CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Hardness of Ni60 Coating on Copper Surface by Plasma Overlay
Literature Overview
This study note investigates the microstructure and hardness characteristics of Ni60 (a nickel-iron-phosphorus hardfacing alloy) coatings deposited on copper surfaces using plasma transferred arc (PTA) powder cladding. The combination of nickel-based overlay on copper substrate is technically challenging due to the significant differences in melting point, thermal conductivity, thermal expansion coefficient, and metallurgical compatibility between the two material systems. Copper's high thermal conductivity creates a strong heat sink effect, while its low melting point (1085 degrees C versus 1400 degrees C for Ni60) leads to high dilution and potential interface defects. The literature provides detailed metallographic analysis, hardness measurements, and process optimization strategies for this challenging material combination.
Core Technical Content
Material System Challenges
| Property | Copper (base) | Ni60 (overlay) | Mismatch Factor |
|---|---|---|---|
| Melting point | 1085 degrees C | 1400 degrees C | 1.29x |
| Thermal conductivity | 390 W/(m.K) | 15 W/(m.K) | 26x |
| Thermal expansion coefficient | 17 x 10^-6 /K | 13 x 10^-6 /K | 1.3x |
| Density | 8.96 g/cm3 | 8.0 g/cm3 | 1.12x |
| Electrical conductivity | 5.96 x 10^7 S/m | 1.4 x 10^6 S/m | 43x |
The extreme thermal conductivity mismatch is the primary challenge. Copper rapidly conducts heat away from the molten pool, requiring significantly higher heat input than would be used for steel substrates. However, excessive heat input causes excessive copper dissolution into the molten Ni60 pool, leading to high dilution and degradation of overlay properties.
Microstructural Analysis
The literature reports the following microstructural features in the Ni60-on-copper PTA overlay:
- Overlay microstructure (low dilution zone): The Ni60 microstructure away from the interface consists of a dendritic nickel solid solution matrix with interdentritic FeNi3 intermetallic compounds and Ni3P phosphides. This microstructure provides hardness of HV 500-600.
- Interface region: At the copper-Ni60 interface, a copper-nickel solid solution zone forms due to mutual diffusion. The width of this zone is typically 20 to 80 micrometers, depending on the dilution rate and solidification conditions. The copper-nickel solid solution exhibits a continuous solid solubility range, which is favorable for metallurgical bonding.
- Dilution gradient: The literature demonstrates a clear hardness gradient from the overlay surface (HV 550-600) to the interface region (HV 300-400), reflecting the progressive increase in copper content from 0 to 30-40 percent by weight.
- Solidification mode: The high cooling rate at the interface (enhanced by copper's thermal conductivity) promotes columnar grain growth from the substrate into the overlay. This columnar structure can be beneficial for crack resistance if the grains are fine enough to prevent crack propagation.
Process Parameters for Optimal Results
| Parameter | Recommended Value | Effect on Interface |
|---|---|---|
| Arc current | 300-500 A | Higher current increases dilution |
| Travel speed | 150-300 mm/min | Higher speed reduces dilution |
| Powder feed rate | 300-600 g/min | Higher feed rate reduces dilution |
| Preheat temperature | 300-500 degrees C | Reduces thermal gradient, may increase dilution |
| Shielding gas | Argon (20-30 L/min) | Prevents oxidation of copper and nickel |
| Single pass thickness | 0.8-1.5 mm | Thinner passes reduce dilution |
| Interpass temperature | Below 200 degrees C | Maintain rapid solidification |
The literature recommends a multi-pass approach with the first pass using a nickel-rich filler to establish a low-dilution bonding layer, followed by subsequent passes of full Ni60 composition.
Hardness Distribution
The literature presents hardness traverse results across the overlay cross-section:
| Position from Surface | Copper Content (wt%) | Hardness (HV) | Microstructure |
|---|---|---|---|
| 0 mm (surface) | 0-2% | 550-600 | Ni matrix + FeNi3 + Ni3P |
| 0.5 mm | 5-10% | 480-530 | Ni matrix + FeNi3 + Ni3P |
| 1.0 mm | 15-20% | 400-450 | Ni-Cu matrix + FeNi3 |
| 1.5 mm (interface) | 25-35% | 300-380 | Cu-Ni solid solution |
| Base (copper) | 100% | 80-120 | Pure copper |
Defect Analysis and Prevention
- High dilution exceeding 35 percent: Results in hardness below HV 350 and loss of wear resistance. Prevention requires reducing heat input, increasing powder feed rate, and using thinner passes.
- Interface cracking: Thermal mismatch and residual stresses can cause cracking at the interface, particularly during cooling. Preheating to 300-500 degrees C reduces the thermal gradient, while controlled interpass temperatures prevent stress accumulation.
- Porosity: Gas porosity can form at the interface due to insufficient shielding or trapped gases at the copper-Ni boundary. Adequate argon shielding and thorough surface cleaning are essential.
- Soft spots: Localized regions of high copper dilution create soft spots in the overlay that are susceptible to preferential wear. Uniform powder distribution and consistent welding parameters are critical.
Study Insights and Reflections
The literature provides a valuable analysis of one of the more challenging overlay combinations in practice: nickel-based alloy on copper. The key insight is that the copper-nickel system benefits from complete solid solubility, which means that metallurgical bonding is inherently favorable. Unlike systems that form brittle intermetallic compounds at the interface (such as iron-titanium or aluminum-nickel), the copper-nickel interface develops a ductile solid solution that provides good toughness and crack resistance.
However, the extreme thermal conductivity mismatch creates a practical paradox: copper's ability to conduct heat away from the molten pool promotes rapid solidification (favorable for fine microstructure) but also increases dilution (unfavorable for overlay properties). The literature demonstrates that this paradox is resolved through careful parameter optimization, particularly by using higher powder feed rates and lower travel speeds to maintain a powder-rich molten pool despite the heat sink effect of the copper substrate.
The hardness gradient observed across the overlay cross-section is an important practical consideration. The transition from HV 550-600 at the surface to HV 300-380 at the interface represents a natural property gradient that provides both wear resistance at the surface and adequate toughness at the interface. This gradient is actually beneficial for the overall performance of the coating, as it prevents the abrupt property change that would exist if a single homogeneous layer were deposited.
The PTA process is clearly superior to conventional arc welding for this application due to its ability to achieve lower dilution rates (10-20 percent versus 30-50 percent for GMAW or SAW) and produce thinner, more controlled deposit layers. The literature confirms that PTA is the process of choice for nickel-based overlays on copper substrates where low dilution and high overlay quality are required.
This study provides a solid foundation for understanding the metallurgical behavior of nickel-copper overlay systems and offers practical guidance for engineers working on copper component protection in marine, electrical, and chemical processing applications. The systematic approach to process optimization, microstructural characterization, and property verification establishes a methodology that can be applied to other challenging substrate-overlay combinations in the field of thermal spray and weld overlay technology.