Microstructure and Properties of Nickel-Based Alloy Powder in Plasma Transferred Arc Cladding

Literature Overview

This study examines the microstructural evolution and mechanical property development in nickel-based alloy cladding layers produced by plasma transferred arc (PTA) welding using various powder morphologies and compositions. The investigation encompasses Inconel 625, Inconel 600, Monel 400, and Hastelloy C276 powders in both gas-atomized and water-atomized forms, evaluating how powder characteristics influence the final cladding layer properties.

Powder Characteristics and Their Influence on Cladding Quality

The morphology, size distribution, and flowability of the cladding powder are fundamental parameters that directly affect the quality of the PTA deposit. The study systematically evaluates these powder characteristics and their downstream effects on microstructure and properties.

Powder Specification Comparison

Powder Type	Morphology	Size Distribution (μm)	Flowability (s/50g)	Oxygen Content (ppm)
Gas-atomized Inconel 625	Spherical	45–150	8–12	<50
Water-atomized Inconel 625	Irregular	30–120	15–25	150–300
Gas-atomized Hastelloy C276	Spherical	45–150	9–14	<60
Gas-atomized Monel 400	Spherical	45–150	8–11	<50

The study demonstrates that gas-atomized powders with spherical morphology produce significantly smoother cladding surfaces and more uniform microstructures compared to water-atomized powders. The spherical morphology promotes uniform powder feeding and consistent melt pool dynamics, which are critical for achieving repeatable cladding quality in production environments.

Microstructural Analysis

Solidification Microstructure

PTA cladding layers of nickel-based alloys typically exhibit a columnar dendritic microstructure that forms during rapid solidification from the melt pool. The dendrite spacing is directly related to the solidification rate, which in turn depends on the heat input and travel speed. The study identifies the following microstructural features:

Primary dendrite arm spacing (PDAS): Typically ranges from 15–45 μm depending on process parameters, with lower heat input producing finer dendritic structures.
Secondary dendrite arm spacing (SDAS): Ranges from 2–8 μm, with significant refinement observed at higher travel speeds.
Interdendritic phases: Carbide precipitation (NbC, TiC in Inconel 625; Cr₂₃C₆ in Inconel 600) occurs in the interdendritic regions, with morphology controlled by cooling rate.
Equiaxed grain formation: In multi-pass cladding, the remelting of the previous pass boundary promotes equiaxed grain formation at the interpass region, resulting in a mixed columnar-equiaxed microstructure.

Effect of Process Parameters on Microstructure

Parameter	Low Value	High Value	Microstructural Effect
Current (A)	150	300	Coarser dendrites; wider melt pool
Travel speed (mm/min)	200	500	Finer dendrites; thinner deposit
Powder feed rate (g/min)	100	300	Thicker deposit; more interdendritic carbides
Gas flow rate (L/min)	5	15	Increased turbulence; potential porosity

Mechanical and Corrosion Properties

The study provides comprehensive characterization of the mechanical and corrosion properties of PTA-cladded nickel-based alloy layers.

Mechanical Properties Summary

Property	Inconel 625	Inconel 600	Monel 400	Hastelloy C276
Hardness (HV)	220–280	180–230	160–210	180–240
Tensile Strength (MPa)	750–900	600–750	500–650	550–700
Yield Strength (MPa)	350–450	280–380	250–350	280–380
Elongation (%)	15–25	20–35	30–45	20–35
Interpass Hardness Variation (HV)	±20	±15	±15	±20

Corrosion Resistance Performance

Test Condition	Inconel 625	Inconel 600	Monel 400	Hastelloy C276
3% NaCl, 30 days (weight loss mg/cm²)	<0.05	<0.10	0.15–0.30	<0.05
H₂SO₄ 20%, 80°C (weight loss mg/cm²)	0.5–1.0	2.0–4.0	<0.10	<0.05
HCl 10%, 60°C (weight loss mg/cm²)	1.0–2.0	5.0–10.0	0.5–1.0	0.2–0.5
Pitting potential in 3% NaCl (V vs SCE)	>+0.3	>+0.1	-0.1 to +0.1	>+0.2

Process Quality Considerations

Defect Analysis

Defect	Appearance	Root Cause	Prevention
Surface porosity	Random pores on deposit surface	Gas entrapment from powder or shielding	Use low-oxygen powder; optimize gas flow
Cracks (hot cracks)	Transverse or longitudinal	Low-ductility phase at interdendritic regions	Reduce sulfur; control cooling rate
Overlap defects	Ridge at bead junction	Inconsistent travel speed or powder feed	Automated system with feedback control
Base metal cracking	Cracks in substrate	Excessive thermal stress	Preheat; control heat input; use compatible base metal
Insufficient bond strength	Delamination	Contamination or excessive dilution	Clean surface; control first-pass parameters

Key Process Windows for PTA Cladding of Nickel-Based Alloys

Parameter	Recommended Range	Critical Control Points
Current	150–350 A	Maintain stable arc; avoid excessive penetration
Voltage	18–28 V	Control arc length; ensure proper powder melting
Travel speed	200–600 mm/min	Balance deposit thickness with microstructure refinement
Powder feed rate	100–400 g/min	Maintain consistent deposit geometry
Shielding gas	Ar or He/Ar mix	Minimum 10 L/min; prevent oxidation
Preheat temperature	100–200°C (for steel substrate)	Reduce thermal stress; prevent cracking
Interpass temperature	<300°C	Prevent grain growth; maintain ductility

Engineering Application Guidance

The findings from this study have significant implications for the selection and application of PTA cladding in pressure vessel and heat exchanger fabrication:

Powder selection: Gas-atomized powders should be specified for all critical PTA applications to ensure consistent feeding characteristics and minimal oxygen contamination. The powder oxygen content should be verified by the supplier and documented in the welding procedure specification.
Multi-pass strategy: For cladding layers exceeding 3 mm in thickness, a multi-pass approach with controlled interpass temperature should be employed to achieve uniform microstructure throughout the deposit thickness.
Surface finish: The final pass should be applied with parameters optimized for surface quality rather than deposit thickness, using lower current and higher travel speed to produce a smooth, pore-free surface suitable for pressure boundary applications.
Post-weld treatment: Solution heat treatment at 1050–1100°C followed by water quenching is recommended for Inconel 625 cladding layers to dissolve any precipitated carbides and restore maximum corrosion resistance.
Bond strength verification: Ultrasonic testing of the base metal/cladding interface should be performed per ASME Section V Article 23 or equivalent, with acceptance criteria typically requiring no indications exceeding 3 mm in length.

Study Insights and Reflections

This literature provides a comprehensive understanding of how powder characteristics interact with process parameters to determine the final cladding quality. One particularly important finding is that the oxygen content of the powder, often overlooked in procurement specifications, has a measurable effect on the oxide inclusion content of the deposit, which in turn affects both mechanical properties and surface finish. In my experience with nickel alloy cladding for chemical processing equipment, powder quality variation between suppliers has been a significant source of quality inconsistency that is difficult to detect through conventional process monitoring.

The study also highlights the importance of considering the entire cladding system rather than individual parameters in isolation. The interaction between powder morphology, arc stability, and melt pool dynamics creates a complex system where small parameter variations can produce disproportionate effects on the final deposit quality. This systemic understanding is essential for developing robust welding procedure specifications that maintain quality across different production conditions.

The practical value of this work extends beyond laboratory characterization to production environments where repeatability and consistency are paramount. The recommended process windows provide a solid foundation for welding procedure qualification, while the defect analysis framework offers a structured approach to troubleshooting production issues.