Ultrasonic Pulse TIG Welding Power Supply Topology and Arc Welding Applicability

Literature Overview

This paper published in the Journal of Beijing University of Aeronautics and Astronautics (2009, Vol. 35, No. 1) by Qi Bojin et al. from Beijing University of Aeronautics and Astronautics presents the design, topology, and operational principles of an ultrasonic direct current pulse TIG welding power supply. The developed power supply can output pulse currents with different frequencies, each featuring steep rising and falling edges with current change rates exceeding 50 A/μs. The authors demonstrate that the power supply is applicable to both thin sheet and medium-to-thick plate welding, and they provide metallurgical evidence through microstructural comparison of 1Cr18Ni9Ti austenitic stainless steel welds produced by conventional DC TIG and ultrasonic pulse DC TIG welding.

Power Supply Topology and Technical Characteristics

The ultrasonic pulse TIG power supply operates on the principle of ultra-high-frequency current modulation, where the pulse frequency exceeds the audible range (typically above 20 kHz). The key technical specifications include:

The steep current rise and fall times are critical for achieving rapid arc ignition and extinction cycles, which in turn influence weld pool dynamics, arc stability, and microstructural refinement. The ability to independently adjust base current, peak current, pulse frequency, and duty ratio provides extensive process flexibility.

Metallurgical Effects on 1Cr18Ni9Ti Stainless Steel

The microstructural comparison between conventional DC TIG and ultrasonic pulse DC TIG welds reveals significant grain refinement in the heat-affected zone (HAZ). Specifically, the coarse grain zone (CGZ) is noticeably narrowed in ultrasonic pulse TIG welds, and grain refinement is pronounced throughout the weld zone.

The grain refinement mechanism is attributed to the rapid thermal cycling induced by ultrasonic pulse frequency. The steep current rise creates a high-intensity arc that rapidly heats the base metal, while the steep current fall allows rapid cooling. This rapid heating-cooling cycle promotes nucleation and suppresses grain growth, resulting in a finer microstructure with potentially improved mechanical properties.

Engineering Applicability Assessment

The demonstrated applicability to both thin and thick plates is particularly noteworthy. For thin sheet welding (common in aerospace and food processing equipment piping), the ultrasonic pulse capability allows precise heat input control, minimizing distortion and burn-through. For medium-thick plates (relevant to pressure vessel and pipeline applications), the high peak current ensures adequate penetration while the pulse modulation maintains arc stability.

From a welding procedure development perspective, the multiple adjustable parameters require systematic process optimization. A Taguchi experimental design or response surface methodology approach would be appropriate to identify the optimal parameter combinations for specific material-thickness combinations. The steep di/dt capability also raises considerations for power supply design, including switching device selection, electromagnetic compatibility, and arc circuit impedance matching.

Key Reflections and Study Insights

The ultrasonic pulse frequency concept represents a significant advancement over conventional low-frequency pulse TIG welding (typically 1–50 Hz). The transition to ultrasonic frequencies (above 20 kHz) fundamentally changes the arc plasma dynamics, as the arc cannot follow the current modulation at such high frequencies in the traditional sense. Instead, the pulse effects manifest through cumulative thermal cycling and modified plasma column behavior.

The grain refinement observed in austenitic stainless steel is particularly valuable for pipeline applications, where 304L and 316L stainless steel piping per ASME B31.3 and ASTM A312/A270 specifications require controlled microstructures for corrosion resistance and mechanical performance. A narrower coarse grain zone in the HAZ translates to reduced susceptibility to intergranular corrosion and improved fatigue resistance.

However, I note that the study focuses primarily on microstructural observations and does not provide comprehensive mechanical property data or corrosion resistance testing. For engineering qualification, complementary testing per applicable codes (ASME Section IX, EN ISO 15614-1) would be necessary to confirm that the grain refinement translates to improved or at least maintained mechanical and corrosion performance.