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Crystals
Volume 16
Issue 1
10.3390/cryst16010003
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Article

Dual-Bifurcation Model and Numerical Analysis of Driving Forces on the Keyhole Boundary in Variable Polarity Plasma Arc Welding

by
Bin Xu
1,
Boyu Xiao
1,
Fan Jiang
1,2,*,
Yongquan Han
2,
Guowei Li
2,
Zhenbang Sun
2,
Shinichi Tashiro
3,
Manabu Tanaka
3 and
Shujun Chen
1
1
College of Mechanical & Energy Engineering, Beijing University of Technology, Beijing 100124, China
2
School of Materials Science and Engineering, Inner Mongolia University of Technology, Hohhot 010051, China
3
Joining and Welding Research Institute, Osaka University, Osaka 5670047, Japan
*
Author to whom correspondence should be addressed.
Crystals 2026, 16(1), 3; https://doi.org/10.3390/cryst16010003 (registering DOI)
Submission received: 14 November 2025 / Revised: 11 December 2025 / Accepted: 16 December 2025 / Published: 21 December 2025
(This article belongs to the Special Issue Welding and Joining of Metallic Materials: Microstructure and Mechanical Properties, 2nd Edition)
Versions Notes

Abstract

Molten pool flow and keyhole status during Variable Polarity Plasma Arc (VPPA) welding directly affect the weld quality and stability. The lack of a clear correlation between them, however, prevents this process approach from being developed further. To investigate the keyhole morphology and liquid metal flow, the experimental examination of fluid flow by the X-ray imaging method and numerical simulation of plasma arc under the effect of the keyhole were carried out. By changing the tungsten electrode setback while keeping all other parameters, it is possible to vary the keyhole status and maintain the consistency of heat input to the base metal. This work establishes a dual-bifurcation flow model to characterize the keyhole molten pool, where the bifurcation point on the keyhole rear wall significantly affects the stability of the keyhole molten pool. The rear wall of the keyhole is divided into three sections from top to bottom, with the arc pressure in the middle section being significantly higher than in the upper and lower sections. As the degree of arc constriction increases—i.e., as arc stiffness or arc force increases—the middle section becomes more vertical. By the calculated distribution of driving forces, the arc pressure has a high possibility of being one of the dominances for the metal flow in keyhole welding of aluminum alloys. Arc pressure is also important for the bifurcation point position, which is closely related to the three welding states: blind keyhole, keyhole, and cutting.
Keywords: variable polarity plasma arc welding; keyhole molten pool; bifurcation point; fluid flow variable polarity plasma arc welding; keyhole molten pool; bifurcation point; fluid flow

Share and Cite

MDPI and ACS Style

Xu, B.; Xiao, B.; Jiang, F.; Han, Y.; Li, G.; Sun, Z.; Tashiro, S.; Tanaka, M.; Chen, S. Dual-Bifurcation Model and Numerical Analysis of Driving Forces on the Keyhole Boundary in Variable Polarity Plasma Arc Welding. Crystals 2026, 16, 3. https://doi.org/10.3390/cryst16010003

AMA Style

Xu B, Xiao B, Jiang F, Han Y, Li G, Sun Z, Tashiro S, Tanaka M, Chen S. Dual-Bifurcation Model and Numerical Analysis of Driving Forces on the Keyhole Boundary in Variable Polarity Plasma Arc Welding. Crystals. 2026; 16(1):3. https://doi.org/10.3390/cryst16010003

Chicago/Turabian Style

Xu, Bin, Boyu Xiao, Fan Jiang, Yongquan Han, Guowei Li, Zhenbang Sun, Shinichi Tashiro, Manabu Tanaka, and Shujun Chen. 2026. "Dual-Bifurcation Model and Numerical Analysis of Driving Forces on the Keyhole Boundary in Variable Polarity Plasma Arc Welding" Crystals 16, no. 1: 3. https://doi.org/10.3390/cryst16010003

APA Style

Xu, B., Xiao, B., Jiang, F., Han, Y., Li, G., Sun, Z., Tashiro, S., Tanaka, M., & Chen, S. (2026). Dual-Bifurcation Model and Numerical Analysis of Driving Forces on the Keyhole Boundary in Variable Polarity Plasma Arc Welding. Crystals, 16(1), 3. https://doi.org/10.3390/cryst16010003

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