Ni interlayer does not merely serve as a bonding layer but fundamentally alters the damage mechanism of the NiTi cladding under cavitation erosion. The transition from brittle intermetallic-dominated failure to ductile deformation-dominated failure represents a qualitative improvement in cavitation erosion resistance. This finding has broader implications for the design of multi-layer cladding systems, where the selection of interlayer materials should be based not only on metallurgical compatibility but also on the specific degradation mechanism of the application. For engineers involved in cladding technology development, this study demonstrates the value of combining advanced materials (NiTi shape memory alloys) with established cladding processes (TIG surfacing) and interlayer strategies to achieve superior performance in demanding service environments.