Keyhole Behaviour in Low-Power Pulsed Laser-Arc Hybrid Welding

Literature Overview

This 2016 study by Li Chenbin, Shi Jipeng, and Liu Liming from the School of Materials Science and Engineering, Dalian University of Technology, published in the Journal of Mechanical Engineering, investigates the keyhole behaviour during low-power pulsed laser-arc hybrid welding. Funded by the National Natural Science Foundation of China (11375038), this research addresses a fundamental aspect of laser welding physics that has direct implications for process control and weld quality. The keyhole is the primary mechanism of deep penetration in laser welding, and understanding its dynamic behaviour is essential for predicting weld geometry, controlling porosity formation, and optimising process parameters. The focus on low-power pulsed operation is particularly relevant for applications where excessive heat input must be avoided, such as the welding of thin-section stainless steel, aluminium alloys, and dissimilar material joints.

Core Technical Content

The authors employ high-speed imaging and numerical simulation to characterise the dynamic behaviour of the keyhole during pulsed laser-arc hybrid welding. The keyhole formation mechanism involves the balance between the laser energy input, which drives the formation of a deep, narrow cavity through vaporisation of the workpiece material, and the surface tension and hydrodynamic forces that tend to collapse the keyhole. In pulsed operation, the keyhole undergoes periodic expansion and contraction, creating a unique fluid dynamic environment in the weld pool that differs significantly from continuous-wave laser welding.

The arc interaction with the keyhole introduces additional complexity. The arc plasma provides supplementary heat input that modifies the keyhole geometry and depth, while the electromagnetic forces from the arc current influence the weld pool flow patterns. The authors demonstrate that the laser-arc interaction creates a stable keyhole regime that is less prone to collapse than either process used alone, which is particularly beneficial for achieving consistent penetration in thin-section materials.

Keyhole Dynamics and Process Windows

The research reveals that the arc plasma acts as a stabilising influence on the keyhole, preventing the periodic collapse that often leads to porosity in pulsed laser welding alone. The electromagnetic stirring from the arc current enhances the fluid flow in the weld pool, promoting the escape of entrapped gas bubbles and reducing porosity. The authors identify a critical welding speed range (0.8-2.5 m/min) below which the keyhole becomes overly deep and unstable, and above which the keyhole becomes too shallow to achieve adequate penetration.

Numerical Simulation and Experimental Validation

The numerical model developed by the authors couples the laser energy deposition, arc heat input, multiphase flow in the weld pool, and keyhole dynamics in a unified framework. The model predicts the keyhole geometry, temperature field, and flow field with good agreement with experimental measurements. The simulation reveals that the keyhole depth is primarily controlled by the balance between the laser power density and the surface tension, while the keyhole width is influenced by the arc current and the laser-arc gap. The model also predicts the formation of a recirculation zone below the keyhole that promotes mixing of the weld pool and uniform solidification.

The experimental validation using high-speed imaging (up to 10,000 fps) captures the dynamic keyhole behaviour during individual pulses. The images show that the keyhole expands rapidly during the laser pulse, reaches a maximum depth, and then contracts during the pulse-off period. The arc maintains a continuous heat input that prevents complete keyhole collapse between pulses, creating a quasi-steady keyhole state that is ideal for consistent penetration.

Engineering Implications for Cladding and Overlay Applications

For cladding and overlay applications, where the penetration depth must be controlled to avoid excessive dilution of the base material, the low-power pulsed laser-arc hybrid process offers significant advantages. The keyhole depth can be precisely controlled by adjusting the laser power and pulse parameters, allowing for overlay layers with controlled dilution rates of 5-20%. The reduced heat input minimises the HAZ width and the risk of base material degradation, which is critical when overlaying corrosion-resistant or wear-resistant alloys on carbon steel or low-alloy steel substrates.

The process is particularly suitable for the overlay of nickel-based alloys (Inconel 625, Hastelloy C276) and stainless steels (308L, 316L) on carbon steel pressure vessels, where the dilution rate must be minimised to maintain the corrosion resistance of the overlay layer. The low power density reduces the risk of cracking in the overlay layer, which is particularly important for nickel-based alloys that are susceptible to solidification cracking due to their narrow freezing range.

Key Questions and Reflections

The research raises important questions about the scalability of the keyhole control strategies from laboratory conditions to production environments. The high-speed imaging and numerical simulation provide detailed insights into the keyhole dynamics, but the practical implementation of these insights requires robust process monitoring and control systems that can maintain the keyhole stability under varying conditions. The authors acknowledge that the process is sensitive to material properties, surface condition, and alignment tolerances, and that additional work is needed to develop adaptive control strategies that can compensate for these variations in real-time.

Study Insights and Engineering Value

This research provides fundamental insights into the keyhole dynamics of pulsed laser-arc hybrid welding that have direct implications for process optimisation and weld quality control. The key finding is that the arc interaction stabilises the keyhole and reduces porosity formation, which is particularly beneficial for thin-section and dissimilar material applications. For engineers involved in cladding and overlay operations, the low-power pulsed hybrid process offers a valuable tool for achieving controlled dilution and minimised HAZ, which are critical for maintaining the performance of the overlay layer. The work underscores the importance of understanding the fundamental physics of the welding process as a prerequisite for achieving consistent, high-quality results in production environments, and demonstrates that even at low power levels, the laser-arc hybrid configuration offers synergistic benefits that neither process can achieve alone.