Pulsed TIG Welding Process Parameters on Microstructure Regulation of High-Temperature Nickel-Based Alloy Welds

Literature Overview

This research, published in the Chinese Journal of Mechanical Engineering (2018) by Zhang Xiaohong and colleagues from Southwest Jiaotong University, investigates how pulsed TIG welding parameters influence the microstructure of high-temperature nickel-based alloy welds. Funded by the National Natural Science Foundation of China (Project No. 51504198), this work addresses the critical challenge of controlling solidification microstructure in nickel-based superalloys, which are essential for gas turbine components, nuclear reactor internals, and high-temperature pressure vessel linings.

Core Technical Content

Nickel-based alloys such as Inconel 625, Inconel 600, Monel 400, and Hastelloy C276 exhibit challenging weldability due to their susceptibility to solidification cracking, sensitization, and hot short cracking. The pulsed TIG welding technique offers unique advantages for these alloys through its ability to independently control heat input, penetration depth, and cooling rate through the manipulation of pulse parameters including peak current, base current, pulse frequency, and duty cycle.

Parameter-Microstructure Relationships

The study systematically examines the effects of key pulsed TIG parameters on the resulting weld microstructure:

The peak current primarily determines the maximum temperature achieved in the weld pool, directly influencing the solidification rate and consequently the dendrite arm spacing. Higher peak currents promote coarser dendritic structures with increased inter-dendritic spacing, which can affect both mechanical properties and susceptibility to hot cracking.

Dendrite Arm Spacing and Solidification Mode

The secondary dendrite arm spacing (SDAS) is a critical microstructural parameter that correlates strongly with mechanical properties and crack susceptibility in nickel-based alloys. The relationship between SDAS and cooling rate follows the well-established Kurz-Fisher equation:

SDAS = k · G^(-1/2) · R^(-1/2)

where G is the temperature gradient and R is the solidification rate. Pulsed TIG parameters allow independent manipulation of both G and R, providing a powerful means of controlling SDAS within the desired range of 20-60 μm for optimal properties.

Precipitation Behavior and Phase Stability

High-temperature nickel-based alloys often contain strengthening precipitates such as γ' (Ni₃(Al,Ti)) and δ (Ni₃Nb) phases. The welding thermal cycle can dissolve these precipitates in the fusion zone and cause coarsening in the heat-affected zone (HAZ). The pulsed TIG parameters determine the peak temperature and cooling rate, which directly influence the extent of precipitate dissolution and the subsequent precipitation kinetics during cooling.

Process Optimization Framework

The study employs a systematic approach to parameter optimization, which can be summarized using the PDCA (Plan-Do-Check-Act) framework:

• Plan: Establish target microstructural characteristics (fine equiaxed grains, SDAS < 40 μm, absence of Laves phase)
• Do: Conduct welding trials across the parameter space with controlled variations
• Check: Characterize microstructure through optical microscopy, SEM, EBSD, and TEM
• Act: Refine parameter combinations based on microstructural feedback

Critical Parameter Windows

For Inconel 625 weld overlay applications, the following parameter windows have been identified as producing optimal microstructural characteristics:

• Pulse frequency above 100 Hz promotes grain refinement through repeated nucleation events
• Duty cycle below 30% limits interpass temperature accumulation, reducing grain coarsening
• Peak-to-base current ratio of 4:1 to 6:1 provides sufficient penetration while maintaining thermal control
• Travel speed above 5 mm/s ensures adequate cooling rates for fine microstructure development

Engineering Practice Integration

For bimetal pressure vessel fabrication where nickel-based alloy cladding is applied to carbon steel substrates, the pulsed TIG approach offers significant advantages over conventional DC TIG welding. The reduced heat input minimizes the thermal distortion of thin nickel-based overlay layers and reduces the risk of intermetallic compound formation at the clad-base metal interface.

The process qualification requirements under NB/T 47014 and ASME IX for pulsed TIG welding of nickel-based alloys require demonstration of weldability across the full range of anticipated parameter variations. The microstructural control demonstrated in this research provides the technical foundation for establishing qualified welding procedure specifications (WPS) for critical overlay applications.

Study Insights and Implications

This research demonstrates that pulsed TIG welding provides a powerful means of microstructural control in high-temperature nickel-based alloys, with implications extending to weld overlay cladding applications. The ability to independently manipulate heat input, cooling rate, and interpass temperature through pulse parameter optimization opens new possibilities for achieving the fine, uniform microstructures required for high-temperature pressure vessel service. The key engineering insight is that microstructural control through process parameter optimization can substitute for or reduce the need for expensive post-weld heat treatment, potentially lowering fabrication costs while maintaining or improving component performance.