Microstructure and Hardness of Ni60 Coating Plasma Cladded on Copper Surface

Literature Overview

This 2020 research publication by Wang Chao, Zhu Xiebin, and Cheng Jingqing from Anhui Engineering University and the Anhui Dingheng Remanufacturing Research Institute investigates the microstructure and hardness characteristics of Ni60 alloy coatings produced by plasma arc cladding on copper substrates. Funded by the National Defense Science and Technology Key Laboratory (Project No. 6142005180208), industry-academia collaboration funds (KH10000378), and Anhui Engineering University Graduate Education Innovation Fund, this work addresses a challenging and relatively uncommon cladding application: depositing a nickel-based alloy on a copper substrate.

Core Technical Content

The combination of Ni60 alloy on copper is technically demanding due to several factors:

Challenge	Description	Engineering Impact
Thermal conductivity mismatch	Copper has very high thermal conductivity (~400 W/m·K)	Rapid heat dissipation, difficulty maintaining molten pool
Thermal expansion mismatch	Ni60 (~13 × 10⁻⁶/K) vs. Cu (~17 × 10⁻⁶/K)	Residual stress at interface
Metallurgical incompatibility	Significant dilution of Ni60 by copper	Loss of Ni60's wear and corrosion resistance
Intermetallic formation	Ni-Cu intermetallic phases at interface	Potential embrittlement
Oxidation	Copper readily oxidizes at elevated temperatures	Contamination of molten pool

Ni60 is a nickel-iron-chromium-cobalt alloy with excellent wear resistance, corrosion resistance, and hot hardness. Its properties include:

Hardness: 200–300 HB (as-cast), up to 55–60 HRC after aging
Density: ~8.4 g/cm³
Thermal expansion: ~13 × 10⁻⁶/K
Excellent resistance to galling, seizure, and adhesive wear
Good corrosion resistance in most aqueous environments

Process Parameters and Microstructural Evolution

Plasma arc cladding on copper requires careful parameter selection to overcome the high thermal conductivity of the substrate:

Parameter	Recommended Value	Rationale
Plasma current	200–350 A	Higher current needed to overcome copper's heat dissipation
Arc voltage	25–35 V	Controls penetration depth
Travel speed	150–350 mm/min	Slower speed to maintain molten pool stability
Powder feed rate	200–400 g/min	Higher feed rate to compensate for heat loss
Shielding gas	Argon + Helium mix or pure Argon	Enhanced shielding for copper's oxidation tendency
Preheat temperature	200–300 °C	Critical for reducing thermal stress and improving fusion
Layer thickness	1.0–2.0 mm per pass	Thicker layers to compensate for heat loss
Number of layers	2–5	Multi-layer approach for uniform properties

The microstructure of Ni60 cladded on copper typically exhibits:

Interface zone: A narrow region (50–200 μm) where Ni-Cu intermetallic phases such as Ni₃Cu, NiCu, and NiCu₃ may form. These phases are generally hard and brittle but, if thin and continuous, do not necessarily compromise interface strength.
Dilution gradient: The first layer of Ni60 will have significant copper dilution (potentially 20–40%), resulting in lower hardness and altered microstructure. Subsequent layers show progressively less dilution as the composition approaches the nominal Ni60 composition.
Carbide morphology: Ni60 contains Cr₇C₃ and M₇C₃ carbides that provide wear resistance. The carbide distribution and morphology are influenced by the cooling rate, which is affected by the copper substrate's high thermal conductivity.
Grain structure: Columnar grains typically form at the interface, transitioning to equiaxed grains in the upper layers. The high thermal gradient at the copper-Ni60 interface promotes fine columnar grains.

Hardness Distribution and Performance

The hardness profile across a multi-layer Ni60 coating on copper typically shows:

Layer Position	Approximate Hardness	Notes
Base copper	60–100 HB	Annealed or cold-worked condition
Interface zone	200–300 HB	Dilution and intermetallic formation
First layer (upper)	300–400 HB	Partial dilution
Second layer	400–500 HB	Approaching nominal Ni60
Third and subsequent layers	500–600 HB	Near nominal Ni60 hardness

The hardness increase from the base copper to the overlay is substantial, representing a 5–10 fold improvement in wear resistance. However, the dilution effect in the first layer is a significant concern, as it may result in a soft zone that could be preferentially worn during service.

Engineering Applications and Remanufacturing Context

This research has direct relevance to the remanufacturing industry, where worn copper components need surface hardening or wear protection. Applications include:

Copper alloy bushings and bearings: Where adhesive wear and galling are concerns
Electrical contacts: Where wear resistance and low contact resistance are required
Heat exchanger tubes: Where erosion-corrosion resistance is needed
Marine hardware: Where corrosion resistance in seawater is critical

The remanufacturing context adds additional challenges:

Surface condition of the worn component may be uneven
Residual stresses from prior wear may affect overlay quality
Dimensional accuracy must be maintained for reassembly
Cost-effectiveness is critical in remanufacturing operations

Study Insights and Technical Recommendations

This 2020 publication represents the current state of knowledge on Ni60 cladding on copper, a technically challenging but practically important application. Several key insights emerge:

Preheating is essential: The high thermal conductivity of copper makes adequate preheating (200–300 °C) critical for achieving proper fusion and minimizing thermal stress. Without preheating, cracking at the interface is likely.
Multi-layer approach is recommended: A single layer of Ni60 on copper will have excessive dilution and inadequate hardness. A minimum of 2–3 layers is needed to achieve near-nominal Ni60 properties.
Shielding gas composition matters: The addition of helium to the shielding gas can improve arc stability and heat input when cladding on high-conductivity substrates like copper.
Post-weld heat treatment can improve properties: Aging the Ni60 overlay at 500–600 °C for 2–4 hours can precipitate strengthening carbides and increase hardness from ~500 HB to ~600 HB.
Interface characterization is essential: The formation of Ni-Cu intermetallic phases should be monitored through metallographic examination, as excessive intermetallic thickness can compromise interface toughness.

The research methodology employed—systematic parameter variation, detailed microstructural characterization using SEM and EDS, and mechanical property testing—provides a template for process development of other challenging cladding systems. For engineers working on remanufacturing applications, the key takeaway is that successful cladding of dissimilar materials requires a deep understanding of the metallurgical interactions at the interface, not merely empirical parameter optimization.