Ni60A-WC Composite Coating by Plasma Overlay Welding with Dual Powder Feeding

Overview and Background

Plasma transferred arc (PTA) welding with dual powder feeding represents an advanced surface engineering technique capable of producing high-quality composite coatings with excellent metallurgical bonding and controlled microstructure. This study note examines the development of a Ni60A-WC composite coating using dual powder feeding technology, analyzing the microstructural evolution, mechanical properties, and performance optimization strategies.

Process Description and Technology

Plasma overlay welding with dual powder feeding involves the simultaneous introduction of two different powders into the plasma arc, allowing for the creation of composite coatings with tailored properties. The process offers superior control over coating composition, dilution, and microstructure compared to conventional single-powder PTA welding.

Process Configuration

Component	Specification	Function
Plasma torch	Non-transferred / transferred arc	Heat source
Powder feeder 1	Ni60A alloy powder	Matrix material
Powder feeder 2	WC (tungsten carbide) powder	Reinforcement phase
Shielding gas	Argon	Arc stabilization, oxidation prevention
Powder delivery	Coaxial / side-stream	Powder injection into arc
Substrate	Carbon steel / low-alloy steel	Base material

Process Parameters

Parameter	Range	Optimized Value	Notes
Arc current	150-300 A	220 A	Controls heat input
Arc voltage	25-35 V	30 V	Arc length control
Travel speed	100-300 mm/min	200 mm/min	Deposition rate control
Ni60A feed rate	50-150 g/min	100 g/min	Matrix formation
WC feed rate	20-80 g/min	50 g/min	Reinforcement content
Powder ratio (Ni60A:WC)	2:1 to 5:1	2:1	Controls composite properties
Preheat temperature	150-300 °C	200 °C	Reduces cracking
Interpass temperature	<300 °C	200 °C	Prevents overheating

Microstructural Analysis

The Ni60A-WC composite coating exhibits a complex microstructure resulting from the interaction between the Ni-based matrix and WC reinforcement particles:

Matrix Microstructure

The Ni60A matrix solidifies as a dendritic structure with primary austenite (γ) and secondary carbide phases
Grain size ranges from 50-150 μm, depending on cooling rate and powder particle size
The matrix contains dissolved alloying elements (Cr, Mo, Fe) that contribute to solid solution strengthening

WC Particle Behavior

WC particles undergo several transformations during the PTA process:

WC State	Condition	Result
Intact WC	Low heat input, rapid cooling	Retains original morphology, high hardness
Partially dissolved	Moderate heat input	Core-shell structure with Ni-rich rim
Fully dissolved	High heat input, slow cooling	Complete dissolution, carbide precipitation on cooling
Reaction products	Extended residence time	Formation of W2C, Ni3W, Ni7W6

Composite Microstructure Features

WC particles are distributed throughout the Ni60A matrix with a slight gradient toward the surface
Particle size distribution ranges from 5-50 μm, with larger particles near the surface due to reduced dissolution
Particle-matrix interfaces show good metallurgical bonding with minimal interfacial reaction
No significant porosity or cracking is observed when process parameters are optimized

Mechanical Properties and Performance

Hardness Distribution

Position	Hardness (HV)	Phase Composition
Surface	1200-1500	Ni60A matrix + intact WC particles
Mid-layer	900-1200	Ni60A matrix + partially dissolved WC
Dilution zone	500-700	Mixed Ni-Fe alloy + reduced WC content
Base metal	200-250	Original substrate structure

Wear Resistance Performance

Wear Test	Base Material	Ni60A Only	Ni60A-WC Composite	Improvement Factor
Dry sliding (steel pin)	2000 mg	300 mg	150 mg	13.3x
Abrasive (SiC paper)	3000 mg	400 mg	200 mg	15.0x
Corrosive wear (3.5% NaCl)	5000 mg	800 mg	400 mg	12.5x
High-temperature wear (400°C)	4000 mg	600 mg	300 mg	13.3x

The wear resistance improvement is attributed to the combined effects of hard WC particles providing abrasive resistance, the Ni60A matrix providing ductility and crack resistance, and the metallurgical bonding ensuring particle retention during wear.

Process Optimization Strategies

Powder Ratio Optimization

The Ni60A:WC powder ratio significantly influences coating properties:

Ratio (Ni60A:WC)	Hardness	Wear Resistance	Cracking Tendency
5:1	800 HV	Moderate	Low
3:1	1000 HV	Good	Low-Moderate
2:1	1200 HV	Excellent	Moderate
1.5:1	1400 HV	Very High	High
1:1	1500 HV	High	Very High

The optimal ratio of 2:1 provides the best balance between hardness, wear resistance, and crack resistance. Higher WC content increases hardness but also increases cracking susceptibility due to the brittle nature of WC and the high thermal expansion mismatch.

Layer Thickness Control

Multi-layer deposition with controlled thickness per pass improves coating quality:

Layer Configuration	Total Thickness	Quality
Single layer	2-3 mm	Acceptable, higher porosity risk
2 layers	3-5 mm	Good, improved bonding
3 layers	4-7 mm	Excellent, uniform properties
4+ layers	5-10 mm	Very good, requires interpass temperature control

Engineering Applications

Ni60A-WC composite coatings are particularly suitable for the following applications:

Oil and gas industry: Downhole tools, valve seats, pump impellers, drill collars
Mining industry: Crusher components, conveyor rollers, bucket teeth, chutes
Power generation: Boiler tubes, heat exchanger tubes, fan blades, turbine components
Chemical processing: Pump impellers, valve components, mixer shafts, agitator blades
Marine engineering: Propeller blades, shafts, rudders, seawater pump components

Study Insights and Implications

The plasma overlay welding process with dual powder feeding for Ni60A-WC composite coatings represents a powerful surface engineering solution for enhancing wear resistance in demanding industrial applications. The key insight from this study is that the dual powder feeding technology enables precise control over the composite microstructure, allowing engineers to optimize the balance between hardness, toughness, and wear resistance for specific service conditions.

The metallurgical bonding between the WC particles and Ni60A matrix is critical for coating performance. Unlike thermal spray processes where particles are mechanically bonded to the substrate, PTA welding achieves true metallurgical bonding, resulting in superior adhesion and wear resistance. The in-situ reaction between WC and the Ni-based matrix creates a composite structure with synergistic properties that exceed those of either material alone.

However, the process also requires careful attention to several critical factors. The high cost of WC powder, the potential for cracking due to thermal stress, and the need for precise process control represent challenges that must be addressed in engineering practice. Additionally, the dilution rate must be carefully monitored to ensure adequate WC content in the coating, as excessive dilution reduces the reinforcement effectiveness.

The dual powder feeding technology offers significant advantages over conventional single-powder PTA welding, including the ability to create composite coatings with tailored properties, improved wear resistance, and enhanced durability. For critical applications where coating performance directly impacts equipment reliability and safety, the investment in PTA welding with dual powder feeding is justified by the extended service life and reduced maintenance costs.