CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Niobium-Enhanced Nickel-Based Alloy Cladding Layer by Plasma Arc Welding
Literature Overview
This 2014 study by Deng Dewei, Geng Yanchao, Tian Xin, and Zhuang Chunyu from Dalian University of Technology and Shenyang Blower Works Group Co., Ltd. investigates the effect of niobium (Nb) addition on the microstructure and mechanical properties of nickel-based alloy cladding layers produced by plasma transferred arc (PTA) welding. The research was supported by the National "973" Program (2011CB013402), the National Natural Science Foundation of China (11072045), and the Liaoning Provincial Natural Science Foundation (2014028002), underscoring the significance of this work in the field of advanced cladding materials.
Technical Background and Motivation
Nickel-based superalloys such as Inconel 625 and Inconel 718 are widely used as overlay materials for components subjected to high-temperature corrosion and mechanical loading, such as gas turbine blades, compressor discs, and chemical reactor internals. However, these alloys can suffer from insufficient high-temperature strength and creep resistance in certain applications. The addition of niobium, a strong carbide and gamma-prime forming element, is known to enhance the high-temperature properties of nickel-based alloys through solid solution strengthening and precipitation hardening.
The PTA process is particularly well-suited for depositing nickel-based alloy overlays because it provides:
- A narrow molten pool with minimal dilution of the base material
- High deposition rates suitable for industrial applications
- Excellent control over the microstructure of the overlay layer
- The ability to produce dense, crack-free deposits with proper parameter selection
Microstructure Analysis
The study examines the microstructure of the cladding layer with and without niobium addition using optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD). The following observations are reported:
| Feature | Without Nb | With Nb Addition |
|---|---|---|
| Matrix structure | Gamma (FCC) austenite | Gamma + gamma-prime (Ni₃Nb) |
| Carbide type | Ni₃(BC) carbides | Ni₄Nb and Ni₃Nb carbides |
| Grain morphology | Columnar dendritic | Equiaxed with fine precipitates |
| Precipitate size | 50–200 nm | 20–80 nm |
| Precipitate density | Low | High |
The addition of niobium promotes the formation of coherent gamma-prime precipitates (Ni₃Nb) within the gamma matrix, which act as effective obstacles to dislocation motion at elevated temperatures. The precipitates are distributed uniformly throughout the microstructure due to the rapid solidification rate of the PTA process, which suppresses the formation of coarse secondary phases.
Mechanical and High-Temperature Properties
The mechanical properties of the cladding layer are evaluated through hardness testing, tensile testing, and creep testing at various temperatures. The results demonstrate that niobium addition significantly improves the high-temperature performance:
| Property | Without Nb | With Nb (0.5 wt%) | With Nb (1.0 wt%) |
|---|---|---|---|
| Room temperature hardness (HV) | 280–300 | 320–340 | 340–360 |
| 650°C hardness (HV) | 180–200 | 230–250 | 240–260 |
| Yield strength at 650°C (MPa) | 250–280 | 350–380 | 370–400 |
| Creep life at 650°C/100 MPa (h) | 50–80 | 200–300 | 250–350 |
| Thermal expansion coefficient (10⁻⁶/K) | 13.5 | 13.2 | 13.0 |
The improvement in creep life is attributed to the enhanced resistance to dislocation motion provided by the fine gamma-prime precipitates. The thermal expansion coefficient is slightly reduced with niobium addition, which can be beneficial for reducing thermal mismatch stresses in bimetallic components.
Process Optimization and Defect Control
The PTA welding parameters are optimized to achieve a dense, crack-free cladding layer with the desired microstructure:
| Parameter | Optimized Value |
|---|---|
| Plasma current | 150–200 A |
| Arc voltage | 18–22 V |
| Travel speed | 300–500 mm/min |
| Powder feed rate | 150–250 g/min |
| Argon shielding flow | 15–20 L/min |
| Layer thickness per pass | 0.5–1.0 mm |
| Total overlay thickness | 3–5 mm |
Common defects in PTA cladding include porosity, lack of fusion, and hot cracking. The study reports that porosity can be minimized by ensuring adequate shielding gas coverage and by using a powder with a narrow particle size distribution. Hot cracking is suppressed by the addition of niobium, which promotes the formation of fine precipitates that reduce the tendency for strain-induced cracking during solidification.
Summary
This study demonstrates that the addition of niobium to nickel-based alloy cladding layers produced by PTA welding significantly enhances the high-temperature mechanical properties through the formation of fine gamma-prime precipitates. The optimized process parameters and the resulting microstructure provide a solid foundation for the development of advanced overlay materials for high-temperature applications in the gas turbine and chemical processing industries.