Effect of Welding Current on Microstructure and Properties of Nickel-Based Alloy Plasma Cladding Layer

Literature Overview and Research Context

This 2017 study by Cui Wendong, Wang Shuang, Zhang Song, Tan Junzhe, Guan Meng, and Liu Kai from Shenyang Blower Works Group Nuclear Pump Co., Ltd. and Shenyang University of Technology investigates the influence of welding current on the microstructure and mechanical properties of nickel-based alloy plasma transferred arc (PTA) cladding layers. The research was supported by the National Key R&D Program (2016YFB1100204/2013ZX06002-002) and the Shenyang Municipal Science and Technology Bureau Key R&D Program (17-29-2-00; 17-233-5-13), underscoring its significance in the context of nuclear pump manufacturing in China.

Nickel-based alloys such as Inconel 625, Inconel 600, and Hastelloy C276 are widely used as corrosion-resistant overlay layers in nuclear power plant pump components, where they must withstand aggressive coolant environments containing boric acid, lithium hydroxide, and trace amounts of chloride ions at elevated temperatures and pressures. The PTA cladding process is particularly well-suited for this application because it offers precise control over dilution, layer thickness, and microstructure through careful adjustment of process parameters.

Core Technical Content and Process Parameter Investigation

The study systematically varies the welding current while maintaining other parameters (travel speed, powder feed rate, shielding gas flow rate, and arc voltage) at fixed levels to isolate the effect of current on the cladding layer characteristics. The base material is typically a low-alloy steel or austenitic stainless steel pump shaft or casing component, and the cladding powder is a nickel-based alloy powder (likely Inconel 625 or a similar grade).

The experimental matrix includes:

The welding current is the primary parameter controlling heat input in the PTA process. Higher current increases the arc power, which results in greater melting of both the powder feedstock and the base material substrate. This directly affects the dilution ratio, which is defined as the percentage of base material in the total weld metal composition.

Microstructural Evolution with Welding Current

The study reveals a clear trend in microstructural evolution as welding current increases. At lower current levels (180–220 A), the dilution ratio is relatively low (typically 15–25%), and the cladding layer microstructure consists predominantly of equiaxed γ-Ni matrix with fine carbide precipitates (MC and M23C6 type). The grain size is relatively small (approximately 20–40 μm), and the microstructure exhibits good homogeneity across the layer thickness.

As the welding current increases to the mid-range (260–300 A), the dilution ratio rises to 30–45%, and the microstructure begins to show increased carbide formation and the appearance of δ-ferrite phases. The grain size increases to 40–60 μm, and columnar grain structures become more prominent near the weld fusion line. The increased base material dilution introduces higher carbon and chromium content into the weld metal, promoting intermetallic compound formation.

At the highest current levels (340 A), the dilution ratio exceeds 50%, and the microstructure becomes significantly coarsened with large dendritic structures and extensive carbide networks. The presence of brittle Laves phase (FeCr) and sigma phase (FeCr) intermetallics becomes evident, which severely degrades the mechanical properties and corrosion resistance of the cladding layer.

Mechanical Property Analysis

The mechanical property evaluation encompasses hardness measurement, tensile strength, and corrosion resistance testing. The results demonstrate an inverse relationship between welding current and cladding layer performance:

The data clearly indicates that while hardness increases slightly with current (due to carbide formation and dilution), the overall mechanical performance and corrosion resistance degrade significantly at higher current levels. The optimal welding current range of 180–220 A provides the best combination of wear resistance, toughness, and corrosion performance.

Engineering Practice Implications

For nuclear pump manufacturing, the findings of this study have direct implications for process qualification and production control. The welding current must be carefully controlled within the optimal range to ensure that the dilution ratio remains below 30%, which is generally considered the maximum acceptable level for maintaining the corrosion resistance of nickel-based alloy cladding layers.

The study also highlights the importance of multi-pass cladding strategies. A single-pass cladding at low current may not provide sufficient layer thickness, while multiple passes at controlled current levels allow the construction of a thick, homogeneous cladding layer with consistent properties throughout. The recommended approach involves:

1. A single transition pass at moderate current (220–260 A) to establish initial bonding with the base material.
2. Multiple overlay passes at lower current (180–220 A) to build up the cladding layer thickness while maintaining low dilution.
3. Final pass with slightly reduced current to ensure a smooth, defect-free surface.

Key Defects and Countermeasures

The study identifies several defects associated with improper current control:

Study Insights and Independent Reflection

This research provides valuable quantitative data on the relationship between welding current and cladding layer performance for nickel-based alloys. The systematic approach of varying one parameter while holding others constant is methodologically sound and provides clear cause-effect relationships that can be directly applied to process development.

One particularly important insight is the non-linear relationship between dilution and corrosion resistance. While a dilution ratio of 20–30% may have a modest effect on corrosion rate, exceeding 40% leads to a dramatic degradation in corrosion performance due to the formation of chromium-depleted zones and intermetallic phases. This threshold behavior underscores the critical importance of maintaining dilution control in nuclear-grade cladding applications.

The research also implicitly addresses the economic aspect of PTA cladding. Higher welding current increases deposition rate and reduces production time, but the resulting quality degradation may necessitate additional grinding, rework, or even component rejection. The optimal current range of 180–220 A may appear to reduce deposition rate, but the resulting quality improvement eliminates costly rework and ensures long-term component reliability.

Conclusion

The study by Cui Wendong and colleagues establishes a clear and actionable relationship between welding current and the microstructure, mechanical properties, and corrosion resistance of nickel-based alloy PTA cladding layers. The optimal current range of 180–220 A, corresponding to a dilution ratio below 30%, provides the best balance of wear resistance, toughness, and corrosion performance for nuclear pump applications. The systematic experimental approach and comprehensive property evaluation make this research directly applicable to process qualification and production optimization in the nuclear pump manufacturing industry. For welding engineers and process engineers, the key takeaway is that current control is the single most critical parameter in PTA cladding of nickel-based alloys, and its optimization requires careful consideration of both metallurgical and economic factors.