Keyhole Porosity Formation Mechanism in Fiber Laser-TIG Hybrid Welding

Literature Overview

This paper by Zhao Zhenjia, Zhu Baoqi, Zou Jianglin, Guo Shihui, and Kong Hua from Beijing University of Technology (Beijing Key Laboratory of Advanced Manufacturing with High-Power and Ultrafast Lasers), published in Chinese Journal of Lasers in 2023 under National Natural Science Foundation of China (51875007) and Beijing Natural Science Foundation (3222004), provides a comprehensive analysis of the formation mechanism of keyhole-type porosity in fiber laser-TIG hybrid welding. Keyhole porosity is one of the most persistent and challenging defects in laser-assisted welding processes, and understanding its formation mechanism is essential for achieving high-quality welds in both structural welding and cladding applications.

Keyhole Porosity: Definition and Classification

Keyhole porosity in laser welding refers to elongated, wormhole-shaped pores that form within the weld nugget, typically aligned with the welding direction. These pores are distinct from gas porosity (spherical pores caused by gas entrapment) and are directly related to the instability of the keyhole cavity during the welding process. In hybrid laser-TIG welding, the interaction between the laser-induced keyhole and the TIG arc creates a complex fluid dynamic environment that can exacerbate keyhole porosity formation.

Porosity Type	Shape	Formation Mechanism	Typical Location
Keyhole porosity	Elongated, wormhole-shaped	Keyhole collapse or gas entrainment	Center of weld nugget
Gas porosity	Spherical	Dissolved gas nucleation and growth	Throughout weld metal
Slag inclusion porosity	Irregular	Slag entrapment during multi-pass welding	Between weld passes
Hydrogen porosity	Small, spherical	Hydrogen absorption from moisture	Near surface or center

Formation Mechanism Analysis

The researchers identified several key mechanisms for keyhole porosity formation in fiber laser-TIG hybrid welding:

Keyhole Instability

The keyhole cavity, formed by vaporization of the base metal under the intense laser beam, is inherently unstable. Pressure fluctuations within the keyhole, caused by variations in laser power density, welding speed, and material properties, can lead to partial or complete collapse of the cavity. When the keyhole collapses, the molten metal rapidly fills the cavity, trapping gas bubbles that were present within the keyhole. These trapped bubbles solidify as elongated pores in the direction of welding.

Arc-Plasma Interaction

In hybrid laser-TIG welding, the TIG arc interacts with the laser-induced plasma plume. This interaction can create turbulence and pressure waves within the keyhole, promoting instability. The arc plasma, which has a lower density than the surrounding shielding gas, can flow into the keyhole region and disturb the vaporization-remelting equilibrium that maintains the keyhole stability.

Weld Pool Surface Tension Effects

The surface tension of the molten metal plays a critical role in keyhole stability. Variations in surface tension along the keyhole wall, caused by temperature gradients and composition variations, can create Marangoni convection flows that destabilize the keyhole. In hybrid welding, the additional heat input from the TIG arc can alter the temperature distribution in the weld pool, affecting the surface tension balance.

Process Parameters and Keyhole Porosity

Parameter	Effect on Keyhole Stability	Recommended Range
Laser power	Higher power increases keyhole depth but also instability	Optimize for minimum power achieving required penetration
Welding speed	Higher speed reduces heat input but increases keyhole aspect ratio	300–1000 mm/min depending on material thickness
Focal position	Negative focus (below surface) increases keyhole depth	-0.5 to +1.0 mm depending on material
TIG current	Higher current increases arc pressure and plasma disturbance	80–150 A for hybrid mode
Laser-TIG combination	Leading arc configuration is more stable than trailing	Leading arc with 1–3 mm offset
Shielding gas flow rate	Insufficient flow causes oxidation; excessive flow causes turbulence	10–20 L/min primary; 5–15 L/min back

Engineering Practice in Cladding Operations

Keyhole porosity is a critical concern in laser cladding operations, where the quality of the cladding layer directly affects the corrosion resistance and mechanical properties of the final product. In cladding applications:

The cladding layer is typically thin (1–5 mm), making keyhole porosity more detrimental as it can penetrate through the entire cladding thickness
Multi-pass cladding is common, and porosity in the root pass can propagate through subsequent passes
The cladding alloy composition may differ significantly from the base metal, affecting keyhole stability
The surface quality of the cladding layer is often critical for functional applications (e.g., corrosion resistance, wear resistance)

To minimize keyhole porosity in cladding operations, the following measures are recommended:

Optimize the laser power and welding speed combination to achieve a stable keyhole without excessive depth
Use a leading arc configuration where the TIG arc precedes the laser beam, providing preheating and reducing thermal shock
Ensure adequate shielding gas coverage to prevent oxidation and reduce gas entrapment
Control the heat input to avoid excessive melt pool size, which increases the risk of turbulence and porosity
Perform thorough surface preparation to remove contaminants that can contribute to gas evolution during welding

Quality Control and Inspection

Detection of keyhole porosity requires appropriate non-destructive testing (NDT) methods:

Radiographic testing (RT) is effective for detecting keyhole porosity but may miss fine pores in thin cladding layers
Ultrasonic testing (UT), particularly phased array ultrasonic testing (PAUT), offers better sensitivity for detecting planar and elongated defects
Computed tomography (CT) scanning provides three-dimensional visualization of porosity but is limited by cost and accessibility
Metallographic examination remains the most reliable method for characterizing porosity morphology and distribution

In accordance with relevant standards such as GB/T 150 and ASME VIII Div.1, the acceptable porosity level depends on the criticality of the application. For pressure vessels and critical cladding applications, zero tolerance for porosity may be required, necessitating rigorous process control and 100% NDT coverage.

Study Insights and Implications

The comprehensive analysis presented in this paper provides valuable insight into the complex physics governing keyhole porosity formation in hybrid laser-TIG welding. The researchers' identification of multiple interacting mechanisms—keyhole instability, arc-plasma interaction, and surface tension effects—underscores the need for a holistic approach to process optimization.

For cladding engineers, the key takeaway is that keyhole porosity cannot be eliminated by adjusting a single parameter but requires systematic optimization of the entire process window. The interaction between laser and arc parameters, combined with material-specific factors such as thermal conductivity and vapor pressure, creates a complex system that must be carefully controlled.

This research also highlights the importance of process monitoring and control in achieving consistent cladding quality. Real-time monitoring of laser power, welding speed, and arc characteristics, combined with adaptive control algorithms, offers the potential to maintain stable keyhole conditions and minimize porosity formation throughout the welding operation.