Microstructure and Properties of Plasma Clad Nickel-Based Alloy Powder

Introduction and Technical Background

Plasma Transferred Arc (PTA) cladding is one of the most advanced thermal spray and overlay technologies for depositing nickel-based alloy coatings. The technology offers exceptional control over dilution, microstructure, and coating properties, making it suitable for critical applications in aerospace, chemical processing, and nuclear industries. The literature under review investigates the microstructural evolution and mechanical properties of PTA-clad nickel-based alloy powders, with particular attention to the relationship between powder characteristics, process parameters, and final coating performance.

Nickel-Based Alloy Powder Systems

The study examines several nickel-based alloy powder systems commonly used in PTA cladding:

Alloy Designation	Ni (%)	Cr (%)	Mo (%)	Fe (%)	Al (%)	Ti (%)	Typical Application
Inconel 625	58	22	9	5	3	4	High-temperature corrosion
Inconel 718	53	19	9	17	2	1	Structural + corrosion
Monel 400	63	2.5	—	28	—	—	Acid corrosion resistance
Hastelloy C276	52	16	16	5	—	—	Reducing acid environments
Custom Ni-Cr-Mo	55	25	12	5	1	2	Extreme corrosion service

Each alloy system offers a unique combination of corrosion resistance, mechanical strength, and thermal stability. Inconel 625 remains the most widely used PTA alloy due to its excellent combination of high-temperature strength (up to 980°C), outstanding resistance to a wide range of corrosive environments, and good weldability.

Process Parameters and Their Effects

The PTA process parameters significantly influence the final microstructure and properties of the clad layer. The following table summarizes the key parameter ranges and their effects:

Parameter	Typical Range	Effect on Microstructure	Effect on Properties
Arc Current	200–400 A	Higher current → deeper penetration, more dilution	Higher dilution → lower Ni content in deposit
Powder Feed Rate	0.3–0.8 kg/min	Higher rate → thinner, more dilute layer	Lower dilution → better corrosion resistance
Travel Speed	50–200 mm/min	Higher speed → thinner layer, less HAZ	Thinner layer → less distortion
Standoff Distance	8–15 mm	Too close → spatter; too far → poor powder melting	Optimal distance → smooth, dense deposit
Shielding Gas Flow	15–25 L/min	Insufficient flow → oxidation, porosity	Proper flow → clean, oxide-free surface

The dilution rate is the most critical factor in PTA cladding. For nickel-based alloy cladding, a dilution rate below 20% is generally required to maintain the corrosion resistance of the deposit. The study demonstrates that dilution can be controlled through:

Using a higher powder feed rate relative to the arc current.
Employing multi-pass techniques with a thin first pass and thicker subsequent passes.
Using a "sandwich" technique where a sacrificial layer is deposited first and then ground off.
Selecting a powder with a slightly higher alloy content than the target composition to compensate for expected dilution.

Microstructural Characteristics

The PTA-clad microstructure is characterized by several distinctive features:

Columnar grains: The rapid solidification from the molten pool produces a strongly columnar grain structure oriented perpendicular to the substrate. Grain spacing is typically 50–200 μm depending on the cooling rate.
Equiaxed grains: In the upper portion of the deposit, where the cooling rate is lower, equiaxed grains may form. This transition from columnar to equiaxed is important for the mechanical properties of the deposit.
Precipitate phases: For alloys like Inconel 625, fine γ' (Ni₃(Al,Ti)) and γ'' (Ni₃Nb) precipitates form during cooling. These precipitates provide solid solution strengthening and are critical for high-temperature strength.
Segregation bands: Microsegregation of chromium and molybdenum can occur along the interdendritic regions, potentially leading to localized corrosion susceptibility if the segregation exceeds the solubility limit.

Mechanical and Corrosion Properties

The study reports the following properties for PTA-clad Inconel 625 deposits:

Property	As-Clad	After Solution Treatment (1050°C/1h)	After Age Treatment (720°C/8h)
Hardness (HV)	280–320	240–270	350–380
Tensile Strength (MPa)	950–1050	900–1000	1100–1200
Elongation (%)	18–22	20–25	12–15
Pitting Resistance (Ecorr, mV)	+250 to +300	+280 to +320	+260 to +300
Intergranular Corrosion	Pass	Pass (after proper PWHT)	Pass

The solution treatment step is important for eliminating microsegregation and homogenizing the composition. However, excessive solution treatment can cause grain coarsening and reduce the precipitation hardening response. The age treatment at 720°C for 8 hours produces the optimal precipitate distribution for maximum strength.

Common Defects and Quality Control

Defect	Cause	Detection Method	Prevention
Porosity	Trapped gas, incomplete powder melting	RT, UT	Optimize gas flow, ensure full powder melting
Cracking	Thermal stress, HAZ embrittlement	MT, PT	Preheat substrate, reduce travel speed
Lack of fusion	Insufficient heat input	UT, TOFD	Increase current, reduce travel speed
Excessive dilution	Low powder feed rate	Spectroscopic analysis	Increase feed rate, use multi-pass
Surface roughness	Uneven powder distribution	Visual, profilometry	Optimize powder feed, maintain standoff

Engineering Practice and Application

PTA cladding of nickel-based alloys is particularly valuable in applications where:

The component is subject to severe corrosion attack (acidic, reducing, or oxidizing environments).
High-temperature creep resistance is required (up to 980°C for Inconel 625).
The component geometry is complex and difficult to clad by other methods.
Minimal dilution and precise composition control are required.

Typical applications include gas turbine blades, chemical reactor internals, heat exchanger tubes, nuclear fuel handling components, and marine propeller shafts. The technology is also used for repair of damaged components where the original material is a nickel-based alloy or where a nickel-based overlay is required for corrosion protection.

Study Insights and Recommendations

The literature confirms that PTA cladding is a highly effective technology for depositing nickel-based alloy coatings with excellent microstructural control and minimal dilution. The key to success lies in optimizing the process parameters to achieve the target dilution rate, maintaining proper shielding gas coverage to prevent oxidation, and applying appropriate post-weld heat treatment to achieve the desired precipitate distribution. Engineers should be aware that the columnar grain structure inherent to PTA deposits can lead to anisotropic mechanical properties, with lower transverse toughness compared to longitudinal properties. For applications where transverse toughness is critical, a multi-pass technique with varying travel directions can be employed to break up the columnar grain structure and produce a more isotropic microstructure. The overall conclusion is that PTA cladding of nickel-based alloys represents a mature, reliable technology that can deliver coatings with properties exceeding those achievable by alternative methods, provided the process is properly controlled and qualified.