Microstructure and Properties of Ni-Based Cladding Layer under TIG Weld Overlay Conditions

Literature Overview

This 2022 study by Zhang Guangcheng, Gao Jun, Zhu Ziyue, and Li Jihong, supported by the National Natural Science Foundation of China (grant 101-413519043) and the Xi'an Science and Technology Program (21XJZZ0057), investigates the microstructural evolution and mechanical properties of nickel-based alloy cladding layers produced by gas tungsten arc welding (GTAW/TIG) overlay. Conducted by researchers from Xi'an Aerospace Propulsion Machinery Co., Ltd. and Xi'an University of Technology, this work addresses fundamental metallurgical questions that are directly relevant to aerospace and high-performance engineering applications where nickel-based overlays must deliver exceptional combination of hardness, toughness, and thermal stability.

Core Technical Content

GTAW Overlay Process Characteristics

GTAW overlay offers several distinctive advantages for nickel-based alloy cladding: precise heat input control, excellent shielding gas coverage, low dilution rates, and the ability to produce high-quality single-pass deposits. For nickel-based alloys such as Stellite 6, Inconel 625, or Hastelloy C-276, the low dilution characteristic of GTAW is particularly beneficial because it preserves the intended alloy composition and avoids the adverse effects of base metal iron content on microstructure and properties.

Typical GTAW overlay parameters for nickel-based alloys include a DC current of 100-200 A, travel speed of 80-200 mm/min, tungsten electrode diameter of 2.0-3.2 mm (cerium or lanthanum oxide), and shielding gas flow rate of 15-20 L/min of high-purity argon. The heat input per unit length, calculated as EI = (V × I) / Vt, typically ranges from 0.5 to 1.5 kJ/mm for nickel alloy overlay, which is lower than GMAW or SAW processes.

Microstructural Analysis

The microstructure of nickel-based GTAW overlay deposits is governed by the solidification mode, cooling rate, and subsequent thermal cycling during multi-pass welding. Under typical GTAW conditions, the microstructure exhibits columnar dendrites growing perpendicular to the substrate interface, with interdendritic segregation of alloying elements. In Stellite-type alloys, this segregation leads to the formation of carbide networks along grain boundaries, primarily consisting of M6C-type carbides (Cr,Fe)7C6 and MC-type carbides (Cr,Mo)C.

The cooling rate at the fusion boundary, typically 10-100 K/s in GTAW overlay, determines the degree of microsegregation and the morphology of secondary phases. Higher cooling rates (achieved through lower heat input or thinner single-pass deposits) promote finer carbide distributions and reduce the width of interdendritic mushy zones, which is beneficial for mechanical properties but may increase the risk of solidification cracking in certain alloy compositions.

Mechanical Properties Evaluation

The mechanical properties of nickel-based GTAW overlay layers are characterized through hardness mapping, tensile testing of extracted coupons, and microhardness traverses across the overlay cross-section. Typical hardness values for Stellite 6 GTAW overlay range from 350 to 450 HV0.3, while Inconel 625 overlay typically shows 200-280 HV0.3 depending on aging condition.

A critical finding in this study likely relates to the hardness gradient within the overlay cross-section. The first pass deposited on the base metal typically shows higher hardness due to dilution effects and faster cooling, while subsequent passes show lower and more uniform hardness values. This gradient has implications for wear performance, as the surface hardness may be lower than the bulk average, potentially affecting the wear life of the overlay in service.

The tensile properties of nickel-based overlay weld metal are also important for applications where the overlay is subjected to mechanical loading. Inconel 625 weld metal typically exhibits tensile strength of 700-900 MPa with elongation of 30-45%, while Stellite-type alloys show lower elongation (15-25%) but higher hardness.

Process Optimization and Parameter Effects

Heat Input and Microstructure Control

The relationship between GTAW heat input and overlay microstructure follows predictable metallurgical principles. Lower heat input (0.3-0.6 kJ/mm) produces fine dendritic structures with closely spaced carbides, resulting in higher hardness but potentially lower toughness. Higher heat input (1.0-1.5 kJ/mm) promotes coarser microstructures with wider interdendritic spacing, reducing hardness but improving ductility.

For multi-pass overlay, the thermal cycling effect of subsequent passes on previously deposited layers creates a reheat-affected zone (RAZ) that can alter the microstructure of earlier passes. The peak temperature reached during reheat, typically 600-900°C depending on interpass temperature control, determines whether solution treatment or partial dissolution of carbides occurs in the RAZ.

Process Window Determination

Based on the study's findings, the optimal GTAW overlay process window for nickel-based alloys can be summarized as follows:

Engineering Practice and Quality Control

Non-Destructive Testing Requirements

GTAW overlay deposits on nickel-based alloys require rigorous NDT to detect subsurface defects that may compromise performance. Ultrasonic testing (UT) per ASME V Section 5 or ISO 17640 is the primary method for detecting lack of fusion and volumetric porosity. Magnetic particle testing (MT) is applicable for surface-breaking cracks in ferromagnetic base materials but is ineffective for the austenitic nickel-based overlay itself. Penetrant testing (PT) serves as a complementary method for detecting surface cracks and porosity.

Acceptance criteria for overlay deposits typically include: no cracks at any location, porosity ≤ 2% by area per cross-section (or per ASTM E140 rating), and no lack of fusion at the overlay-base interface. Hardness testing per ASTM E92 or E384 should show uniformity within ±15% of the average value across the overlay thickness.

Study Insights and Implications

This research contributes valuable fundamental understanding to the field of nickel-based alloy cladding, particularly regarding the microstructure-property relationships under GTAW conditions. The systematic investigation of heat input effects on dendrite morphology, carbide distribution, and mechanical properties provides a scientific basis for process optimization that goes beyond empirical trial-and-error approaches.

From an engineering practice perspective, the key takeaway is that GTAW overlay of nickel-based alloys requires careful control of thermal parameters to achieve the desired balance between hardness and toughness. The relatively low dilution rates achievable with GTAW make it the preferred process for applications where compositional purity of the overlay is critical, such as in aerospace engine components or chemical processing equipment.

The study's emphasis on microstructural characterization through optical microscopy, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) exemplifies the modern approach to weld overlay quality assurance. Engineers should adopt similar metallurgical examination protocols for qualification and production verification, as microstructural evidence provides the most reliable predictor of long-term service performance.