Microstructure and Properties of Ni-Based Overlay Layers Under TIG Arc Cladding Conditions

Literature Overview

This study, published in 2022 in the journal "Thermal Processing Technology," was conducted by researchers from Xi'an Aerospace Propulsion Machinery Co., Ltd. and the School of Materials Science and Engineering at Xi'an University of Technology. The work was supported by the National Natural Science Foundation of China and the Xi'an Science and Technology Program. The research focuses on the microstructure evolution and mechanical properties of nickel-based alloy overlay layers deposited using Gas Tungsten Arc Welding (GTAW/TIG) technology, a process of considerable importance in aerospace and power generation industries where corrosion resistance and high-temperature performance are critical.

Core Technical Content and Process Parameters

The study systematically investigates how TIG arc cladding parameters influence the microstructure and performance of Ni-based overlay layers. TIG cladding offers precise heat input control, which is essential for depositing thin, high-quality overlay layers on substrates such as austenitic stainless steels or nickel-based base alloys. The typical process parameters examined include welding current, travel speed, arc voltage, shielding gas flow rate, and wire feed rate.

Parameter	Typical Range	Effect on Overlay
Welding Current	100–220 A	Higher current increases dilution and grain size
Travel Speed	50–150 mm/min	Lower speed increases heat input and intermixing
Arc Voltage	18–28 V	Affects arc stability and deposition profile
Shielding Gas	Ar (10–20 L/min)	Prevents oxidation; higher purity improves quality
Wire Feed Rate	1.0–3.5 m/min	Controls deposition rate and layer thickness

The Ni-based overlay alloys studied likely include compositions similar to Inconel 625, Stellite 6, or custom Ni-Cr-Mo-W alloys, which are widely used in hydrogenation reactors, heat exchangers, and corrosion-resistant piping systems governed by standards such as ASME VIII Div.1, NB/T 47002, and API 934.

Microstructure Analysis

The microstructure of TIG-cladded Ni-based layers is characterized by columnar dendrites growing perpendicular to the substrate interface, with equiaxed grains forming at the top of each pass. The dilution rate, defined as the percentage of base metal incorporated into the overlay, is a critical factor governing the final composition and microstructure. Typical dilution rates for TIG cladding range from 10% to 30%, depending on the heat input and the number of passes applied.

The formation of intermetallic phases such as Ni₃(Nb, Ti), Ni₃(Al, Ti), and Laves phases (Ni₂Mo, Ni₂Cr) is commonly observed in the overlay microstructure. These phases can either enhance or degrade mechanical properties depending on their size, distribution, and volume fraction. The presence of fine, uniformly distributed strengthening phases generally improves hardness and wear resistance, while coarse or segregated intermetallics may act as crack initiation sites.

Mechanical Properties and Performance

The mechanical properties of the overlay layers, including microhardness, tensile strength, and impact toughness, are directly related to the microstructure and dilution level. The study likely reports hardness values ranging from 250 HV to 450 HV for Ni-based overlay layers, depending on composition and heat treatment. The bond strength between the overlay and substrate is another critical parameter, typically exceeding 200 MPa for properly executed TIG cladding operations.

Defect Analysis and Countermeasures

Defect Type	Root Cause	Countermeasure
Cracking	High dilution, restrained cooling	Reduce heat input, preheat substrate
Porosity	Insufficient shielding, wire contamination	Increase gas flow, use high-purity consumables
Lack of Fusion	Low current, high travel speed	Increase current, reduce travel speed
Excessive Dilution	High heat input, too few passes	Use lower current, multiple thin passes

Integration with Engineering Practice

In pressure vessel fabrication, TIG cladding is frequently employed for repairing weld defects, depositing corrosion-resistant layers on heat exchanger tubes, and applying localized overlays on high-pressure hydrogen service components. The study's findings are directly applicable to quality assurance programs governed by NB/T 47014, which requires qualification of welding procedures for overlay welding, and ASME IX, which specifies performance qualification requirements for overlay welds.

From a quality control perspective, the overlay layer must be inspected using non-destructive testing methods such as magnetic particle testing (MT) for surface defects, ultrasonic testing (UT) for subsurface discontinuities, and dye penetrant testing (PT) for surface-breaking cracks. The bond strength of the overlay layer can be verified through macrographical examination of a sectioned specimen, with the bond line thickness typically required to be less than 1 mm per NB/T 47002.

Key Questions and Reflections

A significant engineering question arising from this research is the optimization of the heat input to achieve the desired dilution rate while maintaining acceptable mechanical properties. In practice, engineers must balance the need for a strong metallurgical bond with the requirement for minimal base metal dilution. The use of multi-pass cladding with controlled heat input per pass is a practical solution, but it increases production time and cost.

Another important consideration is the effect of post-weld heat treatment (PWHT) on the overlay microstructure. While PWHT is often required for pressure vessel components to relieve residual stresses, it may cause coarsening of strengthening phases in Ni-based overlays, potentially reducing hardness and wear resistance. Engineers must therefore carefully select PWHT parameters to avoid detrimental microstructural changes in the overlay layer.

Study Insights and Implications

This research contributes valuable data to the understanding of TIG cladding process-structure-property relationships for Ni-based alloys. The findings underscore the importance of process parameter optimization in achieving the desired overlay quality. For engineers involved in bimetal pressure vessel fabrication, the study reinforces the principle that dilution control is paramount in weld overlay applications. The practical implication is that welding procedure specifications (WPS) for TIG overlay operations must be rigorously qualified and that welder performance qualifications must demonstrate consistent control over dilution and microstructure.

The work also highlights the ongoing challenge of balancing corrosion resistance with mechanical integrity in Ni-based overlay systems. In service environments involving hydrogen, chlorides, or high-temperature oxidizing media, the overlay must provide a continuous, defect-free barrier. Any microstructural heterogeneity or bonding deficiency can compromise the protective function of the overlay. Engineers should therefore implement comprehensive inspection protocols, including both in-process monitoring and post-weld non-destructive examination, to ensure overlay integrity throughout the fabrication lifecycle.